ADSORPTION AND HYDROGENATION OF SIMPLE ALKENES AT PT-GROUP METAL-ELECTRODES STUDIED BY DEMS - INFLUENCE OF THE CRYSTAL ORIENTATION

The adsorption of ethene and cyclohexene on mono-and polycrystalline P t and on polycrystalline Pd electrodes was studied using differential electrochemical mass spectrometry (DEMS). Both molecules are partially hydrated to an oxygen containing species upon adsorption on Pt. In th e case of ethene, this species dissociated to methane and adsorbed CO at negative potentials. Another part of the adsorbed ethene can be cat hodically desorbed as ethane and butane. The ratio of the various spec ies formed strongly depends on crystal orientation and adsorption pote ntial. Contrary to heterogenous gas phase hydrogenation (and also cont rary to some earlier reports on electrochemical hydrogenation), the ra te of the Faradaic hydrogenation reaction is also strongly dependent o n the crystallographic orientation, being faster on Pt(110) or roughen ed surfaces. During hydrogenation, H/D exchange occurs to an appreciab le degree, suggesting the participation of adsorbed intermediates.