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The hydrogen precipitation of vanadium from aqueous solutions O'Brien, Robert Neville

Abstract

It has been found that sodium vanadate in aqueous solutions, can he reduced by gaseous hydrogen, at temperatures of 300 - 400°F, and in the presence of a nickel catalyst, and precipitated as an oxide of lower valence. The following reaction has been proposed: 2 VO₃[superscript -] + 2H₂ → V₂0₃ •+ 2OH[superscript -] + H₂O The kinetics of this reaction have been examined, and the following effects observed. (1) Two distinct stages of reduction are involved.In the overall reaction, VO₃ is first reduced to a soluble tetravalent ion, believed to be V₂O₅[superscript =] followed by further reduction to the trivalent oxide which is precipitated. The latter stage is rate controlling. (2) The kinetics of the reaction are initially of zero order, the rate being independent of the concentration of vanadium down to a value of one gram per litre. Below this value the kinetics become first order. (3) The rate is directly proportional to the amount of nickel catalyst present. (4) The rate is proportional to the square root of the pressure of hydrogen. (5) The activation energy for the reaction is 7850 calories per mole. The mechanism of the reaction is discussed in the light of these kinetic results and a rate controlling step is proposed consisting of a reaction between V₂O₅[superscript =] ions and H atoms, both adsorbed on the surface of the nickel. Such a mechanism is consistent with all the kinetic observations.

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