The Temperature Effect on Iron Nanocrystallites Size Distribution

An industrial pre-reduced iron catalyst for ammonia synthesis was reduced at 773 K and heated in the hydrogen atmosphere at temperatures in the range of 773 K – 973 K. Samples of different mean sizes of nanocrystallites depending on heating temperature were obtained (773 K 20 nm, 823 K 30 nm, 873 K 35 nm, 923 K 38 nm and 973 K 40 nm). The mean sizes of crystallites were determined by XRD method. The chemical method was applied to determine the size distributions of nanocrystallites, using the results of measuring the rate of nanocrystalline iron nitriding reaction with simultaneous measurement of catalytic ammonia decomposition reaction rate. Samples were nitrided with ammonia under atmospheric pressure at 748 K in a tubular reactor equipped with systems enabling simultaneous conducting of continuous thermogravimetric measurements together with analysis of gas phase composition. Along with an increase of mean crystallites sizes both the values of the minimum nitriding potential, at which the nanocrystalline iron nitriding reaction begins, and the nitriding degree increase, but the rate of surface reaction of catalytic ammonia decomposition decreases. Changes in shapes of nanocrystallites size distributions depending on the heating temperature were observed. With the increase of the heating temperature bimodal distribution becomes unimodal.