Investigation Of Ammonia Decomposition With High Activity Iridium Catalyst

Proton exchange membrane fuel cell (PEMFC) uses hydrogen as fuel source, andH₂ is the cleanest fuel ever known and H₂O is the only product of its combustion, andits energy conversion efficiency is 40-60%. Recently, on-site generation of hydrogenhas become an interesting alternative. The use of ammonia as a hydrogen providerappears to be a good choice because the produciton is only hydrogen and nitrogen,there is no COx generation, and the unconverted NH₃ can be reduced to less than2×10^-7 level by means of a suitable absorber. In addition, at room temperature (R.T.)and around 810.4 KPa, NH₃ is in its liquid form and storage is not a problem. Also,the hydrogen storage capacity (17.7 wt.%) and energy density (3000 Wh/kg) ofammonia is higher than those of methanol and other fuels. From an economicalstandpoint, it has been reported that, NH₃ decomposition is a more economical processfor hydrogen generation. In addition, the catalytic reaction of ammonia decompositionis performed for generation of the hydrogen-nitrogen mixture used mainly, instead ofhydrogen, as a reducing atmospherein steel and electronic industries.Iridium has been usually included in systematic studies on the catalytic applicationof groupâ…§transition metals. Iridium has been investigated as catalyst for manyreactions: de-NOx for automobile exhaust gases, hydrazine decomposition for rocket thruster and the hydrogenation of unsaturated hydrocarbons.In this paper, the ammonia decomposition was used for model reaction. Firstly, thepreparation parameter of Ir/SiO₂ catalysts including to calcination temperature,calcinations atmosphere and reduction temperature were investigated. Concerned withthe H₂-TPR and H₂-TPD charaterization, the effects of different preparation conditionson the reduction ability and adsorption-desorption performance of catalysts werediscussed in detail. It was found that the catalysts prepared with actone was better thanthe catalyst prepared with water, because the actone can improve the dispersion of theactive compotent of the catalysts. The calcination temperature and the reductiontemperature were important to the activity of the catalyst. Comparing with the catalystcalcined under air atmosphere, the catalyst calcined with 350℃under hydrogenatmosphere and argon atmosphere had a higher catalytic activity. The H₂-TPDexplained that the hydrogen and argon atmosphere could enhance the interactionbetween the iridium and the supports. Alkali were known to be efficient promoters forcatalysts in ammonia decomposition, and the higher the electronegativity of thepromoter, the lower is the NH₃ conversion of the catalysts. Barium was found to be aeffective promoter as well, which was considered to act either as an electronicpromoter, similarly to an alkali, or as a structural one, that modified the arrangement ofsurface Ir atoms, because the Ir can be trapped by BaO on the surface of supports, soBarium acts as an effective adjuvant for preventing Ir from sintering. At last, Ir-Ni/SiO₂ catalysts were prepared by glow discharge plasma method, which were betterthan the catalysts prepared by conventional impregnation. Because the dispersion ofcatalysts prepared by plasma was higher than that of conventional catalyst, and theformer catalysts has a lower apparent activation energy.