CO Oxidation on Technological Pd-Al2O3 Catalysts: Oxidation State and Activity

The specific CO oxidation activity of palladium versus palladium oxide is still controversially discussed. In this study, 5 wt. % Pd-gamma-Al2O3 catalysts were utilized to investigate the effect of the palladium oxidation state on the CO oxidation activity. Comprehensive in situ and ex situ characterization of different alumina supported PdOx (x = 0-1) phases (by HR-TEM, XRD, and FTIR spectroscopy), combined with kinetic measurements and DFT calculations of CO adsorption, allowed us to assess the catalytic activity of the different PdOx (x = 0-1) species: Supported Pd-0 and substoichiometric PdOx<1 exhibited comparable high activities, due to a rapid reduction of PdOx<1 to Pd upon CO exposure. PdO nanoparticles showed a lower catalytic activity resulting from their higher stability against reduction by CO. The limited extent of oxidation of Pd under typical reaction conditions together with the facile reduction of substoichiometric PdOx<1 suggest that the active phase present under the current reaction conditions is oxygen-covered metallic Pd.