The Effect of CeO₂ on the Catalytic Activity and SO₂ Resistance of the V₂O₅-MoO₃/TiO₂ Catalyst Prepared Using the Ball Milling Method for the NH₃-SCR of NO

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The work paid attention to the weakened sulfur tolerance of VMoTi catalyst in addition to the improved SCR activity. Although a series of characterizations including DRIFTS study were performed, the conclusions are quite superficial. They should point out the deactivation mechanism of VMoCeTi catalysts since the modification of Ce on SCR activity of VTi catalysts has been widely reported.

Other questions:

（1）     The information of SCR activity test should be provided in the Section 2.

（2）     Only NO conversion is shown in Fig. 1, and the data of N2 selectivity should be presented. For example, the addition of ceria would increase the redox property of catalyst and hereby probably accelerate the oxidation of NH3 at high temperatures. This would result in the increase in the NO conversion but decrease in the NOx conversion.

（3）     Why did they only include the XRD patterns of two samples in Fig. 3?

（4）     Please show the Raman spectrum of VMoTi in Fig. 6. Moreover, the characteristic band of Ce-O bond was not found even at a high Ce loading in the 20Ce sample.

（5）     Why did the addition of ceria increase the oxidation state of V?

Author Response

Reviewer #1:

Comments 1: Why was the effect of the amount of CeO₂ on the catalytic activity and SO₂ resistance of the V₂O₅-MoO₃/TiO₂ catalyst not studied?

Response: Thank the Reviewer for the good comment. We prepared CeO₂-modified 3V6MoxCeTiO₂ catalysts using the solid-state ball milling method. NH₃-TPD and H₂-TPR results indicated that the good SCR activity of the CeO₂-modified catalyst was due to its larger number of acid sites and the strong redox ability. XPS analysis showed that the introduction of CeO₂ created new charge transfer pathways (Ce⁴⁺/Ce³⁺ ↔ V⁵⁺/V⁴⁺) in the 3V6MoxCeTiO₂ catalysts, leading to an increase in the V⁵⁺ ratio and chemisorbed oxygen content. This led to improved catalytic activity. Since chemisorbed oxygen on the catalyst surface exhibits better mobility, higher reactivity, and a more significant migration rate, the higher proportion of O_α in the modified catalyst may be beneficial for the progress of "fast" SCR reactions, leading to superior catalytic performance.

Detailed in situ DRIFTs studies confirmed the presence of rich adsorbed   NH₃ and NO_x species on the CeO₂-modified catalyst surface. The adsorbed NH₃ was strongly activated by the CeO₂-modified catalyst, leading to better high-temperature activity. The SCR reaction over the 3V6MoxCeTiO₂ catalysts followed both the E-R and L−H mechanisms.

The addition of CeO₂ may significantly enhance the redox cycling ability of the catalyst. CeO₂ can provide active oxygen species, thereby increasing the reaction activity of the catalyst. However, the presence of SO₂ may react with CeO₂ to form stable sulfides or sulfates, which can cover or deactivate active sites, thus reducing the catalyst's resistance to SO₂ poisoning. Furthermore, in the presence of H₂O, water will also compete for adsorption with the reactant species, reducing the number of active sites available for the reaction, thereby decreasing the catalytic activity.

 

Chapter 2. Materials and Methods.

Comments 2: Line 68

Quote: ...the powder mixture was ground and blended...

Comment: Please specify the grain size after grinding.

Response: Thank the Reviewer for the good comment.

The 3V6MTiO₂ catalyst exhibited increased surface roughness after the addition of CeO₂. This could be due to CeO₂ particles attaching to the catalyst surface or surface changes induced by mechanical forces during the ball milling process. When the CeO₂ addition reached 20%, CeO₂ agglomerated, and some larger particles could be observed in the SEM images. The size of the catalyst is approximately 2 μm, and there was no significant change in the particle size of the sample after the addition of CeO₂.

Modification: Lines 179~185/Page 5: The 3V6MTiO₂ catalyst exhibited increased surface roughness after the addition of CeO₂. This could be due to CeO₂ particles attaching to the catalyst surface or surface changes induced by mechanical forces during the ball milling process. When the CeO₂ addition reached 20%, CeO₂ agglomerated, and some larger particles can be observed in the SEM images. The catalyst size was approximately 2 μm, and there was no significant change in the particle size of the sample after the addition of CeO₂.

Comments 3: Lines 106-107

Quote: Prior to the experiment, the catalyst powder was finely ground...

Comment: Please specify what "finely ground" means-grain size.

Response: Thank the Reviewer for the good comment. We mixed and ground the catalyst with KBr to ensure uniform catalyst dispersion. We obtained the wafer following the usual procedure for infrared testing, and the particle size was not the primary factor to consider. After grinding, we did not perform a sieving operation to determine particle size.

Chapter 3. Results and discussion 

Comments 4: Please show and explain the mechanism of the catalytic action of the tested catalysts

Response: The CeO₂-modified catalyst exhibits excellent selective catalytic reduction (SCR) activity, attributed to its greater number of acid sites and strong redox ability. With the addition of CeO₂, the V⁵⁺ ratio and chemisorbed oxygen content on the catalyst surface increase, and creating new charge transfer pathways (Ce⁴⁺/Ce³⁺ ↔ V⁵⁺/V⁴⁺) in the 3V6MoTiO₂ catalyst. Since chemisorbed oxygen on the catalyst surface shows better mobility, higher reactivity, and a more significant migration rate, the higher proportion of O_α in the modified catalyst may facilitate "fast" SCR reactions, enhancing catalytic performance. The CeO₂-modified catalyst surface has abundant adsorbed NH₃ and NO_x species, and the adsorbed NH₃ is strongly activated by the CeO₂-modified catalyst, thus improving catalytic activity. The SCR reaction on the 3V6MoxCeTiO₂ catalyst follows both the E-R and L-H mechanisms.

 

Chapter 3.1.1. NH₃-SCR activity for NO conversion

Comments 5: Lines 118-130

Please re-examine the interpretation of Fig.1. The interpretation does not reflect the actual values on the graph.

Response: Thank the Reviewer for the good comment. We have made modifications to the manuscript.

Fig. 1 NO conversion over the 3V6MoxCeTiO₂ catalysts.

Reaction conditions: [NO] = [NH₃] = 1000 ppm, [O₂] = 6 vol%, N₂ (balance), total flow rate = 500 mL/min, GHSV=30000 h⁻¹.

Modification: Lines 126~142/Page 3~4: The NH₃-SCR activity of the 3V6MoTiO₂ and CeO₂-modified 3V6MoxCeTiO₂ catalysts prepared by the solid-state ball milling method were shown in Fig. 1a. The 3V6MoTiO₂ catalyst exhibited weak high-temperature catalytic activity, and only 73% NO conversion was achieved at 420 ^oC. However, CeO₂ had a remarkably positive impact on catalyst activity. With the increase in CeO₂ doping, the high-temperature activity of the catalysts showed a significant improvement. However, when the CeO₂ doping reached 30%, the high-temperature activity of the catalyst decreased slightly, but it was still higher than that of the 3V6MoTiO₂ catalyst. This shrinkage in the catalyst's performance (3V6Mo30CeTiO₂) may be due to the over-oxidation of ammonia by the excessive CeO₂ content at high temperatures.

Meanwhile, the addition of CeO₂ broadens the activity temperature window of the catalyst, with the catalyst containing 20% CeO₂ expanding the activity temperature window by nearly 150 ^oC compared to the 3V6MoTiO₂ catalyst.

 

Comments 6: Why is the 3V6Mo20CeTiO₂, catalyst particularly promoted in the interpretation? After all, the 3V6Mo20CeTiO₂ catalyst stands out in the temperature range up to 150 ℃. In the temperature range from about 200 ℃ to about 400 ℃, the NO conversion values are at the same level (only the 3V6MoTiO₂ catalyst without CeO₂ showed lower values-above the temperature of about 340 ℃).

Response: Thank the Reviewer for the good comment. We chose the 3V6Mo20CeTiO₂ catalyst as the one typical sample with the optimal activity, among these catalysts, the temperature range at which the 3V6Mo20CeTiO₂ catalyst achieves ca. 100% activity is 50 °C wider compared to the other catalysts.

 

Chapter 3.1.2. Effect of SO₂, on the NH₃-SCR conversion of NO

Comments 7: Fig 2. Caption under the figure: there is ... (3V6MoCeTi0₂, ...) should be (3V6MoTiO₂, ...).

Response: The related revision has been made, as shown in the modification below.

Modification: Lines 145~146/Page 4: Fig. 2 NO conversion rate of the selected catalysts (3V6MoTiO₂, 3V6Mo5CeTiO₂, 3V6Mo20CeTiO₂) in the presence/absence of SO₂ and H₂O at 200 ^oC.

 

Comments 8: Why were 3V6Mo20CeTiO₂, and 3V6Mo5CeTiO₂, catalysts selected for testing CeO₂ catalysts?

Response: Thank the Reviewer for the good comment. We chose the 3V6Mo20CeTiO₂ and 3V6Mo5CeTiO₂ catalysts for the sulfur oxides and water resistance experiments because the NH₃-SCR activity of these two catalysts stands out across the entire temperature testing range.

 

Comments 9: How do the authors explain that the incorporation of CeO₂ into the V₂O₅-MoO₃/TiO₂ catalyst diminishes its resistance to sulfur and water poisoning (also the influence of CeO₂ amount)?

Response: Thank the Reviewer for the good comment. The addition of CeO₂ may significantly enhance the redox cycling ability of the catalyst. CeO₂ can provide active oxygen species, thereby increasing the reaction activity of the catalyst. However, the presence of SO₂ may react with CeO₂ to form stable sulfides or sulfates due to the ball milling method, which can cover or deactivate active sites, thus reducing the catalyst's resistance to SO₂ poisoning. Furthermore, in the presence of H₂O, water will also compete for adsorption with the reactant species, reducing the number of active sites available for the reaction, and thereby decreasing the catalytic activity.

 

Chapter 3.2. XRD analysis of the catalyst materials

Comments 10: Why did the authors choose for the research a CeO₂ catalyst 3V6Mo20CeTiO₂?

Response: Ceria has the unique property that its electrons can easily shift between reduced and oxidized states (Ce³⁺ ↔ Ce⁴⁺), which is in favor of variable levels of bulk and surface oxygen vacancies over the catalyst. Besides, our research group improved the dispersion of Ce species in CeO₂/TiO₂ catalyst for NH₃-SCR using a simple and convenient ligand-assisted ball milling approach. In this work, we chose the 3V6Mo20CeTiO₂ catalyst as one typical sample with the optimal activity.

Reference:

1. Zhan Z.C.; Liu X.J.; He H.; Song L.Y.; Li J.Z.; Ma D.Z. Fabrication of a flower-like Pd/CeO₂ material with improved three-way catalytic performance. J. Rare. Earth, 2013, 31, 750–759.
2. Yan, J.F.; Qiu, W.G.; Song, L.Y.; Chen, Y.; Su, Y.C.; Bai, G.M.; Zhang, G.Z.; He, H. Ligand-assisted mechanochemical synthesis of ceria-based catalysts for the selective catalytic reduction of NO by NH₃. Chem Commun. 2017, 53, 1321-1324.

 

Comments 11: line 164: jest (3V6MoCeTiO₂) should be (3V6MoTiO₂)

Response: The related revision has been made, as shown in the modification below.

Modification: Lines 170~172/Page 5: Fig. 3 depicts the XRD patterns of the pristine (3V6MoTiO₂) and modified catalyst (3V6Mo20CeTiO₂), with distinct characteristic peaks corresponding to anatase phase (TiO₂) appearing at 2θ values of 25.2°, 37.7°, 47.8°, 53.7°, 55.9°, and 62.5°.

 

Chapter 3.3. Surface area and porosity of the catalysts

Comments 12: Why in order to research catalysts with CeO₂, the catalysts 3V6Mo5CeTiO₂ and 3V6Mo20CeTiO₂ were chosen?

Response: The 3V6Mo5CeTiO₂ and 3V6Mo20CeTiO₂ catalysts were considered as the typical samples to study the effect of CeO₂ addition on the crystallinity of the catalyst and other related information.

 

Comments 13: line 186: jest (3V6MoCeTiO₂...) Should be (3V6MoTiO₂...)

Response: The related revision has been made, as shown in the modification below.

Modification: Lines 197~200/Page 6: As shown in Fig. 4, all three catalysts (3V6MoTiO_2, 3V6Mo5CeTiO_2, and 3V6Mo20CeTiO₂) exhibited type-IV isotherms and H2-type hysteresis loops indicating mesoporosity [14,15]and a narrow pore size distribution [16] in the structure of the catalysts.

 

 

Comments 14: lines 183-185

Quote: According to Table 1, the specific surface area of the catalysts does not show significant differences, suggesting a minimal effect of the cerium species on the specific surface area.

Comment: This is not true. The influence is clear, only the differences in values are not large. The presence of Ce, but also its amount in the catalyst reduces the surface area (73.4>69.0>58.3)

Response: Thank the Reviewer for the good comment. As the amount of CeO₂ introduced increases, it alters the pore structure of the catalyst due to the ball milling method, reducing the size of the pores, which in turn gradually decreases the specific surface area of the catalyst.

Modification: Lines 192~196/Page 6: According to Table 1, as the amount of CeO₂ introduced increases, it alters the pore structure of the catalyst, reducing the size of the pores, which in turn gradually decreases the specific surface area of the catalyst due to the ball milling method. The particle size of CeO₂ is relatively large, which covers and blocks the pores, leading to a reduction in surface area and smaller pore sizes.

Comments 15: Please also refer to the values of pore volume and pore size. After all, the differences in pore volume are significant- even of 300% (0,3 cm³/g, 02 (cm³/g, 01 (cm³/g). There is a visible downward trend in the volume of CeO₂, share: 3V6MoTiO₂>3V6Mo5CeTiO₂, 3V6Mo20CeTiO₂ How do the authors explain this?

Response: As the amount of CeO₂ introduced increases, it alters the pore structure of the catalyst due to the ball milling method, reducing the size of the pores, which in turn gradually decreases the specific surface area of the catalyst.

Modification: Lines 192~196/Page 6: According to Table 1, as the amount of CeO₂ introduced increases, it alters the pore structure of the catalyst due to the ball milling method, reducing the size of the pores, which in turn gradually decreases the specific surface area of the catalyst. The particle size of CeO₂ is relatively large, which covers and blocks the pores, leading to a reduction in surface area and smaller pore sizes.

 

Chapter 3.4. FT-IR and Raman spectral analysis of the catalysts

Comments 16: Why did the authors choose for the research with CeO₂, a 3V6Mo20CeTiO₂, catalyst?

Response: The NH₃-SCR activity of 3V6Mo20CeTiO₂ is optimal. It is necessary to conduct characterization to explore how the addition of CeO₂ enhances the catalyst's activity.

 

Chapter 3.5.H₂-TPR of the catalysts

Comments 17: Why were the catalysts 3V6Mo5CeTiO₂, and 3V6Mo20CeTiO₂, chosen to research CeO₂, catalysts?

Response: Selecting these two catalysts with outstanding NH₃-SCR activity for H₂-TPR characterization is crucial for understanding their redox properties and the nature of active sites.

 

Comments 18: Please discuss Table 2. Please note that the H₂, Consumption (μmol/g) values differ significantly. There is a clear trend related to the presence, but also the amount of CeO₂, in the catalysts.

Response: Thank the Reviewer for the good comment. After the addition of CeO₂ to the catalyst, the amount of reduced species on the catalyst surface decreases, leading to reduced H₂ consumption. The catalytic activity of the catalyst is enhanced, possibly due to the excellent redox cycling ability of CeO₂, which enhances the Ce⁴⁺/Ce³⁺ ↔ V⁵⁺/V⁴⁺ cycle, increases the oxygen species, and improves the redox performance of the catalyst. Moreover, characterization results also indicate that the addition of CeO₂ promotes the dispersion of other active components on the catalyst surface, thereby increasing the number of effective active sites.

Modification: Lines 254~260/Page 9: After the addition of CeO₂ to the catalyst, the amount of reduced species on the catalyst surface decreases, leading to reduced H₂ consumption. The catalytic activity of the catalyst is enhanced, possibly due to the excellent redox cycling ability of CeO₂, which enhances the Ce⁴⁺/Ce³⁺ ↔ V⁵⁺/V⁴⁺ cycle, increases the oxygen species, and improves the redox performance of the catalyst. Moreover, characterization results also indicate that the addition of CeO₂ promotes the dispersion of other active components on the catalyst surface, thereby increasing the number of effective active sites.

 

Chapters 3.6. NH₃-TPD of the catalysts and 3.7.XPS analysis of the catalysts and 3.8.1. Adsorption of NH, and NO and 3.8.3. Transient Reaction of NH, with Pre-adsorbed NO+O₂ and 3.8.4. In-situ DRIFT studies on SO₂ Tolerance

Comments 19: Why did the authors choose for the research with CeO₂ a 3V6Mo20CeTiO₂ catalyst?

Response: Selecting the 3V6Mo20CeTiO₂ catalyst with excellent NH₃-SCR activity for NH₃-TPD, XPS, and In-situ DRIFTs characterization can help better understand how CeO₂ enhanced the activity of the 3V6MoTiO₂ catalyst.

 

Chapter 4. Conclusions

Comments 20: Please rephrase your conclusions. They are imprecise.

The formulations are misleading because the influence of the amount  of CeO₂, on the catalytic activity and SO₂ resistance of the V₂O₅-MoO₃/TiO₂ catalysts were not studied. Please consider the conclusions from the numerical values of the obtained research results of CeO₂ on the catalytic activity and SO₂ resistance of the V₂O₅-MoO₃/TiO₂ catalyst. Please take into account the conclusions from the numerical values of the obtained research results.

Response: Thank the Reviewer for the good comment. Significantly enhanced NH₃-SCR catalytic performance was achieved by modifying the 3V6MoTiO₂ catalyst with CeO₂. The optimal loading of CeO₂ was 20 %, which resulted in over 98% NO efficiency at 420 ^oC. NH₃-TPD and H₂-TPR results indicated that the good high-temperature SCR activity of the CeO₂-modified catalyst was caused by its larger number of strong acid sites and the strong redox ability. XPS analysis showed that the introduction of CeO₂ created new charge transfer pathways (Ce⁴⁺/Ce³⁺ ↔ V⁵⁺/V⁴⁺) in the 3V6MoTiO₂ catalyst, leading to an increase in the V⁵⁺ proportion and chemisorbed oxygen content. This led to improved catalytic activity. Detailed in situ DRIFTs studies confirmed the presence of rich adsorbed NH₃ and NO_x species on the CeO₂-modified catalyst surface. The adsorbed NH₃ was strongly activated by the CeO₂-modified catalyst, leading to better high-temperature activity. The SCR reaction over the 3V6MoxCeTiO₂ catalysts followed both the E-R and L−H mechanisms.

Modification: Lines 437~448 Page 15: Significantly enhanced NH₃-SCR catalytic performance was achieved by modifying the 3V6MoTiO₂ catalyst with CeO₂. The optimal loading of CeO₂ was 20 wt%, which resulted in over 98% NO efficiency at 420 ^oC. NH₃-TPD and H₂-TPR results indicated that the good high-temperature SCR activity of the CeO₂-modified catalyst was caused by its larger number of strong acid sites and the strong redox ability. XPS analysis showed that the introduction of CeO₂ created new charge transfer pathways (Ce⁴⁺/Ce³⁺ ↔ V⁵⁺/V⁴⁺) in the 3V6MoTiO₂ catalyst, leading to an increase in the V⁵⁺ proportion and chemisorbed oxygen content. This led to improved catalytic activity. Detailed in situ DRIFTs studies confirmed the presence of rich adsorbed NH₃ and NO_x species on the CeO₂-modified catalyst surface. The adsorbed NH₃ was strongly activated by the CeO₂-modified catalyst, leading to better high-temperature activity. The SCR reaction over the 3V6MoxCeTiO₂ catalysts followed both the E-R and L−H mechanisms.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors

 The article is interesting. It brings values ​​and scientific knowledge. It is worth recommending. However, please correct the errors and supplement the content with explanations.

Why was the effect of the amount of CeO₂ on the catalytic activity and SO₂ resistance of V₂O₅-MoO₃/TiO₂ catalyst not studied? Without these studies, the possibility of optimizing the composition was excluded.

Detailed remarks

 Chapter 2. Materials and Methods

Line 68

Quote: „…the powder mixture was ground and blended….”

Comment: Please specify the grain size after grinding.

Lines 106-107

Quote: „Prior to the experiment, the catalyst powder was finely ground…..”

Comment: Please specify what "finely ground" means – grain size.

 Chapter 3. Results and discussion

Please show and explain the mechanism of catalytic action of the tested catalysts

 Chapter 3.1.1. NH₃-SCR activity for NO conversion

Lines 118-130

1. Please re-examine the interpretation of Fig.1. The interpretation does not reflect the actual values ​​on the graph.

2. Why is the 3V6Mo20CeTiO₂ catalyst particularly promoted in the interpretation? After all, the 3V6Mo20CeTiO₂ catalyst stands out in the temperature range up to 150⁰C. In the temperature range from about 200⁰C to about 400⁰C, the NO conversion values ​​are at the same level (only the 3V6MoTiO2 catalyst without CeO2 showed lower values ​​- above the temperature of about 340⁰C).

Chapter 3.1.2. Effect of SO₂ on the NH₃-SCR conversion of NO

1.Fig 2.

Caption under the figure: there is„……………………..(3V6MoCeTiO₂,………………)

                                          Should be„………………….(3V6MoTiO₂,…………)

2. Why were 3V6Mo20CeTiO₂and 3V6Mo5CeTiO₂  catalysts selected for testing CeO₂ catalysts?

3. How do the authors explain that the incorporation of CeO₂ into the V₂O₅-MoO₃/TiO₂ catalyst diminishes its resistance to sulfur and water poisoning (also the influence of CeO₂ amount)?

 Chapter 3.2. XRD analysis of the catalyst materials

1.Why did the authors choose for the research a CeO₂ catalyst 3V6Mo20CeTiO₂?

2. line 164: jest (3V6MoCeTiO₂)

                     should be (3V6MoTiO₂)

 Chapter 3.3. Surface area and porosity of the catalysts

1. Why in order to research catalysts with CeO₂, the catalysts 3V6Mo5CeTiO₂ and 3V6Mo20CeTiO₂ were chosen?

2. line 186: jest (3V6MoCeTiO₂……………..)

                    Should be (3V6MoTiO₂…………)

3. lines 183-185

Quote: „According to Table 1, the specific surface area of the catalysts does not show significant differences, suggesting a minimal effect of the cerium species on the specific surface area.”

Comment:  This is not true. The influence is clear, only the differences in values ​​are not large. The presence of Ce, but also its amount in the catalyst reduces the surface area (73,4>69,0>58,3)

 4. Please also refer to the values ​​of pore volume and pore size. After all, the differences in pore volume are significant- even of 300% (0,3 cm³/g, 02 (cm³/g, 01 (cm³/g). There is a visible downward trend in the volume of CeO₂ share: 3V6MoTiO₂>3V6Mo5CeTiO₂. 3V6Mo20CeTiO_2.

How do the authors explain this?

 Chapter 3.4. FT-IR and Raman spectral analysis of the catalysts

Why did the authors choose for the research with CeO₂, a 3V6Mo20CeTiO₂ catalyst?

 Chapter 3.5. H₂-TPR of the catalysts

1. Why were the catalysts 3V6Mo5CeTiO₂ and 3V6Mo20CeTiO₂ chosen to research CeO₂  catalysts?

2. Please discuss Table 2. Please note that the H₂ Consumption (μmol/g) values ​​differ significantly. There is a clear trend related to the presence, but also the amount of CeO₂ in the catalysts.

 Chapters 3.6. NH₃-TPD of the catalysts    and   3.7. XPS analysis of the catalysts    and  3.8.1. Adsorption of NH₃ and NO     and   3.8.3. Transient Reaction of NH₃ with Pre-adsorbed NO+O₂     and     3.8.4. In-situ DRIFT studies on SO₂ tolerance

Why did the authors choose for the research with CeO₂, a 3V6Mo20CeTiO₂ catalyst?

 Chapter 4. Conclusions

Please rephrase your conclusions. They are imprecise.

The formulations are misleading, because the influence of the amount of CeO₂ on the catalytic activity and SO₂ resistance of V₂O₅-MoO₃/TiO₂ catalyst were not studied.

 Please consider the conclusions from the numerical values ​​of the obtained research results

Sformułowania są mylące, gdyż nie badano wpływu ilości of CeO₂ on the catalytic activity and SO₂ resistance of  V₂O₅-MoO₃/TiO₂ catalyst.

Please take into account the conclusions from the numerical values ​​of the obtained research results.

Best regards,

Reviewer

Author Response

Reviewer #2:

Comments 1:

The work paid attention to the weakened sulfur tolerance of the VMoTi catalyst in addition to the improved SCR activity. Although a series of characterizations including the DRIFTS study were performed, the conclusions are quite superficial. They should point out the deactivation mechanism of VMoCeTi catalysts since the modification of Ce on SCR activity of VTi catalysts has been widely reported.

Response: The addition of CeO₂ may significantly enhance the redox cycling ability of the catalyst. CeO₂ can provide active oxygen species, thereby increasing the reaction activity of the catalyst. However, the presence of SO₂ may react with CeO₂ to form stable sulfides or sulfates, which can cover or deactivate active sites, thus reducing the catalyst's resistance to SO₂ poisoning. Furthermore, in the presence of H₂O, water will also compete for adsorption with the reactant species, reducing the number of active sites available for the reaction, thereby decreasing the catalytic activity.

Modification: Lines 448~453/Page 15: However, the presence of SO₂ may react with CeO₂ to form stable sulfides or sulfates, which can cover or deactivate active sites, thus reducing the catalyst's resistance to SO₂ poisoning. Furthermore, in the presence of H₂O, water will also compete for adsorption with the reactant species, reducing the number of active sites available for the reaction, thereby decreasing the catalytic activity.

 

Comments  2: The information of the SCR activity test should be provided in Section 2.

Response: Thank the Reviewer for the good suggestion. The related revision has been made, as shown in the modification below.

Modification: Lines 73~83/Page 2: The fixed-bed quartz reactor was used for NH₃-SCR activity measurements of the 3V6MoxCeTiO₂ catalysts in a state flow mode with 1 mL catalyst (40-60 mesh). Reaction conditions were as follows: 1000 ppm NO, 1000 ppm NH₃, 6 vol% O₂, 100 ppm SO₂ (when needed)，10% H₂O (when needed), and pure N₂ as the balance gas. The total flow rate of the feed gas was 500 mL/min, and the gas hourly space velocity (GHSV) was 30000 h^-1. After the reaction reached a stable condition for 30 min at each testing temperature, the concentration of NO was determined by an exhaust gas analyzer (4000 VM NOx Analyzer, Signal). The NO conversion was calculated using the following equations:

where [NO_x]_in and [NO_x]_out represent the inlet and the outlet concentration of NO_x, respectively.

 

Comments  3: Only NO conversion is shown in Fig. 1, and the data of N₂ selectivity should be presented. For example, the addition of ceria would increase the redox property of catalyst and hereby probably accelerate the oxidation of NH₃ at high temperatures. This would result in the increase in the NO conversion but decrease in the NO_x conversion.

Response: Thank the Reviewer for the good comment. In this study, we used a 4000 VM gas analyzer to analyze the compositions of the flue gases. Because of the limitations of the gas analyzer, only NO, NO₂ and NO_x can be detected. Some other gas components, such as NH₃ and N₂O cannot be detected. And in our neighboring labs, only one gas analyzer that could be used to detect the NH₃ and N₂O signals is available. Due to the malfunction of the equipment in the laboratory, which is difficult to repair in short time, we are quite sorry that we cannot provide the N₂ selectivity results based on the present conditions.

We conducted a literature review and selected reports on the N₂ selectivity of Ce-modified VWT/VMT catalysts to learn the N₂ selectivity of the 3V6MoxCeTiO₂ catalysts. Studies have reported that adding Ce modification to VWT catalysts does not significantly affect the N₂ selectivity at different Ce doping levels. As the amount of Ce increases, the N₂ selectivity of the modified catalyst may fluctuate at various temperatures, and it remains above 95%. Studies have shown that with the increase of Ce addition, the Ce-modified VMT catalysts could maintain a high N₂ selectivity at temperatures below 400 °C.

Reference:

1. Song, L.; Chao, J.; Fang, Y.; He, H.; Li, J.; Qiu, W.; Zhang, G. Promotion of ceria for decomposition of ammonia bisulfate over V₂O₅-MoO₃/TiO₂ catalyst for selective catalytic reduction. Chem. Eng. J. 2016, 303, 275-281.
2. Liu, Z.; Yu, F.; Dong, D.; Gui, R.; Li, W.; Sun, R.; Wan, Y.; Dan, J.; Wang, Q.; Dai, B. Transition-metal‐doped ceria carried on two-dimensional vermiculite for selective catalytic reduction of NO with CO: experiments and density functional theory. Appl. Surf. Sci. 2021, 566, 150704.
3. Li, C.; Shen, M.; Wang, J.; Wang, J.; Zhai, Y. New insights into the promotional mechanism of ceria for activity and ammonium bisulfate resistance over V/WTi catalyst for selective catalytic reduction of NO with NH₃. Appl. Catal. A: General 2018, 560, 153-164.
4. Shen, M.; Xu, L.; Wang, J.; Li, C.; Wang, W.; Wang, J.; Zhai, Y. Effect of synthesis methods on activity of V₂O₅/CeO₂/WO₃-TiO₂ catalyst for selective catalytic reduction of NO_x with NH3. J. Rare Earths 2016, 34, 259-267.
5. Zhang, Y.; Li, J.; Cai, J.; Li, S.; Fan, X.; Song, L.; Guo, R.; Liu, J. Enhancement of low-temperature activity of γ-fe2o3-modified V₂O₅-MoO₃/TiO₂ catalysts for selective catalytic reduction of NO_x with NH3. J. Environ. Chem. Eng. 2024, 12, 112589.

 

Comments  4: Why did they only include the XRD patterns of two samples in Fig. 3?

Response: Thank the Reviewer for the good comment. To study the effect of CeO₂ addition on the crystallinity of the catalyst and other related information, the 3V6Mo20CeTiO₂ and 3V6MoTiO₂ catalyst were chosen as the typical samples. The XRD test results are intended to compare the changes in the crystalline structure of the catalyst before and after adding Ce.

 

Comments  5: Please show the Raman spectrum of VMoTi in Fig. 6. Moreover, the characteristic band of the CeO bond was not found even at a high Ce loading in the 20Ce sample.

Response: Thank the Reviewer for the good comment. In Figure 6, we compare the Raman spectra of TiO₂, 3V6MoTiO₂, and 3V6Mo20CeTiO₂ catalysts. No obvious peak related to the Ce-O bond was found on the 3V6Mo20CeTiO₂ catalyst in the figure, which may be due to its small grain size or disordered structure. We have repeated the Raman test on the 3V6Mo20CeTiO₂ catalyst, however, no any characteristic bands related to Ce in the spectrum were detected. As previous work, MoO₃ in the catalyst may promote dispersion of CeO₂ over surface.

Reference:

Song, L.; Chao, J.; Fang, Y.; He, H.; Li, J.; Qiu, W.; Zhang, G. Promotion of ceria for decomposition of ammonia bisulfate over V₂O₅-MoO₃/TiO₂ catalyst for selective catalytic reduction. Chem. Eng. J. 2016, 303, 275-281.

Modification: Lines 231~234/Page 8: In Figure 6, we compare the Raman spectra of TiO₂, 3V6MoTiO₂, and 3V6Mo20CeTiO₂ catalysts. No obvious peak related to the Ce-O bond was found on the 3V6Mo20CeTiO₂ catalyst in the figure, which may be due to its small grain size or disordered structure [24].

 

Comments  6: Why did the addition of ceria increase the oxidation state of V?

Response: Thank the Reviewer for the good comment. Cerium oxide (CeO₂) is well-known for its high oxygen storage capacity, numerous oxygen vacancies, strong interactions with metals, and its ability to readily convert between Ce³⁺ and Ce⁴⁺. Therefore, the addition of Ce will increase the surface oxygen vacancies and electron transfer of the catalyst, enhance the interaction between Ce and V, Mo, Ti, and thus affect the oxidation state of these species.

Modification: Lines 287~292/Page 10: Cerium oxide (CeO₂) is well-known for its high oxygen storage capacity, numerous oxygen vacancies, strong interactions with metals, and its ability to readily convert between Ce³⁺ and Ce⁴⁺. Therefore, the addition of Ce increase the surface oxygen vacancies and electron transfer of the catalyst, enhance the interaction between Ce and V, Mo, Ti, and thus affect the oxidation state of these species.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

They have answered the questions basically and the work can be accepted.