Structural Properties of Palladium-Manganese Alloys
Abstract: Structural properties of adsorbed overlayers on well-defined single crystals and of Palladium-Manganese surface alloys formed on Pd(100) have been investigated using synchrotron-based photoemission, scanning tunneling microscopy and Fourier-transform infrared spectroscopy. The experimental valence electronic structure of PdMn systems on Pd(100) is compared to density functional calculations (DFT). It is shown that room temperature growth and alloying of Mn on Pd(100) undergoes a two-dimensional to three-dimensional transition. Pd 3d photoemission results show that the 2D c(2x2) alloy contains subsurface Mn and the alloy was found to be terminated by a PdMn mixed layer. Total energy calculations demonstrate that a Pd-terminated Cu3Au structure is energetically more favorable, but the system is not able to reach this state at the used temperature due to kinetic effects. It could be concluded that a flat Mn monolayer formed at 90 K is anti-ferromagnetic in line with earlier predictions. When concerning the annealed 2D c(2x2) alloy, the best agreement was found for a structure with PdMn mixed layers as the two outermost layers. The changes in the NO adsorption properties of Mn modified Pd(100) have been investigated using high resolution core level photoemission. NO adsorption on the alloy films was found to destroy the c(2x2) structure. In the case of a surface partly modified by Mn, dissociation at boundaries was found to be most important. Adsorption and reactions of CO, CO2, O2 and NO on a 3D c(2x2)-PdMn surface was studied using photoelectron spectroscopy. The c(2x2) alloy structure was destroyed by O2 and NO adsorption, which involves formation of MnOx on the surface, and these aggregates are crucial for subsequent reactions. Results are also presented that demonstrate that a pseudomorphic two-layer-thick c(2x2) PdMn alloy can be grown on W(110). Investigations of small molecules adsorbed on Pd(100), Rh(111) and Mo(110) surfaces are also presented. Structural models are proposed for NO adsorption on Pd(100) and for the O and CO coadsorption system on Rh(111). The dissociation of CO adsorbed on Mo(110) was studied in detail by core level photoemission.
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