This article focuses on the clustering effects of admolecules driven by surface diffusion in a heterogeneous energy landscape as a mechanism for Freundlich behavior at low pressures. The idealized preferential adsorption model is presented and the phenomenology of this model is justified in terms of surface diffusion (preferential adsorption model, PAM). Mean field analysis and Monte Carlo simulation of PAM are analyzed. The effects of temperature and of patchwise distributions of adsorption energies are discussed.