Relative rates of various steps of NO-CH4-O2 reaction catalyzed by Pd/H-ZSM-5

Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO-CH4-O-2 reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623-703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N-2 formation in NO-CH4-O-2 vs. NO-CD4-O-2 reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H-D isotopic exchange took place significantly in NO-(CH4+CD4)-O-2 reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C-H dissociation of methane is the only rate-determining step, and show that the C-H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5. A reaction scheme was proposed, in which the relative rates of the three steps ((i)-(iii) below) vary depending on the reaction conditions. [GRAPHICS] Further, in contrast to Co-ZSM-5, NOx-CH4-O-2 reaction was much slower than CH4-O-2 reaction for Pd/H-ZSM-5; the presence of NOx retards the reaction of CH4 over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH4 is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO2 strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO-CH4-O-2 reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5. (C) 1998 Elsevier Science B.V. All rights reserved.