The global temperature has been rising for decades due to the continuous increase in the emission of greenhouse gases (GHGs) into the atmosphere. The reduction and utilization of GHGs had attracted more attention as an effective way to avoid their impact on the environment (Vakili et al., 2020). It was reported that CO2 and CH4 account for 76% and 14% of the global man-made emissions, respectively (Vakili et al., 2020). The reforming of CH4 with CO2, also designated as the dry reforming of methane (DRM), was regarded as an attractive pathway for the conversion of GHGs into syngas (i.e., H2 + CO). Syngas is an important intermediate for the synthesis of various chemicals and fuels, such as ammonia, methanol, and dimethyl ether (DME) (Bouchoul et al., 2020; Cao et al., 2018; Vakili et al., 2020). Despite the optimization of reforming conditions, such as temperature, the composition of the reactant gas, space velocity, and pressure, there had been great efforts to reduce the energy consumption associated with DRM due to the high endothermicity of the reaction. The catalytic DRM was an efficient way to resolve the problem. Thus, numerous catalysts with high activity and stability were developed. The catalysts for DRM are classified into metal-supported and carbon-supported catalysts. The former is further divided into noble metal and transition metal catalysts (Pakhare et al., 2014; Zhang et al., 2018). However, metal-based catalysts possessed a series of limitations, such as limited availability, low cost-efficiency of noble metals, and rapid deactivation of transition metals caused by severe coke deposition. Thus, the use of carbon-based catalysts for DRM is a promising method, which is also advantageous for industrial processes (Li et al., 2016; He et al., 2015; Xu et al., 2014). Carbon-based catalysts are classified into two categories: one is carbonaceous materials are directly served as the catalysts in the reforming process, and the other is metallic components are supported over the carbonaceous materials. Commonly, the latter shows the better catalytic activities (Li et al., 2016; He et al., 2015; Xu et al., 2014; Zhang et al., 2017). In particular, carbon-based catalysts using nickel as the active component have been widely used in the reforming process, mainly due to their low cost and relatively high activity.
The most common carbonaceous materials for the synthesis of carbon-based catalysts include activated carbon (AC), biomass-derived char, coal-derived char, metallurgical coke, and carbon nanofibers (CNF) (Cao et al., 2018; Chinthaginjala et al., 2007; Fidalgo et al., 2010; He et al., 2015; Li et al., 2016; Xu et al., 2014). Due to high surface porosity, large surface area, and cost-efficiency, AC has been extensively studied. AC was also found to have the potential to be the most cost-effective and environment-friendly catalyst for DRM (Xu et al., 2014). It has been reported in the literature that the catalytic activity of AC mainly depends on the concentration of alkaline and alkaline earth metals present in the ash as well as the oxygen-containing functional groups attached to the surface (Xu et al., 2014). Zhang et al. found that the oxygen-containing functional groups on the AC surface, such as carbonyl, carboxyl, and lactone, were the primary active sites for DRM (Zhang et al., 2017a). Xu et al. used AC treated with HNO3 for DRM and reported that the hydroxyl group had a significant role in DRM (Xu et al., 2014). Moreover, AC could serve as a catalyst as well as a microwave absorber in the microwave-assisted DRM process (Fidalgo et al., 2008; Fidalgo et al., 2011; Fidalgo et al., 2012). Despite the high initial catalytic ability, AC can be quickly deactivated (Xu et al., 2014). Conversely, metal-supported carbon-based carriers have better resistance to carbon deposition, thereby prolonging the catalytic activity for the DRM process (Cao et al., 2018; Fidalgo et al., 2011; Li et al., 2017; Li et al., 2019; Zhang et al., 2015). The metallic components, such as Ni, Fe, Co, alkaline and alkaline earth metal, are incorporated onto the carbonaceous carrier through mechanical mixing or impregnation method (Cao et al., 2018; Fidalgo et al., 2011; Li et al., 2017; Li et al., 2019; Zhang et al., 2015). The catalyst developed via mechanical mixing has a combined catalytic effect of the mixed metallic substances, which limits the enhancement of catalytic activity. On the other hand, the catalysts constructed by the impregnation method completely exploit the catalytic effects of the metallic component and carbon-based carrier. The main threats to the catalysts prepared by the impregnation method were the agglomeration of metals and carbon deposition. The catalysts synthesized by the impregnation method often requires a great deal of energy consumption in the calcination process. In addition, the unavoidable carbon deposition and consumption of the carbon existed in the carbon-based catalysts can speed up deactivation of the carbon-based catalysts (Li et al., 2016 ; Li et al., 2017). Therefore, developing a highly active and stable catalyst with low cost and energy consumption is of significance towards the dry reforming of methane, by searching for appropriate carbonaceous supports and preparation methods of the catalysts.
Nano-sized metallic components supported over AC are of great interest as nanosizing increases specific surface areas by highly dispersing metals on the support and thus exposes more active centers and potentially improves the catalyst performance (Pegios et al., 2018; Shang et al., 2017; Xie et al., 2013). Besides, the resistance of the catalyst to carbon deposition is enhanced by controlling the metallic particles at nanometer. Many types of preparation method have been studied for the fabrication of nanoparticles (Zhao et al., 2018). The solid-phase method of grinding is labeled as a green approach for the development of catalysts, and possesses a series of advantages, such as in-situ synthesis, improved synthetic efficiency, less energy consumption, and enhanced reactivity (Lu et al., 2016; Zhang et al., 2017b). The preparation of the catalyst using this method reduces the particle size of the metallic component to the nanometer scale. Wang et al. developed a catalyst based on the commercial carbon substrate using this method and found the corresponding particle size of the metallic component to be around 2 nm (Wang et al., 2006). In this paper, the increase in the interaction between the metallic component and carrier for catalysts developed using the solid-phase method, relative to the impregnation method, has also been demonstrated (Wang et al., 2006). Q.L. Zhang et al. developed a nickel-based catalyst on the SBA-15 substrate by the solid-phase method of grinding. The results indicated a carbon deposition of 6.3% on testing the DRM for 100 h at 700°C (Zhang et al., 2017b). In contrast, the catalysts synthesized by the impregnation method experienced a serious carbon deposition as high as 38.3% under the same conditions (Zhang et al., 2017b).
In the related literature, it is found that the catalysts synthesized using solid-phase method have the metal nanoparticles and exhibit the desirable catalytic effect on the process of DRM (Lu et al., 2016; Zhang et al., 2017b). These studies pay extensive attention to the effect of main preparation parameters on the catalyst activity. To the best our knowledge, the properties of the support are important to ensure the dispersion of metal particles and enhance the catalyst performance. Nevertheless, the connection of the support properties, for instance the surface chemical properties and porous structure with its catalytic activity remains to be further explored, when the catalyst is prepared via solid-phase method and using AC as the support. In our previous paper, we prepared the nickel-supported catalysts over AC by the solid-phase method, and the results show that the catalytic activity of the used catalyst can be strengthened after a certain time in the process of DRM. However, there is no in-depth research on the effect of different supports on the self-reinforced catalytic activity. Herein, a series of nickel nanoparticles supported over different activated carbons is obtained using the solid-phase method in the paper. The prime purpose of this work is to investigate the effect of different activated carbons (ACs) on the catalytic performance of AC supported nickel nanoparticles for dry reforming of methane (DRM). The paper also concerns about the in-depth effect of the support properties on the self-enhancement of catalyst activity in the reforming process. The work will provide valuable information on the synthesis of a highly active and stable catalyst by the solid-phase method.