Combining Carbon Fibers with Ni / γ – Al 2 O 3 Used for Syngas Production : Part A : Preparation and Evaluation of Complex Carrier Catalysts

To promote the adsorption and activation of carbon dioxide in the dry reforming of methane (DRM), Ni and Al2O3 were coprecipitated on activated carbon fibers (ACF). Various characterization methods were adopted in order to investigate the surface characteristics of different catalysts. Chemisorption characterization results, such as H2-temperature programmed reduction (H2-TPR), H2-temperature programmed desorption (H2-TPD), and CO2-temperature programmed desorption (CO2-TPD) illustrated that ACF in a nickel-based catalyst could enhance the basic sites and improve the metal dispersion on a catalyst surface, which is beneficial for the adsorption and activation of feed gas. The coprecipitated coating on ACF proved by scanning electron microscope (SEM) can prevent the carbon of ACF from participating in the reaction, while retain good surface properties of carbon fibers. X-ray diffraction (XRD) patterns illustrated that the ACF in a nickel-based catalyst could decrease the crystallite size of the spinel NiAl2O4, which is beneficial for methane reforming. In addition, the Fourier transform infrared spectroscopy (FTIR) of different catalysts revealed that the added ACF could provide abundant functional groups on the surface, which could be the intermediate product of DRM, and effectively promote the reaction. Different to the catalyst supported on single alumina, the performance evaluation and stability test proved that the catalyst added with ACF exhibited a better catalytic performance especially for CO2 conversion. Moreover, based on the characterization results as well as some related literature, the dry reforming mechanism over optimum catalyst was derived.


Introduction
The syngas (H 2 and CO) commonly used for organic synthesis is mainly obtained by coal gasification and the steam reforming of methane (SRM) [1].In recent years, the carbon dioxide reforming of methane (DRM) has attracted considerable attention [2,3] for its comprehensive utilization of two main greenhouse gases [4,5].
For the moment, the catalyst used for SRM is a nickel-based catalyst supported on alumina, because of its low cost and relatively high activity [6][7][8].However, it is not recommendable to apply into DRM directly, for the easy carbon deposition and sintering [9].To modify the catalyst with a great anti-coking performance is a prerequisite for DRM industrialization.It is known that carbon deposition is due to the active carbon from methane cracking, which cannot promptly react with CO 2 , thus becoming inactive carbon.Compared with steam, carbon dioxide with less carbon removal capacity cannot remove the intermediate carbon in a timely manner.So, the key to eliminating carbon deposition is to promote the activation of CO 2 .It has been proven that the support greatly affects the activation of CO 2 during DRM, due to the varying active surface area and acid base property [10].Related literature indicates that Lewis basic sites on support could promote the chemisorption of CO 2 and accelerate eliminating coke by the reaction of CO 2 with deposited coke to form CO during the DRM reaction [11,12].So, various metal oxides [13][14][15][16][17][18][19] (La 2 O 3 , SiO 2 , MgO, CaO, ZnO, CeO 2 , TiO 2 , ZrO 2 , and so on) are applied for the modification of supports.
Carbon material, as a unique catalyst carrier, has attracted much attention for its great surface characteristics [20][21][22].Much research about nickel-based catalysts supported on carbon materials [23,24] has been reported.For example, Bradford [25] and Ferreira-Aparicio [26] prepared the Ru-based catalyst, supported on carbon black and graphite, respectively, used for DRM, and illustrated that the support serves as a collector of CH X species, which reduces the residence time of carbon species on Ru phase, and therefore leads to a very stable catalyst.Among the numerous carbon materials, the alkaline activated carbon fibers (ACF) draw our attention, for their high specific surface area, rich functional groups, and abundant basic sites on surface [27].However, based on our former research [28], the pure carbon fibers that served as support could easily react with CO 2 and lead to the deactivation of the catalyst, because of the pore structure collapse; although they could restrain carbon deposition.
Herein, we consider the combination of carbon fibers and nickel-based alumina catalyst systems.The great surface characteristics of carbon fibers will contribute to the adsorption and activation of CO 2 .In this work, nickel and alumina would be coprecipitated on the surface of ACF.A series of nickel-based catalysts were configured by combining γ-Al 2 O 3 with ACF powder by coprecipitation.The obtained catalysts would be applied in DRM to illustrate the catalytic performance.Then, the mechanism of reforming would be investigated based on the related characterization results.

Results and Discussion
In this work, to obtain the optimum proportion of ACF to γ-Al 2 O 3 , various catalysts with different mass ratios of ACF/γ-Al 2 O 3 (from 0:10 to 10:0) were prepared by coprecipitation.The catalytic performance of the different prepared catalysts at 650 • C and 750 • C are shown in Figure S1.It is found that the optimum ratio of ACF/γ-Al 2 O 3 is 5:5.So, in the following research, the mass ratio of ACF/γ-Al 2 O 3 for complex carrier catalysts would be 5:5.This kind of catalyst will be named as Ni-γ-Al 2 O 3 /ACF, and the catalysts supported on single γ-Al 2 O 3 and ACF will be named Ni-γ-Al 2 O 3 and Ni/ACF, respectively.

Surface Characterization
To illustrate the surface morphology of the prepared catalysts, SEM was conducted as shown in Figure 1.It can be found that the solid solution of the nickel and alumina uniformly covers the surface of the carbon fibers.The coprecipitated coating can effectively inhibit the carbon of ACF from participating into the reaction, while retaining the good surface properties of the fibers.Meanwhile, the added ACF could also remarkably improve the surface structure of the nickel-based catalyst, especially for the specific surface area, as shown in Table S1, which is favorable for the adsorption of raw gas.In addition, we conducted an energy dispersive spectroscopy (EDS) of the catalysts Ni-γ-Al 2 O 3 /ACF, as shown in Figure S2.Based on the EDX result, the mass content of Ni on the surface of the catalyst is 1.43%.Because of the co-precipitation method, some of the nickel is covered by alumina, which results in the smaller mass content of Ni on the surface of the catalyst than the total Ni loading in the catalyst.

Chemisorption Characterization
To better explain the influence of ACF on the catalyst system, the chemisorption characterizations (H2-temperature programmed reduction (H2-TPR), H2-temperature programmed desorption (H2-TPD), and CO2-temperature programmed desorption (CO2-TPD)) were conducted, as shown in Figure 2. The interaction between the active metal and support was revealed by H2-TPR, as shown in Figure 2a.When the nickel and alumina coprecipitated on the surface of ACF, the reduction peak appears at a higher temperature compared with the nickel-based catalyst coprecipitated with single alumina or ACF.It can be considered that the addition of ACF leads to the stronger metal-support interaction, which is beneficial to anti sintering.This could be attributed to the formation of the spinel of NiAl2O4, which would be proven by the subsequent XRD.To obtain the proportion of Ni in direct contact with alumina (and that is able to produce NiAl2O4 spinel) and the proportion of Ni in direct contact with ACF, we conducted the peak fitting of the H2-TPR curve of the catalyst Ni-Al2O3/ACF, as shown in in Figure S3 and Table S2.With the combined proportion of the peak area with the result of EDS, the proportion of the different nickel on the catalyst surface could be calculated.The proportion of the free nickel, the nickel in direct contact with ACF, and the nickel in direct contact with the alumina is 0.174%, 0.094%, and 1.623%, respectively.
As is known to all, the role of support is to provide a high specific surface area so that the active metal can be maximally dispersed [29].The H2-TPD could illustrate the metal dispersion and active sites.As presented in Figure 2b, the H2 desorption temperature for the catalyst supported on alumina is about 460 °C.The minimum peak area and lower desorption temperature proves the poor metal dispersion of the catalyst supported on the alumina.While for the catalyst supported on ACF, the higher specific surface area results in a much better metal dispersion, which leads to the larger desorption peak area and higher desorption temperature existing at 660 °C.When the nickel and alumina coprecipitated on the surface of ACF, the H2-TPD of the catalyst supported on double supports exhibits two desorption peaks at 580 °C and 890 °C, respectively.It has been proven that there are two kinds of active sites for the catalyst supported on complex supports.The first desorption peak existing at 580 °C is the active site resulting from the combination of nickel and

Chemisorption Characterization
To better explain the influence of ACF on the catalyst system, the chemisorption characterizations (H 2 -temperature programmed reduction (H 2 -TPR), H 2 -temperature programmed desorption (H 2 -TPD), and CO 2 -temperature programmed desorption (CO 2 -TPD)) were conducted, as shown in Figure 2.

Chemisorption Characterization
To better explain the influence of ACF on the catalyst system, the chemisorption characterizations (H2-temperature programmed reduction (H2-TPR), H2-temperature programmed desorption (H2-TPD), and CO2-temperature programmed desorption (CO2-TPD)) were conducted, as shown in Figure 2. The interaction between the active metal and support was revealed by H2-TPR, as shown in Figure 2a.When the nickel and alumina coprecipitated on the surface of ACF, the reduction peak appears at a higher temperature compared with the nickel-based catalyst coprecipitated with single alumina or ACF.It can be considered that the addition of ACF leads to the stronger metal-support interaction, which is beneficial to anti sintering.This could be attributed to the formation of the spinel of NiAl2O4, which would be proven by the subsequent XRD.To obtain the proportion of Ni in direct contact with alumina (and that is able to produce NiAl2O4 spinel) and the proportion of Ni in direct contact with ACF, we conducted the peak fitting of the H2-TPR curve of the catalyst Ni-Al2O3/ACF, as shown in in Figure S3 and Table S2.With the combined proportion of the peak area with the result of EDS, the proportion of the different nickel on the catalyst surface could be calculated.The proportion of the free nickel, the nickel in direct contact with ACF, and the nickel in direct contact with the alumina is 0.174%, 0.094%, and 1.623%, respectively.
As is known to all, the role of support is to provide a high specific surface area so that the active metal can be maximally dispersed [29].The H2-TPD could illustrate the metal dispersion and active sites.As presented in Figure 2b, the H2 desorption temperature for the catalyst supported on alumina is about 460 °C.The minimum peak area and lower desorption temperature proves the poor metal dispersion of the catalyst supported on the alumina.While for the catalyst supported on ACF, the higher specific surface area results in a much better metal dispersion, which leads to the larger desorption peak area and higher desorption temperature existing at 660 °C.When the nickel and alumina coprecipitated on the surface of ACF, the H2-TPD of the catalyst supported on double supports exhibits two desorption peaks at 580 °C and 890 °C, respectively.It has been proven that there are two kinds of active sites for the catalyst supported on complex supports.The first desorption peak existing at 580 °C is the active site resulting from the combination of nickel and The interaction between the active metal and support was revealed by H 2 -TPR, as shown in Figure 2a.When the nickel and alumina coprecipitated on the surface of ACF, the reduction peak appears at a higher temperature compared with the nickel-based catalyst coprecipitated with single alumina or ACF.It can be considered that the addition of ACF leads to the stronger metal-support interaction, which is beneficial to anti sintering.This could be attributed to the formation of the spinel of NiAl 2 O 4 , which would be proven by the subsequent XRD.To obtain the proportion of Ni in direct contact with alumina (and that is able to produce NiAl 2 O 4 spinel) and the proportion of Ni in direct contact with ACF, we conducted the peak fitting of the H 2 -TPR curve of the catalyst Ni-Al 2 O 3 /ACF, as shown in in Figure S3 and Table S2.With the combined proportion of the peak area with the result of EDS, the proportion of the different nickel on the catalyst surface could be calculated.The proportion of the free nickel, the nickel in direct contact with ACF, and the nickel in direct contact with the alumina is 0.174%, 0.094%, and 1.623%, respectively.
As is known to all, the role of support is to provide a high specific surface area so that the active metal can be maximally dispersed [29].The H 2 -TPD could illustrate the metal dispersion and active sites.As presented in Figure 2b, the H 2 desorption temperature for the catalyst supported on alumina is about 460 • C. The minimum peak area and lower desorption temperature proves the poor metal dispersion of the catalyst supported on the alumina.While for the catalyst supported on ACF, the higher specific surface area results in a much better metal dispersion, which leads to the larger desorption peak area and higher desorption temperature existing at 660 • C. When the nickel and alumina coprecipitated on the surface of ACF, the H 2 -TPD of the catalyst supported on double supports exhibits two desorption peaks at 580 • C and 890 • C, respectively.It has been proven that there are two kinds of active sites for the catalyst supported on complex supports.The first desorption peak existing at 580 • C is the active site resulting from the combination of nickel and alumina.The second desorption peak existing at 890 • C is the active site resulting from the combination of nickel and carbon fibers.Compared to the catalysts supported on a single support, the desorption peak of the active site from the combination of the nickel and alumina is enhanced, while the desorption peak of the active site from the combination of nickel and carbon fibers is decreased.This is consistent with the two reduction peaks of the catalysts shown by H 2 -TPR.
Another function of support is to provide the acid or basic sites [29] on the catalyst surface, which is beneficial for the adsorption and activation of feed gas.The CO 2 -TPD of three catalysts was conducted in order to investigate the basic sites on the surface, as presented in Figure 2c.Each catalyst presents three desorption peaks, which correspond with weak, intermediate strength, and strong basic sites, respectively.It is mainly the intermediate strength and strong basic sites on the surface of the catalyst Ni/ACF.While for the catalyst supported on alumina, it is mainly the weak basic site on the surface.As we know, the basic site is favored for the adsorption and activation of carbon dioxide [10][11][12].When adding the ACF into the nickel-based catalyst supported on alumina, the basic sites on the catalyst surface are strengthened.The strengthened intermediate strength and strong basic sites are helpful for the adsorption of acid carbon dioxide, because of the abundance of the OH group.So, in consideration of the results of the specific surface area and pore structure parameters, it is concluded that the ACF could promote the chemisorption and activation of CO 2 .

X-Ray Diffraction
To explain the chemisorption characterization results and illustrate the crystal structure of the prepared catalysts, the XRD for ACF, Ni-Al 2 O 3 , and Ni-Al 2 O 3 /ACF was conducted as shown in Figure 3.The two catalysts were not reduced before XRD.As can be seen in Figure 3, both of the catalysts exhibit three diffraction peaks of spinel NiAl 2 O 4 around 37 • , 45 • , and 66 • .However, because of the addition of ACF, the crystallite size of the spinel in the different catalysts in different.Table 1 shows the crystallite sizes of the spinel at different two-Theta in two catalysts, calculated by the Scherrer equation [30].After adding ACF into the catalyst Ni-Al 2 O 3 /ACF, the crystallite size of the spinel is obviously smaller than that in Ni-Al 2 O 3 .The smaller structure of the spinel NiAl 2 O 4 is beneficial to anti sintering.
alumina.The second desorption peak existing at 890 °C is the active site resulting from the combination of nickel and carbon fibers.Compared to the catalysts supported on a single support, the desorption peak of the active site from the combination of the nickel and alumina is enhanced, while the desorption peak of the active site from the combination of nickel and carbon fibers is decreased.This is consistent with the two reduction peaks of the catalysts shown by H2-TPR.
Another function of support is to provide the acid or basic sites [29] on the catalyst surface, which is beneficial for the adsorption and activation of feed gas.The CO2-TPD of three catalysts was conducted in order to investigate the basic sites on the surface, as presented in Figure 2c.Each catalyst presents three desorption peaks, which correspond with weak, intermediate strength, and strong basic sites, respectively.It is mainly the intermediate strength and strong basic sites on the surface of the catalyst Ni/ACF.While for the catalyst supported on alumina, it is mainly the weak basic site on the surface.As we know, the basic site is favored for the adsorption and activation of carbon dioxide [10][11][12].When adding the ACF into the nickel-based catalyst supported on alumina, the basic sites on the catalyst surface are strengthened.The strengthened intermediate strength and strong basic sites are helpful for the adsorption of acid carbon dioxide, because of the abundance of the OH group.So, in consideration of the results of the specific surface area and pore structure parameters, it is concluded that the ACF could promote the chemisorption and activation of CO2.

X-Ray Diffraction
To explain the chemisorption characterization results and illustrate the crystal structure of the prepared catalysts, the XRD for ACF, Ni-Al2O3, and Ni-Al2O3/ACF was conducted as shown in Figure 3.The two catalysts were not reduced before XRD.As can be seen in Figure 3, both of the catalysts exhibit three diffraction peaks of spinel NiAl2O4 around 37°, 45°, and 66°.However, because of the addition of ACF, the crystallite size of the spinel in the different catalysts in different.Table 1 shows the crystallite sizes of the spinel at different two-Theta in two catalysts, calculated by the Scherrer equation [30].After adding ACF into the catalyst Ni-Al2O3/ACF, the crystallite size of the spinel is obviously smaller than that in Ni-Al2O3.The smaller structure of the spinel NiAl2O4 is beneficial to anti sintering.

Fourier Transform Infrared
To reveal the different functional groups on the catalyst surface, the FTIR spectra of ACF and catalyst Ni-Al 2 O 3 /ACF was conducted, as shown in Figure 4.The broad adsorption peak at 3405 cm −1 could be attributed to the -OH stretching vibration [31], and the weak adsorption peaks at 2931 cm −1 , 2857 cm −1 , and 1160 cm −1 could be ascribed to the stretching vibration of -CH 3 , -CH 2 -, and -CH-, respectively [32].The characteristic adsorption peak at 1615 cm −1 is generated by the C=C or C-O stretching vibration [33].The weak adsorption peak at 1444 cm −1 is due to the stretching vibration of CH 3 O- [34].The FTIR spectra reveals that the added ACF in the catalyst Ni-Al 2 O 3 /ACF provides abundant oxygen functional groups and methyl groups on the surface, which could be the intermediate product of dry reforming methane, and effectively promote the reaction.The abundant functional groups from ACF could provide the basic sites on the catalyst surface, which is favored for the adsorption and activation of CO 2 .

Fourier Transform Infrared
To reveal the different functional groups on the catalyst surface, the FTIR spectra of ACF and catalyst Ni-Al2O3/ACF was conducted, as shown in Figure 4.The broad adsorption peak at 3405 cm −1 could be attributed to the -OH stretching vibration [31], and the weak adsorption peaks at 2931 cm −1 , 2857 cm −1 , and 1160 cm −1 could be ascribed to the stretching vibration of -CH3, -CH2-, and -CH-, respectively [32].The characteristic adsorption peak at 1615 cm −1 is generated by the C=C or C-O stretching vibration [33].The weak adsorption peak at 1444 cm −1 is due to the stretching vibration of CH3O- [34].The FTIR spectra reveals that the added ACF in the catalyst Ni-Al2O3/ACF provides abundant oxygen functional groups and methyl groups on the surface, which could be the intermediate product of dry reforming methane, and effectively promote the reaction.The abundant functional groups from ACF could provide the basic sites on the catalyst surface, which is favored for the adsorption and activation of CO2.

Catalyst Stability Test
To illustrate the influence of the added ACF on the catalytic performance, the stability test of three catalysts (the ratios of ACF/γ-Al2O3 are 5:5, 0:10, and 10:0, respectively) at 750 °C were conducted, as presented in Figure 5. Apparently, combining ACF with γ-Al2O3 is beneficial for the catalytic performance.Compared with the other two catalysts, the catalyst supported on single ACF is easy to inactivate in 900 min.However, when adding ACF into the nickel-based alumina system, the catalytic performance was different.It can be found that the added ACF leads to the remarkable increase in the CO2 conversion ratio at 750 °C for catalyst Ni-γ-Al2O3/ACF.The catalyst supported on the composite supports shows no obvious deactivation in 900 min.This could be ascribed to three reasons.The first is that the coprecipitated coating of Ni and Al2O3 on the ACF surface, proven by the SEM result, could restrain the carbon of the ACF from participating in reforming, while retaining the good surface properties of the fibers.Then, the ACF with abundant basic sites, proven by CO2-TPD, is helpful for the chemisorption and activation of CO2 [10][11][12], and could accelerate the removal of deposited coke to form CO during reforming.Furthermore, the theoretically ideal solid solution [35] of Ni and Al2O3 formed by the coprecipitation method is beneficial to control the particle size of metallic nickel, and could improve the catalytic performance.

Catalyst Stability Test
To illustrate the influence of the added ACF on the catalytic performance, the stability test of three catalysts (the ratios of ACF/γ-Al 2 O 3 are 5:5, 0:10, and 10:0, respectively) at 750 • C were conducted, as presented in Figure 5. Apparently, combining ACF with γ-Al 2 O 3 is beneficial for the catalytic performance.Compared with the other two catalysts, the catalyst supported on single ACF is easy to inactivate in 900 min.However, when adding ACF into the nickel-based alumina system, the catalytic performance was different.It can be found that the added ACF leads to the remarkable increase in the CO 2 conversion ratio at 750 • C for catalyst Ni-γ-Al 2 O 3 /ACF.The catalyst supported on the composite supports shows no obvious deactivation in 900 min.This could be ascribed to three reasons.The first is that the coprecipitated coating of Ni and Al 2 O 3 on the ACF surface, proven by the SEM result, could restrain the carbon of the ACF from participating in reforming, while retaining the good surface properties of the fibers.Then, the ACF with abundant basic sites, proven by CO 2 -TPD, is helpful for the chemisorption and activation of CO 2 [10][11][12], and could accelerate the removal of deposited coke to form CO during reforming.Furthermore, the theoretically ideal solid solution [35] of Ni and Al 2 O 3 formed by the coprecipitation method is beneficial to control the particle size of metallic nickel, and could improve the catalytic performance.Catalyst stability test (a: CH4 conversion ratio; b: CO2 conversion ratio).Gas hourly space velocity (GHSV): 24,000 mL•h −1 •g −1 ; detailed reaction conditions: CH4: 50 mL/min; CO2: 50 mL/min; N2: 100 mL/min; catalyst dosage: 0.5 g; temperature: 750 °C; test time: 900 min.

Carbon Equilibrium in Gas Phase
Our former research proved that the carbon from the support could be involved in reforming for the catalyst supported on the single ACF, which leads to the rapid deactivation [28].The combination of ACF and alumina by coprecipitation could solve the problem effectively.The carbon equilibrium of dry reforming in the gas phase with two catalysts (Ni-γ-Al2O3/ACF and Ni/ACF) is calculated, as presented in Figure 6.For the catalyst supported on a single ACF, the carbon content in the produced gas is nearly 50% higher than that in the feed gas, which could illustrate that the carbon of the ACF is remarkably involved in reforming.While the combined ACF with alumina acts as composite supports, the extra carbon content for the catalyst is decreased remarkably.The covered solid solution of nickel and alumina on the surface, proven by SEM, could effectively prevent the carbon of ACF from participating in the reaction.

Catalytic Performance at Different Temperatures
To further illustrate the unique advantages of ACF, the catalytic performance of two catalysts (Ni-γ-Al2O3/ACF and Ni/ACF) at different temperatures was conducted, as shown in Figure 7. Apparently, adding ACF into the nickel-based catalyst supported on the alumina is beneficial for the catalytic performance at different temperatures, especially for the conversion ratio of CO2.At a higher temperature, the conversion ratio of CO2 for catalyst Ni-γ-Al2O3/ACF is much higher than that without ACF.The added ACF for the catalyst Ni-γ-Al2O3/ACF promotes the chemisorption and activation of CO2, which could accelerate the reaction between the adsorbed CO2 and the intermediate active carbon from methane dissociation [11], which then leads to the higher conversion ratio of CH4 and CO2.Furthermore, with the increase of temperature, the conversion ratio of CO2 decreases first and then increases.For the methane reforming system, the increased temperature is beneficial for the endothermic reactions, including the dry reforming of methane and methane cracking, while it inhibits the exothermic reaction, including the reaction between ACF and

Carbon Equilibrium in Gas Phase
Our former research proved that the carbon from the support could be involved in reforming for the catalyst supported on the single ACF, which leads to the rapid deactivation [28].The combination of ACF and alumina by coprecipitation could solve the problem effectively.The carbon equilibrium of dry reforming in the gas phase with two catalysts (Ni-γ-Al 2 O 3 /ACF and Ni/ACF) is calculated, as presented in Figure 6.For the catalyst supported on a single ACF, the carbon content in the produced gas is nearly 50% higher than that in the feed gas, which could illustrate that the carbon of the ACF is remarkably involved in reforming.While the combined ACF with alumina acts as composite supports, the extra carbon content for the catalyst is decreased remarkably.The covered solid solution of nickel and alumina on the surface, proven by SEM, could effectively prevent the carbon of ACF from participating in the reaction.

Carbon Equilibrium in Gas Phase
Our former research proved that the carbon from the support could be involved in reforming for the catalyst supported on the single ACF, which leads to the rapid deactivation [28].The combination of ACF and alumina by coprecipitation could solve the problem effectively.The carbon equilibrium of dry reforming in the gas phase with two catalysts (Ni-γ-Al2O3/ACF and Ni/ACF) is calculated, as presented in Figure 6.For the catalyst supported on a single ACF, the carbon content in the produced gas is nearly 50% higher than that in the feed gas, which could illustrate that the carbon of the ACF is remarkably involved in reforming.While the combined ACF with alumina acts as composite supports, the extra carbon content for the catalyst is decreased remarkably.The covered solid solution of nickel and alumina on the surface, proven by SEM, could effectively prevent the carbon of ACF from participating in the reaction.Calculation of carbon equilibrium.GHSV: 24,000 mL•h −1 •g −1 ; detailed reaction conditions: CH4: 50 mL/min; CO2: 50 mL/min; N2: 100 mL/min; catalyst dosage: 0.5 g; temperature: 750 °C; test time: 900 min.

Catalytic Performance at Different Temperatures
To further illustrate the unique advantages of ACF, the catalytic performance of two catalysts (Ni-γ-Al2O3/ACF and Ni/ACF) at different temperatures was conducted, as shown in Figure 7. Apparently, adding ACF into the nickel-based catalyst supported on the alumina is beneficial for the catalytic performance at different temperatures, especially for the conversion ratio of CO2.At a higher temperature, the conversion ratio of CO2 for catalyst Ni-γ-Al2O3/ACF is much higher than that without ACF.The added ACF for the catalyst Ni-γ-Al2O3/ACF promotes the chemisorption and activation of CO2, which could accelerate the reaction between the adsorbed CO2 and the intermediate active carbon from methane dissociation [11], which then leads to the higher conversion ratio of CH4 and CO2.Furthermore, with the increase of temperature, the conversion ratio of CO2 decreases first and then increases.For the methane reforming system, the increased temperature is beneficial for the endothermic reactions, including the dry reforming of methane and methane cracking, while it inhibits the exothermic reaction, including the reaction between ACF and

Catalytic Performance at Different Temperatures
To further illustrate the unique advantages of ACF, the catalytic performance of two catalysts (Ni-γ-Al 2 O 3 /ACF and Ni/ACF) at different temperatures was conducted, as shown in Figure 7. Apparently, adding ACF into the nickel-based catalyst supported on the alumina is beneficial for the catalytic performance at different temperatures, especially for the conversion ratio of CO 2 .At a higher temperature, the conversion ratio of CO 2 for catalyst Ni-γ-Al 2 O 3 /ACF is much higher than that without ACF.The added ACF for the catalyst Ni-γ-Al 2 O 3 /ACF promotes the chemisorption and activation of CO 2 , which could accelerate the reaction between the adsorbed CO 2 and the intermediate active carbon from methane dissociation [11], which then leads to the higher conversion ratio of CH 4 and CO 2 .Furthermore, with the increase of temperature, the conversion ratio of CO 2 decreases first and then increases.For the methane reforming system, the increased temperature is beneficial for the endothermic reactions, including the dry reforming of methane and methane cracking, while it inhibits the exothermic reaction, including the reaction between ACF and CO 2 .The combined effects of these reactions lead to this phenomenon.In addition, it can be seen that the selectivity of both H 2 and CO is near 1, which indicates that the main reaction in the feed gas was a dry gas reforming reaction with only a few side reactions.CO2.The combined effects of these reactions lead to this phenomenon.In addition, it can be seen that the selectivity of both H2 and CO is near 1, which indicates that the main reaction in the feed gas was a dry gas reforming reaction with only a few side reactions.Figure 7. Catalytic activity at different temperatures (a: CH4 conversion ratio; b: CO2 conversion ratio; c: H2 selectivity; d: CO selectivity).GHSV: 24,000 mL•h −1 •g −1 ; detailed reaction conditions: CH4: 50 mL/min; CO2: 50 mL/min; N2: 100 mL/min; catalyst dosage: 0.5 g.

Mechanism Analysis for Reforming
The relevant research about the mechanism of dry reforming shows that the CH4 activation (breaking of the C-H bond) favorably occurs on the Ni crystallites in the catalyst, while CO2 is preferentially adsorbed on the support [36].For this study, CO2-TPD illustrated that the CO2 would be adsorbed on Al2O3 and ACF, respectively.The FTIR spectra reveals that the added ACF in the catalyst supports the abundant oxygen functional groups and methyl groups on the surface, which could be the intermediate product of dry reforming methane.So, based on the characterization results and the reaction steps of heterogeneous catalysis (adsorption-surface reaction-desorption), the dry reforming mechanism of catalyst Ni-γ-Al2O3/ACF (ACF:Al2O3 = 1:1) could be derived, as seen in Figure 8. Generally, the adsorbed methane is activated by nickel, and is dissociated to release the CHX and H fragments.The carbon dioxide will be adsorbed by the supports (ACF or Al2O3), followed by activation.Then, the adsorbed CO2 will release the CO and O fragment.The CHX fragment c: H 2 selectivity; d: CO selectivity).GHSV: 24,000 mL•h −1 •g −1 ; detailed reaction conditions: CH 4 : 50 mL/min; CO 2 : 50 mL/min; N 2 : 100 mL/min; catalyst dosage: 0.5 g.

Mechanism Analysis for Reforming
The relevant research about the mechanism of dry reforming shows that the CH 4 activation (breaking of the C-H bond) favorably occurs on the Ni crystallites in the catalyst, while CO 2 is preferentially adsorbed on the support [36].For this study, CO 2 -TPD illustrated that the CO 2 would be adsorbed on Al 2 O 3 and ACF, respectively.The FTIR spectra reveals that the added ACF in the catalyst supports the abundant oxygen functional groups and methyl groups on the surface, which could be the intermediate product of dry reforming methane.So, based on the characterization results and the reaction steps of heterogeneous catalysis (adsorption-surface reaction-desorption), the dry reforming mechanism of catalyst Ni-γ-Al 2 O 3 /ACF (ACF:Al 2 O 3 = 1:1) could be derived, as seen in Figure 8. CO2.The combined effects of these reactions lead to this phenomenon.In addition, it can be seen that the selectivity of both H2 and CO is near 1, which indicates that the main reaction in the feed gas was a dry gas reforming reaction with only a few side reactions.Figure 7. Catalytic activity at different temperatures (a: CH4 conversion ratio; b: CO2 conversion ratio; c: H2 selectivity; d: CO selectivity).GHSV: 24,000 mL•h −1 •g −1 ; detailed reaction conditions: CH4: 50 mL/min; CO2: 50 mL/min; N2: 100 mL/min; catalyst dosage: 0.5 g.

Mechanism Analysis for Reforming
The relevant research about the mechanism of dry reforming shows that the CH4 activation (breaking of the C-H bond) favorably occurs on the Ni crystallites in the catalyst, while CO2 is preferentially adsorbed on the support [36].For this study, CO2-TPD illustrated that the CO2 would be adsorbed on Al2O3 and ACF, respectively.The FTIR spectra reveals that the added ACF in the catalyst supports the abundant oxygen functional groups and methyl groups on the surface, which could be the intermediate product of dry reforming methane.So, based on the characterization results and the reaction steps of heterogeneous catalysis (adsorption-surface reaction-desorption), the dry reforming mechanism of catalyst Ni-γ-Al2O3/ACF (ACF:Al2O3 = 1:1) could be derived, as seen in Figure 8. Generally, the adsorbed methane is activated by nickel, and is dissociated to release the CHX and H fragments.The carbon dioxide will be adsorbed by the supports (ACF or Al2O3), followed by activation.Then, the adsorbed CO2 will release the CO and O fragment.The CHX fragment Generally, the adsorbed methane is activated by nickel, and is dissociated to release the CH X and H fragments.The carbon dioxide will be adsorbed by the supports (ACF or Al 2 O 3 ), followed by activation.Then, the adsorbed CO 2 will release the CO and O fragment.The CH X fragment combined with the O fragment will generate the CH X O fragment, then produce the CO and H fragment.The H fragment could merge to release H 2 .

Preparation of Catalyst
The coprecipitation method is applied for the catalyst preparation.
The chopped polyacrylonitrile-based activated carbon fibers with around 100 meshes were selected to serve as the support.The solutions of aluminum nitrate, nickel nitrate, and sodium carbonate with different proportions were selected as the precursor of precipitant.The catalyst could be prepared by the following steps.Firstly, adding a certain amount of ACF into a beaker with 100 mL deionized water and stirring in a water bath at 40 • C.Then, the solutions of aluminum nitrate, nickel nitrate, and sodium carbonate with different proportions were added into that beaker by peristaltic pumps.The precipitant would be coprecipitated on the surface of the ACF, and was aged 30 min.Then, the filtered precipitant would be washed by deionized water, until the pH was neutral, and was then dried at 105 • C, followed with calcination for 2 h at 600 • C. Finally, the unreduced catalyst would be obtained.A series of nickel-based catalysts were configured, with different mass ratios of γ-Al 2 O 3 to ACF, ranging from 0:10 to 10:0.Based on our former research [28], the optimum mass content of Ni for all of the catalysts is 5%.

Catalysts Evaluation
The catalytic performance of the different catalysts was evaluated in a fixed bed reactor, as shown in Figure S3.Before the reaction, the catalysts would be reduced in a hydrogen atmosphere of 50 mL/min at 650 • C for 2 h.Then, 0.5 g of the catalyst was loaded in a vertical tubular furnace, whose inner diameter is 8 mm.The nitrogen was selected as the carrier gas.The produced gas was collected by a gas bag of 500 mL for analysis, after first being dried by silica gel.
In order to better reflect the catalytic performance, some parameters have been defined.The CH 4 conversion ratio is defined as Equation (1), as follows: where F in, CH 4 is the CH 4 flow rate in the feed gas, and F out, CH 4 is the CH 4 flow rate in the out products.The CO 2 conversion ratio is defined as Equation (2), as follows: where F in, CO 2 is the CO 2 flow rate in the feed gas, and F out, CO 2 is the CO 2 flow rate in the out products.The H 2 selectivity and CO selectivity is defined as Equations ( 3) and ( 4), as follows: where F out, H 2 is the H 2 flow rate in the out products.The carbon equilibrium in gas phase before and after reaction is defined as Equation ( 5), as follows: where F out, CO is the CO flow rate in the out products.
FWHM: full width at half maxima.
F out, CO 2 + F out, CH 4 + F out, CO ) − F in, CO 2 + F in, CH 4 F in, CO 2 + F in, CH 4