XPS study of the surface chemical state of a Pd/(SiO2+TiO2) catalyst after methane oxidation and SO2 treatment

X-ray photoelectron spectroscopy (XPS) was employed to study the changes in the surface composition and electronic structure of a TiO2-promoted Pd/SiO2 catalyst following methane oxidation under lean-burn conditions (up to 600 degrees C) and SO2 treatment at 350 degrees C. Overnight exposure of the activated catalyst to SO2 was found to cause the formation of a layer of palladium sulfate and sulfate/sulfite species on the support surface, leading to its deactivation. A single cycle of methane combustion over the SO2-treated catalyst gave rise to its reactivation due to the effective decomposition of Pd sulfate and desorption of SO2 at relatively low temperatures. In the stabilized and restored states, the catalyst exhibited a high and stable activity, had the largest proportion of metallic versus oxidic palladium species, a high density of Pd 4d states near the Fermi level, and the smallest extent of surface hydroxylation. The catalytic reaction over an as-prepared or SO2-treated catalyst was revealed to cause a partial and reversible 'encapsulation' of Pd particles by TiOx suboxide moieties from the mixed oxide support. The encapsulation resulting from strong metal support interaction was enhanced by the SO2 treatment of the Pd/(SiO2+10 wt% TiO2) catalyst. (C) 2014 Elsevier Inc. All rights reserved.