1 research outputs found
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 <i>g</i> matrix
and vanadium hyperfine parameters of the persistent paramagnetic V<sup>4+</sup> center in “as prepared” and H<sub>2</sub> reduced
SBA-15 supported VO<sub><i>x</i></sub> 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 V<sup>4+</sup> center as a surface
exposed deprotonated vanadium site. The same site topology was found
for oxidized and H<sub>2</sub> 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 V<sup>4+</sup> sites. Hence, application
of 2D EPR enabled us to directly identify the previously proposed
V–O–V structural motif on SiO<sub>2</sub> supported
VO<sub><i>x</i></sub> catalysts for the first time