Photocatalytic decolorization of three commercial dyes using a new heteropolyoxotantalate catalyst

The decolorization of commercial dyes is still a pertinent issue since these azo dyes are relatively resistant to conventional biological treatment methods. It is well known that polyoxometalates can absorb light in UV–Vis range that delivers electrons to the reducible species resulting in the decomposition of organic compounds. In this paper, we present the third heteropolyoxotantalate under conventional synthetic conditions in the presence of hydrogen peroxide. The compound has been thoroughly characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, thermogravimetric analysis and powder X-ray diffraction. The polyanion incorporates two 3-peroxotantalo-2-phosphate clusters that are linked together by four oxygen bridges. In addition, the photocatalytic activities of the title compound 1a were investigated. After 270 min irradiation, about 90% of Rhodamine B (RhB) was removed in the presence of 1a while the degradation of RhB could be negligible in the absence of 1a, indicating it can be a promising catalyst candidate for decolorization of organic dyes. Also, photocatalytic experiment for hydrogen generation was studied, and the results show that the H2 evolution rate is 3383 µmol h−1 g−1 for compound 1a (100 mg) over 6 h with the corresponding turnover number of 432.


22-May-2019
Dear Dr Zhou: Title: Photocatalytic decolorization of three commercial dyes using a new heteropolyoxotantalate catalyst Manuscript ID: RSOS-190015 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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• Acknowledgements Please acknowledge anyone who contributed to the study but did not meet the authorship criteria. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) This manuscript reported the synthesis of a new heteropolyoxotantalate and tested its performance for photocatalytic degradation of three organic dyes in water. The materials are generally well characterized. However, this reviewer has concerns over the rationale for using this homogeneous photocatalyst for dye removal, since the compound is hard to remove from water after photocatalytic test. Actually, this may be the reason why the compound (and other homogeneous photocatalysts) are more suitable for solar energy applications such as hydrogen generation instead of environmental applications such as water purification. To be accepted for publication, the authors need to address the above concern and performance experiments to show that this compound can be easily recovered from the treated water after photocatalyst tests and possibly reused.

Reviewer: 2
Comments to the Author(s) In this manuscript, the authors present the synthesis, structure and its photocatalytic properties of a new HPOTa complex K3[H3P4(TaO2)6(OH)4O20]·12H2O, which was successfully used to degrade the Rhodamine B, Methyl blue and Acid red 1. The characterization and data analysis of this manuscript is sufficient. Therefore, the manuscript can be considered to be accepted after further revision. The following questions needs to be addressed for further improvement: 1. The nature of catalyst is that the composition of catalyst does not change before and after the catalytic reaction. In order to prove that the as-obtained material is a catalyst, please provide the phase data of the material after the catalytic reaction. 2. Dye-degradation catalysts have been studied for decades. Many corresponding catalysts have been developed. Please compare the previous reports on dyestuff degradation catalysts and propose the advantages of using HPOTa complex as a catalyst. The author can refer to relevant research, such as Nanoscale, 2013, 5, 6589-6598;Nanoscale, 2014Nanoscale, , 6, 1124Nanoscale, -1133CrystEngComm, 2012, 14, 3965-3971;Chemical Engineering Journal, 2018, 334, 1537-1549. Please propose the mechanism of catalytic degradation. 4. There are some typo and grammar mistakes. Please revise the manuscript carefully.

Author's Response to Decision Letter for (RSOS-190015.R0)
See Appendix A.

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
In the revised manuscript, my previous concerns are partially addressed. Since this reported dye decolorization by the compounds can be useful in applications other than wastewater treatment, this manuscript can be accepted for publication after minor revision, with the following concerns properly addressed.
1. Figure S17, for the polyoxometalates reported herein, is it more suitable to use HOMO/LUMO or CB/VB in describing the proposed photocatalytic mechanism? 2. Please refrain from using lumped references. For example, in the first three sentences in the Introduction section, the authors used 7 references, but actually, no more than 3 references are needed. There is no need to have 37 papers to support the first two paragraphs of the Introduction. The authors are strongly advised to remove the unnecessary references.

24-Jun-2019
Dear Dr Zhou: Title: Photocatalytic decolorization of three commercial dyes using a new heteropolyoxotantalate catalyst Manuscript ID: RSOS-190015.R1 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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1. Figure S17, for the polyoxometalates reported herein, is it more suitable to use HOMO/LUMO or CB/VB in describing the proposed photocatalytic mechanism? 2. Please refrain from using lumped references. For example, in the first three sentences in the Introduction section, the authors used 7 references, but actually, no more than 3 references are needed. There is no need to have 37 papers to support the first two paragraphs of the Introduction. The authors are strongly advised to remove the unnecessary references. We hope this revised manuscript is acceptable.
Best regards,

Comments from Reviewer 1:
 This manuscript reported the synthesis of a new heteropolyoxotantalate and tested its performance for photocatalytic degradation of three organic dyes in water. The materials are generally well characterized. However, this reviewer has concerns over the rationale for using this homogeneous photocatalyst for dye removal, since the compound is hard to remove from water after photocatalytic test. Actually, this may be the reason why the compound (and other homogeneous photocatalysts) are more suitable for solar energy applications such as hydrogen generation instead of environmental applications such as water purification. To be accepted for publication, the authors need to address the above concern and performance experiments to show that this compound can be easily recovered from the treated water after photocatalyst tests and possibly reused.    As you mentioned above, photocatalytic water-splitting offers a promising way for environmentally friendly solar-hydrogen production in recent years. Therefore, a preliminary photocatalytic study for hydrogen generation has been done, and the results show that the H 2 evolution rate is 3383 μmol h -1 g -1 for compound 1a (100 mg) over 6 h with the corresponding turnover number of 432 (moles of H 2 formed / moles of 1a), which is shown in Figure S16 in the revised Supporting Information.
Also, blank experiments indicate that no H 2 can be detected under the absence of Pt-co-catalyst, sacrificial solvent (CH 3 OH) or cluster 1a. (black) and the absence of compound 1a (red). Experiment condition: 100 mg of 1a and 0.05 mg of H 2 PtCl 6 were dissolved in 100 mL of 10% methanol, which was irradiated under UV using a 500 W mercury lamp.

Comments from Reviewer 2:
 In this manuscript, the authors present the synthesis, structure and its photocatalytic properties of a new HPOTa complex K 3 [H 3 P 4 (TaO 2 ) 6 (OH) 4 O 20 ]• 12H 2 O, which was successfully used to degrade the Rhodamine B, Methyl blue and Acid red 1. The characterization and data analysis of this manuscript is sufficient. Therefore, the manuscript can be considered to be accepted after further revision. The following questions needs to be addressed for further improvement: 1. The nature of catalyst is that the composition of catalyst does not change before and after the catalytic reaction. In order to prove that the as-obtained material is a catalyst, please provide the phase data of the material after the catalytic reaction.
Response: Thank you for your positive comments and good suggestion. As above, the stability of compound 1a dissolved in solution can be proved by both UV and ESI-MS spectra, shown in Figures S10 and S11 in the revised Supporting Information. Whereas the IR spectra of compound 1a remain the same before and after catalysis ( Figure S12 in the revised Supporting Information), indicating that compound 1a can be stable in the solid state.   HPOTa complex as a catalyst. The author can refer to relevant research, such as Nanoscale, 2013, 5, 6589-6598;Nanoscale, 2014Nanoscale, , 6, 1124Nanoscale, -1133CrystEngComm, 2012, 14, 3965-3971;Chemical Engineering Journal, 2018, 334, 1537-1549 Response: Thank you for your good suggestion. POMs is a large family of metal oxo clusters used for various applications as it has a lot of advantages like, structural versatility, accessible redox property, being environmental friendly and chemically stable. A number of investigations on the photocatalytic activity of POM clusters are also studied. As you said, dye-degradation catalysts have been studied for decades. Honestly, we have paid more attention on POMs instead of the other materials in photocatalytic properties.
Thank you for your recommendation! We have carefully read these four researches and cited them in our revised manuscript. However, it is difficult to compare the dye-degradation effect owing to the different experiment conditions, such as dye concentration, hydrogen peroxide concentration, pH, light source (UV light or visible light) and the amount of reported clusters. For example, most experiments were carried out with the help of H 2 O 2 to yield highly reactive hydroxyl radicals that can in turn react with the dyes and lead to dye degradation.
In our experiment, the degradation of RhB could be negligible under 350 W Xenon lamp irradiation in the absence of the title compound 1a, while the degradation rate was greatly enhanced upon introducing compound 1a. After 270 min irradiation, about 90% of RhB was removed in the presence of the 1a samples. However, your suggestion gives us an inspiration to design experiment in the future, we should pay much attention to the related researches and do comparison that maybe be more persuasive.
 3. Please propose the mechanism of catalytic degradation.
Response: Thank you for your good suggestion. As shown in Figure S17 in the revised Supporting Information, the charge separation may occur when the compound absorbed the UV light. The adsorbed oxygen present in the system and then form the active species O 2 · to degrade the RhB. So, we think it might be a one-step mechanism for compound 1a. Figure S17. Schematic illustration for the photocatalytic mechanism of compound 1a.
 4. There are some typo and grammar mistakes. Please revise the manuscript carefully.

Response:
We have modified the English expression and polished the whole manuscript, highlighted in yellow in the revised manuscript. Thank you! Dear Dr Laura Smith, Thank you for your work to our manuscript entitled "Photocatalytic decolorization of three commercial dyes using a new heteropolyoxotantalate catalyst" (Manuscript ID:

RSOS-190015.R2).
We have contacted the CSD team in regards to the DOIs for the data records. The response is: CSD team do not assign DOIs to data pre-publication. They only assign DOIs to data once it has been published. The only solution is to publish our data as a CSD Communication, which will make the data accessible to the public. However, this should not be used if we are planning to publish our data in a scientific article.
Then, we got advice from Editorial Coordinator, Lianne Parkhouse and cited the CCDC datasets appropriately within the revised manuscript.
In addition, we have already published the file we deposited to the Royal Society figshare portal.
We hope this revised manuscript is acceptable.