In situ laser Raman studies of the mechanism of ethanol oxidation over supported molybdenum oxide

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Virginia Tech


"Ethanol oxidation over supported molybdenum oxide was studied by in situ laser Raman spectroscopy, deuterium isotopic substitution, together with steady-state and transient reactivity and kinetic measurements. The combination of techniques allows a complete description of the mechanism of ethanol oxidation on MoO₃ supported on SiO₂, Al₂O₃, and TiO₂, including the quantification of adsorbed ethoxide species. Very interestingly, on the SiO₂-supported catalysts two types of adsorbed ethoxide species were observed. One, associated with Mo=O bonds, was a true reactive intermediate, while the other, associated with Mo-O-Mo bonds, was an unreactive spectator on the surface."

Isotopic substitution experiments established that the same mechanism with two stages occurred over MoO₃ on SiO₂, Al₂O₃ and TiO₂: an equilibrated adsorption step to form adsorbed ethoxide intermediates, followed by either of two rate-determining steps, α-hydrogen abstraction from the ethoxide intermediates to form acetaldehyde or β-hydrogen abstraction to form ethylene.

The activation energy was found to be the same for the different supports at the similar loading levels. The link to reducibility and the existence of a common ethoxide intermediate indicated that the rate was controlled by a term in the preexponential factor. From the similarity in the isotope effect the controlling factor was deduced to be the electronic partition function associated with the density of electron-accepting levels in the molybdate-support complex.

Analysis of the reaction isotherms showed that activation energy decreased with increasing coverage, suggesting a non-uniform surface. Consistent with this, the kinetics could be described by an extended treatment of Temkin’s theory of rates. A full derivation of the theory, which was expanded to accommodate two types of sites, is presented.

Propylene oxidation was studied on MoO₃ on SiO₂, Al₂O₃ and TiO₂. Oxygen chemisorption and temperature programmed reduction (TPR) results indicated that the molybdenum oxide - support interaction decreased in the order: TiO₂ > Al₂O₃ > SiO₂. Temperature programmed surface reaction (TPSR) of adsorbed ethanol was used to characterize the acid-base properties of the catalysts, which played an important role in the formation of oxidation products.

A comparative study of ethanol oxidation over MoO₃ on SiO₂, Al₂O₃, and TiO₂ by O₃/O₂ and O₂ was carried out in a flow reactor. The effect of ozone appeared below a transition temperature which differed according to the support (523 K on SiO₂, 448 K on Al₂O₃ and 443 K on TiO₂). The reactivity below the transition temperature followed the order SiO₂ > TiO₂ > Al₂O₃, while above it the reactivity followed the order TiO₂ > Al₂O₃ > SiO₂. It appeared that the ozone decomposition activity of the catalysts played an important role in ethanol oxidation by O₃/O₂.