Measurements of the relative number of active sites on iron catalyst for ammonia synthesis by hydrogen desorption

Analysis of relative number of active sites of ammonia synthesis iron catalysts was performed by TPD-H2 method. Samples of the industrial pre-reduced catalyst, which were reduced and heated at different temperatures from the range 500 °C to 780 °C in order to modify their specific surface area, were used. Chemical composition of samples to be tested was modified as well by adding sulfur compound or reducing the amount of potassium oxide. The process of hydrogen adsorption was conducted at 25 °C. Then hydrogen adsorption occurs on the surface together with absorption in the volume of iron. On the spectrum of H2-TPD the presence of peaks α1, α2 and α3 was observed at temperatures 130 °C, 300 °C and 450 °C, respectively. Peak α1, associated with adsorption on the surface, is proportional to the total amount of absorbed as well as adsorbed hydrogen. Height of α1 peak varies with the shift of catalysts’ activity. This peak gives the information about the number of active sites on the catalyst's surface. Heights of peaks α2 and α3 depend mainly on the size of sorption of hydrogen in the volume of iron. Changes in activity of the samples did not significantly affect the heights of peaks α2 and α3.