Hyperstoichiometric hydrogen occlusion by palladium nanoparticles included in NaY zeolite

Abstract

Amounts of occluded hydrogen in palladium-containing NaY zeolites increased with the palladium content. Allocation of mols of hydrogen atoms to palladium gave large H/Pd ratios higher than unity at 298 K under 100 kPa of hydrogen. As the palladium content increased, the proportion of the external palladium increased and the H/Pd value decreased to approach that for powdery palladium. The hyperstoichiometric occlusion of hydrogen can be attributed to palladium nanoparticles or clusters inside the pores of zeolite. Whereas the interstitial site per palladium atom was unity for powdery palladium, for the internal palladium species it seemed to be two or more. Before reaching the stoichiometric composition, PdH, the palladium species released substantial heat on hydriding similarly to powdery palladium. After the hydrogen content exceeded the stoichiometry, the heat on hydriding was as low as 1 kJ/mol H 2.