Adsorption measurements
CO2 adsorption isotherms at three different temperatures (0°C, 20°C and 40°C) were measured using an ASAP 2020 (Micromeritics) up to 120 kPa. Each sample was outgassed for 12 h at 150°C under vacuum (P < 10-7 kPa) prior to each CO2 adsorption test.
H2 adsorption capacity was evaluated at – 196°C up to 100 kPa by using a 3Flex automatic device (Micromeritics) equipped with a high-vacuum system and three 0.013 kPa pressure transducers, one per analysis port. The samples were outgassed for 24 h at 120°C under vacuum (P < 10-7 kPa) for at least 24 h before being weighed. An additional outgassing was carried out for at least 6 hours more on the analysis ports at the same temperature and under the same final vacuum as in the previous outgassing step. High-resolution H2 isotherms were obtained by dosing small hydrogen amounts, as low as 3 cm3/g .The determination of the warm and cold volume was carried out after H2adsorption-desorption analysis to avoid any possibility of a helium entrapment in the ultramicroporosity.
Measurements of C2H4 and C2H6 isotherms at – 58 and – 73°C were carried out using again the high-resolution Micromeritics 3Flex adsorption instrument. The latter was equipped with a high-vacuum system, three 0.1 Torr pressure transducers, and was coupled with a cryostat (Cold Edge Technologies). The isotherms of C2H4 and C2H6 as well as CO2 at different temperatures were used to calculate the isosteric heats of adsorption. The calculations were performed using the analytical form of virial equation [34].