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 spectrum of the reduced sample shows a short-distance peak at 2.15 Angstrom and a second peak at 2.75 Angstrom. XANES studies eliminate the possibility of the existence of oxidized species and/or Pt species coordinated 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-distance 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 individual EXAFS functions for a Pt cluster and PtSn alloy resulted in a good fit of the experimental spectrum, accurately describing the structure of the metallic phase.