Detection of Hydrogen Spillover in Palladium-Modified Activated Carbon Fibers during Hydrogen Adsorption

Abstract

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.

Enhanced Hydrogen Dipole Physisorption, Final Report

“…A model-independent, virial-type thermal equation was used to calculate isosteric heats from the experimental isotherms. Consistent with the literature, the KC24 isotherm has an "S" shape due to lattice expansion associated with H2 adsorption [48,49]. The isosteric heat is positively correlated with the size of the alkali metal intercalant.…”

“…Although direct observation of atomic hydrogen has been recently demonstrated by transmission electron microscopy on a single graphene sheet 55 , volumetric measurements of hydrogen uptake are never this sensitive. Investigations using inelastic neutron scattering also recently yielded evidence of the formation of new C-H bonds on the surface of activated carbon fibers after spillover from palladium clusters; however, reversibility was unclear and enhancement attributed to spillover was not tested at high pressures 49 . Interestingly, theoretical models cannot account for the remarkable enhancement of hydrogen uptake capacity in carbon materials due to spillover reported by experimental investigation 49,56 ,57,58 .…”

Influence of CO2 activation on hydrogen storage behaviors of platinum-loaded activated carbon nanotubes

“…Many researchers have investigated with the carbon materials in presence of metal catalysts (Pt, Pd, Ni, and so on), for the purpose of increasing the hydrogen storage capacities, both experimentally and theoretically [22,23,38]. These metal particles play an important role in enhancing of the hydrogen adsorption by the affinity of the metal particles and hydrogen, afterwards metal dissociates the hydrogen molecules and spills them over the metal-carbon interfaces [39][40][41].…”

Adsorption and diffusion of atomic hydrogen on a curved surface of microporous carbon: A theoretical study

“…The hydrogen storage capacity of carbon materials is reported to be enhanced by adding a small amount of metal nanoparticles such as Pt [14][15][16][17][18] or Ni [19]. This phenomenon is explained as follows: hydrogen molecules are dissociatively adsorbed on the surface of Pt particles, and then atomic hydrogen moves to the carbon surface ('hydrogen spillover').…”