Peer review of: Modulating oxygen coverage of Ti3C2Tx MXenes to boost catalytic activity for HCOOH dehydroge (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

It is happy to read this nice work which combine synthesis, characterization and evaluation and DFT 
rationalization. It is nice but not novel, since such type of studies are plentiful in literature both 
homogeneous and heterogeneous catalysis from experiment and theory. To improve the quality or 
novelty, additional experiment and computations should be done 

The major point is the discussion about the effect of formate, the review would like to see the kinetic 
isotopic effect (KIE); and the authors can use DCOOH as substrate. This indeed can be used to check the 
computed mechanism shown in Figure 4. 

In Figure 4, the authors present a nearly mono-molecular concerted mechanism, in which surface 
hydride and H from HCOO combine to get CO2 and H2, and this is quite unusual. Normally, HCOO will 
gives H back to surface via so-called β-hydride elimination, and this is different from the proposed 
mechanism in current work. If this is true, it would be very interesting 

Using HCOO and DCOO one can not only study the KIE and also proof the proposed mechanism (H2, HD 
and D2 formation). There are papers reporting KIE to study the mechanisms. 

The authors have to rationalize their use of atomic element (H, Al F) to study the effect of promotion, 
and this is indeed something non-chemical case. What is the effect if counter ion? 


Reviewer #2 (Remarks to the Author): 

In this work, the authors modulated oxygen coverage on the surface of Ti3C2Tx MXenes to boost the 
catalytic activity toward HCOOH dehydrogenation. Ti3C2Tx MXenes after treating with air at 250 °C 
(Ti3C2Tx-250) significantly increased the amount of surface oxygen atoms, exhibiting high performance 
for HCOOH dehydrogenation. This work has been done well and the results are sound and important. It 
can be accepted after minor revision. 

(1) The Toc graphic contains unintentional lines. 
(2) May give more discussions on the effect of Al on the catalysts surface on the catalysis. 
(3) May give clearer HAADF-STEM images for the claimed nanosheet morphology of the Ti3C2Tx 
materials (Fig.1a). 


Reviewer #3 (Remarks to the Author): 

The presented manuscript is related to utilization of MXenes for the hydrogen production in formic acid 
decomposition. The authors propose novel catalysts for this reaction and perform their careful 
characterization. However, there are some serious problems, which do not allow me to advise this paper 
for publication in Nature Communications. All of them are related to their studies of the reaction 
mechanism. 
They measured BET surface area only for one, the most active sample. Therefore, there is a question, 
whether their activity difference for the samples can be related to the difference of the surface area. 
They say that O-Ti concentration increases with the temperature of the treatment in air and this is a 
reason of the activity increase of their material. However, if to compare the materials treated at 25 and 
150 oC. I see a reversed picture. The activity is doubled, but the XPS O-Ti line intensity, in contrast, 
decreases by a factor of 2. Hence, I cannot relate the activity with these species as they do. 
They use IR spectroscopy and say that they observe the bands of formate species at 2882, 1552, 1455 
cm-1 in 6 spectra for 3 samples. I have experience in IR spectroscopy and do not see reliably that they 
have these 3 lines in their spectra. Additional confirmation could be a study of the OH groups region. 
Some hydroxyls could interact with formic acid and this could be observed in IR spectra. However, the 
authors do not show this region for the in situ studies. 
Also, if they calculate Ea of some steps by DFT can they compare with experimental values of the 
apparent Ea. 
Hence, they need to prove further the presence of formate as this study is a key point for the 
establishment of the mechanism. They also have to provide more evidence for the dependence of the 
activity on the specific oxygen species.