A comparative study of thermal desorption spectra obtained through Monte Carlo simulation for monomers, symmetric dimers, and asymmetric dimers, on heterogeneous surfaces presenting different topographies of two kind of sites, is presented in such a way as to serve as a guide for the interpretation of experimental data.

The possibility of N island formation during the conversion of NO + CO mixtures on Rh(111) and the kinetics observed in thermal desorption experiments from this system were studied by kinetic Monte Carlo simulations. Three models were proposed and tested to account for the behavior observed:  (a) one where the free formation of islands through N diffusion on the surface is combined with nearest-neighbor (NN) attractive and repulsive second- and third-order neighbor interactions. (b) A second where compact islands with size similar to that suggested by experiments are produced by an alternative mechanism such as a preferential NO dissociation rate in the presence of an already dissociated N atom. In this model, thermal desorption is still assumed to be affected by adsorbate−adsorbate interactions as in model a. (c) A third where the nitrogen islands are formed as a result of adsorbate−substrate rather than adsorbate−adsorbate interactions, that is, where a “surface stress” mechanism is operative, and the same interactions affect the thermal desorption process. It was found that the experimental data can only be satisfactorily reproduced by model c.

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