Computational Study of
the Hydrolysis Reactions of
Small MO<sub>2</sub> (M = Zr and Hf) Nanoclusters with Water
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Abstract
Density functional theory (DFT) has been used to study
the hydrolysis
reaction of (MO<sub>2</sub>)<sub><i>n</i></sub> (M = Zr,
Hf, <i>n</i> = 1–4) nanoclusters in the ground singlet
and first triplet states. The reactions for singlet <i>n</i> = 1 were benchmarked at the CCSD(T) level of theory. The reactions
of H<sub>2</sub>O with the metal site having an MO bond and/or
M–O bonds as well as H transfer to both terminal O
atoms and bridge −O atoms have been studied. The partial charge
on M increases as the MO bonds are replaced with M–OH
bonds. The first H<sub>2</sub>O adsorption (physisorption) energies
for these MO<sub>2</sub> nanoclusters are calculated to be −20
to −30 kcal/mol for the singlet state and −15 to −48
kcal/mol for the triplet state. These physisorption energies depend
on the cluster size and the adsorption site, consistent with existing
experimental and computational studies. The first hydrolysis (dissociative
chemisorption) reaction energies of the MO<sub>2</sub> nanoclusters
are calculated to have a much broader range, −30 to −80
kcal/mol for the singlet states and −30 to −100 kcal/mol
for the triplet states. Steric effects play an important role in determining
the physisorption and chemisorption energies, especially for the trimers
and tetramers. The potential energy surfaces for hydrolysis in both
the singlet and triplet states are calculated. The calculated Lewis
acidities (fluoride affinities) correlate with the hydrolysis properties
of the nanoclusters. Our calculations show that H<sub>2</sub>O readily
reacts with both the singlet and triplet states of the MO<sub>2</sub> nanoclusters to form the hydroxides. The reaction barriers are generally
less than 10 kcal/mol for the singlet states, and because the H<sub>2</sub>O physisorption energies are large, the barriers occur below
the (MO<sub>2</sub>)<sub><i>n</i></sub> asymptote
