Surface chemistry studied by in situ X-ray photoelectron spectroscopy

Time-dependent X-ray photoelectron spectroscopy is used to study the kinetics and dynamics of simple surface reactions. Combining high-resolution core level spectroscopy with a supersonic molecular beam in one experimental setup, processes such as the dissociative adsorption of methane on both Pt(111) and Ni(111), the coadsorption of water and CO on Pt( 111), and the oxidation of CO on Pt( 111) have been studied. In the case of methane, the observed vibrational fine structure in C 1s spectra is used to identify the adsorbed species (CH3) and further thermal dehydrogenation steps. While simple dehydrogenation via CH is observed on Pt( 111), a C - C coupling reaction to acetylene is found on Ni( 111). In the coadsorbate phase, CO is found to be able to replace predosed water from the bilayer into multilayers. Water, in turn, leads to a site change of the CO molecules, which are preferably adsorbed at bridge sites in the presence of water, as opposed to on-top adsorption on clean Pt( 111). For the truly bimolecular surface reaction, the CO oxidation on Pt( 111), the ability of the molecular beam to create a relatively high CO pressure was found essential to study the kinetics of the basic step (CO + O --> CO2) without influence of adsorption or diffusion rate. An activation energy of 0.53 eV and a preexponential factor of similar to 5 x 10(6) s(-1) are found.