CO在负载型新型铜基合成甲醇催化剂上活化本质的量子化学

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

基于XPS、ESR和TPD-MS等谱学方法对CO在锰促的钢基负载型催化剂MnO—Cu/SiO_2上吸附和加氢合成甲醇的考察,本文提出了在此催化剂上锰促和非锰促两种活性中心原子簇模型,Cu_4MnO和Cu_5。应用电荷自洽离散变分X_(?)近似计算方法。对CO在Cu_4Mno和Cu_5原子簇上吸附活化的微观本质进行量子化学研究。吸附构型的计算表明CO是以碳端吸附在活性中心模型的~1Cu原子上,并向Mn^(++)倾斜与~1Cu—Mn^(++)轴的法线N成45°。对CO在两种活性中心上吸附态CO-Cu_4Mno和CO-Cu_5的分子轨道波函数、态密度以及在C和O原子上电荷集居数的计算指出,CO均受到活化,但在CO-Cu_4MnO上助催化剂MnO对改善前线轨道HOMO中Cu3d_(xz)轨道与CO2π轨道的匹配。增加Cu3d_(x(?))电子的反馈以及降低C-O间的键级等有显著的效应,促进CO的活化。

Based on the XPS, ESR and TPD-MS spectroscopies investigations into CO adsorption on Cu-bascd catalysts MnO-Cu/ SiO_2, both the Cus cluster model and the Cu_4MnO 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 Ca and Mn to the lowest unfilled CO 2π~* orbital. The relavant mechanism is also obtained for CO adsorption on the Cus cluster like Fig.1, but the π-back-donation of charge is smaller than CO on Cu_4MnO cluster. Moreover, the σ and π overlap populations between the C and O atoms of adsorbed CO on the Cu_5 cluster are less weaken than that on the Cu_4MnO cluster. The calculations also show that the occupied 2(?)~* orbital of the CO-Cu_4MnO 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.