On the characterization of surface VOx-species by XPS: layered oxidic systems as model catalysts

Abstract

The influence of the support on vanadia, immobilized on Au, SiO2, Al2O3 and TiO2, suitable for the selective catalytic reduction (SCR) of nitric oxide by ammonia has been investigated. The layered oxidic substrates, which are deposited onto gold coated silicon wafers, represent model systems for the surfaces of technical SCR catalysts, that are suitable for surface science investigations. The oxidation states of the surface vanadia species immobilized on these samples have been studied by XPS after well-defined sample treatment in both oxygen and hydrogen atmospheres at 573 K and 723 K. From the observed shifts of the V 2p3/2 binding energies, and from peak deconvolution, vanadia layers on gold, silica and alumina are found to exhibit characteristics similar to those of bulk vanadia. This behavior is characterized by the relative stability against a reductive atmosphere at 573 K. Changes in the oxidation states of the surface vanadia species are prominent only after treatment at 723 K. Titania exhibits a unique support interaction with vanadia which allows the reduction of vanadia at temperatures as low as 573 K. This fact may contribute to the outstanding performance of VOx/TiO2 in the selective reduction of nitric oxide by ammonia in waste gases.