Synthesis, Characterization, and Catalytic Activity of Nickel Sulfided Catalysts for the Dehydrogenation of Propane: Effect of Sulfiding Agent and Sulfidation Temperature

Round 1

Reviewer 1 Report

It is an interesting paper about the synthesis of nickel sulfide catalysts dedicated for propane dehydrogenation. The authors investigated the impact of different sulfiding agent and dissimilar sulfidation temperature on the type of Ni-S species formed on NiO/MgAl₂O₄ support. The synthesized samples were tested in propane dehydrogenation to verify, which synthesis procedure is the most efficient for the synthesis of active and stable catalysts in tested reaction. The authors characterized the synthesized catalysts by XRD, N₂ adsorption/desorption, SEM, TEM and TGA. The obtained materials showed the good activity in propane dehydrogenation and achieved ca. 50 % of propane conversion and ca. 80% of selectivity to the main reaction product. It is an interesting work, thus I recommend it for publication in Catalysts journal. Nevertheless I suggest make some major corrections:

• Page 4, line 146 and page 27, line 601. The authors mentioned that the reflexes assign to NiO phase disappeared with increasing temperature of sulfidation for S2 and S3 samples, whereas in Figures 1.1.c) and 1.1.d) that reflexes are well visible and even they are marked in XRD profiles.
• Page 17, line 397. The authors wrote that the calculated particle cluster size for Ni/MgAl₂O₄-S2-550 is 48 nm. Is it correct?
• I would kindly ask to change the all Figures of nitrogen adsorption/desorption isotherms. It will be more legible if authors use the same mark (for instance square, triangle, circle) for one catalyst. The isotherm of adsorption can be marked by filled shape (like filled square), while the isotherm of desorption can be marked by unfilled shape (like unfilled square).
• The crystallite size of Ni₃S₂ shown in Table 2 are different than that mentioned in the main text of manuscript. Which of them are correct? In the main text the authors used the reflex at 2 theta 55º to calculate the Ni₃S₂ crystallite size (page 4, line 160; page 5, line 187). Did the authors use the different technique to calculate the crystallite size shown in Table 2? The description should be added below the Table 2 how it was calculated.
• There is no Supporting data added to the uploaded files, thus I do not have an access to the Figure S2 about which authors mentioned on page 21, line 471.
• I would kindly ask to present the graphs in Figure 20 with the same scale. It will be more legible for readers.
• The authors correlated a higher selectivity towards propylene at 3 minutes for Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550 in comparison to Ni/MgAl₂O₄-S1-200 with the lower amount of unreacted Ni in the earlier samples (page 21, line 481). However, as we can see on page 3 (line 126) the lowest amount of unreacted Ni shows Ni/MgAl₂O₄-S1-200 (2%, in comparison to 11% and 5% for Ni/MgAl₂O₄-S1-400 and Ni/MgAl₂O₄-S1-550, respectively). Please, verify that results.
• Did the authors try to verify the activity of catalysts in longer reaction time than 120 min? In my opinion it will be the added value to the manuscript to show the such experiment.
• The authors mentioned in conclusions that TEM analysis indicated that the particle size of S1 catalysts did not have changed significantly with increasing temperature of sulfidation (page 27, line 620). However, on page 16, line 389 we can notice that particle size was increased significantly from 5 nm to even 11 nm (it is more than 50 %).
• The paragraph 4.4. was copied from reference (32), the another paper published by authors [Tahier, T.; Mohiuddin, E.; Key, D.; Mdleleni, M. M. In-Depth Investigation of the Effect of MgAl₂O₄ and SiO₂ Support on Sulfur Promoted Nickel Catalysts for the Dehydrogenation of Propane. Today 2021, No. December 2020. 798 https://0-doi-org.brum.beds.ac.uk/10.1016/j.cattod.2020.12.028]. Please change this paragraph according to the procedure applied in this paper.
• The several typos and editorial errors appear in the main text.
• Please use the same units of temperature in the manuscript, ºC or K.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

It is a well-structured paper however some aspects need to be improved before  being acceptable for publication.

1. The introduction should be improved,  when it comes to the importance and novelty of the work.
2. Catalyst particle size, bulk density and WHSV of the experiments should be reported.
3. Average residence time  should be mentioned.
4. Given the small catalyst mass used per experiment (1g)  and the reactor diameter the bed height is limited to few mm. How did the authors ensure that no channelling is taking place ? If the bed was diluted with inert material, detailed description should be given.
5. The powder diffraction standards (ID) that were used for peaks  identification should be reported.
6. While using Scherrer equation one should take into consideration the instrument broadening of the peaks.  The authors should describe the procedure in the materials and methods section.
7. Figure 2C: different y-axis scaling is needed for the reader to observe any differences.
8. DTG curves are important to show mass loss  intensity with respect to temperature. 
9. 75% mass loss is a relatively big figure  indicating unreacted sulfidation material in the pore structure of the  material (as also indicated by the N2 physisorption) and thus no direct comparison can be made.
10. The catalysts S1 have a considerable mass loss upon heating. Were they pretreated before the tests that were performed at 600oC? From TG, one can see>25% loss, which indicates a different space velocity for the tests.
11. A selectivity time chart with mainly c3 olefins selectivity is easier for the reader to follow as compared to the bars.
12. XRD data of the spent catalysts should be presented. 
13. What is the crystallite size after the test? Ni/MgAl2O4 catalysts are known to be prone to sintering, which would reduce conversion and affect selectivity.
14. The carbon balance as a function of time should be reported 
15. What is the coke formation for each of the catalytic tests?
16. Physisoprtion data of the spent catalysts would be of interest to evaluate coking and sintering phenomena accompanied by TEM.
17. Subheadings  in the characterization section corresponding to the different techniques would be helpful for the reader

Author Response

Please see the attachment 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The corrections performed by authors improved the manuscript. Thus I recommend it for publiaction in Catalysts journal.