J. T.KUMMER
666
Vol. 60
THE CATALYTIC OXIDATION OF ETHYLENE TO ETHYLENE OXIDE OVER SINGLE CRYSTALS OF SILVER BY J. T. KUMMER The Dou, Chemical Company, Midland, Mich. Received November 86,1066
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 31, 2015 | http://pubs.acs.org Publication Date: May 1, 1956 | doi: 10.1021/j150539a039
The chemical efficiency of ethylene oxidation and to a lesser extent the absolute rate of reaction do not appear to depend on the type of crystal face used as the catalyst surface. Sulfur or halogen atoms on the surface of silver do increase the chemical efficiency of the reaction. The absolute rate of reaction per cm.2 of surface and the energy of activation can be used t o substantiate Twigg's proposed mechanism.
Twigg'~'.~ work on the catalytic oxidation of ethylene has shown that the rate of oxidation of ethylene to carbon dioxide and water is proportional to the square of the concentration of chemisorbed oxygen atoms on the silver surface, which implies that a pair of oxygen atoms are involved, whereas the oxidation of an ethylene molecule to ethylene oxide involves only a single oxygen atom on the surface. Twigg's work leads one to the conclusion that the spatial separation of the chemisorbed oxygen atoms is important in determining the ratio of ethylene oxide to carbon dioxide in the reaction products. If this is so then it would be of interest to study the reaction using different crystallographic planes of silver as the catalytic surface for which the spatial separation of 0 atoms during reaction would be anticipated t o be different on the different crystal faces. This paper describes some experiments in which single crystals of silver having different crystallographic surfaces have been used for the catalytic oxidation of ethylene. Contrary t o expectations all of the crystals gave close to the same absolute rate of total oxidation per square centimeter of surface and the same relative amounts of ethylene oxide and carbon dioxide in the product. However, when the crystal surface is partially poisoned by sulfur or chlorine atoms, there is an increase in the ratio of ethylene oxide to carbon dioxide in the product together with a decrease in the total rate of reaction per square centimeter of surface. Experimental Catalyst Description.-Since commercial catalysts for the oxidation of ethylene to ethylene oxide have small silver surface areas (less than 2 m.2 per gram of silver), i t was thought that small flat sheets of silver would be sufficiently active so that the rate of oxidation of ethylene to the oxide could be conveniently measured. This is the case and a clean silver sheet 1.5 cm. wide and 10 cm. long will cause -5 cc. of ethylene to react with oxygen during one hour at 210". The surface must be free of sulfur and halogens. Se arate experiments on the stability of the surface silver sufide as a function of [HzS]![H~] in the gas phase have shown that reduction of the silver sheet with hydrogen a t temperatures greater than 450" is necessary in order to remove sulfur from the surface in a reasonable time. I n the experiments described here a reduction temperature of 570' was used. Chlorine is more readily removed than sulfur. Fortunately the catalyst is not poisoned by oxyor water, or carbon dioxide, a t reaction temperature. he 35 cm.4 of silver surface of the sheet requires -0.001 cc. S.T.P. of a gas with one sulfur atom per molecule in order to give each silver atom on the surface a sulfur atom. Since -20 cc. S.T.P. of ethylene and -30 cc. S.T.P. of oxygen are used per run, it can be seen that gases of com-
y,
(1) G. H.Twigg, Tram. Faraday Soc., 4a, 284 (1946). (2) W . H. Langwell, ibid., 42, 290 (1946).
mercially available purity (
