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Hydrogen is the primary fuel source for fuel cells. However, the low volume density and difficulty in storing and transporting hydrogen are major obstacles for its practical utilization. Among various hydrogen carries, ammonia is one of the most promising candidates because of its high hydrogen density and boiling point and ease in liquefaction and transportation. The reaction temperature of ammonia cracking into nitrogen and hydrogen is about 500˚C or higher. The hydrogen can be effectively separated by the membrane based on Pd alloy about 500˚C. Currently, the extraction of hydrogen from ammonia is carried out by two step process involving catalytic decomposition of ammonia followed by hydrogen separation to produce high purity hydrogen with less than 1 ppm ammonia. On the other hand, the similar operation temperature ranges of an ammonia cracker and separation system suggest the coupling of both functionalities into single reactor using membrane reactor for ammonia cracking where the hydrogen formed by catalytic cracking is concentrated and purified by a palladium membrane. The integration of these two devices is beneficial in terms of producing pure hydrogen with a simplified overall system. In this study, several catalyst materials for cracking of ammonia and hydrogen separation membranes have been investigated.
Catalysts for decomposition of ammonia have been developed using metal oxide-supported Ni- and Co-based catalysts. It was found that Co3Mo3N catalyst was active for ammonia cracking, especially, the addition of Cs to the catalyst further promoted its activity. The catalytic activities of Ni catalysts have been significantly affected by the kind of support oxides. Among the Ni catalysts investigated, the activity of Ni/Y2O3 and SrO modified Ni/Y2O3 for ammonia cracking was sufficiently high at the temperature around 600˚C. The SrO modified Ni/Y2O3 catalyst showed higher performance than Ni/Y2O3 and achieved complete decomposition at 550ºC. As was represented by these catalysts, basicity control of the support oxides significantly affected the catalytic activity of Ni for ammonia cracking. The Ni/Y2O3 catalyst maintained high activity during operation for 1000 h at 700ºC. These catalysts were selected for usage in combination with a Pd-based membrane reactor for hydrogen extraction process intensification and economic hydrogen production.
Supported thin film palladium and palladium alloy membranes were prepared similarly to methods described in the literature for hydrogen separation from carbon dioxide, carbon monoxide and inert gases as well as ammonia decomposition products separation [1, 2]. The membranes were exposed to representative ammonia/H2/N2 gas mixtures at 450-550°C to demonstrate their chemical and thermal stability as evidenced by the rate of change in ideal H2/N2 selectivity. The performance of a packed-bed catalytic membrane reactor at different temperatures, pressures and gas hourly space velocities was studied. The catalytic membrane reactor with Ni/Y2O3 and SmO based Ni catalysts will be integrated with Pd-based alloy membranes for ammonia decomposition into high purity hydrogen. The results on the catalytic membrane reactor for ammonia decomposition into hydrogen will be presented.
This work was supported by the Japan Cooperation Center Petroleum (JCCP), and Saudi Aramco.
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