Oxygen vacancies on ZnO(00•¯1) were proposed to be the catalytically active sites

for methanol synthesis on pure ZnO. The charge state and thus the chemical

reactivity of such vacancies on this polar O-terminated basal plane of ZnO was

expected to be intimately connected to the degree of its hydroxylation in view of its

Tasker type(3) unstable character. Here, the interplay between hydrogen adsorption

and the thermodynamic stability of O vacancies in various charge states,

corresponding formally to F2+, F+, F0, F− and F2− centers, was investigated using

electronic structure calculations. Assuming thermodynamic equilibrium of the

defective surface with a hydrogen containing gas phase the thermodynamically

most stable O vacancy type was determined as a function of temperature and

pressure. For the adsorption of H2 molecules at O vacancy sites it was found that

the homolytic process leads to energetically more favorable structures than

heterolytic adsorption and hydride formation. By homolytic adsorption and

desorption one can switch between F++, F0, and F−− or between F+ and F−, a process

which was believed to occur during methanol synthesis. However, the barrier for

heterolytic dissociation of H2 at O vacancies was significantly lower compared to

homolytic cleavage. Furthermore, the barrier for transforming hydridic hydrogen,

i.e., ZnH species, to protonic hydrogen, i.e., OH species together with a reduction

of ZnO itself, was quite high. This implied that hydridic H− species created as a

result of heterolytic dissociation might have a long enough lifetime at O vacancies

that they will be available for methanol synthesis. ZnH and OH vibrational

frequencies were computed in order to assist future experimental assignments.

Methanol Synthesis on ZnO(000¯1) - I, Hydrogen Coverage, Charge State of

Oxygen Vacancies and Chemical Reactivity. J.Kiss, A.Witt, B.Meyer, D.Marx:

Journal of Chemical Physics, 2009, 130[18], 184706