Tribocatalysis of Cefuroxime Axetil: Effect of Stirring Speed, Magnetic Rods, and Beaker Material Type

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors synthesized ZnO using both the hydrothermal method and the sol-gel method, and applied it as a tribocatalyst for the degradation of Cefuroxime Axetil. The effects of rotational speed, rotor surface area, and container material on the tribocatalytic efficiency were systematically investigated. This manuscript holds certain reference significance in the field of tribocatalysis; however, the authors are encouraged to enhance the sample characterization and conduct a more in-depth mechanistic analysis.

The sentence in the abstract "In this paper, we successfully utilize a friction route to convert mechanical energy into hydrothermal and sol-gel ZnO tribocatalysts" does not make sense. I carefully reviewed the manuscript and found that the author synthesized ZnO using two methods and used these two types of ZnO as tribocatalysts, rather than directly converting friction energy into catalysts.

In Figure 1, the Zeiss SEM used by the author is why it is written as (SEM, JSM-5510, JEOL, Krefeld, Germany) in the equipment introduction? Additionally, the magnification is too small, making it impossible to clearly see the morphological differences between the two different preparation methods for ZnO. A magnification of 5k-10k should be provided.

The authors mentioned “The ZnO samples were analyzed using energy-dispersive X-ray spectroscopy (EDS). Hydrothermal ZnO (Zn 30.28 at%, O 69.72 at%) and sol-gel ZnO (Zn 48.97 at%, O 51.03 at%) are the two types of tribocatalysts whose atomic percentages of elements are known.” What could be the reason for the significant difference in their atomic compositions? Do these two samples share the same chemical formula?

In Figure 4, why are the intensities of the two different ZnO not the same? The author is recommended to separate the two figures. From the intensity in the XRD, the intensity of the sol-gel is greater and the crystallinity is better, but it contradicts the SEM in Figure 1. The author is requested to carefully verify.

In Table 1, how was the Crystallite size calculated? There should be direct evidence from SEM or TEM or particle size distribution.

In Table 2, what do D and k represent respectively and how were they obtained? It should be explained.

The authors mentioned "When ZnO absorbs mechanical energy during friction, electrons are represented by excited e—, and holes are represented by the resulting h+. During the breakdown of the drug, oxygen molecules interact with electrons to produce O2— superoxide radicals. The holes convert into hydroxyl radicals, or OH•, after interacting with OH—. Efficient electron-hole separation and increased production of O2− and OH• radicals may be responsible for the enhanced activity of the sol-gel ZnO sample. The higher efficiency results from both the increased number of oxygen vacancies and the greater adsorption of hydroxyl ions onto the ZnO surface. The reaction between holes and OH— helps form OH•." The author did not provide evidence for holes, superoxide, and hydroxyl radicals. Moreover, this text has no references.

Figure 9 shows an unclear color contrast. It is recommended to change to other colors.

The authors mentioned "A possible mechanism for cefuroxime axetil degradation, caused by a reaction with hydroxyl radicals, has been reported in the literature []. " This lacks references.

What are the degradation products? No explanation is given, and the degradation mechanism is not clear.

ZnO is a piezoelectric material. Does its piezoelectric effect have any impact on degradation?

Author Response

The manuscript: “Tribocatalysis of Cefuroxime Axetil: effect of stirring speed, magnetic rods, and beaker material type”

by Nina Kaneva.

Manuscript ID: inorganics-3817901

Dear Editors,

Thank you very much for the prompt review process and for the Reviewers' comments and recommendations. Considering all the advice and suggestions, I made the necessary corrections to enhance the manuscript.

The new changes in the paper's text are highlighted in blue. Please see my comments below.

Reviewer #1:

The authors synthesized ZnO using both the hydrothermal method and the sol-gel method, and applied it as a tribocatalyst for the degradation of Cefuroxime Axetil. The effects of rotational speed, rotor surface area, and container material on the tribocatalytic efficiency were systematically investigated. This manuscript holds certain reference significance in the field of tribocatalysis; however, the authors are encouraged to enhance the sample characterization and conduct a more in-depth mechanistic analysis.

The sentence in the abstract "In this paper, we successfully utilize a friction route to convert mechanical energy into hydrothermal and sol-gel ZnO tribocatalysts" does not make sense. I carefully reviewed the manuscript and found that the author synthesized ZnO using two methods and used these two types of ZnO as tribocatalysts, rather than directly converting friction energy into catalysts.

I changed the sentence in the abstract: “In this paper, ZnO tribocatalysts were synthesized using the hydrothermal and sol-gel methods.”, as recommended.

In Figure 1, the Zeiss SEM used by the author is why it is written as (SEM, JSM-5510, JEOL, Krefeld, Germany) in the equipment introduction? Additionally, the magnification is too small, making it impossible to clearly see the morphological differences between the two different preparation methods for ZnO. A magnification of 5k-10k should be provided.

I thank the reviewer for the pointed inaccuracy, I have corrected it: “To investigate the morphology and microstructure of the obtained samples, we used an imaging device a Zeiss Evo 15 microscope (Bruker Resolution 126 eV, Berlin, Germany) and energy-dispersive X-ray spectroscopy (EDS, Tokyo, Japan).”

I have provided 10k magnification SEM images of both samples, as recommended.

The authors mentioned “The ZnO samples were analyzed using energy-dispersive X-ray spectroscopy (EDS). Hydrothermal ZnO (Zn 30.28 at%, O 69.72 at%) and sol-gel ZnO (Zn 48.97 at%, O 51.03 at%) are the two types of tribocatalysts whose atomic percentages of elements are known.” What could be the reason for the significant difference in their atomic compositions? Do these two samples share the same chemical formula?

I thank the reviewer for drawing my attention to the figure related to the EDS analysis. The data I have given in the text are not atomic percentages, but weight percentages. I have given the real values ​​in Figure 2 itself. It contains weight and atomic percentages of the two elements. No significant, huge difference is observed.

In Figure 4, why are the intensities of the two different ZnO not the same? The author is recommended to separate the two figures. From the intensity in the XRD, the intensity of the sol-gel is greater and the crystallinity is better, but it contradicts the SEM in Figure 1. The author is requested to carefully verify.

The intensities of the two different zinc oxides are the same. This is better seen when I separated the two figures (Figure 4a and Figure 4b), as recommended by the reviewer.

In Table 1, how was the Crystallite size calculated? There should be direct evidence from SEM or TEM or particle size distribution.

The crystallite size was calculated using the Scherrer formula. This is described in the Materials and Methods section.

A particle size distribution was made based on the SEM images. A very accurate distribution cannot be made because agglomeration of the particles is observed.

In Table 2, what do D and k represent respectively and how were they obtained? It should be explained.

The text comments on how the rate constant was derived. I only added "k" to indicate that it is a rate constant: “The reaction rate constants (k), shown in Figures 6b and d, were determined using the pseudo-first-order kinetics equation −Ln(C/C₀) = kt, commonly used for catalytic removal [30].”

I added to the text what D is and how it was obtained, as recommended: “Degradation efficiencies (D, D (%) = ((C₀-C)/C₀)*100, where C and C0 are at time t and the initial concentration) for the sol-gel ZnO sample were 60.68%, 78.94%, and 88.82% at 100, 300, and 500 rpm, respectively.”

The authors mentioned "When ZnO absorbs mechanical energy during friction, electrons are represented by excited e—, and holes are represented by the resulting h+. During the breakdown of the drug, oxygen molecules interact with electrons to produce O2— superoxide radicals. The holes convert into hydroxyl radicals, or OH•, after interacting with OH—. Efficient electron-hole separation and increased production of O2− and OH• radicals may be responsible for the enhanced activity of the sol-gel ZnO sample. The higher efficiency results from both the increased number of oxygen vacancies and the greater adsorption of hydroxyl ions onto the ZnO surface. The reaction between holes and OH— helps form OH•." The author did not provide evidence for holes, superoxide, and hydroxyl radicals. Moreover, this text has no references.

Evidence for their existence is Figure 9, “Figure 9. Radical scavenger assay for the tribocatalytic decomposition of Axetine in (a) glass and (b) PTFE beaker with ZnO.”. I added references to the text indicated by the reviewer.

Figure 9 shows an unclear color contrast. It is recommended to change to other colors.

The colors in Figure 9 have been changed as requested by the reviewer.

The authors mentioned "A possible mechanism for cefuroxime axetil degradation, caused by a reaction with hydroxyl radicals, has been reported in the literature []. " This lacks references.

Yes, the reference has been added to the text.

What are the degradation products? No explanation is given, and the degradation mechanism is not clear.

What are the degradation products of a given pollutant is a topic that is extremely interesting to me. Very soon we expect to receive an apparatus through which we can determine the intermediate and final products in the degradation of an organic pollutant. For this reason, in the article we have written and cited a reference for the degradation of axetine by titanium dioxide. We assume that the same intermediate products are present in our case. This will be investigated soon in our future studies.

ZnO is a piezoelectric material. Does its piezoelectric effect have any impact on degradation?

            Indeed – the known piezoelectric properties of ZnO were one of the reasons that we decided to work on it as a base system for these tribocatalytic studies, which are new to our groups. As know, piezoelectric catalysis is based on the polarization characteristics of the material, which generate a polarization electric field under the action of ultrasound or mechanical action drive charges to its surface and trigger redox reactions. However, piezocatalysis depends on particular crystal orientations or texturing of the crystal to allow them (e.g., needle particles that could orient themselves between the stirring bar and the glass beaker surface). We have attempted to measure the d33 value for the ZnO powders and the result came as nil, possibly because they are spherical and lack orientation when pressed in the d33 meter. Hence, this option was not discussed in the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

General Comments

1. The first general problem is that the author mentions tribocatalytic efficiency; however, is this truly the only phenomenon occurring in the system?

2. There are several typographical errors in the text.

Introduction

1. A more comprehensive state of the art for the proposed process is lacking. It is particularly important to address: What properties must the catalyst to be used have? Why ZnO? Can any photocatalyst be susceptible to being used?

Results

1. The author does not consider the most important aspect of the process: the particle size. Even a very rough particle, if it is very small, will not produce the necessary mechanical stresses to activate the process. At a minimum, the particle size should be measured and compared.

2. The difference in the calculated band gap is too small to draw a conclusion; the 0.02 eV difference is practically attributable to the UV measurement process and the inherent error of the Tauc equation and similar methods (it is not mentioned in the text if this was the conversion used, but these transformations have typical errors of up to 0.05 eV). Furthermore, the only possible conclusion is that there is no difference in the Band-Gap. In fact, the assignment of the slope for the hydrothermal sample seems incorrect because it is observed to be practically the same as the sol-gel one.

3. From Figure 6, there is a more important point to analyze: the differences between the rate constants (which are directly proportional to the rate). It is observed that, in general, the rate of the sol-gel particles is approximately 30% higher, which is not explained by the small difference in the band gap or the surface area. The reviewer suggests, once again, analyzing the effect of particle size.

4. The idea that the size of the stirring rod modifies the friction is incorrect. What will modify the friction are the local hydrodynamic conditions, which will produce changes in the trajectory and movement velocity of the particles, and therefore, their friction. For example, the author states that increasing the length increases degradation; however, what would happen if the size of the container were increased? The degradation would decrease. That is, the only way to use the data shown (which is interesting) is for the author to compare not lengths, but rather some hydrodynamic property, suggesting it be the Reynolds number (Re), which can be calculated with the available experimental data. Otherwise, the results in Figure 7 cannot be reported. The phrase; “As the stir rod grows larger, it has a greater surface area to contact the catalyst and the reactor bottom.” is, at the very least, incomplete.

5. The R² values for figure 8 are missing. On the other hand, is it correct to say that one material absorbs more electrons than another? For the current process, isn't charge transfer more important than the charge itself?

6. The process described in lines 226-237 should be explained with a diagram.

7. The phrase “The addition of EDTA-2Na and IPA did not significantly alter the degradation efficiency of axetine. As a result, h+ plays a negligible part in the drug's tribodegradation over ZnO tribocatalyst. But AA showed a stronger inhibition, suggesting that the production of superoxide radicals contributes more to the rate of axetine degradation” must be explained mathematically, i.e., what is the proportion of one mechanism relative to the other, or how much it increases in the case of superoxide. This is because, directly from the figures, the conclusion is not robust. Furthermore, if the superoxide radical is responsible for the degradation, is this consistent with the literature? The decomposition mechanism should already be proposed in references.

8. Finally, how does the proposed process compare to degradation values from conventional photocatalytic processes with ZnO? Search for references with a similar concentration and compare not the degradation, but the reaction rate.

Methods

1. In the sol-gel synthesis, the drying stages (100 °C?, 24 h?) and the thermal treatment (500-600 °C?) are missing.

2. The phrase: “The reactive species that were causing the paracetamol to break down were investigated…” is not clear. What does it refer to? At what wavelength does maximum absorption occur?

3. A diagram of the tribocatalysis process is necessary to understand it.

4. The chemical formula of the model pollutant is missing.

5. Since the process is mechanically activated, it would be expected that particles with greater mass could have a beneficial effect. Why were the sol-gel particles centrifuged? It seems more convenient for the process to use micrometric rather than nanometric particles.

6. After each sampling, was it considered that the test volume decreases?

Comments on the Quality of English Language

There are several typographical and grammatical errors.

Author Response

The manuscript: “Tribocatalysis of Cefuroxime Axetil: effect of stirring speed, magnetic rods, and beaker material type”

by Nina Kaneva.

Manuscript ID: inorganics-3817901

Dear Editors,

Thank you very much for the prompt review process and for the Reviewers' comments and recommendations. Considering all the advice and suggestions, I made the necessary corrections to enhance the manuscript.

The new changes in the paper's text are highlighted in blue. Please see my comments below.

Reviewer #2:

General Comments

The first general problem is that the author mentions tribocatalytic efficiency; however, is this truly the only phenomenon occurring in the system?

There are several typographical errors in the text.

The catalytic tests were performed without catalyst and light. The data showed that the pollutant was degraded by about 5%. Furthermore, the synthesized samples were placed in the dark in the reaction vessel for 30 minutes before stirring was turned on - the aim was to achieve adsorption equilibrium between the tribocatalysts and the cefuroxime axetil solution. For this reason, the only phenomenon is tribocatalytic efficiency.

Typos have been corrected.

 

Introduction

A more comprehensive state of the art for the proposed process is lacking. It is particularly important to address: What properties must the catalyst to be used have? Why ZnO? Can any photocatalyst be susceptible to being used?

Attention has been paid to the three questions posed by the reviewer. The answers to these questions are included in the introduction.

 

 

Results

The author does not consider the most important aspect of the process: the particle size. Even a very rough particle, if it is very small, will not produce the necessary mechanical stresses to activate the process. At a minimum, the particle size should be measured and compared.

A particle size distribution was made based on the SEM images. A very accurate distribution cannot be made because agglomeration of the particles is observed.

 

The difference in the calculated band gap is too small to draw a conclusion; the 0.02 eV difference is practically attributable to the UV measurement process and the inherent error of the Tauc equation and similar methods (it is not mentioned in the text if this was the conversion used, but these transformations have typical errors of up to 0.05 eV). Furthermore, the only possible conclusion is that there is no difference in the Band-Gap. In fact, the assignment of the slope for the hydrothermal sample seems incorrect because it is observed to be practically the same as the sol-gel one.

Figure 5 has been changed, the slope of the hydrothermal sample has been changed.

Yes, the difference in the values ​​of the forbidden zones is almost insignificant. The text has been corrected, the Tauk equation has been described as well.

 

From Figure 6, there is a more important point to analyze: the differences between the rate constants (which are directly proportional to the rate). It is observed that, in general, the rate of the sol-gel particles is approximately 30% higher, which is not explained by the small difference in the band gap or the surface area. The reviewer suggests, once again, analyzing the effect of particle size.

The sentences related to the width of the prohibited zone have been removed: " The higher efficiency of sol-gel ZnO is likely due to its increased specific surface area and narrower band gap (Figure 6), which provides more active sites for Axetine degradation, enhances carrier participation in redox reactions, and promotes better separation of tribogenerated electron-hole pairs [27]. "

New ones related to particle size have been added: “The higher efficiency of sol-gel ZnO is likely due to its surface morphology. The entire surface is uniform with small particles, which improves carrier participation in redox reactions [27]. The morphology of the hydrothermal samples, where agglomeration of particles is observed, is significantly different. As a result of agglomeration, the active surface area of the catalyst decreases, and the likelihood of recombination of the generated electron-hole pairs increases.”

The idea that the size of the stirring rod modifies the friction is incorrect. What will modify the friction are the local hydrodynamic conditions, which will produce changes in the trajectory and movement velocity of the particles, and therefore, their friction. For example, the author states that increasing the length increases degradation; however, what would happen if the size of the container were increased? The degradation would decrease. That is, the only way to use the data shown (which is interesting) is for the author to compare not lengths, but rather some hydrodynamic property, suggesting it be the Reynolds number (Re), which can be calculated with the available experimental data. Otherwise, the results in Figure 7 cannot be reported. The phrase; “As the stir rod grows larger, it has a greater surface area to contact the catalyst and the reactor bottom.” is, at the very least, incomplete.

The size of the stirrer does not change the friction, but the contact surface increases. As this surface area increases, the friction between particles and the rod also increases.

The reviewer suggests investigating the size of the container. My group is relatively new to research in this area, we will also investigate the influence of the size of the container. But I think it will not have a significant impact, since the contact and friction occur between the rod and the catalyst. Based on literature data related to the effect of anchor length (the literature is cited in the text) - we also conducted a similar study with three anchors of different lengths, using hydrothermal and sol-gel zinc oxide.

The reviewer gives a really wonderful suggestion for a hydrodynamic property, calculated with the Reynolds number. I admit that I am not aware of this number. I did a literature review and found that The Reynolds number (Re) formula is Re = (ρvL)/μ. It is a dimensionless number calculated by dividing the fluid's inertial forces by its viscous forces. The variables are: ρ for density, v for velocity, L for characteristic length (like pipe diameter), and μ for dynamic viscosity. The Reynolds number helps predict if the fluid flow around a catalyst will be laminar or turbulent. Turbulent flow generally enhances mass transfer to the catalyst surface, which can significantly increase reaction rates. This is really intriguing and I will pay attention, I will delve deeper. But I need more time, a topic for future research.

The phrase mentioned by the reviewer has been corrected. All these studies have been confirmed and explained by other colleagues, the relevant literature and team have been cited.

 

The R² values for figure 8 are missing. On the other hand, is it correct to say that one material absorbs more electrons than another? For the current process, isn't charge transfer more important than the charge itself?

The R² values ​​have been added to Figure 8b.

Yes, the word "absorb" has been replaced with "transfer". The reviewer is right.

The process described in lines 226-237 should be explained with a diagram.

The process described in lines 226-237 has been modified and explained with a diagram (new Figure 9) as recommended.

The phrase “The addition of EDTA-2Na and IPA did not significantly alter the degradation efficiency of axetine. As a result, h+ plays a negligible part in the drug's tribodegradation over ZnO tribocatalyst. But AA showed a stronger inhibition, suggesting that the production of superoxide radicals contributes more to the rate of axetine degradation” must be explained mathematically, i.e., what is the proportion of one mechanism relative to the other, or how much it increases in the case of superoxide. This is because, directly from the figures, the conclusion is not robust. Furthermore, if the superoxide radical is responsible for the degradation, is this consistent with the literature? The decomposition mechanism should already be proposed in references.

I added in the text how much the percentage of superoxide radical was reduced in both cases compared to EDTA. The superoxide radical is responsible for the degradation of the drug, it is supported by literature references. The mechanism of this drug has been proposed, but titanium dioxide was used as a catalyst. I have cited the reference and I assume that when zinc oxide is used, similar intermediate products are obtained. The absorption spectra prove that no by-products are obtained and at the end of the tribocatalytic process straight lines are observed - the drug should have degraded to carbon dioxide and water.

 

Finally, how does the proposed process compare to degradation values from conventional photocatalytic processes with ZnO? Search for references with a similar concentration and compare not the degradation, but the reaction rate.

I searched for references to compare tribocatalytic results for the degradation of cefuroxime axetil using zinc oxide. Unfortunately, I did not find any photocatalytic processes with zinc oxide that degraded Axetin.

My guess is that the reaction rate will be much faster under UV light irradiation (photocatalysis) than in the dark under stirring (tribocatalysis), regardless of the organic contaminant.

 

 

 

 

Methods

In the sol-gel synthesis, the drying stages (100 °C?, 24 h?) and the thermal treatment (500-600 °C?) are missing.

There are no annealing steps in this paper. Here, hydrothermal and sol-gel materials were prepared, and their structural and catalytic properties were investigated. In a future work, the effect of temperature will be investigated.

The phrase: “The reactive species that were causing the paracetamol to break down were investigated…” is not clear. What does it refer to? At what wavelength does maximum absorption occur?

The reactive species are holes, superoxide and hydroxyl radicals. The next sentence in the text describes them. The wavelength is 290 nm, at which the maximum absorption is observed - it is described in the Materials and Methods section.

A diagram of the tribocatalysis process is necessary to understand it.

A diagram of a possible mechanism of tribocatalysis has been added as recommended (new Figure 9).

The chemical formula of the model pollutant is missing.

The chemical formula of the organic pollutant is listed in the Material and Methods section.

Since the process is mechanically activated, it would be expected that particles with greater mass could have a beneficial effect. Why were the sol-gel particles centrifuged? It seems more convenient for the process to use micrometric rather than nanometric particles.

Thanks to the reviewer for drawing attention to the method used to obtain the sol-gel samples. They are not centrifuged.

The only centrifugation at 6000 rpm is when taking aliquot samples during the tribocatalytic process.

After each sampling, was it considered that the test volume decreases?

After each aliquot sample was taken, the volume was returned. The goal was a constant volume throughout the tribocatalysis process.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

After the manuscript was revised, its quality significantly improved. However, I must also point out one very important error in it.

For ZnO, the theoretical mass fraction of ZnO is approximately Zn = 80.3%, O = 19.7%; the atomic fraction is Zn = 50.0%, O = 50.0%. Due to the limitations of EDS technology in detecting oxygen, the oxygen content in your actual report will be significantly lower than the theoretical value, while the zinc content will be correspondingly higher. So, I don not think your results about EDS are right, please check them again.

Author Response

Actually, these are numbers that the EMF device gives me.

Today, I had the opportunity to use it and ran the two samples again. I changed the percentages, and the reviewer turned out to be right. The atomic percentages of oxygen and zinc are about 50%.

Figure 2 has been corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have completed the requested corrections and the manuscript can be accepted.

Author Response

Thank you very much! Thank you for the valuable recommendations and questions!