CH4 Oxidation Activity in Pd and Pt-Pd Bimetallic Catalysts: Correlation with Surface PdOx Quantified from the DRIFTS Study

For Pd-based catalysts, facile and fast interconversion between Pd and PdO occurs continuously during the CH4 oxidation reaction, which makes it challenging to determine active sites. Herein, we report that the amount of partially oxidized palladium (PdOx) on the catalyst surface shows a linear correlation with the CH4 oxidation activity in a series of Pd/Al2O3 and Pt-Pd/Al2O3 catalysts hydrothermally aged under commercially relevant conditions. We characterized the amount of surface PdOx through diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as a probe molecule. With careful consideration that the redox cycle of Pd particles occurs continuously during the CH4 oxidation reaction, the Pd/Al2O3 catalysts were reoxidized before obtaining the CO adsorption DRIFTS data to minimize the discrepancy between the catalytically relevant phases and the characterized surface composition. The IR spectra of CO adsorption on reoxidized Pd/Al2O3 catalysts contain peaks at 2135-2145 cm(-1), which correspond to PdOx. The steady-state CH4 oxidation activities at 300 degrees C increased proportionally with the normalized surface PdOx. Furthermore, Pt-Pd bimetallic catalysts also showed the same linear correlation between the surface PdOx and CH4 oxidation activity irrespective of composition, preparation method, and support. Our results indicate that the surface PdOx plays a critical role in determining CH4 oxidation activity rather than the electronic properties of Pd. Overall, we report a general correlation between the amount of surface PdOx and the steady-state CH4 oxidation activities in various Pd-based catalysts. This work will greatly help in achieving the fundamental understanding of the CH4 oxidation reaction on the PdOx surface and the further development of Pd-based catalysts for CH4 oxidation with better activity.