Physical adsorption of nitric oxide on low and high energy surfaces

Abstract

Stepwise isotherms for adsorption of krypton and nitrogen on barium fluoride at 77.5 K indicate a homogeneous surface. In contrast, isotherms for nitric oxide adsorbed on BaF 2 at 77.5 and 90.2 K are of BDDT Type I shape, with no indication of stepwise character. These results are discussed in terms of possible dimerization and adsorbate mobility, and are compared with adsorption on a “low energy” surface, i.e., polyvinylidene chloride (PVDC). Nitric oxide on PVDC at 77.5 and 90.2 K gives shallow Type II isotherms, with corresponding values of the isosteric heat of adsorption close to the value of the enthalpy of sublimation of NO. The initial heats of adsorption for NO on PVDC are consistent with dimerization of the adsorbate. The effective molecular cross-sectional areas of NO, found from comparison with nitrogen BET monolâyer amounts, were 0.10 nm 2 on BaF 2 and 0.125 nm 2 on PVDC. Semiempirical calculations of isosteric heats of adsorption are discussed.