Development of nano NixMgyO solid solutions with outstanding anti-carbon deposition capability for the steam reforming of methanol
Graphical abstract
Introduction
In the past few decades, energy consumption increased rapidly worldwide, especially in emerging countries such as China and India [1]. However, the use of fossil fuels to meet the increasing energy demand has already led to severe environmental problems, such as global warming and poor air quality [2]. Since 1990s, as a clean energy carrier, hydrogen, has attracted significant attention globally due to its high energy density, high conversion efficiency and environmental-friendly nature [3]. Currently, hydrogen is produced commercially via processes such as catalytic reforming of natural gas, partial oxidation of heavy oil and coal gasification, etc. [4].
To date, significant amount of work has been carried out to develop catalysts for hydrogen production through catalytic reforming of a variety of raw materials, such as natural gas, biomass, bio-oil and alcohols [5], [6], [7], [8], [9]. Among the transition metal catalysts being studied, Ni-based catalysts have been studied extensively due to their high catalytic performance in the steam reforming of oxygenated hydrocarbons and low cost, which make these catalysts widely used commercially [10], [11], [12]. However, the use of Ni-based catalysts is usually associated with problems during the steam reforming process, such as deactivation of catalysts and reactor occlusion due to oxidation, sintering, and poor carbon resistance [13].
Generally, carbon deposition occurs via following reactions: hydrocarbon decomposition (R1), CO disproportionation (R2) and CO hydrogenation (R3) [14]. However, both (R2) and (R3) are accompanied by reverse water-gas shift reaction (R4) [14], [15].CH4 = C + 2H2 ΔH298k = 75 kJ/mol2CO = C + CO2 ΔH298k = −172 kJ/molCO + H2 = C + H2O ΔH298k = −131 kJ/molCO2 + H2 = CO + H2O ΔH298k = −41 kJ/mol
Normally, carbon deposition during reforming process can be suppressed by introducing more steam into the reaction system to promote reverse reactions of (R3) and (R4) to form more CO2 and less CO, which subsequently inhibits (R2). However, the penalty of such is the increased energy consumption due to the endothermic property of reverse reactions of (R3) and (R4) and the excessive amount of unreacted steam in the reaction system. Another approach to mitigate carbon deposition is to use noble metals such as Pt, Pd, Rh and Ag as active component(s) in the catalysts, which could alleviate the aggregation of Ni particles and subsequently lower the rate of carbon formation [16], [17]. The doping of rare earth elements in reforming catalysts was also found effective in inhibiting carbon deposition [18], [19], [20]. It is reported that CeO2 in the catalyst could provide lattice oxygen for coke removal due to its unique redox properties and high oxygen capacities [18], [19]. The basic La2O3 was also found effective in the activation of CO2 to remove surface carbon species [20]. In addition, low acidity was found suppress carbon formation via the inhibition of direct decomposition of hydrocarbons [21]. Therefore, alkali or alkali earth elements, such as B, K, Mg and Ca, are often added into alumina catalysts to adjust surface acidity of the catalysts [22], [23], [24].
It was also reported that the basic MgO showed high resistance ability for carbon deposition due to its ability to activate CO2 [25], [26]. The high melting point of MgO (3073 °C) contributes to the thermal stability of the catalysts and enables the reaction take place under its hutting temperature, at which metal atoms gain sufficient energy to escape slowly from surface of the crystal [27]. However, the low specific surface area and low catalytic activation of the Ni/MgO catalyst restricted its further development and applications. The NixMgyO solid solutions have been widely studied and have been proved highly effective for the catalytic reforming of natural gas [28], [29], [30], [31]. However, not much work has yet been carried out to understand the catalytic performance of NixMgyO solid solutions in the steam reforming of methanol, and how preparation methods affect the structure of these nano solid solutions and subsequently influence their catalytic performance and anti-carbon deposition capability.
In this study, the main purpose was to develop nano NixMgyO solid solutions with high catalytic performance and outstanding anti-carbon deposition property for the steam reforming of methanol. Different preparation methods were adopted to prepare nano NixMgyO solid solutions with different structures. These catalysts were then tested to evaluate their catalytic performance as well as the propensity in carbon deposition. Fresh and spent catalysts were also characterised to understand the differences in catalytic performance and to reveal the mechanism for the inhibition of carbon deposition.
Section snippets
Preparation of catalysts
All the chemicals used in this research were analytical grade purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
In this study, three different nano NixMgyO solid solutions were prepared using three different methods. The preparation of the first type of NixMgyO catalyst started from the preparation of MgO support by precipitation of Mg(NO3)2·6H2O with ammonia at room temperature followed by drying in air at 105 °C for 12 h and calcination in air at 600 °C for 4 h. The catalyst
XRD analysis
XRD patterns of the fresh, reduced and spent catalysts are shown in Fig. 1. From the XRD spectrum, it is clear that the nickel crystal had peaks close to that of the MgO crystal. This was due to their identical lattice type and similar ionic radius value [37]. The peaks of nickel phase have increments of 0.6°, 0.8° and 0.81°, compared with peaks of MgO at 262.64°, 75.00°, and 79.00° [29]. In Fig. 1(B)–(D), no double structure was found associated with the five major peaks, which were assigned
Conclusion
In this research, it is found that catalysts prepared using different preparation methods showed active phase in different crystal structures and nano NixMgyO solid solutions were found in all the three catalysts studied. The good dispersion of nano-scale Ni species in the NixMgyO solid solution and the ‘isolation effect’ of MgO resulted in the formation of nano Ni crystallites in these catalystsafter reduction. A stable catalytic performance of NixMgyO-hydro (97.4% conversion of methanol and
Acknowledgements
Following funding bodies are acknowledged for partially sponsoring this research: Ningbo Bureau of Science and Technology (Innovation Team Scheme, 2012B82011 and Major Research Scheme, 2012B10042), and Zhejiang Provincial Department of Science and Technology (Innovation Team on SOx and NOx Removal Technologies, 2011R50017), and Ministry of Science and Technology (International Cooperation Programme, 2012DFG91920). The University of Nottingham Ningbo China is also acknowledged for providing the
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