Thermostable Rh Catalysts Supported on Metal Phosphates: Effect of Aging on Catalytic Activity for NO-CO-C3H6-O2 Reactions

The catalytic activity and thennostability of Rh catalysts (0.4% loading) have been studied using various metal phosphates (B, Al, Ga, Y, La, Pr, Zr, and Ce) as support materials. Temperature-programmed reactions of mixtures of NO, CO, C3H6, and O-2 with the stoichiometric A/F ratio (14.6) exhibited a steep rise in conversion efficiency as a common feature of Rh supported on metal phosphates. The activity of Rh/AlPO4 was found to be the highest even alter aging at 900 degrees C for 25 h in a stream of 10% H2O/air. Among four types of AlPO4 polymorphs having different BET surface area, i.e. berlinite (5 m(2) g(-1)), cristobalite (1 m(2) g(-1)), tridymite (82 m(2) g(-1)), and microporous AlPO4-5 (330 m(2) g(-1)), tridymite AlPO4 achieved the lowest light-off temperature. The slow grain growth of tridymite AlPO4 at < 1000 degrees C is a possible reason for the stability against sintering, whereas one-dimensional micropores of AlPO4-5 are unstable in the high-temperature steam.