摘要
本文用EHMO法分别计算了C_2H_4与O_2,C_2H_4与Ag-O_2体系的势能面,从而得出:无Ag时,C_2H_4与O_2形成超氧化物中间体须越过一能量很高的位垒,因此反应是禁阻的;有Ag时,C_2H_4与吸附在Ag上的O_2形成的超氧化物中间体的位垒不高,因此反应是允许的。用轨道对称守恒规则的相关图进行分析,也得到相同的结论,从而对Ag能催化乙烯选择氧化为环氧乙烷的反应作出一种可能的理论解释。
The energy variations in the reaction course of ethyl ene and oxygen to form, a superoxide intermediate, with and without the presence of silver have been obtained respectively using EHMO method.
The model for calculation is as follows: the ethylene molecule is lying on XY plane with its C-C line coincident with X axis and its center of mass coincides with the origin of the coordinates; both O2 and Ag are placed on Z axis with O2 located in-between ethylene and Ag. Calculations have been performed to obtain a series of total energy of the system at different sets of distances among Ag,O, O and C2H4. Then, the potential energy surface for the reaction of ethylene and oxygen with the presence of Ag is obtained,as shown in Fig.6. On removing the Ag in the above calculation, the potential energy surface for the same reaction but with no Ag present is obtained, as shown in Fig. 5 . From Fig. 5, it is seen that the total energy of C2H4-O-O is -510.579eV and the energy at the saddle point in Fig. 5 is -504.0eV. The energy of the reactants ( C2H4 + O2 ) is -507.487 eV. Thus the activation barrier for the reaction is -504.0eV - ( -507.487 eV ) =3.50eV or 338 kJ/mol. Hence the reaction is forbidden. From Fig.6,we obtain that the total energy of C2H4 - O - O - Ag is -635.418eV, and the energy at the saddle point is -63.0eV. The energy of the reactants (Ag-O-O + C2H4 ) is -631.819eV. Thus the activation barrier is -631.0- ( -631.819) =0.8eV or 77 kJ/mol. Hence the reaction is allowed.
In order to verify the above conclusion, an analysis with the rule of orbital symmetry conservation for the reaction system has been made. The correlation diagram of orbital symmetry for the reaction of ethylene and oxygen with(Fig. 9 )and without (Fig.10)silver are given. From Fig. 9, the state correlation diagram for the reaction system is obtained, giving the activation barrier for the reaction the value 4.6eV or 443 kJ/mol. This is in accord, qualitatively, with the results from Fig. 5, the reaction is forbidden. In accord with the conclusion from Fig. 6, it may be seen from Fig. 10 that,when silver is present, reaction of ethylene with oxygen to form superoxide is allowed. The transformation of the forbidden reaction into an allowed one by introducing silver into the reaction system may be understood as follows. The d orbitals of Ag provide moderate energy levels and a symmetry suitable to the reaction, thus make a marked change in the order of the orbital levels in the correlation diagram of orbital symme-try.
出处
《分子催化》
EI
CAS
CSCD
1991年第1期50-58,共9页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金
