Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Promotion Effect of SiO2 on the Catalytic Performance of Ni/CF for Biomass Derived Gas Reforming Yuexing Wei,† Min Song,*,† Lei Yu,† Ruiqi Gao,† Fanyue Meng,† Jun Xiao,† and Yangyang Zhang‡ †
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Ministry of Education of Key Laboratory of Energy Thermal Conversion and Control, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, China ‡ Nanjing University & Yancheng Academy of Environmental Protection Technology and Engineering, Yancheng, Jiansu 224300, China ABSTRACT: A series of nickel-based carbon fiber prepared via ultrasonicassisted impregnation method, were applied for biomass derived gas (CH4 and CO2) reforming. Results illustrated that parts of the carriers were consumed by CO2, followed by the deactivation of catalysts. In order to improve the catalytic activity and stability of the catalyst, a certain amount of silica was introduced into the 15%Ni/CF by hydrothermal synthesis method. The results demonstrated that the presence of silica species plays positive roles by inhibiting the reaction of carbon fiber and improving the metal support interaction. Additionally, hydrogen temperature-programmed reduction (H2-TPR), carbon dioxide temperature-programmed surface reaction (CO2-TPSR), scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to characterize the morphology of the catalysts for the sake of illustrating the promotion mechanism of silica. At last, a process of carbon dioxide reforming methane on the Ni/SiO2−CF catalyst was proposed. Carbon materials, such as activated carbon,11 carbon nanotubes,12 and carbon fibers have been used with certain purpose because of their large specific surface area and pore structure and attention has been shifted to the utilization of carbon materials as catalyst carrier. It can balance the carbon deposition produced during the reforming process and react with the CO2 to increase the dissociation adsorption capacity of CO2.13 Zhang et al.14 has found that the catalytic performance of Co based catalysts can be improved with the supports of a series of activated carbon. In addition, several studies15−17 have also reported the existence of activated fibrous carbon deposition during the beginning of CRM process can promote the catalytic performance through increasing the specific surface area of the catalyst. However, there is no study on using the carbon fiber as a catalyst carrier for the CRM reaction. Moreover, the industry development of using lignin to prepare carbon fiber also makes the source of carbon fiber more extensive and renewable.18,19 Therefore, this study selects the carbon fiber as the support of nickel based catalysts for the sake of improving the carbon dioxide conversion rate in CRM reaction. In our recent research,20 nickel based carbon fiber was utilized for the steam reforming of methane and the results illustrated that the use of carbon fibers at the reforming reaction system could effectively
1. INTRODUCTION The urgent demand of clean energy from renewable and sustainable sources has promoted a large amount of studies, including the thermochemical conversion of biomass.1,2 Thermochemical conversion of biomass converts the volatiles from biomass into combustible gases at high temperature, such as H2, CO, CH4, CO2. One problem of this technology is that the generated combustible gas cannot be widely used because of its poor quality. Moreover, CH4 and CO2 produced from biomass increase the amount of greenhouse gases emission.3 Therefore, with attempts to improve the availability of biomass derived gas and mitigate the greenhouse effect, CO2 reforming CH4 (CRM) reaction has been investigated intensively during the last decades. In CRM reaction, the CH4 and CO2 are primarily converted to syngas (CO and H2), which are further converted to oxygenated derivatives and long chain hydrocarbons through Fischer−Tropsch synthesis reaction,4 further broadening the utilization of biomass derived gas. Catalysts such as Rh, Ru, Pd, loaded on different supports have been used for CRM reaction to increase the catalytic activity. Nickel-based catalysts have also gain attentions due to its high activities and low cost.5,6 The obstacles to the widespread commercial development of the nickel based catalyst are the metal sintering and coke deposition at high temperature, which leads to quick deactivation of catalyst at a short time. Therefore, many studies7,8 have been done focusing on the metal sintering resistance and the coke resistance by using ultrasonic-assisted impregnation method or selecting suitable carriers.9,10 © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
April 4, 2018 July 19, 2018 July 23, 2018 July 23, 2018 DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
the 15%Ni/SiO2 catalyst was prepared by hydrothermal method, and the preparation process was the same as that of 15%Ni/SiO2−CF except the procedure of mixing with carbon fiber. 2.3. Catalysts Characterization. The X-ray diffraction patterns of 15%Ni/CF, 15%Ni/SiO2, and 15%Ni/SiO2−CF were obtained on a diffractometer (Smart Lab, Tokyo, Japan) with Cu Kα radiation under a 0.02° scanning step at 40 kV and 100 mA. The scanning 2θ angle was from 5° to 85°. Scanning electron microscope (Navo Nano SEM450) was used to observe the surface morphology of the fresh and spent catalysts. H2-TPR profiles of fresh samples were carried out on a PCA-1200 chemical adsorption instrument (Beijing, China). After packing 30 mg sample into U-shaped tube, it was pretreated under flowing N2 (30 mL/min) at 300 °C for 90 min, then cooled down to room temperature. Subsequently, 10% H2/He mixture gas with a flow rate of 30 mL/min was switched into the U-tube for 30 min. The outlet signal of the reactor was recorded continuously by a thermal conductivity detector (TCD). Once the detector signal became stable, the reactor was heated up to 900 °C at 8 °C/min in the 10% H2/ He mixture gas. Carbon dioxide temperature-programmed surface reaction (CO2-TPSR) were conducted by the same instrument with an attempt to the evaluation of the carrier carbon participation in the reaction. It is the same pretreatment procedure with H2-TPR. After pretreatment, the sample was reduced at 600 °C in H2 gas with the flow rate of 30 mL/ min for 2h followed by cooling down to room temperature. CO2-TPSR profile was obtained by heating the reactor up to 900 °C at a heating rate of 8 °C/min in the 10% CO2/He mixture. 2.4. Catalytic Experiments. The CRM reaction were performed on a fixed bed reactor under atmospheric pressure. There were two parts, first, in order to determine the appropriate content of the active metal, the four kinds of catalysts 5% Ni/CF, 10% Ni/CF, 15% Ni/CF, and 20% Ni/CF with the dosage of 0.1 g were loaded into a quartz tube (inner diameter is 1 cm) for CRM reaction, respectively. Before reaction, the catalyst was reduced at 600 °C for 2h by pure H2 with flow rate of 30 mL/min. Then the temperature was heated up to 650 °C in nitrogen atmosphere, followed by switching mixture gas (CH4/CO2/N2 = 1:1:1) into the reactor with the flow rate of 30 mL/min, keeping the GHSV was 18 000 mL/g·h. The tail gas was collected at 650 °C, 700 °C, 750 °C, 800 °C, and 850 °C respectively and analyzed by a gas chromatograph equipped with TCD and FID detectors (Agilent 6890N). Second, after the determination of the optimum metal content, it is necessary to determine the amount of proper silica coated onto the surface of nickel based carbon fiber. The lifetime measurements of 15%Ni/xSiO2−CF were carried out at the same bed reactor. Similarly, the samples were first reduced at 600 °C in pure H2 with the flow rate of 30 mL/min, then heated the temperature up to 800 °C in nitrogen atmosphere. Subsequently, mixture gas (CH4/CO2/ N2 = 1:1:1) was introduced into the reactor with the flow rate of 30 mL/min. The tail gas was detected by the same gas chromatograph. In addition to the lifetime test of 15%Ni/ xSiO2−CF, the lifetime test of 15%Ni/CF and 15%Ni/SiO2 were also carried out at the same experiment conditions as a contrast. The conversion rates of CH4 and CO2 were used to evaluate the overall catalytic activity, which was calculated using eqs 1
improve the catalytic activity. However, some of the carbon fibers will be reacted with the reactants, which leads to the reduction of specific surface area of the catalyst, leading to the nickel particle sintering, thus producing coke deposition and reducing the catalytic activity. Studies have shown that coating inorganic layer such as alumina (Al2O3), La2O3 and silica (SiO2) onto the surface of metal supported catalyst is an effective way to stabilize metal particles against sintering at high temperatures.21,22 Zhang et al.23 have synthesized Ni@SiO2 and used it in dry reforming, showing good catalytic performance with high temperature stability and high metal support interaction. It is well-known that silica has high temperature oxidation resistance, which can inhibit the carrier participation in the reaction.24 Therefore, the lifetime of the catalyst as well as the conversion of reactants can be improved. With an attempt to overcome the problem of carbon fiber participation, the idea of introducing silica onto the surface of nickel based carbon fiber was proposed. In this study, the nickel based carbon fibers were prepared through ultrasound- assisted impregnation method and used in CRM reaction. In order to stabilize the nickel particles and inhibit the carrier consumption in the reaction, SiO2 was introduced into the catalyst by hydrothermal synthesis method. H2-TPR, CO2-TPR, SEM, XRD have been used to characterize the fresh and spent catalysts.
2. MATERIALS AND METHODS 2.1. Materials. Nickel nitrate hexahydrate (Ni(NO3)2· 6H2O), tetraethyl orthosilicate (TEOS) and commercial carbon fiber (CF) were purchased from Shen Bo Chemical Co., Ltd. (Shanghai, China). The ultrapure water (resistance above 18 MΩ/cm) that used in the experiments was produced on a Milli-Q purification system (Branstead). The purity of all the gases needed for the experiment, including carbon dioxide, methane, helium, hydrogen, and nitrogen, were 99.999%. 2.2. Catalyst Preparation. A series of nickel based carbon fibers (mass ratio of nickel to carbon fiber were 5%, 10%, 15%, 20%) were prepared by ultrasonic-assisted impregnation method. Commercial carbon fiber support was washed first, and then added into an aqueous solution of Ni(NO3)2·6H2O, followed by ultrasonic treatment for 15 min. Subsequently, the suspension was placed in a water bath at 40 °C overnight. Finally, those samples were labeled as xwt % Ni/CF (x represents the 5, 10, 15, 20) after drying in an oven at 105 °C for 6h and calcining in the flow of nitrogen atmosphere (100 mL/min) at 550 °C for 3h. For the preparation of nickel based carbon fiber with different SiO2 content, 0.2963g nickel nitrate was first dissolved in 10 mL H2O, and then mixed with 0.4 g carbon fiber (mass ratio of nickel to carbon fiber were 15%), followed by ultrasonic treatment for 15 min. The suspensions were denoted as solution A. After that, 1/2/3/4 mL TEOS (mass ratio of Si to CF were 0.5:1, 0.75:1, 1:1, 1.25:1) were added into 15 mL ethanol solution separately, followed by ultrasonic treatment for 15 min. The suspensions were denoted as solution B. The mixture of solution A and B was placed in a reactor with Teflon lining at 160 °C for 24h. After the hydrothermal reaction, the samples were washed by ethanol and ultrapure water alternately to remove the residual sol. Then the samples were put into a tube furnace filled with nitrogen and calcined at 550 °C for 3h with the heating rate of 10 °C/min. The samples obtained above were labeled as 15% Ni/xSiO2−CF (x represents 0.5, 0.75, 1, 1.25). As a contrast, B
DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research and 2. The ratio of hydrogen to carbon (H2/CO) was determined using eq 3. XCH4(%) =
XCO2(%) =
H 2 /CO =
FCH4,in − FCH4,out FCH4,in FCO2,in − FCO2,out FCO2,in
× 100% (1)
× 100% (2)
FH2,out FCO,out
(3)
Where Fi, in or Fi, out is the flow rate of each component in the feed gas or effluent.
3. RESULTS AND DISCUSSION 3.1. Catalytic Activity. Catalytic performance of the four catalysts 5% Ni/CF, 10% Ni/CF, 15% Ni/CF and 20% Ni/CF were evaluated at temperature of 800 °C and GHSV of 18 000 mL/g·h to obtain the optimal nickel content for the followed experiment. The results are shown in Figure 1(a). It is illustrated that the CH4 and CO2 conversion rate increases with the increase of temperature. For catalysts with different nickel content, there is a growing trend for CH4 and CO2 conversion rate with the increase of nickel content from 5% to 15%, which mainly due to the activity sites for CRM reaction are increased as the increase of nickel content. However, the conversion rate of CH4 and CO2 reach to a higher steady state conversion rates (80.39% for CH4 and 90.13% for CO2) when the metal content increases from 15% to 20%. This can be attributed that the impregnation reaches saturation at the metal content of 15% at a certain time for the carbon fiber. Higher amount of metal particles than 15% will not affect the nickel content loaded on the carrier.25 Therefore, 15% was chosen for the further investigation. The catalytic stability of SiO2 coated nickel based carbon fiber are shown in Figure 1(b) and (c) with the reaction time from 10 to 600 min at reaction temperature of 800 °C. Results indicated that the stability enhances as the amount of SiO2 increases, while the best stability were achieved at the silica to carbon ratio of 1. However, when Si/CF reaches to 1.25, the activity of catalyst starts to decrease. The reason might be that the active sites are covered by SiO2, resulting in a slight reduction in the conversion rate of CH4 and CO2.26 Another possible reason is that the pore structures of carbon fibers were occupied by silica, decreasing the specific surface area and further leading to the reduction of conversion rate in both the long and short time.27 Therefore, 15%Ni/SiO2−CF (Si/CF = 1) was apparently more suitable for the further CRM research. The complete details of the SiO2 influence mechanism on the catalytic activity and stability of nickel based carbon fiber will be provided below. In order to investigate the effect of SiO2 on the catalytic performance of the 15%Ni/CF, the stability evaluation of three catalysts 15%Ni/SiO2−CF, 15%Ni/SiO2 and 15%Ni/CF was conducted at 800 °C and GHSV of 18000 mL/g.h. As shown in Figure 2 (a), the initial CH4 conversion rate of 15%Ni/CF is 80.39%, but it decreases to 60.36% after 150 min.This can be attributed that the reaction of carbon fiber with CO2 over 150 min raises the mobility of nickel to a serious sintering, which decreases the active site, thus resulting in a reduction of methane conversion rate.28 For the catalyst of 15%Ni/SiO2,
Figure 1. CH4 and CO2 conversion rate of catalysts with different nickel content (5%, 10%, 15%, 20%) (a) and different silica content (Si/CF = 0.5, 0.75, 1, 1.25) (b) (CH4:CO2:N2 = 1:1:1, GHSV = 18 000 mL/g.h)
the initial CH4 conversion (89.6%) rate is higher than that of 15%Ni/CF. However, the CH4 conversion rate decreases about 10% after 200 min of reaction. This can be attributed to the sintering of the metal active components on the surface of the SiO2, which further reduces the contact area between the reactant and the active site.29 The CH4 conversion rate on 15% Ni/SiO2−CF, has a slight fluctuation and became steady at 84.28% after 600 min. It is possible that the addition of SiO2 inhibits the involvement of the carrier and increases the interaction between the metal and the carrier, which improves its sinter resistance. Figure 2(b) shows the CO2 conversion rate of three catalysts during a 600 min reaction time. It can be seen clearly that the initial CO2 conversion rate of 15%Ni/CF catalyst is higher than 15%Ni/SiO2 with the value over 91.13%. It is further illustrating the fact that the carrier carbon will participate in the dissociation activation of CO2.30 In addition, the mass of the catalyst reduced 30% after the reaction. It may be attributed that a large number of carbon fibers reacted with C
DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
indicates that the carbon dioxide conversion rate decreases within 150 min for the 15%Ni/CF. In contrast, the CO2 conversion rate of 15%Ni/SiO2−CF is higher than that of others and almost remains stable in 600 min. This can be attributed to the inhibition effect of SiO2 on the carrier reaction with CO2. Similarly, the mass of 15% Ni/SiO2−CF after the CRM reaction was weighed, and the mass was almost no changed. H2/CO of the CRM reaction over three catalysts at 800 °C within 600 min were analyzed and the results are illustrated in Figure 2 (c). For 15%Ni/CF, the initial value of H2/CO is about 0.9, however, a rapid decrease was found within 150 min, which mainly because of the CO generation through the reaction of CO2 with carrier carbon.31 3.2. Characterization of Fresh Catalysts. The catalytic performance comparison of three kinds of samples on CRM reaction has revealed that the introduction of SiO2 can inhibit the carrier reaction with CO2 as well as improve the activity and stability of the catalyst. It may be attributed that the addition of SiO2 changes the metal support interaction. In order to test this assumption, a H2-TPR experiment on 15% Ni/SiO2−CF catalyst was carried out. The H2-TPR profiles of 15%Ni/CF and 15%Ni/SiO2 were also studied as control. Results have shown (Figure 3 (a)) that the reduction peaks of 15%Ni/CF were found at 243.81 °C, 370.75 °C, 483.14 °C, 630.27 °C. The reduction peak at 200∼500 °C refers to the reduction of free nickel oxide,32 while the wide reduction peak at high temperature section shows the strong metal support interaction as well as the better metal dispersion on surface. For 15%Ni/SiO2, the reduction peaks located at 220.6 °C, 439.21 °C, 671.65 °C, represent the reduction of free nickel oxide, nickel oxide on the surface of silica and the nickel phyllosilicates species formed during the calcination process, respectively.33 After deposition of SiO2, the low temperature peak disappears for 15%Ni/SiO2−CF, leaving a reduction peak at 678.61 °C that is the highest among all the reduction peaks shown in the Figure 3 (a). Meanwhile, the peak width also increases comparing with that of other two samples, indicating the better metal dispersion and the strong metal support interaction after adding SiO2 onto the surface of nickel based carbon fiber. CO2-TPSR was carried out to assess whether there was an inhibitory effect of SiO2, as shown in Figure 3 (b). The CO2TPSR profiles of 15%Ni/CF and 15%Ni/SiO2 are included in Figure 3(b) as control. For the CO2-TPSR profiles, the area below the curve is the amount of CO2 reacted with carrier. As it shows, there is no reaction between 15%Ni/SiO2 and CO2. The reaction between 15%Ni/CF and the CO2 starts when
Figure 2. Conversions of methane (a) and carbon dioxide (b) and H2/CO ratios (c) of three kinds of catalysts (CH4/CO2/N2 = 1:1:1, GHSV = 18000 mL/g.h, T = 800 °C).
carbon dioxide will decrease the attachment point of nickel particles, resulting in aggregation of a large of nickel particles as well as a serious carbon deposition, further inhibiting the dissociation and activation of carbon dioxide. Figure 2(b)
Figure 3. H2-TPR (a) and CO2-TPSR (b) profiles of fresh catalysts. D
DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 4. XRD patterns of three reduced catalysts (a) and spent catalysts (b): (1) 15% Ni/SiO2; (2) 15% Ni/SiO2−CF; (3) 15% Ni/CF.
temperature went over 434.31 °C. The results of the three samples illustrate that the amount of the reacted carbon dioxide is the largest for 15%Ni/CF during the heating process, especially in the temperature section of CRM reaction (650∼850 °C). It is further explained that most of the carriers are involved in the reaction with CO2. Our results indicate that the addition of SiO2 moves the initial reaction temperature of the carrier to high temperature. Moreover, the area under the curve decreases significantly, demonstrating the inhibitory action of SiO2. Although there is still a small amount of carbon fiber involved in the reaction, it is generally accepted that the existence of a small amount of amorphous carbon have an opposite effect with increasing dissociation of CO2 and the catalytic activity performance.16 It has been described by the CO2-TPSR and H2-TPR results of fresh catalysts that there are promoting effect of silica on catalytic performance by covering a part of the surface of carbon fiber, thus inhibiting the reaction of carrier and increasing the metal support interaction. For further explain the improvement of the catalytic activity and stability by silica, we have performed XRD and SEM characterizations of fresh and spent catalysts (Figure 4 and Figure 5). Figure 4(a) is the XRD diffraction pattern of three fresh catalysts after reduction. There is no evidence of any metallic NiO remaining after reducing, indicating a complete reduction of those three catalysts. For 15% Ni/SiO2−CF, it can maintain high activity and stability over 600 min, while the other two catalysts 15% Ni/CF and 15% Ni/SiO2 were found deactivated after 200 min. Therefore, the catalysts after the CRM reaction over 200 min were selected as the spent catalyst for the XRD analysis and the results are shown in Figure 4(b). As can be seen from the diffraction pattern of the fresh 15% Ni/CF, there is no diffraction peak of graphite carbon. However, a clear wide diffraction peak belongs to the graphite carbon at 22.890° was observed on the spent 15% Ni/CF after reaction, indicating that the catalyst has obvious carbon deposition after reforming.32 Besides, the diffraction peaks at 44.520°, 51.880°, and 76.380° are corresponded to the diffraction peaks of Ni (111), Ni (200), and Ni (220). The nickel crystallite sizes (based on Ni (111) and Ni (200) reflections) estimated by Scherrer equation are provided in Table 1. For 15% Ni/CF, the increases of the nickel crystallite size from 16.54 to 18.22 nm was due to the metal sintering after the reaction.34 In terms of the 15% Ni/SiO2−CF, the broad peak located at 10.247°, 21.604° should be assigned to amorphous silica. There is no reflection peak of graphite carbon appears and the crystallite size of nickel phase remains unchanged after the CRM reaction with the value of 19 nm
Figure 5. SEM diagrams of catalysts: fresh 15% Ni/CF (a); spent 15% Ni/CF (b); fresh 15% Ni/SiO2−CF using magnification of ×20 000 (c) and ×50 000 (d); spent 15% Ni/SiO2−CF using magnification of ×20 000 (e) and ×50 000 (f).
Table 1. Nickel Crystallite Sizes of Three Kinds of Catalysts before and after Reactiona sample
.Ni (111)b
Ni (111)c
Ni (200)b
Ni (200)c
15%Ni/CF 15%Ni/SiO2 15%Ni/SiO2−CF
16.54 12.89 19.73
18.22 16.36 19.99
17.02 13.16 19.79
20.23 16.99 19.69
a
Note: Ni (111) and Ni (200): Nickel size of different crystalline surfaces. bFresh catalyst. cSpent catalyst. All of the nickel size are calculated from the Scherrer equation using XRD profiles.
approximately, indicating its relatively good metal sinter resistance. It can be attributed that the existence of amorphous silica increases the metal support interaction.23 In order to study the effect of silica on the surface morphology of 15% Ni/CF, the SEM characterizations of 15% Ni/SiO2−CF before and after reaction was carried out (Figure 5). As control, the SEM analysis of 15% Ni/CF before and after reaction was also conducted. Figure 5(a) and (b) are E
DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research SEM images of 15% Ni/CF before and after reaction. The surface of carbon fiber is smooth before reaction and the nickel active components can be well dispersed on the surface of fiber. After the CRM reaction, aggregations of the nickel particles and carbon deposition were imaged. Same results have been confirmed by the H2-TPR profile and XRD patterns. In addition, the surface of carbon fiber becomes very rough, which can be attributed to the reaction with CO2. An explanation is that the reaction of a large number of carriers raise the mobility of nickel phase to a serious sintering, then produce a lot of carbon deposition, followed by the catalytic activity decreases rapidly.20 Figure 5(c) and (d) show SEM images of fresh 15% Ni/ SiO2−CF. Two types of silica after modification were observed and the surface of carbon fiber can be covered partially. In addition, Figure 5(d) shows that the nickel active components are mainly loaded at the carbon fiber surface, the silica surface and the junction of silica and carbon fiber. Figure 5(e) and Figure 5(f) are SEM images of 15% Ni/SiO2−CF after 600 min CRM reaction. The surface of carbon fiber is mainly covered by amorphous SiO2 after the CRM reaction, which is similar to the results obtained by XRD. It should be noticed that no obvious metal sintering were found and the structure of carbon fiber does not change dramatically. Although a small amount of carbon deposition appeared, which has also been proved in the previous XRD patterns. However, such small amount of carbon would not affect its catalytic performance.35 A small amount of carbon fiber involved in the reaction can keep a dynamic balance with the deposited carbon, which promotes the CRM reaction.36 Additionally, the results of CH4 and CO2 conversion rates are compared with some other carbon based catalysts, which are shown in Table 2. It is illustrated that the CH4 and CO2
Figure 6. Mechanism of CRM reaction on Ni/SiO2−CF.
CO2 leads to the decline of nickel attachment point on the support surface, which will result in metal sintering and catalyst deactivation at a short time. It has been confirmed that SiO2 plays a important role in promoting the activity and stability. The addition of SiO2 can cover the surface of carrier partially, therefore inhibiting the consumption of the carrier in the CRM reaction. Moreover, the presence of silica also increases the interaction between the metal and the carrier, thus improving the nickel sinter resistance, therefore enhancing the catalytic activity and stability.
4. CONCLUSION In this study, nickel based carbon fiber with different metal contents (5%, 10%, 15%, 20%) were prepared by ultrasoundassisted impregnation method and applied to the biomass derived gas reforming. A certain amount of SiO2 was introduced onto the surface of 15%Ni/CF to overcome the deactivation problems that occurs in other studies. Results demonstrated that the new catalyst improves the catalytic activity and stability during CRM. H2-TPR, CO2-TPSR, XRD, and SEM analysis have shown that the existence of SiO2 suppresses the reaction of carrier and the metal sintering, but enhances the interaction between metal and support that leads to considerable activity and stability during a 600 min stream time. It provides a feasible idea for the preparation of CRM catalyst.
Table 2. Comparison of the Catalytic Activity on Different Carbon Based Catalystsa sample AC Co/AC Co−Zr/ AC Ni-CNT/ MS KMnO4/ AC Ni/CF Ni-SiO2/ CF
temperature/ (°C)
CH4 conversion rate/(%)
CO2 conversion rate/(%)
950 800 750
42.6 57.8 89.5
47.4 68.2 88.6
14 37 38
650
42.1
53.2
39
800
41.4
44.8
37
800
80.39
91.13
800
88.47
95.47
this work this work
reference
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-13770606581. Fax: +86-025-83790986. E-mail:
[email protected]. ORCID
Min Song: 0000-0002-0002-0568
a
Note: AC represents the activated carbon; CNT means the carbon nanotubes.
Notes
The authors declare no competing financial interest.
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conversion rates over Ni/SiO2−CF are comparable when the GHSV used in this manuscript was larger than that recorded in the literature, illustrating the potential application of Ni/SiO2− CF as a catalysts. Based on the above results and discussions, a potential mechanism of carbon dioxide reforming methane on the Ni/ SiO2−CF catalyst is proposed in Figure 6. Highly dispersed nickel particles serve as active sites for methane activation. In addition, the hydrogen free radicals come from methane cracking as well as the carbon fiber facilitate the activation of carbon dioxide. However, the reaction between carrier and
ACKNOWLEDGMENTS We are grateful for the National Natural Science Foundation of China (51576048). This work was sponsored by the Fundamental Research Funds for the Central Universities.
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REFERENCES
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DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.iecr.8b01452 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
