(574d) Aqueous-Phase Reforming of n-BuOH Over Ceramic Oxide Based Catalysts

            Hydrogen, the simplest element in the periodic table can be considered the most environment friendly potential fuel for fuel cells application because of the cleanest emission it produces and yet it has the highest energy content per unit weight of all the fuels, three times the energy of same amount of gasoline. Hydrogen, produced from renewable biomass instead of non-renewable fossil fuel sources is an alternative source of environmentally clean renewable energy [1]. Recently n-Butanol (n-BuOH) has been considered as a potentially significant source of H₂ , because  of it has higher weight % of H content compared to ethanol or methanol, low vapor pressure; so less flammable-easy to handle. Additionally, n-BuOH is a waste byproduct of the classical fermentation process from sugar beet, sugar cane, corn, wheat, lignocellulosic biomass and aqueous fraction of biomass pyrolysis liquids (bio-oil) [2]. It can also be produced through the non-fermentative pathways by manipulating metabolic engineering methods [3] and from macroalgae or seaweeds [4, 5]. There are different methods of producing H₂ from n-BuOH; such as steam reforming [6, 7], partial oxidation [8], dry reforming [9, 10].

            Compared to these, aqueous phase reforming (APR) is a single step and low temperature (£ 225 °C) energy efficient process, which produces hydrogen from water-diluted oxygenated hydrocarbons. The typical operating pressure and low temperature for APR can be helpful for the separation of H₂ and CO₂ from other products that are volatile at atmospheric pressure. Additionally, APR is useful for producing fuel cell grade H₂ with small amounts of CO in a single chemical reactor as a consequence of the water–gas shift (WGS) reaction being thermodynamically favored at lower temperature reaction conditions [11,12].

            Reaction kinetics studied for aqueous-phase reforming of ethylene glycol over various supported metals indicated that Pt and Pd catalysts are selective for production of H₂ and Pt shows higher catalytic activity, but at a very high cost (~$1700/oz as of 17^th Oct, 2010). This coupled with limited availability of Pt make it advantageous to develop catalysts based on less expensive metals, such as Ni (~$10/lb) [13, 14]. Studies show that in order to achieve high H₂ selectivity on a catalyst for the APR of an oxygenated hydrocarbon, a high C-C bond breaking rate, a low C-O breaking rate, and a low methanation reaction rate on metal, and low acidic catalyst supports are required [1]. For Ni, C-C bond breakage is reported to be high with reasonably good water gas shift activity compared to Co, Pt, Pd, Fe, Ir, and Rh [15-17].

            High specific surface area-to-volume ratio, homogeneous dispersion of metals, and precise design of mesopore structure (large pore volume and narrow pore size distribution) combined with proper control of acidity/basicity of the support oxides are other important factors for high catalytic performance. There are many different ways of preparing nano metal/Al₂O₃ based catalysts; such as sol-gel, aerosol, co-precipitation, solution combustion synthesis, etc. [18-23].

            Solution combustion synthesis (SCS) is a fast, simple, and energy efficient technique for the preparation of pure, porous, and small-particle size ceramics generally used as catalysts, phosphors, pigments, etc. [24-26]. We have reported APR of EtOH over alumina supported nano-scale nickel catalysts prepared by a SCS method before. Bimbela et al reported the steam reforming of n-BuOH over Ni/Al₂O₃ catalyst prepared by co precipitation method [7]. According to the knowledge of the present authors no body so far reported on the formation of H₂ by APR of n-BuOH.

            In this paper, we present a study on aqueous-phase reforming of n-BuOH over Ni/CeO₂ and Ni/Al₂O₃ catalysts. Both catalysts were prepared by a SCS route. Over 300hr of run time, the Ni/CeO₂ catalyst lost 25 and 37% of its activity in terms of H₂ and CO₂ yield and 12 and 11.8 % in terms of selectivity to H₂ and CO₂. Ni/Al₂O₃ catalyst lost 41 and 45 % of its activity for H2 and CO₂ yield, and 19.8 and 19.5% in terms of selectivity to H₂ and CO₂. The steady state BuOH conversion and H₂ and CO selectivity increased and CO₂ and alkane selectivity decreased with increasing reactor temperature. The activation energies for H₂ and CO₂ production were 175.3, and 189 kJ/m and 154.4, and 172.7 kJ/m for Ni/CeO₂ and Ni/Al₂O₃ samples, respectively.  The catalytic activity of these catalysts was compared at different temperatures, pressures, feed flow rates, and feed concentrations.

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