Photoemission of ethylbenzene adsorbed on Pt(111) and on epitaxial films of FeO(111) and Fe3O4(111): electronic structure and isosteric heats of adsorption

Abstract The adsorption of ethylbenzene (EB) on Pt(111) and on epitaxial FeO(111) and Fe 3 O 4 (111) films is studied by ultraviolet photoelectron spectroscopy. The spectra are measured under adsorption–desorption equilibrium conditions at constant gas pressures and at temperatures between 370 K and 135 K. Several adsorbed species are identified by comparison with gas phase photoelectron spectra. On the thin oxygen-terminated FeO(111) film, one monolayer (ML) of EB is physisorbed in the first layer (β phase), followed by condensation of EB from the second layer on (α phase). On the iron-terminated Fe 3 O 4 (111) multilayer films, 0.8 ML of EB is chemisorbed in the first layer (γ 1 phase), followed by physisorption of 1 ML of β-EB in the second and condensation from the third layer on. On Pt(111), about 0.5 ML of a different chemisorbed species (δ phase) is observed, followed by physisorption of about 1.5 ML of β-EB and finally by condensation of α-EB. We interpret the δ species on Pt(111) as chemisorbed styrene and hydrogen formed by dissociation of EB. Adsorption isobars are measured and the isosteric heats of adsorption q st ( Θ ) are determined as a function of the EB coverage Θ . For condensed α-EB, q st =52 kJ mol −1 is found on all surfaces. The values of q st for physisorbed β-EB depends on the substrate. On FeO it is 58 kJ mol −1 , on Pt(111) covered by styrene and hydrogen in the first layer it decreases with coverage from 85 to 55 kJ mol −1 . On Fe 3 O 4 (111) covered by γ 1 -EB in the first layer it could not be determined because of structural non-equilibrium and transformations between the physisorbed β phase and the condensed α phase. For chemisorbed γ 1 -EB, q st decreases from 85 to 68 kJ mol −1 with increasing coverage.