Energetics and kinetics of ethylbenzene adsorption on epitaxial FeO(111) and Fe3O4(111) films studied by thermal desorption and photoelectron spectroscopy

Abstract

The adsorption of ethylbenzene (EB) has been studied on thin films of FeO(111) and Fe3O4(111) grown epitaxially on Pt(111) using thermal desorption spectroscopy (TDS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). Applying a threshold analysis of the TDS data, desorption energies Edes and the corresponding frequency factors are deduced. The UPS measurements are performed under adsorption - desorption equilibrium conditions: The spectra are taken at varying sample temperature at constant EB gas phase pressures. From the spectra, the EB-coverages θEB are deduced. From the adsorption isobars obtained in this way, isosteric heats of adsorption qst(θEB) are obtained which are compared to the desorption energies Edes deduced from TDS. On the oxygen-terminated FeO(111) surface, two adsorption states are observed, a physisorbed first layer (β-EB) followed by condensation (α-EB). Their UP spectra are almost identical and very similar to the spectrum of gas phase EB. On Fe3O4(111), a more strongly chemisorbed species (γ1-EB) is adsorbed first, followed by physisorbed β- and condensed α-EB. The chemisorbed phase exhibits a strong shift and split of the highest occupied π orbitals of the phenyl group. This indicates a strong interaction between the substrate and the adsorbed molecules that are adsorbed with the phenyl ring lying flat on the surface. The desorption energies Edes and the isosteric heats of adsorption qst, respectively, are 91 (85) kJ/mol for γ1-, 55 (58) kJ/mol for β- and 50 (52) kJ/mol for α-EB and agree generally well. The differences are discussed in terms of different coverage ranges accessible for both methods, the nonequilibrium character of the TDS method and to the threshold analysis which yields only data for the most loosely bound molecules desorbing first in each desorption track. © 1998 American Institute of Physics.