Hydrogen from urea-water and ammonia-water solutions

The conversion of urea-water into hydrogen was investigated in a downward flow packed-bed reactor using a Ni-Al 2O 3 catalyst. This was conducted at atmospheric pressure under molar steam to carbon ratios (S:C) of 4-7, and at temperatures between 500 and 700°C. The urea and water conversions, selectivity to the hydrogen containing products H 2, CH 4 and NH 3, selectivity to the carbon containing products CO 2, CO and CH 4, and the hydrogen yield, were very close to the calculated equilibrium values at and above S:C of 5 and temperatures at and above 600°C. CO 2 dominated the carbon products, in agreement with equilibrium trends. The selectivity to ammonia decreased abruptly from 20% to below 5% when the temperature increased from 500 to 550°C, and exhibited a small sensitivity to the steam to carbon ratio. High selectivity to NH 3 was accompanied by a low urea conversion to CO, CO 2 and CH 4, and poor hydrogen yield below 500°C. Up to 99.3% of the ammonia generated was easily separated from the syngas by condensation in the excess water. Experiments replacing the Ni-bed with Al 2O 3 pellets showed no significant H 2 yield, while the main H-product was overwhelmingly NH 3. Aqueous ammonia cracking experiments indicated a reaction further away from equilibrium than the equivalent urea-water experiments, indicative of a hydrogen formation mechanism from urea to water that was more than just a sequence of urea decomposition, HNCO hydrolysis and NH 3 cracking. Looking for signs of deactivation, the catalyst was characterised with N 2 adsorption, TEM-EDX, and powder XRD. NiO was shown to be present in negligible amounts after the experiments, while crystallite sizes and surface area were not affected significantly, and no coking was observed, evidencing a robust catalyst for this reaction.