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Issue Date: 8-Feb-2012
Authors: Zito, Roberta
Title: Development of metallic catalysts for hydrogen production from metal hydrides
Abstract: Nowadays proton exchange membrane fuel cell (PEMFC) using hydrogen as fuel is considered as one of the most promising alternative to combustion engines due to its zero emissions. In order to operate a PEMFC successfully, a safe and convenient hydrogen storage and production system is necessary. A stabilized aqueous solution of metal hydride is considered appropriate for use as hydrogen storage material. In particular, NaBH4 is the preferred hydride due to its high hydrogen storage capacity (10.8 wt%). Moreover, sodium borohydride is non-flammable and non-toxic. The by-products of hydrolysis reaction are environmentally friendly and can be recycled in order to re-synthesizing NaBH4. Sodium borohydride reacts with water, forming 4 moles of hydrogen and 1 mol of sodium metaborate, according with the following exothermic reaction: NaBH4 + 2H2O ¡ú NaBO2 + 4H2 + 217 KJ/mol To inhibit the self-hydrolysis reaction, NaBH4 solution must be maintained at pH > 13. At this pH, NaBH4 solution is very stable and the hydrogen release occurs only if the solution is in contact with specific catalysts permitting the ideation of HOD (hydrogen on demand) systems. Considering our preliminary experiments and data of literature showing that catalysts based on ruthenium metal have a good catalytic activity, for this study, ruthenium metal catalysts on different supports such as ¦Ã-Al2O3, CeO2, TiO2, activated carbon (a mineral carbon with high surface area) were prepared. We found that Ru supported on an activated carbon of mineral origin with high surface area, is more active with respect to Ru on other supports. The best performances of Ru/activated carbon were related both to the high surface area of the activated carbon and to its high chemical inertness in the strong basic environment. Considering that activated carbon (mineral carbon with surface area of 1059m2g-1) resulted the more suitable support for the reaction under study, we investigated the NaBH4 hydrolysis over Ru catalysts supported on activated carbons with different origin and morphological characteristics. The influence of two different Ruthenium precursors (RuCl3 or Ru(NO)(NO3)3) was also studied. We found that a higher surface area leads to smaller Ru nanoparticles, whereas the presence of alkali metals on the support (mainly potassium) and the use of RuCl3 as precursor (containing chlorine), promote the formation of larger Ru clusters. In this context, it was found that the best catalytic performance was obtained using the activated carbon of vegetable origin as support and Ru(NO)(NO3)3 as precursor that give Ru clusters with 3nm as diameter size.
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