In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH 3 -SCR during temperature-programmed reduction in NO/NH 3

Ammonia-mediated selective catalytic reduction (NH 3 -SCR) using Cu-exchanged chabazite zeolites as catalysts is one of the leading technologies for NO x removal from exhaust gases, with Cu II /Cu I redox cycles being the basis of the catalytic reaction. The amount of Cu II ions reduced by NO/NH 3 can be quantified by the consumption of NO during temperature-programmed reduction experiments (NO-TPR). In this article, we show the capabilities of in situ X-ray absorption near-edge spectroscopy (XANES), coupled with multivariate curve resolution (MCR) and principal component analysis (PCA) methods, in following Cu II /Cu I speciation during reduction in NO/NH 3 after oxidation in NO/O 2 at 50 °C on samples with different copper loading and pretreatment conditions. Our XANES results show that during the NO/NH 3 ramp Cu II ions are fully reduced to Cu I in the 50–290 °C range. The number of species involved in the process, their XANES spectra and their concentration profiles as a function of the temperature were obtained by MCR and PCA. Mixed ligand ammonia solvated complexes [Cu II (NH 3 ) 3 (X)] + (X = OH − /O − or NO 3 − ) are present at the beginning of the experiment, and are transformed into mobile [Cu I (NH 3 ) 2 ] + complexes: these complexes lose an NH 3 ligand and become framework-coordinated above 200 °C. In the process, multiple Cu II /Cu I reduction events are observed: the first one around 130 °C is identified with the reduction of [Cu II (NH 3 ) 3 (OH/O)] + moieties, while the second one occurs around 220–240 °C and is associated with the reduction of the ammonia-solvated Cu-NO 3 − species. The nitrate concentration in the catalysts is found to be dependent on the zeolite Cu loading and on the applied pretreatment conditions. Ammonia solvation increases the number of Cu II sites available for the formation of nitrates, as confirmed by infrared spectroscopy.