Investigation On Hydrogen Generation From Integrating Ammonia Decomposition Over Nickel-Based Catalysts With Separation By Palladium Membrane

Continuous production of cheapness hydrogen is a principal problem for large scale application of fuel cell. It is a key issue to provide the advanced technology for hydrogen production on site for PEMFC application in order to deliver the cheap hydrogen for hydrogen fueling station and distributed power station. Ammonia is an ideal hydrogen carrier which has a high hydrogen storage density and is easy to be transported to the site. The non-COx hydrogen will be prepared by using ammonia decomposition, which should be used for PEMFC fuel directly. On the other hand, the palladium composite membrane would be the best option for hydrogen separation especially for small scale hydrogen separation instead of PSA. The high performance palladium composite membrane would be used for hydrogen separation and purification from the gas mixtures obtained from the ammonia decomposition, which is significant for the development of technology of hydrogen energy and PEMFC.In this paper, reaction thermodynamic properties of ammonia decomposition was investigated, the equilibrium constant at various temperatures and equilibrium conversions of the reaction at various temperatures and pressures were estimated. The results show that equilibrium conversion of the reaction can reach at 99﹪under appropriate conditions, indicating decomposition of ammonia to produce hydrogen can come true at milder reaction conditions when a high performance catalyst was applied.Four kinds of catalysts such as Ni/La-Al₂O₃, Ni/Al₂O₃, Ni/TiO₂ and Ni/MgO were prepared by impregnation method using dehydrate ethanol as solvent. Their catalytic performance for the ammonia decomposition to hydrogen was discussed and their active order was determined. The result exhibits that 14 mass﹪Ni/La-Al2O3 has higher activities than that of other three catalysts, 97.1﹪conversion of ammonia decomposition and hydrogen generation rate of 222.1 mmol/g(Ni) min were achieved.The Ni/Al₂O₃ and Ni/La₂O₃-Al₂O₃ catalysts were prepared by co-precipitation method and their activity and stability were investigated at various reaction conditions. The Ni/La₂O₃-Al₂O₃ exhibits higher activities than the catalysts prepared by impregnation method and it has the same activity as the precious Ru-based catalyst for ammonia decomposition. The ammonia conversion could be more than 99% under temperature of 550oC and NH3 space velocity of 5000 h-1, which realizes the high efficient process of hydrogen production via ammonia decomposition under low temperatures, as well as good stability. The activity of the catalyst was almost unchangeable when it has been continuously used for 200 h. At the same time, the dependence of the catalytic performance on surface state, composition and structure of the catalyst was explored by means of N₂-adsorption, XRD, TEM, TPD, TPSR, TPR etc. In addition, the influence of assistant reagent lanthanum on the NiO reduction kinetics in the catalyst was also studied by using the temperature-programmed reduction (TPR) method. The result exhibits that incorporation of La destroyed the metastable state of Ni-Al phase and decreased the Ea values by 4⁷ kJ/mol, and thus NiO species was easier to be reduced.The novel method modified by Al(OH)₃ colloid for preparation of high performance palladium composite membrane was established. The hydrogen permeating functions of the prepared membranes has been greatly improved. Hydrogen flux of Al2O3 porous ceramic membrane achieves 44 m3/m2 h with selectivity of permeating hydrogen 12500 at 500 oC and pressure difference of 0.1 MPa; and Hydrogen flux and selectivity to permeating hydrogen of porous stainless steel support membrane are 22.5 m3/m2 h and 4900, respectively. However, it is the first time to integrate the high active catalyst of ammonia decomposition and the high performance palladium membrane to establish highly efficient and economic process for hydrogen production. By this integrated method, the hydrogen flux of 44 m3/m2 h, hydrogen purity of 99.95﹪and NH3 conversion of 99.8﹪were achieved and the stability kept constant during the 120 h testing.In this paper, reaction mechanism of ammonia decomposition and the concentration polarization phenomenon in hydrogen separation from gas mixture by means of the mathematical simulation were also explored. The mathematic model was established and a consistent result of theoretical calculation with experimental data was obtained.