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 VOx and MoOx or VOx and CrOx on Al2O3 were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H-2, and the oxidative dehydrogenation of propane. VOx domains on Al2O3 modified by an equivalent MoOx monolayer led to dispersed binary structures at all surface densities. MOO,, layers led to higher reactivity for VO,, domains present at low VO, 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, is deposited before or after VOx 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 CrOx on Al2O3 led to more reactive VOx domains than those deposited on pure Al2O3 at similar VOx 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 VOx domains; it also led to higher propene selectivities via the selective inhibition of secondary C3H6 combustion pathways, prevalent in VOx-Al2O3, and Of C3H8 combustion routes that lead to low alkene selectivities on CrOx-Al2O3. VOx and CrOx 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 VOx domains can be modified significantly by the formation of binary dispersed structures. VOx-CrOx structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VOx domains present at similar surface densities on pure Al2O3 supports.