Pt and Pt/Al2O3 thin films for investigation of catalytic solid-liquid interfaces by ATR-IR spectroscopy:: CO adsorption, H2-induced reconstruction and surface-enhanced absorption

Model platinum catalysts have been designed to study the platinum-solvent interface in situ using attenuated total reflection (ATR) infrared spectroscopy. Pt and Pt/Al2O3 thin films were evaporated on a Ce internal reflection element (IRE) and characterized by XRD, XPS, AFM, STM, and III spectroscopy. Changes within the adsorbate layer of the Pt catalyst during cleaning with O-2 and H-2 were followed. After cleaning, the catalyst surface was probed by CO adsorption from CH2Cl2. For the Pt/Al2O3 film the spectrum of adsorbed CO showed a band at 2000 cm(-1), which is typical for Pt/Al2O3 catalysts. The stretching vibration of linearly bonded CO exhibited a coverage-dependent frequency shift due to vibrational coupling, thus showing the existence of large clean domains on the reactive catalyst surface even in the presence of an organic solvent. CO adsorption from CH2Cl2 was slow before the cleaning process. However, subsequent admission of Hz resulted in an instantaneous and drastic increase of the CO absorption signal. The origin of this effect is a structural change of the Pt particles induced by dissolved hydrogen, which was directly monitored by ATR spectroscopy using CO as probe molecule. STM investigations showed sintering of the Pt particles upon hydrogen treatment in CH2Cl2 at room temperature, which leads to a surface-enhanced infrared absorption (SEIRA).