Gold, silver and copper catalysts supported on TiO2 for pure hydrogen production

Gold catalysis

Catalysis by gold has rapidly become a hot topic in chemistry, with a new discovery being made almost every week. Gold is equally effective as a heterogeneous or a homogeneous catalyst and in this Review we attempt to marry these two facets to demonstrate this new found and general efficacy of gold. The latest discoveries are placed within a historical context, but the main thrust is to highlight the new catalytic possibilities that gold-catalyzed reactions currently offer the synthetic chemist, in particular in redox reactions and nucleophilic additions to pi systems. Indeed gold has proved to be an effective catalyst for many reactions for which a catalyst had not been previously identified, and many new discoveries are still expected.

Degradation of bisphenol A in aqueous solution by H2O2-assisted photoelectrocatalytic oxidation

A series of titanium dioxide (TiO(2)/Ti) film electrodes were prepared from titanium (Ti) metal mesh by an improved anodic oxidation process and were further modified by photochemically depositing gold (Au) on the TiO(2) film surface as Au-TiO(2)/Ti film electrodes. The morphological characteristics, crystal structure and photoelectroreactivity of both the TiO(2)/Ti and Au-TiO(2)/Ti electrodes were studied. The experiments confirmed that the gold modification of TiO(2) film could enhance the efficiency of e(-)/h(+) separation on the TiO(2) conduction band and resulted in the higher photocatalytic (PC) and photoelectrocatalytic (PEC) activity under UV or visible illumination. To further enhance the TiO(2) PEC reaction, a reticulated vitreous carbon (RVC) electrode was applied in the same reaction system as the cathode to electrically generate H(2)O(2) in the aqueous solution. The experiments demonstrated that such a H(2)O(2)-assisted TiO(2) PEC reaction system could achieve a much better performance of BPA degradation in aqueous solution due to an interactive effect among TiO(2), Au, and H(2)O(2). It may have good potential for application in water and wastewater treatment in the future.

Theoretical Insights on O2 and CO Adsorption on Neutral and Positively Charged Gold Clusters

With the aim of understanding the elementary steps governing the oxidation of CO catalyzed by dispersed or supported gold nanoclusters, the adsorption of molecular species, such as O2 and CO, on model neutral and positively charged clusters (Au(n)(m+) n = 1, 9, and 13; m = 0, 1, and 3) has been studied using an ab initio approach. The computed structural and thermodynamic data related to the binding process show that molecular oxygen interacts better with neutral clusters, acting as an electron acceptor, while CO more strongly binds to positively charged species, thus acting as an electron donor.

Gold catalysts for pure hydrogen production in the water–gas shift reaction: activity, structure and reaction mechanism

The production of hydrogen containing very low levels of carbon monoxide for use in polymer electrolyte fuel cells requires the development of catalysts that show very high activity at low temperatures where the equilibrium for the removal of carbon monoxide using the water-gas shift reaction is favourable. It has been claimed that oxide-supported gold catalysts have the required high activity but there is considerable uncertainty in the literature about the feasibility of using these catalysts under real conditions. By comparing the activity of gold catalysts with that of platinum catalysts it is shown that well-prepared gold catalysts are significantly more active than the corresponding platinum catalysts. However, the method of preparation and pre-treatment of the gold catalysts is critical and activity variations of several orders of magnitude can be observed depending on the methods chosen. It is shown that an intimate contact between gold and the oxide support is important and any preparative procedure that does not generate such an interaction, or any subsequent treatment that can destroy such an interaction, may result in catalysts with low activity. The oxidation state and structure of active gold catalysts for the water-gas shift reaction is shown to comprise gold primarily in a zerovalent metallic state but in intimate contact with the support. This close contact between small metallic gold particles and the support may result in the "atoms" at the point of contact having a net charge (most probably cationic) but the high activity is associated with the presence of metallic gold. Both in situ XPS and XANES appear unequivocal on this point and this conclusion is consistent with similar measurements on gold catalysts even when used for CO oxidation. In situ EXAFS measurements under water gas shift conditions show that the active form of gold is a small gold cluster in intimate contact with the oxide support. The importance of the gold/oxide interface is indicated but the possible role of special sites (e.g., edge sites) on the gold clusters cannot be excluded. These may be important for CO oxidation but the fact that water has to be activated in the water gas shift reaction may point towards a more dominant role for the interfacial sites. The mechanism of the water gas shift reaction on gold and other low temperature catalysts has been widely investigated but little agreement exists. However, it is shown that a single "universal" model is consistent with much of the experimental literature. In this, it is proposed that the dominant surface intermediate is a function of reaction conditions. For example, as the temperature is increased the dominant species changes from a carbonate or carboxylate species, to a formate species and eventually at high temperatures to a mechanism that is characteristic of a redox process. Similar changes in the dominant intermediate are observed with changes in the gas composition. Overall, it is shown that reported variations in the kinetics, structure and reaction mechanism for the water gas shift reaction on gold catalysts can now be understood and rationalised.

Enhanced photocatalysis of pentachlorophenol by metal-modified titanium (IV) oxide

The effects of four metals (Ag, Au, Pt, and Cu) doped on TiO2 on the photocatalysis of pentachlorophenol (PCP) were investigated. The results of this study indicated that all four metals-doped TiO2 catalysts were able to enhance the efficiency of PCP photocatalysis with an optimum metallic content of 0.1 wt%. For the metal-doped TiO2 samples (Au, Pt, and Cu), the patterns of light absorption were significantly extended toward visible light spectra in the wavelengths between 400 and 800 nm. The photocatalysis of PCP was pH dependent with the maximum degradation rate achieved in the solution at pH 3. The formation of chloride ion corresponded with the concentration of PCP degraded which confirmed that dechlorination was the major pathway of PCP photocatalysis. The overall toxicities of PCP samples were reduced with the extension of light exposure using the microtox test. The results of PCP photocatalysis are also discussed based on the characteristics of metal/TiO2 including X-ray differential (XRD) patterns, Brunquer Emmett Teller (BET) specific area analysis, and Ultra Violet (UV)-Vis absorption spectra.

DFT and In Situ EXAFS Investigation of Gold/Ceria−Zirconia Low-Temperature Water Gas Shift Catalysts: Identification of the Nature of the Active Form of Gold

A combined experimental and theoretical investigation of the nature of the active form of gold in oxide-supported gold catalysts for the water gas shift reaction has been performed. In situ extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) experiments have shown that in the fresh catalysts the gold is in the form of highly dispersed gold ions. However, under water gas shift reaction conditions, even at temperatures as low as 100 degrees C, the evidence from EXAFS and XANES is only consistent with rapid, and essentially complete, reduction of the gold to form metallic clusters containing about 50 atoms. The presence of Au-Ce distances in the EXAFS spectra, and the fact that about 15% of the gold atoms can be reoxidized after exposure to air at 150 degrees C, is indicative of a close interaction between a fraction (ca. 15%) of the gold atoms and the oxide support. Density functional theory (DFT) calculations are entirely consistent with this model and suggest that an important aspect of the active and stable form of gold under water gas shift reaction conditions is the location of a partially oxidized gold (Audelta+) species at a cerium cation vacancy in the surface of the oxide support. It is found that even with a low loading gold catalysts (0.2%) the fraction of ionic gold under water gas shift conditions is below the limit of detection by XANES (<5%). It is concluded that under water gas shift reaction conditions the active form of gold comprises small metallic gold clusters in intimate contact with the oxide support.

The enhanced PC and PEC oxidation of formic acid in aqueous solution using a Cu-TiO2/ITO film

The photocatalytic (PC) and photoelectrocatalytic (PEC) activity of Cu-TiO2/ITO films for degrading formic acid in aqueous solution was investigated in this study. Compared with a TiO2/ITO film, the degradation efficiency of formic acid on the Cu-TiO2 films increased markedly in both the PC and PEC oxidation processes. However, it was found that the photodeposited Cu metal on the Cu-TiO2 films could electrochemically dissolute during the PEC reaction, while an electrical bias with the voltage higher than 1.48 V was applied. It is believed this is a common problem occurred for several metals deposition on the TiO2 films, which results in a poor stability of the metal-deposited TiO2 electrode in PEC processes. To improve the stability of the Cu-TiO2 electrode, an alternative process between PC and PEC reactions was investigated. It was found that the dissolute Cu metal during the PEC process was re-deposited on the Cu-TiO2 film again during the PC process. The experiments with repeated runs demonstrated that this alternative process could not only overcome the loss of Cu, but also enhance the PEC oxidation efficiency of the Cu-TiO2 films.

13C NMR characterization of the organic constituents in ligand-modified hexagonal mesoporous silicas: media for the synthesis of small, uniform-size gold nanoparticles

This paper reports the (13)C NMR characterization of functionalized MCM-41's and describes the chemistry that occurs in the pores of these materials in the process of forming gold nanoparticles. Nanoparticles formed on hexagonal mesoporous silica (MCM-41) by hydrogen reduction of chloroauric acid have little affinity for pure silica surfaces. The gold can be removed from the support with very mild treatment, for example, solvent extraction. The loss of gold from the substrate can be prevented using a pore functionalization methodology that entails synthesis of the silica containing polydentate amine functionality chemically bound in the mesopores. The synthetic scheme introduces solvents and templating reagents (surfactants) into the mesopores that are chemically reactive under the conditions required for gold particle formation. Extensive base-catalyzed elimination and nucleophilic substitution reactions involving the tetraalkylammonium surfactant occur during the reduction of chloroauric acid to gold.