I. A Study of the Effects of Dislocations in Silver on the Rate of Oxidation of Carbon Monoxide. II. A Mechanism for the Formation of Dislocations during the Oxidation of Zinc

Citation

Miller, John Wayne (1976) I. A Study of the Effects of Dislocations in Silver on the Rate of Oxidation of Carbon Monoxide. II. A Mechanism for the Formation of Dislocations during the Oxidation of Zinc. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PDYT-7V46. https://resolver.caltech.edu/CaltechTHESIS:09252017-162443679

Abstract

I. Effect of Dislocations in Silver on the Rate of Oxidation of Carbon Monoxide

An investigation was made to determine the effects of the number of dislocations in a silver catalyst on the rate of oxidation of carbon monoxide. The reaction was catalyzed by the (111) face of several silver crystals which had a dislocation density of either 10⁴ cm^-2 or 10⁸ cm^-2. Activation of the catalyst was accomplished by alternate treatments of hydrogen and oxygen at 800°F.

The reaction was carried out in a pyrex recirculation reactor with a reactant mixture of 75% carbon monoxide and 25% oxygen. Gas chromatography was utilized to analyze the reactor gases, and about 1% carbon dioxide was produced at 170°F during the time of reaction. Reactor composition data were fitted as a function of time to an expression which was developed from a proposed model of the surface kinetics. The model used the assumptions that there existed adsorption equilibrium and that the rate of formation of carbon dioxide was proportional to the number of carbon monoxide-oxygen sites on the surface.

Results of the study indicated that the greatest change to the rate of reaction was caused by impurities which adsorbed on the catalyst surface. During the study the chemical-etch-pit count of silver was increased after pretreatment with oxygen. However, other measurements showed the dislocation density had remained the same, so comparison of the reaction rates was made for crystals with different initial dislocation densities. This comparison indicated that dislocations did not alter the rate of reaction, thus dislocations were not considered to be the most important catalytic sites under the conditions of this study.

II. A Mechanism for the Formation of Dislocations during the Oxidation of Zinc

A study was made of the changes which occurred to an oxidized zinc crystal. It was determined that the normal oxide film was thin and epitaxial but this film would form many micro-cracks when exposed to an aqueous environment. The rate of subsequent oxidation of the crystal was increased because the cracks formed low energy paths for the zinc to get to the surface. The increased oxidation rate also increased the number of the vacancies in the solid and the vacancies in excess of the thermodynamic value formed dislocations. The dislocations were in the shape of loops and spirals, and were presumed to result from the coalescence of the excess vacancies. Berg-Barrett topography was utilized to view the dislocations and to investigate the Burgers Vector. The information from both the experimental data and the calculations of the configuration with the lowest energy suggested that the Burgers Vector was a full c.

As an addendem to the work on zinc, a discussion is included of a chemical etching solution that was developed to reveal dislocations which intersected the basal plane. The chemical solution worked well for surfaces whose orientation was within 0.5 of the basal plane. Evidence was presented which suggested a one-to-one correspondence between dislocations and etch pits.

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