Study of HI, HBr and NO2 adsorption on graphite and silver aerosol particles using short-lived isotopes

Radiochemical techniques were used to study in situ the adsorption of halides and nitrogen dioxide on aerosol particles. Using short-lived isotopes, H136I, H86Br, and 13NO2 molecules were synthesized and adsorbed on graphite and silver aerosols. By measuring temperature-dependent desorption of these compounds from the particles, adsorption enthalpies could be determined for adsorption on silver particles, which agree well with data on macroscopic surfaces. The unexpected shapes of the desorption curves from graphite particles are probably due to a distribution of different adsorption sites. The preparation of the aerosol for the desorption study gave also interesting insights into the adsorption process. In spite of rather high adsorption enthalpies the sticking probability of H136I on silver particles seemed to be rather low. It was concluded that the molecules accommodating on the surface in a physisorbed state may either chemisorb or desorb back into the gas. This model could explain the strong temperature dependence of the chemisorption rate.