Interaction of CH4 with NO and NO2 over Pt-ZSM-5 in the absence of O2

Abstract

The reduction of NO and NO2 with CH4 to form N2 catalyzed by Pt-ZSM-5 has been investigated. For both reactions the dependence of conversion on temperature is similar to that of CH4 combustion catalyzed by Pt. The conversion increases slowly before a sharp increase at the ignition temperature (∼300 °C for NO + CH4 and ∼475 ○C for NO2 + CH4). Based on results in which the mole ratios and partial pressures of NOx and CH4 were varied, it is suggested that the oxygen surface coverage determines the catalytic activity of Pt-ZSM-5. It is postulated that NO2 rapidly dissociates on Pt, covering the surface with oxygen adatoms. The interaction of oxygen adatoms with the Pt surface is sufficiently strong that CH4 cannot compete for adsorption sites. Thus, the catalytic activity is low at temperatures less than 475 °C, where oxygen desorption from the surface is unfavorable. However, NO has a lower sticking probability, and the slower rate of N–O bond dissociation results in a lower steady state oxygen coverage 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 °C and back to 350 °C provides evidence that overstoichiometric CH4 dissociation on the Pt surface can occur, and the surface carbon that is formed enhances NO2 reduction to N2.