ADSORPTION AND DEHYDROGENATION OF ETHYLBENZENE ON ULTRATHIN IRON-OXIDE MODEL CATALYST FILMS

Thin iron oxide model catalyst films with defined stoichiometries were grown onto a Pt(111) single crystal substrate. On clean and potassium covered monolayer films with FeO stoichiometry as well as on clean Fe 3O4 and Fe2O3 multilayer films the adsorption of ethylbenzene (EB) at T = 120 K and the catalytic dehydrogenation of EB to styrene was studi ed by temperature programmed desorption (TPD) and stationary mass spec trometry measurements. On all films weakly chemisorbed EB desorbs mole cularly with first order kinetics at temperatures between T = 200 and 250 K. On potassium covered FeO monolayer films the EB desorption temp erature increases to 260 K. Desorption energies and Frequency factors of these adsorption states were determined by a numerical analysis of the TPD curves. Between 2 and 2.5 langmuir (L) exposures these weakly bound states pet saturated. With further increasing exposures condense d EB multilayers desorbing at T = 155 K form and stronger chemisorbed adsorption states are occupied. For 7 L exposure we observe about 0.5 monolayers of EB desorbing between T = 300 and 500 K from the FeO mono layer and Fe3O4 multilayer films and it round T = 900 K from the Fe2O3 films. The latter temperature coincides with the reaction temperature of the technical iron oxide catalyst. Stationary measurements in a wa ter-EB mixture at T = 873 K reveal a catalytic styrene formation only on the Fe2O3 film, demonstrating that only this oxide phase is active for the dehydrogenation of EB. (C) 1997 Elsevier Science B.V.