Anti-Arrhenius Passage of Gaseous Molecules through 2D Membranes

Chemical reactions and other physicochemical phenomena are known to follow the Arrhenius-like kinetics, i.e. they are accelerated upon heating and vice versa. There exist though stepwise processes which rates can decrease with temperature if, for example, adsorbed intermediates are involved. As surface-mediated diffusion occurs in porous materials with nanoscale thickness, lowering the temperature is expected to facilitate the permeation of condensable gases. In this study, we implement temperature-variable experiments in the range from -50 to +50 °C and observe anti-Arrhenius behavior in freestanding carbon nanomembranes. The permeation rate of water vapor is found to drop manyfold with warming, while the passage of ammonia molecules appears to explode when the membrane is cooled down to the dew point. Liquefaction of isobutylene shows no enhancement for its transmembrane flux which is consistent with the material’s pore architecture. The effects are described by the Clausius-Clapeyron relation and highlight the key role of gas-surface interactions in two-dimensional membranes.