In situ characterization of Cu-ZSM-5 by X-ray absorption spectroscopy: XANES study of the copper oxidation state during selective catalytic reduction of nitric oxide by hydrocarbons

Abstract

Reported here is our recent investigation of the mechanism of the selective nitric oxide reduction by hydrocarbons on Cu-ZSM-5 catalysts in an oxygen-rich gas mixture. We studied the copper oxidation state change during the catalytic reaction using the X-ray Absorption Near Edge Structure (XANES) method. We observe that even under strongly net oxidizing conditions, a significant fraction of the copper ions in ZSM-5 is reduced to Cu I at elevated temperature, when propene is present in the reactant stream. XANES spectra show that the Cu I 1s → 4p transition intensity, which is proportional to cuprous ion concentration, changes with the reaction temperture in a pattern similar to the NO conversion activity. For comparison purposes, we also studied the Cu I concentration change using a gas mixture in which propene was replaced by a stoichiometrically equivalent concentration of methane. Unlike propene, methane provides no NO selective reduction pathway over Cu-ZSM-5. No window of enhanced Cu I concentration was observed using methane as the reductant. Our study indicates that, even in a strongly oxidizing environment, cupric ion can be partially reduced by propene to form Cu I, possibly by way of allylic intermediate, which may be a crucial step for effective NO conversion through a redox mechanism.