Nitric oxide reduction with methane over Pd/TiO2 catalysts .2. Isotopic labeling studies using N-15, O-18, and C-13

Reduction of NO with methane was investigated over Pd/TiO2 catalysts using isotopically labeled species. Steady-state as well as unsteady-state isotopic labeling studies have been performed using the switches (NO)-N-14-O-16 --> (NO)-N-15-O-18, O-16(2) --> O-18(2), CH4 + (NO)-N-15 + O-2 --> CH4 + (NO)-N-14 + O-2, CH4 + CH4 + O-2, (CH4)-C-13 --> NO, (CH4)-C-13 + NO --> He, (CH4)-C-13 + NO --> NO, (CH4)-C-13 + NO + O-2 --> NO, (CH4)-C-13 --> O-2, CH4 + (NO)-N-15, CH4 + (NO)-N-15-O-18, and N2O --> He, which have given some important clues regarding the mechanistic aspects of the NO-CH4 interaction. Based on these studies, it was concluded that N-2 is formed through direct participation of CH4 whereas N2O formation is mainly a result of NO decomposition reaction. It was also observed that at least three types of reactions occur simultaneously on the surface, namely NO + CH4, CH4 oxidation and NO decomposition. The extent of these reactions is determined by the oxidation state of palladium on the surface. (C) 1997 Academic Press.