Tailoring Pore Size in Bimetallic Nb-Mn/MCM-41 Catalysts for Enhanced Plasma-Driven Catalytic Oxidation of Toluene

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

This manuscript is a valuable contribution to the field of plasma catalysis with mesoporous materials. It offers insights into how pore size and metal loading affect plasma-assisted toluene oxidation. However, the manuscript has some drawbacks, and a major revision is required.

• The title overstates the scope by using the term "architectural," while the work primarily focuses on tuning pore size and metal loading. Additionally, the Mn valency and loading have significant impact on plasma-driven catalytic oxidation, which the current title does not reflect. Please revise the title to better represent the actual focus and highlight the role of Mn/Nb chemistry.
• Although the metal loadings are reported to be consistent across samples, Nb-Mn/M4 exhibits a substantial increase in pore volume (1.202 cm³/g) relative to its support (0.725 cm³/g). This unusual result should be clarified.
• The XPS data (Table S2) should be included in the main manuscript, not the supplementary material. These results indicate that redox-active Mn⁴⁺ species and lattice oxygen abundance correlate strongly with catalytic performance, suggesting that metal valency plays a crucial role alongside pore size. The current discussion downplays metal valency factors. While Mn's role is well-described, the function of Nb is less developed. Table S2 shows variations in Nb valency, but the discussion does not clearly define whether Nb is involved in catalytic process.
• Page 5, Lines 168–170

The statement that "the larger pore size in Nb-Mn/M3 likely facilitates a greater surface area" is misleading. As shown in Table S1, Nb-Mn/M3 has a lower surface area (981 m²/g) than M1 (1261 m²/g). The optimized pore size in Nb-Mn/M3 (3.73 nm) might improve dispersion and accessibility of active sites, rather than increasing total surface area.

• Page 8, Lines 242–252

 Referring to M1’s pore size (2.7 nm) as “subcritical” and M3’s (3.8 nm) as “critical” may overstate the physical significance of this transition. In fact, 2.7 nm is already sufficiently large for water/small molecule diffusion. The authors should either provide literature support for defining a “critical” pore size in this range or revise this terminology.

• While this study focuses on fundamental insights, it would strengthen the paper to compare the optimal catalyst (Nb-Mn/M3) to commercial or previously reported systems in terms of plasma energy efficiency or turnover performance.
• Figure 1 Label should be revised for clarity.
• Figure 2 Caption is incorrectly labeled as “pore size distribution.”

 

Author Response

Comments 1: The title overstates the scope by using the term "architectural," while the work primarily focuses on tuning pore size and metal loading. Additionally, the Mn valency and loading have significant impact on plasma-driven catalytic oxidation, which the current title does not reflect. Please revise the title to better represent the actual focus and highlight the role of Mn/Nb chemistry.

Response 1: Thank you for your insightful feedback. We agree with your suggestion and have revised the title to more accurately reflect the manuscript's scope. The updated title is now: "Tailoring Pore Size and Mn Valency in Bimetallic Nb-Mn/MCM-41 Catalysts for Enhanced Plasma-Driven Catalytic Oxidation of Toluene". The modified title can be found in Page 1, Lines 2-3 of the revised manuscript.

Comments 2: Although the metal loadings are reported to be consistent across samples, Nb-Mn/M4 exhibits a substantial increase in pore volume (1.202  cm³g^-1) relative to its support (0.725  cm³g^-1). This unusual result should be clarified.

Response 2: We appreciate the reviewer’s keen observation regarding the unexpected increase in pore volume for Nb-Mn/M4 compared to its support. This phenomenon indeed warrants clarification. We have addressed this explanation in lines 89-94 on page 2-3 of the revised manuscript (highlighted in red), with supporting references [38] now included in the updated reference list:

“However, Nb-Mn/M4 exhibited an anomalous increase in pore volume (1.202 cm³g^-1) compared to its support (0.725 cm³g^-1), which may arise from the unique interplay between the expanded pore architecture of M4 (3.98 nm) and the spatial distribution of metal oxides. The larger pore size of M4 likely facilitated the formation of well-dispersed Nb-Mn clusters within the mesochannels without significant pore blockage, while partial structural reorganization during calcination could contribute to localized pore expansion [38].”

Comments 3: The XPS data (Table S2) should be included in the main manuscript, not the supplementary material. These results indicate that redox-active Mn⁴⁺ species and lattice oxygen abundance correlate strongly with catalytic performance, suggesting that metal valency plays a crucial role alongside pore size. The current discussion downplays metal valency factors. While Mn's role is well-described, the function of Nb is less developed. Table S2 shows variations in Nb valency, but the discussion does not clearly define whether Nb is involved in catalytic process.

Response 3: We sincerely thank the reviewer for highlighting the importance of metal valency in catalytic mechanisms. As suggested, we have moved Table S2 into the revised manuscript as Table 1 (Located on Page 6, lines 177, highlighted in red), which now explicitly details the XPS-derived Mn and Nb valence states alongside oxygen species distribution.

We have addressed this explanation in lines 154-160 and lines 168-170 on page 5 of the revised manuscript (highlighted in red):

“As shown in Table 1, the Nb⁵⁺/Nb⁴⁺ ratio increased from 2.80 in Nb-Mn/M1 to 5.28 in Nb-Mn/M4, with Nb⁵⁺ content rising from 73.7% to 84.1%. This mixed-valent niobium configuration exhibited dual functionality: Nb⁵⁺ enriched oxygen vacancies through strong Nb-O bonds. While Nb⁴⁺ promoted charge transfer between Mn centers and adsorbed intermediates [46]. Notably, the highest Nb⁵⁺/Nb⁴⁺ ratio (5.25) in Nb-Mn/M3 coincided with its optimal toluene degradation efficiency (96.8%), suggesting synergistic redox coupling between Nb and Mn.”

“The synergistic interplay between Nb⁵⁺-driven oxygen vacancy formation and Mn⁴⁺-mediated ROS generation underscores the critical role of dual-metal valency in catalytic oxidation.”

Additionally, we have expanded the discussion of Nb’s catalytic function in the Discussion section (Page 9, lines 280-281, lines 287-295, lines 298-301, highlighted in red), with the following additions:

“XPS analysis revealed that pore size evolution modulated the valence state distribution of Mn by altering the strength of the metal-support interaction.”

“Concurrently, the dual oxidation states of Nb (Nb⁴⁺ and Nb⁵⁺) synergistically modulated catalytic behavior. The Nb⁵⁺/Nb⁴⁺ ratio increased from 2.8 (Nb-Mn/M1) to 5.28 (Nb-Mn/M4), with Nb⁵⁺% rising from 73.7% to 84.1% (Table 1). Higher Nb⁵⁺ content (e.g., 84.0% in Nb-Mn/M3) enhanced oxygen vacancy formation through strong Nb-O bonds, while Nb⁴⁺ facilitated electron transfer between Mn⁴⁺ and adsorbed intermediates. This dual functionality of Nb stabilized Mn⁴⁺ via Nb-O-Mn linkages and amplified ROS generation, as evidenced by the optimized O_lat/O_ad ratio (55.5%) and toluene degradation efficiency (96.8%) in Nb-Mn/M3 [46].”

“Despite the highest Nb⁵⁺% in Nb-Mn/M4 (84.1%), the diminished Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio (27.4%) and lower active site density disrupted the Nb-Mn redox synergy, ultimately explaining the reduced carbon balance (Figure 7b).”

The supporting references [46] now included in the updated reference list.

Comments 4: Page 5, Lines 168–170 The statement that "the larger pore size in Nb-Mn/M3 likely facilitates a greater surface area" is misleading. As shown in Table S1, Nb-Mn/M3 has a lower surface area (981 m²/g) than M1 (1261 m²/g). The optimized pore size in Nb-Mn/M3 (3.73 nm) might improve dispersion and accessibility of active sites, rather than increasing total surface area.

Response 4: We sincerely thank the reviewer for identifying this inaccuracy. The original statement regarding surface area was indeed imprecise. We have revised the text in Page 6, Lines 189-193 (highlighted in red) to clarify the role of pore size in Nb-Mn/M3, as follows:

"The optimized pore size in Nb-Mn/M3 (3.73 nm) improves the dispersion and accessibility of active sites (Table S1), despite its lower total surface area (981 m²/g) compared to Nb-Mn/M1 (1261 m²/g). This enhanced active site exposure facilitates oxygen adsorption and promotes the uniform distribution of oxygen species, thereby amplifying redox activity and catalytic performance."

Comments 5: Page 8, Lines 242–252 Referring to M1’s pore size (2.7 nm) as “subcritical” and M3’s (3.8 nm) as “critical” may overstate the physical significance of this transition. In fact, 2.7 nm is already sufficiently large for water/small molecule diffusion. The authors should either provide literature support for defining a “critical” pore size in this range or revise this terminology.

Response 5: We sincerely thank the reviewer for highlighting the potential ambiguity in the terminology. We agree that the terms “subcritical” and “critical” may imply an unwarranted physical threshold. We have revised the text in Page 9, Lines 269-279 (highlighted in red) to adopt more neutral descriptors:

“XRD and TEM characterization revealed that pore size expansion significantly influenced the diffusion kinetics of precursors within the channels. As the pore size increased from a suboptimal range (M1: 2.49 nm) to an optimized range (M3: 3.73 nm), the capillary forces of the impregnation solution within the mesoporous structure weakened substantially. This facilitated uniform distribution of metal precursors along the entire channel axis, effectively mitigating particle aggregation during the subsequent drying process. As a result, the MnO_x crystallite size decreased sharply from 2.5 ± 0.4 nm (Nb-Mn/M1) to 1.5 ± 0.2 nm (Nb-Mn/M3), while the BET surface area increased remarkably, thereby exposing significantly more active catalytic sites. However, when the pore size exceeded this ideal range, the excessive space dramatically reduced the zeolite's ability to anchor metal species, leading to renewed sintering tendencies and partial loss of dispersion control.”

Comments 6: While this study focuses on fundamental insights, it would strengthen the paper to compare the optimal catalyst (Nb-Mn/M3) to commercial or previously reported systems in terms of plasma energy efficiency or turnover performance.

Response 6: We sincerely thank the reviewer for raising this valuable point. We fully acknowledge the importance of benchmarking against commercial or literature-reported systems to contextualize practical performance. In future studies, we plan to systematically compare the plasma energy efficiency and turnover metrics of Nb-Mn/M3 with industrial catalysts. This will enable a rigorous evaluation of the technological advantages conferred by zeolite pore architecture optimization.

Comments 7: Figure 1 Label should be revised for clarity.

Response 7: We sincerely thank the reviewer for highlighting this ambiguity and apologize for the incomplete labeling in the original figure. In the revised Figure 1, we have added critical annotations to clarify the pore size distribution analysis. (Located on Page 3, Line 102, Figure 1)

Comments 8: Figure 2 Caption is incorrectly labeled as “pore size distribution.”

Response 8: We sincerely apologize for the oversight and deeply appreciate the reviewer’s diligence in identifying this critical labeling discrepancy. We confirm that the corrected Figure 2 caption now reads: “Figure 2. XRD patterns of different pore structure of catalysts.” (Highlighted in red on Page 4, Line 124 of the revised manuscript).

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Editor,

please find attached my review of the manuscript "Architectural Modulation of MCM-41 Mesopores for enhancing Bimetallic Nb-Mn/MCM-41 catalyst in Plasma-Driven Catalytic Oxidation of Toluene" submitted by Yao et al. aiming on the engineering of the pore structure to improve conversion of toluene. In the current state due to the following reasons I cannot recommend a publication of the manuscript:

• The authors please take care of the basics; abbreviations are properly introduced when used for the first time, parameters are stated in italics, there is a space between value and unit, the figure caption gives a correct description of the figure
• None of the determined values have quantified measurement uncertainties, hence it is impossible if the reported changes are significant or not
• What is the criterion to define the "optimum" M4+/(Mn2+-Mn3+) ratio? Why is it 36.8% and not 35.9856%?
• Why is ozone formation bad? Please be specific.
• Properly introduce MCM-41 with highlighting the benefits compared to ZSM-5 and 13X
• Which modification of Nb2O5 is received in XRD, there are several modifications of Nb2O5 existing. How does this finding match with the dominance of Nb4+ in XPS?
• The XRD plots should be done with log intensity axis and better data quality is needed. It looks like that the overall intensity is decreasing when going from M1 to M4 sample, hence, a more detailed analysis of the intensity changes and potential changes in  phase fractions is needed. Additionally, the statement that a decrease in size of the crystallites results in a reduced intensity is fundamentally wrong, because the the reflection is just getting broader and hence the maximum value drops, but the intensity is the area underneath the reflection and proportional to the number of electrons involved. So, if there is not difference in the composition, there is not difference in intensity. The composition should be analyzed by ICP-OES, since it is anyway claimed but not shown.
• Does XRD crystallite size match TEM determination?
• A precise description of the samples is missing, what is behind the different sample labels? What is there composition?
• Do the phase fractions from XRD match the XPS determined oxidation states of Mn?
• How many crystallites were analyzed for the distribution?
• The XPS peak asignment needs to be supported by literature
• What is the explanation for usual Nb4+ presence?
• Figure 4 has different legend styles. For Nb 3d two sets of peaks are expected. Most likely Nb4+ and Nb5+ are asigned in the wrong way. It will be important to discuss in detail the storage and potential treatment conditions of the sample before XPS.
• How can a desorption of H2O be excluded, in particular at around 100 °C?
• XRD and TEM cannot resolve a change in the diffusion kinetics, only in the crystal and pore structure
• Line 253-257: To me there is a contradiction, since if Mn4+ bonds stronger (it will bind to the substrate also via an oxide ion), it hardly can enhance reducibilit of lattice oxygen.
• There is no proof for a catalytic operation regime. How many cycles can be done? What is the TON? etc.
• The mechanism is barely supported and more sophisticated analysis is required
• The experimental section does not comply with good scientific pratice, since there is no information given allowing for a reproduction of the results, neither for sample preparation nor for sample characterization, e.g., what is the mixing ratio of the chemicals? which wavelength was used in XRD? Which machines were used? Are the results reproducible even by the authors?

Author Response

Comments 1: The authors please take care of the basics; abbreviations are properly introduced when used for the first time, parameters are stated in italics, there is a space between value and unit, the figure caption gives a correct description of the figure.

Response 1: Thank you for your careful review and valuable suggestions. We sincerely apologize for the oversight in formatting details and have now carefully revised the manuscript to comply with the journal's guidelines. The following corrections have been made: All abbreviations (e.g., VOCs, ROS, MvK) are now properly introduced upon their first mention in the text. A space has been consistently added between numerical values and their corresponding units throughout the manuscript. The caption for Figure 2 has been revised to accurately describe the figure content. It now reads: "Figure 2. XRD patterns of different pore structure of catalysts." (Highlighted in red on Page 4, Line 124 of the revised manuscript).

Comments 2: None of the determined values have quantified measurement uncertainties, hence it is impossible if the reported changes are significant or not.

Response 2: We sincerely appreciate the reviewer’s insightful feedback. To address the concern regarding measurement uncertainties, we have now incorporated quantified uncertainties for all nanoparticle size distributions derived from TEM analysis. The uncertainties represent the standard deviations calculated from statistical analysis of the particles per catalyst using Image J software. These additions (Highlighted in red on Page 1 (Line 16), Page 4 (Lines 133-135), and Page 9 (Lines 275-276)) explicitly demonstrate the significance of differences in particle sizes across catalysts.

Comments 3: What is the criterion to define the "optimum" Mn⁴⁺/(Mn²⁺-Mn³⁺) ratio? Why is it 36.8% and not 35.9856%?

Response 3: We sincerely appreciate the reviewer’s critical evaluation of the term "optimal" in our original statement. Upon reflection, we agree that "optimal" might imply a universally absolute or theoretically derived "best value," which could be misinterpreted without explicit justification. To address this concern, we propose the following revisions: We have revised the text in Page 11, Lines 340-342 (highlighted in red) to adopt more neutral descriptors: “The Nb-Mn/M3 catalyst, exhibiting optimized Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio (36.8%) and lattice oxygen abundance (O_lat/O_ad = 55.5%)(Table 1)”. The term "optimal" has been revised to "optimized" to emphasize that the Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio (36.8%) and lattice oxygen abundance (55.5%) were experimentally determined to maximize catalytic performance among the tested catalysts. The value 36.8% reflects the highest ratio observed in the catalyst series, correlating with its superior toluene degradation efficiency. As noted, Mn⁴⁺ promotes O₂^- stabilization and O^- radical generation, which drive C-H bond cleavage [54]. Thank you for highlighting the need for precision in terminology and data interpretation.

Comments 4: Why is ozone formation bad? Please be specific.

Response 4: Ozone formation in plasma-catalytic systems can negatively impact the degradation process and practical applications for the following reason. Competitive Consumption of ROS: While ozone contributes to replenishing O_lat via decomposition, excessive ozone competes with other ROS for active sites on the catalyst surface. This competition may reduce the availability of atomic oxygen radicals, which are critical for C-H bond cleavage in toluene (as highlighted in Figure 8 and Ref. [36]). In our system, the Nb-Mn/MCM-41 catalyst mitigates ozone-related drawbacks by (1) rapidly decomposing ozone into reactive O⁻ via Mn⁴⁺-mediated pathways, and (2) leveraging Nb⁴⁺/⁵⁺ redox pairs to buffer and stabilize oxygen species, thereby minimizing gas-phase ozone accumulation. This design ensures that ozone primarily serves as an O_lat replenishment source rather than a harmful residual.

Comments 5: Properly introduce MCM-41 with highlighting the benefits compared to ZSM-5 and 13X

Response 5: We sincerely appreciate the reviewer’s constructive comment. As suggested, we have expanded the discussion on MCM-41’s structural advantages compared to ZSM-5 and 13X in the revised manuscript. We have added the content in lines 63-68 on page 2 of the revised manuscript (highlighted in red), with supporting references [36] now included in the updated reference list:

“Unlike microporous ZSM-5 (pore size: 0.5-0.6 nm) and 13X (pore size: 0.7-0.8 nm), MCM-41’s ordered hexagonal mesopores (2-10 nm) enable superior mass transfer of toluene and intermediates, mitigate pore-blocking, and enhance accessibility to active sites. Additionally, MCM-41’s high specific surface area and tunable surface acidity optimize metal dispersion and redox cycle stability under plasma conditions [36].”

Comments 6: Which modification of Nb₂O₅ is received in XRD, there are several modifications of Nb₂O₅ existing. How does this finding match with the dominance of Nb⁴⁺ in XPS?

Response 6: We appreciate the reviewer's valuable comments. The XRD diffraction peak at 33.12° corresponds to orthorhombic Nb₂O₅ (JCPDS 30-0873), which is conventionally associated with Nb⁵⁺. The observed coexistence of Nb⁴⁺ (26.3-15.9%) and Nb⁵⁺ in XPS arises from surface-specific redox interactions between Nb₂O₅ and manganese oxides during calcination. While bulk crystalline structure detected by XRD predominantly reflects Nb⁵⁺ in the Nb₂O₅ lattice, XPS probes surface composition where partial reduction occurs through electron transfer at Nb-Mn interfaces. This surface reduction mechanism is consistent with the Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio variation across pore sizes, as enhanced Mn⁴⁺ content in larger pores (Nb-Mn/M3) promotes stronger redox coupling with adjacent Nb species, explaining the Nb⁴⁺ presence despite bulk Nb₂O₅ crystallinity.

Comments 7: The XRD plots should be done with log intensity axis and better data quality is needed. It looks like that the overall intensity is decreasing when going from M1 to M4 sample, hence, a more detailed analysis of the intensity changes and potential changes in phase fractions is needed. Additionally, the statement that a decrease in size of the crystallites results in a reduced intensity is fundamentally wrong, because the the reflection is just getting broader and hence the maximum value drops, but the intensity is the area underneath the reflection and proportional to the number of electrons involved. So, if there is not difference in the composition, there is not difference in intensity. The composition should be analyzed by ICP-OES, since it is anyway claimed but not shown.

Response 7: We sincerely appreciate the reviewer’s insightful comments and suggestions, which have significantly improved the quality of our manuscript. We fully agree with the reviewer’s points regarding the misinterpretation of XRD intensity changes and the necessity of composition verification. In response to the recommendation, we have removed the erroneous statement that reduced crystallite size directly causes decreased intensity. Furthermore, ICP-OES data confirming the loading of Nb/Mn across all catalysts are included in Table S1 of the Supplementary materials, aligning with our claims of compositional uniformity. We have added the following contents in lines 109-111 and lines 118-122 on page 3 of the revised manuscript (highlighted in red):

“Notably, the progressive broadening of MnO, Mn₂O₃, and MnO₂ reflections in Nb-Mn/M1 to Nb-Mn/M4 catalysts indicates a reduction in crystallite size with increasing MCM-41 pore diameter.”

“ICP-OES results (Table S1) confirmed a nominal Nb/Mn molar ratio of 1:1 across all catalysts (deviation < 3%), ruling out compositional variation as a cause of reflection broadening. The intensity-area invariance further supports that phase fractions remain consistent, with observed broadening attributed solely to crystallite size effects.”

Comments 8: Does XRD crystallite size match TEM determination?

Response 8: We thank the reviewer for this important question. The XRD reflection broadening trends align qualitatively with TEM-derived particle size distributions, both indicating smaller crystallites in larger-pore catalysts. TEM statistically samples individual particles, whereas XRD reflection broadening is sensitive to crystallite size effects, explaining potential minor quantitative differences. This consistency supports the proposed pore-size-dependent growth mechanism.

Comments 9: A precise description of the samples is missing, what is behind the different sample labels? What is there composition?

Response 9: We sincerely thank the reviewer for highlighting this ambiguity. In the revised manuscript (Page 11, Lines 380-384 of Section 4.2), we have added the following clarification:

“The labels Nb-Mn/M1 to Nb-Mn/M4 denote Nb-Mn/MCM-41 catalysts with incrementally enlarged pore diameters (specific values in Table S1), synthesized using MCM-41 supports of distinct pore architectures. All samples maintain a nominal Nb/Mn molar ratio of 1:1, as verified by ICP-OES (Table S1).”

Comments 10: Do the phase fractions from XRD match the XPS determined oxidation states of Mn?

Response 10: We thank the reviewer for this critical observation. While XRD identifies bulk MnO, Mn₂O₃, and MnO₂ phases (Figure 2), XPS reveals surface Mn³⁺/Mn⁴⁺ enrichment (Table 1) due to calcination-induced oxidation. The systematic increase in Mn⁴⁺/ (Mn²⁺+Mn³⁺) ratios with pore size (up to 36.8% in Nb-Mn/M3) aligns with enhanced catalytic redox activity, though bulk composition remains unchanged. This apparent disparity arises from XPS’s surface sensitivity (~5 nm depth) versus XRD’s bulk averaging, highlighting complementary insights: surface Mn valency tuning (XPS) governs reactivity, while bulk phase uniformity (XRD) ensures structural consistency. The synergy between surface Mn⁴⁺ enrichment and pore-dependent redox enhancement underpins the catalytic mechanism.

Comments 11: How many crystallites were analyzed for the distribution?

Response 11: We thank the reviewer for emphasizing statistical rigor. Over 200 nanoparticles per catalyst were analyzed via TEM (Figure 3) to ensure representative size distributions. Detailed counts. This robust sampling minimizes random errors and validates the observed pore-size-dependent crystallite growth trend.

Comments 12: The XPS peak asignment needs to be supported by literature

Response 12: We thank the reviewer for this suggestion. In Page5, Line 148 and 154, references [37] [44] [45] have been added to support the XPS peak assignments: Mn²⁺/Mn³⁺/Mn⁴⁺ (640.4/641.2/642.8 eV) [44] and Nb⁴⁺/Nb⁵⁺ (205.2/206.0 eV) [37, 45], aligning with reported metal oxide binding energies.

Comments 13: What is the explanation for usual Nb⁴⁺ presence?

Response 13: We thank the reviewer for this query. The coexistence of Nb⁴⁺ and Nb⁵⁺ arises from partial reduction of Nb⁵⁺ during calcination, mediated by MCM-41’s silanol groups acting as weak reductants. Nb⁴⁺ facilitates charge transfer between Mn centers and intermediates (enhancing redox cycling), while Nb⁵⁺ stabilizes oxygen vacancies via strong Nb-O bonds. This dual-valent synergy optimizes catalytic activity, as evidenced by Nb-Mn/M3’s highest Nb⁵⁺/ Nb⁴⁺ ratio (5.25) correlating with superior toluene degradation (96.8%). Revised §3 Discussion (Located on Page 9, Lines 287-301 highlighted in red) clarifies this mechanism:

“Concurrently, the dual oxidation states of Nb (Nb⁴⁺ and Nb⁵⁺) synergistically modulated catalytic behavior. The Nb⁵⁺/Nb⁴⁺ ratio increased from 2.8 (Nb-Mn/M1) to 5.28 (Nb-Mn/M4), with Nb⁵⁺% rising from 73.7% to 84.1% (Table 1). Higher Nb⁵⁺ content (e.g., 84.0% in Nb-Mn/M3) enhanced oxygen vacancy formation through strong Nb-O bonds, while Nb⁴⁺ facilitated electron transfer between Mn⁴⁺ and adsorbed intermediates. This dual functionality of Nb stabilized Mn⁴⁺ via Nb-O-Mn linkages and amplified ROS generation, as evidenced by the optimized O_lat/O_ad ratio (55.5%) and toluene degradation efficiency (96.8%) in Nb-Mn/M3 [46]. Notably, the excessively large pore size (M4), while further improving mass transfer efficiency, resulted in a reduced surface area (1015→760 m²/g). This led to a decreased loading density of active components; and an excessively fast desorption rate of intermediate products, reducing opportunities for secondary oxidation. Despite the highest Nb⁵⁺% in Nb-Mn/M4 (84.1%), the diminished Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio (27.4%) and lower active site density disrupted the Nb-Mn redox synergy, ultimately explaining the reduced carbon balance (Figure 7b).”

Comments 14: Figure 4 has different legend styles. For Nb 3d two sets of peaks are expected. Most likely Nb⁴⁺ and Nb⁵⁺ are asigned in the wrong way. It will be important to discuss in detail the storage and potential treatment conditions of the sample before XPS.

Response 14: We sincerely thank the reviewer for their meticulous observation and valuable feedback. We acknowledge the error in the original assignment of Nb⁴⁺ and Nb⁵⁺ peaks in Figure 4 and we have corrected the XPS spectrum labels in the revised manuscript (Located on Page 5, Line 174, Figure 4b).

Comments 15: How can a desorption of H₂O be excluded, in particular at around 100 °C?

Response 15: We thank the reviewer for highlighting this concern. H₂O desorption at ~100 °C was excluded by pre-treating catalysts at 150 °C under N₂ for 2 h to remove physisorbed water.

Comments 16: XRD and TEM cannot resolve a change in the diffusion kinetics, only in the crystal and pore structure

Response 16: We appreciate the reviewer’s valid critique. While XRD/TEM directly resolve crystallite/pore structures, the inferred diffusion effects are based on structural trends: larger pores (M4) enable uniform metal dispersion (TEM, Figure 3), shortening diffusion paths, whereas confined pores (M1) restrict precursor mobility, aligning with crystallite size differences (XRD).

Comments 17: Line 253-257: To me there is a contradiction, since if Mn⁴⁺ bonds stronger (it will bind to the substrate also via an oxide ion), it hardly can enhance reducibilit of lattice oxygen.

Response 17: We appreciate the reviewer’s keen observation. While Mn⁴⁺ forms stronger bonds, its presence promotes oxygen vacancy generation (via charge compensation between Mn⁴⁺ and Nb⁵⁺ in Nb-Mn/M3), lowering the energy barrier for lattice oxygen release (O₂-TPD: 0.222 mmol/g mobile oxygen). This is further evidenced by the enhanced O_lat/O_ad ratio (55.5%, Table 1), where oxygen vacancies accelerate oxygen mobility. Thus, Mn⁴⁺’s role in vacancy-mediated redox cycling overcomes its intrinsic bond strength, reconciling the apparent contradiction.

Comments 18: There is no proof for a catalytic operation regime. How many cycles can be done? What is the TON? etc.

Response 18: We sincerely thank the reviewer for raising this important concern regarding the catalytic operation regime. We have conducted a 60-hour continuous stability test (newly added as Figure S3 in the Supplementary Materials) under plasma-catalytic conditions. The results demonstrate that both Nb-Mn/M3 and Nb-Mn/M1 exhibit robust stability. We have added the following contents in lines 262-266 on page 8 of the revised manuscript:

“The long-term stability of the catalysts was evaluated through a 60-hour continuous plasma-catalytic oxidation test (Figure S3). Both Nb-Mn/M3 and Nb-Mn/M1 maintained high toluene degradation efficiencies with minimal deactivation, highlighting their potential for industrial-scale applications.”

Comments 19: The mechanism is barely supported and more sophisticated analysis is required

Response 19: We appreciate the reviewer’s critical feedback. While the proposed mechanism is primarily inferred from structural (XRD/TEM) and surface-state (XPS) correlations with catalytic activity. Current evidence-such as Mn⁴⁺ enrichment (XPS) correlating with enhanced oxygen mobility (O₂-TPD) and pore-dependent performance trends-collectively supports the role of metal-valency-mediated ROS generation.

Comments 20: The experimental section does not comply with good scientific pratice, since there is no information given allowing for a reproduction of the results, neither for sample preparation nor for sample characterization, e.g., what is the mixing ratio of the chemicals? which wavelength was used in XRD? Which machines were used? Are the results reproducible even by the authors?

Response 20: We sincerely appreciate the reviewer’s emphasis on reproducibility. In the revised §4.3. Catalyst characterization (Page 12, Lines 387-413), we have comprehensively detailed:

“Textural properties including specific surface area, pore volume and architecture were analyzed by N₂ physisorption using a Micromeritics ASAP 2020 analyzer (Micromeritics, USA). Prior to measurements at 77.3 K, approximately 300 mg of each sample underwent degassing treatment at 300 °C for 6 h under vacuum conditions. The specific surface area and pore size distribution were calculated using the Brunauer-Emmett-Teller (BET) and Barrett Joyner Halenda (BJH) pore distribution methods, respectively.

Crystalline phase identification was performed by X-ray diffraction (XRD) using an ARL X'TRA diffractometer (Thermo Scientific, USA) with Cu Kα radiation (λ = 0.15406 nm) operated at 30 kV and 30 mA. Diffraction patterns were recorded in the 2θ range of 5 ° to 90 ° with a scanning rate of 4 ° min⁻¹.

X-ray photoelectron spectroscopy (XPS) measurements were conducted on a PHI 5000 VersaProbe III spectrometer (ULVAC-PHI, Japan) equipped with monochromatic Al Kα X-ray source (300 W, 14 kV). All spectra were charge-referenced to the adventitious carbon C 1s peak at 284.6 eV and subsequently deconvoluted using the XPS Peak 4.1 software package with Shirley-type background subtraction.

Transmission electron microscopy (TEM) observations were carried out using a JEOL JEM-200CX instrument (JEOL, Japan) operated at 200 kV. Specimens were prepared by dispersing the powder samples in ethanol through ultrasonic treatment, followed by deposition onto carbon-coated copper grids and subsequent drying under infrared lamp illumination.

Oxygen temperature-programmed desorption (O₂-TPD) experiments were performed on a Micromeritics AutoChem II 2920 system (Micromeritics, USA) equipped with thermal conductivity detector (TCD). The protocol included: 100 mg catalyst pretreated in ultrapure He (99.999%, 30 mL min^-1) at 150°C for 2 h. Exposed to 2 vol% O₂/He mixture (50 mL min^-1) at 30 °C for 2 h after cooling to room temperature. He flow (50 mL min^-1) for 0.5 h to stabilize baseline. Temperature ramped from 30 °C to 800 °C at 10 °C min^-1 under He flow, with desorbed oxygen monitored by TCD.”

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed most of my concerns and I have no other comment on the revised manuscript. Thanks

Author Response

We are deeply grateful for the reviewer’s time and constructive feedback, which significantly improved our manuscript. We sincerely appreciate your acknowledgment of our revisions and your positive assessment of the revised version.

Reviewer 2 Report

Comments and Suggestions for Authors

The reviewer appreciates the efforts undertaken by the authors to improve their manuscripts and recommends it for publication after the following minor corrections.

• In context of response 3. Please add the boundary conditions of the optimization to the manuscript. This means that the optimized ratios observed are based on the data points of this study.
• Please include the content of response 4 as a brief and concise version to the manuscript
• It will be good to add to the Supporting Information a table with the determined crystallite sizes from XRD or at least the FWHM
• Please add the argumentation provided in response 10 in an appropriate manner to the manuscript
• Please add the (indirect) correlation between XRD/TEM and diffusion effects outline in response 16 to the manuscript to make it clear.
• Please also include the most important aspects of response 17 to the manuscript
• For the description of the XRD device the given wavelength suggests that Cu-Ka1 radiation was used, however, the text just states Cu-Ka (which normally implies that no monochromator was used and Cu-Ka1 and Cu-Ka2 are present and often a Ni filter is used to remove Cu-Kb radiation). Please adjust the description accordingly.

Author Response

Comments 1: In context of response 3. Please add the boundary conditions of the optimization to the manuscript. This means that the optimized ratios observed are based on the data points of this study.

Response 1: We sincerely appreciate the reviewer's insightful suggestion regarding the clarification of boundary conditions for the optimization. We fully agree that explicitly stating the experimental constraints is crucial for proper interpretation of the "optimized" ratios. In response, we have modified the manuscript text (Page 11, Lines 354-358) as follows (changes highlighted in red):

“The Nb-Mn/M3 catalyst, exhibiting optimized Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio (36.8%) and lattice oxygen abundance (O_lat/O_ad = 55.5%) within the tested catalyst series (Nb-Mn/M1 to Nb-Mn/M4, Table 1), which were synthesized under consistent conditions (incipient wetness impregnation, 500 °C calcination) with a fixed Nb/Mn molar ratio of 1:1.”

Comments 2: Please include the content of response 4 as a brief and concise version to the manuscript.

Response 2: We appreciate the reviewer's suggestion to concisely incorporate the ozone-related discussion into the manuscript. In response, we have added the following brief statement to the Discussion section (Page 10, Lines 342-346, highlighted in red):

"While ozone formation in plasma-catalytic systems may compete with reactive oxygen species for active sites, the Nb-Mn/MCM-41 catalyst effectively mitigates this effect through Mn⁴⁺-mediated ozone decomposition into reactive O⁻ species and Nb⁴⁺/Nb⁵⁺ redox stabilization of oxygen species, minimizing gas-phase ozone accumulation while maintaining efficient O_lat replenishment."

Comments 3: It will be good to add to the Supporting Information a table with the determined crystallite sizes from XRD or at least the FWHM.

Response 3: We sincerely appreciate the reviewer's valuable suggestion regarding the inclusion of crystallite size information. In response to this comment, we have added “Table S3. XRD peak analysis of Nb-Mn/M1-M4 catalysts” to the Supporting Information.

Comments 4: Please add the argumentation provided in response 10 in an appropriate manner to the manuscript.

Response 4: We sincerely appreciate the reviewer’s insightful suggestion. As requested, we have added the following paragraph to the Discussion section (Page 10, Lines 305-313, highlighted in red):

“The XRD identifies bulk MnO, Mn₂O₃, and MnO₂ phases (Figure 2), XPS reveals surface Mn³⁺/Mn⁴⁺ enrichment (Table 1) due to calcination-induced oxidation. The systematic increase in Mn⁴⁺/ (Mn²⁺+Mn³⁺) ratios with pore size (up to 36.8% in Nb-Mn/M3) aligns with enhanced catalytic redox activity. This apparent disparity arises from XPS’s surface sensitivity (~5 nm depth) versus XRD’s bulk averaging, highlighting complementary insights: surface Mn valency tuning (XPS) governs reactivity, while bulk phase uniformity (XRD) ensures structural consistency. The synergy between surface Mn⁴⁺ enrichment and pore-dependent redox enhancement underpins the catalytic mechanism.”

Comments 5: Please add the (indirect) correlation between XRD/TEM and diffusion effects outline in response 16 to the manuscript to make it clear.

Response 5: We sincerely appreciate the reviewer's suggestion to clarify the correlation between XRD/TEM structural data and diffusion effects. In response, we have added the following paragraph to the Discussion section (Page 9, Lines 280-283, highlighted in red):

“XRD/TEM directly resolve crystallite/pore structures, the inferred diffusion effects are based on structural trends: larger pores (M4) enable uniform metal dispersion (TEM, Figure 3), shortening diffusion paths, whereas confined pores (M1) restrict precursor mobility, aligning with crystallite size differences (XRD, Table S3).”

Comments 6: Please also include the most important aspects of response 17 to the manuscript.

Response 6: We sincerely appreciate the reviewer’s insightful observation regarding the apparent contradiction in our original discussion. We fully agree with the reviewer’s point: The modified description now reads (Page 9, Lines 284-289, highlighted in red):

XPS analysis revealed that pore size evolution modulated the valence state distribution of Mn by altering the strength of the metal-support interaction. When the pore size increased to 3.73 nm, the surface Mn⁴⁺/(Mn²⁺+Mn³⁺) ratio reached 36.8% (Table 1). The high Mn⁴⁺ content not only stabilized the lattice oxygen framework, as evidenced by O₂-TPD analysis, but also significantly improved the generation efficiency of ROS by promoting plasma-activated pathways.

Comments 7: For the description of the XRD device the given wavelength suggests that Cu-Ka1 radiation was used, however, the text just states Cu-Ka (which normally implies that no monochromator was used and Cu-Ka1 and Cu-Ka2 are present and often a Ni filter is used to remove Cu-Kb radiation). Please adjust the description accordingly.

Response 7: We sincerely appreciate the reviewer's careful reading and valuable suggestion regarding the XRD experimental details. We have revised the text to provide more precise information about the X-ray source and filtering conditions. The modified description now reads (Page 12, Lines 411-414, highlighted in red):

“Crystalline phase identification was performed by X-ray diffraction (XRD) using an ARL X'TRA diffractometer (Thermo Scientific, USA) with Cu Kα1 radiation (λ = 0.15406 nm) operated at 30 kV and 30 mA. Diffraction patterns were recorded in the 2θ range of 5 ° to 90 ° with a scanning rate of 4 ° min⁻¹.”

Author Response File: Author Response.pdf