In situ X-ray absorption study of Cu species in Cu-CHA catalysts for NH3-SCR during temperature-programmed reduction in NO/NH3

Ammonia-mediated selective catalytic reduction (NH3-SCR) using Cu-exchanged chabazite zeolites as catalysts is one of the leading technologies for NOx 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/NH3 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/NH3 after oxidation in NO/O-2 at 50 degrees C on samples with different copper loading and pretreatment conditions. Our XANES results show that during the NO/NH3 ramp Cu-II ions are fully reduced to Cu-I in the 50-290 degrees 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(NH3)(3)(X)](+) (X = OH-/O- or NO3-) are present at the beginning of the experiment, and are transformed into mobile [Cu-I(NH3)(2)](+) complexes: these complexes lose an NH3 ligand and become framework-coordinated above 200 degrees C. In the process, multiple Cu-II/Cu-I reduction events are observed: the first one around 130 degrees C is identified with the reduction of [Cu-II(NH3)(3)(OH/O)](+) moieties, while the second one occurs around 220-240 degrees C and is associated with the reduction of the ammonia-solvated Cu-NO3- 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.

Automated summary

This study investigated the oxidation-reduction states of copper ions during NO/NH3 reduction process using techniques such as X-ray absorption near-edge spectroscopy, with results showing complete reduction of Cu(II) to Cu(I) between 50-290°C. Multiple Cu(II)/Cu(I) reduction events were observed, with the first one associated with Cu(II) hydroxy/ammonia complexes and the second one with ammonia-solvated Cu-nitrate species. Nitrate concentration in the catalysts was found to depend on zeolite Cu loading and pretreatment conditions, with ammonia solvation increasing the number of available Cu(II) sites for nitrate formation.