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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

Catalysts 2024, 14(11), 794; https://doi.org/10.3390/catal14110794
by Xuehong Zi 1, Jingtong Ye 1, Yao Cheng 2, Shuangye Li 2, Xiangru Li 2, Xingtong Li 2, Wenge Qiu 1 and Liyun Song 2,*
Reviewer 1: Anonymous
Reviewer 2:
Catalysts 2024, 14(11), 794; https://doi.org/10.3390/catal14110794
Submission received: 30 September 2024 / Revised: 27 October 2024 / Accepted: 5 November 2024 / Published: 7 November 2024
(This article belongs to the Section Catalytic Materials)

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 Oα 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 NOx 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 Oα in the modified catalyst may facilitate "fast" SCR reactions, enhancing catalytic performance. The CeO2-modified catalyst surface has abundant adsorbed NH3 and NOx 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 (3V6MoTiO2, 3V6Mo5CeTiO2, 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 NOx 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 NOx 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. 3V6Mo20CeTiO2.

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 [NOx]in and [NOx]out represent the inlet and the outlet concentration of NOx, 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 NOx 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 NOx 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 NOx 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 NOx 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.

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