With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO(2) and CH(4) to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH(2), H(2)PO(3), NO(2), and CH(3). Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO(2) to exceed that of CH(4) by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH(2), H(2)PO(3), NO(2), and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO(2)/CH(4) selectivity are discussed.

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Chloride-based salt hydrates form a promising class of thermochemical materials (TCMs), having high storage capacity and fast kinetics. In the charging cycles of these hydrates however hydrolysis might appear along with dehydration. The HCl produced during the hydrolysis degrades and corrodes the storage system. Our GGA-DFT results show that the enthalpy charge during proton formation (an important step in hydrolysis) is much higher for CaCl2·2H2O (33.75 kcal/mol) than for MgCl2·2H2O (19.55 kcal/mol). This is a strong indicator that hydrolysis can be minimized by appropriate chemical mixing of CaCl2 and Mg Cl2 hydrates, which is also confirmed by recent experimental studies. GGA-DFT calculations were performed to obtain and analyze the optimized structures, charge distributions, bonding indicators and harmonic frequencies of various chemical mixtures hydrates and compared them to their elementary salts hydrates. We have further assessed the equilibrium products concentration of dehydration/hydrolysis of the chemical mixtures under a wide range of operating conditions. We observed that chemical mixing leads to an increase of the onset hydrolysis temperature with a maximum value of 79 K, thus increasing the resistance against hydrolysis with respect to the elementary salt hydrates. We also found that the chemical mixing of CaCl2 and MgCl2 hydrates widens the operating dehydration temperature range by a maximum value of 182 K (CaMg2Cl6·2H2O) and lowers the binding enthalpy with respect to the physical mixture by ≈65 kcal/mol for TCM based heat storage systems.

To understand the effect of the in situ carbon deposition and permeation process on cobalt-catalyzed heterogeneous reactions, the thermodynamics and kinetics of the carburization process of ten different surfaces of hexagonal close-packed (hcp) and face-centered cubic (fcc) Co and the resulting evolution of crystalline morphology, electronic structure, and barriers of typical surface reactions are explored theoretically using density functional theory (DFT) and atomistic thermodynamics methods. The exposed facets of hcp-Co and fcc-Co crystallites showed distinct thermodynamic and kinetic sensitivity to the permeation of carbon into subsurface positions. The formation of surface carbides depends on both the working condition and the type of crystalline facets. Two types of carbon permeation mechanisms were discovered with distinct diffusion paths and barriers. The morphology of cobalt catalysts was found to be greatly modulated by carbon deposition under operando conditions, favoring (0001) of the hcp phase and (111) of the fcc phase at high carbon chemical potentials. As the carbon coverage increases, the d-band centers of the carburized Co surface approach the values of bulk cobalt carbides. Evaluation of a representative Fischer–Tropsch side reaction (CH3–H coupling in methane formation) shows that the barrier is highly influenced by the degree of carbide formation on the cobalt surfaces. Our study indicates that it is crucial to consider the chemical potential of gas-phase carbon to understand the surface carbon adsorption and permeation phenomenon on the skin layer of the catalyst, the overall cobalt morphology, and the observed catalytic performance of cobalt catalysts in syngas conversion.

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