Comparative NOx Reduction Behavior of Pt, Pd, and Rh Supported Catalysts in Simulated Exhaust Gases as a Function of Oxygen Concentration

NO _x reduction activity on Pt and Pd catalysts had a maximum for S value as stoichiometry number at a fixed temperature, and the S value at the maximum NO _x conversion increased with decreasing temperature. NO _x conversion on Rh catalyst increased with decreasing S value, but independent of temperature. As for the effect of HC on NO _x reduction behavior, it was concluded that, for Pt and Pd catalysts, HC adsorbs strongly on the catalysts surface to cause the self-inhibition. Increasing O₂ concentration lead to oxidation of HC, but decreased the value of NO/O₂ ratio. The balance point of the two factors generated a maximum NO _x conversion. For Rh catalyst, the strongly adsorbed oxygen is more reactive with decreasing S value, and thus NO _x conversion is increased.