BLANC - Blanc

Alliages métalliques complexes: Surfaces et Réactivité – ASURE

Submission summary

The project is a bottom-up approach designed to investigate the physical and chemical parameters influencing the surface reactivity of a new class of transition metal aluminides known as Complex Metallic Alloys (CMA). The crystallographic structure of CMA differs from that of usual alloys by the huge number of atoms in their unit cell and the occurrence of highly-symmetric clusters as alternative structural units. Specific electronic structure derives from this complexity that propagates up to the surface. To test the reactivity of CMA surfaces as a function of their structural complexity and electronic structure, a dual approach based on experimental observations supported by ab initio density functional theory (T = 0K) and semi-empirical Monte Carlo Metropolis (T # 0K) calculations has been chosen. The project can be described in a 2-dimensional framework. Horizontally, a substitution of the transition metal (TM) in complex binary phases of similar structural complexity should allow us to correlate the position of the TM d band centre with the surface reactivity. Vertically, the Al-TM system will be fixed and here the reactivity will be investigated on different phases of increasing structural complexity. Again, the position of the TM d band centre will vary as a function of local environments. To this end, five complex metallic alloys have been selected within the Al-TM (TM= Fe, Co, Cu) systems. They are the monoclinic Al13Fe4 (102 atoms/cell), the orthorhombic Al13Co4 (102 atoms/cell), the hexagonal Al5Co2 (28 atoms/cell), the cubic Al4Cu9 (52 atoms/cell) and the tetragonal Al2Cu (12 atoms/cell) samples. Experimentally, the electronic and atomic surface structures will be investigated for all samples using scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and photoemission spectroscopy. A surface atomic model will be proposed. Complementarily, the theoretical approach will determine first the bulk electronic structure then the surface energies and electronic structures of all five alloys. Then, STM images will be calculated and compared to experimental ones. In addition, bulk and surface Monte Carlo Metropolis simulations will be performed at different temperatures to investigate the possible segregation and surface reconstruction at CMA surfaces. With the above tasks completed, the initial stages of adsorption of O2, CO and NO molecules on the selected CMA surfaces will be studied. The determination of the nucleation sites and the geometry of the adsorbed molecules will be analysed using STM and ion scattering spectroscopy (ISS). The adsorption/desorption energies will be extracted from thermodesorption spectroscopy (TDS) measurements. From the theoretical side, the total energy and the adsorption energy for each of the corresponding systems will be systematically calculated. This approach will provide a map of the potential energy surface and thus identify the most favourable nucleation sites. These calculations will be compared with experimental findings. Due to the progress already made on the surface characterisation of three of those CMA surfaces, reactivity measurements and calculations can be undertaken. Consequently, all workpackages described above can run in parallel hence maximising the work done. Finally it has to be mentioned that this proposal is part of a larger effort within the European Network of Excellence CMA to investigate the potential of these new materials as catalysts. New understanding will be gained on the role of chemistry and TM d band position on the CMA surface reactivity. Hence, this study is perfectly timed with the actual scientific demand and will be at the forefront of research in this field.

Project coordination

Julian LEDIEU (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Help of the ANR 626,874 euros
Beginning and duration of the scientific project: - 36 Months

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