Exploring the structure of paramagnetic centers in SBA-15 supported vanadia catalysts with pulsed one- and two-dimensional Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR)

Using pulsed EPR and ENDOR, the full set of g matrix and vanadium hyperfine parameters of the persistent paramagnetic V4+ center in "as prepared" and H-2 reduced SBA-15 supported VOx catalysts has been measured. The determination of relative signs of the vanadium hyperfine tensor elements by ENDOR using orientation selection allowed an unambiguous extraction of the isotropic part of this interaction. This allowed for identification of the persistent V4+ center as a surface exposed deprotonated vanadium site. The same site topology was found for oxidized and H-2 reduced catalysts, thus indicating that the identified sites represent catalytically active centers. Hyperfine interaction with distant protons indicates formation of an oligomeric structure even for samples with vanadium loadings of less than 2 wt %. This conclusion is confirmed by applying 2D EPR for measuring the hyperfine interaction with neighboring vanadium atoms, covalently linked to reduced V4+ sites. Hence, application of 2D EPR enabled us to directly identify the previously proposed V-O-V structural motif on SiO2 supported VOx catalysts for the first time.