THE RATE-DETERMINING STEP IN THE SELECTIVE REDUCTION OF NITRIC-OXIDE BY METHANE OVER A CO ZSM5 CATALYST IN THE PRESENCE OF OXYGEN

The relative reaction rates of CH4 and CD4 with NO in the presence of excess oxygen over a Co-ZSM5 catalyst have been determined. The rate r atio, CH4 consumption versus CD4 consumption, is nearly 2.4 when using a dry feed at 375 degrees C. Experiments at 431 degrees C using a fee d containing 1.6% H2O show a rate ratio of 2.05. These values are clos e to those expected for a primary kinetic isotope effect at the respec tive temperatures, and it is therefore concluded that the rate-determi ning step is the breaking of a carbon hydrogen bond. The kinetic isoto pe effect is slightly less for the selective reaction with NO than for the concurrent direct oxidation of methane by oxygen. The two reactio ns have very similar kinetic orders in methane, suggesting that a comm on intermediate is involved in the two pathways. Hydrogen-deuterium ex change between CD4 and H2O to make CH3D is negligible under reaction c onditions. Similarly, mixed feeds of CH4 and CD4 show very little exch ange. One interpretation consistent with the results is that the rate- determining step involves detachment of hydrogen from methane by adsor bed nitrogen dioxide to form a methyl species which reacts further wit h a nitrogen oxide in one pathway but with oxygen in the other. (C) 19 95 Academic Press, Inc.