Comparative loss of alkali promoter by desorption from two catalysts for the dehydrogenation of ethyl benzene to styrene

Abstract

Using molecular-beam and mass-spectrometric methods, the comparative desorption of potassium as K atoms and as excited-state (K^2*) species was studied from two commercial iron oxide catalysts (A and B, of which the latter was known to be superior) for the styrene process. The desorption of potassium from B was only observed at temperatures considerably higher than that from A. The measured activation energy values for emission are: (a) for K atoms, 1.0 eV (96 kJ/mol) and 3.1 eV (300 kJ/mol) from A and B resp.; (b) for K^2* species, 1.7 eV (160 kJ/mol) and 0.84 eV (81 kJ/mol) from A and B resp. The temperature ranges where these values are measured are (a) (A) 830-920 K, (B) 970-1090 K and (b) (A) 810-1060 K, (B) 910-1060 K. The lower emission barrier for the excited state species from B may perhaps be responsible for the better and more stable performance of this catalyst in the styrene process. The excited states observed in emission here are believed to be formed also under normal operating conditions on the catalyst surfaces.