The Effect of ZrO₂ as Different Components of Ni-Based Catalysts for CO₂ Reforming of Methane and Combined Steam and CO₂ Reforming of Methane on Catalytic Performance with Coke Formation

Round 1

Reviewer 1 Report

The work deals with the hot topic of finding suitable promoters to Ni-based catalysts for the dry reforming of methane reaction to produce syngas. The authors prepared a series of ZrO₂-promoted catalysts at different ZrO₂ loadings via two different methods, namely the co-precipitation and the sequential impregnation. They conclude that the "10 wt.% Ni/Al₂O₃ with ZrO₂ as a coactive component (10 wt.%)" is the most active and stable catalysts.

Although the work is sound, I wonder how it relates to the following papers, which were not cited by the authors in the submitted manuscript:

• Synthesis gas production from dry reforming of methane
  over Ni/Al₂O₃ stabilized by ZrO₂, International Journal of Hydrogen Energy 33 (2008) 991–999;
• Design of stable Ni/ZrO₂ catalysts for dry reforming of methane, Journal of Catalysis 356 (2017) 147–156.

It appears as though the authors took a lot of "inspiration" from the aforementioned papers, especially from the first one, which is, by the way, from the same institution of some of the authors. The similarities are many, starting from the experimental approach down to the very same conclusions--the Ni-Al2O₃ catalyst promoted with 10-15 wt.% of ZrO₂ and prepared by sequential impregnation procedure is the most active and most stable against coking. The little more characterization that may differentiate in some way the present work from that one, was already preformed in the second paper I listed before. And, again, the conclusions are the same--the Ni-ZrO₂ interaction at the boundaries is critical to enhancing the oxygen mobility therefore suppressing the coke formation.

That being said, I'd like to hear from the authors if and how they justify all these "similarities".

Author Response

We appreciate the reviewerʹs helpful comment. It is a very good comment that helps us to be aware of any similarities and the citation in our article. Please let us inform you of the real objective behind this work. In the beginning, we have been interested in the use of ZrO₂ for developing Ni‐based catalyst in order to be a promising catalyst for the CSCRM process (the combined steam and CO₂ reforming of the methane) using low steam to carbon ratio (our work for this process: https://www.mdpi.com/2073‐4344/10/12/1450) not exactly for CO₂ reforming of methane (CRM). Then, we were looking for information that fulfils the understanding of the requirement of the catalyst design for CSCRM that we
can apply the conventional preparation method(s). However, we did not actually find the answers that we looking for. Then, we came up with the study from the simpler process (CRM) to the more complicated process (CSCRM) and the target is the control parameters derived from the properties of the catalyst which is not actually the preparation method. With this purpose, then, we designed to use such a very different ZrO₂ content and the way to gain a different pathway using the different interactions. Because our own focus is to gain all control parameters, we did not do the review carefully.

To avoid the similarity and to show respect to the researchers who worked on those two papers, we added more works for catalytic characterizations as well as CSCRM performance and cited those two papers in our revised manuscript. Accordingly, a new control parameter has been revealed and the different point of view between CRM performance and CSCRM performance has been disclosed in the revised version. In the revised manuscript, the red characters are rewritten/added for catalyst characterization of CRM part and the blue characters are the addition of CSCRM part. We hope that you find that our work makes sharing the important information for methane reforming.

Please see the attached file for the point-to-point responses followed by the revised manuscript. 

Author Response File: Author Response.pdf

Reviewer 2 Report

The present manuscript claims “The effect of ZrO2 as different components of Ni-based catalysts for dry methane reforming on catalytic performance and coke formation”. This is a very well written manuscript with meaningful results. The authors did excellent job in explaining the role of ZrO2 as different components of Ni-based catalysts for DMR. I accept this manuscript with no revision.

Author Response

We really appreciate your time. Your comment is meaningful for our group.

Author Response File: Author Response.pdf

Reviewer 3 Report

Manuscript Number: catalysts-1322342

 

Article type: Research Paper

 

Full Title: The effect of ZrO₂ as different components of Ni-based catalysts for dry methane reforming on catalytic performance and coke formation

 

Remarks to the Authors:

 

The authors studied the effect of ZrO₂ loading using an impregnation, co-impregnation, and sequential methods on Ni-based (10 wt.%) catalyst in the reaction of dry methane reforming (DMR) at 620 °C under atmospheric pressure for 10 h. In addition, the high catalytic activity and coke resistance were found over the 10 wt.% Ni/Al₂O₃ catalyst with 10 wt.% ZrO₂ loading synthesized via a sequential method. I can recommend this paper for publication in MDPI Catalysts but only after a major revision. Please see my comments below.

 

1. Unfortunately, from the Abstract it is not clear for me what the authors try to say about the promoter with 1 wt.%ZrO₂ and a coactive component with 10 wt.% ZrO₂. This statement should be elucidated, maybe more discuss in the Introduction section. Why 10 wt.% of ZrO₂ can not be a promoter too? The authors said: “As promoters, 1 wt.% ZrO₂ and 10 wt.% Ni were co-impregnated onto the Al2O3 support. As a coactive component, 10 wt.% ZrO2 and 10 wt.% Ni were loaded onto the Al2O3 support using co-impregnation and the sequential impregnation method to generate the different interactions of Ni-ZrO₂ on the prepared catalysts”. First, the authors said that the 10 wt.% Ni is promoter but in the next sentences, they have informed us that 10 wt.% Ni acts like a coactive component.

 

2. The authors should include achieved catalytic results (selectivity, conversion, catalyst stability, etc.) in their Abstract and Conclusion sections. It will help the journal readership better understand the main achievements of the current work.

 

3. The real catalysts composition should be provided (ICP-OES or XRF analyses) and included in Table 1.

 

4. The authors should explain how they obtained/calculated SF (stoichiometry factor) and σ_m (cross-sectional area of one metal atom) and include this information in the Experimental section.

 

5. The authors should better (in more detail) discuss the observed high dispersion over the 10Ni-10ZrO2/Al2O3 (SI) catalyst. Only one sentence (Lines 112-114) about the formation of the ZrO₂-Al₂O₃ is not enough.

 

6. Where are the scales with numbers for axis-Y in Figure 2a and Figure 2b? The authors must present them. In addition, the N₂ adsorption-desorption isotherms should be compared with each other on one scale.

 

7. The authors should compare their achieved catalytic activity results (conversion, selectivity, H2/CO ratio, catalyst weight, TOS, etc.) with well-known literature data (recently published) and presented it in a table format in section 2.4. Catalytic Activity.

 

8. How the authors can explain the increasing trend of CH₄ and CO₂ conversion over the 10Ni-10ZrO₂/Al₂O₃ (SI) catalyst after 200 min? How the authors can explain H₂/CO ratio decreasing for all the studied samples (excluding 10Ni-1ZrO₂/Al₂O₃ (CI)) at the first 20 minutes of time-on-stream?

 

9. How the authors can explain so significant difference between DTG results (axis-Y2) for the 10Ni/ZrO2 catalyst and others samples in Fig. 6a?

 

10. The English language of this manuscript should be polished by a native speaker.

 

Author Response

We really appreciate your time for reviewing our article that help us to improve our manuscript. We have responded to every comment and we also inserted each detail in the revised manuscript. Moreover, we have changed the name of dry methane reforming (DMR) to CO₂ reforming of methane (CRM). It is because we have added the data and discussion according to a catalytic performance for combined steam and CO₂ reforming of methane (CSCRM) on the selected catalysts from this catalyst set due to the comment of reviewer 1. Thus, you will also find the information of CSCRM process in our revised manuscript that we have discussed all the detail that we added.

Please find our responses to your points in the attached file for point-to-point response followed by the revised manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors significantly improved their manuscript. I can recommend accepting this paper in the present form.