Computational study of H2 and O2 production from water splitting by small (MO2)n clusters (M = Ti, Zr, Hf).

Coupled cluster [CCSD(T)] theory and density functional theory (DFT) have been used to study the production of H2 and O2 from hydrolysis products generated from H2O addition to (MO2)n (M = Ti, Zr, Hf, n = 1-3) clusters on both the lowest singlet and triplet potential energy surfaces (PESs). H2 production occurs via the formation of an M-H containing intermediate followed by H-H recombination and H2 desorption from M(n)O(2n)(OH)2 and M(n)O(2n+2). The hydrogen transfer reactions to form the M-H bond are the rate determining steps and can be considered to be proton coupled, electron transfer (PCET) reactions with one or two electrons being transferred. Oxygen is produced by breaking two weak M-O bonds in an atomic oxygen saturated metal oxide from an M(n)O(2n)•O2 intermediate. On the triplet PES, the activation energies for the first and second H transfer to the metal are calculated to be ~10 to 50 kcal/mol and ~75 to 90 kcal/mol depending on the size of the clusters and the metal. The barriers on the singlet surface for the first and the second H transfer are predicted to be 110 to 140 kcal/mol, in general larger than the H-O bond dissociation energy. The activation barriers for the step of H-H recombination are 15 to 50 kcal/mol, and the H2 desorption energies are less than 10 kcal/mol on the singlet and triplet PESs. The oxygen desorption energies follow the order Ti < Zr < Hf for the triplets and Ti < Zr ≈ Hf for the singlets. The oxygen desorption energy is approximately independent of the size of the cluster for the same metal. The water splitting reactions prefer to take place on the triplet surface. A low excess potential energy is needed to generate 2H2 and O2 from 2H2O after the endothermicity of the reaction is overcome on the triplet PES.