Abstract
Sum frequency generation spectroscopy (SFG) is a nonlinear optical, surface sensitive and pressure independent method allowing to acquire vibrational spectra in a pressure range from ultrahigh vacuum (UHV) up to about 1 bar. During this thesis work an ultrahigh vacuum chamber with high pressure cell and two SFG spectrometers based on different picosecond laser systems (Ti:Sa and Nd:YAG) were developed for preparation and investigation of model catalysts. The suitability of the setup was demonstrated in studies of the adsorption of carbon monoxide and ethylene on a multitude of model catalysts in the pressure range mentioned above and under reaction conditions.
In SFG spectra of the system CO/Pt(111) at pressures up to 500 mbar no high pressure species were detected - contrary to results obtained by other groups. The necessity of performing a compensation for gas-phase absorption is treated in detail.
Extensive measurements of CO/Pd(111) up to 1000 mbar showed, that adsorbate structures prepared at low temperatures in high vacuum can also be obtained at high surface temperatures - at accordingly high pressures. Determination of the tilt angle of CO on Pd(111) in polarization dependent measurements showed a strong dependency on the model used.
In first SFG measurements of CO adsorption on supported Palladium nanoparticles (Pd/Al2O3/NiAl(110)) of different size (< 10 nm) and morphology with the Ti:Sa spectrometer, in high vacuum the signal of on-top CO on disordered particles was bigger than that on faceted particles. The frequency of the bridge signal on both kinds of nanoparticles corresponded to that found on defective Pd(111) surfaces. At high pressures of a few hundred mbar a higher population of on-top sites could be reached also for the faceted particles.
With the better resolution of the Nd:YAG spectrometer in addition hollow CO and a second bridge species attributed to CO on (111) facets were detected on faceted particles, in high vacuum and at high pressure. Thus, in spectra of faceted particles a component was identified that behaves in high vacuum at low temperatures and at high pressure analogously to CO/Pd(111). Differences to the Pd(111) surface are caused by the heterogeneity of the particles. The nanoparticle spectra recorded with the two spectrometers showed different lineshapes (absorption vs. dispersion curves). This results from the difference in the non resonant background, that is probably caused by the differing (vis-)laser wavelengths. In measurements with the Nd:YAG spectrometer also an dependency of the lineshape on the polarization combination (ppp or ssp) used was found.
The suitability of the experimental setup for investigations of heterogeneous reactions with SFG spectroscopy and simultaneous gas-phase analysis by gas chromatography was demonstrated by measurements of CO hydrogenation and ethylene hydrogenation. | 
