Effect of thermal treatment on various characteristics of undoped and V2O5-doped Co3O4/TiO2 catalysts

The thermal treatment of undoped and V₂O₅-doped Co₃O₄/TiO₂ catalysts was studied in the temperature range, 330–600° C both in vacuum and in air. The wide difference in the catalytic behaviour of the two catalysts could be attributed to surface as well as bulk diffusion of the active cobalt oxide particles. Although in both cases the total Co³⁺ ions of various energy states were considered to be the active species for the given reaction, the distribution of various cobalt species, namely Co-t and Co-o, occupying tetrahedral and octahedral sites in the support-defective structure, seemed to be seriously affected by doping with V₂O₅. This dopant was supposed to have two-fold effect: part is incorporated into the surface Co₃O₄ crystallites leading to smaller more mobile particles, easily reducible and more dispersed, and another part diffuses a few atomic layers deeper in the support causing the redistribution of cobalt species. Upon heating, the increased mobility and the increased availability of the support tetrahedral sites may be responsible for the deactivation behaviour. The bulk diffusion enhanced by doping might cause some modification in the porosity characteristics of the titania support.