The Demonstration of the Superiority of the Dual Ni-Based Catalytic System for the Adjustment of the H₂/CO Ratio in Syngas for Green Fuel Technologies

Round 1

Reviewer 1 Report

This paper is aimed to the demonstration of higher performances of the dual Ni-base catalytic system compared to other catalysts’ performance retrievable in the scientific literature. The experimental investigation is aimed to the determination of the optimal temperature and feed ratio for the adjustment of H₂/CO ratio in syngas. Since the catalytic reactions and applying heterogeneous catalysis have high impact on the preparation of eco-friendly and renewable energy sources, this investigation deserves attention.

 

The paper is well developed and deals interesting and up-to-date topic, but some comments are to be made:

1. Regarding catalyst preparation, section 2.1.1., solution B is ‘stirred at 80°C to 90°C for 1 h’ (line 117). Is it possible to clarify better at which temperature the solution is stirred? Similarly, in section 2.1.2. (line 132), is possible to determine the exact temperature of the refluxed solution?
2. In section 2.2. I suggest describing in a chronological pattern how surface area, pore volume and size is determined (e.g. place sentence at line 147-148 before the explanation of the analysis).
3. For clarity sake, I suggest splitting section 2.2. into two subsequent sections describing the morphological and the physical characterization. I suggest also to split section 3.1. according to section 2.2. Furthermore, to improve readability, split long paragraph into shorter one (e.g. section 3.1. paragraph at lines 196-210 could be split into two shorter paragraphs).
4. Please provide references for eq. (1) and (2).
5. To avoid repetition of the H₂O/(CH₄+CO₂) ratio, please consider defining it with an acronym.
6. In the results section some information regarding measurement uncertainties should be provided.
7. A list of acronyms used in the paper should be given.
8. There are some typing and formatting mistakes such as:
   1. in fractional measurement units there is no space between the single units (e.g. p. 3 line 133 °Cmin^-1 instead of °C min^-1, p. 4 line 141 m²g^-1 instead of m² g^-1), and be consistent through all the paper;
   2. at p. 3 line 114 avoid the use of the article ‘the’ before ‘ambient temperature’
   3. in p. 6 line 215 there is H3 instead of H₂;
   4. in p. 7 line 233, please reformat Table 1 in a more compact way, and with attention to the footnotes;
   5. 8 line 244 the reference should be placed just after the name of the first author;
   6. 11 line 339 ‘reached to 2’ instead of ‘reached 2’
   7. please check reference 37;
   8. please uniform the formatting style all over the manuscript (e.g. paragraph 2. Materials and methods should look like paragraph 1. Introduction);

therefore, a thoroughly revision of the manuscript, in order to avoid this kind of mistakes, is suggested.

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 1 Comments

 

First of all, according to the style of Catalysts journal, we would like to inform reviewers that “Results and Discussion” section appears before the “Material and Methods” section, thus the References, Figures, Tables and Equations in these parts of revised manuscript was reordered.

 

 

Point 1: Regarding catalyst preparation, section 2.1.1., solution B is ‘stirred at 80°C to 90°C for 1 h’ (line 117). Is it possible to clarify better at which temperature the solution is stirred? Similarly, in section 2.1.2. (line 132), is possible to determine the exact temperature of the refluxed solution?

Response 1: We thank the reviewers for this comment. We have clarified the exact temperatures in red. The solution is stirred at 80 °C (line 329) and the saddle brown solution was refluxed at 70 °C (line 343);

Revise :

line 329

At the same time, aluminum isopropoxide (Al[OCH(CH₃)₂]₃, Sigma Aldrich) was dissolved in distilled water (denoted as solution B) and continually stirred at 80 °C for 1 h.

line 343

The saddle brown solution was refluxed at 70 °C for 2 h to form the gel product.

 

Point 2: In section 2.2. I suggest describing in a chronological pattern how surface area, pore volume and size is determined (e.g. place sentence at line 147-148 before the explanation of the analysis).

Response 2: We agree to your comment. Accordingly, we rearranged the H₂-TPD part by placing the sentence of how to determine the metal dispersion and the metal particle size (line 360-368) before the explanation of the analysis (line 369-373):

Revise :

line 360-373

According to the stoichiometry of 1:1 for the chemisorbed hydrogen atom on the Ni surface, the metal dispersion ( , %) and the metal particle size ( , nm) were calculated from Equations (5) and (6), respectively using the H₂-TPD results [64,65]. In these equations,  is an amount of hydrogen desorption (cm³);  is a stoichiometry factor;  is an atomic weight of metal (g mol^-1);  is a sample weight (g);  is a wt % of supported metal content;  is a crosssectional area of one metal atom (nm²); and is a density of metal (g cm^-3).

                                                                                                                                 (5)

                                                                                                                          (6)

Before analysis, 50 mg of the calcined catalyst were reduced in situ at 600 °C in a H₂ flow of 50 mL min^-1 for 2 h, followed by cooling to 100 °C in Ar flow of 50 mL min^-1. Consequently, H₂ was isothermally chemisorbed on the surface of the sample at 100 °C for 0.5 h and the sample was cooled to ambient temperatures in Ar flow of 50 mL min^-1. The desorbed H₂ was measured by a TCD during the temperature programmed from 40 °C to 900 °C under Ar flow of 50 mL min^-1.

 

Point 3: For clarity sake, I suggest splitting section 2.2. into two subsequent sections describing the morphological and the physical characterization. I suggest also to split section 3.1. according to section 2.2. Furthermore, to improve readability, split long paragraph into shorter one (e.g. section 3.1. paragraph at lines 196-210 could be split into two shorter paragraphs).

Response 3: We agree that your suggestion for the management of section 2 will improve readability of our article.

1. We split section 2.2. (the revised manuscript reordered to section 3.2) into two subsequent sections describing the morphological (starting at line 350) and the physical characterization (starting at line 359):

Revise:

3.2. Catalyst characterization

3.2.1. Morphological characterization

The crystalline phases of the catalyst samples were examined by XRD analysis using an X-ray diffractometer (PANalytical X’Pert-Pro) with nickel-filtered Cu K  ( = 1.54178 , 2  range from 10° to 80°), a monochromatized radiation source, operated at 40 kV and 30 mA, having the scanning rate of 0.02° with 0.5 s per step.

The specific surface area (S_BET, m² g^-1), pore volume (Vp, cm³ g^-1), and average pore size diameter (nm) were characterized by N₂ adsorption/desorption isotherms, which were measured at -196 °C using BELSORP-mini II instrument. The pore size distribution curve was calculated from the analysis of the desorption branch of the isotherm by the BJH method.

3.2.2. Physical characterization

According to the stoichiometry of 1:1 for the chemisorbed hydrogen atom on the Ni surface, the metal dispersion ( , %) and the metal particle size ( , nm) were calculated from Equations (5) and (6), respectively using the H₂-TPD results [64,65]. In these equations,  is an amount of hydrogen desorption (cm³);  is a stoichiometry factor;  is an atomic weight of metal (g mol^-1);  is a sample weight (g);  is a wt % of supported metal content;  is a crosssectional area of one metal atom (nm²); and is a density of metal (g cm^-3).

                                                                                                                                 (5)

                                                                                                                          (6)

Before analysis, 50 mg of the calcined catalyst were reduced in situ at 600 °C in a H₂ flow of 50 mL min^-1 for 2 h, followed by cooling to 100 °C in Ar flow of 50 mL min^-1. Consequently, H₂ was isothermally chemisorbed on the surface of the sample at 100 °C for 0.5 h and the sample was cooled to ambient temperatures in Ar flow of 50 mL min^-1. The desorbed H₂ was measured by a TCD during the temperature programmed from 40 °C to 900 °C under Ar flow of 50 mL min^-1.

The reducibility of the calcined catalyst was evaluated via the H₂-TPR technique performed in the BELCAT-basic system. In this analysis, 50 mg of the calcined catalyst were degassed at 220 °C for 1 h in Ar flow of 30 mL min^-1, followed by cooling to 40 °C. After, the sample was reduced in the temperature programmed from 40 °C to 900 °C under 5%H₂/Ar flow of 50 mL min^-1. The H₂ consumption was detected by the TCD.

The quantity and nature of the deposited carbon over the spent catalyst was measured by TGA and DTG using a METTLER TOLEDO thermogravimetric analyzer. The catalyst sample weight loss and the derivative thermogravimetric curve of the weight loss versus temperature were collected continuously under flowing air up to 800 °C with a heating rate of 10 °C min^-1.

2. We also split section 3.1 (the revised manuscript became section 2.1). into two subsequent sections according to section 2.2 (the revised manuscript became section 3.2). The Morphological characterization starts at line 125 and the Physical characterization starts at line 168)
3. We spit paragraph at lines 196-210 in the last manuscript into two shorter paragraphs (line 126-133 and line 134-140) and split long paragraph into shorter one through all the paper:

Revise:

2. Results and discussion

2.1. Catalyst characterization

2.1.1 Morphological characterization

The diffraction patterns of the calcined NiCo/Mg-Al and the Ni/5Ce-Al catalysts were investigated through the XRD as displayed in Figure 1. The NiCo/Mg-Al catalyst revealed characteristic diffraction peaks of MgAl₂O₄ spinel at 2  angles of 36.8° 44.8°, and 65.3° (JCPDS 77-0435); these peaks could also be assigned to the spinel phases of NiAl₂O₄ (JCPDS 78-0552) and CoAl₂O₄ (JCPDS 82-2243) because Ni²⁺, Co²⁺, and Mg²⁺ can incorporate into the identical lattice with Al₂O₃ [33]. However, these diffraction peaks were not easily to be identified because of either lower calcination temperatures or an existing overlap between the diffraction peaks of NiAl₂O₄ (or CoAl₂O₄) and the peaks of MgAl₂O₄ [34,35].

The Ni/5Ce-Al catalyst showed characteristic diffraction peaks of γ-Al₂O₃ at 2  angles of 37.5°, 45.6°, and 66.6° (JCPDS 50-0741). At the same time, the broad peaks at 2  angles of 28.5° and 47.5° (JCPDS 34-0394) are attributed to the cubic fluorite type structure of CeO₂. Diffraction peaks that correspond to the crystalline species of NiO (JCPDS 89-7131) and Co₃O₄ (JCPDS 76-1802) in cubic structures were not observed for all calcined catalysts. This occurrence indicates that the active metal species transformed into the spinel structure (especially the NiCo/Mg-Al catalyst) or the high dispersion of the active metal species on the surface of the support [36].

Figure 1. XRD patterns of calcined catalysts.

The N₂ adsorption-desorption isotherms of all catalysts are illustrated in Figure 2. According to the IUPAC classification, the NiCo/Mg-Al and Ni/5Ce-Al catalysts showed type IV isotherm curves with different hysteresis loops. The loop of NiCo/Mg-Al catalyst indicates a H3 hysteresis behavior associated with solids containing aggregates or agglomerations of particles, representing slit-like pores (plates or edged particles like cubes) with a non-uniform size and/or shape [2]. The H2 hysteresis behavior observed on Ni/5Ce-Al catalyst refers to pores with narrow mouths and an ink-bottle shape [37,38]. The pore size distributions of the NiCo/Mg-Al and the Ni/5Ce-Al catalysts comprise a mesoporous material with a pore diameter range of 3 to 30 nm and 5 to 10 nm, respectively.

The structural properties of calcined catalysts are summarized in Table 1. The NiCo/Mg-Al catalyst has a surface area of 130 m² g^-1 and a pore volume of 0.4 cm³ g^-1, and an average pore size diameter of 11.2 nm, which is in agreement with other studies [2,39]. The Ni/5Ce-Al catalyst presented the surface area of 183 m² g^-1 with a pore volume of 0.6 cm³ g^-1 and an average pore size diameter of 12.5 nm, which is located in the same range compared to other published research [40–42]. Moreover, these two catalysts disclosed the small metal particle size due to the high metal dispersion as explained by the XRD results.

Figure 2. N₂ adsorption-desorption isotherms and BJH pore distributions of calcined

• NiCo/Mg-Al and (b) Ni/5Ce-Al catalysts.

 

Table 1. Physicochemical properties of calcined catalysts.

Catalysts	SBET (m2 g-1) a	Vp (cm3 g-1) a	Average pore size diameter (nm) a	%  b	(nm) b	H2-uptakes (µmol g-1)
						Actual c	Theoretical d
NiCo/Mg-Al	130	0.4	11.2	32.2	2.0	1616	1700
Ni/5Ce-Al	183	0.6	12.5	23.8	2.7	1428	1704

^a calculated by the BET Equation with about 5% systematic error; ^b calculated from H₂-TPD results with about 8% systematic error; H₂ consumption calculated experimentally (Actual ^c) from TPR profiles (with about 8% systematic error) after complete reduction at T = 900 °C and theoretical values (Theoretical ^d) determined based on metal loading.

2.1.2 Physical characterization

Figure 3 presents the H₂-TPR profiles of NiCo/Mg-Al and Ni/5Ce-Al catalysts. The TPR profile of NiCo/Mg-Al catalyst displayed shoulder peaks at lower temperatures (a range of 150 °C to 450 °C), which correlate to the reduction of Co₃O₄ to CoO and CoO to metallic Co⁰. The broad peak centered at 520 °C relates to the reduction of Ni²⁺ to Ni⁰ [3]. The peak at a high temperature of around 840 °C indicates the reduction of Ni or Co species with strong interactions due to the metal alloy effect and/or SMSI effect regarding the NiCo-based catalyst [43]. A similar trend of H₂-TPR behavior was also reported in Li et al. [44], indicating high reduction temperatures because of the formation of NiCo alloy phases. Furthermore, the reduction peak at a high temperature (825 °C) with a NiCo catalyst has been assigned as the reduction of small active metal particles and possibly the reduction of nickel and cobalt aluminate-like compounds (NiAl₂O₄ and CoAl₂O₄ spinel structures) [45].

For the TPR profile of the Ni/5Ce-Al catalyst, small broad peaks from 200 °C to 400 °C were observed. The peak at around 270 °C can be ascribed to the reduction of the CeO₂ and the NiO interacting with the partial bulk CeO₂ while the peak at around 370 °C correlates to the reduction of free NiO on the catalyst support [46]. In accordance with the XRD analyses from Figure 1, the high intensity peak at 700 °C illustrates the reduction of Ni²⁺ ions in the amorphous spinel phases with non-stoichiometry of nickel aluminate (NiAl_xO_y) and stoichiometry of nickel aluminate (NiAl₂O₄) [47]. Actual H₂-uptakes calculated from H₂-TPR profiles for the prepared catalysts were close to the theoretical H₂-uptake, are listed in Table 1. The result implies that the Ni²⁺ species were fully reduced. Although the TPR profiles suggest a reduction temperature of over 650 °C, the temperature for the reduction of all catalysts was limited at 650 °C to avoid the agglomeration of active metal according to the calcination temperature of the NiCo/Mg-Al catalyst [48].

Figure 3. H₂-TPR profiles of calcined catalysts.

 

Point 4: Please provide references for eq. (1) and (2).

Response 4: We agree with the relevance of this reference, reference numbers  [64,65] are added for eq. (1) and (2). However, in the revised manuscript regarding the style of catalysts journal, these equations became eq. (5) and (6) (line 360-362):

Revise:

According to the stoichiometry of 1:1 for the chemisorbed hydrogen atom on the Ni surface, the metal dispersion ( , %) and the metal particle size ( , nm) were calculated from Equations (5) and (6), respectively using the H₂-TPD results [64,65].

 

Point 5: To avoid repetition of the H₂O/(CH₄+CO₂) ratio, please consider defining it with an acronym.

Response 5: We think so. Then, we use S/C ratio instead of the H₂O/(CH₄+CO₂) ratio as an acronym in the revised manuscript.

 

Point 6: In the results section some information regarding measurement uncertainties should be provided.

Response 6: We have done the test for systematic error. As both TPD and TPR are tested in the same instrument, the systematic error is about 1% for temperature and about 8 % for the quantity of gaseous substances. The systematic error for BELSORP-mini II instrument is about 5%. We provided these measurement uncertainties below Table 1:

Table 1. Physicochemical properties of calcined catalysts.

Catalysts	SBET (m2 g-1) a	Vp (cm3 g-1) a	Average pore size diameter (nm) a	%  b	(nm) b	H2-uptakes (µmol g-1)
						Actual c	Theoretical d
NiCo/Mg-Al	130	0.4	11.2	32.2	2.0	1616	1700
Ni/5Ce-Al	183	0.6	12.5	23.8	2.7	1428	1704

^a calculated by the BET equation with about 5% systematic error; ^b calculated from H₂-TPD results with about 8% systematic error; H₂ consumption calculated experimentally (Actual ^c) from TPR profiles (with about 8% systematic error) after complete reduction at T = 900 °C and theoretical values (Theoretical ^d) determined based on metal loading.

 

 

Point 7: A list of acronyms used in the paper should be given.

Response 7: We thank for the reviewer's comment. We added a list of acronyms used in the paper after conclusions:

Nomenclature:

List of acronyms

BET                                Brunauer-Emmett-Teller

BJH                       Barrett-Joyner-Halenda

CRM                    CO₂ reforming of methane

CSCRM               combined steam and CO₂ reforming of methane

DCP                     dual Ni-based catalytic process

DTG                     derivative thermogravimetric analysis

EISA                     evaporation-induced self-assembly

FTs                       Fischer-Tropsch synthesis

GHSV                  gas hourly space velocity

GTL                      Gas-to-liquids

H₂-TPD                hydrogen temperature programmed desorption

H₂-TPR                hydrogen temperature programmed reduction

RWGS                  Reverse water gas shift

S/C ratio              steam-to-carbon (H₂O/(CH₄ + CO₂) ratio

SMSI                    strong metal support interaction

TCD                     thermal conductivity detector

TGA                     thermogravimetric analysis

UHT-WGS          ultra high temperature water-gas shift

XRD                     X-ray diffraction

 

Point 8: There are some typing and formatting mistakes such as:

1. in fractional measurement units there is no space between the single units (e.g. p. 3 line 133 °Cmin^-1instead of °C min^-1, p. 4 line 141 m²g^-1 instead of m² g^-1), and be consistent through all the paper;
2. at p. 3 line 114 avoid the use of the article ‘the’ before ‘ambient temperature’
3. in p. 6 line 215 there is H3 instead of H₂;
4. in p. 7 line 233, please reformat Table 1 in a more compact way, and with attention to the footnotes;
5. 8 line 244 the reference should be placed just after the name of the first author;
6. 11 line 339 ‘reached to 2’ instead of ‘reached 2’
7. please check reference 37;
8. please uniform the formatting style all over the manuscript (e.g. paragraph 2. Materials and methods should look like paragraph 1. Introduction);

therefore, a thoroughly revision of the manuscript, in order to avoid this kind of mistakes, is suggested.

Response 8: We thank the reviewer very much for pointing this out. We have checked the typing and formatting mistakes through the manuscript and revised accordingly. For comment (g) “in p. 6 line 215 there is H3 instead of H₂”, we keep the content of H3 because it is the characterization of the hysteresis loop that we used for explanation of the catalyst property. For the bibliography (g), according to the Mendeley, a free reference management software, that suggests on the website of catalysts journal, we cannot change the uppercase.

 

Reviewer 2 Report

This work deals with the development of a dual Ni-based catalytic system for the DCP process in order to achieve a H₂/CO ratio of 2 in the syngas produced. Although the idea behind the study represent an interesting and novel concept, I would suggest a major revision before its publication. Indeed, a better and more clarified explanation of the advantages gained with the adoption of this system could be furnished by the authors, thus particularly adjusting the abstract, the introduction and the keywords. I would suggest to the authors to better identify and present the actual issue and present their solution in a more schematic way.

Some doubts concerning the materials and methods part are listed below:

• The two catalysts, CRM and UHT-WGS, have been calcined at different temperatures 650 and 600°C respectively, while the in-situ hydrogen reduction took place until 900°C. Since the reaction temperatures were chosen as 500°C, 550°C and 600°C, why the authors adopted different calcination temperatures for the two samples?
• Through the preparation part is not clearly exposed to what are referred the percentages of the metals, it would need to be better clarified.
• A scheme of the used plant, or at list a description of it, would be useful to better understand how the activity tests were performed, since at the present only a description of the used reactor is present.

Suggestion and doubts concerning the experimental section are presented:

• The first part written in the DCP section. concerning the presentation of the reaction sistem, I would suggest putting it in the introduction section since, in this section, only the obtained results should be presented.
• In the Figures 5a and 5b values of CH₄ and CO₂ conversion comparable or even higher are present at 500°C compared to those obtained at 550°C and 600°C, could the authors provide a discussion of these results?
• Which are the conditions used for the tests? No values of GHSV or WHSV are present.

Typing errors:

1. There is a mistake in the title, I supposed that should be “Ni-based” instead of “Ni-base”.
2. Line 61: “The authors concluded” instead of the “authors conclude”.
3. Line 133: A space is missing in 3 °Cmin ^-1.
4. Line 136: A space is missing in 3 °Cmin ^-1.
5. Line 171: “Catalytic test” would be more appropriate than “catalyst tests”.
6. Line 201: “these diffraction peaks were not easily to identified” should be “these diffraction peaks were not easily to be identified”.
7. In the bibliography all the superscripts and subscripts are not present.

Author Response

Response to Reviewer 2 Comments

First of all, according to the style of Catalysts journal, we would like to inform reviewers that “Results and Discussion” section appears before the “Material and Methods” section, thus the References, Figures, Tables and Equations in these parts of revised manuscript was reordered.

 

Point 1: I would suggest a major revision before its publication. Indeed, a better and more clarified explanation of the advantages gained with the adoption of this system could be furnished by the authors, thus particularly adjusting the abstract, the introduction and the keywords. I would suggest to the authors to better identify and present the actual issue and present their solution in a more schematic way.

Response 1: We thank very much for your suggestions that truly improve our article. Therefore, we particularly adjusted the abstract, the introduction and the keywords in order to make a better and more clarified explanation of the advantages gained with the adoption of this system.

We adjusted the abstract in line 19-31, the keywords in line 33-34 and the introduction (line 38-40 and line 88-121, the revised text writes as follows;

Revise:

line 19-31:

Abstract: A novel dual Ni-based catalytic process (DCP) to control the H₂/CO ratio of 2 in the syngas product within one step at temperature < 700 °C was created and constructed. With the sequence of the catalysts located in the single reactor, the endothermic combined steam and CO₂ reforming of methane (CSCRM) reaction and the exothermic ultra high temperature water-gas shift (UHT-WGS) reaction work continuously. During the process, the H₂/CO ratio is raised suddenly at UHT-WGS after the syngas is produced from CSCRM and CSCRM utilizes the heat released from UHT-WGS. Because of these features, DCP is more compact, enhances energy efficiency, and thus, decreases a capital cost compared to reformers connecting with shift reactors. To prove this propose, the DCP tests were demonstrated in a fixed-bed reactor under various conditions (temperature = 500 °C, 550 °C, and 600 °C; the feed mixture (CH₄, CO₂, H₂O, and N₂) with H₂O/(CH₄ + CO₂) ratio = 0.33, 0.53, and 0.67). According to the highest CH₄ conversion (around 65%) with carbon tolerance, the recommended conditions for producing syngas with the H₂/CO ratio of 2 as a feedstock of Fischer-Tropsch synthesis include the temperature of 600 °C and the H₂O/(CH₄ + CO₂) ratio of 0.53.

line 33-34:

Keywords: dual catalytic process; low temperature syngas production; methane reforming; Ni-based catalysts; ultra high temperature water-gas shift; Fischer-Tropsch synthesis

 

Introduction

line 38-40:

The ongoing contamination of air by greenhouse gases has been a critical issue as it causes adverse environmental impacts around the world. GTL is a process that is considered as an alternative energy processing technology to convert natural gas into clean burning-liquid fuels such as gasoline, jet fuel, and diesel [1–3].

line 88-121:

In previous work [31], 5wt%Ni5wt%Co/MgO-Al₂O₃ (NiCo/Mg-Al) showed the higher metal dispersion, smaller metal particle size, and a high reducibility due to the effect of the metal-metal interaction. As a result, a valuable CRM catalytic activity in terms of CH₄ and CO₂ conversion was attained. The maintainability of the UHT-WGS catalytic performance at the temperature range of 500-600 °C over the catalyst with the composition of 10wt%Ni/5wt%CeO₂-Al₂O₃ (Ni/5Ce-Al) was also successful because of the high surface area, high metal dispersion, and practical Ni-Ce-Al interaction [32]. Using these developed Ni-based catalysts, the novel concept of the superior DCP that converts CH₄ and CO₂, the main greenhouse gases, into syngas with the H₂/CO ratio of about 2 at relatively low temperatures (< 700 °C) was developed in this work. In DCP, the Ni/5Ce-Al catalyst is located next after the NiCo/Mg-Al catalyst. As a result, the CSCRM and the UHT-WGS are operated continuously in a single reactor. During the operation, the syngas is produced on the catalyst for the CSCRM and then the H₂/CO ratio in the syngas is increased on the catalyst for the UHT-WGS. Moreover, the exothermic WGS can supply energy to the endothermic reactions above that take place on the CSCRM zone. Consequently, this original DCP is an alternative compact catalyst system that provides a very favorable H₂/CO ratio for syngas product associated with energy efficiency. Based on the reactants in the feed (CH₄:CO₂:H₂O:N₂) and the thermodynamic favorability, the possible reactions are demonstrated in the Equation (1) to (4).

                                                                                     (1)

                                                                                        (2)

                                                                                     (3)

                                                                                            (4)

Regarding Equation (1) to (4), three representative reactions are endothermic except for Equation (4). The reaction temperature and the feed composition are significant to control the activity as well as the composition of the syngas product.

In this work, the DCP was constructed and demonstrated. All supported catalysts were prepared by the impregnation method. The physicochemical properties of prepared catalysts were characterized. The CRM and UHT-WGS catalytic performances were separately tested with temperature programmed from 500 °C to 600 °C. According to the well-known stoichiometry of the CSCRM Equation (3CH₄ + 2H₂O + CO₂  8H₂ + 4CO) [6,14,19], the H₂/CO ratio of 2 can be achieved when managing the composition of the gas feed with S/C ratio close to 0.5 whereas performing under the severe temperature (≥ 700 °C). Then, the effect of the operating condition adjustment on the DCP catalytic performance and the H₂/CO ratio was examined for various the temperature (500 °C, 550 °C, and 600 °C) and the feed composition (CH₄:CO₂:H₂O:N₂ molar ratio = 1:0.5:x:1; x = 0.5, 0.8, and 1 reflecting the S/C ratio of 0.33, 0.53, and 0.67, respectively).

 

Some doubts concerning the materials and methods part are listed below:

 

Point 2: The two catalysts, CRM and UHT-WGS, have been calcined at different temperatures 650 and 600°C respectively, while the in-situ hydrogen reduction took place until 900°C. Since the reaction temperatures were chosen as 500°C, 550°C and 600°C, why the authors adopted different calcination temperatures for the two samples?

Response 2: We thank you for your comment. First of all, the TPR test was done with the temperature programmed from room temperature until 900 °C because we would like to investigate the reducibility as well as the interaction on the surface (metal-metal interaction and metal-support interaction). The strong interaction can be indicated at temperature  ³ 600 °C. Moreover, the stronger interaction are represented by the higher reduction temperature. Consequently, the TPR measurement was performed until 900 °C. However, to be used in the real process, the metal agglomeration at high temperature must be avoided. Moreover, the high-calcined temperature of CeO₂ must be aware due to its stability. In addition, the sintering can be minimized by choosing the reduction temperature ≤ reaction temperature [Int. J. Hydrogen Energy 2013, 38, 13938–13949, doi:10.1016/j.ijhydene.2013.08.029; Journal of Energy Chemistry 23(2014)633–638, doi: 10.1016/S2095-4956(14)60194-7]. Employing all these formation in the literature, we designed to use the calcined temperature close to the reduction temperature (650 °C) for NiCo/Mg-Al catalyst and slightly lower (600 °C) for Ni/Ce-Al catalyst.

 

                           

Point 3: Through the preparation part is not clearly exposed to what are referred the percentages of the metals, it would need to be better clarified.

Response 3: We thank for your comment. We provided the the percentages of the metals as explain in catalyst preparation section (line 333 and 346).

line 333-336:

The 5wt%Ni5wt%Co/MgO-Al₂O₃ (NiCo/Mg-Al) catalyst was prepared by co-impregnation using nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, Sigma Aldrich) and cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, Sigma Aldrich) as precursors for the nickel solution and cobalt solution, respectively.

line 345-347:

Secondly, an amount of nickel (II) nitrate hexahydrate corresponding to 10 wt% of Ni was impregnated onto the calcined support followed by drying at 50 °C for 2 h and calcination at 600 ^oC for 6 h using the heating rate of 3 °C min^-1.

 

Point 4: A scheme of the used plant, or at list a description of it, would be useful to better understand how the activity tests were performed, since at the present only a description of the used reactor is present.

Response 4: We agree with the reviewer for the valuable comment. We embed a scheme of the used plant after the explanation of the activity test.

Figure 8. Schematic diagram of the experimental setup.

 

Suggestion and doubts concerning the experimental section are presented:

 

Point 5: The first part written in the DCP section. Concerning the presentation of the reaction system, I would suggest putting it in the introduction section since, in this section, only the obtained results should be presented.

Response 5: This suggestion helps to improve our article. We have moved the first part written in the DCP section, concerning the presentation of the reaction system, to the introduction in order to clarify the idea of this system.

Revise:

line 88 -111:

In previous work [31], 5wt%Ni5wt%Co/MgO-Al₂O₃ (NiCo/Mg-Al) showed the higher metal dispersion, smaller metal particle size, and a high reducibility due to the effect of the metal-metal interaction. As a result, a valuable CRM catalytic activity in terms of CH₄ and CO₂ conversion was attained. The maintainability of the UHT-WGS catalytic performance at the temperature range of 500-600 °C over the catalyst with the composition of 10wt%Ni/5wt%CeO₂-Al₂O₃ (Ni/5Ce-Al) was also successful because of the high surface area, high metal dispersion, and practical Ni-Ce-Al interaction [32]. Using these developed Ni-based catalysts, the novel concept of the superior DCP that converts CH₄ and CO₂, the main greenhouse gases, into syngas with the H₂/CO ratio of about 2 at relatively low temperatures (< 700 °C) was developed in this work. In DCP, the Ni/5Ce-Al catalyst is located next after the NiCo/Mg-Al catalyst. As a result, the CSCRM and the UHT-WGS are operated continuously in a single reactor. During the operation, the syngas is produced on the catalyst for the CSCRM and then the H₂/CO ratio in the syngas is increased on the catalyst for the UHT-WGS. Moreover, the exothermic WGS can supply energy to the endothermic reactions above that take place on the CSCRM zone. Consequently, this original DCP is an alternative compact catalyst system that provides a very favorable H₂/CO ratio for syngas product associated with energy efficiency. Based on the reactants in the feed (CH₄:CO₂:H₂O:N₂) and the thermodynamic favorability, the possible reactions are demonstrated in the Equation (1) to (4).

                                                                                     (1)

                                                                                        (2)

                                                                                     (3)

                                                                                            (4)

Regarding Equation (1) to (4), three representative reactions are endothermic except for Equation (4). The reaction temperature and the feed composition are significant to control the activity as well as the composition of the syngas product.

 

Point 6: In the Figures 5a and 5b values of CH₄ and CO₂ conversion comparable or even higher are present at 500°C compared to those obtained at 550°C and 600°C, could the authors provide a discussion of these results?

Response 6: As the reviewer commented, we provided the discussion of values of CH₄ and CO₂ conversion in the revised manuscript (line 221-223).

Revise:

line 220-229

Figure 5 (a) and (b) exhibit the CH₄ and CO₂ conversions against time-on-stream with different operating temperatures (500 °C, 550 °C, and 600 °C) at a fixed S/C ratio of 0.67. As seen in Figure 5 (a), the CH₄ conversion increases when the operating temperature increases because the CSCRM is the endothermic process and high temperature encourages the reactions [12]. This trend is also found for CO₂ conversion (Figure 5 (b)), the extent of the CO₂ conversion rises from ~ 15% at 500 °C to ~ 35% at 600 °C. It should be noted that the appearance of the low CO₂ conversion at 500 °C could involve the fact that CO₂ is a product of the WGS reaction. Nevertheless, these CH₄ and CO₂ conversions were reliable compared to the results at a similar operating temperature range demonstrated by previously published works [2,12,56]. The obtained H₂/CO ratios for different operating temperatures, displayed in Figure 5 (c), were close to 2 for all operating temperatures according to these conditions.

 

Point 7: Which are the conditions used for the tests? No values of GHSV or WHSV are present.

Response 7: We thank you for pointing this out. We have considered and provided values of GHSV for the conditions used for the tests.

Revise:

line 384-404

Catalytic tests were carried out in a stainless steel tubular fixed-bed reactor at atmospheric pressure. Before the DCP reaction, the catalytic performances of CRM and UHT-WGS catalysts were demonstrated separately with a continued reaction temperature programmed (500 °C, 550 °C, and 600 °C); each temperature was held for 8 h. Prior to the CRM test with the composition of CH₄:CO₂:N₂ = 1:1.7:1.3 molar ratio with GHSV of 1.8 x 10⁴ mL g_cat^-1 h^-1, the 200 mg of CRM catalyst diluted with 1000 mg of fused silica were packed (diluted catalyst height of 1.4 cm) and the in situ reduced was at 650 °C for 6 h under a H₂ flow of 30 mL min^-1; the temperature was then decreased to the reaction temperature in N₂ at a flow rate of 30 mL min^-1. For the UHT-WGS tests using the GHSV of 2.0 x 10⁵ mL g_cat^-1 h^-1 with H₂O/CO ratio of three, the 30 mg of UHT-WGS catalyst diluted with 1000 mg of fused silica (diluted catalyst height of 1.2 cm) were preactivated and cooled using a similar reduction condition to the CRM catalyst.

For the DCP reaction, the diluted UHT-WGS catalyst as previously mentioned was charged first and the quartz wool was then placed on the top of the UHT-WGS catalyst. Subsequently, the diluted CRM catalyst as mentioned was loaded second. When the effect of the operating temperature was evaluated, the different reaction temperatures (500 °C, 550 °C, and 600 °C) were used under the feed composition with the S/C ratio of 0.67. The effect of H₂O content in the feed composition was investigated using the feed composition of CH₄:CO₂:H₂O:N₂ molar ratio = 1:0.5:x:1; x = 0.5, 0.8, and 1 (corresponding to the S/C ratio of 0.33, 0.53, and 0.67, respectively) with the fixed reaction temperature of 600 °C employing the GHSV rang of 1.6 x 10⁴-1.8 x 10⁴ mL g_cat^-1 h^-1. The conversions of CH₄ and CO₂ and the H₂/CO ratio were calculated using the following Equations (Eq. (7)-(9)).

 

Point 8: Typing errors:

1. There is a mistake in the title, I supposed that should be “Ni-based” instead of “Ni-base”.
2. Line 61: “The authors concluded” instead of the “authors conclude”.
3. Line 133: A space is missing in 3 °Cmin ^-1.
4. Line 133: A space is missing in 3 °Cmin ^-1.
5. Line 171: “Catalytic test” would be more appropriate than “catalyst tests”.
6. Line 201: “these diffraction peaks were not easily to identified” should be “these diffraction peaks were not easily to be identified”.
7. In the bibliography all the superscripts and subscripts are not present.

Response 8: We revised the typing errors (1-6) following the reviewer comments and we have checked the typing and formatting mistakes through the manuscript and revised accordingly. For the bibliography (7), we cannot type the superscripts and subscripts because we use the Mendeley, a free reference management software, that suggest on the website of catalysts journal.

Reviewer 3 Report

Please reconsider the following comments.

1. The reviewer is very curious if the Ni/5Ce-Al catalyst has no catalytic activity in CO2 reforming of methane (CRM). The author should provide the reaction results in CRM.
2. Page 4, Section 2.3. Catalytic activity test: The author must describe the reactor dimension, the height of catalyst bed, the position of thermocouple in details.
3. Page 8, line 256-259: The reviewer doesn’t agree with the reviewer’s opinion that the catalysts were reduced at 650 ^oC to avoid the active metal sintering. The NiCo/Mg-Al catalyst showed the main reduction peak at 850 ^oC in Fig. 3. However, the Ni/5Ce-Al catalyst was partially reduced at 650 ^oC. For this reason, the reviewer strongly suggest that the author should provide the reaction results of Ni/5Ce-Al catalyst in CRM.
4. Fig. 4 and Fig. 6: The author should provide the thermodynamic equilibrium values of CRM and WGS for a better understanding of readers.
5. Page 13, line 370-373: In general, a high steam-to-carbon(S/C) ratio prevents the coke formation. The author should explain clearly the reaction results at H2O/(CH4+CO2) ratio of 0.67.

Author Response

Response to Reviewer 3 Comments

First of all, according to the style of Catalysts journal, we would like to inform reviewers that “Results and Discussion” section appears before the “Material and Methods” section, thus the References, Figures, Tables and Equations in these parts of revised manuscript was reordered.

 

Point 1: The reviewer is very curious if the Ni/5Ce-Al catalyst has no catalytic activity in CO2 reforming of methane (CRM). The author should provide the reaction results in CRM.

Response 1: We thank you for this point of suggestion. It would have been interesting to explore this aspect. We understand your concerning. However, we apologize indeed that we cannot complete the CRM experiment on the Ni/5Ce-Al catalyst within one week according to the time for catalyst preparation and the different temperature experiments.

 

Point 2: Page 4, Section 2.3 (the revised manuscript reordered to section 3.3). Catalytic activity test: The author must describe the reactor dimension, the height of catalyst bed, the position of thermocouple in details.

Response 2: We thank for your suggestion. We have placed the reactor dimension and the position of thermocouple in the schematic diagram of the used plant and added the height of catalyst bed information in Figure 8 located in section 3.3.

Revise:

3.3. Catalytic activity test

Catalytic tests were carried out in a stainless steel tubular fixed-bed reactor at atmospheric pressure. Before the DCP reaction, the catalytic performances of CRM and UHT-WGS catalysts were demonstrated separately with a continued reaction temperature programmed (500 °C, 550 °C, and 600 °C); each temperature was held for 8 h. Prior to the CRM test with the composition of CH₄:CO₂:N₂ = 1:1.7:1.3 molar ratio with GHSV of 1.8 x 10⁴ mL g_cat^-1 h^-1, the 200 mg of CRM catalyst diluted with 1000 mg of fused silica were packed (diluted catalyst height of 1.4 cm) and the in situ reduced was at 650 °C for 6 h under a H₂ flow of 30 mL min^-1; the temperature was then decreased to the reaction temperature in N₂ at a flow rate of 30 mL min^-1. For the UHT-WGS tests using the GHSV of 2.0 x 10⁵ mL g_cat^-1 h^-1 with H₂O/CO ratio of three, the 30 mg of UHT-WGS catalyst diluted with 1000 mg of fused silica (diluted catalyst height of 1.2 cm) were preactivated and cooled using a similar reduction condition to the CRM catalyst.

For the DCP reaction, the diluted UHT-WGS catalyst as previously mentioned was charged first and the quartz wool was then placed on the top of the UHT-WGS catalyst. Subsequently, the diluted CRM catalyst as mentioned was loaded second. When the effect of the operating temperature was evaluated, the different reaction temperatures (500 °C, 550 °C, and 600 °C) were used under the feed composition with the S/C ratio of 0.67. The effect of H₂O content in the feed composition was investigated using the feed composition of CH₄:CO₂:H₂O:N₂ molar ratio = 1:0.5:x:1; x = 0.5, 0.8, and 1 (corresponding to the S/C ratio of 0.33, 0.53, and 0.67, respectively) with the fixed reaction temperature of 600 °C employing the GHSV rang of 1.6 x 10⁴-1.8 x 10⁴ mL g_cat^-1 h^-1. The conversions of CH₄ and CO₂ and the H₂/CO ratio were calculated using the following Equations (Eq. (7)-(9)). The scheme of the experimental setup presented in Figure 8.

                                                                                                                                  (7)

                                                                                                                                  (8)

                                                                                                                                  (9)

 

Figure 8. Schematic diagram of the experimental setup.

 

Point 3: Page 8, line 256-259: The reviewer doesn’t agree with the reviewer’s opinion that the catalysts were reduced at 650 ^oC to avoid the active metal sintering. The NiCo/Mg-Al catalyst showed the main reduction peak at 850 ^oC in Fig. 3. However, the Ni/5Ce-Al catalyst was partially reduced at 650 ^oC. For this reason, the reviewer strongly suggest that the author should provide the reaction results of Ni/5Ce-Al catalyst in CRM.

Response 3: We thank for your comment. According to the literature [Int. J. Hydrogen Energy 2013, 38, 13938–13949, doi:10.1016/j.ijhydene.2013.08.029; Journal of Energy Chemistry 23(2014)633–638, doi: 10.1016/S2095-4956(14)60194-7], the sintering can be minimized by choosing the reduction temperature ≤ reaction temperature. We used long time (6 h) reduction with the chosen reduction temperature to avoid the sintering as well as to increase the degree of reduction at this temperature. We also used this reduction temperature (for 2 h) for H₂-TPD measurement of the Ni/5Ce-Al catalyst, the results were reasonable (reported in Fuel 2019, 252, 488–495, doi:10.1016/J.FUEL.2019.04.157). We also understand your concerning of the reaction results of Ni/5Ce-Al catalyst in CRM and apologized for this incomplete data.

 

Point 4: Fig. 4 and Fig. 6: The author should provide the thermodynamic equilibrium values of CRM and WGS for a better understanding of readers.

Response 4:  We have added the the thermodynamic equilibrium values of CRM and WGS to the revised manuscript for a better understanding of readers at the results and discussion section in the catalytic performance part (line 200, line 206, line 251-252 and line 258-259).

Revise:

line 197-201 (for CRM)

As seen in Figure 4 (a), the CH₄ conversion increased from ~ 29% at 500 °C to ~ 52% at 600 °C because of a highly endothermic process by nature. Furthermore, the sustainable CH₄ conversion as functions of time-on-stream for each studied temperature was observed and also close to the thermodynamic equilibrium conversions (23% at 500 °C, 39% at 550 °C, and 59% at 600 °C) calculated using the reactivity test conditions.

 

line 203-207 (for UHT-WGS)

The catalytic activity of the Ni/5Ce-Al catalyst in the UHT-WGS reaction is presented in Figure 4 (b). It was found that the CO conversion performance decreased from ~ 65% at 500 °C to ~ 55% at 600 °C due to an exothermic reaction. Although the CO conversions obtained from UHT-WGS tests are different from the thermodynamic equilibrium conversions (92% at 500 °C, 89% at 550 °C, and 86% at 600 °C), a similar tendency was observed in the case of increasing operating temperature.

 

line 250-252 and line 258-263 (for DCP)

Nevertheless, the experimental CH₄ conversion for all feed compositions in are slightly higher than those of the thermodynamic equilibrium (45% for S/C ratio of 0.33, 51% for S/C ratio of 0.53, and 55% for S/C ratio of 0.67).

 

Meanwhile, the CO₂ conversion in the thermodynamic equilibrium (35% for S/C ratio of 0.33, 17% for S/C ratio of 0.53, and 6% for S/C ratio of 0.67) decrease with the increase of the S/C ratio and lower than experimental CO₂ conversions for all S/C ratios. The difference between the experimental and the thermodynamic equilibrium results could be caused by the side reactions (RWGS reaction) considering the similar reaction temperature [57].

 

Point 5: Page 13, line 370-373: In general, a high steam-to-carbon(S/C) ratio prevents the coke formation. The author should explain clearly the reaction results at H₂O/(CH₄+CO₂) ratio of 0.67.

Response 5: We thank very much for your suggestions that truly improve our article. We also concerned about this point before we submitted that last manuscript. Normally, a high steam-to-carbon(S/C) ratio (>1) should be excellent for preventing the coke formation. However, our work performed the DCP test at the relative low S/C ratio. Therefore, the catalyst should be high active toward the dissociative adsorption of H₂O in order to maximize oxygen species to remove the surface carbon. According to the TGA data, it was found that NiCo/Mg-Al (catalyst for CRM, less active to the dissociative adsorption of H₂O compared to Ni/Ce-Al) required a relative long time for the dissociative adsorption of H₂O. Therefore, less active sites for the oxidizing agents were available for the S/C ratio of 0.67 than for the S/C ratio of 0.53, resulting into less oxygen species on the surface, allowing more deposited carbon to polymerization on the surface of Ni for the NiCo/Mg-Al catalyst. This explanation was rewritten in the TGA part (line 303-309).

Revise:

line 298-309

As seen in Figure 7 (a), the weight loss of the spent NiCo/Mg-Al catalyst decreased from 65% to 27% with an increasing S/C ratio from 0.33 to 0.53 and then increased from 27% to 33% with an increasing S/C ratio from 0.53 to 0.67. These combined results indicate that the S/C ratio of 0.53 has a trendency to remove a deposited carbon in the case of a low S/C ratio, which can reduce the graphitic carbon. The formation of graphitic carbon existed when adjusting the S/C ratio to equal 0.67. This evidence reflected the longer time of deposited carbon grown on the surface of the NiCo/Mg-Al catalyst for the S/C ratio of 0.67 compared to the S/C ratio of 0.53. It means that the dissociative adsorption of H₂O on the NiCo/Mg-Al catalyst requires time before the gasification step to contribute oxygen species in order to remove carbon on the surface. Therefore, less active sites for the oxidizing agents were available for the S/C ratio of 0.67 than for the S/C ratio of 0.53, resulting into less oxygen species on the surface, allowing more deposited carbon to polymerization on the surface of Ni.

Round 2

Reviewer 2 Report

The paper has been properly modified according to the reviewer's suggestions, since all the confusing aspects have been clarified. 

I would just give to the authors a trip concerning the adoption of superscripts and subscripts with the use of mendeley: indeed, after the bibliography is created, the authors can paste it as a text and, after that, modify all the characters.

Anyway, I recommend this manuscript for its publication on catalysts. 

Reviewer 3 Report

All comments were reflected well in the manuscript.