Effects of temperature on water adsorption on controlled microporous and mesoporous carbonaceous solids

Water adsorption isotherms in porous carbons were measured at temperatures in the range 263 and 298 K. To investigate the role of porous structure, we used RF-50, RF-100, RF-200, derived from resorcinol–formaldehyde cryogels, and an activated carbon (B-AC) derived from bamboo. RF-50 is microporous while RF-100 and RF-200 are micro- and mesoporous with non-overlapping distributions of micropores and mesopores. The isotherms of RF-50 exhibit only one step at all temperatures, and they are independent of temperature when plotted against the reduced pressure. On the other hand, the isotherms of RF-100 and RF-200 have two-step uptake at 298 K, while they exhibit only one-step uptake at 263 K; the first step of the 298 K-isotherms is practically the same as the 263 K-isotherm. We attributed the first step to adsorption in micropores and the second one to mesopores. The isotherms of B-AC, showing a single broad hysteresis loop, could be viewed as the merging of the two steps that we observed with RF-100 and RF-200 because of the broad pore size distribution of B-AC. From the analysis of all isotherms we developed a universal mechanism for water adsorption in porous carbon, which is based on the temperature dependence of the clustering of water molecules.