CVD OF SILICA ON A PALLADIUM-LOADED ZEOLITE - A METHOD OF CANCELING ACTIVITY RETARDATION BY WATER-VAPOR IN THE REDUCTION OF NO

The activity of NO reduction with methane in the presence of oxygen on Pd/HZSM-5 is suppressed by the addition of water vapor: however, it i s recovered by the deposition of silica on the zeolite by the chemical vapor depositon of Si(OCH,),. It was found from a transient experimen t that water vapor retarded the activity both reversibly and irreversi bly. The irreversible deactivation for NO reduction was similar to tha t in the oxidation of methane. The irreversible retardation of methane oxidation was caused by the deactivation of palladium due to water va por in the presence of oxygen. A water adsorption experiment showed th at the equilibrium amount of adsorbed water was reduced by the deposit ion of silica on the external surface. Based on these experimental fin dings, a scheme of the deactivation of Pd by the presence of water and a recovery of activity by the deposited silica is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.