Exchange Reactions of Nitrogen Isotopes on Iron and Tungsten Surfaces

Abstract

The activities of iron synthetic ammonia catalysts and of tungsten filaments in promoting the reaction between N230 and N228 have been studied in the range from room temperature to 725°C with iron and to 900° on tungsten. In contrast to the exchange reaction of molecular hydrogen isotopes, the exchange reaction of nitrogen molecules is very slow, first measurable above 450°C. The reaction is accelerated by the presence of hydrogen. The evidence indicates that the exchange involves, as the slow process, either the rupture of valence bonds of absorbed molecular nitrogen on the surface or migration of nitrogen atoms over the surface, and that hydrogen accelerates such processes. The temperature coefficient of exchange in a N2 : 3H2 mixture is ∼50 kcal. The data suggest that measurements of the velocity of nitrogen adsorption at ca. 400°C on synthetic ammonia catalysts are measurements of true activated adsorption.