Patent Application
20160296909
A. Field of the Invention
The invention generally concerns photocatalysts that can be used to produce hydrogen from water in a photocatalytic reaction. The photocatalysts include SrTiO3 or CeO2 as the photoactive material and graphene (e.g., graphene oxide or reduced graphene oxide) as the conductive material.
B. Description of Related Art
Hydrogen production from water offers enormous potential benefits for the energy sector, the environment, and the chemical industry (See, for example, Kodama et al. Chem Rev. 2007, 107:4048; Connelly et al., in Green Chemistry. 2012, 14:260; Fujishima et al. in Nature, 1972, 238, 1972; Kudo et al. in Chem Soc Rev 38:253, 2009; Nadeem et al. in Nanotechnology 2012, 9:121; and Maeda et al. in Nature, 2006, 440:295). While methods currently exist for producing hydrogen from water, many of these methods can be costly, inefficient, or unstable. For instance, photo electrochemical (PEC) water splitting requires an external bias or voltage and a costly electrode (e.g., Pt-based) for electrolysis of water.
With respect to photocatalytic electrolysis of water from light sources, while many advances have been achieved in this area (See, for example, Connelly et al., in Green Chemistry. 2012, 14:260; Fujishima et al. in Nature, 1972, 238, 1972; Kudo et al. in Chem Soc Rev 38:253, 2009; Nadeem et al. in Nanotechnology 2012, 9:121; and Maeda et al. in Nature, 2006, 440:295), most materials are either unstable under realistic water splitting conditions or require considerable amounts of other components (e.g., large amounts of sacrificial hole or electron scavengers) to work, thereby offsetting any gained benefits. By way of example, a semiconductor photocatalyst is a material that can be excited upon receiving energy equal to or higher than its electronic band gap. Upon photo-excitation, electrons are transferred from the valence band (VB) to the conduction band (CB), resulting in the formation of an electron (in the CB) and a hole (in the VB). In the case of water splitting, electrons in the CB reduce hydrogen ions to H2 and holes in the VB oxidize oxygen ions to O2. One of the main limitations of most photocatalysts is the fast electron-hole recombination; a process that occurs at the nanosecond scale, while the oxidation-reduction reactions are much slower (microsecond time scale). Over 90% of photo-excited electron-hole pairs disappear before reaction by radiative and non-radiative decay mechanisms (See, for example, Yamada et al. in Appl Phys Lett., 2009, 95:121112-121112-3). Current photocatalysts such as those that utilize noble metals dispersed on the surface of a photoactive material suffer from these inefficiencies.
A solution to the aforementioned inefficiencies surrounding current water-splitting photocatalysts has been discovered. In particular, the solution resides in using graphene nanostructures as the conductive material and either SrTiO3 or CeO2 microstructures or larger as the photoactive material. In one particular aspect, a relatively strong attachment of the graphene to the photoactive material is obtained by precipitation of an aqueous solution of the photoactive material in the presence of graphene. Without wishing to be bound by theory, it is believed that the specific combination of graphene nanostructures and SrTiO3 or CeO2 microstructures or larger reduce the likelihood that an excited electron would spontaneously revert back to its non-excited state (i.e., the electron-hole recombination rate can be reduced or suppressed). This provides for a more efficient use of the excited electrons in water-splitting applications. Further, this improved efficiency allows for a reduced reliance on additional materials such as sacrificial agents as well as electrically conductive noble metals, thereby decreasing the complexity and costs associated with photocatalytic water-splitting systems.
In one aspect of the present invention, there is disclosed a photocatalyst comprising graphene (e.g., graphene oxide or reduced graphene oxide or a combination thereof) nanostructures or combinations thereof attached to the surface of a photoactive metal oxide semiconductor selected from SrTiO3 or CeO2, wherein the photoactive metal oxide semiconductor is a microstructure or larger. Conductive material “attached” to the surface of a photoactive metal oxide semiconductor includes embodiments wherein the conductive material is chemically or physically bonded to the surface, and embodiments wherein the conductive material is dispersed or distributed on the surface of a photoactive metal oxide. In a preferred embodiment, the graphene is attached to the surface of the photoactive metal oxide semiconductor via precipitation of the photoactive metal oxide semiconductor from an aqueous solution comprising the graphene. In certain aspects, the nanostructure has a size ranging from 1 to less than 1000 nm, or 1 to 500 nm, or 1 to 100 nm, or 1 to 50 nm, or 1 to 25 nm, or 1 to 10 nm. In particular instances, the graphene is a nanowire, nanoparticle, nanocluster, or nanocrystal, or any combination thereof In even more particular instances, the graphene is not a graphene platelet or a graphene sheet (i.e., a sheet of carbon atoms arranged in a honeycomb lattice that has two opposing planar/substantially planar surfaces). The photoactive metal oxide semiconductor can be a particle such as a microparticle or larger. In particular embodiments, it was found that low amounts of conductive materials can be used and still efficiently split water and create hydrogen gas. Such amounts can be less than 5, 4, 3, 2, or 1 wt. % of the total weight of the photocatalysts. Also, the conductive material can cover less than 50, 40, 30, 20, 10, or 5% of the surface area of the photoactive metal oxide semiconductor, or can cover from about 0.0001 to 5% of the total surface area of the photoactive material, and still efficiently produce hydrogen from water. In particular aspects, the photocatalyst can be in particulate or powdered form and can be added to water. With a light source, the water can be split and hydrogen and oxygen gas formation can occur. In particular instances, a sacrificial agent can also be added to the water so as to further prevent electron/hole recombination. Notably, the efficiency of the photocatalyst of the present invention allows for one to avoid using or to use substantially low amounts of sacrificial agent when compared to known systems. In one instance, 0.1 to 5 vol. % of the photocatalyst and/or 0.1 to 5 g/L % of the sacrificial agent can be added to water. Non-limiting examples of sacrificial agents that can be used include methanol, ethanol, ethylene glycol propanol, iso-propanol, n-butanol, iso-butanol, ethylene glycol, propylene glycol, glycerol, or oxalic acid, or any combination thereof In particular aspects, ethanol is used or ethylene glycol is used or a combination thereof. The photocatalyst can be self-supported (i.e., it is not supported by a substrate) or it can be supported by a substrate (e.g., glass, polymer beads, metal oxides, etc.). As noted above, the photocatalysts of the present invention are capable of splitting water in combination with a light source. No external bias or voltage is needed to efficiently split said water. In one non-limiting embodiment, the photocatalyst is capable of producing hydrogen gas from water at a rate of 1×10−7 to 30×10−7 mol/gCatal min. or from about 1×10−7 and 10×10−7 mol/gCatal min, or from about 1×10−7 and 5×10−7 mol/gCatal min, or from about 2×10−7 and 3×10−7 mol/gCatal min.
Also disclosed is a system for producing hydrogen gas and/or oxygen gas from water. The system can include a container (e.g., transparent or translucent containers or opaque containers such as those that can magnify light (e.g., opaque container having a pinhole(s)) and a composition that includes photocatalyst of the present invention, water, and optionally a sacrificial agent. The container in particular embodiments is transparent or translucent. The system can also include a light source for irradiating the composition. The light source can be natural sunlight or can be from a non-natural source such as a UV lamp. As noted above, the system does not have to include an external bias or voltage.
In another embodiment, there is disclosed a method for producing hydrogen gas and/or oxygen gas from water, the method comprising using the aforementioned system and subjecting the composition to the light source for a sufficient period of time to produce hydrogen gas and/or oxygen gas from the water. The photocatalyst can be heated to between 200° C. and 400° C. prior to addition of the photocatalyst to the water. The hydrogen gas and/or oxygen gas can then be captured and used in other down-stream processes such as for ammonia synthesis (from N2 and H2), for methanol synthesis (from CO and H2), for light olefins synthesis (from CO and H2), or other chemical production processes that utilize H2 etc. In one non-limiting aspect, the method can be practiced such that the hydrogen production rate from water can be modified as desired by increasing or decreasing the amount of light or light flux that the system is subjected to. By way of example, a light source having a flux of about 0.1 mW/cm2 to 30 mW/cm2 can be used to produce hydrogen at a rate of about 1×10−7 to 30×10−7 mol/gCatal min.
The following includes definitions of various terms and phrases used throughout this specification.
“Water splitting” or any variation of this phrase describes the chemical reaction in which water is separated into oxygen and hydrogen.
“Inhibiting,” “preventing,” or “reducing” or any variation of these terms, when used in the claims or the specification includes any measurable decrease or complete inhibition to achieve a desired result. By way of example, reducing the recombination of an excited electron encompasses a situation where a decrease in the amount of recombination occurs in the presence of a photocatalyst of the present invention when compared with a situation where, for example, a photocatalyst is used that does not have the graphene nanostructure attached to the surface of a metal oxide semiconductor.
“Effective” or any variation of this term, when used in the claims or specification, means adequate to accomplish a desired, expected, or intended result.
“Nanostructure” refers to an object or material in which at least one dimension of the object or material is equal to or less than 100 nm (e.g., one dimension is 1 to 100 nm in size). In a particular aspect, the nanostructure includes at least two dimensions that are equal to or less than 100 nm (e.g., a first dimension is 1 to 100 nm in size and a second dimension is 1 to 100 nm in size). In another aspect, the nanostructure includes three dimensions that are equal to or less than 100 nm (e.g., a first dimension is 1 to 100 nm in size, a second dimension is 1 to 100 nm in size, and a third dimension is 1 to 100 nm in size). The shape of the nanostructure can be of a wire, a particle, a sphere, a rod, a tetrapod, a hyper-branched structure, or mixtures thereof. In some aspects, the nanostructure of the present invention can be a graphene platelet or a graphene sheet (i.e., a sheet of carbon atoms arranged in a honeycomb lattice that has two opposing planar/substantially planar surfaces), while in other instances it can exclude such graphene platelets or sheets.
“Microstructure” refers to an object or material in which at least one dimension of the object or material is between 0.1 and 100 μm and in which no dimension of the object or material is 0.1 μm or smaller. In a particular aspect, the microstructure includes two dimensions that are between 0.1 and 100 μm (e.g., a first dimension is 0.1 to 100 μm in size and a second dimension is 0.1 to 100 μm in size). In another aspect, the microstructure includes three dimensions that are between 0.1 and 100 μm (e.g., a first dimension is 0.1 to 100 μm in size, a second dimension is 0.1 to 100 μm in size, and a third dimension is 0.1 to 100 μm in size).
The terms “about” or “approximately” are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.
The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.”
The words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.
The photocatalysts and photoactive materials of the present invention can “comprise,” “consist essentially of,” or “consist of” particular components, compositions, ingredients, etc. disclosed throughout the specification. With respect to the transitional phase “consisting essentially of,” in one non-limiting aspect, a basic and novel characteristic of the photoactive catalysts and materials of the present invention are their ability to efficiently use excited electrons in water-splitting applications to produce hydrogen.
Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
While hydrogen-based energy has been proposed by many as a solution to the current problems associated with carbon-based energy (e.g., limited amounts and fossil fuel emissions), the currently available technologies are expensive, inefficient, or unstable. The present application provides a solution to these issues. The solution is predicated on the use of photocatalysts that employ a photoactive metal oxide semiconductor selected from SrTiO3 or CeO2 in combination with a graphene nanostructure attached to the surface of said photoactive metal oxide. These photocatalysts can be used for efficient hydrogen production by splitting water via a light source such as sunlight or a UV lamp.
These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.
The graphene nanostructures 17 can be used as conductive material for the excited electrons to ultimately reduce hydrogen ions to produce hydrogen gas. The graphene can be graphene oxide or it can be graphene oxide that has been reduced. Graphene nanostructures 17 are conductive materials with very low resistivity, making them well suited to act in combination with a photoactive metal oxide 12 in photoactive catalyst of the present invention (e.g., 10) to facilitate fast transfer of excited electrons to hydrogen before the electron-hole recombination. The graphene 17 nanostructures have at least one dimension that measures 100 nm or less. In some embodiments, the nanostructures can have two or three dimensions that measure 100 nm or less. In some embodiments, the nanostructures can have one or two dimensions that measure more than 100 nm. The nanostructures can be of any shape suitable for use in the photoactive catalytic systems of the present invention, including but not limited to nanowires, nanoparticles, nanoclusters, nanocrystals, or combinations thereof.
The photoactive metal oxides 12 of the present invention are commercially available from a wide range of sources (e.g., Sigma-Aldrich® Co. LLC (St. Louis, Mo., USA); Alfa Aesar GmbH & Co KG, A Johnson Matthey Company (Germany)). Alternatively, they can be made by any process known by those of ordinary skill in the art (e.g., precipitation/co-precipitation, sol-gel, template/surface derivatized metal oxide synthesis, solid-state synthesis of mixed metal oxides, microemulsion technique, solvothermal, sonochemical, combustion synthesis, etc.). In a non-limiting aspect, the metal oxides 12 can be made by creating aqueous solutions of metal ions and precipitating metal oxides out of solution. This precipitation can take place in the presence of graphene 17, resulting in the nanostructures 17 being attached to at least a portion of the surface of the photoactive metal oxides 12.
Graphene nanostructures 17 are commercially available from a wide range of sources (e.g., Sigma-Aldrich® Co. LLC (St. Louis, Mo., USA); Graphenea S. A. (Donostia-San Sebastian, Spain)). Alternatively, they can be made by any process known by those of ordinary skill in the art (e.g., mechanical exfoliation, chemical vapor deposition, sonication, cutting open carbon nanotubes, reduction of graphite oxide, etc.). In a non-limiting aspect, graphene oxide 17 can be produced from graphite by oxidizing graphite to form graphite oxide, followed by stirring, sonication, or both to exfoliate graphene oxide monolayers from multi-layer graphite oxide. Graphene oxide 17 can then be reduced using a number of methods, including but not limited to exposure to hydrogen plasma, thermal treatment under hydrogen, exposure to strong pulse light, heating in distilled water, mixing with an expansion-reduction reagent such as urea followed by heating, directly heating in a furnace, linear sweep voltammetry, and exposure to a reducing agent such as, for example, N2H4.
Attachment of graphene nanostructures 17 to the surface of photoactive metal oxides 12 can be accomplished by any process known by those of ordinary skill in the art. Attachment can include dispersion and/or distribution of the nanostructures 17 on the surface of the photoactive metal oxides 12. Attachment can be accomplished, for example, by precipitating metal oxides 12 out of solution in the presence of graphene nanostructures 17, followed by drying and calcination. As another non-limiting example, metal oxide 12 and graphene 17 can be mixed in a volatile solvent. After stirring and sonication, the solvent can be evaporated off. The dry material can then be ground into a fine powder and calcined. Calcination (such as at 300° C.) can be used to further crystalize the metal oxides 12.
Once the photocatalysts of the present invention are prepared, they can be placed in a transparent container containing an aqueous solution and used in a water splitting system. Referring again to
The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.
Graphene oxide (GO) was produced from graphite using a modified Hummers method (Hummers & Offeman, 1958). In a dry 500 mL round bottom flask equipped with a magnetic stirrer, graphite powder (1 g), sodium nitrate (1 g, 11.76 mmol), and sulphuric acid (46 mL) were combined and stirred in an ice bath. To the resulting reaction mixture, KMnO4 (6 g, 37.96 mmol) was slowly added. Once mixed, the reaction flask was transferred to an oil bath and vigorously stirred for 1 h at 40° C. To the resulting brown paste, 80 ml of water was added, and the slurry was stirred for additional 1 h while the temperature was raised to 90° C. Finally, 200 mL of water was added, followed by the slow addition of 6 mL of H2O2 (30%), turning the color of the solution from dark brown to brownish-yellow. The product was filtered off (while warm), washed with excess water, and dried under reduced pressure.
Reduced GO (RGO) was made by placing into a 250 mL round bottom flask a suspension of the GO (0.3 g) in water (100 mL), followed by the addition of hydrazine monohydrate (0.1 mL). The mixture was then stirred for 24 h at 80° C. The resulting black powder was filtered off, sequentially washed with water, HCl (10%), and acetone. The product was finally dried under vacuum.
RGO was also made by placing a dry sample (0.1 g) of GO in a quartz tube furnace. The tube containing the GO sample was purged with nitrogen gas for 10 min. prior to heat treatment. The sample was then heated up to 1000° C. under flowing nitrogen. The heat treatment was performed as follows: 1) heat for 18 min to reach 1000° C., 2) maintain at 1000° C. for 5 min., 3) slowly cool to 20° C. over 200 min., 4) allow to reach room temperature over 50 min.
Preparation of Graphene/SrTiO3 and Graphene/TiO2 catalysts:
To produce graphene/SrTiO3, graphene (2 wt. %) prepared in accordance with paragraph [0033] above was mixed with SrTiO3. Ethanol (100 ml) was then added and the mixture was sonicated in a water bath for 2 hours to get a homogeneous mixture. The mixture was then gently stirred at room temperature for 12 hours to allow for slow evaporation of the solvent. The obtained solid material was then ground to a fine powder. The resulting catalyst was then calcined at 300° C. for 5 hours.
Graphene/SrTiO3 was also be prepared by dissolving Sr(NO3)2 and TiCl4, Ti((CH3)3CO)4, or Ti(CH3CH2O)4 in water. Graphene (3 wt. %) is then added to the solution and the whole mixture was sonicated for 30 min. SrTiO3 is precipitated using NH4OH. The mixture was then washed several times and dried overnight, followed by calcination at 500° C. for 5 hours.
Graphene/CeO2 was prepared from ceric ammonium nitrate (CeH8N8O18) (3.18 g), which was charged into a 100 ml round bottom flask. Water (10 ml) was then added to form a solution. Graphene (3 wt %, 30 mg) was then added to the solution and the whole mixture was sonicated for 30 min. CeO2 was precipitated using NH4OH. The mixture was then washed several times and dried overnight, followed by calcination at 500° C. for 5 hours.
The prepared catalyst from Example 1 (20 mg, powder) was charged into a batch reactor. The catalyst was then reduced at 300° C. for one hour. The reactor was purged with nitrogen gas for 30 min. Water (25 ml) was then injected into the reactor. The mixture was stirred under UV-irradiation. Gas samples were collected using a syringe and analysed by using GC-TCD equipped with a Porapak Q column at different time intervals.
This application claims the benefit of U.S. Provisional Application No. 61/911,805, filed Dec. 4, 2013. The contents of the referenced patent application are incorporated into the present application by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2014/066567 | 12/3/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61911805 | Dec 2013 | US |
