Using the Zn_4O_4 clusters as model and combining quantum chemical calculations with counterpoise method, we study the states of CO absorbed on polar (0001), (000-1) and nonpolar (10-10) surfaces of ZnO and evaluate binding energies of the states. It can be infered that the cations Zn~(2+) on any surface of ZnO are the strongest active sites and the bond which carbon atom coordinated to a surface Zn is the strongest one, which are in accordance with experimental results. Although the similar binding energies for all surfaces of ZnO were found in experiments by some researchers, which might be attributed to that initially CO populates preferentially defect sites. A weak interaction between absorbent and adsorbate can be described well by counterpoise-method. No binding for either end of the CO molecule coordinates to a surface oxygen atom of ZnO. When the CO molecule is parallel to the surface of ZnO and coordinates to the surface oxygen atom, there occurs a weak binding only.

Key words: CARBON MONOXIDE, ZINC OXIDE, CRYSTALS, ADSORPTION, AB INITIO CALCULATION