Oxidation of CO on a Pt/Al2O3 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 Osads) formed by O2 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 Osads 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 Tr<273 K and peak profiles for Tr⩾300 K. The CO2 production at Tr<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): Osads+L→CO2ads, with a rate constant k3b=ν3bexp(−E3b/RT) and E3b=65 kJ/mol at θOsads≈1. For Tr>300 K, mass transfer processes contribute to the apparent CO2 production rate. At high θOsads values, they compete with the surface reactions for 273 K360 K. However, kinetic studies can be performed at Tr>300 K after a significant decrease in θOsads due to the increase in E3b: E3b=110 kJ/mol at θOsads=0.4. Several conclusions of the present study are in very good agreement with the reduction of Osads species on Pt single crystals using a CO molecular beam under UHV conditions.