Kinetics of Complex Double Salts [Co(A)₃][Fe(C₂O₄)₃]∙xH₂O (A=2NH₃, En (Ethylenediamine))

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents interesting data, but there are many aspects that require clarification or improvement, as detailed below. Therefore, I recommend that the article be accepted only after major revisions.

 

It is essential to use correct scientific terminology. Some terms used are inaccurate or inappropriate:

• The term “thermal destruction” should be replaced with “thermal decomposition”, “thermal degradation”, or “pyrolysis”.
• Replace “thermokinetics” with “kinetics”.
• Use “isoconversional” instead of “isoconversion”.
• Replace “model methods” with “model-fitting methods”.
• Use “dwelling time” instead of “holding time”.
• Use “endothermic process” instead of “endoeffect”.
• Use “enxothermic process” instead of “enxoeffect”

Several important references are missing, especially regarding the thermal degradation of [Со(NH3)6][Fe(C2O4)3] and [Со(en)3][Fe(C2O4)3]:

• Yusenko, K.V., Pechenyuk, S.I., Vikulova, E.S. et al. Isostructurality and Thermal Properties in the Series of Double Complex Salts [M1(NH3)6][M2(C2O4)3]·3H2O (M1 = Co, Ir, M2 = Fe, Cr). J Struct Chem 60, 1062–1071 (2019). https://doi.org/10.1134/S0022476619070060
• Pechenyuk, Yu.P. Semushina, G.I. Kadyrova, Solid-state transformations by thermal decomposition of [Co(en)3][Fe(C2O4)3] in an inert atmosphere, Thermochimica Acta, 687, 2020, 178578, https://doi.org/10.1016/j.tca.2020.178578.

These studies should be cited.

Also the results presented in these two works together with ref 25 (Pechenyuk, S.I.; Semushina, Y.P.; Mikhailova, N.L.; Ivanov, Y.V. Ru. J. C. Chem. 2015, 41(3), 175-181) should be compared with the current work. The comparison with these studies is relevant, as there are differences in the observed TG curves that must be discussed.

The meaning of "en" (ethylenediamine) must be clearly indicated when first mentioned.

The acronym STA-MS must be defined when first used (page 3).

The identification of volatile products is not convincing since the signals shown in Figure 2 are weak and the identification based solely on the coincidence of three or more reference lines. Note that different compounds may exhibit the same abortions related to the same vibrational state. To support the proposed assignments, the authors should provide selected full-range FTIR spectra of the volatiles. This will allow the reader to better assess the reliability of the identifications.

The derivative TG (dTG) curve is just as important as the TG curve itself. It provides relevant information, such as the temperatures at which the decomposition rate reaches its maximum, and it enables the identification of the different stages. Therefore, a thorough kinetic analysis should include dTG curves. Moreover, since the dTG signal is proportional to the transformation rate, it is highly relevant to compare the dTG and DSC signals, as this can offer deeper insight into the kinetic aspects of the decomposition processes.

The authors analyse each decomposition stage separately. For Sample I, this is reasonable due to the presence of plateaus in the TG curve. However, Sample II shows no such plateaus, indicating that decomposition processes are overlapping. How did the authors separate these overlapping processes for kinetic analysis? Was a global model-fitting approach used instead of analyzing each stage individually? This point requires explicit clarification in the manuscript.

There is a mismatch between the caption of Figure 1, the content of the figure, and the main text. Based on the figure, it seems that Figure 1a corresponds to Sample I and Figure 1b corresponds to Sample II. Please check the caption to ensure consistency and clarity.

It is important to emphasize that obtaining a good fit with a kinetic model does not imply that the actual reaction mechanism has been identified. This is a common misconception in thermal analysis, which is, by nature, an indirect method. The fact that the kinetic data can be fitted using a model such as Cnm does not constitute proof that the reaction follows that specific mechanism, there is no direct experimental evidence to support it. The only valid conclusion is that the proposed model reproduces the experimental data accurately, not that it reflects the true mechanism. Furthermore, each stage is analyzed assuming a single reaction model with a single activation energy, suggesting that each stage proceeds via a single-step process. However, the isoconversional analysis presented in the supplementary information shows non-constant activation energies, which is typically indicative of complex, multi-step mechanisms rather than single-step kinetics. This inconsistency between the isoconversional and model-fitting approaches requires clarification.

In addition, the kinetic analysis is overly lengthy, and certain sections resemble a technical report rather than a scientific discussion. Some parts lack scientific novelty or relevance to the core findings. The most important aspects of the kinetic analysis are already highlighted in the conclusions of the paper, and the rest should be significantly condensed.

In summary, I strongly recommend that the authors clarify the contradiction between the isoconversional and model-fitting results, and reduce the kinetic analysis discussionto focus on the most relevant and novel findings.

Minor Comments

Please check the abbreviations of the cited journals. For instance, Thermochimica Acta should be cited as "Thermochim. Acta", not "Tc. A", and Russian Journal of Coordination Chemistry should appear as "Russ. J. Coord. Chem."

Adding page and line numbers to the manuscript would greatly facilitate the review process.

There are instances where Cyrillic characters (e.g., ‘К’ and ‘№’) are used instead of the correct Latin equivalents, which should be corrected throughout the text.

Some numerical subscripts are missing and should be formatted appropriately.

On page 5, the symbol ‘q’ appears to be used where ‘θ’ (theta) is intended; the character ‘_’ should be replaced with ‘°’ (degree symbol). Also, there are two typographical errors: ‘ona’ should read ‘on a’, and ‘Kasource’ should be corrected to ‘K source’.

On page 6, it is stated that “Figure F1 provides the results of STA with IR analysis of the evolving gases,” yet no IR data appears in Figure 1. This should be clarified or corrected.

In Table 5, it should be explicitly stated that the composition of the final product is determined via elemental analysis.

Comments on the Quality of English Language

A thorough language revision is recommended. Although there are no major spelling errors, the construction of some sentences is awkward and hinders readability. Moreover, there are typographical issues, for example:“...the release of H₂O and tn is also recorded for...” or “...15.8% for ene and 0.1% for NH₃...”. Please revise the entire manuscript carefully for clarity and consistency.

Author Response

Reviewer 1

 

This manuscript presents interesting data, but there are many aspects that require clarification or improvement, as detailed below. Therefore, I recommend that the article be accepted only after major revisions.

 

We thank the distinguished reviewer for his valuable comments.

 

• It is essential to use correct scientific terminology. Some terms used are inaccurate or inappropriate:
• The term “thermal destruction” should be replaced with “thermal decomposition”, “thermal degradation”, or “pyrolysis”.
• Replace “thermokinetics” with “kinetics”.
• Use “isoconversional” instead of “isoconversion”.
• Replace “model methods” with “model-fitting methods”.
• Use “dwelling time” instead of “holding time”.
• Use “endothermic process” instead of “endoeffect”.
• Use “enxothermic process” instead of “enxoeffect”

 

Thank you for your comment, the changes have been made to the text.

 

• Several important references are missing, especially regarding the thermal degradation of [Со(NH₃)₆][Fe(C₂O₄)₃] and [Со(en)₃][Fe(C₂O₄)₃]:
• Yusenko, K.V., Pechenyuk, S.I., Vikulova, E.S. et al. Isostructurality and Thermal Properties in the Series of Double Complex Salts [M1(NH3)₆][M2(C₂O₄)₃]·3H₂O (M1 = Co, Ir, M2 = Fe, Cr). J StructChem 60, 1062–1071 (2019). https://doi.org/10.1134/S0022476619070060
• Pechenyuk, Yu.P. Semushina, G.I. Kadyrova, Solid-state transformations by thermal decomposition of [Co(en)₃][Fe(C₂O₄)₃] in an inert atmosphere, ThermochimicaActa, 687, 2020, 178578, https://doi.org/10.1016/j.tca.2020.178578.

These studies should be cited.

Also the results presented in these two works together with ref 25 (Pechenyuk, S.I.; Semushina, Y.P.; Mikhailova, N.L.; Ivanov, Y.V. Ru. J. C. Chem. 2015, 41(3), 175-181) should be compared with the current work. The comparison with these studies is relevant, as there are differences in the observed TG curves that must be discussed.

 

Thank you for your comment. New literary references have been added, additions have been made to the text of the article.

• The meaning of "en" (ethylenediamine) must be clearly indicated when first mentioned.

The acronym STA-MS must be defined when first used (page 3).

 

Additions have been made to the text of the article.

 

• The identification of volatile products is not convincing since the signals shown in Figure 2 are weak and the identification based solely on the coincidence of three or more reference lines. Note that different compounds may exhibit the same abortions related to the same vibrational state. To support the proposed assignments, the authors should provide selected full-range FTIR spectra of the volatiles. This will allow the reader to better assess the reliability of the identifications.

The necessary explanations are included in the text of the article. The STA-IR method is relatively specific. However, this method makes it possible to identify various gaseous products in a gas stream with the necessary reliability by three values of absorption bands in the range of 4000-400 cm^-1.

Additionally, we apply the figure, describing the CO₂ for [Co(NH₃)₆][Fe(C₂O₄)₃]·2H₂O during pyrolysis in argon. We also apply the reference spectrum of gaseous CO₂ (Nicolet/Aldrich Vapor Phase library, spectrum number in the library 4678). We apologize, but due to the limitations of the program, it is only possible to preserve the library spectrum in this form.

The most intense bands were selected using the library CO₂ spectrum: 648, 675, 3595, 2360 cm^-1. All 4 spectra are plotted in the "time - transmittance” axes. For the graphs shown in the article, additional mathematical processing was carried out to rearrange the graphs in the "temperature - transmittance" axes. It should be noted that the spectrum for frequency 2360 cm^-1 has been mathematically reduced in intensity by three times for the convenience of viewing the figure. It can be seen that the course of the spectra for all four wavelengths completely coincides. Therefore, CO₂ is present in the composition of gaseous decomposition products.

 

• The derivative TG (dTG) curve is just as important as the TG curve itself. It provides relevant information, such as the temperatures at which the decomposition rate reaches its maximum, and it enables the identification of the different stages. Therefore, a thorough kinetic analysis should include dTG curves. Moreover, since the dTG signal is proportional to the transformation rate, it is highly relevant to compare the dTG and DSC signals, as this can offer deeper insight into the kinetic aspects of the decomposition processes.

The authors analyse each decomposition stage separately. For Sample I, this is reasonable due to the presence of plateaus in the TG curve. However, Sample II shows no such plateaus, indicating that decomposition processes are overlapping. How did the authors separate these overlapping processes for kinetic analysis? Was a global model-fitting approach used instead of analyzing each stage individually? This point requires explicit clarification in the manuscript.

 

We agree with the reviewer that DTG carries information about the rate of mass change. It was these data that were used to separate the decomposition curves into separate stages for kinetic analysis. DTG curves have been added to SI.

 

• There is a mismatch between the caption of Figure 1, the content of the figure, and the main text. Based on the figure, it seems that Figure 1a corresponds to Sample I and Figure 1b corresponds to Sample II. Please check the caption to ensure consistency and clarity.

 

We apologize for this unfortunate mistake. The changes have been made to the text.

 

• It is important to emphasize that obtaining a good fit with a kinetic model does not imply that the actual reaction mechanism has been identified. This is a common misconception in thermal analysis, which is, by nature, an indirect method. The fact that the kinetic data can be fitted using a model such as Cnm does not constitute proof that the reaction follows that specific mechanism, there is no direct experimental evidence to support it. The only valid conclusion is that the proposed model reproduces the experimental data accurately, not that it reflects the true mechanism. Furthermore, each stage is analyzed assuming a single reaction model with a single activation energy, suggesting that each stage proceeds via a single-step process. However, the isoconversional analysis presented in the supplementary information shows non-constant activation energies, which is typically indicative of complex, multi-step mechanisms rather than single-step kinetics. This inconsistency between the isoconversional and model-fitting approaches requires clarification.

 

We agree with the reviewer's comment. However, it should be noted that we do not draw any far-reaching conclusions about the mechanism of the process based on the results of kinetic analysis. The models used to describe kinetics are exclusively mathematical tools that allow for the analysis of the observed results and can provide a predictive device for selecting optimal parameters for thermolysis in industrial conditions. In most cases, the results of isoconversion approaches, taking into account the error in determining parameters depending on the degree of conversion, are within the error range within one stage of decomposition. This allows us to consider the described processes as single-stage.The mention of the complexity of the processes at some stages is given in the text. In addition, the simultaneous description of all stages, taking into account the uncertainty of the model choice, leads to an exponential increase in possible variations in the combination of models, which leads to a significant complication of data analysis.

 

• In addition, the kinetic analysis is overly lengthy, and certain sections resemble a technical report rather than a scientific discussion. Some parts lack scientific novelty or relevance to the core findings. The most important aspects of the kinetic analysis are already highlighted in the conclusions of the paper, and the rest should be significantly condensed.

In summary, I strongly recommend that the authors clarify the contradiction between the isoconversional and model-fitting results, and reduce the kinetic analysis discussionto focus on the most relevant and novel findings.

 

Determining the kinetic parameters of the thermolysis process of various compounds, especially those with complex structures, is not always a trivial task. In this regard, the paper provides an analysis of the obtained kinetic parameters and their changes depending on the stage of decomposition. We are confident that this part is an integral part of the work and is important for subsequent modeling of technological processes.

 

Minor Comments

• Please check the abbreviations of the cited journals. For instance, ThermochimicaActa should be cited as "Thermochim. Acta", not "Tc. A", and Russian Journal of Coordination Chemistry should appear as "Russ. J. Coord. Chem."

 

The changes have been made to the text.

 

• Adding page and line numbers to the manuscript would greatly facilitate the review process.

There are instances where Cyrillic characters (e.g., ‘К’ and ‘№’) are used instead of the correct Latin equivalents, which should be corrected throughout the text.

 

The changes have been made to the text.

 

• Some numerical subscripts are missing and should be formatted appropriately.

 

The changes have been made to the text.

 

• On page 5, the symbol ‘q’ appears to be used where ‘θ’ (theta) is intended; the character ‘_’ should be replaced with ‘°’ (degree symbol). Also, there are two typographical errors: ‘ona’ should read ‘on a’, and ‘Kasource’ should be corrected to ‘K source’.

 

The changes have been made to the text.

 

• On page 6, it is stated that “Figure F1 provides the results of STA with IR analysis of the evolving gases,” yet no IR data appears in Figure 1. This should be clarified or corrected.

 

The changes have been made to the text.

 

• In Table 5, it should be explicitly stated that the composition of the final product is determined via elemental analysis.

 

The changes have been made to the text.

 

• A thorough language revision is recommended. Although there are no major spelling errors, the construction of some sentences is awkward and hinders readability. Moreover, there are typographical issues, for example:“...the release of H₂O and tn is also recorded for...” or “...15.8% for ene and 0.1% for NH₃...”. Please revise the entire manuscript carefully for clarity and consistency.

The changes have been made to the text.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Thermokinetics of [Co(NH3)6]][Fe(C2O4)3].2H2O and [Co(en)3][Fe(C2O4)3] have been studied in detail, and the differences between the two DCS compounds have been explained, giving some new insights. However, there are many points that need to be significantly improved. Below are some points to be reconsidered.

1) In Figure 1, the data for compounds I and II appear to be swapped.

2) The compounds in this paper are the same as those in ref. 25, and the TG curves show similar results, so it is likely that the compounds synthesized by the authors in this study are the same. Because the experimental section states that elemental analysis was performed, the synthesis method and elemental analysis values are recommended to be provided as supplementary data.

3) In the title, the compounds are described as [Co(A)][Fe(C2O4)3].xH2O (A=2NH3, en), which differs from Ref. 25. Regarding “en,” it should be specified in the title that “en” stands for ethylenediamine.

4) The abstract should concisely summarize the results obtained in this study.

5) The introduction feels redundant. It should be written in a more concise manner.

6) The analytical model equations should also be included in the text. For example, in Ref. 20 by the authors, the model equations are provided. The explanations in the text are rather complicated. Can the decomposition process be explained using chemical equations?

7) Tables should be formatted more carefully. Some words are broken up in the middle of the words, making it difficult to read.

8) Is the higher stability of [Co(en)₃][Fe(C₂O₄)₃] compared with  [Co(NH₃)₆][Fe(C₂O₄)₃].2H₂O related to the fact that en is a chelating ligand? Please explain whether similar trends are observed in other DCSs. 

9) There are many careless mistakes, so please revise the manuscript more carefully. 

Author Response

Reviewer 2

 

Thermokinetics of [Co(NH3)6]][Fe(C2O4)3].2H2O and [Co(en)3][Fe(C2O4)3] have been studied in detail, and the differences between the two DCS compounds have been explained, giving some new insights. However, there are many points that need to be significantly improved. Below are some points to be reconsidered.

 

We thank the distinguished reviewer for his valuable comments.

 

• In Figure 1, the data for compounds I and II appear to be swapped.

 

The changes have been made to the text.

 

• The compounds in this paper are the same as those in ref. 25, and the TG curves show similar results, so it is likely that the compounds synthesized by the authors in this study are the same. Because the experimental section states that elemental analysis was performed, the synthesis method and elemental analysis values are recommended to be provided as supplementary data.

 

The results of the elemental analysis have been added to the article.

 

• In the title, the compounds are described as [Co(A)][Fe(C₂O₄)₃]·xH₂O (A=2NH₃, en), which differs from Ref. 25.

 

DCS [Co(A)₆][M(C₂O₄)₃] (A = NH₃, 1/2C₂H₈N₂, M = Fe, Cr) were investigated in Ref 25 (ref 23 in new article). In this work, the salts of [Со(A)₃][Fe(C₂O₄)₃]∙xH₂O (A=2NH₃, en). The difference in names can be explained as follows. Chromium compounds have not been studied by us. By performing mathematical operations, it can be seen that both articles consider [Со(NH₃)₆][Fe(C₂O₄)₃] and [Со(en)₃][Fe(C₂O₄)₃].

 

• Regarding “en,” it should be specified in the title that “en” stands for ethylenediamine.

 

The transcript has been added to the article.

 

• The abstract should concisely summarize the results obtained in this study.

 

The abstract has been enlarged according to your recommendation.

 

• The introduction feels redundant. It should be written in a more concise manner.

The introduction has been shortened according to your recommendation.

 

• The analytical model equations should also be included in the text. For example, in Ref. 20 by the authors, the model equations are provided. The explanations in the text are rather complicated. Can the decomposition process be explained using chemical equations?

 

Literary reference to the Ref.20 has been added to the experimental part. In the new version, this is the reference [18].

 

• Tables should be formatted more carefully. Some words are broken up in the middle of the words, making it difficult to read.

 

The tables have been completely rebuilt.

 

• Is the higher stability of [Co(en)₃][Fe(C₂O₄)₃] compared with  [Co(NH₃)₆][Fe(C₂O₄)₃]·2H₂O related to the fact that en is a chelating ligand? Please explain whether similar trends are observed in other DCSs. 

 

The necessary explanations have been added to the corrected text.

 

• There are many careless mistakes, so please revise the manuscript more carefully. 

Thank you for your comment, the changes have been made to the text.

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

There are still relevant issues from my previous report that have not been properly addressed. Therefore, my recommendation remains major revision.

 

Specific comments:

Page 2, lines 26 and 48: Replace “thermal destruction” with “thermal decomposition”, “thermal degradation”, or “pyrolysis” (in inert conditions).

Page 4, line 17: Correct “Cu-Ka source” to “Cu-Kα source”.

Page 4, line 17: Correct “(l = 1.5418 Å)” to “λ = 1.5418 Å”.

Page 4, line 50: Replace “model-based methods” with “model-fitting methods”.

Page 5, line 18: Figure 2 is cited before Figure 1. Figure numbering should follow the order of appearance. In addition, the quality of Figure 2 is far from excellent.

Major concern (still unresolved):

One of my main concerns raised in the previous report remains unaddressed. Specifically:

“The identification of volatile products is not convincing since the signals shown in Figure 2 are weak and the identification based solely on the coincidence of three or more reference lines. Note that different compounds may exhibit the same abortions related to the same vibrational state. To support the proposed assignments, the authors should provide selected full-range FTIR spectra of the volatiles. This will allow the reader to better assess the reliability of the identifications.”

 

In their reply, the authors included the IR spectrum of CO2 from a library. However, what I requested was the FTIR spectra from their own measurements, not reference spectra from a library. The authors must clearly indicate how each volatile was identified, since several compounds may exhibit similar absorptions. Identification of CO2 is straightforward with FTIR; the challenge lies in the reliable identification of the other volatiles. I therefore repeat my request for convincing evidence supporting the assignments of volatile products.

In page 13 the authors present the ‘equation’ of thermal decomposition, but what introduce is not an equation not a chemical equation and not a stoichiometric balance, it is just the identification of reactants and products. The use of the term equation is misleading.

Additional comments:

Page 13: The authors present what they call the “equation” of thermal decomposition. However, what is shown is neither a chemical equation nor a stoichiometric balance, but rather a simple listing of reactants and products. Referring to this as an “equation” is misleading.

In my previous report, I also remarked: “the kinetic analysis is overly lengthy, and certain sections resemble a technical report rather than a scientific discussion.” The authors’ reply: “ Determining the kinetic parameters of the thermolysis process of various compounds, especially those with complex structures, is not always a trivial task”—does not address this point. The complexity of the task is irrelevant; the issue is the novelty of the methodology. Their kinetic analysis may indeed be challenging, but it is not novel. The methods are well known and some parts rely on a commercial software. This is why the technical details can be shortened or moved to the supplementary information. At this stage, I leave it as a suggestion: if the authors prefer to retain all the details in the main text, that is their choice.

I was unable to open the file SI-v2.docx, as it contained an error that made it impossible to access.

Author Response

We thank the esteemed reviewer for his valuable comments. The necessary amendments have been made to the text of the paper. We ask you to study the archive of documents.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been well revised and is considered to be ready for publication. Minor errors are noted below.

1) Page 5, line 11: Figure S8 appears before Figures S1-S7 (Page 7, line 21).

2) Page 3, line 30: C₂O₄ is not written with a subscript 4.

3) Page 3, line 33: A space is needed between “en” and “in”.

Author Response

We thank the esteemed reviewer for his valuable comments. The necessary amendments have been made to the text of the paper.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

In several parts, the authors still use the term destruction to refer to decomposition.

The authors have not understood what I am requesting regarding the IR spectra. The methodology they describe in its main aspects was already clear to me. What I am asking for is much simpler: to validate the identification of the volatiles, it is sufficient to select a few spectra obtained at particular temperatures, generally those where gas emission is most intense, and, for these spectra, identify the characteristic absorptions of the identified volatiles. This is something we can see in several published works; it is not an unusual or extraordinary request. It is simply the identification of the volatiles in an IR spectrum acquired at a selected temperature. This allows the reader to validate the identification of the volatiles, since this is not an obvious task, and when different volatiles overlap, the identification may not be unique.

Author Response

Reviewer 1

 

We thank the esteemed reviewer for his valuable comments. The necessary amendments have been made to the text of the paper and in the Supplementary Materials (figure S2). Please find our responses in the attachment.

Author Response File: Author Response.pdf

Round 4

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have satisfactorily addressed my comments.