Oxygen Adsorption and Activation on Cobalt Center in Modified Keggin Anion-DFT Calculations
Round 1
Reviewer 1 Report
This study looks at the ability of Co(II) ions to activate (adsorb) molecular oxygen when Co(II) is in different environments including heteropolyacids. It is important especially in O2 adsorption applications and potentially for catalysis (oxidation). The study is presented well. Being completely honest, I do not see much originality of this study - the ability of various Co(II) complexes to activate and chemisorb O2 has been known for over a hundred years, but not too many studies look at those things using DFT - and this is the strength of the study. Overall, the work is nicely done. However, there are a few points I would like the authors to address. It is well-known that Co(II) in aqua environment forms hexa-acqua complex which is completely inert to oxygen adsorption (one water ligand has to be expelled in order for oxygen which simply does not happen). Only when the ligands are replaced with, for example, nitrogen containing ligands do the Co(II) complexes begin to adsorb/chemisrob oxygen. The authors should clearly state it. What happens when acetonitrile enters the Co ligand sphere? please comment. (does it adsorb O2 or not in reality, this information should be available online. please comment and add.the required info. I would also like the authors to add the following recent reference in which it was shown that binuclear NxCo(OH)2Nx complexes were excellent at O2 adsorption (and reversible binding) over N2 even in the aqueous solution due to superoxo Co-O2 stabilization presumably through hydrogen bonding https://pubs.rsc.org/en/content/articlehtml/2018/cc/c8cc01457d. If the authors could add some calculations for such relevant systems, would be even better. Good Luck!Author Response
"Please see the attachment."
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors conducted DFT study of oxygen adsorption onto a Co atom under four specific chemical environments. This is interesting to various fields of oxygen catalysis. In addition, it advances theoretical understanding of complicated chemistry. Thus I recommended publication, but following mistakes should be checked.
Figures 5(g)-(i), need to check the numeric subscript of \phi and \Delta, i.e. which of 1 or 2 is correct.
line 273, "shorter wavelengths" --> "longer wavelengths"
Author Response
Response to Reviewer 2 Comments
Point 1: Figures 5(g)-(i), need to check the numeric subscript of \phi and \Delta, i.e. which of 1 or 2 is correct. 

Response 1:
Figure 5 has been corrected. The subscriptions in Figures 5g, 5h, 5i have been changed from 1 to 2. Of course, 5g-i drawings illustrate the second channel.
In the whole drawing, instead of the Greek letter Ï•, the letter φ was introduced in the description. Thank you to the reviewer for finding an error.
Point 2: line 273, "shorter wavelengths" --> "longer wavelengths"
Response 2:
Thank you to the reviewer for finding an error. Line 273 has been corrected.
