Co-production of hydrogen and carbon nanotube-silica fiber composites from ethanol steam reforming over an Ni-silica fiber catalyst
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Nickel supported on silica fiber (Ni-SF) catalysts was successfully synthesized by sol–gel-assisted electrospinning of SFs followed by the conventional impregnation of the Ni salt and calcining. Their activity for the co-production of hydrogen (H2) and carbon nanotube-silica fiber (CNT-SF) composites in the ethanol steam reforming (ESR) process was investigated. The effects of the ESR reaction temperature, steam-to-carbon ratio (S/C), space–time (W/F), and Ni loading level on the reaction activity as well as H2 production and CNT-SF characteristics of the Ni-SF catalyst were investigated. The Ni-SF catalyst was highly effective at the simultaneous production of H2 and CNT. The optimized condition of the ESR in terms of the ethanol conversion and H2 yield was achieved at 600 °C, an S/C ratio of 9, and W/F of 18 gcat h mol−1 with a maximum H2 yield of 55%, while the best quality and quantity of the CNT (36%) formed along with a H2 yield of 29% was obtained at an Ni loading of 30 wt%, S/C ratio of 1, and W/F of 9 gcat h mol−1. The novel CNT-SF composite obtained from ESR exhibited a relatively high surface area and easy accessibility, making it a promising catalyst support for various processes.
KeywordsNickel-based catalyst Steam reforming of ethanol Silica fiber Carbon nanotubes
Hydrogen (H2) is considered as a clean energy. It can be directly utilized in a fuel cell to generate electricity and is also a potential feedstock in the oil refinery and chemical industry. Currently, the conventional H2 production is by the steam reforming of fossil fuel, but this releases extra undesirable greenhouse gases to the atmosphere [1, 2], as well as depletes the non-renewable fossil fuel stocks. Therefore, new renewable feedstocks for replacing the conventional fossil fuels are receiving more attention. Ethanol is one of the interesting sources for H2 production, since it is a clean source that is derived from renewable biomass . The successful production of H2 via ethanol steam reforming (ESR) over various catalysts has been reported in many research groups. The ESR is an endothermic reaction, which produces H2 and numerous by‐products such as acetaldehyde, acetone, ethylene, CO, CO2, CH4, and carbon deposition. The origin of the deactivation by carbon deposition on the catalysts during the ESR is attributed to the following routes: (a) the polymerization of ethylene formed by ethanol dehydration on the acid sites of the catalyst; (b) the decomposition of methane; and (c) the Boudouard reaction . According to the literature , there are two types of carbon deposited occurring during the ESR: amorphous carbon and filamentous carbon (CNT and CNF). The valuable CNT product is receiving considerable attention, since the CNT has unique structural properties such as a high specific surface area, high thermal stability, and excellent electrical conductivity. These advantages allow it to be used in various applications, especially in composite materials for improving the physical and chemical properties of other materials [6, 7]. The CNT can be synthesized by various methods such as arc discharge, pyrolysis, laser ablation, plasma-assisted deposition, and thermal chemical vapor . There are a few reports in relation to the simultaneous production of hydrogen and carbon nanotubes from the ESR; therefore, the ESR is an interesting approach in converting ethanol into the co-production of H2 and CNT.
Previous studies have investigated the ESR for the production of H2 over various active metals including noble metal-based metals (Ru, Rh, Pd, and Pt) and transition metals (Ni, Co, and Fe). The noble metal-based metals exhibited excellent activity and stability in the ESR; however, their application in large scales is limited by their high costs . Among transition metal catalysts, Ni-based catalysts have been widely used in the ESR because of its inexpensive and inherent activity on C–C and C–H bond cleavage. The Ni-based catalysts have been dispersed on the external and internal surfaces of porous materials such as silica (SiO2) and alumina (Al2O3) . However, the active sites deep inside the pores of the Ni catalysts are less accessible by the reactants which limits their applications. Fibrous materials, such as silica fiber (SFs), have been shown to be a good support for Ni-based catalysts due to their extremely open morphology and ease of accessibility. The use of Ni-SFs for the steam reforming of glycerol has been reported , where Ni-SF catalysts exhibited a higher catalytic activity and stability than similar porous supported Ni catalysts. However, SFs have a low surface area, since they are non-porous, whereas the high surface area and electron transfer properties of CNT or CNF make them promising composite materials for improving the physical and chemical properties of other materials . For this reason, the combination of the SFs and CNT as a composite is of great interest. Hybrid composites of CNT and conventional porous Al2O3 were developed as an alternative support for a cobalt catalyst in Fischer–Tropsch synthesis , and were found to have a remarkable catalytic activity and stability as well as outperforming other supports.
In this work, we proposed a novel clean process for the synthesis of a co-production of hydrogen and CNT on the SF by ESR. The effects of the reaction temperature, steam-to-carbon ratio (S/C), space–time (W/F), and Ni loading level on the gas production (including H2) and CNT deposition were investigated. The CNT hybridized onto the SFs combined the advantages of a hollow carbon nanostructure and the extremely open structure of SFs which could be used as a support for the Ni catalyst in steam reforming processes to solve the mass transfer limitation problem and to enhance the catalytic activity and stability.
Results and discussion
Textural properties of the SF support and Ni-SF catalysts
Surface area/m2 g−1
Pore volume/cm3 g−1
The textural properties of the supports and catalysts are also presented in Table 1. The actual contents of Ni as determined by EDS analysis were fairly similar to the theoretical Ni loading levels. The surface area (3 m2 g−1) and pore volume (0.0003 cm3 g−1) of the SF support were quite low, suggesting that the obtained SF support was a non-porous structure. The surface area and pore volume increased with increasing Ni contents, especially at 30 wt% Ni, up to the highest surface area of 19 m2 g−1 and pore volume of 0.0754 cm3 g−1 for the 30Ni-SF, since at higher Ni loading levels, the NiO particles covered all of the SF surfaces (Fig. 1), and so increased the surface area .
Hydrogen obtained from the ESR
However, the H2 yield and CO selectivity showed almost no change when the W/F was further increased from 18 to 36. The maximum H2 yield was 27%.
The effect of temperature on the ethanol conversion level and product distribution in the ESR over the 10Ni-SF catalyst was studied at 400, 500, and 600 °C (Fig. 4c). The ethanol conversion level continuously increased from 52% up to 100% by increasing the reaction temperature from 400 to 600 °C. For the ESR product distribution, the H2 yield and CO2 selectivity also clearly increased, with a maximum of H2 yield (28%) being achieved at 600 °C. In contrast, the ethane and CH4 selectivity tended to decrease slightly with increasing reaction temperature. In the ESR, system involves the formation of an ethoxy species through the dissociative adsorption of ethanol. The ethoxy species would be oxidized to form an aldehyde intermediate. This process was facilitated by water or the OH groups. The acetaldehyde intermediate could either transform to ethane and water by the cleavage of the C–O bond of acetaldehyde and addition of hydrogen or further be oxidized to acetate species by hydroxyl groups from the support. The acetaldehyde and acetate species can further decompose through C–C bond cleavage to form CH4 and CO. The formation of acetone can also occur in the ERS via possible two routes: (a) the coupling of two acetate molecules and (b) the disproportionation of two acetyl species . In experiment, at 400 °C, a negligible level of C2 products (such as acetaldehyde, acetone, and ethane) was detected. The presences of acetaldehyde and acetone were only noticeable at 400 °C with relatively low selectivity of 0.4 and 0.002%, respectively; hence, it was not expressed in Fig. 4. The capability of Ni to cleave C–C bonds was weaker at lower temperatures and the ethanol dehydrogenation [Eq. (2)] to acetaldehyde followed by decomposition of the latter to CO and CH4 [Eq. (3)] was favored, giving a low ethanol conversion level. With increasing temperature, the ability of breaking the C–C bond of Ni was improved, which promoted acetaldehyde steam reforming. Furthermore, the decrease in CH4 selectivity and the simultaneous increased in H2 yield and CO2 selectivity observed were attributed to the CH4 steam reforming and water gas shift (WGS) reactions [Eqs. (5), (6)] .
The influence of the S/C molar ratio on the ESR catalytic performance over the 10Ni-SF catalyst was studied at 600 °C with S/C ratios of 1, 3, 6, 9, and 12 (Fig. 4d). The ethanol conversion level was 100% at all S/C ratios studied, but the H2 and CO2 yields increased progressively as the S/C ratio increased. The highest H2 yield (55%) was achieved at an S/C ratio of 9. In contrast, the CO and CH4 selectivity decreased with increasing S/C ratios. Thus, the addition of more water into the feed exhibited a positive effect on the H2 yield. The WGS reaction was not flavored at a low S/C ratio, and so a high selectivity to CO and CH4 was obtained. Increasing the S/C ratio to 9 promoted the WGS and CH4 steam reforming reactions, which resulted in an increased H2 and CO2 concentration, associated with a gradual decrease in the CH4 and CO concentrations . However, further increasing the S/C ratio from 9 to 12 did not obviously improve the H2 yield or change the concentration of CO and CO2. The unchanged H2 yield was attributed to that higher water content led to the competition between ethanol and water to be adsorbed on the active sites .
Synthesis of CNTs on the Ni-SF catalysts during the ESR reaction
Textural properties of the spent Ni-SF catalysts and formed CNT-SF composites
Surface area/m2 g−1
Pore volume/cm3 g−1
As mentioned above, the systematically effect of various conditions (S/C molar ratios, W/F, reaction temperatures, and Ni loadings) on the co-production of hydrogen and CNTs from the ESR over the Ni-SF catalysts was discussed. The results can be summarized as follows: the higher reaction temperature (600 °C) and S/C molar ratio (9) favored the production of H2 with a yield of 55%, which was accompanied by 14% yield of CNTs. In terms of the carbon production, the highest yield of carbon (36%) along with an H2 yield of 29% was obtained at 600 °C with a lower S/C (1) and W/F (9 gcat h mol−1) and a higher Ni loading (30 wt%). It is clear to achieve the co-production yield of H2 and CNTs using the same reaction.
The specific surface area of the spent 30Ni-SF catalyst was 116 m2 g−1, while that of the CNT-SF after removing Ni was 157 m2 g−1 (Table 2). The higher specific surface area after treatment was because the treatment to remove Ni opened the carbon tube tip, which allowed the N2 adsorption into the intertubular pores of the CNT resulting in an increased derived surface area .
The successful synthesis of SFs was attained by sol–gel-assisted electrospinning, and they were then used as a support for the impregnated and calcined NiO catalyst in the ESR reaction to produce H2 and CNT. The effect of the reaction temperature, S/C ratio, W/F, and Ni loading level was investigated, with an optimized ESR condition, in terms of the ethanol conversion level and H2 yield, being established as an Ni loading of 10 wt%, 600 °C, S/C of 9, and W/F of 18 gcat h mol−1. At this condition, H2 and CNT yields of 55 and 14%, respectively, were simultaneously observed, while the highest yield of CNT (36%) along with an H2 yield of 29% was obtained at 600 °C with an Ni loading of 30 wt%, S/C of 1, and W/F 9 gcat h mol−1. The novel CNT-SF composites produced in ESR were uniformly distributed with an outer diameter of 20–30 nm. The CNT-SF composites offered the advantages of a large surface area and an entirely open structure, making them appealing for use as a catalyst support in various reactions.
The Ni-SF catalyst was synthesized through the combination of sol–gel and electrospinning techniques followed by the conventional impregnation . In the synthesis of the SF support, tetraethyl orthosilicate (TEOS) was used as the Si source. First, 18 cm3 of TEOS was dissolved in 3 cm3 of distilled water (DW) and stirred for 5 min, followed by the addition of 1 g of concentrated HCl. Then, 9.4 cm3 of ethanol was added into the mixed solution with continuous stirring for an additional 5 min. After that the solution was heated to 55 °C with continuous stirring for approximately 30 min. Finally, the mixed solution was transferred into a disposable syringe equipped with a 0.4 mm diameter needle. The electrospinning condition used for the synthesis of the SF support was a voltage of 15 kV, a feed rate of 5 cm3 h−1, and a tip-to-collector distance of 15 cm. The obtained SFs were dried overnight at 110 °C in an oven and then calcined at 500 °C for 2 h.
The Ni-SF catalysts were prepared by impregnation using Ni(NO3)2·6H2O as the Ni precursor. Ni(NO3)2·6H2O was dissolved in DW and then impregnated onto the SF support, dried at 110 °C overnight, and calcined at 500 °C for 2 h.
Nitrogen (N2) physisorption via BET method (Brunauer–Emmett–Teller) on Micromeristics ASAP 2020 system was used to measure the surface area, pore diameter, and pore volume of the material, while X-ray diffraction (XRD) patterns were measured with D8 ADVANCE X-ray diffractometer (Bruker) equipped with Cu Kα radiation (λ = 1.5406 Å) with the radiation source CuKα, operating in the 2θ angle range of 5°–80° with a resolution of 0.02°. The patterns were identified using the Joint Committee on Powder Diffraction Standards (JCPDS cards). Scherrer’s equation was used to determine the average particle size of Ni metal. Temperature programmed reduction of hydrogen (H2-TPR) was conducted using a Micromeristics TPD/TPR 2920 system to determine the reducibility of the catalysts using a quartz reactor under 5% (v/v) H2 (N2 balance). The morphology of each fiber material was observed in field emission scanning electron microscopy (FESEM) using a JEOL JSM-7610F system. The average diameter and particle size of the Ni metal on the SF were estimated from the FESEM images using the SemAfore program. The actual metal content in the catalysts was determined by energy dispersive X-ray spectrometry (EDS; Oxford X-MaxN). Transmission electron microscopy (TEM; JEOL 2100F) was used to observe the surface morphology appearance of the carbon deposited on the Ni-SF catalyst from the ESR reaction. Thermal gravimetric analysis (TG/DTG) was performed on a METTLER TOLEDO STARe system under oxygen atmospheres at a heating rate of 5 °C min−1 from room temperature to 900 °C to study the weight changes and evaluate the yield of carbon deposited.
The co-production of H2 and CNTs was synthesized by ESR over the Ni-SF catalyst. The catalytic performance of the Ni-SF catalyst was conducted in a fixed-bed tube reactor. Before ESR, 0.2 g of the catalyst sample was reduced in situ with 10% (v/v) H2 (N2 balance) at 400 °C for 1 h. The ESR was performed at atmospheric pressure using N2 as the carrier gas for 360 min. A water–ethanol mixture was fed into the reactor via an HPLC pump. The effects of the reaction temperature (400, 500, and 600 °C), S/C molar ratio (1, 3, 6, 9, and 12), W/F (9, 12, 18, and 36 gcat h mol−1), and Ni loading level (10, 20, and 30 wt. %) on the gas production and CNTs deposition were investigated. Analysis of the products was performed by gas chromatography (GC) using a flame ionization detector (FID) to analyze the reaction organic products (methane, ethane, acetaldehyde, ethanol, and acetone) and a thermal conductivity detector (TCD) to analyze the gaseous products including H2, CO2, CH4, and CO.
After the reaction, the CNTs obtained from the optimum condition were washed with a mixture of 3 mol dm−3 HCl and 1 mol dm−3 HNO3 solution under sonication at 40–50 °C for 4 h, then washed with DW to pH 7, and dried overnight at 120 °C to obtain the CNTs-SF composites. The morphology and physical properties of the CNTs-SF composites were investigated by TEM and N2 physisorption.
The authors are very grateful for financial support to Science Achievement Scholarship of Thailand (SAST), Overseas Academic Presentation Scholarship for Graduate Students, Graduate School, Chulalongkorn University, the research fund from the Thailand Research Fund (IRG5780001), The National Research Council of Thailand with the National Natural Science Foundation of China (NRCT-NSFC2558-104), and Science and Technology Research Partnership for Sustainable Development (SATREPS), Japan Science and Technology Agency (JST)/Japan International Cooperation Agency (JICA). The authors also acknowledge the support of HORIBA (Thailand) Limited for catalysts characterization by Raman model XploRA PLUS.
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