Adsorption and Photocatalytic Reduction of Carbon Dioxide on TiO₂

Round 1

Reviewer 1 Report

Dear Authors, 

only few annotations.

row 93 (Carlo Erba) instead of (Calro Erba) 

3.2. Photocatalytic and adsorption properties of TiO2 samples

Is Inset in fig.6 necessary?  That inset is not described in the text or caption

Conclusions

row 266: Adsorbability of electromagnetic radiation must be corrected. Probably absorption of electromagnetic..... is better

 

Author Response

Dear reviewer,

Thank you for valuable comments and suggestions, which helped us to improve the quality of the article. Our responses to the comments are described below in a point-to-point manner.

    1. row 93 (Carlo Erba) instead of (Calro Erba)

Corrected (page 3, line 93)

2. Photocatalytic and adsorption properties of TiO2 samples

Corrected (page 6, line 189)

3. Is Inset in fig.6 necessary? That inset is not described in the text or caption

We would like to remain the Fig 6 as it shows the differences in desorption rates among photocatalysts. This Figure is mentioned in the text of manuscript (page 7, line 223)

4. row 266: Adsorbability of electromagnetic radiation must be corrected. Probably absorption of electromagnetic..... is better

“Adsorbability of” has been changed to “absorption of” (page 9, line 265)

Reviewer 2 Report

Dear authors,

I am not sure what is new from this study. TiO2 photocatalytic reduction of CO2 is well-studied for years.

The title is too big, like a review title. And the conclusion is not quite clear. The summarized conclusion is similar to the one from your previous publication.

Also, the English of this manuscript is quite hard to understand. Too many long sentences.

Author Response

Dear reviewer,

Thank you for valuable comments and suggestions, which helped us to improve the quality of the article. Our response to the comments are described below in a point-to-point manner.

1. I am not sure what is new from this study. TiO2 photocatalytic reduction of CO2 is well-studied for years.

Indeed, the photocatalytic reduction of CO₂ over TiO₂ has been extensively studied for years. That being said, we showed the importance of considering the sorption properties (especially if the reaction is performed at ambient temperature). Moreover, we demonstrated the rutile is more photocatalytically active than rutile-anatase mixture which is in contradiction with result reported in the literature before.

2. The title is too big, like a review title. And the conclusion is not quite clear. The summarized conclusion is similar to the one from your previous publication.

In our opinion, the title is as short and straightforward as possible with regards to the subject discussed, it is quite hard to make it shorter without losing the context.  In our previous publication we have just mentioned the possibility of deactivation of photocatalyst due to product accumulation but did not discuss it in detail. More profound investigations on adsorption-desorption factors (as a function of semiconductor properties) were conducted in the current work.

3. Also, the English of this manuscript is quite hard to understand. Too many long sentences.

We have deleted/edited long sentences (p. 4, lines 147 – 158)

Reviewer 3 Report

This manuscript reports that the activity of TiO2 in the gas-phase reduction of CO2 is governed mainly by the desorption rate of the reaction intermediates and final products phootcatalysts.

It was shown that rutile exhibits higher photocatalytic activity than anatase/rutile mixture mainly due to its high efficiency in the visible portion of the electromagnetic spectrum.

The important literatures were well referred. Accordingly, my opinion is that the manuscript may be publishable
after the minor revision of the following points:

Compare the morphology of the all utilized catalyst. Maybe TEM analysis would be useful. 

Stability is also important for the effective utilization of photocatalyst. Therefore, the stability should also be studied for at least 3 cycles. Compare XRD data of the all forms of TiO2.

Author Response

Dear reviewer,

Thank you for valuable comments and suggestions, which helped us to improve the quality of the article. Our responses to the comments are described below in a point-to-point manner.

1 – 2. Compare the morphology of the all utilized catalyst. Maybe TEM analysis would be useful. Stability is also important for the effective utilization of photocatalyst. Therefore, the stability should also be studied for at least 3 cycles. Compare XRD data of the all forms of TiO2.

Unfortunately, we are not able to perform TEM measurements before revision deadline as we do not have TEM at our disposal. We did photocatalytic activity measurements of investigated samples (at 100 °C) for a longer period (up to 25 h on stream) but did not observed any significant differences among photocatalysts (XRD, TOC). Therefore, the results were not included in the publication.

Reviewer 4 Report

The importance of tuning of the adsorption-desorption processes of reaction products originated during CO2 photocatalytic reduction on TiO2 is discussed within this paper. The paper is well organized; the introduction provide the necessary background information related to presented results.

I have following comments:

1. page 3, line 98. RIR is assigned as “reference intensive ratio”, should be “reference intensity ratio”.
   Other comment to XRD – The Scherrer equation was used for calculation of grain size. What does it mean exactly? It would be better to use “crystallite size”
2. The reference [42] deals with carbon-doped TiO2 and the authors of this paper report different Eg values than the authors list in this manuscript. Please check if the references are aligned with the information you provide within the manuscript.
3. The discussion related to the page 4, lines 147 – 160 should be improved in general since it is difficult to understand which text is related to own results and which one is related to cited papers.
4. The Figure 2 is not mentioned in the text.
5. Could be Lc values obtained from Scherrer equation used for discussion about the agglomeration?
6. Figure 5, X axis should be wavenumber.
7. Please change the Fig 5b) as Fig 5a) since this figure is discussed before the Figure 5a) in text.
8. Please indicate the bands which are compared in Figures 5 and provide the list of groups associated to these bands.
9. The text of the discussion on Page 9 is not well understandable since for example the comment on activity of TiO2-C1000 is after the discussion of the results in Figure 8 and this sample is not included in Figure 8.

Author Response

Dear reviewer,

Thank you for valuable comments and suggestions, which helped us to improve the quality of the article. Our responses to the comments are described below in a point-to-point manner.

1. page 3, line 98. RIR is assigned as “reference intensive ratio”, should be “reference intensity ratio”. Other comment to XRD – The Scherrer equation was used for calculation of grain size. What does it mean exactly? It would be better to use “crystallite size”

“reference intensive ratio” has been changed to “reference intensity ratio” (page 3, line 98)

“grain size” has been changed to “crystallite size” (page 4: lines 144, 146,148, 153; page 3: line 97; page 5: lines 176)

1. The reference [42] deals with carbon-doped TiO2 and the authors of this paper report different Eg values than the authors list in this manuscript. Please check if the references are aligned with the information you provide within the manuscript.

The sentence with this reference was removed (p. 4, lines 148 – 150)

1. The discussion related to the page 4, lines 147 – 160 should be improved in general since it is difficult to understand which text is related to own results and which one is related to cited papers.

We have edited sentences (p. 4, lines 147 – 160)

1. The Figure 2 is not mentioned in the text.

Picture reference was added (page 5, line 169)

1. Could be Lc values obtained from Scherrer equation used for discussion about the agglomeration?

The Lc values obtained from Scherrer equation are used in the discussion of the agglomeration (p. 4, line 153- 158)

1. Figure 5, X axis should be wavenumber.

Corrected (p. 7)

1. Please change the Fig 5b) as Fig 5a) since this figure is discussed before the Figure 5a) in text.

The purpose of the Figure was to demonstrate the fast adsorption of CO₂  (Fig 5a) and slow desorption (Fig 5b) of reaction product. This is the reason of such sequence. We understand your point but would like to remain the Figure as it is.

1. Please indicate the bands which are compared in Figures 5 and provide the list of groups associated to these bands.

We indicated the IR bands in Figures 5 and provide the description of groups (p. 6, line 196; p.7, line 217 – 218)

1. The text of the discussion on Page 9 is not well understandable since for example the comment on activity of TiO2-C1000 is after the discussion of the results in Figure 8 and this sample is not included in Figure 8.

First, we tried to explain the prime factors which influence the photocatalytic activity (most active samples) and, then, the secondary factors (least active samples). Therefore, we started the discussion with rutile (C-700) and then shifted to C-500 and C-1000. To avoid misunderstanding we added reference to the Figure. 8 in the text (p. 9, line 256).

 

Round 2

Reviewer 2 Report

Thank you for your reply and modifications. I think the updated manuscript is good to be published.

Author Response

Dear Reviewer, 

Thank you for your revision

Reviewer 4 Report

The authors responded my comments, but the uploaded paper, the one which I can download now with the changes yellow highlighted, does not consist important parts of the actions the authors declare they did.

There is still reference [42], in my review report 1 I mentioned, "The reference [42] deals with carbon-doped TiO2 and the authors of this paper report different Eg values than the authors list in this manuscript. Please check if the references are aligned with the information you provide within the manuscript." The correction of X axis ? - there is still wavelength. The indication of the bands in Figure 5 ? and the description of the groups - I cannot find relevant text on page 6, line 196 as well as page 7 217 - 218.

These are the reasons I cannot recommend the paper to be published.

Author Response

I am sorry for inconvenience I did not upload the latest version of review. In this version all your remarks are considered