Interpretation of temperature-programmed desorption spectrum broadening

A molecular model of broadening of temperature-programmed desorption (TPD) spectra for desorption from a nonuniform surface is developed. For nondissociative desorption from a nonreconstructing surface, the positions of TPD peaks, their FWHMs, and asymmetry factors are analyzed as functions of the initial surface coverage. The parameters of the correlation functions depend on the distribution of adsorption sites over the adsorption heat. As the degree of nonuniformity increases, a low-temperature peak arises. Lateral interactions mask the effect of nonuniformity: repulsive and attractive interactions broaden and narrow the TPD peaks, respectively. Analysis of the TPD spectra for the CO/Rh system shows that the TPD spectra for polycrystalline Rh cannot be represented as a superposition of the TPD spectra for the Rh(111) and Rh(110) surfaces, and it is necessary to consider the contribution from sites with a high adsorption heat, which are typical of polycrystalline Rh.