Oxidation of CO on a Pt/Al2O3 catalyst:: From the surface elementary steps to lighting-off tests -: III.: Experimental and kinetic model for lights-off tests in excess CO

The adsorption of CO (1% CO/He mixture) at 300 K on a 2.9% Pt/Al2O3 catalyst leads to the detection of a strong IR band at 2075 cm(-1) associated with weak and broad IR bands in the range 1900-1700 cm(-1) ascribed to linear (denoted L) and multibound (bridged and threefold coordinated) CO species, respectively. In parts I and II of the present study it was shown that the L CO species is oxidized at T < 350 K according to the surface elementary step denoted S3, L + O-wads --> CO2, where O-wads is a weakly adsorbed oxygen species formed without competition with the L CO species by the dissociative chemisorption of O-2. In the present study the coverage of the L CO species as well as the conversion of CO into CO2 are determined during the increase in the reaction temperature T-r from 300 to 740 K (lighting-off tests) using several 1% CO/x% O-2/He mixtures, with x less than or equal to 0.5. In an excess of CO (x < 0.5), it is shown that the experimental curves theta(L) = f(T-r) can be fitted by a kinetic model by considering that theta(L) is determined by the equilibrium between the rates of adsorption, oxidation, and desorption of the L CO species. The parameters used in the kinetic model are those previously determined by studying (a) the adsorption equilibrium of the L CO species in the temperature range 300-740 K and (b) its oxidation by 02 at T < 350 K (in the absence of CO). The evolution of the experimental turnover frequency (TOFex) during the increase in T-r is determined and compared to the theoretical TOFth for low CO conversions. It is shown that TOFex = f(T-r) is in very good agreement with a Langmuir-Hinshelwood mechanism via step S3, with the kinetic parameters used to fit the curves theta(L) f(T-r). (C) 2002 Elsevier Science (USA).