Transient CO Oxidation as a Versatile Technique to Investigate Cu2+ Titration, Speciation and Sites Hydrolysis on Cu–CHA Catalysts: The Cu Loading Effect

The investigation of the ZCu2+(OH)− and Z2Cu2+ ions modifications during NH3-SCR on Cu–CHA catalysts is a key aspect to clarify the still-debated low-T redox SCR mechanism. In previous works, the dry transient CO oxidation protocol has been employed to identify the generation of dinuclear Cu2+ structures under conditions representative of the low-T SCR–RHC: NH3 solvation promotes the inter-cage mobility and coupling of ZCu2+(OH)−, acting as the catalytic centers for the CO oxidation process, while Z2Cu2+ results inactive. Herein the same protocol, with pre-stored NH3, has been applied to a set of Cu–CHA catalysts with variable Cu loading (0.7–2.4% w/w) but fixed Si/Al: an increasing Cu content produced a net positive effect on the CO2 production, coherent with a growing ZCu2+(OH)− population, while a further enhancement was observed in the presence of H2O. The analysis of the integral CO2 production enabled to predict the maximum CO conversion, corresponding to the titration of the whole ZCu2+(OH)− content for each catalyst under dry condition, verifying the initial mechanism. Conversely, in the presence of water, the analysis evaluated an asymptotic titration of the total catalyst Cu2+ contents. This finding permits to generalize a recent study where combined TRMs, DFT and FTIR were used to probe the complete reversible Cu2+ sites hydrolysis and pairing in the presence of H2O and NH3, thus activating the participation of Z2Cu2+ species, too. These results also highlight the versatility and effectiveness of the CO oxidation protocol as a multi-purpose technique to study the Cu2+ ions in Cu–CHA catalysts.


Introduction
The prominent deNOx performances of Cu-promoted zeolites catalysts, in particular for the chabazite framework (Cu-CHA), in parallel to their resistance to harsh hydrothermal conditions, scarce N 2 O selectivity, and robust hydrocarbon poisoning resistance, have posed them as ideal materials for NH 3 -Selective Catalytic Reduction (NH 3 -SCR) applications to effectively control the NOx emissions in lean-burn heavy-duty engines exhausts [1][2][3][4][5]. However, the worldwide stringent emission regulation requirements during the cold start phase promoted extensive research to disclose the SCR low temperature (low-T) chemistry on Cu-CHA, in order to improve the technology abatement efficiency. The presence of two isolated Cu 2+ cations in the zeolite framework have been extensively probed, namely ZCu 2+ (OH) − and Z 2 Cu 2+ , characterized by different zeolite crystalline position, coordination (Z) and thermodynamic stability [6][7][8][9]. These ions have been recognized as the low-T SCR active sites precursors, which proceed following a redox chemistry where copper is cyclically reduced and oxidized (Cu 2+ ↔ Cu + ) during the two respective half cycles (RHC and OHC) depending on the dynamic operative conditions [10][11][12]. However, their nature as single site or dual-site based mechanisms on the copper ions remains debated, alongside with the evolution of two different Cu 2+ ions in each scheme. During realistic low-T SCR conditions, the Cu cations are solvated by 1 3 NH 3 that detaches them from the zeolite framework, forming respectively Cu 2+ (OH) − (NH 3 ) 3 and Cu 2+ (NH 3 ) 4 species. Their formation has been initially proposed by density functional theory (DFT) and successively experimentally proven relying on NH 3 -TPD analysis: upon full catalyst saturation, Z 2 Cu 2+ resulted able to coordinate up to four NH 3 ligands, while three could be bound to ZCu 2+ (OH) − [6,9]. The mobility granted by the NH 3 ligands confer to the system similarities to an homogeneous catalysis scenario [6,9,[13][14][15]. The kinetic analysis for the low-T RHC over several Cu-CHA catalysts suggests a quadratic dependance on the overall oxidized copper content, indeed implying the formation of Cu 2+ pairs originating from both the originally isolated cations [16]. Conversely, DFT ascribed an intracage confined mobility for the Cu 2+ (NH 3 ) 4 species, due to the electrostatic tethering, whereas only the coupling of two Cu 2+ (OH) − (NH 3 ) 3 resulted favored [17]. A recent work from some of us, consisting in an integrated analysis combining transient response methods (TRMs), DFT and insitu FTIR over a Cu-CHA catalyst sample, has rationalized these experimental findings by suggesting the activation of an NH 3 -assisted hydrolysis which reversibly interconverts all the Cu cations into Cu 2+ (OH) − (NH 3 ) 3 [18].
In this context, the present work extends the investigation of the Cu 2+ multinuclear structures formation and the Cu 2+ sites hydrolysis to a broader range of Cu-CHA catalysts, by means of the transient CO oxidation protocol. Initially proposed as a probe reaction for copper dimers in Cu-ZSM-5, its application has been carried over to Pd and Cu promoted chabazites [19][20][21]. Accordingly, this simple method has been employed in recently published works from us to unravel the Cu nuclearity under conditions representative of the low-T SCR-RHC [18,22,23]. The CO to CO 2 oxidation indeed requires a two-electron exchange and an oxygen donor species, and the proposed mechanism involves the participation of an NH 3 -solvated ZCu 2+ (OH) − pair, as a two-proximate or dinuclear complex in a single cage. This chemistry results affected by copper nuclearity modifications or by ZCu 2+ (OH) − ions evolution and in fact enabled to detect clear promotions in the carbon dioxide formation in the presence of NH 3 pre-stored on the catalyst under dry conditions, in agreement with its crucial role as cation mobilizing agent, and with both NH 3 and water, following the aforementioned sites hydrolysis [18,22,23]. Its application herein confirmed the concurrent positive effects of an increasing catalyst Cu content and of the presence of water, as both promote the number of catalyst active sites [18,22]. CO 2 production kinetic analyses unveiled the involvement of different Cu 2+ site populations in the dimer formation, or Cu 2+ pairing, formation, depending on the experimental conditions. The original ZCu 2+ (OH) − resulted the only active species under dry conditions, confirming the proposed CO oxidation mechanism, while the whole copper content was found to be active in the presence of water. Such coherent results permit to generalize the complete site hydrolysis and Cu 2+ coupling occurrence under the presence of water and NH 3 , which are indeed both present under realistic SCR low-T conditions, regardless of the Cu content of the catalysts and in agreement with other works [16,18]. Furthermore, the work emphasizes the CO oxidation ductility to probe different phenomena and highlights it application as a valid, simple alternative titration method for the Cu ions in Cu-CHA catalysts.

Cu-CHA Powder Samples Characterization and Experimental Methods
The experimental activity of this study was carried out in collaboration with Johnson Matthey on a set of three powder research Cu-promoted chabazite (Cu-CHA) catalysts, characterized by a variable copper loading (0.7, 1.7 and 2.4%) and a constant Si/Al ratio equal to 12.5 evaluated by ICP-MS analysis. As shown in previous activities, a direct correspondence was found between the total Cu content and the reducible Cu 2+ [9,16,24]. Each fresh powder sample has been sieved and heated in air at 120 °C to remove the moisture content and obtain a controlled average particle sized of 90 microns. 32 mg were then diluted with cordierite, similarly pre-treated, up to 130 mg and located in a vertical quartz microflow reactor tube, between two inert quartz wool layers and in direct contact with a K-type thermocouple. The mixing of the gas stream was favored by quartz grains which filled the upstream and downstream sections of the catalyst bed. The reactor was positioned in an electrical oven and connected to the inlet and outlet section of a dedicated experimental apparatus already described elsewhere [9,18,23,24]. Briefly, the reactant concentrations were regulated by mass flow controllers (Brooks Instruments), with Ar and He respectively employed as tracer and dilutant. A pair of 6-way fast valves enabled feed step changes. to ensure complete catalyst cleaning and Cu oxidation, followed by a cooling step in oxygen to reach the test thermal condition. ZCu 2+ (OH) − quantification: spectroscopic studies have shown the NO 2 molecule ability to be adsorbed on ZCu 2+ (OH) − ions only, following a disproportionation mechanism and forming stable surface nitrates, which can decompose during a temperature ramp releasing NO 2 with a 1:1 stoichiometry [25][26][27]. Therefore, to characterize the ion population per samples, it has been adopted a previously developed 3-step protocol composed by an isothermal NO 2 adsorption and desorption phase at 150 °C, followed by a temperature programmed desorption (TPD), considering a total flow of 450,000 cm 3 /h/g cat (STP) [9,18]. In accordance with the proposed mechanism, the integration of the nitrogen dioxide chemical desorption enabled a straightforward sites evaluation. The integral and fractional ZCu 2+ (OH) − ions, alongside with the Cu 2+ content per sample, have been reported in the supporting information (Table SI.1). The residual Cu 2+ fraction is herein assigned to the Z 2 Cu 2+ ions. Indeed, a similar batch of Cu-CHA catalysts has been previously characterized and the combined outcome of UV-Vis and TRMs converged to identify the predominant presence of two cation populations (ZCu 2+ (OH) − and Z 2 Cu 2+ ), with negligible CuOx clusters or unexchanged Cu [9]. The NO 2 TPD of Fig.SI1 depicted a positive Cu loading effect, as known by both DFT analysis and transient tests [6,9]. A further increment for the 2.4% Cu fresh with respect to its conditioned counterpart was observed, this has been reconciled with the onset of a mild hydrothermal aging which activated an irreversible site transformation with the conversion of ZCu 2+ (OH) − into Z 2 Cu 2+ , without altering the overall Cu 2+ content [28,29].

Transient Isothermal CO Oxidation -Experimental Protocol and Kinetic Analysis
This protocol was adopted to assess the effect of the catalyst formulation and water on the multinuclear Cu species formation, utilizing at a constant total flow rate of 266,250 cm 3 /h/ g cat (STP) and an operative temperature of 200 °C. Three main steps were present: (i) NH 3 adsorption till saturation to ensure solvation and mobility to the Cu ions, (ii) 90 min Cu reduction by CO with the production of CO 2 , (iii) NO + NH 3 co-feed to reduce the remaining Cu 2+ and verify the Cu material balance. Further information on the experimental protocol was reported in earlier works [18,22,23]. The protocol has been replicated under wet conditions and a 5% v/v of water flux was constantly fed throughout the experimental steps, starting from the end of the cooling phase.
To corroborate the proposed CO oxidation chemistry, several kinetic models have been herein incorporated. In accordance with previous experimental findings, under the hypothesis of differential reactor conditions due to the scarce CO conversion, a quadratic dependance on the oxidized Cu content was proposed for the carbon dioxide rate expression [22]. Time integration led to the following equation (Eq. 1), where [Cu 2+ ] 0 corresponds to the initial copper ion population active in the CO oxidation catalysis, while k app is the apparent 2° order rate constant [18]: Eq. 1 has been successfully implemented to describe the time evolution of the CO 2 integral production under both dry and wet conditions in a previous work [18]. The maximum theoretical CO 2 conversion can be estimated by considering the asymptotic limit (t → ∞) of Eq. 1 ([Cu 2+ ] 0 /2), thus this has been labeled as the asymptotic approach. The active sites involved in the process can be estimated by accounting for twice such an asymptote.
A second order kinetic analysis of dry CO oxidation data on an early Cu-CHA catalysts set was effectively realized, demonstrating the dual-site mechanism nature on ZCu 2+ (OH) − ions. Accounting for the same assumptions just adopted and defining σ as the fraction of residual unreduced Cu 2+ ions (Eq.2), a linear relation able to describe the experimental data was derived (Eq. 3). More information for its derivation can be found elsewhere [22].
These two expressions (Eq. 2 + Eq. 3) were used to propose a further linearized expression for the integral CO 2 production, therefore labeled as the linearized approach: The linear fit of the integral experimental data, according to Eq. 4, permitted an alternative straightforward quantification of [Cu 2+ ] 0 using the intercept (1/a, a = 1/[Cu 2+ ] 0 ).
The dry and wet CO 2 productions evaluated over the 1.7% Cu sample, alongside with the integral analysis applying Eq. 1, already published, have been herein integrated to emphasize the effect of the Cu loading and further analyzed with additional kinetic models to confirm the mechanistic understanding [18]. (1)

Transient CO Oxidation -Cu Loading and Water Effects
The CO to CO 2 oxidation process, as anticipated in the Introduction section, requires the proximity of two reducible copper cations to satisfy the 2-electron exchange requirement. However, the metal ions would not be able to participate in such a reaction, as initially isolated in each cage and coordinated to the crystalline framework. In a realistic low-T reacting environment NH 3 is present and solvates them forming the aforemenioned Cu 2+ (OH) − (NH 3 ) 3 and Cu 2+ (NH 3 ) 4 complexes, now characterized by greater mobility with respect to their cage-confined counterparts [6,9,13,15]. The existence of a thermodynamic driving force has been probed by DFT analysis, which favors the exergonic pairing of two adjacent Cu 2+ (OH) − (NH 3 ) 3 in a cage, forming a two-proximate structure called Two-P [18]. This complex, akin to a Cu 2+ dinuclear species, acts as the CO oxidation active catalytic center, supplying the required oxygen atom for the carbon dioxide formation. The global stoichiometry comprehends two main events, which can be condensed in the following reactions (Eqs. 5, 6) [18,23]: Therefore, the process chemistry permits to use the CO 2 release to directly probe the formation of multinuclear Cu 2+ species, originating from the pairing of two isolated ZCu 2+ (OH) − , as supported by recent UV-Vis analysis [21]. Based on these considerations, dry and wet CO oxidation protocols have been applied to the variable Cu loading Cu-CHA samples in the presence of a NH 3 saturation step, prior to the CO feed, to guarantee the Cu 2+ mobility and pairing. The transient CO 2 release for both cases is illustrated in Fig. 1.
Under dry conditions, a net positive Cu loading effect was observed on the carbon dioxide formation as depicted in Fig. 1A. Indeed a growing copper content determined a rise in the ZCu 2+ (OH) − population, as reported in Table  SI.1, thus increasing the active centers availability for the process, in agreement with the expected proposed chemistry. For the 0.7% Cu sample an integral CO 2 production of 0.35 µmol was evaluated, as shown in Table 1, which resulted more than fivefold greater for the highest loading 2.4% Cu sample. Similarly to the NO 2 TPD comparison reported in Fig. SI1, an additional carbon dioxide increase was observed for the 2.4% Cu fresh with respect to the conditioned sample. The conditioning pre-treatment may be regarded as a mild hydrothermal aging process that activates an irreversible ZCu 2+ (OH) − decrease, as discussed   Table 1), assessed as twice the integral CO 2 production, with respect to the catalyst cations content [22]. As shown, such a fraction showed a minimum (41%) in correspondence to the 0.7% Cu sample, while the upper limit was found on the 2.4% Cu fresh sample (63%). Indeed, the 1.7% and 2.4% Cu catalysts presented a similar reduction extent (57-58%), despite the different integral CO 2 release, coherent with the dual-site nature of the process whereby catalysts with similar ZCu 2+ (OH) − fractions should exhibit similar relative reduction extents. Noteworthily, the fraction assessed over the 0.7% Cu sample results coherent with the one evaluated for a similar catalyst despite the lower loading (42%-16 mg) as reported in a previous work, further confirming the dependence of the CO 2 production on the relative ZCu 2+ (OH) − population [22]. To corroborate the Cu 2+ reduction by CO, and close the metal ions balance, an NO + NH 3 reduction step has been performed successive to the CO exposure, as the ability of the two reactants to fully convert Cu 2+ to Cu + is well-known in the literature [9, 10, 16-18, 24, 28]. The relative reduction extent during the two phases (Cu 2+ red. -CO oxi., Cu 2+ red. -NO + NH 3 red.) has been evaluated comparing twice the CO 2 release and the average NO consumption/N 2 production to the overall catalyst Cu content (Cu 2+ :CO 2 = 1:2; Cu 2+ :NO:N 2 = 1:1:1), similarly to a recently published work [9, 10, 16-18, 23, 24, 28]. Table SI.2 reports the assessed values, alongside with the overall Cu 2+ balance as the sum of the two contributions: a close to 100% reduction extent is obtained over all catalysts, thus validating the CO capacity to interact with the oxidized Cu following a dual-site mechanism.
The same protocol has been replicated in the presence of a constant water feed on the three variable Cu loading catalysts, and the transient carbon dioxide profiles are shown in Fig. 1B. The experimental data underlined a clear favourable H 2 O impact on the CO 2 formation, shared across the three considered samples. A quantitative analysis reported in Table 1 certified this enhancement in the CO 2 production, corresponding to values ranging from twice up to almost three times higher than their dry counterparts.
Such a substantial effect was firstly observed and reported for the 1.7% Cu sample and was rationalized by a change in the amount of sites capable to catalyse the CO 2 formation. The evidence collected by the application of simple TRMs, namely the dry/wet CO oxidation and SCR-RHC protocols, in conjunction with first-principles calculations of DFT, ab initio molecular dynamics (AIMD) and in situ FTIR spectroscopy, enabled to infer the following. In the presence of pre-stored NH 3 and water, directly fed or produced in-situ (e.g. during the RHC), a complete reversible site hydrolysis occurred, with the conversion of the previously inactive Cu 2+ (NH 3 ) 4 species into Cu 2+ (OH) (NH 3 ) 3 (Eq. 7) [18]. A similar conclusion arose analyzing the NH 3 -TPD successive to an ammonia adsorption step in the presence of water over a variety of Cu-CHA catalysts with different SAR and Cu loadings [30].
This phenomena, combined with the pairing process (Eq. 5) and the presence of Two-P scavenging reactions (e.g. RHC or CO oxidation), enables a thermodynamically and kinetically favoured Cu 2+ pair mediated reduction pathway where all the copper content acts as a single site population in dimer-like structures. This interpretation can be tentatively extended to the other catalysts included in this research (0.7 and 2.4% Cu), reconciling the growing wet CO 2 production with a site transformation. Under the preliminary hypothesis of complete cations conversion, we can evaluate an additional relative index for the quantitative results comparison, the Cu 2+ reduction extent (Cu 2+ red. in Table 1), akin to the previous one applied in the absence of water, now considering twice the CO 2 production with respect to the overall catalyst reducible Cu 2+ , listed in Table  SI.1. All the samples displayed higher fractions with respect to their dry counterpart, in agreement with the increased availability of catalytic sites. Moreover, an evident Cu loading positive effect was detected from the CO 2 profiles observed under wet conditions, and the quantitative analysis showed again how the 0.7% Cu sample was associated with the lowest released (0.89 µmol), which increased more than four times with the 2.4% Cu. The favourable impact was observed also on the Cu 2+ reduction extent, which increased from 47 to 64% considering the lowest and mid loading samples, respectively, while an apparent negligible effect was detected for the 2.4% Cu. Akin to the dry experiments, the terminal NO + NH 3 step titrated a residual Cu 2+ fraction which complied well with the expected remaining quantity successive to the CO exposure, confirming the complete oxidized copper reduction also in the presence of water, over the three samples (Table SI.2). It is worthwhile mentioning how the ZCu 2+ (OH) − reduction extent for the wet data, if evaluated without accounting for an active site population change, would have led to fractions ≥ 100%, in contradiction with a continuous CO 2 release after the 90 min phase duration, for all the three considered catalysts.
According to the collected evidence, the feedwater was thus involved in the hydrolysis reaction only (Eq. 7), increasing the available Two-P structures and consequently enhancing the CO oxidation (Eq. 6). A negligible impact on the (Table SI.2) and the NH 3 capacity to solvate and mobilize the metal ions has been observed, in accordance with previously reported results [16,17].

Integral CO 2 Kinetic Analysis
The qualitative data analysis clearly showed an incomplete CO conversion at the considered experimental conditions, as CO 2 concentration never reached a stationary zero value for all the samples but rather monotonically decreased. This is reflective of the limited Cu 2+ cations reduction extent evaluated in the earlier section, where the highest assessed values were 63/64%. The intrinsically slow CO oxidation required the implementation of a kinetic model to predict the theoretical carbon dioxide release trend and, by extension, to estimate the population of the related active sites. Two kinetic models (Eqs. 1, 4), reported in the Materials and Methods section of this work as the asymptotic and linearized approach, have been adopted to describe the integral CO 2 time evolution. Their application to the experimental data in absence of water is illustrated in Fig. 2, while the fit details have been reported in Tables 2 and 3 and in Table SI. 3.
The asymptotic approach model accurately captured the integral carbon dioxide release, predicting an asymptotic trend closely approaching half of the catalysts ZCu 2+ (OH) − population as depicted in Fig. 2A [18]. Such findings confirm the involvement of these cations only in the CO oxidation mechanism in the absence of water, regardless of the catalyst Cu loading. The evaluation of the theoretical active ions in the CO oxidation (Table 2: [Cu 2+ ] 0 -asymptotic app.) by the application of Eq. 1 verified such considerations, assessing a close match with the number of sites quantified by NO 2 adsorption/TPD tests across all the samples. Figure 2B illustrates the application of the linearized approach as an alternative method based on linearization, herein applied in this context for the first time. By rearranging the CO 2 integral production data following Eq. 4, we clearly observed the expected trend, thus verifying the underlying hypothesis of a dual-site mechanism on Cu 2+  Comparison between the expected ZCu 2+ (OH) − content and the predicted maximum CO 2 production using the asymptotic and the linearization approach for the dry CO 2 integral production analysis of the different Cu-exchange samples  Table 3 Comparison between the expected Cu 2+ content and the predicted maximum CO 2 production using the asymptotic and linearization approach for the wet CO 2 integral production analysis of the different Cu-exchange samples sites. A simple linear fit permitted to evaluate the intercept and, consequently, the expected number of active sites, in a similar manner to the previous model. These estimates are also shown in Table 2 ([Cu 2+ ] 0 -linearized app.), illustrating a high degree of consistency with the quantification of the asymptotic approach and the expected ZCu 2+ (OH) − content, in line with the expected system stoichiometry.
The previous kinetic models were applied also to the wet data set, as shown in Fig. 3 (kinetic fit details in Tables 2 and 3 and Table SI.3). Like in the previous observations, the asymptotic approach described effectively the initial experimental trend (Fig. 3A). However, with respect to dry conditions, the asymptote was increased in the presence of water for all the samples, and was now coherent to half of the overall catalyst copper content, as reported in Table 3. This relevant finding thus confirmed the previously tentative proposal of complete Cu 2+ sites conversion due to the occurrence of a reversible hydrolysis, which resulted notably not affected by the Cu-CHA catalyst Cu loading, and was thus shared among the considered catalysts. A similar conclusion was reached by the application of the linearized approach: the data shown in Fig. 3B verified the validity of the linear relation in the presence of H 2 O regardless of the Cu loading. In addition, the evaluated population of active sites of Table 3 resulted in agreement with the involvement of the whole catalyst Cu 2+ content. These coherent findings permitted to clearly assess the central role of water, in the presence of NH 3 , as a facilitator to Cu sites conversion and pairing, and the negligible effect of the considered catalyst Cu loading. Furthermore, the cations quantification accuracy herein achieved by the two modelling approaches allows to propose the dry and wet CO oxidation as an alternative, simple but effective method for their titration.

2nd Order Kinetic Analysis
While the previous integral models were employed to assess the involvment of different Cu 2+ sites, depending on the catalyst formulation and on the reaction environment, a second order kinetic analysis has been further applied to unravel the kinetic features of the CO oxidation process [22]. Originally proposed to challenge the validity of the dual-site mechanism based on ZCu 2+ (OH) − in early dry CO oxidation tests, its use has been herein expanded. The different integral CO 2 production plots, elaborated according to Eq. 3 reported in the Materials and Methods section, are represented in Fig. 4.
All the four samples tested in absence of H 2 O followed the theoretical linear trend through the origin predicted by a second order rate law in ZCu 2+ (OH) − , as clearly evident in Fig. 4A. In the expression herein used, σ is defined as the residual Cu 2+ cations fraction at each time. Indeed, this evidence confirmed the dual-site nature of the CO oxidation mechanism and the sites quantification by NO 2 adsorption/ TPD tests. A simple linear fit permitted the estimation of the apparent 2nd order rate constant k app for each catalyst, whose values have been reported both in the panel and in Table 4. A Cu loading kinetic promotion was clearly detected: the 0.7% Cu sample resulted characterized by a value of 1.75 × 10 − 4 s − 1 , which almost doubled for the 2.4% Cu fresh, as representative of the rising ZCu 2+ (OH) − content. The comparable constants estimated for the 1.7 and 2.4% Cu catalysts was reflective of the similar cations contents for both, in agreement with the comparable reduction extent assessed early in the work.  Figure 4B shows the application of the same analysis to wet data, and the definition of σ accounts in this case for the prior finding of the complete Cu 2+ site hydrolysis for all catalysts. Following this approach, we retrieved also here a clear linear trend. The apparent 2nd rate constant for the 1.7 and 2.4% Cu were positively affected by the addition of water, as representative of faster kinetics due to the increased reactants available, namely the Cu 2+ cations. Again, a major Cu loading impact was observed ranging from the 0.7% to the 1.7% Cu sample, with a rate constant which almost doubled (Table 4), emblematic of the strong difference in the two catalysts copper contents, while only a minor further increment was detected for the 2.4% Cu.
To complete this kinetic analysis, an evaluation of the intrinsic second order rate constant was carried out employing the collective data here gathered. As k app was estimated hypothesizing a dual-site mechanism, it should depend linearly on the concentration of ZCu 2+ (OH) − and Cu 2+ ions, respectively under dry and wet conditions. Therefore, in the Table 4 below, the corresponding ratios have been quantified: the values were close in all cases, ranging between 0.4 and 0.3, with and without water, thus supporting the validity of such an analysis. Only the 0.7% Cu sample, under dry conditions, showed a different value, however, interestingly, once water was present it decreased to reach the previous reported range. While this may be representative of specific different catalyst kinetics, the authors do not exclude the possible impact of marginal experimental errors, which were magnified by the scarce ZCu 2+ (OH) − and CO 2 release present in absence of site hydrolysis for this specific sample. The rate constants herein assessed have been compared to the corresponding ones retrieved from the application of the previously discussed integral models of the integral CO 2 production. As shown in Table 4 last columns, all these values resulted  in a close match, unraveling the internal coherency and consistency of the analysis.

Conclusions
The identification of the Cu-CHA catalyst formulation effect (Cu loading: 0.7-2.4%) on CO titration experiments and the role of water in the multinuclear Cu 2+ complexes formation during the low temperature SCR reduction half cycle were analyzed in this research. The transient release of CO 2 during the carbon monoxide feed was associated to an NH 3 -solvated ZCu 2+ (OH) − pair mediated reduction pathway and enabled the indirect observation of such species presence. The main outcomes can be summarized as follows: • An increasing Cu content promoted the CO 2 formation, in agreement with a growing population of ZCu 2+ (OH)active complexes. The presence of 5% v/v water in the gas feed mixture further enhanced the carbon dioxide release and this effect was rationalized with the occurrence of an NH 3 -assisted hydrolysis, reversibly activating the inert Z 2 Cu 2 species by converting them into redox active ZCu 2+ (OH) -. This determined a growing availability of Cu 2+ pairs to catalyze the CO oxidation process. • Two approaches have been applied for the CO 2 integral analysis. Both provided an accurate description of the experimental trends, and the predicted asymptotic CO 2 production resulted in a close match with half of the ZCu 2+ (OH) − population of the respective catalysts, under dry conditions, in agreement with the expected CO oxidation mechanism. The asymptotic CO 2 productions changed in the presence of water, approaching half of the overall Cu 2+ population for each sample. This finding suggests the participation of all the copper ions in the CO oxidation process, implying the complete site hydrolysis regardless of the Cu loading. The accurate prediction of the Cu 2+ and ZCu 2+ (OH) − contents with such approaches also posed the CO oxidation protocol as an alternative, but effective, transient titration method. • A 2nd order kinetic analysis highlighted the validity of the quadratic dependency of the CO oxidation rate on ZCu 2+ (OH) − and Cu 2+ , respectively under dry and wet conditions. All the experimental data followed the predicted theoretical relation, with a Cu loading positive effect on the apparent rate constants. The results converged to propose a single intrinsic rate constant for all the catalyst samples regardless of the Cu loading, with and w.out water.