Selective Reduction of NO over Pt-Mo Catalysts under Lean Static Conditions

The selective reduction of NOx on various PtMoX-catalysts (X: Li, Na, K, Mg and Ca) loaded on metal oxides such as SiO2, Al2O3, TiO2, ZSM-5 and silicalite was studied in simulated exhaust gases from an automotive lean-burn engine and a stoichiometric engine and compared with that on PtMo/SiO2, PtNa/SiO2, and Pt/SiO2 catalysts. The temperature window for the selective reduction of NOx on the PtMoLi-, PtMoNa- and PtMoK-catalysts loaded on SiO2 was found to be wider and to shift to a higher temperature than that on the other catalysts under lean static conditions. The redox ratio window, in which three-way activity occurred on a PtMoNa catalyst loaded on SiO2, was also found to be wider than that on PtMo/SiO2, catalysts and Pt/SiO2 around the stoichiometric point. On the other hand, XPS, IR and CO adsorption data indicated that the oxidation of Pt on PtMoNa/SiO2 catalysts was depressed by the added Mo and Na even under excess oxygen conditions, so that the reaction characteristics of the PtMoNa/SiO2 catalysts were improved