Interaction of CH4 with NO and NO2 over Pt-ZSM-5 in the absence of O-2

The reduction of NO and NO2 with CH4 to form N-2 catalyzed by Pt-ZSM-5 has been investigated. For both reactions the dependence of conversion on tempe rature is similar to that of CH4 combustion catalyzed by Pt. The conversion increases slowly before a sharp increase at the ignition temperature (simi lar to 300 degreesC for NO + CH4 and similar to 475 degreesC for NO2 + CH4) . Based on results in which the mole ratios and partial pressures of NOx an d CH4 were varied, it is suggested that the oxygen surface coverage determi nes the catalytic activity of Pt-ZSM-5. It is postulated that NO2 rapidly d issociates on Pt, covering the surface with oxygen adatoms. The interaction of oxygen adatoms with the Pt surface is sufficiently strong that CH4 cann ot compete for adsorption sites. Thus, the catalytic activity is low at tem peratures less than 475 degreesC, where oxygen desorption from the surface is unfavorable. However, NO has a lower sticking probability, and the slowe r rate of N-O bond dissociation results in a lower steady state oxygen cove rage and, in turn, a higher activity in the NO + CH4 reaction. Experiments in which the CH4 + NO2 reaction temperature was cycled from 350 to 500 degr eesC and back to 350 degreesC provides evidence that overstoichiometric CH4 dissociation on the Pt surface can occur, and the surface carbon that is f ormed enhances NO2 reduction to N-2.