Adjacent single-atom irons boosting molecular oxygen activation on MnO2

Efficient molecular oxygen activation is crucial for catalytic oxidation reaction, but highly depends on the construction of active sites. In this study, we demonstrate that dual adjacent Fe atoms anchored on MnO2 can assemble into a diatomic site, also called as MnO2-hosted Fe dimer, which activates molecular oxygen to form an active intermediate species Fe(O = O)Fe for highly efficient CO oxidation. These adjacent single-atom Fe sites exhibit a stronger O2 activation performance than the conventional surface oxygen vacancy activation sites. This work sheds light on molecular oxygen activation mechanisms of transition metal oxides and provides an efficient pathway to activate molecular oxygen by constructing new active sites through single atom technology.

The authors claimed that they demonstrated that dual adjacent single Fe atoms anchored on MnO2 can assemble into a binuclear site, which activates molecular oxygen to form an active intermediate species Fe(O=O)Fe for highly efficient CO oxidation. It was concluded that binuclear Fe sites exhibited a stronger O2 activation performance than the conventional surface oxygen vacancy activation sites. However, the data provided seems not enough to support their claims. I understood that Fe/MnO2 showed better CO oxidation performance compared to pure MnO2 but unfortunately not convinced the formation of binuclear Fe sites and L-H type reaction. The following is the reasons why I consider it is inadequate, including questions and comments. I hope this will help improve the quality of this author's work and contribute further understanding the physics behind this seminal catalytic reaction.
In Fig.1a, TEM with atomic resolution are shown and Fe and Mn atoms were identified by line profile in Fig. 3b. I assume that they were distinguished by the intensity differences of the bright spots from the line profile. If so, I suggest the authors to show TEM for MnO2 to confirm that there is no intensity differences among the bright spots and prove intensity difference is inherently from Fe and Mn in What is the meaning of "molecular oxygen activation" and "activation behavior of oxygen" in line 32 and 181? Is it mentioning low dissociation energy of oxygen molecules? I suggest the authors define the term and share the physical image clearly before it is used for the readership of this multidisciplinary journal. In line 24, in introductory part, author described that high concentration of oxygen in air or high temperature will inevitably lead to the refilling of oxygen vacancy by referencing ORR study and not CO oxidation. I don't think this is appropriate citation. There are many studies for oxygen vacancy involved CO oxidation studies. I suggest authors to present discussions based on more relevant literatures. For the same reasons, reference 1 to 3 seems inappropriate (they are electrochemical ORR studies and there should be more relevant references).
Injection of Cl2 was performed. Is it necessarily be Cl2? Why Cl2? I recommend authors to add explanation why this way is good for investigating/identifying the CO oxidation mechanism, probably with citing some articles.
In Supplementary Fig. 11 STEM images were shown but two of them are labeled the same. I recommend that you carefully check the other data to make sure there are no misrepresentations. Unit error was found in Line 108 and it should be 167 C, not cm-1.
In Supplementary Fig. 6, fitted light off curve for MnO2 is awkward. Fitting needs to be performed based on the provided data and how you fit needs to be mentioned with reliability factor.
Reviewer #3 (Remarks to the Author): In this work, the authors report on a novel "single-atom" binuclear iron site embedded in a MnO2 host lattice. Overall, this work is of very high quality and the findings are corroborated with theoretical study. This kind of earth-abundant material may well serve as the next generation of catalyst for a variety of chemical technologies and this type of fundamental study will be valuable in identifying paths forward in both improving and utilizing such structures. A few minor suggestions that might improve this manuscript: 1) While I understand the usage in comparison to previous work, the portion of the title "Binuclear single-atom irons" is a bit problematic as the binuclear and plural of iron both seem to contradict the "single-atom" term. This term is of course somewhat problematic for all "single-atom" sites since local structure can strongly influence key binding sites but even more so here where 2 distinct atoms are serving as the localized active site. A similar issue occurs at the end of the discussion on page 12 where the term "monatomic binuclear active sites" is used. 2) Oxygen activation is of course key to a wide variety of (electro)chemical reactions. A sentence on some of the most technologically important applications and some additional discussion on the importance of CO  CO2 in particular would help highlight the importance of this work. 3) On a technical level, the denticity/binding motif of the bound O2 as brought up on page 7 and Figure  S10 could be strengthened somewhat. In particular, it seems odd to not have considered and reported on the energetics of the O2 with 1 O bonded to each of the Fe atoms, even if it is indeed higher energy than the O2 bridging the 2 Fe with a single O. Also, based on equation 5, it appears that binding does not include vibrational contributions to the free energy including zero-point energies. Given how close some of the competing binding sites are for the O2, these effects may well shift binding energetics, especially for different binding motifs which should be addressed. The authors claimed that they demonstrated that dual adjacent single Fe atoms 55 anchored on MnO2 can assemble into a binuclear site, which activates molecular 56 oxygen to form an active intermediate species Fe(O=O)Fe for highly efficient CO 57 oxidation. It was concluded that binuclear Fe sites exhibited a stronger O2 activation 58 performance than the conventional surface oxygen vacancy activation sites.

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However, the data provided seems not enough to support their claims. The following 60 is the reasons why I consider it is inadequate, including questions and comments. I 61 hope this will help improve the quality of this author's work and contribute further 62 understanding the physics behind this seminal catalytic reaction.   What is the meaning of "molecular oxygen activation" and "activation behavior of 96 oxygen" in line 32 and 181? Is it mentioning low dissociation energy of oxygen 97 molecules? I suggest the authors define the term and share the physical image clearly 98 before it is used for the readership of this multidisciplinary journal.

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In line 24, in introductory part, author described that high concentration of oxygen in 101 air or high temperature will inevitably lead to the refilling of oxygen vacancy by 102 referencing ORR study and not CO oxidation. I don't think this is appropriate citation.

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There are many studies for oxygen vacancy involved CO oxidation studies. I suggest 104 authors to present discussions based on more relevant literatures. For the same 105 reasons, reference 1 to 3 seems inappropriate (they are electrochemical ORR studies 106 and there should be more relevant references).

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Injection of Cl2 was performed. Is it necessarily be Cl2? Why Cl2? I recommend 109 authors to add explanation why this way is good for investigating/identifying the CO 110 oxidation mechanism, probably with citing some articles.  In this work, the authors report on a novel "single-atom" binuclear iron site embedded 1) While I understand the usage in comparison to previous work, the portion of the 132 title "Binuclear single-atom irons" is a bit problematic as the binuclear and plural of 133 iron both seem to contradict the "single-atom" term. This term is of course somewhat 134 problematic for all "single-atom" sites since local structure can strongly influence key 135 binding sites but even more so here where 2 distinct atoms are serving as the localized 136 active site. A similar issue occurs at the end of the discussion on page 12 where the 137 term "monatomic binuclear active sites" is used.

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2) Oxygen activation is of course key to a wide variety of (electro)chemical reactions.   form. 168 We thank the Reviewer for confirming the important significance of our findings and 169 for approving our extensive catalysis studies, and for making many insightful    During the revision, Figure R1 and Table R1 were added as the new Supplementary 202 Figure S16 and Table S2 respectively. Meanwhile, we also added more discussions in     atoms distribution on the surface of three catalysts ( Figure R1 and     Figure R11). The 398 results revealed that the intensity and the distance of 2.9 Å between the atoms were in 399 accordance with the theoretical calculation model of MnO2.

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(2) Based on the strength of atoms and the distance between adjacent atoms, we     Figure R1.    Figure R13). It was found that the characteristic

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During the revision, Figure R13 and R15 were added as new Figure S6 and Figure S3  Response: We thank the reviewer for these valuable questions and would like to reply 508 the comments as follows.

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(1) We adopted DFT calculation to check why the MvK mechanism on MnO2 surface 510 was hindered by Fe atoms. First of all, O2 located at the unsaturated Fe sites possessed 511 a higher energy (-2.00 eV in Figure R5a) than that at the oxygen vacancy (-0.83 eV in 512 Figure R5d). Then, the calculation results of the transition state revealed that the 513 energy (0.19 eV) for CO reacting with O2 adsorbed on the two adjacent Fe sites was 514 lower than that (0.37 eV) of O2 adsorbed in the oxygen vacancy ( Figure R16).

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Therefore, the MvK mechanism on bare MnO2 was inhibited by the high energy 516 demand.

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(2) In accordance with the reviewer's suggestion, we calculated the oxygen adsorption   In Supplementary Fig. 6, fitted light off curve for MnO2 is awkward. Fitting needs to 611 be performed based on the provided data and how you fit needs to be mentioned with 612 reliability factor.

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Response: We appreciate the reviewer for the valuable comment. Figure R18 614 corresponds to the activity spectrum after the normalization of specific surface area.

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The relation between C/(C0*S) and T was used to plot the spectrum, rather than the 616 result of kinetic curve fitting.  Zero-point energy correction (Table R2) was carried out for each adsorption   687 configuration, and the corresponding adsorption energies was recalculated ( Figure   688 R5).

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During the revision, Figure R19 and Table R2 was added as the new Supplementary 690 Figure S17 and Table S8, respectively.  The authors have provided more experimental evidence and calculation results to address my previous concerns, and have revised the manuscript and its SI accordingly. The whole manuscript has been improved drastically. Therefore, I would endorse its acceptance.
Reviewer #2 (Remarks to the Author): The authors made very good effort for revising manuscript. The results structural characterizations provided in the revised manuscript and supporting information are satisfactory and I'm convinced by the authors' explanation. My one last suggestion is to include the information regarding specific activity at T50 in addition to the activation energies reported already in the manuscript since the mass of Fe is now clear through this revision thanks to the authors' effort for identifying the amount of Fe atoms on MnO2, and discuss it with reported values (see for example, https://www.nature.com/articles/s41929-019-0282-y which summarizes the specific activities of single and few atom catalysts). I consider that the specific activity is one of the important figure of merit of catalyst to consider the practical applications.
Reviewer #3 (Remarks to the Author): I will defer to the other reviewers on their comments, though they have made some very good points regarding the experimental data and its interpretation. I very much appreciate the authors' work to address the Reviewer 3 comments but I still have some reservations regarding the work as presented though these are fairly minor and should be easily addressed. First, the new term "adjacent single-atom iron" is still somewhat problematic. MnO2-hosted Fe dimer seems like a much more appropriate term given that in the pathways presented, both iron atoms play some direct role, making the "single-atom" phrase inappropriate. I don't understand the continued inclusion in this manuscript. It also looks from the figures that the Fe is integrated into the MnO2 oxide which means these are likely substitutional defects and not adatom like structures. This needs to be made more explicitly clear since "adjacent Fe atoms anchored on MnO2" sounds like dimer pairs of adatoms which does not appear to be the structures considered since oxygen atoms are above the Fe (e.g., Figs. R19 and R5).
Also, more detail is required on the zero-point energy correction (e.g., in what limit was it calculated/ methodology used and which atoms were utilized for calculating… likely only the adsorbing molecule based on values but this should be explicitly stated Response: We thank the reviewer for the constructive advices and would like to reply to the comments as follows. (1) In order to avoid the confusion, we added "also called as MnO2-hosted Fe dimer," in the abstract to further explain the structure of catalyst developed in this study.
(2) Zero-point energy correction was obtained from vibrational frequencies by applying normal-mode analysis through density functional theory calculations [J. Am.
(3) We added the configuration of oxygen molecule adsorbed on the two adjacent Fe sites with end-on mode, and Bader charge calculation results revealed the two oxygen atoms had different charges (-0.37 and -0.06) ( Figure R1a). The different charges constantly changed the dipole moment of O2 in the vibration process, accounting for the infrared characteristic absorption peak of Fe(O=O)Fe. However, for the side-on configuration ( Figure R1b), two oxygen atoms got the same number of electrons, and the electron density was evenly distributed among the adsorbed oxygen molecule.
Therefore, the dipole moment of oxygen molecules did not change during the vibration process. As a result, the adsorption configuration of oxygen on Fe sites was determined with end-on mode.