Hydrogen from urea-water and ammonia-water solutions

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Abstract

The conversion of urea–water into hydrogen was investigated in a downward flow packed-bed reactor using a Ni–Al2O3 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 H2, CH4 and NH3, selectivity to the carbon containing products CO2, CO and CH4, 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. CO2 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 NH3 was accompanied by a low urea conversion to CO, CO2 and CH4, 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 Al2O3 pellets showed no significant H2 yield, while the main H-product was overwhelmingly NH3. 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 NH3 cracking. Looking for signs of deactivation, the catalyst was characterised with N2 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.
Original languageEnglish
Pages (from-to)304-315
Number of pages11
JournalApplied Catalysis B: Environmental
Volume106
Issue number3-4
DOIs
Publication statusPublished - 11 Aug 2011

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