Methane oxidation on Pd–Ceria: A DFT study of the mechanism over PdxCe1−xO2, Pd, and PdO

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 Pdδ+ 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 PdxCe1−xO2(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 PdxCe1−xO2(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 PdxCe1−xO2(1 1 1). The low barrier over the PdxCe1−xO2(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.

The methane activation barrier over the PdxCe1−xO2(1 1 1) surface is lower than that over CeO2(1 1 1), Pd(1 1 1), or PdO(1 0 0). Methane activation is rate limiting in all cases; thus, the low barrier over PdxCe1−xO2 evidences the unique hydrocarbon oxidation activity of Pd/ceria mixed oxides.Figure optionsDownload high-quality image (131 K)Download as PowerPoint slideResearch highlights
► The DFT + U method was used to examine methane oxidation on model CeO2, PdxCe1−xO2, and PdO surfaces.
► The methane activation barrier is lower over PdxCe1−xO2(1 1 1) surface than over CeO2(1 1 1), Pd(1 1 1), or PdO(1 0 0).
► Methane activation is rate limiting; thus, the low barrier over PdxCe1−xO2 indicates unique methane oxidation activity.
► PdxCe1−xO2(1 1 1) mixed oxide sites may account for high activity, despite being a minority site.