The effect of (001) TiO 2 anatase support on the electronic and catalytic properties of a V 2 O 5 monolayer is analyzed using density functional theory (DFT). The catalyst is represented by both clusters and periodic slabs. Using two experimentally relevant models of monolayer V 2 O 5 /TiO 2 (anatase) catalyst, both weak and strong interactions between a V 2 O 5 monolayer and the TiO 2 support have been investigated. In the first model, where a crystallographic (001) V 2 O 5 layer is placed on top of the (001) TiO 2 support, the weak interaction between vanadia and titania does not result in a major reconstruction of the active phase. Nevertheless, the changes in the electronic properties of the system are evident. The deposition of the vanadia monolayer on the titania substrate results in charge redistribution, enhancing the Lewis acidity of vanadium and the chemical hardness above the vanadyl oxygen, and in a shift of the Fermi level to lower binding energies accompanied by a reduction in the band gap. In the second model, where the (001) titania anatase structure is extended with a VO 2 film terminated by half a monolayer of vanadyl oxygen, apart from a similar electronic effect, the strong interaction of the vanadia phase with the titania support resulting from a high order of epitaxy has an important effect on the structure of the active phase. Atomic hydrogen adsorption is most favorable on the vanadyl oxygen of all the investigated surfaces, while the adsorption energy on this site increases by ∼10 kJ/mol due to the weak interaction between vanadia and titania and is further increased by ∼50 kJ/mol as a stronger interaction between the two phases is achieved, all in agreement with the increase in the negative electrostatic potential above the vanadyl site. The observed trends in the reactivity of the oxygen sites in H adsorption for the different catalyst models are successfully explained in terms of a frontier orbital analysis. IntroductionTransition metal oxides are widely used as catalysts for the oxidation of hydrocarbons. Especially vanadium oxide based catalysts are among the most active for the oxidation of both aliphatic and aromatic hydrocarbons. An important example of such a catalyst is V 2 O 5 /TiO 2 (anatase), which is used in industry as an effective catalyst for the production of phthalic anhydride from o-xylene.1,2 For the anatase support, although the (101) surface is thermodynamically more stable, the (100) and (001) faces are found in the industrial TiO 2 powders.3 Earlier experimental studies indicate that this catalytic system is of high performance when it consists of a monolayer of V 2 O 5 upon a TiO 2 (anatase) substrate, showing activity and selectivity not observed in the unsupported V 2 O 5 or TiO 2 anatase.2,4 The enhanced catalytic performance can be attributed to a synergetic effect between the active phase and the support. In order to unravel the role of the TiO 2 support on a V 2 O 5 monolayer catalyst, ab initio methods can help to provide a better understanding of the rel...

Vanadium oxides based materials are well known to play an active role as catalysts in many chemical processes of technological importance like for example hydrocarbon oxidation reactions or selective catalytic reduction of NO x in the presence of ammonia. Usually the (010) surface is pointed out as the most important, however one has to underline that other low-indices surfaces are by far less studied. In the present study the electronic structure of V 2 O 5 (001) and (100) surfaces are determined by ab initio DFT methods using gradient-corrected RPBE exchange-correlation functional. Detailed analyses of the electronic structure of each cluster are performed using charge density distributions, Mayer bond orders, electrostatic potential maps, character of frontier orbitals, and density of states (total as well as partial, atom projected). Results of the calculations show that overall negative charge of the surface oxygen sites scales with their coordination independent of the surface orientation. Terminal oxygen O(1) is charged the least negatively while doubly coordinated atoms -O(2) and O e (2) have charge twice as large. This indicates that bridging (for (001) and (100) netplanes) and edging (only for (001) netplane) oxygen sites are more nucleophilic than terminal vanadyl sites, which becomes important in view of the reactivity of the different sites for surface chemical reactions. Vanadium atoms present at these surfaces are positively charged (electrophilic) and may play a role of electron acceptors. The unsaturated surfaces show a strong tendency to surface relaxation that manifest by large relaxation energies.

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