Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study

TL;DR

This study uses density functional theory to analyze how different chemical functionalizations on graphene edges affect the adsorption of CO2 and CH4 gases, revealing potential pathways to enhance gas storage and separation.

Contribution

It provides a comparative analysis of various functional groups on graphene edges and their impact on gas binding energies and orientations, informing material design for gas separation.

Findings

CO2 binds roughly twice as strongly as CH4.

Certain functional groups significantly enhance gas binding.

In-plane binding correlates with dipole moments of functional groups.

Abstract

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.