Sheth Priyam A, Neurock Matthew, Smith C Michael
Department of Chemical Engineering and Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4741, USA.
J Phys Chem B. 2005 Jun 30;109(25):12449-66. doi: 10.1021/jp050194a.
The effect of alloying Pd with Ag on the hydrogenation of acetylene is examined by analyzing the chemisorption of all potential C(1) (atomic carbon, CH, methylene, and methyl) and C(2) (acetylene, vinyl, ethylene, ethyl, ethane, ethylidene, ethylidyne, and vinylidene) surface intermediates and atomic hydrogen along with the reaction energies for the elementary steps that produce these intermediates over Pd(111), Pd(75%)Ag(25%)/Pd(111), Pd(50%)Ag(50%)/Pd(111), and Ag(111) surfaces by using first-principle density functional theoretical (DFT) calculations. All of the calculations reported herein were performed at 25% surface coverage. The adsorption energies for all of the C(1) and C(2) intermediates decreased upon increasing the composition of Ag in the surface. Both geometric as well as electronic factors are responsible for the decreased adsorption strength. The modes of adsorption as well as the strengths of adsorption over the alloy surfaces in a number of cases were characteristically different than those found over pure Pd (111) and Ag (111). Adsorbates tend to minimize their interaction with the Ag atoms in the alloy surface. An electronic analysis of these surfaces shows that there is, in general, a shift in the occupied d-band states away from the Fermi level when Pd is alloyed with Ag. The s and p states also appear to contribute and may be responsible for small deviations from the Hammer-Nørskov model. The effect of alloying is more pronounced on the calculated reaction energies for different possible surface elementary reactions. Alloying Pd with Ag reduces the exothermicity (increases endothermicity) for bond-breaking reactions. This is consistent with experimental results that show a decrease in the decomposition products in moving from pure Pd to Pd-Ag alloys.(2-5) In addition, alloying increases the exothermicity of bond-forming reactions. Alloying therefore not only helps to suppress the unfavorable decomposition (bond-breaking) reaction rates but also helps to enhance the favorable hydrogenation (bond-forming) reaction rates.
通过分析所有可能的C(1)（原子碳、CH、亚甲基和甲基）和C(2)（乙炔、乙烯基、乙烯、乙基、乙烷、亚乙基、次乙基和亚乙烯基）表面中间体以及原子氢的化学吸附，以及在Pd(111)、Pd(75%)Ag(25%)/Pd(111)、Pd(50%)Ag(50%)/Pd(111)和Ag(111)表面上产生这些中间体的基本步骤的反应能量，研究了Pd与Ag合金化对乙炔氢化的影响，采用第一性原理密度泛函理论(DFT)计算。本文报道的所有计算均在25%的表面覆盖率下进行。随着表面Ag组成的增加，所有C(1)和C(2)中间体的吸附能均降低。几何因素和电子因素都导致了吸附强度的降低。在许多情况下，合金表面的吸附模式和吸附强度与纯Pd(111)和Ag(111)表面的吸附模式和吸附强度有显著差异。吸附质倾向于最小化它们与合金表面Ag原子的相互作用。对这些表面的电子分析表明，一般来说，当Pd与Ag合金化时，占据的d带态会远离费米能级移动。s和p态似乎也有贡献，可能是导致与哈默-诺尔施科夫模型出现小偏差的原因。合金化对不同可能表面基元反应的计算反应能量的影响更为显著。Pd与Ag合金化降低了断键反应的放热性（增加了吸热性）。这与实验结果一致，实验结果表明，从纯Pd到Pd-Ag合金，分解产物减少。(2 - 5)此外，合金化增加了成键反应的放热性。因此，合金化不仅有助于抑制不利的分解（断键）反应速率，而且有助于提高有利的氢化（成键）反应速率。
