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 could switch between F2+, F0 and F2- 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. Kiss, J., Witt, A., Meyer, B., Marx, D.: Journal of Chemical Physics, 2009, 130[18], 184706