Insights into Oxygen Exchange Between Gaseous O2 and Supported Vanadium Oxide Catalysts via 17O NMR
Christopher A. Klug*†‡, Scott Kroeker§, Pedro M. Aguiar§, Min Zhou†, Donald F. Stec† and Israel E. Wachs#
Vanadium oxide reference compounds, KVO3 and V2O5, and supported vanadium oxide catalysts (Al2O3, TiO2, and SiO2) were investigated using magic angle sample spinning 17O NMR. All samples were 17O-enriched using gas−solid exchange. Extraction of chemical shift and quadrupolar coupling information for the model compounds KVO3 and V2O5 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 17O signal from the catalyst oxide supports. Spectra obtained via rotor-synchronized spin echoes revealed additional wide lines for Al2O3 and TiO2 supported vanadia catalysts that arise from 17O in the surface vanadia species of the catalysts. Additional 17O−51V TRAPDOR (TRAnsfer of Populations in DOuble Resonance) experiments support this assignment. The wide lines suggest that the local environments of the 17O 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 17O 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 O2 with supported metal oxide catalysts, especially supported vanadia catalysts, is a much more complex process than originally perceived.