UC Santa Barbara

Vanadia catalysts supported on TiO2 belong to a class of mixed-metal oxides capable catalyzing the oxidative dehydrogenation (ODH) of alkanes into high commodity olefins. As olefins are central to many important industrial processes and their demand expected to increase, improving the reactivity and yield of vanadia catalysts has become an active area of research. Supported vanadia display unique catalytic properties not observed in their pure oxide counterparts; this unique reactivity and selectivity has been attributed to the complex local bond-coordination present at the interface. Thus, understanding the role of each metal oxide has on the reactive interface is critical to designing better catalysts.

Previous investigations into vanadia catalysts have found the activity for methanol ox- idation to formaldehyde increases with reducibility (sub-stoichiometric) of the support, however the nature in which support defects contribute to this synergy is unknown. This is due to the fact that the support is typically oxidized in the growth of vanadia catalysts. We circumvent this problem by soft-landing mass-selected cationic VO+ clusters onto single crystal rutile TiO2 110) - (1 × 1) surfaces to create well-defined model in-terfaces. By integrating temperature programmed desorption/reaction (TPD/R) studies with variable temperature scanning tunneling microscopy (VT-STM), we can correlate ensemble surface reactivity with sub-nanometer structures, reserved here for VO clusters and surface point-defects. First, we identify sub-surface Ti interstitial defects in bare reduced TiO2-x as the origin of a new methanol disproportionation reaction to methane and formaldehyde. These Ti interstitial defects are implicated as electron donors and thus contribute to the reduction of methanol. We find that the inclusion of mass-selected VO clusters results in a VO-mediated disproportionation reaction to formaldehyde and methanol at high temperature. Our STM results confirm the VO-mediated disproportionation reaction pathway by observation of a VO-(OCH3)n precursor intermediate. Furthermore, our methanol TPD/R and STM results indicate methanol adsorbs favor- ably over VO and five-fold coordinated Ti sites in-lieu of surface bridging oxygen vacancy defects and Ti interstitial defects. We propose VO clusters scavenge electrons from Ti interstitial and bridging oxygen vacancy defects, resulting in an effectively oxidized surface. The VO/TiO2 interface investigated here represents an interesting model towards understanding the synergistic effects of mixed-metal oxides catalysts.