Dynamics of the Reversible Inhibition during Methane Oxidation on Bimetallic Pd-Pt Catalysts Studied by Modulation-Excitation XAS and DRIFTS

Water and NO reversibly inhibit catalytic methane oxidation to CO2 over Pd/Al2O3 and Pd-Pt/Al2O3 at temperatures near 50 % conversion. Modulation-excitation X-ray absorption spectroscopy (ME-XAS) was used to characterize the interaction of the inhibitors with the Pd and Pt components. Simulated X-ray absorption near-edge structure (XANES) spectra of Pd and Pt with various surface species were used to verify the findings of ME-XAS measurements. Diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) allowed to correlate methane oxidation with surface formate species. An interaction of water with active Pd sites and water coverage of the Al2O3 support caused a drastic drop in formate production, correlated with the observed inhibition. In a similar manner, nitrite groups formed from gas-phase NO inhibited the production of formates. At higher temperatures, the inhibition was reversed into a promoting effect by surface nitrates formed from NO that oxidized gas-phase methane. This approach is not only interesting for emission control but in general for catalysts where dynamic structural changes occur.