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44674-AC10
Three-Dimensional Reconstruction of Mesoporous Materials from Gas Adsorption and Structure Factor Data
We are developing a general method for the three-dimensional reconstruction of mesoporous materials by evolutionary optimization against target data. In work to date, the method is applied specifically in reconstruction of amorphous material models using gas adsorption data, structure factor data, or a combination of both data. A recently introduced lattice-gas approach is used to model adsorption in these calculations, and a high-pass limited Fourier representation is used to facilitate evolution of large-scale structures during the optimization. Reconstructions are made of several material models which mimic real silica materials obtained either by phase separation and etching or by sol-gel processing.
Analysis of the reconstructions provides considerable insight into the type and quantity of structural information probed by gas adsorption and small-angle scattering experiments. We find that reconstructions based only on structure factors tend to underestimate the mean pore size. We also find that in many cases excellent reconstructions can be obtained using only adsorption-branch data, and that in all cases reconstructions based jointly on both types of data are superior to those based only on one, suggesting that these measures contain "complementary" information. In most cases the use of desorption data is not warranted, and the use of adsorption data taken at many temperatures will not improve reconstructions. The reproducibility of the method is shown to be satisfactory. The method can be computationally expensive if gas adsorption data are used but it is easily parallelized and therefore results can still be obtained in reasonable time. Results from this study were presented at the 2006 AIChE Annual Meeting, and the work was published in Langmuir in 2007.
We are now considering several possible improvements to the adsorption model underlying the reconstructions. In the first, we have attempted to introduce a more realistic description of the dynamics of fluid flow into and out of porous materials, which is thought to affect the shape of the adsorption isotherm in materials of limited connectivity ("pore blocking effects".) While the equations for these dynamics are reasonably straightforward, they seem to be quite computationally expensive, which needs to be addressed before they can be applied to large systems.
Secondly, we are working on a strategy for using finer-scale lattices in the adsorption model that could provide greater realism in the descriptions of the gas and of the adsorbate surface. In this approach, correlation functions extracted from bulk-phase (exact) Monte Carlo simulations of the homogeneous fluid would be used in a density-functional-like calculation of the free energy of the adsorbed fluid, replacing the more typical weighted density functionals obtained from continuum fluid theory. If this scheme can be made to work efficiently, it would enable adsorption-based reconstructions of microporous materials such as activated carbons, as well as the mesoporous glasses discussed above.
