Quantum Chemistry Study on the Mechanism of the CO Activation aver Cu-based Catalysts for Methanol Synthesis

Based on the XPS, ESR and TPD-MS spectroscopies investigations into CO adsorption on Cu-bascd catalysts MnO-Cu/ SiO₂, both the Cu₅ cluster model and the Cu₄MnO cluster model for the CO adsorbed on the active center of catalysts have been proposed The nature of CO activation on this catalysts for methanol synthesis has been studied with the DV-X_α quantum chemistry calculations.;
The calculations of the total energy of adsorption system show that the CO molecule is linearly bonded to the Cu atom in a minner shown in Fig.1, in which the CO is titled with its O atom toward the Mn atom, forming an angle of 45° with normal of the Cu-Mn bond. The activation of CO on Cu MnO cluster is caused through 0.58 electrons transfer from the σ molecular orbitals of CO to the Cu and Mn atoms and back-donation of 0.66 electrons from the Cu and Mn to the lowest unfilled CO 2π^* orbital. The relavant mechanism is also obtained for CO adsorption on the Cu₅ cluster like Fig.1, but the π-back-donation of charge is smaller than CO on Cu₄MnO cluster. Moreover, the σ and π overlap populations between the C and O atoms of adsorbed CO on the Cu₅ cluster are less weaken than that on the Cu₄MnO cluster. The calculations also show that the occupied \tilde \pi^* orbital of the CO-Cu₄MnO cluster is primarity formed by the mixing of CO 2π^* orbital with Cu3d and Mn3d orbitals, the main atomic components are Cu 1.21%, Mn: 79.21%, C: 2.46% and O: 9.81%, i.e., the 2π^* orbital of adsorbed CO has some electrons. The above results indicate that the effects of promoter MnO on CO activation improve the ability of metal atoms to backdonation electrons into 2π^* orbital of adsorbed CO molecule.