Letter pubs.acs.org/journal/estlcu
Surface Condensation of CO2 onto Kaolinite H. T. Schaef,* V.-A. Glezakou, A. T. Owen, S. Ramprasad, P. F. Martin, and B. P. McGrail Pacific Northwest National Laboratory, Richland, Washington 99354, United States ABSTRACT: The fundamental adsorption of CO2 onto poorly crystalline kaolinite (KGa-2) under conditions relevant to geologic sequestration has been investigated using a quartz crystal microbalance (QCM) and density functional theory (DFT) methods. The QCM data indicated linear adsorption of CO2 (0−0.3 mmol of CO2/g of KGa-2) onto the kaolinite surface up through the gaseous state (0.186 g/cm3). However, in the supercritical region, the extent of CO2 adsorption increases dramatically, reaching a peak (0.9−1.2 mmol of CO2/g of KGa-2) near 0.40 g/cm3, before declining rapidly. DFT studies of interactions of CO2 with kaolinite surface models confirm that surface adsorption is favored up to ∼0.34 g/cm3 of CO2, showing distorted T-shaped CO2−CO2 clustering, typical of supercritical CO2 aggregation over the surface at higher densities. Beyond this point, the adsorption energy gain for any additional CO2 becomes smaller than the CO2 interaction energy (∼0.2 eV) in the supercritical medium, resulting in the desorption of CO2 from the kaolinite surface.
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housed inside an oven for maximal thermal control (±0.1 °C). Polished gold-coated 10 MHz quartz crystals (International Crystal, Oklahoma City, OK) were coated with 4 Å, at the density where the highest adsorption energy is observed (∼0.34 g/cm3), although this value may slightly vary with the choice (slab size) of model. Both experiment and theory converge on a critical desorption density of approximately 0.3−0.4 g/cm3. On the basis of molecular simulations, CO2 intercalation in kaolinite is unlikely and interactions are strictly limited to surface adsorption.
the experimental unit cell parameters, with an added vacuum space in the z direction. The model cell was decided after a series of exploratory calculations such that the experimentally relevant range of densities could be covered, while allowing for enough vacuum space to simulate adsorption conditions. The following supercell dimensions were used for the simulations: A = 10.298 Å, B = 17.868 Å, C = 20.522 Å, α = β = 90°, and γ = 89.791° (including an 8 Å vacuum slab). The resulting CO2 densities covered the range from 0.05 to ∼0.5 g/mL. Densities of up to 0.198 g/cm3 correspond to gaseous densities and from then on to supercritical densities. At each density, the system was relaxed and equilibrated at 50 °C, corresponding to the experimental temperature, and then thermally annealed and optimized to obtain an equilibrium structure. In all simulations, only the bottom layer of silicon atoms was kept frozen and all other atoms were allowed to relax. A similar approach was used for N2 adsorption studies to help extract correction parameters of surface roughening. Validation of the rms correction factor for the QCM and molecular modeling with N2 and the kaolinite surface were conducted under relevant experimental conditions. Results of these simulations indicate the maximal adsorption at ∼0.18 eV/ N2, occurring at approximately 0.13 g/cm3 of adsorbed N2 (Figure 3, purple curve). Equilibrium structures at the different
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SUMMARY Experimentally derived high-pressure adsorption isotherms for N2 and He on KGa-2 produced similarly shaped profiles in which the total amounts of absorbed gas did not increase significantly with an increase in pressure. Measured sorption of CO2 onto the kaolinite surface produced distinctively different isotherms, in which CO2 concentrations increase dramatically until reaching a maximum at a density corresponding to ∼0.40 g/cm3. Subsequently, surface concentrations of CO2 decreased with an increase in pressure. At the maximal CO2 adsorption (0.40 g/cm3), the coverage reached ∼0.9−1.2 mmol of CO2/g of KGa-2. Density functional theory calculations showed that initial adsorption steps are energetically favorable until the first CO2 layer is formed, up to densities of ∼0.34 g/cm3. As more CO2 layers form, the adsorption energy decreases beyond the stabilization energy of the supercritical phase (∼0.2 eV) and desorption becomes the principal mechanism.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 3. Adsorption energy of CO2 on kaolinite (red) and concomitant change in the adsorption energy (blue) with respect to the density. The purple line shows the adsorption energy for N2. The green line marks an estimate of the CO2−CO2 interaction energy per mole of CO2 in the supercritical medium.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Fossil Energy. The simulations were possible through a user proposal from EMSL, a national scientific user facility at Pacific Northwest National Laboratory (PNNL) that is managed by the DOE’s Office of Biological and Environmental Research. PNNL is operated for the DOE by Battelle Memorial Institute under Contract DE-AC06-76RLO-1830.
densities show that the N2 molecules organize in a single layer perpendicular to the surface. The shortest N2−N2 contacts are ∼5.5 Å apart (at the highest density), while the N−H (surface) contacts are almost uniformly ∼2.2 Å. As the N2 density increases, the N2 molecules continue to form a single layer but start also adopting a distorted T-shaped orientation, with the shortest N2−N2 distances nearing 3.6 Å, while maintaining the N−H (surface) contacts at 2.2−2.3 Å. On the basis of these results, the rms value allowing for a single layer of N2 to form on the kaolinite surface was established and subsequently used to remove the roughness from the CO2 QCM data. The density functional calculations showed an increase in the adsorption energy of CO2 on the kaolinite surface (Figure 3). These simulations indicate that although initial CO2 (gas, I) adsorption is unfavorable, as the number of CO2−CO2 interactions increases the overall interactions with the mineral
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