Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. II. Diffusion limited kinetics in amorphous solid water

The adsorption, desorption, and diffusion kinetics of N2 on thick (up to approximately 9 microm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption. Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 microm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films (>1 microm), N2 adsorption at 27 K results in a nonuniform distribution, where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is approximately 7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Preadsorption of Kr preferentially adsorbs onto the highest energy binding sites, thereby preventing N2 from trapping in the outer region of the film which facilitates N2 transport deeper into the porous film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor mediated, and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed second layer N2 species on top of the first surface bound layer.