High Pressure Carbon Monoxide Oxidation over Platinum (111)

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.