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Palladium-modified activated carbon fibers (Pd-ACF) are being evaluated for adsorptive hydrogen storage at near-ambient conditions because of their enhanced hydrogen uptake in comparison to Pd-free ACF. The net uptake enhancement (at room temperature and 2 MPa) is in excess of the amount corresponding to formation of β-Pd hydride and is usually attributed to hydrogen spillover. In this paper, inelastic neutron scattering was used to investigate the state of hydrogen in Pd-containing activated carbon fibers loaded at 77 K with 2.5 wt % H2. It was found that new C−H bonds were formed, at the expense of physisorbed H2, during prolonged in situ exposure to 1.6 MPa hydrogen at 20 °C. This finding is a postfactum proof of the atomic nature of H species formed in presence of a Pd catalyst and of their subsequent spillover and binding to the carbon support. Chemisorption of hydrogen may explain the reduction in hydrogen uptake from first to second adsorption cycle.
Contescu et al. (Thu,) studied this question.