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

Abstract

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% O2/z% Ar/He mixtures (x and z in the range 0.5–4). CO and O2 adsorption measurements show that the L CO species and a strongly adsorbed oxygen species (denoted Osads) formed by the dissociative chemisorption of O2 are adsorbed on the same sites of the freshly reduced Pt particles. L CO and Osads 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 Owads) according to the elementary step (denoted S3): L+Owads→CO2 (rate constant, k3). 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 Owads species and that its adsorption equilibrium is rapidly attained and (b) that an Osads species is adsorbed for each L CO species removed by oxidation. This leads to a Pt surface where the coverage of the LCO species decreases while that of the Osads species increases with the duration of the oxidation. However, it is shown that the rate of the reaction (denoted S3a), L+Osads species, is significantly lower than that of step S3 and does not contribute to the CO2 formation in the presence of O2. A comparison is presented with the literature data on Pt single crystals (UHV studies) and supported Pt catalysts.