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Reduction of thio-molybdate in aqueous solutions Okita, Yoshiaki


The high temperature behaviour of the molybdenum (VI) -sulphur (-II) -water system in the presence of an ammoniacal buffer was studied. At 150°C. all species of the form MoO(4-x)S(2¯/x) were shown to exist and the stability constants of mono-, di-, tri-, and tetra-thiomolybdate were 2.3 x 10(2) M.(-1), 3.5 x 10(5) M.(-2), 2.7 x 10(8) M.(-3), 7.0 x 10(10) M.(-4), respectively. There were strong indications of the formation of protonated species, Mo(SH)(6), in solutions containing low concentration of free ammonia. Application of reducing gases to this system produced a mixture of a sulphide and an oxide of molybdenum, whose composition depended on the initial composition of the solution. Under hydrogen, the reduction reaction was autocatalytic, rate being first order in product amount and hydrogen pressure. A mechanism was proposed in which the rate determining step was heterogeneous activation of hydrogen on the surfaces of precipitates followed by two paths, one to produce the sulphide and the other to produce the oxide. The proportion of the sulphide to the oxide was dependent on the solution composition, the higher the fractional distribution of tetra-thiomolybdate and the higher the concentration of hydrogen ion, the more the sulphide being produced. Under carbon monoxide the reduction reaction was found to have an induction period. The molybdenum in solution then followed a linear decrease in concentration with time. The slope of this plot showed Langmuir type of dependence on both molybdenum concentration and pressure. A mechanism was proposed in which the rate determining step was a slow decomposition of some complex between thiomolybdates and carbon monoxide adsorbed strongly on catalytic precipitate which was produced during the induction period.

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