Heterogeneous Photocatalytic Chlorination of Methylene Blue Using a Newly Synthesized TiO2-SiO2 Photocatalyst
Round 1
Reviewer 1 Report
This article presents the synthesis of UV-treated TiO2-SiO2 hydrid nanostructures and their application in photocatalytic MB dye degradation reaction under various conditions. I suggest some minor addition to this article before it's acceptance in Catalysts:
- If possible for authors, TEM images of UV-treated TiO2-SiO2 hydrid nanostructures would be nice addition to this manuscript, as it is the most efficient photocatalyst.
- I request authors to cite these recent literatures during the explanation of photocatalytic reaction mechanism: ACS Catalysis 2019, 9, 2674−2684; J. Mater. Chem. A 2021,9, 5915-5951.
Author Response
in attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript presents the results on sunlight-assisted photocatalytic degradation of Methylene Blue (MB) dye over TiO2-SiO2 catalysts combined with the action of HOCl as the source of reactive chlorine species (RCS). This work is in the focus of Catalysts journal. Photodegradation of MB over titania-based catalysts is a widely studied research area. This means that the current problem should be formulated on the basis of the published results and achievements for similar photocatalytic systems and the obtained data should be compared with the published indicators. It is difficult to understand the novelty of the work. This is the main reason for major revision of the manuscript.
List of remarks
1 The effectiveness of the proposed approach for improving the photocatalytic activity of titania by the modification with an adsorbent layer should be discussed in the Introduction. Is this the first time using the TiO2-SiO2 photocatalyst? What is about concentration of SiO2?
2 The Introduction should contain available information about combination of TiO2-based photocatalytic process with the RCS agent. The advantages and disadvantages of this approach for the process under study need to be discussed.
3 The method (or reference) describing the preparation of SiO2 solution from TEOS should be done on Page 3 (line100) or in Supplementary Material. The solvent used for UV treatment of TiO2-SiO2 should be pointed out (Page 3, Lines 105-108). The concentration of chemical scavengers added in the reaction medium should be also indicated (Page 3, Line 129). I recommend that the authors also indicate the accuracy of the spectrophotometric method for measuring concentration of MB. The problem is the lack of information on the content of SiO2 in the TiO2-SiO2 photocatalyst.
4 Page 4, Lines 175-177. The authors wrote that “The UV-vis DRS data demonstrated that SiO2 incorporation does not remarkably influence the light absorption properties of the P-25 nanoparticles [17]”. The other information was done in the Introduction (Page 2, Lines 69-71) that the “Deposition of SiO2 onto TiO2 caused an increase in the band-gap value of the TiO2- SiO2 composite, which is due to the quantum-size effect resulting in an increase in the band-gap value”.
5 I strongly recommend the authors to point out the presence of HOCl in the reaction medium in the captions of figures. As I understand it, the data in Figure 3 were obtained without chlorination of the reaction medium. Is it so? This should be done for the convenience of the readers.
6 The figure caption of Fig.4 should be revised. All experimental details for each of the four discussed processes should be shown. Process designations should be clarified. For example, what is RCS in this figure? Is this chlorination with HClO without irradiation? And so for each process…
7 Page 6, Line 207. The authors wrote that “The enhancement of photocatalytic activity of UV-treated TiO2-SiO2 is attributed to enhanced surface area…”. But, such measurements of surface area were not done. Phrases about an increase in the specific surface area are found further in the text, they should be corrected. For example, the same is on Page 11, Lines 410.
8 There are some typos in the text. For example, the typo is in the phrase (Page 6, Line 254) as “schematic diagram displaying the change in the surface change of the UV-treated TiO2-SiO2”.
9 There is unlucky phrase. For example (Page 9, Lines 309-310) -“slowing the generation of ROS by the UV-treated TiO2-SiO2 and the generation of RCS by HOCl photolysis and the degradation reaction [36]” and etc.
10 My main remark! The achieved indicators of purification of the aqueous medium from the MB should be compared with the literature data. It is convenient to do this in the form of a separate table indicating the details of the experiment (concentration of MB, amount of catalyst, type and power irradiation and etc.).
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
In their work, the authors have employed sol gel process to incorporate SiO2 to the commercial De-gussa-P25 TiO2. The synthesized catalyst was used for the degradation of model pollutant MB degradation.
Som major comments are listed below:
1-Add standard XRD patters for both TiO2 and SiO2, also the indexation of the diffractogram must be carried out.
2-All the figures' quality must be improved
3-It is recommended that the authors show a simple schematic figure of the reactor to facilitate readers' understanding.
4-According to the band gap energies, the authors have stated that "SiO2 incorporation does not remarkably influence the light absorption properties of the P- 25 nanoparticles", how about the absorbance? was there an effect? It is recommended to show the absorbance curves
5-Authors are invited to discuss the origin of the observed PL? is it only from tghe recombination of charge carriers?
6-From Figure 3, it looks that 40 min was not enough to reach the adsorption-desorption equilibrium between the catalyst and the pollutant!!
7-It is recommended to add the oxidation potential of the different RCS
8-The point zero charge was calculated or taken from literature? if so, add references
9-Chlorine evolution reaction can take place which is undesirable reaction, have the authors took this into account?
10-What is the TOC removal of the system
11-During the study of the dye initial concentration effect, authors did not test concentrations above 8 ppm, why this choice?
12-The legend of Fig.S6 is incomplete, it lacks time units.
13-Fig.S7 shows the mineralization products ( CO2, H2O), however, this was not confirmed in the work
14-The authors should make a comparative analysis with other works in literarure.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
In the Part 2.2 the authors wrote that "The TiO2-SiO2 nanocomposite was prepared as explained here. In the specified method, 0.2 g of P25 nanoparticles were put into the prepared SiO2 solution and then diluted with ethanol. " I asked for clarification on the preparation of SiO2 solution, but no answer was received. The authors did not give information on Si concentration in the prepared TiO2-SiO2 composite. SiO2 supporting was confirmed only by the infrared spectroscopy. I believe it is important to provide detailed information on TiO2-SiO2 in the revised manuscript.
Author Response
We apologised for overlooking this question : In the Part 2.2 the authors wrote that "The TiO2-SiO2 nanocomposite was prepared as explained here. In the specified method, 0.2 g of P25 nanoparticles were put into the prepared SiO2 solution and then diluted with ethanol. TEOS, a liquid silicon source , was used as a deposition source for silicon dioxide. The process is mentioned in detail in ref 18 and 19 in the manuscript and our review paper (ref 11 at page 2). Thank you.
Reviewer 3 Report
The authors have answered the asked comments, the revised manuscript can be accepted for publication.
Author Response
Thank you so much for the reviewing efforts.
Round 3
Reviewer 2 Report
The manuscript was revised and may be publised.
