Potassium promoter in industrial ammonia synthesis catalyst: Studies by surface ionization

Abstract

Industrial ammonia synthesis catalysts, as fresh (pre-reduced and passivated) and used (passivated) flat samples were studied in a molecular beam UHV apparatus, using a surface ionization detector. The desorption of the K promoter from the catalyst at 870–1080 K could be characterized, by measuring angular distributions and simultaneous Arrhenius plots, as a function of the angle around the sample. The two activation energies observed for K desorption from the fresh catalyst were 260 kJ/mol (2.7 eV) and 330 kJ/mol (3.4 eV); the used catalyst had only one, much smaller activation barrier at 164 kJ/mol (1.70 eV). The chemical state of potassium in the catalyst changes from a stronger bound ionic, probably in the form of FeOK, to a weaker, more covalent one during the industrial process. A large difference in work functions between the fresh (3.9 eV) and used (1.7 eV) catalyst was observed. This seems to be due to formation of K oxides on the surface of the used catalyst, caused by enrichment of potassium at the surface during the industrial use.