Effect of the Addition of Alkaline Earth and Lanthanide Metals for the Modification of the Alumina Support in Ni and Ru Catalysts in CO₂ Methanation

Round 1

Reviewer 1 Report

I see the paper has some sentences/word in red colour, possibly due to a previous revision.

The paper is very interesting and contains important results and should be published in Catalysts. However, it needs some (further) revisions first.

Some references were lost and in the paper there are many places where it is read "Error! Reference source not found.". Please check and correct.

In Figures 1 and 2, TPR, the profiles of alumina and Ba, Ca, Mg, Ce and La-alumina should also be shown for comparison. The same comment is valid for TPD, XPS, XRD if possible. And naturally, also for measurement of catalytic activity. If results of those catalysts without Ru or Ni are not shown, how can one know their effect?

Author Response

I see the paper has some sentences/word in red colour, possibly due to a previous revision.

The paper is very interesting and contains important results and should be published in Catalysts. However, it needs some (further) revisions first.

Some references were lost and in the paper there are many places where it is read "Error! Reference source not found.". Please check and correct.

In Figures 1 and 2, TPR, the profiles of alumina and Ba, Ca, Mg, Ce and La-alumina should also be shown for comparison. The same comment is valid for TPD, XPS, XRD if possible. And naturally, also for measurement of catalytic activity. If results of those catalysts without Ru or Ni are not shown, how can one know their effect?

 

he authors appreciate the reviewer's comments, as it will help to improve the paper. References have been corrected and error messages removed. In the graphs of TPD, XPS, XRD, along with the rest of the graphs that appear in the text, all the study catalysts have been included to identify all the physico-chemical properties.

 

However, catalysts based only on alumina and Ba, Ca, Mg, Ce and La have not been analysed, due to the absence of hydrogenation metals such as Ni and Ru avoids the hydrogenation reactions. Therefore, we do not expect any conversion in this reaction using the catalysts without Ru or Ni. For this reason, the incorporation of these catalysts in this work has not been contemplated, in order to avoid dispersing the reader's attention and focusing it on the stated objective.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have performed an extensive study on the effect of alkaline earth and lanthanide modification of the alumina support in Ni catalysts for CO₂ methanation. The effect of Ru addition to form bimetallic Ni-Ru catalysts is also studied. The fresh and used catalysts are sufficiently characterized via XRD, N2 physisorption, NH₃- and CO₂-TPD, as well as XPS. The attempt to improve the activity of Ni-based CO₂ methanation catalysts via the support modification or Ru-addition is a hot topic in the literature. However, many parts of the manuscript need to be revised by the authors before publication. Therefore, I recommend acceptance of this manuscript after major revisions.

The recommended revisions are listed hereafter:

1. Page 2: Synthetic methane or synthetic natural gas (SNG) produced via the CO₂ methanation reaction would normally not be considered as a biofuel, except for the case where the reaction is used to convert the CO₂-content of biogas into methane and thus yielding biomethane.
2. Page 3: The authors should revisit Ref. [29] for that argument. In Ref. [29] it is stated that NiO-CeO₂ nanoparticles achieve a better CO2 methanation performance than 3DOM NiO-CeO₂. The catalytic performance follows the order NiO-CeO₂ (np) > NiO-CeO₂ (ref) > NiO-CeO₂ (3DOM).
3. In the introduction part, the authors discuss the effect of alkaline earth and lanthanide addition on Al₂O₃ to promote CO₂ methanation, but fail to discuss works that focus on the effect of Ru addition on Ni-based catalysts, which is a big part of their work. The authors can enrich their introduction part via adding a paragraph where they discuss the effect of noble metal addition (e.g. Ru) on Ni-based catalysts to improve their methanation performance. The following works are suggested to be incorporated in this part:

• Tsiotsias, A.I.; Charisiou, N.D.; Yentekakis, I.V.; Goula, M.A. Bimetallic Ni-Based Catalysts for CO₂Methanation: A Review. Nanomaterials 2021, 11, 28.
• Polanski, J.; Lach, D.; Kapkowski, M.; Bartczak, P.; Siudyga, T.; Smolinski, A. Ru and Ni—Privileged Metal Combination for Environmental Nanocatalysis. Catalysts 2020, 10, 992.
• Zhen, W.; Li, B.; Lu, G.; Ma, J. Enhancing catalytic activity and stability for CO₂ methanation on Ni-Ru/-Al₂O₃ via modulating impregnation sequence and controlling surface active species. RSC Adv. 2014, 4, 16472–16479.
• Renda, S.; Ricca, A.; Palma, V. Study of the effect of noble metal promotion in Ni-based catalyst for the Sabatier reaction. J. Hydrogen Energy 2020, in press.
• Bustinza, A.; Frías, M.; Liu, Y.; García-Bordejé, E. Mono- and bimetallic metal catalysts based on Ni and Ru supported on alumina-coated monoliths for CO₂ Catal. Sci. Technol. 2020, 10, 4061–4071.

4. Page 5: Regarding the ICP-OES analyses: It is expected that Ni and Ru content will vary a little due to experimental errors during the catalyst synthesis. However, the authors report high deviations from the targeted 13% Ni and 1% Ru loadings in many of their catalysts. The argument that these deviations have to do with the promoter metal used does not seem plausible, since some of the works they cite in this part, e.g. Ref. [39], do not report such high deviations. At first, the authors should state whether the ICP-OES was performed on calcined or reduced catalysts. Moreover, the authors should consider these high deviations in Ni and Ru loadings when discussing the activity tests for these catalysts (e.g. 13Ni/10Ba-Al₂O₃)
5. Page 8: The reduction profiles suggest that Ca addition improves the reducibility of Ni in 13Ni/10Ca-Al₂O₃, just like the case of 13Ni/14La-Al₂O₃, since the Ni-O-Al peak is observed at lower temperatures than in 13Ni/Al₂O₃.
6. Page 10: Maybe it would be better to discuss the acidity first and the basicity second. The authors could also discuss the effect of the population of medium-strength basic sites on the catalytic performance as reported by Liu et al.:

• Liu, K.; Xu, X.; Xu, J.; Fang, X.; Liu, L.;Wang, X. The distributions of alkaline earth metal oxides and their promotional effects on Ni/CeO₂ for CO₂ J. CO₂ Util. 2020, 38, 113–124.

 

7. Pages 15 and 16: Regarding the XRD: The authors should include fresh and used 13Ni/Al₂O₃ diffraction pattern, if measured, in Figures 9 and 10 for comparison purposes.

 

8. Page 16, Table 4: By AlLa₃ do the authors mean an alloy phase? Also, metallic Ba and AlLa₃ alloy phases are unlikely to occur after reduction at just 673 K. Please reconsider the occurrence of such phases. By the way, the authors in Ref. [27] mention the occurrence of a LaAlO₃ perovskite phase in their XRDs, far form a AlLa₃ alloy one.

 

9. Page 17, Table 5: Does Table 5 refer to the crystallite size of these phases in all catalysts, or an average value between them? Do all catalysts have exactly the same crystallite size of these phases, as calculated via the Scherrer equation? Moreover, it would be better if the authors just try to compare the Ni and Ru crystallite sizes (which are the ones serving as CO₂ methanation active sites) between the different catalysts, instead of listing all the possible phases observed in XRD.

 

10. Generally about the catalyst characterization section: Which are the “used” catalysts the authors refer to? Are they after activity test, or after H₂S exposure? Please state the exact treatment the used catalysts were subjected to.
11. Generally about the catalyst characterization section: Many Figures in this section lack a vertical axis title.
12. Page 17, Figure 11: Vertical axis title misses the (%) after CH₄ yield.
13. Did the authors observe 100% CH₄ selectivity in all their experiments? If yes, this is a highly rare phenomenon in the literature and the authors should state whether this is an effect of the high pressure used etc.. If not, and there are other by-products produced such as CO, the authors should discuss CH₄ selectivity along with their other results during the activity tests.
14. Pages 18-20: When discussing the results referred to in Figure 13, the authors should also mention the differences in Ni content as measured by ICP-OES. Same for Ru content in Figure 15.
15. Page 19: Did the authors prepare any catalysts supported on rGO that they fail to mention in the experimental section, or is there a typing error?
16. Page 22: The existence of Ru-Ni bimetallic particles has not been proven by the authors due to the lack of more advanced characterization techniques such as TEM and EDS mapping. Seperate Ni and Ru metallic phases could also exist in their NiRu bimetallic catalysts.
17. Page 22: The authors should also include some literature works on their argument about H₂S deactivation and the effect of lanthanide (La and Ce) addition to the delay of this deactivation.
18. Some spelling errors have also been spotted:

• Page 2: Either [12-22] or [12,22] would be correct
• Page 2: increases the alkalinity ..., facilitates the chemisorption ..., increases (the population) of oxygen vacancies ... and improves the dispersion ....

 

Author Response

The authors have performed an extensive study on the effect of alkaline earth and lanthanide modification of the alumina support in Ni catalysts for CO₂ methanation. The effect of Ru addition to form bimetallic Ni-Ru catalysts is also studied. The fresh and used catalysts are sufficiently characterized via XRD, N2 physisorption, NH₃- and CO₂-TPD, as well as XPS. The attempt to improve the activity of Ni-based CO₂ methanation catalysts via the support modification or Ru-addition is a hot topic in the literature. However, many parts of the manuscript need to be revised by the authors before publication. Therefore, I recommend acceptance of this manuscript after major revisions.

The recommended revisions are listed hereafter:

1. Page 2: Synthetic methane or synthetic natural gas (SNG) produced via the CO₂ methanation reaction would normally not be considered as a biofuel, except for the case where the reaction is used to convert the CO₂-content of biogas into methane and thus yielding biomethane.

 

Authors agree with the reviewer comment, eliminating this term from the text to facilitate its understanding:

 

“The methanation of CO₂, studied by Sabatier (Eq. 1), proposes a sustainable way to transform residual CO₂ into methane as a biofuel”.

 

1. Page 3: The authors should revisit Ref. [29] for that argument. In Ref. [29] it is stated that NiO-CeO₂ nanoparticles achieve a better CO2 methanation performance than 3DOM NiO-CeO₂. The catalytic performance follows the order NiO-CeO₂ (np) > NiO-CeO₂ (ref) > NiO-CeO₂ (3DOM).

 

Following the suggestion made by the reviewer, to clarify the argument, the text has been changed and a new paragraph has been added:

 

“Jomjaree et al. [29] prepared Ni/CeO₂ nanostructured catalysts with tuneable CeO₂ morphology/structure determining the influence of this in increasing oxygen vacancy and oxygen storage capacity and their influence on catalytic activity enhancement between 473 and 773 K under atmospheric pressure. Cardenas-Arenas et al. [29] prepared 3D ordered macroporous structure of NiO-CeO₂ mixed oxide (NiO-CeO₂ (np)) to achieve high CO₂ methanation conversion due to its high specific surface area if compared with a reference catalyst without size control (NiO-CeO₂ (Ref))”.

 

1. In the introduction part, the authors discuss the effect of alkaline earth and lanthanide addition on Al₂O₃ to promote CO₂ methanation, but fail to discuss works that focus on the effect of Ru addition on Ni-based catalysts, which is a big part of their work. The authors can enrich their introduction part via adding a paragraph where they discuss the effect of noble metal addition (e.g. Ru) on Ni-based catalysts to improve their methanation performance. The following works are suggested to be incorporated in this part:

• Tsiotsias, A.I.; Charisiou, N.D.; Yentekakis, I.V.; Goula, M.A. Bimetallic Ni-Based Catalysts for CO₂Methanation: A Review. Nanomaterials2021, 11, 28.
• Polanski, J.; Lach, D.; Kapkowski, M.; Bartczak, P.; Siudyga, T.; Smolinski, A. Ru and Ni—Privileged Metal Combination for Environmental Nanocatalysis. Catalysts 2020, 10, 992.
• Zhen, W.; Li, B.; Lu, G.; Ma, J. Enhancing catalytic activity and stability for CO₂methanation on Ni-Ru/-Al₂O₃ via modulating impregnation sequence and controlling surface active species. RSC Adv. 2014, 4, 16472–16479.
• Renda, S.; Ricca, A.; Palma, V. Study of the effect of noble metal promotion in Ni-based catalyst for the Sabatier reaction. Hydrogen Energy 2020, in press.
• Bustinza, A.; Frías, M.; Liu, Y.; García-Bordejé, E. Mono- and bimetallic metal catalysts based on Ni and Ru supported on alumina-coated monoliths for CO₂ Sci. Technol. 2020, 10, 4061–4071.

 

Authors thank the suggestion made by the reviewer in order to improve the quality of this paper. Following the recommendation, a paragraph has been included discussing the effect of adding alkaline earth metals and lanthanides on the alumina support, based on the recommended literature:

 

“Renda et al. [25] has studied the effect of incorporating different amounts of platinum and ruthenium (0.5; 1 and 3 wt%) on a catalyst based on ceria as support and nickel (10 wt%) as promoter metal was studied. It is observed that the most suitable amount of ruthenium to improve the activity of the nickel catalyst is 1 wt%, with the best value being 0.5 wt% in the case of platinum, reaching a higher conversion at a lower temperature. A similar conclusion was found by Bustinza et al. [26], by incorporating ruthenium in different proportions and order on a nickel (12.7 wt%) catalyst supported in alumina coating monolith. On this occasion, the best conversion in the methanation reaction was achieved for catalysts with 1 wt% ruthenium”.

 

1. Page 5: Regarding the ICP-OES analyses: It is expected that Ni and Ru content will vary a little due to experimental errors during the catalyst synthesis. However, the authors report high deviations from the targeted 13% Ni and 1% Ru loadings in many of their catalysts. The argument that these deviations have to do with the promoter metal used does not seem plausible, since some of the works they cite in this part, e.g. Ref. [39], do not report such high deviations. At first, the authors should state whether the ICP-OES was performed on calcined or reduced catalysts. Moreover, the authors should consider these high deviations in Ni and Ru loadings when discussing the activity tests for these catalysts (e.g. 13Ni/10Ba-Al₂O₃)

 

Following the kind reviewer recommendation, the state of the catalyst on which it is analysed in ICP has been clarified in the text, the discussion has been incorporated in the activity text according to the results obtained in ICP for both catalysts that incorporate alkaline earth metals, such as those that incorporate ruthenium:

 

The chemical composition of the calcined catalysts employed in the CO₂ methanation was measured by ICP-OES analyses.

 

According to what is observed in the characterization of these catalysts by ICP, it is observed that in those catalysts whose metallic content of Ni is lower, they present worse performances than those, whose content of Ni. Thus, the Mg catalyst (Ni content 17.52 wt%) has better activity values than Ca (Ni content 15.73 wt%), and both better than Ba, whose Ni content is 8.83 wt%.

The incorporation of Ru into the 13Ni/14La-Al₂O₃ catalyst produced a clear improvement in the methanation rate with respect to the other catalysts, despite the decrease in the amount of Ru produced with the incorporation of Ni, as observed in the ICP results. At 581 K the methane yield reached by this catalyst was 74%, reaching a maximum value of 89% at 651 K. Therefore, although the contribution of Ru appears to be similar to that of Ni, the effect of Ni is greater than that of Ru, especially in combination.

 

1. Page 8: The reduction profiles suggest that Ca addition improves the reducibility of Ni in 13Ni/10Ca-Al₂O₃, just like the case of 13Ni/14La-Al₂O₃, since the Ni-O-Al peak is observed at lower temperatures than in 13Ni/Al₂O₃.

 

The authors appreciate the reviewer suggestion, that is why a paragraph has been included justifying the effect of the addition of calcium on the activity of the catalyst:

 

“The contribution of Ca is due to the presence of CaO in the catalyst, which is related to the reduction at low temperatures,strengthening the interaction with NiO, improving the reducibility of Ni [28]”.

 

1. Page 10: Maybe it would be better to discuss the acidity first and the basicity second. The authors could also discuss the effect of the population of medium-strength basic sites on the catalytic performance as reported by Liu et al.:

• Liu, K.; Xu, X.; Xu, J.; Fang, X.; Liu, L.;Wang, X. The distributions of alkaline earth metal oxides and their promotional effects on Ni/CeO₂for CO₂ CO₂ Util. 2020, 38, 113–124.

The authors appreciate the reviewer interesting recommendations. Thus, the order of the discussion of acidity has been modified so that it is in first place and to facilitate the understanding and coherence of the text. In addition, a paragraph has been included that justifies the effect of moderate basic centres on activity, in reference to what is observed in the bibliography:

“In ceria-supported catalysts, moderate base centres show significant benefit in catalytic activity, as demonstrated by Liu et al. [61]. The contribution of these moderate basic centres in catalysts supported on alumina adds to the effect of the strong basic centres, both effects being those that lead to an increase in the interaction with CO₂, to facilitate the methanation reaction”.

1. Pages 15 and 16: Regarding the XRD: The authors should include fresh and used 13Ni/Al₂O₃ diffraction pattern, if measured, in Figures 9 and 10 for comparison purposes.

 

Following the kind reviewer recommendation, the Ni/Al₂O₃ catalyst has been included in Figures 9 and 10 to facilitate comparison and understanding of the discussion:

 

 

1. Page 16, Table 4: By AlLa₃ do the authors mean an alloy phase? Also, metallic Ba and AlLa₃ alloy phases are unlikely to occur after reduction at just 673 K. Please reconsider the occurrence of such phases. By the way, the authors in Ref. [27] mention the occurrence of a LaAlO₃ perovskite phase in their XRDs, far form a AlLa₃ alloy one.

 Following the suggestion made by the reviewer, the text was corrected in the Table 4:

Metallic species	JCPDS code	Value (2q)	Bibliography
Al2O3	077-0396	19.7, 34, 37.5, 44.5, 57, 61, 67.4, 85	[62,63]
Ni	087-0712	44.5, 51.6, 76.7	[17]
Ru	088-1734	38.5, 42.3, 44.2, 78.6, 84.9	[64,65]
Ba	006-0235	24.1, 34.5	[28,53]
CaO	086-0402	32.2, 37.3, 53.8, 64.1, 67.3	[34,67]
Mg0.4Ni0.6O	034-0410	37.4, 43.5, 63, 75.5, 79.4	[68,69]
MgO	087-0653	37.5, 43.5, 63, 75.7, 79.2	[68,69]
CeO2	075-0076	28.9, 33.4, 47.7, 56.6, 76.8	[64,70]
Al22La2O36	028-0502	25.8, 34.1, 37.5, 44.5, 45.9, 51.6	[29]
LaAlO3	031-0022	25.3, 30.8, 44.1, 51.3, 56.7	[29]
BaAl2O4	017-0306	19.7, 22.2, 28.4, 34.5, 37.5, 40.3, 42.2, 45.1, 45.9, 54.7, 58, 61.5, 67.2, 76.4	[34,53,71]
BaCO3	005-0378	19.7, 24, 24.4, 34.5, 40.3, 42.2, 44.6, 45.1, 54.7, 61.5, 76.4	[34,71]
La2O2CO3	025-0424	26, 30.5, 33.74, 44.6, 51.82	[72]

 

1. Page 17, Table 5: Does Table 5 refer to the crystallite size of these phases in all catalysts, or an average value between them? Do all catalysts have exactly the same crystallite size of these phases, as calculated via the Scherrer equation? Moreover, it would be better if the authors just try to compare the Ni and Ru crystallite sizes (which are the ones serving as CO₂ methanation active sites) between the different catalysts, instead of listing all the possible phases observed in XRD.

 Authors thank the suggestion made by the reviewer in order to improve the paper. The particle size value of all species has been removed from Table 5, with the exception of ruthenium and nickel crystals, which will make the text easier to understand.

Species		Peak position (º)		Crystal size approximation of reduced catalyst (nm)	Crystal size approximation of used catalyst (nm)
Al2O3		67.3		5	5
CeO2		28.8		7	10
MgO		62.9		7	10
BaO	BaAl2O4	24.2	24.2	20	50
	BaCO3		28.6		50
Ru		44.2		20	5 and 20 (in 1Ru/Al2O3)
Ni		51.9		< 5	15
CaO				10	25
Al22La2O36/AlLa3	La2O2CO3	25.4	30.6	10	60

 

1. Generally about the catalyst characterization section: Which are the “used” catalysts the authors refer to? Are they after activity test, or after H₂S exposure? Please state the exact treatment the used catalysts were subjected to.

 

The authors appreciate the indication of the error in the text, which has been corrected:

 

Used catalyst (after activity test and H₂S exposure) showed also three peaks in 13Ni/14La-Al₂O₃ and 1Ru-13Ni/14La-Al₂O₃, at 855.1, 857.5 and 862.8 eV, corresponding to Ni²⁺ from NiO, Ni²⁺ from NiAl₂O₄, and Ni²⁺ satellite respectively, and only two in the others at 855.8-856.6 and 862.0-863.0 eV, corresponding to Ni²⁺ from NiAl₂O₄, and Ni²⁺ satellite respectively, as observed in Fig. 5 and Fig. 6.

 

 

 

1. Generally about the catalyst characterization section: Many Figures in this section lack a vertical axis title.

 

 

Authors kindly appreciate this reviewer suggestion. However, the axes in these graphs do not improve the understanding of the figures, since they are axes with random units. That is why these graphs have been unified to maintain the scale, not including the vertical axis to avoid confusing the reader.

 

 

1. Page 17, Figure 11: Vertical axis title misses the (%) after CH₄ yield.

 

Following the reviewer’s indication, to facilitate the understanding, the graph was corrected:

 

 

1. Did the authors observe 100% CH₄ selectivity in all their experiments? If yes, this is a highly rare phenomenon in the literature and the authors should state whether this is an effect of the high pressure used etc.. If not, and there are other by-products produced such as CO, the authors should discuss CH₄ selectivity along with their other results during the activity tests.

 

Authors appreciate this reviewer comment, adding the required explanation in the text:

 

All the catalysts studied reach 100% CH₄ selectivity due to the pressure of 10 bar and WSHV conditions to which the reactor was configured, detecting an amount of CO as a by-product that was close to zero. The maximum methane yield value was reached and stablished by the thermodynamic equilibrium in the temperature range, for an amount of catalyst of 200 mg.

 

 

1. Pages 18-20: When discussing the results referred to in Figure 13, the authors should also mention the differences in Ni content as measured by ICP-OES. Same for Ru content in Figure 15.

 

 

 

According to what is observed in the characterization of these catalysts by ICP, it is observed that in those catalysts whose metallic content of Ni is lower, they present worse performances than those, whose content of Ni. Thus, the Mg catalyst (Ni content 17.52 wt%) has better activity values than Ca (Ni content 15.73 wt%), and both better than Ba, whose Ni content is 8.83 wt%.

 

 

1. Page 19: Did the authors prepare any catalysts supported on rGO that they fail to mention in the experimental section, or is there a typing error?

 

Following the reviewer’s indication, to facilitate the understanding, the text was corrected:

 

Thus, 60 mg of catalyst (128.75 h^-1), and a temperature range between 598 and 773 K, were used to compare the improvement in the activity of catalysts based on Ru, Ni and La supported on alumina on rGO.

 

1. Page 22: The existence of Ru-Ni bimetallic particles has not been proven by the authors due to the lack of more advanced characterization techniques such as TEM and EDS mapping. Seperate Ni and Ru metallic phases could also exist in their NiRu bimetallic catalysts.

 

Authors agree with the reviewer statement. For this reason, the text has been modified so as not to confuse the reader by indicating that it is a bimetallic catalyst, since it has not been effectively verified, eliminating these indications and modifying the text:

 

Ru dispersed as RuO₂ and RuO_x across the surface of the support in contact with the NiO present, being able to give Ru-Ni bimetallic particles.

 

1. Page 22: The authors should also include some literature works on their argument about H₂S deactivation and the effect of lanthanide (La and Ce) addition to the delay of this deactivation.

 

Following the kind reviewer recommendation, more bibliography has been included to facilitate understanding of the results, and to compare them with other studies. However, it has not been possible to include references that justify the effect observed for lanthanum in this study, with other studies, since this is one of the novel points that this study contributes with respect to others based on the same line of research.

 

The recovered catalyst was used in a reaction stage, at 773 K. It was observed that the catalysts activity as methane yield was not recovered [37,74].

 

 

1. Some spelling errors have also been spotted:

• Page 2: Either [12-22] or [12,22] would be correct
• Page 2: increases the alkalinity ..., facilitates the chemisorption ..., increases (the population) of oxygen vacancies ... and improves the dispersion ....

 

Authors truly appreciate the reviewer comment. The modifications indicated by the reviewer have been included in the text, correcting those errors that persisted in it.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I am satisfied that the authors have significantly improved their manuscript, which now merits publication

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This paper focuses on an important topic that is both timely and of interest to a broad range of the catalytic research community by examining the effect of the modification of Al2O3 support using alkaline earth (Ba, Ca, Mg) or lanthanide (Ce or La) for Ni or Ru catalysts. The work is well thought off and executed and the conclusions reached will be of benefit to other researchers in the field. The use of literature is also appropriate, although the incorporation of some recent Review papers in the introduction will strengthen the discussion (e.g., https://doi.org/10.1016/j.cattod.2020.07.023, https://doi.org/10.1016/j.matpr.2020.07.191, https://doi.org/10.3390/catal10070812).   

The main weakness if this work is in its use of English, which should be substantially improved. This is an important point as the work contains numerous grammatical mistakes, which devalue what is otherwise an excellent contribution.

Reviewer 2 Report

Article title	Effect of the addition of alkaline earth and lanthanide metals for the modification of the alumina support in Ni and Ru catalysts in CO2 methanation.
Authors	Méndez-Mateos et al.

 

Méndez-Mateos et al. report in this work the effects of incorporating promoters (alkali earth or lanthanides) in the formulation of Ni and Ru-containing alumina catalysts for carbon dioxide methanation reaction. Interestingly, they also evaluated the effect of incorporating H₂S in the reaction feed.

Despite the significant amount of work presented, the reduced clarity of the text in some parts of the manuscript (which suggests that some parts were written by different authors without being fully revised by one/two authors prior to the submission to produce an homogeneous and systematic document), the poor quality of the figures and the limitations of the comparisons established when taking into account the differences in terms of real (determined by ICP) catalysts formulations motivate my decision of suggesting the rejection of this work.

To my point of view, authors should try to divide this work into two articles, simplifying the text and improving at the same time the focus, results analysis and discussion and conclusions. Apart from this, I recommend taking into account my comments (presented below) for future submissions:

 

1. In the preparation conditions, authors refer they prepared the catalysts by “wet wetness impregnation”. They should correct this to wet impregnation (as written for the case of Ni incorporation method).

 

2. Authors should explain how did they choose the used active metals and promoters’ loadings. Regarding La and Ce, is it due to Garbarino et al. findings?

 

3. Table 1 should be used to summarize all the samples from this work in terms of Name, preparation conditions (including precursor salt characteristics) and metal loadings, among all.

 

4. Did authors perform H₂-TPD analysis? (“The temperature programmed reduction and desorption of H₂ (H₂-TPR)” is written in the Experimental section).

 

5. Authors should clarify how did choose the pre-reduction temperature. Even if they write a paragraph in the end of TPR results discussion, Ru samples usually present reduction processes at lower temperatures, which motivates the use of lower reduction temperatures (avoiding unnecessary sintering processes).

 

6. Authors should indicate the flowrate of the reactants for the catalytic tests and clearly refer if they use a pure H₂/CO₂ stream or include inert gases in the mixture.

 

7. Authors say “The stoichiometry proportion for the methanation reaction was employed to obtain biogas”. What do they mean? Biogas is a mixture of CH₄ and CO₂ (with other impurities) resulting from biomass degradation. If authors carried out this work in the context of biogas upgrading via CO₂ methanation, the introduction section could be improved in this way.

 

8. Authors say “The reactor was heated until the reaction temperature at a rate of 10 K/min under N₂ flow, studying the catalyst activity”. How are they studying the catalytic activity in absence of reactants? This must be corrected.

 

9. Authors state they analyse the products of the reaction in a Peltier condenser. They should correct this information, as the Peltier is used for removing water from the stream.

 

10. The synthesis of samples whose ICP analysis differs significantly from the target must be repeated. No conclusions could be established if real compositions are very different among the studied series (e.g. if Ni loading is different the differences in the catalysts performances cannot be only attributed to the modifications induced by the promoter nature).

 

11. Table 2: Where are the TOF numbers? How can authors present values for Ni crystallite sizes in samples only containing Ru? 7 out of 9 samples present exactly the same average crystallite sizes after reduction and tests. Is this correct? Based on Figure 7 and Figure 8 patterns, the low intensity and broad peaks found for Ni diffraction peaks indicate a high metallic dispersion. TEM microscopy should be used for obtaining the Ni⁰ average particle sizes in a more accurate way.

 

12. Figure 1: Not clear. Authors should split this Figure to improve its clarity, as it is not possible to see the reduction processes occurring in many samples. Authors should include the reduction temperature used in the tests in the graph, to show the readers which is the expected amount of reducible species at the chosen temperature. Also, line 273: authors attribute peaks to “metallic nickel” instead of nickel oxide.

 

13. Both TPR and TPD profiles cannot be clearly analysed. Figures must be split.

 

14. XPS: this technique was not included in the Experimental section.

 

15. The discussion regarding spent samples characteristics (XPS, XRD) should be placed after the catalytic tests in the manuscript. This will allow establishing structure-reactivity relationships.

 

16. Table 6: As already pointed out regarding Table 2, are nickel crystallite sizes equal in all samples after reduction and test? TEM must be performed.

 

17. Catalytic performances: Some samples present higher performances than the equilibrium. This must be revised. Also, the presentation of the results is not clear and the legends must be improved, as some figures present exactly the same text. Comparing samples at a specific temperature using a bar graph will be more interesting.

 

Overall, English must be fully revised and corrected (many incorrect words like syntering or lantanides can be seen in the document). Authors should also pay attention and correct the decimal signs (e.g. 1093 K instead of 1.093 K). Also, decimal numbers should be revised (e.g. Table 3 data must be presented without decimal numbers if the accuracy of the values is not high enough).

Reviewer 3 Report

In my opinion, it is a very nice paper. The topic is important and the results are interesting. Also the paper is well written and clear, in a good english. The characterisation of the materials is exhaustive and very complete.

I only have a very few minor comments:

1) Not important at all, but usually Table captions are places above the tables, not below. But this is not even an issue.

2) Figures 9-14 - It might only be seen as a minor detail, but it is important Do not connect the points wth straight lines. Like that you give the wrongly impression that the curves are continuous, just put the experimental points in larger size and then if you want add a smoothed dashed line connecting them, but make clear in the figure caption that the points are the experimental values obtained and the lines are only a guidance to see the tendency of the curves.