(67a) Effect of Oxide Supports on Activity of Pdo Catalysts during Hydrothermal Aging in CH4 Oxidation

The present study compares the effect of hydrothermal aging (HTA) of PdO dispersed on high surface area supports (γ-Al₂O₃ and SiO₂) versus low area supports (α-Al₂O₃ and SnO₂) for CH₄ oxidation at conditions relevant to natural gas vehicle exhaust gas converters. Pd supported catalysts, prepared by incipient wetness impregnation, with similar Pd loadings were hydrothermally aged at 973 K in air/H₂O for various periods of time and tested using Temperature-programmed CH₄ oxidation. The catalyst performance before and after HTA is compared and possible deactivation mechanisms are discussed. A comparison of catalyst properties reveals that the PdO catalysts sinter during HTA and PdO crystallites are occluded by the support as confirmed by XPS and HRTEM studies. PdO occlusion by the support plays a key role in deactivating the PdO/SiO₂ catalysts during HTA but does not significantly affect the performance of PdO on other supports. The deactivation of PdO/γ-Al₂O₃ and PdO/SnO₂ catalysts is mostly related to PdO sintering and PdO to Pd⁰ transformation. XRD and XPS results confirm the coexistence of PdO and Pd⁰ after HTA in these catalysts, associated with the presence of H₂O. PdO/α-Al₂O₃ was found to have the highest catalyst stability during HTA at 973 K for up to 65 h. Less PdO sintering and absence of a Pd⁰ phase in PdO/α-Al₂O₃ suggests stronger Pd-support interactions, leading to higher catalyst stability during HTA.