Microscopic theory of hysteretic hydrogen adsorption in nanoporous materials

(Chemical Equation Presented) Understanding gas adsorption confined in nanoscale pores is a fundamental issue with broad applications in catalysis and gas storage. Recently, hysteretic H₂ adsorption was observed in several nanoporous metal - organic frameworks (MOFs). Here, using first-principles calculations and simulated adsorption/desorption isotherms, we present a microscopic theory of the enhanced adsorption hysteresis of H ₂ molecules using the MOF Co(1,4-benzenedipyrazolate) [Co(BDP)] as a model system. Using activated H₂ diffusion along the small-pore channels as a dominant equilibration process, we demonstrate that the system shows hysteretic H₂ adsorption under changes of external pressure. For a small increase of temperature, the pressure width of the hysteresis, as well as the adsorption/desorption pressure, dramatically increases. The sensitivity of gas adsorption to temperature changes is explained by the simple thermodynamics of the gas reservoir. Detailed analysis of transient adsorption dynamics reveals that the hysteretic H₂ adsorption is an intrinsic adsorption characteristic in the diffusion-controlled small-pore systems.