The composition and structure of Pd-Au surfaces Journal of Physical Chemistry B, 2005, 109, C. W. Yi, K. Luo, T. Wei, and D. W. Goodman Bimetallic catalysts Pd-Au alloys are used for CO oxidation, synthesis of vinyl acetate monomer, hydrogenation of hydrocarbon,… The activity and selectivity of Pd catalysts can be significantly enhanced by the addition of Au. Pd and Au are completely miscible as a solid solution,so overlayers of Pd on Au(111) and Au on Pd(111) can be prepared. Surface of Pd-Au alloy differs from the corresponding bulk concentration. The Pd-Au surface has significant enrichment of the surface in Au compared to the bulk.

Low energy ion scattering spectroscopy (LEISS) is the preferred technique to analyze the surface composition as it detects only the topmost surface layer. LEISS, XPS, IRAS, and TPD are the other techniques that are used for to investigate the surface structure, the Pd-Au surface ensembles (isolated Pd surrounded by Au), and the corresponding variation in chemical properties. To facilitate the preparation of well-characterized mixed-metal compositions, a Mo(110) was used to support multilayers of Au and Pd deposited by physical vapor deposition. This study demonstrates a method for studying the surface versus bulk composition of mixed-metal phases using multilayer metal films on a refractory metal substrate.

XPS Upon annealing to 800 K, the Au 4f 7/2 and Pd 3d 3/2 peak position shift to lower binding energies (alloy formation) With a further increase in the annealing temperature, the core level of Pd 3d 3/2 shifts to higher binding energy because of desorption of Au.

ION SCATTERING SPECTROSCOPY When a beam of ions hits a solid surface part of the projectiles will be scattered back into the vacuum after one or more collisions with target atoms of the top few layers. Measurement of the energy of the backscattered particles can be used to identify the mass of the these atoms. The technique is called Ion Scattering Spectroscopy (ISS). The term encompasses actually several techniques depending on the energy of the primary ion beam. LEIS (Low Energy Ion Scattering) spectroscopy is referred to primary energies in the range of 100 eV to 10 keV.

Upon annealing to 600 K, the Pd peak intensity gradually decreases with a corresponding increase in the Au peak intensity. Au- Pd interdiffusion is clearly apparent. LEISS

Between 700 and 100 K, Au 0.8 Pd 0.2 remains constant in order to minimize the surface free energy as the surface free energy of Pd is higher than of Au. Production of this stable alloy is independent of the metal deposition sequence