Effect of Ceria Doping in Different Impregnation Steps on Ni-Based Catalysts Loading on TiO2-SiC for CO Methanation
Round 1
Reviewer 1 Report
The manuscript describes the preparation of TiO2-SiC supported Ni-based catalysts doped with ceria which was introduced into the catalyst in different steps of the impregnation process (introducing Ce simultaneously with Ni-solution (co-impregnation 10Ce-Ni/TiO2-SiC(co)), after drying (10Ce-Ni/TiO2-SiC(ad)), and after calcination (step-impregnation 10Ce-Ni/TiO2-SiC(st)). The authors test the catalytic activity for CO methanation of Ce-doped samples and compare it to that of non doped Ni/TiO2-SiC. They use several characterization techniques (XRD, H2-TPR, Nitrogen adsorption-desorption, TGA) in order to correlate the catalytic tests results with the physico-chemical properties of the solids. Even if the scientific approach is meaningful, there are some issues that should be carefully solved before accepting the manuscript for publication.
It is not understandable why for the catalyst 10Ce-Ni/TiO2-SiC(ad), the selectivity to CH4 decreases from 100% at to almost 80 in the temperature range of 290-340 oC and then increases to 90%.
In fig. 4b the diffraction lines for Ni(111) and Ni(200) are placed differently cmpared to fig 4a. The same for CeO2(200) and CeO2(220). In both figures 4a, 4b the diffraction line corresponding to TiO2 anatase is not visible in the pattern of the sample 10Ce-Ni/TiO2-SiC(st) since it is covered by the legend. However it seems that the support is more likely Ti-SiC not TiO2-SiC since the line corresponding to TiO2 is too weak compared to those of TiC.
On figure 5 what is the significance of HT used ?
At line 215 the authors make a comment about the loss of Ni. How is the Ni lost?
Considering the results showed in Table 4, the catalyst 10Ce-Ni/TiO2-SiC(st) has the lowest Tm (574) for γ-NiO and its concentration (54.6%) is similar to that of the sample 10Ce-Ni/TiO2-SiC(ad) (55.2%). Hence why10Ce-Ni/TiO2-SiC(st) does not have a higher activity than 10Ce-Ni/TiO2-SiC(ad)
Besides that the authors should make a thorough revision of English usage in the manuscript.
Author Response
Response to Reviewer 1#
The manuscript describes the preparation of TiO2-SiC supported Ni-based catalysts doped with ceria which was introduced into the catalyst in different steps of the impregnation process (introducing Ce simultaneously with Ni-solution (co-impregnation 10Ce-Ni/TiO2-SiC(co)), after drying (10Ce-Ni/TiO2-SiC(ad)), and after calcination (step-impregnation 10Ce-Ni/TiO2-SiC(st)). The authors test the catalytic activity for CO methanation of Ce-doped samples and compare it to that of non doped Ni/TiO2-SiC. They use several characterization techniques (XRD, H2-TPR, Nitrogen adsorption-desorption, TGA) in order to correlate the catalytic tests results with the physico-chemical properties of the solids. Even if the scientific approach is meaningful, there are some issues that should be carefully solved before accepting the manuscript for publication.
Response: The authors are grateful to the reviewer 1# for your appreciation of our work and constructive comments. The comments are well taken to improve our manuscript.
Comment 1：It is not understandable why for the catalyst 10Ce-Ni/TiO2-SiC(ad), the selectivity to CH4 decreases from 100% at to almost 80 in the temperature range of 290-340 oC and then increases to 90%.
Response: Thanks for this comment. The authors have double checked the original data of this point, and found the calculation of this point used a wrong response factor obtained from calibration gas. We have recalculated it and found the selectivity of CH4 at 350 oC over catalyst 10Ce-Ni/TiO2-SiC(ad) was 79.32%. The new data was updated in Figure 1. (Page, Line)
Comment 2：In fig. 4b the diffraction lines for Ni(111) and Ni(200) are placed differently cmpared to fig 4a. The same for CeO2(200) and CeO2(220). In both figures 4a, 4b the diffraction line corresponding to TiO2 anatase is not visible in the pattern of the sample 10Ce-Ni/TiO2-SiC(st) since it is covered by the legend. However it seems that the support is more likely Ti-SiC not TiO2-SiC since the line corresponding to TiO2 is too weak compared to those of TiC.
Response: The authors improve the quality of the figure. In these XRD patterns, figure 4a showed the results of catalysts before reduction (fresh), while figure 4b showed results of spent catalysts. In figure 4a, the diffraction lines were not the metallic Ni but NiO, while in Figure 4b, the diffraction line was metallic Ni. So, they have different response peaks.
For CeO2, the peak of CeO2 (200) was too tiny to identify in figure, however, the diffraction peaks for CeO2 (111) and CeO2 (220) could be identified and consistent with the results mentioned in references. [1-4] The manuscript has revised on Page 9, Line 230 to Line 233.
“Interestingly, in both XRD results of fresh and used sample NCTS(st), two diffraction peaks at 28.5 °, and 47.4 ° appeared, which belonged to CeO2 (JCPDS65-2925), which was consistent with the results mentioned in literatures. [23, 27-28]
For the support, this support was a commercial support from SICAT, of which the properties had been reported. [5] The authors have compared the results from their report with our experimental results, and found the support used in this manuscript was TiO2-SiC. As mentioned in their paper, after an oxidative treatment at 600 oC, the precursor TiC transferred to both anatase and rutile TiO2. The manuscript has revised on Page 8, Line 216 to Line 218 and Page 9, Line 224 to Line 226.
“Meanwhile, the shoulder peaks at 60.5 ° and reflection at 76 ° belonged to TiC (JCPDS65-0971), which was consistent with the results of SICAT published. [36]”
“As mentioned in the report of SICAT, [36] when the precursor TiC-SiC underwent an oxidative treatment at 600 °C, the TiC would transfer to both anatase and rutile TiO2.”
Comment 3：On figure 5 what is the significance of HT used?
Response: The significance of HT used sample is to supply a directly evidence that no carbon deposition occurs on the catalysts. The TG-DSC results indicate the support TiO2-SiC can effectively avoid the occurrence of carbon deposition, suggesting that carbon deposition is not the reason for the decrease of the catalyst activity. A short discussion of the significance of HT used has added in revised manuscript on Page 15, Line 303 to Line 306.
“Carbon deposition on the catalyst surface was more likely to occur at high reaction temperatures. The TG-DSC results of the HT used sample showed that even under the reaction conditions of 550 °C, no surface carbon formation was detected on the catalyst.”
Comment 4：At line 215 the authors make a comment about the loss of Ni. How is the Ni lost?
Response: Thanks for this comment. In the presence of CO, metallic nickel will generate nickel carbonyl, and nickel carbonyl is gaseous under the present reaction conditions. Although nickel carbonyl can be re-decomposed into metallic nickel and CO at high temperature, it still causes irreversible nickel loss under the continuous reaction conditions of high space velocity. A short discussion of the formation of nickel carbonyl and Ni loss has added in revised manuscript on Page 15, Line 311 to Line 315.
“It could be attributed by the loss of nickel. In the presence of CO, metallic nickel could generate gaseous nickel carbonyl under the present reaction conditions, which would cause irreversible nickel loss under the continuous reaction conditions. This phenomenon could be explained by the loss of nickel and matched with our experimental results at stability test (2.1.2).”
Comment 5：Considering the results showed in Table 4, the catalyst 10Ce-Ni/TiO2-SiC(st) has the lowest Tm (574) for γ-NiO and its concentration (54.6%) is similar to that of the sample 10Ce-Ni/TiO2-SiC(ad) (55.2%). Hence why10Ce-Ni/TiO2-SiC(st) does not have a higher activity than 10Ce-Ni/TiO2-SiC(ad)
Response: As mentioned in manuscript, comparing to α-NiO and β-NiO, the γ-NiO has a stable activity at high temperature. Therefore, in figure 1a, the sample NCST(st) did not show a better performance than the sample NCST(ad). However, as shown in revised Figure 2, when comparing with the results of long term stability test of sample NCST(co), NCST(ad) and NCST(st), it could be found that the stability of NCST(ad) and NCST(st) was better than NCST(co), which had a lowest concentration of γ-NiO. The revised discussion of stability test has added in revised manuscript on Page 6, Line 156 to Line 174.
“To investigate the CO Methanation long-term stability of samples with CeO2 doped, a group of experiments was conducted at 300 °C, which was the initial stage of CO completely conversion, and kept more than 55 hours. The results were shown in Figure 2. From the Figure 2, the methanation activity of all the samples was maintained at a high level (CO conversion of more than 95%) in the first 12 hours. At this stage, the order of activity of these samples was NCTS(ad)> NCTS(co)> NCTS(st), although the difference was less than 2%. However, with the reaction carried out after 36 hours, CO conversion dropped to around 91% of both NCTS(ad) and NCTS(st), while the CO conversion of NCTS(co) decreased to 77%. When the test finished at TOS of 55 hours, the CO conversion of NCTS(ad) and NCTS(st) simultaneously reduced to 82%. At the same condition, the performance of NCTS(co) was worst, only left a CO conversion of less than 60%, which meant the stability of sample NCTS(ad) and NCTS(st) was much higher than sample NCTS(co). The reasons for the deactivation of the catalyst could be concluding to sintering of active metal particles, oxidation of the catalyst, poisoning or carbon deposition on the surface of the catalyst. In addition, the nickel-based catalysts could generate gaseous nickel carbonyl in the presence of CO, which leaded to the loss of nickel and the decrease of the activity. The reaction gases used in this work were all high-purity cylinder gases, and the possibility of catalyst poisoning was very low. Other deactivation factors need to be inferred from catalyst characterization results.”
Comment 6：Besides that the authors should make a thorough revision of English usage in the manuscript.
Response: The manuscript has been revised and checked carefully by the authors and sent for professional editing as well. All the changes have been highlighted in the revised manuscript.
Reviewer 2 Report
The current manuscript deals with "Effect of ceria doping in different impregnation steps on Ni-based catalysts loading on TiO2-SiC for CO Methanation." The author show that addition of ceria to the Ni/TiO2-SiC improves its stability and catalytic performance, and that the stage that the ceria is added is also very important.
The catalytic results are interesting and supported by different characterizations. Yet, the introduction is not sufficient and the discussion in the results is not satisfactory. For instance, the stability test was performed just for 10Ce-Ni/TiO2-SiC(ad), as it showed the best results. Yet, as the authors claimed that one of the reasons to the different between the different preparations might be the difference in stability, they have to show the stability test to all the catalysts. Furthermore, the XRD results shows lower crystalline size for the 10Ce-Ni/TiO2-SiC(ad) catalyst, but it was not discussed at all. Finally, as the addition of CeO2 to the catalyst improved performances. It will be interesting also to study the effect of ceria loading.
In addition, the English should be improved, and different typos should be corrected, for instead: row 55- lording instead of loading, row 85-CeO2 and not Ce.
Author Response
Response to Reviewer 2#
The current manuscript deals with "Effect of ceria doping in different impregnation steps on Ni-based catalysts loading on TiO2-SiC for CO Methanation." The author show that addition of ceria to the Ni/TiO2-SiC improves its stability and catalytic performance, and that the stage that the ceria is added is also very important. he catalytic results are interesting and supported by different characterizations. Yet, the introduction is not sufficient and the discussion in the results is not satisfactory.
Response: We highly appreciate the reviewers’ helpful and insightful comments regarding our manuscript. The Reviewers comments are welcomed, and we will incorporate them in order to improve our manuscript.
Comment 1：T For instance, the stability test was performed just for 10Ce-Ni/TiO2-SiC(ad), as it showed the best results. Yet, as the authors claimed that one of the reasons to the different between the different preparations might be the difference in stability, they have to show the stability test to all the catalysts.
Response: Thank you for this useful comment. The authors has revised the stability test part of manuscript and added the results from sample NCST(co) and NCST(st) in figure 2. The revised discussion of stability test has added in revised manuscript on Page 6, Line 156 to Line 174.
“To investigate the CO Methanation long-term stability of samples with CeO2 doped, a group of experiments was conducted at 300 °C, which was the initial stage of CO completely conversion, and kept more than 55 hours. The results were shown in Figure 2. From the Figure 2, the methanation activity of all the samples was maintained at a high level (CO conversion of more than 95%) in the first 12 hours. At this stage, the order of activity of these samples was NCTS(ad)> NCTS(co)> NCTS(st), although the difference was less than 2%. However, with the reaction carried out after 36 hours, CO conversion dropped to around 91% of both NCTS(ad) and NCTS(st), while the CO conversion of NCTS(co) decreased to 77%. When the test finished at TOS of 55 hours, the CO conversion of NCTS(ad) and NCTS(st) simultaneously reduced to 82%. At the same condition, the performance of NCTS(co) was worst, only left a CO conversion of less than 60%, which meant the stability of sample NCTS(ad) and NCTS(st) was much higher than sample NCTS(co). The reasons for the deactivation of the catalyst could be concluding to sintering of active metal particles, oxidation of the catalyst, poisoning or carbon deposition on the surface of the catalyst. In addition, the nickel-based catalysts could generate gaseous nickel carbonyl in the presence of CO, which leaded to the loss of nickel and the decrease of the activity. The reaction gases used in this work were all high-purity cylinder gases, and the possibility of catalyst poisoning was very low. Other deactivation factors need to be inferred from catalyst characterization results.”
Comment 2：Furthermore, the XRD results shows lower crystalline size for the 10Ce-Ni/TiO2-SiC(ad) catalyst, but it was not discussed at all.
Response: The smaller crystallite size of NCST(co) might be attributed by the "crystallization-partial dissolution-recrystallization" process of nickel compounds. During the impregnation process, there is a part of water-soluble nickel compound in the catalyst before calcination after drying. It dissolves again during the introduction of CeO2 and recrystallizes together with CeO2. Thus, the size of NiO crystals in NCST(co) generated after final calcination is reduced. The discussion of NiO crystal size has added in revised manuscript on Page 9, Line 243 to Line 256.
“As shown in the table, all the crystallite sizes of catalysts with CeO2 doping were smaller than sample NTS, which indicated the introduced CeO2 could effectively re-duce the crystallinity of Ni. Moreover, among the catalysts with CeO2 doping, the sample NCTS(ad) showed the smallest crystallite size. This phenomenon may be caused by the "crystallization-partial dissolution-recrystallization" process of nickel compounds. During the impregnation process, there is a part of water-soluble nickel compound in the catalyst before calcination after drying. It dissolves again during the introduction of CeO2 and recrystallizes together with CeO2. Thus, the size of NiO crystals generated after final calcination is reduced. Indeed, this reaction is sensitive to the catalysts’ structure property, suggesting that the nickel dispersion degree can have an impact on the activity of catalysts. Smaller nickel crystallite size contributes to higher dispersion degree, thereby obtaining better methanation performance [39]. In addition, from the table, the crystallite size of the fresh and used samples was ap-proximately equal, which indicated that no serious sintering happened in the CO Methanation reaction.”.
Comment 3: Finally, as the addition of CeO2 to the catalyst improved performances. It will be interesting also to study the effect of ceria loading.
Response: Thank you for this interesting suggestion. In fact, there are some other works mentioned introducing CeO2 into Ni-based catalysts, including CeO2 as a support and as a promoter. However, few works have systematically investigated the effect of CeO2 loading on this reaction. We could certainly study this topic but that would be future work.
Comment 4：In addition, the English should be improved, and different typos should be corrected, for instead: row 55- lording instead of loading, row 85-CeO2 and not Ce.
Response: The manuscript has been carefully revised according the reviewers’ comments and checked carefully by the authors and professional editor. All the mistypes have been corrected in our revised manuscript.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The article reports interesting data on Ce dopet Ni catalyst for CO methanation.
Authors write a good introduction and well desribe their methods.
No words on the statistical analysis on the data are reported. Were experiments repeated? how many times? the steady-state data were obtained after how much time?
The discussion will improve with some surface chemical characterization, done with SEM/EDS (with the added value of imaging the sample) or XPS.
These new data would better support the conclusions.
In general is a good manuscript
Comments for author File: Comments.pdf
Author Response
Response to Reviewer 3#
The article reports interesting data on Ce doped Ni catalyst for CO methanation.
Authors write a good introduction and well describe their methods.
Response: The authors are grateful for the positive comments from the reviewer and have revised the manuscript carefully according to the reviewer’s comments.
Comment 1: (Page 3, Line 101) No words on the statistical analysis on the data are reported. Were experiments repeated? how many times? the steady-state data were obtained after how much time?
Response: Thanks for this remind. Some of experiments were repeated to verify the repeatability of catalyst performance. In this work, the activity test of sample NTS and all the test including activity test, stability test and high temperature stability test of sample NCTS (ad) were repeated twice. The results of the repeated experiments were so close that the authors considered the experimental results to be credible and reproducible.
All the experimental results of activity test and high temperature stability test were the average value of 3 steady-state results obtained after at least 4 hours running. A discussion of data reproducibility and credibility was added to the revised manuscript on Page 3, Line 92 to Line 95:
“To verify the repeatability and reliability of the experimental results, some experiments, including sample NTS and NCTS(ad), were repeated twice. The results of repeated experiments were very close. Therefore, we consider the response data to be credible and reproducible.”
And Page 17, Line 402 to Line 404:
“All the experimental results of activity test and high temperature stability test were the average value of 3 steady-state results obtained after at least 4 hours running.”
Comment 2: (Page 4, Line 127 to Line 129) The discussion will improve with some surface chemical characterization, done with SEM/EDS (with the added value of imaging the sample) or XPS.
These new data would better support the conclusions.
Response: The authors are grateful to this constructive comment. The main reasons for the reduction or deactivation of the catalyst are sintering of active metal particles, oxidation of the catalyst, poisoning or carbon deposition on the surface of the catalyst. In addition, the nickel-based catalysts can generate gaseous nickel carbonyl in the presence of CO, which leads to the loss of nickel and the decrease of the activity. The comparison of the characterization results of the chemical properties of the catalyst surface is the most directly evidence that the catalyst loss leads to deactivation. Unfortunately, the authors are currently unable to perform SEM-EDS or XPS characterization. Therefore, in the absence of directly evidence, the authors have adjusted the discussion of the causes of catalyst deactivation in the revised manuscript on Page 6, Line 168 to Line 174:
“The reasons for the deactivation of the catalyst could be concluding to sintering of active metal particles, oxidation of the catalyst, poisoning or carbon deposition on the surface of the catalyst. In addition, the nickel-based catalysts could generate gaseous nickel carbonyl in the presence of CO, which leaded to the loss of nickel and the de-crease of the activity. The reaction gases used in this work were all high-purity cylinder gases, and the possibility of catalyst poisoning was very low. Other deactivation factors need to be inferred from catalyst characterization results.”
And Page 9, Line 236 to Line 239:
“It was worth noting that no characteristic diffraction peaks of NiO were detected in the XRD patterns of all spent catalysts. This result indicated that almost no metallic nickel was oxidized during the reaction.”
And Page 9, Line 254 to Line 256:
“In addition, from the table, the crystallite size of the fresh and used samples was ap-proximately equal, which indicated that no serious sintering happened in the CO Methanation reaction.”
And Page 15, Line 303 to Line 306:
“Carbon deposition on the catalyst surface was more likely to occur at high reaction temperatures. The TG-DSC results of the HT used sample showed that even under the reaction conditions of 550 °C, no surface carbon formation was detected on the catalyst.”
And Page 15, Line 311 to Line 315:
“It could be attributed by the loss of nickel. In the presence of CO, metallic nickel could generate gaseous nickel carbonyl under the present reaction conditions, which would cause irreversible nickel loss under the continuous reaction conditions. This phenom-enon could be explained by the loss of nickel and matched with our experimental re-sults at stability test (2.1.2)”
Other comments:
Comment 3: (Page 1, Line 17) state how much
Response: The sentence has been revised to “The experimental results showed that the performance of all CeO2 doping samples (more than 80% of CO conversion) was obviously better than the sample without CeO2 (around 20% of CO con-version).”
Comment 4: (Page 1,Line 17) delete world “obviously”
Response: The word “obviously” has been deleted.
Comment 5: (Page 1, Line 18) “CO completely conversion” should be revise to “complete CO conversion”
Response: The “CO completely conversion” has been revised to “complete CO conversion”
Comment 6: (Page 2, Line 65 to Line 70) more literature on Ce oxides, and its anti-cocking feature, which is well known
Response: Thanks for this comment. Some literatures about CeO2 have been added in revised manuscript on Page 2, Line 66 to Line 81.
“Cerium, a rare earth elementary, has been wildly used in the preparation of catalysts as support or promoter, [23, 25-28] because it can improve the activity and stability of catalysts, and can efficiently resist carbon deposition. As reported by Zhou et al., [26] the Ni-based catalysts supported on CeO2 showed an excellent reactivity of CO2 Methanation at 340 °C and atmospheric pressure. While, Yang et al. reported that CeO2-Al2O3 was an excellent support for reverse water-gas shift reaction helping to achieve high degrees of CO2 conversions. [27] As a promoter, CeO2 also showed excellent properties in Ni-based catalysts. As mentioned in Santamaria et al.’s work, [28] the stability of catalyst Ni/Al2O3 for steam reforming of biomass pyrolysis volatiles had been greatly improved by incorporating CeO2, because it enhanced the gasification of coke precursors. Moreover, Liu et al. considered the promoter CeO2 could enhance interaction between Ni and SiC and improved dispersion of Ni active species, which improved catalytic activity and stability for high temperature CO Methanation. [23] In Xavier’s study [29], adding 1.5 wt. % CeO2 into Ni/Al2O3 catalyst could improve the reducibility and low-temperature activity of the catalyst, and the main reason for increased catalytic activity was considered to be the electronic interaction between Ni and CeO2.”
Comment 7: (Page 2, Line 81 to Line 83) delete sentence
Response: The sentence has been deleted.
Comment 8: (Page 3, Line 99 to Line 100) this discussion is not clear, maybe SEM or TEM iages may help understanding the effect
Response: Since the authors are currently unable to develop new characterizations, we have revised the confusing formulation on manuscript Page 3, Line 107 to Line 108:
“The experimental results showed that the introduction of CeO2 at different stages of impregnation leaded to significant changes in the reactivity of CO Methanation.”
Comment 9: (Page 4, Line 128) nickel lost?
Response: The wrong phrase has been revised.
Comment 10: (Page 4, Line 138) the other
Response: The word “another” has been revised to “the other”.
Comment 11: (Page 7, Line 198) improve the figure
Response: The quality of figures has been improved.
Comment 12: (Page 9, Line 263) specific surface area of
Response: The “surface” has been revised to “specific surface area”.
Comment 13: (Page 9, Line 270) cooling at wich rate? max T held for how long?
Response: The description of catalysts calcination has been revised to “After these ten hours, the precursors of the catalyst were baked in a muffle furnace heated from room temperature to 600 °C at a rate of 120 °C per hour, and then maintained at 600 °C for 5 hours. After calcination, the catalyst was naturally cooled to room temperature for use.”
Comment 14: (Page 9, Line 274 to Line 275) improve nomenclature, difficult to follow in the text
Response: Thanks for this suggestion. All the nomenclature of samples has been revised.
Comment 15: (Page 10, Line 279 to Line 280) how can you tell? did you run blank tests?
Response: In fact, authors had run the blank test by loading support into reactor tube. There is a thermocouple guard inner the reactor, so that we can easily obtain the axial temperature distribution in the reactor, thereby obtaining the constant temperature zone in the reactor. Moreover, based on the results of blank test, we can confirm that the feed gas had been sufficiently preheated.
Comment 16: (Page 10, Line 284) at atmospheric P?
Response: At the catalyst reduction process, the pressure was 1 bar on gauge, while in reaction process, the pressure was atmospheric pressure. The detail pressure information has been added in the manuscript.
In general is a good manuscript
Response: The authors are grateful to the reviewer #3 for your appreciation of our work and constructive comments.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors answered to all my queries. Hence I consider that the manuscript can be published now.
Reviewer 2 Report
The manuscript was revised properly.
Reviewer 3 Report
Authors tried to reply to all comments. Even though no surface characterisation is present, I think the work can be published
