STATE OF PD ON H-ZSM-5 AND OTHER ACIDIC SUPPORTS DURING THE SELECTIVEREDUCTION OF NO BY CH4 STUDIED BY EXAFS XANES/

The selective reduction of NO by methane has been studied on a series of Pd catalysts. When supported on non-acidic materials, palladium is totally unselective for the conversion of NO and very active for the c ombustion of methane. By contrast, lowloading Pd catalysts supported o n acidic (protonic) materials (H-ZSM-5, H-Mordenite, sulfated zirconia ) exhibit much higher selectivity. X-ray absorption studies (XANES and EXAFS) were conducted on several Pd catalysts before and after the re duction of NO in the presence of oxygen. These studies have demonstrat ed that, upon exposure to the reaction mixture (CH4 + NO + O-2), Pd ge ts oxidized. The morphology of the oxidized Pd species strongly depend s on the metal loading and the acidity of the support. On low Pd loadi ng catalysts over acidic supports, the metallic Pd particles, initiall y present, are rapidly transformed in Pd2+ ions by the reaction mixtur e. By contrast, on the non-acidic materials, the Pd particles are tran sformed into PdO clusters. These clusters have high catalytic activity for the methane combustion, resulting in low SCR selectivities. (C) 1 997 Elsevier Science B.V.