Structure of catalytically active gold nanoparticles by XAFS spectroscopy

Abstract

A series of Au/Al2O3 catalysts prepared by the different procedures and series of model system Au/Si(100), Au/Si(111) differing in the mean Au particle sizes (from 2 nm up to 30 nm) were studied by EXAFS and XANES techniques. No visible changes in electron states of Au for all samples prepared by different methods in comparison with bulk material were detected. Oxygen atoms were not detected around Au atoms within the detection limits of our study (a few per cents of surrounding atoms). A gradual reduction of the Au-Au bond length and first shell Au-Au coordination number and an increase in the Debye-Waller factor are observed as the size of supported Au particles decreases. The significant increase of structural disorder for the smallest Au particles comparatively to the bulk Au metal and sizeable particles was detected. These variations in micro-structural parameters of Au nanoparticles are in line with an increase in their catalytic activity in CO oxidation. For model systems some coordination of Au- atoms from islands and Si- atoms from crystal surface was detected for thin Au "films".