Methane oxidation on Pd–Ceria: A DFT study of the mechanism over Pd xCe 1−xO 2, Pd, and PdO

Abstract

Palladium/ceria exhibits unique catalytic activity for hydrocarbon oxidation; however, the chemical and structural properties of active sites on the palladium–ceria surface are difficult to characterize. Strong interactions between palladium and the ceria support stabilize oxidized Pd δ + species, which may contribute to the significant activity of Pd/ceria for methane oxidation. We present a density functional theory (DFT + U) investigation into methane oxidation over Pd/ceria and quantify the activity of the Pd x Ce 1− x O 2(1 1 1) mixed oxide surface in comparison with the PdO(1 0 0) and Pd(1 1 1) surfaces. The methane activation barrier is lowest over the Pd x Ce 1− x O 2(1 1 1) surface, even lower than over the Pd(1 1 1) surface or low coordinated stepped or kinked Pd sites. Subsequent reaction steps in complete oxidation, including product desorption and vacancy refilling, are considered to substantiate that methane activation remains the rate-limiting step despite the low barrier over Pd x Ce 1− x O 2(1 1 1). The low barrier over the Pd x Ce 1− x O 2(1 1 1) surface demonstrates that mixed ceria-noble metal oxides offer the potential for improved hydrocarbon oxidation performance with respect to dispersed noble metal particles on ceria.