Pt and Pt/Al2O3 thin films for investigation of catalytic solid-liquid interfaces by ATR-IR spectroscopy: CO adsorption, H-2-induced reconstruction and surface-enhanced absorption

Model platinum catalysts have been designed to study the platinum-solvent i nterface in situ using attenuated total reflection (ATR) infrared spectrosc opy. Pt and Pt/Al2O3 thin films were evaporated on a Ce internal reflection element (IRE) and characterized by XRD, XPS, AFM, STM, and III spectroscop y. Changes within the adsorbate layer of the Pt catalyst during cleaning wi th O-2 and H-2 were followed. After cleaning, the catalyst surface was prob ed by CO adsorption from CH2Cl2. For the Pt/Al2O3 film the spectrum of adso rbed CO showed a band at 2000 cm(-1), which is typical for Pt/Al2O3 catalys ts. The stretching vibration of linearly bonded CO exhibited a coverage-dep endent frequency shift due to vibrational coupling, thus showing the existe nce of large clean domains on the reactive catalyst surface even in the pre sence of an organic solvent. CO adsorption from CH2Cl2 was slow before the cleaning process. However, subsequent admission of Hz resulted in an instan taneous and drastic increase of the CO absorption signal. The origin of thi s effect is a structural change of the Pt particles induced by dissolved hy drogen, which was directly monitored by ATR spectroscopy using CO as probe molecule. STM investigations showed sintering of the Pt particles upon hydr ogen treatment in CH2Cl2 at room temperature, which leads to a surface-enha nced infrared absorption (SEIRA).