Hydrothermal Aging of Pd/γ-Al2O3 Catalysts for Lean CO Oxidation

Hydrothermal aging significantly affects the performance of Pd-based oxidation catalysts employed in lean-burn exhaust gas aftertreatment systems. In this study, powder catalysts with a Pd load of 2 wt.% were hydrothermally exposed to 650, 750, 850 and 950 °C, followed by comprehensive physicochemical characterization and evaluation of lean CO oxidation activity under steady-state conditions in the presence of H2O. The catalytic performance was correlated with the number of accessible Pd surface sites quantified by temperature-programmed desorption of H2 and scanning transmission electron microscopy. A marked loss of accessible Pd surface sites was observed after hydrothermal aging above 750 °C, accompanied by a corresponding decline in CO oxidation efficiency. Steady-state kinetic analysis revealed a progressive increase in the apparent activation energy from 36 kJ mol−1 for the fresh catalyst to 74, 82 and 135 kJ mol−1 after aging at successively higher temperatures. X-ray diffraction and X-ray photoelectron spectroscopy indicated dynamic changes in the Pd oxidation state during CO conversion to CO2. The increase in activation energy was attributed to the combined effects of an increasing fraction of metallic Pd, Pd particle growth and phase transformations of the alumina support with increasing aging temperature. These findings demonstrate the pronounced structure sensitivity of lean CO oxidation on Pd/Al2O3 catalysts and highlight the critical impact of hydrothermal aging at temperatures of 750 °C and above on the physicochemical properties driving catalytic activity.