Microwave Spectroscopic Investigation of the Kinetics of the Heterogeneous Ammonia-Deuterium Exchange

Abstract

A method is described for analyzing a mixture of gases using microwave spectroscopy. Relatively simple microwave measurements are employed on single resolved collision broadened spectral lines. For binary mixtures, only the peak absorption coefficients need to be measured if the microwave collision diameters are known. Mixtures of isotopically substituted molecules can also be analyzed because the microwave absorption lines of the isotopic species are easily resolved. The technique is applied to determining the mole fraction of NH₃ in mixtures of deuterated ammonias, and to a study of the kinetics of the NH₃—D₂ isotopic exchange reaction on a singly promoted iron catalyst. It is found that rates of desorption of molecules from surfaces are markedly increased on adsorption of other molecules, a fact that can be explained only in terms of strong interactions between adsorbed molecules. The rate of the exchange reaction is proportional to the square root of the deuterium pressure. With increasing ammonia pressure the rate passes through a maximum, showing that the mechanism involves reaction between adsorbed ammonia and adsorbed deuterium atoms. The activation energy diminishes significantly with increasing ammonia pressure, providing additional evidence for the existence of strong interactions between adsorbed molecules.