Surface vibrational spectroscopy from ultrahigh vacuum to atmospheric pressure: adsorption and reactions on single crystals and nanoparticle model catalysts monitored by sum frequency generation spectroscopy

Abstract

Vibrational sum frequency generation (SFG) spectroscopy has been developed to a stage of surface sensitivity that is comparable to the classical surface science methods. SFG has been successfully employed to study a variety of adsorbate–substrate interfaces and due to its surface-specificity it allows the study of adsorbates from submonolayer coverages up to atmospheric gas pressure. A number of case studies is presented including adsorption, co-adsorption and reactions on single crystal surfaces and supported nanoparticles. Studies of CO adsorption on Pt(111) and Pd(111) from 10⁻⁷ to 1000 mbar have shown that the high pressure adsorbate structures were comparable with saturation structures obtained at low temperature in ultrahigh vacuum. No evidence for pressure-induced surface rearrangements was found. However, pronounced differences in the CO adsorbate structure on supported Pd nanoparticles and on Pd(111) were detected. It is further shown how polarization dependent SFG can be employed to determine molecular orientations of CO and NO and how SFG is carried out during ethylene hydrogenation. Broadband techniques allow the performance of time-resolved pump–probe SFG experiments and to take “snapshots” of the transient vibrational spectrum.