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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3We report a mechanistic DRIFTS in-situ study of NO 2 , NO + O 2 and NO adsorption on a 4 commercial Cu-CHA catalyst for NH 3 -SCR of NOx. Both pre-reduced and pre-oxidized catalyst 5 samples were investigated with the aim to clarify mechanistic aspects of the NO oxidation to NO 2 6 as a preliminary step towards the study of the Standard SCR reaction mechanism at low 7 temperatures. Nitrosonium cations (NO + , N oxidation state = +3) were identified as key surface 8 intermediates in the process of NO (+2) oxidation to NO 2 (+4) and nitrates (+5). While NO + and 9 nitrates were formed simultaneously upon catalyst exposure to NO 2 , nitrates evolved consecutively 10 to NO + when the catalyst was exposed to NO + O 2 , suggesting that nitrite-like species, and not NO 2 , 11 are formed as the primary products of the NO oxidative activation over Cu-CHA. Upon catalyst 12 exposure to NO only, i.e. in the absence of gaseous O 2 , NO + and then nitrates were formed on a 13 preoxidized sample but not on a prereduced one, which demonstrates the red-ox nature of the NO 14 oxidation mechanism. The negative effect of H 2 O on NO + and nitrates formation was also clearly 15 established. Assuming small Cu clusters, in the form of Cu dimers, as the active sites for NO 16 oxidation to NO 2 , we propose a mechanism which reconciles all the experimental observations. In 17 particular, we show that such a mechanism also explains the observed kinetic effects of H 2 O, O 2 18 and NO 2 on the NO oxidation activity of the investigated Cu zeolite catalyst. 19 20 21

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