Molecular Conformation of Styrene on Ag(100):  Relevance to an Understanding of the Catalytic Epoxidation of Terminal Alkenes

Fast XPS and NEXAFS have been used to investigate the adsorption, desorption, and molecular conformation of styrene adsorbed on Ag(100) in order to determine whether single-crystal studies on the catalytic oxidation of this molecule do indeed provide valid information relevant to a fundamental understanding of the Ag-catalyzed epoxidation of terminal alkenes, especially ethene. Styrene adsorbs and desorbs from the metal surface without decomposition and three states are discerned in the desorption spectra, depending on the coverage. The strongest and weakest bound states are associated with molecules in multilayer and submonolayer environments, respectively. XPS data indicate that a desorption peak that appears at intermediate coverages is associated with tilted molecules in the contact layer. In the coverage regime relevant to the catalytic epoxidation reaction, the phenyl and vinyl groups are coplanar, and the molecule lies flat on the metal surface. These observations validate the use of styrene as a model adsorbate with which to investigate the catalytic chemistry of terminal alkenes.