Emission and loss of potassium promoter from styrene catalysts: studies by ultrahigh vacuum/molecular-beam and laser techniques Original Research Article

The main cause of deactivation of potassium-promoted iron-oxide catalyst for the styrene process is the migration and loss of potassium from the catalyst under process conditions. Ultrahigh vacuum (UHV)/molecular-beam and laser techniques have been employed to monitor this loss of K at typical process temperatures from commercial styrene catalyst samples. The emission of alkali species in the form of neutral atoms, ions, excited species and clusters can be identified and estimated separately. The activation energies of desorption can be determined by monitoring the increasing flux of the desorbing species on increasing the temperature of the catalyst sample, and by fast desorption kinetics studies. Further studies have focussed on the characterization of the highly excited long-lived states, the so-called Rydberg states, indicated as K∗, of the K promoter formed, which are K atoms with the valence electron excited to very high Bohr orbits. By comparison with the well studied energetics for desorption of K atoms, ions and Rydberg states from basal graphite surfaces, a general energy diagram is proposed for the interaction of K atoms with the styrene catalyst surface, based on numerous thermal desorption and desorption kinetics studies. Thus, the kinetics and dynamics of the alkali promoter on the catalyst surface can be studied and understood through the Rydberg state concept.