Metal Loading Affects the Proton Transport Properties and the Reaction Monitoring Performance of Fe-ZSM-5 and Cu-ZSM-5 in NH3-SCR

Metal-promoted zeolites are widely applied as catalysts for the selective catalytic reduction of nitrogen oxide by NH 3 (NH 3 -SCR). Here, we describe a systematic study on the proton transport properties of Fe-ZSM-5 and Cu-ZSM-5 NH 3 -SCR catalysts with different metal loadings. By means of in situ impedance spectroscopy (IS), we revealed the different influence of exchanged Fe and Cu on the NH 3 -supported proton transport in different atmospheres (N 2 and NO/O 2 ) and, consequently, on the monitoring of the NH 3 -SCR reaction using the zeolite catalysts directly as sensors. On the one hand, the mobility of adsorbed NH 3 species as proton carriers, determined by the NH 3 –zeolite interaction, was influenced differently by Fe and Cu and their loadings. While the increase of Fe loading weakened slightly the NH 3 –zeolite interaction, a higher Cu loading enhanced significantly the NH 3 –zeolite interaction. On the other hand, highly proton-conducting ammonium ion (NH 4 + ) intermediates, as unveiled by IS combined with DRIFTS, formed on Cu-ZSM-5 due to NO–NH 3 coadsorption and influenced the proton transport properties significantly in a complex manner. This systematic study thus not only clarifies several mechanistic aspects for NH 3 -SCR monitoring using zeolite catalyst directly as sensor but also provides new perspectives to understand mechanistically the NH 3 -SCR reaction over Fe– and Cu–zeolite catalysts.