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
<p>I. Effect of Dislocations in Silver on the Rate of Oxidation of Carbon Monoxide</p> <p>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)...
| Summary: | <p>I. Effect of Dislocations in Silver on the Rate of Oxidation of Carbon Monoxide</p>
<p>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 10<sup>4</sup> cm<sup>-2</sup> or 10<sup>8</sup> cm<sup>-2</sup>.
Activation of the catalyst was accomplished by alternate treatments of
hydrogen and oxygen at 800°F.</p>
<p>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.</p>
<p>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.</p>
<p>II. A Mechanism for the Formation of Dislocations during the Oxidation of Zinc</p>
<p>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.</p>
<p>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.</p>
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