Photocatalytic 4-Nitrophenol Reduction by Hydrothermally Synthesized Mesoporous Co- and/or Fe-Substituted Aluminophosphates

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript entitled “Photocatalytic 4-nitrophenol reduction by hydrothermally synthesized mesoporous Co and/or Fe substituted aluminophosphates” is focused on the environmental remediation application using mesoporous Co and/or Fe substituted aluminophosphates. The theme of the manuscript is interesting as of current need for clean atmosphere but the manuscript has some drawbacks as given below:

1. The manuscript needs to improve the introduction part by keeping consistent formatting style in the manuscript.

2. No need to put the optical photographs in the manuscript. It is enough to mention the color in text.

3.  XRD patterns should be indexed with hkl values (Figure 3).

4. Electron microscopic images do not provide the concluding remark because the images are not cleared. This is recommended that the electron microscopic data should be repeated for better imaging.

5. Is there any change in morphology and oxidation state before and after photocatalysis? Post-catalytic structural and morphological characterizations should be discussed for the stability check of the catalysts.

6. The authors have quoted 111 references in the manuscript that is too much. I will recommend that only the appropriate references should be quoted.

Comments on the Quality of English Language

Moderate editing of English language required

Author Response

The manuscript entitled “Photocatalytic 4-nitrophenol reduction by hydrothermally synthesized mesoporous Co and/or Fe substituted aluminophosphates” is focused on the environmental remediation application using mesoporous Co and/or Fe substituted aluminophosphates. The theme of the manuscript is interesting as of current need for clean atmosphere but the manuscript has some drawbacks as given below:

1. The manuscript needs to improve the introduction part by keeping consistent formatting style in the manuscript.

A: The introduction has been shortened and revised as suggested.

2. No need to put the optical photographs in the manuscript. It is enough to mention the color in text.

A: The optical images have been excluded from the main manuscript.

3. XRD patterns should be indexed with hkl values (Figure 3).

A: The hkl values for the major peaks were already provided in the Fig. 3 and S2 (ESI). Nevertheless, we also provide it in the revised Fig. 2 as suggested.

4. Electron microscopic images do not provide the concluding remark because the images are not cleared. This is recommended that the electron microscopic data should be repeated for better imaging.

A: We regret that the TEM and SEM data could not provide clear information. We could not redo the experiments within the timeframe of review deadline. Moreover, the non-conducting nature of all three samples (along with high disordered surface) limits good resolution and the clear lattice fringes are not observed, and thus, repeat experiments would be futile.

5. Is there any change in morphology and oxidation state before and after photocatalysis? Post-catalytic structural and morphological characterizations should be discussed for the stability check of the catalysts.

A: As suggested, we carried out XRD and XPS assessment for the CoAPO-1000 sample, post photocatalytic 4-nitrophenol reduction. The results are provided in Fig. S8. ESI. The crystallographic phase is retained, although some peaks are significantly diminished. The XPS results confirm similar oxidation states compared to fresh sample (with slight shift in peak position and relative intensity of 3+ and 2+ states for Co). The structure of the catalyst is mostly intact and the slight changes can be due to some surface reconstruction during the catalytic reaction. Since SEM images only show aggregated structures, morphological evidence is not useful.

5. The authors have quoted 111 references in the manuscript that is too much. I will recommend that only the appropriate references should be quoted.

A: Several references have been excluded in the revised version.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this work, Swetha et al. synthesized mesoporous Co and/or Fe substituted aluminophos-phates catalysts for the photocatalytic reduction of 4-nitrophenol. They observed that Co based samples performed the best although they have lower surface areas. Further, they suggested that the induction time can be ascribed to the initial adsorption of the reactants onto the catalyst surface. Overall, the manuscript is well prepared, thus can be published after solving the following issues.

 

1.      Parameters for different characterization techniques, such as XRD, TEM, TGA etc., should be provided.

2.      XRD diffraction peaks should be labeled in Figure 3.

3.      The assignment of Co 2p XPS spectrum is not correct. The peak at around 788 eV should be the satellite peak. Reference: 10.1016/S1381-1169(01)00216-3.

4.      Some recent studies about supported metal and bimetallic catalysts should be cited.

5.      In line 388, the authors claimed that “The XPS spectra of FeAPO-1000 and CoFeAPO-1000 have substantial noise and low intensity peaks, owing to their more amorphous nature.” This is incorrect, as the amorphous state of the metal does not influence their XPS signal intensity. It is possible that Fe and Co were coated by the carbon layer. C 1s spectra can be provided and analyzed.

6.      The influence of Co, Fe and CoFe on the catalytic performance should be compared and discussed in detail.

Comments on the Quality of English Language

Minor revision of English language is required.

Author Response

Comments 2

In this work, Swetha et al. synthesized mesoporous Co and/or Fe substituted alumino-phosphates catalysts for the photocatalytic reduction of 4-nitrophenol. They observed that Co based samples performed the best although they have lower surface areas. Further, they suggested that the induction time can be ascribed to the initial adsorption of the reactants onto the catalyst surface. Overall, the manuscript is well prepared, thus can be published after solving the following issues.

1. Parameters for different characterization techniques, such as XRD, TEM, TGA etc., should be provided.

A: All details are already provided in the experimental section.

“All the as-synthesized and calcined samples were characterized using analytical and spectroscopic techniques. The powder X-ray diffraction (XRD) patterns were recorded on 'PANalytical' X-Ray diffractometer. Rietveld refinement was performed to understand the doping of the transition metals in the APO lattice. Scanning electron micrographs were imaged using 'Ultra 55' electron microscope. Transmission electron micrographs (TEM) were imaged on a 'JEOL' FETEM (300kV). ⁵⁷Fe Mössbauer spectra were recorded at room temperature using (SEE Co, USA) spectrometer in transmission geometry using a ⁵⁷Co source (Rh-matrix). Thermogravimetry analysis (TGA) measurements were performed in a 'TA Q50' system in a nitrogen atmosphere (40 ml min^-1) at a heating rate of 10 °C min⁻¹. The net surface area was determined through Brunauer–Emmett–Teller (BET) isotherms, while the pore size in the samples was determined by the Horvath–Kawazoe (HK) method. Ultraviolet-Visible Diffuse Reflectance Spectra (UV-DRS) were recorded on a Perkin Elmer Lambda 750 spectrophotometer in the wavelength range of 250-800 nm with barium sulfate as the standard.”

1. XRD diffraction peaks should be labeled in Figure 3.

A: The hkl values are provided in the revised Fig. 2.

2. The assignment of Co 2p XPS spectrum is not correct. The peak at around 788 eV should be the satellite peak. Reference: 10.1016/S1381-1169(01)00216-3.

A: We thank the reviewer for pointing out our mistake. The Co2p spectrum has been refit and the reference cited.

3. Some recent studies about supported metal and bimetallic catalysts should be cited.

A: Some recent references have been included.

5. In line 388, the authors claimed that “The XPS spectra of FeAPO-1000 and CoFeAPO-1000 have substantial noise and low intensity peaks, owing to their more amorphous nature.” This is incorrect, as the amorphous state of the metal does not influence their XPS signal intensity. It is possible that Fe and Co were coated by the carbon layer. C 1s spectra can be provided and analyzed.

A: We have revised the sentence as suggested. As the calcination was done in air at 1000 °C, the possibility of carbon coating is negligible. Some residual carbon atoms could be present in the APO matrix but its quite unlikely. The colour of the calcined samples (Fig. S1, ESI) also suggests that the carbon coating has been removed at high temperature. Since, a clear reason for the low intensity of the XPS signals is not understood at present, we would like to exclude the previous phrase “owing to their more amorphous nature”.

6. The influence of Co, Fe and CoFe on the catalytic performance should be compared and discussed in detail.

A: The detailed comparative performance and analysis has already been provided in the manuscript (Section 4, Fig. 12-15 and Table 5).

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript can now be accepted for publication in this journal.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have revised the manuscript properly. Thus, this manuscript can be published after addressing the following minor issues.

1.      The resolution of Figure 4 can be improved.

2.      There are still several XRD diffraction peaks not labeled in Figure 3.

3.      It is recommended to cite some very recent studies about the synthesis of nano catalysts: 10.1038/s44160-023-00387-3; 10.1038/s44160-024-00486-9.