The role of destabilization of palladium hydride in the hydrogen uptake of Pd-containing activated carbons

This paper reports on differences in stability of Pd hydride phases in palladium particles with various degrees of contact with microporous carbon supports. A sample containing Pd embedded in activated carbon fibre (2 wt% Pd) was compared with commercial Pd nanoparticles deposited on microporous activated carbon (3 wt% Pd) and with support-free nanocrystalline palladium. The morphology of the materials was characterized by electron microscopy, and the phase transformations were analysed over a large range of hydrogen partial pressures (0.003-10 bar) and at several temperatures using in situ x-ray diffraction. The results were verified with volumetric hydrogen uptake measurements. Results indicate that higher degrees of Pd-carbon contacts for Pd particles embedded in a microporous carbon matrix induce efficient 'pumping' of hydrogen out of β- PdH_x. It was also found that thermal cleaning of carbon surface groups prior to exposure to hydrogen further enhances the hydrogen pumping power of the microporous carbon support. In brief, this study highlights that the stability of β- PdH_x phase supported on carbon depends on the degree of contact between Pd and carbon and on the nature of the carbon surface.