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The decomposition of ammonia in the presence of hydrogen, catalyzed by evaporated iron films, was followed using a mass spectrometer. With reaction mixtures of small ammonia content, initial rates of decomposition to hydrogen and nitrogen, without apparent loss of the latter to the catalyst, gave an activation energy of 38.8 kcal/mole and were proportional to pK&sp,Fp",.Reaction mixtures having a higher ammonia pressure reacted with the iron catalyst to form the nitride Fe4N, confirmed by X-ray diffraction analysis. Activation energies for the formation 38, decomposition 38.6, and hydrogenation 14 kcal/mole, of the nitride phase were obtained. From this information and the kinetic data, desorption of nitrogen can be identified as the rate-determining step in ammonia decomposition.Where iron nitride was formed, the influence of temperature on the rate of production of nitrogen showed three distinct regions corresponding to decomposition of ammonia on iron, on a surface which is becoming increasingly nitrided, and finally on an iron nitride surface. The results obtained are discussed in relation to the anomalous behaviour reported for the bulk catalyst no. 954 used by Love and Emmett.4 922

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