Reactivity of (TiO2)N Clusters (N = 1−10): Probing Gas-Phase Acidity and Basicity Properties

We present B3LYP calculations on a selection of small-size clusters with (TiO 2 ) N stoichiometry ( N = 1−10) built on previous works on TiO 2 and SiO 2 or derived by kinship with stable clusters of different sizes. Their reactivity is analyzed as a function of size and electronic structure. Gas-phase acidity is probed by H + interaction with the oxygen sites, while basicity is tested by interaction of molecular NH 3 with titanium sites. Correlation with size, topology, or electronic properties is observed for some systems. In general, the correlation with electronic levels (highest occupied and lowest unoccupied molecular orbitals, HOMO and LUMO) is good criteria of reactivity, although this is not always observed. The calculated values generally decrease with the size. The HOMO−LUMO gaps show oscillations and a general decrease with the size. Coordination of the active site influences both the levels of the frontier orbitals and their effect upon reactivity. The protonation testing the cluster basicity is found to be higher for clusters with N = 3 and N = 9 in accordance with the high HOMO values. A remarkable exception is found for N = 4 for which the most stable protonated structure is different from the most stable naked cluster. We have not tested here the flexibility of the naked cluster; however, this case means that structure reorganization should be considered for reactivity. Regarding ammonia adsorption testing cluster acidity, clusters with N = 3 and N = 8 present the highest adsorption energy toward NH 3 in accordance with a low LUMO value for the former but because of the local topology of the adsorption site for the latter. Acidic and basic character decreases for N = 8−10 probably because of the increase in cohesive energy. A structure with N = 9 emerges as the strongest base with the largest protonation energy. A tetrahedral structure with N = 10 is remarkably stable and presents the lowest adsorption energy values.