Gold clusters ranging in diameter from 1 to 6 nanometers have been prepared on single crystalline surfaces of titania in ultrahigh vacuum to investigate the unusual size dependence of the low-temperature catalytic oxidation of carbon monoxide. Scanning tunneling microscopy/spectroscopy (STM/STS) and elevated pressure reaction kinetics measurements show that the structure sensitivity of this reaction on gold clusters supported on titania is related to a quantum size effect with respect to the thickness of the gold islands; islands with two layers of gold are most effective for catalyzing the oxidation of carbon monoxide. These results suggest that supported clusters, in general, may have unusual catalytic properties as one dimension of the cluster becomes smaller than three atomic spacings.

Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties

Size- and support-dependency in the catalysis of gold

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices:  (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utiliza...

Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion

Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO2 nanoparticles under steady-state and laser pulse excitation. Equilibration with the C60/C60- redox couple provides a means to determine the apparent Fermi level of the TiO2−Au composite system. The size-dependent shift in the apparent Fermi level of the TiO2−Au composite (20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge-transfer steps from UV-excited TiO2 → Au → C60 has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and siz...

Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration

The influence of the preparation methods on the catalytic activity for CO oxidation was markedly large for Au-TiO2 and negligible for Pt-TiO2 catalysts. Platinum and gold were deposited on TiO2 by deposition-precipitation (DP), photodeposition (FD) and impregnation (IMP). The DP method gave the most active catalysts for both Pt and Au. Gold catalysts prepared by DP were active at temperatures below 273 K and showed a much greater activity than Pt catalysts.

The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation

Photocatalytic H 2 production from aqueous solutions of ethylene glycol was studied using a catalyst consisting of gold supported on TiO 2 . The catalysts were prepared by the deposition-precipitation method, using TiO 2 powder and an aqueous solution of chloroauric acid. Transmission electron microscopy showed that Au was deposited as highly dispersed particles with mean diameters of 2 to 5 nm, depending on the method of pretreatment of the precursor. The effects of precursor calcination temperature, duration of irradiation, gold loading, concentration of ethylene glycol, initial pH and temperature on H 2 production were investigated. The gas phase products of the reaction were mainly H 2 and CO 2 . The precursor calcined in air at 673 K proved to be the most active photocatalyst. The yield of H 2 exhibited a weak maximum at a gold loading of about 2 wt.%. The rate of H 2 production was strongly dependent on the initial pH of the suspension; it increased with pH up to neutral pH and decreased thereafter. Increasing the concentration of ethylene glycol and the temperature had a positive effect on the H 2 yield. The activation energy of H 2 production, as given by an Arrhenius plot over the temperature range 284–340 K, was 15 kJ mol −1 .

G.R. Bamwenda

Papers

The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation

Photoinduced hydrogen production from an aqueous solution of ethylene glycol over ultrafine gold supported on TiO2