Molecular and Dissociative Adsorption of Water on
(TiO<sub>2</sub>)<sub><i>n</i></sub> Clusters, <i>n</i> = 1–4
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Abstract
The low energy structures of the
(TiO<sub>2</sub>)<sub><i>n</i></sub>(H<sub>2</sub>O)<sub><i>m</i></sub> (<i>n</i> ≤ 4, <i>m</i> ≤ 2<i>n</i>) and (TiO<sub>2</sub>)<sub>8</sub>(H<sub>2</sub>O)<sub><i>m</i></sub> (<i>m</i> = 3, 7, 8) clusters were predicted using
a global geometry optimization approach, with a number of new lowest
energy isomers being found. Water can molecularly or dissociatively
adsorb on pure and hydrated TiO<sub>2</sub> clusters. Dissociative
adsorption is the dominant reaction for the first two H<sub>2</sub>O adsorption reactions for <i>n</i> = 1, 2, and 4, for
the first three H<sub>2</sub>O adsorption reactions for <i>n</i> = 3, and for the first four H<sub>2</sub>O adsorption reactions
for <i>n</i> = 8. As more H<sub>2</sub>O’s are added
to the hydrated (TiO<sub>2</sub>)<sub><i>n</i></sub> cluster,
dissociative adsorption becomes less exothermic as all the Ti centers
become 4-coordinate. Two types of bonds can be formed between the
molecularly adsorbed water and TiO<sub>2</sub> clusters: a Lewis acid–base
Ti–O(H<sub>2</sub>) bond or an O···H hydrogen
bond. The coupled cluster CCSD(T) results show that at 0 K the H<sub>2</sub>O adsorption energy at a 4-coordinate Ti center is ∼15
kcal/mol for the Lewis acid–base molecular adsorption and ∼7
kcal/mol for the H-bond molecular adsorption, in comparison to that
of 8–10 kcal/mol for the dissociative adsorption. The cluster
size and geometry independent dehydration reaction energy, <i>E</i><sub>D</sub>, for the general reaction 2(−TiOH)
→ −TiOTi– + H<sub>2</sub>O at 4-coordinate Ti
centers was estimated from the aggregation reaction of <i>n</i>Ti(OH)<sub>4</sub> to form the monocyclic ring cluster (TiO<sub>3</sub>H<sub>2</sub>)<sub><i>n</i></sub> + <i>n</i>H<sub>2</sub>O. <i>E</i><sub>D</sub> is estimated to be −8
kcal/mol, showing that intramolecular and intermolecular dehydration
reactions are intrinsically thermodynamically allowed for the hydrated
(TiO<sub>2</sub>)<sub><i>n</i></sub> clusters with all of
the Ti centers 4-coordinate, which can be hindered by cluster geometry
changes caused by such processes. Bending force constants for the
TiOTi and OTiO bonds are determined to be 7.4 and 56.0 kcal/(mol·rad<sup>2</sup>). Infrared vibrational spectra were calculated using density
functional theory, and the new bands appearing upon water adsorption
were assigned
