PHOTOCATALYTIC WATER SPLITTING WITH COBALT OXIDE-TITANIUM DIOXIDE-PALLADIUM NANO-COMPOSITE CATALYSTS

Abstract

Photocatalysts and methods of using the same for producing hydrogen and oxygen from water are disclosed. The photocatalysts include photoactive titanium dioxide loaded with 0.5 wt. % to 4 wt. % of a hole-scavenging material comprising cobalt oxide and 0.1 wt. % to 1 wt. % of palladium (Pd) and/or a PdCo alloy.

Claims

1. A water-splitting photocatalyst comprising photoactive titanium dioxide loaded with 0.5 wt. % to 4 wt. % of a hole-scavenging material comprising cobalt oxide and 0.1 wt. % to 1 wt. % of palladium (Pd).

2. The water-splitting photocatalyst of claim 1, comprising 1.5 wt. % to 2.5 wt. % cobalt oxide and 0.2 wt. % to 0.4 wt. % Pd.

3. The water-splitting photocatalyst of claim 2, comprising about 2 wt. % cobalt oxide and about 0.3 wt. % Pd.

4. The water-splitting photocatalyst of claim 1, wherein the cobalt oxide is cobalt (II) oxide, cobalt (III) oxide, or cobalt (II, III) oxide.

5. The water-splitting photocatalyst of claim 4, wherein the cobalt oxide is cobalt (II) oxide.

6. The water-splitting photocatalyst of claim 1, further comprising a palladium-cobalt alloy.

7. The water-splitting photocatalyst of claim 1, wherein the titanium dioxide is anatase, rutile, or brookite, or any combination thereof.

8. The water-splitting photocatalyst of claim 7, wherein the titanium dioxide is anatase.

9. The water-splitting photocatalyst of claim 7, wherein the titanium dioxide is a mixed-phase comprising anatase and rutile.

10. The water-splitting photocatalyst of claim 9, wherein the weight ratio of anatase to rutile is 1.5:1 to 10:1.

11. The water-splitting photocatalyst of claim 1, wherein photoactive titanium dioxide, the hole-scavenging material, and the Pd are each in particulate form.

12. The water-splitting photocatalyst of claim 11, further comprising a palladium (Pd)-cobalt (Co) alloy that is in particulate form.

13. The water-splitting photocatalyst of claim 12, wherein the photoactive titanium dioxide, the hole-scavenging material, the Pd, and the PdCo alloy are each nanostructures or sub-nanostructures, said nanostructure or subnanostructures having at least one dimension equal to or less than 1000 nm.

14. The water-splitting photocatalyst of claim 13, wherein the nanostructures are nanowires, nanoparticles, nanoclusters, or nanocrystals, or combinations thereof.

15. The water-splitting photocatalyst of claim 1, wherein the hole-scavenging material is deposited on the surface of the photoactive titanium dioxide.

16. The water-splitting photocatalyst of claim 15, wherein the Pd is deposited on the surface of the photoactive titanium dioxide or the surface of the hole-scavenging material, or both surfaces.

17. The water-splitting photocatalyst of claim 7, wherein the titanium dioxide is anatase and brookite.

18. An aqueous composition comprising the water-splitting photocatalyst of claim 1.

19. The aqueous composition of claim 18, wherein the aqueous composition has a pH of 7 to 13

20. A method for producing oxygen (O.sub.2) and hydrogen (H.sub.2) from water, the method comprising: obtaining the aqueous composition of claim 18; and subjecting the aqueous composition to a light source for a sufficient period of time to produce O.sub.2 and H.sub.2 from the water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIGS. 1A-C: (a) Schematic of a cross-sectional view of a photocatalyst of the present invention where photoactive titanium dioxide is impregnated with cobalt oxide. (b and c) Schematics of a cross-sectional view of photocatalysts of the present invention where photoactive titanium dioxide is impregnated with cobalt oxide and palladium.

[0032] FIG. 2: Schematic of a water splitting system of the present invention where the photoactive catalyst is dispersed in an aqueous solution.

[0033] FIGS. 3A and 3B: (3A) UV-Vis absorption spectra of TiO.sub.2 photocatalyst with different loadings of Co (in wt. %) (i) 0 (ii) 0.5 (iii) 1 (iv) 2 and (v) 4%. (3B) Plots of Tauc units versus (eV) for same series of photocatalysts.

[0034] FIGS. 4A-C: (4A) XRD spectra of TiO.sub.2 with different loadings of Co (in wt %). (4B) XPS spectra of Co 2p peak in CoO.sub.x-loaded TiO.sub.2 containing 1.0 wt % cobalt before and after Ar ions sputtering. (4C) Valence band of the same samples of (4B), before sputtering (red line) and after sputtering (black line). The inset in (4C) represents the corresponding Ti2p and O2s lines.

[0035] FIGS. 5A-D: (5A) H.sub.2 production as a function of time over TiO.sub.2 with different loadings of Co (in wt %). (5B) H.sub.2 production rates (extracted from (5A)) as function of Co loading. Reaction conditions: 4 mg catalyst, 30 mL H.sub.2O and 5 vol % glycerol under UV lamp (375 nm) at a flux of 4 mW/cm.sup.2 (measured after a Pyrex with a similar thickness to that of the reactor). (5C) O.sub.2 evolution from TiO.sub.2 with different loadings of Co (in wt %) using 0.05 M AgNO.sub.3 solutions (5B) O.sub.2 evolution rates as function of Co loading. (5D) O.sub.2 production rates (extracted from (5C)) as function of Co loading.

[0036] FIGS. 6A-D: (6A) H.sub.2 production as a function of time of TiO.sub.2 photocatalysts with different loadings of Co (in wt. %). (6B) H.sub.2 production rates as a function of Co loading. Reaction conditions: 4 mg catalyst, 30 mL H.sub.2O and 5 vol % glycerol under Xenon lamp (250-650 nm) with a total flux of 26 mW/cm.sup.2 (UV about 3.3 mW/cm.sup.2, visible about 22.7 mW/cm.sup.2). (6C) H.sub.2 production rates (normalized to UV flux) for UV lamp versus UV plus Visible lamp as function of Co loading. (6D) % drop in activity on using 1% glycerol as function of Co loading.

[0037] FIG. 7: Proposed mechanism for the photocatalytic hydrogen evolution over Coo loaded TiO.sub.2 photocatalyst.

[0038] FIGS. 8A and 8B: (8A) H.sub.2 production as a function of time of 2 wt. % CoTiO.sub.2 photo-catalyst with different loadings of Pd (in wt. %). (8B) H.sub.2 production rates of 2 wt. % CoTiO.sub.2 with different loadings of Pd (in wt. %).

[0039] FIGS. 9A-C: (9A) high angle annular dark field imaging (HAADF) in STEM mode of TiO.sub.2 (anatase) particles. (9B) high angle annular dark field imaging (HAADF) in STEM mode of 0.3 wt. % Pd-2 wt. % CoO/TiO.sub.2 particles; the inset presents EDS from two distinct particles. (9C) high angle annular dark field imaging (HAADF) in STEM mode of 0.3 wt. % Pd-2 wt. % CoO/TiO.sub.2 particles; inset shows the particle size distribution.

DETAILED DESCRIPTION OF THE INVENTION

[0040] 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 invention provides a solution to these issues. The solution is predicated on the discovery that titanium dioxide particles impregnated with certain amounts of cobalt oxide and palladium or palladium-cobalt alloy can dramatically enhance hydrogen and oxygen production rates from a water-splitting reaction. Without wishing to be bound by theory, it is believed that when cobalt oxide and palladium or PdCo alloy are each used in amounts of 0.5 wt. % to 4 wt. % and 0.1 wt. % to 1 wt. % by weight of the total catalyst, respectively, an increase in the charge carrier life time of the electrons and holes occurs, which leads to an increase in hydrogen and oxygen production rather than an electron-hole recombination event. As illustrated in non-limiting embodiments in the Examples and Figures, maximum H.sub.2 and O.sub.2 production can be obtained when cobalt oxide is used in amounts of 1.5 wt. % to 2.5 wt. %, preferably about 2 wt. %, and palladium (or a combination of Pd and PdCo) is used in amounts of 0.2 wt. % to 0.4 wt. %, preferably about 0.3 wt. %, of the total weight of the catalyst. Stated plainly critical amount ranges for both of cobalt oxide and palladium have been discovered in the present invention. Still further, it was also discovered that cobalt oxide can act as a hole scavenging agent much like sacrificial agents (e.g., glycerol and ethylene glycol) do. Therefore, the catalysts of the present invention, when used in a water-splitting reaction, do not need to rely on the presence of solubilized sacrificial agents in the aqueous solution to reduce the likelihood of electron-hole recombination events. This can make the catalysts of the present invention more cost efficient and less complicated to use when compared with other known water-splitting catalysts.

[0041] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Photoactive Catalysts

[0042] Referring to FIGS. 1A and 1B, water-splitting photoactive catalysts 10 of the present invention are illustrated through non-limiting illustrations. The size and shape of the catalysts 10, which include photoactive titanium 11, cobalt oxide 12, and palladium 13 are non-limiting and used only for illustration purposes. In one instance, the catalyst 10 can include a photoactive titanium dioxide particle 11 that is impregnated with cobalt oxide 12 such that cobalt oxide 12 is present on the surface of the titanium dioxide 11. This can allow for the formation of interfaces between the titanium dioxide particle 11 and the cobalt oxide particles 12 (See, FIG. 1A). Alternatively, the catalyst 10 can be further impregnated with palladium 13 such that palladium particles 13 are present on the surfaces of the titanium dioxide particle 11 and/or on the surface of the cobalt oxide particle 12 (See, FIG. 1B). This can allow for the formation of interfaces between palladium particles 13 and titanium dioxide particle 11 or palladium particles 13 and cobalt oxide particles 12. In some instances, an individual cobalt oxide particle 13 can contact both surfaces of the titanium dioxide particle 11 and a cobalt oxide particle 12 at the same time. The size and shapes of each of the titanium dioxide particle 11, the cobalt oxide particles 12, and the palladium particles 13 can be modified as desired. In particular instances, each of the particles 11, 12, and 13 are nanostructures. In preferred aspects, the titanium dioxide particle 11 is substantially spherical in shape and has a diameter of 7 to 10 nm, while the palladium particle 13 is also substantially spherical in shape with a diameter of 1 to 2 nm, and the cobalt oxide particle 12 is typically sub-nanometers in size up to particles having a diameter of 3 nm or 1 to 3 nm.

[0043] One of the discoveries of the present invention are the weight percentage ranges of cobalt oxide particles 12 and palladium particles 13 that are included in the catalyst 10. In particular, cobalt oxide particles 12 can be present in an amount of 0.5 wt. % to 4 wt. % based on the total weight of the catalyst 10. In preferred instances, the cobalt oxide particles 12 are present in an amount of 1.5 wt. % to 2.5 wt. %, and most preferably about 2 wt. %, based on the total weight of the catalyst 10. The palladium particles 13 can be present in an amount of 0.1 wt. % to 1 wt. % based on the total weight of the catalyst 10. Still further, the palladium particles 13 can be a combination of Pd particles and PdCo alloy particles, both of which are represented as element 13. The Pd particles can be separate from the PdCo alloy particles. In preferred instances, the palladium particles 13 are present in an amount of 0.2 wt. % to 0.4 wt. %, and preferably about 0.3 wt. %, based on the total weight of the catalyst 10. It is believed that these weight percentage ranges provides for maximum catalytic activity of the catalysts 10 of the present invention. Still further, the discovery of the cobalt oxide particles 12 acting as hole scavengers allows for one to limit or avoid the use of sacrificial agents during a water-splitting reaction.

[0044] Still further, the catalysts 10 of the present invention can be further impregnated with additional materials that further enhance the efficiency of the water-splitting reaction and ultimate production of H.sub.2 and/or O.sub.2. By way of example, further impregnation with metals or oxides or alloys thereof can assist in reducing or preventing electron/hole recombination events. Non-limiting examples of such metals include silver, platinum, gold, rhodium, ruthenium, rhenium, iridium, nickel, or copper, or any combinations or oxides or alloys thereof. These additional metals can be nanostructures, preferably nanoparticles having a substantially spherical shape.

[0045] 1. Materials Used

[0046] The photoactive titanium dioxide 11 can be capable of being excited by ultraviolet and/or visible light. The 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. Anatase can be more efficient than rutile in the charge transfer, but may not be 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, 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). In preferred instances, the titanium dioxide 11 is pure anatase or a mixed phase of anatase and rutile.

[0047] The cobalt oxide particles 12 (CoO.sub.x) can be in the form of cobalt(II) oxide (CoO), cobalt(III) oxide (Co.sub.2O.sub.3), or cobalt(II,III) oxide (Co.sub.3O.sub.4). In preferred instances, the cobalt oxide 12 particles are in the reduced form of CoO. The cobalt oxide 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 (U.S.A.) and Alfa Aesar GmbH (Germany) 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.

[0048] The palladium particles 13 can be palladium or an alloy that includes palladium. In preferred instances, the palladium is used. Palladium 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 (U.S.A) and Alfa Aesar GmbH (Germany) 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 palladium particles can be prepared using co-precipitation or deposition-precipitation methods. The palladium particles 13 can be used as conductive material for the excited electrons to ultimately reduce hydrogen ions to produce hydrogen gas. Palladium particles 13 are highly conductive materials, making them well suited to act in combination with the photoactive material 11 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.

[0049] 2. Process of Making the Photocatalysts

[0050] Non-limiting examples for making photocatalysts are disclosed in the Examples of the present specification. Generally, the following steps can be used to manufacture catalysts of the present invention.

[0051] The water-splitting photoactive catalysts 10 of the present invention can be prepared from the aforementioned titanium dioxide particles 11, the cobalt oxide particles 12, and the palladium particles 13 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 10 includes formation of an aqueous solutions of titanium dioxide particles 11 in the presence of cobalt oxide particles 12 followed by precipitation where the cobalt oxide particles 12 are attached to a least a portion of the surface of titanium dioxide particles 11 (e.g., precipitated titanium dioxide crystals or particles 11, 12). Deposition or impregnation of palladium particles 13 can be obtained by mixing the titanium dioxide-cobalt oxide composite with aqueous solutions of palladium or salt forms or precursors thereof, followed by precipitation, where the palladium particles 13 become attached to at least a portion of the surface of titanium dioxide-cobalt oxide composite. Alternatively, the palladium particles 13 can be deposed on the surface of the composite titanium dioxide-cobalt oxide composite material by any process known by those of ordinary skill in the art. Deposition can include attachment, dispersion, and/or distribution of the palladium particles 13 on the surface of the titanium dioxide particle 11, the cobalt oxide particles 12, or both. As another non-limiting example, the titanium dioxide-cobalt oxide composite material can be mixed in a volatile solvent with the palladium particles 13. 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 10 of the present invention. Calcination (such as at 300 C.) can be used to further crystalize the titanium dioxide-cobalt oxide composite material. FIG. 1C is a schematic representation of the photocatalyst that includes the titanium dioxide particle 11, the cobalt oxide particle 12, and the palladium particle 13. The titanium dioxide particle 11 is in contact with the cobalt oxide particle 12. The palladium particle is in contact with both of the titanium dioxide particle 11 and the cobalt oxide particle 12.

[0052] B. Water-Splitting System

[0053] Referring to FIG. 2, a non-limiting representation of a water-splitting system 20 of the present invention is provided. The system includes a plurality of the photocatalysts 10, a light source 21, and container or reaction vessel 22 that can be used to hold aqueous solutions or water 23. The plurality of photocatalysts 10 can be suspended in the aqueous solution 23. Although not shown, the system 20 can also include at least one inlet for the aqueous solution/water 23 and at least one or more outlets for produced hydrogen and oxygen formed during the water-splitting reaction. Although not shown, the photocatalyst 10 can be coated onto the walls of the container 22 or can be packed in a bed (or plurality of beds), which is then immersed in the aqueous solution 23.

[0054] The container 22 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 10 can be used to split water to produce H.sub.2 and O.sub.2. The light source 21 can includes either one of or both of visible and (400-600 nm) and ultraviolet light (280-400). The light can excite the photoactive titanium dioxide 11 to excite an electron in the valence band 24 to the conductive band 25. The excited electrons (e.sup.) leave a corresponding hole (h.sup.+) when the electrons move to the conductive band. 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. In particular, the cobalt oxide 12 can act as a hole scavenger to oxidize oxygen anions and produce O.sub.2. Palladium 13 can act as an electron sink to reduce protons and produce H.sub.2. The hydrogen gas and the oxygen gas can then be collected and used in down-stream processes. Due to the hole scavenging properties of the cobalt oxide and conductive properties of palladium, 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. The system 20 does not require the use of an external bias or voltage source. Further, the efficiency of the system 20 as well as the hole scavenging properties of cobalt oxide 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 23. The presence of the sacrificial agent can increase the efficiency of the system 20 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.

[0055] 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 10 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 MoPt cathodes (See, International Journal of Hydrogen Energy, June 2006, Vol. 31, issue 7, pages 841-846) or MoS.sub.2 cathodes (See, International Journal of Hydrogen Energy, February 2013, Vol. 38, issue 4, pages 1745-1757).

EXAMPLES

[0056] 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

Preparation of Photocatalysts

[0057] The CoO.sub.xTiO.sub.2 photocatalysts were prepared by wet impregnation. Anatase TiO.sub.2 from Hombikat was used as the support catalyst. Different loadings of Co (0, 0.5, 1, 2 and 4 wt. %) on TiO.sub.2 support were prepared by adding known amount of Co(NO.sub.3).sub.2.6H.sub.2O salt solution to 500 mg of TiO.sub.2 support. Excess water was evaporated to dryness under constant stirring with slow heating at 80 C. The dried photocatalysts was calcined at 350 C. for 5 hours to improve the crystallinity.

[0058] Photocatalysts with dual co-catalysts of Pd and CoO.sub.x were prepared by co-impregnating the Pd and Co salt solutions in the same way. Pd acetate as well as Pd chloride were both used as precursors for Pd metal deposition. Both gave Pd metal of about the same particle size.

Example 2

Characterization Data

[0059] UV-VIS absorbance spectra of the powdered catalysts was collected over the wavelength range of 250-900 nm on a Thermo Fisher Scientific spectrophotometer equipped with praying mantis diffuse reflectance accessory. Absorbance (A) and reflectance (% R) of the samples were measured. The reflectance (% R) data was used to calculate the band gap of the samples using the Tauc plot (Kubelka-Munk function). The crystal structure and phase of our photocatalysts was characterized using X-ray diffraction (XRD). XRD spectra was recorded using a Bruker D8 Advance X-ray diffractometer. A 2 interval between 20 and 90 was used with a step size of 0.010 and a step time of 0.2 sec/step. X-ray photoelectron spectroscopy (XPS) was used to study the elemental composition and electronic state of our photo-catalysts. XPS was conducted using a Thermo scientific ESCALAB 250 Xi. The base pressure of the chamber was typically in the low 10.sup.9 to high 10.sup.10 mbar range. Charge neutralization was used for all samples. Spectra were calibrated with respect to C1s at 285.0 eV. Quantitative analyses were conducted using the following sensitivity factors: Co2p (3.8), Ti2p (1.8), and O1s (0.66).

[0060] The band gaps and absorption properties of the prepared photocatalysts were studied using diffuse reflectance UV-Vis spectroscopy. The UV-Vis spectra of CoO.sub.xTiO.sub.2, recorded in the range of 250 nm to 900 nm as shown in FIG. 3A. The UV-Vis spectra shows typical absorption from anatase TiO.sub.2 with a band edge around 370 nm (E.sub.g about 3.35 eV) due to the charge-transfer from the valence band formed by 2p orbitals of the oxide anions to the conduction band formed by 3d t2g orbitals of the Ti.sup.4+ cations (See, Sadanandam et al., Cobalt doped TiO2: A stable and efficient photocatalyst for continuous hydrogen production from glycerol:Water mixtures under solar light irradiation, Int. J. Hydrogen. Energ. 2013, 38, 9655-9664 (Sadanandam), Yan et al., Noble metal-free cobalt oxide (CoOx) nanoparticles loaded on titanium dioxide/cadmium sulfide composite for enhanced photocatalytic hydrogen production from water, Int. J. Hydrogen. Energ. 2014, 39, 13353-13360) (Yan)). Absorption spectra of CoO.sub.xTiO.sub.2 nanocomposite photocatalysts showed a red shift in the absorption with broad absorption in visible light. The nanocomposite catalysts also show an absorption peak in the region of 500 nm (about 2.5 eV) which can be attributed to Co.sup.2+.fwdarw.Ti.sup.+ charge-transfer interaction, consistent with earlier reports (see Sadanandam and Yan). The other absorption peak near 800 nm (about 1.5 eV) is caused by the transition of electrons from the occupied Co 3d states below the Fermi level to the uncopied Co 3d states which form the conduction band of CoO.sub.x (See, Deori et al., Morphology oriented surfactant dependent CoO and reaction time dependent Co3O4 nanocrystals from single synthesis method and their optical and magnetic properties, CrystEngComm 2013, 15, 8465-8474, and Liao et al., Efficient solar water-splitting using a nanocrystalline CoO photocatalyst, Nat. Nanotechnol. 2014, 9, 69-73).

[0061] The Kubelka-Munk function, F(R)=(1R)2/(2R), was used to calculate the band gap of the materials. Since TiO.sub.2 (anatase) is known to be an indirect band gap semiconductor, the Tauc plot of the quantity (F(R)E).sup.1/2 was plotted against the radiation energy and is shown in FIG. 3(b). Pure anatase TiO.sub.2 has a band gap of 3.3 eV and with Co loading it slightly shifts to 3.2 eV. With increasing Co loading, there is no significant change in band gap of TiO.sub.2 but at the same time the composite photocatalyst shows increased visible light absorption due to the presence of CoO.sub.x.

[0062] The effect of Co loading on crystal structure of TiO.sub.2 support was studied using XRD. FIG. 4A shows the X-Ray diffraction patterns of TiO.sub.2 photocatalyst with different loadings of Co (in wt. %). The XRD patterns clearly show the characteristic planes of anatase phase at 2=25.5 (101), 37.7 (004) and 48.2 (200). The XRD pattern does not show any cobalt phase (up to 4 wt. % loading), indicating that cobalt ions are uniformly dispersed on the TiO.sub.2 support. This was also observed in earlier reports where at low loadings the CoO diffraction peaks could not be detected (See, Sadanandam and Yan). The XRD peaks positions of anatase TiO.sub.2 also do not show any change upon Co loading, confirming there is no change in structure/crystal phase of TiO.sub.2 or doping of Co ions into TiO.sub.2. A broadening of the TiO.sub.2 diffraction peaks was observed with addition of Co, indicated by the larger FWHM. This broadening could be due to smaller TiO.sub.2 crystallites and/or lattice strain on TiO.sub.2 due to the presence of CoO nanoparticles.

[0063] In order to further analyze chemical composition of CoO and electronic state of the composites, the CoO.sub.x-loaded TiO.sub.2 sample was analyzed by XPS. FIG. 4B shows the Co 2p spectra from 1 wt. % CoO.sub.xTiO.sub.2 samples calcined at 400 C. for 5 hours. Co2p of Co.sup.2+ has its characteristic satellites, reduction of Co.sup.2+ leads to Co.sup.0 which results in a shift in the binding energy by about 2 eV. The binding energy of Co.sup.3+ is very close to that of Co.sup.2+ but Co.sup.3+ satellites are much more attenuated and therefore the presence of strong satellites can gauge the extent of Co.sup.2+ contribution (See, Idriss et al., Reactions of Acetaldehyde on CeO.sub.2 and CeO.sub.2-Supported Catalysts, J. Catal. 1995, 155, 219-237). In FIG. 4B XPS Co2p before and after Ar ion sputtering is presented. The binding energies for Co2p.sub.3/2 and Co2.sub.1/2 appear at 781.4 eV and 797.1 eV. A spin orbit splitting of 15.5 eV and satellites presence at about 7 eV above the main peaks (about 788 and 804 eV) are also observed. These structures are consistent with those reported for Co.sup.+2 of CoO (See, Sadanandam). Argon ions sputtering results in the preferential removal of oxygen anions (See Idriss et al., Characterization of TiO.sub.2 surfaces active for novel organic synthesis, Catal. Lett. 1994, 26, 123-139) and consequently the reduction of metal cations to lower oxidation states. This can be seen in FIGS. 4(b) and 4(c). In FIG. 4B, a shoulder at the lower binding energy side is seen at about 778 eV that is attributed to CoO. The appearance of CoO is associated with the decrease of the signal of Co.sup.2+. In FIG. 4C, the valance band region is presented for the fresh and Ar ions sputtered surfaces. The appearance of the lines at about 1 eV below the Fermi level is indicative of 3d electrons due to both Ti cations in reduced states (See, Idriss et al., Two routes to formaldehyde from formic acid on TiO2 (001) surfaces, Surf Sci., 1996, 348, 39-48) and metallic Co (See, Biesinger et al., Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni, Applied Surface Science, 2011, 257:2717-2730, and Riva et al., Metal-support interaction in Co/SiO2 and Co/TiO2, Applied Catalysis A: General, 2000, 196, 111-123). The inset in FIG. 4C presents the Ti3p and O2s for the same samples. The broad structure at the low binding energy side of the Ti3p is due to the presence of Ti cations in lower oxidation state than +4. Quantitative analyses of the Co2p, Ti2p, O1s indicated that Co is present is present in about 0.1 at. % on the TiO.sub.2 support.

Example 3

Photocatalytic Activity

[0064] The photocatalysts were evaluated for H.sub.2 production in a 135 mL volume Pyrex glass reactor. The catalyst sample (4 mg) was introduced into the reactor. Milli-Q (Millipore Corp., U.S.A.) deionized water (30 mL) and glycerol (5 vol. %, 1.5 mL) as sacrificial agent was added. The final slurry was purged with N.sub.2 gas to remove any O.sub.2 and subjected to constant stirring. The reactor was then exposed to the UV light; a 100 Watt ultraviolet lamp (H-144GC-100, Sylvania par 38) with a flux of about 5 mW/cm.sup.2 at a distance of 5 cm. Product analysis was performed by gas chromatograph (GC) equipped with thermal conductivity detector (TCD) connected to Porapak Q packed column (2 m) at 45 C. and N.sub.2 was used as a carrier gas.

[0065] The H.sub.2 production activity of CoO.sub.xTiO.sub.2 photo-catalysts under UV lamp from water-glycerol (5 vol. %) mixtures is shown in FIG. 5A. The photocatalytic activity from the composite photocatalysts was evaluated over 24 hours and was stable and reproducible. Pure anatase TiO.sub.2 calcined at 350 C. showed H.sub.2 production rates of about 10 molg.sup.1min.sup.1. The loading of CoO.sub.x resulted in a substantial improvement in the H.sub.2 evolution. The highest H.sub.2 production rates of about 47 molg.sup.1min.sup.1 was achieved when the Co metal concentration was 2 wt. % relative to TiO.sub.2. The H.sub.2 production rates as a function of Co loading is plotted in FIG. 5(b). One can notice that increasing the Co loading above 2 wt. %, decreased the photocatalytic activity. In order to further check for this activity, O.sub.2 evolution activity of the catalysts was also analyzed under UV excitation condition and in the same reactor but with 0.05 M AgNO.sub.3 solutions to scavenge electrons. As shown in FIG. 5C, the O.sub.2 evolution was linear as a function of time and was observed to be in a stoichiometric ratio of 1:2 to the H.sub.2 production seen earlier with glycerol as the sacrificial agent. The O.sub.2 evolution also showed a similar trend to the H.sub.2 production as shown in FIG. 5D with the 2 wt. % Co loading had the highest activity (about 21 molg.sup.1min.sup.1).

[0066] To further investigate the contribution of CoO.sub.x in the enhancement of photocatalytic activity, the reaction was also tested under UV plus visible light irradiation under a total flux of about 26 mW/cm.sup.2 (UV about 3.3 mW/cm.sup.2 and visible about 22.7 mW/cm.sup.2). As shown in FIG. 6A, similar to the trend under UV lamp, the loading of cobalt on TiO.sub.2 resulted in a substantial improvement in the H.sub.2 evolution. The highest H.sub.2 production rates was also achieved when the Co concentration was 2 wt. % as seen in FIG. 6B. FIG. 6C presents the H.sub.2 production rates normalized to UV flux where similar trends of activity, under UV light only and UV plus visible light, are seen. This indicates that there is no contribution of visible light charge carriers in the photocatalytic water splitting process. The results indicate that any charge carriers being generated in CoO.sub.x from visible light do not participate in the photocatalytic water splitting process. It is highly likely that in this case Co.sup.2+ are transformed to Co.sup.3+ during the hydrogen reduction process; in other words, this is a stoichiometric and not a catalytic reaction.

[0067] It is possible that the enhanced photocatalytic activity of the composite catalysts is due to the formation of a Schotkky type heterojunction leading to efficient charge carrier separation. The high valence band edge in CoO.sub.x is ideal for trapping photogenerated holes in TiO.sub.2. The proposed mechanism is shown in FIG. 7 where the CoO.sub.x nanoparticles act as oxidation co-catalyst. This suggests that UV may aid in the excitation of TiO.sub.2. To confirm this hypothesis, the photocatalysts were tested by changing the concentration of the hole scavenging sacrificial agent. In particular, photocatalytic activity under the same conditions by lowering the glycerol concentration from 5 vol % to 1 vol. % was analyzed. As shown in FIG. 6D, in pure anatase TiO.sub.2, the H.sub.2 evolution rates drop by about 42% when glycerol concentration is reduced to 1 vol. %. In contrast, samples with 2 wt. % of CoO.sub.x show better activity, with a drop of about 10%. This result may indicate that CoO.sub.x nanoparticles function similar to the sacrificial agent, i.e., as an oxidation co-catalyst/hole trapping agent.

[0068] H.sub.2 production activity of 2 wt. % CoO.sub.xTiO.sub.2 photocatalysts impregnated with Pd metal is shown in FIG. 8A. It was observed that upon loading Pd metal, H.sub.2 evolution can be further improved. The highest H.sub.2 production rates was achieved when the Pd concentration was 0.3 wt. % as seen in FIG. 8B, with H.sub.2 production rates of about 180 molg.sup.1min.sup.1. This illustrates that a system where a dual semiconductor-based co-catalyst, i.e., CoO.sub.x as an oxidation co-catalyst and Pd as reduction co-catalyst, can function remarkably well and remain stable during extended periods of use.

[0069] To further investigate the role of Pd, Transmission Electron Microscopy (TEM) was performed on the catalyst having 0.3 wt. % Pd-2 wt. % CoO/TiO.sub.2. Characterization by high resolution TEM (HRTEM) did not yield images where cobalt and/or palladium particles were visible; therefore it was necessary to use high angle annular dark field imaging (HAADF) in STEM mode, which is suited to better identify nanoparticles with higher atomic number than the support. The reason for that is that the nanoparticles are very small, so the number of atomic planes is low and lattice fringes are difficult to observe.

[0070] FIG. 9A shows a representative general image of the sample, which is constituted by TiO.sub.2 particles very homogeneous in size. At high magnification (FIG. 9B), individual nanoparticles are recognized well dispersed over the titania support (some of them are marked by arrows in FIG. 9B). Analysis by energy dispersive X-ray spectroscopy (EDS) on individual nanoparticles shows the existence of both cobalt and palladium. As representative examples, the EDS spectra of two nanoparticles (marked as a and b in FIG. 9B) are included, both showing the common occurrence of Co and Pd and being Co more abundant than Pd according to the composition of the sample. This result indicate that regions of TiO.sub.2 contain CoO, other regions may contain Pd/CoOTiO.sub.2 where Pd is alone or in an alloy form with a fraction of Co (originating from CoO). It is important to add that the EDS analysis of the titania support did not show the appearance of either cobalt or palladium. FIG. 9C shows another representative image of the sample together with the nanoparticles size distribution histogram obtained using more than one hundred nanoparticles. The mean particle size is centered at 2.8 nm, and the particles had a substantially homogenous particle size.

[0071] In summary, nano-composite photocatalysts by impregnating anatase TiO.sub.2 with different amounts of Co salt solutions was prepared, characterized and tested. The presence of CoO enhances the activity of TiO.sub.2 with optimal loading determined to be ca. 2.0 wt. %, and the average rate of hydrogen evolution was about 5 times higher than that of TiO.sub.2 alone. The increasing activity was not due to increasing absorption of the visible light but most likely due to the role of CoO nanoparticles as hole scavengers at the interface with TiO.sub.2. The addition of Pd (as hydrogen ion reduction sites) further improved the reaction rate about 4 times compared to that of the composite system, to 180 molg.sup.1min.sup.1. A fraction of Pd appeared to be in the form of PdCo alloy dispersed on the CoO/TiO.sub.2 semiconductor support. No catalytic deactivation was seen for prolonged reaction time (up to about 24 hours).