Adsorption of hydrogen on porous materials of activated carbon and zeolite NaX crossover critical temperature

Hydrogen adsorption properties of typical porous materials, activated carbon and zeolite NaX, are studied through isotherm measurements at pressure up to 7 MPa crossover the critical temperature. The Soave–Redlich–Kwong equation of state is selected for calculation of compressibility factor of hydrogen. The two adsorbents exhibit remarkably different hydrogen uptake capacities, which are attributed to their framework structures and porous surfaces. At near critical temperatures, significant drops and recoveries of excess adsorption are observed on the isotherms. The multilayer adsorption and associated reduced density and increased volume of the adsorbed phase are derived from the extents of adsorption drops. The recovery is attributed to the bias of compressibility factor from the assumed equation of state. The negative adsorption is observed at subcritical temperature, and explained by the conception of pore critical temperature and relevant calculation. The isosteric heats of adsorption are determined as 2.1 kJ/mol for activated carbon and 4.4 kJ/mol for NaX, respectively.

Graphical abstractHydrogen adsorptions of activated carbon and zeolite NaX are studied through isotherm measurements crossover critical temperature. Some unique adsorption features are observed and explained.Figure optionsDownload full-size imageDownload as PowerPoint slide