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LEED crystallographic studies for chemisorption on rhodium and zirconium surfaces

University of British Columbia

The work in this thesis includes crystallographic investigations with low-energy electron diffraction (LEED) for the surface structures designated Rh(111)-(√3x√3)30°-S, Rh(111)-(2x2)-0, Zr(0001)-(1x1)-0 and Zr(0001)-(1x1)-N. In each case intensity-versus-energy (I(E)) curves for a set of diffracted beams were measured with a video LEED analyzer, and then compared with the results of multiple scattering calculations made for various structural models. Levels of correspondence between experimental and calculated 1(E) curves were assessed with the reliability index proposed by Pendry, and surface geometries were determined by the conditions for the best correspondence. The LEED intensity analyses for both the Rh(111)-(√3x√3)30°-S and Rh(111)-(2x2)-0 surface structures indicate that S and 0 atoms adsorb respectively 1.53A and 1.23A above the "expected" hollow sites of three-fold coordination. These values correspond to nearest-neighbor Rh-S and Rh-0 bond distances equal to 2.18 and 1.98A respectively. For the Zr(0001)-(1x1)-0 and Zr(0001)-(1x1)-N surface structures studied, the multiple-scattering analyses suggest that the first involves 0 atoms occupying octahedral holes between successive bulk Zr layers, and that the substrate Zr layers undergo a fee type reconstruction. By contrast the N atoms in Zr(0001)-(1x1)-N appear to just occupy octahedral holes between the first and second layers of hep zirconium, exactly as reported by Shih et al. for the analogous structure formed on titanium. The LEED-determined Zr-0 and Zr-N bond distances are 2.30 and 2.27A respectively, in very close agreement with the values determined by X-ray crystallography for bulk ZrO (2.31A) and bulk ZrN (2.29A). A preliminary study of oxygen chemisorption on the Zr(0001) surface has been made in the low-exposure regime with Auger electron spectroscopy (AES) and with measurements of the width of a half-order LEED beam. Some observations and conclusions are: (i) the diffusion of 0 atoms to the bulk effectively starts at around 236°C; (ii) oxygen adsorbs in a disordered state at room temperature but orders sufficiently to show a (2x2)-type LEED pattern on heating to 220°C; (iii) with increasing 0 exposure, 1/4, 1/2 and 3/4 of the available sites can be systematically filled, prior to the establishment of an ordered (1x1)-0 surface; (iv) the process in (iii) can be reversed by starting with the (1x1)-O surface and heating above 236°C. LEED and AES have also been used to compare the adsorption and coadsorption of 0₂ and H₂S on the Zr(0001) surface for exposures in the one to five Langmuir regime. The new observations made are: (i) sulfur forms a stable (3x3) surface structure after heating to 600°C; (ii) the Zr(0001) surface with high 0 coverage can still adsorb H₂S, whereas the Zr(0001) surface with high S coverage does not adsorb oxygen in detectable amounts; (iii) for surfaces with adsorbed H₂S the 150 eV to 92 eV Auger peak ratio suddenly increases on heating to 530°C. Observation (iii) has been tentatively interpreted in terms of hydrogen desorption. Finally, a set of 1(E) curves were measured for normal incidence on the Zr(0001)-(3x3)-S surface.

Text

2010-08-20

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http://hdl.handle.net/2429/27568

Chemistry

University of British Columbia

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