Physical adsorption of nitric oxide on low and high energy surfaces》.pdfVIP

Physical Adsorption of Nitric Oxide on low and High Energy Surfaces P. B. BARRACLOUGH, C. E. BROWN AND P. G. HALL Department of Chemistry, The University of Exeter, Exeter EX4 4QD, Devon, England Received January 15, 1973; accepted August 15, 1973 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~ 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 dimerizafion and adsorbate mobility, and are com- pared 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 monolayer amounts, were 0.10 nm2 on BaF~ and 0.125 nm 2 on PVDC. Semi- empirical calculations of isosteric heats of adsorption are discussed. INTRODUCTION and Woodward and Jonathan (5) reinterpreted A previous investigation (1) showed that the results of Granville and Hall (6) as support- ing dirnerization of NO on KC1