Iron-based Composite Nanostructure Catalysts Used To Produce CO_x-free Hydrogen From Ammonia

With more emphasis on the concept of substantial development by people, the development and utilization of new energy has drawn the attention of the world. As a secondary energy carrier, hydrogen is an effective way to solve this problem due to its advantages such as clean, efficient and recyclable. Though the direct use of hydrogen as fuel has many problems both in terms of storage and transport, on-line hydrogen production technology can overcome such problems and thus attracted extensive attention. Ammonia is an ideal hydrogen carrier which has a high hydrogen storage density and is easy to be liquifided. Therefore, the on line hydrogen production with ammonia decomposition has been widely concerned.At present, the research on ammonia decomposition catalysts mainly includes two categories, the precious metal catalysts and transition metal catalysts. Though precious metal catalysts usually have excellent catalytic activity, they also have high corrosion resistance requirements of equipment. Meanwhile, the synthesis of transition metal catalysts is mainly based on impregnation method. So they have poor stability under the high temperature condition of ammonia decomposition which results in low catalytic activity. Thus, it is significant to synthesize a catalyst with high activity and high stability.Iron-based catalyst has been studied extensively due to its moderate activity and low cost. Its catalytic activity and stability can be enhanced greatly by the addition of Ce or other additives. In this work, a series of iron-based composite catalysts were synthesized and their structures were characterized by XRD, TEM, BET and ICP. Their catalytic activity of ammonia decomposition was also investigated to obtain iron-based catalyst with high activity and stability. Concrete research contents are as follows:The Fe3O4 sphere was synthesized via solvothermal method. The iron-based composite nanostructures those have ceria or titania as shell coating on the naked iron spheres were successfully synthesized by different coating methods and used as catalysts for ammonia decomposition. The structure and texture of fresh and used catalysts were characterized by various techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), in-situ X-ray diffraction(in-situ XRD), temperature programmed reduction by hydrogen (H2-TPR) and N2 adsorption/desorption. For the ammonia decomposition reaction, the iron-based composite catalyst by coated with cerium and titanium show excellent catalytic activity compared with the naked iron sphere catalyst, giving nearly 100% ammonia conversions at 650℃ and better stability during the catalytic test (for 60 h) at the temperature of 600℃ with a space velocity of 24,000 cm3gcat-1h-1. Though the active component of the naked iron sphere catalyst is the highest, its poor stability at high temperature leads to its worst catalytic activity. The results show that the addition of cerium and titanium played a key role in improving the ammonia decomposition catalytic activity and give a good thermal stabilityA series of Fe-Ce and Fe-La composite catalysts with different molar content of iron were synthesized via hydrothermal method. The structure and texture of fresh and used catalysts were characterized by various techniques including elemental analysis (ICP-AES), transmission electron microscopy (TEM), X-ray diffraction (XRD) and N2 adsorption/desorption. Characterization results showed iron was successfully incorporated into the lattice of CeO2 and La(OH)3. For the ammonia decomposition reaction, the catalytic activity improved gradually with the increase of iron content. The activity of the catalyst has negligible increase when the molar content of iron reaches 15%. The particle size of catalyst samples after the reaction become larger with the increase of iron content. Compared with the iron samples synthesized via same method, which showed the addition of Ce and La is beneficial to improve the catalytic activity of the active species. Meanwhile, the stability statistics of the catalyst showed that the catalytic activity of 15% Fe-Ce and Fe-La composite catalyst keep stable in the stability test (for 70h).