Adsorption of supercritical CO2 in aerogels as studied by small-angle neutron scattering and neutron transmission techniques

Abstract

Small-angle neutron scattering (SANS) has been used to study the adsorption behavior of supercritical carbon dioxide (CO2) in porous Vycor glass and silica aerogels. Measurements were performed along two isotherms (T=35 and 80 degrees C) as a function of pressure (P) ranging from atmospheric up to 25 MPa, which corresponds to the bulk fluid densities ranging from rho(CO2) approximately 0 to 0.9 gcm3. The intensity of scattering from CO2-saturated Vycor porous glass can be described by a two-phase model which suggests that CO2 does not adsorb on the pore walls and fills the pore space uniformly. In CO2-saturated aerogels an adsorbed phase is formed with a density substantially higher that of the bulk fluid, and neutron transmission data were used to monitor the excess adsorption at different pressures. The results indicate that adsorption of CO2 is significantly stronger in aerogels than in activated carbons, zeolites, and xerogels due to the extremely high porosity and optimum pore size of these materials. SANS data revealed the existence of a compressed adsorbed phase with the average density approximately 1.07 gcm3, close to the density corresponding to closely packed van der Waals volume of CO2. A three-phase model [W. L. Wu, Polymer 23, 1907 (1982)] was used to estimate the volume fraction phi3 of the adsorbed phase as a function of the fluid density, and gave phi3 approximately 0.78 in the maximum adsorption regime around rho(CO2) approximately 0.374 gcm3. The results presented in this work demonstrate the utility of SANS combined with the transmission measurements to study the adsorption of supercritical fluids in porous materials.

Supercritical Fluids in Confined Geometries

“…6.9.2, the volume fraction (ϕ 3 ) of the adsorbed phase at different temperature and pressure can be determined provided the scattering invariant measured using SAS is supplemented by additional information on the average density of the adsorbed phase (ρ 3 ) at the same (T,P) [31,32,41]. Typical Kratky plots of Q 2 I(Q) vs. Q, based on SANS data [31] from CO 2 saturated aerogel at T ¼ 35 C and different fluid densities are shown in Fig. 10.9.…”

“…The described approach was used in [31,32] to measure excess adsorption of SC CO 2 in silica aerogels as a function of pressure and temperature. Base-catalyzed silica aerogels with a nominal density ρ aerogel ¼ 0.1 g/cm 3 , corresponding to $96 % porosity, were obtained from Oscellus Technologies, Livermore, CA.…”

Adsorption-induced deformation of nanoporous materials—A review

“…Thus, ordered mesoporous materials are very suitable for studies of fluid adsorption. Template-grown silica and other self-ordered mesoporous substrates, such as ordered porous alumina, were intensively explored by in situ X-ray 22,[56][57][58][59][60] and neutron scattering [61][62][63] studies during adsorption of various fluids.…”

Pore lattice deformation in ordered mesoporous silica studied by in situ small-angle X-ray diffraction

“…Whereas pore wall deformations are difficult to measure in the case of amorphous materials (SiO 2 ), deformations of the pore lattice of ordered materials such as MCM-41 and SBA-15 can be investigated by small-angle diffraction. So far, only a few in situ sorption experiments of fluids in mesoporous materials have been conducted by X-ray (Mitropoulos et al, 1995;Albouy & Ayral, 2002;Hofmann et al, 2005;Zickler et al, 2006) and neutron diffraction (Smarsly et al, 2001;Hoinkis, 2004;Melnichenko et al, 2006). None of these were focused specifically on the effect of lattice deformations, although a slight change of the pore lattice parameter upon capillary condensation has already been noted by Albouy & Ayral (2002).…”