On the Mechanism of NO Selective Catalytic Reduction by Hydrocarbons over Cu-ZSM-5 via X-ray Absorption Spectroscopic Study

Abstract

An understanding of the catalytic mechanism of NOx reduction is critical for the development of next-generation high-fuel efficiency, low-emission vehicles. This paper compiles our investigations in recent years on the mechanism of NO selective catalytic reduction (SCR) by hydrocarbon over Cu-ZSM-5. The studies were focused on the oxidation state and coordination chemistry of the exchanged Cu as the active site during the catalytic reaction using X-ray absorption spectroscopic (XAS) techniques, mainly XANES and EXAFS. Our experiment demonstrated the existence of a redox mechanism which involves cyclic switching of the oxidation states between Cu(II) and Cu(I) in an oxygen-rich gas mixture under elevated temperature. We also observed the coordination structural change of copper ion in ZSM-5 accompanying the change of oxidation state. A correlation between cuprous ion concentration and catalytic activity was found in NO SCR by propene. The impact of another two hydrocarbons, propane and methane, on the copper redox behavior also appears to correlate to catalytic activities in the respective mixtures. Discussions on the nature of the active sites and the mechanism of SCR are presented based on the XAS data analysis. The similarity and difference of the physical properties of copper ion between NO catalytic decomposition and NO SCR are also discussed.