Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃:  Structural Characterization and Catalytic Function

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO_x and MoO_x or VO_x and CrO_x on Al₂O₃ were examined using Raman and UV−visible spectroscopies, the dynamics of stoichiometric reduction in H₂, and the oxidative dehydrogenation of propane. VO_x domains on Al₂O₃ modified by an equivalent MoO_x monolayer led to dispersed binary structures at all surface densities. MoO_x layers led to higher reactivity for VO_x domains present at low VO_x surface densities by replacing V−O−Al structures with more reactive V−O−Mo species. At higher surface densities, V−O−V structures in prevalent polyvanadates were replaced with less reactive V−O−Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV−visible data indicate that polyvanadates predominant on Al₂O₃ convert to dispersed binary oxide structures when MoO_x is deposited before or after VO_x deposition; these structures are less reducible and show higher UV−visible absorption energies than polyvanadate structures on Al₂O₃. The deposition sequence in binary Mo−V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO_x on Al₂O₃ led to more reactive VO_x domains than those deposited on pure Al₂O₃ at similar VO_x surface densities. At all surface densities, the replacement of V−O−Al or V−O−V structures with V−O−Cr increased the reducibility and catalytic reactivity of VO_x domains; it also led to higher propene selectivities via the selective inhibition of secondary C₃H₆ combustion pathways, prevalent in VO_x−Al₂O₃, and of C₃H₈ combustion routes that lead to low alkene selectivities on CrO_x−Al₂O₃. VO_x and CrO_x mix significantly during synthesis or thermal treatment to form CrVO₄ domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO_x domains can be modified significantly by the formation of binary dispersed structures. VO_x−CrO_x structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO_x domains present at similar surface densities on pure Al₂O₃ supports.