Temperature-Programmed Reduction of MoO3 and MoO2

Article abstract of DOI:10.1021/j100267a021

The reduction of MoO3 and MoO2 is studied by temperature-programmed reduction (TPR).TPR patterns appear to be highly dependent on H2O content of the reducing mixture, sample size, precalcination temperature, and heating rate.Activation energy values for reduction have been calculated from TPR series with various heating rates.A consistent interpretation of the complex TPR results is constructed, in which the extent of H2 dissociation catalyzed by low-valent Mo atoms plays a central role.The formation of these Mo catalytic sites is found to depend on H2O pressure, concentration of surface defects, and surface area.Reduction of MoO3 to MoO2 and of MoO2 to Mo metal both can be catalyzed, in which case the rate-determining step is either H2 dissociation, when the formation of catalytic sites is limited, or Mo-O bond breaking, when excess catalytic sites are present.Reduction of MoO3 to MoO2 can also take place noncatalyzed, with oxygen diffusion as rate determining step.MoO2 and Mo metal are also formed, as intermediates, in temperature-programmed sulfiding (TPS) of MoO3.MoO2 production is faster in H2/H2S than in H2 due to a different reduction mechanism involving H2S as primary reactant.The formation of metal is also faster, due to a higher porosity of the MoO2 formed in situ.