Supercritical adsorption of ‘permanent’ gases under corresponding states on various carbons Original Research Article

Adsorption isotherms at 298 K have been measured on four different carbons for six gases: argon, carbon monoxide, hydrogen, methane, nitrogen and oxygen at pressures up to 7 MPa. For all the carbons, methane, on a molar basis, was adsorbed to a greater extent than all the other gases and hydrogen the least. These same gases were then adsorbed using the same carbons at temperatures corresponding to a reduced temperature of 2.36 up to reduced pressures of about 1.0. Under those conditions, five gases presented very similar uptakes on any given carbon although the porous nature of these carbons was very different. The exceptional gas was hydrogen. For every carbon, hydrogen was adsorbed to a much greater extent than the other gases. In all cases, the isotherms were reversible suggesting that only physisorption was involved. It would appear that hydrogen can penetrate into pores within the structure of carbons that go undetected by conventional pore or surface characterisation techniques. The carbon with the highest 77 K N2 D-R micropore volume, AX-21, still showed the greatest hydrogen uptake under these reduced conditions.