The Catalytic Effect of Pt on Lignin Pyrolysis: A Reactive Molecular Dynamics Study

Round 1

Reviewer 1 Report

Reviewer Comments from Manuscript ID: sustainability-2915565

Re: Manuscript Number: sustainability-2915565

Title: The Catalytic effect of Pt on lignin pyrolysis: a reactive molecular dynamics study

This manuscript discusses the impact of a Pt catalyst on lignin pyrolysis, a significant component of biomass. Using molecular dynamics simulations, the study found that Pt catalysts attract lignin molecules, lowering their decomposition temperatures. Additionally, the catalyst exhibits strong adsorption capacity for H radicals and reduces the activation energy of the reaction. This research provides insights into biomass pyrolysis and the industrial use of metal catalysts.

Reviewer Comments:

·      One of the key findings of this research lies in the observation that H radicals exhibit the strongest adsorption capacity on the Pt catalyst. However, the simulation only considered H radicals. Why were OH radicals not included as well, given that water was present in the reaction system simulated at the studied temperatures?

·      In Figure 2 a comparison was made between the experimental results from the literature and the results found in the present study using dynamic molecular simulations. The authors claim that the trend of curves in the experimental and simulation results is basically the same, especially the mass fraction of gas is very close. However, no explanation is offered concerning the differences of the simulation and experimental results. (Possible causes resides in the mass transport phenomena that may take place within the experimental results).

·      Furthermore, in Figure 2, results of MD simulations in the temperature range of 2000-3200 K are presented. However, it is important to question why the simulation data was compared over a temperature range from 2000 to 3200 Kelvin when the experimental data only ranges from 600 to 1000 Kelvin. What is the rationale behind comparing such disparate temperature ranges? The authors should provide an explanation for this discrepancy to ensure a reasonable comparison.

·      Based on the findings depicted in Figure 4, illustrating the distribution of gaseous species resulting from lignin pyrolysis at various temperatures with and without a platinum catalyst, it is crucial to compare these results with thermodynamically predicted values for lignin decomposition under identical conditions. This comparison would provide insight into the potential maximum limit of this decomposition process, allowing for a comparison between theoretical (thermodynamic) values and those obtained through simulation, both with and without the catalyst.

·      In the conclusions section it is stated that the catalyst has an obvious attraction effect on lignin macromolecules, and the lignin begins to decompose during the attraction process, so the lignin starts to break down at a lower temperature. At least an explanation or hypothesis should be provided regarding the reason behind this effect between the lignin macromolecules and the platinum catalyst bed.

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

This article investigates the impact of platinum (Pt) catalyst on the lignin pyrolysis process, which is a significant component of biomass. Molecular dynamics simulations using the ReaxFF method were employed. By analyzing the configuration changes of lignin molecules in the presence and absence of the catalyst, the authors draw conclusions regarding the catalyzed pyrolysis mechanism.

However, the article requires several explanations regarding the rationale behind the simulated process conditions:

1. Why was such a high final temperature (5000 K) included in the simulations? In the case of high-temperature pyrolysis, process conditions typically reach 1200/1300 K. The temperature would rather suggest plasma pyrolysis. It needs clarification how this served the purpose, in reference to real-world practices.
2. It should be clarified whether dissociation of products at high temperatures, adiabatic temperature, was taken into account.
3. There is also a lack of description of the properties of the catalyst subjected to simulations.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Development of eco-friendly technologies of product and energy production is an important task of modern scientific activities. In this regards, lots of studies focus on renewable resources like biomass, containing lignin and other substances. One of the widely used techniques to convert lignin to value-added products and energy sources is its pyrolysis with or without catalysts. In the latter case, many studies are undertaken, however, with no detail insight into the catalytic processes involved. The present work exploring the mechanism and detail parameters of catalytic reaction of lignin pyrolysis using Pt as one of the most efficient catalysts, via MD simulation, partly fills this gap. Comparing the process of pyrolysis with the catalyst with that without catalyst, the authors have demonstrated several advantages the catalyst offers. For instance, it is found that the reaction temperature notable decreases with the catalyst, which is due to the fact that lignin molecules are more readily adsorbed on the surface of the catalyst, thus promoting their decomposition. Moreover, reduction of the activation energy of molecular decomposition dramatically affecting the kinetics of the process of molecular decomposition has been found that makes the process industrially relevant. An interesting finding is also the evolution and interplay between molecular and atomic hydrogen at the surface and subsurface region. 

The strength of the work: Very detail molecular-level modeling of the catalytic process, enabling quite novel and convincing data. High applied relevance of the work.

The weakness: It is known that the catalytic effect is strongly dependent on the atomic structure and morphology of the catalyst’s surface. Therefore, apart from the used three-layer model of 98 Å ×98 Å Pt atoms in a cell box, other models prototyping different structures and morphologies of the Pt catalyst would be useful. 

In general, the manuscript is scientifically sound with the appropriate design of the models to address the issues under consideration. It is clear, relevant for the field and presented in a well-structured manner. The manuscript provides sufficient details to reproduce results. Referencing is relevant, quite comprehensive and up-to-dated. The manuscript is suitable for publication in Sustainability in its present form.

Author Response

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Reviewer 4 Report

This research delves into simulating the influence of a Pt catalyst on the pyrolysis dynamics of hardwood lignin using reactive force field (ReaxFF) molecular dynamics. By contrasting systems with and without catalysts, a conspicuous affinity between the catalyst and lignin molecules was observed, leading to their decomposition at diminished temperatures. Analyzing kinetics enabled the determination of reaction activation energy, revealing a decrease with catalyst incorporation. This study sheds light on the role of Pt catalysts in lignin pyrolysis. However, it has some minor flaw commented below:

In the study is mentioned that is used hardwood lignin, please add the tree source of the examined hardwood.

Lines 139-140: “It can be seen that the trend of curves in the experimental and simulation results 139 is basically the same, especially the mass fraction of gas is very close.” Completely disagree with the observation, since the values of mass fraction for tar is completely different from experiment and the simulation. Please correct it.

Please add in Conclusions also the negative part of using the Pt catalyst, is the lower activation energy enough reason to use Pt catalyst? Please make discussion about this.

Please correct all the sentences which are written in the first person plural (ex. we, us, our..). It degrades the overall quality of the written article.

Author Response

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Round 2

Reviewer 2 Report

Dear Authors,

The difficulty remains in understanding why a temperature of 3200 K was meant to correspond to an experiment conducted under conditions of 1000 K. The translation still appears unclear.

Calculating kinetics for 2000 K I consider unjustified, as in such conditions it is physical processes rather than chemical kinetics that will determine the reaction rate.

Author Response

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Round 3

Reviewer 2 Report

Thank you for the provided response.

I can understand the strategy of virtually increasing the simulation temperature above the physical temperatures typically representing physical experiments in order to decrease the computational time by numerically increasing the kinetics, specifically for very low integration timesteps (0.2 fs).

However that needs further explanation, specifically when comparing to experiments. The authors need to explain the methodology of selecting the MD-Temperature range in Figure 2. Why 2000 – 3200 K range was selected and not perhaps 1500 – 4000 K? That needs further clarification. 

The TGA provides the macroscopic information about the entire sample, while the ReaxFF MD gives microscopic, atom-level resolution over time. Please provide specific mathematical equations explaining the physical meaning of the temperature in the ReaxFF model (“simulation temperature” in the paper Hong, D. et al, Energy 2022 that the authors indicate) and how that corresponds to the temperature of the experiment.

Author Response

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