Temperature Dependence of Hydrogen Adsorption on Pd-Modified Carbon Blacks and Their Enthalpy-Entropy Changes

Metal-carbon composites have recently gained attention as potential hydrogen storage materials. In the present investigation, carbon blacks (CBs) with 0.6 mass %, 4.9 mass %, and 9.3 mass % of Pd were prepared to investigate the cooperative effect together with Pd and CB for hydrogen storage. The hydrogen adsorption isotherms were measured at 77 K, 98 K, 123 K, 148 K, 173 K, 223 K, and 273 K under mild pressures below 1 MPa. The lower temperature gave the higher hydrogen content. Almost all the hydrogen contents of Pd-modified CBs exceeded the sum of the adsorption contents of CB and the occluded amounts of the assumed hydride, PdH0.6. The highest hydrogen content was recorded for Pd 0.6 mass %-modified CB at 77 K. At temperatures above 77 K, CBs with the higher Pd contents adsorbed more hydrogen than Pd 0.6 mass %-modified CB, and they indicated an increase in the absolute values of adsorption enthalpy with the progress of adsorption. Pd was thought to be at first blocking deep potential sites, with accessibility to hydrogen acceptable sites gradually increasing as adsorption progressed.