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

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We use dry CO oxidation to probe CuII dimers in Cu−CHA catalysts in the presence of ammonia. Samples with different Cu/Al and Si/Al ratios were exposed to CO in He at 200 °C after either one of two catalyst pre‐treatments: (i) pre‐oxidization in 8 % O2/He at 550 °C for 1 hour; (ii) pre‐oxidation in 8 % O2/He at 550 °C for 1 hour followed by NH3 saturation at 200 °C in O2. Pre‐adsorbed NH3 was necessary to observe a significant CO2 evolution, the rate of CO2 formation being second order in the CuIIOH fraction of the CuII sites. This is direct evidence that NH3, not involved in the CO oxidation chemistry, favours the formation of CuII dimers with removable oxygen by ligating with and mobilizing isolated CuIIOH ions, otherwise inactive. These findings are relevant for the low‐temperature NH3−SCR redox mechanism over Cu−CHA, in which CuII dimers may play a role in parallel or in alternative to the isolated CuII sites so far proposed for the reduction half cycle of standard SCR.

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Cu2+ cations complexed by linear polyamines have been studied as structure-directing

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