WO2018020344A1

WO2018020344A1 - Hydrogen production from aqueous alcohol mixtures over noble metal titanium dioxide supported photo-catalysts

Info

Publication number: WO2018020344A1
Application number: PCT/IB2017/054152
Authority: WO
Inventors: Geoff Waterhouse; Zakiya AL ALZRI; Hicham Idriss
Original assignee: Sabic Global Technologies B.V.
Priority date: 2016-07-26
Filing date: 2017-07-10
Publication date: 2018-02-01

Abstract

Disclosed is a water-splitting photo-catalyst, and methods for its use, that includes: (a) photoactive nanomaterial containing titanium dioxide nanoparticles having an average size of 10 nanometers (nm) to 100 nm; and (b) metal nanoclusters comprising platinum, palladium, or gold, or a combination thereof, deposited on the surface of the photoactive nanomaterial, wherein: (i) the platinum, palladium, or gold nanoclusters having an average size of 0.5 nm to 1 nm, 1 nm to 3 nm, and 3 nm to 7 nm, respectively; (ii) the amount of the platinum, palladium, or gold nanoclusters deposited on the surface of the photoactive nanomaterial is 0.25 to 1 wt.%, 0.25 to 1 wt.%, or 1 to 3 wt.%, respectively, based on the total weight of the combination of the photoactive nanomaterial and the metal nanoclusters; and (iii) 0.1 % to 2 % of the surface area of the photoactive material is covered with the platinum, palladium, or gold nanoclusters to maximize light absorption by the titanium dioxide, minimize surface defects and optimize charge transfer reactions.

Description

HYDROGEN PRODUCTION FROM AQUEOUS ALCOHOL MIXTURES OVER NOBLE METAL TITANIUM DIOXIDE SUPPORTED PHOTO-CATALYSTS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent Application No. 62/366,886 filed July 26, 2016, and U.S. Provisional Patent Application No. 62/370,790 filed August 4, 2016. The entire contents of each of the above-referenced disclosures are specifically incorporated herein by reference without disclaimer.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention generally concerns a water-splitting photo-catalyst. In particular, the invention concerns a water-splitting photo-catalyst that contains a photoactive nanomaterial having titanium dioxide nanoparticles and platinum, palladium, or gold nanoclusters deposited on the surface of the photoactive nanomaterial.

B. Description of Related Art [0003] Hydrogen (H₂) production from water offers enormous potential benefits for the energy sector, the environment, and the chemical industry. While some methods that currently exist for producing hydrogen and oxygen from water are costly, inefficient, or unstable, water splitting and alcohol photo-reforming using sunlight and semiconductor photo-catalysts represent approaches for H₂ production. These photo-catalysts absorb photons with E > Eg (where E_g is the semiconductor electronic band gap) that can promote electrons from the semiconductor valence band into the conduction band, creating charge carriers in the form of valence band holes and conduction band electrons that can migrate to the catalytic surface to drive photoreactions. In most semiconductors, fast electron-hole recombination (occurring on microsecond timescales) follows photo-excitation, which decreases the concentration of charge carriers available for photoreactions, thereby limiting the overall photocatalytic efficiency of the reactions. Enhancing charge separation mobility and the lifetime of photo-generated charge carriers are essential for achieving efficient photocatalytic activity. An approach for enhancing the activity of semiconductors (e.g., Ti0₂) for water splitting and alcohol photo-reforming can be to functionalize the semiconductor surface with oxidation and/or reduction co-catalysts. The roles of co-catalysts include: i) providing trapping sites for the photogenerated charges, thereby facilitating charge separation; ii) improving the photostability of the photo-catalysts by consuming the photogenerated charges, particularly the holes; and iii) catalyzing the reactions by lowering the activation energy. Further, adding an alcohol (or other oxygenates) as a sacrificial hole scavenger to water can dramatically enhance photocatalytic H₂ production rates.

[0004] Recent academic studies have provided insights on the effect of metals on Ti0₂ performance for H₂ production in alcohol-water mixtures. By way of example, Jovic et al (J. Catal. 305 (2013) 307-317) evaluated the activity of a series of Au/Ti0₂ photo-catalysts (Au loadings = 0-10 wt.%) for H₂ production in 80 vol.% ethanol under UV irradiation and observed that the optimal Au loadings was in the range 0.5-2 wt.%. The quantum yield of these reactions was low (-25 %). In another example, Al-Azri et al. {Int. J. Nanotechnol. 11 (2014) 695-703) describes that Pt/Ti0₂ and Pd/Ti0₂ photo-catalysts for H₂ production at metal loadings of 0.5 wt. % for Pt and 2 wt. % for Pd when reduced in a H₂ atmosphere at about 300 °C.

[0005] Many of the aforementioned catalysts suffer in that they are costly to manufacture and have limited chemical reactivity, light scattering, surface area, light absorption spectrum, and/or recombination suppression properties. These deficiencies make the catalysts inefficient for solar energy conversion, and in particular, water splitting applications.

SUMMARY OF THE INVENTION

[0006] A solution to the to solves some of the problems associated with the currently available water-splitting photo-catalysts has been discovered. In particular, the critical parameters of a M/Ti0₂ (M = Pd, Pt, Au, or alloys thereof) photo-catalyst for optimal production of hydrogen from water and/or alcohol-based/water mixtures have been discovered. The critical parameters, as demonstrated throughout the specification and in the Examples, that allow for maximum production of hydrogen from aqueous solutions include: (a) the size of photoactive nanomaterial (Ti0₂ nanoparticles) used; (b) the size of metal nanoclusters (platinum, palladium, or gold, or a combination thereof) deposited on the surface of the photoactive nanomaterial; (c) the amount (by weight %) of the metal nanoclusters deposited on the surface of the photoactive nanomaterial; and (d) the amount of surface area coverage of the photoactive nanomaterial by the metal nanoclusters. In particular, these parameters include: (a) titanium dioxide nanoparticles having an average size of 10 to 100 nm; (b) the nanoclusters having an average size of 0.5 nm to 7 nm; (c) 0.25 to 3 wt.%) nanocluster deposited on the surface of the photoactive nanomaterial and (d) 0.1% to 2% of the surface area of the photoactive material covered by the nanoclusters. These critical amounts are believed to maximize electron acceptors "electron sink" and charge carriers (h or e^") available for photoreactions on Ti0₂ surfaces. While a metal is needed as an electron sink (as well as for hydrogen-hydrogen recombination centers) its presence, however, creates defects states (at the interface between the semiconductor and the metal), which act as charge carriers recombination centers. This can result in decreasing the overall reaction rate. Therefore, a finite balance is needed between these two parameters. Notably, maximum light absorption by the semiconductor, minimum surface defects and optimal charge transfer reactions can occur when 0.1% to 2% of the surface area of the photoactive material is covered with the platinum, palladium, or gold nanoclusters. By way of example, it was found that photo-catalysts having these critical ranges produced higher amounts of hydrogen (e.g., at least 25 mmol g^_1catai h^"1 when subjected to an ultraviolet source having a flux of about 6 mW) as compared to similar catalysts that did not have such critical ranges. Stated another way, the discovery of the aforementioned critical ranges or parameters allows for maximum hydrogen production from a photocatalytic water splitting reaction. Without wishing to be bound by theory, it is believed that these critical ranges maximize the electronic properties of the reduced metals (e.g., the d-center position of the Pd, Pt, Au, or alloy thereof and work function), thereby maximizing the water splitting reaction.

[0007] In a particular aspect of the invention, a water-splitting photo-catalyst is described. The water-splitting photo-catalyst can include: (a) photoactive nanomaterial that includes titanium dioxide nanoparticles having an average size of 10 nanometers (nm) to 100 nm; and (b) metal nanoclusters that include platinum, palladium, or gold, or a combination thereof, dispersed on the surface of the photoactive nanomaterial. The platinum, palladium, or gold nanoclusters can have an average size of 0.5 nm to 1 nm, 1 nm to 2 nm, and 3 nm to 7 nm, respectively. The amount of the platinum, palladium, gold nanoclusters deposited on the surface of the photoactive nanomaterial can be 0.25 wt.% to 1 wt.%, 0.25 wt.% to 1 wt.%, or 1 wt.% to 3 wt.%), respectively, based on the total weight of the combination of the photoactive nanomaterial and the metal nanoclusters. In some embodiments, the water- splitting photo-catalyst can include a nanocluster that is an Au-Pd or Au-Pt alloy. Au-Pd or Au-Pt alloy nanoclusters can have an average size of 0.5 nm to 7 nm and/or the amount of Au-Pd or Au-Pt alloy loaded on the surface of the photoactive nanomaterial can be 0.1 wt.%> to 1 wt.%), preferably about 0.04 wt.%>. In one instance, the Au-Pd alloy can include about 0.125 wt.% Pd and about 0.25 wt.% Au. About 0.1% to 2% of the surface area of the photoactive material can include the platinum, palladium, gold or alloys thereof nanoclusters or nanoparticles. This very narrow window of metal surface coverage provides for maximum activity, which is in contrast to structure insensitive thermal catalytic reactions where the reaction rate linearly scales with the amount of metal deposited on the semiconductor. Furthermore, this narrow window is also sensitive to the nature of the metal. Without wishing to be bound by theory, it is believed that the dependence on such a very narrow window is not related to geometric effect, but stems from intrinsic electronic properties between the metal and the semiconductor. In one aspect, the platinum metal nanoclusters can (i) have an average size of 1.0 nm to 1.5 nm and/or (ii) be loaded onto the surface of the photoactive nanomaterial in an amount of 0.3 to 0.7 wt.%, preferably about 0.5 wt.%, based on the total weight of the photo-catalyst. In another aspect, the palladium metal nanoclusters can (i) have an average size of 1.5 nm to 2.5 nm and/or (ii) be loaded onto the surface of the photoactive nanomaterial in an amount of 0.3 to 0.7 wt.%, preferably about 0.5 wt.%, based on the total weight of the photo-catalyst. In another aspect, the gold metal nanoclusters can (i) have an average size of 4 nm to 6 nm and/or (ii) be loaded onto the surface of the photoactive nanomaterial in an amount of 1.5 wt.% to 2.5 wt.%, preferably about 2 wt.%, based on the total weight of the photo-catalyst. In a particular aspect of the current invention, the titanium dioxide nanoparticles can be mixed phase titanium dioxide nanoparticles having a ratio of anatase to rutile that ranges from 1.5: 1 to 10: 1, 3 : 1 to 8: 1, preferably 5: 1 to 7: 1, or more preferably 5.5: 1 to 6.5: 1. In some aspects, the photocatalytic water-splitting catalyst can be further included in an aqueous solution. The aqueous solution can include an organic sacrificial agent and water having a volume percent (vol.%) ratio of 30:70 to 1 :99 of sacrificial agent to water. Specifically the vol.% ratio of sacrificial agent to water can be 20:80, preferably 5:95.

[0008] Also described is a method for producing hydrogen gas (H₂) from water. The method can include (a) obtaining an aqueous solution containing the photocatalytic water- splitting catalyst of the present invention, a sacrificial agent, and water, and (b) subjecting the mixture to a light source for a sufficient period of time to produce H₂ from the water. The aqueous solution can have a volume % ratio of sacrificial agent to water of 30:70 to 1 :99 or 20:80 to 1 :99, preferably 5:95. H₂ can be produced at a rate of at least 30 mmol g^_1catai h^"1, preferably 30 to 50 mmol g^_1catai h^"1, or more preferably 40 to 45 mmol g^_1catai h^"1, when subjected to a UV flux of about 6 mW cm^"2, and when platinum is deposited on the surface of the photoactive material. In another embodiment, H₂ can be produced at a rate of at least 30 mmol g^_1catai h^"1, preferably 30 to 50 mmol g^_1catai h^"1, or more preferably 40 to 50 mmol g^" ^Xcatai h^"1, when subjected to a UV flux of about 6 mW cm^"2, and when palladium is deposited on the surface of the photoactive material. In some embodiments, H₂ can be produced at a rate of at least 25 mmol g^_1catai h^"1, preferably 25 to 35 mmol g^_1catai h^"1, or more preferably 30 to 35 mmol g^_1catai h^"1, when subjected to a UV flux of about 6 mW cm^"2, and when gold is deposited on the surface of the photoactive material. In a further aspect, the light source used in the method can be sunlight or a UV light source or a combination thereof.

[0009] Non-limiting examples of sacrificial agents that can be used in the methods of the present invention include alcohols, diols, polyols, dioic acids, or any combination thereof. Non-limiting examples of particular sacrificial agents include methanol, ethanol, propanol, iso-propanol, n-butanol, iso-butanol, ethylene glycol, propylene glycol, glycerol, or oxalic acid, or any combination thereof. In preferred instances, ethanol or ethylene glycol can be used, with ethanol being most preferred.

[0010] "Water splitting" or any variation of this phrase describes the chemical reaction in which water is separated into oxygen gas (0₂) and hydrogen gas (H₂). [0011] "Nanomaterial" refers to an object or structure in which at least one dimension of the object or structure is equal to or less than 1000 nm (e.g., one dimension is 1 to 100 nm in size). In a particular aspect, the nanomaterial includes at least two dimensions that are equal to or less than 1000 nm (e.g., a first dimension is 1 to 1000 nm in size and a second dimension is 1 to 1000 nm in size). In another aspect, the nanomaterial includes three dimensions that are equal to or less than 100 nm (e.g., a first dimension is 1 to 1000 nm in size, a second dimension is 1 to 100 nm in size, and a third dimension is 1 to 1000 nm in size). The shape of the nanomaterial can be of a wire, a particle, a sphere, a rod, a tetrapod, a hyper-branched structure, or mixtures thereof. A "nanocluster" is a grouping of a number of atoms of a one or more metals. By way of example, and as noted above, nanoclusters can include platinum, palladium, or gold or combinations thereof.

[0012] The terms "wt.%", "vol.%", or "mol.%" refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component.

[0013] The terms "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. 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%.

[0014] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%. [0015] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/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.

[0016] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result. [0017] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" 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."

[0018] 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.

[0019] The photo-catalysts of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, 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 catalyze water-splitting applications to produce hydrogen. [0020] 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. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS [0021] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings.

[0022] FIG. 1 is a schematic of a water-splitting system using the photo-catalysts of the present invention. [0023] FIG. 1 A is an enlargement of the catalyst of FIG. 1.

[0024] FIGS. 2A-2G are Transmission Electron Microscopy (TEM) images for (2A) 0.5 wt.% Pd/TiC-2, (2B) 0.4 wt.% Au-Pd/Ti0₂ photo-catalysts, (2C) 0.5 wt.% Pt/Ti0₂, (2D) 2 wt.% Pt/Ti0₂, (2E) 0.5 wt.% Au/Ti0₂, (2F) 2 wt.% Au/Ti0₂, and, 2(G) 2 wt.% Pd/Ti0₂.

[0025] FIG. 2H is a high resolution TEM image of the Au-Pd/Ti0₂ photo-catalyst of FIG. 2G.

[0026] FIG. 3A shows XRD patterns for Pd foil (reference, bottom pattern), P25 Ti0₂ (top pattern), and Pd/Ti0₂ catalysts with Pd loadings of 4 wt.%, 2 wt.%, 1 wt.%, 0.5 wt.%, and 0.25 wt.%, respectively from bottom to top.

[0027] FIG. 3B shows XRD patterns for Pt foil (reference, bottom pattern), P25 Ti0₂ (top pattern), and Pt/Ti0₂ catalysts with Pt loadings of 4 wt.%, 2 wt.%, 1 wt.%, 0.5 wt.%, and 0.25 wt.%), respectively from bottom to top.

[0028] FIG. 3C shows XRD patterns for Au foil (reference, bottom pattern), P25 Ti0₂ (top pattern), and Au/Ti0₂ catalysts with Au loadings of 4 wt.%, 2 wt.%, 1 wt.%, 0.5 wt.%, and 0.25 wt.%, respectively from bottom to top. [0029] FIGS. 4A, 4B and 4C show the XPS data for Pd/Ti0₂ (4A), Pt/Ti0₂ (4B), and Au/Ti0₂ (4C) photo-catalysts, respectively, at metal loading of 0.5 wt.% and 2 wt.%. [0030] FIG. 5 is a graphical representation of XPS Metal/Ti ratio as a function of nominal metal loading (0-4 wt.%) as determined from the data in FIGS. 4A-4C.

[0031] FIGS. 6A-C shows photoluminescence spectra for (0-4 wt.%) metal loadings, (6A) Pd/Ti0₂, (6B) Pt/Ti0₂, and (6C) Au/Ti0₂ photo-catalysts. [0032] FIG. 7 is a graph of metal loading in weight percentage versus normalized photoluminescence (PL) data from FIGS. 6A-6C.

[0033] FIG. 8 are summarized H₂ production rates in 80 vol.% (UV flux of about 6.0 mWcm^"2) as a function of nominal metal loading (0-4 wt.%).

[0034] FIG. 9 shows plots of H₂ production rates per metal per metal atom (molecules of H₂ Matom^"1 s^"1) versus rates of H₂ production per metal particle (molecules of H₂ Mparticie^"1 s^"1) for M/Ti0₂ photo-catalysts (M = Pd, Pt or Au) in 80 vol.% ethanol (UV flux ~ 6.0 mWcm^"2). The points on the plot represent the various metal loading of M/Ti0₂ photo-catalysts starting from 0.25 wt.%, at the top towards 4 wt.% at the last bottom.

[0035] FIG. 10 shows a plot of (a) H₂ production rates versus ethanol concentration (0- 100 vol.%) for 0.5 wt.% Pd/Ti0₂, 0.5 wt.% Pt/Ti0₂, and 1 wt.% Au/Ti0₂ (UV flux of about 6.0 mWcm^"2)

[0036] FIG. 11 shows oxidation potentials (versus HE) of ethanol photo-reforming in the absence and presence of water with respect to the conduction band (CB) and valence band (VB) of Ti0₂. [0037] FIGS. 12A and 12B show plots of H₂ production lots versus time for selected M/Ti0₂ (M = Pd, Pt or Au) photo-catalysts of the present invention at the same molar amount of metal in (12A) 10 vol.% ethanol, and (12B) 80 vol.% ethanol (UV flux of about 6.0 mWcm^"2).

[0038] FIG. 13 are bar graphs showing the comparison of H₂ production rates for M/Ti0₂ photo-catalysts of the present invention of FIGS. 12A and 12B.

[0039] FIGS. 14A and 14B show plots of H₂ production rates versus the work function for selected photo-catalysts M/Ti0₂ (M = Pd, Pt, or Au) of the present invention and comparative catalyst M/Ti0₂ (M = Ni or Ag) of the same metal loading at (14A) 0.5 wt.%; and (14B) 7.5 x 10^"5 mol_M in 10 vol.% and 80 vol.% ethanol (UV flux ~ 6.0 mWcm^"2). [0040] FIGS. 15A and 15B show plots of H₂ production rates versus the d-band centre, s , for selected photo-catalysts M/Ti0₂ (M = Pd, Pt, or Au) of the present invention and comparative catalyst M/Ti0₂ (M = Ni or Ag) of the same metal loading at (15A) 0.5 wt.%, and (15B)7.5 x 10^"5 mol_M in 10 vol.% and 80 vol.% ethanol (UV flux of about 6.0 mWcm^"2).

[0041] FIG. 16 are bar graphs showing H₂, 0₂, C0₂, C_xH_y (i.e., methane and C₂ hydrocarbons) production rates in gas phase for the M/T1O2 photo-catalysts (M = Pd, Pt or Au, metal loading 1.5 x 10^"3 ΓΠΟΙΜ) in 10 vol.% ethanol (UV flux of about 6.8 mWcm^"2).

[0042] FIG. 17 shows plots of the molar ratio of H₂/ethanol consumed (assuming ethanol consumption equals the total sum of C-containing products in the gas phase: C0₂/2 + CH₄/2 + C₂) for (a) 0.5 wt.% Pd/Ti0₂; (b) 1 wt.% Pt/Ti0₂; and (c) 1 wt.% Au/Ti0₂ in 10 vol.% ethanol (UV flux of about 6.8 mWcm^"2). [0043] FIG. 18 shows a plot summarizing H₂, C0₂ and C_xH_y production rates in gas phase versus ethanol concentration (0-10 vol.%) for 0.5 wt.% Pd/Ti0₂ photo-catalyst (UV flux ob about 6.8 mWcm^"2).

[0044] FIG. 19 shows a plot of hydrogen product for M-Ti0₂ (M= Pt, Pd, Au, Au-Pd) and comparative Ag/Ti0₂ and Ni/Ti0₂ photocatalysts. [0045] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0046] 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 either expensive, inefficient, or unstable. By way of example, many of the currently available photo-catalysts for water-splitting processes have poor activity and/or stability.

[0047] A solution to these and other problems associated with photocatalytic water- splitting reactions has been discovered. The solution is premised on metal loading, particle size or dispersion, tuned co-catalyst (e.g., Ti0₂ and noble metal) electronic properties, especially the <i-band center position of the noble metals, and/or work function. In particular, the solution is predicated on the identification of critical ranges for these parameters (i.e., (a) titanium dioxide nanoparticles having an average size of 10 to 100 nm; (b) platinum, palladium, or gold nanoclusters having an average size of 0.5 nm to 1 nm, 1 nm to 2 nm, and 3 nm to 7 nm, respectively; (c) 0.25 to 1 wt.% platinum, 0.25 to 1 wt.% palladium, or 1 to 3 wt.% gold deposited on the surface of the photoactive nanomaterial and (d) 0.1 %> to 2 %> of the surface area of the photoactive material covered by the platinum, palladium, or gold nanoclusters). These critical ranges provide an elegant way to tune photo-catalyst performance based on the nature and the amounts of the materials used along with sacrificial agent to water ratios. The end result is maximum production of hydrogen from a water- splitting reaction. These and other non-limiting aspects of the present invention are discussed in further detail in the following sections with reference to the Figures.

A. Photoactive Catalysts

[0048] The photoactive catalysts of the present invention include a photoactive nanomaterial having an optimal amount of metal catalyst (co-catalyst) dispersed on 0.1%> to 2%o of the surface of the photoactive material. As exemplified in the Examples, the amount of metal or metal species dispersed on the surface of the photoactive nanomaterial is critical to the maximum production of hydrogen gas from water and/or sacrificial agent/water mixtures. It was surprisingly found that increasing metal loading to amounts outside of 0.25 wt.%) to 1 wt.%, 0.25 wt.%) to 1 wt.%, or 1 wt.%> to 3 wt.%, of platinum, palladium and gold, respectively, or 0.1 wt.%> to 1 wt.%> of an alloy thereof, did not increase the production of hydrogen under photocatalytic conditions. Rather, it was discovered that these loading ranges for each noble metal are critical ranges that maximize hydrogen production from a photocatalytic water-splitting reaction. Going outside of these ranges can have a detrimental effect on the efficiency of hydrogen production from water.

1. Photoactive Nanomaterial

[0049] The photoactive nanomaterial includes any 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 having an average particle size of 10 nanometers (nm) to 100 nm and all average sizes greater than, equal to, or between any two of 10, 11, 12, 13 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62 ,63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, and 100 nm. 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 Ti0₆ octahedra, their phases differ in that anatase octahedrons are arranged such that four edges of the octahedrons are shared, while in rutile, two edges of the octahedrons are shared. These different crystal structures result in different density of states (DOS). This 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 these phases. 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 nanopowder and titanium (IV) oxide rutile nanopowder 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, Florida 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). A mixture of anatase and rutile titanium dioxide in a ratio of 6: 1 is sold under the tradename of P25 Ti0₂ support. [0050] In one aspect of the invention, mixed phase titanium dioxide anatase and rutile may be a 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 Ti0₂ anatase nanoparticles that are transformed into mixed phase Ti0₂ nanoparticles have a surface area of about 45 to 80 m²/g, or 50 m²/g to 70 m²/g, or preferably about 50 m²/g. The particle size of these single phase Ti0₂ 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 Ti0₂ 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 TiCl₄, solvothermal/hydrothermal methods, chemical vapor deposition, and physical vapor deposition methods. Using a ratio of anatase to rutile of 1.5: 1 or greater can substantially increase the photocatalytic activity of the semiconductor material. In a particular aspect of the current invention, the titanium dioxide nanoparticles are mixed phase titanium dioxide nanoparticles having a ratio of anatase to rutile that ranges from 1.5 : 1 to 10: 1, 3 : 1 to 8: 1, preferably 5 : 1 to 7: 1, or more preferably 5.5 : 1 to 6.5 : 1 or about 6: 1. 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 carriers in the anatase phase have an increased chance of being transferred to the metal conducting materials rather than undergoing an electron-hole recombination event.

2. Electroconductive Material

[0051] In another aspect, the photoactive catalysts of the present invention include electroconductive nanostructured material that can include noble metals (e.g., platinum (Pt), palladium (Pd), or gold (Au)). Electroconductive material or metals can function with the titanium dioxide support to catalyze water-splitting reactions or alcohol (e.g., ethanol) photo- reforming reactions. Pd, Pt, and Au have high work functions that are the most effective in promoting H₂ production from water over Ti0₂ surfaces under UV excitation as compared to other noble or transition metals (e.g., silver and nickel). The nanostructures can be nanowires, nanoparticles, nanoclusters, or nanocrystals, or combinations thereof. In preferred embodiments, the platinum, palladium, or gold nanomaterial is in the form of nanoclusters. The nanoclusters can have an average size of 0.5 nm to 7 nm, or greater than, equal to, or between any two of 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, and 7 nm. A platinum nanocluster can have an average size of 0.5 to 1 nm, or 0.75 nm to 1.75 nm, 1 nm to 1.5 nm, or 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1 nm, or any value there between. A palladium nanocluster can have an average size of 1 nm to 2 nm, 1.25 to 2.75 nm, 1.5 nm to 2.5 nm, 1.75 nm to 2 nm, or 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, 2 nm or any value there between. A gold nanocluster can have an average size of 3 nm to 7 nm, 3.25 nm to 6.75 nm, 3.5 nm to 6.5 nm, 3.75 nm to 6.25 nm, 4 nm to 6 nm, 4.25 nm to 5.75 nm, 4.5 nm to 5.5 nm, 4.75 to 5.25 nm, or 3 nm, 3.1 nm, 3.2 nm, 3.3 nm, 3.4 nm, 3.5 nm, 3.6 nm, 3.7 nm, 3.8 nm, 3.9 nm, 4 nm, 4.1 nm, 4.2 nm, 4.3 nm, 4.4 nm, 4.5 nm, 4.6 nm, 4.7 nm, 4.8 nm, 4.9 nm, 5 nm, 5.1 nm, 5.2 nm, 5.3 nm, 5.4 nm, 5.5 nm, 5.6 nm, 5.7 nm, 5.8 nm, 5.9 nm, 6 nm, 6.1 nm, 6.2 nm, 6.3 nm, 6.4 nm, 6.5 nm, 6.6 nm, 6.7 nm, 6.8 nm, 6.9 nm or 7 nm or any value or range there between. Nanocluster sizes can be determined by transmission electron microscopy (TEM). The amount of the platinum nanoclusters deposited on the surface of the photoactive nanomaterial can be 0.25 wt.% to 1 wt.%, 0.3 wt.% to 0.95 wt.%, 0.4 wt.% to 0.8 wt.%, 0.5 wt.% to 0.7 wt.% or any value or range there between (e.g., 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95 or 1 wt.%), based on the total weight of the combination of the photoactive nanomaterial and the metal nanocluster. The amount of the palladium nanoclusters deposited on the surface of the photoactive nanomaterial can be 0.25 wt.% to 1 wt.%, 0.3 wt.% to 0.95 wt.%, 0.4 wt.% to 0.8 wt.%, 0.5 wt.% to 0.7 wt.% or any value or range there between (e.g., 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95 or 1 wt.%), based on the total weight of the combination of the photoactive nanomaterial and the metal nanocluster. The amount of gold nanoclusters deposited on the surface of the photoactive nanomaterial can be 1 to 3 wt.%, 1.5 wt.% to 2.5 wt.%, or 1 wt.% to 2 wt.% or any value or range there between (e.g., 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.55, 1.60, 1.65, 1.70, 1.75, 1.80, 1.85, 1.90, 1.95, 2, 2.25, 2.30, 2.35, 2.40, 2.45, 2.50, 2.55, 2.60, 2.65, 2.70, 2.75, 2.80, 2.85, 2.90, 2.95, or 3 wt.%), based on the total weight of the combination of the photoactive nanomaterial and the metal nanocluster. The amount of the Au-Pd or Au-Pt alloy nanoclusters deposited on the surface of the photoactive nanomaterial can be 0.1 wt.% to 1 wt.%, 0.3 wt.% to 0.95 wt.%, 0.4 wt.% to 0.8 wt.%, 0.5 wt.% to 0.7 wt.% or any range or value there between (e.g., 0.1, 0.15, 0.2, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95 or 1 wt.%), based on the total weight of the combination of the photoactive nanomaterial and the metal alloy nanocluster. In one instance, the Au-Pd alloy can include 0.1 to 0.15 wt.% Pd, and 0.1 to 0.5 wt.% Au, or about 0.125 wt.% Pd and about 0.25 wt.% Au.

[0052] About 0.1% to 2% and any value there between (e.g. 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0%) of the photoactive material surface area can include platinum, palladium, or gold nanoclusters. In one aspect, the metal nanoclusters are platinum and the platinum metal nanoclusters have an average size of 1.0 nm to 1.5 nm and/or the amount of platinum loaded on the surface of the photoactive nanomaterial is 0.3 to 0.7 wt.%, preferably about 0.5 wt.%. In another aspect, the metal nanoclusters are palladium and the palladium metal nanoclusters have an average size of 1.5 nm to 2.5 nm and/or the amount of palladium loaded on the surface of the photoactive nanomaterial is 0.3 to 0.7 wt.%, preferably about 0.5 wt.%. In another aspect, the metal nanoclusters are gold and the gold metal nanoclusters have an average size of 4 nm to 6 nm and/or the amount of gold loaded on the surface of the photoactive nanomaterial is 1.5 to 2.5 wt.%), preferably about 2 wt.%. The nanoclusters can be in any form or shape (e.g., nanorods, particle, and the like). Electroconductive material precursors (i.e., platinum, gold, and palladium salts) can be obtained from a variety of commercial sources (e.g., By way of example, Sigma-Aldrich® Co. LLC and Alfa Aesar GmbH & Co KG offer such products) and can be transformed into metal nanoclusters using the procedures exemplified in the Examples and throughout the specification.

B. Methods of Making the Photo-Catalyst

[0053] The photo-catalyst can be prepared by a deposition-precipitation method, which results in the formation of adsorbed ion species on the Ti0₂ surface. The absorbed ion species can then be partially reduced under H₂ atmosphere causing agglomeration of metal ions into metal nanoclusters. By way of example, a metal source of Pd, Pt, Au or combinations thereof can be dissolved in ultrapure water (e.g., water purified through a Milli- Q® (Millipore, U.S.A.) system to remove all ions from the water). In some embodiments, acid (e.g., IN HC1) can be added to assist dissolution. A templating agent and titanium dioxide can be then added to the solution, and heated under agitation to effect precipitation of the metals onto the surface of the titanium dioxide. Preferably the deposition-precipitation is performed in the presence of an urea, 1,3-diphenylurea (DPU), l,3-diethyl-l,3-diphenylurea or 3 -methyl- 1, 1-diphenylurea. The resulting precipitate can be separated from the water using known separation techniques (e.g., vacuum filtration, centrifugation, gravity filtration, and the like). The precipitate can be optionally washed with ultrapure water, and then air dried at a temperature of 40 °C to 60 °C, or 45 °C to 55 °C, or 50 °C. The dried powders can be heated at 300 °C to 600 °C, 325 °C to 550 °C, or 350 °C to 500 °C in the presence of an oxygen source (e.g., static air) for a time sufficient (e.g., 1 to 4 hours, or 2 hours) to reduce the metal surface to zero valence and/or remove the templating agent from the photo-catalyst. In certain embodiments, a metal alloy can be formed by heating the metal alloy precipitate (e.g., Au-Pd/Ti0₂) to 450 to 600 °C, or 475 to 575 °C, or 500 to 550 °C, or about 500 °C in a reducing flow of reducing gas (e.g., H₂ atmosphere) to reduce the Au⁺³ to Au and allow metal alloy formation. This heating process avoids or inhibits initial reduction of surface Au⁺³, which is formed when the precipitate is calcined in static air. This heating process provides the reducing environment so both Au⁺³ and Pd⁺² can be reduced at the same temperature and therefore poised to make the alloy upon inter-particle diffusion. In other embodiments, the precipitate (e.g., Pt/Ti0₂ or Pd/Ti0₂ precipitates) can be calcined in a step-wise manner. For example, the Pt/Ti0₂ or Pd/Ti0₂ precipitates can be heated at 300 °C to 400 °C, 325 °C to 375 °C, or 350 °C in the presence of an oxygen source (e.g., static air) for a period of time (e.g., 1 to 4 hours, or 2 hours), and subsequently heated at 400 °C to 600 °C, 425 °C to 575 °C, or 500 °C in a H₂/N₂ flow to reduce adsorbed metal cationic species (palladium oxide form or platinum oxide form) to their metallic form. The photo-catalyst can be in particulate form or powdered form. C. Systems and Methods of Using the Photo-catalyst

[0054] The photo-catalysts described throughout the specification can be used to generate hydrogen from water under photolytic conditions. FIG. 1 is a schematic of an embodiment of water-splitting system 100. Water-splitting system 100 includes container 102, photo- catalyst 104, light source 106, and water or sacrificial -water mixture 108. Adding an organic sacrificial agent, such as an alcohol (or other oxygenates) as a sacrificial hole scavenger to water can enhance photocatalytic H₂ production rates, since alcohols irreversibly react with photo-generated holes to suppress e^"-h⁺ recombination. The sacrificial agent and water can have a volume % ratio of 30:70 to 1 :99 and all ratios there between (e.g. 29:71, 28:73, 26:74, 25:75, 24:76, 23 :77, 22:78, 21 :79, 20:80, 19:81, 18:82, 17:83, 16:84, 15:85, 14:86, 13 :87, 12:88, 11 :89, 10:90, 9:91, 8:92, 7:93, 6:94, 5:95, 4:96, 3 :97, or 2:98). Specifically, the volume % ratio of sacrificial agent to water can be 20:80, preferably 5:95. In some embodiments, the sacrificial agent can be an alcohol, a diol, a polyol, a polybasic carboxylic acid such as a dioic acid and corresponding water soluble salts, or any combination thereof. Non-limiting examples of sacrificial agents include methanol, ethanol, propanol, iso- propanol, «-butanol, z^'so-butanol, ethylene glycol, propylene glycol, glycerol, oxalic acid, malonic acid, succinic acid, maleic acid, malic acid, tartaric acid, citric acid and their water soluble salts, or any combination thereof, preferably ethanol or ethylene glycol, most preferably ethanol. Ethanol can be easily produced from biomass feedstocks and has a high H:C ratio of 3 and high hydrogen content per unit volume in the liquid state. Further as a liquid fuel, ethanol is safe to handle and distribute.

[0055] Referring back to FIG. 1, container 102 can be translucent or even opaque such as those that can magnify light (e.g., opaque container having a pinhole(s)). Photo-catalyst 104 is one or more of the photo-catalysts described herein and is shown as single nanoparticles dispersed in the media. Light source 106 can be sunlight, a UV lamp, or an Infrared (IR) lamp. An example of a UV light is a 100 Watt ultraviolet lamp with a flux of about 6 mW/cm² to 7 mW/cm². The UV flux from the Sun is up to 5 mW/cm². For the opaque container light is provided using fiber optic cables through pinholes in the reactor. The light source can be at a distance of 5 cm to 20 cm, or 8 to 10 cm. The UV lamp can be used with a 360 nm and above filter. Such UV lamps are commercial available from, for example, Sylvania. Referring to FIG. 1A, a schematic of an enlarged view of photo-catalyst 104 particle depicts the water-splitting process to produce H₂ and 0₂. Light source 106 contacts photo-catalyst 104, thereby exciting electrons (e-) from their valence band to their conductive band to initiate the photochemical process. The excited electrons can reduce hydrogen ions to form hydrogen gas, and the holes generated from the electrons leaving their valence band can oxidize oxygen ions to oxygen gas. The resulting hydrogen gas and the oxygen gas can then be collected and used in down-stream processes. When the organic (carbon-based) sacrificial agent was ethanol and was present in sufficient amount (e.g., 0.5 vol.% to 80 vol.%), it was surprisingly found that the higher quantities of hydrogen was produced than the theoretical amount of hydrogen from ethanol. Without wishing to be bound by theory, it is believed that the increased production of hydrogen is formed through the following series of photoreactions. Upon excitation the molecular or dissociatively adsorbed ethanol in the presence of H₂0 may inject an electron in the valence band (hole trapping) via oc-H abstraction to give a- hydroxyl radicals on Ti0_2.

CH₃CH₂-OH(a) + h⁺(VB)→ CH₃CH^'-OH(a) + H⁺ (1)

OH(a) + h⁺(VB)→ ^'OH(a) (2)

[0056] H⁺ is nominally an OH(a) but for simplicity it is kept as H⁺. This reaction is faster than that of surface hydroxyls to OH radical. A second injection of an electron into the valence band from (1) results into the formation of acetaldehyde while a second injection of an electron from the OH radical would give molecular 0₂:

CH₃CH^»-OH(a) + h +(VB)→ CH₃CHO + H+ (3)

•OH(a) + h+(VB)→ ½ 0₂ (g) + H+ (4)

[0057] Electrons at the conduction band of Ti0₂ become trapped at metal nanoparticles (Pd, Pt or Au), which are then transferred to reduce protons to form molecular H₂:

2H+ + 2e- (M)→ 2H(ads)→ H₂(g) (5)

[0058] The stoichiometry of the reaction is not maintained in the above equations. Acetaldehyde formed in (3) will either decompose into CO and CH₄ or react with water to form acetic acid. CH₃CHO(g)→ CO + CH₄ (6)

CH₃CHO + H₂0 + 2h+→ CH₃COOH + 2H+ (7) CH₃COOH→ CH₃COO- + H+ (8)

CH₃COO- + h+→ CH₃COO→ CH₃ ^» + C0₂ (CH₃ radical injection)

2CH₃ ^» C₂H₆ (coupling of CH₃ radicals) (10)

[0059] CO from (6) participates in the water-gas shift reaction:

CO + H₂0→ C0₂ + H₂ (rapid reaction) (11).

[0060] In addition to being capable of catalyzing water splitting without an external bias or voltage, the photo-catalysts 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.

EXAMPLES [0061] 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.

MATERIALS

[0062] Palladium chloride (>99% purity), chloroplatinic acid hexahydrate (98% purity), tetrachloroauric acid trihydrate (>99% purity), 1,3-diphenylurea (DPU, >99.5% purity), hydrochloric acid (37 wt.%), and ethanol (>99.5% purity) were all obtained from Sigma- Aldrich and used without further purification. P25 Ti0₂ (85 wt.% anatase, 15 wt.% rutile, >99.5%) was obtained from Evonik Industries, Germany. Ultrapure water (Milli-Q® (Millipore, U.S.A.) water, 18.2 ΜΩ-cm resistivity) was used for preparation of all photo- catalysts and in the photocatalytic H₂ production tests.

Example 1

(Preparation of M/T1O2 Photo-catalysts) [0063] M/T1O₂ (M = Pd, Pt, Au, or alloys thereof) photo-catalysts were prepared by the deposition-precipitation with DPU method. HAuCl₄.3H₂0 (1.654 g, 0.0042 moles), PdCl₂ (0.745 g, 0.0042 moles) or H₂PtCl₆»6H₂0 (2.175 g, 0.0042 moles) were separately dissolved in ultrapure water (1 L) to give metal stock solutions of concentration 4.2 x 10^"3 mol L^"1. To assist with the dissolution of PdCl₂, HC1 (37 wt.%, 1 mL) was first added to the PdCl₂ in 20 mL of water, after which the solution was made up to a final volume of 1 L with ultrapure water. For the preparation of the 0.25, 0.5, 1, 2 and 4 wt.% Pd/Ti0₂ photo-catalysts, Pd²⁺ stock solution (16.8, 33.7, 67.8, 137.0, 279.7 mL, respectively) was diluted with ultrapure water to a total volume of 300 mL. To obtain the alloy, stock solutions of the above- described solutions were mixed. DPU (7.50 g, 0.035 moles) and P25 Ti0₂ (3 g, 0.038 moles) were then added to each of the solutions under vigorous mechanical stirring, after which the solutions were heated to 80 °C and held at this temperature for 8 h. The resulting powders were collected by vacuum filtration, washed repeatedly with ultrapure water, and then air dried at 50 °C overnight. The same procedure was used for the preparation of the 0.25, 0.5, 1, 2 and 4 wt.% Pt/Ti0₂ and Au/Ti0₂ photo-catalysts using corresponding Pt⁴⁺ or Au³⁺ stock solutions (9.1, 18.6, 36.8, 74.4, 151.8 mL, respectively, diluted to 300 mL with ultrapure water). The same procedure was used for the preparation of the 0.375 wt.% Au-Pd /Ti0₂ photo-catalysts. The Au/Ti0₂ powders were calcined at 350 °C in static air for 2 h to reduce cationic gold the Au³⁺ to Au°. The PdO/Ti0₂ and PtO/Ti0₂ powders were calcined in static air at 350 °C, and subsequently heated to 500 °C for 2 h in a H₂/N₂ flow (5 vol.% H₂, 100 mLmin^"1) to reduce adsorbed oxide species to their metallic form. The Au-Pd/Ti0₂ photo-catalysts were immediately heated to 500 °C for 2 h in a H₂/N₂ flow (5 vol.% H₂, 100 mLmin^"1) without any prior calcination in static air to avoid initial reduction of surface Au³⁺ to Au and therefore allow significant Au-Pd alloy formation. Example 2

(M/Ti0₂ Photo-catalyst Characterization)

[0064] Transmission Electron Microscopy (TEM). TEM images were taken on a JEOL 2012F TEM operating at 200 kV, equipped with an Oxford Instruments ISIS energy- dispersive X-ray spectroscopy (EDS) system for qualitative analysis of elements present in each sample. Powders samples were dispersed in ethanol and then several drops of the resulting dispersion placed on holey carbon coated copper TEM grids for analysis. Table 1 lists the physiochemical data for 0 to 4 wt.% M/Ti0₂ photo-catalysts (M = Pd, Pt or Au).

Table 1

[0065] FIGS. 2A-G are TEM images for (2A) 0.5 wt.% Pd/Ti0₂; (2B) 0.375 wt.% Pd- Au/Ti0₂ photo-catalysts; (2C) 0.5 wt.% Pt/Ti0₂; (2D) 2 wt.% Pt/Ti0₂; (2E) 0.5 wt.% Au/Ti0₂; (2F) 2 wt.% Au/Ti0₂; (2G) 2 wt.% Pd/Ti0₂. FIGS. 2H AND 21 are high resolution TEM and STEM spectra of the 0.375 wt.% Pd-Au/T/Ti0₂ photocatalysts. All photo-catalysts exhibited similar topographies under TEM apart from the increase in the number and size of separate metal nanoparticles (appears as small dark spots in the TEM images). From the images, it was determined that Pd/Ti0₂ and Pt/Ti0₂ photo-catalysts exhibited an average particle size of about 2 nm to 1.5 nm and about 1.2 nm to 1.5 nm, respectively, which was evidence for strong metal-support interactions. Au nanoparticles were of average size (about 5.5 nm, Table 1) and pseudo-spherical morphologies with metal-support contact angles of greater than 90 degrees provided evidence for a relatively weak metal-support interaction (less Au nanoparticle supported on Ti0₂). From the images, it was also determined that the Pd and Pt nanoparticles scattered on both anatase and rutile Ti0₂ support particles and the Au nanoparticles preferentially developed at the interface between two Ti0₂ crystallites. The average particle size of metal nanoparticle followed the trend: Pt < Pd < Au. Without wishing to be bound by theory, it is believed that this trend was due to minimization of the total surface free energy, which played a crucial part in size, shape and uniformity control of these metals. The zero valent metal atoms of Pd, Pt and Au nanoparticles were formed during nucleation and growth processes under reducing an atmosphere (H₂ reduction in the case of Pd and Pt, while only calcination in air for Au). Upon calcination conditions, these metal atoms attained thermal kinetic energy and became mobile and coalesced with each other to produce small clusters. The resulting metal clusters were thermodynamically unstable and dissolved before they reached a critical size or overcame a critical free energy barrier and become thermodynamically stable nuclei. Free atoms, or unstable small metal clusters, became consumed by these stable nuclei to eventually grow into nanoparticles. The experimental values of the surface free energies (γ) for Pt, Pd and Au metals were determined to be: 2.48, 1.88 and 1.50 J m^"2, respectively, which followed the same trend of their average particle sizes. Large surface free energies required higher minimization of the surface free energy, resulting in difficulty in nucleation. In the case of Pt, for example, only small numbers of clusters could reach a critical size to minimize the excess free energy by partial wetting of Ti0₂ support phases. Hence, Pt was more favorable to display higher wetting due to its high free surface energy and hence spread through the surface support, evidence for the strong-metal support interaction (contact angle < 90°), and thus having a smaller particle than Pd, which has a smaller particle size than Au. The deposition-precipitation method with urea induced no change in the original particle size of all metal nanoparticles. For the P25 Ti0₂ support, the small crystallites seen in the figures were generally classified as anatase (20-30 nm) and the large angular crystallites classified as rutile (40-70 nm).

[0066] X-ray Diffraction Analysis (XRD). Powder XRD patterns were collected on a Philips PW-1130 diffractometer, equipped with a Cu anode X-ray tube and a curved-graphite filter monochromator. Data was taken from 2Θ = 2-100° (0.02°, 2°min^"1) using Cu Ka X-rays (λ = 1.5418 A). The rutile:anatase ratio in the samples was determined by

%Rutile =

where A was the peak area for the anatase (101) reflection at 2Θ = 25.3°, and R was the peak area for the rutile (110) reflection at 2Θ = 27.4°.

[0067] The above observations obtained from TEM results were further corroborated by the XRD patterns for Pd/Ti0₂, Pt/Ti0₂ and Au/Ti0₂ catalysts with various metal loadings are shown in FIGS. 3A, 3B and 3C, respectively. From the XRD patterns, it was determined that all catalysts had characteristic reflections of nanocrystalline anatase and rutile in the P25 Ti0₂ support. Pd/Ti0₂ and Pt/Ti0₂ showed only peaks characteristic for anatase and rutile in the P25 Ti0₂ support due to the very small size and high degree of dispersion of the supported Pd and Pt crystallites. Au/Ti0₂ showed additional broad and weak XRD feature at 2Θ at about 38.2° assigned to Au (200) that intensified with the nominal Au loading (some of these features were partially concealed by Ti0₂ reflections). The position and relative intensity of these peaks were consistent with face centered cubic (fee.) gold particles on Ti0₂ support. Identification of metallic gold of size about 5 nm by XRD for Au/Ti0₂ was consistent with TEM data.

[0068] Ultraviolet- Visible (UV-Vis) Analysis. UV-Vis absorbance spectra were collected over the wavelength range 230-1200 nm on a Shimadzu UV-2600PC scanning spectrophotometer fitted with a Shimadzu ISR-260 integrating sphere attachment. BaS0₄ powder was used as a reference. The band gap was determined from Tauc plots of the UV- VIS data were all in the range 3.1-3.2 eV and are were similar to those determined for P25 Ti0₂. [0069] X-ray Photoelectron Spectroscopy (XPS). XPS data were taken on the soft X- ray beamline of the Australian Synchrotron, equipped with a hemispherical electron energy analyzer and an analysis chamber of base pressure ~1 ^χ 10^"10 Torr. Spectra were excited at a photon energy of 1486.7 eV (Al Koc equivalent), and calibrated against the C Is signal of adventitious hydrocarbons at 285.0 eV. M/Ti0₂ powders were gently pressed into thin pellets of -0.1 mm thickness for the analyses. Survey scans were collected at a pass energy of 40 eV over the binding energy range 1200-0 eV, while core level scans were collected with a pass energy of 20 eV.

[0070] FIGS. 4A, 4B and 4C show the XPS data for Pd/Ti0₂, Pt/Ti0₂ and Au/Ti0₂ photo- catalysts, respectively, at metal loading of 0.5 and 2 wt.%. FIG. 5 is a graphical representation of XPS Metal/Ti ratio as a function of nominal metal loading (0-4 wt.%) as determined from the data in FIGS. 4A-4C. Results for quantitative XPS analyses are presented in Table 1. The metal content in the samples measured as atomic %, increased with the nominal metal loading. The total nominal metal/Ti ratio showed a typical linear increase at low loading (See, FIG. 5). Pd 3d XPS analysis of 0.5 and 2 wt.% Pd/Ti0₂ (FIG. 4A) was dominated by peaks at 335.4 eV and 340.7 eV (3 :2 area ratio), typical for metallic Pd (Pd_5/2 and Pd_3/2 transitions, respectively). A further set of Pd 3d features seen at 336.7 and 342.0 eV (also in a 3 :2 area ratio) was assigned to a Pd¹¹ species, likely PdO or Pd(OH)₂. Deconvolution of the spectrum through curve-fitting revealed the Pd°:Pdⁿ ratio to be approximately 3 : 1. Pt 4f XPS spectrum of 0.5 and 2 wt.% Pt/Ti0₂ (FIG. 4(B)) showed peaks at 71.3 and 74.7 eV in a 4:3 area ratio, characteristic for metallic Pt° (Pt 4f_7/2 and Pt 4f_5/2, respectively). Pt 4f peaks for metallic Pt were inherently asymmetric, but a minor contribution from Pt(OH)₂ or PtO (Pt 4f_7/2 = 74.0 eV) or Pt0₂ (Pt 4f_7/2 = 74.8 eV) species could not be excluded. Au 4f XPS for 0.5 and 2 wt.% Au/Ti0₂ (FIG. 4C) showed only single sets of Au 4f peaks at about 83.6 eV and 87.3 eV, respectively, in a 4:3 peak area ratio. These peaks were assigned to the Au 4f_7/2 and Au 4f_5/2 peaks, respectively, of supported Au° nanoparticles on Ti0₂. The difference in the number of sets of peaks was attributed to the cationic Pd and Pt species being formed through exposure of the H₂-reduced PdO/Ti0₂ and PtO/Ti0₂ photo-catalysts to air and the Au metal being formed by directly from reduction of the cationic form.

[0071] Photoluminescence data were collected on a Perkin-Elmer LS-55 Luminescence Spectrometer. Spectra were excited at 310 nm, and photoluminescence spectra recorded over the range of 330- 600 nm using a standard photomultiplier. A 290 nm cut-off filter was used. [0072] FIGS. 6A-C show photoluminescence (PL) data for P25 Ti0₂ (top graph) as well as for Pd/Ti0₂, Pt/Ti0₂ and Au/Ti0₂, respectively, at metal loadings of 4 wt.%, 2 wt.%, 1 wt.%), 0.5 wt.%), and 0.25 wt.%, from bottom to top, respectively. FIG. 7 is a graph of metal loading in weight percentage versus normalized PL intensity from FIGS. 6A-6C. Under 310 nm excitation, the un-modified P25 Ti0₂ support gave an intense photoluminescence signal due to radiative of e^"-h⁺ recombination upon photoexcitation. The PL signal consisted primarily of direct transitions and phonon-assisted indirect transitions of anatase the P25 Ti0₂ support (transitions from rutile phase are swamped). Weaker features at energies of 2.80, 2.70, 2.56 and 2.34 were ascribed to oxygen vacancies (Vo ) and defects on the Ti0₂ support. Metal deposition strongly attenuated the photoluminescence signal of P25 Ti0₂, with the extent of suppression following the order Pt > Au > Pd up to 4 wt.%> loading {See, FIG. 7). This data confirmed that each metal co-catalyst functioned as an electron acceptor "electron sink" therefore providing an increased number of charge carriers (h⁺ or e^") available for photoreactions on Ti0₂ surfaces. Because attenuation occurred over the wide range of the metals % investigated, the relative changes due to the number of particles at each % was neglected. It was determined from linking the attenuation to electron captured by the metal that Pt/Ti0₂ catalyst pumped electrons faster than Au/Ti0₂ catalyst, which in turn was faster than Pd/Ti0₂ catalyst. [0073] N₂ physisorption isotherms. N₂ phy si sorption isotherms were determined at liquid nitrogen temperature (-195 °C) using a Micromeritics Tristar 3000 instrument (Micromeritics, U.S.A.). Specific surfaces areas were calculated from the N₂ adsorption data according to the Brunauer-Emmett-Teller (BET) method using P/P₀ values in the range 0.05- 0.2. Cumulative pore volumes and pore diameters were calculated from the adsorption isotherms by the Barrett- Joy ner-Halenda (BJH) method. Samples were degassed at 100 °C under vacuum for 1 h prior to the N₂ physisorption measurements. Surface area values are listed in Table 1.

Example 3

(Hydrogen Production Using the M/Ti0₂ Photo-catalysts General Procedure)

[0074] Photocatalytic hydrogen production tests on the M/Ti0₂ photo-catalysts of Example 1 were carried out in a tubular pyrex reactor (105 mL volume) containing ethanol - water mixtures (0-100 vol,% ethanol). Photo-catalyst (0.0065 g) was placed in the reactor and evacuated under a nitrogen flow for 20 min to remove oxygen. Water (20 mL) or alcohol -water mixture (20 mL) was then injected into the reactor through a rubber septum. The resulting suspension was then sonicated for 5 min to ensure good photo-catalyst dispersion and then stirred continuously in the dark for 30 min. The reactor was then exposed to UV light, supplied from a Spectraline model SB-1000P/F lamp (200 W, 365 nm) at a distance of 8 cm from the reactor. The photon flux at the sample was approximately 6.0 mW cm^"2. Reaction temperature was at about 35-40 °C. Hydrogen evolution was monitored by taking gas head space samples (1 mL) at 20 min intervals and injecting these into a Shimadzu GC 20 14 equipped with a TCD detector and Carboxen-1010 plot capillary column (L x ID. 30 m x 0.53 mm, average thickness 30 μπι). H₂ produced during the photoreaction was quantified against an external calibration curve. Photocatalytic data presented had a mean + standard deviation of 3 replicate runs. For the analysis of the reaction products, the liquid phase was purged carefully for 20 min under N₂ prior the start of the reaction to remove any dissolved 0₂ in solution. Total product evolution was monitored by taking gas head space samples (1 mL) at 1 hour intervals and injecting these into Agilent GC-TCD equipped with a Porapak Q packed column (3 feet ^χ 1/8 inch in external diameter ^χ 2 m long) at 45 °C and N₂ was used as a carrier gas (ca. 20 mL/min). The products observed were eluted in the following order: hydrogen, methane, carbon dioxide, ethylene, ethane, acetaldehyde and finally ethanol. Oxygen was separately monitored using GC-MS connected to a molecular sieve 5A mesh column (8 feet x 1/8 inch in external diameter x 2 m long). Example 4

(Effect of Metal Loading)

[0075] Using the procedure of Example 3, the photocatalytic reactions were systematically conducted on all M/Ti0₂ (M= Pd, Pt, Au) catalysts at metal loading (0.25-4 wt.%) under UV- irradiation at 80 vol.% ethanol. Table 2 lists the normalized H₂ production rates for (0-4) wt.% M/Ti0₂ photo-catalysts (M = Pd, Pt or Au) in 80 vol.% ethanol and apparent quantum yield of hydrogen gas.

Table 2

* QY is the quantum yield of H₂.

[0076] P25 Ti0₂ had negligible H₂ production activity because less photo-excited electrons were available to reduce hydrogen ions (H⁺) to H₂ due to rapid e^"-h⁺ recombination and weak H-H recombination rates. Loading metal co-catalysts increased the H₂ production activity significantly. It was observed that highest H₂ production rates were achieved at metal loadings of 0.25-1 wt.% for Pd/Ti0₂ catalysts, 0.25-0.5 wt.% for Pt/Ti0₂ catalysts; and 1-2 wt.%) for Au/Ti0₂ catalyst (See, FIG. 8). At higher loadings in each set, the activity decreased progressively. It was considered that the rate of H₂ production increase slowly with loading up to the critical metal loading, which was the point when particles touch and maximize the number of active site, then metal particles merge reducing the active sites and thereafter reducing the H₂ production rate. Also at higher metal loadings, metal particles performed as recombination centers, which increased the e^"-h⁺ recombination rate. This was related to the decrease of the average distance between trapping sites with increasing the number of metal atoms confined within a particle. Furthermore, the integrative effect of access metal nanoparticles that blocked the active sites of Ti0₂ as well as a shadowing effect when less light reached the slurry reaction was due to the increase in co-catalyst densities and light scattering. FIG. 8, further shows that the photocatalytic activity per unit weight of catalyst (mmol g^"1 h^"1) decreased in the order Pd/Ti0₂ > Pt/Ti0₂ > Au/Ti0₂ up to 1 wt.% loading while similar activity in all cases was achieved at higher loadings. This was due to the i) the intrinsic electronic properties (work function and <i-band centre) of the different metal co-catalysts; and ii) their size and degree of dispersion over Ti0₂ support.

Example 5

(Effect of Metal Particle Size)

[0077] The effect of metal particle size of the M/Ti0₂ (where M stands for Au, Pd or Pt) catalyst of Example 1 on catalytic rates was determined by analyzing normalized H₂ production rates per: unit weight of metal loading (mol gjyf¹

moles of metal catalyst (mol ITLOIM^"1 h^"1), and surface area (mol m_M ^"2 h^"1) extracted from the data of FIG. 8 are listed Table 3.

Table 3

*M stands for metal co-catalyst (Pd, Pt or Au)

[0078] When the rate was expressed as per unit weight, the following activity trend was observed: Pd ~ Pt > Au up to 1 wt.%, while all metals gave similar activity at higher loading. Yet, when the activity was compared per mol of metal co-catalyst Pd/Ti0₂ was slightly less active than Pt/Ti0₂ or Au/Ti0₂. The activity of Au/Ti0₂, however, was superior to that of Pd/Ti0₂ and Pt/Ti0₂ at all metal loadings when normalized per metal co-catalyst surface area. From these results, it was determined that the larger metal nanoparticle size observed in the Au/Ti0₂ system did not impact detrimentally the photocatalytic H₂ production activity. Likewise, much higher metal dispersions achieved in both the Pd/Ti0₂ and Pt/Ti0₂ did not result in considerable increase in activity. FIG. 9 shows a plot of the rate of H₂ molecules produced normalised to the nominal total number of atoms as a function of the same rate normalised to the number of metal particles (particle sizes were taken from TEM results) at each metal loading (Table 2). From this analysis it was determined that the rate was highest (per metal particle or per metal atom) at the lowest loading for all metals. It was also determined that the overall reaction was sensitive to both particle size and particle number. For example, at a 0.25 wt. % Pd the catalyst produced as much hydrogen as 4 wt. % of Au, per particle. Yet a 0.25 wt. % of Au produced about 10 times more hydrogen than an loading of Pd, again per particle. It was also determined that the fastest time per metal particle of Au was about 0.5 ms for each hydrogen molecule to be made while it was much larger for Pd and Pt. Without wishing to be bound by theory, it is believed that this is due to the total number of electrons that a particle can hold during the reduction of hydrogen ions.

Example 6

(Effect of Metal Loading and Ethanol Concentration)

[0079] The effect of ethanol concentration on H₂ production rates under UV irradiation was compared over selected catalysts made from the procedure of Example 1 : 0.5 wt.% Pd/Ti0₂, 0.5 wt.% Pt/Ti0₂ and 1 wt.% Au/Ti0₂. Linear, stable and reproducible H₂ production rates were observed over a 4 hour period. The rates of H₂ production (normalized against catalyst mass, mmol g^"1 h^"1) in a wide range of ethanol concentration (in vol.%) are shown in FIG. 10. In the absence of ethanol, H₂ was detected from water-splitting. Adding a low amount of ethanol (0.5 vol. % ethanol) was sufficient to increase the amount of H₂ produced. The activity of Pd/Ti0₂ and Pt/Ti0₂ increased from 0.5 up to 90 vol.% ethanol gave maximum rates of 51.3 mmol g^"1 h^"1 and 47.3 mmol g^"1 h^"1, respectively while that of Au/Ti0₂ showed a maximum at about 80 vol.% ethanol. Above this level, the activity dropped to reach one-half of the catalyst's maximum rate for pure ethanol. Hydrogen production from pure ethanol produced acetaldehyde, which in turn decomposed to CO and CH₄. Adding water resulted in more hydrogen being produced because CO reacted to give hydrogen and C0₂. The driving force for such a reaction was determined by the potentials of charge carriers on Ti0₂ electronic bands (conduction band electrons, EC_B = -0.5 V, and valence band holes, EV_B = +2.70 V) relative to the redox potential of adsorbed ethanol molecules (in the presence or absence of H₂0) on the surface of Ti0₂. FIG. 11 shows that the standard oxidation potential of ethanol in the presence of water (photo-reforming, C₂H₅OH + 3H₂0 + 12 h⁺→ 2C0₂ + 12H⁺) increases due to the increase in the Gibbs free energy (AG⁰) compared to that in the absence of water (photo-dehydrogenation, C₂H₅OH + 2h⁺→ CH3CHO + 2H⁺) with respect to the conduction band (CB) and valence band (VB) of Ti0₂. The larger the potential separation between ethanol oxidation process and the Ti0₂ VB, the higher the H₂ production rate. Water inhibits the dehydration of ethanol, but can facilitate the decomposition of acetaldehyde. These results suggested that the rate of H₂ production from ethanol can be increased in the presence of water and vice versa. In this example, the surface active sites remained constant per fixed gram of catalyst, while the number of adsorbed ethanol molecules increased, which led to saturation of the catalyst. FIGS. 12A and 12B show plots of H₂ production versus time for selected M/Ti0₂ photo-catalysts of similar mol of M/Ti0₂ (~ 7.5 10^"5 mol_M in 0.25 wt.% Pd, 0.5 wt.% Pt and 0.5 wt.% Au while 1.5 10^"3 mol_M in 0.5 wt.% Pd, 1 wt.% Pt and 1 wt.% Au) at 10 vol.% and 80 vol.% ethanol, respectively. The rates of H₂ production are shown in FIG. 13. At 10 vol.% ethanol, all photo-catalysts at different loadings gave similar activity under test conditions (except 0.5 wt.%) Au photo-catalyst). At 80 vol.%> ethanol, the catalytic activity become more disparate meaning that more H₂ was produced via ethanol photo-reforming when ethanol decomposition become thermodynamically more favorable than H₂0 decomposition.

[0080] The intrinsic electronic properties of metal nanoparticles (M), such as the work function and electronic density of states near their Fermi levels, influenced their catalytic activity. Theoretically, the larger the difference between the metal work function, O_m> and that of the support (in this case Ti0₂) the more effective the metal should be at accepting photo-excited electrons in Ti0₂. The plot log (rate of H₂ production, mmol g^"1 h^"1) at 10 and 80 vol.%) ethanol versus < _m for selected M/Ti0₂ (Pd, Pt, Au) photo-catalysts of the present invention and comparative M/T1O₂ (Ni, Ag) photo-catalyst at the same metal loading 0.5 wt.%) and 7.5 x lO^"5 mol is shown in FIGS. 14A and 14B, respectively. The rates for Ni/Ti0₂ and Ag/Ti0₂ photo-catalysts were taken using similar test conditions (reactor configuration, catalytic set up, UV-flux, power supply and type/sensitivity of GC used). At 80 vol.%> ethanol, a linear correlation between the logarithm of the exchange current density (j₀) of the electrolytic H₂ evolution reaction and the work function of a set of transition metals well was observed while at lower ethanol concentration (10 vol.%>) the results are slightly scattered at which the activity become not easy to compare as mentioned above. FIGS. 15A and 15B are plots of the rates of H₂ production (mmol g^"1 h^"1) at 10 and 80 vol.%> ethanol versus metal co- catalysts <i-band centre (e_d-E_F). Using the known <i-band center model, which defines the relationship between the log (j₀) and metal work function, < _m as arising from the dependence of heat of adsorption (i.e. the bond strength between metal and H, D_MH), which in turn depends on the number of holes present in <i-band of the metals. From this model, it is believed that the higher the ^-states are in energy relative to the Fermi level (8d-E_F), the more empty the antibonding states and the stronger the adsorption bond.

[0081] From analysis of FIGS. 15A and 15B, it was determined that a volcano shaped relationship between catalytic rates and <i-band centre was present. One possible explanation for data like those of FIGS. 15A and 15B, is that the closer the <i-band centre was to the Fermi level of a metal, the higher the adsorption energy it applied on an adsorbate until reaching the critical value, where they bind neither too weekly nor too strongly (Sabatier principle), at Pd and Pt. From the results, the activity of metals of the present invention and the comparative metals for H₂ production follow the order: Pd ~ Pt > Au ~ Ni > Ag and clearly showed a volcano- curve with respect to the corresponding <i-band centres. It is worth noting in both FIG. 14A and FIG. 15 A, if the rate was compared by the same weight of metal (0.5 wt.%), Pd gave the highest activity. Yet, 0.5 wt.% Pt become the most active catalyst when the rate was compared by similar number of moles of metal (7.5 x 10^"5 mol) as shown in FIG. 14B and FIG. 15B. This phenomenon can also be related to electron transfer system as the current catalytic reaction was governed by excited state electron transfer and not a ground state. Example 7

(Analysis of Products From Photo-reforming of Ethanol/Water Mixtures

Using M/T1O₂ Photo-catalysts of the Present Invention)

[0082] Photo-reforming of a 10 vol.% ethanol in water mixture to produce H₂ production was performed using a 0.5 wt.% Pd/Ti0₂ photo-catalyst of the present invention, a 1 wt.% Pt/Ti0₂ photo-catalyst of the present invention and a 1 wt.% Au/ Ti0₂ photo-catalyst of the present invention. The photo-catalysts were made using the Example 1 procedure. The procedure of Example 2 was used with the exception that a UV flux ~ 6.8 mWcm^"2 was used.

[0083] The produced products included H₂, C0₂, 0₂, CH₄, C₂H₄ and C₂H₆. Unquantifiable amounts of acetaldehyde were observed in the gas phase. Summarized rates of all reaction products in this study are illustrated in FIG. 20 and Table 4. Table 4 is a summary of H₂ and reaction by-products rates for selected M/Ti0₂ photo-catalysts (M = Pd, Pt or Au, metal loading = 1.5 x 10^"3 mol_M) of Example 1 from 10 vol.% ethanol. Results show that the rates of formation of all analyzed products in the study follow the trend H₂ > 0₂ > C0₂ > C_xH_y for M/Ti0₂ photo-catalysts. Specifically, for a 0.5 wt.% Pd/Ti0₂ the trend of produced products was: H₂ » 0₂ > C0₂ > C₂H₄ » CH₄ > C₂H₆, a 1 wt.% Pt/Ti0₂, the trend of produced products was: H₂ » 0₂ > C0₂ » C₂H₄ ~ C₂H₆ > CH₄, and for 1 wt.% Au/Ti0₂ the trend of produced products was H₂ » 0₂ » C0₂ > C₂H₄ > » CH₄, no C₂H₆ detected. Table 4

not detected, * value + 0.15

[0084] From the data in Table 4, it was determined that the ratio of H₂ to C0₂ product yield rates was 15: 1 to 23 : 1 for all photo-catalysts of the present invention, while the ratio of H₂ to 0₂ product yield rates ranged from 8: 1 to 11 : 1. Since the H₂ to C0₂ ratio was larger than the theoretical amount of 3 for the photo-reforming in the presence of water, it was determined that a large amount of H₂ was produced directly from water splitting. The rate of 0₂ was almost consistence over Pd, Pt and Au photo-catalysts. It should be noted that molecular oxygen can capture electrons from the conduction band and can oxidize ethanol to produce C0₂.

[0085] Based on gas phase products concentration, the amount of ethanol consumed can be estimated and plotted as a function of reaction time. FIG. 17 shows plots of H₂ to ethanol consumption for the three photo-catalysts of the present invention. The H₂/ethanol consumed decreased in the case of Pd/Ti0₂ and Pt/Ti0₂ photo-catalysts, while remained unchanged in the case of Au/Ti0₂. An asymptote is reached for the three catalysts for H₂/ethanol consumption of about 20. Deviation at the beginning of the reaction was believed to be due to the properties of the three metals where Au is in its metallic state while both Pt and Pd contain some oxide. At the end of the experiment, the pH was about 4.4 to 4.5 for all three experiments and within experimental error of the calculated pH value, thus the amount of dissolved C0₂ was not sufficient to affect the overall reaction. . Using the Table 4 data, it was determined that the production of other reaction products (CH₄, C₂H₄ and C₂H₆) was 10 to 100 times less than that of hydrogen, and therefore contributed marginally to the overall reaction network. Example 8

(Ethanol Photo-reforming at Various Ethanol Concentrations Using the Photo-catalysts of the Present Invention)

[0086] Using the procedure of Example 6 the ethanol photo-reforming reactions were performed various ethanol to water ratios and a 0.5 wt.% Pd/Ti0₂. The products distributions from water-splitting in various ethanol/water rations are shown in FIG. 18 and summarized in

Table 5.

Table 5

[0087] From the results, it was determined that the rates of both H₂ and C0₂ increased as ethanol concentration increases. The second order rate constants for hydrogen and C0₂ production were extracted and found to be about 0.03 and 0.002 L g^_1catai. h^"1 respectively

[0088] From the above Examples, it was shown that the effect of metal loading, metal size or dispersion, the concentration of the sacrificial agent and the intrinsic electronic properties of metal type on the photoactivity of M/Ti0₂ (M = Pd, Pt, Au) for H₂ production in ethanol/water mixtures are critical to the production of hydrogen gas from water or sacrificial agent/water compositions. At 80 vol.% ethanol, it was observed that highest H₂ production rates were achieved at metal loadings of 0.25-1 wt.% for Pd/Ti0₂ catalysts (ca. φ_Η2 = 28- 32 %), 0.25-0.5 wt.% for Pt/Ti0₂ catalysts (ca. φ_Η2 = 25-31 %); and 1-2 wt.% for Au/Ti0₂ catalyst (ca. c m = ~ 23 %). Higher metal loading of each metal increase rate of H₂ production. The activity of M/Ti0₂ (M = Pd, Pt, Au) photo-catalysts for H₂ production under UV excitation in ethanol/water mixtures decreased generally in the order Pd/Ti0₂ > Pt/Ti0₂ > Au/Ti0₂ » Ti0₂ due to metal intrinsic electronic properties. From the distribution of reaction products it was determined that in photo-reforming of ethanol into H₂ and C0₂ also occurs, which is favored as the amount of ethanol in the reaction mixture increased. Example 9

(Ethanol Photo-reforming at Various Alcohols Using

the Photo-catalysts of the Present Invention) [0089] Using the procedure of Example 6 the photo-reforming reactions were performed using glycerol, ethylene glycol and methanol and Ti0₂ photocatalysts of the present invention that included nanoclusters of Pd, Pt, Au and Pd-Au alloy, and comparative Ti0₂ photocatalysts with Ag and Ni loading. It was found that the Pd-Au/Ti0₂ H₂ production over Au-Pd was found to be the highest. FIG. 19 shows a graph of the hydrogen production.

Claims

A water-splitting photo-catalyst comprising:

(a) photoactive nanomaterial comprising titanium dioxide nanoparticles having an average size of 10 nanometers (nm) to 100 nm; and

(b) metal nanoclusters comprising platinum, palladium, or gold, or a combination thereof, deposited on the surface of the photoactive nanomaterial, wherein:

(i) the nanoclusters having an average size of 0.5 nm 7 nm;

(ii) the amount of the nanoclusters deposited on the surface of the photoactive nanomaterial is 0.25 to 3 wt.% based on the total weight of the combination of the photoactive nanomaterial and the metal nanoclusters; and

(iii) 0.1 % to 2 % of the surface area of the photoactive material comprises the nanoclusters.

The water-splitting photo-catalyst of claim 1, wherein the metal nanoclusters are platinum.

The water-splitting photo-catalyst of claim 2, wherein the platinum metal nanoclusters have an average size of 1.0 nm to 1.5 nm and/or the amount of platinum loaded on the surface of the photoactive nanomaterial is 0.3 to 0.7 wt.%, preferably about 0.5 wt.%.

The water-splitting photo-catalyst of claim 1, wherein the metal nanoclusters are palladium.

The water-splitting photo-catalyst of claim 4, wherein the palladium metal nanoclusters have an average size of 1.5 nm to 2.5 nm and/or the amount of palladium loaded on the surface of the photoactive nanomaterial is 0.3 to 0.7 wt.%, preferably about 0.5 wt.%.

The water-splitting photo-catalyst of claim 1, wherein the metal nanoclusters are gold.

The water-splitting photo-catalyst of claim 6, wherein the gold metal nanoclusters have an average size of 4 nm to 6 nm and/or the amount of gold loaded on the surface of the photoactive nanomaterial is 1.5 to 2.5 wt.%, preferably about 2 wt.%.

The water-splitting photo-catalyst of claim 1, wherein the nanocluster is an Au-Pd alloy. The water-splitting photo-catalyst of claim 8, wherein the Au-Pd alloy nanoclusters have an average size of 0.5 nm to 7 nm and/or the amount of Au-Pd alloy loaded on the surface of the photoactive nanomaterial is 0.1 to 1 wt.%, preferably about 0.4 wt.%.

The water-splitting photo-catalyst of any one of claims 1 to 9, wherein the titanium dioxide nanoparticles are mixed phase titanium dioxide nanoparticles having a ratio of anatase to rutile of 1.5: 1 to 10: 1.

The water-splitting photo-catalyst of any one of claims 1 to 10, further comprised in an aqueous solution comprising an organic sacrificial agent and water having a volume % ratio of 30:70 to 1 :99 of sacrificial agent to water.

The water-splitting photo-catalyst of any one of claims 10 to 11, wherein the sacrificial agent comprises an alcohol, a diol, a polyol, a dioic acid, or any combination thereof.

The water-splitting photo-catalyst of claim 12, wherein the sacrificial agent comprises methanol, ethanol, propanol, iso-propanol, n-butanol, iso-butanol, ethylene glycol, propylene glycol, glycerol, or oxalic acid, or any combination thereof, preferably ethanol or ethylene glycol, most preferably ethanol.

The water-splitting photo-catalyst of any one of claims 1 to 13, wherein the catalyst is capable of producing H₂ at a rate of:

(i) at least 30 mmol g^_1catai h^"1, preferably 30 to 50 mmol g^_1catai h^"1, or more preferably 40 to 45 mmol g^_1catai h^"1, when subjected to a UV flux of about 6 mW cm^"2, and when platinum is deposited on the surface of the photoactive material;

(ii) at least 30 mmol g^_1catai h^"1, preferably 30 to 50 mmol g^_1catai h^"1, or more preferably 40 to 50 mmol g^_1catai h^"1, when subjected to a UV flux of about 6 mW cm^"2, and when palladium is deposited on the surface of the photoactive material; or

(iii) at least 25 mmol g^_1catai h^"1, preferably 25 to 35 mmol g^_1catai h^"1, or more preferably 30 to 35 mmol g^_1catai h^"1, when subjected to a UV flux of about 6 mW cm^"2, and when gold is deposited on the surface of the photoactive material.

A method for producing hydrogen gas (H₂) from water, the method comprising: (a) obtaining an aqueous solution comprising the water-splitting photo-catalyst of any one of claims 1 to 9, a sacrificial agent, and water, wherein the aqueous solution has a volume % ratio of 30:70 to 1 :99 of sacrificial agent to water; and

(b) subjecting the mixture to a light source for a sufficient period of time to produce H₂ from the water.

The method of claim 15, wherein H₂ is produced at a rate of:

The method of any one of claims 15 to 16, wherein the volume % ratio of sacrificial agent to water is 20:80 to 1 :99, preferably 5:95.

The method of any one of claims 15 to 17, wherein the sacrificial agent comprises an alcohol, a diol, a polyol, a dioic acid, or any combination thereof.

The method of claim 18, wherein the sacrificial agent comprises methanol, ethanol, propanol, iso-propanol, n-butanol, iso-butanol, ethylene glycol, propylene glycol, glycerol, or oxalic acid, or any combination thereof, preferably ethanol or ethylene glycol, most preferably ethanol.

The method of any one of claims 15 to 19, wherein the light source is sunlight or a UV light source or a combination thereof.