NO reduction by CO over Rh/Al2O3 and Rh/AlPO4 catalysts: Metal-support interaction and thermal aging.

Rhodium was impregnated on γ-Al2O3 and AlPO4 followed by treating in a wet air at 1050°C for 5h. The catalytic activities of the catalysts were evaluated by temperature-programmed reactions in a stoichiometric mixture of NO and CO. The fresh Rh/Al2O3 and Rh/AlPO4 catalysts exhibit similar catalytic activities. After the hydrothermal aging, Rh/Al2O3 experiences a significant deactivation, whereas Rh/AlPO4 shows even higher activity than the fresh sample. A series of structural and surface characterizations were performed to explore different aging mechanisms of these two catalysts. The fresh Rh/AlPO4 catalyst shows superior low-temperature redox property, but its NOx adsorption is weakened due to the acidic nature of the support. Thus, two fresh catalysts present similar catalytic activities. The hydrothermal aging at high temperature leads to the formation of inert Al5Rh2 alloy. Contrarily, no such strong interaction occurs between rhodium and AlPO4. Furthermore, large Rh particles, which facilitate the dissociation of NO, are formed on the aged Rh/AlPO4 catalyst. Thus, a superior high activity for NO-CO is achieved over the aged Rh/AlPO4.