Low energy electron diffraction and work function change studies of the adsorption of substituted aromatic molecules on the (111) and (100) crystal faces of platinum

Abstract

The chemisorption of toluene, m-xylene, mesitylene, n-butylbenzene, t-butylbenzene, aniline, nitrobenzene and cyanobenzene were studied on the (111) and (100) crystal faces of platinum at low pressures (10 −9 to 10 −7 torr) and at temperatures of 20 to 300 °C by low energy electron diffraction and work function change measurements. After adsorption, reorientation of the molecules in the adsorbed layer is necessary to form the ordered structures. Molecules that have either higher rotational symmetry (mesitylene) or have only small size substituents on the benzene rings exhibit better ordering if the adsorption is carried out at low incident flux. The adsorbed layers are more ordered on the (111) crystal face than on the (100) crystal face of platinum. The work function changes upon adsorption rangs from −1.4 eV for nitrobenzene to −1.8 eV for aniline. Both the diffraction and work function change data indicate that, under the conditions of these experiments, all of the molecules chemisorb with their benzene ring parallel to the surface and interact with the metal surface primarily via the π-electrons in the benzene ring. The substituent groups play an important role in determining the ordering characteristics of the overlayers but do not markedly effect the strength of the chemical bond between the substrate and the adsorbate.