Studies of the ammonia decomposition over a mixture of α-Fe(N) and γ′-Fe4N

An industrial pre-reduced iron catalyst for ammonia synthesis was nitrided in a differential reactor equipped with the systems that made it possible to conduct both the thermogravimetric measurements and hydrogen concentration analyser in the reacting gas mixture. The nitriding process, particularly the catalytic ammonia decomposition reaction, was investigated under an atmosphere of ammonia-hydrogen mixtures, under the atmospheric pressure, at 475 degrees C. The nitriding potentials were changed gradually in the range from 19.10(-3) to 73.10(-3) Pa-0.5 in the reactor for an intermediate area where two phases exist simultanously: Fe(N) and gamma'-Fe-4. In the area wherein P > 73.10(-3) Pa-0.5, approximately stoichiometric composition of gamma'-Fe4N phase exists and saturating of that phase by nitrogen started. The rate of the catalytic ammonia decomposition was calculated on the basis of grain volume distribution as a function of conversion degree for that catalyst. It was found that over gamma'-Fe4N phase in the stationary states the rate of catalytic ammonia decomposition depends linearly on the logarithm of the nitriding potential. The rate was decreasing along with increase in the nitriding potential. For the intermediate area, the rate of ammonia decomposition is a sum of the rates of reactions which occur on the surfaces of both Fe(N) and gamma'-Fe4N.