Preparation of Metal Organic Framework (MOF) Derived Bimetallic Catalyst for Dry Reforming of Methane

In the past decade, efforts have been focused on development of catalyst to show high activity for dry reforming of methane (DRM). The development of catalyst has been crucial to be carried out as this may significantly reduce the concentration of most common greenhouse gases, namely methane (CH₄) and carbon dioxide (CO₂) in the atmosphere. In present work, a series of varying molar ratio of Ni:Ce metal organic framework (MOF) derived catalysts were grown on alumina in one step. The synthesis steps were in accordance to reported solvothermal method for the syntheses of NH₂-MIL-88B with slight modification. This was followed by reduction at 500°C in hydrogen environment for 1 h. The physical and chemical properties of the catalysts were probed by powder XRD, BET surface area analysis, EDX, ICP, CO₂-TPD and H₂-TPR. XRD showed that diffraction patterns were in agreement with the diffraction pattern of MOF synthesized in previous work, thus confirmed the successful formation of the MOF structure. The variation in the molar ratio of Ni:Ce did not show significant difference in the diffraction pattern of the MOF-derived catalysts. For reduction phase, sharp diffraction peaks were detected at 2? = 44.5°, 51.85°, and 76.37°, which can be indexed to (1 1 1), (2 0 0) and (2 2 0) planes of face-centered cubic (FCC) metallic Ni, respectively. The addition of Ce promoted smaller particle size of Ni, ranging from 4.6 nm to 6.88 nm. The presence of CeO₂ was observed at 2? = 28.6°, 33.0°, and 56.4°. Elemental distribution was compared between EDX and ICP-OES. ICP-OES and EDX analyses indicated that weight percent of bimetallic metal of Ni and Ce was consistent, in which the amount of respective metal obeyed the ratio trend of the metal precursors added during the MOF synthesis. This suggested the homogeneity of the catalyst, even though EDX showed relatively higher weight percent than ICP-OES. The catalytic performance of catalysts showed that 1Ni1Ce exhibited better conversion of CH₄ and CO₂, with 63.5% and 86.8% respectively at 800^oC, and the conversion tend to increase at a higher temperature. The results were convincing for the design of a performing catalyst for DRM process.

Alumina support; Dry reforming of methane; Metal organic framework; Nickel-cerium

Based on the 2013 Intergovernmental Panel on Climate Change (IPCC) report, anthropogenic greenhouse gases, namely GHG emissions since the early Industrial Revolution, have led to global warming of 1.0°C (Anderson et al., 2016). The evidence and statistics show that GHG emissions are strongly related to observable effects on global warming (Solomon et al., 2017).  CO₂ and CH₄ were reported to be the most common GHG in Earth’s atmosphere, in terms of emission quantity and total impact on the global warming (Smith et al., 2013). Hence, it is crucial to develop method to mitigate this problem, as both the concentration of GHGs and warming rates are expected to increase drastically in the future. Rather than capturing CO₂ and CH₄, the consumption of two natural abundant GHGs is undoubtedly an attractive route to reduce the GHG in an environmental-friendly way. In this regard, dry reforming of methane (DRM) was introduced by reacting both GHGs, forming a mixture of hydrogen (H₂) and carbon monoxide (CO) generally known as syngas (Winanti et al., 2014). This is a feasible way to reduce GHGs without complicated and expensive separation process ( San-José-Alonso et al., 2009; Rubin et al., 2015). Furthermore, low CO/H₂ ratio associated with syngas from DRM is necessary for certain downstream applications, such as Fischer Tropsch process and synthesis of oxygenated materials (Tristantini Budi et al., 2015; Nataj et al., 2019).

In the past decade, well-documented literature has been focused on development of high activity and stability catalyst for DRM. In summary, transition metal (e.g., Ni, Co and Fe) and noble metal (Rh, Pt) were found to have a promising catalytic performance in terms of conversion and selectivity for reactants (Pakhare & Spivey, 2014). Due to its wide availability and low cost, alumina-supported Ni catalysts were most widely used in large-scale industrial contexts (Souza et al., 2004). However, the process of rapid catalyst deactivation by sintering and poisoning via coke formation is still a serious concern for Ni metal-based catalyst (Kim et al., 2016).

As a result, this work has focused on the use of perovskite-liked material, such as metal-organic framework (MOF). Composed of geometrically well-defined structures of central metal atoms connected by organic linkers, the MOF can be used as a structure-by-design precursor to produce highly-dispersed metallic particles (Rogge et al., 2017). The additional of CeO₂ promotes higher activity with more stable basic sites, and inhibits the coke deposition due to its high oxygen mobility, high oxygen storage capacity and constraint on particle size, as reported in previous literature (Loc et al., 2017; Movasati et al., 2017)

In this study, a facile approach focused on the effects of Ni:Ce molar ratio on catalytic performance of an alumina-supported catalyst in DRM. A series of bimetallic catalysts with various Ni/Ce ratios were prepared through solvothermal method, and characterized to study the physical and chemical properties of the MOF-derived catalysts. A catalytic test was carried out, ranging from 300 to 900°C in order to evaluate their activity and stability of MOF-derived catalysts. To our knowledge, there is no literature data about the derivation of DRM catalyst for MOF precursor. 

MOF-derived catalysts prepared by solvothermal method exhibited convincing results for the design of a performing catalyst for the DRM process. The results from TPR justify the utilization of MOF as a precursor to produce highly-dispersed metallic particles. In addition, the promotion of Ce showed significant influence on the catalytic performance of the Ni-based catalyst. Increasing Ce molar loading enhanced a smaller Ni cluster size and provided more basic sites, in agreement with XRD and CO₂-TPD characterisation, respectively. The addition of Ce did not show a significant change in the reduction temperature of Ni active phase. However, the reduction of bulk oxygen from CeO₂ was observed at higher Ce loading. Within the range of the studied molar ratios, 1Ni2Ce showed the highest CH₄ and CO₂ conversion at 63.5% and 86.8%, respectively at 800^oC, as well as greater stability within a 7-hour time frame, further supporting the promotion of Ce.

This research is funded by Universiti Tunku Abdul Rahman under grant number 6220/C18. The authors gratefully acknowledge the laboratory facilities and analytical services from Universiti Tunku Abdul Rahman and National Tsing Hua University.

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