Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5) and (+4) Species Over Low-Coverage VOX/TiO2 (101) and (001) Anatase Catalysts
2015
Arnarson, Logi | Rasmussen, Søren B. | Falsig, Hanne | Lauritsen, Jeppe V. | Moses, Poul Georg
We investigate the structural properties and coordination of VOX species supported on the facets of anatase TiO₂ as the oxidation state changes during the catalytic cycle of selective catalytic reduction (SCR) of NOX. Density functional theory (DFT) calculations reveal how the molecular structures of hydrated, low-coverage V sites depend on the surface facet of the TiO₂ (anatase) support. Thus, the support effect is not limited to only disperse the active phase but also dictates its shape and ultimately its reactivity. We find that hydrated VOX species, where V is present in oxidation state +5, are present in distorted tetrahedral configurations on the most stable TiO₂(101) facet, whereas on the less stable TiO₂(001) facet they are present in distorted octahedral configurations. Furthermore, we find the reduced states of the same species where V is present in oxidation state +4 also exhibit unchanged local coordination with the vanadyl unit V═O intact. We support the findings by electron paramagnetic resonance spectroscopy (EPR) data on a vanadia–titania catalyst partly reduced in situ by ammonia exposure. These results clarify the inconsistencies in the literature and interrelate the spectroscopic observations by Raman and IR with EPR and NMR.
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