Wastewater Contaminated with Hydrazine as Scavenger Agent for Hydrogen Production by Cu/Ti Nanostructures

Round 1

Reviewer 1 Report

The manuscript entitled “Wastewater contaminated with hydrazine as scavenger agent for hydrogen production by Cu/Ti nanostructures” presents the data on the modification of TiO₂ with copper for enhanced water splitting and hydrogen production. The manuscript is good written but the following concerns should be addressed before consideration for publication:

1. The sentence “Just at 500 °C, Cu2O (JCPDS 05-0667) becomes present with the crystalline plane (111) located at 41°, see inset in Figure 1b, simultaneously with the CuO oxide (JCPDS 45-0937), which is evident with the presence of the crystalline plane (111) located at 37° of 2 Theta” does not correspond to the XRD patterns in figure and actual data.
2. The sentence “it is found that the surface area decreases with the increasing copper content from 64.6 to 35.2 m2/g, where the highest value corresponds to the highest copper content” does not correspond to the actual data.
3. Instead of EDS, XRF or ICP-MS method should be employed for the estimation of actual copper content.
4. TEM analysis should also be made to show the local structure of copper species, and the effect of TiO2 doping or/and its surface modification at various Cu contents should be more discussed.
5. Supplementary material does not contain the description for estimation of the optical band gap.
6. Interpretation of XPS data for Ti states for various samples (Fig.5 and Table 2) is incorrect (BE value for Ti(3+) should be higher than for Ti(4+)).
7. The role of hydrazine as a scavenger is not discussed.
8. “Also these materials are cost-effective catalysts due to the use of copper, which is as effective as expensive transition metals.” Provide a comparison with, e.g., Pt/TO2 under the same conditions to support this statement.

Author Response

Thank you very much for the comments. We hope that the corrections made satisfy the reviewer's observations.

The sentence “Just at 500 °C, Cu2O (JCPDS 05-0667) becomes present with the crystalline plane (111) located at 41°, see inset in Figure 1b, simultaneously with the CuO oxide (JCPDS 45-0937), which is evident with the presence of the crystalline plane (111) located at 37° of 2 Theta” does not correspond to the XRD patterns in figure and actual data.

The authors acknowledge the observation, we made a mistake, the current manuscript version is now corrected:

Just at 500 °C, Cu₂O (JCPDS 05-0667) becomes present with the crystalline plane (111) located at 37.0°, see inset in Figure 1b, simultaneously with the CuO oxide (JCPDS 45-0937), which is evident with the presence of the crystalline plane (111) located at 38.7° of 2 Theta.

The sentence “it is found that the surface area decreases with the increasing copper content from 64.6 to 35.2 m2/g, where the highest value corresponds to the highest copper content” does not correspond to the actual data.

It was a typo mistake. This information was corrected, as follows: “it is found that the surface area decreases with the increasing copper content from 64.6 to 35.2 m²/g, where the highest value corresponds to the lowest copper content”.

Instead of EDS, XRF or ICP-MS method should be employed for the estimation of actual copper content.

The authors appreciate the observation. XRF measurements have been performed in the past for the determination of copper. In the new manuscript version are included with the equipment information.

A wavelength dispersive x-ray fluorescence (WDXRF) spectrometer Rigaku ZSX Primus II model (rhodium X-ray tube; 4 kW maximum power), equipped with an automatic sampler for 12 pellets was used for a quantitative copper content in all the Cu/Ti photocatalysts.

The copper content was measured by X-Ray Fluorescence technique, see Table 1. In the 1.0 Cu/Ti material, the actual copper content correspond to 0.95 wt.%, the 2.5 Cu/Ti material, the actual copper content correspond to 2.30 wt.%, while for the 5.0 Cu/Ti photocatalyst, 4.60 wt.% of copper was found. Slightly variation was found in the nominal and actual copper contents.

In this sense, the Table 1 was modified with the nominal and actual copper loadings.

HRTEM analysis should also be made to show the local structure of copper species, and the effect of TiO2 doping or/and its surface modification at various Cu contents should be more discussed.

We acknowledge the observation. In order to complement the SEM information, the 1.0 Cu/Ti 500 photocatalyst was examined by TEM in a Tecnai FEI 300 transmission electron microscope operated at 300 kV, in order to elucidate the structural copper species. Including the follow text and modified image.

On the other hand, the HRTEM image reveals semi-spherical copper nanoparticles of around 2 to 4 nm on the TiO₂ surface. The measured value of the crystalline interplanar distance was 2.4 Å, and according to the JCPDS 05-0667and 45-0937 cards are in good agreement with Cu₂O oxide.

Figure 2. a) SEM image, b) TEM image for the 1.0 Cu/Ti 500 photocatalyst.

Supplementary material does not contain the description for estimation of the optical band gap.

We acknowledge the observation. This information was added to the new version of the manuscript.

The band gap of the photocatalysts was calculated by linearization of the slope to the X axis (wavelength, nm) with the Y axis (absorbance) equal to zero. For practical purposes, the band gap energy for the different samples was calculated using the follow equation.

Eg= 1239 * m / -b

Where m and b are obtained by the linear fit (y=mx+b) of the flat section of the UV–Vis spectrum.

[REF] R. López, R. Gómez. Band-gap energy estimation from diffuse reflectance measurements on sol-gel and commercial TiO₂: a comparative study. Journal of Sol-Gel Science and Technology. 6 (202) 1–7.

Interpretation of XPS data for Ti states for various samples (Fig.5 and Table 2) is incorrect (BE value for Ti(3+) should be higher than for Ti(4+)).

Thank you very much for the suggestion. We made a mistake when assigning species during the realization of the graphs, however, we performed the corresponding deconvolutions again, the assignments were corrected for Ti³⁺ and Ti⁴⁺ taking the NIST book as a reference. Please, check the XPS section corrected.

The peak located at 461 and 466 eV corresponds to Ti 2p_3/2 and Ti 2p_1/2, respectively, related to the presence of Ti⁴⁺ species for the TiO₂-Cu materials. In addition, the peaks centered at 457 and 463 eV, corresponding to Ti 2p_3/2 and Ti 2p_1/2, respectively, indicate the presence of Ti³⁺ species for the TiO₂-Cu materials in agreement with previous studies [Ref].

The peak fitting for each element was done by using XPSPEAK 41 free software with Shirley background and using the database of the NIST handbook; for Ti 2p splitting-value as oxide (Δ_oxide=5.7eV).

Also, the information of the Table 2 was updated according to the new data of deconvolution of Ti species.

[REF]     Z. Zhang, J. T. Yates. Band bending in semiconductors: Chemical and Physical consequences at surfaces and interfaces. Chemical Reviews 112 (2012) 5520-5551.

The role of hydrazine as a scavenger is not discussed.

Thank you for the observation. The role of the hydrazine results as an oxygen scavenger that drives the hydrogen production from water, and its reaction profits gradually, removing dissolved oxygen from aqueous environment at room temperature in synergy with the Cu species, e.g. 4CuO + N₂H₄ 2Cu₂O + 2H₂O + N₂.

 “Also these materials are cost-effective catalysts due to the use of copper, which is as effective as expensive transition metals.” Provide a comparison with, e.g., Pt/TO2 under the same conditions to support this statement.

Thank you for the observation. In the follow table are presented some comparative examples of hydrogen production. It is important to mention that the 577.9 µmol g^-1 h^-1 of hydrogen produced are in correspondence with the literature. Other copper materials produce 427.8 µmol g^-1 h^-1 but using ethanol as scavenger, or other materials with precious metals as Pt produces a similar quantity of 525.7 µmol g^-1 h^-1. On the other hand, gold catalysts produces a grant quantity of hydrogen of 2488 µmol g^-1 h^-1 by using similar experimental conditions. In this sense the results here obtained represent a great strategy to produce hydrogen by a hydrazine-wastewater.

[i] https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0926337319303972

[ii] https://sci-hub.se/https://pubs.acs.org/doi/pdf/10.1021/acscatal.9b01786

[iii] https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0920586119304419

[iv] https://www.tandfonline.com/doi/abs/10.1080/01496395.2020.1788598

[v] https://sci-hub.se/https://www.sciencedirect.com/science/article/abs/pii/S0360319919315617

Thank you very much for the comments. We hope that the corrections made satisfy the reviewer's observations.

Author Response File: Author Response.pdf

Reviewer 2 Report

There are some problems to addresss with this paper. Someone needs to go through the paper and correct the mistakes in the use of English. 43, To catalyze. 47, through. Figure 1, assign all peaks. Table 1, define the - entries. Table 1, the surface area data have too many significant figures. Table 2, why does the first line have missing entries? Figure 7, give error bars on all data points. Table 3, what are the blank spaces in the table? Figure 8, give error bars. 

Author Response

We acknowledgment all the suggestions and observations of the referee in order to improve the quality of this manuscript.

The surface area data in the new manuscript version do not have significant figures.

The --- in the Table 1 were update for the nominal and actual copper contents corresponding for the 0.0 Cu/Ti 500 sample of 0.0 and 0.0 wt.%

Error bars were added to Figures 7 and 8 according to the experimental data obtained by Gas Chromatograph.

Table 2 and 3 the missing entries corresponding to copper contents for the 1.0 Cu/Ti 450 sample was not possible to determine this copper loading because is in the limit of the XPS instrument exceeding noise in the XPS measurements. In this sense, due to the short time for review and COVID-19 outbreak we are unable to perform the XPS measurements of this sample since our research institution is under strict confinement measures and all kind of activities are performed from home at the time.

Author Response File: Author Response.pdf

Reviewer 3 Report

It is a very good study with overall adequate presentation of experimental results. Some additions are needed:

1) Authors should further emphasize on the novelty of their work.

2) Some minor typos, grammar and syntax errors should be carefully revised and corrected accordingly.

3) Reference can be even more updated (more recent relative works).

4) Errors bars are needed in Figures.

Author Response

Thank you very much for the comments. We hope that the performed manuscript corrections satisfy the Reviewer’s observations.

1) Authors should further emphasize on the novelty of their work.

2) Some minor typos, grammar and syntax errors should be carefully revised and corrected accordingly.

The authors are grateful with the observation. The new manuscript version was carefully revised, the novelty was remarked in abstract and manuscript and improved by a native English speaker.

3) Reference can be even more updated (more recent relative works).

Some references are updated preserving the aim of the introduction and discussion of our research

4) Errors bars are needed in Figures.

It is important to highlight that our device is continuously calibrated, and readings are taken every 60 min by triplicate. Our calibration curve has a correlation coefficient (R²=0.9995) for the equation y=0.0293x+0.0767, where x corresponds to the area under the curve (chromatographic data) and y corresponds to the hydrogen concentration.”

Figure. Calibration curve for hydrogen quantification in our water splitting system.

Nevertheless, the errors bars were added to the graphs of photocatalytic hydrogen production, according to our experimental data registered by the gases chromatograph.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

All my comments of the initial submission have been correctly replied and included in the revised manuscript. The quality of this work has been drastically improved after revision and therefore I recommend its publication as it is.