(132a) Application of Do & Do Models to Single-Walled Carbon Nanotubes: Insight into Water Adsorption In Microporous Carbons

This paper describes the application of a water-activated carbon isotherm model to study the effect of oxidation and operating temperature on water adsorption data collected for single-walled carbon nanotubes (SWNTs). Water adsorption isotherms in carbon adsorbents are commonly curve-fitted by several adsorption isotherm equations. Our previous study showed that of these isotherm equations the Do and Do equation modified by Marban et al. (2004) was the most suitable equation for interpreting the experimental water isotherms collected for single-walled carbon nanotubes (SWNTs). This model is able to deconvolute the total experimentally-observed isotherm with great accuracy into two components: micropore filling and adsorption on primary sites. We applied the modified Do model to better understand water adsorption mechanisms on SWNTs exposed to ozonation and different temperatures. Water adsorption isotherms were obtained using a customized high sensitivity gravimetric balance technique at different temperatures from 5 to 55 oC. SWNT samples exposed to ozone were analyzed using x-ray photospectroscopy (XPS) and presented the gradually increasing oxygenated functional groups with increasing ozone concentrations. Water adsorption isotherm of the ozone treated SWNT samples showed the water adsorption by functional groups at lower relative pressure (P/Po) gradually increased with increasing ozone concentrations. Parameters of curve-fitting by modified Do model showed that functional groups (So) also increased as ozone concentration increased which was identical to XPS result whereas water cluster size (m) diffusing into micropore decreased. Overall, the oxidation of nanocarbons enhanced the water uptake at lower vapor pressures, and reduced the size of water cluster that grows on primary sites and the water cluster that migrates into the micropores. Water adsorption isotherms at different temperatures on SWNT samples showed that as temperature increased the relative pressure at which water uptake abruptly increased. Parameters of curve-fitting by modified Do model showed that as temperature increased the functional group concentrations were almost constant, micropore volume decreased and the size of cluster size penetrating into micropore increased which is due to the thermal expansion of water clusters. Exploring the applicability of water-activated carbon models to water-carbon nanotubes is a goal unique to our research. This work should help identify those physical and chemical properties of microporous carbons that are most influencing in uptake of water vapor from air and process streams.