Assessment of Agricultural Residue to Produce Activated Carbon-Supported Nickel Catalysts and Hydrogen Rich Gas
, Feridun Esmaeilzadeh 2, María del Carmen Recio-Ruiz 3, Ramiro Ruiz-Rosas 3,*, Juana M. Rosas 3, José Rodríguez-Mirasol 3 and Tomás Cordero 3Round 1
Reviewer 1 Report
The work of Hosseinzaei et al. may be interesting for publication because the essence is the use of a biomass to get both the catalyst support and bio-oil to produce hydrogen. However, the manuscript lacks a storyline to make it more convincing. At first, one sees that there are many characterization results focused on the support (active carbon obtained from biomasses) that at the end are not tied up with the catalytic activity. The authors must improve the following aspects of their manuscript to make it suitable for publication:
1) Although there is an European policy to make hydrogen an energy vector, the past, present and future in the massive use of hydrogen was, is and will be as a (petro)chemical raw material. Then, the main use of hydrogen a raw material must be emphizised in the introduction.
2) This phrase should be cautiously rethought: “To the best of our knowledge, the initial materials for synthesizing the catalyst supports and the feedstock for the supercritical water gasification process are reported for the first time”. Even if it is true, the core of the work from a strictly catalytic viewpoint (you are intending to publish in a catalysis journal) is using active carbon as a support (regardless of the origin), and this is not new. Here you must think about the real novelty, like if you really were able to obtain a much better active carbon material in comparison with others already studied and reported. It would be also worthy to mention that you are getting both the catalyst support (with good properties?) and bio-oil from the same biomass to produce hydrogen, which could have some advantages…
3) In general, there are a lot of characterization results and fair catalytic test results but the authors did not correlate the catalyst properties (amply studied by different characterization techniques) with the reaction performance in the supercritical water gasification of bio-oil. For example, looking at Table 1 and 4, it could be inferred that the surface area may determine the catalytic activity (the higher the surface area of the support, the higher the gas yield). The authors just mentioned the correlation of the activity with the dispersion in the conclusions but a further explanation is needed in the results section. There may be other properties determining the catalytic activity as well. The authors must do this correlation between catalyst properties and activity to tied up all the results.
4) I do not understand what the authors meant in the first paragraph of section 2.3.4 (catalytic test), in part because of the very bad English writing. I guess they are trying to justify why they did not do blank experiments (same reaction tests without catalyst to see the possible thermal conversion). However, this excuse is not acceptable since they claim to have done this blank before for the steam reforming of acetone, which is very different from the reactions they are presenting in this work. The authors must do the experiments without catalyst for this reaction (supercritical water gasification of bio-oil) since the conversion of real/raw bio-oil may greatly differ from that of “model” compounds.
5) Also, the explanations of this section (catalytic test) are very vague and basic. Can the authors present their own thermodynamic equilibrium calculations for this system? For this, you need the bio-oil composition, which by the way, it is not in this work. Presenting their own thermodynamic calculations (which is possible by simulating the bio-oil composition) would provide information on the goodness of these catalysts to approach the equilibrium (if the contact time/velocity is sufficiently high/low for this).
6) Furthermore, and maybe more important, the catalytic test results must be compared with those obtained with other catalysts (for example a conventional Ni/Al2O3 catalyst) to really see the advantage (if any) of this active carbon supports. This comparison would bring up opportunities to highlight the benefits of this catalyst formulation with active carbon (if any).
7) Nothing is mention about the catalyst stability. Would this catalyst formulation with an active carbon support be stable over time and over usages/cycles? Do you detect coke formation? How can coke deposits be analyzed? Can coke be removed from this catalyst without damaging the active carbon support?
8) There are many bad/inappropriate uses of English and some phrases do not even make sense. For example:
- “Regarding the problems arising from non-renewable energies such as depletion of energy resources and the increase in environmental pollution due to the emission of greenhouse gasses, nowadays, in the energy field; many researches are shifted towards utilizing renewable sources of energy including biomass” (this is very long and bad organized).
- “Accordingly, those conditions didn't repeat again in this study under the SCWG process”.
There are many others. You must carefully rereading your work making appropriate corrections and if necessary look for assistance for checking the language.
Author Response
Dear editor
First, we would like to express our highest appreciation and gratitude for all the time and effort that the respected referees and Associate Editor have dedicated to providing such an insightful review and the required guidance to mention errors and weak points in our manuscript. We have tried to do our best to correct the manuscript according to the referee's comments. Furthermore, since one of the reviewers recommended us to revise the manuscript in terms of English language, The English language has been revised by a native speaker. In the following section, we have addressed all the concerns provided by the referees one by one and made any necessary changes in the revised manuscript. The applied changes are marked up using “track changes” function. Please, find below our response to the reviewers.
Reviewer 1:
1) Although there is an European policy to make hydrogen an energy vector, the past, present and future in the massive use of hydrogen was, is and will be as a (petro)chemical raw material. Then, the main use of hydrogen a raw material must be emphizised in the introduction.
In response to this question, we have added the following sentences in the page 2 of the paper text:
“The increasing world’s population not only leads to nonrenewable fossil sources overconsuming, which increases their cost and generates geopolitical problems, but also emit greenhouse and harmful gases (COx, NOx, SOx). Hence, a replacement of fossil fuels by renewable energy sources is needed in the near future. In this line, hydrogen could be regarded as clean energy carrier (since it only releases H2O during its combustion), which has the potential to replace the traditional fossil fuels for stationery and heavy transportation (mass transport, aviation and naval sectors) applications. Apart from that, hydrogen is a very important industrial feedstock, whose worldwide demand reached 95 Mts/year in 2021, and is expected to increase to 115 Mts/year in 2030, being currently satisfied by steam reforming of natural gas or steam cracking of naphtha.
Governments around the world are promoting policies and measures for greening hydrogen production [1]. Clean hydrogen can be generated from fossil fuels with carbon capture, biomass, or water through thermal, electrolytic, or photolytic processes. Biomass can be thermally processed through gasification or pyrolysis to produce hydrogen [2-5] through industrial processes and can be a complementary, reliable backup source of renewable hydrogen for hydrogen produced by electrolysis using solar or wind.”
2) This phrase should be cautiously rethought: “To the best of our knowledge, the initial materials for synthesizing the catalyst supports and the feedstock for the supercritical water gasification process are reported for the first time”. Even if it is true, the core of the work from a strictly catalytic viewpoint (you are intending to publish in a catalysis journal) is using active carbon as a support (regardless of the origin), and this is not new. Here you must think about the real novelty, like if you really were able to obtain a much better active carbon material in comparison with others already studied and reported. It would be also worthy to mention that you are getting both the catalyst support (with good properties?) and bio-oil from the same biomass to produce hydrogen, which could have some advantages…
According to the reviewer's suggestion, the statement has been corrected and the following text added to the revised version of the manuscript. Please see page 3:
“The novelty of the article is based on the use of novel carbon catalysts and supports obtained by chemical activation with H3PO4. To our knowledge, the use of AC obtained via activation with H3PO4 for the preparation of Ni catalysts for SCWG is reported for the first time. The high surface area and the development of mesoporosity of H3PO4-Acs can improve the performance of previously Ni-AC catalysts used in this process. Moreover, the process would work in a closed loop; catalysts were prepared from the same biomass being used as the feedstock to produce bio-oil for the supercritical water gasification process.”
3) In general, there are a lot of characterization results and fair catalytic test results but the authors did not correlate the catalyst properties (amply studied by different characterization techniques) with the reaction performance in the supercritical water gasification of bio-oil. For example, looking at Table 1 and 4, it could be inferred that the surface area may determine the catalytic activity (the higher the surface area of the support, the higher the gas yield). The authors just mentioned the correlation of the activity with the dispersion in the conclusions but a further explanation is needed in the results section. There may be other properties determining the catalytic activity as well. The authors must do this correlation between catalyst properties and activity to tied up all the results.
We thank the reviewer for bringing our attention to this point. We tried to solve this issue by establishing additional relationships between porosity, chemical speciation and metal distribution and the activity of the catalysts. The following explanations in the results and discussion section on pages 13 and 14:
“The total gas amount, gas composition in volume percent and mmol/g bio-oil along with LHV values obtained at different experimental conditions were summarized in Table 4. As can be seen, the total gas yield ranged from 0.33 to 7.87 mmol/gr bio-oil. The lowest amount was achieved for blank test, performed at 500 ˚C under similar operational conditions. This shows that the catalyst improved the gasification of the feedstock and even resulted in higher total gas yield at lower temperature, 400 ˚C. Furthermore, the total gas yield was found to be highly affected by the temperature under catalytic tests. Temperature plays a key role in gas production because most of the reactions involved in SCWG like hydrolysis, decomposition, and steam reforming are endothermic so increasing the temperature enhances the gasification [45]. As to the catalytic effect, the Ni/AC(PS) led to a higher total gas amount compared to the Ni/AC(OP) and Ni/AC(SP) samples. Furthermore, the Ni/AC(OP) catalyst showed better performance in gas production at 400 and 500 ˚C than the Ni/AC(SP) catalyst. As it was previously discussed, Ni/AC(PS) developed the highest apparent surface area and the highest mesoporosity, among the three catalysts, while AC(SP) gave rise to the lowest porosity development. The presence of a broader porosity is expected to minimize the impact of diffusional constraints in Ni/AC(PS) and Ni/AC(OP) samples. Similarly, Ni/AC(PS) showed the smallest average nanoparticle size, Figure 7, which improves the activity of the active phase. Nevertheless, the varying catalytic activities can largely associate to the existence of distinct active phases. Specifically, the main active phase in Ni/AC(OP) and Ni/AC(SP) is nickel phosphide, according to XPS and XRD measurements. Nickel phosphides are not able to catalyze the steam gasification of bio-oil, however, they are known to be active for hydrogenation reactions [46], and can shift the gas product distribution once hydrogen is formed. In the same line, the presence of a larger amount of active metallic nickel in Ni/AC(PS) is also confirmed by the higher CO-evolution related to carbothermal reduction of nickel during TPD, Figure 6. Therefore, the total gas production can be directly related to the higher porosity development and the largest amount and dispersion of active metallic nickel showed by Ni/AC(PS). The total gas production obtained in this catalyst is within the highest reported for similar conditions, being only surpassed by the one obtained in the work of Nanada et al. [47], which investigated the subcritical and supercritical water gasification of lignocellulosic biomass impregnated with nickel nanocatalysts for hydrogen production. The total gas produced under similar operational conditions at 500 °C was reported to be 7.1 mmol/g.
As for the product distribution, according to results depicted in the Table 4, the gas composition under the blank test are mainly composed of CO2 along with low amounts of CO, CH4 and H2. This clearly indicates that the application of the catalyst for hydrogen generation is critically essential. Regarding Ni/ACs, both Ni/AC(PS) and Ni/AC(OP) showed better catalytic performance in H2 gas production. As expected, the gas composition in terms of CH4 and CO2 are far from those predicted by the methanation and water gas shift equilibriums. Hydrogen production increases with temperature in a larger extent for PS-derived catalyst. The increase in hydrogen amount at higher temperatures is related to the improvement of the radical reactions, which are enhanced at high temperatures [45]. CH4 is the primary product formed at 400°C, being formed as a result of the decomposition of oxygenates, but its concentration diminishes as the gasification temperature increases [48]. Still, methane production is enhanced by Ni/AC(PS) with temperature. Metallic nickel is known to catalyze methanation at mild steam reforming conditions, and temperatures higher than 600 ºC are needed to suppress CH4 formation via steam reforming of the freshly produced methane [49]. The observed temperature-dependent generation of CO aligns with theoretical predictions based on the prevalence of the reverse water gas shift reaction at elevated temperatures. Note that the temperature had little effect for Ni/AC(PS) and Ni/AC(SP) samples on the yield of CO2. Similar trends in the CO2 yield was observed by Duan et al, during the SCWG of microalgae with Ru and Rh/AC [50]. However, the larger CO2/CO ratio shown by Ni/AC(OP) could be indicative of a larger contribution of the WGS for this catalyst.
There are few examples in the literature to directly compare the effectiveness of the catalysts of this work [24,51]. Remon et al. [24] investigated sub- and supercritical water gasification of bio-oil obtained from the fast pyrolysis of pinewood using Ni-Co/Al-Mg catalyst under the operational conditions of temperature (310-450 ˚C), pressure (20-26 MPa), catalyst/bio-oil mass ratio (0-0.25 g catalyst/g bio-oil) and reaction time (0-60 min). They reported that under supercritical conditions at temperature of 339 ˚C, pressure of 20 MPa and catalyst/biooil ratio 0.2 g/g for 60 min, the highest amount of H2 production from bio-oil was 30 vol.%. In another study, Osada et al [51] studied the supercritical water gasification of sugarcane bagasse over Ru/AC and Ru/TiO2. They reported that under the gasification conditions of 0.1 g of bagasse, 0.3 g of catalyst, reaction temperature of 400 ˚C, and reaction time, 15 min, the volume fraction of H2 were, respectively, 9.1 and 14.4 vol.%, respectively. In this work, the volume fraction of H2 at the same temperature are 15.9, 12.7 and 11.5 vol.%, which highlights the outstanding performance of Ni/ACs catalysts obtained from chemical activation with H3PO4 .
Regarding to LHV value, this parameter increased with the process temperature (see Table 4), which is related to the higher contribution of CO and H2 in the gas product distribution. Moreover, compared to Ni/AC(OP) and Ni/AC(SP), the Ni/AC(PS) delivered an enhanced LHV value.”
4) I do not understand what the authors meant in the first paragraph of section 2.3.4 (catalytic test), in part because of the very bad English writing. I guess they are trying to justify why they did not do blank experiments (same reaction tests without catalyst to see the possible thermal conversion). However, this excuse is not acceptable since they claim to have done this blank before for the steam reforming of acetone, which is very different from the reactions they are presenting in this work. The authors must do the experiments without catalyst for this reaction (supercritical water gasification of bio-oil) since the conversion of real/raw bio-oil may greatly differ from that of “model” compounds.
The comment of the reviewer is correct. Since the results under steam reforming of model compounds might be quite different from real bio-oil under supercritical water gasification. According to the reviewer’s suggestion, we performed one experiment at 500 ˚C without using catalyst as a blank test. We have not enough time to carry out additional blank test at 400 and 550 ºC, however, we believe that this result highlights the significance of utilizing a catalyst for achieving high conversions and hydrogen yields levels. The result was included in Table 4 and the explanation related to total gas amount was added in page 13 as follow:
“The total gas amount, gas composition in volume percent and mmol/g bio-oil along with LHV values obtained at different experimental conditions were summarized in Table 4. As can be seen, the total gas yield ranged from 0.33 to 7.87 mmol/gr bio-oil. The lowest amount (0.21 mmol/gr bio-oil) was achieved for blank test, performed at 500 ˚C under similar operational conditions. This shows that the catalyst improved the gasification of the feedstock and even resulted in higher total gas yield at lower temperature, 400 ˚C.”
We also added the following explanation for distribution of gas products on page 14.
“the gas composition under the blank test are mainly composed of CO2 along with low amounts of CO, CH4 and H2. This clearly indicates that the application of the catalyst for hydrogen generation is critically essential.”
5) Also, the explanations of this section (catalytic test) are very vague and basic. Can the authors present their own thermodynamic equilibrium calculations for this system? For this, you need the bio-oil composition, which by the way, it is not in this work. Presenting their own thermodynamic calculations (which is possible by simulating the bio-oil composition) would provide information on the goodness of these catalysts to approach the equilibrium (if the contact time/velocity is sufficiently high/low for this).
We are sorry about not reporting the bio-oil elemental composition in this manuscript. We have included it, and we have also added the estimated gas composition for the thermodynamic equilibrium of the main reactions taking part at the operating conditions. See the next paragraphs on page 13:
From the thermodynamic point of view, the supercritical water gasification of bio-oil is a complex process where many reactions occur simultaneously. The overall chemical conversion can be expressed by the following main reactions, which accounts for the bio-feedstock decomposition into H2, CO, CO2, and CH4.
Steam reforming:
CxHyOz+(2x-z) H2O→xCO2+(y/2+2x-z)H2 (1)
Water gas shift reaction:
CO+H2 O↔CO_2+H2 (2)
Methanation reaction:
CO+3H2↔CH4+H2 O (3)
In this study, the weight percentages of C, H and O elements present in the bio-oil were 33.1%, 6.6% and 58.7%, respectively. Based on these values and considering the stoichiometry of Eq. (1), steam reforming reaction can be rewritten as follows (4):
CH2.4O1.3+0.7 H2 O→CO2+1.9H2 (4)
The reforming reaction of bio-oil (Eq. 4) is highly endothermic and the water gas shift reaction (Eq. 2) is moderately exothermic, giving an overall endothermic process. Therefore, based on Le Chatelier’s principle, high temperatures shift the equilibrium of the endothermic reaction in the forward direction (Eq. (4)), while lower reaction temperatures favor for methane production (Eq. 3). Hence, at high reaction temperatures, hydrogen formation should predominate over methane formation [43]. The thermodynamic equilibrium composition for the gas phase was calculated at the working operation conditions using a Gibbs reactor in AspenOne ® 12.1, finding an estimated composition of 3.0%, 0.4%, 34.4% and 62.2% for H2, CO, CO2 and CH4, respectively, at 500 ºC, which shifted towards higher H2 and CO production at 550 º C (composition of H2, CO, CO2 and CH4 of 4.6%, 0.8%, 34.0%, and 60.7%, respectively).
6) Furthermore, and maybe more important, the catalytic test results must be compared with those obtained with other catalysts (for example a conventional Ni/Al2O3 catalyst) to really see the advantage (if any) of this active carbon supports. This comparison would bring up opportunities to highlight the benefits of this catalyst formulation with active carbon (if any).
We agree with the reviewer. In order to clarify the potential benefits of the Ni/ACs prepared on this work, the behavior of the catalysts has been compared with those obtained by other research groups under similar operation conditions, and has been reflected in the next paragraph in the manuscript, see page 14:
“Only a few published works may directly compare to the effectiveness of this work. For example, Remon et al [23] investigated sub- and supercritical water gasification of bio-oil obtained from the fast pyrolysis of pinewood with the use of Ni-Co/Al-Mg catalyst under the operational conditions: temperature of 310-450 ˚C, the pressure of 20-26 MPa, catalyst/bio-oil mass ration of 0-0.25 g catalyst/g bio-oil and reaction time of 0-60 min. They obtained the highest amount of H2 by 30 vol.% that was produced under supercritical conditions at a temperature of 339 ˚C, pressure of 20 MPa, and catalyst/bio-oil ratio 0.2 g/g after 60 min. from bio-oil. While in the present work, the maximum amount of obtained H2 was 41 vol.%”
“In another study, the Osada et al [48], studied the supercritical water gasification of sugarcane bagasse over Ru/AC and Ru/TiO2. They reported that under the gasification conditions of reactant, 0.1 g; catalyst, 0.3g; reaction temperature, 400 ˚C; and reaction time, 15 min, the volume fraction of H2 was respectively 9.1 and 14.4 vol.% over Ru/AC and Ru/TiO2 catalysts. In this work the volume fraction of H2 at the same temperature were 15.9, 12.7 and 11.5 vol.%, which highlight the superiority of catalytic performance of the Ni/ACs compared to Ru/AC.”
7) Nothing is mention about the catalyst stability. Would this catalyst formulation with an active carbon support be stable over time and over usages/cycles? Do you detect coke formation? How can coke deposits be analyzed? Can coke be removed from this catalyst without damaging the active carbon support?
We would like to thank the reviewer for this accurate comment. Resistance to coke deactivation is one of the main issues that needs to be improved in Ni-catalysts for supercritical gasification and steam reforming. The nature of the coke formed in the catalyst can be characterized by Raman, XRD, TEM and TPO. Before analyzing the coke formed in the catalyst, we had to develop a separation procedure based on organic solvent washing to separate the charred biomass from the AC catalyst, which would allow to test cyclability of the catalysts. At this point, we have been able to successfully recover the catalyst from the solid fraction obtained at the end of the experiment using Soxhlet extraction with THF and subsequent filtration. Unfortunately, we have been not able to produce the results on cyclability topics due to time limitations. In addition, the techniques needed for characterizing the coke are unavailable for the researchers at Shiraz University, and the samples are to be submitted to University of Malaga for completing their study. Consequently, the study of the formed coke is not covered in this manuscript. Still, we firmly believe that it is relevant to report the improvements found in the catalytic activity when sustainable mesoporous H3PO4-activated ACs are used as supports for Ni-catalysts, and how it is possible to increase the sustainability of the process by using the same feedstocks for both the preparation of the catalytic support and as raw material for the hydrogen production.
In accordance with the reviewer comment, we will continue studying these catalysts and similar ones in the near future, and in subsequent works we will be reporting the coke forming analysis.
8) There are many bad/inappropriate uses of English and some phrases do not even make sense. For example:
We are deeply sorry about the bad grammar use and abundant typos found in the previous version of the manuscript. We have made a strong effort to improve the English in this new version by consulting a native English speaking about how to improve the text, and we have amended all the mistakes indicated by both reviewers and any other found in the revision of the text. We hope that the reviewer can find this time our work suitable for publication.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript (catalysts-2336270) described the application of different activated carbon as support for the water gasification of bio oil. The topic is interesting, however, the following concerns should be addressed before the consideration for publication.
1. Please pay attention to the statement in the abstract ". The BET results showed that the ACs developed considerable amounts of pore structure, , containing both micro and meso pores" since BET is the model used for the evaluation of specific surface area, which can not indicate the pore structure.
2. There are many mistakes such as "Figure 1 represents the N2 adsorption–desorption isotherms at 196 °C for three activated carbons.....", it should be -196 oC. Please carefully check the whole manuscript to modify these kinds of mistakes.
3. There are also mistakes for the images, such as the vertical coordinate content in Figure 2, the unit is missing for Figure 3.
4. It is strange that the particle size distribution for Ni/AC(OP) is not obeying the normal distribution, please explain this.
5. Please make a deeper insights into the intrinsic reason for the catalytic performance, the influence of pore structure, the chemical state, the particle size etc. should be analyzed.
6. How about the cycling stability of the catalyst?
7. The following manuscripts are recommended for the citation, especially in the analysis of pore structure: Journal of Hazardous Materials, 452 (2023) 131319, Nano Research, 2022, DOI: doi.org/10.1007/s12274-022-5250-1
Author Response
Reviewer 2:
Please pay attention to the statement in the abstract ". The BET results showed that the ACs developed considerable amounts of pore structure, containing both micro and meso pores" since BET is the model used for the evaluation of specific surface area, which cannot indicate the pore structure.
According to the reviewer’s comment, the sentence has been corrected in the revised version of the manuscript as follows:
“The adsorption results showed that the ACs developed both micro- and mesopores,”
.
2. There are many mistakes such as "Figure 1 represents the N2 adsorption–desorption isotherms at 196 °C for three activated carbons", it should be -196 oC. Please carefully check the whole manuscript to modify these kinds of mistakes.
We would like to thank the reviewer for helping us to improve the English language of the paper. We tried to read carefully the text and correct it technically and grammatically. We have uploaded a version of the manuscript where control of changes is activated, so that he/she can check all the amendments inserted in the text.
There are also mistakes for the images, such as the vertical coordinate content in Figure 2, the unit is missing for Figure 3.
We are sorry about this mistake. In response to this question, we checked the figures and revised or added the required units and labels.
It is strange that the particle size distribution for Ni/AC(OP) is not obeying the normal distribution, please explain this.
We thank the reviewer for noting this mistake in the frequency distribution of the particle sizes. In order to solve this issue, the particle size is measured again, and the revised figure is replaced. The result is improved and distribution is showing a better marching to the normal distribution.
Please see page 14 in the manuscript.
Please make a deeper insight into the intrinsic reason for the catalytic performance, the influence of pore structure, the chemical state, the particle size etc. should be analyzed.
We are thankful to the reviewer for this comment. We tried to go deeper to answer this question. The following explanations are added to the paper on pages 13 and 14.
“The total gas amount, gas composition in volume percent and mmol/g bio-oil along with LHV values obtained at different experimental conditions were summarized in Table 4. As can be seen, the total gas yield ranged from 0.33 to 7.87 mmol/gr bio-oil. The lowest amount was achieved for blank test, performed at 500 ˚C under similar operational conditions. This shows that the catalyst improved the gasification of the feedstock and even resulted in higher total gas yield at lower temperature, 400 ˚C. Furthermore, the total gas yield was found to be highly affected by the temperature under catalytic tests. Temperature plays a key role in gas production because most of the reactions involved in SCWG like hydrolysis, decomposition, and steam reforming are endothermic so increasing the temperature enhances the gasification [45]. As to the catalytic effect, the Ni/AC(PS) led to a higher total gas amount compared to the Ni/AC(OP) and Ni/AC(SP) samples. Furthermore, the Ni/AC(OP) catalyst showed better performance in gas production at 400 and 500 ˚C than the Ni/AC(SP) catalyst. As it was previously discussed, Ni/AC(PS) developed the highest apparent surface area and the highest mesoporosity, among the three catalysts, while AC(SP) gave rise to the lowest porosity development. The presence of a broader porosity is expected to minimize the impact of diffusional constraints in Ni/AC(PS) and Ni/AC(OP) samples. Similarly, Ni/AC(PS) showed the smallest average nanoparticle size, Figure 7, which improves the activity of the active phase. Nevertheless, the varying catalytic activities can largely associate to the existence of distinct active phases. Specifically, the main active phase in Ni/AC(OP) and Ni/AC(SP) is nickel phosphide, according to XPS and XRD measurements. Nickel phosphides are not able to catalyze the steam gasification of bio-oil, however, they are known to be active for hydrogenation reactions [46], and can shift the gas product distribution once hydrogen is formed. In the same line, the presence of a larger amount of active metallic nickel in Ni/AC(PS) is also confirmed by the higher CO-evolution related to carbothermal reduction of nickel during TPD, Figure 6. Therefore, the total gas production can be directly related to the higher porosity development and the largest amount and dispersion of active metallic nickel showed by Ni/AC(PS). The total gas production obtained in this catalyst is within the highest reported for similar conditions, being only surpassed by the one obtained in the work of Nanada et al. [47], which investigated the subcritical and supercritical water gasification of lignocellulosic biomass impregnated with nickel nanocatalysts for hydrogen production. The total gas produced under similar operational conditions at 500 °C was reported to be 7.1 mmol/g.
As for the product distribution, according to results depicted in the Table 4, the gas composition under the blank test are mainly composed of CO2 along with low amounts of CO, CH4 and H2. This clearly indicates that the application of the catalyst for hydrogen generation is critically essential. Regarding Ni/ACs, both Ni/AC(PS) and Ni/AC(OP) showed better catalytic performance in H2 gas production. As expected, the gas composition in terms of CH4 and CO2 are far from those predicted by the methanation and water gas shift equilibriums. Hydrogen production increases with temperature in a larger extent for PS-derived catalyst. The increase in hydrogen amount at higher temperatures is related to the improvement of the radical reactions, which are enhanced at high temperatures [45]. CH4 is the primary product formed at 400°C, being formed as a result of the decomposition of oxygenates, but its concentration diminishes as the gasification temperature increases [48]. Still, methane production is enhanced by Ni/AC(PS) with temperature. Metallic nickel is known to catalyze methanation at mild steam reforming conditions, and temperatures higher than 600 ºC are needed to suppress CH4 formation via steam reforming of the freshly produced methane [49]. The observed temperature-dependent generation of CO aligns with theoretical predictions based on the prevalence of the reverse water gas shift reaction at elevated temperatures. Note that the temperature had little effect for Ni/AC(PS) and Ni/AC(SP) samples on the yield of CO2. Similar trends in the CO2 yield was observed by Duan et al, during the SCWG of microalgae with Ru and Rh/AC [50]. However, the larger CO2/CO ratio shown by Ni/AC(OP) could be indicative of a larger contribution of the WGS for this catalyst.
There are few examples in the literature to directly compare the effectiveness of the catalysts of this work [24,51]. Remon et al. [24] investigated sub- and supercritical water gasification of bio-oil obtained from the fast pyrolysis of pinewood using Ni-Co/Al-Mg catalyst under the operational conditions of temperature (310-450 ˚C), pressure (20-26 MPa), catalyst/bio-oil mass ratio (0-0.25 g catalyst/g bio-oil) and reaction time (0-60 min). They reported that under supercritical conditions at temperature of 339 ˚C, pressure of 20 MPa and catalyst/biooil ratio 0.2 g/g for 60 min, the highest amount of H2 production from bio-oil was 30 vol.%. In another study, Osada et al [51] studied the supercritical water gasification of sugarcane bagasse over Ru/AC and Ru/TiO2. They reported that under the gasification conditions of 0.1 g of bagasse, 0.3 g of catalyst, reaction temperature of 400 ˚C, and reaction time, 15 min, the volume fraction of H2 were, respectively, 9.1 and 14.4 vol.%, respectively. In this work, the volume fraction of H2 at the same temperature are 15.9, 12.7 and 11.5 vol.%, which highlights the outstanding performance of Ni/ACs catalysts obtained from chemical activation with H3PO4.
Regarding to LHV value, this parameter increased with the process temperature (see Table 4), which is related to the higher contribution of CO and H2 in the gas product distribution. Moreover, compared to Ni/AC(OP) and Ni/AC(SP), the Ni/AC(PS) delivered an enhanced LHV value.”
How about the cycling stability of the catalyst?
We would like to thank the reviewer for this accurate comment. Resistance to coke deactivation is one of the main issues that needs to be improved in Ni-catalysts for supercritical gasification and steam reforming. Indeed, we are currently working on the characterization and recovery of the catalysts used in this work. We are developing a separation procedure based on organic solvent washing to separate the charred biomass from the AC catalyst. Unfortunately, we have been not able to produce the results on these topics due to time limitations, and so the stability study is not covered in this manuscript. We still believe that it is relevant to report the improvements found in the catalytic activity when sustainable mesoporous H3PO4-activated ACs are used as supports for Ni-catalysts, and how it is possible to increase the sustainability of the process by using the same feedstocks for both the preparation of the catalytic support and as raw material for the hydrogen production.
In accordance with the reviewer comment, we will continue studying these catalysts and similar ones in the near future, and in subsequent works we will be reporting their stability under different conditions, cycling stability and coke forming analysis.
The following manuscripts are recommended for the citation, especially in the analysis of pore structure: Journal of Hazardous Materials, 452 (2023) 131319, Nano Research, 2022, DOI: doi.org/10.1007/s12274-022-5250-1
We thank again the reviewer for providing us these valuable manuscripts. We read them and include them as references, being cited as 25, 26 in the references lists.
Please see page 18 in the manuscript.
With kind regards,
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors made lot of changes in the text taking into consideration most of my comments. They improved the explanations for the reaction test results and the correlations between the catalyst characterization and the catalytic performance. So that, the manuscript is now suitable for publication.
Reviewer 2 Report
The author has well addressed the comment. The manuscript is suggested for the acceptance.
