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The photocatalytic reduction of gold on a colloidal Ti02 sample in the presence of i-PrOH has been studied by in situ EXAFS. This colloidal sample shows a quantum size effect in its band gap UV-vis absorption and a photocatalytic activity for the formation of Ti3+ species. An experimental setup has been used that consists of a photochemical reactor connected to a peristaltic pump and an EXAFS transmission cell specially designed for liquid samples. The results show that it is possible to follow the two-step mechanism of the photoreduction of noble metals on titania, i.e. the nucleation and the growth of the metallic crystallites. At the same time, a surprisingly low intensity of the threshold feature at the Au LIII edge has been found for low irradiation times, when small gold particles are formed. This effect has been interpreted as a result of a negative charge injection into the metal particles which is favored by the presence of the alcohol acting as a hole trap. For comparison, an in situ XPS analysis has been carried out to follow the photoreduction of gold in a Au3+-Ti02 material in the presence of i-PrOH at the gas-solid interface.

The electronic structure of transition metal clusters is of great interest for a wide range of investigations including the properties of catalytic systems. In order to determine the electronic structure of transition metal dimers, calculations of total energies and geometry optimizations have been carried out on Pd2, Rh2, Ru2, Au2, and Pt2 dimers as well as PdPt, RhPt, RuPt, and PdAu dimers using a method based on density functional theory (DFT) with nonlocal (NLDFT) correction. The typical applications of DFT for the determination of the general features of electronic structure of Pd4, Rh4, Au4, and Pt4 clusters have also been performed at the level of NLDFT using the norm-conserving pseudopotential method (NCPP). The results of these calculations are in good qualitative agreement with those of the previous DFT calculations. The Mulliken charge analysis has indicated that a charge transfer occurs between the atoms of Pd and Pt, as well as between Rh and Pt atoms, Ru and Pt atoms, and Pd and Au atoms in the corresponding bimetallic dimers. Finally, this DFT method is expected to be very helpful to predict the electronic structure of small metal cluster systems.

The effect of polydispersity and inhomogeneity of supported bimetallic catalysts on the EXAFS analysis is investigated with some simple model calculations. These show that EXAFS is very insensitive to polydispersity. Polydispersity and inhomogeneous distribution of the metals over the particles however have only limited influence on the ability to distinguish between core-shell particles and particles with random distribution of both metals.