Catalytic Effect of CO2 and H2O Molecules on •CH3 + 3O2 Reaction
Round 1
Reviewer 1 Report
Comments for author File: 
Comments.pdf
Author Response
Responses to Reviewer: 1
General Comments
The reactions between CH3 and O2 are important both in tropospheric and combustion conditions. In this theoretical study, the authors calculated the rate constants for the formation of formaldehyde and OH, at different pressures and temperatures.
The density functional theory with M06-2X functional with dispersion correction and 6-311++G(3df,3pd) basis set, followed by CCSD(T)/6-311++G(3df,3pd) single point calculations, were used to estimate thermodynamics and kinetics. The rate constants were estimated in the range 500-1500K. The pressure dependent rate constants were calculated between 0.0001 and 10 atm by using the RRKM theory. Tunneling correction was included by asymmetric Eckart model.
Because the TS2s (TS2, TS2-c, and TS2-h), which are the rate determining steps, lay abundantly above the reactants, and because the following steps have very small barriers, I believe the RRKM theory is unnecessary. Probably, the simple TST theory would give the same rate constants. The lack of any pressure dependence is an a posteriori demonstration
Anyway, the level of calculations is adequate to appropriately describe the potential energy surface and to estimate the rate constants. The paper is well written, the results are carefully analyzed and compared with the available experimental and theoretical data.
I recommend the publication on Catalysts, after minor revision
Author Response: We sincerely appreciate the reviewer for their thoughtful and careful comments on our paper. The reviewer provided very useful and insightful comments on several topics that required addressing and/or further discussion and we feel that our paper is stronger and has increased citation potential. We have incorporated their suggested corrections into the current, revised version of the paper. We feel that the revised version of the paper has improved a lot. We believed that the present revised paper should be of interest to gas-phase Catalysts and Physical Chemistry, computational chemistry readers especially those interested in chemical kinetics, ab initio/DFT method, CO2 and H2O catalysis on Combustion Reaction systems.
Suggestion:
In general, the RRMK theory assumes a rapid intramolecular vibrational energy redistribution, faster than any chemical reaction. This redistribution is reasonably achieved for the reaction of CH3 with O2.
However, for the reactions with CO2 and H2O, because the weak intermolecular bonds, the intramolecular vibrational relaxation (IVR) could be slow. In this case, a non-RRKM behavior is expected. The authors should add some comments about this aspect.
Author Response: We thank the reviewer for pointing out the important discussion related to RRKM and non-RRKM behavior. As discussed the details for CH3+O2 reaction in previous studies [8-11] and the current study the RRKM/ME simulation was used to compute the temperature and pressure-dependent rate constants. We have carried out the calculation at the same condition and the same level to understand the effect of CO2 and H2O to compute the pressure-dependent and temperature-dependent rate constants, As suggested by the reviewer, and observed in the current study, for the effect of CO2 and H2O are pressure-independent and can be considered as a non-RRKM method. Now we have added the details of RRKM and non-RRKM behavior in our discussion " It is interesting to mention that the formation CH2O is unfavorable in RRKM/ME simulation and the calculated rate constants are independent of the pressure, therefore we can say that the effect of CO2 on CH3+O2 reaction shows non RRKM behavior, which can be treated using simple Canonical Transition State Method. However, the result reported in the current study in the high-pressure limit condition can be treated as a CTST method"
Minor comment 1: Line 119: Did the authors use the D3 version of Grimme’s dispersion?
Author Response: Yes, we have used D3 versions of Grimme dispersion corrections. Now have added a sentence in the computational method for more clarity as" The Grimme empirical dispersion correction= GD3 was used to account for the Van der Waal complexes.
Minor comment 2: Line 131. Was the same keyword (Guess=mix) used for CCSD(T) calculations too? Restricted CCSD(T), Unrestricted CCSD(T) or RO-CCSD(T): what kind of approach was used in the Coupled Cluster calculations?
Author Response: Only the TS related to the addition reaction was used for the GUESS=MIX for the optimization and frequency calculations using the M02X method. The optimized structure was used to calculate the energy using CCSDT without the GUESS=MIX method. We calculated the CCSD(T) energies on these optimized geometries. For energy calculation, we used RCCSD(T) method for closed-shell species, such as CH2O and H2O, CO2. For all open-shell species such as RCs, Ints, and PCs was used UCCSD(T) method. Now we have added these lines to the computational section.
Minor comment 2: RRMK: how were the internal hindered rotations treated?
Author Response: In the RRKM calculations, we first calculate the density of states using densum program for each vibrational mode and 1-D rotor of Intermediate and transition states and treat them HO approximately. The vibrational mode which corresponds to the HR rotor was treated as HR approximation in the densum input file. The rotation potential energy of those modes can be calculated using relaxed optimization and which can be used in the HR calculation. We have added the sentences related to this confusion in the revised version.
Minor comment 3 : Line 203, Figure 1. The meaning of the red dashed line, connecting the reactants to Int-1, should be explained in the caption.
Author Response: We have added the statement about the red-dashed line as “The red dashed line pathway is based on earlier studies which are more feasible reaction pathways than the formation of RC first as "The rate constants are also calculated for the new reaction mechanism i.e., R→RC→Int-1→Int-2→PC→CH2O+OH as shown in Figure 1. In this case, we have observed that the reaction is independent of pressure and the rate constant is at least 2 order magnitude smaller than R→Int-1→Int-2→PC→CH2O+OH"
Minor comment 4: Line 215, Table 1. There is a mistake in the first two lines: HOO···CH3 instead of OO···CH3.
Author Response: We have replaced the HOO•••CH3 to OO•••CH3 in the table 1.
Minor comment 5: There are some typos in the Reference section
Author Response: We have corrected the typos in the reference sections
Author Response File: Author Response.pdf
Reviewer 2 Report
The manuscript “Catalytic Effect of CO2 and H2O Molecules on CH3 + 3O2 Reaction“ by Mohamad Akbar Ali et al. deals with the influence of CO2 and H2O molecules on the rate coefficient of the reaction between the methyl radical and the oxygen molecule. The topic is important and interesting for the readers of Catalysis. I cannot recommend publication of the manuscript in the current form and would like to ask the authors to consider the following suggestions and questions.
- Some expressions are strange, for example “catalytic mechanic“, or „chemical catalysis behavior with the effect of temperature and pressure“.
- The statement “When CO2 and H2O molecule is introduced, the reaction becomes more thermodynamically favourable“ is not correct, as the catalyst changes the reaction path, not the thermodynamic parameters of the products with respect to the reactants.
- Page 3, line 127: the abbreviation M0-p5 is not introduced.
- Page 4, line 141: could the authors comment on the optical isomers they mention?
- What is “reaction critical energy“?
- Why were parameters for Lennard-Jones potential used if nitrogen molecules are not considered?
- Page 5, line 191: it is written that TS2’s energy is 15.9 kcal/mol higher than the one of the reactants. But more importantly, the activation energy for this rate determining step equals around 46 kcal/mol.
- The term “reaction energy“ should be defined.
- What is "active thermochemical value"
Author Response
Responses to Reviewer: 2
General Comments
Comments and Suggestions for Authors
The manuscript “Catalytic Effect of CO2 and H2O Molecules on CH3 + 3O2 Reaction“ by Mohamad Akbar Ali et al. deals with the influence of CO2 and H2O molecules on the rate coefficient of the reaction between the methyl radical and the oxygen molecule. The topic is important and interesting for the readers of Catalysis. I cannot recommend publication of the manuscript in the current form and would like to ask the authors to consider the following suggestions and questions.
Author Response: We sincerely appreciate the reviewer for their thoughtful and careful comments on our paper. The reviewer provided very useful and insightful comments on several topics that required addressing and/or further discussion and we feel that our paper is stronger and has increased citation potential. We have incorporated their suggested corrections into the current, revised version of the paper. We have corrected the grammar-related problem in the revised paper. The revised sentences are now in the yellow color and more clear. We feel that the revised version of the paper has improved a lot. The point-by-point responses to the reviewer's comments are given below;
Comment 1: Some expressions are strange, for example “catalytic mechanic“, or „chemical catalysis behavior with the effect of temperature and pressure“.
Author Response: We thank reviewer for pointing out the problem related to chemical kinetic expression, now we have corrected the typos related to this and explain all the term in the details.
For pressure-dependent reactions, Rice−Ramsperger−Kassel−Marcus (RRKM)/master equation (ME) was used and the energy-dependent specific unimolecular rate constants k(E) is given by [39]
where is the reaction path degeneracy; and are the external rotation symmetry numbers of the transition state and reactant; and m are the numbers of optical isomers (mirror image) of the transition state and reactant, respectively, h is Planck’s constant; is the density of states of the reactant molecule; is the sum-of-states of the transition state; E0 is the reaction threshold energy, and ρ(E) is the density of states of the reactant molecule. The sums and densities of states were computed using DenSum as implemented in MultiWell Program. The internal energy E is measured relative to the zero point energy of the reactant molecule and the reaction threshold energy (critical energy) is the difference between the zero point energies of reactant and transition state.
Comment 2" The statement “When CO2 and H2O molecule is introduced, the reaction becomes more thermodynamically favourable“ is not correct, as the catalyst changes the reaction path, not the thermodynamic parameters of the products with respect to the reactants.
Author Response: We thank reviewer for pointing out the problem related to CO2 and H2O effects on CH3+O2 reaction. We have modified the sentences in the revised version as" when CO2 and H2O molecule is introduced in •CH3 + 3O2 reaction, the reactive complexes, intermediatse, transition states, post complexes become thermodynamically more favorable.
Comment 3: Page 3, line 127: the abbreviation M0-p5 is not introduced.
Author Response: We have corrected this typo in the revised version as "M0-p"
Comment 4: line 141: could the authors comment on the optical isomers they mention?
Author Response: Now we have added the explain the optical isomers, which is a mirror image of the species.
Comment 5: What is “reaction critical energy“?
Author Response: Now we have explained the critical energy in the revised version as" E0 is the reaction threshold energy. The internal energy E is measured relative to the zero point energy of the reactant molecule and the reaction threshold energy (critical energy) is the difference between the zero point energies of reactant and transition state.
Comment 6: Why were parameters for Lennard-Jones potential used if nitrogen molecules are not considered?
We have used The Lennard-Jones parameters and discussed as " for collider gases (N2) εkB, σ(N2) = 3.74 Å, ε/kB(N2) = 82 K were used in our calculation based on previous studies.[28,32] The bimolecular rate constant describing collisions between N2 and the complex , intermediate (“Well”) was based on Lennard-Jones collisions with net parameters obtained using the usual combining rules from parameters for the two collision partners: N2 gas (σ = 3.74 Å and ε/kB = 82 K)52 and the Well (σ = 5.20 Å and ε/kB = 212 K, used for all Wells).
- Comment 7 : Page 5, line 191: it is written that TS2’s energy is 15.9 kcal/mol higher than the one of the reactants. But more importantly, the activation energy for this rate determining step equals around 46 kcal/mol.
Author response: We thank reviwer for the pointing out the confussion related to energy of TS2. Now we have modified the senteces.
Comment 8: The term “reaction energy“ should be defined.
Now we have defined the reaction energy as "Internal energy E is measured relative to the zero point energy of the reactant molecule.
Comment 9: What is "active thermochemical value"
Author response: We have added the details about the active thermochemical values in the revised paper as "The literature thermochemical values (ATcT) are a new paradigm of how to develope a most accurate thermochemical data given by Argonne National Lab.
Author Response File: Author Response.pdf
Reviewer 3 Report
Herein, the authors explored the rate constants for reactions •CH3 + 3O2 and the effect of CO2 and H2O molecules. Their results are also in good agreement with previous studies. This study is potentially important to design and analyze possible new forms of nonpetroleum-based fuel or fuel components for use in combustion engines.
Overall, it is a good study, and it will be worth being published in Catalysts after minor revisions pointed as follows.
1. While I see the potential importance of this study, it is also important to articulate the importance of the study to the readers. In the Introduction, the authors have given enough information about the previous investigation on the •CH3 + 3O2 reaction; however, it is not clear from the introduction what the novelty of this work is. In addition, in the Result and Discussion and Conclusions, the authors have mentioned the consistency of their results with previously published data. These could give a negative impact on the novelty of the work.
So, I recommend revising the Introduction section and dedicating a paragraph (ideally at the end of the Introduction) about the rationale behind their work and what additional information will this work provide.
2. Did the authors use complete basis set (CBS) extrapolation with their CCSD (T) calculations? If not, could the author comment on why they didn’t use CBS because the energy at CCSD(T)/CBS with those computed at CCSDT(Q)/6-311G** shows an average error of 0.9%.
Author Response
Responses to Reviewer: 3
Comments and Suggestions for Authors
Herein, the authors explored the rate constants for reactions •CH3 + 3O2 and the effect of CO2 and H2O molecules. Their results are also in good agreement with previous studies. This study is potentially important to design and analyze possible new forms of nonpetroleum-based fuel or fuel components for use in combustion engines.
Overall, it is a good study, and it will be worth being published in Catalysts after minor revisions pointed as follows.
Author Response: We sincerely appreciate the reviewer for their thoughtful and careful comments on our paper. The reviewer provided very useful and insightful comments on several topics that required addressing and/or further discussion and we feel that our paper is stronger and has increased citation potential. We have incorporated their suggested corrections into the current, revised version of the paper. We have corrected the grammar-related problem in the revised paper. The revised sentences are now in the yellow color and more clear. We feel that the revised version of the paper has improved a lot.
Comment 1. While I see the potential importance of this study, it is also important to articulate the importance of the study to the readers. In the Introduction, the authors have given enough information about the previous investigation on the •CH3 + 3O2 reaction; however, it is not clear from the introduction what the novelty of this work is. In addition, in the Result and Discussion and Conclusions, the authors have mentioned the consistency of their results with previously published data. These could give a negative impact on the novelty of the work. So, I recommend revising the Introduction section and dedicating a paragraph (ideally at the end of the Introduction) about the rationale behind their work and what additional information will this work provide.
Author Response: We have incorporated their suggestions into the current, revised version of the paper "There are no theoretical chemical kinetic details for the catalytic effect of the CO2 and H2O molecules on •CH3 + 3O2reaction in the gas phase. In this work, for the first time, we have investigated the rate constant for the catalytic of CO2 and H2O molecules on the most important atmospheric and combustion prototype reaction i.e., •CH3 + 3O2.The temperature and pressure-dependent rate constants were estimated in the temperature range between 500 and 1500 K and 0.0001 to 10 atm using the RRKM/ME simulation for all the three reaction systems. In order to assess the accuracy of our results, we have compared the energies and rate constants for all three reaction systems. We hope this study is potentially important to design and analyze the possible forms of nonpetroleum-based fuel or fuel components in combustion engines."
Comment 2. Did the authors use complete basis set (CBS) extrapolation with their CCSD (T) calculations? If not, could the author comment on why they didn’t use CBS because the energy at CCSD(T)/CBS with those computed at CCSDT(Q)/6-311G** shows an average error of 0.9%.
Author Response: We thank the reviewer for pointing out the confusion related to the basis sets. Extrapolation to the complete basis set (CBS) limit is an effective theoretical tool that allows the removal of a significant fraction of the finite basis set error. We have used a reasonably high polarized and diffuse function of the Pople basis set 6-311++G(3df,3pd), which is reasonably good for the system studies here. We have used 6-311++G(3df,3pd) for a similar system in our earlier calculations [26-28] and used by many other researchers for the rate constant calculations. The thermodynamics and kinetic results were in very good agreement with the experimental measure value and with ATcT value. To improve the functional and basis set we have also added Grimme functional for the non-bonding interaction. We have also mentioned that the method is good within the level of accuracy of 1 kcal/mol, which is ~20% error in the computed rate constant at >1000K. Therefore we believe the calculation provided in the paper is reasonably good.
Author Response File: Author Response.pdf
