Effect of Precious Metals on NO Reduction by CO in Oxidative Conditions
, Pirault-Roy Laurence 2, Damien P. Debecker 3, François Devred 3, Renaud Cousin 1 and Christophe Poupin 1,*
Round 1
Reviewer 1 Report
This is an interesting MS with useful results. There are some suggestions for improving the quality of this MS, as shown in follows.
- The keywords did not exist in the abstract, please recheck it again.
- In pages 2 and 3, lines 85-93, it is confused to know why authors want to present. Please recheck these sentences.
- In the section 2, the number of sub-title is wrong, please revise these mistakes.
- In table 1, what is the mean of "acac"? Please explain this term below table 1.
Author Response
Dear Reviewer,
We thank you for your careful reading of this manuscript and for your valuable remarks and suggestions.
* The keywords did not exist in the abstract, please recheck it again.
Answer:
The abstract was modified and the keywords exist now.
* In pages 2 and 3, lines 85-93, it is confused to know why authors want to present. Please recheck these sentences.
Answer:
We agree with the reviewers: these sentences must be removed. They are in the Applied Science template to indicate what to do. We apologized for this error.
* In the section 2, the number of sub-title is wrong, please revise these mistakes.
Answer:
The wrong number of sub-title has been fixed in the new version of the article.
* In table 1, what is the mean of "acac"? Please explain this term below table 1.”
Answer:
The meaning of acetylacetonate (acac) was added for table 1.
Reviewer 2 Report
This article reports synthesis of a series of noble metal nanoparticle catalysts supported on titania for NO reduction by CO in order to purify CO2. This reaction is of importance for CO2 reforming reactions, making the work certainly of potential broad interest to readers of Applied Sciences. However, the manuscript suffers from lack of clarity in much of the text (see specific comments below) and multiple issues with the use of English. More importantly, the TEM images presented in Fig 2 do not support the conclusion of ~2 nm small particle size distributions reported in the accompanying histograms. Moreover, the lack of discussion about why Pt performs best at NO reduction makes what should be the obvious novelty of the results not apparent. Therefore, I recommend a major revision before acceptance.
Specific comments/questions:
line 35: What does "impact of 79%" mean? Does it refer to the relative contribution of CO2 to climate change? Also, a reference for this statistic should be added.
line 49: How does the separation of O2 from air result in flue gas being dominated by CO2? Is it because the NOx side reactions are eliminated? This statement needs clarification.
line 56: The statement that NO is a poor reductant of CO over noble metal catalysts based on ref. [4] seems to contradict the subsequent conclusion that the NO/CO system could purify CO2.
line 79: "SCR reaction" has not been previously defined.
line 85-93: remove the template text
line 108: define BET, BJH methods
line 130: the equation to calculate particle size doesn't make sense. Were the particle sizes directly measured from the TEM images? Was software such as ImageJ used or were the particle diameters measured manually? Why was a cubic morphology chosen? Most metal particles on supported catalysts typically have spherical morphology (and that's what is seen in Figure 2).
line 131: it is unclear what "correlation between the metallic accessibility determined by hydrogen adsorption..." means. What correlation? Is the particle size also needed to calculate the metallic accessibility reported in Table 3?
line 145: were the catalysts sieved to remove both large and small grains or just to remove large grains? Was this to ensure that similar catalyst surface area was present for all reactions?
line 152: What type of analyzer is used for NO? Is it IR, mass spec,...? and why is a separate analyzer used for NO as compared to N2 and CO2?
line 183: Figure 2 does not support the conclusion of "homogeneous distribution of particle size." First off, obvious metal nanoparticles are not even visible in the TEM image for Pd. This may arise from the reported small particle sizes in the distribution, but a clear image showing the ~2 nm particles must be included. Second the TEM image for the Pt particles appears to show some large ~20 nm particles (assuming the Pt are the black regions). Again, no small particles are visible in the image. In the Rh sample, are the white dots in the image the Rh particles? I would expect them to be darker than the support (and there are some dark small particles). If not, what are the white spots from? Finally, a TEM image of Ru sample should be included since particle size estimation from TEM is reported in Table 3.
line 211: does the lack of Pd reduction peak at 80 C indicate that there was no Pd hydride formed?
line 214: the lack of Pt reduction peak seems unexpected from the statement "generally a reduction peak is found by TPR". Why is there no reduction peak? Does this just mean the reduction step went to completion during synthesis? In that case, why do most studies see a reduction peak?
line 220: Is the IrO2 or Ir0 phase the active phase for CO oxidation? Not clear from the text.
line 228: What is a "light-off curve"? What are the products of the three reactions listed in Fig. 4?
line 235: Was NO present in the reaction mixture for the results shown in Figure 5?
line 241: how does the CO oxidation activity trend of Pt>Pd... etc. compare to what has been reported in the literature?
line 252: is the CO oxidation activity reported in each panel of figure 6 the same data as in figure 5?
line 289: the discussion of better activity of Ir as compared to Pd seems to be consistent with previous literature, so it is unclear what, if anything, is new. In contrast, there is no mention of why Pt would be better than any other element. Since this seems to be a key new result of the paper, discussion of why Pt is best should be included.
Author Response
Dear Reviewer,
We thank you for your careful reading of this manuscript and for your valuable remarks and suggestions.
The manuscript has been proofread and corrected by all the authors for greater clarity and the errors in English should be corrected. The specific comments are all taken into account and the answers are in the following specific questions.
Specific comments/questions:
- line 35: What does "impact of 79%" mean? Does it refer to the relative contribution of CO2 to climate change? Also, a reference for this statistic should be added.
Answer:
The meaning of ”impact of 79%” i.e. representing 79% of greenhouse gases emitted was added as well as a reference from literature.
- line 49: How does the separation of O2 from air result in flue gas being dominated by CO2? Is it because the NOx side reactions are eliminated? This statement needs clarification.
Answer:
The statement was simplified to clarify. In the oxyfuel system, there is indeed only small amount of NOx formation.
- line 56: The statement that NO is a poor reductant of CO over noble metal catalysts based on ref. [4] seems to contradict the subsequent conclusion that the NO/CO system could purify CO2.
Answer:
Although NO is a poor reductant of CO over noble metal catalysts, the NO/CO reaction is possible with the advantage that no additional reducers have to be introduced. This is a challenge that we tried to lift up.
- line 79: "SCR reaction" has not been previously defined.
Answer:
"SCR reaction" was defined.
- line 85-93: remove the template text
Answer:
The template text was removed. We apologized for this error.
- line 108: define BET, BJH methods
Answer:
The meaning of BET and BJH methods were added: “Brunauer-Emmett-Teller (BET) and Barett–Joyner–Halenda (BJH) methods”.
- line 130: the equation to calculate particle size doesn't make sense. Were the particle sizes directly measured from the TEM images? Was software such as ImageJ used or were the particle diameters measured manually? Why was a cubic morphology chosen? Most metal particles on supported catalysts typically have spherical morphology (and that's what is seen in Figure 2).
Answer:
The equation to calculate average particle size hasn’t any sense because sub and superscripts were missing; however, the equation was removed as we added that we used ImageJ software. The particle sizes were measured from the TEM images by imageJ. and compared to those calculated from H2 chemisorption. For chemisorption, the classical cubic morphology was chosen in order to estimate the particle size assuming that the particles are homogeneous. When the dispersion is important (particles smaller than 2 nm) no significant change in the particles sizes was been observed whatever the model used (cubic or spherical) but the lowest the dispersion is, the highest the difference between the two model is.
For example for platinum considering that the accuracy of H2 chemisorption is around 3%, the size is around 2.3 ± 0.2 nm using the cubic model and 2.8 ± 0.2 nm using the spherical model, For ruthenium, the mean particle size is 3.8 ± 0.5 nm using the cubic model and 4.6 ± 0.5 nm using the spherical model, In general, the cubic model is preferred as it is close to the shape of particles which are never spherical. As the authors evidenced (Baletto, F.; Ferrando, R.; Fortunelli, A.; Montalenti, F.; Mottet, C. Crossover among structural motifs in transition and noble-metal clusters. J. Chem. Phys. 2002, 116, 3856−3863 and ref Le Valant et al, J. Phys. Chem. C 2016, 120, 46, 26374-26385) many molecular dynamics calculations for metal particles show that, for very small sized Pt clusters (<100 atoms) the noncrystalline icosahedron structure is the most stable shape. Then, beyond a certain cluster size (>100 atoms), the equilibrium shape changes from an icosahedron to a noncrystalline truncated decahedron structure (Marks decahedron and perfect truncated decahedron, respectively), and finally, for a large size (>6500 atoms), the equilibrium shape changes from a noncrystalline truncated decahedron structure to an fcc-crystalline truncated octahedron structure. Then, it appears that the cuboctahedron shape (ref Le Valant et al, J. Phys. Chem. C 2016, 120, 46, 26374-26385) can perfectly mimic the evolution of surface atoms which are involved in H2 chemisorption, as a function of the cluster size. Le Valant et al (ref Le Valant et al, J. Phys. Chem. C 2016, 120, 46, 26374-26385) developed a more complex model to determine the particles size from chemisorption using H/Pt value with a serious added value for very small particles. The model was generalized to other noble metals (Pd, Rh, Ir) (Epron et al, Materials, MDPI, 2018, ⟨10.3390/ma11050819⟩) . For larger particles, as the ones on our catalysts, the cubic model gave results quite similar to the ones of this new model.
- line 131: it is unclear what "correlation between the metallic accessibility determined by hydrogen adsorption..." means. What correlation? Is the particle size also needed to calculate the metallic accessibility reported in Table 3?
Answer:
To consolidate the results, the mean particles size was determined using two different and independent techniques: H2 chimisorption and TEM. From TEM, the mean particles size is obtained by calculating the surface average diameter. For H2 chemisorption, a model should be applied (here the cubic one) to deduce the mean particles size from the accessibility values.
- line 145: were the catalysts sieved to remove both large and small grains or just to remove large grains? Was this to ensure that similar catalyst surface area was present for all reactions?
Answer:
The catalysts were sieved in order to retain grains with diameters between 0.315 and 0.500 mm and diluted to a constant volume by SiC to avoid intra- or extra-granular diffusion limitation effects
- line 152: What type of analyzer is used for NO? Is it IR, mass spec,...? and why is a separate analyzer used for NO as compared to N2 and CO2?
Answer:
The Xentra 4900C (Servomex) is an IR analyzer. This information is now mentioned in the text.
- line 183: Figure 2 does not support the conclusion of "homogeneous distribution of particle size." First off, obvious metal nanoparticles are not even visible in the TEM image for Pd. This may arise from the reported small particle sizes in the distribution, but a clear image showing the ~2 nm particles must be included. Second the TEM image for the Pt particles appears to show some large ~20 nm particles (assuming the Pt are the black regions). Again, no small particles are visible in the image. In the Rh sample, are the white dots in the image the Rh particles? I would expect them to be darker than the support (and there are some dark small particles). If not, what are the white spots from? Finally, a TEM image of Ru sample should be included since particle size estimation from TEM is reported in Table 3.
Answer:
It is well known that metallic particles are not easily observable on alumina by TEM. Therefore the homogeneity is not so clear and then we put:“ quite homogeneous distribution of particle size”. We put a better image for the Pd sample. For the Pt sample, black small particles are visible, and the white dots on Rh and Ru images correspond to the small metallic particles. The TEM image and the size distribution of the Ru/Al2O3 sample were added in Fig.2.
- line 211: does the lack of Pd reduction peak at 80 C indicate that there was no Pd hydride formed?
Answer:
The lack of a negative Pd reduction peak at 80°C is a clear indication that there was no Pd hydride formed.
- line 214: the lack of Pt reduction peak seems unexpected from the statement "generally a reduction peak is found by TPR". Why is there no reduction peak? Does this just mean the reduction step went to completion during synthesis? In that case, why do most studies see a reduction peak?
Answer:
All the catalysts were reduced at the end of the preparation and stored under air. According to their oxidation resistance and their particles size, some metals can be re-oxidized at room temperature under air. So, it is important to check the oxidation state of the metals before the NO/CO reaction using TPR technique. As it concerns platinum, the lack of reduction peak means that the reduction step went to completion during synthesis and that no reoxidation occurred under air at room temperature or so weakly that the Ar treatment before TPR can remove the oxygen(ref. 21). This result is important because “these Pt° nanoparticles should then be especially active for CO oxidation.”
The text was modified to be clearer.
- line 220: Is the IrO2 or Ir0 phase the active phase for CO oxidation? Not clear from the text.
Answer:
It seems that the IrO2 phase is easily reducible (see TPR) so Ir (0) must be the active phase in our reaction. M. Haneda et al. (Journal of Molecular Catalysis A: Chemical 256 (2006) 143–148) have shown that NO reduction by CO occurs on stable iridium metal sites.
line 228: What is a "light-off curve"? What are the products of the three reactions listed in Fig. 4?
Answer:
The light-off curves are conventional conversion curves in temperature rise given for oxidation reactions such as that of CO.
- line 235: Was NO present in the reaction mixture for the results shown in Figure 5?
Answer:
Yes, NO was present in the reaction mixture for the results shown in Figure 5 as it’s mentioned in the given feedstream composition.
- line 241: how does the CO oxidation activity trend of Pt>Pd... etc. compare to what has been reported in the literature?
Answer:
The oxidation of CO was investigated on noble metal catalysts (Pt, Pd, Ir, Rh and Au) supported on TiO2 by V.P. Santos et al. (Applied Catalysis B: Environmental 99 (2010) 198–205) and they found on metallic particles between 2-4 nm the activity order: Pt > Pd > Ir >Rh
- line 252: is the CO oxidation activity reported in each panel of figure 6 the same data as in figure 5?
Answer:
Yes, the CO oxidation activity reported in each panel of fig 6 is the same data as in fig 5.
- line 289: the discussion of better activity of Ir as compared to Pd seems to be consistent with previous literature, so it is unclear what, if anything, is new. In contrast, there is no mention of why Pt would be better than any other element. Since this seems to be a key new result of the paper, discussion of why Pt is best should be included.
Answer:
To highlight the novelty of the results the text was changed at the end of section 3.4
”The results reported in Fig. 7 evidenced that Pt/Al2O3 is the best catalyst for reducing NO by CO in oxidizing environment. Moreover, this interesting performance occurs at a lower temperature than those of the other catalysts. The better performances of Pt and in a lesser extent Ir could be correlated with the TPR results where Pt and Ir were evidenced to be less oxidized than the other catalysts. As expected, the more metallic the species is and remains, the better the activity for NO reduction is despite these oxidizing operating conditions.”
And in the conclusion:
“The better performances for NO reduction were obtained for Pt/Al2O3 catalyst for which the metallic state of particles was evidenced in TPR. The results clearly demonstrated that the more metallic the species is and remains, the better the activity for NO reduction is despite these oxidizing operating conditions. The work is still in progress but it has already proven that NO/CO reaction could be achieved without injection of additional reducers such as ammonia, which can facilitate the implementation of the process and lower the operating cost.”
Reviewer 3 Report
The work appears to be sound. Only minor revisions may be required, but the English, including spelling and punctuation, requires correction.
Specific comments
Line 130
Proper subscripting is required.
Line 215
The statement is unclear. Pt NPs are usually covered by oxide under ambient conditions, so it is no surprise that they can be reduced with H2. Why would this affect the activity for CO oxidation.
Line 220
See previous comment.
Line 228
What is the meaning of "light-off?" It would be better to use a more standard term.
Author Response
Dear Reviewer,
We thank you for your careful reading of this manuscript and for your valuable remarks and suggestions.
The specific comments are all taken into account and the answers are in the following specific questions.
Specific comments
Line 130
Proper subscripting is required.
Answer:
In the equation to calculate average particle size, sub and superscripts were missing; However, the equation was removed as we added that we used ImageJ software. We apologized for this error.
Line 215
The statement is unclear. Pt NPs are usually covered by oxide under ambient conditions, so it is no surprise that they can be reduced with H2. Why would this affect the activity for CO oxidation.
And Line 220
See previous comment.
Answer:
The statement was modified. Platinum species easily reduced are often more active for oxidation reactions.
Line 228
What is the meaning of "light-off?" It would be better to use a more standard term.
Answer:
The light-off curves are conventional conversion curves in temperature rise given for oxidation reactions such as that of CO.
Reviewer 4 Report
The paper deals with the identification of the best metal catalysts for CO2 purification in 300 oxidative conditions. The authors focused on the NO reduction on alumina supported precious metal using carbon monoxide as reduction agent in oxidizing conditions. However, the paper should be revised, because the main chapters are poor detailed and is not easy to understand what the authors wanted to explain.
In the introduction, a model from the Instructions for the authors remained in the manuscript and should be removed (p2 and p3). At Experimental part, the synthesis of catalysts are not detailed at all.Is difficult to understand how the catalysts have been synthesised. This chapter should be presented in detail.
Also, the discussion of the results are not adequately presented. They should be detailed and the authors should put into evidence the main important points of each results they obtained.
The conclusions are too poor. this chapter should be expanded.
Based on these comments, I suggest to the authors a major revision of the paper and to re-submit the revised paper.
As final remark, I suggest to the authors to major revise the paper and to clear answer at the main chapter of the paper.
Author Response
Dear Reviewer,
We thank you for your careful reading of this manuscript and for your valuable remarks and suggestions.
The manuscript has been proofread and corrected by all the authors for greater clarity. The specific comments are all taken into account and the answers are following.
* However, the paper should be revised, because the main chapters are poor detailed and is not easy to understand what the authors wanted to explain.
Answer:
The introduction, results part and conclusion were developed to highlight the novelty of the work and the important points resulting from the study.
* In the introduction, a model from the Instructions for the authors remained in the manuscript and should be removed (p2 and p3).
Answer:
The template text was removed. We apologized for this error.
* At Experimental part, the synthesis of catalysts are not detailed at all. Is difficult to understand how the catalysts have been synthesised. This chapter should be presented in detail.
Answer:
More details were added in the experimental part. The catalysts were prepared by a classical wet impregnation. You can see for example reference “C. Poupin, R. Maache, L.Pirault-Roy, R.Brahmi, C.T.Williams, Applied Catalysis A: General, Volume 475, 5 April 2014, Pages 363-370”.
* Also, the discussion of the results are not adequately presented. They should be detailed and the authors should put into evidence the main important points of each results they obtained.
Answer:
The TPR results were discussed and correlated with the activity observed for NO reduction (see the end of section 3.4)
* The conclusions are too poor. this chapter should be expanded.
Answer:
The conclusion was developed to highlight the novelty of the work and the important points resulting from the study.
Round 2
Reviewer 2 Report
The authors have addressed my major comments in their revision. The only issue that hasn't been completely fixed is that the TEM images in Figure 2 still do not clearly show the small nanoparticles. It would be helpful to have an inset that shows the small particles for clarity.
Author Response
Dear Reviewer,
We thank you for your valuable suggestion.
We have found clearer pictures of Pt, Rh and Ru / Al2O3. The photo of the Pt at 50 nm has therefore been changed to that at 20 nm where the small particles are more visible. For the both Rh and Ru / Al2O3, we put images with the dark original particles without the white pointing squares from ImageJ. The TEM images in Figure 2 are now homogeneous and clearly show the small nanoparticles.
Best regards
Reviewer 4 Report
The paper has been improved a lot. The authors corrected the english, added many clarifications and removed the non-useful details.
I appreciate that this paper could be published in the present version.
Author Response
Dear Reviewer,
We thank you again for your careful reading of this manuscript and for your valuable remarks and suggestions.
We have still found clearer pictures of Pt, Rh and Ru / Al2O3. The TEM images in Figure 2 are now more homogeneous and clearly show the small nanoparticles.
Best regards
