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Non-oxidative coupling of methane (NOCM, 2CH4 → C2H6 + H2) is a reaction that can directly produce ethane and hydrogen at the same time, and gallium oxide (Ga2O3) powder has been reported as an effective photocatalyst for NOCM at room temperature. In this study, we investigated the reaction conditions for Pd-loaded Ga2O3 photocatalysts to improve the production rate of C2H6 and H2. We found that the 0.1 wt% Pd/Ga2O3 exhibited high selectivity of C2H6 (75.8%, carbon-based) under the conditions of steam reforming of methane. Photocatalytic NOCM seems to proceed in the presence of small amount of water. An increase in water vapor pressure (PH2O) was essential for the steady production of C2H6 and H2. The C2H6 production rate was 0.79 μmol min−1 for 50 mg of Pd/Ga2O3 powder at PH2O = 3.6 kPa. The apparent quantum efficiency (AQE) for C2H6 production was 5.1%, which is much higher than that of conventional photocatalytic NOCM in the absence of water vapor. The importance of water adsorbates on the photocatalyst surface was suggested by water vapor adsorption isotherm and Fourier transform infrared (FT-IR) spectroscopy. It is revealed that multilayered water molecules adsorbed on the photocatalyst surface play a role as a reaction field that promotes the dehydrogenative coupling of CH4.

TiO2 nanotube (TNT) photoanodes exhibit activity and stability for the oxygen evolution reaction (OER) by photoelectrochemical water oxidation. However, the location of the OER site by the photogenerated holes has not been clarified for the TNT photoanodes, unlike well-studied macrocrystalline photocatalysts. In this study, we performed reactions of TNT photoanodes in a 0.1 M Pb(NO3)2 under UV irradiation. The photoelectrochemically deposited PbO2 particles were observed through scanning electron microscopy in the backscattered electron mode. We found that β-PbO2 was deposited on the nanotubes with photocurrent decay and that the reaction site was located on the upper part (∼1 μm) of the TNT array with ∼3 μm length. The photocurrent decay implies the selective deposition of PbO2 on the catalytic site for water oxidation. The PbO2 nanoparticles were deposited on the inner and outer surfaces of the tube walls. This result is consistent with the mechanism of charge separation at the space charge layers formed on both surfaces of the walls. We also confirmed that the OER site changes depending on the wavelength of the incident light due to the change in the light penetration depth.

Bismuth vanadate (BiVO4) is a promising material for visible-light-induced photoelectrochemical (PEC) water splitting. The strategies of cation doping and formation of heterojunctions with tungsten...

The layer of electrocatalytic mixed oxides is known as a dimensionally stable anode (DSA) used in many industrial electrochemical processes. A layer of iridium oxide (IrO2) and tantalum oxide (Ta2O...

Large scale production of H2 using renewable energy can be realized by photoelectrochemical (PEC) splitting of water vapor in the gas phase. Air humidity can be a water source for producing solar H2. The concept is based on photoelectrochemistry in a gaseous environment using a proton exchange membrane (PEM) as a solid polymer electrolyte and a macroporous semiconductor electrode wrapped in a perfluorosulfonic acid ionomer. A prototype vapor-fed PEC cell using a TiO2 gas-diffusion electrode as an O2-evolving photoanode and a carbon-supported platinum catalyst cathode achieved water splitting to produce O2 and H2, which are physically separated from each other by the PEM. Herein, we demonstrate a PEM-PEC system consisting of a strontium titanate (SrTiO3) nanocrystalline layer decorated on titanium microfiber felt to achieve overall water splitting into H2 and O2 at a ratio of 2 : 1 on each electrode at an applied voltage of only 0.3 V under gaseous conditions. In comparison with TiO2 electrodes, the SrTiO3 gas-diffusion electrode decreased the external voltage and suppressed the gradual decomposition of the ionomer coated on the photoanode. These differences are because of the cathodic shift of the photocurrent onset potential and improvement of the Faraday efficiency of O2 evolution, respectively. The interface between the SrTiO3 surface and the ionomer thin film exhibited fast kinetics for O2 evolution from water adsorbates supplied from the gas phase and durability under ultraviolet light irradiation.

Pt/Ga2O3 exhibited high activity for dehydrogenative coupling of methane into ethane (2CH4 → C2H6 + H2) in a fixed-bed flow reactor at 25 °C under 254 nm UV irradiation. The C2H6 formation was negligible at CH4 pressure of 10 kPa, but it was linearly increased with an increase in the pressure to 300 kPa.

Anatase TiO2 is a widely investigated photocatalyst; however, it can only work under ultraviolet (UV) light with wavelengths less than 390 nm (band gap 3.2 eV). Rutile TiO2 can absorb visible light at wavelengths less than 410 nm (band gap 3.0 eV); however, its photocatalytic activity is not high. Herein, we activated rutile TiO2, which was prepared from Evonik TiO2 P 25 through calcination at 800 °C using hydrogen reduction treatment at 700 °C. The photocatalytic activity of the hydrogen-treated TiO2 was as high as P 25 under UV irradiation at 380 nm, which was significantly higher than P 25 under violet light irradiation at 405 nm for the oxidative decomposition of acetic acid in water. Electron spin resonance studies indicate that charge separation is enhanced in reduced TiO2, and their oxygen reduction pathways differ between anatase and rutile. The formation of H2O2 was observed on rutile TiO2; however, it was consumed during photocatalysis to accelerate acetic acid decomposition.

The importance of electron concentration in rutile TiO2 photocatalysts motivated us to apply H2 reduction treatment to visible-light-responsive TiO2 photocatalysts sensitized by chromium ion doping. We found that H2 reduction treatment of rutile TiO2 particles codoped with Ta and Cr (TiO2:Ta/Cr) enhanced photocatalytic activity for O2 evolution by water oxidation under visible irradiation (>2.2 eV). The enhanced visible light activity of H2-treated TiO2:Ta/Cr was attributed to the increase of electron concentration, which was confirmed by UV–vis diffuse reflectance and electron spin resonance (ESR) spectroscopy. The H2-treated TiO2:Ta/Cr photocatalyst was repeatedly used in aqueous media in spite of the presence of doped electrons. Photoluminescence and transient absorption spectroscopies revealed that electron doping with H2 treatment decreased the midgap states working as deep traps of photoexcited electrons and increased the accumulation of the photoexcited electrons in the conduction band. The optimiz...

Direct conversion of methane (CH4) into valuable chemicals with low-energy input is an important goal in the sustainable chemical industry. Herein, we report a photoelectrochemical activation of CH4 in the gas phase under visible light irradiation at room temperature. The proof-of-concept study revealed that homocoupling of CH4 to form ethane (C2H6) with high selectivity of 54% was induced by photogenerated holes over a tungsten trioxide (WO3) gas-diffusion photoanode coated with a proton-conducting ionomer in the presence of water vapor. The gas–electrolyte–solid triple-phase boundary enables the oxidation of the inert carbon–hydrogen bond of CH4, and the formation of carbon oxides and ethane with a carbon–carbon bond. The gas-phase photoelectrochemical system shows incident photon-to-current conversion efficiency of 11% under blue light at an applied voltage of 1.2 V. This work is also the first demonstration of a visible-light-driven hydrogen evolution from CH4. The hydrogen is separated from CH4 and o...

Photoelectrochemical (PEC) decomposition of water at room temperature and separation of hydrogen and oxygen has been achieved using a solid-state cell with a proton exchange membrane (PEM). The PEM–PEC cell can use water vapor instead of liquid aqueous electrolytes for solar hydrogen production. In this study, we aimed to evaluate the performance of thermally oxidized rutile TiO2 layers on titanium microfiber felt for the photoanodic oxidation of water vapor in the gas phase. We investigated the effects of perfluorosulfonic acid ionomer coating, the wavelength and intensity of incident UV light, and external electric bias on the photocurrent density of TiO2 photoanodes in a humidified gaseous condition. The gas-phase PEC performance of the rutile TiO2 thin layer electrode was much higher than that of the anatase TiO2 nanotube array electrode under 385 nm UV illumination at high irradiance (40 mW cm−2). The H2 evolution rate in the PEM–PEC cell using a rutile TiO2 layer as an oxygen-evolving photoanode was 330 μmol h−1 under 385 nm UV (irradiation area 16 cm2) at an applied voltage of 1.2 V.

A photocatalytic reaction mechanism that involves multielectron reduction of molecular oxygen (O 2 ), as a key step, in photocatalytic decomposition of acetic acid by pristine bismuth tungstate (Bi 2 WO 6 ; BWO) particles is proposed on the basis of results of studies on (i) comparison of photocatalytic activity trends of BWO, tungsten(VI) oxide and titanium(IV) oxide photocatalyst powders with or without platinum deposits in three photocatalytic reaction systems (oxidative decomposition of acetic acid in aerobic aqueous suspensions, methanol dehydrogenation in deaerated aqueous suspensions and O 2 liberation from deaerated silver salt solutions) and (ii) effects of morphology and structural properties of BWO particles on photocatalytic activities.

The mechanism of the formation of flake ball-shaped (FB) bismuth-tungstate (BWO) particles was studied with samples prepared by hydrothermal reactions under standard conditions (tungsten/bismuth ratio of 0.55 in feed; 433 K; 20 h) and other modified conditions, and the structural characteristics of the samples were analyzed by powder X-ray diffraction (composition and crystal structure), field emission-type scanning electron microscopy (FE-SEM) (morphology), focused ion-beam etching followed by FE-SEM (inner void structure), ultrahigh-voltage transmission electron microscopy (inner void structure) and low-voltage scanning electron microscopy with energy-dispersive X-ray spectroscopic analysis (morphology and elemental distribution). Based on the results, a novel two-step mechanism of (1) formation of aqueous bismuth-oxyhydroxide droplets covered by tungstic acid (BOx@HWO) with fluidity and (2) reaction of inner bismuth and outer tungsten components at the BOx@HWO surface to yield BWO flakes is proposed as a one of the most plausible pathways that are consistent with the hollow structure of FB BWO and observations of the products prepared with shorter hydrothermal reaction. Photocatalytic activities of FB BWO samples in relation to their structure were studied.

Crystalline defects of photocatalyst particles may be considered to be the recombination center of photoexcited electrons and holes. In this study, we investigated the photocatalytic activity of cation-doped rutile TiO2 photocatalysts for O2 evolution from an aqueous silver nitrate solution under ultraviolet light irradiation. The photocatalytic activity of rutile TiO2 was enhanced by donor doping of Ta5+ and Nb5+ with a valence higher than that of Ti4+, regardless of increased density of electrons and Ti3+ species (an electron trapped in Ti4+ sites). Conversely, acceptor doping of lower valence cations such as In3+ and Ga3+ decreased photocatalytic activity for O2 evolution by water oxidation. The doping of equal valence cations such as Sn4+ and Ge4+ hardly changed the activity of non-doped TiO2. This study demonstrates that Ti3+ species, which is a crystalline defect, enhanced the photocatalytic activity of semiconductor oxides, for example rutile TiO2 with large crystalline size.

Bismuth vanadate (BiVO4) and tungsten trioxide (WO3) are promising materials for photoelectrochemical (PEC) water splitting. To date, the heterojunction composed of both materials garners great attention due to the superior performance and the potential to capture visible light. However, the compatibility in different experimental conditions is limited by its low stability. Some factors which can impact the stability are pH of aqueous media, electrolytes, conductive substrates, and light irradiation. The mechanism of the corrosion process, which decays PEC performance as a function of time, has been studied. In this article, we prepared WO3/Mo-doped BiVO4 on a three-dimensional macroporous titanium (Ti) felt, which is a potential substrate for different types of PEC, and compare its stability with the heterostructure prepared on a conventional fluorine-doped tin oxide (FTO)-coated glass. The PEC performance and the stability of single layer WO3, Mo-doped BiVO4, and the heterojunction are investigated at low and moderate pH. The characterization and PEC measurements determined that pH has a significant effect on chemical dissolution and photocorrosion, directly impacting the stability of photocurrent generated from WO3/Mo-doped BiVO4 heterojunction in both substrate structures. Besides, WO3/Mo-doped BiVO4 on Ti felt shows higher and more stable PEC performance at low and moderate pH values.

Ti microfibers with a diameter of 20 μm were used as a three dimensional conductive back contact for a tungsten oxide (WO3) photoelectrode. We coated the microfibers with WO3 nanoparticles by a solution-based dip coating step and heat treatment at 650 °C. A high loading of WO3 nanoparticles was achieved over a large surface area of the porous microfibers through the use of a high viscosity solution for the dip coating. The prepared WO3-coated Ti microfibers exhibited enhanced photoelectrochemical properties. Visible light response was improved in the conductive Ti microfibers owing to increased absorption of visible light and efficient extraction of photoexcited electrons at the large interface.

The photoelectrochemical properties of titanium-doped iron(III) oxide (α-Fe 2 O 3 , hematite) nanoparticulate thin films are investigated using two photoelectrode reactions, the oxidation of water and iodide ions (I − ) to elucidate the role of doped Ti 4+ species in the enhanced photocurrent. Increase in the calcination temperature of titanium-doped Fe 2 O 3 thin films from 450 °C to 550 °C provides significant enhancement in their efficiency for water oxidation to evolve O 2 , which is four-electron transfer slow reaction. In contrast, such a calcination effect is not remarkable for oxidation of I − to I 3 − , which is two-electron transfer fast reaction. The electrochemical impedance spectroscopy (EIS) study indicates that the surface reaction rates feature heterogeneity only for water oxidation. This heterogeneity in EIS responses decreased with increasing calcination temperature. This shows that calcination creates photoactive surface sites with doped Ti 4+ species especially for kinetically limited water oxidation. Long-lived photoholes might be effectively captured in the vicinity of these sites to suppress surface recombination and induce multi-electron transfer water oxidation.

Although TiO2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H2 reduction treatment on the water oxidation activity of rutile TiO2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO2 owing to strong oxidation, but its initial activity was restored by H2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the high-temperature calcination created O•– radicals (trapped hole in oxygen lattice site), while the H2 reduction treatment created Ti3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H2 treatment temperature. Measurements of the sheet...

Degussa (Evonik) TiO2 P25 consisting of a mixture of anatase and rutile crystallites is a well-known commercial material with high photocatalytic efficiency. The mixture of anatase and rutile phases in P25 is reportedly more active than the individual polymorphs of TiO2. Contrary to this viewpoint, we demonstrate that H2-reduced rutile TiO2 is much more active than mixed-phase P25 for photocatalytic H2 evolution under ultraviolet irradiation. An important factor to improve the photocatalytic activity of rutile TiO2 is H2 reduction treatment. H2-reduced rutile TiO2 outperforms anatase-rich TiO2 because of the wider absorption range caused by its smaller band gap (3.0 eV for rutile and 3.2 eV for anatase). The apparent quantum efficiency for H2 evolution of H2-reduced rutile TiO2 was estimated to be 46% under 390 nm irradiation, which was 3.3 times higher than that of mixed-phase P25. This highly efficient rutile photocatalyst is easily fabricated from P25 by H2 reduction treatment at 700 °C for 2 h.

Charge-carrier dynamics during photoelectrochemical (PEC) water splitting over WO3 particulate photoanode deposited on titanium sheet (Ti/WO3) was studied using intensity-modulated photocurrent spectroscopy (IMPS). The WO3 particulate film calcined at 650 °C showed high PEC performance for water oxidation to evolve O2. In linear sweep voltammetry, the photocurrent density proportionally increased with incident light intensity (I0). This means that the incident photon-to-current conversion efficiency (IPCE) of Ti/WO3 was not dependent on I0 in voltammetry. In contrast, there was no proportional relation between I0 and the photocurrent density in chronoamperometry at a constant potential, indicating that the IPCE at steady state decreased with an increase of I0. To elucidate the effect of light intensity on the steady-state IPCE, we investigated the IMPS response of Ti/WO3 photoanodes by applying a simple branching model with respect to the photogenerated holes (h+). It was found that the recombination rate constant (krec) rather than the charge transfer rate constant (kct) significantly depends on I0 at the steady-state condition. The pseudo-first-order krec increased with an increase of I0, suggesting that recombination is increased by the accumulation of photogenerated carriers during continuous PEC water oxidation. This is the reason for the decrease in the steady-state IPCE of Ti/WO3 under high-intensity light irradiation. The effects of applied potentials on the kinetic parameters (kct and krec) were also evaluated for Ti/WO3 under different I0.

Cuprous oxide (Cu2O) is a candidate for low-cost p-type semiconductor oxides for photoelectrochemical solar energy conversion. The Cu2O crystalline films electrodeposited on conductive substrates have been extensively studied as photocathodes because of their moderate photoelectrochemical performance, but their incident photon-to-current conversion efficiency (IPCE) was still limited because of the incompatibility between minority carrier diffusion length and light absorption depth in planar structures. One of the strategies to solve this issue is to decrease the thickness of the Cu2O films with keeping the loading amount. Herein, we used a sintered titanium microfiber felt as a three-dimensional macroporous scaffold for Cu2O crystalline thin films because it exhibits a large surface area compared with the conventional flat substrate. The effect of the Cu2O film thickness on the photocathode properties was investigated using methyl viologen as an acceptor of the photoexcited electrons. The Cu2O thin films deposited on Ti microfibers were superior to that of the Cu2O films deposited on dense planar substrates such as fluorine-doped tin oxide (FTO)-coated glass and titanium sheets. It was revealed that the large Cu2O–electrolyte and Cu2O–substrate interfaces resulted in the enhancement of IPCE for the photoelectrochemical cathodic reaction.

Photoelectrochemical (PEC) water vapor splitting by using n-type semiconductor electrodes with a proton exchange membrane (PEM) enabled pure hydrogen production from humidity in ambient air. We proved a design concept that the gas-electrolyte-semiconductor triple-phase boundary on a nanostructured photoanode is important for the photoinduced gas-phase reaction. A surface coating of solid-polymer electrolyte on a macroporous titania-nanotube array (TNTA) electrode markedly enhanced the incident photon-to-current conversion efficiency (IPCE) at the gas-solid interface. This indicates that proton-coupled electron transfer is the rate-determining step on the bare TNTA electrode for the gas-phase PEC reaction. The perfluorosulfonate ionomer-coated TNTA photoanode exhibited an IPCE of 26 % at an applied voltage of 1.2 V under 365 nm ultraviolet irradiation. The hydrogen production rate in a large PEM-PEC cell (16 cm2 ) was 10 μmol min-1 .

In this study, unique WO3-based bifunctional electrodes were prepared with the aim of achieving optimal photocatalytic and electrocatalytic activities of WO3 nanoplatelets and Pt nanoparticles, respectively. WO3 nanoplatelets with a high visible light response were directly grown on both sides of a tungsten substrate via a facile hydrothermal method. Pt nanoparticles were deposited on one side of the WO3 electrode, where they served as the electrocatalyst. The prepared electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDS) and cyclic voltammetry. Our experimental results revealed that the synthesized WO3 electrodes possessed high photochemical activity under visible light exposure, whereas the deposition of Pt nanoparticles with a large electrochemical active surface area significantly enhanced the electrocatalytic activity of the electrode. Various sets of photodegradation reactions involving Rhodamine B (RhB) were conducted to evaluate the efficacy of the WO3 electrodes, prior to and following the deposition of Pt nanoparticles under both visible light and applied potential. The bifunctional electrode exhibited far greater activity as compared to the WO3 electrode as the sole photocatalyst and WO3–Pt as the sole electrocatalyst, very promising for energy and environmental applications.

The mechanism of formation of hierarchical “flake-ball”-structured bismuth tungstate (BTO) particles was studied on the basis of structural analyses and was revealed to be the growth of plate-like ...

Rutile titania (TiO 2 ) is an efficient photocatalyst for oxidizing water to O 2 . The photocatalytic activity of particulate rutile for water oxidation was significantly improved by H 2 reduction at 700 °C, after calcination at 1100 °C. The improved activity was due to an increase in crystalline size during calcination, and an increase in conduction band electron concentration by the creation of oxygen vacancies. In contrast to the consideration that oxygen vacancy increases the recombination of electron and holes, the hydrogenated TiO 2 exhibited high apparent quantum efficiency for O 2 evolution, 41% under irradiation at 365 nm. It was found that H 2 treatment improved the photocatalytic activity per unit of surface area not only for O 2 evolution but also for H 2 evolution and acetic acid decomposition. The effect of H 2 reduction treatment was obtained only if the rutile particle was previously calcined at temperatures higher than 1000 °C. This suggests that space charge layer in large crystalline particles is involved in the activation mechanism of hydrogenated rutile TiO 2 particles.

The photoelectrochemical properties of electrodeposited p-type copper(I) oxide (Cu 2 O) films were investigated using methyl viologen (MV 2 + ) as an electron acceptor. The pristine Cu 2 O films were deactivated during the photocathodic reaction as a result of self-reduction, whereas the (111)-oriented Cu 2 O films treated in an aqueous solution containing hexamethylenetetramine at pH 5 and 90 °C exhibited stable photocurrent for MV 2 + reduction into the cation radical. Scanning electron microscope images showed that the treated films contained smaller crystal grains than untreated ones. X-ray photoelectron spectroscopy revealed that the treatment etched the thin layer of copper(II) oxide from the Cu 2 O polycrystalline surface. Etching of the film surface enhanced the stability and steady-state photocurrent for photocathodic reduction of MV 2 + , suggesting that the crystalline composition and structures exposed on the outermost surface of Cu 2 O polycrystalline films have a considerable influence on the selectivity for the photocathodic reaction over self-reduction.

The particle size of a photocatalyst is one of the factors that determine the photon utilization efficiency. Here, we report the effects of the particle size on the photocatalytic activity of tungsten trioxide (WO3) for water oxidation to evolve molecular oxygen in the presence of sacrificial silver nitrate. For the water oxidation, the photocatalytic activity of large aggregates consisting of particles with a small specific surface area (3 m2 g–1) was greater than that of fine particles with a larger specific surface area (11 m2 g–1). Transient infrared absorption spectra revealed that the recombination of photoexcited electrons in the large particles was slower than that in the fine particles. It was found that the fast recombination in the fine particles mostly occurred on the surface. The photocatalytic activity for water oxidation increased with increasing particle size for spherical diameters smaller than 200 nm. These results showed that large particles with a low surface area-to-volume ratio were ...

This chapter has discussed the preparation of micrometer-sized spherical particles of bismuth tungstate (Bi2WO6) with the hierarchical architecture of “flake-ball” shape for photocatalytic application. The particles, which are assemblies of polycrystalline flakes composed of square-shaped laminar plates with a lateral size of a few hundred nanometers and thickness of 20–35 nm, are prepared by a facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. An excess amount of a tungstate precursor (10 %) and an acidic condition (pH 1.2) during the hydrothermal reaction are required to obtain a high yield of uniform particles with the flake-ball architecture. The control of the hierarchical assemblies of two-dimensional nanostructures provides high crystallinity, large surface area (19 m2 g−1), and large pore volume. The flake-ball particles are promising as a photocatalyst for oxidative decomposition of organic pollutants to carbon dioxide, for example, in water purification systems because of the high level of photocatalytic activity, the response to visible light, and feasible separation from suspensions by sedimentation and filtration.

Colloidal gold (Au) nanoparticles were prepared and successfully loaded on titanium(IV) oxide (TiO(2)) without change in the original particle size using a method of colloid photodeposition operated in the presence of a hole scavenger (CPH). The prepared Au nanoparticles supported on TiO(2) showed strong photoabsorption at around 550 nm due to surface plasmon resonance (SPR) of Au and exhibited a photocatalytic activity in mineralization of formic acid in aqueous suspensions under irradiation of visible light (ca. 520 nm). A linear correlation between photocatalytic activity and the amount of Au loaded, that is, the number of Au nanoparticles, was observed, indicating that the activity of Au/TiO(2) plasmonic photocatalysts can be controlled simply by the amount of Au loading using the CPH method and that the external surface area of Au nanoparticles is a decisive factor in mineralization of formic acid under visible light irradiation. Very high reaction rates were obtained in samples with 5 wt % Au or more, although the rate tended to be saturated. The CPH method can be widely applied for loading of Au nanoparticles on various TiO(2) supports without change in the original size independent of the TiO(2) phase. The rate of CO(2) formation also increased linearly with increase in the external surface area of Au. Interestingly, the TiO(2) supports showed different slopes of the plots. The slope is important for selection of TiO(2) as a material supporting colloidal Au nanoparticles.