Oxidation of CO on a Pt/A12O3 catalyst:: from the surface elementary steps to light-off tests IV.: Kinetic study of the reduction by CO of strongly adsorbed oxygen species

Transient experiments using mass and FTIR spectroscopy as detectors are performed at 300 K with a reduced 2.9% Pt/Al2O3 catalyst to study the reduction of strongly adsorbed oxygen species (denoted O-sads) formed by O-2 chemisorption using several y% CO/z% Ar/He mixtures (y and z in the range 0.5-10). During the first seconds of the reaction C mass balances reveal that the CO consumption is mainly due to the formation of a strongly adsorbed CO species identified as a linear CO species (denoted L) interacting with the O-sads species (IR band at 2084 cm(-1)). The evolution of the CO2 production rate with time on stream presents different profiles according to the reaction temperature: decreasing exponential at T-r < 273 K and peak profiles for T-r greater than or equal to 300 K. The CO2 production at T-r < 273 K is in agreement with a kinetic model considering two elementary steps: the adsorption of the L CO species without competition with Osads followed by a L-H elementary step (denoted S3b): O-sads + L --> CO2ads, with a rate constant k(3b) = v(3b) exp(-E-3b/RT) and E-3b = 65 kJ/mol at theta(Osads) approximate to 1. For T-r > 300 K, mass transfer processes contribute to the apparent CO2 production rate. At high theta(Osads) values, they compete with the surface reactions for 273 K < T-r < 360 K and finally dominate the CO2 production at T-r > 360 K. However, kinetic studies can be performed at T-r > 300 K after a significant decrease in theta(Osads) due to the increase in E-3b: E-3b = 110 kJ/mol at theta(Osads) = 0.4. Several conclusions of the present study are in very good agreement with the reduction of O-sads species on Pt single crystals using a CO molecular beam under UHV conditions. (C) 2003 Elsevier Inc. All rights reserved.