Pd掺杂的Ni催化剂表面CH4解离吸附的理论研究

Theoretical study on the dissociative adsorption of CH_4 on Pd-doped Ni surfaces

使用密度泛函理论研究了Pd掺杂的Ni(111),Ni(100)和Ni(211)表面最稳定的结构,同时考察了干净的和Pd掺杂的Ni表面催化CH4解离反应的活性.结果表明,由Pd原子取代最外层Ni原子而形成的表面Pd掺杂的Ni表面在热力学上最为稳定,亚表面Pd掺杂的Ni表面在热力学上都不稳定;而对于表面Pd吸附的Ni表面,只有Pd/Ni(211)表面是稳定的.表面掺杂的Pd/Ni表面上CH4解离中间体(CH4,CH3,CH,C,H)吸附能的计算结果表明,Pd的掺杂在不同程度上减弱了除CH4之外各解离中间体的吸附能.另外,CH4和CH均优先在Ni(211)和Pd/Ni(211)台阶面上解离,其次是在比较开阔的Ni(100)和Pd/Ni(100)表面上.Pd的掺杂不同程度上提高了CH4和CH解离的能垒,对于活性最高的Ni(211)面,Pd的掺杂使得CH脱氢的能垒较CH4脱氢的高,改变了其速率控制步骤,从而抑制了积碳的生成.

Density functional theory (DFT) was employed to predict the most stable structure of Pd-doped Ni(111), Ni(100), and Ni(211) surfaces, and the activity for CH4 dissociation on pure and Pd-doped Ni surfaces. We predict that the thermodynamically most stable structures are the surface-doped Pd/Ni surfaces, where a surface Ni atom is replaced by a Pd atom; subsurface-doped Pd/Ni surfaces are thermodynamically unstable. Of the surface-adsorbed Pd/Ni surfaces, only the Pd/Ni(211) surface is thermodynamically stable. From the calculated adsorption energies of CH 4 dissociation intermediates (CH4 , CH3 , CH, C and H) on surface-doped Pd/Ni surfaces, we find Pd-doping to reduce the adsorption energy for all species except for CH4 . In addition, from the calculated activation bar-riers for CH4 and CH dissociations, we predict CH4 and CH to dissociate predominately on Ni(211) and Pd/Ni(211) step surfaces, followed by the broad Ni(100) and Pd/Ni(100) surfaces. Pd-doping raises the activation barriers for CH4 and CH dissociations. For the most active Ni(211) surface, Pd-doping causes the CH dissociation step to have a higher activation barrier than the CH 4 dissocia-tion step, which changes the rate limiting step, and helps reduce carbon deposition.