The study of thermochemical hydrogen preparation. VI. A hydrogen-evolving step through the H2S-CO cycle.

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A two-step decomposition process of hydrogen sulfide was studied. The process is constituted from a hydrogen-evolution step (CO+H2S\oversetca.200°C→H2+COS) and a carbonyl sulfide-decomposition step (COS\oversetca.830°C→CO+1⁄x·Sx. Almost an equilibrium amount of hydrogen could be produced in the first reaction (ca. 40%, 200 °C, 1 atm, CO/H2S=2.2) with a cobalt sulfide or nickel sulfide catalyst. In the second reaction, an almost equilibrium amount of carbonyl sulfide (45%) could also be decomposed into carbon monoxide and elementary sulfur at 830 °C under 1 atm with a sufficient reaction rate and selectivity without using any catalysts. Carbon dioxide was formed as a by-product in the second reaction: however, its yield was only 5% of the carbon monoxide produced. From these results, it may be concluded that this two-step decomposition process of hydrogen sulfide can produce more hydrogen than that produced by the direct decomposition of hydrogen sulfide. In the second reaction, however, the recombination of carbon monoxide with elementary sulfur can not be neglected. Thus, a rapid separation of sulfur from carbon monoxide is required.