Thermodynamic study of adsorption of NO on copper-based catalysts

Abstract

The adsorption properties of NO on copper-based catalysts stabilized on crystalline (ZSM-5) and amorphous (SiO ₂ –Al ₂ O ₃ ) matrices prepared by different methods have been studied. The catalysts were prepared by ionic exchange on the two matrices and also by impregnation and pyrolytic mixture on the amorphous one. The oxidation state and surface concentration of copper, in terms of Cu/(Al+Si), were determined by X-ray photoelectron spectroscopy (XPS) analyses. The nature of the NO adsorbed species and their energy of interaction with the adsorption sites, as well as the number of the Cu adsorption sites, were studied by temperature-programmed desorption (TPD), adsorption calorimetry and adsorption isotherms collected at different temperatures (40–100°C), using NO as adsorbate. A thermodynamic model applied to the NO adsorption isotherms was utilized to describe the behaviour of the adsorbate–adsorbent system in terms of surface energy distribution of the copper sites (number of Cu sites vs. Δ _a H). The characteristic parameters of adsorption and the new parameter ‘half-coverage temperature at unit pressure’ for each type of site were obtained. The SiO ₂ –Al ₂ O ₃ catalysts showed more energetic interaction with NO (-110<Δ _a H/kJ mol ^-1 <-10), with higher heterogeneity, than copper zeolite (-50<Δ _a H/kJ mol ^-1 <-10).