CO在Ni(100)表面活化及硫中毒机理的ASED—MO研究

本文应用原子交迭和电子离域-分子轨道(ASED-MO)理论,研究了CO在Ni(100)表面活化及硫中毒机理。计算结果表明,CO顶位吸附比四度中心位吸附有更大的结合能。当CO分子被吸附于顶位时,其2π反键轨道将获得0.72个电子,解离能将从自由CO分子的11.1eV降到2.15eV。当吸附于四度中心位时,2π轨道将获得1.22个电子,解离能进一步降到1.85eV。由此看来,吸附于四度中心位的CO分子将具有更大的活性。当Ni(100)面上同时存在S原子吸附时,不同相对位置CO吸附结合能的计算结果表明,一个吸附S原子将“堵塞”四个最近邻顶位和四个最近邻中心位对CO的吸附作用,而对更远一些的吸附位则没有什么影响。这些结果支持了被吸附S原子对CO吸附的影响主要是近程性“结构效应”的观点。被吸附S原子的上述“堵塞”作用,吸附S原子后CO激活吸附位的减少,及可能存在的其它因素,构成了Ni表面的硫中毒。 关键词：

A ASED-MO STUDY OF THE MICRO MECHANISM OF ACTIVATION OF CO ADSORBED ON Ni(100)AND POISONING BY THE SUBMONOLAYER S ATOM

Using an atom superposition and electron delocalization molecular orbital (ASED-MO) theory, the micro-mechanism of activation of CO adsorbed on Ni(l00) and of poisoning by submonolayer S atom is studied. CO is found to bond to the top site more strongly than on the fourfold site. When CO is adsorbed on the top site, it will get 0.72 e on its antibonding 2π orbital, and its dissociation energy will reduce from 11.1 eV for free CO molecule to 2.15 eV. If CO is adsorbed on the fourfold site, the electrons on its 2π orbital will be 1.22 e and dissociation energy 1.85 eV. It seems likely that CO adsorbed on fourfold site is more active. Our binding energy calculations have verified that one S atom adsorbed on Ni(l00) will inhibit 4 nearest top sites and 4 nearest fourfold sites from adsorbing CO molecule, but will not give any important affection on farther sites. These results support the idea that the dominant effect of adsorbed S atom is "structural effect". These "blockage" effect by the adsorbed S atom, i.e. the reduction of the active sites, and some other factors, result in the poisoning of Ni sur-faces by submonolayer S atom.