Speciation of Cu Cations in Cu-CHA Catalysts for NH3-SCR: Effects of SiO2/AlO3 Ratio and Cu-Loading Investigated by Transient Response Methods

We develop an experimental protocol to quantify the speciation of Cu ions (ZCuIIOH and Z2CuII) in Cu-CHA catalysts for the NH3 Selective Catalytic Reduction of NOx (NH3-SCR). Toward this end, we performed four transient tests, namely H2-TPR, NO + NH3 TPR, NO2 adsorption + TPD, and NH3 adsorption + TPD, over two sets of Cu-CHA research catalysts characterized by different Cu contents (0-2.1% w/w) and SiO2/Al2O3 (SAR) ratios (10-25). Preliminary H2-TPR tests on the samples with the extreme SAR and Cu loading values were used to identify the variability range of the fractions of ZCuIIOH and Z2CuII species in these catalysts. The ZCuIIOH fraction was found to vary between 0.55 (at Cu/Al = 0.11) and 0.79 (at Cu/Al = 0.29). NO+NH3 TPR runs demonstrated that the NO + NH3 mixture is a much stronger reducing agent than H2: full reduction of all the Cu was obtained already at lower temperature, and differences in the reducibility of ZCuIIOH and Z2CuII were strongly attenuated. Both the integral NO consumption and the integral N2 release were found to be effective estimators of the reducible Cu in Cu-CHA, matching the total Cu from ICP measurements. NO2 adsorption + TPD tests pointed out that NO2 is adsorbed in the form of nitrates on ZCuIIOH ions only, the nitrates storage capacity being therefore dependent on SAR and Cu loading: on increasing both parameters, the amount of stored NOx increased, as well as their stability. Both the NO released during isothermal NO2 adsorption and the NO2 released during the following TPD can be used to directly estimate the number of ZCuIIOH ions in Cu-CHA. Finally, NH3-TPD provided information on the acid sites in the Cu-CHA samples. From the NH3 stored on Lewis sites, it was possible to evaluate the number of NH3 molecules coordinated to each Cu atom: a decrease of the NH3/Cu ratio on increasing both SAR and Cu content was observed. This behavior is explained by the changes in the distribution of ZCuIIOH and Z2CuII sites in Cu-CHA, as a result of varying the Cu/Al ratio. In accordance with literature results, we found that Cu ions are able to ligate either 3 (ZCuIIOH) or 4 (Z2CuII) NH3 molecules, when gaseous NH3 is present, the NH3/Cu ratios estimated from our experiments falling close to this range. When only preadsorbed NH3 was present, however, (no gaseous ammonia), the NH3/Cu ratio dropped to either 1 (ZCuIIOH) or 2 (Z2CuII). On the basis of these elements, NH3 TPD can also be used to quantify the two Cu species in Cu-CHA. We recommend however the more straightforward approach based on (i) NO + NH3 TPR (for direct quantification of the overall reducible Cu) and (ii) NO2 TPD (for direct quantification of the ZCuIIOH species).