Abstract
Palladium-modified activated carbon fibers (Pd-ACF) were synthesized by meltspinning,
carbonization and activation of an isotropic pitch carbon precursor premixed
with an organometallic Pd compound. The hydrogen uptake at 25 oC and 20 bar on Pd-
ACF exceeded the expected capacity based solely on Pd hydride formation and hydrogen
physisorption on the microporous carbon support. Aberration-corrected scanning
transmission electron microscopy (STEM) with sub-Ångstrom spatial resolution provided
unambiguous identification of isolated Pd atoms occurring in the carbon matrix that
coexist with larger Pd particles. First principles calculations revealed that each single Pd
atom can form Kubas-type complexes by binding up to three H2 molecules in the pressure
range of adsorption measurements. Based on Pd atom concentration determined from
STEM images, the contribution of various mechanisms to the excess hydrogen uptake
measured experimentally was evaluated. With consideration of Kubas binding as a viable
mechanism (along with hydride formation and physisorption to carbon support) the role
of hydrogen spillover in this system may be smaller than previously thought.
