Published: October 28, 2011 r2011 American Chemical Society 24133 dx.doi.org/10.1021/jp204726b | J. Phys. Chem. C 2011, 115, 24133–24142 ARTICLE pubs.acs.org/JPCC Dynamics of Hydration in VanadiaTitania Catalysts at Low Loading: A Theoretical and Experimental Study Anna E. Lewandowska, †,# M onica Calatayud, ‡,§,∞ Frederik Tielens, ||,^ and Miguel A. Ba~ nares* ,† † Catalytic Spectroscopy Laboratory, Institute of Catalysis and Petroleum Chemistry, CSIC, E-28049-Madrid, Spain, ‡ UPMC Univ Paris 06, UMR 7616, Laboratoire de Chimie Th eorique, Institut Universitaire de France, F-75005 Paris, France § CNRS, UMR 7616, Laboratoire de Chimie Th eorique, F-75005, Paris, France ∞ Institut Universitaire de France ) Laboratoire de R eactivit  e de Surface, UPMC Univ Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France ^ Laboratoire de R eactivit  e de Surface, CNRS, UMR 7197, 4 Place Jussieu, 75252 Paris Cedex 05, France b S Supporting Information 1. INTRODUCTION Supported metal oxide catalysts are widely used in the chemical, petrochemical, and environmental catalytic processes. 1 Among these, the titania-supported vanadium oxide materials exhibit ex- cellent catalytic properties for several selective oxidation processes, such as o-xylene oxidation to phthalic anhydride, 24 methanol oxidation, 57 the oxidation of toluene to benzoic acid, 8,9 and the selective catalytic reduction of NO (DeNOx process). 1012 The catalyst efficiency is related to the vanadium loading on the support, TiO 2 -anatase. Vanadium oxide dispersed on a titania surface forms monomeric and/or polymeric species depending on the vanadia coverage. In addition, hydration has proven to have an impact on the catalytic processes. Thus, water vapor addition to reactant stream has a promoting effect on the catalytic pro- perties of the vanadiatitania system in the selective oxidation of hydrocarbons. For instance, steam accelerates the formation of pyridine-3-carbaldehyde intermediate and subsequent formation of nicotinic acid in the oxidation of 3-picoline. Water increases the vanadiatitania catalyst activity and selectivity. 13 The presence of water vapor in toluene oxidation provides a rise in the catalyst activity and enhances the selectivity to benzoic acid. 14 The pro- moting role of water is related to the formation of V-OH hydroxyl groups and generation of mobile protons. Since water is the coproduct in the oxidation reactions and selective catalytic reduction, and since it acts as a promoter in some of the selective oxidation reactions, it is critical to understand the interaction of water with the catalyst surface. The knowledge of water impact on the structure of surface vanadium oxide species, the vanadium coordination sphere, the ratio between polymeric and mono- meric species, and their domain size is still unknown and needs a systematic study. The higher stability of the long-lived industrial vanadiatitania catalyst, compared to the other vanadia-supported systems, gen- erates an interest to widen the knowledge on the transformations Received: May 21, 2011 Revised: October 28, 2011 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)O 2 monomers with high affinity for water. Interestingly, OVO 3 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. Interconver- sion 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.