Reviewers' comments on your work have now been received. You will see that they have raised sufficient major issues with your paper such that I must reject it for publication in Catalysis Communications.
For your guidance, I append the reviewers' comments below.
Thank you for giving us the opportunity to consider your work.
Angelika Brückner, PhD
Reviewer #1: The paper describes the photocatalytic activity of Cu-modified mesoporous TiO2 materials for formaldehyde degradation to hydrogen. The authors pretend to have discovered novel photocatalytic materials, and argue about a possible size-dependency of the activity which maximizes at some Cu loading. In reality, several papers have already described what the authors report (for example: J. Yu, Y. Hai, M. Jaroniec. Journal of Colloid and Interface Science, 357 (2011) 223-228; and, even more importantly, Anna V. Korzhak Journal of Photochemistry and Photobiology A: Chemistry 198 (2008) 126-134). There is not one detail that can be considered novel, and the discovery that metallic Cu is the active phase is based on a quite doubtful ex-situ procedure (DDT protection, when it's known that DDT can reduce metallic cations if used in excess to form several metal-thiol species).
There are doubts whether this paper can be published anywhere, since it reports things that have been described in the literature already.
Reviewer #2: The authors have completed an interesting study of the Cu/TiO2 system in relation to photocatalytic H2 production from aqueous formaldehyde solutions. I believe that this manuscript could be considered for publication pending the following changes.
1) The english in the introduction section is loose and of a conversation style rather than a scientific style. For example, the first sentences should be modified to read "H2 is expected to be one of the most important energy carriers in the future. Currently, approximately 95% of H2 is produced from hydrocarbons and coal."
2) The authors need to give more information about the formaldehyde solution they are using. Was the 2 wt.% solution of formaldehyde obtained by dilution 37 wt.% formaldehyde (which also contains methanol as a stabiliser), or was it obtained by dissolving paraformaldehyde in water?
3) The reduction of Cu(II) to Cu metal occurs in the absence of TiO2. Regarding the synthesis of Cu/TiO2, what evidence is there that all Cu(II) in solution is deposited on the TiO2? The authors present Cu 2p XPS data. but no supporting quantification information about the relative surface abundances of Cu, Ti, O, C and S. Does the optimum 1 wt.% nominal loading actually correspond to a 1 wt.% actual loading. XPS or XRF Cu/Ti ratios would have been useful in this regard.
4) UV-Vis absorbance spectra would have been useful for probing the presence of Cu(II), Cu(I) or Cu(0) on the surface of TiO2. Much of the evidence in this paper to confirm the presence of Cu metal was performed in the presence of a capping agent, dodecanethiol. I would like to see XPS and TEM images for the or the Cu/TiO2 photocatalysts in the absence of the capping agent as a comparison, or to at least have some text describing that data.
Overall, this is good work that needs a bit more fine tuning and elaboration before publication. If the authors are willing to consider and the address the above points, I think this paper will be acceptable for publication in CATCOM.
Reviewer #3: The authors report about a H2 evolution reaction from aqueous formaldehyde over in-situ deposited copper nanoparticles onto mesoporous TiO2. Unfortunately, I cannot recommend the present manuscript for publishing in CATCOM due to the following reasons:
1. In my opinion, the reported results lack in innovative value, since such UV-light induced H2 generation over similar photocatalytic systems have been published earlier, also with focus on the Cu salt concentration and the resulting particle sizes (see e.g. P. Gomathisankar et al. "Enhanced photocatalytic hydrogen production from aqueous methanol solution using ZnO with simultaneous photodeposition of Cu", Int. J. Hydr. Ener. 38 (2013) 11840-11846 or Choi et al. "Hydrogen production from methanol/water decomposition in a liquid photosystem using the anatase structure of Cu loaded TiO2", Int. J. Hydr. Ener., 32 (2007) 3841-3848).
Concerning the practical application, it would be interesting to test also the visible-light activity e.g. by use of optical filters as the Cu nanoparticles exhibit plasmonic absorption and might be used as visible-light photocatalyst as well (demonstrated in Gomathisankar et al.).
2. The manuscript is not well structured as within the first part of photoactivities the authors often refer to the structural properties of the catalysts described in the second part. Besides, English spelling and grammar should be improved as well.
3. The light absorption properties (UV-vis spectra) of the catalysts would be interesting as well, especially a comparison of the protected (DDT) and non-protected samples in order to demonstrate the effect of absorption changes under catalytic conditions. The data of at least one unprotected sample should also be added to the XRD and XPS results. In case of XPS, often lower binding energies are detected due to a fast reduction of Cu(II) by introduction of the sample into the high vacuum chamber (see Irie et al. "Visible light-sensitive Cu (II)-grafted TiO2 photocatalysts: activities and X-ray absorption fine structure analyses." The Journal of Physical Chemistry C 2009, 113 (24), 10761-10766).
4. At last, from my point of view one should focus on the main results, so that minor important facts such as the influence of anion are mentioned only in the supporting information.