applied catalysis B ELS EV l ER Applied Catalysis B: Environmental 4 (1994) 155-165 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 Di-Jia Liu*, Heinz J. Robota Allied Signal Inc., Research and Technology, 50 E. Algonquin Roa~ Des Plaines, IL 60017-5016,USA Received 30 November 1993; revised 25 February 1994; accepted 5 March 1994 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 t at elevated temperature, when propene is present in the reactant stream. XANES spectra show that the Cu x 1s ~ 4p transition intensity, which is proportional to cuprous ion concentration, changes with the reaction temperature 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 ~ 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 l, possibly by way of allylic intermediate, which may be a crucial step for effective NO conversion through a redox mechanism. Keywords: Cu-ZSM-5; in situ characterization; methane; propene; NO; selective reduction; XANES 1. Introduction The emission of nitrogen oxides, NOx, has caused serious damage to terrestrial and aquatic ecosystems. Developing a catalytic system to remove NOx at its sources *Corresponding author. Tel. ( + 1-708)3913703, fax. ( + 1-708)3913773, e-mail: L 1U@RESEARCH.ALLIED.COM. 0926-3373/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10926-3 37 3 ( 94 ) 00012-4