Evolution of Pd Species for the Conversion of Methane under Operation Conditions

Abstract

A Pd model catalyst was tested under dynamic operation conditions to investigate the active site for the conversion of CH4 from natural gas vehicle (NGV) exhausts. It was demonstrated that in situ pretreatment in reaction gas significantly improved the CH4 removal activity. X-ray photoelectron spectroscopy results revealed that the metallic Pd phase was much more active than PdO for CH4 conversion under stoichiometric conditions. Interestingly, under rich conditions (lambda = 0.98), the formation of inactive PdCx led to considerable deactivation of catalyst. Increasing the O2 concentration can efficiently convert it into active Pd0; thereby, the activity was recovered to the original level. By contrast, under stoichiometric conditions (lambda = 1.00), CH4 conversion was stable, probably owing to the maintenance of the active Pd0 phase. However, further increasing the O2 amount (lambda = 1.02) resulted in reduced activity, which may be due to the formation of PdO from the oxidation of metallic Pd. These results imply that Pd species under dynamic working conditions are evolvable.