PHOTOCATALYTIC HYDROGEN PRODUCTION FROM WATER OVER CATALYSTS HAVING P-N JUNCTIONS AND PLASMONIC MATERIALS

Abstract

A photocatalyst and a method for producing hydrogen and oxygen from water by photocatalytic electrolysis are disclosed. The photocatalyst includes a photoactive material and metal or metal alloy material (15)—e.g. pure particles or alloys of Au, Pd and Ag—capable of having plasmon resonance properties deposited on the surface of the photoactive material. The photoactive material includes a p-n junction (17) formed by contact of a n-type semiconductor material (10), such as mixed phase TiO2 nano particles (anatase to rutile ratio of 1.5 to 1 or greater), and a p-type semiconductor material (16), such as CoO or Cu2O.

Claims

1. A photocatalyst comprising: a photoactive material comprising a n-type semiconductor material and a p-type semiconductor material, wherein a p-n junction is present at an interface of the p-type and n-type materials; and a metal or metal alloy material having surface plasmon resonance properties in response to visible light and/or infrared light, wherein the metal or metal alloy material is deposited on the surface of the photoactive material.

2. The photocatalyst of claim 1, wherein the n-type semiconductor material comprises titanium dioxide or zinc oxide.

3. The photocatalyst of claim 2, wherein the n-type semiconductor material comprises titanium dioxide that has an anatase to rutile ratio of greater than or equal to 1.5:1.

4. The photocatalyst of claim 1, wherein the p-type semiconductor material comprises cobalt (II) oxide or copper (I) oxide.

5. The photocatalyst of claim 1, wherein the n-type semiconductor material comprises titanium dioxide having an anatase to rutile ratio of greater than or equal to 2:1 and the p-type semiconductor material comprises cobalt (II) oxide.

6. The photocatalyst of claim 1, wherein the metal or metal alloy material is gold, silver-gold-palladium alloy, or silver-palladium alloy, respectively, or a mixture thereof.

7. The photocatalyst of claim 1, wherein the n-type material, the p-type material, and the metal or metal alloy material are each in particulate form.

8. The photocatalyst of claim 7, wherein the n-type material, the p-type material, and the metal or metal alloy material are each nanostructures, wherein the nanostructures are nanowires, nanoparticles, nanoclusters, or nanocrystals, or combinations thereof.

9. The photocatalyst of claim 1, comprising less than 5 wt. % of the metal or metal alloy material.

10. The photocatalyst of claim 1, wherein the metal or metal alloy material does not cover more than 30% of the surface area of the photoactive material.

11. The photocatalyst of claim 1, wherein the ratio of the n-type semiconductor material to the p-type semiconductor material is 75 to 25.

12. The photocatalyst of claim 1, wherein the photocatalyst is deposited onto a substrate such as an indium tin oxide substrate, a stainless steel substrate, silicon oxide, aluminum oxide, zirconium oxide, or magnesium oxide.

13. The photocatalyst of claim 1, wherein the photocatalyst is capable of catalyzing the splitting of water.

14. (canceled)

15. The photocatalyst of claim 1, wherein the photocatalyst is capable of catalyzing the photocatalytic oxidation of an organic compound.

16. A composition comprising the photocatalyst of claim 1.

17. The composition of claim 16, comprising 0.1 to 2 g/L of the photocatalyst.

18. The composition of claim 16, further comprising water, a sacrificial agent or both.

19. The composition of claim 18, comprising 1 to 10 w/v % or 2 to 7 w/v % of the sacrificial agent.

20. (canceled)

21. A method of producing hydrogen gas by photocatalytic water-splitting, the method comprising irradiating an aqueous solution comprising the with light photocatalyst of claim 1 under conditions suitable to split water molecules to form hydrogen and oxygen.

22-24. (canceled)

25. The photocatalyst of claim 1, wherein the ratio of the n-type semiconductor material to the p-type semiconductor material is 80 to 20.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIGS. 1A-D are a schematic of photoactive material comprising anatase and rutile phase particles in which the particles are in contact with one another: (A) larger anatase particles; (B) larger rutile phase particles; (C) similar sized particles; (D) films of anatase and rutile.

[0033] FIGS. 2A-C are Transmission Electron Microscopy (TEM) images of different titanium dioxide based catalyst with anatase, brookite and rutile phases, respectively.

[0034] FIG. 3 is a schematic of a photoactive catalyst of the present invention.

[0035] FIG. 4 is a schematic of a water splitting system of the present invention.

[0036] FIG. 5 is a ultra violet/visible absorption spectrum of CoO—TiO.sub.2 semiconductor material with 2 wt. % cobalt oxide and 0.5 wt. % cobalt oxide, respectively.

[0037] FIG. 6 is a ultra violet/visible absorption spectrum of Ag—Pd/CoO—TiO.sub.2 semiconductor photocatalyst of the present invention with 0.1 wt. % silver, 0.4 wt. % palladium, 2 wt. % cobalt oxide on TiO.sub.2 and TiO.sub.2.

[0038] FIG. 7 is a graph of hydrogen production versus time for a Ag—Pd/CoO/TiO.sub.2 semiconductor photocatalyst of the present invention with 0.1 wt. % silver, 0.4 wt. % palladium, 2 wt. % cobalt oxide on TiO.sub.2 and the TiO.sub.2.

[0039] FIG. 8 is a graph of hydrogen production versus time for a comparative CoO/TiO.sub.2 photocatalyst with 2 wt. % of cobalt oxide

[0040] FIG. 9 is a graph of hydrogen production versus time for the comparative Ag—Pd/TiO.sub.2 photocatalyst having 0.1 wt. % silver and 0.4 wt. % palladium.

DETAILED DESCRIPTION OF THE INVENTION

[0041] While hydrogen-based energy from water 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, and/or unstable. The present application provides a solution to these issues. The solution is predicated on the use of photocatalysts that employ a p-type semiconductor material, a n-type semiconductor, wherein a p-n junction is formed between the p-type and n-type materials, and a metal or a metal alloy that have surface plasmon resonance properties in response to visible light. This combination results in efficient production of hydrogen and oxygen by reducing electron/hole recombination events and increasing water splitting reactions. Further, the increased water-splitting efficiency of the photocatalysts of the present invention allows a reduction in or avoidance of other costly materials such as sacrificial agents that are used in water splitting reactions.

[0042] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Photoactive Catalysts

[0043] The photoactive material includes any n-type semiconductor material able to be excited by light in a range from 360-600 nanometers. In a preferred embodiment, the photoactive material is titanium dioxide or zinc oxide. Titanium dioxide can be in the form of three phases, the anatase phase, the rutile phase, and the brookite phase. Anatase and rutile phases have a tetragonal crystal system, whereas the brookite phase has an orthorhombic crystal system. While anatase and rutile both have a tetragonal crystal system consisting of TiO.sub.6 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. These different crystal structures resulting in different density of states may account for the different efficiencies observed for transfer of charge carriers (electrons) in the rutile and anatase phases and the different physical properties of the catalyst. For example, anatase is more efficient than rutile in the charge transfer, but is not as durable as rutile. Each of the different phases can be purchased from various manufactures and supplies (e.g., Titanium (IV) oxide anatase Nano powder and Titanium (IV) oxide rutile Nano powder 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)); all phases of titanium dioxide from L.E.B. Enterprises, Inc. (Hollywood, Fla. USA)). They can also be synthesized using known sol-gel methods (See, for example, Chen et al., Chem. Rev. 2010 Vol. 110, pp. 6503-6570, the contents of which are incorporated herein by reference).

[0044] Referring to FIG. 1, n-type semiconductor material 10 of the present invention can take a variety of different forms. By way of example only, anatase particles 11 can be larger than rutile phase particles 12 (FIG. 1A). Alternatively, rutile particles 12 can be larger than anatase particles 11 (FIG. 1B). Still further, anatase 11 and rutile 12 particles can be substantially the same size (FIG. 1C). Brookite particles 14 can also be included in the n-type semiconductor material 10 (FIG. 1C). While the particles in FIG. 1 are illustrated as spheres, other shapes such as rod-shaped and irregularly shaped particles are contemplated. In other instances, the anatase 11 and rutile 12 phases can be formed into sheets or films (FIG. 1D). FIGS. 2A-2C depict TEM of titanium dioxide catalysts. FIG. 2A is a TEM of titanium dioxide anatase particles. The titanium dioxide anatase particles are about 15 nm in size. FIG. 2B is a TEM of titanium dioxide brookite particles with gold particles deposited on the top. FIG. 2C is a TEM of titanium dioxide rutile particles (grey area) with platinum (dark spots) deposited on the surface.

[0045] In one aspect of the invention, mixed phase titanium dioxide anatase and rutile may be the transformation product obtained from heat-treating single phase titanium dioxide anatase at selected temperatures. Heat-treating the single phase titanium dioxide anatase nanoparticle produces small particles of rutile on top of anatase particles, thus maximizing the interface between both phases and at the same time allowing for a large number of adsorbates (water and ethanol) to be in contact with both phases, due to the initial small particle size. Single phase TiO.sub.2 anatase nanoparticles that are transformed into mixed phase TiO.sub.2 nanoparticles have a surface area of about 45 to 80 m.sup.2/g, or 50 m.sup.2/g to 70 m.sup.2/g, or preferably about 50 m.sup.2/g. The particle size of these single phase TiO.sub.2 anatase nanoparticles is less than 95 nanometers, less than 50 nm, less than 20, or preferably between 10 and 25 nm. Heat treating conditions can be varied based on the TiO.sub.2 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. Other methods of making mixed phase titanium dioxide materials include flame pyrolysis of TiCl.sub.4, solvothermal/hydrothermal methods, chemical vapor deposition, and physical vapor deposition methods. A non-limiting example of transforming nanoparticles of TiO.sub.2 anatase nanoparticles to mixed phase TiO.sub.2 anatase and rutile nanoparticles includes heating single phase TiO.sub.2 anatase nanoparticles isochronally at a temperature of 700-800° C. for about 1 hour to transform the nanoparticles of TiO.sub.2 anatase phase to nanoparticles of mixed phase TiO.sub.2 anatase phase and rutile phase. In a preferred embodiment, titanium dioxide anatase is heated to a temperature of 780° C. to obtain mixed phase titanium dioxide containing about 37% rutile. Without wishing to be bound by theory, it is believed that this ratio and the particle structure may allow for the 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 percentage of anatase to rutile in the titanium dioxide polymorph can be determined using powder X-ray diffraction (XRD) techniques. For example, a Philips X'pert-MPD X-ray powder diffractometer may be used to analyze powder samples of titanium dioxide polymorphs. Using the areas of these peaks the amounts of rutile phase in the titanium dioxide polymorph can be determined using the following equation:

[00001]$\begin{array}{cc}\%.Math..Math.\mathrm{rutile}=\frac{1}{\left(\frac{A}{R}\times 0.884\right)+1}\times 100& \left(1\right)\end{array}$ [0046] where A is the area of anatase peak (such as that of the (101) plane); R is the area of rutile peak (such as that of the (101) plane); as determined by XRD; and 0.884 is a scattering coefficient.

[0047] Notably, it was discovered that when a ratio of anatase to rutile of 1.5:1 or greater is used, the photocatalytic activity of the n-type semiconductor material (10) can be substantially increased. The mixed phase TiO.sub.2 nanoparticles of the present invention can have a ratio of anatase and rutile phase ranges from 1.5:1 to 10:1, from 6:1 to 5:1, from 5:1 to 4:1, or from 2:1. As explained above, it is believed that this ratio allows for the 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.

[0048] The plasmon resonance material can be metal or metal alloys. The metal or metal alloys can be obtained from a variety of commercial sources in a variety of forms (e.g., particles, rods, films, etc.) and sizes (e.g., Nano scale or Micro scale). By way of example, each of 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. In a non-limiting aspect, the metal particles (element 15 in FIG. 3) can be prepared using co-precipitation or deposition-precipitation methods (Yazid et al.). The metal particles 15 can be used as conductive material for the excited electrons to ultimately reduce hydrogen ions to produce hydrogen gas. The metal particles 15 can be substantially pure particles 15 of Au, Pd, and Ag. The metal particles 15 can also be binary or tertiary alloys of Au, Pd, and/or Ag. The metal particles 15 are highly conductive materials, making them well suited to act in combination with the photoactive material 10 to facilitate transfer of excited electrons to hydrogen before an electron-hole recombination event occurs or by increasing the time that such an event occurs. The metal particles 15 can also enhance efficiency via resonance plasmonic excitation from visible light, enabling capture of a broader range of light energy. The metal particles 15, when in contact with a p-type material of the invention, can inhibit or substantially inhibit the oxidation of the p-type material because of the reducible nature of the metal particles 15. The metal particles 15 can be of any size compatible with the n-type semiconductor material 10. In some embodiments, the metal particles 15 are nanostructures. The nanostructures can be of any form suitable for use in the photoactive catalytic systems of the present invention, including but not limited to nanowires, nanoparticles, nanoclusters, nanocrystals, or combinations thereof.

[0049] The p-type semiconductor material (i.e., cobalt or copper) can also be obtained from a variety of commercial sources in a variety of forms (e.g., particles, rods, films, etc.) and sizes (e.g., Nano scale or Micro scale). By way of example, each of 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 such as precipitation or impregnation methods. The p-type materials can be cobalt or copper oxides in their reduced state (for example, CoO (Co II) and Cu.sub.2O (Cu I). These metals when in contact with the n-type material forms a p-n-junction. The p-type materials can be of any size compatible with the n-type semiconductor material and the plasmon resonance materials. In some embodiments, the metal oxides are nanostructures. The nanostructures can be of any form suitable for use in the photoactive catalytic systems of the present invention, including but not limited to nanowires, nanoparticles, nanoclusters, nanocrystals, or combinations thereof.

[0050] Referring to FIG. 3, the photoactive catalysts 30 of the present invention can be prepared from the aforementioned n-type material 10, the metal particles 15, and the p-type material 16 by using the process described in the Examples section of this specification. A non-limiting example of a method that can be used to make the photoactive catalysts 20 of the present invention includes formation of an aqueous solutions of titanium dioxide particles 11, 12 in the presence of CoO particles 16 followed precipitation where the metal particles CoO particles 16 are attached to a least a portion of the surface of the n-type semiconductor material 10 (e.g., precipitated titanium dioxide crystals or particles 11, 12). The n-type and p-type particles can be mixed with aqueous solutions of plasmonic resonance metals, (for example, Au, Ag, and Pd precursors), followed by precipitation, where the metal particles 15 are attached to at least a portion of the surface of the precipitated n-type and p-type materials. Alternatively, the metal particles 15 can be deposed on the surface of the n-type and p-type materials by any process known by those of ordinary skill in the art. Deposition can include attachment, dispersion, and/or distribution of the metal particles 15 on the surface of the photoactive active material or TiO.sub.2—CoO particles. As another non-limiting example, the photoactive material (e.g., TiO.sub.2—CoO particles) can be mixed in a volatile solvent with the metal particles 15. After stirring and sonication, the solvent can be evaporated off. The dry material can then be ground into a fine powder and calcined (such as at 300° C.) to produce the photoactive catalysts 30 of the present invention. Calcination (such as at 300° C.) can be used to further crystalize the TiO.sub.2—CoO particles. FIG. 3 is a schematic representation of the photocatalyst that includes the metals 15 and a photoactive material containing the n-type material 10 and the p-type material 16. The n-type material (for example, TiO.sub.2) 10 is in contact with the p-type material (CoO) 16. The metals (for example, Au or Ag—Pd) 15 are in contact with both the n-type material 10 and the p-type material 16. Contact of the n-type material 12 with the p-type material 16 forms p-n junction 17. The electric field 18 is generated by the plasmon resonance materials in response to visible light.

B. Water-Splitting System

[0051] Referring to FIG. 4, a non-limiting representation of a water-splitting system 40 of the present invention is provided. The system includes the photocatalyst 30, a light source 41, and container 42. The photocatalyst includes the photoactive material and the metal particles 15 attached to the surface of n-type semiconductor material 10 and p-type semiconductor material 16 of the photoactive material. The container 42 can be a transparent, translucent or even opaque such as those that can magnify light (e.g., opaque container having a pinhole(s)). The photocatalyst 30 can be used to split water to produce H.sub.2 and O.sub.2. The light source 41 includes visible and (400-600 nm) and ultraviolet light (360-410). The ultraviolet light excites both the n-type material 10 and the p-type material 16, while the visible light excites the p-type material and by “resonance” electrons from Au and Ag atoms (plasmonic excitation) both affect the width of the depletion width at the p-n junction 17. The exciting electrons (e.sup.−) from their valence band 43 to their conductive band 44 of both p- and n-type semiconductors, thereby leaving a corresponding hole (h.sup.+). The excited electrons (e.sup.−) are used to reduce hydrogen ions to form hydrogen gas, and the holes (h.sup.+) are used to oxidize oxygen ions to oxygen gas. The hydrogen gas and the oxygen gas can then be collected and used in down-stream processes. Due to the p-n junction 17 and the highly conductive metal particles 15 dispersed on the surface of the photoactive material 10, excited electrons (e.sup.−) are more likely to be used to split water before recombining with a hole (h.sup.+) than would otherwise be the case. Notably, the system 40 does not require the use of an external bias or voltage source. Further, the efficiency of the system 40 allows for one to avoid or use minimal amounts of a sacrificial agent such as methanol, ethanol, propanol, butanol, isobutanol, methyl tert-butyl ether, ethylene glycol, propylene glycol, glycerol, oxalic acid, or any combination thereof. In certain aspects, however, 0.1 to 10 w/v %, or preferably 2 to 7 w/v %, of a sacrificial agent can be included in the aqueous solution. The presence of the sacrificial agent can increase the efficiency of the system 40 by further reducing the likelihood of hole/electron recombination via oxidation of the sacrificial agent by the hole rather than recombination with the excited electron. Preferred sacrificial agents ethylene glycol, glycerol, or a combination thereof is used. 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 energy or light flux. For example, the photoactive catalyst 20 can be used as the anode in a transparent container containing an aqueous solution and used in a water-splitting system. An appropriate cathode can be used such as Mo—Pt cathodes (See, International Journal of Hydrogen Energy, June 2006, Vol. 31, issue 7, pages 841-846, the contents of which are incorporated herein by reference) or MoS.sub.2 cathodes (See, International Journal of Hydrogen Energy, February 2013, Vol. 38, issue 4, pages 1745-1757, the contents of which are incorporated herein by reference).

EXAMPLES

[0052] 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.

Example 1

Production of Photocatalysts of the Present Invention

[0053] CoO—TiO.sub.2 Substrate.

[0054] The CoO—TiO.sub.2 substrate was made using a co-impregnation method to obtain the CoO loading listed in Table 1 on the TiO.sub.2 substrate. The TiO.sub.2 semiconductor was either prepared by a sol-gel method (see, for example, Chen et al., Chem. Rev. 2010 Vol. 110, pp. 6503-6570) or purchased as Hombikat TiO.sub.2 from a commercial source (for example, Sigma-Aldrich®, USA, Sachtleben Chemie GmbH, Germany). The TiO.sub.2 was in an anatase phase or an anatase-rutile mixed phase as listed in Table 1. The TiO.sub.2 was placed into a mixed (170 rpm) with a stock solution of Co(NO.sub.3).sub.2 6H.sub.2O at 80° C. for 12 to 24 hours, until a pasted formed. The amount of Co(NO.sub.3).sub.2 6H.sub.2O used was determined based on the amount of cobalt to be loaded on the titanium dioxide substrate. The paste was dried for at greater than 4 hours at 120° C., and then calcined at 350° C. for 5 hours with a ramp temperature of 10° C./min. The calcined substrate was crushed using a mortar and pestle to obtain small particles of CoO—TiO.sub.2 semiconductor material listed in Table 1. FIG. 5 is a ultra violet/visible absorption spectrum of CoO—TiO.sub.2 anatase rutile phases semiconductor material with 2 wt. % Co (Sample 3, data line 500) and 0.5 wt. % Co (data line 502), respectively. The absorption of TiO.sub.2 was about 3.0-3.2 electron volts and the CoO was 2.2-2.7 electron volts.

TABLE-US-00001 TABLE 1 Sample TiO.sub.2 Cobalt Oxide on No. (g) Phase of TiO.sub.2 TiO.sub.2 (wt. %) 1 10 Anatase + Rutile 0.5 2 1 Anatase + Rutile 1 3 2 Anatase + Rutile 2 4 5 Anatase + Rutile 5 5 2 Anatase 2

[0055] Ag—Pd CoO—TiO.sub.2 Photocatalyst—Sample 6.

[0056] The Ag—Pd/CoO—TiO.sub.2 photocatalyst was made using a co-impregnation to obtain the Ag and Pd loading 0.1 wt. % Ag, 0.4 wt. % Pd on a 2 wt. % CoO on TiO.sub.2 anatase support (Sample 5 in Table 1), The metal precursors AgNO.sub.3 and Pd(CH.sub.3COO).sub.2 were obtained from Sigma Aldrich® and had a purity of 100% to 99.9%, respectively. A reactor equipped with a stirring apparatus and a condenser was charged with CoO—TiO.sub.2 semiconductor material (2 grams) a stock solution of aqueous AgNO.sub.3 and a stock solution of Pd(CH.sub.3COO).sub.2 to obtain the metal loading of 0.1 wt. % Ag and 0.4 wt. % Pd, polyvinyl alcohol (PVA to metal ratio of 10 wt/wt), and ethylene glycol (15 mL). The mixture was heated with stirring to 180 to 200° C. for 12 to 24 hours. The condenser was removed and the mixture was heated under stirring under the mixture solidified. The resulting solid was dried at 100 to 110° C. for 12 hours, and then calcined at 350° C. for 5 hours to obtain the Ag—Pd/CoO—TiO.sub.2 photocatalyst of the present invention (Sample 6). FIG. 6 is a ultra violet/visible absorption spectrum of the Ag—Pd/CoO—TiO.sub.2 semiconductor material with 0.1 wt. % Ag, 0.4 wt. % Pd, 2 wt. % Co on TiO.sub.2 anatase (data line 600) and TiO.sub.2 (data line 602). The absorption of Ag—Pd/CoO—TiO.sub.2 was observed to be higher than the TiO.sub.2 substrate. This increased adsorption was attributed to the plasmonic resonance effect of the metals with the CoO—TiO.sub.2 semiconductor material.

Example 2

Use of the Photocatalysts of the Invention in Water-Splitting Reactions

[0057] Water-Splitting Reaction Using Sample 6.

[0058] Catalytic reactions were conducted in a borosilicate (Pyrex®, Corning) glass reactor having a capacity of 100 mL. Photocatalyst was added to the glass reactor in a concentration of 0.1 g/L (Sample No. 6, 10 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/cm.sup.2. 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. FIG. 7 is a graph of hydrogen production versus time for the 0.1 wt % Ag, 0.4 wt. % Pd on 2 wt. % CoO/TiO.sub.2 (anatase). The molar ratio of Ag to Pd was 0.25 and the rate of hydrogen production was 2×10.sup.−4 mole g.sub.Catal.sup.−1 min.sup.−1.

Example 3

Comparative Examples

[0059] General Procedure.

[0060] 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 (10 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/cm.sup.2. 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/cm.sup.2 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.

[0061] Water-Splitting Using a Comparative CoO/TiO.sub.2 Photocatalyst.

[0062] Mixed phased titanium dioxide anatase and rutile particles were doped with cobalt oxide to produce the CoO/TiO.sub.2 photocatalyst having the wt. % of cobalt oxide per total weight of catalyst listed in Table 3. These catalyst were used generate hydrogen and oxygen from water in water-splitting reactions. The rates of hydrogen production (moles of H.sub.2 per gram of catalyst per min) from the splitting of water using the CoO/TiO.sub.2 catalysts are listed in Table 2. FIG. 8 is a graph of hydrogen production versus time for a comparative CoO/TiO.sub.2 catalyst with 2 wt % CoO.

TABLE-US-00002 TABLE 2 Cobalt Oxide Rate Sample No. (wt. %) (mol/g.sub.cat .Math. min) C1 0.5 2 × 10.sup.−5 C2 1 4 × 10.sup.−5 C3 2 5 × 10.sup.−5 C4 5 4 × 10.sup.−5

[0063] Water-Splitting Using a Ag—Pd/TiO.sub.2 Photocatalyst.

[0064] Mixed phase titanium dioxide anatase and rutile particles were doped with silver and palladium metals to produce the Ag—Pd/TiO.sub.2 photocatalyst having 1 wt. % of total metals (silver and palladium) in the catalyst and the molar ratio of the silver to palladium listed in Table 3. These catalyst were used generate hydrogen and oxygen from water in water-splitting reactions. The rates of hydrogen production (moles of H.sub.2 per gram of catalyst per min) from the splitting of water using the Ag—Pd/TiO.sub.2 catalysts are listed in Table 4.

TABLE-US-00003 TABLE 3 Ag—Pd Rate Sample No. (ratio) (mol/g.sub.cat .Math. min) C5 0.25 9 × 10.sup.−5 C6 0.66 7 × 10.sup.−5 C7 1 6 × 10.sup.−5 C8 1.33 11 × 10.sup.−5  C9 4 7 × 10.sup.−5

[0065] Water-Splitting Using a Ag—Pd/TiO.sub.2 Photocatalyst.

[0066] Titanium dioxide particles were doped with silver and palladium metals to produce the Ag—Pd/TiO.sub.2 photocatalyst having 0.5 wt. % of total metals (C) in the catalyst. These catalysts were used generate hydrogen and oxygen from water in water-splitting reactions. The rates of hydrogen production (moles of H.sub.2 per gram of catalyst per min) from the splitting of water using the Ag—Pd/TiO.sub.2 catalysts are listed in Table 4. FIG. 9 is a graph of hydrogen production versus time for the Ag—Pd/TiO.sub.2 having 0.5 wt. % of metals and a molar ratio of Ag to Pd of 0.66.

TABLE-US-00004 TABLE 4 Ag—Pd Rate Sample No. (ratio) (mol/g.sub.cat .Math. min) C10 0.25 14 × 10.sup.−5 C11 0.66 15 × 10.sup.−5 C12 1  9 × 10.sup.−5 C13 1.33  9 × 10.sup.−5 C14 4 12 × 10.sup.−5

[0067] The production of hydrogen (2×10.sup.−4 mole per gram of catalyst per min) of the bi-metallic catalyst (Sample No. 6, Ag—Pd CoO/TiO.sub.2) was found to be about 1.5 times higher than the Comparative sample No. C10 doped with the same amount of Ag and Pd in the same ratio. Thus, the photocatalyst of the invention has improved efficiency of hydrogen production as compared to conventional photocatalysts. It is to be noted that in Sample 6 a longer induction period than Comparative Sample C10 and C11 (shown in FIG. 9). Without wishing to be bound by theory it is believed that the induction period was due to reduction of the metal particles at the interface with CoO because the latter is more prone to donating its oxygen to the metal when compared to TiO.sub.2.