Understanding the Hydrophilicity and Water Adsorption Behavior of Nanoporous Nitrogen-Doped Carbons

Through molecular simulation we exposed the anomalous water adsorption behavior in slit-shaped and disordered nitrogen doped carbons. Instead of Langmuirian, water adsorption proceeds via mechanisms analogous to crystal nucleation: birth and spread of (poly) nucleation sites in one, two, and three dimensions forming water nanowires/pillars and water clusters before complete pore filling via capillary condensation. The adsorption of water and the adsorption hysteresis in N-doped carbons are strongly influenced by the pore-size. The smaller the pore-size, the smaller is the pressure at which the above-mentioned process tends to occur in N-doped carbons. The nucleation analogues are clearly visible in a pore that has a homogeneous pore structure, whereas in disordered pore structures, the complex and distributed pore-sizes disturb these nucleation analogue patterns especially at lower relative pressures. The effect of the adsorbed water molecules on the connectivity of the available pore volume is discussed. Adsorption at zero loading confirmed water molecules preferentially adsorb over specific zones, which corresponds to regions with a high local density of N atoms rather than specific sites or type of N (such as graphitic or pyridinic). Simulated adsorption isotherms showed the hydrophilicity introduced to the carbon pore via N doping is sensitive to impurities such as water, such that it can affect the ability of the carbon framework to host another guest molecule such as CO2, a prime fluid involved in flue gas.