The rate processes occurring at catalytic surfaces are complicated by various factors, and the corresponding kinetic models are diverse. One of the common ingredients of these models is the assumption that binding sites are vacant or occupied by one adsorbed particle. Double occupation is, however, also possible although energetically not favourable. With this background, the author presents general equations describing coadsorption isotherms and kinetics of molecular adsorption, desorption, and Langmuir–Hinshelwood reaction in the situations when double occupation of binding sites is allowed for one of the adsorbates. The results of the corresponding calculations indicate that the conventional kinetic pathways including vacant sites or sites occupied by one adsorbed particle dominate up to high coverage, roughly at θ ≤ 0.9. With increasing pressure, one can, however, reach the high-coverage limit, θ ≥ 0.9, where the kinetic pathways including double occupation of sites can be dominating. This finding identifies and clarifies one of the likely reasons or complicating factors of the pressure-gap problem in heterogeneous catalysis.
