Localization of Hydrogen in Porous Space of Activated Carbon
Adsorption of hydrogen on activated carbons (ACs) was studied in quasi-isobaric conditions (P = 1.1·10 <sup> 5 </sup> Pa) using low-temperature <sup> 1 </sup> H NMR spectroscopy. The ACs were prepared by carbonization of porous phenol-formaldehyde resin beads with subsequent activation by CO2 with the burn-off degree of 86% (AC-86) and 47% (AC-47). It has been shown that the hydrogen adsorption increases with increasing concentration of water pre-adsorbed in pores of AC-86 (specific surface area 3463 cm <sup> 2 </sup> /g). In the case of the water adsorption in the amounts of 40 mg/g, the hydrogen adsorption reaches 1.4 mg/g at T = 200 K. The adsorbed hydrogen was localized in the slit-shaped pores where the zero shielding effect of the surface was shown. The results obtained are explained by the existence of slit-shaped pores with three minima in the cross section curve of the potential energy of adsorption, and two of them are localized near the pore walls (where the shielding effect of the surface is larger) and the third one is in the middle of pores (where the shielding effect is close to zero). All adsorbates with exception of hydrogen (water, saturated hydrocarbons) are localized mainly at the pore walls. A part of water can be moved to the middle of pores with decreasing temperature that reduces the screening effect of the surface. Adsorbed hydrogen is localized in the middle space of the pores with the presence of co-adsorbates or in the narrowest pores where co-adsorbates cannot be adsorbed.