True and Spectator Intermediates in Catalysis:  The Case of Ethanol Oxidation on Molybdenum Oxide As Observed by in Situ Laser Raman Spectroscopy

A high-throughput laser Raman spectrometer is used to detect adsorbed ethoxide species during the catalytic oxidation of ethanol over supported molybdenum oxide at atmospheric pressure. Two different species are observed, a true reaction intermediate and a relatively inert spectator on the surface. Steady-state and dynamic experiments indicate that the reactive species is associated with Mo O terminal bonds on the surface, while the spectator is associated with Mo−O−Mo bridging bonds. Deuterium isotopic substitution experiments indicate that the active intermediate is formed in an equilibrated adsorption step, and decomposes by a rate-determining α-C−H bond-breaking step to form acetaldehyde.