Dynamics of Hydration in Vanadia–Titania Catalysts at Low Loading: A Theoretical and Experimental Study

Abstract

The hydration process of dehydrated vanadia–titania catalysts at low loading is investigated using periodic DFT calculations. We focus on the early stages of the hydration process of the vanadia–titania in order to shed light onto the structural and dynamical changes occurring at the molecular level. Hydration is modeled by addition of successive water molecules to the dehydrated models. Special attention is paid to the V–OH bond formation and the transformation between different surface species. It is found that at low vanadia coverage the predominant surface species are OV(OH)O2 monomers with high affinity for water. Interestingly, OVO3 pyramids are stable only under severe dehydrating conditions, and hydroxylated species are expected to be present even at low water content. While low water content leads to water dissociation and supports hydration, higher content leads to a dynamic equilibrium between hydrated vanadia surface species. Interconversion between different surface species is fast and depends on the water coverage, through a fast hydrogen transfer mechanism. Leaching of OV(OH)3 species is observed in the case of high water content. The number of adsorbed water molecules depends on temperature, but even at high temperature, water adsorption is preferred, which is relevant to the state of titania-supported catalysts during reaction conditions in which water is fed or generated during reaction. Calculated harmonic vibrations are provided for the most stable surface species; their redshift upon coordination with water and their blueshift upon progressive dehydration are experimentally confirmed by in situ Raman spectra in dry and humid air at increasing temperatures.