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Title: Non-linear effects in adsorbate-induced restructuring : processes on Pt{100} and Ir{100}
Author: Ali, T.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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The quantitative results from a recent molecular beam study have been used to formulate an improved model which accurately reproduces the observed kinetic oscillations during CO oxidation on Pr {100}. The model includes an experimentally determined power-law which manifests as a flux-dependent sticking probability describing the CO-induced lifting of the hex reconstruction. The calculations reveal that the inclusion of the non-linear power-law in the modelling is critical in order to accurately reproduce the experimentally observed oscillations. The supersonic molecular beam scattering apparatus was used to extend the recent studies in adsorbate-induced restructuring processes on Pr{100} to the structurally similar Ir{100} surface. To this effect, the adsorption kinetics of CO, oxygen and hydrogen chemisorption on both the (1 x 5) and (1 x 1) surfaces of Ir{100} have been studied. Due to competition between desorption from the (1 x 5) phase and growth of (1 x 1) islands, the sticking probability of CO on the initial Ir{100}-(1 x 5) surface is strongly flux dependent at surface temperatures in the range 480 - 510 K. It is shown that, as with Pt{100}, this is due to a strongly non-linear dependence of the growth rate on the local CO coverage on the (1 x 5) substrate, with an apparent reaction order of around 5. The heat of adsorption of CO on both surface phases of Ir{100} has been determined. The zero coverage adsorption energy on the (1 x 1) substrate is 196 kJ/mol and on the (1 x 5) substrate it is around 150 kJ/mol. The formation of (1 x 1) islands during oxygen adsorption on Ir{100} occurs between two limiting surface coverages. Between 350 and 600 K the local oxygen coverage on the (1 x 1) is about 0.28 ML during the prevailing phase transformation, whereas it is 0.20 ML in the temperature range 700 to 900 K. This "biphasic" behaviour is explained by the enhancement of surface diffusion at temperatures above 650 K. The adsorption kinetics of oxygen on Ir{100} at 1080 K also how a pronounced flux dependence of the sticking probability due to a non-linear growth law for the formation of (1 x 1) islands. The average apparent reaction order for this process is around 4.5.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available