The average Pd oxidation state in Pd/SiO2 quantified by L3-edge XANES analysis and its effects on catalytic activity for CO oxidation

Abstract

The palladium oxidation state of an SiO₂-supported palladium catalyst was quantitatively determined by Pd L₃-edge XANES (X-ray absorption near-edge structure) analysis. By changing the time of CO-reduction pre-treatment at 673 K, a series of 5 wt% Pd-loaded SiO₂ catalysts (PdO_x/2/SiO₂) containing different amounts of the Pd metal and PdO phases were prepared, and the average oxidation number (x) was estimated from the number of CO₂ molecules formed during the CO-reduction treatment. L₃-edge XANES spectra of these samples and a reference sample (Pd powder) were recorded, and the white line area of the spectra was evaluated. A good linear relationship was confirmed between the white line area intensity and the average oxidation number (x), indicating that the oxidation state of Pd in structurally unknown Pd samples could be quantitatively determined by a simple XANES analysis. To demonstrate the utility of this method in a catalytic study, the effect of the oxidation number (x) on the CO oxidation activity of PdO_x/2/SiO₂ was also examined, and metallic Pd⁰ sites in PdO_x/2/SiO₂ were shown to be active species.