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+Owads→CO2, where Owads is a weakly adsorbed oxygen species formed without competition with the L CO species by the dissociative chemisorption of O2. 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 Tr from 300 to 740 K (lighting-off tests) using several 1% CO/x% O2/He mixtures, with x≤0.5. In an excess of CO (x<0.5), it is shown that the experimental curves θL=f(Tr) can be fitted by a kinetic model by considering that θL 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 O2 at T<350 K (in the absence of CO). The evolution of the experimental turnover frequency (TOFex) during the increase in Tr is determined and compared to the theoretical TOFth for low CO conversions. It is shown that TOFex=f(Tr) is in very good agreement with a Langmuir–Hinshelwood mechanism via step S3, with the kinetic parameters used to fit the curves θL=f(Tr).