MONITORING SURFACES ON THE MOLECULAR-LEVEL DURING CATALYTIC REACTIONSAT HIGH-PRESSURE BY SUM-FREQUENCY GENERATION VIBRATIONAL SPECTROSCOPYAND SCANNING-TUNNELING-MICROSCOPY

Sum frequency generation (SFG), using non-linear laser optics, detects vibrational spectra of submonolayer amounts of adsorbates with excell ent energy and time resolution. Scanning tunneling spectroscopy (STM) is sensitive to the atomic surface structure; readily imaging defects, steps and kinks as well as stationary adsorbed species. Both of these techniques can be used during reactions at high pressures and tempera tures to obtain molecular information in situ. We report studies of pr opylene hydrogenation over Pt(111) crystal surfaces at atmospheric pre ssures and 300 K using SFG and STM. Four surface species (2-propyl, pi -bonded propylene, di sigma-bonded propylene, and propylidyne) were id entified; the first two being implicated as reaction intermediates. Th e platinum surface structure remains unchanged during the reaction, co nsistent with the structure insensitive nature of olefin hydrogenation . Propylene decomposition-induced substantial surface reconstruction.