The kinetics of the CO oxidation over a Pt(111) thin film was studied in situ under CO and O2 pressures in the millibar range, using infrared spectroscopy and in situ surface X-ray diffraction to follow the actual structural and chemical nature of the catalyst surface. The total pressure of the gas mixture and the composition of the reacting gases were monitored online with a capacitive gauge and a quadrupole mass spectrometer, respectively. Under mildly oxidizing reaction conditions with excess O2, the initial reaction is catalyzed by a Pt(111) surface, which is covered by mixed CO and O overlayer, revealing an apparent activation energy of 45 kJ/mol in the temperature range 626–750 K. If the CO content is partially depleted by the ongoing reaction so that the gaseous reaction mixture turns into a strongly oxidizing one, the catalytic activity increased by an order of magnitude. This active state is characterized by a vanishingly small surface CO coverage according to in situ RAIRS, while the surface structure is assigned to a mixture of an oxidic and chemisorbed O covered Pt(111) on the basis of in situ SXRD. An apparent activation energy of about 0 kJ/mol point toward mass transfer limitation in the active phase.

A series of microstructured, supported platinum (Pt) catalyst films (supported on single-crystal yttria-stabilized zirconia) and an appropriate Pt catalyst reference system (supported on single-crystal alumina) were fabricated using pulsed laser deposition and ion-beam etching. The thin films exhibit area-specific lengths of the three-phase boundary (length of three-phase boundary between the Pt, support, and gas phase divided by the superficial area of the sample) that vary over 4 orders of magnitude from 4.5 × 102 to 4.9 × 106 m m–2, equivalent to structural length scales of 0.2 μm to approximately 9000 μm. The catalyst films have been characterized using X-ray diffraction, atomic force microscopy, high-resolution scanning electron microscopy, and catalytic activity tests employing the carbon monoxide oxidation reaction. When Pt is supported on yttria-stabilized zirconia, the reaction rate clearly depends upon the area-specific length of the three-phase boundary, l(tpb). A similar relationship is not observed when Pt is supported on alumina. We suggest that the presence of the three-phase boundary provides an extra channel of oxygen supply to the Pt through diffusion in or on the yttria-stabilized zirconia support coupled with surface diffusion across the Pt.

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