Stability and excitation of potassium promoter in iron catalysts - the role of KFeO2 and KAlO2 phases

Well-characterized catalyst model compounds of KAlO2 and KFeO2 are investig ated by thermal desorption of potassium from the material. The desorbing fl uxes of ions, atoms and highly excited states (field ionizable Rydberg stat es) were studied with surface and field ionization detectors in a vacuum ap paratus. From the Arrhenius plots the activation energies for desorption of K and K+ were determined. The chemical state of potassium at the surfaces is concluded to be: ionic on KAlO2 (with the K desorption barrier of 1.76 e V) and covalent on KFeO2 (barrier of 2.73 eV). These results agree with the data obtained earlier for industrial catalysts for ammonia and styrene pro duction. They are interpreted in terms of the Schottky cycle, which is comp leted for KAlO2 and fails for KFeO2. This failure indicates a non-equilibri um desorption process. K Rydberg states are only found to desorb from KFeO2 , in agreement with the suggestion that such states in some way are respons ible for the catalytic activity.