An Operando Raman, IR, and TPSR Spectroscopic Investigation of the Selective Oxidation of Propylene to Acrolein over a Model Supported Vanadium Oxide Monolayer Catalyst

The selective oxidation of propylene to acrolein was investigated over a well-defined supported V2O5/Nb2O5 catalyst, containing a surface vanadia monolayer, with combined operando Raman/IR/MS, temperature programmed surface reaction (TPSR) spectroscopy and isotopically labeled reactants (O2 and C3D6). The dissociative chemisorption of propylene on the catalyst forms the surface allyl (H2CdCHCH2*) intermediate, the most abundant reaction intermediate. The presence of gas phase molecular O2 is required to oxidize the surface H* to H2O and prevent the hydrogenation of the surface allyl intermediate back to gaseous propylene (Langmuir-Hinshelwood reaction mechanism). The O2 labeled studies demonstrate that only lattice 16O is incorporated into the acrolein reaction product (Mars-van Krevelen reaction mechanism). This is the first time that a combined Langmuir-Hinshelwood-Mars-van Krevelen reaction mechanism has been found for a selective oxidation reaction. Comparative studies with allyl alcohol (H2CdCHCH2OH) and propylene (H2CdCHCH3) reveal that the oxygen insertion step does not precede the breaking of the surface allyl C-H bond. The deuterium labeled propylene studies show that the second C-H bond breaking of the surface allyl intermediate is the rate-determining step. These new observations have been incorporated in the derivation of the overall kinetics for propylene oxidation to acrolein with the model supported V2O5/Nb2O5 catalyst.