Temperature programmed adsorption and desorption of nitrogen on iron ammonia synthesis catalysts, and consequences for the microkinetic analysis of NH3 synthesis

Iron catalysts with and without potassium promoter were studied in a combined temperature programmed reaction and flow micro reactor system connected to a mass spectrometer. The temperature programmed desorption (TPD) studies demonstrated a significant shift of the N-2 peak to lower temperatures with the addition of K to the catalyst. The isobars suggested that this change can be interpreted as a destabilization of N-(*). The N-2 adsorption results were in good agreement with earlier studies of singly promoted catalysts. Thus the rate was found to be slow and activated. The rate constants were not in any disagreement with transition state theory. The addition of K had very little effect on the temperature programmed adsorption (TPA) curves, while for multiply promoted catalysts there was a significant shift to lower temperatures of the main adsorption dip. A microkinetic model for the NH3 synthesis reaction based on isobars for H-2 and N-2 and on the TPD results for N-2 displayed a good agreement with many of the features of the experimental data. An interesting example is that the destabilization of N-(*) and NH-(*) by K can explain the difference in the promoting effect at high and low pressures. However, the conclusion is that a simple Langmuir type of kinetic model cannot result in a perfect fit of a reasonably extended experimental set of data.