In the present paper we study the reactivity of model Pt nanoparticles supported on glassy carbon. The particle size effect is rationalized for CO monolayer oxidation exploring electrochemical methods (stripping voltammetry and chronoamperometry) and modelling. Significant size effects are observed in the particle size interval from ca. 1 to 4 nm, including the positive shift of the CO stripping peak with decreasing particle size and a pronounced asymmetry of the current transients at constant potential. The latter go through a maximum at low COads conversion and exhibit tailing, which is the longer the smaller the particle size. Neither mean field nor nucleation & growth models give a coherent explanation of these experimental findings. We, therefore, suggest a basic model employing the active site concept. With a number of reasonable simplifications a full analytical solution is obtained, which allows a straightforward comparison of the theory with the experimental data. A good correspondence between experiment and theory is demonstrated. The model suggests restricted COads mobility at Pt nanoparticles below ca. 2 nm size, with the diffusion coefficient strongly dependent on the particle size, and indicates a transition towards fast diffusion when the particle size exceeds ca. 3 nm. Estimates of relevant kinetic parameters, including diffusion coefficient, reaction constant etc. are obtained and compared to the literature data for extended Pt surfaces.

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