Effect of Aging Atmosphere on Thermal Sintering of Modern Commercial TWCs

The effect of aging atmosphere on the sintering behavior of commercial Pd and Rh catalysts as well as the TWC performance thereof has been investigated under straight oxidizing, reducing and periodic cycling aging conditions, in search of useful guidance in the optimum design of thermally durable TWCs for advanced gasoline engines equipped with the deceleration fuel cutoff technology. Pd and Rh catalysts individually exhibit an opposite trend in the thermal sintering behavior with respect to the aging atmosphere. Under oxidizing conditions, the Pd catalyst becomes more resistant to sintering in higher O-2 concentrations, whereas the Rh catalyst reveals the opposite behavior, regardless of the aging temperature. Physicochemical characterizations by using TGA and CO chemisorption have indicated that the state of Pd (PdO vs. Pd-0) and Rh (Rh2O3 vs. Rh-0) on the catalyst surface formed during the thermal aging plays an important role for determining the trend of thermal sintering; the positive effect of the O-2 concentration on Pd sintering is attributable primarily to the formation of PdO, while the main cause for severe deactivation of the Rh catalyst under oxidizing conditions is the diffusion of Rh2O3 into the support along with the agglomeration of Rh particles.