Ni-based catalysts for the steam reforming of ethanol: surface acidity and catalytic activity

Concept A series of Ni-based catalysts supported on TiO2, SiO2 and ZrO2 (10 wt% Ni) were prepared by an innovative flame pyrolysis (FP) method to achieve proper thermal resistance and to tune metal dispersion. Every sample was characterised by various techniques, in among which infrared spectroscopy (FT-IR). The latter allowed to assess samples acidity and to define the nature of the Ni species present on catalyst surface. Catalytic activity for the steam reforming of ethanol was tested on a bench scale continuous plant under atmospheric pressure at different temperatures, i.e. 500, 625 and 750°C, with 3:1 mol/mol water/ethanol ratio. Motivations and Objectives The steam reforming of biofuels such as ethanol represents a hot research topic in the last few years. Different metals have been proposed as active phase, e.g. Ni, Co and Cu, to restrict the field to the less expensive non-noble metals. The most interesting results have been obtained with Co and Ni [1-3]. The latter seems very promising, though some drawbacks remain unsolved due to sintering and coking [1,4,5], especially when Ni particles are very dispersed [6]. From what above reported it seems that the thermal resistance of the sample is one of the key points for these catalytic materials and that another important feature is Ni dispersion and its interaction with the support. The aim of the work was then the design, the synthesis and the characterisation of heterogeneous catalysts to be used for the steam reforming of ethanol. Results and Discussion The FP technique proved an interesting method for the preparation of steam reforming catalysts, especially for use at high temperature (≥625°C). The titania supported catalyst showed higher activity and, above all, superior stability, with respect to similar samples prepared by coprecipitation. The advantages of the FP synthesis were less evident when dealing with silica and zirconia supported samples. Medium Lewis acidity due to exposed support ions was detected over the titania and zirconia based catalysts, whereas over the Ni/SiO2 catalyst, Lewis acidity could be induced by the metal phase itself. However, catalyst acidity did not seem really connected to coking. Based on H2 productivity and on the selected operating conditions, the best results were obtained with the silica-supported sample at 625°C, a temperature sufficient to achieve full ethanol conversion and 100% C balance. By contrast, coke formation was usually observed at 500°C, though in general not related to irreversible catalyst deactivation. An optimization of the reaction conditions is therefore required to further decrease the operating temperature, e.g. a higher water/ethanol ratio. Supposing to use such catalyst for the production of 7 Nm3/h of H2, suitable to feed a 5 kWel+5 kWth fuel cell, ca. 1.5 kg of catalyst would be needed, working at 625°C. References [1] V. A. Kirillov, V. D. Meshcheryakov, V. A. Sobyanin, V. D. Belyaev, Yu. I. Amosov, N. A. Kuzin, A. S. Bobrin, Theoretical Foundations of Chemical Engineering, 42 (2008) 1 [2] M. Benito, R. Padilla, A. Serrano-Lotina, L. Rodríguez, J.J. Brey, L. Daza, J. Power Sourc., 192 (2009) 158 [3] L.J.I. Coleman, W. Epling, R.R. Hudgins, E. Croiset, Appl. Catal. A: General, 363 (2009) 52 [4] A.J. Vizcaíno, A. Carrero, J.A. Calles, Int. J. Hydrogen Energy, 32 (2007) 1450 [5] J. Xuan, M.K.H. Leung, D.Y.C. Leung, M. Ni, Renewable and Sustainable Energy Reviews, 13 (2009) 1301 [6] S.Q. Chen, Y. Liu, Int. J. Hydrogen Energy, 34 (2009) 4735