Hydrogen adsorption isosteric enthalpies

Fig. 4. Differential heats of adsoiption of hydrogen on A7. Keys circle, adsorbed at 298 K triangle, 243 K square, 223 K diamond, 203 K broken line, isosteric enthalpy of adsorption.

While the isosteric enthalpy of adsorption values reported so far for hydrogen adsorption on porous materials are too low for significant adsorption at ambient temperatures, high values would make hydrogen desorption very difficult. An intermediate weak nondissociative coordination or other weak chemisorption with higher isosteric enthalpy of adsorption values with facile adsorption and desorption characteristics is required for storage applications. 

The design of porous materials with improved adsorption capacity under ambient conditions involves enhancing hydrogen surface site interactions. The maximum enthalpy of hydrogen adsorption obtained experimentally for MOFs is 12-13 kj moU while for boron doped carbon it is 12.5 kJ mol and for zeolites it is 18 kJ moU. The MOFs with the highest isosteric enthalpies of adsorption obtained currently are Zn3(bdc)3.Cu(pyen) and Ni2(dhtp) and both of these materials have open metal centres. The adsorption thermodynamic results, and structural and spectroscopic observations of hydrogen adsorbed in pores with open metals centres in MOFs show that the interactions observed so far are much weaker than those observed for... 

For the adsorption of hydrogen, it may be readily shown that = —SR for a variety of adsorbents [18]. For the delivery cycle reasonable values of adsorption and desorption pressures may be taken as Pi = 30 bar and P2 = 1.5 bar respectively, which upon substitution in Eq.(2) yield = —15.1 kJ/mole at r = 298 K. Thus, for optimum delivery of hydrogen between pressures of 30 bar and 1.5 bar at 298 K, an adsorption enthalpy change of -15.1 kJ/mole is desired. The isosteric heat of adsorption of hydrogen on carbons is substantially less, typically about 5.8 kJ/mole. However, if cryogenic conditions are acceptable then one may determine an optimum teu terature of operation in the case of activated carbon, for which delivery is maximized. Following Eq.(2), this ten terature is obtained as... 

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