Hydration Dynamics for Vanadia/Titania Catalysts at High Loading: A Combined Theoretical and Experimental Study
Résumé
Periodic DFT calculations are used to simulate the early stages of hydration of dehydrated high-loading vanadia/titania catalysts. The hydration of molecularly dispersed vanadia is studied by successive additions of water molecules to initially dehydrated models. Special attention is paid to the formation and transformation between different surface species, monovanadates and polyvanadates, and the role of V-OH in the hydration process. It is found that two physical surface processes occur at high vanadia coverage and that their importance depends on the surface water content. First, under mild hydration conditions, a polymerization process increases the number of polyvanadates chains. Polyvanadates formation is preceded by an initial generation of monomers with OV(OH)O-2 and OVO3 pyramidal structures. Second, with higher number of water molecules, a solvation process increases the coordination number of vanadium. The interconversion between different surface vanadium oxide species occurs through a fast hydrogen transfer mechanism and depends on water content. Theoretical results are combined with in situ Raman spectra acquired at several temperatures in dry and humid environment during stepwise dehydration. The experimental data indicate that the redshift of the vanadyl band upon exposure to increasing humidity and decreasing temperature is associated to such progressive interaction with water molecules, which weakens the vanadyl V=O bond.