Carbon formation and its influence on ethanol steam reforming over Ni/Al2O3 catalysts
Introduction
In a few decades, the continuous rise of energy demand and the shortage of petroleum reserves may result in a critical situation, with exponential increase in energy cost. Hydrogen fuel cells are the most promising systems for energy production, since they are more efficient and environmentally clean than conventional thermal machines.
Natural gas is still the main source of hydrogen, due to its abundance. However, the development of alternative routes for hydrogen production employing renewable sources is of great interest due to economic and environmental reasons. In this context, ethanol steam reforming has attracted the attention of many researcher groups in the last years. It may become an important industrial process, especially for sugarcane producing countries. It is a cleaner process than methane steam reforming, because the CO2 “free” cycle, which is generated in the reforming of ethanol and consumed by sugarcane photosynthesis.
Many studies have suggested Ni as the most suitable metal [1], [2], [3], [4] for ethanol steam reforming. Although comparative studies [4], [5], [6] have shown better performance on basic supports, such as MgO and La2O3, alumina in γ or α phase is interesting due to its industrial use. The high surface area of γ-Al2O3 provides higher metal dispersion, while α-Al2O3 presents better mechanical resistance.
The coke deposition on Ni/Al2O3 catalysts is the main cause for deactivation during ethanol steam reforming [3], [4]. The routes for carbon formation include Boudouard reaction, methane decomposition and polymerization of ethene, the latter originated from ethanol dehydration over Al2O3 acidic sites. Carbon deposition is thermodynamically unfavorable at high temperatures and higher water/ethanol ratios [7], [8], [9].
The direct deactivation of catalysts occurs predominantly by covering active phases, due to encapsulating carbon. Nevertheless, carbon may deposit over catalysts without deactivation [10], in a mechanism that includes: carbon deposition over metal surface, migration of carbon containing species to the bulk phase of metal, saturation of these species and condensation of carbon [11], [12], [13]. This mechanism results in the formation of filamentous carbon, which does not cause direct catalyst deactivation, but accumulates continuously, blocking the bed or breaking pellets.
It has been reported that the formation of filamentous carbon can result in a better catalyst activity performance. In another study by Schmal and co-workers [14], the method of reduction with hydrogen was compared with an activation method, using a mixture CH4/O2 in the ratio 2/1, for methane dry reforming. This method of activation resulted in filamentous carbon formation, which improved the catalytic performance for dry reforming.
To our knowledge, there is no study reporting the formation of different types of carbon, and its influence in ethanol steam reforming over Ni catalysts. The aim of this work is to study the deactivation of Ni/Al2O3 catalyst during ethanol steam reforming, and the influence of different forms of deposited carbon on catalyst activity and stability.
Section snippets
Catalyst preparation
The catalysts were prepared by wet impregnation, using a Ni(NO3)2·6H2O (Acros Organics) aqueous solution. After impregnation, catalysts were dried at 398 K during overnight and calcined by heating (10 K min−1) in air (30 mL min−1) to 823 K and holding for 3 h. Catalysts were prepared with 8 and 16% metallic loadings. NiO bulk was prepared from calcination of Ni(NO3)2·6H2O at 773 K under air flow.
Catalyst characterization
The surface area and pore volume were obtained by nitrogen physisorption (Micromeritcs ASAP, model 2000).
Thermodynamic analysis
Thermodynamic analysis of carbon formation was done by minimization of Gibbs free energy as a function of temperature and various H2O/EtOH ratios, as displayed in Fig. 1. According to the literature [7], [8], [9], the graphitic carbon formation is unfavorable at higher H2O/EtOH ratios, as shown in Fig. 1, and is maximized around 773–873 K. This behavior suggests that the endothermic reaction of methane decomposition prevails at that temperature:CH4 ↔ C + 2H2 (ΔH = 74 kJ/mol)
The carbon gasification and
Conclusions
The activity of Ni supported on γ-Al2O3 catalyst is higher than on supported α-Al2O3, because, according to XRD results, Ni particles are more dispersed compared to Ni crystallites on α-Al2O3. In addition, XRD in situ measurements after different time exposure of reactants prove the presence of nickel particles, allowing determining the dispersion of Ni during the ethanol reforming itself.
The higher surface area of γ-Al2O3 promotes the formation of ethene during ethanol steam reforming, which
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