Low Temperature Methanation of CO₂ on High Ni Content Ni-Ce-ZrO_δ Catalysts Prepared via One-Pot Hydrothermal Synthesis

Round 1

Reviewer 1 Report

This manuscript is deal with low temperature methanation of CO2 on high Ni content Ni-Ce-ZrO catalysts prepared by hydrothermal synthesis. The result obtained from such study reflects good chemistry and physics. The manuscript is written well and has good enough data for publication. This result show that   catalysts may be tailored to obtain efficiently designed methanation catalysts. Reviewer think that present manuscript can be accepted for publication .

Author Response

Thank you very much for your kind comments.

Reviewer 2 Report

In this work that focuses on the conversion of carbon dioxide to methane, the authors synthesized Ni-Ce-Zr-O catalysts with different Ni loading. The authors reported catalysts with catalysts with high surface area and high Ni loading. The catalyst with the highest Ni loading was found to be highly active, selective and stable for CO2 methanation. Notably, the authors correlated catalytic activity to medium basic sites. The introduction was written well, thereby demonstrating the author’s fundamental understanding in this hot topic. The references are also diverse and included studies of different groups, but some of the references are not up-to-date. In general, the study was performed in a systematic approach and the correlation of catalytic activity to medium basic sites is the highlight of this work. However, there are missing details and several scientific arguments that I am not convinced about:

Ni elemental loading was reported in Table 1, but no elemental analysis was documented in the experimental section of catalyst characterization. If this is a theoretical loading, it should be mentioned. The experimental section of CO2 methanation tests have to be further elaborated. From Supplementary Figure S2, it is deduced that the temperature variation of each catalyst is performed in a single experiment. This should be mentioned in detail as it is not clear how the results in Figure 5 was obtained. The incorporation of Ni into the Ce-Zr lattice is probably to obtain single Ni atoms, and the authors demonstrated with XRD that it is possible with Ni loading less than 25 wt.%. It will be highly relevant in this case to obtain TEM characterization on the differences between catalysts high and low Ni loading. My guess is that with a high loading of Ni (i.e. 71.5 wt. %), large Ni particles will be segregated and not supported on Ce-Zr oxide. In-situ reduction was performed prior to catalysis to reduce Ni oxide to metallic Ni, which is the active site for CO₂ Thus I am not sure about the argument on the amount of OH groups and surface oxygen species with increasing Ni loading (line 130). With increasing Ni loading, I would expect the catalyst to behave more like a bulk Ni-O catalyst. It is surprising that the catalyst with lowest Ni loading to resemble that of a bulk Ni-O catalyst (Figure 5). Stability of the catalyst could not be determined because the test (Figure S2) was carried out at full conversion. Thus, it could be argued that the full catalyst bed was not fully utilized during the test and deactivation was not observed. A catalytic test of the catalyst Ni6.0 should be carried out at lower conversions, e.g. 50 %, to determine the stability of this catalyst. Spent catalyst characterization is missing and it is critical and have to be included.

Author Response

We have to thank the reviewer for very useful comments. We have altered the manuscript to clarify the ambiguous points.

 

Table 1 was amended to give the details of theoretical loading and also add information regarding TOF in Table 2.

XRD patterns were corrected for the incorrect symbols and information

The catalytic activity test was given in more details.

Regarding the stability test, we have tried to increase the space velocity but could not bring the conversion down at this temperature. However, TEM images were also included showing no sign of carbon formation and metal sintering.

Reviewer 3 Report

I do not recommend the paper for publication. The quality is low compared to the journal standard. The introduction is superficial and not completed, while the discussion is not always supported by the results.

Firstly, the characterization of CeZrOx system without nickel should be added to support some conclusions reported in the paper. XRD discussion is confusing. The peaks in Figure 1 are associated to fluorite type cubic structure of ceria-zirconia mixed oxide. On the contrary the authors report separately the peaks for CeO2 and ZrO2.  Then, the authors discuss that the addition of nickel influences the lattice parameter reported in Table 1. However, the crystallite size is reported in Table 1 and not the lattice parameter.

TPR discussion has issue. Partial reducible oxide as CeZrOx are characterized by broad peak during the TPR, the addition of small amount of nickel increases their reduction, so this peak is not just the nickel reduction in the structure as the authors claim.

XPS are not properly discussed, no table are reported about the relative composition of Ce, Zr and Ni. The authors claim that at lower loading nickel is in the structure, so why they don’t show the nickel concentration on the surface at different loading? SEM/EDX should be carried out to compare the bulk composition with the surface. The sentence at line 130-132 has no sense.

In the activity data is not clear what catalyst is NiO, is it an unsupported nickel catalyst? I think it would be more interesting if the authors would have compared the performance of one of their catalysts with a nickel supported on CeZrO2, this would give some indication if the technique used to prepare the catalyst is advantageous. Finally, the discussion about mechanism is not clear.

Author Response

We have to thank the reviewer for very useful comments. We have altered the manuscript to clarify the ambiguous points.

 

Table 1 was amended to give the details of theoretical loading and also add information regarding TOF in Table 2.

XRD patterns were corrected for the incorrect symbols and information. We have added the reference for CeZrOx typical planes according to our previous study.

For TPR analysis, the surface oxygen reduction of CeZrOx at about 300 ^oC can be improved by the addition of Ni. For clarification we have amended this section.

NiO catalyst is an unsupported catalyst. And we have made a comparison on the catalytic activity with the data obtained from the literature.

Mechanism section was clarified.

XPS is tabulated in supplementary and more discussion has been made.

Reviewer 4 Report

In this manuscript, Meeyoo et al. prepared Ni-Ce-Zr-O catalysts via one-pot hydrothermal synthesis for CO2 methanation reaction. As one would expect, the sample with highest Ni-content showed the highest CO2 conversion and CH4 selectivity. Authors, also characterized the sample using few standard methods (BET, XRD, H2-TPR, CO2-TPD, and XPS)

Overall, the results are clearly presented, pleasant to read, and contains relevant information for further studies. I have few suggestions for the authors,

1) What does the ‘substituted syngas’ means in introduction? Is that a specific  type of syngas? Please state clearly.

2) Can the authors explain more why there is no correlation between the surface area and nickel loading? In the section 3.1, at first authors show that surface area decreases with increase in nickel content but then authors argue that ‘surface nickel species are richer with high nickel contents, which result in a better activity.’ This is rather contradictory.

3) Please estimate the number of active sites and calculate the turn-over frequency. I would really like to see a comparison with literature TOF values.

4) Authors argue that Ni6.0 possessed the highest activity due to higher amount of OH groups and surface oxygen species that enable to form bidentate formate and monodentate formate intermediates easily. Also, authors argue that increasing Ni content provided more adsorption sites for the migration of intermediate species, enabling the H2 assisted CO dissociation process and hence leading a high activity. In a recent study (ACS Catalysis, 6 (2016) 3660-3664), OH species was shown to act as hydrogenating species by easily donating protons for hydrogenation of O* atom of the RC-O molecules. Can the authors comment on such pathway?

Author Response

We have to thank the reviewer for very useful comments. We have altered the manuscript to clarify the ambiguous points.

 

The word “substituted syngas” was removed and we have slightly modified the introduction part.

This one-pot preparation technique is simple. However, the results cannot be compared to those of the impregnation method in terms of the surface area. This technique yields much higher surface area materials along with Ni incorporated in the Ce-Zr lattice when compared to the impregnation method.

 

We have also reviewed our discussion on the mechanism. It is believed that the mechanism of CO₂ methanation was initially through the formate formation not similar to what reported in ACS Catalysis, 6 (2016) 3660-3664. We have omitted the claim on H₂ assisted CO dissociation process.

Round 2

Reviewer 2 Report

According to the authors, GHSV was increased but conversion did not decrease. This points to under utilization of the catalyst bed and the stability of the catalyst should not be discussed. Please refer to the perspective of Schuth et al. (Chem. Mater. 2018, 30, 11, 3599-3600) for common pitfalls in catalysis manuscripts, in particular on deactivation studies. As the highlight of this work is on the synthesis of catalysts and not on presenting the most stable catalyst, this work is still worthy of publication after the improvement made by the authors. However, the authors should exclude the claim of 'remarkably stable catalyst' as this cannot be verified based on the reaction conditions.

Author Response

I am very appreciated with your comments and suggestion. We will adopt this suggestions to our new manuscript. 

 

With the manuscript we omit the word "remarkably stable catalyst"

 

 

Reviewer 3 Report

The authors have improved the paper and I recommend for publication after minor revision. TOF has been added in Table 2, thus some comparison with the literature should be done.

Author Response

We have included the comparison of TOF values with the available data from the literature. (Table 2)