Advancements on Basic Working Principles of Photo-Driven Oxidative Degradation of Organic Substrates over Pristine and Noble Metal-Modified TiO₂. Model Case of Phenol Photo Oxidation

Round 1

Reviewer 1 Report

The author has experimentally investigated the photo-driven response of a pristine and metal modified TiO2 in phenol aqueous solution. Although the manuscript would not add any new information to the current knowledge in this field, I would recommend the publication of this manuscript. I personally believe that the information in this manuscript would help the readers to quantitatively observe the photocatalytic response of a semiconductor (here TiO2) in an aqueous medium. This information would be useful in different fields of research regarding the semiconductor photo-response.  To further improve the quality of this manuscript, I would have a general question for the author and some minor suggestions:

1- One of the main issues with using a powder form agents for organic degradation is the secondary contamination that they cause. In other words, the agent’s remnant in the aqueous solution is somehow more damaging to the environment than the organic compound. I would suggest the author to mention this problem in the manuscript and discuss his/her suggestions.

2-The author mentioned a direct and indirect mechanism of charge transfer to the reactant substrate. The indirect pathway is the common mechanism, where the author came to it in the conclusion. On the other hand, I could not find any reference regarding the direct charge transfer from a semiconductor to an organic molecule which causes the damage or degradation. I would suggest the author to clarify this or simply take it out.

3- Regarding Figure.6, I would recommend to show the whole PL spectrum (including the visible range) as there is important information laying in this range.

4- At last, TiO2 partially absorbs light in the visible range (check the literatures for reference). It is recommended not to be distinct about the UV absorption of the TiO2 to sound scientifically right.

Good luck.

Author Response

1. As the reviewer suggested, we mentioned that "the metal-modified photocatalyst in powder form dispersed in water can be harmful to the environment".
2. The reviewer observation right. It is difficult to find in literature clear evidences for  "direct reaction mechanism" in case of oxidative degradation of organic compounds. Therefore we have eliminated all mentions on this issue.  
3. The PL spectra of the investigated photocatalysts include the visible domain, extending up to 490 nm. The  dependence of PL maxima on metal choice was best observed at around 428 nm, as it can be observed in Fig. 6.  Above 500 nm (for 255 nm excitation wavelength) we could not observe any feature which could help the discussion about charge separation. 
4. It's true that tinny amount of light is absorbed in visible domain by TiO2. Large number of published papers suggest various approaches to decrease TiO2 band gap. However, in our specific case we could not observe any activity of bare TiO2 under visible light. 

All modifications operated are highlighted in manuscript. 

Reviewer 2 Report

The article investigates the role of some moble metal nanoparticles in the photocatalytic process of phenol using titania. Although the destruction of the polutant slows down by the use of the nanoparticles by generating intermediate oxygenated compounds, this method as the authors claim can be useful for this very property when selective oxidation of the organic compound is needed. 

The article is well presented and investigates thoroughly the role of the noble metals in the potocatalysis process by titania. I suggest publication after some minor corrections. See attached file.

Comments for author File: Comments.pdf

Author Response

We amended the manuscript according to reviewer suggestions. All the changed operated are highlighted.