Electronic and Geometric Structures of Supported Platinum, Gold, and Platinum−Gold Catalysts

Abstract

X-ray absorption spectroscopy (XAS) on the Pt and Au L3 edges showed that the structure of platinum and gold nanoclusters differs from that of the bulk metals. The interatomic distances are contracted and the s, p, and d orbitals rehybridize, which results in less filled 5d valence bands. The XAS spectra of the monometallic clusters were used to interpret the effect of alloying platinum and gold. Preparation of SiO2- and TiO2-supported PtAu catalysts from a Pt2Au4(C⋮CBut)8 precursor resulted in well-mixed bimetallic clusters. Gold was preferentially located on the surface of the small clusters. X-ray absorption near-edge spectroscopy (XANES) showed that the electronic structure of platinum and gold differed from that in the monometallic clusters. The Fermi levels of platinum and gold shifted in opposite directions: the position of the Pt L3 edge shifted to higher energy and the Au L3 edge shifted to lower energy. Furthermore, the white-line intensity increased for gold and decreased for platinum. These changes originate from interatomic charge transfer from the Au 5d to the Pt 5d band, accompanied by intra-atomic charge redistributions. Charge transfer affected the energy positions of the 5d valence bands. Analysis of the XANES spectra, aided by full multiple-scattering calculations, revealed that the changes in the electronic structures are due to alloying and distance effects.