Catalytic Behaviour of Ce-Doped Ni Systems Supported on Stabilized Zirconia under Dry Reforming Conditions
, Yasir Arafat 1,*, Ahmed Aidid Ibrahim 1, Samsudeen Olajide Kasim 1, Abdulrahman Alharthi 2, Anis Hamza Fakeeha 1, Ahmed Elhag Abasaeed 1, Giuseppe Bonura 3 and Francesco Frusteri 3Round 1
Reviewer 1 Report
The manuscript describe the synthesis of Ce-doped Ni systems on zirconia for methane dry reforming. There are important changes to perform before being accepted since despite the amount of characterization performed for the catalysts. The role of the different modifications performed on the material on the final catalytic activity is not clearly established.
Some major corrections:
Line 136: It is claimed an upgraded Ni dispersion. The authors need to provide HRTEM to prove this highly dispersed uniformity of the metal.
Line 96: It is stated that a better catalyst for the reaction is highly surface area ZrO2, which is one of the objectives to accomplish in this manuscript. However, Table 1 shows the highest surface are of around 65 m2/g. Then what is the typical value for a non mesoporous high surface area ZrO2?
Lines 266-268 It seems that the incorporation of La2O3 improves the overall stability of the system as stated in line 267, this is mechanical stability?, BET analysis was performed to support this statement? How many recicles the structure withhold?. In these lines it is also stated that the addition of Ceria leads to a general improvement of the catalytic stability. What that this statement means?. It is not clear what is the separate role of the La2O3 and the Ceria in the reaction. Please clarify.
Author Response
Reviewer #1:
The authors greatly thank the reviewers for their valuable comments. All the points have been thoroughly considered, certainly improving the manuscript quality. The MS was deeply revised and new details and comments added, as requested. The literature was updated with some new references. We hope that the provided responses are satisfactory for the reviewers, wishing that now the MS is worthy to be published.
Reviewer #1:
Comment 1
Line 136: It is claimed an upgraded Ni dispersion. The authors need to provide HRTEM to prove this highly dispersed uniformity of the metal.
Response 1
We determined Ni dispersion using H2-TPD (Table 3). It is evident that Ni dispersion significantly enhanced after ZrO2 modification by La2O3.
Comment 2
Line 96: It is stated that a better catalyst for the reaction is highly surface area ZrO2, which is one of the objectives to accomplish in this manuscript. However, Table 1 shows the highest surface are of around 65 m2/g. Then what is the typical value for a non mesoporous high surface area ZrO2?
Response 2
The effective use of zirconia as catalyst and catalyst support requires a high specific surface area with suitable pore structure. However, as reported, “commonly available ZrO2 possesses rather smaller surface area (viz. <50 m2/g)”. These details were already present in the MS (see lines 70-72). A new reference [17] about the conventional features of ZrO2 was also added for clarity.
Comment 3
Lines 266-268 It seems that the incorporation of La2O3 improves the overall stability of the system as stated in line 267, this is mechanical stability?, BET analysis was performed to support this statement? How many recicles the structure withhold?. In these lines it is also stated that the addition of Ceria leads to a general improvement of the catalytic stability. What that this statement means?. It is not clear what is the separate role of the La2O3 and the Ceria in the reaction. Please clarify.
Response 3
Actually, ZrO2 modification by La2O3 endowed the catalyst structural (mechanical) stability. It can be witnessed by the phase analysis (XRD: Fig.1) of used catalyst that t-ZrO2 phase of Ni/Zr catalyst had almost completely transformed into m-ZrO2 phase after thermal treatment. However, when La2O3 incorporated into ZrO2 support, t-ZrO2 phase was protected and prominently stabilized (which is recognized to render high catalytic activity and stability), leading to the superior catalytic performance (see lines 260-264). To better clarify the other points dealing with catalyst stability, we rephrased all the period (see lines 267-273). On The other hand, the role of Ceria may be denoted by the Ni particle size reduction besides, its metal surface area and Ni dispersion improvements coupled with oxygen storage vacancy.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript “catalysts-499381” describes an interesting work concerning the influence of modification of ZrO2 support by La2O3 and the doping of Ni phase by CeO2 on the physicochemical characteristics and catalytic performance of the corresponding DRM catalysts. The work is well organized but the manuscript needs major revision in order to be accepted for publication.
Specific comments
1. Line 39: the words “paraffin, olefins” have been written twice.
2. Line 50: “constrain” should be written instead of “conatain”.
3. Line 68: two commas.
4. Figure 1: XRD patterns of the supports (after identical thermal treatment to the corresponding catalysts) should be also presented in Figure1.
5. Table 1: Textural properties of the supports should be included in Table 1. Decimal points have not meaning in the BET values.
6. Line 126: 39 should be written instead of 111.
7. Line 135: The phrase “Therefore, the high surface area of the catalysts led to the strong metal support interaction, which upgraded the Ni dispersion and curbed the sintering of metallic species.” is not supported by the results presented up to this point. Thus, it should be omitted.
8. Figure 3: TPR peaks appeared at temperature higher than 700 oC in the TPR profiles of Ni/Zr and Ni-Ce/Zr samples have not been commented. They probably indicate the formation of NiO-ZrO2 solid solutions (Int. J. Hydrogen Energy 2017, 42, 13724–13740.) Modification of zirconia support by lanthana seems to inhibit the formation of NiO-ZrO2 solid solutions.
9. Line 152: The text "Table 2 ...//... of metallic species." should be modified. I think that as already mentioned in my previous comment the modification of ZrO2 with La2O3, on one hand, inhibits the formation of solid solutions between the two oxides and on the other hand, increases the active phase support interaction. The latter increase is depicted by the decrease of hydrogen consumption by 22% from Ni/Zr to Ni/La-Zr and by 17% from Ni-Ce/Zr to Ni-Ce/La-Zr catalyst. The increase of active phase - support interaction is expected to be accompanied by improved active phase dispersion. Thus, I think that Table 2 should be omitted and the values of H2 consumption to be included in Table 1.
10. Line 163: The phrase "After doping ...//... of three stages." is confusing.
11. Line 174: the word "concentration" is written twice.
12. Line 177: “N” has not meaning before μmol unit.
13. XPS experimental details have not been given in the Experimental part of the manuscript. What was the catalyst pretreatment (reduction conditions) before XPS analysis?
14. Line 218: The authors write that "The O1s XPS data also display a little amount of COx species, which implies that COx species are present as reaction intermediates on the catalyst surface." The XPS analysis was performed on the used samples?
15. Catalyst Preparation: Experimental details concerning the preparation of the supports are missing. If the authors have used commercial samples, they have to write the company from which they have purchased them. What was the La2O3 content in the modified zirconium oxide?
16. N2 Physisorption: It is not clear whether the textural properties of the catalysts have been determined in their oxide or reduced stage.
17. Section 3.3.5: CO2 TPD measurements were performed over catalysts reduced at 400 oC, while their catalytic evaluation was done at 700 oC. Why?
18. Conclusions have to be rewritten in a clearer manner.
Author Response
Reviewer #2:
The authors greatly thank the reviewers for their valuable comments. All the points have been thoroughly considered, certainly improving the manuscript quality. The MS was deeply revised and new details and comments added, as requested. The literature was updated with some new references. We hope that the provided responses are satisfactory for the reviewers, wishing that now the MS is worthy to be published.
Reviewer #2:
Comment 1
Line 39: the words “paraffin, olefins” have been written twice.
Response 1
Text revised as suggested.
Comment 2
Line 50: “constrain” should be written instead of “conatain”.
Response 2
Text revised as per the advice.
Comment 3
Line 68: two commas.
Response 3
One comma omitted.
Comment 4
Figure 1: XRD patterns of the supports (after identical thermal treatment to the corresponding catalysts) should be also presented in Figure1.
Response 4
The diffraction dashed lines in Fig. 1 refer to the phases of reference ZrO2. No change proposed.
Comment 5
Table 1: Textural properties of the supports should be included in Table 1. Decimal points have not meaning in the BET values.
Response 5
Decimal points in the BET values are omitted and the BET value of the support is included in Table 1.
Comment 6
Line 126: 39 should be written instead of 111.
Response 6
The referee’s right. That was a mistake. Value changed.
Comment 7
Line 135: The phrase “Therefore, the high surface area of the catalysts led to the strong metal support interaction, which upgraded the Ni dispersion and curbed the sintering of metallic species.” is not supported by the results presented up to this point. Thus, it should be omitted.
Response 7
The sentence omitted as per the advice.
Comment 8
Figure 3: TPR peaks appeared at temperature higher than 700 oC in the TPR profiles of Ni/Zr and Ni-Ce/Zr samples have not been commented. They probably indicate the formation of NiO-ZrO2 solid solutions (Int. J. Hydrogen Energy 2017, 42, 13724–13740.) Modification of zirconia support by lanthana seems to inhibit the formation of NiO-ZrO2 solid solutions.
Response 8
We are grateful to the reviewer for this valuable comment. We have included this finding in the manuscript (see lines 150 -163).
Comment 9
Line 152: The text "Table 2 ...//... of metallic species." should be modified. I think that as already mentioned in my previous comment the modification of ZrO2 with La2O3, on one hand, inhibits the formation of solid solutions between the two oxides and on the other hand, increases the active phase support interaction. The latter increase is depicted by the decrease of hydrogen consumption by 22% from Ni/Zr to Ni/La-Zr and by 17% from Ni-Ce/Zr to Ni-Ce/La-Zr catalyst. The increase of active phase - support interaction is expected to be accompanied by improved active phase dispersion. Thus, I think that Table 2 should be omitted and the values of H2consumption to be included in Table 1.
Response 9
The text modified as per the advice and Table 2 omitted while including the values of H2 consumption values in Table 1.
Comment 10
Line 163: The phrase "After doping ...//... of three stages." is confusing.
Response 10
The sentence was modified for clarity (see lines 166-167).
Comment 11
Line 174: the word "concentration" is written twice.
Response 11
Comment 12
Line 177: “N” has not meaning before μmol unit.
Response 12
We agree with the reviewer, so the unit of measure changed as suggested.
Comment 13
XPS experimental details have not been given in the Experimental part of the manuscript. What was the catalyst pretreatment (reduction conditions) before XPS analysis?
Response 13
As per the advice of reviewer, experimental details have been included in the new Par. 3.3.8. with reduction conditions.
Comment 14
Line 218: The authors write that "The O1s XPS data also display a little amount of COx species, which implies that COx species are present as reaction intermediates on the catalyst surface." The XPS analysis was performed on the used samples?
Response 14
The text was changed for clarity (see caption to the Fig. 5) and new Par. 3.3.8.
Comment 15
Catalyst Preparation: Experimental details concerning the preparation of the supports are missing. If the authors have used commercial samples, they have to write the company from which they have purchased them. What was the La2O3 content in the modified zirconium oxide?
Response 15
New details about catalyst formulation are now added in the text (see Par. 3.1).The commercial samples of the support are obtained from DAIICHI KIGAKU KOGYO CO.; LTD, Japan.
Comment 16
N2 Physisorption: It is not clear whether the textural properties of the catalysts have been determined in their oxide or reduced stage.
Response 16
Actually, textural properties of fresh (oxide stage) catalysts were determined.
Comment 17
Section 3.3.5: CO2 TPD measurements were performed over catalysts reduced at 400 oC, while their catalytic evaluation was done at 700 oC. Why?
Response 17
The CO2-TPd measurements are independent of the catalyst reduction. For instance, Liu et al.[1] reduced their catalyst at 500°C for CO2-TPd measurements, while their catalysts were activated at 800° and 900°C; Similarly, Al fatesh et al; [2] reduced their catalyst at 200°C for CO2-TPd measurements, while their catalysts were activated at 800°
[1] Lui et al; ‘Natural clay-based Ni-catalysts for dry reforming of methane at moderate temperatures’, Catalysis Today 306 (2018) 51–57
[2] Al-Fatesh et al. ‘Rh promoted and ZrO2/Al2O3 supported Ni/Co based catalysts: High activity for CO2 reforming, steameCO2 reforming and oxyeCO2 reforming of CH4’ international journal of hydrogen energy 43 (2018) 12069 -12080
Comment 18
Conclusions have to be rewritten in a clearer manner.
Response 18
Conclusions have been rewritten and modified as suggested.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
None. The manuscript was greatly improved after the fist review.
Reviewer 2 Report
Ref 12 is the same with Ref 27
