Kinetics of isotope exchange between hydrogen and liquid ammonia on heterogeneous catalysts

Abstract

The isotope exchange between hydrogen and liquid ammonia has been studied in the temperature range between −60 ° and +25 °C at pressures up to 150 atm in the presence of various catalysts, in particular supported platinum catalysts. Comparative exchange experiments were carried out with gaseous ammonia at pressures from 25 Torr up to the saturation pressure (8.7 atm at 20 °C). (1) Group eight metals catalyze the H D exchange with liquid ammonia even at temperatures below −60 °C. The activity decreases in the order: Pt, Pd, Ni, and Fe. (2) For a platinum-carbon catalyst suspended in liquid ammonia the rate of exchange increases proportionally to the amount of catalyst in the range from 10 to 100 g/liter. (3) Platinum catalysts supported on carbon and on silica gave the same exchange constants per unit weight and resulted in the same activation energy irrespective of the widely different surface areas. (4) The following apparent activation energies were obtained: Pt C , 10.0 ± 0.5; Pt SiO 2 , 9.6 ± 0.6; Pd C , 11.3 ± 0.7; Raney-Ni, 11.5 ± 0.8 kcal/mole. (5) Comparative measurements with potassium amide indicated that this homogeneous catalyst has a considerably higher activity and a lower activation energy (5.4 ± 0.6 kcal/mole) than the heterogeneous catalysts. Contrary to exchange on these catalysts transport processes are rate determining in the potassium amide system under the conditions investigated. (6) While the rate of exchange in liquid ammonia increases proportionally to the hydrogen pressure in the case of potassium amide a dependence on the square root of the hydrogen pressure has been found for the platinum/carbon catalyst. (7) The experimental results support the following mechanism: Isotope exchange occurs between chemisorbed hydrogen atoms and a chemisorbed ammonia molecule, i.e. according to a Langmuir-Bonhoeffer-Hinshelwood mechanism. In principle the mechanism is the same in the gas as well as in the liquid phase. The number of atoms exchanged per unit time is, however, considerably smaller in the liquid, since a large portion of the surface is blocked up for hydrogen chemisorption by adsorbed ammonia molecules. If any, there is only a very small contribution from an ionic mechanism to the total exchange in liquid ammonia.