Pd model catalysts: Effect of aging environment and lean redispersion

Abstract

The performance of automotive three-way catalysts (TWC) deteriorates with time, temperature and aging environment. Engine control methods are needed to minimize the extent of catalyst deactivation and provide an environment capable of partially redispersing noble metal catalyst particles. In this study, palladium-based model powder catalysts on ceria-zirconia or alumina supports were exposed to three different exhaust compositions, lean-only, rich-only and redox, each at 700°C for 16h. Catalyst activity was determined by CO oxidation with the water gas shift (WGS) reaction and oxygen storage capacity (OSC) measurements to probe the contact between the noble metal and support at a given state of catalyst deterioration. Lean catalyst treatments at 550°C and 700°C were applied to determine the effect on measured Pd size and catalyst activity. The lean-only gas environment was above the PdO decomposition condition, yet showed slightly deteriorated catalyst activity from the fresh state. The rich-only and redox gas environments significantly deteriorated catalytic activity through a combination of metal oxidation state effects and support/additive interactions, both with various degrees of reversibility depending on lean treatment time and temperature. The insight gained from this work could be used to develop engine control and after treatment design strategies to track (or infer) the aging process on the vehicle, avoid severe aging modes and actively intervene at various points to regenerate the catalyst.