Indexed on: 17 Apr '17Published on: 05 Apr '17Published in: Journal of Physical Chemistry C
A new method, entitled chemical potential programmed reaction, for determining the physicochemical properties of iron ammonia synthesis catalyst has been proposed. Two model reactions were applied: nitriding of the iron catalyst and reduction of the obtained nitrides. Measurements of the rates of those reactions were carried out at 350 °C in a differential tubular reactor. The reactor is equipped with a system that allows us to perform simultaneous thermogravimetric measurements and a catharometric system to determine hydrogen concentration in the gas phase. The reactor was fed with a mixture of ammonia and hydrogen of varying composition, which was changing in a controlled way. Different accelerations of the nitriding potential change were applied. During the processes of nitriding of nanocrystalline iron and reduction of the obtained nanocrystalline iron nitrides rates of these processes were measured. The minimum nitriding potential, at which the phase transformation of nanocrystallites of a certain size took place, was determined. As a result, the relative nanocrystallite size distribution related to the active surface of nanocrystallites was calculated. Then, making use of the mean size of nanocrystallites the absolute size distribution was obtained. Bimodal size distribution of nanocrystallites in test samples was observed. The dependence of the minimum nitriding potential on the mass of crystallites was determined. During the reduction of iron nitrides, similarly as in the iron nitriding process, nanocrystallites underwent a phase transition in their entire volume in the order of the largest to the smallest in size.