Adsorption and Nitrogen Isotope Exchange on Metallic Osmium Catalysts

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Abstract
Osmium catalysts prepared by decomposition‐reduction of ammonium osmium chloride with hydrogen at 573°K show activated adsorption of hydrogen from 80 to 537°K. At the lowest temperature the adsorption is in part van der Waals adsorption. Van der Waals adsorption of nitrogen on the same catalyst decreases continuously from 80°K. The nitrogen isobar at 1 atmos. pressure shows a minimum at 350°K and then shows increasing adsorption to a maximum at 428°K. The increase is attributed to activated adsorption of nitrogen which velocity measurements confirm. From a nitrogen adsorption isotherm at 80°K the area per gram of catalyst is computed to be 30.8 square meters. Only 4 percent of this area is covered at 1 atmos. pressure by nitrogen at 428°K. The nitrogen adsorption is less than 10 percent of the hydrogen adsorption at 1 atmos. pressure in the temperature range of 273–573°K. Such osmium catalysts show progressive sintering when heated at 673°K. The nitrogen isotope exchange reaction N228+N230 = 2N229 on these catalysts is just measurable at 473°K and very rapid at 573°K, with an activation energy of 21.8 kcal. The reaction is not second‐order and is measurably inhibited by even 3 percent of hydrogen in the gas. With hydrogen concentrations of 50 percent and upwards there is no measurable exchange at 550°K. Oxygen poisons the exchange reaction which has an activation energy of 26 kcal. on the poisoned catalyst. Ammonia decomposes on the catalyst at 573°K at least 6 times as fast as the exchange reaction occurs, yielding an N2 : 3H2 mixture. The data on exchange point to a mechanism in which the slow process is the migration of atomically bound nitrogen on a surface on which active centers are few. Repeated adsorption and desorption of nitrogen from configurationally more complex active centers is a possible alternative mechanism for the exchange. Experiment cannot yet decide whether the breaking of bonds in the adsorption of the nitrogen molecule is the rate‐determining process.

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