Catalytic Decomposition of CH₄ to Hydrogen and Carbon Nanotubes Using the Pt(1)-Fe(30)/MCM-41 Catalyst

Round 1

Reviewer 1 Report

The author investigated the CH4 decomposition performance based on Pt(1)-Fe(30)/MCM-41 catalyst, various characterization methods were used to give a reasonable explanation for the different performance of the catalysts and the activity-structure relationship was also established. My question is as follows.

1.       All of the figures are blurry, it should be modified to be clear.

2.       The standard XRD pattern should be given in Fig 5 and 8.

3.       In Fig 6b, one line goes out of the base line. Therefore, there must be some mistakes in the fitting process.

Please check the whole manuscript.

Author Response

Revised according to comments in the paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors used group VIII metals which are fairly well-known for functions to dissociate C-H via various mechanisms, and to produce H2 and carbon materials at high temperatures. The following may need to be clarified in the discussions. 

1. Because of a heating rate very high for the thermal conductivity of a quartz reactor and a SiO2 support, the transient at the beginning of reactions may be partially related to over-heating which is nearly impossible to prevent with PID control loop well-known for slow responses. 

2. Intuitively, it would be unlikely that CNTs can continue growing over a long period on transition metals inside pores, unless the growth happen to be along the pores. Hence could the authors comment on the location of the elements and CNT growth? Along with the Raman spectra, how did the authors eliminate the possibility of other forms of sp2 carbon materials with irregularity? 

3. In the O 1s XPS spectra, should not the most prominent contribution be the O as in SiO2? The two bands could possibly be related to FeOx and SiO2? 

4. In the Fe 2p spectra, deconvoluted peaks may be challenging to distinguish by around 0.1 eV for such broad bands. 0.1 eV is possibly minor compared to large FWHMs. Were the catalysts reduced before the reactions? The author discussed the presence of Pt4+, then could the authors comment on the possibility of Fe4+ presence? Could the authors clarify whether water formed at the beginning of a run? 

6. Can the author comment on the possibility of Pt3Fe NPs presence? 

Great. Only a few minor typos. 

Author Response

Revised according to comments in the paper.

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript entitled ‘ Catalytic Decomposition of CH₄ to Hydrogen and Carbon Nanotubes Over the Pt(1)-Fe(30)/MCM-41 Catalyst’ submitted by Ho Joon Seo demonstrated the production of H2 and carbon nanotube from CH₄ using Pt(1)-Fe(30)/MCM-41 catalyst. Different catalysts were characterized by XPS, XRD, SEM, TEM etc spectroscopic techniques.

Abstract:

1- The author should write the full name of XPS, XRD etc technique in the beginning of the manuscript.

2- The author should write the full form of MCM-41.

3-    Need to revise the abstract thoroughly.

Introduction:

The introduction needs to elaborate more about the application part and more references in this section.

Result and discussion:

(a)   Activity of catalyst: 1- Authors did not mention about the presentation of Fe(30)/MCM-41, Ni(20)/MCM-41, Pt(1)/MCM-41, Pt(1)-Fe(30)/MCM-41, and Pt(1)Ni(20)/MCM-41 catalysts. What are the values in brackets, please mention them clearly so that the reader can understand it clearly. Data obtained due to this catalyst must be shown in Tabular form so that the reader can understand clearly each part's application for hydrogen production as shown in Figures 1 and 2. Figures 1 and 2 data plots should be represented in colorful mode.

(b)  At high reaction temperatures of 973 and 1023K, the methane conversion quickly increased to 46.17 and 64.40%, while the hydrogen yield gradually increased to 31.68 and 37.94%. What is the probable reason for the low yield % of hydrogen?

(c)    Line number 239 needs to be revised ‘depect’ should be a defect.

(d) The author did not mention the catalytic hydrogen production mechanism as well as the recyclability of the catalyst.

(e) The author did not elaborate on the manuscript to understand the role of MWCN formation and mechanism with Pt(1)-Fe(30)/MCM-41

(f)   Using CH₄ gas to produce the hydrogen production yield % is low even though all production is happening at incredibly high temperature.

(g)  It is a point to note that, the author did not use Silicon (Si) in the methodology, however, Table-1 XPS it is mentioned with 82.34 atomic %. In conclusion, also it is mentioned. Need to clarify.

(h)  Figure 7 of FESEM images are bad resolution pic need to revised thoroughly.

Material and methods:

1-    What is MCM-41, please elaborate the abbreviation.

2-    What is the reactor, need to elaborate to understand where authors took 0.002 g, 0.295 g, and 0.124 g of Pt(NO3)2·xH2O and Fe(NO3)3·9H2O, Ni(NO3)2·6H2O respectively, in ethanol. Why did the author use ethanol solvent? All these metal nitrites are soluble in water also, please justify.

3-    The catalyst was employed by reducing the hydrogen temperature to 773 K for 5 h at a flow rate of 20 ml/min before the reaction, and then increasing the reaction temperature at a rate of 283 K/min. What is the meaning of reducing the hydrogen temperature to 773 K?

4-    Reaction temperature employment is too high to decompose methane gas, what is the role of the catalyst here?

 Conclusion:

The conclusion is not properly written to understand the core findings of the research as well as application data. Some data points are mentioned here like band gap energies which are not mentioned in the result and discussion (Diffuse reflectance spectroscopy (DRS) was not performed as not mentioned).

Need revision

Author Response

Revised according to comments in the paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

In the revised manuscript, the higher-resolution version of Figures 9c and 9d provides strong evidence for the existence of CNTs. Unfortunately, other revision and responses do not lower the concerns on potential over-interpretation of data from reaction at early time on stream and certain characterization methods. Also, to paraphrase into a different manner the major concern in points 2 & 6 from the previous comments: 

MCM-41 has a much higher surface area than typical supports for catalysts to produce CNTs, for example, as listed in reference #2 in this manuscript. By the addition of Pt into a support with a very high surface area, a fraction of the metals may form NPs in pores with methane coupling and aromatization functions, and they are known to form coke which usually has aromatic domains. They may have been assembled into other forms of carbon nanostructures or just fragmented graphene-like structures. The excess Fe may form typical catalyst(s) on the external surfaces of the support to produce CNTs. 

Above is an alternative speculation and in this manuscript, I do not find enough evidence that eliminates the possibility and supports the other claims. As such, for a non-typical, potentially multi-functional catalyst, I hold the previous recommendation for this work without further evidence. 

Author Response

As the reviewer commented,"The addition of Pt to a very high surface area support allows some of the metal to form NPs in the pores with methane coupling and aromatization functions, which are generally known to form cokes with aromatics domains." I agree with that. Therefore, we would like to revise the content of the paper as follows.

It is believed that MWCNTs are formed as supersaturated carbon dissolved in Fe or Pt nanoparticles located on the surface of the Pt(1)-Fe(30)/MVM-41 catalyst.

→

It is believed that MWCNTs may be formed as supersaturated carbon dissolved in Fe or Pt nanoparticles located on the external surfaces of the MCM-41 carrier.

Reviewer 3 Report

No comment further. Thanks   

NA

Author Response

Thank you for your comment.