Molecular studies of catalytic reactions on crystal surfaces at high pressures and high temperatures by infrared-visible sum frequency generation (SFG) surface vibrational spectroscopy

Infrared-visible sum frequency generation (SFG) is a surface-specific vibra tional spectroscopy that can operate in a pressure range from ultrahigh vac uum (uhv) to atmospheric pressures. SFG is therefore one of the few surface science techniques that permits atomic scale monitoring of surface species during catalytic reactions at high pressures (around 1 atm) and high tempe ratures. Using single-crystal surfaces of transition metals, platinum and r hodium, reaction rates can be simultaneously determined by gas chromatograp hy, and correlations between the concentration of adsorbates under reaction conditions and the observed turnover numbers can help to elucidate the rea ction mechanism. To bridge the gap to traditional surface science experimen ts, SFG is also employed under uhv or low pressures. The technique has been successfully applied to the adsorption and oxidation of CO, hydrocarbon co nversion such as ethylene hydrogenation and cyclohexene hydrogenation and d ehydrogenation on Pt(111). The experiments demonstrate that the key interme diates of high-pressure catalytic reactions are not present under low-press ure (uhv) conditions. Furthermore, the identification of active intermediat es and their concentration at ambient conditions allows calculation of turn over frequencies per active surface species rather than simply per surface metal atom.