Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5) and (+4) Species Over Low-Coverage VOX/TiO2 (101) and (001) Anatase Catalysts

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.