Water vapour adsorption and contrast-modified SAXS in microporous polymer-based carbons of different surface chemistry

Abstract

The adsorption of water vapour on highly microporous activated carbons with different surface chemistry is investigated by small angle X-ray scattering (SAXS) as well as by adsorption isotherms. The water changes the intensity of the SAXS in a way that depends on how the pores are filled. With wetting liquids such as hexane, a pseudo binary model can be assumed in which pore-filling in reciprocal space q is described by a density function p(q). For water, clusters develop, even in the most oxidized carbon, creating a fully ternary system. In the Porod scattering region, however, the final slope is insensitive to the liquid-vapour interfaces. In this region, for the less oxidized samples, p(q) shows reasonable agreement with the adsorption isotherms. At low relative pressure P /P 0 , however, the SAXS results indicate a small degree of filling (about 10%) that is not reflected in the isotherms. The highly oxidized sample attains a degree of filling of about 70% that, unlike the corresponding isotherm, is constant for P /P 0 > 0. These differences may be due to kinetic effects and/or ageing, involving either redistribution of the water molecules or modifications of the surface groups.

Structural behaviors in water adsorbing phenomena on nanoporous carbon fabricated from waste rice husk

“…The absence of sharp diffraction peaks in this region signifies that the sample is free from microcrystalline or impurities. 24) Figure 7 presents the SAXS curves from the NPC at various water contents. Several features are notable.…”

“…Also, the scattering intensity from XRD and SAXS contains additional, complementary structural information (the regularity of order assemble structure and the electron deviation occurred by the adsorbent model). [24][25][26] In this study, therefore, the present work focuses on the exploitation of XRD and SAXS for characterizing the structural behavior of the NPC and the water model during the desorption to confirm the structure behavior in different adsorption processes. Water vapor was selected as adsorbate, considering the autonomous desorption of gas species ridden off the pressure conditions.…”