Prediction of Cu Zeolite NH₃-SCR Activity from Variable Temperature ¹H NMR Spectroscopy

Round 1

Reviewer 1 Report

In this manuscript Eric and coworkers report the Prediction of Cu Zeolite NH₃-SCR Activity from Variable Temperature ¹H NMR Spectroscopic studies.  They have demonstrated how variable temperature ¹H NMR spectroscopy reveals the Cu induced generation of sharp ¹H resonances linked with a low concentration of sites on the zeolite. They have also revealed that onset temperature for the appearance of  these signals strongly correlates with the  NH₃-SCR activity. This manuscript is well written and should be accepted for publication in Molecules in its current form. 

English needs to be improved

Author Response

Dear referee,

 

We would like to thank you, both for your positive appreciation for our work and for your efforts to ensure the language used in the manuscript is accessible to a wide audience. In response to your last comment we asked a native English speaker to proofread and correct the manuscript. This resulted in the correction of some typos and in the rephrasing of few sentences to render them more accessible for non-native English speakers. The changes to the manuscript have been highlighted in the revised document sent to the editors.

 

Regards,

 

Eric Breynaert,

on behalf of all co-authors

Reviewer 2 Report

molecules-2557072

 

Prediction of Cu Zeolite NH3-SCR Activity from Variable Temperature 1H NMR Spectroscopy

Comments: The following questions will help stimulate discussion and provide valuable insights for the authors to further clarify and expand upon their research findings.

Scientific Questions:

1. How does the generation of a multi-electron donating active site play a role in the reduction of NOx to N₂ with Cu zeolite NH3-SCR catalysts? Are there specific molecular mechanisms proposed for this process?
2. Could authors elaborate on the factors that contribute to the reversible appearance of sharp 1H resonances associated with low concentrations of sites on the zeolite? What is the underlying cause of this phenomenon?
3. What are the implications of the observed correlation between the onset temperature of the sharp 1H resonances and the NH3-SCR activity? How does this correlation shed light on the catalyst's behavior?
4. Can authors explain the relationship between the presence of CuII ions and the broadening of the NH₄ resonance? How does this interaction contribute to the catalytic activity?
5. The authors mentioned the impact of paramagnetic relaxation enhancement (PRE) and paramagnetic shift (PS) on spectral broadening and shifting of NMR resonances. How can these effects be leveraged to gain insights into catalyst behavior?
6. Could authors discuss the potential significance of the temperature-induced ligand exchange process as an explanation for the appearing signals? What other evidence could support or refute this hypothesis?
7. What are the theoretical predictions and mechanisms that explain the mobility of Cu²⁺ ions in zeolites, particularly at low temperatures? How does this mobility impact the overall catalytic process?

Technical Challenging Questions:

 

1. How did the authors ensure that the observed sharp resonances were indeed associated with the generation of framework defects and not artifacts of the experimental setup or other phenomena?
2. Could authors elaborate on the methods used to determine the concentration of the local chemical environments associated with the new resonances? How accurate and reliable are these concentration estimates?
3. In cases where the peak narrowing transition occurred outside the accessible temperature window of the NMR probe head, how might this limitation impact the overall conclusions drawn from the study?
4. The authors mentioned the challenge of determining Tonset for some samples due to experimental limitations. How might these limitations affect the accuracy of the correlation between Tonset and catalytic activity?
5. What are the potential implications of Cu atom mobility at such low temperatures for practical applications of Cu zeolite SCR catalysts in real-world scenarios?
6. Could authors discuss the potential drawbacks or limitations of using variable temperature NMR as a predictive tool for catalyst activity? Are there scenarios where this approach might not be as effective? 

Minor editing of English language required.

Author Response

We thank the reviewer for the effort spent on assessing our manuscript in such high detail and for providing us with a stimulus to further improve the text. Below we provide a point-by-point response to the remarks and questions (in grey) that were raised. The discussion also includes a detailed description of the changes made to the manuscript. 

Scientific Questions:

How does the generation of a multi-electron donating active site play a role in the reduction of NOx to N₂ with Cu zeolite NH3-SCR catalysts? Are there specific molecular mechanisms proposed for this process?

The reviewer raises an interesting, yet complicated question. The mechanism of SCR catalysis has been a topic of debate for a considerable time and several pathways have been proposed to explain the reaction mechanism. Numerous experimental and theoretical studies implicate a complex multi-electron reaction mechanism involving multiple Cu atoms and two redox half cycles. Starting from Cu²⁺, a reduction of Cu²⁺ to Cu⁺ occurs with the simultaneous participation of NH₃ and NO (Cu²⁺/Cu⁺ half cycle). In the Cu⁺/Cu²⁺ (oxidation) half-cycle, Cu⁺ is re-oxidized in a reaction with NO₂ (fast SCR) or by reaction of NO + O₂ (standard SCR).

Recent literature on this topic have been added to the main text on page 1, line 38 to better guide the reader to the appropriate literature. Please see references below:

7. Shi, Z.; Peng, Q.; E, J.; Xie, B.; Wei, J.; Yin, R.; Fu, G. Mechanism, Performance and Modification Methods for NH3-SCR Catalysts: A Review. Fuel 2023, 331, 125885, doi:10.1016/J.FUEL.2022.125885.
8. Zhang, S.; Chen, J.; Meng, Y.; Pang, L.; Guo, Y.; Luo, Z.; Fang, Y.; Dong, Y.; Cai, W.; Li, T. Insight into Solid-State Ion-Exchanged Cu-Based Zeolite (SSZ-13, SAPO-18, and SAPO-34) Catalysts for the NH3-SCR Reaction: The Promoting Role of NH4-Form Zeolite Substrates. Appl. Surf. Sci. 2022, 571, 151328, doi:10.1016/J.APSUSC.2021.151328.
9. Khivantsev, K.; Kwak, J.H.; Jaegers, N.R.; Koleva, I.Z.; Vayssilov, G.N.; Derewinski, M.A.; Wang, Y.; Aleksandrov, H.A.; Szanyi, J. Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts. Chem. Sci. 2022, 13, 10383–10394, doi:10.1039/D2SC00350C.
10. Chen, L.; Janssens, T.V.W.; Vennestrøm, P.N.R.; Jansson, J.; Skoglundh, M.; Grönbeck, H. A Complete Multisite Reaction Mechanism for Low-Temperature NH3-SCR over Cu-CHA. ACS Catal. 2020, 10, 5646–5656, doi:10.1021/acscatal.0c00440.

Could authors elaborate on the factors that contribute to the reversible appearance of sharp 1H resonances associated with low concentrations of sites on the zeolite? What is the underlying cause of this phenomenon?

This comment refers to the discussion on page 6, lines 208 to 242 in the original manuscript. We apologize this discussion was unclear. As a result of this comment, we have rephrased and clarified the text. We further explain the potential causes of the reversible appearance of the sharp resonances below and have indicated the changes to the manuscript.

"Evaluating the observations, only few mechanisms could potentially explain the sudden appearance of a low concentration of sharp resonances: ‘

We first highlight that quantitative ¹H MAS NMR provides us with an absolute concentration of the ¹H nuclei involved, from which a site concentration can be derived. Re-evaluating the above sentence it became clear that it is necessary to re-specify that we are dealing with a low concentration of sites. ‘ a low concentration’ was therefore inserted into the sentence. Whereas the determination of the site concentration is rather straightforward, the appearance of these signals can originate in multiple mechanisms, which are discussed below together with the likelihood for each mechanism to have impacted the spectra. Since, for now, we cannot experimentally exclude either mechanism, we leave it up to the reader to form their own conclusion.

original text: “Cu^II ligation: The appearing signals could originate from a temperature induced ligand exchange process. Molecules originally adsorbed onto the zeolite could coordinate to Cu²⁺ and exhibit a chemical shift impacted by a paramagnetic shift effect. The new resonances are however sharper than most other resonances in the spectra, indicting they are not broadened by PRE effects. The chemical shift of the new resonances also appears to be independent of temperature. The combination of both observations renders it very unlikely that the signals are derived from a ligand of a paramagnetic Cu^II ion.”

In response to referee comment, additional details on the potential ligands of Cu in the dehydrated zeolite catalyst were added to the discussion.

revised text:“Cu^II ligation: The appearing signals could originate from a temperature induced ligand exchange process. Molecules originally adsorbed onto the zeolite could coordinate to Cu^II and exhibit a chemical shift impacted by a paramagnetic shift effect. Since the Cu-zeolite was evacuated at 473 K prior to the measurement, the only possible ligands would be water chemisorbed ammonia (δ(¹H) 6 – 8 ppm)[43], strongly adsorbed water (δ(¹H) 4 – 6 ppm)[44,45] or the zeolite framework protons i.e Bronsted acid site protons (δ(¹H) 3.6 – 7 ppm) or defect protons (δ(¹H) 0 – 3 ppm)[44,45]. The new resonances are however sharper than most other resonances in the spectra, indicating they are not broadened by PRE effects. The chemical shift of the new resonances also appears to be independent of temperature. The combination of both observations renders it very unlikely that the signals are derived from a ligand of a paramagnetic Cu^II ion.”

original text: “Cu^II reduction: If a Cu^II ion suddenly reduces to Cu^I, previously paramagnetically blinded proton resonances associated to proton spins present in close vicinity of this Cu atom, either on the exchanger or on a Cu ligand, could suddenly become visible. In this case, the new signals should also be affected by a ¹H-{⁶³Cu} TRAPDOR NMR experiment. As shown in Figure S4, this is not the case readily excluding this option.”

In response to the referee comment the discussion on the Cu^II reduction mechanism was clarified as indicated below.

revised text: “Cu^II reduction: If a Cu^II ion suddenly reduces to Cu^I, proton resonances previously blinded by paramagnetically enhanced T₁ and/or T₂ relaxation can suddenly become visible as the paramagnetic effect disappears. The impacted proton spins should be present in close vicinity of this Cu^II atom, either on the exchanger or on a Cu ligand to exhibit such effects. This also implies the new signals should be affected by a ¹H-{⁶³Cu} TRAPDOR NMR experiment, which specifically exploits the vicinity of the quadrupolar Cu atom to induce enhanced relaxation of nearby ¹H spins, thus impacting the area of the resonances associated with these spins. As shown in Figure S4 this is not the case, readily excluding this option.”

original text: “Cu^II dimerization: When two monomeric Cu species form a dimer with exhibiting antiferromagnetic or weak ferromagnetic coupling between the Cu^II unpaired electron spins, previously blinded ¹H resonances could suddenly appear. Similar to the case of Cu^II reduction above, this would also imply the new resonance should react to a ¹H-{⁶³Cu} TRAPDOR NMR experiment. As this is not the case (Figure S4), also this option can be excluded.”

In response to the referee’s comment the discussion on the Cu^II dimerization mechanism was clarified as indicated below.

revised text: “Cu^II dimerization: When two monomeric Cu species form a dimer exhibiting antiferromagnetic or weak ferromagnetic coupling between the Cu^II unpaired electron spins, previously blinded ¹H resonances of species in close vicinity of Cu could suddenly appear because the paramagnetic effects of the Cu atoms diminish or completely vanish. As for the case of Cu^II reduction (supra), this would also imply the new resonance should react to a ¹H-{⁶³Cu} TRAPDOR NMR experiment. As this is not the case (Figure S4), also this option can be excluded.”

original text: “Framework defects: An alternative explanation, considering the chemical shift of the new resonances, is that these signals originate from the reversible generation of framework defects. The presence of Cu could catalyze hydrolysis of the siloxane bonds in the zeolite framework generating aluminol and/or silanol defects. As for most zeolite framework defects, this would give rise to proton nests hence explaining the DQ correlations observed for the new signals. The previously calculated very low concentration of these sites, as compared to the exchange site concentration and the Cu loading, suggests that the generation of such defects would exhibit a limited effect on the overall Cu speciation or on the framework as a whole. The sharpness of the signals, in combination with the absence of a ¹H-{⁶³Cu} TRAPDOR (Figure S4) response suggests that even though the generation of the defects is clearly dependent on the presence of Cu, once generated the Cu atoms reside at a distance far enough from the defect to minimize their paramagnetic influence. The occurrence of Cu mobility at such low temperatures might appear surprising, but it is in full agreement with the theoretically predicted mobility of Cu²⁺ ions in zeolites, even in absence of ligating molecules.[19]”

In response to the referee’s comment the discussion on the Cu^II dimerization mechanism was clarified as indicated below.

revised text: “Framework defects: An alternative explanation, considering the chemical shift of the new resonances, is that these signals originate from framework defects, either reversibly generated or reversibly forming a surface complex with Cu²⁺. In the former option, the presence of Cu induces strain in the zeolite framework, catalyzing reversible hydrolysis of siloxane bonds with temperature. In this case the hydrolysis either occurs at suitable distance for Cu²⁺  not to blind the resonances associated with these defects or Cu²⁺ should exhibit a high enough mobility in the pore space to average out its paramagnetic relaxation enhancement effects. In the latter case, defects generated during the synthesis of the catalyst are blinded at low temperature due to PRE effects resulting from their association with Cu²⁺. Raising temperature, enhanced Cu²⁺ mobility would then again average out its PRE effects, causing the re-appearance of the respective resonances. In zeolites, silanol or aluminol groups associated with framework defects always occur in proton nests. This would explain the double quantum (DQ) correlations observed for the new signals. The previously calculated very low concentration of these sites, as compared to the exchange site concentration and the Cu loading, also suggests that if the generation of such defects would impact the Cu speciation, its impact would be very limited. The sharpness of the signals, in combination with the absence of a ¹H-{⁶³Cu} TRAPDOR (Figure S4) response suggests that even though the generation of the defects is clearly dependent on the presence of Cu, once generated, the Cu atoms either reside at a distance far enough from the defect to minimize their paramagnetic influence or their mobility is high enough to on average diminish or cancel out their PRE and TRAPDOR effects. The occurrence of Cu mobility at such low temperatures might appear surprising, but is nevertheless in full agreement with the theoretically predicted mobility of Cu²⁺ ions in zeolites, even in absence of ligating molecules.[19]”

 

What are the implications of the observed correlation between the onset temperature of the sharp 1H resonances and the NH3-SCR activity? How does this correlation shed light on the catalyst's behavior?

While the nature of the appearing resonances is still subject to several hypotheses, most of them imply mobility of the Cu in the zeolite pore system. Copper mobility has previously been suggested as a vital aspect of to the catalytic performance of Cu zeolites in SCR catalysis. In this respect we also refer to our response on comment 1. The clear correlation between the onset temperature and the low-temperature catalytic activity suggests that the lower the T requirements are for Cu atoms to become mobile, the better the catalysts will perform under operando conditions, confirming what has been theorized in literature.

 

Can authors explain the relationship between the presence of CuII ions and the broadening of the NH₄ resonance? How does this interaction contribute to the catalytic activity?

 

There appears to be a confusion. We correlate the catalytic activity to the appearance of sharp ¹H resonances with increasing temperature and not with a broadening of the NH₄ resonance. In the rather unlikely case that the sharp resonances originate from NH₄ exhibiting a ¹H resonance affected by a pseudo-contact shift, it would still be the associated Cu mobility which enables the re-appearance of these signals and it would therefore also be the Cu mobility that impacts the catalytic performance.

The authors mentioned the impact of paramagnetic relaxation enhancement (PRE) and paramagnetic shift (PS) on spectral broadening and shifting of NMR resonances. How can these effects be leveraged to gain insights into catalyst behavior?

As this appears to be a reformulation of the previous remarks, we kindly refer the reviewer to our answers to questions 1, 2, 3 and 4.

Could authors discuss the potential significance of the temperature-induced ligand exchange process as an explanation for the appearing signals? What other evidence could support or refute this hypothesis?

The hypothesis that the appearing signals come from ligands that are liberated from Cu ions from specific temperatures onwards, is a fair one. In this work, the samples were measured in conditions where potential ligands are limited to the zeolite frame and molecular ligands which can be ammonia- or water-based. Water in zeolites typically generates ¹H NMR resonances in the range of 4 to 6 ppm. Chemisorbed and physisorbed ammonia can be found at 6-8 and 3.5 ppm. This was published by us in Physical Chemistry Chemical Physics in 2018:

• Vallaey et al., Reversible room temperature ammonia gas absorption in pore water of microporous silica-alumina for sensing applications, Phys. Chem. Chem. Phys., 2018, 20, 13528-13536, DOI: 10.1039/C8CP01586D

The appearing signals had chemical shifts between 0 and 2 ppm. This range typically contains signals associated with the zeolite framework and more specifically with framework defects such as silanol and aluminol. The presence of Cu is however mandatory for the appearing signals to be observed as they are never observed in the parent zeolite prior to Cu loading.

In response to this comment we added this reference along with two other references about chemical shift of different ¹H species in a zeolite to the discussion on page 6, lines 214 - 216. We kindly also refer the reviewer to our response to remark 2, in response of which we already clarified and elaborated on the discussion in the manuscript.

43. Vallaey, B.; Radhakrishnan, S.; Heylen, S.; Chandran, C.V.; Taulelle, F.; Breynaert, E.; Martens, J.A. Reversible Room Temperature Ammonia Gas Absorption in Pore Water of Microporous Silica-Alumina for Sensing Applications. Phys. Chem. Chem. Phys. 2018, 20, doi:10.1039/c8cp01586d.
44. Hunger, M. Brønsted Acid Sites in Zeolites Characterized by Multinuclear Solid-State NMR Spectroscopy. Catal. Rev. - Sci. Eng. 1997, 39, 345–393, doi:10.1080/01614949708007100.
45. Haase, F.; Sauer, J. 1H NMR Chemical Shifts of Ammonia, Methanol, and Water Molecules Interacting with Brønsted Acid Sites of Zeolite Catalysts: Ab-Initio Calculations. J. Phys. Chem. 1994, 98, 3083–3085, doi:10.1021/j100063a006.

What are the theoretical predictions and mechanisms that explain the mobility of Cu²⁺ ions in zeolites, particularly at low temperatures? How does this mobility impact the overall catalytic process?

There are multiple reports on the theoretical calculations and potential mechanisms of explaining the impact of Cu ion mobility on SCR catalysis. The group of Göltl et al (references 23 and 24 in the main text) concluded that thermal motion of Cu ions can already occur at room temperature, even in the absence of ligating molecules. This aspect is already discussed in the manuscript on page 2, lines 59 -To guide the reader to the appropriate literature the references below are cited in the manuscript.

23. Göltl, F.; Sautet, P.; Hermans, I. The Impact of Finite Temperature on the Coordination of Cu Cations in the Zeolite SSZ-13. Catal. Today 2016, 267, 41–46, doi:10.1016/j.cattod.2015.10.028.
24. Göltl, F.; Sautet, P.; Hermans, I. Can Dynamics Be Responsible for the Complex Multipeak Infrared Spectra of NO Adsorbed to Copper(II) Sites in Zeolites? Angew. Chemie - Int. Ed. 2015, 54, 7799–7804, doi:10.1002/anie.201501942.
25. Zhang, R.; McEwen, J.S.; Kollár, M.; Gao, F.; Wang, Y.; Szanyi, J.; Peden, C.H.F. NO Chemisorption on Cu/SSZ-13: A Comparative Study from Infrared Spectroscopy and DFT Calculations. ACS Catal. 2014, 4, 4093–4105, doi:10.1021/cs500563s.

Specifically concerning the potential impact of Cu mobility on the low temperature SCR catalysis, the following references are being cited in the manuscript:

15. Paolucci, C.; Parekh, A.A.; Khurana, I.; Di Iorio, J.R.; Li, H.; Albarracin Caballero, J.D.; Shih, A.J.; Anggara, T.; Delgass, W.N.; Miller, J.T.; et al. Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in Ssz-13 Zeolites. J. Am. Chem. Soc. 2016, 138, 6028–6048, doi:10.1021/jacs.6b02651.
16. Krishna, S.H.; Goswami, A.; Wang, Y.; Jones, C.B.; Dean, D.P.; Miller, J.T.; Schneider, W.F.; Gounder, R. Influence of Framework Al Density in Chabazite Zeolites on Copper Ion Mobility and Reactivity during NOx Selective Catalytic Reduction with NH3. Nat. Catal. 2023 63 2023, 6, 276–285, doi:10.1038/S41929-023-00932-5.
17. Paolucci, C.; Khurana, I.; Parekh, A.A.; Li, S.; Shih, A.J.; Li, H.; Di Iorio, J.R.; Albarracin-Caballero, J.D.; Yezerets, A.; Miller, J.T.; et al. Dynamic Multinuclear Sites Formed by Mobilized Copper Ions in NOx Selective Catalytic Reduction. Science (80-. ). 2017, 357, 898–903, doi:10.1126/science.aan5630.
18. Oda, A.; Shionoya, H.; Hotta, Y.; Takewaki, T.; Sawabe, K.; Satsuma, A. Spectroscopic Evidence of Efficient Generation of Dicopper Intermediate in Selective Catalytic Reduction of NO over Cu-Ion-Exchanged Zeolites. ACS Catal. 2020, 10, 12333–12339, doi:10.1021/acscatal.0c03425.
19. Liu, C.; Kubota, H.; Amada, T.; Kon, K.; Toyao, T.; Maeno, Z.; Ueda, K.; Ohyama, J.; Satsuma, A.; Tanigawa, T.; et al. In Situ Spectroscopic Studies on the Redox Cycle of NH3−SCR over Cu−CHA Zeolites. ChemCatChem 2020, 12, 3050–3059, doi:10.1002/cctc.202000024.
20. Hu, W.; Selleri, T.; Gramigni, F.; Fenes, E.; Rout, K.R.; Liu, S.; Nova, I.; Chen, D.; Gao, X.; Tronconi, E. On the Redox Mechanism of Low‐Temperature NH3‐SCR over Cu‐CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle. Angew. Chemie Int. Ed. 2021, 60, 7197–7204, doi:10.1002/anie.202014926.

Technical Challenging Questions:

How did the authors ensure that the observed sharp resonances were indeed associated with the generation of framework defects and not artifacts of the experimental setup or other phenomena?

The appearance of sharp resonances with temperature was exclusively observed on Cu-loaded zeolite samples, while the ammonium form of these zeolites measured with the same hardware and using the same experimental parameters did not exhibit these signals. This confirms the appearance is not an experimental artifact. We would also like to stress that the temperature at which the sharp signals appears not only is dependent on the sample and its Cu loading, this temperature also correlates with experimentally measured catalytic performance of each Cu zeolite.

Could authors elaborate on the methods used to determine the concentration of the local chemical environments associated with the new resonances? How accurate and reliable are these concentration estimates?

The method used to determine the maximum concentration of the local chemical environments associated with these new resonances is based on absolute quantification in MAS NMR. This is a method developed at NMRCoRe, which is fully described, discussed and applied to multiple systems and nuclei in the literature listed below:

Houlleberghs, M.; Hoffmann, A.; Dom, D.; Kirschhock, C.E.A.; Taulelle, F.; Martens, J.A.; Breynaert, E. Absolute Quantification of Water in Microporous Solids with 1 H Magic Angle Spinning NMR and Standard Addition. Anal. Chem. 2017, 89, 6940–6943, doi:10.1021/acs.analchem.7b01653.

Vanderschaeghe, H.; Houlleberghs, M.; Verheyden, L.; Dom, D.; Chandran, C.V.; Radhakrishnan, S.; Martens, J.A.; Breynaert, E. Absolute Quantification of Residual Solvent in Mesoporous Silica Drug Formulations Using Magic-Angle Spinning NMR Spectroscopy. Anal. Chem. 2022, 95, 1880–1887, doi:10.1021/acs.analchem.2c03646.

Radhakrishnan, S.; Colaux, H.; Chandran, C.V.; Dom, D.; Verheyden, L.; Taulelle, F.; Martens, J.; Breynaert, E. Trace Level Detection and Quantification of Crystalline Silica in an Amorphous Silica Matrix with Natural Abundance 29Si NMR. Anal. Chem. 2020, 92, 13004–13009, doi:10.1021/acs.analchem.0c01756.

In cases where the peak narrowing transition occurred outside the accessible temperature window of the NMR probe head, how might this limitation impact the overall conclusions drawn from the study?

With our current hardware, we are limited to perform the variable temperature NMR experiments in a temperature range of 173 K to 373 K. The bulk of the samples investigated in this study exhibit the narrowing transition in temperature range experimentally accessible by our hardware. Based on the correlation derived from all these samples, the onset temperature for the very few samples that did not exhibit the transition within our experimental window was estimated based on their catalytic performance. This exercise readily revealed that these few samples, exhibiting either an extremely high or low activity, would have a T_onset outside the temperature range accessible by our hardware. As the conclusions were made based on the samples that did show the transition in our experimentally accessible temperature window, the conclusions are in no way impacted by this window. We invite groups with access to other hardware to in the future extend our study with datapoints in more extreme temperature conditions.

The authors mentioned the challenge of determining Tonset for some samples due to experimental limitations. How might these limitations affect the accuracy of the correlation between Tonset and catalytic activity?

This appears to be a reformulation of technically challenging comment number 3. We therefore kindly refer the reviewer to our response to that comment.

What are the potential implications of Cu atom mobility at such low temperatures for practical applications of Cu zeolite SCR catalysts in real-world scenarios?

While Cu atom mobility has been theorized to positively impact low temperature SCR catalytic performance, the experimental determination of this mobility has always remained elusive. With our method and results, Cu atom mobility now becomes an experimentally accessible parameter which can be expected to assist in the design of future low temperature SCR catalysts. It is especially in the low temperature region that current real-world catalysts have trouble to provide performance requested by legislation.

Could authors discuss the potential drawbacks or limitations of using variable temperature NMR as a predictive tool for catalyst activity? Are there scenarios where this approach might not be as effective? 

At present, the only limitation that could potentially impact the assessment of real-world SCR catalysts is related to the temperature range experimentally accessible by the NMR hardware. This aspect is already discussed in the manuscript in the section dealing with the onset temperature of samples exhibiting a catalytic performance associated with onset temperatures that fall outside the range experimentally accessible by our hardware.

Comments on the Quality of English Language

Minor editing of English language required.

The revised manuscript has been proofread by a native English speaker to ensure that all linguistic issues associated with the first draft have been resolved.

Reviewer 3 Report

This manuscript reports on the results of the 1H NMR study of a number of Cu- zeolites obtained from the NH4+ form. The work is very interesting, a correlation has been established between the spectral characteristics and the catalytic activity of the samples. However, in the process of reading, some questions and comments arose. They are formulated below.

 

1. Lines 117-119  “…appearance around 300 K of sharp 1H NMR resonances with a chemical shift between 1 and 2 ppm, exclusively in the Cu-exchanged catalyst. The sudden appearance of these resonances is fully reversible and is never observed in the purely NH4-exchanged material”. This looks like with temperature increasing NH4+, that  present as a charge compensating cation in the zeolite, decomposes into NH3 and a proton with formation hydroxyl. And this process is reversible. I.e., the temperature onset that is the appearance of this signal = the decomposition temperature of NH4+. Then the correlation between this temperature and catalytic activity may indicate that both the decomposition of NH4+ and the catalytic conversion of NOx are determined by the same sites (the lower the decomposition temperature of ammonium, the higher the activity). But what these sites are is not entirely clear. See the following remark. It seems to me that the authors should consider the possibility of such an explanation of the observed phenomenon.

 

2. The authors focused on the study of 1H NMR of various zeolite samples with different topology of the zeolite framework and different Si/Al ratio. Table A.1 shows the data of the elemental analysis. However, as follows from the 1H NMR spectra and the text of the manuscript, there is extra-framework Al in the samples (and it may also contribute to catalytic conversion). But in what amount? Have the 27Al NMR spectra been recorded to estimate the amount of this extra-framework Al?

 

3. About the paramagnetic ion Cu2+ and its reduction. The samples are dehydrated. It is known from EPR studies that during the evacuation of water, the copper cations Cu2+ approache the negatively charged zeolitic wall and charge transfer from oxygen to copper occurs. As a result, the EPR signal from Cu2+ disappears. This is not exactly "reduction" in the usual sense, but copper becomes invisible to EPR. During hydration, the EPR signal is restored. This was observed on Cu-modnenite, but obtained from a sodium form. It would be very interesting to make similar EPR studies for this series of samples: is there a charge transfer effect in ammonium samples? If so, some of the copper ions are not involved in the formation of complexes with ammonium. This assumption, by the way, is consistent with the authors' conclusions that “only a limited fraction of the Cu atoms in the sample is contributing to the catalytic conversion” (line 167)

 

As a conclusion,  I believe that after revising the manuscript accounting for the comments above it can be published in Molecules.

 

Author Response

We thank the reviewer for the positive assessment of our work and for the detailed analysis of the manuscript. Below we provide a point-by-point response to the questions raised and remarks formulated by the reviewer. The reviewer comments written in grey text, the responses are in black text. 

Lines 117-119  “…appearance around 300 K of sharp 1H NMR resonances with a chemical shift between 1 and 2 ppm, exclusively in the Cu-exchanged catalyst. The sudden appearance of these resonances is fully reversible and is never observed in the purely NH4-exchanged material”. This looks like with temperature increasing NH4+, that  present as a charge compensating cation in the zeolite, decomposes into NH3 and a proton with formation hydroxyl. And this process is reversible. I.e., the temperature onset that is the appearance of this signal = the decomposition temperature of NH4+. Then the correlation between this temperature and catalytic activity may indicate that both the decomposition of NH4+ and the catalytic conversion of NOx are determined by the same sites (the lower the decomposition temperature of ammonium, the higher the activity). But what these sites are is not entirely clear. See the following remark. It seems to me that the authors should consider the possibility of such an explanation of the observed phenomenon.

We thank the reviewer for suggesting an alternate hypothesis. The experimental evidence can however not be aligned with this hypothesis. The new resonances are exclusively observed in Cu-zeolites indicating a process associated to the presence of Cu. The mechanism suggested by the reviewer, decomposition of a charge compensation NH₄⁺ ion into NH₃ and a Bronsted acid proton would not only occur in the Cu form, but also in the NH₄ form of the zeolite. In addition, this mechanism would also imply that free or physisorbed NH₃ is formed at low temperature. From our previous work, we not only know the chemical shift of physisorbed NH₃, we also have demonstrated that physisorbed NH₃ does not survive vacuum drying at 200°C. Since all samples were subjected to vacuum drying at 200°C prior to the variable temperature ¹H NMR measurements, which is significantly higher than the maximum temperature in the VT experiments, all ammonia that could have been formed by the suggested decomposition would already have been removed from the catalysts. The suggested Bronsted acid sites should then invariably be visible at every temperature. To guide the reader to the relevant literature concerning NH₄⁺ decomposition, NH₃ removal and spectroscopic identification of chemi- and physorbed NH₃ and chemical shifts of different moieties in a zeolite matrix, the sentence given below and the following references were cited in the manuscript on page 6, lines 216 - 220:

“Since the Cu-zeolite was evacuated at 473 K prior to the measurement, the only possible ligands would be water ammonia (chemisorbed δ(¹H) 6 – 8 ppm or physisorbed δ(¹H) 3.5 ppm)[43] or the zeolite framework protons i.e Bronsted acid site protons (δ(¹H) 3.6 – 7 ppm) or defect protons (δ(¹H) 0 – 3 ppm)[44,45].”

42. Vallaey, B.; Radhakrishnan, S.; Heylen, S.; Chandran, C.V.; Taulelle, F.; Breynaert, E.; Martens, J.A. Reversible Room Temperature Ammonia Gas Absorption in Pore Water of Microporous Silica-Alumina for Sensing Applications. Phys. Chem. Chem. Phys. 2018, 20, doi:10.1039/c8cp01586d.
43. Hunger, M. Brønsted Acid Sites in Zeolites Characterized by Multinuclear Solid-State NMR Spectroscopy. Catal. Rev. - Sci. Eng. 1997, 39, 345–393, doi:10.1080/01614949708007100.
44. Haase, F.; Sauer, J. 1H NMR Chemical Shifts of Ammonia, Methanol, and Water Molecules Interacting with Brønsted Acid Sites of Zeolite Catalysts: Ab-Initio Calculations. J. Phys. Chem. 1994, 98, 3083–3085, doi:10.1021/j100063a006.

The authors focused on the study of 1H NMR of various zeolite samples with different topology of the zeolite framework and different Si/Al ratio. Table A.1 shows the data of the elemental analysis. However, as follows from the 1H NMR spectra and the text of the manuscript, there is extra-framework Al in the samples (and it may also contribute to catalytic conversion). But in what amount? Have the 27Al NMR spectra been recorded to estimate the amount of this extra-framework Al?

We thank the reviewer for this remark. ²⁷Al spectra are by default recorded for all zeolite samples, specifically to assess the speciation of Al in the samples. In all these catalysts, the concentration of extraframework Al is negligible and at no point in the study any correlation to the concentration of extraframework Al was observed. To address this comment, the following sentence was added to the manuscript on page 4, lines 164 – 166..

“²⁷Al MAS NMR spectra were recorded for all samples and indicated the concentration of extraframework Al was not only negligible, there also was no correlation between this concentration and the observed T_onset or the catalytic performance.”

Also, as an example, ¹H decoupled ²⁷Al NMR spectrum of CHA-1 in NH₄-form and partially Cu-exchanged form was added to supplementary information (Figure S8), showing the absence of significant amount of extra framework aluminum in the sample.

Figure S8. ¹H decoupled ²⁷Al NMR spectrum of CHA-1 in NH₄-form (blue trace) and partially Cu-exchanged form (red trace) recorded at 295 K.

About the paramagnetic ion Cu2+ and its reduction. The samples are dehydrated. It is known from EPR studies that during the evacuation of water, the copper cations Cu2+ approache the negatively charged zeolitic wall and charge transfer from oxygen to copper occurs. As a result, the EPR signal from Cu2+ disappears. This is not exactly "reduction" in the usual sense, but copper becomes invisible to EPR. During hydration, the EPR signal is restored. This was observed on Cu-modnenite, but obtained from a sodium form. It would be very interesting to make similar EPR studies for this series of samples: is there a charge transfer effect in ammonium samples? If so, some of the copper ions are not involved in the formation of complexes with ammonium. This assumption, by the way, is consistent with the authors' conclusions that “only a limited fraction of the Cu atoms in the sample is contributing to the catalytic conversion” (line 167)

We thank to reviewer for this valuable suggestion and agree that this study should in the future be extended with an associated EPR study, even if only to provide further evidence that the catalytic conversion is mainly determined by a limited concentration of Cu atoms. We however do not have access to EPR hardware which can provide the accuracy required to draw conclusions from such measurements. We therefore consider such experiments outside the scope of the present work, but invite collaboration with research groups that could perform complementary measurements in future extensions to the present study.

Round 2

Reviewer 2 Report

Revised manuscript can be accepted 

Reviewer 3 Report

The paper can be accepted in the present form