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

Abstract

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).

Low‐T CO Oxidation over Cu−CHA Catalysts in Presence of NH3: Probing the Mobility of CuII Ions and the Role of Multinuclear CuII Species

“…Recently, we proposed an experimental protocol which allows to quantify directly the number of ZCu II OH and Z 2 Cu II ions by applying simple transient response methods based on the dry Cu II reduction by NO + NH 3 and on the nitrates storage/desorption via NO 2 adsorption + TPD. [4] For several different CuÀ CHA samples, such an approach gave estimates of the two Cu species consistent with those generated by H 2 À TPR and NH 3 adsorption/desorption tests, i. e. the established and most popular techniques for the Cu speciation, which however may require complex deconvolutions of the experimental signals. [1][2][3][4][5] Although ZCu II OH and Z 2 Cu II ions are generally regarded as the active sites for the standard SCR mechanism, some literature works considered the possibility that other Cu species, like dimeric copper-oxo species or CuOx nanoclusters, could play a role in redox catalytic mechanisms over Cu-zeolites.…”

“…[4] For several different CuÀ CHA samples, such an approach gave estimates of the two Cu species consistent with those generated by H 2 À TPR and NH 3 adsorption/desorption tests, i. e. the established and most popular techniques for the Cu speciation, which however may require complex deconvolutions of the experimental signals. [1][2][3][4][5] Although ZCu II OH and Z 2 Cu II ions are generally regarded as the active sites for the standard SCR mechanism, some literature works considered the possibility that other Cu species, like dimeric copper-oxo species or CuOx nanoclusters, could play a role in redox catalytic mechanisms over Cu-zeolites. [6][7][8][9][10] Da Costa et al [6] studied a series of CuÀ ZSM5 samples with varying Cu/Al ratios and reported evidence for the involvement of Cu II dimers with one bridging extra framework oxygen atom in the direct NO decomposition to N 2 .…”

“…Indeed, NH 3 ligands confer mobility to Cu II ions, forming Cu II À NH 3 complexes that are completely detached from the zeolite framework. [1,3,4,5,11] In O 2 -containing mixtures, isolated Cu II ions with high mobility may eventually combine to form multinuclear Cu species with bridging extra framework (and removable) oxygen atoms. To demonstrate this hypothesis, however, a probe reaction which can follow the Cu reduction unambiguously is critically necessary.…”

“…More experimental details are given in Section S1 of the Supporting Information, while detailed characterization data for the same samples are provided in. [4] Three experimental protocols were adopted to examine the role of NH 3 in the transient reduction of Cu II sites by CO (CO titration), namely: In all protocols, the final step was the isothermal Cu reduction by a NO + NH 3 (500 ppm each) mixture in He at 200°C: by measuring the NO consumption and the N 2 production (NO : N 2 : Cu I = 1 : 1 : 1) we thus estimated the number of Cu sites which were not reduced by CO in the previous step, [4] closing the Cu II balance. Further details about the experimental protocols are provided in Section S2 of the Supporting Information.…”

An Aging Model of NH3 Storage Sites for Predicting Kinetics of NH3 Adsorption, Desorption and Oxidation over Hydrothermally Aged Cu-Chabazite

“…The fractional coverages of Z 2 W, ZCu1, ZCu2 and ZB for the degreened sample were, therefore, 0.60, 0.21, 0.12 and 0.07, respectively. The stoichiometry between a certain active site and NH 3 [18,47,56] was lumped into to the reaction rate parameters, similar to the prior literature [30,44]. Figure 5 shows the NH 3 concentration at the outlet of the reactor during NH 3 adsorption, isothermal desorption and TPD along with the fractional coverages obtained using the 4-site transient kinetic model for the degreened Cu-CHA.…”