Methane oxidation over Pd/Al2O3 under rich/lean cycling followed by operando XAFS and modulation excitation spectroscopy

Abstract

Rich/lean cycling is used to study the total oxidation of methane over a Pd/Al2O3 catalyst at different oxidation states. Time-resolved energy-dispersive X-ray absorption fine structure is used in a modulation excitation approach to monitor the chemical state of the palladium nanoparticles during the cycling, and the resulting spectra are demodulated using phase sensitive detection. Cycling is performed using oxygen pulses with a concentration of 0.15, 0.25, and 1.5% over a constant flow of 0.1% methane. For the two lowest oxygen concentrations the methane conversion is generally low but increases at the switches between the rich and the lean periods, while for the highest oxygen concentration the conversion is highest during the lean periods. The oxidation state of Pd changes rapidly but to a limited extent for the two lowest oxygen concentrations, whereas for the high oxygen concentration the oxidation appears to proceed via a two-step process, where the first step is rapid and the second step is slower. EXAFS fitting of the demodulated spectra is used to quantify the Pd oxidation state, the first rapid oxidation step is assigned to surface oxidation, while the second step is assigned to bulk oxidation of Pd. A low methane conversion is observed when the Pd nanoparticles are covered with chemisorbed oxygen or surface oxide, the methane conversion is higher when the Pd nanoparticles are bulk oxidized.