Mechanochemical Reduction of V₂O₅: Alkali Metals vs. Alkali Metal Hydrides—Which Are the More Suitable Reducing Agents?

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reports a systematic comparison of alkali metals and alkali metal hydrides as reducing agents for the mechanochemical reduction of V2O5. The combination of experimental investigation, pressure evolution analysis, PXRD/Rietveld refinement, and theoretical calculations provides valuable insights into the reaction behavior and product evolution. The topic is relevant to mechanochemistry and inorganic materials chemistry. The manuscript is generally well organized, and the conclusions are supported by the experimental results. Some minor revisions are suggested to further improve the clarity and presentation of the manuscript.

1. The authors correlate the calculated reaction enthalpies with the experimentally observed ignition behavior and reduction degree. The discussion could be slightly expanded to clarify the relationship between thermodynamic driving force and actual mechanochemical reaction pathways.

2. The self-propagating reaction behavior is an important feature of this work. A brief discussion on the possible contribution of local temperature increase and mechanical activation during milling would help readers better understand this phenomenon.

3. Hydrogen evolution is an important difference between hydride and elemental metal reducing agents. The authors are encouraged to further clarify its influence on reaction safety and possible effects on the milling process.

4. The Rietveld refinement results are important for supporting the phase evolution discussion. Please provide refinement reliability factors (such as Rwp and χ2 values) in the Supporting Information to improve the transparency of the analysis.

5. The influence of the ductility of sodium metal on milling efficiency is interesting. A short discussion regarding particle deformation and possible cold welding effects would further strengthen this explanation.

6. The title suggests a direct comparison of “better reducing agents”. Since the manuscript also highlights the practical advantages of hydrides in terms of handling and processability, the wording of the title could be reconsidered to better reflect the balance between reducing ability and practical applicability.

7. Since this work extends the previous study on NaH reduction of V2O5, the authors should more clearly emphasize the new insights obtained from the comparison among NaH, Na, LiH, and Li systems in the Introduction.

8. The experimental section is generally clear. However, a concise summary of the different milling conditions, especially for room-temperature and cryogenic milling, would facilitate comparison among different systems.

9. The Bell–Evans–Polanyi principle is used to explain the relationship between reaction enthalpy and activation energy. The authors may briefly mention that this relationship is empirical and should be interpreted qualitatively in the present system.

10. Some conclusions regarding the preference of elemental metals or hydrides for different systems appear somewhat broad. The authors are encouraged to slightly moderate these statements and restrict them to the investigated mechanochemical reduction systems.

11. Please carefully check the consistency of chemical formulas, phase labels, and oxidation state descriptions throughout the manuscript.

12. The Supporting Information provides essential details for phase analysis and theoretical calculations. Please ensure that all relevant information required for reproducing the refinement and calculation results is clearly presented.

Author Response

Comment 1: The authors correlate the calculated reaction enthalpies with the experimentally observed ignition behavior and reduction degree. The discussion could be slightly expanded to clarify the relationship between thermodynamic driving force and actual mechanochemical reaction pathways.

Response 1: We added a statement addressing this issue in the introduction of the revised manuscript.

 

Comment 2: The self-propagating reaction behavior is an important feature of this work. A brief discussion on the possible contribution of local temperature increase and mechanical activation during milling would help readers better understand this phenomenon.

Response 2: A corresponding statement has been added to the introduction.

 

Comment 3: Hydrogen evolution is an important difference between hydride and elemental metal reducing agents. The authors are encouraged to further clarify its influence on reaction safety and possible effects on the milling process.

Response 3: Hydrogen evolution is indeed a relevant aspect of reactions involving hydride reducing agents and requires appropriate safety measures. However, under the milling conditions employed in this study, no observable effect of the generated hydrogen on the reaction progress or product formation was detected. To clarify this point, additional text has been included in Section 2.2 and the Experimental Section discussing hydrogen evolution, the associated safety considerations, and its lack of apparent influence on the milling process and reaction outcome.

 

Comment 4: The Rietveld refinement results are important for supporting the phase evolution discussion. Please provide refinement reliability factors (such as Rwp and χ2 values) in the Supporting Information to improve the transparency of the analysis.

Response 4: Reliability factors are given in Tables S4 – S6 in the Supporting Information.

 

Comment 5: The influence of the ductility of sodium metal on milling efficiency is interesting. A short discussion regarding particle deformation and possible cold welding effects would further strengthen this explanation.

Response 5: A corresponding statement has been added to page 6.

 

Comment 6: The title suggests a direct comparison of “better reducing agents”. Since the manuscript also highlights the practical advantages of hydrides in terms of handling and processability, the wording of the title could be reconsidered to better reflect the balance between reducing ability and practical applicability.

Response 6: The world “better” has been changed to “more suitable” in the title.

 

Comment 7: Since this work extends the previous study on NaH reduction of V₂O₅, the authors should more clearly emphasize the new insights obtained from the comparison among NaH, Na, LiH, and Li systems in the Introduction.

Response 7: The introduction has been modified.

 

Comment 8: The experimental section is generally clear. However, a concise summary of the different milling conditions, especially for room-temperature and cryogenic milling, would facilitate comparison among different systems.

Response 8: The experimental section has been rephrased to make it more understandable.

 

Comment 9: The Bell–Evans–Polanyi principle is used to explain the relationship between reaction enthalpy and activation energy. The authors may briefly mention that this relationship is empirical and should be interpreted qualitatively in the present system.

Response 9: A corresponding statement has been added to page 5.

 

Comment 10: Some conclusions regarding the preference of elemental metals or hydrides for different systems appear somewhat broad. The authors are encouraged to slightly moderate these statements and restrict them to the investigated mechanochemical reduction systems.

Response 10: It has been specified that these observations are valid for the investigated systems.

 

Comment 11: Please carefully check the consistency of chemical formulas, phase labels, and oxidation state descriptions throughout the manuscript.

Response 11: All formula, phase labels, and oxidation state descriptions have been checked and corrected if necessary.

 

Comment 12: The Supporting Information provides essential details for phase analysis and theoretical calculations. Please ensure that all relevant information required for reproducing the refinement and calculation results is clearly presented.

Response 12: We have carefully reviewed the Supporting Information and confirmed that all relevant parameters, procedures, and data required to reproduce the phase analysis, refinement, and theoretical calculations are clearly documented.

Reviewer 2 Report

Comments and Suggestions for Authors

The present manuscript is an interesting study on the mechanochemical reactivity of vanadium oxide with lithium and sodium metals and hydrides. It combines theoretical calculations with experimental work, albeit the latter could have been a little more elaborated. After improving the points mentioned below, the manuscript could be published.

1. The introduction does not provide any introduction into mechanically induced self-propagating reactions, although it clearly reports it. This term as a whole is mentioned for the first time in Conclusions. I recommend providing some introduction (e.g. from https://www.sciencedirect.com/science/article/abs/pii/S0079642501000020).
2. The authors report formation enthalpies, but it would be great if they could add information about adiabatic temperatures, as it is known that this value is important when assessing whether MSR can be actually expected.
3. It should be clarified why both Tables 1 and 2 are necessary.
4. The authors should at least mention that the MSR can be influenced by various properties such as metals morphology or their particle size (as discussed e.g. in https://pubs.rsc.org/mr/article/1/1/94/814008/Mechanically-induced-self-propagating-reactions), and is not only governed by the chemical and thermodynamic factors.
5. What is the impact of NaCl of the MSR occurrence? I could not find the actual result of this experiment in the paper. Does not behave as inert additive and thus changes the mechanism from MSR to gradual? This can change with the amount introduced. Have the authors not considered such experiments?
6. Was MSR also observed when cryomilling was used? Cryogenic milling was performed in MM500 nano. Was the jar suspended in liquid nitrogen during milling?
7. The authors mention the use of Easy GTM system, which records both pressure and temperature. However, the authors report just pressure. What about temperature? The authors should provide T curves in the ESI and discuss them.
8. Have the authors tried the experiments in air? Do the same products form and what about MSR? Does it happen sooner/later and is of higher/lower intensity?
9. In Fig. 2, at least the main phases should be marked directly in the figure, as it is not comfortable to look into the very long caption to look for the exact symbols for each phase.
10. The authors report the situation after 10 min of milling. However, a comparison of the phase composition directly after MSR with the one obtained after 10 min would be beneficial.
11. In line 236, most probably NaH is meant.
12. XPS measurements would be beneficial to shed more light on the oxidation states of metals (mainly vanadium) in the products.
13. What is meant by a “large product mixture” in the conclusions? Was it supposed to mean that a mixture of phases is formed?
14. In this context, the products are mixtures of various phases. Is such a mixture not a problem for subsequent application in energy storage and conversion? Should the phase not be pure?
15. The main message in the last paragraph in conclusions should be written more clearly. From my opinion, the results when using metals bring more added value than hydrides.
16. The provided ESI in part 3 contains only info about NaH system and misses others and is thus incomplete. Moreover, it also contains comments in German.
17. If the authors use the graphs from their previous study in Inorg. Chem. (ref. 21), they should ask for a permission to reproduce them.

Author Response

Comment 1: The introduction does not provide any introduction into mechanically induced self-propagating reactions, although it clearly reports it. This term as a whole is mentioned for the first time in Conclusions. I recommend providing some introduction (e.g. from https://www.sciencedirect.com/science/article/abs/pii/S0079642501000020).

Response 1: We added a section about mechanically induced self-propagating reactions (MSRs) to the introduction.

 

Comment 2: The authors report formation enthalpies, but it would be great if they could add information about adiabatic temperatures, as it is known that this value is important when assessing whether MSR can be actually expected.

Response 2: Information about the adiabatic temperature of MSRs has been added to the introduction.

 

Comment 3: It should be clarified why both Tables 1 and 2 are necessary.

Response 3: In response to the reviewer’s comment, Tables 1 and 2 have been moved to the Supporting Information, as they primarily provide supporting data and are not essential for the main discussion in the manuscript.

 

Comment 4: The authors should at least mention that the MSR can be influenced by various properties such as metals morphology or their particle size (as discussed e.g. in https://pubs.rsc.org/mr/article/1/1/94/814008/Mechanically-induced-self-propagating-reactions), and is not only governed by the chemical and thermodynamic factors.

Response 4: We agree that the occurrence and characteristics of mechanically induced self-propagating reactions (MSRs) are influenced not only by chemical and thermodynamic factors but also by physical parameters such as particle size, particle morphology, and the degree of contact between reactants. To acknowledge these additional influences, a corresponding statement, including the suggested reference, has been added to the Introduction.

 

Comment 5: What is the impact of NaCl of the MSR occurrence? I could not find the actual result of this experiment in the paper. Does not behave as inert additive and thus changes the mechanism from MSR to gradual? This can change with the amount introduced. Have the authors not considered such experiments?

Response 5: We would expect that the MSR is delayed when milling with NaCl as an inert additive. However, we have always observed a strong cementation (independent of the added amount), which completely hindered the reaction. This was already observed visually since the ochre color was still visible. In the end, adding an inert additive is also not the most desirable solution if the product could be used for further application since an additional step for the removal of the inert additive would always be necessary.

 

Comment 6: Was MSR also observed when cryomilling was used? Cryogenic milling was performed in MM500 nano. Was the jar suspended in liquid nitrogen during milling?

Response 6: Since we don’t have the possibility to measure the evolution of temperature and pressure during cryomilling, it is difficult to prove that MSR is occurring. Based on our experience and a similar product composition after milling at room temperature and under cryogenic conditions, it is very likely that MSR occur even under cryogenic conditions.

 

Comment 7: The authors mention the use of Easy GTM system, which records both pressure and temperature. However, the authors report just pressure. What about temperature? The authors should provide T curves in the ESI and discuss them.

Response 7: In all cases, a delayed temperature increase of a few degrees was observed, as shown in our previous publication for NaH. Since a certain percentage of the heat is absorbed by the milling jar and since the entire atmosphere inside the jar also must first warm up, the detection of a temperature rise takes longer and does not give valuable insights into further analyzing the occurring reaction. We therefore decided not to show the temperature curves, but a corresponding sentence has been added to page 5.

 

Comment 8: Have the authors tried the experiments in air? Do the same products form and what about MSR? Does it happen sooner/later and is of higher/lower intensity?

Response 8: We have not performed the reactions under air, as handling alkali metals and alkali metal hydrides in the presence of air poses significant safety risks, particularly under mechanochemical conditions where milling can substantially increase their reactivity. Consequently, we cannot experimentally assess the influence of air on product formation or on the occurrence, timing, or intensity of the MSR.

While it is conceivable that an MSR could still occur if the reducing agent reacts preferentially with the vanadium oxide rather than with atmospheric oxygen or moisture, any discussion of its characteristics under air would remain speculative. Likewise, although similar reaction products might be expected, the presence of air could affect the extent of reduction and potentially result in higher vanadium oxidation states. To avoid unsupported conclusions, we have not included such speculations in the manuscript and instead note that these experiments were not conducted due to safety considerations.

 

Comment 9: In Fig. 2, at least the main phases should be marked directly in the figure, as it is not comfortable to look into the very long caption to look for the exact symbols for each phase.

Response 9: We are aware that the figure contains a lot of information. However, we chose to leave it as it is because only the main reflections are marked. In our opinion, not marking any reflections would not make the figure easier to understand.

 

Comment 10: The authors report the situation after 10 min of milling. However, a comparison of the phase composition directly after MSR with the one obtained after 10 min would be beneficial.

Response 10:

We have done such a comparison in our previous publications and the phase compositions directly after MSR compared to after 10 min were very similar. Therefore, we did not perform this experiment again, but a similar result can be expected for all investigated reducing agents.

 

Comment 11: In line 236, most probably NaH is meant.

Response 11: Thank you for noticing, we corrected it.

 

Comment 12: XPS measurements would be beneficial to shed more light on the oxidation states of metals (mainly vanadium) in the products.

Response 12: Since PXRD can measure the bulk of the sample, unlike XPS which can only measure the surface, we chose not to perform XPS measurements. This is because the products are crystalline and analyzable with PXRD, and the large number of different oxidation states would complicate XPS measurements.

 

Comment 13: What is meant by a “large product mixture” in the conclusions? Was it supposed to mean that a mixture of phases is formed?

Response 13: We thank the reviewer for this comment. To improve clarity and avoid ambiguity, we have rephrased the sentence referring to a “large mixture of different phases.”

 

Comment 14: In this context, the products are mixtures of various phases. Is such a mixture not a problem for subsequent application in energy storage and conversion? Should the phase not be pure?

Response 14: Ideally, the phase would be pure for applications in energy storage and conversion, meaning that product mixtures would most likely be hindering. However, our main focus is the behavior of transition metal oxides in the ball mill instead of subsequent applications due to rather scarce literature on that topic.

 

Comment 15: The main message in the last paragraph in conclusions should be written more clearly. From my opinion, the results when using metals bring more added value than hydrides.

Response 15: We thank the reviewer for this comment. In response, we have rewritten the final paragraph of the Conclusions to more clearly highlight the key findings of this study and to better emphasize the advantages and scientific value associated with the use of elemental metals as reducing agents compared to hydrides.

 

Comment 16. The provided ESI in part 3 contains only info about NaH system and misses others and is thus incomplete. Moreover, it also contains comments in German.

Response 16: We thank the reviewer for pointing out this issue. The Supporting Information has been revised and corrected to ensure completeness. Information for all investigated systems has now been included, and the remaining comments in German have been removed to ensure consistency and clarity throughout the document.

 

Comment 17: If the authors use the graphs from their previous study in Inorg. Chem. (ref. 21), they should ask for a permission to reproduce them.

Response 17: We thank the reviewer for this comment. Permission to reproduce the graphs from our previous publication was obtained, and the manuscript has been updated accordingly where appropriate.

Reviewer 3 Report

Comments and Suggestions for Authors

This study systematically investigates the mechanochemical reduction of V₂O₅ using alkali metal hydrides (NaH, LiH) and their elemental counterparts (Na, Li) as reducing agents. Through a combination of pressure evolution monitoring, PXRD with Rietveld refinement, and DFT calculations, the authors demonstrate that both hydrides and elemental alkali metals follow similar self-propagating reaction pathways, yielding mixtures of various alkali metal vanadium oxides. The work provides useful insights into the trade-offs between hydrides (easier handling, but risk of H₂ evolution) and elemental metals. This offers practical guidance for mechanochemical synthesis of reduced vanadium oxide phases. However, before the manuscript can be accepted for publication, the following issues need to be properly addressed.

1. A full experimental gradient of 1, 2, 3 and 4 equivalents of reducing agent was set up, yet Tables 1 and 2 only present balanced reactions corresponding to 1, 2 and 4 mol of reducing agent, with no overall reaction enthalpies available for the intermediate 3 equivalent ratio.
2. The manuscript lacks XPS characterization for vanadium valence states. The proportions of V³⁺, V⁴⁺ and V⁵⁺ are merely inferred indirectly from XRD phase identification, while no direct XPS measurement was performed to quantify the valence distribution of vanadium species.
3. The Rietveld refinement results indicate that multiple phases coexist in all products, and the authors attribute this to the complex thermodynamic landscape. However, the refinement quality is not reported for most samples. Please provide these figures of merit and discuss whether the phase fractions are reliably distinguished given the large number of phases and peak overlaps.
4. The observation that 1 eq. LiH shows no reaction after 10 min of milling while 1 eq. NaH reacts readily but not sufficiently explained. The authors should provide a more detailed mechanistic discussion of why LiH exhibits this anomalous behavior at low concentrations, and whether longer milling times would eventually induce the reaction.
5. For the cryogenic milling experiments with 4 eq. Na, the authors report powder characteristics. However, no comparative PXRD or Rietveld data are shown for the cryo-milled sample versus the room-temperature product. Please include this comparison and discuss whether cryomilling also alters the phase composition or only improves handling.
6. In the experimental section, the authors explicitly state that, for safety reasons to avoid exceeding the pressure limit, the total batch mass for the 3 and 4 eq. LiH reactions was reduced from the standard 3 g to 2 g and 1.5 g, respectively. Whether the reduced batch mass could potentially delay the induction period of the LiH reactions or alter the final phase distribution of the products; and if such effects exist, whether complementary experiments under identical ball‑to‑powder ratios and filling ratios should be performed to ensure the reliability of the conclusions.

Author Response

Comment 1: A full experimental gradient of 1, 2, 3 and 4 equivalents of reducing agent was set up, yet Tables 1 and 2 only present balanced reactions corresponding to 1, 2 and 4 mol of reducing agent, with no overall reaction enthalpies available for the intermediate 3 equivalent ratio.

Response 1: We thank the reviewer for this valuable comment. The reaction involving 3 equivalents of reducing agent is included as Reaction 15 in Table S3 of the Supporting Information.

 

Comment 2: The manuscript lacks XPS characterization for vanadium valence states. The proportions of V³⁺, V⁴⁺ and V⁵⁺ are merely inferred indirectly from XRD phase identification, while no direct XPS measurement was performed to quantify the valence distribution of vanadium species.

Response 2: Since PXRD can measure the bulk of the sample, unlike XPS which can only measure the surface, we chose not to perform XPS measurements. This is because the products are crystalline and well analyzable with PXRD, and the large number of different oxidation states would complicate XPS measurements.

 

Comment 3. The Rietveld refinement results indicate that multiple phases coexist in all products, and the authors attribute this to the complex thermodynamic landscape. However, the refinement quality is not reported for most samples. Please provide these figures of merit and discuss whether the phase fractions are reliably distinguished given the large number of phases and peak overlaps.

Response 3: The figures of merit for all Rietveld refinements are provided in the Supporting Information. To make this information easier to locate, the Supporting Information has been revised accordingly.

 

Comment 4. The observation that 1 eq. LiH shows no reaction after 10 min of milling while 1 eq. NaH reacts readily but not sufficiently explained. The authors should provide a more detailed mechanistic discussion of why LiH exhibits this anomalous behavior at low concentrations, and whether longer milling times would eventually induce the reaction.

Response 4: We thank the reviewer for this insightful comment. We did not observe a significant reaction for the 1 eq. LiH system even when longer milling times were applied. Instead, pronounced cementation of the milling powder was observed, which is known to impede efficient mixing and energy transfer during mechanochemical processing and can therefore hinder reaction progress. In addition, the reaction may be less thermodynamically favored under the investigated conditions. Among the reactions considered, the system containing 1 eq. LiH exhibits the lowest calculated exothermicity, suggesting a reduced driving force for reaction initiation compared to the other hydride-containing systems. While the exact origin of the observed behavior cannot be conclusively determined from the available data, both the limited reaction enthalpy and the occurrence of cementation are likely contributing factors. A corresponding discussion has been added to page 4 of the revised manuscript.

 

Comment 5. For the cryogenic milling experiments with 4 eq. Na, the authors report powder characteristics. However, no comparative PXRD or Rietveld data are shown for the cryo-milled sample versus the room-temperature product. Please include this comparison and discuss whether cryomilling also alters the phase composition or only improves handling.

Response 5: The corresponding data is given in the Supporting Information (Page S7).

 

Comment 6. In the experimental section, the authors explicitly state that, for safety reasons to avoid exceeding the pressure limit, the total batch mass for the 3 and 4 eq. LiH reactions was reduced from the standard 3 g to 2 g and 1.5 g, respectively. Whether the reduced batch mass could potentially delay the induction period of the LiH reactions or alter the final phase distribution of the products; and if such effects exist, whether complementary experiments under identical ball‑to‑powder ratios and filling ratios should be performed to ensure the reliability of the conclusions.

Response 6: We thank the reviewer for this important comment. Based on our previous work, we have found that variations in milling conditions, including milling time and milling equipment, exert only a limited influence on the final phase composition. As demonstrated in our earlier publication, extending the milling time beyond the onset of the reaction resulted in only minor changes in the reaction products. Therefore, even if the reduced batch mass affected the induction period through a modified ball-to-powder ratio, no significant changes in the final product distribution would be expected. Furthermore, reducing the batch mass leads to a lower filling degree of the milling jar, falling below the range recommended by the manufacturer (Fritsch) for efficient milling. Additional experiments at lower batch masses would therefore not necessarily provide more representative conditions. Considering these factors, as well as the desire to minimize unnecessary wear of the milling equipment, no further complementary experiments were performed.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors successfully addressed the majority of my comments; however, they did not conduct additional experiments, provide further characterization, or implement the requested changes to the manuscript. I still believe these changes are necessary, despite the authors' contrary opinion. The persistent comments are listed below:

 

Comment 5: Still no results for the experiments performed with NaCl are shown. Showing e.g. PXRD patterns in the ESI is suggested.

Comment 7: Showing temperature curves in the ESI is suggested.

Comment 9: Marking phases in the XRD pattern is suggested.

Comment 10: Situation directly after MSR vs after 10 min- at least a note that this was investigated in the previous work and that the patterns were the same should be added.

Comment 12: XPS measurements were not conducted.

Author Response

Comment 5: Still no results for the experiments performed with NaCl are shown. Showing e.g. PXRD patterns in the ESI is suggested.

Response 5: We have included the PXRD patterns for the experiment performed with NaCl in the ESI (Figure S2).

 

Comment 7: Showing temperature curves in the ESI is suggested.

Response 7: We have included the evolution of pressure and temperature during milling for V₂O₅ with 2 eq. of (a) NaH, (b) Na, (c) LiH and (d) Li for 10 min in the ESI (Figure S3). We have also added a comment to the manuscript stating that all product phases had already formed at the time of maximum pressure, as shown in the previous paper.

 

Comment 9: Marking phases in the XRD pattern is suggested.

Response 9: In the revised manuscript, the crystalline phases identified by the Rietveld refinements have been marked in the XRD patterns using phase reference markers (Figure 2).

 

Comment 10: Situation directly after MSR vs after 10 min- at least a note that this was investigated in the previous work and that the patterns were the same should be added.

Response 10: We have added a note in the revised manuscript stating that the phase composition immediately after the MSR was investigated in our previous work and that the corresponding XRD patterns were found to be identical to those obtained after 10 minutes of milling.

 

Comment 12: XPS measurements were not conducted.

Response 12: We thank the reviewer for raising this point. We respectfully maintain our view that XPS measurements are not essential to support the conclusions of the present study. The primary objective of this work is to elucidate the mechanochemical reducing behavior of alkali metal hydrides in comparison with the corresponding elemental metals. In this context, the key findings are derived from phase analysis and quantitative Rietveld refinements, which already provide clear evidence for the reaction pathways and the coexistence of multiple vanadium-containing phases.

While XPS could provide additional information on the surface oxidation states of vanadium, we expect the measurements to mainly confirm the presence of a mixture of different oxidation states, which is already evident from the XRD results. Given the multiphase nature of the products and the fact that the manuscript does not focus on application-oriented properties, we believe that XPS data would not substantially strengthen the scientific conclusions of the study.

In addition, the investigated samples are highly air-sensitive, requiring specialized handling and transfer procedures for reliable XPS analysis. Considering the limited additional insight expected from these measurements, we do not consider their inclusion justified within the scope of the present work. We nevertheless appreciate the reviewer’s suggestion and intend to incorporate complementary oxidation-state analyses, including XPS and potentially XANES measurements, in future studies where such investigations are more directly relevant to the scientific objectives.