原位红外光谱研究Ni/Al_2O_3催化剂上CO的吸附态 Ⅰ.CO的倒式吸附态

IN SITU INFRARED STUDIES OF ADSORBED CO ON Ni/AI_2O_3 CATALYST Ⅰ.THE POSSIBLE EXISTENCE OF ADSORBED“LYING CO”

CO在过渡金属上的吸附方式一般认为可分成三类:线式吸附态,桥式吸附态和多重键吸附态.这些吸附态的C—O伸缩振动具有红外活性,共振动频率分别位于～2050,～1950和～1700cm^(-1).但也有人认为CO在过渡金属表面上可能会形成倒式吸附态,即C—O键向表面倾斜,甚至平行于表面.例如,Dijk等曾推测在Ni/SiO_2催化剂上CO可能形成倒式双点吸附态(即C,O原子同时与表面成键).Krasser等用喇曼光谱研究CO在Ni/SiO_2上的吸附态时发现在330cm^(-1)处有一喇曼峰。

An unusually adsorbed CO with infrared inactive on Ni/Al2O3 catalyst was identified by in situ infrared spectia. Based on the 'metal--surface selection rule', the unusual adsorption state of CO is tentatively attributed to the adsorbed CO lying horizontally on the nickel surface of the catalyst. The side-on adsorbed CO can convex t into the upright CO (linear and bridged CO) when adsorbed hydrogen is present and can be recovered after pumping the adsorbed hydrogen, i. e. the transformation of the adsorbed lying CO is reversible. The adsorbed lying CO can be formed at room temperature and it is difficult to be removed even at 190 C in vacuo. It is believed that the lying CO may be a common species which can be formed on transition metal surface at least on nickel surface.