Nickel catalysts supported on ordered mesoporous SiC materials for CO2 reforming of methane
Graphical abstract
Introduction
CO2 reforming of methane (DRM) reaction could convert CH4 and CO2, which contributes to greenhouse gas abatement [1], into more valuable syngas (CO and H2). Not only can this reaction protect the environment, but its product can be directly used as a raw material since its low H2/CO ratio for the synthesis reaction such as carbonyl and liquid fuels [2]. As a consequence, DRM has attracted increasing attentions in recent years.
It was reported the noble metals exhibit excellent activity and can restrict carbon deposition capacity. However the high cost and limited availability inhibit their industrial application, thus transition metals are considered as active components because of their high catalytic activity and low cost [3,4]. The Ni-based catalyst is one of the supreme catalytic systems with excellent catalytic performance for DRM [5], however, rapid catalyst deactivation caused by carbon deposition and metal sintering restricts the industrial application of Ni-based catalyst [6,7]. In order to overcome these negative impact on catalyst activity, lots of investigations were made on the catalyst components, including the reinforcing effect of additives [8,9] and the incorporation of nickel ions within well-defined solid solutions [[10], [11], [12]] etc. Moreover, some new work focusing on the confinement structures of ordered mesoporous/microporous material is also reported as an effective way to inhibit the metal aggregation and carbon deposition, thus further inhibits the rapid deactivation of catalyst [[13], [14], [15], [16]].
In comparison with traditional non-porous materials, it is believed that ordered mesoporous materials are excellent support material for catalytic reactions due to their high specific surface area, regular pore array and tunable pore size [17]. Furthermore, mesoporous silica materials (such as KIT-6, SBA-15, MCM-41) are one of the most widely used catalyst supports [[18], [19], [20]]. The large pore size of mesoporous silica provides excellent active sites for the catalytic reactions of many larger molecules, and the uniform nano channels show significant spatial confinement effects in the catalytic reactions of some macromolecules. However, the industrial application of mesoporous silica materials are restricted due to their poor stability at high temperature [21,22].
Non-oxide ceramics, for instance, silicon carbides are especially considered as effective catalyst support with high thermal conductivity, good thermal stability, excellent mechanical strength, and chemical inertness for many reactions in harsh environments [[23], [24], [25], [26]]. However the specific surface of great majority ordered mesoporous SiC material are below 100 m2/g, highly restricting the application of silicon carbides in catalysis. And it was difficult to fabricate ordered mesoporous SiC structures with high surface area and adjustable pores size due to the extremely high synthesis temperature and the lack of proper precursors [27]. The successful mesoporous SiC synthesis strategy including chemical vapor infiltration [28], gel injection molding [[29], [30], [31]], thermal reduction [32] and template synthesis [[33], [34], [35]] methods. For example, Park et al. [36] penetrated a low viscosity SiC precursor into the silica pellet template with diameter of 20–100 nm, and then obtained an unordered mesoporous SiC with specific surface area larger than 200 m2/g after removing the silica template. Krawiee et al. [37] used dimethyldichlorosilane as SiC precursor and hydrogen as the carrier gas to introduce the SiC into the ordered mesoporous molecular sieves MCM-48 and SBA-15 by chemical vapor infiltration process. A drawback of this method is that the formed SiC has a relatively wide pore size distribution. In addition, Shi et al. [38,39] used the mesoporous SBA-15 and MCM-41 as templates, and introduced the precursor into template channel by capillary condensation process, finally obtained the reversed copy structure by in situ transformation via nanocasting. Compared with these methods, the nanocasting process is relatively simple to synhtesize SiC material with highly ordered mesopore structure.
As far as we know, there are few examples using mesoporous SiC as the catalyst support. Hoffmann et al. [40] prepared a disordered mesoporous SiC material with better activity and stability using fumed silica as template via casting process and the prepared catalysts showed good temperature stability for DRM. However, the spatial confinement effect of SiC support is weak due to fumed silica does not possess ordered mesopores.
In this paper, highly ordered mesoporous silicon carbide materials were synthesized using polycarbosilane (PCS) as precursor and ordered mesoporous silicon oxide (such as KIT-6, SBA-15, MCM-41) as hard template by a one step nanocasting process. Thus the Ni active component was loaded into the pores of the mesoporous SiC material by impregnation method for DRM reaction. More importantly, the confinement effects of the SiC support with ordered mesoporous channels on metal active sites and their structural-performance relationship were thoroughly investigated.
Section snippets
Synthesis of mesoporous silicon carbide
KIT-6, SBA-15 and MCM-41 were synthesized following the procedure reported by Kleitz et al. [41], Zhao et al. [42] and Karnjanakom et al. [43]. Triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer Pluronic P123 was used as the structure directing agent for SBA-15 and KIT-6, while the cethyltrimethy lammonium bromide (CTAB) was used for MCM-41. Tetraethyl orthosilicate was used as silica precursor.
Mesoporous SiC was synthesized following the nanocasting
N2 adsorption
The textural properties of prepared mesoporous templates, SiC supports and corresponding Ni-based catalysts were characterized by N2 adsorption, and their isotherm plots and pore size distributions are depicted in Fig. 1. All these isotherms exhibit type IV isotherm curves, showing the typical ordered mesoporous structures (Fig. 1a, c and e) [45]. As shown in Fig. 1a, the isotherm hysteresis loops of three hard templates are H1 type, indicating that the synthesized silica materials have uniform
Conclusions
The ordered mesoporous Ni-based SiC are prepared by a novel nanocasting method through using three common ordered mesoporous hard templates (KIT-6, SBA-15 and MCM-41). Characterization results demonstrate that the prepared SiC supports retain the ordered mesoporous structure of its corresponding hard template, and Ni metal particles are successfully introduced into the ordered mesoporous SiC support. For Ni/SiC-MCM-41 with a worm-like accumulation pore structure, the weak confinement effect
Acknowledgment
This work has been supported by the grant from the National Natural Science Foundation of China (21603127, 21673132), the Natural Science Foundation of Shanxi Province (201601D202020), and the Talent Development Funds of Shanxi University.
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