Pd or PdO: Catalytic active site of methane oxidation operated close to stoichiometric air-to-fuel for natural gas vehicles

Abstract

A Pd catalyst supported on La-modified Al2O3 (La-Al) with and without doping CeO2-ZrO2 (CZ) is prepared via incipient wetness impregnation method. The catalysts are used as model catalyst for natural gas vehicles (NGVs) to investigate the active sites of methane oxidation within the range of air/fuel ratios (λ values) close to stoichiometric value (λ=1). Both the two catalysts show high activity for methane oxidation under rich conditions. Under lean conditions, the methane oxidation activity over the dopant-free catalyst depend on the direction of λ change, and this catalyst displays relatively lower activity when the λ changes from richer values to lean ones. By contrast, the catalyst with dopant under the same lean conditions can nearly maintain its high activity regardless of the change direction. The results of X-ray powder diffraction (XRD) and temperature programmed reduction under H2 (H2-TPR) indicate the existence of the interactions between the dopant and Pd species, and these interactions are beneficial to retain Pd in oxidized state. Ex situ X-ray photoelectron spectra (XPS) experiments are explored to correlate the catalytic activity of the methane oxidation with the oxidized valent state of Pd. The results confirm that the dopant can preserve most of the Pd species in PdO even under the rich conditions. The results also suggest that both metallic Pd and PdO can act as the active sites for methane oxidation under much richer conditions, and that under lean conditions, PdO instead of metallic Pd plays a role in methane oxidation.