Oxidation of CO on a Pt/Al_2O_3 catalyst:from the surface elementary steps to light-off tests IV.Kinetic study of the reduction by CO of strongly adsorbed oxygen species

摘要

Transient experiments using mass and FTIR spectroscopy as detectors are performed at 300 K with a reduced 2.9% Pt/Al_2O_3 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 0.5-1.0).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 and 2084 cm~(-1)).The evolution of the CO_2 production rate with time on stream presents different profiles according to the reaction temperature:decreasing exponential at T_r<273K and peak profiles for T_r>=300 K.The CO_2 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 O_(sads) followed by a L-H elementary step (denoted S3b):O_(sads)+L->CO_(2ads),with a rate constant k_(3b)=v_(3b) exp(-E_(3b)/RT) and E_(3bA)=65 kJ/mol at delta_(Osads) approx =1.For T_r>30 K,mass transfer processes contribute to the apparent CO_2 production rate.At high delta_(Osads) values,they compete with the surface reactions for 273 K360K.However,kinetic studies can be performed at Tr>300 K after a significant decrease in delta_(Osads) due to the increase in E_(3b):E_(eb)=110 kJ/mol at delta_(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.