Hydration dynamics for vanadia/titania catalysts at high loading: A combined theoretical and experimental study

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)O2 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. © 2013 American Chemical Society.The authors thank Spanish Ministry project PRO-CTQ2011-25517 and ESF COST Action D36, COST CM1104. A.E.L. acknowledges the European Commission, Capacities Area, Research Infrastructures for HPC-EUROPA2 project (nr: 228398) and Spanish Ministry of Education and Science for ″Juan de la Cierva″ postdoctoral contract. M.C. and F.T. thank HPC resources from GENCI-CINES/IDRIS (Grants 2010-x2010082131 and 2011-x2011082022), the CCRE-DSI of Université P. M. Curie, and Institut Universitaire de France for a junior position.Peer Reviewe