Influence of the preparation conditions on the structure of the active phase and catalytic properties of Ni-Co-molybdate propane oxydehydrogenation catalysts

The effect of the pH conditions on the structure and composition of the active phase in Ni0.5Co0.5MoO4 propane oxy-dehydrogenation (ODH) catalyst is investigated by using different characterization techniques: XRD, XPS, laser Raman spectroscopy, chemical composition and BET-surface area. Increasing precipitation pH from 6.0 to 8.5 produces the following effects: i) BET surface area progressively increases, ii) Mo loading decreases, iii) all samples show higher Co loading than Ni in the bulk, iv) the Ni content increases strongly from pH 6.0 to 7.5 and remains almost constant between 7.5-8.5 pH, v) Co content is almost constant in the pH range 6.0-7.5, but strongly increases at pH 8.5, vi) surface composition varies as follows: at pH 6.0: Mo >Co >Ni, at pH 7.5: Mo congruent to Ni > Co, at pH 8.5: Ni > Co congruent to Mo. MoO3 phase was detected only in the sample prepared at pH 6.0. XRD patterns showed, for the catalysts prepared at pH 6.0 and 7.5, diffraction lines characteristics of both beta-NiMoO4 and beta-CoMoO4-like phases. The catalyst prepared at pH 8.5 is a solid solution of beta-NiMoO4 and beta-CoMoO4 in a common molybdate structure and probably NiO and Co3O4 separated phases. Activity measurements conducted at different space-time indicated that the sample prepared at pH 8.5 is the most effective for the propylene production