CO2 Adsorption Thermodynamics over N-Substituted/Grafted Graphanes: A DFT Study

Abstract

This work examines CO2 adsorption over various N-substituted/grafted graphanes to identify the promotional effects of various N-functionalities have on the adsorption characteristics using DFT. CO2 adsorbs weakly on a graphane surface functionalized with a single, isolated substituted N- or grafted NH2-sites. The presence of coadsorbed H2O on the surface promotes CO2 adsorption on both N- and NH2-sites, with highly exothermic adsorption energies (∼-50 kJ mol(-1)). Directly grafted -NH2 or -OH functional groups on C atoms adjacent to C atoms which have a -NH2 group grafted suffer from geometrical restrictions preventing dual stabilization of formed carbamate upon adsorption of CO2. CO2 adsorption can be greatly enhanced with the presence of a -OH group or second -NH2 group in the proximity of a -NH2 site on graphane, and only if a n(-CH2-) (n ≥ 1) linker is introduced between the -NH2 or -OH and graphane surface (adsorption energies of -58.8 or -43.1 kJ mol(-1) at n = 2). The adsorption mechanistics provided by DFT can be used to guide the atomic-level rational design of N-based graphane and carbon adsorbents for CO2 capture.