110th Anniversary: Kinetics and X-ray Absorption Spectroscopy in Methane Total Oxidation over Alumina-Supported Pt, Pd, and Ag–Pd Catalysts

Abstract

The kinetics of methane oxidation over Pt/Al2O3, Pd/Al2O3, and Ag–Pd/Al2O3 is investigated as a function of the O2/CH4 ratio. We show that as the oxygen fraction increases from low values, three distinct kinetic regimes exist that are nearly first order, negative order, and zero order with respect to the oxygen concentration. At O2/CH4 < 3, Pd and Ag–Pd are superior catalysts, whereas at higher oxygen concentrations, Pt is superior. Hysteresis with bistability is demonstrated over all three catalysts with two distinct activity regimes at identical feed compositions: higher activity upon increasing oxygen concentration from low values and lower activity upon decreasing oxygen concentration from high values. In situ X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) over the Ag–Pd catalyst suggest that the bistability stems from different oxidation states of the catalyst as well as a core–shell structure in the reduced state.