Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

Abstract

The identity of reactive intermediates and of active sites and the details of redox cycles and oxygen removal pathways during NO decomposition on well-characterized Cu−ZSM5 were examined by combining previous spectroscopic and steady-state kinetic studies with measurements of the rate of evolution of NO, N2O, NO2, N2, and O2 during isothermal and nonisothermal kinetic transients. The oxygen coverages measured during reversible isothermal switches from He to NO/He mixtures showed that NO decomposition involves bimolecular reactions of two NO molecules adsorbed on vicinal Cu+ species with the formation of N2O as the initial product near ambient temperature. These vicinal Cu+ species form via oxygen removal from {Cu2+−O2-−Cu2+}2+ to form {Cu+−□−Cu+}2+ using NO2 as an oxygen carrier among distant oxidized dimers. Adsorbed nitrate (NO3*) is the kinetically relevant intermediate in the formation of O2 during NO decomposition. This NO3* decomposition reaction is one of the steps involved in the equilibrated formation of NO2 observed during NO decomposition. These NO-mediated oxygen removal pathways, in which NO acts both as a reductant and as an oxidant, are significantly more rapid than recombinative desorption steps. The desorption of products and of unreacted NO during reactions of preadsorbed NO with increasing catalyst temperature confirmed the bimolecular nature and the low activation energy for N2O formation from NO. The facile nature of this reaction and the unfavorable NO adsorption thermodynamics as temperature increases combine to give the observed decrease in NO decomposition rates at high temperatures. These findings are consistent with some reported mechanism-based steady-state rate expressions and with previous infrared detection of the reaction intermediates proposed here based on isothermal and nonisothermal transients. These pathways appear to be relevant also to N2O decomposition reactions, which occur after the initial formation of N2O from NO and involve reactions of N2O with {Cu+−□−Cu+}2+ and removal of oxygen via NO-mediated desorption pathways. This study brings consensus and some clarification into the mechanistic details for NO and N2O decomposition reactions and resolves some remaining discrepancies and some contradictory conclusions in previous reports.