Insights into Oxygen Exchange Between Gaseous O2and Supported Vanadium Oxide Catalysts via17O NMR

Vanadium oxide reference compounds, KVO 3 and V 2 O 5, and supported vanadium oxide catalysts (Al 2 O 3, TiO 2, and SiO 2 ) were investigated using magic angle sample spinning 17 O NMR. All samples were 17 O-enriched using gas−solid exchange. Extraction of chemical shift and quadrupolar coupling information for the model compounds KVO 3 and V 2 O 5 was performed via the simulation of MAS spectra obtained in one-pulse experiments and the observations were consistent with their known bulk structures. For the supported vanadia catalysts, it was found that the oxygen exchange process is dominated by 17 O signal from the catalyst oxide supports. Spectra obtained via rotor-synchronized spin echoes revealed additional wide lines for Al 2 O 3 and TiO 2 supported vanadia catalysts that arise from 17 O in the surface vanadia species of the catalysts. Additional 17 O− 51 V TRAPDOR (TRAnsfer of Populations in DOuble Resonance) experiments support this assignment. The wide lines suggest that the local environments of the 17 O nuclei associated with the dehydrated surface vanadia species are extremely heterogeneous and fall in the range of oxygen in singly (V═O) and/or doubly coordinated environments (V−O−V or V−O−Support). The relatively small total amount of 17 O associated with the surface vanadia species contrasts with oxygen exchange models which commonly assume only the surface vanadium oxide layer is involved. These results demonstrate that the isotopic exchange of molecular O 2 with supported metal oxide catalysts, especially supported vanadia catalysts, is a much more complex process than originally perceived.