Surface kinetics and thermodynamic processes
- Oxygen-ruthenium by Lattice-gas Hamiltonian
- Oxygen-silver(111) by ab inito thermodynamics
Oxygen-ruthenium by Lattice-gas Hamiltonian
Towards a First-Principles Theory of Surface Thermodynamics
Understanding of the complex behavior of particles at surfaces requires detailed knowledge of both macroscopic and microscopic processes that take place; also certain processes depend critically on temperature and gas pressure. To link these processes we combine state-of-the-art microscopic, and macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001) system and calculate thermal desorption spectra, heat of adsorption, and the surface phase diagram. The agreement with experiment provides validity for our approach which thus identifies the way for a predictive simulation of surface thermodynamics and kinetics. The outline is sketched below: [C. Stampfl, H.J. Kreuzer, S.H. Payne, H. Pfnüur, M. Scheffler, Phys. Rev. Lett. 83, 2993 (1999)]
Lattice gas Hamiltonian: the E are the adsorption energies of the isolated O atom in the fcc and hcp sites and the V are the interaction parameters between neighboring O atoms.
Calculated adsorption structures: for purely fcc sites as well as purely hcp sites and hcp-fcc mixtures.
The
results, namely (left) the heat of formation at different temperatures
as a function of coverage, and (right) the calculated and experimental
temperature programmed desorption spectra (TPD). The peaks and dips in
the heat of adsorption indicate the energetic preference for the
formation of ordered phases with coverages 0.25, 0.5, 0.75, 1ML as in
complete agreement with experiment. The TPD spectra show a clear shift
to lower temperatures for higher O coverages reflecting the strongly
repulsive O-O interactions.
Oxygen-silver (111) by ab initio thermodynamics
Why is a noble metal Catalytically Active? The role of the O-Ag interaction in the function of silver as an oxidation catalyst
Extensive density-functional theory calculations, and taking into account temperature and pressure through consideration of the oxygen chemical potential, affords a comprehensive picture of the behavior and interaction of oxygen and Ag(111), and provides valuable insight into the function of silver as an oxidation catalyst. The obtained phase-diagram reveals the most stable species present in a given environment and thus identifies (and excludes) possibly active oxygen species. In particular, for the conditions of ethylene epoxidation, a thin oxide-like structure is most stable (orange region in the below phase diagram), suggesting that such atomic O species are actuating the catalysis, in contrast to hitherto proposed molecular-like species. [W.-X. Li, C. Stampfl, and M. Scheffler, Phys. Rev. Lett. in press.]
The temperature-pressure phase diagram showing the energetically most favorable structure for a give pressure and temperature: from red to blue, bulk disilver oxide, thin (4x4) surface oxide, low coverage of chemisorbed oxygen, and the clean Ag(111) surface.
The same as the above P-T phase diagram except showing more structures and only two pressures are considered, namely, those corresponding to typical ultra high vacuum (UHV) conditions and to atmospheric pressures, typical of catalysis. The colored panels indicate the regions of phase stability.
