Nickel-Based Single-Atom Alloys for Methane Dehydrogenation and the Effect of Subsurface Carbon: First-Principles Investigations
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe presentation of this manuscript was fine in terms of structure but methodology and language needs to improve. Moreover, the paper is subjected to improvement with following comments:
1. Define the concentration of C used in this study.
2. Remove the use of first persons (our, we) from the manuscript even in abstract.
3. define 'C' in sub-title.
4. Methodology and datasets are completely missing.
Comments on the Quality of English LanguageModerate editing of English language required
Author Response
Thank you for the referee report. Please find below our responses:
- Define the concentration of C used in this study.
We have now done this. In particular, we write “1 ML is defined to have the same number of adsorbates as there are nickel atoms in the (111) layer.” Then it is clear that e.g. ¼ ML means there will be one carbon atom per 4 Ni atoms on the surface.
- Remove the use of first persons (our, we) from the manuscript even in abstract.
We have done this.
- define 'C' in sub-title.
We have now done this.
- Methodology and datasets are completely missing.
We have put the methodology section just after section 4, as advised by the Editor. We have included a Supplementary Information/Material document with further details about the calculations.
Reviewer 2 Report
Comments and Suggestions for AuthorsI think this paper should be accepted after the following modifications:
1. The background of the paper must be expanded through an addition of state-of-the-art references. This will help to understand better the novelty and the importance.
2. The novelty must be clearer explained and more expanded. Authors need to work more on this point.
3. Please double-check English style, there are some minor errors.
Comments on the Quality of English LanguagePlease double-check English style, there are some minor errors.
Author Response
We thank the referees for the comments. We have provided answers below:
- The background of the paper must be expanded through an addition of state-of-the-art references. This will help to understand better the novelty and the importance.
We thank the referee for the comment, in response, we have added the following paragraph after the following text “Adding promoters (dopant atoms) to Ni-based catalysts is another avenue to enhance the catalytic activity, stability and coking resistance, such as alkaline earth metal oxides,[7] as well as novel single-atom alloys (SAA) [8,9].”
“By alloying different metal atoms into the surface of another provides a way to tune the surface electronic structure and hence tailor its performance. In particular, SAAs [1] have demonstrated enhanced properties e.g. in reducing CO poisoning [2] and selective heterogeneous hydrogenations [3].”
[1] M. T. Greiner et al. “Free-atom-like d states in single-atom alloy catalysts” Nature Chem. 10, 1008 (2018).
[2] J. Liu et al. “Tackling CO poisoning with single-atom alloy catalysts” J. Am. Chem. Soc. 138, 6396 (2016).
[3] G. Kyriakou et al. “Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations” Science, 335, 1209 (2012).
- The novelty must be clearer explained and more expanded. Authors need to work more on this point.
Stimulated by the referee’s comment, we have added the below text after the sentence “In the present work we investigate a series of Ni-based SAA for methane decomposition using first-principles calculations.”
“Our strategy is to investigate a range of metal dopants with different character, from platinum group metals with partially filled d-bands, to those with full d-states, those which form magnetic elements, and finally, free electron metals. In this way we aim to identify what electronic nature of the doped atoms yields improved performance for the various reaction steps involved in the dehydrogenation of methane.”
- Please double-check English style, there are some minor errors.
We have done this.
Reviewer 3 Report
Comments and Suggestions for Authors
In this computational study the authors investigated the role of doping Ni(111) with TM on the methane dehydrogenation processes. A set of SAAs was investigated and results rely to thermodynamic and kinetic reaction barriers.
The topic is relevant and the results are found. However, some parts need to be revised, according to the points reported below. I would glad to recommend publication pending the following major revisions:
1) Calculations were done at the level of PBE, which is a common choice. It is known that SAAs and not only (this is a general point of single atom catalyst) retain some atomic magnetization to the metal site, which is at the basis of their atomic-like character [Nat. Chem., 2018, 10, 1008–1015]. In this respect, it is known that the self-interaction error may have a role, and a way to tame this issue is to invoke hybrids or the PBE+U correction [Adv. Theory Simul.2023,6, 220051; ACS Catal. 2022, 12, 10, 5846–5856]. I recommend to perform benchmark calculations with PBE+U for thermodynamic barriers of some SAAs having a residual atomic magnetization.
2) Related to the first point, the atomic magnetization should be reported. Also, some words should be devoted to their binding energy to the Ni(111).
3) It should be specified if calculated TSs were checked by means of imaginary frequency search.
4) Do reaction energies include entropy and zero-point energy correction?
5) Some additional information could be extracted from free energy diagrams refereed to the equilibrium reaction potential, to gain some insight on the role of the dopant to the reaction limiting energy.
Author Response
We thank the referee for the report. We provide responses below and accordingly, we have made changes to the manuscript:
- Calculations were done at the level of PBE, which is a common choice. It is known that SAAs and not only (this is a general point of single atom catalyst) retain some atomic magnetization to the metal site, which is at the basis of their atomic-like character [Nat. Chem., 2018, 10, 1008–1015]. In this respect, it is known that the self-interaction error may have a role, and a way to tame this issue is to invoke hybrids or the PBE+U correction [Adv. Theory Simul.2023,6, 220051; ACS Catal. 2022, 12, 10, 5846–5856]. I recommend to perform benchmark calculations with PBE+U for thermodynamic barriers of some SAAs having a residual atomic magnetization.
We thank the referee for the comment. This is indeed important for systems where the doped atom maintains strong atomic-like character when doped into the host system such as when situated in the center of pyridinic 4-nitrogen doped graphene (as in the two papers above, Adv. Theory, and ACS Catal.) and as well as in similar molecular structures of iron, manganese, and cobalt porphyrins, where the description of spin states and the binding properties depends sensitively on the functional used [e.g. P. Morgante and R. Peverati, “Comparison of the Performance of Density Functional Methods for the Description of Spin States and Binding Energies of Porphyrins” Molecules, 28, 3487 (2023).
Motivated by the referee’s question, we have inspected the density-of-states the systems investigated in the present work to establish the extent to which the doped atoms exhibit a strongly single-atom (more atomic)-like nature. We find that there is considerable hybridization with the host nickel states, and therefore we do not expect that higher level approaches would yield significantly different results for the present systems. We have added PDOS in the supplementary information to illustrate this.
- Related to the first point, the atomic magnetization should be reported. Also, some words should be devoted to their binding energy to the Ni(111).
We have now included a sentence about atomic magnetization in the manuscript after the sentence “These are created by substitutionally adsorbing different metal atoms in the Ni(111) surface; specifically, Cu, Fe, Pt, Pd, Zn and Al were considered.”
“We find that the Ni atoms exhibit a magnetization of between 0.65 and 0.71 bohr magneton and that the non-magnetic atoms acquire a small magnetization of around 0.2 bohr magneton, while Fe has a substantial value of 2.95 bohr magneton. The doped atoms furthermore bind exothermically into the host material.”
- It should be specified if calculated TSs were checked by means of imaginary frequency search.
We did not calculate the frequencies.
We used the climbing image approach where the highest energy image is driven up to the saddle point. This image does not feel the spring forces along the band. Instead, the true force at this image along the tangent is inverted. In this way, the image tries to maximize it’s energy along the band, and minimize in all other directions. When this image converges, it will be at the exact saddle point.
- Do reaction energies include entropy and zero-point energy correction?
No. Since the systems studied are very similar in nature (doped atoms in the host nickel) we expect similar values for the zero-point corrections. We are predominantly interested in trends (i.e. differences in) across the series of doped atoms we expect this would not change the conclusions. We have now stated this in the manuscript.
- Some additional information could be extracted from free energy diagrams refereed to the equilibrium reaction potential, to gain some insight on the role of the dopant to the reaction limiting energy.
Our main interest is in the dry reforming of methane to produce syngas and in the relative trends across the SAA nickel-based structures. The role of the dopant is directly evident from the change in the reaction barrier heights.
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors addressed the raised points
