Investigation of Cobalt (II)-Tungsten (VI)-Oxide Reduction in Hydrogen: Part I

Abstract

CoWO₄ reduction in dry hydrogen, at temperatures lower than 550°C brings about an intermediate unstable phase of cubic structure, (a = 10.846Å) and composition Co₄W_2.5O; the tungsten which is not retained in this phase appears in the form ofβ- W (A-15 structure,a = 5.039Å). Another phase (noted ϕ@#@) which does not give any X-ray diffraction pattern forms by further reduction of Co₄W_2.5O. The whole set of these transformations is characterized by a kinetic global law described by a Johnson-Mehl equationξ = 1- exp(-kt) ^3/2 and an activation energy, 24 kcal/Mole, determined byβ-W continuous growth. The rate of C03W crystallization fromϕ becomes substantial, only from around 700°C, temperature at whichβ-W is also converted into α-W. The final alloy Co₇W₆ is formed from about 800°C on, through reaction of Co₃W and α-W. In wet hydrogen, WO₂ forms instead ofβ- W and Co₄W_2.5O—which may be stabilized under certain conditions-is directly reduced into Co₃W after WO₂ reduction into α-W.