CHEMICAL DISPLACEMENT OF MOLECULES ADSORBED ON COPPER SURFACES - LOW-TEMPERATURE STUDIES WITH APPLICATIONS TO SURFACE-REACTIONS

Previous experiments have demonstrated that displacement of a molecule adsorbed on a metal surface by an impinging gas-phase molecule can be quite a facile process. The generality of this process for an enthalp ic driving force as small as I kcal/mol is demonstrated here using the displacement of a weakly binding alkene, cyclopentene, by a series of more strongly binding alkenes on Cu(100). Surface structure sensitivi ty in the process is also demonstrated by a comparison of benzene and cyclopentene coadsorption on Cu(100) and Cu(110). This work also shows the utility of conducting the displacement process below the temperat ure at which the displaced molecule desorbs from the multilayer so tha t temperature-programmed desorption can be used to quantify the surfac e coverage of displaced molecules. It is also shown that one can readi ly determine the kinetics of adsorbate bond dissociation and bond form ation reactions by combining these chemical displacement measurements of surface coverage with an anneal/quench protocol, This approach is d emonstrated through chemical displacement experiments that determine t hat the C-Br bond in vinyl bromide adsorbed on Cu(100) dissociates nea r 157 It and that the formation of toluene from reaction between methy l iodide and coadsorbed phenyl groups on eu(110) occurs below 160 K. T he relative importance of enthalpy and entropy in chemical displacemen t is also discussed.