Quantum Chemstry Study on the Nature of the CO and H₂ Activation on CuO-ZnO Catalysts for Methanol Synthesis

In this paper, the SCC-DV-X_α（Self-Consistent-Charge-Decrete-Variation-X_α） quantum chemical calculations were used to study the nature of activation of CO and H₂ on CuO -ZnO catalysts for methanol synthesis. Based on the XPS, EPR and TDP—MS investigations into CO and H₂ activation adsorption, the model of CO adsorption on the cluster of CuO-ZnO and the model of H₂ adsorption on the cluster of （10 \overline1 0） surface of ZnO have been proposed. The results show that CO is terminelly bonded to the Cu of CuO-ZnO cluster and its activation is caused through 0.30 electrons transfer from the molecular orbitals of CO to the Cu and back-donation 0.23 electrons from Cu to the lowest unfilled orbital 2π^* of adsorbed CO molecule. Moreover, DV-X_α calculations for the Zn₈O₈ clustre model on the （10 \overline1 0） surface of ZnO for the adsorption of hydrogen species proposed show that the state density of Zn₈O₈ is in good agreement with known UPS results. The LUMO of cluster model is mainly made of surface states Zn4s+4p, which is made of linear combination of ss hybridized orbitals centerred on the surface Zn²⁺ ions and directed away from the surface along tetrahedral direction and the HOMO is mainly made of 2p_x+2p_y+2p_z surface orbital on O^2-, both frontier orbitals play a role in H^- or H⁰ adsorption on Zn²⁺ and H⁺ or H⁰ adsorption on O^2-. The calculation also show that H₂ adsorbs heterlytically on ZnO, yielding Zn²⁺-H^- and O^2--H⁺, which is consistest with the infrared spectra investigations.