Catalytic Ammonia Decomposition during Nanocrystalline Iron Nitriding at 475 °C with NH3/H2 Mixtures of Different Nitriding Potentials

Ammonia is now regarded as an effective source of hydrogen containing no CO. In this paper, we have studied the nitriding of nanocrystalline iron accompanied by catalytic ammonia decomposition reaction. The chemical processes were carried out in a tubular differential flow reactor at 475 degrees C. NH3/H-2 gas mixtures of different compositions viz, different nitriding potentials were let into the reactor. At each nitriding potential the stationary states were observed, when mass of solid and rate of ammonia decomposition were constant. In the whole range of nitriding potentials three regions of different phase composition of solid were observed: alpha-iron, mixture of alpha-iron and Fe4N nitride, and Fe4N nitride. It occurred surprisingly that along with increase in nitriding potential the rate of ammonia decomposition decreased when Fe4N nitride phase was present. A model of the process in question was proposed to explain that phenomenon. The decrease in decomposition reaction rate, related with decrease in iron surface coverage degree, resulted from changes in segregation enthalpy of nitrogen. The model allowed the determination of such parameters as bulk concentration of nitrogen, enthalpy of segregation, and surface coverage degree.