Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany.
Phys Chem Chem Phys. 2011 Jun 14;13(22):10741-54. doi: 10.1039/c1cp00009h. Epub 2011 May 6.
The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.
采用程序升温脱附（TPD）和高分辨率电子能量损失谱（HREELS）研究了结构明确的 PdAg/Pd(111) 表面合金与 CO 的相互作用，以揭示和理解电子应变、电子配体和几何整体效应的贡献。TPD 测量表明，CO 在表面合金的 Ag 位上不能吸附（在 120 K 时），并且 CO 在 Pd 位上的结合强度随 Ag 浓度的增加而显著降低。与之前关于表面合金中 Pd 和 Ag 原子分布的扫描隧道显微镜（STM）数据进行比较表明，这种修饰主要是由于几何整体效应，因为在非修饰的 Pd(111) 上，CO 吸附的首选 Pd(3) 团簇不再可用于富 Ag 表面。因此，随着 Ag 含量的增加，CO 的优先吸附位从 Pd(3)三聚体通过 Pd(2)二聚体变为 Pd 单体，CO 吸附的强度也随之逐渐减弱。此外，表面合金的 CO 吸附性质也受到电子配体和应变效应的影响，但影响较小。结果与之前关于 PdAg 体相合金、负载型 PdAg 催化剂和 PdAu/Pd(111) 模型体系的研究结果进行了讨论。
