Photo-assisted production of hydrogen and ammonia.

Abstract

The synthesis of ammonia and cleavage of water have been studied using different photo-catalysts in fluidised bed reactors. Ammonia has been photo-synthesised over iron-doped titanium dioxide mixed with alumina. Irradiation was carried out with a 100 watt medium pressure mercury lamp. The synthesis reaction was carried out at 84°C and atmospheric pressure. High purity nitrogen saturated with water was used for the synthesis reaction in a gas-solid flat fluidised bed reactor. Methods of preparation, TiO2 to alumina ratio and the concentration of doped metals such as iron, platinum and ruthenium were varied to find the most suitable catalyst formulation. Flow rate, particle size, reactor temperature and the position of the lamp were varied to obtain the optimum production of ammonia. A comparative study was made between fixed bed and fluidised bed reactors. The cleavage of water and fixation of nitrogen have been successfully performed using titanium exchanged zeolites irradiated by visible light. The cations in Laporte type 3A, 4A and 5A zeolites were exchanged with titanium ions using titanium trichloride solution. ESP analysis showed the presence of Ti3+ in the exchanged zeolites. When titanium exchanged zeolites were immersed in water and exposed to visible light irradiation, hydrogen was produced. The Ti3+ ions were depleted probably because they combined with oxygen produced from the water cleavage reaction to form titanium dioxide. The Ti3+ ions were replenished by a re-exchanging process. Introducing nitrogen into the reactor resulted in the formation of ammonia. The reaction was carried out in a gas-solid-liquid fluidised bed reactor. Ammonia was also obtained when air instead of nitrogen was used for fluidisation. Concentration of the exchange solution, duration, pH inside the reactor, and particle size, were varied to find the optimum conditions for ammonia production. Production of hydrogen and ammonia increased during successive exchanges. These results were not affected by the cutting off of UV irradiation, thus raising the potential of utilising solar irradiation. Small scale-up studies showed that production of hydrogen and ammonia increased when a four pass reactor was used instead of a single pass reactor. Flow rate was varied to investigate the advantages and/or disadvantages of fluidisation. A reaction mechanism has been proposed.

Date of Award	1982
Original language	English
Awarding Institution	University of Bath