Usefulness of CO2 adsorption at 273 K for the characterization of porous materials

摘要

Characterization of microporous solids has been carried out by N2 (subatmospheric pressures) and CO2 adsorption (atrnsubatmospheric and high pressure) at 77 K and 273K, respectively. Because the relative pressure range covered by our CO2 studyrnis similar to the one covered with N2 at 77 K, a suitable comparison of both adsorptives has been possible. The results show thatrncarbon molecular sieves, with very narrow microporosity can not be characterized by N2 at 77 K due to the existence ofrndiffusional problems. This is also extensive to many other carbon materials, such as carbon fibers and activated carbons with lowrndegree of activation. In addition, the characteristic curves corresponding to N2 adsorption on microporous carbons have therncommon feature of presenting a large downwards deviation for low values of relative pressures. As a consequence of theserndiffusional limitations, N2 adsorption at 77 K cannot be used to determine the micropore volume of the narrowest porosity and thernmicropore size distribution (MPSD) can not be obtained from N2 adsorption data. Thus, in this work, we analyze and encouragernthe use of CO2 (I.e. at 273 K) as a complement to N2 adsorption. For samples that do not have diffusional limitations, it is shownrnthat the MPSD obtained from both adsorptives (N2 at low pressures and CO2 at low and high pressures) are very similar, whichrnreinforces the similarities between CO2 and N2, and confirms the validity of CO2 to characterize the microporosity. In addition, itrnhas been shown that by using CO2 adsorption, the micropore volume and the MPSD of samples with very narrow MPSD can bernobtained. Finally, the usefulness of CO2 for the characterization of activated carbons with different activation degree has beenrndemonstrated.