Effect of Nitrogen Doping on the CO2 Adsorption Behavior in Nanoporous Carbon Structures: A Molecular Simulation Study

Nitrogen (N) doping is considered an effective design strategy to improve CO2 adsorption in carbon materials. However, experimental quantification of such an effect is riddled with difficulties, due to the practical complexity involved in experiments to control more than one parameter, especially at the nanoscale level. Here, we use molecular simulations to clarify the role of N doping on the CO2 uptake and the CO2/N2 selectivity in representative carbon pore architectures (slit and disordered carbon structures) at 298 K. Our results indicate that N doping shows a marginal improvement on the CO2 uptake, although it can improve the CO2/N2 selectivity. CO2 uptake and CO2/N2 selectivity are predominantly controlled by the pore architecture as well as ultra-micropores; the tendency of linear CO2 molecules to lie flat on the carbon surface favors the CO2 uptake in slit pore architectures rather than disordered carbon pore structures. We also demonstrated through molecular simulations that the N doping effect may be difficult to exemplify experimentally if the material has a disordered pore architecture and complex surface chemistry (such as the presence of other functional groups).

氮掺杂（N doping）被认为是改善碳材料二氧化碳吸附性能的有效设计策略。然而，由于实验中需同时调控多个参数（尤其是纳米尺度下）的实际复杂性，对该效应进行实验定量表征往往面临诸多挑战。本研究采用分子模拟（molecular simulations）方法，在298开尔文条件下，阐明了氮掺杂对典型碳孔结构（狭缝型与无序碳结构）内二氧化碳吸附量（CO₂ uptake）以及二氧化碳/氮气选择性（CO₂/N₂ selectivity）的调控作用。研究结果表明，尽管氮掺杂可提升二氧化碳/氮气选择性，但对二氧化碳吸附量的改善效果十分有限。二氧化碳吸附量与二氧化碳/氮气选择性主要由孔结构与超微孔（ultra-micropores）共同决定；线性二氧化碳分子在碳表面呈平躺吸附的趋势，更有利于狭缝型孔结构中的二氧化碳吸附，而非无序碳孔结构。本研究同时通过分子模拟证实，若材料具备无序孔结构与复杂表面化学（surface chemistry）环境（如存在其他官能团（functional groups）），则氮掺杂的效应很难通过实验得以验证。