Porous materials hydrogen adsorption

At the plateau the maximum uptake value of the adsorbent is therefore reached. The type I or Langmuir isotherm describes the formation of a single monolayer on the surface of the porous material. No multilayers of hydrogen are formed at 77 K, because the interaction strength between single layers is too weak at temperatures higher than the critical temperature of H2. Therefore a type I isotherm is typically obtained for porous materials at adsorption temperatures higher than the critical temperature of the gas. 

Fast adsorption/desorption kinetics and relatively small (<10 kj/mol) adsorption enthalpies are observed for hydrogen adsorption on many porous materials, which indicates that physisorption on porous materials is suitable for fast recharging with hydrogen [81,82], The narrowest pores make the biggest contribution to hydrogen-adsorption capacity, whereas mesopores contribute to total pore volume, but little to hydrogen capacity, and are detrimental for the overall volumetric capacity. Hence, porous materials with very narrow pores or pore-size distributions are required for enhanced hydrogen capacity at low pressures. 

Thomas, K.M., Hydrogen adsorption and storage on porous materials. Catal. Today 120,389-398, 2007. 

Physisorption (i.e., adsorption of hydrogen) of molecular hydrogen by weak van der Waals forces to the inner surface of a highly porous material. Adsorption has been studied on various nanomaterials, e.g., nanocarbons, metal organic frameworks and polymers. 

Heat of adsorption of hydrogen on different classes of porous materials. 

The absolute adsorption is predicted from theoretical calculations and is the amount of hydrogen which is adsorbed in the porous material, not considering the gas phase. The relation between the excess adsorption (N and the absolute adsorption (N ds) can be esily derived considering a typical adsorption experiment [21] The amount of gas adsorbed on a sample (Nads) is expressed as the total amount of gas introduced in the sample cell (N ) minus the free molecules in the gas phase. (2.4) ... 

At room temperature only a small amount of hydrogen is stored in porous materials and the adsorption isotherm typically does not show any saturation described by a plateau. Instead a linear increase in the hydrogen uptake is observed up to relatively high pressure. Therefore, at room temperature the adsorption is better described by a linear Henry type isotherm [Ij. 

The activation of char, obtained from the pyrolysis of post-consumer PET bottles, with carbon dioxide at 925°C leads to highly porous materials. After a burn-off of 76% a BET area of 2500 m is reached. This material shows similar or better hydrogen adsorption properties than high-tech carbon materials such as nanotubes [40]. It was possible to charge the carbon with 2.3 wt% hydrogen at — 196°C. This fact opens the way for the use of this low-cost material as hydrogen storage. 

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