Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction
Round 1
Reviewer 1 Report
Reviewer Report
The current manuscript entitled “The activity and stability of Pd bimetallic catalysts for catalytic nitrate reduction” examined the series of Pd-based catalysts for nitrate reduction. The author’s synthesized the Sn/In promoted Pd catalysts with various ratios of the weight loading on Al2O3 by the impregnation method and evaluated its catalytic properties for the nitrate reduction. In addition, the authors investigated the effect of metal loading, the impregnation order, and varying the promoter on catalytic nitrate reduction. The activity and stability of the catalyst were evaluated using commercial drinking waters spiked with NO3- to establish a relationship between water composition and catalyst deactivation. The subject is of particular importance from the practical point of view and toward the rational design of catalysts for selective nitrate reduction.
I consider, however, the following questions need to be addressed before being accepted for publication.
The details revision are as follows.
- Authors claimed the formation of bimetallic catalysts. However, it did not show any evidence of the bimetallic formation. Although the Pd and Sn/Ir promoter weight loadings are less and difficult to get a good XRD pattern, especially with alumina support, it's better to change the terminology from bimetallic to Sn/Ir- promoted Pd/Al2O3.
- Is there any role of the Pd particle size on nitrate reduction and selectivity? The particle size/particle size distribution on alumina support would have given a better explanation of the nitrate conversion/selectivity.
- In Figure 3, XPS Pd0 binding energy varies from one to another; the authors should explain why there is a change in Pd0 binding energy. Do ions in the water influence the Pd0 binding energy.? If so, the authors should compare any co-relation between Pd0 binding energy shift and nitrate conversion/selectivity.
- Why there is a difference in catalyst selectivity with the order of impregnation? The authors should clearly discuss this point.
- Authors should discuss in more detail why the adequate composition of metal to promoter ratio is good for the reaction.?
- Authors should discuss the role of Al2O3 in this reaction? Why do authors choose Alumina over other supports?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors investigated the nitrate removal from water using Pd bimetallic catalysts. The authors did sufficient activity tests for this reaction, but the paper lacks scientific discussions. It is not clear what is the novelty of this work and what new insights are provided. This is a catalyst journal, but this work seriously lacks catalyst characterization and insight. Hence this work cannot be accepted in its present form.
1. “Aristizábal et al. [34] observed slight differences when the Pt–Ag/AC catalyst was first impregnated with Pt; it presented a larger particle size (4.1 and 1.7 nm with Pt(NH3)4(NO3)2 and H2PtCl6 as the Pt precursor, respectively) than did the catalyst impregnated with Ag prior to Pt (3.9 and 1.4 nm, respectively).” In this statement, there is no significant difference between the particle size, a difference of 0.3 nm is well within the error due to catalyst preparation, treatment, and particle size measurement.
2. In the abstract, please write what ratio is this (mole or wt) ? “with Pd:Sn ratios of 1:10 and 1:2”
3. In the introduction section, report these numbers in terms of molarity (50, 0.1, and 0.5 mg L-1 for NO3-, NO2-, and NH4+). It will be much easier to compare them with molarity.
4. The control experiment with monometallic Pd, In, and Sn is missing. Please provide those results
5. Do a control experiment with only the support and without any catalyst or support.
6. In Table 1, are the selectivity reported at the same conversion? The product selectivity should be compared at the same conversion.
7. The title mentions Pd bimetallic catalyst:
· No proof of bimetallic formation is provided
· How are In and Sn affecting the Pd bimetal activity?
8. Please provide the CO-uptake values, and use them to get the TOF values to compare the catalyst activity.
9. “The C, N, and S contents of the catalyst did not change after 60 h on stream. However, the H content was higher in the catalyst used with DW1 and DW3, which contained a higher HCO3- concentration” If the H content increased due to HCO3-, then why did the content of C not increase. How was the Hydrogen contribution from adsorbed water removed?
10. Table 4 title is missing. In table 4, what is the unit of concentrations of DW components
11. Authors claimed that the increase in the NH4+ selectivity with drinking water (DW) 1,2, and 3 was due to the presence of HCO3-. But the NH4+ selectivity for all the three DW samples was almost similar, while the concentration of HCO3- was significantly low in the DW 2 samples. Then how can this claim be made?
12. “Regarding the metal concentration on the catalyst surface, a significant Pd intensity loss after the reaction was observed in the spectra, regardless of the treated water. This decrease in the Pd intensity could be caused by fouling of the catalyst surface since a significant loss of total Pd content was not observed in the TXRF results”
· In this statement, it is not ideal for comparing the XPS peaks intensities of different spectra. The XPS peak intensities can change due to a lot of factors. But the atomic % analysis in Table 3 shows that the Pd % was almost constant in the fresh and the spent catalyst. Why are authors correlating the low Pd intensity to Pd site fouling?
13. “Thus, the deposition of the noble metal over the promoter metal, especially when the ratios were 10:1 and 2:1, favored the hydrogen spillover rate, increased the H concentration on the catalyst surface”
· In this statement, authors did not cite any previous work or present any data to support this statement. How is the Hydrogen spillover rate increasing?
14. “These results differ from those reported by Pintar et al. [35], who observed that impregnation with the promoter metal followed by the deposition of noble metal enhanced N2 production”
· What is the reason for this difference?
15. Please provide more catalyst characterization and try to correlate it with the catalyst activity.
16. The reviewer appreciates the work done by the authors and believes that this work can be improved to intrigue the readers.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
This article is devoted to obtaining catalysts for nitrate reduction. The article is relevant and interesting. Many of the authors' assumptions are well substantiated and solid, which significantly increases the quality of this article. Detailed analysis by physicochemical methods helps to understand the composition of these catalysts. Catalyst recycling experiments are important for catalyst research and also improve this work. The article may be of interest to specialists in the field of catalysis, physical chemistry and the transformation of nitro substances. I encourage the authors to clarify the following points:
1. Table 1. For the Sn1–Pd0.5 catalyst, many characteristics (for example, A(BET)) differ from other catalysts. Is it due to an experimental error or to the structure of this catalyst?
2. Did the authors make experiments on reproducibility? at least at one point.
3. Table 3. Is it possible, together with the atomic concentration, to give in this article the mass percentages for this catalyst?
4. It is desirable for the authors to point out in more detail the advantages of their bimetallic catalyst in comparison with those known from the literature.
5. What is the advantage of these particular catalysts over monometallic palladium catalysts?
6. What is the role of the support in this process?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I think the authors have adequately addressed the comments made by the reviewers in the revised version of the manuscript. Therefore, I have no further comments.
Reviewer 2 Report
Accept in present form
