Low-Pressure Hysteresis in Adsorption: An Artifact?

Hysteresis phenomena are usually observed in the characterization of porous solids using gas adsorption of polar and nonpolar probe molecules. Commonly, hysteresis phenomena take place at high relative pressures due to the presence of metastable states associated with the capillary condensation of the probe molecule on mesopores. However, low-pressure hysteresis phenomena have also been reported for porous solids such as ordered mesoporous silicas, zeolites, and activated carbons. Unfortunately, the reason behind these processes taking place at low pressure is still unclear. Here we prove that the low-pressure hysteresis is rather an artifact associated with the lack of equilibrium in the adsorption isotherm and/or the lack of proper outgassing, mainly found in porous solids where narrow pore constrictions are expected.