Oxidation of CO on a Pt/Al2O3 catalyst:: From the surface elementary steps to lighting-off tests -: II.: Kinetic study of the oxidation of adsorbed CO species using mass spectroscopy

Experiments in the transient regime using a mass spectrometer as a detector are performed at T = 300 K on a 2.9% Pt/Al2O3 catalyst to study the oxidation of the linearly adsorbed CO species (denoted L) with several x% O-2/z% Ar/He mixtures (x and z in the range 0.5-4). CO and 02 adsorption measurements show that the L CO species and a strongly adsorbed oxygen species (denoted O-sads) formed by the dissociative chemisorption of O-2 are adsorbed on the same sites of the freshly reduced Pt particles. L CO and O-sads species have high heats of adsorption: 115 and 175 kJ/mol, respectively, at full coverage of the Pt surface and do not desorb in helium at a temperature lower than 350 K. Moreover, it is shown that at 300 K a preadsorbed L CO species is not displaced by oxygen adsorption but is converted into CO2 by a weakly adsorbed oxygen species (denoted O-wads) according to the elementary step (denoted S3): L + O-wads --> CO2 (rate constant, k(3)). This confirms the conclusion of a previous study performed using FTIR spectroscopy. O and C mass balances during the transient regime reveal (a) that during the first seconds of the transient the oxygen consumption is mainly due to the formation of the O-wads species and that its adsorption equilibrium is rapidly attained and (b) that an O-sads species is adsorbed for each L CO species removed by oxidation. This leads to a Pt surface where the coverage of the L CO species decreases while that of the O-sads species increases with the duration of the oxidation. However, it is shown that the rate of the reaction (denoted S3a), L + O-sads species, is significantly lower than that of step S3 and does not contribute to the CO2 formation in the presence Of O-2. A comparison is presented with the literature data on Pt single crystals (UHV studies) and supported Pt catalysts. (C) 2002 Elsevier Science (USA).