CO<sub>2</sub> Adsorption Thermodynamics over N‑Substituted/Grafted Graphanes: A DFT Study

This work examines CO<sub>2</sub> adsorption over various N-substituted/grafted graphanes to identify the promotional effects of various N-functionalities have on the adsorption characteristics using DFT. CO<sub>2</sub> adsorbs weakly on a graphane surface functionalized with a single, isolated substituted N- or grafted NH<sub>2</sub>-sites. The presence of coadsorbed H<sub>2</sub>O on the surface promotes CO<sub>2</sub> adsorption on both N- and NH<sub>2</sub>-sites, with highly exothermic adsorption energies (∼−50 kJ mol<sup>–1</sup>). Directly grafted −NH<sub>2</sub> or −OH functional groups on C atoms adjacent to C atoms which have a −NH<sub>2</sub> group grafted suffer from geometrical restrictions preventing dual stabilization of formed carbamate upon adsorption of CO<sub>2</sub>. CO<sub>2</sub> adsorption can be greatly enhanced with the presence of a −OH group or second −NH<sub>2</sub> group in the proximity of a −NH<sub>2</sub> site on graphane, and only if a <i>n</i>(−CH<sub>2</sub>−) (<i>n</i> ≥ 1) linker is introduced between the −NH<sub>2</sub> or −OH and graphane surface (adsorption energies of −58.8 or −43.1 kJ mol<sup>–1</sup> at <i>n</i> = 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 CO<sub>2</sub> capture.