INTERFERENCE PHENOMENA IN THE EXAFS SPECTRA OF PT-SN BIMETALLIC CATALYSTS

A detailed structural analysis of a bimetallic Pt-Sn/SiO2 catalyst has been obtained. The uncorrected Fourier transform (FT) of the EXAFS sp ectrum of the reduced sample shows a short-distance peak at 2.15 Angst rom and a second peak at 2.75 Angstrom. XANES studies eliminate the po ssibility of the existence of oxidized species and/or Pt species coord inated to Cl that could be responsible for the short-distance peak. A theoretical analysis of the structure of Pt clusters and Pt-Sn alloys has been conducted. When the EXAFS functions of pure Pt and PtSn alloy were combined, the resulting FT exhibited two main peaks at 2.18 and 2.78 Angstrom, in good correspondence with the experimental FT. Using theoretical references, it has been demonstrated that this short-dista nce peak arises from an interference phenomenon in the EXAFS spectra, resulting from the coexistence of pure Pt clusters and PtSn alloy. The presence of this peak is not only a fingerprint for Pt-Sn interaction but also evidence for the existence of unalloyed Pt. Adding the indiv idual EXAFS functions for a Pt cluster and PtSn alloy resulted in a go od fit of the experimental spectrum, accurately describing the structu re of the metallic phase.