Physical adsorption of nitric oxide on metal oxides

Adsorption isotherms have been measured at 77.5 K for nitric oxide and nitrogen on Al₂O₃, MgO, ZnO and NiO, and at 90.2 K. for nitric oxide on A1₂O₃ and NiO. Three isotherm measurements at 77.5 K were made on the Al₂O₃ sample for each adsorbate to examine the effect of different degrees of surface dehydroxylation. The latter was assessed by means of infrared absorption studies on an Al₂O₃ disc. Isosteric heats for NO adsorption on Al₂O₃ and NiO increase from ca. 8 kJ mol^?1 and 6 kJ mol^?1 (respectively) at half monolayer coverage to near the value of the enthalpy of sublimation (16.6 kJ mol^?1) at monolayer completion. These results are discussed in terms of adsorbate dimerisation. Anomalous adsorption-desorption behaviour for the NONiO system is discussed. Effective adsorbate molecular cross-sectional areas and results for N2 adsorption on preadsorbed NO do not support the existence of either localisation or micro-porosity.