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First-principles calculations of the initial incorporation of carbon into flat and stepped Pd surfaces

机译:最初将碳掺入平坦和阶梯式Pd表面的第一性原理计算

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We employ density-functional-theory calculations to examine carbon adsorption and diffusion in Pd bulk, and on Pd(111) and Pd(211) surfaces. Different possible subsurface and on-surface structures are explored and the most stable structures are analyzed. We calculate various diffusion paths: lateral diffusion on a surface, migration to a subsurface region, and within the first interlayer. Our calculations show in accordance with the earlier theoretical results that on Pd(111) carbon prefers to adsorb on octahedral interstitial sites. On Pd(211) the fourfold hollow site under the step is energetically the most favorable one and the second best sites are the octahedral sites. The calculations indicate that migration into the first interlayer is more favorable than diffusion on the Pd(111) surface and migration into the second interlayer is already highly activated with barrier height close to those obtained in bulk. Nearly nonactivated diffusion paths into the first interlayer are found at the step edge of bare Pd(211) but carbon is found to diffuse easily from the first interlayer to the fourfold hollow site on Pd(211) thus leading to the decoration of step edges with carbon. Preadsorbed carbon increases the surface-subsurface diffusion barrier but it remains smaller than the corresponding value on bare Pd(111). At higher carbon concentration the mixed surface-subsurface structures are the most stable ones and carbon atoms tend to sit as far away from each other as possible.
机译:我们采用密度泛函理论计算来检查碳在Pd块中以及Pd(111)和Pd(211)表面上的吸附和扩散。探索了不同的可能的地下和地面结构,并分析了最稳定的结构。我们计算各种扩散路径:表面上的横向扩散,迁移到地下区域以及第一夹层内。我们的计算根据较早的理论结果表明,在Pd(111)上,碳更喜欢吸附在八面体间隙位置上。在Pd(211)上,台阶下的四个空心位置在能量上是最有利的一个,次佳的位置是八面体位置。计算表明,向第一中间层的迁移比在Pd(111)表面上的扩散更有利,并且向第二中间层的迁移已经被高度激活,并且势垒高度接近于本体获得的势垒高度。在裸露的Pd(211)的台阶边缘发现几乎未激活的扩散路径进入第一中间层,但发现碳易于从第一中间层扩散到Pd(211)上的四重空心位置,从而导致台阶边缘的装饰碳。预先吸附的碳增加了表面下表面的扩散壁垒,但仍小于裸Pd(111)上的相应值。在较高的碳浓度下,混合的表面-地下结构是最稳定的结构,并且碳原子趋于彼此尽可能远离。

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