The Efficiency of Carbon Conversion and Hydrogen Production from Tar Steam Reforming of Biomass Using Ni-Based Catalysts with Alkaline Earth Promoters

Round 1

Reviewer 1 Report (Previous Reviewer 1)

This is the second time I read this paper. As I said before, the paper presents an interesting study over the design of a new catalyst to produce syngas from tars. However, I still think that the authors are making asseverations that are not supported by the presented results. Authors included the base material for the La-supported catalyst but not for the others and this seems odd. I still think that you need to present the analysis for Al and Zr-based catalysts. I recommend either reducing the analysis just the La-base catalyst or including the other needed data to support the current affirmations. Additionally, I make the following general comments:

 

1.     Abstract. Although there was an improvement, the abstract still needs development in terms of the language and the impact of the study.

2.     Why did the authors test these three supports? The explanation is missing in Introduction.

3.     Fig. 1. Authors say in line 97: “Results also show that catalysts without promoters have a greater weight loss than other catalysts with promoters due to their role in increasing thermal stability by controlling size, reducing oxidation and forming a barrier effect.” However, I see that NiLa loss less weight than NiLaMgCa, so I do not understand this affirmation.

4.     Table 2 needs more discussion supporting the interesting results you have obtained.

5.     Fig. 7. What does the authors propose that this gas can be used for based on the low H2/CO? This because energy applications generally require H2/CO ratios higher than 3.

6.     Fig. 9. I do not think that this figure can present concluding results. Surface area seems not to be an important parameter on this catalytic activity and catalysts dispersion are very similar to propose an effect. Additionally, carbon conversion to gas and H2 yield are not that different. The only thing that seems to be clear from this figure is that Zr is negatively affecting H₂ yield; why? But NiLaMgCa and NiAlMgCa are pretty similar to me.

7.     Line 274: I think you do not have data to say: “The addition of MgO and CaO as a promoter can enhance the dispersion of Ni on La2O3 support by reducing the particle size of Ni.” Again, you would need the dispersion of Ni/La2O3.

8.     Based on my comments on 4 y 5, I recommend deleting the analysis of Fig 9 because it sees very speculative.

9.     I do not understand the reactions presented in line 329, please explain them better.

10.  The line 331 must be separated from the rest of the paragraph and can be better supported.

11.  Conclusions: I suggest to re-write them by a clear introduction and a more organized way of presenting the most important results. Also, I insist that it is too bold to talk about stability when you only presented a 300 min TOS reaction.

12.  There are several mistakes on the manuscript that must be carefully checked, such as the use of Hours instead of h (please use ISO standars), points dislocated, colons where they do not belong, etc.

Author Response

Dear Dr./ Mr./Ms Editor,

Thank you for allowing me to submit a revised draft of my manuscript titled “Efficiency of Carbon Conversion and Hydrogen Production from Tar Steam Reforming of Biomass using Ni-Based Catalysts with Alkaline Earth Promoters” to MDPI Catalyst. We appreciate the time and effort that you and the reviewers have dedicated to providing your valuable feedback on this manuscript.  We are grateful to the reviewers for their insightful comments on our paper. We have been able to incorporate changes to reflect most of the suggestions provided by the reviewers. We have highlighted the changes within the manuscript. Here is a point-by-point response to the reviewers’ comments and concerns.

 

REVIEWER 1

 

Comments and suggestions

This is the second time I read this paper. As I said before, the paper presents an interesting study over the design of a new catalyst to produce syngas from tars. However, I still think that the authors are making asseverations that are not supported by the presented results. Authors included the base material for the La-supported catalyst but not for the others and this seems odd. I still think that you need to present the analysis for Al and Zr-based catalysts. I recommend either reducing the analysis just the La-base catalyst or including the other needed data to support the current affirmations. Additionally, I make the following general comments:

1.      Abstract. Although there was an improvement, the abstract still needs development in terms of the language and the impact of the study.   Response: Thank you for pointing this out. We have accordingly revised the abstract by adding some important points for this research.      “Catalytic steam reforming of biomass tar is the process that involves heating the tar in the presence of steam and a catalyst, which facilitates the chemical reaction of breaking down the tar molecules and producing H2 and carbon dioxide (CO2) as byproducts. The H2 produced can be used as a clean and renewable energy source for a variety of applications, including fuel cells, internal combustion engines, and chemical production. In this research, a multi-compound of biomass tar model including phenol, toluene, naphthalene and pyrene was undergo a steam reforming process.  Various types with 10 wt% of nickel-based..”   “..enhancing the stability and activity of the catalyst by promoting the formation of active sites and improving the acid-base balance of the catalyst surface. This can lead to an increase in the con-version of tar to hydrogen gas and a decrease in the production of undesired byproducts, such as CH4 and CO.  NiLaMgCa produced the most carbon to gas conversion (86.27 mol %) and H2 yield (51.58 mol%) after 5 h hours of reaction compared to other catalysts. Additionally, the filamentous carbon coke that is deposited on the spent catalyst of NiLaMgCa does not impact the catalyst activity..”

2.      Why did the authors test these three supports? The explanation is missing in Introduction.   Response: Thank you for pointing this out. We have accordingly added some explanations supported by previous study literature regarding the nickel-based catalyst that has been supported by La2O3, Al2O3 and ZrO2.   ” The catalyst's support is also a crucial component. Support influences not just the dispersion of active metal particles, but also the catalytic reaction. Pore structure, acidity-basicity, and metal support interaction are the important support material aspects to consider in excellent catalyst development. In this study, Ni based catalysts had been supported with 3 different types of support including lanthana (La2O3), alumina (Al2O3) and zirconia (ZrO2). These oxides are thermally stable and can withstand the high temperatures required for steam reforming reactions and have a high surface area, which helps to increase the number of active sites available for catalytic reactions. According previous study, showed that Al2O3 and La2O3 supported catalysts to produce a low level of CH4 gas due to their excellent ability in reforming produced CH4. Even though nickel-based catalysts with La2O3 support have a small BET surface area, the basicity of it was essential to promote the catalytic activity in steam reforming [20]. Studies regarding the nickel-based catalyst supported by ZrO2 illustrate that large pore size improved bio-oil bulky molecule accessibility to Ni sites, resulting in improved catalytic activity [21]..”

3.      Fig. 1. Authors say in line 97: “Results also show that catalysts without promoters have a greater weight loss than other catalysts with promoters due to their role in increasing thermal stability by controlling size, reducing oxidation and forming a barrier effect.” However, I see that NiLa loss less weight than NiLaMgCa, so I do not understand this affirmation.   Response: Thank you for pointing this out. We have accordingly restructured the phrase in line 97.   “Results also show that catalysts with promoters have a greater weight loss than other catalysts without promoters…”

4.      Table 2 needs more discussion supporting the interesting results you have obtained.   Response: Thank you for pointing this out. We have accordingly revised the manuscript regarding the nickel-based catalyst with various support that has been stated in Table 2.     “Nickel-based catalysts supported with La2O3, Al2O3, and ZrO2 can effectively catalyze the steam reforming of biomass tar, producing high hydrogen yields and having good catalytic activity. These catalysts are also known for their thermal and chemical stability, and resistance to poisoning and sintering, which improves the durability of the catalyst…”

5.      Fig. 7. What does the authors propose that this gas can be used for based on the low H2/CO? This because energy applications generally require H2/CO ratios higher than 3.   Response: Thank you for pointing this out. We have accordingly revised the manuscript by adding some discussion and supported by the previous study regarding the H2/CO ratio.   “The gas may undergo incomplete conversion and catalysts may not be able to convert all of the biomass tar into syngas, resulting in a lower H2/CO molar ratio. Syngas with a low H2/CO ratio can be used as feedstock for the production of chemicals such as ethanol. [58] To improve the H2/CO ratio, adjustments can be made to the steam reforming process, such as increasing the steam to biomass ratio (S/C ratio) [59-61]….”

6.      Fig. 9. I do not think that this figure can present concluding results. Surface area seems not to be an important parameter on this catalytic activity and catalysts dispersion are very similar to propose an effect. Additionally, carbon conversion to gas and H2 yield are not that different. The only thing that seems to be clear from this figure is that Zr is negatively affecting H2 yield; why? But NiLaMgCa and NiAlMgCa are pretty similar to me.   Response: Thank you for pointing this out.  As suggested by the reviewer in comment no (8) by deleting Figure 9, this part of the discussion has also been deleted from the manuscript to avoid speculative information.

7.      Line 274: I think you do not have data to say: “The addition of MgO and CaO as a promoter can enhance the dispersion of Ni on La2O3 support by reducing the particle size of Ni.” Again, you would need the dispersion of Ni/La2O3.   Response: Thank you for pointing this out. As suggested by the reviewer in comment no (8) by deleting Figure 9, this part of the discussion has also been deleted from the manuscript to avoid speculative information.

8.      Based on my comments on 4 y 5, I recommend deleting the analysis of Fig 9 because it sees very speculative.   Response: Thank you for pointing this out. As suggested by the reviewer, we have deleted Figure 9 to avoid any speculative information from the manuscript.

9.      I do not understand the reactions presented in line 329, please explain them better.   Response: Thank you and we found your comments extremely helpful and have revised them accordingly by adding more discussion regarding equation 4 and equation 5 that has been stated in the manuscript for better understanding.   “Additionally, Equation 4 describes the reduction of metal oxide (MO2) to reduced metal oxide (MO2-x) with the release of oxygen gas (O2). This reaction further shows the redox capability of the ZrO2 support, as the transfer of electrons from MO2 to MO2-x is indicative of a redox process. In Equation 5, carbon (C) is oxidized by lattice oxygen (Ox) on the support surface to form carbon monoxide (CO) and a reduced site of the support (Ox-1). This reaction helps prevent the carbon from depositing on the catalyst surface as coke, which would otherwise reduce its effectiveness….”

10.  The line 331 must be separated from the rest of the paragraph and can be better supported.   Response: Thank you for pointing this out. The whole information had been separated as a new paragraph and a discussion has been added to support the paragraph (Line 365-384)

11.  Conclusions: I suggest to re-write them by a clear introduction and a more organized way of presenting the most important results. Also, I insist that it is too bold to talk about stability when you only presented a 300 min TOS reaction.   Response: Thank you for pointing this out. As mentioned above, we have accordingly revised the conclusion for a better understanding and comprehensibility.   “Steam reforming of gasified biomass tar is a process used to convert tar produced from biomass gasification into valuable chemicals and fuels. To improve the efficiency of this process, the catalyst used must have certain properties, such as a strong metal-support interaction, high basicity, and a mesoporous structure. Adding alkaline earth metal promoters, such as MgO and CaO, to the catalyst can improve these properties by strengthening the interaction between the metal and support, as well as increasing the basicity of the catalyst. This results in the formation of a mesoporous structure with an active metallic Ni phase, which is essential for catalytic activity. Among the various catalysts tested, NiLaMgCa was found to be the most effective, exhibiting high catalytic performance and stability in steam reforming of gasified biomass tar. Furthermore, the buildup of filamentous carbon coke on the spent catalyst did not negatively impact its activity, making NiLaMgCa a suitable catalyst for long-term use in the steam reforming process. NiLaMgCa produced the most carbon to gas conversion (86.27 mol %) and H2 yield (51.58 mol%) after 5 h of reaction compared to other catalysts Overall, the addition of alkaline earth metal promoters to the catalyst enhances its properties and makes it more effective in the steam reforming of gasified biomass tar…”

12.  There are several mistakes on the manuscript that must be carefully checked, such as the use of Hours instead of h (please use ISO standard), points dislocated, colons where they do not belong, etc.   Response: Thank you for pointing this out. The errors in abbreviation, punctuation marks, and spelling have been revised and corrected in the manuscript.

 

 

 

Author Response File: Author Response.docx

Reviewer 2 Report (Previous Reviewer 3)

I recommend the acceptance of this paper.

Author Response

Dear Dr./ Mr./Ms Editor,
Thank you for allowing me to submit a revised draft of my manuscript titled “Efficiency of Carbon Conversion and Hydrogen Production from Tar Steam Reforming of Biomass using Ni-Based Catalysts with Alkaline Earth Promoters” to MDPI Catalyst. We appreciate the time and effort that you and the reviewers have dedicated to providing your valuable feedback on this manuscript.  We are grateful to the reviewers for their insightful comments on our paper. We have been able to incorporate changes to reflect most of the suggestions provided by the reviewers. We have highlighted the changes within the manuscript. Here is a point-by-point response to the reviewers’ comments and concerns.

REVIEWER 2

I recommend the acceptance of this paper.
Response: Thank you very much.

Author Response File: Author Response.docx

Reviewer 3 Report (Previous Reviewer 2)

The modified manuscript has been improved but there are still criticisms to be solved before publication.

·         In order to assess that the reaction occurs under kinetic limitations, Prater criteria must be used. This is fundamental to arguing about the relationship between catalytic performance and physico-chemical features.

 

 

Author Response

Dear Dr./ Mr./Ms Editor,

Thank you for allowing me to submit a revised draft of my manuscript titled “Efficiency of Carbon Conversion and Hydrogen Production from Tar Steam Reforming of Biomass using Ni-Based Catalysts with Alkaline Earth Promoters” to MDPI Catalyst. We appreciate the time and effort that you and the reviewers have dedicated to providing your valuable feedback on this manuscript.  We are grateful to the reviewers for their insightful comments on our paper. We have been able to incorporate changes to reflect most of the suggestions provided by the reviewers. We have highlighted the changes within the manuscript. Here is a point-by-point response to the reviewers’ comments and concerns.

 

 

 

 

 

REVIEWER 3

Comments and suggestions

The modified manuscript has been improved but there are still criticisms to be solved before publication.

1.      In order to assess that the reaction occurs under kinetic limitations, Prater criteria must be used. This is fundamental to arguing about the relationship between catalytic performance and physicochemical features.   Response: Thank you for this suggestion.  It would have been interesting to explore Prater criteria as suggested by the reviewer. However, in the case of our study, the tar model is multiple components (toluene, pyrene, phenol, naphthalene) and we have difficulties obtaining quantitative product and reactant analysis. Since the support is known as a small pore size with a low surface area, not like zeolite, so the assumption of internal diffusion is not critical. La2O3 as a support in this study likely provides a stable and chemically resistant platform for the catalytic reaction, while its small pore size and low surface area would not significantly impact the reaction rate through internal diffusion.

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report (Previous Reviewer 1)

I think the authors still must answer the following issues from my previous revision:

 

1.     Authors included the base material for the La-supported catalyst but not for the others and this seems odd. I still think that you need to present the analysis for Al and Zr-based catalysts. I recommend either reducing the analysis just the La-base catalyst or including the other needed data to support the current affirmations.

2.     Abstract. Although there was an improvement, the abstract still needs development in terms of the language and the impact of the study.

 

3.     There are several mistakes on the manuscript that must be carefully checked regarding English style.

Author Response

Dear Editor,

Thank you for giving us the opportunity to submit a revised draft of the manuscript “The efficiency of Carbon Conversion and Hydrogen Production from Tar Steam Reforming of Biomass using Ni-Based Catalysts with Alkaline Earth Promoters” for publication in the Journal of MDPI catalyst. We appreciate the time and effort you and the reviewers dedicated to providing feedback on our manuscript and are grateful for the insightful comments and valuable improvements to our paper. We have incorporated the suggestions made by the reviewers. Those changes are highlighted within the manuscript.

 

1.      Authors included the base material for the La-supported catalyst but not for the others and this seems odd. I still think that you need to present the analysis for Al and Zr-based catalysts. I recommend either reducing the analysis just the La-base catalyst or including the other needed data to support the current affirmations.   Response: Thank you for pointing this out. We found your comments extremely helpful and have revised them accordingly by adding more discussion in the manuscript regarding the other catalyst without the promoter (NiAl and NiZr), as suggested by the reviewer.

2.      Abstract. Although there was an improvement, the abstract still needs development in terms of the language and the impact of the study.   Response: Thank you for pointing this out. We have revised the abstract by adding some critical points for this research (Line17 to line 45)

3.      There are several mistakes on the manuscript that must be carefully checked regarding English style.   Response: Thank you for pointing this out. The spelling and English style in the manuscript has been corrected and revised as suggested by the reviewer.

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

The paper presents an interesting study over the design of a new catalyst to produce syngas from tars. I agree with the authors when they stated that “NiLaMgCa” is an interesting catalyst for this reaction. However, the data that the authors are presenting to explain the behavior of this catalysts when comparing with the other tested materials is not enough for publishing the paper. The authors must include catalyst characterizations and catalytic tests of the catalysts without the oxides to better understand the effect of them on the catalysts’ properties. Additionally, I make the following general comments:

 

1.     Abstract. I recommend including more results and their impact.

2.     TGA. If they were calcined before reaction to 750 °C, how can we explain such a dramatic weight loss in TGA? The authors need to show TGA of the catalysts samples without the oxides to clearly see the effect of them on the stability of the materials.

3.     Line 95. How can the authors say: “reduction peak in the H2-TPR study shifts toward a higher temperature range with the addition of MgO and CaO promoters.” If they did not present TPRs of catalysts without these oxides?

4.     Line 99. It seems to be a mistake, the phase is not comprehensible

5.     TPR, TPD, BET also need to present the materials without the oxides to understand their effect on the catalysts reducibility.

6.     How the authors build the Fig. 6?

7.     TGA of spent catalysts: How is it possible than the catalyst loses less weight after reaction than fresh, calcined?

8.     Fig. 9a: I think the variation in carbon conversion to gas need more explanation.

9.     Conclusions: The following statement is impossible to support if you did not present catalytic activity of the materials without the oxides: “The ability for anti-deactivation and the metal-support interaction were both enhanced by the addition of MgO and CaO promoter.”

10.  Conclusions: I suggest avoiding this kind of affirmation. You only have a min TOS test: “As a result, it has been demonstrated that NiLaMgCa may sustain catalytic activity throughout the steam reformation of gasi-fied biomass tar.”

Reviewer 2 Report

The manuscript reports a study on Ni-based catalysts for steam reforming of tar from biomass gasification. The topic is interesting and current. However, the manuscript suffers from severe lacks suggesting its rejection. Main (not all) criticisms are reported below.

-The main lack is related to the results of catalytic activity. As shown in Figure 7, conversions are very similar, the differences lie within the experimental error as suggested by the error bars in the same figure. Actually, the Authors reported different H2 yield while gas yield is practically constant. However, in Figure 8 the selectivities are constant and H2/CO ratio varies between 0.74 and 0.77 (limited difference). These results suggests that also H2 yield is slightly dependent on the catalyst composition. Generally, catalytic results such as these are related to mass transfer limitations rather than intrinsic catalyst activity; the high reaction temperature favors the shift from kinetic to mass transfer limitations. Accordingly, no relationship between the catalytic performance and the physico-chemical features of the catalysts can be argued.

-No analysis of residual liquid is reported. The Authors used a complex model tar blend (with an overall composition over than 100% (50%+30%+50%+5%=135%)), but no evidence of different components conversion is reported.

- About TGA on spent catalysts, the measurements were performed in air; accordingly, no CO2 is fed and, thus, no weight gain can be assigned to reactions involving carbon dioxide. If CO2 is adsorbed onto the catalysts, its weight is already measured. Conversely, decarbonation, occurring at high temperatures, can be related to the weight loss rather than coke oxidation.

- About carbonation during reaction tests, CO2 adsorption enhances catalytic performance, as reported by the Authors. However, it is a transient phenomenon, occurring up to surface saturation. This is reflected in transient catalytic activity (generally, the performance decreases with time on stream; this is not the case).

-About Zr, its reduction under the current reaction conditions is very difficult. In ref 53, Zr is in solid solutions with Ce (Ce has redox properties); in ref. 63. reduction takes place with methane at 900 °C.

-Reading the manuscript was not simple due to several typos and errors.

Reviewer 3 Report

In this manuscript, the authors demonstrate the findings of steam reforming of biomass tar mixture using nickel-based catalyst with MgO and CaO promoters for carbon conversion and hydrogen production. The resultant catalyst was well studied to understand the contributing factors and mechanism of catalytic activity. The results are useful for designing highly efficient catalysts steam reformation of gasified biomass tar. This work is recommended to be published after addressing the following comments.

1. Are there any other products (CH3OH, HCOOH, ethane, ethylene) generated in the current system?

2. The N2 physisorption isotherms are missing.

3. The structure of NiLaMgCa catalyst after the catalytic CO2 reduction reactions is needed to be characterized.

4. Only filamentous carbon was evident on the spent NiLaMgCa catalyst. How to refresh the catalyst?

5. Could the author compare the catalytic activities of NiLaMgCa in this work with other commercial catalysts and catalyst previoully reported in other papers? An overview table is helpful.

6. In Figure 10, number 7 is missing. What does (12) GC-TC mean?

7. Check the manuscript carefully and revise some errors/typos in the manuscript.

Page 7, Line 200, all the catalysts testedIt is shown

Page 8, Line 223, over timeOver the time of the 300 minutes of reaction.

Page 14, Line 429, Ni/Al 2 O 3

2.3. Characterisation spent Catalyst