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].