Fischer–Tropsch Synthesis: Effect of CO Conversion over Ru/NaY Catalyst

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Fischer-Tropsch synthesis (FTS) is still one of the important techniques to convert the syngas derived from various carbon resources such as coal, biomass, natural gas, organic waste, and CO₂. The submitted manuscript presents that the CO conversion effect on CH₄ and C₅+ selectivities on Ru is the same as that on Fe and Co catalysts, while the effect on olefin formation is opposite. Although the innovation is not sufficient, the work is relatively systematic. I think this manuscript can meet the standard after minor revision, and the following issues should be carefully considered.

1) The manuscript describes that CH₄ selectivity decreases and C₅+ selectivity increases with increasing CO conversion on both Fe and Co catalysts. This expression is inappropriate. In FTS, the relationship of CH₄ and C₅+ selectivity with CO conversion is complex.

2) Are products such as water, oil and wax analyzed using the same chromatography as tailgas?  

3) What’s the fundamental reason for the different CO conversion effect on olefin formation over Ru in comparison with Co and Fe catalysts?

Author Response

Fischer-Tropsch synthesis (FTS) is still one of the important techniques to convert the syngas derived from various carbon resources such as coal, biomass, natural gas, organic waste, and CO₂. The submitted manuscript presents that the CO conversion effect on CH₄ and C₅+ selectivities on Ru is the same as that on Fe and Co catalysts, while the effect on olefin formation is opposite. Although the innovation is not sufficient, the work is relatively systematic. I think this manuscript can meet the standard after minor revision, and the following issues should be carefully considered.

Response: Thank you for the reviewer’s comments. As noted, while the discovery that 'the effect on olefin formation is opposite' may not represent a significant finding on its own, it raised an important scientific question regarding water adsorption on Fe, Co, and Ru surfaces. Specifically, it suggested that water adsorption on Ru is stronger than on Fe or Co, which, in turn, explains the differing olefin trends with conversion on Ru. We dedicated considerable effort in this paper to verify this observation.

• The manuscript describes that CH₄selectivity decreases and C₅+ selectivity increases with increasing CO conversion on both Fe and Co catalysts. This expression is inappropriate. In FTS, the relationship of CH₄ and C₅+ selectivity with CO conversion is complex.

 

Response: Thank you for the comment. This typically refers to the catalytic nature and behavior of the system. If there are mass transfer limitations or experimental errors, the issue can become more complex. To avoid confusion, we have revised the statement to specify that intrinsic CH4 selectivity decreases, while intrinsic C5+ selectivity increases with increasing CO conversion on both Fe and Co catalysts (please see line 70). This clarification is based on our CO conversion effect experiments, which were conducted in a CSTR under conditions that minimize external and internal diffusion (>750 rpm, <170 mesh size).

2) Are products such as water, oil and wax analyzed using the same chromatography as tailgas?  

Response: In our study of the effect of CO on conversion, analyzing the water phase was unnecessary. Although our lab is equipped to perform water phase analysis, we utilized a different gas chromatograph (GC) for tailgas analysis compared to the one typically used for water analysis.

3) What’s the fundamental reason for the different CO conversion effect on olefin formation over Ru in comparison with Co and Fe catalysts?

Response: Olefin formation is influenced by the water concentration in the reactor. This relationship is illustrated in Figure 9, which presents the water co-feeding study, and in Section 3.2.3, which discusses H₂O adsorption on Fe, Co, and Ru catalysts using H₂O-TPD analysis. Addressing this relationship is one of the primary objectives of this study.

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript ID: reactions-3574233

 

Title:  Fischer-Tropsch Synthesis: Effect of CO Conversion Over 2 Ru/NaY Catalyst

 

This manuscript presents the FT, effect of Ru dispersion supported on NaY on the conversion and selectivity of methane, olefin/paraffins and compared with different other metal and supports. They concluded that the Ru favored the olefin distribution compared to Co, Fe supported on Al₂O₃ with different promoters. The reaction was performed in a slurry reactor, long life times. The manuscript presented also important results using HRTEM, EXAFS, TPD to explain the effect of Ru on the product distribuition.

Comments:

1. The manuscript discussed the importance of Ru on the FT performance. As noticed in Table 1 the BET surface area is very high for NaY and decreased significantly with the addition of Ru content, for 7.5% more than half of the support. Moreover the dispersion of Ru is relatively higher than expected. For 1% only 20% dispersion and for 7.5% only 2%. For lower contents one expects high dispersion, which is in contradiction with the literature. In fact, these dispersions evidence very large Ru particles. Are these metallic Ru⁰? If so, the activity must be low. The TEM images show large particles, but can be used to determine the dispersions and compared with the chemisorptions (H2) to confirm these results and then calculate the activity by determining the TOF values, and compare the activity. Indeed, Fig.6 presents the CO conversions and olefins selectivity. The conversions in fig.a show different initial values for Ru contents. One expects that for higher dispersions the conversions are higher then for low dispersions. Here is the opposite. All results indicate that for the 1%Ru the selectivities are lower than for 7.5%. Moreover, it shows big deactivation, that evidence sintering, coke. No results were presented or discussed hiere   Please explain.
2. Figures 7 show the selectivities of all olefin HC and the O/P ratios with increasing conversion. They were compared with different other catalysts and supports, which is not possible. The Ru catalyst shows opposite results. In fact, the supports are different and are difficult to compare. Bur fig.8 shows the influence of particle sizes on the O/P ratios and for different conversions, 22 and 55%. These results indicate that the effect occurs only for 10nm. Higher sizes do not influence this ratio. It is important to explain, because it influences the olefin in the range of C3-C4. Please explain.
3. Finally, Fig.9 shows the effect of water. One expects that water influence negatively the performance, but here it is the opposite. In fact, it favors the RWGS reaction. The presentation is confusing, too long, runs 400h, and has less active catalysts. Please include other references in the discussion.

Author Response

This manuscript presents the FT, effect of Ru dispersion supported on NaY on the conversion and selectivity of methane, olefin/paraffins and compared with different other metal and supports. They concluded that the Ru favored the olefin distribution compared to Co, Fe supported on Al₂O₃ with different promoters. The reaction was performed in a slurry reactor, long life times. The manuscript presented also important results using HRTEM, EXAFS, TPD to explain the effect of Ru on the product distribuition.

Response:  Thanks for the comments. We appreciate the reviewer’s comments and constructive criticism.

Comments:

1. The manuscript discussed the importance of Ru on the FT performance. As noticed in Table 1 the BET surface area is very high for NaY and decreased significantly with the addition of Ru content, for 7.5% more than half of the support. Moreover the dispersion of Ru is relatively higher than expected. For 1% only 20% dispersion and for 7.5% only 2%. For lower contents one expects high dispersion, which is in contradiction with the literature. In fact, these dispersions evidence very large Ru particles. Are these metallic Ru⁰? If so, the activity must be low. The TEM images show large particles, but can be used to determine the dispersions and compared with the chemisorptions (H2) to confirm these results and then calculate the activity by determining the TOF values, and compare the activity. Indeed, Fig.6 presents the CO conversions and olefins selectivity. The conversions in fig.a show different initial values for Ru contents. One expects that for higher dispersions the conversions are higher then for low dispersions. Here is the opposite. All results indicate that for the 1%Ru the selectivities are lower than for 7.5%. Moreover, it shows big deactivation, that evidence sintering, coke. No results were presented or discussed hiere   Please explain.

 

Response: Thank you for the reviewer’s comments. The discussion regarding the effect of Ru loading and deactivation on catalyst performance can be found in lines 299-323. Regarding the reviewer’s statement, “one expects that for higher dispersions the conversions are higher than for low dispersions,” the authors partially disagree. High dispersion alone does not guarantee higher conversion; rather, it is the combination of high dispersion and a high reduction degree resulting in a population of active sites that leads to higher conversion.

 

 

1. Figures 7 show the selectivities of all olefin HC and the O/P ratios with increasing conversion. They were compared with different other catalysts and supports, which is not possible. The Ru catalyst shows opposite results. In fact, the supports are different and are difficult to compare. Bur fig.8 shows the influence of particle sizes on the O/P ratios and for different conversions, 22 and 55%. These results indicate that the effect occurs only for 10nm. Higher sizes do not influence this ratio. It is important to explain, because it influences the olefin in the range of C3-C4. Please explain.

 

Response: The conversion effect is not influenced by the type of support or the size of the active metal particles. Regardless of the support type or particle size, the conversion effect is present. For instance, metal particles larger than 10 nm exhibit stable olefin content (as shown in Figure 7). This does not imply the absence of the conversion effect. The conversion effect persists across a range of particle sizes, as demonstrated in Figure 7: at 20–70 nm, with a conversion rate of 22%, the C₃ olefin/paraffin ratio is 1.2. Similarly, at a conversion rate of 55%, the C₃ olefin/paraffin ratio increases to 1.4, indicating the ongoing conversion effect.

 

1. Finally, Fig.9 shows the effect of water. One expects that water influence negatively the performance, but here it is the opposite. In fact, it favors the RWGS reaction. The presentation is confusing, too long, runs 400h, and has less active catalysts. Please include other references in the discussion.

 

• Response: Thank you for the feedback. Water plays two roles in FTS: it can oxidize catalysts and has an intrinsic effect on the reaction. The term "water effect" typically refers to its intrinsic role in FTS and excludes deactivation caused by water oxidation. The intrinsic effect of water positively influences the activity of cobalt catalysts, as noted by S. Loegdberg et al. (Appl. Catal. A: Gen. 393, 2011, p.109). Deactivation of cobalt catalysts by water is primarily due to oxidation of small Co crystallites and water-induced sintering. To accurately measure the intrinsic effect of water, its deactivation role must be minimized or eliminated. The study utilized highly active cobalt catalysts, and the low conversion at high space velocity should not be interpreted as reduced catalyst activity. The authors believe that the water co-feeding experiment shown in Fig. 9 sufficiently demonstrates the effect of water on olefin formation, without the need for additional references

 

 

 

Reviewer 3 Report

Comments and Suggestions for Authors

A series of 1-7.5%Ru/NaY catalysts were prepared for the CO conversion effect on C1 and C5+ hydrocarbon formation and olefin formation. Major revision is suggested.

• It can be seen in Table 1 that the crystallite size of Ru (or RuO2) is larger than 60 nm over 7.5% Ru/NaY, which is believed exceeding the pore size of the zeolite. Please clarify the location of the precious metal on the zeolite.
• What kind of phase do the three dark dots represent in Fig. 2? The Ru peaks are not found in the figure. How about the RuO2 related peak? It is well-known that metallic Ru can be readily oxidized to RuO2 at low temperatures such as 350 degree C.
• Most of the figures are not clear, especially the FFT profiles in Fig. 3.
• It cannot simply relate the Ru cluster size to the O/P ratios in Fig. 8, since the catalysts adopted have different metal loadings.
• It is really strange to find Ag-Co-Al catalyst in Fig. 7 and Pt-Co-Al catalyst in Fig. 10.

Author Response

 

A series of 1-7.5%Ru/NaY catalysts were prepared for the CO conversion effect on C1 and C5+ hydrocarbon formation and olefin formation. Major revision is suggested.

Response: We value the reviewer's insights and feedback.

• It can be seen in Table 1 that the crystallite size of Ru (or RuO2) is larger than 60 nm over 7.5% Ru/NaY, which is believed exceeding the pore size of the zeolite. Please clarify the location of the precious metal on the zeolite.

 

Response: The reviewer's observation appears correct: the presence of particles larger than 60 nm on the 7.5% Ru/NaY was noted. This size exceeds the pore size of the NaY support, which is approximately 2 nm. Consequently, we hypothesize that these particles are located on the exterior surface of the NaY support. Please refer to the additions highlighted in yellow in lines 215–216 of the text.

 

• What kind of phase do the three dark dots represent in 2? The Ru peaks are not found in the figure. How about the RuO2 related peak? It is well-known that metallic Ru can be readily oxidized to RuO2 at low temperatures such as 350 degree C.

 

Response:  The reviewer should focus on Figure 1 instead of Figure 2. Specifically, three dark dots located at 27.5°, 35.1°, and 54.9° are identified as RuO₂. This identification stems from the fact that metallic Ru readily oxidizes to RuO₂ at relatively low temperatures, such as 350°C. For further details, refer to the highlighted additions in lines 232-234.

•  
• Most of the figures are not clear, especially the FFT profiles in Fig. 3.

Response: The figures appear clear on our computer.

 

• It cannot simply relate the Ru cluster size to the O/P ratios in Fig. 8, since the catalysts adopted have different metal loadings.

Response: Figure 8 highlights the effects of cluster size across all ranges, showing that the conclusion regarding the CO conversion effect on olefin formation remains consistent. The plot is based on data we obtained, which supports the idea that cluster size influences olefin formation. This relationship has also been discussed by others, who observed that active sites on smaller clusters tend to favor olefin formation.

 

• It is really strange to find Ag-Co-Al catalyst in Fig. 7 and Pt-Co-Al catalyst in Fig. 10.

Response:  The reviewers are correct: the Ag promoter is used in Figure 7, while the Pt promoter is used in Figure 10. However, this difference does not affect the role of the Co catalyst (specifically the Co metal site) in CO conversion. Additionally, both Ag and Pt promoters serve a similar function in promoting the reduction of Co, as reported in previous studies.

 

Reviewer 4 Report

Comments and Suggestions for Authors

This manuscript examines the effect of CO conversion on product selectivity over Ru/NaY catalysts, in comparison with Co- and Fe-based systems. The authors highlight how H₂O generated during the reaction affects hydrogenation activity, particularly on Ru, where water adsorption is significant. The experimental results are generally coherent, and the trends in C5+ selectivity, methane suppression, and olefin distribution are interpreted logically. The work presents meaningful insights and could contribute to understanding selectivity control in Fischer–Tropsch systems.

One concern is the comparability of the catalysts. While the Ru catalyst is supported on NaY, the Fe and Co catalysts are supported on SiO₂ and Al₂O₃, respectively. Given that water adsorption and catalytic behavior are influenced by the support, the observed trends may not solely reflect the nature of the active metals. It would be more convincing if Fe and Co were tested on NaY, or if relevant literature using such systems were cited to justify the comparisons. In addition, the manuscript does not describe how the Fe and Co catalysts were synthesized. Including even a brief note on their preparation would help ensure reproducibility and proper comparison.

There also appears to be inconsistency in the composition of the Co catalyst across figures. Figure 7 refers to a Co–Ag catalyst containing 0.27% Ag, whereas Figure 10 involves a Co–Pt catalyst with 0.5% Pt. If these are different materials, this should be clearly stated. If they are intended to be equivalent, the rationale for using two different promoters needs clarification, as it may affect water adsorption behavior.

Some definitions and expressions in the figures lack clarity. The terms 1-C4 and 2-C4 olefins are not defined in the manuscript, though they appear throughout the data discussion. Additionally, the y-axis in Figure 7 is labeled as "content (%)" without specifying the basis—whether it refers to selectivity, molar ratio, or carbon-group fraction. Including equations used to calculate conversion and selectivity would also improve transparency and reproducibility.

A particularly problematic point is the use of unpublished data as Reference [44] to support the proposed mechanism involving water interaction. Citing unpublished results in a peer-reviewed manuscript, especially when used to justify key claims, is not appropriate. The authors should consider removing this reference or replacing it with their own reproducible data or a suitable publication.

Several figures would benefit from consistent formatting and labeling. For example, the layout and labeling in Figure 4 are not uniform. Minor typographical issues also appear in several places and should be corrected. Despite these issues, the core message of the manuscript is scientifically interesting and the data trends are generally consistent. However, the concerns raised above should be addressed to enhance the clarity and scientific reliability of the manuscript.

Author Response

 

This manuscript examines the effect of CO conversion on product selectivity over Ru/NaY catalysts, in comparison with Co- and Fe-based systems. The authors highlight how H₂O generated during the reaction affects hydrogenation activity, particularly on Ru, where water adsorption is significant. The experimental results are generally coherent, and the trends in C5+ selectivity, methane suppression, and olefin distribution are interpreted logically. The work presents meaningful insights and could contribute to understanding selectivity control in Fischer–Tropsch systems.

Response: We are truly grateful that the Reviewer recognized our contribution.

One concern is the comparability of the catalysts. While the Ru catalyst is supported on NaY, the Fe and Co catalysts are supported on SiO₂ and Al₂O₃, respectively. Given that water adsorption and catalytic behavior are influenced by the support, the observed trends may not solely reflect the nature of the active metals. It would be more convincing if Fe and Co were tested on NaY, or if relevant literature using such systems were cited to justify the comparisons. In addition, the manuscript does not describe how the Fe and Co catalysts were synthesized. Including even a brief note on their preparation would help ensure reproducibility and proper comparison.

Response: We conducted water adsorption experiments on the Al₂O₃-150 and NaY supports. Due to the minimal water adsorption observed on these supports, the effect was deemed to be negligible. The CO conversion effect is primarily attributed to the active metals Fe, Co, or Ru, although the supports may also influence activity to some extent. Fe and Co catalysts have been extensively studied across various conversion levels, and no significant effect of support changes on CO conversion was observed. We plan to investigate the impact of the NaY support in future tests. A brief description of the Co and Fe catalyst preparation has been included in the text; please refer to lines 103–106 highlighted in yellow.

There also appears to be inconsistency in the composition of the Co catalyst across figures. Figure 7 refers to a Co–Ag catalyst containing 0.27% Ag, whereas Figure 10 involves a Co–Pt catalyst with 0.5% Pt. If these are different materials, this should be clearly stated. If they are intended to be equivalent, the rationale for using two different promoters needs clarification, as it may affect water adsorption behavior.

Response: Thank you for pointing this out. The type of promoter does not significantly affect the CO conversion effect investigated in this work. Rather, the promoter impacts the number of cobalt active sites primarily by facilitating Co oxide reduction. The differences between the catalysts are explained in line 104.

Some definitions and expressions in the figures lack clarity. The terms 1-C4 and 2-C4 olefins are not defined in the manuscript, though they appear throughout the data discussion. Additionally, the y-axis in Figure 7 is labeled as "content (%)" without specifying the basis—whether it refers to selectivity, molar ratio, or carbon-group fraction. Including equations used to calculate conversion and selectivity would also improve transparency and reproducibility.

Response: We have included definitions in lines 384-390. 

A particularly problematic point is the use of unpublished data as Reference [44] to support the proposed mechanism involving water interaction. Citing unpublished results in a peer-reviewed manuscript, especially when used to justify key claims, is not appropriate. The authors should consider removing this reference or replacing it with their own reproducible data or a suitable publication.

Response: Thank you. As the data in question were never published, Ref 44 has been removed.

Several figures would benefit from consistent formatting and labeling. For example, the layout and labeling in Figure 4 are not uniform. Minor typographical issues also appear in several places and should be corrected. Despite these issues, the core message of the manuscript is scientifically interesting and the data trends are generally consistent. However, the concerns raised above should be addressed to enhance the clarity and scientific reliability of the manuscript.

Response: Thank you for your review comments. The 1%Ru/NaY catalyst was not included in Figure 4, as it is difficult for us to obtain. However, this omission does not impact the overall conclusions of the paper. Regarding typographical issues, we have addressed the ones that we could find in proofreading the revised version. We appreciate your positive feedback on the paper.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript and responses are satisfactory. I can recommend this final version for publication

Author Response

The authors value the Reviewer's positive feedback.

Reviewer 3 Report

Comments and Suggestions for Authors

• As I have pointed out, as the catalysts adopted have different metal loadings, simply relating the Ru cluster size to the O/P ratios in Fig. 8 is not correct, even there have been some other reports to do the similar things.
• If they wanted to make Ag-Co-Al and Pt-Co-Al catalyst as reference catalysts, more detailed information and characterizations would be helpful.

Author Response

• The authors appreciate Reviewer 3's additional comments. First, as demonstrated in Figure 7, the CO conversion effect on 0.27Ag–25%Co/Al was presented. Figure 10 illustrated water adsorption on 0.5%Pt–25%Co/Al2O3. These findings primarily highlight the CO conversion effects influenced by water adsorption on cobalt metal. A minor amount of Ag or Pt acts as highly effective reduction promoters, playing equivalent roles in cobalt reduction (refer to the following articles: Gary Jacobs et al., Group 11 (Cu, Ag, Au) promotion of 15%Co/Al2O3 Fischer–Tropsch synthesis catalysts, Applied Catalysis A: General, Volume 361, Issues 1–2, 20 June 2009, Pages 137–151; Thani Jermwongratanachai et al., Fischer–Tropsch synthesis: Comparisons between Pt and Ag promoted Co/Al2O3 catalysts for reducibility, Local Atomic). The water effects or water adsorption contributions are mainly attributed to cobalt metals. The CO effects caused by these promoters can be disregarded, as they do not participate in the Fischer–Tropsch reaction.
•  

Regarding the relationship between Ru cluster size and the O/P ratio, the reviewer's arbitrary conclusion claiming that Ru cluster size directly impacts the O/P ratio is unsupported. We cannot accept their perspective without evidence. Moreover, the results in Figure 8 were derived entirely from our reaction data. It is important to note that the influence of smaller metal particles on olefin formation has been widely reported by numerous authors. For example, the paper by Ahmed E. Rashed et al. (ACS Omega 2022, 7, 8403−8419), titled "Fe Nanoparticle Size Control of the Fe-MOF-Derived Catalyst Using a Solvothermal Method: Effect on FTS Activity and Olefin Production," highlights that a smaller particle size (6 nm TEA catalyst) led to higher activity and olefin yield. Specifically, this catalyst demonstrated 94% CO conversion and a higher olefin yield of 24% at a lower reaction temperature of 280 °C and a pressure of 20 bar with an H2/CO ratio of 1.

Similarly, Ru, Co, and Ni particle sizes have comparable effects on FTS activity and selectivity.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors have responded carefully and revised the manuscript appropriately. I have no further comments and recommend the manuscript be accepted for publication in its current form.

Author Response

The authors value the Reviewer's positive feedback.