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The Structure and Reactivity of Size-selected VxOy Clusters on the TiO2(110)-(1x1) Surface of Varying Oxidation State

  • Author(s): Buffon, Joshua Woodbridge
  • Advisor(s): Buratto, Steven K
  • et al.
Abstract

The study of the oxidative dehydrogenation of methanol to formaldehyde by vanadia/TiO2 model catalysts has received a great deal of interest by the scientific community in recent years. However, due to the wide variety of catalyst preparation methods, there have been conflicting reports with formaldehyde production occurring anywhere from room temperature to 660 K. This is further complicated by the fact that the methods used often couple the oxidation state of the TiO2 support to that of the vanadia, creating many structures on the surface with uncertain stoichiometries.

In order to overcome these challenges, we have studied the oxidation chemistry of mass-selected Vx and VxOy clusters deposited on TiO2 (110) surfaces as a function of size and composition of the cluster as well as the oxidation state of the support. The cluster-decorated surface was characterized by scanning tunneling microscopy (STM) to determine the structure of the active catalyst and the chemistry was probed using temperature-programmed reaction (TPR). Density functional theory (DFT) was used to calculate the structure of the clusters on the surface and compared with experimental results.

We have shown that both V2O6 and V3O9 clusters are highly active in the oxidative dehydrogenation of methanol. V2O6 clusters catalyze the production of formaldehyde and methyl formate, the latter representing a previously undiscovered reaction path in the reaction of methanol and oxygen on supportedvanadia catalysts. Structural models from DFT and STM images have allowed us insight into the mechanism of this reaction. The models indicate that on a

stoichiometric surface, the V2O6 cluster contains a peroxyl group that is active for the production of methyl formate.

TPR has revealed that the production of methyl formate is highly dependent on the oxidation state of the surface. For the case of V2O6 clusters on the reduced surface, only formaldehyde production is observed. STM results also show a variation in the cluster structure as a function of the oxidation state of the TiO2, with the clusters on the reduced surface containing only vanadyl oxygen atoms due to the interaction with surface oxygen vacancies to dissociate the peroxyl group. The vanadyl oxygen has been previously shown to be active for production of formaldehyde by examining post-oxidized size selected VO clusters. In addition, we show that oxidative dehydrogenation of methanol on V3O9-decorated TiO2 surfaces results in methyl formate production at a lower temperature, and formaldehyde production with a significant increase in signal over that observed for V2O6 clusters.

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