A Comprehensive Study of Coke Deposits on a Pt-Sn/SBA-16 Catalyst during the Dehydrogenation of Propane

Round 1

Reviewer 1 Report

Ruelas-Leyva et al. developed bimetallic Sn-Pt/SBA-16 catalysts (containing 0.5 wt% of Pt and 0, 1 or 2 wt% of Sn) for the propane dehydrogenation, and studied deactivation of the most active one during this reaction due to coke formation and active phase sintering. The use of several characterization methods (TGA, Raman, XPS and TEM) made it possible to distinguish two types of formed coke - soft and hard, which had different effects on the activity of the investigated catalyst. The manuscript is quite interesting, but it contains some ambiguities that need to be explained before possible acceptation for publication in the Catalysts journal.

1. How can you explain the large deficits in the Pt content in the tested catalysts compared to the intended values? The temperature of the heat treatment (400°C) was too low to expect Pt sublimation. On the other hand, the Sn content in the 0.5-Pt-2-Sn/SBA-100 sample is also lower than expected. This effect was not explained.
2. Demonstration of adsorption isotherms would be interesting for a full understanding of the differences in porosity. Furthermore, the manuscript does not actually explain the influence of the temperature of the hydrothermal treatment on the structure of the SBA-16 material. The analysis of the added isotherms could help clarify this issue.
3. No attention was paid to selectivity during catalytic studies. The process of propane dehydrogenation is never 100% selective to propylene. Significant amounts of by-products are always formed. Hence, it is very important to analyze the reaction selectivity profile. From this point of view, an interesting issue would be the change of selectivity with time-on-stream.
4. The catalytic parameter which Authors call "yield" is actually "selectivity to propene". I am asking for the correction not to use incorrect terminology in the catalytic journal. The propene yield means an amount of propene formed related to an amount of propene that could theoretically be produced.
5. Why was hydrogen used as an additive in the catalytic tests? It should be remembered that the dehydrogenation process is controlled by thermodynamic equilibrium, and the introduction of one of the products of this reaction causes a reduction in the possible yield of propylene. From this point of view, should the tests not be carried out in the presence of water vapor, which is usually used as a heat carrier and a component reducing the partial pressures of reactants in dehydrogenation processes?
6. What is the role of the individual components of the active phase in the catalyzed process?
7. It is not possible to reliably determine the value of binding energies with accuracy of 0.01 eV. Therefore, please correct all Eb values presented in the text to one decimal place.
8. The statement "There was not peak of C 1s for the fresh catalyst" (line 280) is very imprecise. In the XPS spectrometer there is always a certain amount of hydrocarbons which, after adsorption on the measuring surface of the sample, give the C 1s photoemission signal. This signal is also visible for the fresh sample (cf. Figure 11a). By the way, how was the binding energy scale calibrated in the measured spectra?
9. The "Real Sn / Pt (wt% / wt%)" column in Table 1 is useless as it duplicates the information from the previous column.
10. Pluronic is a trade name and should be written with a capital letter (“Pluronic”) as such.
11. Correct “408.96 g and distilled water” (line 393).
12. The manuscript contains a lot of linguistic errors and should be deeply checked by a native speaker before submitting the revised version.

Author Response

Ruelas-Leyva et al. developed bimetallic Sn-Pt/SBA-16 catalysts (containing 0.5 wt% of Pt and 0, 1 or 2 wt% of Sn) for the propane dehydrogenation, and studied deactivation of the most active one during this reaction due to coke formation and active phase sintering. The use of several characterization methods (TGA, Raman, XPS and TEM) made it possible to distinguish two types of formed coke - soft and hard, which had different effects on the activity of the investigated catalyst. The manuscript is quite interesting, but it contains some ambiguities that need to be explained before possible acceptation for publication in the Catalysts journal.

1- How can you explain the large deficits in the Pt content in the tested catalysts compared to the intended values? The temperature of the heat treatment (400°C) was too low to expect Pt sublimation. On the other hand, the Sn content in the 0.5-Pt-2-Sn/SBA-100 sample is also lower than expected. This effect was not explained.

Answer: In our method used to prepare the catalysts first the theoretical quantities of booth precursor are diluted in the same solution, and then added dropwise with manual stirring to the support (SBA-16). The water used to dilute the precursors is the minimum required to fill the pore volume, and in occasions the solution resulting was highly concentrated. Therefore, the walls of the materials (beaker, spatula and syringe) were easily impregnated with the metallic precursors instead of the support resulting in a minor impregnation than the calculated.  

2- Demonstration of adsorption isotherms would be interesting for a full understanding of the differences in porosity. Furthermore, the manuscript does not actually explain the influence of the temperature of the hydrothermal treatment on the structure of the SBA-16 material. The analysis of the added isotherms could help clarify this issue.

Answer: A brief discussion (with figure) about adsorption isotherms was added to the manuscript as follows:

“N₂ adsorption isotherms are presented in Figure 1 and textural properties of the catalysts are summarized in Table 1. Both, the textural properties and the hysteresis loops depend strongly on the hydrothermal temperature used during the synthesis of SBA-16. In the case of 100 °C, the adsorption isotherm (Type IV), the hysteresis width and the textural properties suggest a cage-like pore shape characteristic of well-ordered SBA-16 structure [16]. The surface area (SBET) for the support was greater when the hydrothermal treatment temperature was 100 °C. The average pore size (Dp) and the total pore volume (Vp) increased when treatment temperature increased to 140 °C, but the hysteresis loop changed from H2 type to H1 type, associated to agglomerates of uniform spheres in regular array [17]. In the case of supported catalysts, the addition of 0.5 wt. % of Pt and 1 wt. % of Sn to SBA-100 had a small effect on the textural properties. In all cases the SBA-16 structure was preserved. The SBET, Dp, and Vp of the catalysts 0.5-Pt-1-Sn/SBA-60, -100 and -140 were practically the same as those of the support synthesized at their corresponding hydrothermal pretreatment temperature. For samples 0.5-Pt-0-Sn/SBA-100 and 0.5-Pt-2-Sn/SBA-100 the SBET, Dp, and Vp were reduced significantly from the values of their corresponding support. In the case of 0.5-Pt-2-Sn/SBA-100 the loss in textural properties was 28% in SBET and Vp with respect to SBA-100. This is due to the agglomeration of Sn particles.”

3- No attention was paid to selectivity during catalytic studies. The process of propane dehydrogenation is never 100% selective to propylene. Significant amounts of by-products are always formed. Hence, it is very important to analyze the reaction selectivity profile. From this point of view, an interesting issue would be the change of selectivity with time-on-stream.

 

Answer: The selectivity was redefined and discussed in the following question

 

4- The catalytic parameter which Authors call "yield" is actually "selectivity to propene". I am asking for the correction not to use incorrect terminology in the catalytic journal. The propene yield means an amount of propene formed related to an amount of propene that could theoretically be produced.

 

Answer: The definition of yield was changed to selectivity. The authors would like to state that this is a parameter commonly defined in different ways in the literature; and when comparing with other works the definition used has to be carefully analyzed (https://doi.org/10.1016/j.jngse.2016.02.014; https://sci-hub.se/10.1021/acscatal.7b01584; https://doi.org/10.1016/j.apcata.2011.03.057; DOI: 10.1039/C4CY00951G). 

Action: the equation was renamed, the Figure 2 was modified and the word yield was changed in the text by selectivity.

 

5- Why was hydrogen used as an additive in the catalytic tests? It should be remembered that the dehydrogenation process is controlled by thermodynamic equilibrium, and the introduction of one of the products of this reaction causes a reduction in the possible yield of propylene. From this point of view, should the tests not be carried out in the presence of water vapor, which is usually used as a heat carrier and a component reducing the partial pressures of reactants in dehydrogenation processes?

 

Answer: The authors agree that the dehydrogenation process is controlled by thermodynamic equilibrium, and the introduction of one of the products of this reaction may cause a reduction in the yield of propylene as stated by the reviewer. Nonetheless, there are several side reactions occurring simultaneously making difficult to elucidate the role of the H₂. From previous work of our group, the C₃H₈/H₂ proportion has been investigated and we determined that a small amount of H₂ is required for an adequate performance of the catalysts. At last, in several Pt-based catalysts for this reaction H₂ is used in feed composition even in greater proportions (https://doi.org/10.1016/S1872-2067(19)63360-7).

 

6- What is the role of the individual components of the active phase in the catalyzed process?

 

Answer: The exact role of the individual components is still on debate. In our catalytic system, the active site responsible for the dehydrogenation of propane to propylene is the Pt-Sn alloy, nevertheless other alloys has been proposed as active site, for example the alloy Pt₃-Sn  (https://doi.org/10.1515/ijcre-2017-0247; https://doi.org/10.1515/ijcre-2017-0237; https://doi.org/10.1016/j.cattod.2013.10.064).

 

7- It is not possible to reliably determine the value of binding energies with accuracy of 0.01 eV. Therefore, please correct all Eb values presented in the text to one decimal place.

 

Answer: All the binding energies has been changed to one decimal place

 

8.A- The statement "There was not peak of C 1s for the fresh catalyst" (line 280) is very imprecise. In the XPS spectrometer there is always a certain amount of hydrocarbons which, after adsorption on the measuring surface of the sample, give the C 1s photoemission signal. This signal is also visible for the fresh sample (cf. Figure 11a).

 

Answer: Authors agree with the observation of the sentence “There was not peak of C 1s for the fresh catalyst" is imprecise. It has been changed into the document. “There is a small peak of C 1s for the fresh catalyst due to contamination in the air”.

 

8.B- By the way, how was the binding energy scale calibrated in the measured spectra?

 

Answer:  The sample surface was cleaned by 3 keV Ar+ ions for 3 min. Binding energies of all the peaks were corrected (or calibrated) using C 1s energy at 284.6 eV corresponding to adventitious (ubiquitous) carbon in addition to the charge compensation by the flood gun associated with the spectrometer.

 

9- The "Real Sn / Pt (wt% / wt%)" column in Table 1 is useless as it duplicates the information from the previous column.

 

Answer: The column was removed from Table 1.

 

10- Pluronic is a trade name and should be written with a capital letter (“Pluronic”) as such.

Answer: The modification has been made in the manuscript.

11- Correct “408.96 g and distilled water” (line 393).

Answer: The sentence “408.96 g and distilled water” was modified in the manuscript by “and 408.96 g of distilled water”

12- The manuscript contains a lot of linguistic errors and should be deeply checked by a native speaker before submitting the revised version.

 

Answer: The English was deeply checked and the modifications are marked.

Reviewer 2 Report

The process of propane dehydrogenation has been studied for decades, therefore, the authors must have found anything original either in catalyst preparation or performance. The only novelty item found by the authors is related to the study of different forms of coke deposits. The process is industrially important and the reasons behind the catalyst deactivation and possibilities of catalyst regeneration are vital for designing a benign catalytic system. The authors demonstrated that two froms of coke exist in the catalyst at different stages of the process and at different temperatures. In general, the paper is well presented and all necessary methods are used to reveal the working state of the catalyst. What is missing is the test with catalyst regeneration. It would be desirable to include such tests when regeneration is performed with "mild" and "hard" coke deposits.

Author Response

The authors completely agree with the reviewer about the relevance in the experiments of catalyst regeneration, nonetheless, the main objective of the work was to gain understanding in the nature of coke deposits during reaction. In addition, the authors are planning to assess this topic in a future study.

Round 2

Reviewer 1 Report

All my comments have been taken into account and the manuscript has been revised accordingly.

Reviewer 2 Report

The authors revised the manuscript according to the comments. Now it can be accepted for publication.