采用广义梯度近似(GGA)的密度泛函理论(DFT)并结合平板模型,研究了CH_4在清洁Pd(111)及O改性的Pd(111)表面发生C-H键断裂的反应历程.优化了裂解过程中反应物、过渡态和产物的几何构型,获得了反应路径上各物种的吸附能及反应的活化能.结果表明,CH_4采用一个H原子指向表面的构型在Pd(111)表而的顶位吸附,CH_3的最稳定的吸附位置为顶位,OH,O和H的最稳定吸附位置均为面心立方.CH_4在清洁Pd(111)表面裂解的活化能为0.97eV,低于它在O原子改性(O没有参与反应)的Pd(111)表面的活化能1.42eV,说明表面氧原子抑制了CH_4中C-H键的断裂.当亚表而O原子和表面O原子(O参与反应)共同存在时,C-H键断裂的活化能为O.72eV,低于只有表层氧存在时的活化能(1.43 eV),说明亚表面的O原子对CH_4分子的活化具有促进作用.CH_4在O原子改性的Pd(111)表面裂解牛成CH_3和H,以及生成CH_3和OH的反应活化能分别为1.42和1.43 eV,说明CH_4在O原子改性的Pd(111)表面发生这两种反应的难易稗度相当.
The reaction pathway of C-H bond breaking of methane on the clean Pd(111)and O-modified Pd(111)surfaces was investigated by the fwst-principles density functional theory generalized gradient approximation calculations with the slab model.Geometries of reacrants,transition states,and products were calculated.Adsorption energy of possible species and activation energy barriers of the reaction were also obrained.The calculated results show that methane favors such a configuration that one hydrogen points towards the surface in the top site.Methyl is adsorbed in the top site,and hydroxyl,oxygen,and hydrogen are all adsorbed in the fee site.On the clean Pd(111)surface,the activation energy of 0.97 eV is smaller than that of 1.42 eV in the case ofoxygen-modified(oxygen atom acts as a"spectator")Pd(111) surface.which indicates that the presence of oxygen atom inhibited the C-H bond cleavage.Compared with the case that only the surface oxygen atom exists(oxygen atom participates in the reaction),the activation energy decreased from 1.43 to 0.72 eV when the subsurfaceoxygen atom exists.This suggests that the subsurface oxygen atom promotes the activation of methane molecule.On the oxygen-modifiedPd(111),the activation energy of the reactions forming methyl and hydrogen,and methyl and hydroxyl is 1.42 and 1.43 eV,respectively,which indicates that the reaction possibility is equivalent.
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