Most knowledge on catalytic reaction mechanisms has up until now been gathered with UHV-based techniques. The extrapolation of such obtained reaction parameters to the pressure range of real application conditions is called the pressure gap. The nonlinear optical technique Second Harmonic Generation (SHG) allows studies covering the whole pressure range between UHV and application pressures. Of special importance for this purpose is the possibility to distinguish coverages of different adsorbates. In this work we show how the application of phase sensitive SHG can be used to determine coverages of different adsorbates in-situ allowing the determination of important reaction parameters. Our choice of reaction is the catalytic water formation on platinum which is already widely investigated in the literature.
For in-situ investigations of catalytic reactions with second harmonic generation (SHG) it is necessary to distinguish the adsorbate induced signal due to the presence of different species. A further requirement is the quantitative determination of coverages of the reactants and products. We present in this work, how one can distinguish different adsorbates by the use of phase sensitive signal detection and how their coverages can be determined. It turns out that the relative sensitivity for the adsorbates can be varied by changing the azimuthal angle of the sample. Systematic SHG studies of the effect of coadsorbing oxygen and hydrogen show that nonlocal effects are influencing the SHG signal strongly. In contrast these nonlocal effects are negligible during experiments using only one adsorbate. The coadsorption studies are carried out under controlled UHV conditions at a temperature of T equals 100 K. At such low temperatures no reaction between the adsorbates occurs. The experiments are necessary to calibrate the SHG signal to varying coverages of the reactants coadsorbed for the later use during the in-situ study of the catalytical reaction H2 PLU 1/2O2 yields H2O.