Experimental Study on Sulfur Deactivation and Regeneration of Ni-Based Catalyst in Dry Reforming of Biogas
Round 1
Reviewer 1 Report
This study by Chein, Chen, and Chen attempts to learn about how Ni-based catalysts in equimolar dry reforming conditions behave and regenerate owing to sulfur poisoning. While the topic of reserach is interesting and the authors performed many dry reforming experiments, I cannot recommend publication of this manuscript owing to lack of post-reaction characterization which are at the core of their claims. Both carbon accumulation and particle sintering are certainly occuring in these catalysts and this point is "hand waved" and ignored with a citation.
The authors categorically state that their deactivation is only due to sulfur poisoning and ignore completely the impact carbon accumulation and sintering may have on the catalyst.
The following line:
"The carbon deposition also results in catalyst activity loss. As ompared with the catalyst activity loss due to carbon deposition reported by the studies by Luna and Iriarte [34] and Rosset et al., it can be concluded that H2S poison is the dominant factor in controlling the loss of catalyst activity."
is completely unacceptable since carbon accumulation can dramatically impact the activity over short periods of time, and indeed low rates of carbon deposition can dramatically lower the catalyst activity.
The conclusions that the authors draw may be somewhat true, but they must do much more work to show their conclusions more conclusively. The fact that they cited [34,35] above seems to have gotten them out of many key experiments which are critical for any dry reforming paper.
I cannot recommend publication until the authors have run the following experiments
1) TEM identification of carbon accumulation. Graphitic or MWCNT?Decscribe the structure and amount relative to position of the Ni
2) Thermographic analysis to measure the amount of carbon accumulated per mass of nickel per time
3) Particle size distributoin of the via TEM imaging both before and after DRM. The authors run DRM for very long times at very high temperatures and completely ignored goemetric stability, particle coarsening and growth. This may explain why catalysts which show recovery in oxygen do not show a FULL recovery.
4) XPS before and after for at least one sample would also be helpful to see how the Ni oxidation state changes either by oxidation or sulfidization, although this point is less critical compared to comments 1-3.
Author Response
Review 1
This study by Chein, Chen, and Chen attempts to learn about how Ni-based catalysts in equimolar dry reforming conditions behave and regenerate owing to sulfur poisoning. While the topic of reserach is interesting and the authors performed many dry reforming experiments, I cannot recommend publication of this manuscript owing to lack of post-reaction characterization which are at the core of their claims. Both carbon accumulation and particle sintering are certainly occuring in these catalysts and this point is "hand waved" and ignored with a citation.
The authors categorically state that their deactivation is only due to sulfur poisoning and ignore completely the impact carbon accumulation and sintering may have on the catalyst.
The following line:
"The carbon deposition also results in catalyst activity loss. As compared with the catalyst activity loss due to carbon deposition reported by the studies by Luna and Iriarte [34] and Rosset et al., it can be concluded that H2S poison is the dominant factor in controlling the loss of catalyst activity."
is completely unacceptable since carbon accumulation can dramatically impact the activity over short periods of time, and indeed low rates of carbon deposition can dramatically lower the catalyst activity.
The conclusions that the authors draw may be somewhat true, but they must do much more work to show their conclusions more conclusively. The fact that they cited [34,35] above seems to have gotten them out of many key experiments which are critical for any dry reforming paper.
Answer:
From Figure 2, the results for DRM without H2S effect are shown for CH4/CO2=1/1. In this case, the catalyst deactivation is expected to be mainly due to carbon deposition. The results shown in Figure 2 indicate that stable results can be obtained at T=800 and 900°C for the test time. It is noted that the 20wt%Ni-5wt%CeO2/Al2O3 catalyst is used in this study. With Ce contained in the catalyst, it is expected that carbon deposition resistance ability can be enhanced [1]. For T=700°C, CH4 and CO2 conversions decrease slightly as test time increases. Because there is no H2S effect, the decrease in CH4 and CO2 conversions is dominated by the carbon deposition as pointed out by the reviewer.
The same experiment as that shown in Figure 2 was carried out except that H2S was added in the feedstock and the results are shown in Figure 3. With the effect of H2S, it is seen that linearly decreases in CH4 and CO2 conversions are resulted due to catalyst poison by H2S at T=700°C. It is expected that CH4 and CO2 conversions will drop to zero, which means the total loss of active nickel sites is exhibited. For T=800°C, slight decreases in XCH4 and XCO2 resulted in early reaction time. As the reaction time increases, exponentially decay in both CH4 and CO2 conversions are resulted. As reaction time increases further, both CH4 and CO2 conversions approach to steady-state values, indicating that the sulfur coverage on the catalyst surface reaches a saturated condition. For T=900°C, the catalyst poisoned by H2S is to a smaller extent as compared with T=700°C and 800°C under the same reaction time. That is, the sulfur coverage decreases with increased reaction temperature. It is also expected that the H2S coverage on the catalyst surface will reach a saturated condition when the reaction time is increased.
By comparing the results shown in Figures 2 and 3, it can be realized that the catalyst deactivation is dominated by sulfur poison. The same conclusion was also made in the study by Appari et al. [2]. Moreover, the Ce contained in the catalyst enhances the capability of carbon deposition resistance. Therefore, the effect of carbon deposition on catalyst deactivation would be to less extent as compared with the sulfur poison.
Another evidence for supporting the conclusion made above is from Figure 7 in which the effect of reaction temperature on catalyst regeneration is shown. From Figure 7, it can be seen that after removing H2S, the CH4 conversion can be recovered to a value approaching 78% for regenerating the sulfur-poisoned catalyst at 800°C as the reaction time increases further. For T=900°C, the CH4 conversion can be recovered to a value of 96% which is the result from the H2S-free test listed in Table 2. Based on the discussions above, it again demonstrates that sulfur poison is the dominant factor in catalyst deactivation. In the study by Izquierdo et al. [3], tri-reforming of biogas using Ni-based catalyst was studied. From the TGA analysis, they identified that the carbon deposition amount on the catalyst was low. From the quick catalyst deactivation, they also made the conclusion that the catalyst deactivation was mainly due to sulfur poison.
The above results are for the CH4/CO2=1/1 case. In Figure 4, the effect of CO2 content is studied. From thermodynamics, carbon formulation can be enhanced by reducing the CO2 amount in DRM [4, 5]. In the study by Castro Luna and Iriarte [6], the DRM catalyzed by various modified Ni/Al2O3 catalysts was studied. The feed was CH4/CO2=1/1. They concluded that the introduction of 0.5 wt% of K in the catalyst, when compared to the original catalyst Ni/Al2O3, showed a slightly lower activity and lower carbon deposition after the same period of operation in the dry reforming of methane. This study is not particularly related to the present study and is deleted in the revised manuscript. In the study by Rosset et al. [7], Ni-Al catalysts were studied aiming to suppress the carbon deposition in the dry reforming of biogas. The DRM was carried out with unreduced and reduced catalysts in a fixed bed tubular reactor using synthetic biogas having 60% CH4 and 40% CO2 and the reaction temperature in the range of 500~750°C. They showed that carbon deposition in biogas dry reforming could be controlled using the unwashed and unreduced Ni-Al catalyst. Again, this study is also not particularly related to the present study. Two references that relate to the increased carbon formation due to reduced CO2 amount in DRM from thermodynamic analysis are cited in the revised manuscript [4, 5].
In Figure 4, the effects of both CO2 amount and H2S on DRM performance were presented. For the case of a higher CO2 amount, i.e., the CH4/CO2=1/2 case, the DRM performance follows the same trend as the CH4/CO2=1/1 case. For the CH4/CO2=1/0.5 case, due to the presence of more carbon formulation, it can be seen that variation trends for CH4 and CO2 conversions are different from the CH4/CO2=1/1 and 1/2 cases. It is seen that CH4 conversion drops immediately in the early time and then reaches a steady-state value. For the CO2 conversion, linearly decrease after an initial exponentially decay when test time increases.
Based on the results shown in Figure 4 and the above discussion, the conclusion that the sulfur poison is the dominant catalyst deactivation factor in DRM.
I cannot recommend publication until the authors have run the following experiments
Answer:
We realized that the catalyst characteristics is an important tool to understand the effect of chemical reaction on the catalyst. The reason we did not carry out these characterizations is that the results of the activity test have clearly identified the effect of H2S on catalyst capability during the DRM. We have compared syngas production by using various feedstocks before and after the DRM.
It is also noted that the catalyst activity loss by carbon deposition and H2S poison is not new. They can be found in syngas production from CH4-based reforming which has been studied extensively. Among these studies, catalyst characterization has also been reported extensively. References for catalyst characterization will be described in the following and added in the revised manuscript.
1) TEM identification of carbon accumulation. Graphitic or MWCNT?Decscribe the structure and amount relative to position of the Ni
Answer:
TEM is used to examine the catalyst surface and to examine the sample morphologies and estimation of metal particle size. In the study by Izquierdo et al. [2], TEM micrographs were used to analyze the carbon deposition on Ni-based catalysts used in tri-reforming with biogas as feed. They identified the types of carbon deposited onto the catalyst surface. They found that the encapsulated carbon was predominant. Carbon filaments appearing as carbon nanotubes and growing from the Ni particle could also be found.
2) Thermographic analysis to measure the amount of carbon accumulated per mass of nickel per time
Answer:
I think it is the “thermogravimetric analysis” instead of “thermographic analysis” for measuring the amount of carbon accumulation. As reported by Chen et al. [8] in their study of DRM using biogas as feed, the TGA test shows that the weight of spent catalyst was increased due to attributed to the oxidation of Ni−S. TGA by an air stream. Ni−S could be oxidized to nickel sulfate and decomposed to nickel oxide under an air stream. Based on the TGA test, they also pointed out that sulfur poisoning blocked the active metal and inhibited carbon gasification during the DRM test.
3) Particle size distributoin of the via TEM imaging both before and after DRM. The authors run DRM for very long times at very high temperatures and completely ignored goemetric stability, particle coarsening and growth. This may explain why catalysts which show recovery in oxygen do not show a FULL recovery.
Answer:
TEM is used to examine the catalyst surface examine the sample morphologies and estimation of metal particle size. In the study by Yang [9], Transmission electron microscopy (TEM) was performed to examine the catalyst morphologies before and after the steam-reforming reaction test. The TEM images and the particle size distributions showed that the freshly reduced catalyst retained small nickel particles without distinct agglomerate, representing fine dispersion of metallic nickel. The Ni particles on the surface of the catalyst had an average particle size of about 11.9 nm. In the H2S poisoned catalyst, a larger agglomerate of nickel particles was observed. The Ni particles have an averaged particle size of about 19.8 nm.
We agree with the reviewer’s point of view that the full recovery cannot be achieved due to increased nickel particle size
4) XPS before and after for at least one sample would also be helpful to see how the Ni oxidation state changes either by oxidation or sulfidization, although this point is less critical compared to comments 1-3.
Answer:
XPS is a technique that allows the study of the species present on the surface of the catalyst and their chemical state. In the study by Chen et al. [8], XPS analysis was used to determine the surface composition and chemical state of the nickel and sulfur species of the deactivated and regenerated catalysts in their DRM test using biogas as feed. Their results confirmed the formation of Ni−S in the spent catalyst that causing the catalyst deactivation.
References
[1] Chein R-Y, Fung W-Y. Syngas production via dry reforming of methane over CeO2 modified Ni/Al2O3 catalysts. International Journal of Hydrogen Energy. 2019;44:14303-15.
[2] Appari S, Janardhanan VM, Bauri R, Jayanti S. Deactivation and regeneration of Ni catalyst during steam reforming of model biogas: An experimental investigation. International Journal of Hydrogen Energy. 2014;39:297-304.
[3] Izquierdo U, García-García I, Gutierrez Á, Arraibi J, Barrio V, Cambra J, et al. Catalyst Deactivation and Regeneration Processes in Biogas Tri-Reforming Process. The Effect of Hydrogen Sulfide Addition. Catalysts. 2018;8:12.
[4] Chein RY, Chen YC, Yu CT, Chung JN. Thermodynamic analysis of dry reforming of CH4 with CO2 at high pressures. Journal of Natural Gas Science and Engineering. 2015;26:617-29.
[5] Nikoo MK, Amin NAS. Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation. Fuel Processing Technology. 2011;92:678-91.
[6] Castro Luna AE, Iriarte ME. Carbon dioxide reforming of methane over a metal modified Ni-Al2O3 catalyst. Applied Catalysis A: General. 2008;343:10-5.
[7] Rosset M, Féris LA, Perez-Lopez OW. Biogas dry reforming over Ni-Al catalyst: Suppression of carbon deposition by catalyst preparation and activation. International Journal of Hydrogen Energy. 2020;45:6549-62.
[8] Chen X, Jiang J, Yan F, Li K, Tian S, Gao Y, et al. Dry Reforming of Model Biogas on a Ni/SiO2 Catalyst: Overall Performance and Mechanisms of Sulfur Poisoning and Regeneration. ACS Sustainable Chemistry & Engineering. 2017;5:10248-57.
[9] Yang X. An experimental investigation on the deactivation and regeneration of a steam reforming catalyst. Renewable Energy. 2017;112:17-24.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The performance of DRM was studied including the catalyst poison by the presence of H2S. Authors show that the catalyst poison at presence of H2S depends on both reaction temperature and time and аутхорсшроте тхат the H2S coverage onto the catalyst surface decreases with increased reaction temperature. However surface is covered by sulfur but not by H2S. Authors have to change the text in manuscript. This one example – “ Due to the stronger chemisorption of H2S onto catalyst as compared to O2 or H2O, ….”
Authors show that the H2S poisoned catalyst can be regenerated using a high-temperature oxidation. However it is very important to show how catalyst regeneration cycles effect on catalyst activity. Authors have to age by H2S and regenerate catalyst several times and then text DRM. Try to avoid activity test close to the thermodynamic equilibrium.
Manuscript can be accepted after revision.
Author Response
Comments and Suggestions for Authors
The performance of DRM was studied including the catalyst poison by the presence of H2S. Authors show that the catalyst poison at presence of H2S depends on both reaction temperature and time and аутхорсшроте тхат the H2S coverage onto the catalyst surface decreases with increased reaction temperature. However surface is covered by sulfur but not by H2S. Authors have to change the text in manuscript. This one example – “ Due to the stronger chemisorption of H2S onto catalyst as compared to O2 or H2O, ….”
Answer:
We have revised the H2S poison to sulfur poison based on the reviewer’s suggestions.
Authors show that the H2S poisoned catalyst can be regenerated using a high-temperature oxidation. However it is very important to show how catalyst regeneration cycles effect on catalyst activity. Authors have to age by H2S and regenerate catalyst several times and then text DRM. Try to avoid activity test close to the thermodynamic equilibrium.
Answer:
From industrial applications, the regeneration of the catalyst with several cycles is required to show the capability of catalysts. Because this study has demonstrated that sulfur poison is the major factor causing catalyst deactivation and it is a reversible reaction. Theoretically, the catalyst can be regenerated without age problem. However, this needs to be proved in future studies.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The manuscript is acceptable for publication after English editing
Reviewer 2 Report
Accept in present form
