CO Oxidation on Technological Pd−Al₂O₃Catalysts: Oxidation State and Activity

The specific CO oxidation activity of palladium versus palladium oxide is still controversially discussed. In this study, 5 wt. % Pd-γ-Al 2 O 3 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 PdO x ( 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 PdO x ( x = 0−1) species: Supported Pd 0 and substoichiometric PdO x <1 exhibited comparable high activities, due to a rapid reduction of PdO x <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 PdO x <1 suggest that the active phase present under the current reaction conditions is oxygen-covered metallic Pd.