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

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.