Hydrogen storage on platinum nanoparticles doped on superactivated carbon

The equilibrium and kinetics of hydrogen storage on Pt nanoparticles doped on AX-21 superactivated carbon were studied. The Pt/AX-21 sample was prepared by ultrasound-assisted impregnation of H2PtCl6 in a solution of acetone. The dispersion of platinum was determined by CO/H-2 chemisorption and high-resolution transmission electron microscopy (HRTEM). Hydrogen adsorption isotherms and kinetics were measured at 298-348 K and up to 10 MPa. HRTEM images showed that Pt was well dispersed with a uniform particle size similar to 2 nm. CO chemisorption results indicated that the dispersion was 58%. By doping 5.6 wt % Pt, the hydrogen storage capacity of AX-21 was enhanced to 1.2 wt % at 298 K and 10 MPa. Furthermore, the isotherm was totally reversible and rechargeable at 298 K. The overall isosteric heats of adsorption (-23 to -15 kJ/mol) and the apparent activation energy for surface diffusion (7.6 kJ/mol) were determined from the temperature dependence of, respectively, equilibrium isotherm and diffusion time constant. Diffusion of the spiltover H atoms on carbon surface at room temperature was a slow process, which resulted in slow hydrogen uptake and desorption rates especially at higher pressures (higher surface concentrations). Desorption appeared to follow a reverse spillover process. The simple mechanistic model that we reported previously was capable of interpreting all experimental data in this study.