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Density functional theory calculations were performed to obtain an atomistic understanding of facet formation on Ir(210). We determined geometries and energetics of clean and oxygen-covered surfaces of planar Ir(210) as well as Ir(311) and two types of Ir(110) surfaces, which are involved in faceting by forming three-sided nanopyramids. Using the energies together with the ab initioatomistic thermodynamics approach, we studied the stability of substrate and facets in the presence of an oxygen environment. Our results show that facets are stable over the entire temperature range at which oxygen is adsorbed on the surface at coverages ≥0.45 physical ML, supporting the picture of a thermodynamic driving force. We also investigated the dependence of the phase diagram on the choice of the exchange-correlation functional and obtained qualitatively the same behavior. Finally, this work helps to better understand reactivity and selectivity of O-covered planar and faceted Ir surfaces in catalysis.

The stability of the various facets in oxygen-induced faceting of Re(123̅1) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(123̅1) is annealed at 800–1200 K in oxygen (10 −7Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (112̅1) facet competes with the (101̅1) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O−Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.

A potent greenhouse gas, trifluoromethyl sulfur pentafluoride (SF5CF3), has been discovered in the Earth's upper atmosphere. There are no known natural sources of SF5CF3, and it has been reported that high-energy electron irradiation (5keV) of condensed mixtures of SF6and CF4forms SF5CF3. This paper focuses on a search for the electron-induced formation of SF5CF3at low electron energies and for the mechanism of formation of SF5CF3. Temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD) are used to search for products formed under electron irradiation of ultrathin condensed films (∼3 ML) of SF6and CF4.The major evidence for the formation of SF5CF3via irradiation is the observation in ESD of ionic fragments containing S−C bonds, which are also seen in the mass spectra of SF5CF3. The maximum yield of SF5CF3products in the condensed film is induced by ∼8 eV electron irradiation; the mechanism is associated with low-energy dissociative electron attachment (DEA) of SF6and CF4followed by radical reactions to form the product molecules.

We report results on the catalytic oxidation of carbon monoxide (CO) over clean Ir surfaces that are prepared reversibly from the same crystal in situ with different surface morphologies, from planar to nanometer-scale facets of specific crystal orientations and various sizes. Our temperature-programmed desorption (TPD) data show that both planar Ir(210) and faceted Ir(210) are very active for CO oxidation to form CO2. Preadsorbed oxygen promotes the oxidation of CO, whereas high coverages of preadsorbed CO poison the reaction by blocking the surface sites for oxygen adsorption. At low coverages of preadsorbed oxygen (≤0.3 ML of O), the temperature Ti for the onset of CO2 desorption decreases with increasing CO coverage. At high coverages of preadsorbed oxygen (>0.5 ML of O), Ti is <330 K and is independent of CO coverage. Moreover, we find clear evidence for structure sensitivity in CO oxidation over clean planar Ir(210) versus that over clean faceted Ir(210):  the CO2 desorption rate is sensitive to the surface morphological differences. However, no evidence has been found for size effects in CO oxidation over faceted Ir(210) for average facet size ranging from 5 to 14 nm. Energetically favorable binding sites for O/Ir(210) are characterized using density functional theory (DFT) calculations.