Chen, Mingyang
; Straatsma, Tjerk P.
; Dixon, David A.
- Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

In the low energy structures of the (TiO

_{2})

_{n}(H

_{2}O)

_{m} (n ≤ 4, m ≤ 2n) and (TiO

_{2})

_{8}(H

_{2}O)

_{m} (m = 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

_{2} clusters. Dissociative adsorption is the dominant reaction for the first two H

_{2}O adsorption reactions for n = 1, 2, and 4, for the first three H

_{2}O adsorption reactions for n = 3, and for the first four H

more » _{2}O adsorption reactions for n = 8. As more H _{2}O’s are added to the hydrated (TiO _{2})n cluster, dissociative adsorption becomes less exothermic as all the Ti centers become 4-coordinate. Furthermore two types of bonds can be formed between the molecularly adsorbed water and TiO _{2} clusters: a Lewis acid–base Ti–O(H _{2}) bond or an O···H hydrogen bond. The coupled cluster CCSD(T) results show that at 0 K the H _{2}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, ED, for the general reaction 2(-TiOH) → -TiOTi– + H _{2}O at 4-coordinate Ti centers was estimated from the aggregation reaction of nTi(OH) _{4} to form the monocyclic ring cluster (TiO _{3}H _{2}) _{n} + nH _{2}O. E _{D} is estimated to be -8 kcal/mol, showing that intramolecular and intermolecular dehydration reactions are intrinsically thermodynamically allowed for the hydrated (TiO _{2}) _{n} clusters with all of the Ti centers 4-coordinate, which can be hindered by cluster geometry changes caused by such processes. Finally by bending force constants for the TiOTi and OTiO bonds are determined to be 7.4 and 56.0 kcal/(mol·rad ^{2}). Infrared vibrational spectra were calculated using density functional theory, and the new bands appearing upon water adsorption were assigned.« less
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