MEIS investigations of adsorbate induced segregation at the bimetallic surfaces of single crystal alloys and supported nanoparticles

使用双金属催化剂的多相催化反应是一种广泛存在的反应，可用于生产大量高价值的工业化学品。一个例子是在负载型Pd/Au催化剂上由乙烯和醋酸反应合成乙酸乙烯单体(VAM)。近年来，世界各地的许多研究小组都在研究Au在这一反应中的促进作用。研究表明，VAM的形成反应对表面成分和双金属表面的局域原子排列极为敏感。我们已经确定，与大多数其他表面分析技术不同，中能离子散射(MEIS)即使在吸附物的影响下也能够量化表面成分。我们最近用MEIS证明了Pd/Au表面的组成本身受醋酸吸附的影响。在以前的双金属催化剂研究中，这种吸附诱导分离的现象在很大程度上被忽略了，尽管这种影响是基于热力学基础上预测的。在这个项目中，我们将使用MEIS来量化单晶PdAu表面吸附诱导偏析的温度、成分和晶面依赖关系。此外，我们将通过使用MEIS来表征在平面氧化物表面生长的双金属Pd/Au纳米粒子的合金化、烧结和吸附诱导偏析，从而增加模型系统的相关性。我们将利用扫描隧道显微镜、反射吸收红外光谱和程序升温脱附来提供关于双金属纳米颗粒的尺寸、形状和吸附性能的补充信息。

Heterogeneously catalysed reactions using bimetallic catalysts are widespread and responsible for the production of large quantities of high value industrial chemicals. One example is the synthesis of vinyl acetate monomer (VAM) from the reaction of ethylene and acetic acid over supported Pd/Au catalysts. The promoting role of Au in this reaction has been investigated by a number of research groups worldwide in recent years. It has been proposed that the VAM forming reaction is extremely sensitive to the surface composition and the local atomic arrangement at the bimetallic surface. We have established that medium energy ion scattering (MEIS), unlike most other surface analytical techniques, is capable of quantifying the surface composition even under the influence of the adsorbate. We have recently used MEIS to show that the surface composition of Pd/Au surfaces is itself influenced by the adsorption of acetic acid. This phenomenon of adsorbate induced segregation has been largely ignored in previous studies of bimetallic catalysts despite the fact that such effects are predicted on thermodynamic grounds. In this project, we will use MEIS to quantify the temperature, composition and crystal face dependence of adsorbate induced segregation at single crystal PdAu surfaces. Furthermore, we will increase the relevance of our model systems by using MEIS to characterise alloying, sintering and adsorbate induced segregation at bimetallic Pd/Au nanoparticles grown on a planar oxide surface. We will utilise scanning tunnelling microscopy, reflection absorption infrared spectroscopy and temperature programmed desorption to provide complementary information on the size, shape and adsorption properties of the bimetallic nanoparticles.