Activity of PdO/SiO2 catalysts for CH4 oxidation following thermal treatments

The effect of thermal treatment of PdO/SiO2 catalysts in air versus air/water at temperatures <973 K, is reported. PdO sinters during thermal aging in the presence of air (dry aging) and air/water (hydrothermal aging), but the growth and restructuring of the PdO crystallites is suppressed in the presence of added water, as observed in both continuous aging and cyclic aging experiments. The hydrothermally aged catalysts (PdO size 3 nm) showed significantly lower CH4 oxidation activity than the catalysts thermally aged in air (PdO size 14 nm). A significant decrease in the Pd/Si ratio as measured by XPS, compared to a small growth in PdO size after hydrothermal aging, suggests PdO occlusion and hence a significant loss in activity following hydrothermal aging. The PdO occlusion and suppressed PdO particle sintering is attributed to SiO2 overlayers on the PdO that are a consequence of hydroxyl mobility of the support, enhanced by the presence of added water. (C) 2014 Elsevier B.V. All rights reserved.