The H2O Effect on Cu Speciation in Cu-CHA-Catalysts for NH3-SCR Probed by NH3 Titration

The present work is focused on the effect of water on NH3 adsorption over Cu-CHA SCR catalysts. For this purpose, samples characterized by different SAR (SiO2/Al2O3) ratios and Cu loadings were studied under both dry and wet conditions. H2O adversely affects NH3 adsorption on Lewis acid sites (Cu ions) over all the tested catalysts, as indicated by the decreased NH3 desorption at low temperature during TPD. Interestingly, the NH3/Cu ratio, herein regarded as an index for the speciation of Cu cations, fell in the range of 3–4 (in the presence of gaseous NH3) or 1–2 (no gaseous NH3) in dry conditions, in line with the formation of different NH3-solvated Cu species (e.g., [CuII(NH3)4]2+ and [CuII(OH)(NH3)3]+ with gaseous NH3, [Z2CuII(NH3)2]2+ and [ZCuII(OH)(NH3)]+ without gaseous NH3). When H2O was fed to the system, on the contrary, the NH3/Cu ratio was always close to 3 (or 1), while the Brønsted acidity was slightly increased. These results are consistent both with competition between H2O and NH3 for adsorption on Lewis sites and with the hydrolysis of a fraction of Z2CuII species into ZCuIIOH.


Introduction
NH 3 /urea SCR is recognized worldwide as an effective technology in controlling automotive emissions to meet increasingly stricter NOx emission regulations [1,2]. Amongst state-of-the-art metal-promoted zeolite catalysts used in this process, Cu-exchanged chabazite (Cu-CHA) shows excellent activity and extraordinary hydrothermal stability over a broad temperature range under SCR conditions, triggering intensive research on the relationship between its unique structure and its DeNO x performance [3][4][5][6][7][8][9][10][11]. In this regard, the SiO 2 /Al 2 O 3 ratio (SAR) and the Cu loading content are important structure indexes, which could affect the nature of Cu cations existing in the catalyst framework and, based on their nature, location and redox properties, further impact the SCR activity [3][4][5][6][7][8][9]. Two Cu species are recognized to be present in the Cu-CHA catalyst: Z 2 Cu II (doubly coordinated to the framework) and ZCu II OH (balanced by one framework negative charge), whose relative ratio can be predicted based on Si/Al and Cu/Al ratios [5,6,11]. The proportion of such Cu species could also be affected by several factors, for example, hydrothermal aging [4,6,12] or hydrated/dehydrated conditions [5,6]. It was also proposed, based on DFT calculations, DRIFTS, XAS and NH 3 -TPD tests [4,5,8,13], that these two species are able to coordinate a different number of NH 3 molecules, thus influencing, in turn, the NH 3 storage capacity, a relevant parameter for the SCR process. In line with Paolucci et al. [5], our recent paper concluded that upon full catalyst saturation, the Z 2 Cu II species could coordinate up to four NH 3 molecules, while the hydroxylated Cu II site could bind only three molecules [8]. The NH 3 adsorption chemistry on the Lewis acid sites can be summarized by the following reactions [13]: Z 2 Cu II (s) + 4NH 3 (g) → Z 2 Cu II (NH 3 ) 4 (s) (1) ZCu II OH (s) + 3NH 3 (g) → ZCu II OH(NH 3 ) 3 (s) (2) When Cu-CHA catalysts were not fully saturated and Cu ions were not able to detach from the zeolite framework, however, we observed a stoichiometry different from reaction (1) and (2) for the two copper species: Z 2 Cu II ions adsorb two NH 3 molecules while only one NH 3 is bound to ZCu II OH, in agreement with Luo et al. [4].
NH 3 can also be adsorbed on the Brønsted acid sites typical of the zeolite framework, according to the adsorption chemistry reported below [12]: Herein, we carried out NH 3 adsorption + TPD tests under both dry and wet conditions over several Cu-CHA samples with different SARs and Cu loadings, which therefore contain distinct fractions of the two Cu species populations and different Brønsted sites [5,8]. The aim of the present investigation is to verify if H 2 O affects the Lewis and Brønsted acidity in chabazite-based materials and the possibility to use NH 3 -TPD as a probe technique for Cu speciation. In the literature, this method was implemented to characterize Cu-CHA catalysts in dry environments only in [8] or in the presence of water only in [4,12]. On the contrary, to assess the role of H 2 O in NH 3 adsorption, we compared the performance of different samples under both wet and dry conditions.

Results and Discussion
An example of an NH 3 adsorption/desorption run is provided in Figure 1A, depicting the three stages of the test, namely isothermal adsorption, isothermal desorption, and TPD. The run was performed after having pre-oxidized the sample at 550 • C. The aim was not only to evaluate the NH 3 storage capacity, but more specifically to identify the different sites present in the catalyst [8].
Catalysts 2021, 11, x FOR PEER REVIEW 2 of 11 full catalyst saturation, the Z2Cu II species could coordinate up to four NH3 molecules, while the hydroxylated Cu II site could bind only three molecules [8]. The NH3 adsorption chemistry on the Lewis acid sites can be summarized by the following reactions [13]: ZCu II OH (s) + 3NH3 (g) → ZCu II OH(NH3)3 (s) (2) When Cu-CHA catalysts were not fully saturated and Cu ions were not able to detach from the zeolite framework, however, we observed a stoichiometry different from reaction (1) and (2) for the two copper species: Z2Cu II ions adsorb two NH3 molecules while only one NH3 is bound to ZCu II OH, in agreement with Luo et al. [4].
NH3 can also be adsorbed on the Brønsted acid sites typical of the zeolite framework, according to the adsorption chemistry reported below [12]: (3) Herein, we carried out NH3 adsorption + TPD tests under both dry and wet conditions over several Cu-CHA samples with different SARs and Cu loadings, which therefore contain distinct fractions of the two Cu species populations and different Brønsted sites [5,8]. The aim of the present investigation is to verify if H2O affects the Lewis and Brønsted acidity in chabazite-based materials and the possibility to use NH3-TPD as a probe technique for Cu speciation. In the literature, this method was implemented to characterize Cu-CHA catalysts in dry environments only in [8] or in the presence of water only in [4,12]. On the contrary, to assess the role of H2O in NH3 adsorption, we compared the performance of different samples under both wet and dry conditions.

Results and Discussion
An example of an NH3 adsorption/desorption run is provided in Figure 1A, depicting the three stages of the test, namely isothermal adsorption, isothermal desorption, and TPD. The run was performed after having pre-oxidized the sample at 550 °C. The aim was not only to evaluate the NH3 storage capacity, but more specifically to identify the different sites present in the catalyst [8]. For this purpose, we analyze the TPD phase. As shown in Figure 1A, the temperature ramp featured a release of NH3 together with a small N2 peak (about 25 ppm). The integral of the N2 released (i.e., ≈ 0.68 µmol) is in line with the full reduction in Cu II ions (i.e., 4.3 µmol of Cu loaded) by adsorbed NH3 according to the following stoichiometries [13,14]: 6Cu II (s) + 2NH3 (g) → N2 (g) + 6H (s) + 6Cu I (s) For this purpose, we analyze the TPD phase. As shown in Figure 1A, the temperature ramp featured a release of NH 3 together with a small N 2 peak (about 25 ppm). The integral of the N 2 released (i.e., ≈0.68 µmol) is in line with the full reduction in Cu II ions (i.e., 4.3 µmol of Cu loaded) by adsorbed NH 3 according to the following stoichiometries [13,14]: 6Cu II OH (s) + 2NH 3 (g) → N 2 (g) + 6H 2 O (g) + 6Cu I (s) Catalysts 2021, 11, 759 3 of 11 Figure 1B displays the NH 3 and N 2 traces during the TPD, whereas the corresponding integral balances are reported in Table 1. The N balance error improves significantly if the production of nitrogen is taken into account. In line with this result, Borfecchia et al. showed by means of XAS spectroscopy and UV-vis-NIR experiments that at the end of the TPD, after NH 3 adsorption, the Cu sites in their Cu-CHA catalysts were almost completely reduced [13]. In line with the literature, the TPD curve in Figure 1B showed two distinct NH 3 release peaks, ascribed to energetically different populations of acidic sites [4,8,15] (Figure 2). The physical nature of the different sites was estimated via Gaussian deconvolution of the NH 3 -TPD trace, to which the contribution of N 2 according to the reactions discussed above (4) and (5) was added (green line in Figures 1B and 2). The N 2 trace was not measured in our previous investigation [8]; however, it is important to include this contribution to quantify more accurately the various Cu sites taking part in the SCR chemistry.
Catalysts 2021, 11, x FOR PEER REVIEW 3 of 11 6Cu II OH (s) + 2NH3 (g) → N2 (g) + 6H2O (g) + 6Cu I (s) (5) Figure 1B displays the NH3 and N2 traces during the TPD, whereas the corresponding integral balances are reported in Table 1. The N balance error improves significantly if the production of nitrogen is taken into account. In line with this result, Borfecchia et al. showed by means of XAS spectroscopy and UV-vis-NIR experiments that at the end of the TPD, after NH3 adsorption, the Cu sites in their Cu-CHA catalysts were almost completely reduced [13]. Table 1. Integral balances for the NH3 adsorption + TPD test over 1.7Cu-CHA25 shown in Figure 1.

1.7Cu-CHA25
In line with the literature, the TPD curve in Figure 1B showed two distinct NH3 release peaks, ascribed to energetically different populations of acidic sites [4,8,15] ( Figure  2). The physical nature of the different sites was estimated via Gaussian deconvolution of the NH3-TPD trace, to which the contribution of N2 according to the reactions discussed above (4-5) was added (green line in Figures 1B and 2). The N2 trace was not measured in our previous investigation [8]; however, it is important to include this contribution to quantify more accurately the various Cu sites taking part in the SCR chemistry. As established in several works within the literature [4,8,12,16], the first peak in Figure 1B is related to the NH3 adsorbed onto both Cu species and a small amount of Al extra-framework (quantified in [8] and reported in Table S1), while the second one is attributed to NH3 desorption from Brønsted sites associated with the bare zeolite framework. It is notable that the nitrogen production in Figure 1A was well aligned with the low temperature peak of the NH3-TPD curve, implying that mostly Lewis NH3 is responsible for the reduction in Cu II cations into Cu I according to (4)(5). As established in several works within the literature [4,8,12,16], the first peak in Figure 1B is related to the NH 3 adsorbed onto both Cu species and a small amount of Al extra-framework (quantified in [8] and reported in Table S1), while the second one is attributed to NH 3 desorption from Brønsted sites associated with the bare zeolite framework. It is notable that the nitrogen production in Figure 1A was well aligned with the low temperature peak of the NH 3 -TPD curve, implying that mostly Lewis NH 3 is responsible for the reduction in Cu II cations into Cu I according to (4) and (5). Figure 3 shows the Cu loading effect on the NH 3 -TPD data collected over each catalyst under both dry and wet conditions. Figure 3A refers to the simplest case, i.e., the TPD profiles over the unpromoted chabazite catalyst with SAR 25. The curves obtained in the presence and absence of H 2 O are almost overlapped, showing a single desorption peak centered at about 440 • C with a similar intensity in both tests. This peak corresponds to the desorption of NH 3 stored onto the Brønsted acid sites of the zeolite [11,[17][18][19]. The small shoulder visible at 250 • C under dry conditions (black line) is linked to the extra-framework aluminum species, which confers a slight Lewis acidity to the catalyst [6,[17][18][19][20][21]. This peak disappeared under wet conditions, in accordance with findings in the literature [6,20,21].

H 2 O Effect with Changes in Cu Loading
Interestingly, H 2 O significantly affects the weakly bound NH 3 , since H 2 O molecules are stored preferentially on Lewis acid sites. A clear confirmation of this behavior was obtained comparing the NH 3 desorption profiles of the copper-exchanged samples under dry and wet conditions. Figure 3B-D show the TPD results for the zeolite catalysts with the same SAR (25) and loaded with 0.7, 1.7 and 2.1% w/w copper, respectively. The presence of H 2 O considerably lowered the NH 3 storage capacity of all catalysts, affecting mostly the low-T peak, associated predominantly with Cu ions. The high-temperature Brønsted NH 3 peak was less affected by H 2 O. The negative effect of H 2 O on the NH 3 storage capacity of Lewis sites was confirmed by the faster dynamics during the catalyst saturation in the isothermal adsorption phase, when the samples were previously treated with H 2 O (not shown): indeed, the dead time recorded during this phase was reduced from 530-860 s under dry conditions to 380-595 s under wet conditions. In addition, the integral of the TPD curves (NH 3 + 2N 2 ) was lower in the presence of H 2 O even considering the negligible contribution of the extra-framework Al sites (Tables S1 and S2).  Figure 3 shows the Cu loading effect on the NH3-TPD data collected over each catalyst under both dry and wet conditions.

Water Effect with Changes in SAR
Results were also collected over Cu-CHA catalysts with fixed Cu loading (1.7% w/w) and different SARs (10,13,22,25): the corresponding TPD curves are shown in Figure 4. Under dry conditions, the differences in the relative intensities of the low and high temperature peaks indicate that on increasing the SAR, the overall stability of the adsorbed NH 3 molecules decreased [8]. In fact, in line with our previous studies, intensities of the low-T peaks were lower than those of the high-T peaks for high SARs (22,25), and the opposite trend was seen at low SARs (10,13), where the intensities of the low-T peaks were higher [8]. In all cases, the overall amount of chemisorbed NH 3 calculated from the integration of the TPD curves was found fairly constant upon changing the SAR, in the range of 950-1120 µmol/g. integral of the TPD curves (NH3 + 2N2) was lower in the presence of H2O even considering the negligible contribution of the extra-framework Al sites (Tables S1 and S2).

Water Effect with Changes in SAR
Results were also collected over Cu-CHA catalysts with fixed Cu loading (1.7% w/w) and different SARs (10,13,22,25): the corresponding TPD curves are shown in Figure 4. Under dry conditions, the differences in the relative intensities of the low and high temperature peaks indicate that on increasing the SAR, the overall stability of the adsorbed NH3 molecules decreased [8]. In fact, in line with our previous studies, intensities of the low-T peaks were lower than those of the high-T peaks for high SARs (22,25), and the opposite trend was seen at low SARs (10,13), where the intensities of the low-T peaks were higher [8]. In all cases, the overall amount of chemisorbed NH3 calculated from the integration of the TPD curves was found fairly constant upon changing the SAR, in the range of 950-1120 µmol/g. A similar effect was observed in presence of H2O (light-blue lines). Comparing the TPD under wet and dry conditions, it is apparent that H2O reduced ammonia storage (Figure 4). Similarly to the data presented in the previous section, the low-temperature desorption feature diminished as a result of H2O addition. Furthermore, the overall chemisorbed NH3 decreased to 830-880 µmol/g for all the catalysts. Accordingly, the deadtime recorded during the adsorption phase was affected by the presence of water (not shown), decreasing from about 800 to 650 s. A similar effect was observed in presence of H 2 O (light-blue lines). Comparing the TPD under wet and dry conditions, it is apparent that H 2 O reduced ammonia storage (Figure 4). Similarly to the data presented in the previous section, the low-temperature desorption feature diminished as a result of H 2 O addition. Furthermore, the overall chemisorbed NH 3 decreased to 830-880 µmol/g for all the catalysts. Accordingly, the deadtime recorded during the adsorption phase was affected by the presence of water (not shown), decreasing from about 800 to 650 s.

H 2 O Effect on Cu Speciation
At this point, we used Gaussian deconvolution of the bimodal TPD profiles to compare the amount of NH 3 to Cu atoms and to Brønsted acid sites in wet and dry atmosphere. Notice that the NH 3 legated to the Al extra-framework was completely lost in the presence of water, as apparent in Figure 3A for H-CHA (SAR 25) and in Figure S1 for H-CHA (SAR 13).
The results in Figure 5 and in Table S1 indicate that by cofeeding water: (i) the quantity of NH 3 stored on Brønsted sites slightly increased, while (ii) the Lewis NH 3 decreased. This trend could be related to the hydrolysis of Z 2 Cu II species, reaction (6) [6,22]: Catalysts 2021, 11, x FOR PEER REVIEW 6 of 11 At this point, we used Gaussian deconvolution of the bimodal TPD profiles to compare the amount of NH3 to Cu atoms and to Brønsted acid sites in wet and dry atmosphere. Notice that the NH3 legated to the Al extra-framework was completely lost in the presence of water, as apparent in Figure 3A for H-CHA (SAR 25) and in Figure S1 for H-CHA (SAR 13).
The results in Figure 5 and in Table S1 indicate that by cofeeding water: (i) the quantity of NH3 stored on Brønsted sites slightly increased, while (ii) the Lewis NH3 decreased. This trend could be related to the hydrolysis of Z2Cu II species, reaction (6) [6,22]: Z2Cu II (s)+ H2O (g)→ ZCu II OH (s)+ ZH (s) (6) In fact, according to this reaction, the transformation of Z2Cu II into ZCu II OH implies a loss in Cu-NH3 ligands. Indeed, the Z2Cu II species can bind up to four NH3 molecules, while the hydroxylated Cu ions can coordinate only three molecules [5,8,23].
According to our deconvolution results, however, the increment in Brønsted NH3 in wet conditions was less than the decrease in Lewis NH3 for some samples. This behavior could be explained by a further loss of Lewis NH3 due to the competitive adsorption of water on the Cu sites [20], as indicated by the decrease in the overall NH3 storage capacity (i.e., the sum of Brønsted and Lewis NH3 in Figure 5).   In fact, according to this reaction, the transformation of Z 2 Cu II into ZCu II OH implies a loss in Cu-NH 3 ligands. Indeed, the Z 2 Cu II species can bind up to four NH 3 molecules, while the hydroxylated Cu ions can coordinate only three molecules [5,8,23].
According to our deconvolution results, however, the increment in Brønsted NH 3 in wet conditions was less than the decrease in Lewis NH 3 for some samples. This behavior could be explained by a further loss of Lewis NH 3 due to the competitive adsorption of water on the Cu sites [20], as indicated by the decrease in the overall NH 3 storage capacity (i.e., the sum of Brønsted and Lewis NH 3 in Figure 5).
The fraction of Z 2 Cu II transformed into ZCu II OH was estimated by the following equation: Amount of Z 2 Cu II hydrolised initial amount of Z 2 Cu II ( Table 2) = NH 3 Brønsted wet − NH 3 Brønsted dry initial amount of Z 2 Cu II ( Table 2) % (7) From the results in Table 2 it can be noticed that only a fraction of Z 2 Cu II (40-70%) was hydrolyzed after the wet NH 3 adsorption. Furthermore, even if 1.7Cu-SAR25 and 1.7Cu-SAR22 are characterized by similar speciation and Cu/Al ratio, they display different behaviors in a wet atmosphere, more specifically the 1.7Cu-SAR22 shows a more enhanced hydrolysis of the Z 2 Cu II sites. One possible parameter affecting this reaction could be the distribution of the Al atoms in the zeolite framework. In addition, two different types of the Z 2 Cu II sites are viable, with para and metal configuration, respectively, which show distinct features in EPR spectra and distinct reactivity under Standard SCR conditions [24]. In principle, these species could be associated with a different response to the hydrolysis reaction. A dedicated investigation of the hydrolysis reaction is ongoing. At this point we calculated the NH 3 /Cu ratio, taking into account the area under the low-T TPD peak, proportional to the NH 3 stored only on Cu cations, and the number of copper atoms known from the weight fraction of Cu in each sample (Table 3). Figure 6 illustrates how this ratio varies with the Cu/Al ratio. Under dry conditions, the experimental data were in accordance with the literature [8,23]: the ratio was in the range 1-2 when only the Cu-NH 3 desorbed during the TPD was considered, and in the 3-4 range when the physisorbed NH 3 was also considered (area highlighted in orange, Figure 1A). In particular, the increase in Cu/Al lead to a decrease in the NH 3 /Cu ratio, from a value close to 4 (or 2 considering only the TPD phase) for the catalysts with the lowest SAR and Cu loading, to a value closer to 3 (or 1 considering only the TPD phase) for the other tested samples ( Figure 6). When water was added to the system, we observed a shift of all the NH 3 /Cu ratios evaluated from the TPD to values Under dry conditions, the experimental data were in accordance with the literature [8,23]: the ratio was in the range 1-2 when only the Cu-NH3 desorbed during the TPD was considered, and in the 3-4 range when the physisorbed NH3 was also considered(area highlighted in orange, Figure 1A). In particular, the increase in Cu/Al lead to a decrease in the NH3/Cu ratio, from a value close to 4 (or 2 considering only the TPD phase) for the catalysts with the lowest SAR and Cu loading, to a value closer to 3 (or 1 considering only the TPD phase) for the other tested samples ( Figure 6). When water was added to the system, we observed a shift of all the NH3/Cu ratios evaluated from the TPD to values closer to 3 (or 1 considering only the TPD phase). These results are consistent both with competition between H2O and NH3 adsorption onto Lewis sites and with the hydrolysis of a fraction of Z2Cu II species into ZCu II OH according to reaction (6), as discussed above.
At this stage of the discussion, our opinion is that analysis of the NH3-TPD data is not the best method to probe the Cu speciation. First, quantification of the Lewis and Brønsted acid sites requires deconvolution of the NH3-TPD curves. Second, during the At this stage of the discussion, our opinion is that analysis of the NH 3 -TPD data is not the best method to probe the Cu speciation. First, quantification of the Lewis and Brønsted acid sites requires deconvolution of the NH 3 -TPD curves. Second, during the temperature ramp the Cu atoms are reduced, producing N 2 ; hence, it would be important to include its trace in the TPD plot to accurately analyze the experimental data. Finally, under wet conditions the Cu species are modified by the hydrolysis reaction. The occurrence of these phenomena complicates the interpretation of experimental TPD traces, thus it is suggested not to solely use this method to quantify the fraction of Z 2 Cu II and ZCu II OH species in the catalysts. The Cu speciation could be quantified by means of simpler techniques such as, for example, NO 2 adsorption + TPD and H 2 TPR, as discussed in [8,11,25]. However, NH 3 -TPD runs under wet conditions provide useful experimental evidence of the hydrolysis reaction over Cu-CHA materials.

Material and Methods
For this study, we investigated two sets of Cu-exchanged chabazite (Cu-CHA), research catalysts supplied in the form of powders by Johnson Matthey. One set was purposely characterized by different SiO 2 /Al 2 O 3 (SAR) ratios (10,13,22,25), with fixed Cu loading = 1.7% w/w. The other had different Cu loadings (0, 0.7, 1.7, 2.1% w/w, with fixed SAR = 25). The textural properties of some samples, comprising the BET surface area and the micropore volume, are reported in Table S3 and are typical of commercial CHA based materials [26,27]. The samples were conditioned by heating them up to 600 • C at 5 • C/min and holding the maximum temperature for 5 h, while 10% O 2 and 10% H 2 O were fed in to remove all the impurities and the species that may be formed on the sample due to contact with atmosphere.
Sixteen milligrams of catalyst powder, sieved to obtain an average particle size of 90 micron and diluted up to 130 mg with cordierite, was loaded into a quartz flow reactor (6 mm ID) inserted in a vertical electric furnace. The temperature was controlled by a K-type thermocouple directly immersed in the catalyst bed.
Helium was used as balance gas in all the micro-reactor runs. Gases were controlled using mass flow controllers (Brooks Instruments, Hatfield, PA, USA). Water vapor was fed in by means of a saturator and the water concentration was controlled by adjusting the saturator's temperature according to Antoine's Law. NH 3 was fed in using a 6-way valve, enabling step changes in the reactant concentrations. Temporal evolution of the species at the reactor outlet was followed by a mass spectrometer (QGA Hiden Analytical, Warrington, UK) and a UV analyzer (ABB LIMAS 11 HW, Zurich, Switzerland) arranged in a parallel configuration, which allowed the simultaneous measurement of all the species involved.
The interaction between NH 3 molecules and the catalyst was studied by performing NH 3 adsorption + temperature programmed desorption (TPD) runs. The catalysts were saturated using 500 ppm of NH 3 and balance He (GHSV = 266,250 cm 3 /(g cat *h) STP), both under dry and wet conditions (5% w/w H 2 O), at 150 • C. After that, the catalysts were purged with He (or He + H 2 O) for 1 h to desorb weakly bound ammonia, and then heated in He only (or He + H 2 O) from 150 • C up to 550 • C at a rate of 15 • C/min to release the strongly adsorbed molecules. Prior to each experiment, in order to control the oxidation state of the catalysts, the samples were pre-treated at 550 • C, feeding 8% O 2 in He for 1 h, then the adsorption temperature was reached by cooling down under the same gas feed, with a final purge in He for 15 min.
The catalysts were characterized in our previous work [8], the amount of Cu loaded and the Cu speciation is reported in Table 3. In particular, NO + NH 3 TPR and ICP-MS analysis were used to quantify the number of reducible Cu II sites. Meanwhile, the NO 2 adsorption -TPD protocol was implemented to assess the amount of ZCu II OH species [8]. The samples show the typical deNOx activity of CHA based materials as apparent from the NO conversions in Standard SCR conditions reported in Table 3 (i.e., GHSV = 937,500 cm 3 /h/g cat (STP), g cat = 0.016 g, NH 3 = 500 ppm, NO = 500 ppm, H 2 O = 5%, O 2 = 8%, T = 225 • C).

Conclusions
Herein, we have studied the effect of H 2 O on the NH 3 storage onto different Cu-CHA catalysts, using standard NH 3 adsorption/TPD experiments. According to our preliminary results, during the temperature ramp of the TPD run the adsorbed NH 3 fully reduces the initially oxidized Cu sites, releasing a stoichiometric amount of N 2 . This effect is often overlooked in the literature but impacts the quantification of Cu-related and Brønsted NH 3 adsorption sites.
On increasing both the SAR and Cu loading, we found the adsorbed NH 3 /Cu ratio decreased from 4 to 3 when H 2 O was absent from the gas mixture (and from 2 to 1 in the absence of gaseous NH 3 ), which correlates nicely with the corresponding expected increase in the proportion of ZCu II OH over Z 2 Cu II species. Upon addition of water to the feed stream, all the tested catalysts showed NH 3 /Cu ratios closer to 3 (or to 1 in the absence of gaseous NH 3 ). These results are consistent both with the hydrolysis of a fraction of Z 2 Cu II species to form ZCu II OH, even though competition between H 2 O and NH 3 for adsorption onto Lewis sites cannot be ruled out.
This work is novel because it includes both the Cu reduction and the hydrolysis reaction in the analysis of the NH 3 -TPD data. However, the occurrence of these phenomena complicates the interpretation of the experimental TPD runs, thus it is suggested not to solely use this method to probe the Cu speciation in Cu-CHA catalysts [8].
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/catal11070759/s1, Figure S1: Comparison between dry and wet NH 3 -TPD after adsorption at 150 • C on H-CHA catalysts with SAR = 13, Table S1: NH 3 adsorbed on different sites as estimated from NH 3 -TPD deconvolution under dry conditions, Table S2: NH 3 adsorbed on different sites as estimated from NH 3 -TPD deconvolution under dry conditions.