Reaction path analysis of propene selective oxidation over V ₂O ₅ and V ₂O ₅/TiO ₂

The selective oxidation of propene on the vanadyl and bridging oxygen sites of the fully oxidized (0 0 1) V ₂O ₅ surface and of an epitaxial vanadia monolayer supported on (0 0 1) TiO ₂ anatase is analyzed using periodic density functional theory (DFT). The formation of several oxygenated products, that is, allyl alcohol, propene oxide, acrolein, formaldehyde and acetaldehyde, is investigated. Selective oxidation proceeds via a Mars-van Krevelen redox mechanism and its elementary steps on the vanadia surface are identified. Propene activation occurs preferentially on the vanadyl oxygen sites, with allyl CH and CC bond activation being equally favorable. Supporting a vanadia monolayer on titania strongly enhances both bond activations as compared to unsupported V ₂O ₅, yielding a lower activation energy. No dominant pathway is found on V ₂O ₅, with allyl alcohol, propene oxide, and acrolein formation being almost equally favorable, while acrolein formation is expected on V ₂O ₅/TiO ₂. However, propene oxidation to acrolein is overall less preferable on V ₂O ₅/TiO ₂ than V ₂O ₅, due to the reduced activity of the supported catalyst in oxygenation reactions. The latter is linked to the creation of V ³⁺ sites during propene oxidation over V ₂O ₅/TiO ₂. On the other hand, the electrons injected in the empty V 3d states during propene oxidation over V ₂O ₅ lead to the creation of V ⁴⁺ sites.