(141b) In Situ and Operando X-Ray Absorption Spectroscopy On Cu-Zeolite Catalysts for Fast SCR by Ammonia
NOx removal by selective catalytic reduction (SCR) has been studied extensively by the exhaust aftertreatment community. Cu-zeolites, especially Cu on ZSM-5, have been found to be particularly well suited for low temperature SCR . Despite a large body of research on Cu-ZSM-5, the debate about the active Cu species is still ongoing. Our recent measurements of the Cu X-ray Absorption Near Edge Spectra (XANES) in a custom-built, operando reactor using glassy carbon tubes have produced intriguing results. In situ XANES spectra were collected over a range of Cu-ZSM-5 catalysts, from loadings of 1-7 wt.% Cu, under NO oxidation, dry SCR, wet SCR and pure ammonia gas conditions and a 2.7 wt.% Cu was tested operando under wet and dry NO oxidation and wet SCR reaction conditions. During NO oxidation, the Cu was split between isolated Cu2+ species and Cu2+ in the form of bulk-like CuO, present in small amounts at lower loadings and becoming the dominant species at high loadings where the Cu over-exchange was around 300%. Nevertheless, the Cu was fully oxidized. Upon the introduction of the reducing agent, ammonia, for both dry and wet SCR conditions, a significant portion of Cu reduced to its 1+ oxidation state. Similar to the Cu2+, however, this 1+ species was an isolated Cu1+, entirely different from the Cu1+ found in bulk Cu2O. In fact, there was never any identifiable amount of Cu1+ in a bulk-like Cu2O form during any phase of our experiments. Lower Cu wt.% loadings (50-100% exchanged) had higher proportions of reduced Cu where the lowest Cu loading showed the presence of very little Cu2+. The amount of reduced Cu1+ ranged from 5-70% going from high to low Cu loadings. Furthermore, at low weight loadings, some Cu2+ was identified as Cu coordinated with NH3 where the higher wt.% Cu samples appeared to be coordinated with H2O instead. Implications of the effects of the presence of reduced states of Cu on SCR and NO oxidation kinetics will be discussed.
 S. Brandenberger, O. Kröcher, A. Tissler, R. Althoff. Catalysis Reviews, 50:4, 492-531, 2008