Photocatalytic Ammonia Decomposition Using Dye-Encapsulated Single-Walled Carbon Nanotubes
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis study investigated the decomposition of an aqueous ammonia solution into H2 and N2 using 1@CNT/C60-dendron nanohybrid as a photocatalyst in the presence of RuCl3 under visible light irradiation at room temperature. However, the following points need to be clarified or revised before this paper is published
1- Fig2 a, what is the aim of putting this photo in the manuscript?
2- The photocatalytic investigation part is not complete, the effect of effective parameters such as the Initial concentration of NH3, amount of photocatalyst, temperature, and… has not been investigated.
3- How did you find the optimum condition for the photocatalytic reaction?
4- Recycling is crucial for heterogeneous photocatalysts in practice. Therefore photocatalyst renewability and stability experiments should be carried out.
5- Some related latest literature should be updated, such as:
Li J, Sheng B, Chen Y, Yang J, Wang P, Li Y, Yu T, Pan H, Qiu L, Li Y, Song J. Utilizing full-spectrum sunlight for ammonia decomposition to hydrogen over GaN nanowires-supported Ru nanoparticles on silicon. Nature Communications. 2024 Aug 27;15(1):7393.
Ofuchi Y, Mitarai K, Doi S, Saegusa K, Hayashi M, Sampei H, Higo T, Seo JG, Sekine Y. Hydrogen production by NH 3 decomposition at low temperatures assisted by surface protonics. Chemical Science. 2024.
6- On what scale did you use this method? What problems do you face on a large scale?
7- Compare your method with other reported methods in terms of time, efficiency, and cost in the table
8- The conclusion has generic statements. It is recommended that authors add detail with more robust content.
9- Some English correction is needed throughout the manuscript.
Comments on the Quality of English Language
Some English correction is needed throughout the manuscript.
Author Response
We have revised the manuscript according to the suggestions made by the reviewers. Now I would like to send back you our revised manuscript (catalysts-3185524). The revised portions are colored in red in the manuscript. The detailed replies to the reviewer’s comments are listed as follows:
1) Fig 2a, what is the aim of putting this photo in the manuscript?
The purpose of including the photograph in Figure 2a is to visually demonstrate the color change of the solution upon encapsulation of dye 1 within the CNTs. While dye 1 is inherently insoluble in water, the 1@CNT composite allows for its successful incorporation into the photocatalytic system. Although the absorption spectra in Figure 2b also indicate changes attributed to the dye, these changes are relatively subtle. Therefore, the photograph provides a more readily apparent visual confirmation of dye 1 integration.
To ensure clarity on this point, we have revised the manuscript to explicitly state the purpose of the photograph (lines 77-83 on page 3).
2 and 3) The photocatalytic investigation part is not complete, the effect of effective parameters such as the Initial concentration of NH3, amount of photocatalyst, temperature, and… has not been investigated. How did you find the optimum condition for the photocatalytic reaction?
We appreciate the reviewer's insightful comment regarding the importance of investigating and optimizing reaction parameters such as initial NH3 concentration, photocatalyst amount, and temperature. It is important to note that the photocatalytic system reported in this manuscript is synthesized via physical modification of dye-encapsulated CNTs. The crucial dye@CNT/C60 heterojunction, responsible for the observed photocatalytic activity, is constructed solely through physical adsorption, resulting in a system that is not highly robust. Therefore, the primary focus of this work is to report the first example of dye-sensitized ammonia decomposition, establishing its feasibility. A comprehensive investigation into the long-term stability of the photocatalyst and the optimization of reaction parameters for enhanced efficiency will be the subject of a separate, future publication. In that work, we will thoroughly explore the influence of parameters such as initial NH3 concentration, photocatalyst amount, temperature, and others, particularly when considering large-scale applications.
Regarding the photocatalytic reaction reported here, it is crucial to understand that the amount of hydrogen generated is directly proportional to the number of absorbed photons. The efficiency of light absorption by the catalyst can vary depending on the reaction vessel's shape, even at the same catalyst concentration. Consequently, careful consideration is required when optimizing catalyst concentration. We have previously reported on the optimization of photocatalyst concentration for a water splitting hydrogen evolution reaction (ref. 14). We employed the same optimized concentration in the present study. To ensure transparency, we have added a statement clarifying this point in the experimental section (lines 212-217 on page 7-8). Furthermore, as shown in Fig 6, the ammonia decomposition reaction proceeds only at pH values above 11. Moreover, even at a constant pH, the hydrogen evolution rate varies with NH3 concentration. We acknowledge that several challenges remain to be addressed before large-scale implementation of ammonia decomposition can be realized. We have added a discussion of these challenges to the conclusion section, highlighting the need for future research focused on catalyst optimization and stability.
4) Recycling is crucial for heterogeneous photocatalysts in practice. Therefore photocatalyst renewability and stability experiments should be carried out.
Thank you very much for your valuable comment. We acknowledge the importance of photocatalyst recyclability and stability for practical applications. In the present study, we used a very small amount of the 1@CNT/C60-dendron composite (0.07 mg) in a relatively large volume of aqueous solution (150 mL). This makes it technically challenging to recover the photocatalyst after the reaction through filtration using a membrane filter. However, as demonstrated in Fig 6, the photocatalytic activity can be reproduced after stopping and restarting the ammonia decomposition reaction. This suggests that the photocatalyst may possess potential for recyclability. We agree that a thorough investigation of photocatalyst recyclability and long-term stability is essential. However, due to the aforementioned practical limitations in the current experimental setup, we have decided to defer a detailed discussion of these aspects to a future publication. In that work, we will explore the scalability of the photocatalytic system and optimize the reaction conditions for enhanced efficiency. The recyclability and stability of the photocatalyst under optimized conditions will be a central focus of that investigation.
5) Some related latest literature should be updated, such as:
- Li J, Sheng B, Chen Y, Yang J, Wang P, Li Y, Yu T, Pan H, Qiu L, Li Y, Song J. Utilizing full-spectrum sunlight for ammonia decomposition to hydrogen over GaN nanowires-supported Ru nanoparticles on silicon. Nature Communications. 2024 Aug 27;15(1):7393.
- Ofuchi Y, Mitarai K, Doi S, Saegusa K, Hayashi M, Sampei H, Higo T, Seo JG, Sekine Y. Hydrogen production by NH 3 decomposition at low temperatures assisted by surface protonics. Chemical Science. 2024.
Thank you for bringing these relevant and recent publications to our attention. We have incorporated both references (as ref #4 and #5) into the revised manuscript. Furthermore, we have taken the opportunity to thoroughly revise the introduction section to reflect the current state of the field and better contextualize our work within the latest advancements in ammonia decomposition for hydrogen production, including the studies highlighted by the reviewer. We believe that the updated introduction provides a more comprehensive and up-to-date overview of the field, strengthening the manuscript's overall contribution.
6) On what scale did you use this method? What problems do you face on a large scale?
The reaction was performed in a 150 mL aqueous solution containing 13.8 mmol of ammonia and approximately 0.07 mg of the CNT-based photocatalyst. The light-receiving area of the reaction vessel was 11.80 cm². These details are provided in the experimental section of the manuscript.
Scaling up this photocatalytic system presents several challenges. Firstly, a larger reaction vessel with a significantly increased light-receiving area would be required. This necessitates modifications to the reactor design and potentially the use of specialized light sources.
Furthermore, transitioning from a particle dispersion system to a more robust and practical configuration, such as a photocatalyst-sheet device with immobilized photocatalyst, would be advantageous for large-scale applications. This could involve techniques like coating the photocatalyst onto a suitable substrate or developing thin-film photocatalytic materials.
Additionally, enhancing the stability of the photocatalyst is crucial for long-term operation in a scaled-up system. As mentioned in our response to a previous comment, strategies such as coating the dye@CNT/C60 core with an inorganic semiconductor shell could improve robustness and prevent degradation. We have added a discussion of these scalability considerations and potential solutions to the conclusion section of the revised manuscript.
7) Compare your method with other reported methods in terms of time, efficiency, and cost in the table
We appreciate the reviewer's suggestion to compare our method with other reported methods in a tabular format. Frankly, the current photocatalytic system is still in its early stages of development and requires further optimization before a meaningful comparison of efficiency and cost can be made with established methods. However, in response to the reviewer's comment, we have included a brief comparison in the conclusion section with the work of Zhou et al., which, to our knowledge, reports the highest efficiency for photocatalytic ammonia decomposition to date (ref. 4). We have specifically focused on comparing the solar-to-hydrogen (STH) efficiency, a key metric for evaluating the overall performance of solar-driven hydrogen production systems.
8) The conclusion has generic statements. It is recommended that authors add detail with more robust content.
We appreciate the reviewer's feedback regarding the conclusion section. We have carefully revised the conclusion to provide more specific details and robust content, focusing on the key findings of the study, its limitations, and future research directions.
9) Some English correction is needed throughout the manuscript.
Thank you for pointing out the need for English language corrections. We have carefully reviewed the entire manuscript and made the necessary revisions to improve grammar, clarity, and overall readability.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript presents a potential catalyst for the photocatalytic ammonia decomposition process. The authors have proved the high potential of dye1@/CNT/C60-dendron in hydrogen production from NH3. However, some data must be added and discussed to support the authors’ evaluation.
1. The role of the C60-dendron needs to be discussed and elucidated.
_ In Fig. 3, the activity of 1@CNT should be included.
_In Fig. 4, the UV-vis spectrum of 1@CNT needs to be shown.
2. In Fig. 5, with C60-dendron addition, the authors mentioned that the H2 evolution is through photoinduced electron transfer from 1 to C60 and Ru3+. The evidence for this comment needs to be included.
3. The equation to calculate the STH efficiency must be shown.
4. In Fig.6, after 26 h of reaction, the pH was recovered to 11.2 with KOH after dropping to 10.4; however, the activity cannot recover as the initial. The authors need to give a discussion about this matter.
Author Response
We have revised the manuscript according to the suggestions made by the reviewers. Now I would like to send back you our revised manuscript (catalysts-3185524). The revised portions are colored in red in the manuscript. The detailed replies to the reviewer’s comments are listed as follows:
1) The role of the C60-dendron needs to be discussed and elucidated.
Thank you for highlighting this important point. We agree that the role of C60-dendron in the photocatalytic system requires further clarification. Following the reviewer's suggestion, we have expanded the discussion on the role of C60-dendron in the text (lines 102-113 on page 4). As you correctly pointed out, the photocatalytic reaction does not proceed without the presence of C60-dendron. The amphiphilic nature of C60-dendron allows it to physically adsorb onto the surface of the dye-encapsulated CNTs. This adsorption serves two crucial purposes:
*1. Dispersion of CNTs in water: The hydrophobic CNTs become effectively dispersed in the aqueous solution due to the hydrophilic dendron moiety of C60-dendron.
*2. Formation of a heterojunction: The adsorption of C60-dendron creates a dye@CNT/C60 heterojunction, which is essential for efficient charge separation and photocatalysis. This p-i-n type heterojunction facilitates the extraction of photoexcited electrons from the encapsulated dye to the C60 moiety. This electron transfer process generates the electron-hole pairs necessary for driving the photocatalytic ammonia decomposition reaction.
While we have previously reported on the details of this photoinduced electron transfer mechanism, we acknowledge that it was not explicitly described in the current manuscript. To avoid any confusion for the reader, we have now included a more detailed explanation in the revised text.
-) In Fig. 3, the activity of 1@CNT should be included.
We appreciate the reviewer's suggestion to include the activity of 1@CNT in Figure 3 to highlight its lack of activity. As mentioned previously, using 1@CNT alone, i.e., without C60-dendron, results in no photocatalytic activity. This is because the 1@CNT, lacking the amphiphilic C60-dendron, does not disperse well in water and fails to form the necessary charge-separated state required for the photocatalytic reaction. This observation aligns with our previous findings in photocatalytic water splitting (ref. 14), where the presence of C60-dendron was crucial for activity. While including the data for 1@CNT in Figure 3 could visually demonstrate its inactivity, we believe that emphasizing this crucial finding through a detailed textual explanation will be more impactful and avoid potential clutter in the figure. Therefore, we have opted to strengthen the textual description instead.
We have revised the text (lines 102-113 on page 4) to clearly articulate the lack of activity observed when using 1@CNT alone and connect it directly to the essential dual role of C60-dendron in the photocatalytic system.
-) In Fig. 4, the UV-vis spectrum of 1@CNT needs to be shown.
We appreciate the reviewer's suggestion to include the UV-vis spectrum of 1@CNT in Figure 4 for a more comprehensive comparison. Following the reviewer's recommendation, we have revised Figure 4 to include the absorption spectrum of 1@CNT.
2) In Fig. 5, with C60-dendron addition, the authors mentioned that the H2 evolution is through photoinduced electron transfer from 1 to C60 and Ru3+. The evidence for this comment needs to be included.
We appreciate the reviewer's request for further evidence supporting the proposed photoinduced electron transfer mechanism from dye 1 to C60 and subsequently to Ru(III) in the presence of C60-dendron, as shown in Figure 5. As mentioned in the manuscript, our previous studies on photocatalytic water splitting using a similar system have provided strong evidence for this mechanism (ref. 14). In those studies, we employed various spectroscopic techniques, including transient absorption spectroscopy and electrochemical measurements, to demonstrate the photoinduced electron transfer from the dye to C60 and the subsequent electron transfer to the Ru(III) co-catalyst. Furthermore, in the current study on ammonia decomposition, we have observed that the reaction does not proceed in the absence of either C60-dendron or the Ru co-catalyst. This observation further supports the proposed mechanism, as it highlights the essential roles of both C60 as an electron acceptor and the Ru complex as the catalytic center for Hâ‚‚ evolution.
To strengthen the manuscript and provide further clarity, we have added a more detailed explanation of the evidence supporting the proposed mechanism in the text (lines 102-113 on page 4).
3) The equation to calculate the STH efficiency must be shown.
We appreciate the reviewer's comment regarding the calculation of the solar-to-hydrogen energy conversion (STH) efficiency. Following the reviewer's suggestion, we have added the equation used to calculate the STH efficiency in the experimental section of the manuscript (lines 223-232 on page 7-8). We have also included a brief explanation of the parameters involved in the calculation to ensure clarity and transparency. Furthermore, to enhance the reproducibility of our results, we have provided the specific values used for each parameter in the STH efficiency calculation for the reported experiment.
4) In Fig.6, after 26 h of reaction, the pH was recovered to 11.2 with KOH after dropping to 10.4; however, the activity cannot recover as the initial. The authors need to give a discussion about this matter.
We appreciate the reviewer's insightful comment regarding the observed decrease in activity after the pH was adjusted back to 11.2 following a drop to 10.4 during the ammonia decomposition reaction, as shown in Figure 6. We agree that this observation is crucial and warrants further discussion. We believe that the main reason for the reduced activity after pH adjustment is the gradual deactivation of the RuCl₃ co-catalyst under alkaline conditions. While the rate of deactivation is relatively slow when the ammonia concentration is high, it appears to accelerate as the ammonia concentration decreases. Similar deactivation behavior has been observed in our previous studies on photocatalytic water splitting using Ru(III) co-catalysts.
To address the reviewer's comment and highlight the importance of this observation, we have added a discussion of this phenomenon in the manuscript (lines 174-180 on page 6).
Round 2
Reviewer 1 Report
Comments and Suggestions for Authors-
Comments on the Quality of English LanguageMinor editing of English language required.
Author Response
1) Minor editing of English language required.
Response: We appreciate the reviewer's suggestion regarding English language improvements. We have thoroughly reviewed the entire manuscript and made necessary revisions to enhance grammar, clarity, and overall readability.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript is improved after revision. The current manuscript can be accepted for publication.
Author Response
We are grateful for the reviewer's positive assessment of our revised manuscript. We sincerely appreciate the time and effort invested in reviewing our work, which has undoubtedly contributed to improving the quality of our paper. We are delighted to hear that the current version of the manuscript is deemed suitable for publication. Thank you for your valuable input throughout this process.
