Ni/CeO2 Catalyst Prepared via Microimpinging Stream Reactor with High Catalytic Performance for CO2 Dry Reforming Methane
Round 1
Reviewer 1 Report
Comments attached in a word doc file as an attachment.
Comments for author File: 
Comments.pdf
Author Response
We thank the reviewer for the positive comments and we have addressed each of the specific comments as an attachment. Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 2 Report
1. The paper addresses important environmental and scientific issues.
2. The paper compares the physicochemical and catalytic properties of two types of catalysts, obtained by using a commercial CeO2 support and one obtained by precipitation in a stream reactor. The authors highlight the favourable properties of the obtained support. It is a pity that the conditions under which the commercial carrier was obtained are not stated. Therefore, it is difficult to unambiguously assess the benefits of the method used. In the literature, one can find information on methods of CeO2 synthesis in the batch system, which also lead to the production of particles of even smaller size. Therefore, it seems to me that it is better to treat the compared catalytic systems as two model systems with different properties, and one should avoid formulations such as e.g. in line 174 "traditional coprecipitation and microreaction synthesis".
3. Some language changes are required. For example, the term "particle "is used in a double role - to indicate the size of the support particles and to indicate the size of NiO or Ni crystallites. Therefore, e.g. lines 174-175 should more clearly indicate which „particles” are under discussion. Line 188 - The term „Raman” needs to be supplemented, e.g. „Raman spectroscopy studies”. There are also other stylistic errors in the text which should be corrected, (e.g. „spectrogram” –> „spectrum” or „spectra”. Line 243 – rather DTA peak (TG studies do not show thermal effects). Line 287 - It is not clear what the author meant by "Ni structure" - another crystallographic structure (other than Ni(fcc), shape, morphology or electronic properties of nickel crystallites? Equation 10 - the lack of stoichiometry.
4. Line 225. It was stated that „In addition, with the assistance of oxygen vacancies, Ni/CeO2-M catalyst could provide more oxygen species to remove coke deposition which is also helpful to maintain the high reaction activity over a long period of reaction time” However, the direct causes of this phenomenon are not indicated here.
5. Line 228. It is worth mentioning here, that lowering the H2/CO ratio below 1 may be directly related to the contribution of reactions causing the formation of carbon deposits.
6. The loss of activity was mainly attributed to the catalysts' carbonisation. Other causes, such as sintering of the catalysts, were not indicated. It is difficult to find information on changes in crystallite size under reaction conditions, or at least a discussion of this topic.
Author Response
We thank the reviewer for the positive comments and we have addressed each of the specific comments as an attachment. Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors have studied the synthesis of CeO2 support using a microimipinging stream reactor. This support was used for the synthesis of Ni/CeO2 catalyst for CO2 reforming of methane. The experiments were designed nicely, and most of the findings and conclusions were discussed properly. But yet, there are some gaps that need to be addressed, and hence I would suggest the authors work on the following comments, before accepting this paper.
- In the catalyst preparation section, please provide the flow rates of precursor solution and precipitating agents. Also provide information on the dimensions (dia., length) of these tubes in the SI. What was the heating rate used for the calcination?
- Was the commercial CeO2 support also calcined at 500 C, both the CeO2-M and CeO2-C supports have to be treated under similar conditions? If the CeO2-C support was not treated similar to CeO2-M, then during the reaction at 800 C, the CeO2-C support will undergo severe changes, that could affect its activity.
- Since the CeO2 support was calcined at 500 C, it is acceptable to calcine the Ni/CeO2 catalyst at 400 C. But in almost all of the reactions, the Ni/CeO2 catalyst was treated at 800 C, hence the reaction structure of CeO2 and Ni might change rapidly. Hence, please provide information about the treatment of the catalyst before, the XRD, BET, TEM and XAFS measurements. The catalysts have to be treated under H2 at 800 C, before their characterization to have a fair result.
- In equation 5, the unit of flow rate is in mol s-1 whereas the CO uptake values are in umol g-1, the both should have the same unit. Please correct this.
- Provide more information about the BET measurements, degassing temperature and time, and region of P/P0 used for the BET area calculation.
- Which software was used for the EXAFS analysis? Provide a table showing the R factor, coordination number, absorption edge energy, distance between the scattering bonds. For the EXAFS measurement, provide the experimental and fitted EXAFS oscillations. Provide information about the R range used for the EXAFS fitting. Again, how were these catalysts treated prior to these measurements? The authors talk about air-free method, what is it and how was it done?
- The intensity of the white line in the XANES region shows that the Ni in Ni/CeO2 is in a partially reduced state, but in the EXAFS analysis, only Ni-Ni scattering is present and the distance between the Ni-Ni looks similar to that of the Ni foil. How is this possible? I’m unable to see the authors' claim about the strong Ni and CeO2 interaction based on the EXAFS result.
- Please also provide the characterization results of both the CeO2
- One major concern that I have is regarding the difference in the surface areas of both the CeO2 The surface area of CeO2-C is close to four times lower that the CeO2-M. The low surface area of CeO2-M means that the Ni in CeO2-M will be prone to more agglomeration and hence more deactivation. So this is not a fair apple to apple comparison, it is not possible to obtain CeO2 support with surface area close to 100 m2 g-1? If the commercial CeO2 had high surface area, will it behave similar activity to CeO2-M. We need this information and experiments to compare the performance of CeO2-M catalyst
- Provide and compare the TOF of Ni supported CeO2-C and CeO2-M, for TOS experiments and do a temperature-dependent study for both of these catalysts and then use the TOF to find the activation energies. A comparison of the activation energies and TOF will give more information on the active sites and catalyst performance. Also make a table and do the comparison of TOFs of previously reported ceria catalysts in the literature for DRM.
- The reaction temperature is very high and homogeneous activation of methane is possible, hence providing the activity result with both the CeO2 supports and with quartz (packed similar to the Ni/CeO2 catalyst)
- Provide XPS results of the catalyst after reduction and passivation. The difference in the binding energies of Ni might help to understand the strong interaction of Ni with ceria. For this experiment also do the XPS of Ni/SiO2 under similar conditions as a control experiment
The TPSR experiments were conducted nicely.
- In Fig 6 (a) it is difficult to see any CO production with the CeO2-C catalyst. Also if possible do the CO2 and CH4-TPSR for the CeO2
I like the way the authors did these TPSR and TSR experiments. Overall the paper was easy to read, but few improvements can be done to the English writing. The work and the way these experiments were conducted are good, but there is still room for more improvements.
Comments for author File: Comments.pdf
Author Response
We thank the reviewer for the positive comments and we have addressed each of the specific comments as an attachment. Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
This paper can be accepted in its present form, no further revision needed
