A Study of the Effects of Dislocation Density and Crystal Orientation on the Kinetics of the Catalyzed Oxidation of Ethylene over a Single Crystal of Silver

Citation

Woodward, Joe William (1965) A Study of the Effects of Dislocation Density and Crystal Orientation on the Kinetics of the Catalyzed Oxidation of Ethylene over a Single Crystal of Silver. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/G7TT-6X89. https://resolver.caltech.edu/CaltechETD:etd-02102004-111444

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. A study of the effects of dislocation density and crystal orientation on the kinetics of the catalyzed oxidation of ethylene over a single crystal of silver was carried out in a continuous-flow microreactor. The reactor was constructed so the reacting gas stream contacted only gold surfaces and the surface of the silver crystal. A high-sensitivity gas chromatograph was developed and calibrated for the analysis of the exhaust gas from the reactor. The effect of diffusion to and from the catalyst surface on the observed kinetics was determined by solution of the partial differential equations which described diffusion and flow in the reactor geometry. Tests of the reactor indicated ethylene oxide was oxidized on the gold surfaces. The rate was independent of the ethylene oxide concentration and had an apparent activation energy of 9.0 ? 1.3 Kcal./gm. mole. The rate of production of carbon dioxide from the oxidation of 0 ethylene oxide at 260 [degrees] C with 80% oxygen in feed gas and a pressure of 740 mm was 2.1 X 10[^-8] gm. moles/min. [centimeter squared]. The gold surface was also active for the oxidation of ethylene With a feed of 20% ethylene and 80% oxygen at 260[degrees] C and a pressure of 740 mm, the rate of production of carbon dioxide was 2. 8 X 10[^-8] gm. moles/min. [centimeter squared] with an apparent activation energy of 26.0 ? 4.0 Kcal ./gm . mole. A feed composition of approximately 20% ethylene and 80% oxygen was used for all reactor runs with silver crystals, and the temperature was varied from 250 to 350 [degrees] C to determine the effects of dislocation density and crystal orientation on the temperature dependence of the reaction rate. No significant differences were observed in the rates of production of carbon dioxide on non-strained crystals with (100), (110), and (111) crystal planes parallel to the surface. In addition no significant differences were observed in the rates of production of carbon dioxide on crystals with (100), (110), and (111) planes parallel to the surface which had been strained 0.8% by rolling parallel to the axis at 45[degree] increments around the circumference of the crystals. An apparent activation energy of 23.1 ? 1.8 Kcal./gm. mole was obtained for the strained and non-strained crystals, but the rate decreased from 6. 3 x 10[^-8] gm. moles/min. centimeter squared for the non-strained crystals to 3. 2 x l0[^-8] gm. moles/min. centimeter squared for the strained crystals, with a feed of 20% ethylene and 80% oxygen at 260 [degrees] C and 740 mm pressure. The results of chemical etching of the crystal surfaces to reveal dislocations were not considered conclusive, but if a one-to-one correspondence between etch pits and dislocations is assumed, then the dislocation density increased about 40% with the 0.8% strain. As a result it was concluded that the dislocations did not act as active sites for the oxidation of ethylene to carbon dioxide but interfered with the oxidation reaction in some manner. Photographic materials on pp. 79-87 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

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