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

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.