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

Abstract

The selective oxidation of propylene to acrolein was investigated over a well-defined supported V 2O 5/Nb 2O 5 catalyst, containing a surface vanadia monolayer, with combined operando Raman/IR/MS, temperature programmed surface reaction (TPSR) spectroscopy and isotopically labeled reactants ( 18O 2 and C 3D 6). The dissociative chemisorption of propylene on the catalyst forms the surface allyl (H 2CCHCH 2*) intermediate, the most abundant reaction intermediate. The presence of gas phase molecular O 2 is required to oxidize the surface H* to H 2O and prevent the hydrogenation of the surface allyl intermediate back to gaseous propylene (Langmuir−Hinshelwood reaction mechanism). The 18O 2 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 (H 2CCHCH 2OH) and propylene (H 2CCHCH 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 2O 5/Nb 2O 5 catalyst.