Hydrogen adsorption and spill-over effects on H-Y and Pd-containing Y zeolites - An experimental and theoretical investigation

The adsorption of hydrogen on H-Y and Pd(0)-H-Y zeolites was investigated by IR (in the 300-20 K temperature range) and EXAFS spectroscopies and the energy barriers for diffusion of hydrogen in the internal voids of the faujasite structure evaluated by ab initio calculations. On H-Y, H-2 is adsorbed at 20 K inside the super-cages giving rise to 1:1 -OH center dot center dot center dot H-2 hydrogen bonded complexes with the Bronsted acid groups, as well as less specific interaction with the cage walls. Due to the high energy barrier for crossing the framework hexagonal apertures, H-2 cannot enter the sodalite-cages. On Pd(0) containing zeolites, characterized as far the dispersion of the metal is concerned by TEM, EXAFS and IR spectroscopy of adsorbed CO, hydrogen is adsorbed in atomic form to give a metal-hydride phase; subsequent spill-over effects allow confinement of hydrogen also in the small sodalite-cages, as demonstrated by H/D isotopic substitution experiments.