Chemical reactivity of supported gold : III. Atomic binding and coordination of gold from X-ray absorption fine structure spectroscopy

Abstract

Preparations of Au and Pt supported on Al2O3, MgO, and SiO2 were subjected to analysis by extended X-ray absorption fine structure spectroscopy (EXAFS). The samples contained 0.20, 5.00, and 8.00 wt% Au, and 5.00 wt% Pt. Metal dispersion ranged between 0.3 to 45%. Anisotropic growth was detected in the larger particles. The liii X-ray absorption coefficient was normalized to the same atom basis and Fourier transformed to yield the radial structure function for each preparation. From bond distance comparison with reference materials, assignments were made to specific peaks of the radial structure function. The analysis of the results showed that the metals were present in two distinct phases: metallic with coordination and metal-to-metal distance characteristic of bulk Au, and Pt (particulate phase), and a highly disperse one (dissolved phase). It was concluded that the latter consisted of metal atomically dispersed, or condensed in small, flat clusters of a few atoms. It was further estimated that the majority of Au and Pt was present in the highly disperse form, i.e., 60% for Au and 86% for Pt. Au atoms did not simply substitute Mg in the MgO matrix to form the dissolved Au-MgO phase and they carried an average charge +1.