A. Field of the Invention
The invention generally concerns mixed phase titanium dioxide nanoparticles that can be used to produce hydrogen and oxygen from water in photocatalytic reactions. In particular, the nanoparticles can have a mean particle size of 95 nanometers (nm) or less and a ratio of anatase to rutile of at least 1.5:1.
B. Description of Related Art
Hydrogen production from water offers enormous potential benefits for the energy sector, the environment, and the chemical industry. While methods currently exist for producing hydrogen and oxygen from water, many of these methods can be costly, inefficient, or unstable. For instance, photoelectrochemical (PEC) water splitting requires an external bias or voltage and a costly electrode (e.g., Pt-based).
With respect to photocatalytic electrolysis of water from light sources, while many advances have been achieved in this area, 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 excited 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. To increase the electron life time metal deposition on the semiconductor surfaces are routinely used while organic compounds such as alcohols and glycols are added to the aqueous media to increase the hold lifetime. Current photocatalysts such as those that utilize photoactive materials having a uniform phase structure 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 mixed-phase TiO2 nanoparticles having a mean particle size of 95 nanometers (nm) or less and a ratio of anatase to rutile of at least 1.5:1 as photocatalysts. The mixed phase titanium dioxide nanoparticles are the reaction or transformational product of single phase titanium dioxide anatase nanoparticles having a mean particles size of 95 nm or less that have been subjected to heat. It has unexpectedly been found that these transformed photocatalysts demonstrate increased hydrogen production when compared with similar catalysts made from microparticles rather than the nanoparticles of the present invention. Without wishing to be bound by theory, it is believed that subjecting the nanoparticles to heat results in a higher degree of crystallinity, which can then reduce the likelihood that an excited electron will spontaneously revert back to its non-excited state (i.e., the electron-hole recombination rate can be reduced or delayed for a sufficient period of time). Further, it is believed that the anatase to rutile ratio of at least 1.5:1 allows for the efficient transfer of charge carriers (electrons) from the rutile phase to the anatase phase, thereby further reducing the likelihood that an electron-hole recombination event would occur. The improved efficiency of the transformed photocatalysts of the present invention allows for a reduced reliance on additional materials such as sacrificial agents, thereby decreasing the complexity and costs associated with using the photocatalysts in water-splitting applications and systems.
In one aspect of the present invention, there is disclosed a photocatalyst that includes TiO2 The TiO2 includes mixed phase titanium dioxide nanoparticles having mean particle size of 95 nanometers (nm) or less and a ratio of anatase and rutile phases of at least 1.5:1. The ratio of anatase and rutile phases in the nanoparticles can range from 1.5:1 to 10:1, is about 5:1, or is about 4:1. Electrically conductive material may be deposited on the surface of the titanium dioxide. The mixed phase titanium dioxide nanoparticles are the reaction product single phase titanium dioxide anatase nanoparticles having a mean particle size of 95 nm or less and heat. The single phase TiO2 anatase nanoparticles can be heated isochronally at a desired temperature ranging from about 700° C. to about 800° C. to form the mixed phase TiO2 nanoparticles. In some instances, the single phase TiO2 anatase nanoparticles are heated isochronally at a temperature ranging from about 740° C. for one hour. The surface area of the mixed phase titanium dioxide nanoparticles is at least 15 m2/g, or from about 15 m2/g to about 30 m2/g. The mean particle size of the mixed phase titanium dioxide nanoparticles ranges from about 10 nm to about 80 nm, from about 15 nm to about 50 nm, from about 20 nm to about 40 nm, or from about 15 nm to about 20 nm.
Electrically conductive material dispersed on the surface of the nanoparticles may increase the efficiency of water splitting reactions. The metal material may include a metal or metal compound of silver (Ag), rhodium (Rh), gold (Au), platinum (Pt), palladium (Pd), or any combination thereof In a preferred embodiment, the electrically conductive material is platinum. The photocatalyst may include about 0.05 wt. % to about 5 wt. % of the electrically conductive material. Such amounts can be less than 5, 4, 3, 2, 1, or 0.5 wt. % of the total weight of the photocatalyst. The electroconductive material may be impregnated to the mixed phase titanium dioxide. The TiO2 nanoparticle photocatalyst has a band gap between about 3.0 electron volts (eV) and 3.2 eV. A Ti2p3/2 binding energy of the mixed phase TiO2 photocatalyst falls in between that of single phase TiO2 anatase particle and a single phase TiO2 rutile particle.
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 water. The hydrogen production rate from water can be modified as desired by subjecting the system to different amounts of light or light flux. In particular aspects, the photocatalysts of the present invention can be used in water splitting systems to provide a hydrogen production rate from water between 1×10−4 and 3×10−3 mol/gCatal min under direct sunlight. It was surprisingly found that the photocatalyst is capable of producing hydrogen (H2) from water at an increased rate as compared to production of H2 from water under the same conditions and using a similar mixed phase titanium dioxide microparticle photocatalyst. In some instances, the photocatalysts of the present invention are capable of catalyzing the photocatalytic oxidation of an organic compound.
Also disclosed is a composition that includes a photocatalyst of the invention, water, and a sacrificial agent that can be used for water splitting. With a light source, the water can be split and hydrogen and oxygen gas formation can occur. In particular instances, the sacrificial agent may further prevent electron/hole recombination. In some instances, the composition includes 0.1 to 5 g/L of the photocatalyst. Notably, the efficiency of the photocatalysts of the present invention allows for one to use substantially low amounts (or none at all) of sacrificial agent when compared to known systems. In particular instances, 0.1 to 10 w/v %, or preferably 2 to 7 w/v %, of the sacrificial agent may be included in the composition. Non-limiting examples of sacrificial agents that can be used include methanol, ethanol, propanol, n-butanol, iso-butanol, iso-methyl tert-butyl ether, ethylene glycol, propylene glycol, glycerol, oxalic acid, or any combination thereof. In particular aspects, ethanol, ethylene glycol, glycerol, or a combination thereof is used.
In another aspect of the present invention, there is disclosed 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 or artificial source such as a UV lamp. While the system may use an external bias or voltage, such an external bias or voltage is not needed due to the efficiency of the photocatalysts of the present invention. In particular aspects, the method can be practiced such that the hydrogen production rate from water is between 1×10−4 to 3×10−3 mol/gCatal min with direct sunlight.
In the context of the present invention embodiments 1-41 are described. Embodiment 1 is a photocatalyst that includes (a) mixed phase titanium dioxide nanoparticles having a mean particle size of 95 nanometers (nm) or less and having a ratio of anatase to rutile of at least 1.5:1; and (b) an electrically conductive material deposited on the surface of the titanium dioxide nanoparticles, wherein the mixed phase titanium dioxide nanoparticles are the reaction product of single phase titanium dioxide anatase nanoparticles having a mean particle size of 95 nm or less and heat. Embodiment 2 is the photocatalyst of embodiment 1, wherein a surface area of the mixed phase titanium dioxide nanoparticles is at least 15 m2/g. Embodiment 3 is the photocatalyst of embodiment 1, wherein a surface area of the mixed phase titanium dioxide nanoparticles ranges from about 15 m2/g to about 30 m2/g. Embodiment 4 is the photocatalyst of any one of embodiments 1 to 3, wherein the ratio of anatase and rutile phase ranges from 1.5:1 to 10:1. Embodiment 5 is the photocatalyst of any one of embodiments 1 to 3, wherein the anatase phase to rutile phase ratio is about 5:1. Embodiment 6 is the photocatalyst of any one of embodiments 1 to 3, wherein the anatase phase to rutile phase is ratio about 4:1. Embodiment 7 is the photocatalyst of any one of embodiments 1 to 6, wherein the mean particle size ranges from about 10 nm to about 80 nm, or from about 15 nm to about 50 nm, or from about 20 nm to about 40 nm, or from about 15 nm to about 20 nm. Embodiment 8 is the photocatalyst of any one of embodiments 1 to 7, wherein the Ti2p3/2 binding energy as determined by X-Ray PhotoElectron Spectroscopy (XPS) falls in between that of single phase TiO2 anatase particle and a single phase TiO2 rutile particle. Embodiment 9 is the photocatalyst of any one of embodiments 1 to 8, wherein the electrically conductive material includes a metal or a metal compound thereof. Embodiment 10 is the photocatalyst of any one of embodiments 1 to 9, wherein the electrically conductive material includes silver (Ag), rhodium (Rh), gold (Au), platinum (Pt), palladium (Pd) or any combination thereof. Embodiment 11 is the photocatalyst of any one of embodiments 1 to 9, wherein the electrically conductive material includes Pt. Embodiment 12 is the photocatalyst of any one of embodiments 1 to 11, wherein the photocatalyst includes from about 0.05% to about 5% by weight of the electrically conductive material. Embodiment 13 is the photocatalyst of any one of embodiments 1 to 12, wherein the photocatalyst includes about 1% by weight of Pt. Embodiment 14 is the photocatalyst of any one of embodiments 1 to 13, wherein the single phase TiO2 anatase nanoparticles have been heated isochronally at a temperature ranging from about 700° C. to about 800° C. for a desired period of time. Embodiment 15 is the photocatalyst of any one of embodiments 1 to 13, wherein the single phase TiO2 anatase particles have been heated at a temperature of about 740° C. for one hour. Embodiment 16 is the photocatalyst of any one of embodiments 1 to 15, wherein the photocatalyst has a band gap between about 3.0 electron volts (eV) and 3.2 eV. Embodiment 17 is the photocatalyst of any one of embodiments 1 to 16, wherein the photocatalyst is capable of catalyzing the photocatalytic splitting of water. Embodiment 18 is the photocatalyst of any one of embodiments 1 to 17, wherein the photocatalyst is capable of producing H2 from water at an increased rate as compared to production of H2 from water under the same conditions and using a mixed phase titanium dioxide photocatalyst having a substantially same amount of anatase and rutile phases and a particle size of greater than 100 nm. Embodiment 19 is the photocatalyst of any of embodiments 17 or 18, wherein the photocatalyst is comprised in a composition that includes the water. Embodiment 20 is the photocatalyst of embodiment 19, wherein the composition further includes a sacrificial agent. Embodiment 21 is the photocatalyst of embodiment 20, wherein the sacrificial agent includes one or more alcohols, diols, polyols, dioic acids, and any combination thereof. Embodiment 22 is the photocatalyst of any one of embodiments 20 or 21, wherein the sacrificial agent includes methanol, ethanol, propanol, iso-propanol, n-butanol, iso-butanol, ethylene glycol, propylene glycol, glycerol, or oxalic acid, or any combination thereof. Embodiment 23 is the photocatalyst of any one of embodiments 20 or 21, wherein the sacrificial agent is ethanol or ethylene glycol. Embodiment 24 is the photocatalyst of any one of embodiments 20 to 23, wherein the composition includes 0.1 to 5 g/L of the photocatalyst and/or 0.1 to 5 vol. % of the sacrificial agent. Embodiment 25 is the photocatalyst of any one of embodiments 17 to 24, wherein the H2 production rate from water is 1×10−4 to 3×10−3 mol/gcatal min under direct sunlight.
Embodiment 26 is a method of producing a photocatalyst of any one of embodiments 1-25, that includes (a) heating single phase titanium dioxide anatase nanoparticles having a mean particle size of 95 nanometers (nm) or less; (b) forming mixed phase titanium dioxide nanoparticles having a mean particle size of 95 nm or less, wherein the mixed phase titanium dioxide nanoparticles includes anatase and rutile phases at a ratio of at least 1.5:1; (c) depositing an electroconductive material on the surface of the mixed phase titanium dioxide nanoparticles. Embodiment 27 is the method of embodiment 26, wherein a surface area of the mixed phases titanium dioxide nanoparticles ranges from about 15 m2/g to about 30 m2/g. Embodiment 28 is the method of any one of embodiments 26 or 27, wherein the electroconductive material includes silver (Ag), rhodium (Rh), gold (Au), platinum (Pt), palladium (Pd) or mixtures thereof. Embodiment 29 is the method of any one of embodiments 26 to 27, wherein the electroconductive material includes platinum (Pt) or a compound thereof. Embodiment 30 is the method of any one of embodiments 26 to 29, wherein depositing the electroconductive material includes contacting the mixed phase TiO2 nanoparticles with an acidic aqueous solution that includes a salt of the electroconductive material. Embodiment 31 is the method of any one of embodiments 26 to 30, wherein heating includes heating the single phase titanium dioxide anatase nanoparticles isochronally in a temperature range from about 700° C. to about 800° C. for one hour. Embodiment 32 is the method of any one of embodiments 26 to 31, further including calcining mixed phase titanium dioxide anatase nanoparticles.
Embodiment 33 is a system for producing H2 from H2O that includes (a) a container that includes a mixture of photocatalyst of any one of embodiments 1 to 25, water and a sacrificial agent; and (b) a light source configured to provide light to the mixture. Embodiment 34 is the system of embodiment 33, wherein the light source includes sunlight. Embodiment 35 is the system of any one of embodiments 33 or 34, wherein the light source includes an ultra-violet light. Embodiment 36 is the system of any one of embodiments 33 to 35, wherein an external bias is not used to produce the H2. Embodiment 37 is the system of any one of embodiments 33 to 36, wherein the container is transparent.
Embodiment 38 is a method for producing H2 from water, that includes (a) obtaining a system of any one of embodiments 33 to 37; and (b) subjecting the mixture to the light source for a sufficient period of time to produce the H2 from the water. Embodiment 39 is the method of embodiment 38, further including producing oxygen (O2) from the water. Embodiment 40 is the method of any one of embodiments 38 or 39, wherein the light source is sunlight and H2 is produced at a rate from about 1×10−4 to about 3×10−3 mol/gCatal min. Embodiment 41 is the method of any one of embodiments 38 to 40, wherein the H2 is produced at an increased rate as compared to a production of H2 under the same conditions and using a mixed phase titanium dioxide photocatalyst having a substantially same amount of anatase and rutile phases and a particle size of 95 nm or more.
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.
“Nanoparticle” refers to particles having a mean particle size of less than 100 nanometers.
“Microparticle” refers to particles having a mean particle size of 100 nm or more.
“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 likelihood for an excited electron in the conductive band to recombine with a hole in the valence band encompasses situations where a decrease in the number of electron/hole recombination events occurs or an increase in the time it takes for an electron/hole recombination event to occur such that the increase in time allows for the electron to reduce hydrogen ions rather than to recombine with its corresponding hole. In either instance, the photocatalysts of the present invention can be compared with photocatalysts of mixed phase TiO2 anatase particles to rutile phase microparticles (i.e., particles having a mean particle size of greater than 100 nm).
“Effective” or any variation of this term, when used in the claims or specification, means adequate to accomplish a desired, expected, or intended result.
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 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 from water has been proposed 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, and/or unstable. The present application provides a solution to these issues. The solution is predicated on the use of heat-treated mixed phase titanium dioxide nanoparticles having a mean particle size of 95 nanometers (nm) or less and a ratio of anatase to rutile of at least 1.5:1 as photocatalysts. It has been unexpectedly found that photocatalyst of the invention produces higher amounts of hydrogen in photocatalytic water-splitting reactions than similar photocatalysts made from microparticles. This higher hydrogen production rate is attributed to a synergistic effect between the phase ratio and the particle size of the titanium dioxide nanoparticles.
These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.
The photocatalyst is composed of titanium dioxide particles having two main polymorphs, anatase and rutile. The particles have different properties and different photocatalytic performance. Combination of these properties provides for a photocatalyst having better physical and electron transfer properties. While anatase and rutile both have a tetragonal crystal system consisting of TiO6 octahedra, their phases differ in that anatase octahedras are arranged such that four edges of the octahedras are shared, while in rutile, two edges of the octahedras are shared.
In one aspect of the invention, mixed phase titanium dioxide anatase and rutile may be the reaction (transformation) product obtained from heat-treating single phase titanium dioxide anatase at selected temperatures. Single phase TiO2 anatase nanoparticles can be purchased from various manufacturers and suppliers (e.g., Titanium (IV) oxide anatase nanoparticles in a variety of sizes and shapes can be obtained from Sigma-Aldrich® Co. LLC (St. Louis, Mo., USA) and from Alfa Aesar GmbH & Co KG, A Johnson Matthey Company (Germany)); L.E.B. Enterprises, Inc. (Hollywood, Fla. USA)). A surface area of the single phase TiO2 anatase nanoparticles ranges from about 45 m2/g to about 80 m2/g, or from 50 m2/g to 70 m2/g, or preferably about 50 m2/g. The particle size of the single phase TiO2 anatase nanoparticles is less than 95 nanometers, less than 50 nm, less than 20, or preferably between 10 and 25 nm. Reaction conditions can be varied based on the TiO2 anatase particle size and/or method of heating (See, for example, Hanaor et al. in Review of the anatase to rutile phase transformation, J. Material Science, 2011, Vol. 46, pp. 855-874), and are sufficient to transform single phase titanium dioxide to mixed phase titanium dioxide anatase and rutile. For example, titanium dioxide anatase can be transformed into a mixed phase polymorph by flame pyrolysis of TiCl4, solvothermal/hydrothermal methods, chemical vapor deposition, and physical vapor deposition methods. A non-limiting example of transforming nanoparticles of TiO2 anatase nanoparticles to mixed phase TiO2 anatase and rutile nanoparticles includes heating single phase TiO2 anatase nanoparticles isochronally at a temperature of 700-800° C. for about 1 hour to transform the nanoparticles of TiO2 anatase phase to nanoparticles of mixed phase TiO2 anatase phase and rutile phase (See, for example
In an aspect of the invention, it was surprisingly found that the percentage in the change of surface area of mixed phase titanium dioxide anatase and rutile nanoparticles was significantly different than the percentage change in the surface area of the mixed phase titanium dioxide anatase and rutile microparticles relative to the respective starting materials. For example, the surface area of the titanium dioxide microparticles decreased by about 40% at a 25% conversion to rutile phase relative to the surface area of the starting material as determined by Brunauer-Emmett-Teller (BET) methods. In contrast, the surface area of the titanium dioxide nanoparticles decreased by about 70% at a 29% conversion to rutile phase relative to the surface area of the staring material. In particular aspects of the invention, a surface area of the mixed phase TiO2 nanoparticles may decrease by a factor of at least 0.1, at least 0.4, or at least 0.5. The resulting mixed phase titanium dioxide nanoparticles have a surface area of about 15 m2/g, or preferably from 15 m2/g to 30 m2/g. Without wishing to be bound by theory, it is believed that the decrease in the nanoparticle surface area as compared to the microparticle surface area demonstrates that less sintering has occurred on the catalyst surface and a higher degree of crystallinity has been obtained. A higher degree of crystallinity leads to minimum perturbation of the titanium dioxide wave function, which allows enhanced migration of electrons from the bulk portion of the titanium dioxide particle to the surface of the titanium dioxide particle and less recombination of electrons.
Further, during heating, the particle size of the pure anatase changes from a single modal distribution to a bimodal distribution, where the anatase and rutile phases have different particle sizes. The overall particle size distribution, however, of the resulting TiO2 remains less than 100 nm. During heat treatment, the particle size of the original anatase phase increases by a factor of at least 1.5, at least 2, or at least 0.45, while the particle size of the formed rutile phase is from about 0 nm to less than 100 nm depending on the temperature used to form the rutile phase (See, for example, the d values for anatase and rutile phases in Table 1 of the Examples). Even though an increase in particles size is observed during heating, the mean particle size of the mixed phase TiO2 nanoparticles is less than 100 nm. The mixed phased nanoparticles of the invention have a mean particle size of less than 95 nm, from about 10 nm to about 80 nm, from about 15 nm to about 50 nm, from about 20 nm to about 50 nm, or from about 15 nm to about 20 nm.
In a further aspect of the invention, transforming the single phase TiO2 anatase to a mixed phase TiO2 having a anatase to rutile phase ratio of at least 1.5:1 changes the binding energy and the band gap relative to single phase TiO2 anatase and single phase TiO2 rutile. This change in binding energy and band gap is believed to allow efficient transfer of charge carriers (electrons) from the rutile phase to the anatase phase, where said charge carries in the anatase phase have an increased chance of being transferred to the metal conducting materials rather than undergoing an electron-hole recombination event. The TiO2 nanoparticles of the present invention have a Ti2p3/2 binding energy as determined by X-ray photoelectron spectroscopy (XPS) that is in between that of single phase TiO2 anatase and single phase TiO2 rutile. The mixed phase TiO2 nanoparticles also have a band gap between about 3.0 electron volts (eV) and 3.2 eV.
Electroconductive material may be deposited on the surface of the mixed phase TiO2 nanoparticles to increase the photocatalytic activity of the TiO2. The electroconductive material includes highly conductive materials, making them well suited to act in combination with the photoactive material to facility transfer of excited electrons to hydrogen before an electron-hole recombination event occurs or by increasing the time that such an event occurs. The electroconductive material can also enhance efficiency via resonance plasmonic excitation from visible light, enabling capture of a broader range of light energy. The electroconductive materials include noble metals such as, for example, platinum, gold, silver and palladium as metals or metal salts. Electroconductive material (i.e., platinum, gold, silver, and palladium) can be obtained from a variety of commercial sources in a variety of forms (e.g., solutions, particles, rods, films, etc.) and sizes (e.g., nanoscale or microscale). By way of example, Sigma-Aldrich® Co. LLC and Alfa Aesar GmbH & Co KG offer such products. Alternatively, they can be made by any process known by those of ordinary skill in the art. The electrically conducting material may be deposed on the surface of the mixed phase titanium dioxide nanoparticles. Deposition can include attachment, dispersion, and/or distribution of the metal particles on the surface of the photoactive material or TiO2 particles. A non-limiting example of depositing the electrically conductive material on the photoactive material includes impregnating the mixed phase TiO2 nanoparticles with a solution of metal salt. Impregnation may include contacting (for example, spraying or mixing) the mixed phase TiO2 nanoparticles with an acidic aqueous metal salt solution to form a mixture. The mixture may be stirred at a temperature of about 70° C. to 80° C. for about 10 h, 12 h, or longer. After stirring, the water may be evaporated off to form a dry material. The dry material may be calcined at a temperature of 200° C. to 400° C., or preferably at 350° C. for at least 2 h, at least 4 h, or preferably at least 5 h under atmospheric conditions. The resulting TiO2 photocatalyst has a total electroconductive material content of about 1 wt. % to about 5 wt. % or about 2 wt. % to about 4 wt %.
In addition to being capable of catalyzing water splitting without an external bias or voltage, the photocatalysts of the present invention may be included in an anode of an electrochemical cell capable of forming oxygen and hydrogen by electrolysis of water. In a non-limiting example, light energy may be provided to a photocell and from the light energy a voltage between the anode and the cathode is produced and water molecules are split to form hydrogen and oxygen. The method can be practiced such that the hydrogen production rate from water can be modified as desired by subjecting the system to various amounts of light or light flux.
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.
Synthesis of mixed phase TiO2 nanoparticle samples A-E. Single phase titanium dioxide anatase nanopowder was commercially purchased (Sigma Aldrich®). The nanopowder had a surface area of about 55 m2/gcatalyst and a particle size of about 20 nm. The nanopowder was annealed isochronally for 1 hour at different temperatures in the range of 700° C. to 800° C. to obtain mixed phase TiO2 nanoparticle samples A-E. The temperatures and amounts of rutile phase in the samples are listed in Table 1. Table 1 also lists the surface area and particle size of the anatase phase and the rutile phase in the samples. The amount of rutile phase was determined using XRD as described above. The particle size was determined using the Scherrer equation based on the main diffraction line.
Synthesis of mixed phase TiO2 microparticle comparison samples F-L. Single phase titanium dioxide anatase micropowder was commercially purchased (Fisher Scientific). The micropowder had a surface area of about 10 m2/gcatalyst and a particle size of about 100 nm. The micropowder was annealed isothermally at 1000° C. from 1 to 10 hours to obtain mixed phase TiO2 microparticle samples F-L. The temperature and amounts of rutile phase in the samples is listed in Table 1. Table 1 also lists the surface area and the rutile phase in microparticle samples F-L. The amount of rutile phase was determined using XRD as described above.
Deposition of Pt on mixed phase TiO2 materials. The mixed phase TiO2 nanoparticles and mixed phase TiO2 microparticles were impregnated with platinum. The platinum precursor solution was prepared by dissolving a calculated amount of platinum chloride (PtCl2) in 1 normal hydrogen chloride. The calculated amount of precursor solution was then contacted with each of samples A-L. The impregnated mixtures were subjected to stirring and were left at 70-80° C. overnight. The resulting slurries were then dried at 100° C. for 24 hours, followed by calcination at 350° C. for 5 hours in air. The resulting nanoparticle photocatalysts (photocatalysts A-E) and microparticle comparison photocatalysts (photocatalysts F-L) had an elemental platinum content of 1 wt. % based on the total weight of the catalyst.
Characterization of Photocatalysts: Characterization of the produced photocatalysts was performed with BET surface areas determination, X-Ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy.
X-Ray Diffraction (XRD): Powder XRD patterns of samples A-L were recorded on a Philips X′pert-MPD X-ray powder diffractometer. A 2 θ interval between 10 and 90 θ was used with a step size of 0.10 θ and a step time of 0.5 seconds. The X-ray was a Ni-filtered Cu Kα radiation source (K═=1.5418 Å), operated at 45 mA and 40 KV. The percentages of rutile were calculated using Equation (1) above and listed in Table 1. The anatase to rutile ratio was calculated by taking the intensity of anatase phase (101) peak at 2 θ=25.30° and rutile phase (110) peak at 2 θ=27.40°. Peak positions of anatase (101) and rutile (110) for nanoparticles A-E were shifted with increasing annealing temperature. Peak shifts of 0.3 degrees were observed in 2 θ values of anatase (101) and rutile (110) from 720° C. to 780° C. A reduction in lattice constants “a” (0.047 Å) and “c” (0.13.1 Å) was observed with an increase in annealing temperature from 720° C. to 780° C. (crystallite size of 45 nm). Peak positions of anatase (101) and rutile (110) for microparticles F-L were in agreement with reported values except for samples I and L, which had a shift in the peaks at lower 2 θ angles.
UV Absorption: UV-Vis absorbance spectra of the powdered catalysts were collected over the wavelength range of 250-900 nm on a Thermo Fisher Scientific UV-Vis spectrophotometer equipped with praying mantis diffuse reflectance. Samples were grounded using mortar and pestle before being introduced into the praying mantis chamber using a sample cup. Reflectance (% R) of the samples was measured. A 100 Watt ultraviolet lamp (H-144GC-100, Sylvania par 38) was used as a UV source with a flux of up to 3 mW/cm2, depending on the distance from the source, with the cut off filter (360 nm and above). The Kubelka-Munk function, F(R)=(1−R)2/(2R), was used to calculate the optical absorbance from the reflectance (R) of the samples compared to the standard. Band gap was estimated from the Tauc plot of the quantity (F(R) E)1/2 against the radiation energy.
Based on the observed band gap data, one would expect that the comparison photocatalysts containing mixed phase TiO2 microparticles having up to 78% rutile and the photocatalyst of the invention having mixed phase TiO2 nanoparticles having up to 38% rutile would have similar hydrogen production rates in a water-splitting process.
X-Ray Photoelectron Spectroscopy (XPS): XPS was conducted using a Thermo scientific ESCALB 250 Xi. The base pressure of the chamber ranged from 10-10 to 10-11 mbar. Charge neutralization was used for all samples. Spectra were calibrated with respect to C1s at 285.0 eV, Pt4f, O1s, Ti2p, C1s, and valence band energy regions were scanned for all materials. Typical acquisition conditions were as follows: pass energy=30 e V and scan rate=0.1 e V per 200 ms. Argon ion bombardment was performed with an EX06 ion gun at 1 kV beam energy and 10 rnA emission current; sample current was typically 0.9-1.0 nA. Self-supported oxide disks of approximately 0.5 cm diameter were loaded into the chamber for analysis.
Based on the data obtained from XPS, the photocatalysts of the invention have a band gap between about 3.0 eV and 3.2 eV.
Experimental Set-Up: Catalytic reactions were conducted in a borosilicate (Pyrex®, Corning) glass reactor having a capacity of 100 mL. For each experiment, a photocatalyst was added to the glass reactor in a concentration of 0.1 g/L (25 mg in 21 mL total volume). The photocatalyst was reduced under hydrogen flow at 350° C. for 1 h followed by purging with nitrogen gas for 30 minutes. Deionized water (20 mL) and sacrificial agent (ethanol, 5 v/v % based on total water, 1 mL) were added to the reactor. The reaction mixture was irradiated with sunlight, with a light flux at the front side of the reactor of between 0.3 and 1 mW/cm2. The mixture containing photocatalyst, water and sacrificial agent was stirred constantly under dark conditions to disperse the catalyst and sacrificial agent in the water. The reactor was then exposed to a UV light source (100 Watt UV lamp (H-144GC-100, Sylvania par 38) with a flux of about 2 mW/cm2 at a distance of 10 cm with the cut off filter (360 nm and above). Product analysis of the produced gas was done using a gas chromatography (Porapak™ Q (Sigma Aldrich) packed column 2 m, 45° C. (isothermal), with nitrogen as a carrier gas) with a thermal conductivity detector. Hydrogen production rates for reactions run with photocatalysts A-L were normalized with respect to BET surface area of each catalyst.
As shown in
This application claims benefit to U.S. Provisional Patent Application No. 62/022,962 titled, “PHOTOCATALYTIC HYDROGEN PRODUCTION FROM WATER OVER MIXED PHASE TITANIUM DIOXIDE NANOPARTICLES”, filed Jul. 10, 2014. The entire contents of the referenced application is incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/IB2015/054928 | 6/30/2015 | WO | 00 |
Number | Date | Country | |
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62022962 | Jul 2014 | US |