AnOperandoRaman, 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 V 2 O 5 /Nb 2 O 5 catalyst, containing a surface vanadia monolayer, with combined operando Raman/IR/MS, temperature programmed surface reaction (TPSR) spectroscopy and isotopically labeled reactants ( 18 O 2 and C 3 D 6 ). The dissociative chemisorption of propylene on the catalyst forms the surface allyl (H 2 C CHCH 2 *) intermediate, the most abundant reaction intermediate. The presence of gas phase molecular O 2 is required to oxidize the surface H* to H 2 O and prevent the hydrogenation of the surface allyl intermediate back to gaseous propylene (Langmuir−Hinshelwood reaction mechanism). The 18 O 2 labeled studies demonstrate that only lattice 16 O 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 (H 2 C CHCH 2 OH) and propylene (H 2 C CHCH 3 ) 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 V 2 O 5 /Nb 2 O 5 catalyst.