Patent Application
20250178893
The present invention relates to compositions and methods for the degradation of polymers and hydrogen production.
Global energy consumption has been increasing rapidly in recent decades due to population and economic growth. According to recent statistics, ˜60% of the U.S. energy requirement is satisfied by fossil fuel reserves such as natural gas and coal, which amplifies greenhouse gas emissions. However, global annual greenhouse gas emissions are targeted to be reduced by 85% by 2050. Even though the conventional eco-friendly renewable energy methods (e.g., wind, thermal, sunlight, and hydropower) contribute to the overall energy demand in the U.S., those methods cannot act as the replacement for fossil fuel production due to the high production and maintenance costs along with the limited supply.
Clean energy has been the focus of significant research with hydrogen being identified as a green alternative energy source. There are several methods of producing hydrogen including electrolysis, thermal processes, biological methods, and photocatalytic processes that use domestic resources like natural gas, biomass, and renewable resources such as wind and solar power.
Hydrogen (“H2”) is a valuable alternative energy source to the increasing energy demand. Hydrogen is identified as a key contributor to future energy systems due to its production capability using renewable energy sources with zero greenhouse gas emissions. With the increasing global warming and climate changes, the green hydrogen economy may be beneficial in terms of its wide application in power generation, transportation, industry, building heating, and energy storage. The current natural gas leakage from the distributed system for the U.S. is estimated as 0.69 Tg-2.9 Tg CH4/year, and it is predicted that replacing natural gas with hydrogen could substantially reduce global warming.
Renewable energy sources such as solar power, biomass, wind, geothermal, and hydropower are used to generate H2 under different technologies. Photonic energy, biochemical energy, thermal energy, and electric energy generated from renewable sources are applied in H2 production systems. Photonic energy from sunlight is applied in photocatalytic and photo-reforming H2 generation processes. Biochemical energy generated utilizing biomass and solar power may be applied in H2 generation by enzymatic, metabolic, anaerobic, and fermentation processes. H2 production from thermochemical, thermolysis, and thermochemical gasification processes are powered by thermal energy generated by solar energy. The electrical energy generated by renewable sources such as wind, geothermal, hydro, and solar power may be applied in H2 generation systems such as electrolysis, H2S cracking, and hybrid thermo-chemical systems.
The most common hydrogen production method is steam reforming of hydrocarbons (methane in natural gas or coal) which is considered grey hydrogen and emits high carbon dioxide amounts to the environment. Blue hydrogen is introduced after applying carbon capture and storage or carbon capture and utilizing technologies to lower greenhouse gas emissions. Hydrogen may also be generated by electrolysis of water, photolysis of water, and thermolysis processes. When such technologies use the energy generated by clean, renewable sources, such as hydro, wind, or solar, the hydrogen is termed “green hydrogen”. Additionally, hydrogen produced using nuclear energy is defined as purple hydrogen, and there are other forms of hydrogen produced using thermochemical processes due to the high temperatures of the nuclear reactor. The turquoise hydrogen is generated by splitting hydrocarbons. The plasma process used for producing carbon black and hydrogen is the most developed turquoise hydrogen production method, and there are other techniques, such as cold plasma, methane catalytic conversion, and molten metal pyrolysis via thermal splitting.
Water electrolysis is categorized into alkaline water electrolysis, proton exchange membrane electrolysis, solid oxide water electrolysis, and alkaline anion exchange membrane water electrolysis. Photocatalysis is considered an attractive process of hydrogen production as it converts solar energy into chemical energy. Photocatalysis is generally divided into up-hill reactions and down-hill reactions, where in down-hill reactions, photon energy absorbed by photocatalysis is used to induce thermodynamically favored reactions, while in up-hill reactions, photon energy is converted into chemical energy. Water splitting into hydrogen and oxygen falls under up-hill reactions (where ΔG0=237 KJ/mol corresponds to 1.23 eV), and is further divided into three groups, namely thermochemical water splitting, photobiological water splitting, and photocatalytic water splitting.
Solar energy is the most significant renewable energy source that may be used in green hydrogen production. Solar thermal, solar photovoltaic, and photoelectrochemical energy are the three primary forms of solar energy that may be utilized as energy sources in hydrogen production systems. The solar spectrum is comprised of <5% ultraviolet (“UV”) light, ˜45% visible light, and ˜50% infrared (“IR”) light. It is desired to extend the light absorbance of catalysts from the UV to visible or IR region to achieve improved solar light utilization. The bandgap of a photocatalyst determines the range of light wavelengths it may absorb. Bandgap engineering is performed to match the bandgap of the catalyst with the solar spectrum. Surface area and morphology, incorporation of cocatalysts such as noble metals, and carbon dots, application of tandem catalysts, incorporation of quantum dots, and surface passivation are considered in the bandgap engineering.
As an energy conversion technology, photocatalytic water splitting using sunlight provides an efficient and promising method. The semiconductor photocatalysis process begins with photon absorption and consequently converts solar energy into chemical fuels. The irradiation of semiconductor particles with incident light of an equal or greater energy than the bandgap of the material excites the valence band (“VB”) electrons to the conductive band (“CB”), a phenomenon called optical excitation. Photo-reforming is a process that harnesses the redox ability of photocatalysts upon illumination, to simultaneously drive the reduction of hydrogen ion (“H+”) into hydrogen gas and the oxidation of organic compounds. Photo-reforming of plastic waste has been identified as an efficient method of recovering H2. Photo-reforming requires four components such as photocatalyst, substrate, sunlight, and water. An additional step is required to recover catalyst from the suspended photoreactive solutions. However, the hybrid photocatalytic membrane process does not require a complicated recovery of photocatalysts after water treatment. This hybrid technology uses stationary nanostructured photocatalysts to enhance the absorption of photons and reactants so that the catalyst does not need to be suspended in solution. In the process of converting solar energy into chemical fuels, the photocatalyst should follow the following sequential main process.
The sun emits radiation ranging from X-rays to radio waves, but the most intense solar radiation occurs in the visible light range (43% of the solar energy reaching the earth's surface is from 400 to 700 nm). Therefore, it is crucial to focus on photocatalysts that may harvest ultraviolet and visible light. When a photon hits the photocatalyst, an electron-hole pair is generated, and the electron moves from the VB to CB. For a photocatalyst to split water and generate hydrogen, it should have an appropriate bandgap and properly located conduction bands and valence bands for oxidation/reduction reactions, and an effective electron-hole pairs separation. The instability caused by the charge pair separation tends to influence the recombination of the electron-hole. The electron-hole recombination may occur at the surface of the catalyst. Hence, the photocatalyst should undergo the charge transport efficiently and effectively to the surface of the catalyst. In the CB site, the H+ extracts the electron and forms an H radical where the site undergoes a further termination reaction by producing H2. The negatively charged microplastics undergo the oxidation process by transferring the charged moiety to the catalyst, thereby completing the photo-reforming cycle.
Photo-reforming is a process to generate green H2 using the redox ability of photocatalysts under illumination. During photo-reforming, photogenerated electrons are used to reduce aqueous H+ ions to produce H2, while generated holes (the vacancy created due to the electron movement from valance to conduction band) trigger oxidation reactions of organic compounds or biomass. One of the significant advantages of photo-reforming reactions is that the catalyst could have a relatively lower valance band energy depending on the organic oxidation reaction pathways despite the bandgap energy to reduce H+ and water oxidization. In contrast, water splitting requires a photocatalyst with sufficient bandgap to energize the abovementioned reactions. This opens a variety of options when selecting the catalyst, including the tunable to the highest intensity natural light frequency, and cost-effective composite catalyst. Additionally, photo-reforming reactions may form superoxide radicals that compete with the H+ reduction reaction in an oxygen-rich environment. In an anoxic environment, oxidation reactions may happen in two pathways, including direct hole transfer and the formation of highly oxidizing hydroxyl radicals. Photo-reforming offers the potential to both provide a supply of H2 and to remove and degrade plastic. Photo-reforming of common plastic wastes, such as polyethylene terephthalate (PET), in an alkaline slurry system, produces green hydrogen using renewable energy and without greenhouse gas emissions.
Photo-reforming efficiency may be optimized by changing the process conditions to increase hydrogen generation. The concentration of dissolved organic matter, reaction temperature, pH of the reactants, reactor configuration (slurry type, packed bed, or thin film), reactor material, irradiation wavelength, and more importantly, an appropriate catalyst with suitable active sites and chemical properties, are main factors that affect the efficiency of hydrogen production. Dissolved organic concentration may be correlated to the kinetics of the reaction oxidation pathway by the oxidation selectivity. Furthermore, the organic concentration in the solution regulates the adsorption capacity of the catalyst. The reaction temperature and contact time are other important parameters that influence the reaction efficiency by regulating the stability of the intermediate species, determining the residence time of adsorbed species remaining on the catalyst surface, and thereby defining the selectivity towards different reaction pathways. The pH of the reaction, which defines the relative charge of the hydrated species, may impact the reaction rate and selectivity towards oxidation products. However, changing the pH will make the reaction selectivity more complex due to the effect on band potentials, adsorption, desorption kinetics, and physicochemical properties.
When designing the reactors for photo-reforming, the adsorption of reactant species onto the catalyst surface has also been shown to influence the efficiency of hydrogen production. The sequence of photocatalytic reactions is initiated when reactants and transient species are adsorbed on the catalyst surface under light irradiation. The photo-absorption, charge separation, and surface charge consumption efficacies are defining factors of the reaction efficiency, which may be optimized by designing the reactor to achieve optimum light exposure while providing an efficient mass transport facility. Hence, the incident photon flux, photon energy, and the light source are also important parameters. Reactor material, its size, dimension, and shape are some other considerations when designing a reactor because the irradiation wavelength absorbed at the catalyst surface may depend on the reactor material as the intermediate medium between the illumination source and the catalyst surface. Hence, quartz, pyrex, and vycor have been previously tested and considered favorable reactor materials.
Understanding the important design parameters of the photo-reforming system assists in addressing the challenges in the hydrogen production system, such as potential competition between H2O oxidation and organic oxidation, the requirement of constant separation of targeted products to prevent overoxidation into undesirable products, and the complexity of adsorption and desorption of reactant, intermediate, and product species.
The removal and degradation of plastic in water pose an environmental challenge. A significant amount of plastics accumulates in the food chain and causes adverse effects on living organisms. Membrane bioreactors, retro filtration, bacterial oxidation, rapid sand filter, disc filter, coagulation process, and magnetic extraction are currently used for plastic removal from water. However, a much greener approach is required for removing and degrading plastics in water.
Photodegradation of polymers is a process of breaking the polymeric chain by absorption of photons induced by the light source. Polymer undergoes both physical and chemical changes through this process. Therefore, activation energy is of prime importance in photodegradation. Specific polymers absorb a certain wavelength at which they become activated when photo-exposed.
The plastics released into the natural water bodies undergo embrittlement, fragmentation, and photodegradation. It takes many years to form microplastics and nano plastics. The microplastics (less than 5 mm) are mainly categorized into primary and secondary microplastics. The primary and secondary microplastics may be further categorized based on the shapes (fibers, films, foams, foils, fragments, pellets, and spheres), the composition, and the chemical composition (acrylic polyamide, polyester, high-and low-density polyethylene (“HD/LD-PE”), polyethylene terephthalate (“PET”), polypropylene (“PP”), polystyrene (“PS”), and polyvinyl chloride (“PVC”)).
The primary microplastics are the raw materials of plastics while the fragmentation of larger plastics due to degradation are considered as secondary microplastics. The plastic microbeads in facial and body cleansers, and synthetic fibers in air and water created through abrasion are the primary sources of microplastics. Further, paints and tire dust are the most common sources that produce tiny plastics, and they release into waterways. Several studies reported that sewage treatment plants are significant sources of microplastics and the sludge or biosolid applications in agricultural systems present a major contribution to microplastics in the environment.
Plastics within the range of 1 nm to 1000 nm are defined as nano plastics that formed because of the breakdown and degradation of microplastics. These particles show a colloidal behavior in water with highly polydispersed physical properties and heterogeneous composition. Dispersion and aggregation of nano-plastics are influenced by the solution chemistry parameters such as pH, divalent cations, and natural organic matter. The nano-sized plastics have higher surface curvature, surface area, and relatively small surface structures which alters their chemical and biological interactions. The main properties that define nano-plastics are summarized in Table 1.
Translocation of microplastics (0.2 mm and 150 mm) across the gastrointestinal tract in the mammalian gut and into the lymphatic system has been demonstrated in studies involving humans. The physical presence of microplastics may be toxic due to their inherent ability to induce intestinal blockage or tissue abrasion. In earthworms (Eisenia ndrei), fibrosis, congestion, and inflammatory infiltrates were observed after exposure to microplastics (from 62 mg/kg to 1000 mg/kg of PE). In sea bass (Dicentrarchus labrax), moderate to severe histopathological alterations of the intestine were measured, after 30 days to 90 days of exposure to PVC microplastic through ingestion. The health impacts may vary with the types of plastics (Table 2).
Toxicity studies of nano-plastics suggest that acute toxicity and physical damage to Daphnia Magna are associated with the solution chemistry parameters and the particle surface modification. Further, the more complex the solution conditions are, the more toxicity the plastic nanoparticle may cause.
A recorded amount of plastics (4.8 to 12.7 million metric tons in 2010) contaminating the sea from the total production of plastics (275 million metric tons in 2010), and the environmental pollution will increase with population due to the broad applications for plastics and the massive amounts of plastic-contained products. Research on the South African coast suggests that the average concentration of microplastics in sea trawls may be 4.01±3.28 plastics/100 m2. When considering freshwater, three major contamination routes are identified, namely, effluent discharged from wastewater treatment plants, overflow of wastewater sewers during high rain events, and runoff from sludge applied to agricultural land. Due to the ineffective removal of plastics during sewage treatment, it is found that 15,000 to 4.5 million microplastics are released into the environment daily.
There is direct evidence that nano-plastics may accumulate in plants, depending on their surface charge. The positively charged nano plastics may accumulate at relatively low levels in the root tips, but these nano-plastics were found to induce a higher accumulation of reactive oxygen species and inhibit plant growth and seedling development more strongly than negatively charged sulfonic-acid-modified nano-plastics. By contrast, the negatively charged nano plastics were observed frequently in the apoplast and xylem. Plant accumulation of nano-plastics may have both direct ecological effects and implications for agricultural sustainability and food safety.
The removal of microplastics from aqueous media has been studied using different technologies such as membrane bioreactors, retro filtration, and bacterial oxidation. Final stage wastewater treatment techniques such as membrane bioreactor, rapid sand filter, and disc filter were studied to identify their suitability to remove microplastics, and treatment efficiencies were reported as 99.9%, 97%, and 95% for membrane bioreactor, rapid sand filter, and disc filter respectively. Filtration was found to be unsuccessful due to clogging and the isolation of particles. Acidic digestion also failed due to excessive solid loads.
The coagulation process, such as Al-based coagulants, was found to be more effective in removing microplastics than Fe-based coagulants. Ionic strength, the concentration of natural organic matter, and turbidity level showed a lower influence on removal efficiency. However, membrane fouling was induced after coagulation with Al-based salts at a conventional dosage, especially for the large particle size. The removal of microplastics by coagulation and ultrafiltration processes indicates application potential for drinking water treatment.
Photocatalytic H2 production technologies have advanced in the last decades. Among the different ways to modify the photocatalyst, namely dye sensitizing, doping, semiconductor compounding, and noble metal deposition, noble metal deposition has attracted considerable attention due to its unique characteristics that greatly improve the photocatalytic properties of semiconductor materials. Metal dopants such as Au and Pt may boost the H2 production efficiency of TiO2 through Schottky barrier formation, surface plasmon resonance, and generation of gap states by interaction with TiO2 valance band states.
The main groups of photocatalysts used for photocatalytic H2 production technologies are titanium dioxide (“TiO2”), cadmium sulfide (“CdS”), zinc oxide/sulfide (“ZnO/ZnS”), and other metal oxide-based photocatalysts. Gold-cadmium sulfide (“Au—CdS”) has the best performance among individual photocatalysts when considering the bandgap, quantum yield, and H2 production rates. However, low light absorption range, electron-hole recombination, slow oxidation, and reduction kinetics, and photo-corrosion are the drawbacks of metal-based photochemical devices. Some semiconductor materials act as photocatalysts, and their properties are shown in Table 3.
Doping is a method to enhance the light absorption spectrum to the visible region for wide bandgap semiconductors and improve their photocatalytic activity. Doping ions are functional in modifying the electronic and optical properties of semiconductors and improving conductivity and charge carriers transport and separation. Metals with very high work function (e.g., platinum (“Pt”), gold (“Au”), palladium (“Pd”)), generally showed better performance as a TiO2 co-catalyst, due to efficient utilization of electrons in the co-catalytic sites. However, high quantities of doping elements could act as recombination centers for photogenerated charges that limit photocatalytic activity.
TiO2 is a prominent material as a photocatalyst because of its high photocatalytic activity, non-toxicity, and biological and chemical inertness. The larger bandgap value (3.2 eV) gives numerous advantages in terms of photocatalytic water splitting. The TiO2 has many polymorphs including anatase, rutile, and brookite. The anatase phase is more photoactive than rutile, but when they are used in combination with specific ratios, the activity is enhanced.
However, TiO2 alone is inadequate for optimal photocatalytic activity due to the fast recombination rates of charge carriers generated at the conduction and valence bands of TiO2 upon photoexcitation. In terms of hydrogen production rate per gram catalyst, gold-titanium oxide (“Au—TiO2”) has the highest rate and the introduction of Au nanoparticles dispersed onto TiO2 has been found to increase reaction efficiency by facilitating electron transfer. Therefore, doping Au into the TiO2 matrix may inhibit the electron-hole recombination and reduce the overpotentials for H2 generation.
The photocatalytic reforming of organics highly depends on the ability of some organic species to donate electrons to the positive holes of the illuminated photocatalyst and be oxidized, generating proton ions, while photogenerated electrons reduce the latter to produce H2. The VB and CB of TiO2 are bent when a noble metal is added because of the formation of a Schottky barrier, arising from the difference in the Fermi levels between the metal and the semiconductor. The higher the work function, the greater the Schottky barrier in the metal-semiconductor heterojunction, resulting in a more efficient charge separation, which is a critical step in photocatalytic processes.
The selection of a proper hole scavenger (sacrificial agent) is crucial in the photo-reforming process since the organic oxidation pathway influences the energy barrier of the reaction. The most common hole scavengers are methanol, ethanol, and glycerol, and biomacromolecules such as monosaccharides, disaccharides, and polysaccharides are used in successful hydrogen production via photo-reforming. Despite higher structure stability and low solubility, carbohydrates, lignin, food waste, cardboard, paper, and plastic waste have also been tested as sacrificial agents for hydrogen production.
Adding organic compounds in the photocatalytic system enhanced the H2 evolution efficiency by contributing to the redox reactions. The organic compounds (sacrificial agents or hole scavengers) act as reducing agents (electron donors) and utilize the photogenerated holes, thus oxidizing similarly to the oxygen generation process in the water-splitting process while promoting H2 generation efficiency. The sacrificial agent may reduce the carbon to hydroxide (“C:OH”) ratio and enhance the photocatalytic activity of the photo-reforming process. Properties of the sacrificial agent, such as polarity and oxidation potential, as well as the surface properties of the catalyst, adsorption, and electron donation ability, affect H2 production efficiency.
In general, photo-reforming of plastics shows lower H2 production due to the polymeric nature of long chain structure, low water solubility, and poor biodegradability. Polyethylene, polystyrene, and isoprene rubber have been reported to produce 25.0 μmol, 19.4 μmol, and 36.7 μmol H2 per gram catalyst per hour (“μmolgcat−1h−1”), respectively, with 10 M NaOH at 70° C. using Pt 0.5% (w/w) deposited CdOx/CdS/SiC composite materials. Furthermore, polyethylene terephthalate and polylactic acid were used as sacrificial agents to produce 111 μmol H2 per gram of substrate (“μmolgsub−1”) and 211 μmolgsub−1 of H2, respectively, using CNx/Ni2P catalyst. Despite low H2 production compared to more easily degradable organic compounds, utilizing waste plastics as sacrificial agents is a cost-effective, versatile, and environmentally friendly option. However, using plastics as sacrificial agents has the drawbacks of limited stability, potential side reactions, requirement of additional pretreatment, and difficulty in separation after the reaction.
The plastic degradation process is important when using them as sacrificial agents because their effect on the efficiency and selectivity of the reaction influences the photo-reforming process. There are several methods of plastic pretreatment, such as hydrolysis, alcoholysis, aminolysis, methanolysis, glycolysis, and pyrolysis, which vary by the reagent and technique used in the process. Among the pretreatment processes, hydrolysis is the widely applied method that may perform under mild conditions and tolerate highly contaminated post-consumer waste while yielding higher amounts of monomers and oligomers with high purity. The rate of hydrolysis depends on the type of functional group, backbone structure, morphology, degree of crystallinity, porosity of the polymer as well as the operating temperature and pH.
Hydrolysis may be further categorized as neutral, acidic, and alkaline hydrolysis. Hydrolysis of polyethylene terephthalate (“PET”), low-density polyethylene (“LDPE”), and polystyrene (“PS”) yields different oligomers and monomers depending on the hydrolysis conditions. PET hydrolysis yields major monomeric compounds of ethylene glycol and terephthalic acid as shown in Structural Formulae (1) and (2). Soluble PET fragments such as mono-(2-hydroxyethyl) terephthalate and bis(2-hydroxyethyl) terephthalate, have been reported. Acidic hydrolysis of PET produces a high yield of terephthalic acid (“TPA”) and affects the purity of ethylene glycol (“EG”) while the neutral hydrolysis of PET affects the purity of TPA. The PET hydrolysis under alkaline conditions reported a higher depolymerization rate (˜95%) under mild conditions. Weight loss of LDPE was studied under alkaline and neutral hydrolysis, and faster degradation and morphology changes were observed under alkaline hydrolysis. LDPE hydrolysis produces alcohol groups (C—OH) by associating a proton and ketone group (C═O) by associating electron. Furthermore, hydroperoxide groups are often evident in LDPE oxidation. Acidic hydrolysis of LDPE yields glutaric acid and succinic acid, and further photo-oxidation leads to liquid byproducts such as carboxylic acid and formic acid. Alkaline hydrolysis of PS produces the styrene monomer by breaking the chain due to its characteristic of the free radical mechanism, and further oxidation of styrene forms substances with carboxyl and carbonyl groups.
The degradation and hydrolysis of PET forms by-products such as bis (2-hydroxyethyl) terephthalate, ethylene glycol, and terephthalic acid, which are valuable and revenue generation materials as shown in Table 4. Ethylene glycol is an important organic compound and chemical intermediate used in many industrial processes such as energy, plastic, automobiles, and chemicals. The global production and consumption of ethylene glycol are about 20 million metric tons in 2010 with an estimated increase of 5%-10% per year. TPA is one of the most important chemicals that is used in the synthesis of PET and has a global annual production of 50 million tons (in 2014) with an annual growth rate of 5%. With the exhaustion of non-renewable resources, including oil, coal, and natural gas, the production of TPA from organic wastes, especially PET waste, has gained significant attention.
There is a necessity to standardize and harmonize methods when treating microplastic contaminated water. Conventional water treatment methods including coagulation, flocculation, and filtration were found to be not fully applicable to address this issue. High energy demand, high cost, and toxic byproduct formation are concerns with traditional treatment methods for microplastic removal. Microplastics as solid waste are still environmentally harmful even with the high-efficient removal from water. Therefore, removal and efficient degradation of microplastics are emerging concerns with a much greener approach. Hence, what is needed is to: develop high-performance photocatalysts that utilize visible light; degrade microplastics in water and quantify the degradability; and photo-reform and produce hydrogen utilizing microplastics in the substrate. The compositions and methods of the present invention improve photo-reforming system efficiency for H2 production through photocatalysis, polymer hydrolysis, and chemical reactions.
Embodiments of the present invention relate to a method for photo-reforming a polymer, the method comprising: contacting the polymer with an aqueous solution comprising a base and an alcohol; elevating the temperature; contacting the polymer with a nanocomposite; adjusting the pH of the aqueous solution to a more acidic pH; contacting the nanocomposite with radiation; and oxidizing the polymer. In another embodiment, the method further comprises adjusting the pH of the aqueous solution to 7. In another embodiment, the method further comprises elevating the temperature to between about 20° C. to about 48° C.
In another embodiment, the nanocomposite comprises a metal oxide semiconductor and a transition metal catalyst. In another embodiment, the transition metal catalyst is contacted with the metal oxide semiconductor in the absence of light. In another embodiment, the metal oxide semiconductor comprises titanium dioxide. In another embodiment, the transition metal catalyst comprises platinum. In another embodiment, the platinum is in its zero oxidation state. In another embodiment, the transition metal catalyst comprises gold. In another embodiment, the alcohol comprises ethanol. In another embodiment, the polymer comprises polyethylene terephthalate. In another embodiment the base comprises hydroxide.
In another embodiment, the method further comprises forming disodium terephthalate. In another embodiment, the method further comprises forming terephthalate ion. In another embodiment, the method further comprises forming terephthalic acid. In another embodiment, the method further comprises forming ethylene glycol. In another embodiment, the method further comprises forming hydrogen. In another embodiment, the ratio of the polymer to the nanocomposite is 1:1. In another embodiment, the ratio of the polymer to the nanocomposite is 2:1. In another embodiment, the polymer and aqueous solution are contacted for between about 3 hours to about 6 hours.
Further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
Embodiments of the present invention relate to a composition for producing hydrogen and/or polymer degradation. The composition may comprise a semiconductor, and a metal, wherein single metal atoms or metal clusters are attached to the surface of the semiconductor. The metal may include, but is not limited to, platinum, zinc, gold, or a combination thereof. The single metal atoms and/or plurality of metal atoms may not be in contact with another metal atom and/or plurality of metal atoms.
Embodiments of the present invention also relate to a method of hydrogen production, the method comprising: a semiconductor; and a plurality of metal atoms attached to a surface of the semiconductor, wherein each of the plurality of metal atoms is not in contact with another metal atom attached to the surface of the semiconductor. The plurality of metal atoms may comprise a platinum group metal, a noble metal, a precious metal, a base metal or a combination thereof. The metal may comprise platinum, gold, zinc, or a combination thereof. In another embodiment, the platinum group metal comprises platinum. The semiconductor may comprise a metal oxide, a silicon oxide, silicon, germanium, nitrides, sulfides or a combination thereof. The semiconductor may comprise titanium oxide.
Embodiments of the present invention also relate to method for making a nanocomposite, the method comprising: heating a semiconductor nanoparticle; contacting the semiconductor with a first alcohol; contacting the semiconductor with a metal in the absence of light to attach the metal to the semiconductor; and drying the semiconductor. The metal may further comprise contacting the metal attached to the semiconductor with a second alcohol. The first alcohol may comprise methanol. The second alcohol may comprise ethanol. The metal may comprise chloroplatinic acid (“H2PtCl6”). The metal may be attached to the semiconductor as an individual metal atom or plurality of metal atoms.
Embodiments of the present invention also relate to method for pretreating a polymer, the method comprising: contacting a polymer with a reagent to form a polymer and reagent solution; adjusting the temperature of the polymer; and adjusting the pH of the polymer and reagent solution. The reagent may comprise a base. The base may comprise potassium chloride, sodium chloride, ethanol, sulfuric acid, or a combination thereof. The adjusting the temperature may comprise increasing the temperature. Adjusting the pH of the polymer and reagent solution may comprise adjusting the pH to a neutral pH or an acidic pH.
Embodiments of the present invention relate to a method of hydrogen production, the method comprising: contacting a polymer with a nanocomposite; and contacting the polymer with radiation. The method may further comprise pretreating the polymer. The method may further comprise degrading and/or photo-reforming the polymer.
The method may use polymer waste as a resource to generate hydrogen by using a photo-reforming process. The method may be driven by high wavelength energy from solar and/or UV energy and nanocomposites used as the catalyst to increase the efficiency of the process. The production of H2 and degradation of a polymer may yield oxidizing and/or other chemical conditions that may disinfect and/or remove biological and/or organic matter from the environment.
The composition and method may be used for multiple purposes, including but not limited to, treatment of hydrocarbon spills; removal of pathogens from plasma, agricultural products, and/or medical-grade reagents; removal of polymers in blood; removal of biological matter from sewage for wastewater treatment; removal of polymers from an aqueous solution, including, but not limited to, salt water, fresh water, grey water, sea water, produced water, brines, drinking water, or a combination thereof.
The terms “nanocomposite”, “nanocomposite catalyst”, “catalyst”, and “photocatalyst” are used interchangeably in the specification and drawings.
The terms “degrade” and “photo-reform” are used interchangeably in the specification and drawings and are defined to mean to change and/or break down the carbon-hydrogen bonds, carbon-oxygen bonds, and/or hydrogen-oxygen bonds of a polymer.
The terms “metal” or “metals” are defined herein as a compound, mixture, or substance comprising a metal atom. The term “metal” or “metals” includes, but is not limited to, metal hydroxides, metal oxides, metal salts, elemental metals, metal ions, non-ionic metals, minerals, or a combination thereof.
The terms “polymer” or “polymers” as used herein means a substance that has a molecular structure comprising a plurality of similar units bonded together. The polymer may comprise, but is not limited to acrylic polyamide, polyester, high-and low-density polyethylene (“HD/LD-PE”), polyethylene terephthalate (“PET”), polypropylene (“PP”), polystyrene (“PS”), and polyvinyl chloride (“PVC”), terephthalic acid (“TPA”), ethylene glycol (“EG”), or a combination thereof.
The terms “elevating the temperature” or “elevated the temperature” as used herein mean to raise the temperature above ambient temperature. For example, the ambient temperature may be around the standard room temperature of approximately 20° C.
Turning now to the figures,
In
The nanocomposite may comprise a metal and semiconductor. The semiconductor may comprise a metal oxide, a metal sulfide, a graphene oxide, a silicon oxide, silicon (“Si”), germanium (“Ge), or a combination thereof. The metal may comprise a precious metal, a noble metal, a base metal, a transition metal, a platinum group metal, or a combination thereof. The metal may include, but is not limited to, gold (“Au”), platinum (“Pt”), copper (“Cu”), iron (“Fe”), or a combination thereof. The metal may be attached to the surface of the semiconductor including but not limited to, the metal oxide, a metal sulfide, a graphene oxide, a silicon oxide, silicon (“Si”), germanium (“Ge”), or combination thereof. The metal may also be disposed into a lattice of the metal oxide, a metal sulfide, a graphene oxide, a silicon oxide, Si, Ge, or combination thereof. The metal oxide may include, but is not limited to, TiO2, ZnO2, or a combination thereof. The nanocomposite may include, but is not limited to, Au/TiO2, Pt/TiO2, Pt/ZnO, or a combination thereof. The nanocomposite may act as a photocatalyst.
The polymer may comprise carbon-hydrogen bonds. The polymer may also comprise polymer chains. The polymer may include, but is not limited to, low-density polyethylene (“LDPE”), high-density polyethylene (“HDPE”), PET, PS, or a combination thereof. The polymer may be derived from plastic waste, including, but not limited to, beverage bottles, low-density polyethylene bags, polystyrene plates, or a combination thereof. The average molecular weight for the polymer may be at least about 20,000 g/mol, about 20,000 g/mol to about 30,000 g/mol, about 21,000 g/mol to about 29,000 g/mol, about 22,000 g/mol to about 28,000 g/mol, about 23,000 g/mol to about 27,000 g/mol, about 24,000 g/mol to about 26,000g/mol, or about 30,000 g/mol. The polymer may be washed with deionized (“DI”) water and/or ethanol, and/or air-dried before pretreatment.
The radiation may comprise solar radiation, UV radiation, concentrated solar radiation (e.g., solar radiation from a solar concentrator), visible light radiation, or a combination thereof.
The method of producing a nanocomposite catalyst may comprise contacting a precursor with a semiconductor. The precursor may comprise a metal including, but not limited to, H2PtCl6·6H2O. The precursor may be directly deposited onto the semiconductor using a precipitation method and/or dark (e.g., in the absence of light) deposition method. The metal from the precursor may be deposited onto the semiconductor as an individual metal atom or plurality of metal atoms. The precursor may be at a concentration of at least about 0.001 mM, about 0.001 mM to about 0.1 M, about 0.01 mM to about 0.05 mM, or about 0.1 mM. The loading percentages of the metal may be at least about 0.1% (w/w), about 0.1% (w/w) to about 8% (w/w), about 0.3% (w/w) to about 6% (w/w), about 0.7% (w/w) to about 1.5% (w/w), or about 8% (w/w).
The precursor dosage may be controlled to deposit the single metal atoms on the semiconductor surface. High precision and quality in deposition, minimal noble metal loading, and maximal surface-to-volume ratio may be advantages of the dark deposition method. Synthesis under direct deposition may avoid solvent contamination and may produce uniform-quality, pure nanoparticles with high crystallinity.
Dark direct deposition of single atom deposition may comprise heating metal oxide nano powder. The nano powder may be heated to at least about 400° C., about 400° C. to about 600° C., about 450° C. to about 550° C., or about 600° C. The nano powder may be contacted with a MeOH solution comprising a precursor. The nano powder and precursor may be mixed in the dark and may be mixed for at least about 12 hours, about 12 hours to about 36 hours, about 18 hours to about 32 hours, about 22 hours to about 28 hours, or about 36 hours to form a nanocomposite. The nanocomposite may be washed and/or dried. The nanocomposite may be washed with ethanol.
Dark direct deposition may improve the electron storage capacity of the catalyst by trapping electrons. Trapped electrons may reduce the cocatalyst ions into their metallic form and may allow for the growth and aggregation of the deposited single atoms to be controlled. The deposited single atoms may be uniformly distributed onto the semiconductor surface.
The photocatalytic oxidation ability may be improved by adjusting the pH of a solution comprising the polymer and the nanocomposite. The pH may be adjusted to from basic to neutral, acidic to neutral, neutral to basic, neutral to acidic, or a combination thereof when the nanocomposite is in contact with the polymer.
The nanocomposite may produce H2 at a constant rate for at least about 5 days, about 5 days to about 50 days, about 10 days to about 45 days, about 15 days to about 40 days, about 20 days to about 35 days, about 25 days to about 30 days, or about 50 days. The concentration of hydrogen produced may be at least about 4000 μmol, about 4000 μmol to about 10000 μmol, about 5000 μmol to about 9000 μmol, about 6000 μmol to about 8000 μmol, or about 10000 μmol after five days. The nanocomposite may also produce oxygen. The ratio of hydrogen to oxygen produced may be at least about 40:1, about 40:1 to about 10:1; about 30:1 to about 20:1, or about 10:1.
Pt4+ may be reduced to Ptδ+ (surface trapped) and Ti3+ may be oxidized to Ti4+. Attached Ptδ+ may act as an electron relay, aiding electron trapping and promoting H2 formation.
The semiconductor with attached Ptδ+ may act as a catalyst without catalytic loss over time. Ptδ+ (approximately Pt2+) may act as an electron sink (intermediate Pt red-ox states may act as an electron sink in relation to the TiO2 conduction band).
Pt4++2e−→Pt2+ (1),
Ti3+→Ti4++e− (2),
Ptδ++e−→Ptδ−1 (3),
Ptδ−1+H+→2 1/2H2+Ptδ+ (4).
The polymer to nanocomposite ratio may be at least about 0.5:2:5, about 1:2, about 1:1, about 2:1, or about 2.5:0.5. The polymer to nanocomposite ratio may be maintained regardless of polymer type and/or PH level.
The polymer may be pretreated before being contacted with the nanocomposite catalyst. The polymer may be of any size including, but not limited to, at least about 1 μm×1 μm to about 1 m+ to 1 m, about 1 mm×1 mm to about 1 dm×to 1 dm, or about 1 μm×1 μm to about 1 m+ to 1 m in width and length. The polymer may be pretreated by exposing the polymer to basic conditions. Pretreating the polymer may comprise contacting the polymer with a caustic including, but not limited to, NaOH, KOH, or a combination thereof. The molarity of the caustic may be at least 1 M, about 1 M to about 10 M, about 2 M to about 9 M, about 3 M to about 8 M, about 4 M to about 7 M, about 5 M to about 6 M, or about 10 M. Optionally, the polymer may be pretreated under neutral conditions and/or without contacting the polymer with a caustic. The polymer may be pretreated for at least about 1 hour, about 1 hour to about 48 hours, about 4 hours to about 44 hours, about 8 hours to about 40 hours, about 12 hours to about 36 hours, about 16 hours to about 32 hours, about 20 hours to about 28 hours, or about 48 hours. The polymer may be pretreated in a closed system. The polymer may be pretreated at a temperature of at least about 25° C., about 25° C. to about 700° C., about 50° C. to about 600° C., about 100° C. to about 500° C., 200° C. to about 500° C., 300° C. to about 400° C., or about 700° C. with or without mixing. The pretreated polymer may be kept in the dark until used for photo-reforming by contact with the nanocomposite catalyst and UV and/or visible radiation. Pretreatment of the polymer may break down long-chain polymers.
The polymer may be hydrolyzed by pretreatment. Hydrolysis may occur under alkaline or acidic pretreatment. Hydrolysis of the polymer degrades to polymer to increase hydrogen production by the nanocomposite, which further degrades to the polymer to produce hydrogen.
The polymer may be contacted with a base to be pretreated. The base may be at a concentration of at least about 2 M, about 2 M to about 12 M, about 4 M to about 10 M, about 6 M to about 8 M, or about 12 M. The polymer and/or base may be at a temperature of at least about 20° C., about 20° C. to about 50° C., about 25° C. to about 45° C., about 30° C. to about 40° C., or about 50° C. The polymer may be hydrolyzed by contact with a base.
The polymer may be contacted with ethanol to be pretreated. The polymer may be contacted with ethanol and/or a base including, but not limited to, sodium hydroxide (“NaOH”). The polymer, base, and/or ethanol may be at a temperature of at least about 20° C., about 20° C. to about 40° C., about 21° C. to about 39° C., about 22° C. to about 38° C., about 23° C. to about 37° C., about 24° C. to about 36° C., about 25° C. to about 35° C., about 26° C. to about 34° C., about 27° C. to about 33° C., about 28° C. to about 32° C., about 29° C. to about 31° C., or about 40° C. The base may be at a concentration of at least about 1% (w/w), about 1% (w/w) to about 10% (w/w), about 2% (w/w) to about 9% (w/w), about 3% (w/w) to about 8% (w/w), about 4% (w/w) to about 7% (w/w), about 5% (w/w) to about 6% (w/w), or about 10% (w/w). The polymer may be hydrolyzed by contact with ethanol.
The polymer may be contacted with an acid to be pretreated. The acid may be at a concentration of at least 0.5 M, about 0.5 M to about 2 M, about 1 M to about 1.5 M, or about 2 M and form a polymer and acid mixture. The pH of the mixture may be adjusted to pH 7 and may be adjusted by adding caustic. The polymer and acid mixture may be contacted with Na2TP. The Na2TP may catalyze acidic hydrolysis. The polymer and/or acid may be at a temperature of at least about 20° C., about 20° C. to about 30° C., about 21° C. to about 29° C., about 22° C. to about 28° C., about 23° C. to about 27° C., about 24° C. to about 26° C., or about 30° C. The polymer may be hydrolyzed by contact with an acid.
Pretreated polymers may comprise unmodified and/or more stable functional groups. Bands corresponding to the more stable functional groups, such as the terephthalic group and stretching of the C═O group, may remain the same despite their absorbance changes. The stability of the C═O group may be analyzed by calculating the carbonyl index (CO index). Exemplary analysis is presented in Table 5. The CO index depends on the absorbance of the carbonyl group band and the methylene scission band. The CO index is predicted to increase if degradation occurs during the pretreatment since the formation of new acid end groups and scissoring of CH2 bonds are expected. Table 5 shows no considerable difference in the carbonyl indexes after pretreatment of the PET compared to virgin PET particles. Therefore, significant degradation is not observed after pretreatment of the PET.
An Au/TiO2 nanocomposite may be synthesized by doping a TiO2 nanoparticle with a gold nanoparticle. The gold nanoparticle may be at least partially disposed on the surface of the TiO2 nanoparticle using a chemical deposition method. An Au solution may be prepared using chloroauric acid (“HAuCl4”). The molarity of the Au solution may be at least about 1×10−3 M to about 1×10−7, about 1×10−4 M to about 1×10−6, or about 1×10−4 M. The pH of the Au solution may be at least about 4, about 4 to about 13, about 5 to about 12, about 6 to about 11, about 7 to about 10, about 8 to about 9, or about 13. The pH may be adjusted using caustic including, but not limited to, NaOH. The Au loading and/or doping may be at least about 0.1%, about 0.1% to about 10%, about 0.5% to about 9%, about 1% to about 8%, about 2% to about 7%, about 3% to about 6%, about 4% to about 5%, or about 10% (w/w). TiO2 nanoparticles may be contacted with the prepared Au solution to form an Au and TiO2 solution. The Au and TiO2 solution may be at a concentration of at least about 0.1 g/50 ml, about 0.1 g/50 ml to about 10 g/50 ml, about 0.5 g/50 ml to about 9 g/50 ml, about 1 g/50 ml to about 8 g/50 ml, about 2 g/50 ml to about 7 g/50 ml, about 3 g to about 6 g/50 ml, about 4 g/50 ml to about 5 g/50 ml, or about 10 g/50 ml. The Au and TiO2 solution may be stirred and/or maintained at a temperature of about 293 K. The pH of Au and TiO2 solution may be about 8 to about 10, or about 9. The Au and TiO2 solution may be formed into a suspension and be left at room temperature and optionally stirred for up to 24 hours. The Au and TiO2 suspension may also be heated to at least about 300 K, about 300 K to about 575 K, about 325 K to about 550 K, about 350 K to about 525 K, about 375 K to about 500 K, about 400 K to about 475 K, or about 575 K, and optionally stirred for up to 24 hours to form the Au/TiO2 nanocomposite. The Au and TiO2 suspension may be cooled to the room temperature, filtered, and solids may be removed with a solvent including, but not limited to, deionized water. The filtered and washed Au and TiO2 suspension may be dried, e.g., by vacuum-drying for up to 48 hours or until dry. The drying may occur at room temperature.
A Pt/TiO2 nanocomposite may be synthesized by doping a TiO2 nanoparticle with a platinum nanoparticle. The platinum nanoparticle may be at least partially disposed on the surface of the TiO2 nanoparticle using a chemical deposition method. A Pt solution may be prepared using H2PtCl6 solution. The molarity of the Pt solution may be at least about 1×10−3 M to about 1×10−7, about 1×10−4 M to about 1×10−6, or about 1×10−4 M. The pH of the Pt solution may be at least about 7, about 7 to about 12, about 6 to about 11, about 7 to about 10, about 8 to about 9, or about 12. The pH may be adjusted using caustic including, but not limited to NaOH. The Pt loading and/or doping may be at least about 0.1%, about 0.1% to about 10%, about 0.5% to about 9%, about 1% to about 8%, about 2% to about 7%, about 3% to about 6%, about 4% to about 5%, or about 10% (w/w). The Pt solution may be contacted with dry TiO2 to wet the TiO2 to form a Pt and TiO2 slurry. The slurry may be left at room temperature and optionally stirred for up to 24 hours. The Pt and TiO2 slurry may also be heated to at least about 300 K, about 300 K to about 575 K, about 325 K to about 550 K, about 350 K to about 525 K, about 375 K to about 500 K, about 400 K to about 475 K, or about 575 K, and optionally stirred for up to 24 hours to form the Pt/TiO2 nanocomposite. The slurry may be dried. Drying may occur at a temperature of at least about 30° C., about 30° C. to about 75° C., about 35° C. to about 70° C. about 30° C. to about 50° C. about 30° C. to about 50° C. for up to 24 hours or until dry. Drying may be performed by an air oven, vacuum desiccator, or a combination thereof.
A Pt/ZnO nanocomposite may be synthesized by doping a ZnO nanoparticle with a platinum nanoparticle. The platinum nanoparticle may be at least partially disposed on ZnO nanoparticle surface using a chemical deposition method or within a lattice of ZnO nanoparticles. A reaction solution may be prepared by contacting zinc acetate (“Zn(CH3COO)2”)·2H2O with methanol at a temperature of up to 60° C. A methanolic solution of KOH may be added to the reaction solution to form ZnO nanoparticles. A Pt solution may be prepared using H2PtCl6 solution. The Pt solution may be added to the ZnO dispersed nanoparticles. The Pt and ZnO solution may be contacted with a sodium borohydride (“NaBH4”) aqueous solution to form the Pt/ZnO nanocomposite.
Zn-doped TiO2 nanoparticles, e.g., the nanocomposite, may also be synthesized by sol-gel method. The sol-gel method may comprise forming a solution of ethanol (“EtOH”) and isopropanol and contacting the solution with titanium (IV) butoxide. The solution may then be contacted with an aqueous solution of zinc nitrate hexahydratecetyltrimethylammonium bromide (“CTAB”) to form a nanocomposite. Heat may be applied to the solution and excess solution may be evaporated from a water bath with continuous stirring. The nanocomposite may then be dried.
The method may produce a byproduct. The byproduct may comprise disodium terephthalate (“Na2TP”) and terephthalic acid (“TPA”) after the pretreatment of a polymer. The byproducts may be extracted and re-applied in the photo-reforming (hydrogen production) method. Hydrogen may be produced from byproducts acting as polymers according to the methods described herein. The byproduct may be at a concentration of at least about 35 mg/g, about 35 mg/g to about 75 mg/g, about 40 mg/mg to about 70 mg/g, about 45 mg/g to about 65 mg/g, about 50 mg/g to about 60 mg/g, or about 75 mg/g of polymer.
The semiconductor photocatalysts are capable of absorbing visible light and producing H2 from water. Titanium oxide (TiO2) is a promising catalyst with broad applications, but it requires UV light to activate the redox reaction process. Au and Pt are considered as promising doping materials for TiO2 to produce H2 from water in the photo-reforming process. Light flux may be measured using a solar power meter.
The nanocomposite may degrade a polymer in the presence of radiation. The surface morphology of the polymer may be changed to display cracks that may increase with increased irradiation and/or time. The polymer edges may be eroded due to catalytic activity. A change of color of the polymer may occur and/or the transparency of the polymer may be reduced.
The size of the semiconductor including, but not limited to, the metal oxide, a metal sulfide, a graphene oxide, a silicon oxide, or combination thereof, may be at least about 8 nm, about 8 nm to about 50 nm, about 10 nm to about 45 nm, about 15 nm to about 40 nm, about 20 nm to about 35 nm, about 25 nm to about 30 nm, or about 50 nm. The size of the metal may be at least 1 atom, about 1 atom to about 10 nm, about 1 nm to about 9 nm, about 2 nm to about 8 nm, about 3 nm to about 7 nm, about 4 nm to about 6 nm, or about 10 nm.
A polymer in the presence of the nanocomposite catalyst may be contacted with visible and/or UV radiation. Polymer contacted in the presence of the nanocomposite catalyst and contacted with visible and/or UV radiation may generate hydrogen (“H2”), oxygen (“O2”), nitrogen (“N2”), or a combination thereof. H2 may be generated after visible and/or UV radiation.
Embodiments of the present invention provide a technology-based solution that overcomes existing problems with the current state of the art in a technical way to satisfy an existing problem for plastic waste removal from aqueous solutions and for hydrogen production. Embodiments of the present invention achieve important benefits over the current state of the art, such as plastic waste removal, polymer degradation, and/or hydrogen production at ambient temperature and pressure using solar light. Some of the unconventional steps of embodiments of the present invention include a nanocomposite with individual metal atoms attached to a semiconductor and method using a nanocomposite to degrade polymers while simultaneously producing hydrogen gas with polymer pretreated in dark conditions.
The invention is further illustrated by the following non-limiting examples. The materials and methods used in examples 1 to 21 are summarized in Table 6 and further discussed in the following sections.
Table 6 shows the methods used including, but not limited to, PET, transmission electron microscopy (“TEM”), X-ray diffraction spectroscopy (“XRD”), attenuated total reflectance furrier transformation spectroscopy (“ATR-FTIR”), X-ray photoelectron spectroscopy (“XPS”), and gas chromatography with thermal conductivity detector (“GC-TCD”).
All the materials used in examples 1 to 21 are analytical-grade chemicals. Details of the chemicals are listed in Table 7.
An Au/TiO2 catalyst was synthesized. Gold nanoparticles were doped on a TiO2 nanoparticle surface using the chemical deposition method. The 1×10−4 M Au solution was prepared using HAuCl4 (99.99% assay), and its pH was adjusted to 13 (from 4) by using a 0.5 M NaOH solution. TiO2 nanoparticles were added to the prepared Au solution until the Au loading was 5% (w/w). The Au and TiO2 solution (1 g/50 ml solution) was stirred at 300 RPM using a magnetic stirrer and the temperature was maintained at 293 K. The pH was monitored before and after adding the TiO2 for each sample preparation. The final pH was maintained at 9 by adjusting the use of the 0.5 M NaOH solution. The resulting suspension was heated to 343 K and vigorously stirred at 300 RPM for 1 hour. After cooling the solution to the room temperature the solution was filtered and the solids were washed thoroughly with 2 L of deionized water (DI) and then vacuum-dried at room temperature for 24 hours.
A Pt/TiO2 catalyst was synthesized. Hexachloroplatinic acid solution (H2PtCl6) with a molarity of 1×10−4 M was prepared and solution PH was raised to 9 by adding 3 M NaOH. The 50 ml of the salt solution of the metal for doping was added to the 1 g of the catalyst (P-25 TiO2) in such a way that they wet the catalyst completely. The slurry was stirred for 24 hours at ambient temperature and dried in an air oven at 50° C. for 12 hours. The dried powder was then cooled to room temperature and the prepared catalysts were stored in a vacuum desiccator.
A Pt/TiO2 was synthesized using a modified atomic deposition technique to create a uniform Pt distribution on the P25 Degussa TiO2 surface with Pt loading percentages (w/w) of 0.3%, 0.7%, 1.5%, and 6%. This approach significantly enhances charge separation efficiency. The loading percentages were tuned to generate atomic deposition of Pt on TiO2 and consider synthesizing more economical catalysts. Pt/TiO2 catalyst was synthesized using hexachloroplatinic acid solution (8 wt % H2PtCl6 in H2O) with a volume of 4.7, 23.5, 62.5, and 250 μL and H2PtCl6 was added to 200 mL of deionized (DI) water to obtain 0.3%, 0.7%, 1.5%, and 6% Pt loading percentages (w/w) on TiO2, respectively. The solution pH was increased to 10 by adding 3 M NaOH (ACS reagent). Then, the solution temperature was raised to 100° C., and 1 g of the P25 TiO2 (P25 with 21 nm primary particle size, ≥99.5%) was added. The solution mixture was agitated at a speed of 400 rpm for 3 h and then was cooled down to ambient temperature. Then, the mixture was filtered, vacuum-dried, and stored in a vacuum desiccator.
A Pt/ZnO catalyst was synthesized. In a typical synthesis procedure, Zn(CH3COO)2·2H2O (4 mmol) was added to methanol (70 mL) in a 100 mL round bottom flask, and the temperature was raised to 60° C. After a few minutes, methanolic solution of KOH (500 mg in 10 mL) was added dropwise to the reaction solution and the stirring (300 rpm) was continued for 2 hours at 60° C. The color of the solution became turbid at the initial stages and became colorless in 30 minutes. In 2 hours, the solution slowly turned to a white color (the particle size also depends on the size of the magnetic bead and rpm of stirring). Formed ZnO nanoparticles were precipitated out by the addition of water and excess zinc ions were removed by centrifugation. For loading of platinum nanocrystals, 2.3 mL of (8.66 mM) H2PtCl6 aqueous solution was added dropwise to the preformed ZnO nanocrystals dispersed in a fresh methanolic solution, followed by 0.5 mL of NaBH4 (40 mM) aqueous solution. The solution was stirred for 10 minutes and the solution color became black due to the formation of platinum on ZnO nanoparticles. Excess zinc ions were removed by centrifugation and the solid product was separated and used in the photodegradation experiment after drying them at 50° C. for 24 hours.
The photocatalytic activity of catalysts was evaluated. Photocatalytic activity of the prepared catalysts was measured using 30 mg/L Rhodamine B solution. Rhodamine B was used as an indicator organic compound which originally gives a pink color and is colorless after degradation. About 0.05 g of catalysts were suspended in 50 ml of DI water and ultrasonicated for 10 minutes. 10 ml of Rhodamine B solution (30 mg/L) was then added into the suspensions and irradiated by UV light for 1 hour. 10 ml samples were collected in 15-minute intervals. Change of the solution color as an indicator of photocatalytic degradation of the dye solution was quantified using an ultraviolet-visible spectrophotometer (UV-vis). The absorbance at 554 nm was measured to quantify the Rhodamine B degradation using a single wavelength scan of the UV-vis spectroscopy.
The structure and morphology of the TiO2 and TiO2 doped with Au were characterized by a TEM. The samples for examination by TEM were prepared by placing the dry catalyst powder on a holey carbon film supported by a 300-mesh copper TEM grid. TEM imaging showed the shape, size, lattice fringes and calculation of particle size histograms of Au/TiO2. TEM analysis also allowed for the measurement of the nano-particle size. This may be very useful for the characterization of a catalyst where particle size is a key factor. Once the nanoparticle's diameter is determined, a particle size histogram may be created which displays the number and size of particles of each size, as well as allow for the generation of average particle size and standard deviation.
XRD technique is based on the Bragg's law:
where λ corresponds to the wavelength of the x-ray radiation, d is the interplanar distance, and θ is the angle of diffraction. XRD technique was used to determine the composition of the TiO2 sample using the characteristic peaks given for different phases of TiO2 and to clarify any other components present using their respective peaks. Change in the peak intensities, peak areas, as well as the absence of certain peaks, indicated the crystal structure of TiO2 and its surface modification with Au. Intercalated structures resulted in increased d spacing. This caused the angle θ to shift to lower values due to the inverse relationship with d. In contrast, the Au doping on TiO2 structures was characterized by the presence of new peaks.
Characterization was performed using a CuKα radiation (λ=1.54 Å) XRD diffractometer. One gram of TiO2 and Au/TiO2 samples were placed on the cradle using a glass slide with double sticky tape and scanned at 0.039 deg/s rate in reflection over the range of 2θ=5°-80°. The intensity/CPS was plotted against the diffraction angle (2θ) in degrees. The d values for peaks were calculated using the VESTA software. The resulted XRD spectra were compared with the data available in the literature.
A wavelength scan of the catalysts (Au/TiO2 and pure TiO2) within the range of 190 nm to 800 nm was done. 0.05 g of catalyst was dispersed in 50 ml of DI water and a full wavelength scan was done using UV spectrophotometer. Depending on the wavelength scan results, bandgap analysis was done, thus the feasibility of the catalytic activity was evaluated. Tauc plot method was used in the calculations and analysis.
XPS analysis of the P25 TiO2 and Au/TiO2 nanoparticles was conducted. All survey spectra were collected in constant analyzer energy mode with an analyzer pass energy of 160 eV. Individual regions were collected at an analyzer pass energy of 20 eV using the hybrid lens mode which features a combination of electrostatic and magnetic immersion electron collection optics. The maximum analysis area is 700 μm×300 μm using the slot aperture. Photoelectrons are amplified and detected using a microchannel plate stack in a z-configuration with a position-sensitive delay-line detector divided into 127 channels. Typical base pressure during analysis is <5×10−9 torr. Charge compensation/neutralization of surface charging was done using a low kinetic energy electron flux to the surface of the sample.
All fittings were done using processing software. Background subtraction was done using linear or integral backgrounds as well as peak fitting typically using a combination of Gaussian and Lorentzian hybrid models. Samples were embedded in carbon tape in a thin layer to maximize contact with the carbon. Some charge shift is in the raw data but the C 1 s was used to correct the energy scale to C 1 s at 284.6 eV binding energy (adventitious carbon).
Characterization of plastic degradation using ATR-FTIR was performed. In FTIR transmittance bands were observed due to molecular vibrations. These vibrations are of two types, stretching and bending. FTIR spectra for plastics vary from before and after the reaction. Therefore, it was used as a fingerprint to identify the type of reaction that plastic undergoes. Transmittance bands were observed for various vibrations that are characteristic of the plastics and the new band that occurs due to the photo-reformation. This reflected through various bands characteristic to the functional groups in the material.
The IR spectra of TiO2 and Au/TiO2 were obtained. All samples were well powdered and placed on the crystal and the compression clamp was used to obtain good contact between the sample and the crystal to obtain the spectra.
Plastic polymers were pre-treated. The PET particles of size 2 mm×2 mm were used as the polymer in the experiments. The 50 mg of PET was soaked in 1.0 ml of 2 M KOH solution in a sealed vial for 24 h at 40° C. with stirring at 300 RPM. The solution was kept in the dark until used for photo-reforming as below.
A photo-reforming process was performed. Five hundred mg of catalyst was dispersed in 50 ml of DI water and ultrasonicated (pulses of 30 s at 100% amplitude followed by 5 s paused) for 10 minutes. Then 500 mg of pre-treated polymer in 3 M aqueous semiconductor grade KOH was added to the sample. The temperature was maintained at 273 K and pH was maintained at 9. Equations 7 and 8 describe the reactions of the PET hydrolysis and photo-reforming of EG.
PET hydrolysis: C10H8O4+2H2O→C2H6O2+C8H6O4, ΔG°=66 KJ mol−1 (6),
Photo-reforming of EG: C2H6O2+2H2O→5H2+2CO2, ΔG°=9.2 KJ mol−1 (7)
A 50 ml quartz round bottom flask was used in UV irradiation and the used light sources include a high-pressure UV mercury vapor lamp (160W PUV-10), a low-pressure UV lamp (39W T5), and a visible light source provided by a fluorescent lamp (40W F40T12/DX). The irradiance of the high-pressure UV lamp concentrates on both UV (minor peaks at 290 nm, 315 nm, 335 nm, and a dominant peak at 365 nm) and visible light (405 nm, 435 nm, and 545 nm) wavelength ranges. The irradiance of a low-pressure UV lamp is primarily at 253.7 nm. The experiment was carried out for 60 hours.
Plastic degradation was characterized. The plastic degradation and characterization of PET were carried out using an optical microscope and FTIR analysis. Surface morphology of the plastics before and after UV irradiation was observed at 4×, 10×, 20×, and 40× magnification.
The accumulation of H2 was measured via a gas chromatograph equipped with a thermal conductivity detector and HP-5 molecular sieve column using N2 as the carrier gas. The instrument was calibrated using the known source of pure H2. Then 1 mL of headspace gas was directly injected into the inlet of the gas chromatograph. Purge time was selected as 1 minutes and chromatography was collected for 10 mins after injection. The hydrogen production at 18 h, 24 h, 30 h, 42 h, and 48 h was measured.
The performance of the synthesized catalysts was investigated under different pH conditions, an operating parameter not extensively explored in previous studies. The impact of pH on hydrogen production efficiency was studied using a 1:1 polymer-to-catalyst mass ratio for both the Au/TiO2 and Pt/TiO2 catalysts. Two different tests at pH 7 and ambient solution pH (>13) without adjusting pH were conducted. The effect of the polymer-to-catalyst ratio was analyzed in the system by testing three different ratios, 1:1, 2:1, and 1:2. When changing the polymer-to-catalyst ratios, pH remained constant at >13 without further adjustment. Each of the above tests was continued for 48 h with a duplicate while exposing the samples to high-pressure UV mercury vapor lamp irradiation. The maximum yield of H2 production from the photo-reforming process was calculated via mass balance considering the ethanol-induced pretreatment of PET to compare experimental and theoretical yields.
Photo-degradability of different catalysts were evaluated. Photocatalytic activity of synthesized Au/TiO2, Pt/TiO2, and Pt/ZnO was tested to evaluate their performance under UV light. Rhodamine B solution was used as an indicator organic compound and absorbance of the samples was measured after UV irradiation. Results were compared with the degradation of Rhodamine B when unmodified TiO2 was used as the catalyst under similar conditions.
Photocatalytic activity of modified TiO2 with Au deposition was higher than the other catalysts. Degradation efficiency was the highest in Au/TiO2, that is 98%, and pure TiO2 and Pt/TiO2 had similar degradation efficiency of 91%. Rhodamine B degradation efficiency with Pt/ZnO was 89% which is the lowest compared to other photo activities. With these results, the Au/TiO2 catalyst was considered for further characterization.
The catalyst Au/TiO2 was characterized. The TiO2 before and after modification with Au was observed. At a glance, the TiO2 was a white-colored powder, and due to the Au doping, the TiO2 changed the color into pinkish purple.
TEM analysis of the catalyst was performed. The particle size analysis and morphological study of the surface-modified Au/TiO2 and pure TiO2 were carried out using TEM with 100 k, 200 k, and 300 k magnification. The presence of the Au particles on the TiO2 surface confirms the successful surface modification carried out.
The average size of TiO2 nanoparticles before surface modification with Au was 20.34 nm, and the particle size increased to 20.52 nm after carrying out the surface modification. Hence, the surface modification did not affect the photocatalyst size significantly. The P25-TiO2 was known to comprise small spherical anatase crystallites and larger angular rutile crystallites. The anatase crystallite particle size varied from 8 nm to 34 nm while the particle size varied from 34 nm to 50 nm for rutile crystallites. Therefore, the TEM results showed that the majority of the P25-TiO2 sample was anatase but, both crystalline phases of anatase and rutile TiO2 were present.
The average Au particle size deposited on TiO2 surface is 2.1 nm and the particle sizes range from 1 nm to 3.5 nm. The Au particles doped vary from 1 nm to 5 nm and exhibit higher photo-, electro-, as well as chemical catalytic activity than larger-sized particles. The Au nanoparticles of the size of 2 nm to 5 nm showed especially high activity for oxidation of CO and propylene. Hence, the doped Au particles were expected to enhance the photocatalytic activity of TiO2. At pH 9, Au particles are exceedingly small (2 nm) compared to those formed at pH 6 (10 nm). The main reason for the larger size at the lower pH is the retention of chloride ions in the gold complex that exists in solution at this pH and that are attached to the surface in some way or other. Such species are more mobile than those at higher pH that is devoid of chlorine. The Au nanoparticles aggregate into larger clusters during drying and thereby form quite big particles of metallic Au.
XRD analysis of TiO2 and Au/TiO2 was performed. The XRD data for TiO2 and Au/TiO2 was smoothed using a Savitzky-Golay (“SG”) filter. The SG used a process known as convolution, which fits successive subsets of adjacent data points with a low-degree polynomial by the method of linear least squares.
The A and R in the reported value column in Table 8 are the reported anatase crystalline phases and rutile crystalline phases respectively.
The characteristic peaks of anatase are at 25.10°, 37.79°, 48.03°, 53.88°, 55.05°, and 62.68° indexed to (101), (004), (200), (105), (211), and (204) crystal faces, respectively (see, Table 13). The diffraction peak at 2θ angles of 27.0° is assigned to the (110) plane of TiO2 rutile crystalline phases. Hence, the TiO2 XRD results were consistent with the TEM particle size analysis results such that the TiO2 contained both anatase and rutile crystalline phases. Furthermore, the XRD results of TiO2 were in good agreement with the reported values of anatase and rutile crystalline phases results (see, Table 13).
The XRD patterns for the Au/TiO2 were dominated by the peaks of anatase and rutile crystalline phases of TiO2 due to the low Au loading (˜5%). The TiO2 weight fraction of each phase in all samples was determined to be 85% (w/w) of anatase and 15% (w/w) of rutile crystalline phases. For the Au/TiO2 samples, additional broad and weak signals were detected at 38.2, 44.3, 65.6, and 78.9 20 values that intensified with the nominal Au loading (˜5% (w/w)). The position and relative intensity of 38.2°, 43.4°, 65.6°, and 79.8° peaks were consistent with face-centered cubic (“fcc”) Au particles and assigned to Au (111), Au (200), Au (220), and Au (311) reflections, respectively. Hence, the Au/TiO2 retained an fcc structure since the particle size is close to 2 nm as found in TEM analysis. Furthermore, due to the Au doping, 43.4°, 65.6°, and 79.8° new peaks were observed in Au/TiO2, confirming the successful doping of Au nanoparticles.
Absorbance spectroscopy analysis of TiO2 and Au/TiO2 was conducted. The UV-Vis absorption spectrum of TiO2 showed the maximum absorption at 345 nm, indicating the TiO2 nanoparticles had photocatalytic activity in the UV region. However, due to the Au doping, the absorption maxima is shifted to 318 nm and a secondary peak occurs at 586 nm visible region. The absorption maxima shift confirms that Au doping reduced the bandgap of TiO2 since bandgap is inversely proportional to the wavelength according to equation (9). The absorption wavelength is λmax:
where Egap is the bandgap between VB and CB, h is Planck's constant, and c is the speed of light. The bandgap analysis was carried out using the Tauc method, which showed that the bandgap energy of pure TiO2 is around 3.1 eV and due to the Au doping, the bandgap was reduced to 2.6 eV.
The additional absorption band of the surface modified TiO2 samples (Au/TiO2) corresponded to the purple color of the photocatalysts. Such a color matrix obtained by Au/TiO2 was due to the plasmonic absorption band. The surface plasmon absorption band was produced by the movement of the conduction electrons in the particles as a consequence of the incident electric field light, which resulted in a displacement of the negative and positive charges in the metal. The peak maximum corresponding to the plasmon excitation of gold typically occurs around 520 nm in organic solvents. The peak redshift occurred because the gold cluster surface was covered with an oxide layer, which showed that the TiO2 nanoparticles undergo chemisorption due to the Au doping. Furthermore, the surface plasmon absorption of Au nanoparticles deposited on TiO2 surface has a redshift compared to that of Au nanoparticles immersed into water. The different dielectric constant of the environment surrounding the Au nanoparticles causes the redshift. Hence, the absorption, typical of well-separated Au nanoparticles in a colloidal suspension, was particularly evident in the absorption spectrum of Au/TiO2 exhibiting a flatter absorption pattern extending in the visible region.
FTIR analysis of TiO2 and Au/TiO2 was performed. The surface modification of TiO2 with Au resulted in a new band, which occurs in the positions at 3808, 2941, and 1635 cm−1. The band occurring at 3308 cm−1 corresponds to the hydrogen-bonded hydroxyl groups on the surface of the catalyst and is prominently observed in the Au-loaded catalysts. Also, the wavenumbers at 2852 to 2941 cm−1, the bands resulting from the water crystallization, were seen in the Au/TiO2, while they were not observed in the unmodified TiO2 catalyst. The surface adsorbed water is responsible for the band observed to 1635 cm−1 in the Au-loaded TiO2. The observed band in TiO2 and Au/TiO2 at 500 cm−1 and 533 cm−1 corresponded to the Ti—O stretching and Ti—O—Ti bridging vibrations respectively. All bands were broadened as a result of surface modification TiO2. The loading of Au nanoparticles caused surface defects which resulted in broader bands in the vibration and bending modes.
XPS analysis was performed. XPS analysis was used to determine the chemical state of the catalyst before and after gold doping. The C 1 s peak at 286 eV was due to adhesive tape used in sample mounting in XPS measurements. The peak at 531.3 eV indicated the O state (“O 1 s”) of the crystal and it was assigned to oxygen in Ti—O—Ti. Binding energies of 567 eV, 449, 61, and 39, were assigned to Ti states of 2 s, 2 p, 3 s, and 3 p respectively. These peaks were observed in both spectra. Peaks for Au nanoparticles were found at 83.2 and 87.1eV, corresponding to Au 4f7/2 and Au 4f5/2, indicating a slight shift of the Au 4 f peaks toward lower binding energies compared with the peaks located at 84.0 and 87.7 eV, respectively, which is due to the intimate contact between the Au and the TiO2 substrate which led to a change in the electronic properties. Only the Au 4 f peaks and no oxidized Au peaks were seen, which suggested the Au deposited is in a metallic state on the TiO2. The amount of Au measured by XPS was 1.5% by atomic concentration and 5% by mass loading. XPS spectra results confirmed the Au loading on the TiO2 lattice and mass percentage was the same as expected in the synthesis process.
The photocatalytic activity was evaluated. To investigate the photocatalytic activity, the Au/TiO2 contained Rhodamine B (“RB”) solution was exposed to UV radiation and the remaining RB concentration was analyzed by finding the absorption at wavelength 554 nm (λmax for RB). The maximum absorption wavelength for RB was obtained by a full wavelength scan in the range of 450 nm to 650 nm. To confirm the λmax for RB, full wavelength scan was carried out before and after exposure to the UV light. The full wavelength scan of RB with Au/TiO2 catalyst showed the decrease of absorption with increasing exposure time, which led to an increase in RB degradation, verified by the decreasing concentration. The RB absorption results for Au/TiO2 catalyst were compared against TiO2 under similar conditions to evaluate the efficiency and effectiveness of the catalysts. The RB absorption results indicated that photocatalytic activity of modified TiO2 with Au deposition was higher than the TiO2 catalysts. A color change was observed, and pink colored solution become colorless with time.
The experiment was repeated 5 times for verification of the results. The standard deviation of testing trials was 0.1157 and 0.0495 for TiO2 and Au/TiO2, respectively. The concentration of RB decreased gradually to a value near zero after 1 h proportional to absorbance. The degradation efficiency of Au/TiO2 was higher at 98% (w/w) compared to TiO2. The observed degradation rate constant (kobs) was 0.0723 minute−1 for Au/TiO2 and 0.0369 minute−1 for TiO2. Hence, the results indicated Au/TiO2 had higher photocatalytic activity over TiO2, concluding that the Au doping to TiO2 surface enhanced the photocatalytic activity of TiO2.
Microscopic images of PET were taken. The optical microscope was used to analyze the surface changes during the photo-reforming process. The surface morphology of PET was examined at 0 hours, 12 hours, 24 hours, 40 hours, 60 hours of the photo-reforming process, under 4 times, 10 times, 20 times, and 40 times magnifications. The surface morphology of PET plastics changed over time and cracks were observed in PET particles. Crack density and depth of cracks increased with irradiation time. Edges of the plastics were eroded due to catalytic activity. A slight change of color of the plastics was observed and the transparency of the PET was reduced with UV exposure.
FTIR spectrum analysis of PET particles was conducted. High polymer materials such as plastics, rubber, and fibers underwent accelerated oxidative degradation due to the photo-reforming process. This degradation was manifested as an accelerated weakening of the mechanical and/or dielectric strength of the material. Since the oxidized sites had characteristic signatures in the infrared region, FTIR spectroscopy was used as an effective means for investigating the state of deterioration of these materials. Hence, the PET FTIR bands were compared before and during the photo-reforming process and the results were compared with the reported values. For the PET, five main peaks were identified at wavenumbers 731, 1050 to 1100, 1408, 1725, 2907, and 2970 cm−1, corresponding to C—H bending ethyl, C—O ester, C—C phenyl ring, C═O ester, H—C═O group, and C—H ethyl stretching vibration modes of PET. The used PET bands had an overall agreement of 99.46% with reported values of PET bands solidifying the reliability of the results.
The IR band for carbonyl (C═O) stretching mode occurred at 1725 cm−1 and broadening of the carbonyl band toward lower wavenumbers was observed (such as 1675 cm−1) due to the formation of the terminal carboxylic acid group. 1245 cm−1, 1125 cm−1, 1340 cm−1, 975, and 845 cm−1 bands were responsible for the various vibration modes corresponding to trans conformer at carbonyl groups and the benzene ring, ether group, and methylene group in PET. Further, the bands at 1370 cm−1, 1096 cm−1, 1040 cm−1, and 900 cm−1 were due to various vibrations corresponding to the gauche conformer of the ethylene glycol unit. These IR bands were indicators of degradation as both UV light and moisture directly attacked these specific chemical bonds and caused chain scissions and changes in the chain conformation and morphology. The IR band at 975 cm−1, was due to asymmetric stretching of the trans-oxy-ethylene (O—CH2) group in the ethylene glycol unit, and was specifically examined as an indicator of changes in the morphology by transitioning from gauche to trans conformer. Furthermore, broadening below the carbonyl band at 1714 cm−1 and the formation of a new peak at 775 cm−1 were known to be generated by degradation byproducts.
The hydroxyl group at 2900 cm−1 increased over time as a clear indication of the PET degradation process. The band transmittance decreased at 1714 cm−1, 1245 cm−1, 1100 cm−1, and 870 cm−1, indicating the disappearance of ketones and aromatic nature in PET as a process of degradation. A more significant change can be observed at the 845 cm−1, where the band decrease confirms the weakening of the aromatic structure.
PET degradation under UV and visible light irradiation was carried out with the same amount of catalyst and same PET weight to understand the influence of the visible light over the Au doped TiO2. The transmittance of most of the bands decreased (1714 cm−1, 1245 cm−1, 1370 cm−1, and 1040 cm−1) with the UV irradiance and the formation hydroxyl group (at 2900 cm−1) confirmed the PET was subjected to the photo-reforming process. However, such significant changes were not present in the PET that was exposed to visible light irradiation with Au/TiO2. Further analysis of the FTIR spectrums carbonyl index and the vinyl index was conducted. The carbonyl index (ICO) was calculated using:
where A1740 and A1835 are the stretching vibration of the carbonyl group, Absorption (A) at 1740 cm−1 and 1835 cm−1 respectively, are calculated using equation 9, where t is the thickness of the sample (in mm), and T is the transmittance (%) of the band.
The Vinyl index (IV) was calculated using:
The ratio of absorbance of the deformation band at 909 cm−1 (A909), and the stretching vibration of the vinyl group (CH2═CH−) at 2020 cm−1 (A2020), were calculated using equation 10.
The reduction of the ICO and the IV after UV and visible light irradiation (Tables 10 and 11) showed that the UV and visible light was capable of degrading PET. To further analyze the photodegradation of PET, the ratio of the stretching and bending band areas of different functional groups and their variation with time were taken into account.
The BAC=C/BAC−H and BAO−H/BAC−H decreased with time while BAC=O/BAC−H and BAC−O—C/BAC−H increased up to 12 hours with rapid decrease after 12 hours (Table 12). In general, all the BAs with respect to BAC−H, slightly decreased during a 40 hour period (Table 12). Therefore, the overall decrease of BA rations proved the disappearance of bands was due to degradation of PET.
Gas analysis from the reactor was performed. In the initial experiments, air bubbles were observed under the plastic surface and inside of the reactor. Gas production was evident with the bubble generation in the reactor. Gas identification and quantification was the next step of the process. A closed reactor was designed to collect the generated gas and a gas chromatograph with a thermal conductivity detector (“GC-TCD”) was used to characterize and measure the gas composition.
The headspace gas of the photo-reforming process was analyzed using a GC-TCD. The profile of the gas composition was analyzed at different time intervals. A major portion of the gas in the head space was O2 while a significant amount of H2 was produced in the photo-reforming process. The H2 was not detected in the initial stages of the reaction process (after 30 minutes, 1 hour, 2 hours, 3 hours, 6 hours), however, H2 was detected in the gas matrix after the 3 hours interval. The photo-reforming process was carried out and the materials were exposed to the UV light. Two sets of experiments were carried out using pure TiO2 as the catalyst and Au/TiO2 as the catalyst.
The peak in the range of 3.5 mins to 4.8 mins represented H2 gas and the peak between 7.1 to 8.5 minutes occurred due to the presence of O2 gas in the headspace. The presence of H2 gas was evidence that the photo-reforming process had taken place successfully in the reaction chamber. The constant amount of O2 further solidified the PET degradation. The maximum amount of H2 generation was observed at the 6 h time interval with a volume of 21.4 μmol/g of catalyst. Hydrogen generated gradually increased up to 6 h.
The photo-reforming activity of Au/TiO2 was compared with TiO2 catalyst under the same conditions. A pure TiO2 system did not produce H2 or produced an untraceable amount of H2, which verified that Au doping of TiO2 enhanced photocatalytic activity.
Liquid phase dark deposition method of single atom deposition was performed. Annealed P25 Degussa TiO2 nano powders at 500° C. were immersed in 100 mL MeOH (50 vol %) solution containing diluted H2PtCl6·6H2O solution in a volumetric flask and sealed. The solution was stirred for 24 hours in the dark. After staying in the dark for 24 hours, the nano powders were then filtered and washed with EtOH and deionized water for 15 min each. Subsequently, the samples were dried in an oven at 120° C.
Liquid phase direct deposition/deposition precipitation method of single atom deposition was performed. Controlled deposition of metal cocatalysts on the semiconductor and/or metal oxide surface under ambient conditions was performed and precursor concentration and reaction time influence the particle size and distribution.
PET was converted into ethylene glycol (“EG”) and disodium terephthalate (“Na2TP”) under the photo-reforming process. Valuable byproducts formed after the pretreatment and, subsequently, the photo-reforming step. TPA was identified as one such valuable product that may be extracted from the solution after pretreatment. Alkaline and acidic hydrolysis under different conditions was applied in the pretreatment method, as in Table 13. Then the degradation was analyzed considering weight reduction, amount of generated TPA, and extracted TPA amount from the solution mixture. The weight reduction percentage of the PET after the pretreatment process indicated the hydrolysis efficiency of different pretreatment methods. Alkaline hydrolysis methods, such as KOH and EtOH-induced pretreatment, showed higher PET conversion efficiency compared to acidic hydrolysis, such as H2SO4 pretreatment. The highest PET conversion of 92.4% was observed with EtOH-induced pretreatment at 40° C., whereas the PET conversion efficiency reduced to 70.8% when decreasing the temperature to 25° C. Increased KOH concentration increased the PET conversion by 2.8 times and 4.1 times under 25° C. and 40° C., respectively.
The dissolved TPA content in the substrate after pretreatment was evaluated using the HPLC method. The highest dissolved TPA content was observed when EtOH-induced pretreatment was applied at 25° C. Dissolved TPA content increased in all the solutions with increasing reaction temperature. The highest TPA amount was extracted from pretreatment with EtOH at 40° C., which was 200% higher than the extracted TPA under the same method at 25° C. Higher PET conversion resulted in a higher generation of TPA. The added terephthalate ions when PET was converted using catalyzed H2SO4 with Na2TP resulted in a higher TPA concentration in the solution and extracted TPA. Therefore, the observed results suggested the feasibility of PET conversion and TPA extraction even after 24 hours of pretreatment reaction.
The generated dissolved TPA content and resulting H2 production were analyzed to understand the statistical correlation between them. Increased TPA content in the solution increased H2 production since it is an indicator of PET hydrolysis.
Table 14 shows the TPA content in the reaction mixture after the photo-reforming process of 48 h and extracted TPA from the mixture. 0.7% (w/w) Pt/TiO2 was applied as the catalyst, and a UV mercury vapor lamp was used as the light source. The highest extracted TPA content was detected from the reaction mixture pretreated with the EtOH-induced pretreatment method, which is 55 mg TPA per g PET. However, the highest dissolved TPA content in the reaction mixture was measured when 10 M KOH was applied as a pretreatment precursor, even though the extracted TPA from that solution was 40 mg/g of PET. However, results indicated that 48 hours of photo-reforming successfully converted PET, and a significant amount of TPA was extracted from the solution mixture, which could then be reused in H2 production or in polymer synthesis.
Photo-reforming H2 evolution process was applied using three TiO2-based catalysts: Au/TiO2, Pt/TiO2, and Zn/TiO2. Further, Pt/TiO2 was synthesized with Pt loading percentages of 0.3% (w/w), 0.7% (w/w), 1.5% (w/w), and 6% (w/w). Pt and Au are noble metals with excellent catalytic properties, and composite nanomaterials of Pt/TiO2 and Au/TiO2 were identified as effective catalysts in photo-reforming reactions due to their ability to facilitate the necessary redox reactions. The doping percentages were tuned to generate atomic deposition of Pt on TiO2 and consider synthesizing more economical catalysts. The application of Zn/TiO2 was to assess the ability of replacing noble metals with low-cost metal cocatalysts. Preparation of Au/TiO2 was performed following the method discussed in our previous study. The hexachloroplatinic acid solution (H2PtCl6, 8 wt. % in H2O) with a volume of 4.7 μl, 23.5 μl, 62.5 μl, and 250 μl was added to 200 ml of deionized (“DI”) water to obtain 0.3% (w/w), 0.7% (w/w), 1.5% (w/w), and 6% (w/w) Pt doping percentages on TiO2 respectively.
Zn-doped TiO2 nanoparticles were synthesized by sol-gel method. A solution containing 160 ml of ethanol (EtOH) and 20 ml of isopropanol was mixed slowly with 20 ml (50 mmol) of titanium (IV) butoxide under sonication. The resultant mixture was slowly added to 100 ml of deionized water containing 0.1 g of zinc nitrate hexahydrate and 2 g of cetyltrimethylammonium bromide (“CTAB”) to form a white precipitate. The system was kept under constant stirring at 80° C. for 4 hours. The excess water was removed by evaporation on the water bath with continuous stirring. The resultant precursor was then dried at 110° C. for 12 hours and then finally calcined at 500° C. for 4 hours in a high-temperature muffle furnace.
Transmission electron microscopy was used for imaging Au/TiO2, Zn/TiO2, and Pt/TiO2. Pt/TiO2 was further analyzed using a TEM equipped with 3rd-order spherical aberration corrector for the scanning TEM (“STEM”) probe. Software was used for further analysis of the TEM images. The crystalline composition of catalysts was characterized with CuKα radiation (λ=1.54 Å) using an X-ray diffraction system. The bandgap of the catalysts was determined by a full wavelength scan within the range of 190 nm to 1100 nm using a UV spectrophotometer. The Tauc plot method was applied to derive the bandgap energies of the catalysts following our previous study. X-ray photoelectron spectroscopy (“XPS”) equipped with a monochromatized Al K-alpha X-ray source with a photon energy of 1486.6 eV and operated at 15 keV and 20 mA emission current was used to characterize the electronic properties of the catalysts and to determine the loading percentages. The loading percentages were further verified using inductively coupled plasma optical emission spectroscopy (“ICP-OES”) following Environmental Protection Agency (“EPA”) 200.7 method. The ICP-OES analysis was followed by the microwave digestion of the sample following the EPA 3051A method without using HF.
PET was hydrolyzed during a pretreatment process using two methods. During the first method, KOH was used as a precursor at two different concentrations of 3 M and 10 M. KOH solution (10 ml) was added to 0.5 g of finely cut PET and stirred at 300 rpm for 24 hours at two different temperatures (25° C. and 40° C.) on different occasions. After 24 hours, the solution was diluted by adding 40 ml of deionized water, and the photo-reforming process was started after adding the catalyst in a sealed quartz reactor.
When using the ethanol-induced pretreatment method, 6 ml of EtOH was added to finely cut 0.5 g of PET with 4 ml of deionized water and 0.5 g of NaOH was added to the solution and stirred for 3 hours and 6 hours at 25° C.
When testing the influence of the pretreatment processes, the polymer-to-catalyst ratio was maintained at 1:1. Photo irradiation continued for 2 days, and hydrogen production was measured at 6-hour intervals.
A photo-reforming experiment under UV-irradiation was conducted using a high-pressure UV mercury vapor lamp with minor peaks at 290, 315, 335 nm, and a dominant peak at 365 nm to simulate solar light. Light flux was measured using a solar power meter. Gas generated during the reaction was analyzed using a gas chromatograph (“GC”) equipped with a thermal conductivity detector (“TCD”) and HP-5 molecular sieve column using N2 as the carrier gas as described in our previous study.
When analyzing the effect of pH on hydrogen production efficiency, a 1:1 polymer-to-catalyst ratio was maintained, Au/TiO2 and Pt/TiO2 were used as the photocatalysts.
Two different tests at pH 7 and existing pH (>13) without adjusting were conducted. The effect of polymer to catalyst ratio was analyzed in the system by testing three different ratios 1:1, 2:1, and 1:2. When changing the polymer-to-catalyst ratios, pH remained constant at >13 without further adjustment, and two different pre-treatment conditions with two catalysts were tested. Each of the above tests was continued for 48 hours with a duplicate under irradiation of a high-pressure UV mercury vapor lamp, and hydrogen production at 18 hours, 24 hours, 30 hours, 42 hours, and 48 hours was measured.
Computational simulation of the photocatalytic activity of the catalysts was performed. Density functional theory (“DFT”)-based calculations were performed. The lattice parameters of DFT periodic boundary calculation of anatase TiO2, Au/TiO2, and Pt/TiO2 were a=3.800 Å and c=9.708 Å. The surface of TiO2 and composite materials structures were modeled using a (2×2) supercell. The supercell contained four Ti layers and twice as many O layers in a slab, with 5 Å vacuum separating the surface slabs. In addition, in metal composite material, an Au or Pt atom was placed on the Au/TiO2 and Pt/TiO2 surface DFT calculation, respectively, of the TiO2 surface.
The initial structures were optimized using Perdew-Burke-Ernzerh of nonlinear core-correction Rappe Rabe Kaxiras Joannopoulos (“PBE-n-rrkjus”) and pseudopotential plane wave approach. The calculations were carried out using plane wave functions with a kinetic energy cutoff of 30 Ry and a 60 times larger cutoff for charge density with a k-points grid of 8×8×8. During the optimization, two bottom Ti layers and corresponding O atoms were fixed until the forces acting on atoms were less than 0.001 eV Å−1 in any direction to reduce the complexity of the calculation. These optimized structures were used to obtain the Density of states (“DOS”) for further structural analysis.
XRD analysis was performed on the nanocomposite catalysts. X-ray diffraction analysis of TiO2, Pt/TiO2, and Au/TiO2 was performed to identify crystalline changes because of doping Au and Pt. Compared with pristine TiO2, there were few changes in the distribution and intensity of XRD peaks in Pt/TiO2 and Au/TiO2. All the indexes showed in TiO2 may be observed in XRD patterns of Pt/TiO2 and Au/TiO2 which indicated that the crystalline structure of the TiO2 was not affected by the chemical precipitation process of Au and Pt doping. The Au (111) lattice index overlaps with the TiO2 Anatase (103) lattice index, which confirmed the surface deposition of the Au on TiO2. However, there were no significant changes in the XRD patterns due to smaller particle sizes and minimum doping amounts of metal on TiO2. The characteristic peak of the Pt (111) lattice index was visible at two theta values of 38.9o in 6%, 1.5%, and 0.7% Pt/TiO2 XRD patterns but imperceptible in 0.3% Pt/TiO2 XRD spectra due to undetectable level of Pt.
Example 33
XPS analysis was performed on the nanocomposite catalysts. The elemental composition was determined by XPS high-resolution scans over C 1 s, O 1 s, Ti 2 p, and Pt 4 f spectra regions. The XPS analysis was performed to investigate the electronic structure of the elements (Pt, Au, Ti, and O) on the surface of catalysts before and after the synthesis. The energy scale was corrected to a binding energy of Ti 2 p at 458.5 eV and the referenced C 1 s peak at 284.6 eV for surface adventitious carbon. The peaks at 72.0 eV, and 76.0 eV are responsible for Pt 4f7/2 and Pt 4f5/2, respectively. Furthermore, peak deconvolutions indicate the peak positions at 72.6, 74.0, 76.1, and 77.8 eV, corresponding to the presence of Pt2+ and Pt4+ species which may exist in the form of PtO and PtO2 on the surface of the catalyst. However, the calculated ratio of Pt0/Pt2+ concerning peak areas is 5.9, indicating that the Pt0 species are dominant compared to Pt2+ on the surface of the catalyst. The oxidation state of Pt is a crucial factor when improving photocatalytic efficiency. Even though the bandgap energy is higher in PtO/TiO2 compared to Pt/TiO2, better hydrogen production via water-splitting was observed in the presence of PtO/TiO2. TiO2 doped with Pt0 exhibits superior catalytic activity compared to TiO2 platinized with Pt2+ and Pt4+ for degrading organic contaminants.
The peak at 458.4 eV in Pt/TiO2 occurred due to Ti 2p3/2, as identified in P25 TiO2 spectra. However, the Ti 2p3/2 peak in Au/TiO2 is around 458.0 eV. The peak of the O1 s species is visible at 529.0 eV in all spectra, and an additional hydroxide peak is present at 531.0 eV in Au/TiO2 and Pt/TiO2 because water molecules are adsorbed onto the surface. The theoretical weight percentages are lower than the derived weight percentages analyzed using XPS data; all Pt atoms or ions in the TiO2 surface may cause a low theoretical weight. The calculated weight percentages are further validated using inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. The comparison of XPS and ICP-OES analytical results with the values expected from synthesis methods shows that the derived values from the XPS analysis are higher than the predicted values and ICP-OES analytical results.
The comparison of XPS and ICP-OES analytical results with the values expected from synthesis methods shows that the derived values from the XPS analysis are higher than the predicted values and ICP-OES analytical results. This is probably because XPS functions as a method for characterizing surfaces, and the doped metals are deposited on the surface of TiO2 particles. The ICP-OES method quantitatively assessed the elemental makeup of all the catalysts as opposed to the other methods. The detection limit of the ICP-OES instrument for Pt is 0.003 mg/L; hence, the accuracy of the test is higher even though the digestion is limited due to the lower degradability of TiO2. Overall, the XPS and ICP-OES results indicate successful doping of Pt to TiO2, especially when considering the expected 1.0% doped TiO2, which achieved 0.8% dopant content in terms of ICP-OES analysis.
A KOH polymer pretreatment method was performed. Different pretreatment methods were applied to enhance the hydrolysis of PET particles. KOH concentration and the pretreatment temperature were considered key parameters that affect the hydrolysis efficiency. Two KOH concentrations of 3 M and 10 M were experimented with temperatures at 25° C. and 40° C. Hydrogen production concentration in the PET hydrolysis process since the catalytic efficiency was dominant compared to polymer hydrolysis efficiency. However, there was a slight increase in hydrogen production using Zn/TiO2 as the catalyst when increasing the KOH concentration in the pretreatment process. When using Au/TiO2 as the catalysts, hydrogen production with 10 M KOH pretreated PET solution was about 3.5 times the H2 production with 3 M pretreated PET. However, H2 production with 10 M KOH was 1.2 times compared to 3 M KOH pretreatment at 25° C. when using Pt/TiO2 as the catalyst.
An EtOH-induced polymer pretreatment method was performed. The EtOH-induced pretreatment process was applied to enhance the hydrolysis of the PET particles such that more ethylene glycol yield could be produced, which acts as the sacrificial agent during the photo-reforming process. During the pretreatment process, the temperature remained constant at 25° C., and 5% NaOH was added to promote the reaction and to achieve a higher conversion yield of monomers. A mixture of 60% EtOH and 40% deionized water was found to be the most efficient ratio as a co-solvent in PET hydrolysis. The photo-reforming results indicate that longer pretreatment time increased hydrogen production by 30% and 15% with Au/TiO2 and Pt/TiO2, respectively. Increasing the pretreatment time could increase PET monomer yield, thus resulting in higher H2 production.
The polymer-to-catalyst ratio was evaluated. When comparing the H2 production concerning polymer-to-catalyst ratios, the highest H2 yield was observed when using Pt/TiO2 as the catalyst at a ratio of 2:1, where 0.5 g of pretreated PET was mixed with 0.25 g of the catalyst. Increasing the catalyst dosage from 0.25 g to 1.0 g resulted in a 37% and 64% reduction in H2 yield with Pt/TiO2 and Au/TiO2, respectively. High catalyst density in the slurry system might act as an obstacle to lower light penetration, resulting in lower photocatalytic efficiency. Therefore, decreasing catalyst dosage (for both Au/TiO2 and Pt/TiO2) led to an increase in H2 production in the EtOH-pretreated PET system.
The Pt/TiO2 to PET ratio in the KOH-pretreated system behaved as an analog to Pt/TiO2 EtOH-pretreated PET system, showing higher H2 production at the 2:1 polymer-to-catalyst ratio. However, for Au/TiO2 KOH-pretreated system, the highest H2 yield was achieved at a 1:1 polymer-to-catalyst ratio. The lowest H2 yield (˜183 μmol/gcat) was observed when 1.0 g of Au/TiO2 was added with 0.5 g of KOH-pretreated PET. Increasing the catalyst dosage (by a factor of 2) increased the H2 yield (by ˜1.4 times) but further increasing catalyst dosage beyond the optimal ratio reduced hydrogen production due to inhibited light absorption and utilization.
Pretreatment of plastics was performed. PET, LDPE, and PS were collected from waste plastic beverage bottles (the average molecular weight was 24,800 g/mol), low-density polyethylene bags, and polystyrene plates, respectively. The plastic materials were thoroughly washed with deionized water and ethanol (EtOH) and air-dried before pretreatment. Three different pretreatment methods were applied to acquire the monomers and oligomers of the plastics. KOH was used to promote hydrolysis by adding 0.5 g of finely cut plastic into 10 ml of 3 M KOH solution and stirring for 24 hours. The ethanol pretreatment process applied ethanol (EtOH) and deionized water in a 3:2 ratio, with 5% (w/w) NaOH used as the precursor with 0.5 g of finely cut plastic in 10 mL solution (EtOH-induced pretreatment method). Acidic hydrolysis was applied, where 0.5 g of finely cut plastic was added to 10 mL of 1 M H2SO4. The mixture was stirred at 25° C. for 24 hours, and the pH of the resulting supernatant was adjusted to 7 using 10 M KOH solution.
PET hydrolysis was studied under two major processes, including alkaline and acidic pretreatment. The applicability of PET as a sacrificial agent was further studied by optimizing the pretreatment under five different pretreatment conditions. An additional alkaline hydrolysis method was applied using a 10 M KOH solution. Further, the acidic hydrolysis was catalyzed by adding 0.01 g of Na2TP to the solution. These pretreatments were applied into 0.5 g of finely cut polymer with 10 mL of total solution at 25° C. for 24 hours. Byproduct formation under different PET pretreatment conditions was quantified by high-performance liquid chromatography analysis, and the residuals were characterized using ATR-FTIR analysis and TGA after each pretreatment and after photo-reforming process.
The EtOH-induced pretreatment process was studied due to its higher H2 production ability and higher hydrolysis rate under mild conditions. The pretreatment process was performed in five steps. The baseline study was conducted using 30 ml of absolute EtOH mixed with 20 ml of deionized water as the first step. The finely cut PET 0.5 g was added to the
EtOH and deionized water mixture, followed by 5% (w/w) NaOH, and pretreated for 24 hours. The resulting mixture was filtered with 0.1 μm filters, and the filtrate was used in the second step of the photo-reforming experiment. In the third step of the experiment, the same mixture was used without filtering. The supernatant extracted during the second step was dissolved in 50 ml of deionized water and used in the 4th step. The PET contained EtOH and deionized water mixed with 5% (w/w) NaOH 24 hours pretreated solution was exposed to dropwise addition of 1 M H2SO4. The resulting white precipitate was extracted by centrifugation and used in the photo-reforming experiment during the 5th step after suspending in DI water. Each photo-reforming experiment was conducted after adding 0.5 g of the catalyst, adjusting the total volume of the reactor to 50 ml, and a 60 mW/cm2 UV mercury vapor lamp was used for irradiation.
Photo-reforming experiments were performed in sealed 50 ml quartz reactors under simulated light of a high-pressure UV mercury vapor lamp with minor peaks at 290 nm, 315 nm, 335 nm, and a dominant peak at 365 nm. The flux of the light source was measured using a solar power meter as 60 mW/cm2. A volume of 10 ml of pretreated polymer solution was added to the reactor with 0.5 g of the catalyst. The preparation and characterization of Au/TiO2 and Pt/TiO2 nanocomposite photocatalyst were discussed in our previous study. H2 production was measured using a gas chromatograph (“GC”) equipped with a thermal conductivity detector (“TCD”) and a molecular sieve column using N2 as the carrier gas and 1 mL of headspace gas was directly injected into the inlet of the gas chromatography. Purge time was selected as 1 min, and chromatography was collected for 10 mins after injection. The sample peak retention time (appearance time) and area were compared to standard H2 samples to calculate the gas concentration.
Polymer degradation was analyzed by comparing ATR-FTIR of residual plastics after pretreatment and photo-reforming process. Plastic pieces were washed with deionized water and dried before acquiring the ATR-FTIR spectrum. The polymer degradation was reflected by the formation of carbonyl group (C═O) hence the carbonyl index was an indicator used to measure the chemical oxidation of polymers. The carbonyl index (CO index) was calculated using Equation 11 and compared with that of virgin (non-degraded) PET. The CO index was calculated from the ratio between the integrated band absorbance of the carbonyl (C═O) peak from 1,850 cm−1 to 1,650 cm−1 and that of the methylene (CH2) scissoring peak from 1,500 cm−1 to 1,420 cm−1.
TGA was performed to evaluate the degradation process and decomposition potential after pretreatment of residual PET particles. Pt pans for furnaces were loaded with the washed and dried residual PET particles (˜10 mg) filtered after the pretreatment process after taring the small Pt pans. A carrier gas (N2) was applied at a flow rate of 40 mL/min in the temperature range from 30° C. to 800° C. with a heating rate of 20° C./min. Related weight loss bends, plotting, and analysis were completed using resulting TGA curves.
High-performance liquid chromatography (“HPLC”) equipped with a column was employed to analyze the liquid byproducts formed after pretreatment and photo-reforming of PET. The UV/vis detector was set at 256 nm, and the reverse phase C18 column was used. A methanol/water 80/20 (v/v) was used as the mobile phase at a flow rate of 1 ml/min and at injection sample volume of 10 μl. Terephthalic acid was identified and quantified using the calibration with standard TPA (benzene-1,4-dicarboxylic acid).
PET weight reduction was evaluated after pretreatment. PET weight reduction after pretreatment was analyzed using 0.25 g of initial weight (“W0”) of PET in a tared glass vial. Then 5 ml of solution was added and pretreated for 24 hours. The reaction mixture was filtered with a 0.5 mm sieve after 24 hours, and residual PET particles were washed with deionized water. The resulting residual particles were oven-dried at 30° C. for 24 hours, and the final weight (“Wf”) was recorded. The reduced weight percentage was calculated using Equation 12.
TPA extraction was performed. The filtrate after the pretreatment was collected and 0.5 ml of 1 M H2SO4 was added dropwise to the solution to precipitate the dissolved terephthalate ions to TPA. The resulting solution was stirred at 300 rpm for 20 mins and vacuum filtered using tared 0.1 μm filters. The filtered suspension was air-dried for 48 h at room temperature and weighed. The weight of the extracted TPA amount was the increased weight of the filter.
The preliminary cost estimate was performed by comparing different pretreatment methods in this study. Then, a complete techno-economic analysis was performed for the selected optimized system for treating 1 kg of PET per day, considering all the cost factors, including capital costs for materials, supplies, and equipment to build the photo-reforming systems and labor costs. The lifetime of the hydrogen production system was assumed to be 20 years for the ethanol-induced pretreatment method. PET conversion was considered 60%, with 8 h of daily operation using sunlight, sunlight with solar concentrator, and simulated light as the energy source under three separate cases. Hydrogen production using natural light was assumed to be 80% of the H2 production under simulated light, considering the impact of weather. The cost of planning, consulting, administration, construction and labor, and investment interest was assumed to be 20% of the total capital cost. The H2 compression and storage costs were not included in this estimation. Terephthalic acid (TPA) was generated and may be extracted as a byproduct in this process. The revenue generated from H2 production and TPA was included in the cost assessment. The daily operational costs were calculated considering the reuse of NaOH and ethanol for 30 days during the pretreatment process. Based on experimental results, the catalysts were assumed to be recycled for 132 days with 8-h daily operation in 4 regenerated cycles. Costs for power, material, and equipment are included in the cost analysis.
The cost was calculated based on the following Equation 13:
where Ccapital—construction and capital cost ($), Cop—the cost of daily operation ($/day), Ccon—the cost of daily consumables ($/day), H—daily H2 production (kg/day), t—operation life of the plant (days).
The carbon footprint was calculated based on the following Equation 14:
where Fcapital—carbon footprint of construction and capital infrastructure (g CO2-eq), Fop—the carbon footprint of daily operation (g CO2-eq), Fcon—the carbon footprint of daily consumables (g CO2-eq), Fprod—the daily production of CO2 from photo-reforming (g), H—daily H2 production (kg), t—operation life of the plant (days).
The techno-economic analysis considered the landfill release of CO2 from PET disposal and the reduced CO2 emission by reusing PET in the system to be negative carbon emissions. Further, the direct CO2 production from photo-reforming in the optimized system was measured at zero.
Scanning transmission electron microscopy (STEM) was used to image and compare the particle size distribution and morphology of the pristine TiO2 with the metal-doped TiO2. The TEM particle size analysis illustrates that the average size of metal composite TiO2 particles remains the same as 20 nm, indicating that surface modification did not substantially impact the size of the particles. Random distribution of Pt single atoms on TiO2 surface was observed. TEM detected Au particles with an average size of 2.1 nm were deposited on TiO2 nanoparticles surface, and Au particles were randomly distributed on the TiO2 surface.
The bandgap energy is an essential parameter for determining the suitable wavelength range of the light source to power the photo-reforming process. Using the normalized UV-Vis spectrum of the catalysts dispersed in DI water, the bandgap energy was calculated following the Tauc plot method, assuming direct and indirect electron transition. Bandgap values derived under the assumption of direct electron transition are higher in most cases than when considering the indirect electron transition. The calculated TiO2 bandgap value of 3.20 eV aligns well with the range of previously reported values from 3.2 eV to 3.35 eV. The bandgap of Au/TiO2 was 3.09 eV, and it decreased by 0.10 eV because of doping TiO2 with 5% Au, leading to a bandgap range of 3.09 eV to 3.10 eV. The range of bandgap energy derived demonstrates that Au/TiO2 may be activated by light with wavelengths above 400 nm, from near UV to the visible light region of the solar spectrum.
Doping Pt to the TiO2 matrix observed the same trend as Au/TiO2, resulting in a significant shrink in the bandgap. The average bandgap energy of Pt/TiO2 decreased from 2.86 eV to 2.06 eV as the Pt loading percentages increased, compared to 3.20 eV of TiO2. Furthermore, doping Pt on TiO2 reduced the bandgap energy by 10% to 40%, depending on the Pt concentration. The average bandgap energy of 0.3% and 0.7% Pt/TiO2 was calculated to be ˜2.8 eV, and the insignificant difference could be due to the smaller Pt dosages. The significant decrease in bandgap energy in Pt/TiO2 allows the photo-reforming process to occur under visible light, specifically in the range of 400 nm to 500 nm. The doping of noble metals like Pt and Au retarded the carrier recombination rate by trapping the electrons, thereby reducing the electron-hole recombination rate and the bandgap energy. Additionally, the Schottky barrier formed at the metal-TiO2 interface resulted in enhanced catalytic efficiency. In contrast to Au and Pt doping onto TiO2, Zn/TiO2 shows a bandgap increase of 0.15 eV compared to TiO2. This could be due to the different synthesis processes of Zn/TiO2. Under this condition, Zn/TiO2 behaved similarly to pristine TiO2; thereby, the activation of the photocatalyst was possible only under the far UV and near UV range wavelengths. Therefore, based on the analysis of the bandgap structure, and without being limited by any particular theory, we hypothesized that Pt/TiO2 will exhibit enhanced photo-reforming performance, followed by Au/TiO2, with Zn/TiO2 and pristine TiO2 showing little to no change in performance.
This prediction is corroborated by computational simulations, which indicate a reduction in the bandgap of Au/TiO2 and Pt/TiO2 due to the incorporation of Au and Pt into the TiO2 matrix. Additionally, according to the DFT calculations, the density of states in the conduction band increases significantly because of Pt doping compared to Au/TiO2. Therefore, more excited electrons may be occupied by the conduction band, leading to an efficient reduction step that may be expected in the photo-reforming process, thus resulting in higher hydrogen production.
Hydrogen production correlated well with the respective bandgap energies of catalysts. Among different photocatalysts, 0.7% Pt/TiO2 reached the highest efficiency for hydrogen production. The efficiency of Zn/TiO2 in photocatalysis for hydrogen production was significantly lower than Pt/TiO2 and Au/TiO2. However, unlike the other catalysts, Zn/TiO2 was not synthesized using P25 Degussa TiO2, which could also be a reason for its lower performance.
The comparison of various metal-doped TiO2 composites showed that Pt-doped TiO2 performed the best; the research was expanded to investigate the impact of the different doping levels at 0.3%, 0.7%, 1.5%, and 6.0% Pt/TiO2. Hydrogen production ability increased when increasing the Pt content on TiO2. When the doping percentage increased from 0.7% to 1.5%, the H2 production increased by 146% after five days. However, Pt doping percentage and H2 production are not linearly correlated because the higher charge recombination rate due to high defect availability in the catalyst at high Pt dosage reduced the efficiency of H2 production. Even though the Pt amount is four times larger in the 6% Pt/TiO2 compared to 1.5% Pt/TiO2, there is only a slight increase of 3% in hydrogen production. Without being limited to any particular theory, at lower Pt percentages doped in the TiO2, the increase in catalytic activity might be connected to the generation of electronic defects associated with the rutile phase of TiO2. With the increasing percentage of Pt, catalyst nanoparticles may contain metalized Pt, which improves catalytic activity by improving charge separation and reducing charge recombination efficiency. The normalized hydrogen production irradiating UV and visible light was 0.019 μmol and 0.018 μmol per hour per gram catalyst per mW/cm2, respectively, using the Au/TiO2 nanocomposite catalysts.
Two 3 M and 10 M KOH concentrations were applied to pretreat PET at 25° C. and 40° C. Variations in the KOH concentration during the PET hydrolysis process had minimal impact on hydrogen production using Zn/TiO2 since the catalytic efficiency outweighed the efficiency of polymer hydrolysis. However, when the KOH content is increased in the pretreatment process, there is a modest increase in hydrogen production utilizing Zn/TiO2 as the catalyst. Hydrogen generation with 10 M KOH pretreated PET solution was approximately 3.5 times higher than hydrogen production with 3 M prepared PET when Au/TiO2 catalysts were used. On the contrary, when utilizing Pt/TiO2 as the catalyst, hydrogen production with 10 M KOH was 1.2 times greater than that with 3 M KOH pretreatment at 25° C.
When applying Au/TiO2 as the catalysts and increasing the pretreatment temperature of 3 M KOH solutions from 25° C. to 40° C., the hydrogen production was enhanced by 40%. Moreover, the H2 yield in 10 M pretreated PET enhanced by 30% and 4% when Au/TiO2 and Pt/TiO2 were used as catalysts, respectively. Even though there was an overall slight decrease in H2 production when the temperature was raised using a 3 M KOH pretreatment, there was an increase in H2 yield when the temperature was raised using a 10 M KOH pretreatment when Pt/TiO2 was present. Higher pretreatment temperatures were generally associated with a modest increase in hydrogen generation. Without being limited to any particular theory, this was attributed to enhanced hydrolysis and the synthesis of higher yields of monomers and oligomers, which engage in the oxidation process while improving the H+ reduction process and increasing the kinetics activity of the reactions (according to Arrhenius equation), collectively resulting in larger yields of H2.
EtOH-induced pretreatment of PET was used to increase the production of ethylene glycol. Pretreatments time was optimized. The percentage of hydrogen produced increases to 15% for Au/TiO2 and 30% for Pt/TiO2 after prolonged pretreatment time. Without being limited to any particular theory, extending the pretreatment period could increase the yield of PET monomers, ultimately increasing the amount of hydrogen produced. Using the EtOH-induced pretreatment approach, an extremely high hydrogen yield was measured compared to the hydrogen yield from 10 M KOH pretreatment at 25° C., regardless of the catalyst. The hydrogen production was approximately 2.5 times greater than 10 M KOH pretreatment when using EtOH-induced pretreatment for three hours. When applying 6 h of pretreatment, the amount of hydrogen produced was approximately three times greater than when using 10 M KOH pretreatment with Pt/TiO2. While the hydrogen yield from Au/TiO2 was not as high as that from Pt/TiO2, the EtOH-induced pretreatment significantly increased the hydrogen yield. The pretreatment time substantially impacts hydrogen production when using difficult-to-hydrolyze sacrificial agents such as PET with EtOH-induced process.
When comparing the impact of polymer-to-catalyst ratios, the catalyst of Pt/TiO2 resulted in the highest yield of hydrogen observed at a ratio of 2:1, where 0.5 g of pretreated PET was mixed with 0.25 g of the catalyst. Increasing the catalyst amount from 0.25 g to 1.0 g resulted in a 37% and 64% reduction in hydrogen yield with Pt/TiO2 and Au/TiO2, respectively. Without being limited to any particular theory, high catalyst density in the slurry system might act as an obstacle to lower light penetration, resulting in lower photocatalytic efficiency. Therefore, decreasing catalyst dosage (for both Au/TiO2 and Pt/TiO2) increases hydrogen production in the EtOH-pretreated PET system.
The Pt/TiO2 to PET ratio in the KOH-pretreated system behaved as an analog to the Pt/TiO2 EtOH-pretreated PET system, illustrating higher hydrogen production at the 2:1 polymer-to-catalyst ratio. However, for the Au/TiO2 KOH-pretreated system, the highest hydrogen yield was achieved at a 1:1 ratio. The lowest hydrogen yield (˜183 μmol/g) was observed when 1.0 g of Au/TiO2 was added with 0.5 g of KOH-pretreated PET. Generally, increasing the catalyst dosage (by a factor of 2) increased the H2 yield (by ˜1.4 times), but further increasing the catalyst dosage beyond the optimal ratio reduced hydrogen production due to inhibited light absorption and utilization.
Hydrogen production with and without the pH adjustment in the EtOH-pretreated PET and KOH-pretreated photo-reforming systems was determined. Tests were conducted for 48 h, and hydrogen production at 18 h, 24 h, 30 h, 42 h, and 48 h was measured. Each test was duplicated, and standard errors were calculated. When pH reduced from >13 to 7, the hydrogen yield improved significantly for both catalysts in reactors with EtOH-induced pretreated PET. Pt/TiO2 resulted in a 110% increase in hydrogen production, and when PET was treated with the EtOH-induced pretreatment method, hydrogen yield was approximately 100% increased using Au/TiO2. Among the EtOH-induced pretreated systems, the highest hydrogen yield of ˜2800 μmol/g was measured using Pt/TiO2 as the catalyst at pH 7.
However, pH reduction has a negative impact on the reaction mixture with KOH-pretreated PET systems. KOH plays a vital role in PET hydrolysis; therefore, lower pH might cause a reduction in hydrolysis efficiency, yielding a lower concentration of monomers. Hence, hydrogen yield drastically reduced from 255 to 11 μmol/g when tested with Au/TiO2, and a 38% reduction of hydrogen yield was observed with Pt/TiO2 catalyst. The Pt/TiO2 catalyst demonstrated a ˜100% higher hydrogen yield under neutral pH conditions than alkaline conditions in the EtOH-induced method, contrasting with previous studies primarily focused on alkaline environments. This finding suggests a new pathway for optimizing photocatalytic efficiency.
The cost estimation results showed that the catalyst's material cost is substantial. The costs of 1 kg of 5% (w/w) Au/TiO2 and 0.7% (w/w) Pt/TiO2 were estimated at $0.09/day considering an 8 h daily operation of the hydrogen production system, including power input and material for catalyst preparation and regeneration, and capital cost for equipment. However, the cost for metal precursors HPtCl4 and HAuCl3 was negligible compared to the cost of P25 Degussa TiO2. Hence, a similar production cost for both catalysts was observed. Our studies demonstrated that the catalysts may be recycled for 132 days with 8 h of daily operation in 4 regenerated cycles in the continuous operation of the photo-reforming hydrogen production system.
Pretreatment cost is an essential factor due to chemical and energy usage. Hence, the costs were calculated to compare different pretreatment methods, including the costs of electricity and chemicals consumed during the pretreatment processes. The costs of the different optimization conditions with Au/TiO2 and Pt/TiO2 processes are compared with the revenue generation from H2 production. The cost of the catalyst is eliminated during this analysis since the catalyst is reusable. Overall cost analysis illustrates that the revenue generation from H2 is higher when applying Pt/TiO2 as the catalyst. The most cost-effective method for producing hydrogen is the EtOH-induced pretreatment process with a reactor pH of 7, as it results in higher H2 production and lower production costs for both catalysts. PET pretreatment with 10 M KOH at 40° C. is the least economical process, which shows an economic loss of around $0.10 per gram of catalyst. The KOH pretreatment process is more expensive than the EtOH-induced process owing to higher energy requirements and slower reaction time. Even though KOH pretreatment is costly, the average revenue generation is approximately 44% and 28% compared to the EtOH-induced pretreatment method with Au/TiO2 and Pt/TiO2, respectively.
Techno-economic analysis was performed, considering optimized conditions under the EtOH-induced pretreatment method with 0.7% (w/w) Pt/TiO2 and 5% (w/w) Au/TiO2 under separate cases. More than 62% of the capital was dedicated to equipment costs. The cost of power without the light source was compensated for more than 76% of daily operational costs. Three different scenarios were considered in the cost analysis, including applying a simulated light source, direct sunlight, and sunlight with a solar concentrator as the light irradiator.
The carbon emission was estimated considering the carbon emission from all the materials and equipment used in the production process. No CO2 was detected in the proposed process of H2 generation; hence, the carbon emission should be very low. Electricity usage during the operation using a simulated light source accounts for the highest carbon emission. Replacing the simulated light source with natural sunlight, assuming it reduces the H2 production by 20%, slightly reduces the production cost, and carbon emission is drastically reduced to a negative value. Implementing a solar concentrator to the system provided that the H2 production is restored to the original capacity and production cost is reported to be significantly reduced with negative carbon emission.
The photo-reforming process developed in this study provides a more promising alternative solution due to the ability to convert waste plastic to valuable hydrogen resources, easy operation, and simple reactor design. Further, the normalized H2 yield from our study is higher compared to the photo-reformed H2 rate from most of the other recent studies utilizing waste PET. The H2 production from this study was twice that of the 5% Au/TiO2 catalyst system despite the lower Pt concentration applied in this system. Hence, the results illustrate the improvement of catalytic activity and subsequent H2 production ability in single or sub-nano Pt doped catalytic systems. However, CdS/CdOx quantum dots indicate a higher H2 yield due to their unique structure and mechanism compared to the rest of the metal-doped catalytic systems. Cd is a highly toxic, carcinogenic, and non-biodegradable heavy metal with a half-life of 25-30 years in plants and animals. In contrast, TiO2-based catalysts developed in our study are environmentally friendly and do not pose toxic effects on human beings and ecological systems. The harmful impact of CdS-based catalysts may be mitigated by recycling the catalysts and considering strategies to avoid photo corrosion and increase longevity.
The cost of the optimum hydrogen production system was calculated and compared with existing H2 production systems. Due to their different catalytic efficiencies, the cost of hydrogen generation under the ethanol-induced hydrogen production system depends on the applied catalyst. The cost of hydrogen production in this study with both catalysts is much lower than that of hydrogen via photo-reforming of waste PET using other pretreatment methods. The carbon footprint of the proposed optimized method is negative without utilizing carbon capture and storage techniques. Moreover, the practical use of natural light as the energy source and the ability to reuse the catalyst creates a decentralized, portable, environmentally friendly hydrogen production system.
The key factors influencing the production cost are the energy costs during PET pretreatment and operation. If the system is fully powered by natural solar energy, electricity usage will be reduced, and subsequent cost reductions will occur. 30.17 kg of CO2 is saved per 1 kg of H2 production via the proposed photo-reforming process by converting waste PET using sunlight.
The preliminary cost and carbon footprint estimations presented in the study were based on a bench-scale photo-reforming hydrogen production system. Cost and carbon footprint for power generation are identified as significant factors compared to that of applied chemicals. The performance of the catalysts may be improved by applying the dark deposition method while minimizing the usage of precious metals such as platinum. The proven recyclability of the catalysts further reduces catalyst usage, and deactivated catalysts may be recovered to prevent environmental contamination.
Nanoparticles of TiO2 doped with Zn were produced through the sol-gel approach. A 160 ml of ethanol (EtOH, ACS reagent, 99.5%) solution and 20 ml of isopropanol (natural, ≥98%) were slowly combined with 20 mL (50 mmol) of titanium (IV) butoxide (Ti(OC4H9)4, >97%) while sonicating. The resultant mixture was gradually added to 100 mL of DI water that contained 0.1 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥98%) and 2 g of cetyltrimethylammonium bromide (CTAB, C19H42BrN, ≥98%) to form white precipitates. CTAB served as a surfactant to influence the morphology of Zn2+ on TiO2. The mixture was continuously stirred at 80° C. for 4 h. The extra water was evaporated in a water bath while being stirred. Then, the resulting precursor was dehydrated at 110° C. for 12 h and subsequently calcinated at 500° C. for 4 h in a high-temperature muffle furnace.
To better understand the correlation between PET and hydrogen production, we developed a theoretical mass balance for H2 production through ethanol (EtOH, C2H6O)-induced pretreatment of PET (C10H8O4).
When PET reacts with EtOH under basic conditions, it breaks down into terephthalic acid (TPA, C10H10O4) and ethylene glycol (C2H6O2), which contributes to hydrogen production.
When evaluating the activity of TiO2-based nano-photocatalysts, the following balance equation may be derived:
In this case, the active catalyst is denoted by ‘*’. In one of our ongoing studies, we observed that the quantum efficiency of Pt/TiO2 is 7%. Therefore, the H2 production may be calculated as follows:
E is the energy of the photon, h is Planck's constant (6.626×10−34 Js), c is the speed of light (3.0×108 m/s) λ is the wavelength of the UV light (assume 400 nm for Pt/TiO2). Hence E=4.97×10−19 J/photons. Photon Flux (F) may be calculated as follows:
With a quantum efficiency of 7%, the effective number of photons contributing to hydrogen production is: Effective photons for H2=0.07×3.22×1020=2.25×1019 photons/s Assuming each photon produces one molecule of hydrogen, the rate of hydrogen production in moles per second is:
For 6 hour run:
So, after running the process for 6 hours with a 160 W lamp, approximately 1.62 g of hydrogen may be theoretically produced.
The preceding examples may be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
Note that in the specification and claims, “about” or “approximately” means within twenty percent (20%) of the numerical amount cited. The terms, “a”, “an”, “the”, and “said” mean “one or more” unless context explicitly dictates otherwise.
Although the invention has been described in detail with particular reference to these embodiments, other embodiments may achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.
This application is a continuation-in-part of U.S. patent application Ser. No. 18/378,586, entitled “METHOD OF HYDROGEN MANUFACTURE”, filed on Oct. 10, 2023, which claims priority to and the benefit of the filing of U.S. Provisional Patent Application No. 63/414,332, entitled “METHOD OF HYDROGEN MANUFACTURE”, filed on Oct. 7, 2022.
| Number | Date | Country | |
|---|---|---|---|
| 63414332 | Oct 2022 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 18378586 | Oct 2023 | US |
| Child | 19035486 | US |
