Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work reviewed the structural and surface properties of aluminum oxides and hydroxides supported nickel catalysts, and summarized the industrial applications of these materials, as well as mechanisms for reactants molecules activation in different reactions. It is well written and can be considered for publication in the journal.

(1)   The sulfation of nickel species occurs not only in the presence of H2S, but also with SO2 and SO3 in O2-rich atmospheres, which should be included in Section 2.3.2. Moreover, sulfation would also happen over nickel oxides.

(2)   How abut the physical poisoning of formed carbon on nickel species by covering effect when considering influence of carbon in Section 2.3.3. Actually, the encapsulation by carbon is presented as one of main causes of catalyst deactivation in Line 560-561.

(3)   They stated in the first paragraph in Section 3.2 that the exact temperature for the H2-TPR peaks acts may significantly differ as an effect of the actual conditions of the experiment (feed composition, flowrate, heating rate, contact time, etc.). It is suggested that the influence of flowrate is closely associated with contact time, while the latter also depends importantly on other conditions such as catalyst bed.

(4)   They mentioned in Line 631-632 that more than 90% adsorption species on the Ni(111) surface is the carbonate, whereas the Ni(100) surface is mainly covered by adsorbed CO and graphitic carbon. Please list the corresponding literature.

(5)   They should quote the corresponding references in the caption of every figures.

(6)   In a review paper, one table and five figures appear to be few.

Author Response

This work reviewed the structural and surface properties of aluminum oxides and hydroxides supported nickel catalysts, and summarized the industrial applications of these materials, as well as mechanisms for reactants molecules activation in different reactions. It is well written and can be considered for publication in the journal.

• The sulfation of nickel species occurs not only in the presence of H2S, but also with SO2 and SO3 in O2-rich atmospheres, which should be included in Section 2.3.2. Moreover, sulfation would also happen over nickel oxides.

This has been now mentioned in the new chapter concerning deactivation and regeneration.

(2)   How abut the physical poisoning of formed carbon on nickel species by covering effect when considering influence of carbon in Section 2.3.3. Actually, the encapsulation by carbon is presented as one of main causes of catalyst deactivation in Line 560-561.

A short chapter concerning deactivation and regeneration has been added, and this has been cited now .

(3)   They stated in the first paragraph in Section 3.2 that the exact temperature for the H2-TPR peaks acts may significantly differ as an effect of the actual conditions of the experiment (feed composition, flowrate, heating rate, contact time, etc.). It is suggested that the influence of flowrate is closely associated with contact time, while the latter also depends importantly on other conditions such as catalyst bed.

We modified the text, accordingly.

(4)   They mentioned in Line 631-632 that more than 90% adsorption species on the Ni(111) surface is the carbonate, whereas the Ni(100) surface is mainly covered by adsorbed CO and graphitic carbon. Please list the corresponding literature.

Done

(5)   They should quote the corresponding references in the caption of every figures.

The figures are all original from  our laboratory. We added a reference of papers where the subject has been discussed.

(6)   In a review paper, one table and five figures appear to be few.

We made new figures up to 8

 

Reviewer 2 Report

Comments and Suggestions for Authors

This is a useful thorough catalytic review of the industrial applications of reduced and oxidized Ni and its fundamental chemical, and physical properties, dispersed on Al2O3. Although it is sometimes dispersed on other carriers such as high surface area activated carbon or unsupported Raney Ni. The article certainly is a great resource for those searching for catalytic materials that are effective for a number of reactions, but often not as effective as more expensive precious metals. 

I have a few suggestions that may make the review more enjoyable and instructive to read by chemistry professionals. Naturally, a review is limited in length so the authors may want to write book on Ni and catalysis.

1. Some paragraphs are very limited in detail. I would suggest some typical reaction equations be incorporated to make it easier for readers to directly see examples of its use and while making it less boring to the reader. For example, Ni is commonly studied for methane dry reforming but forms coke. Perhaps some examples of this as a limitation for the use of Ni or how to suppress coke formation minimizing deactivation. The hydrogenation of  vegetable oils is briefly mentioned but the reactions are not shown in Table 1 nor in the text.  This deserves some additional attention. 

2. Addition of some figures showing the uniqueness in carbon (coke) formation with the growth of carbon fibers emanating from the Ni surface is an interesting phenomenon would be interesting. 

3. Poisoning and deactivation of Ni by sulfur compounds is mentioned. Are there procedures for regenerating both coked and sulfur poisoned Ni catalysts? This would show its ability to be re used or recycled.  

4. The climate change literature has been focused on CO2 capture from air and flue gas from fossil combustion where Ni is used with the addition of H2, to captured CO2 to form CH4 and CO via RWGS. Inclusion of this in the review would add new relevant material reflecting current trends.  What are the limitations of Ni as a catalytic agent when CO2 is captured from air or an O2-containing flue gas with the goal is ultimately to producing CH4 or CO? 

5. Are there any examples for Ni being more active and/or selective than precious metals. This would be a reason to choose Ni over PGMs other than for price.  

Author Response

This is a useful thorough catalytic review of the industrial applications of reduced and oxidized Ni and its fundamental chemical, and physical properties, dispersed on Al2O3. Although it is sometimes dispersed on other carriers such as high surface area activated carbon or unsupported Raney Ni. The article certainly is a great resource for those searching for catalytic materials that are effective for a number of reactions, but often not as effective as more expensive precious metals.

 

I have a few suggestions that may make the review more enjoyable and instructive to read by chemistry professionals. Naturally, a review is limited in length so the authors may want to write book on Ni and catalysis.

 

1. Some paragraphs are very limited in detail. I would suggest some typical reaction equations be incorporated to make it easier for readers to directly see examples of its use and while making it less boring to the reader. For example, Ni is commonly studied for methane dry reforming but forms coke. Perhaps some examples of this as a limitation for the use of Ni or how to suppress coke formation minimizing deactivation. The hydrogenation of vegetable oils is briefly mentioned but the reactions are not shown in Table 1 nor in the text. This deserves some additional attention.

A chapter concerning deactivation and regeneration of these catalysts has been added. Data on the full hydrogenation of vegetable oils have been added in the 5.2.3, now 6.2.3, section.

2. Addition of some figures showing the uniqueness in carbon (coke) formation with the growth of carbon fibers emanating from the Ni surface is an interesting phenomenon would be interesting.

The point has been deeply discussed by researchrs from Topsoe and this work is cited in the text.

3. Poisoning and deactivation of Ni by sulfur compounds is mentioned. Are there procedures for regenerating both coked and sulfur poisoned Ni catalysts? This would show its ability to be re used or recycled.

A short chapter concerning deactivation and regeneration of these catalysts has been added.

4. The climate change literature has been focused on CO2 capture from air and flue gas from fossil combustion where Ni is used with the addition of H2, to captured CO2 to form CH4 and CO via RWGS. Inclusion of this in the review would add new relevant material reflecting current trends. What are the limitations of Ni as a catalytic agent when CO2 is captured from air or an O2-containing flue gas with the goal is ultimately to producing CH4 or CO?

As reported in the text, Ni-Al2O3 catalysts are excellent for CO2 hydrogenation to methane, and, fro low loading catalysts, to CO. Obviously, CO2 must be previously separated from air or flue gases. Oxygen cannot be present.

5. Are there any examples for Ni being more active and/or selective than precious metals. This would be a reason to choose Ni over PGMs other than for price.

We do not know examples of reaction where nickel is definitely more reactive than noble metals.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have produced an extensive review of nickel-alumina catalysis with a wide range of reactions/applications covered, with a commendable inclusion of actual industrial examples. The manuscript also covers comprehensive discussion on physical characteristics of nickel, alumina, and nickel aluminates that are relevant to these catalytic reactions. Certain statements and sections can be improved for clarity and inclusion of currently significant nickel catalyzed processes.

 

Specific comments for this article are as follows:

Line 30: Suggestion to change to "Among non-PGM transition elements, nickel is frequently..." for clarity

 

Table 1 and content of review: Are these examples limited to large scale industrially applied reactions? There are some reactions of interest in current literature concerning Ni catalysts that are not included. Can the authors comment on their non-inclusion of these reactions, e.g. hydrodeoxygenation/hydroconversion (https://doi.org/10.1016/j.apcata.2024.119889, https://doi.org/10.1016/j.apcata.2018.10.014) and cyclic alkane dehydrogenation (https://doi.org/10.1016/j.apcata.2013.09.047, https://doi.org/10.1016/j.cattod.2008.07.020, although these can be related to the reverse reaction of benzene hydrogenation, or included in Section 4.7) 

 

Table 1: A suggestion to the authors to include a column that identifies references for each entry on thermodynamic characteristics of specific reactions.

 

Lines 183 to 185: This statement suggests the Ni is dispersed as a non-oxide below 20% NiO weight loading. This statement may be revised to say that "Techniques agree that no NiO phase is detected by X-ray diffraction." In addition, are there references that ascertain this limit that confirm no NiO is formed (detected)? There are studies that suggest otherwise, where phases of Ni and NiO are detectable below this weight loading (see https://doi.org/10.1016/j.apcata.2024.119889, https://doi.org/10.1016/j.apcata.2018.10.014, and others). Perhaps this statement is only applicable to a specific set of synthesis conditions.

 

 

Lines 318-320: Is there any reference to support this statement regarding preference to higher loadings? Some anecdotal statements, from industrial sources, say that higher loadings are preferred as they produce more metal surface, providing more active sites and prolonged catalyst lifetime or resistance to deactivation. The authors may also refer the reader to section 5.1 for the discussion relating to this statement.

 

Lines 364-365: Should this statement say "Nickel nanoparticles can adsorb more hydrogen the smaller the particle size..."?

 

Line 540: Repeated word (The adsorption of CO adsorption...)

 

Section 4.2, on CO dissociation / Boudouard disproportionation: The authors are suggested to include a statement or reference(s) on the Ni ensemble size (dispersion) effect on the CO dissociation as found by Gałuszka et. al. (https://doi.org/10.1016/0021-9517(81)90050-6). Possibly as citation for lines 557-558.

 

Section 4.7: This section can include other non-oxidative dehydrogenation of alkanes (see above comment for Table 1).

 

Line 883: Could this be a typographical error "... purified by impurities..." corrected to "... purified of impurities..." or just deletion of "by impurities"

 

Section 5.2.3 The authors may include hydrodeoxygenation examples in this section (see above comment for Table 1).

 

In section 5.3.5 the authors are encouraged to include current interest in the use of liquid organic hydrogen carriers for the transport/production of hydrogen (https://doi.org/10.1021/cen-10205-buscon13, https://cen.acs.org/business/Business-Roundup/102/i2) where Ni is a suitable catalyst (https://doi.org/10.1016/j.cattod.2008.07.020, https://doi.org/10.1016/j.crcon.2023.03.007). Alternatively, that may be included in section 5.3.6 to combine with ammonia as a hydrogen carrier or section 5.3.9 to combine with other chemical looping processes.

 

Comments on the Quality of English Language

Minor clarifications and corrections are needed.

Author Response

The authors have produced an extensive review of nickel-alumina catalysis with a wide range of reactions/applications covered, with a commendable inclusion of actual industrial examples. The manuscript also covers comprehensive discussion on physical characteristics of nickel, alumina, and nickel aluminates that are relevant to these catalytic reactions. Certain statements and sections can be improved for clarity and inclusion of currently significant nickel catalyzed processes.

Specific comments for this article are as follows:

Line 30: Suggestion to change to "Among non-PGM transition elements, nickel is frequently..." for clarity

OK Corrected

Table 1 and content of review: Are these examples limited to large scale industrially applied reactions? There are some reactions of interest in current literature concerning Ni catalysts that are not included. Can the authors comment on their non-inclusion of these reactions, e.g. hydrodeoxygenation/hydroconversion (https://doi.org/10.1016/j.apcata.2024.119889, https://doi.org/10.1016/j.apcata.2018.10.014) and cyclic alkane dehydrogenation (https://doi.org/10.1016/j.apcata.2013.09.047, https://doi.org/10.1016/j.cattod.2008.07.020, although these can be related to the reverse reaction of benzene hydrogenation, or included in Section 4.7)

The subjects were incidentally considered. Now some of these references have been added I the revised paper and the topics treated a little bit more extensively.

Table 1: A suggestion to the authors to include a column that identifies references for each entry on thermodynamic characteristics of specific reactions.

Thermodynamic data come from calculations based on tabulated enthalpy of formation of the molecules. Now the reference is given. 

 Lines 183 to 185: This statement suggests the Ni is dispersed as a non-oxide below 20% NiO weight loading. This statement may be revised to say that "Techniques agree that no NiO phase is detected by X-ray diffraction." In addition, are there references that ascertain this limit that confirm no NiO is formed (detected)? There are studies that suggest otherwise, where phases of Ni and NiO are detectable below this weight loading (see https://doi.org/10.1016/j.apcata.2024.119889, https://doi.org/10.1016/j.apcata.2018.10.014, and others). Perhaps this statement is only applicable to a specific set of synthesis conditions.

We slightly modified the text. These data are for catalysts with S_BET near 180 m2/g (from aluminas with S_BET near 190 m²/g) and well dispersed.

 Lines 318-320: Is there any reference to support this statement regarding preference to higher loadings? Some anecdotal statements, from industrial sources, say that higher loadings are preferred as they produce more metal surface, providing more active sites and prolonged catalyst lifetime or resistance to deactivation. The authors may also refer the reader to section 5.1 for the discussion relating to this statement.

We agree with the reviewer. We modified the text accordingly.

 Lines 364-365: Should this statement say "Nickel nanoparticles can adsorb more hydrogen the smaller the particle size..."?

We modified the text to be clearer.

Line 540: Repeated word (The adsorption of CO adsorption...)

Thanks, corrected

 Section 4.2, on CO dissociation / Boudouard disproportionation: The authors are suggested to include a statement or reference(s) on the Ni ensemble size (dispersion) effect on the CO dissociation as found by Gałuszka et. al. (https://doi.org/10.1016/0021-9517(81)90050-6). Possibly as citation for lines 557-558.

Done, thanks.

 Section 4.7: This section can include other non-oxidative dehydrogenation of alkanes (see above comment for Table 1).

We included a chapter on paraffin dehydrogenation in this section.

 Line 883: Could this be a typographical error "... purified by impurities..." corrected to "... purified of impurities..." or just deletion of "by impurities"

Corrected thanks

 Section 5.2.3 The authors may include hydrodeoxygenation examples in this section (see above comment for Table 1).

Hydrodeoxygenation was mentioned, now is more emphasized.

In section 5.3.5 the authors are encouraged to include current interest in the use of liquid organic hydrogen carriers for the transport/production of hydrogen (https://doi.org/10.1021/cen-10205-buscon13, https://cen.acs.org/business/Business-Roundup/102/i2) where Ni is a suitable catalyst (https://doi.org/10.1016/j.cattod.2008.07.020, https://doi.org/10.1016/j.crcon.2023.03.007). Alternatively, that may be included in section 5.3.6 to combine with ammonia as a hydrogen carrier or section 5.3.9 to combine with other chemical looping processes.

Done, thanks.

The authors acknowledge the reviewer for the many useful comments.

Reviewer 4 Report

Comments and Suggestions for Authors

Please see the comments in the review report.

Comments for author File: Comments.pdf

Author Response

The manuscript is a critical review of nickel and alumina-based catalytic materials, focusing on their bulk and surface properties, mechanisms of reactant activation, and industrial applications.

 

The review emphasizes the growing interest in nickel-alumina catalysts as alternatives to noble metals due to their abundance and resource distribution, suggesting an expanding role in future industrial chemistry and energy sectors. This manuscript is suitable for publication with minor revisions as detailed below.

 

 

1. Figure 1 shows a small peak centered around 36 degrees, indicated by arrows. Given that these arrows denote NiO peaks, it is unclear why the pure Al2O3 sample also exhibits a signal corresponding to NiO.

The accidental superposition of XRD peaks occurs sometimes. In this case maybe the superposition is not so accidental, taking into account that the oxygen packing of rock salt (NiO) and spinel (Al2O3) structures is the same, with very similar spacings.

 

2. While nickel-based catalysts are renowned for their efficacy in steam reforming and other processes, the authors also highlight their susceptibility to sulfur poisoning. This raises concerns about their practical application in industrial settings where sulfur compounds may be present. The viability of these catalysts under such conditions is questionable. Has any research been conducted to address this issue?

A chapter has been added concerning deactivation and regeneration of these catalysts.

3. Sections 2.1.2 and 2.1.3 provide an overview of commonly used alumina oxides in catalysts and Ni-Al oxide compounds. I am interested in understanding how different alumina phases, such as γ-Al2O3 and θ-Al2O3, influence the stability and catalytic performance of NiAl catalysts.

 

As mentioned in the text, the stability of aluminas is growing in the order γ-Al2O3 < d -Al2O3 < θ-Al2O3 < a-Al2O3, and this generates the same trend for Ni metal catalysts supported on them