The decomposition of thiophen, furane and pyrrole on molybdenum disulphide catalysts occurs by two-point adsorption, which leads to a half-hydrogenated state from which the hetero-atom is then broken off. Retardation is caused by molecules also capable of giving two-point adsorption. Tetrahydrothiophen and tetrahydrofurane are decomposed on the same catalysts, but the mechanism is probably simpler. On nickel subsulphide catalysts, the tetrahydro-derivatives are decomposed much more easily than the unhydrogenated molecules, and they are held by single-point adsorption on the 111-plane of Ni
3
S
2
on which pairs of hydrogen atoms are also adsorbed. Thiophen and furane decompose by reaction with metallic nickel produced by reduction of Ni
3
S
2
. The nickel sulphide or oxide thus formed are subsequently reconverted to metal. Retardation of these changes arises from any reactant which can compete for nickel atoms, or which can cover the subsulphide surface and prevent its conversion to metal.
The catalytic decomposition of simple heterocyclic compounds II. Reaction kinetics and mechanism