Catalytic activity, crystal structure and adsorptive properties of evaporated metal films

Abstract

Metal films of high and reproducible catalytic activity were obtained by condensation of their vapours on glass at any desired temperature. The catalytic activity was measured by the hydrogenation of ethylene. The crystal structure of these films was investigated by electron diffraction. By controlling the pressure of an inert gas (nitrogen, argon, etc.) during evaporation of the metals, unoriented and oriented films could be produced at will, and their catalytic activities were compared. Completely oriented nickel films were obtained with an inert gas pressure of 1 mm., the (110) plane, the least dense of the planes, lying parallel to the backing and the two remaining axes showing random distribution. Iron films were oriented with their (111) plane parallel to the backing, again the least dense plane thus oriented. Low-pressure adsorption bf hydrogen at room temperature and of carbon monoxide at liquid-air temperature revealed that the oriented gas-evaporated nickel films have twice the available surface per gram of randomly oriented high-vacuum films but ten times the activity. The oriented films have therefore fivefold the activity of unoriented films. Oriented films of an available surface equal to unoriented but of fivefold activity could also be obtained in high vacuum by evaporation on to oriented films previously produced by evaporation in an inert gas. The activity per unit weight of the films was constant, indicating ready accessibility to the interior of the film by the reacting gases. Adsorption of hydrogen was found to be immeasurably fast in all cases. Adsorption isotherms on nickel films were obtained for ethylene, carbon monoxide, nitrogen, hydrogen and oxygen; and the effect of catalyst poisoning by carbon monoxide and oxygen, as well as the effect of sintering, was studied. Loss of activity, decrease of hydrogen adsorption, and amount of poison were found to be proportional. The most extensive studies were made on nickel films, but films of iron, cobalt, palladium, platinum and copper were also investigated, and with the exception of copper similar results were obtained. The enhanced activity of oriented films appears to be associated with the larger distances in the (110) plane of nickel or the (111) plane of iron. The bearing of the results on the definition of active centres and on the general problem of adsorption is discussed.