Physisorption of Hydrogen on Carbons

As for aU electrostatic interactions, the strength of vdW bond should depend on charges, which in turn depend on the number of electrons per atom or molecule hence the vdW interaction should be stronger between carbon atoms than carbon and hydrogen atoms and even stronger for interactions between atoms of multielectron 

There is also another topological limit on C H ratio. Physisorption proceeds only in the monolayer above the boiling point of hydrogen, and the adsorption must follow the Langmuir isotherm. Hence, the storage capacity depends on the pressme of Hj gas above the carbon surface at a fixed temperatme. It is also greatly limited by available SSA of carbon phase as provided by porous stmctme. 

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M. Rzepka, P. Lamp, M.A. de la Casa-LUlo, Physisorption of hydrogen on microporous carbon and carbon nanotubes. J. Chem. Phys. B, 102 (1998) 10894. 

As an example, Fig. 6 shows the experimental isotherms of the adsorption and desorption of hydrogen on carbon fiber Busofit-M8 and wood-based activated carbon WAC 3-00 at the nitrogen temperature. Absence of an appreciable hysteresis confirms that reversible physisorption exclusively takes place with all investigated materials. 

Benard, P, R. Chahine, Storage of hydrogen by physisorption on carbon and nanostructured materials. Scr. Mater. 56,803-808,2007. 

The low-temperature physisorption (type I isotherm) of hydrogen in zeolites is in good agreement with the adsorption model mentioned above for nanostructured carbon. The desorption isotherm followed the same path as the adsorption, which indicates that no pore condensation occurred. The hydrogen adsorption in zeolites depends linearly on the specific surface areas of the materials and is in very good agreement with the results on carbon nanostructures [24]. 

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