  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 CO2 and CH4 to model zigzag graphene
nanoribbons edge-functionalized with COOH, OH, NH2, H2PO3, NO2, and CH3.
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 CO2 to exceed that of CH4 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 NH2, H2PO3, NO2, and COOH functional
groups can significantly enhance gas binding with respect to a
hydrogen-passivated edge, 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 CO2/CH4 selectivity are discussed.
