Supercritical CO2 Mediated Incorporation of Pd onto Templated Carbons: A Route to Optimizing the Pd Particle Size and Hydrogen Uptake Density

Palladium nanoparticles are deposited onto zeolite template carbon (ZTC) via supercritical CO2 (scCO(2)) mediated hydrogenation of a CO2-phillic transition metal precursor. The supercritical fluid (SCF) mediated metal incorporation approach enabled the decoration of ZTC with 0.2-2.0 wt % of well-dispersed Pd nanoparticles of size 2-5 nm. The resulting Pd-doped ZTCs exhibit enhanced hydrogen uptake and storage density. The ZTC (with surface area of 204-6 m(2)/g) had a hydrogen storage capacity (at 77 K and 20 bar) of 4.9 wt %, while the Pd-ZTCs had uptake of 4.7-5.3 wt. % despite a surface area in the range 1390-1858 m(2)/g. The Pd-ZTCs thus exhibit enhanced hydrogen storage density (14.3-18.3 mu mol H-2/m(2)), which is much higher than that of Pd-free ZTC (12.0 mu mol H-2/m(2)). The hydrogen isosteric heat of adsorption (Q(st)) was found to be higher for the Pd doped carbons (67 kJ/mol) compared to the parent ZTC (53 kJ/mol). The deposition of small amounts of Pd (up to 2 wt %) along with well Pd nanoparticles of size of 2-5 nm is essential for the enhancement of hydrogen uptake and illustrates the importance of optimizing the balance between metal loading/particle size and surface area to achieve the best metal/porous carbon composite for enhanced hydrogen uptake.