First-principles Study Of Carbon Dioxide Adsorption By Modified Graphene

The increase in CO₂ gas emissions will causes environmental problems such as the greenhouse effect and sea level rise,but CO₂ gas is also an important chemical fuel,it is industrially important to study the capture and separation of CO₂ gas by adsorbents.Among the many methods for capturing and separating carbon dioxide,adsorption is widely used due to its low energy consumption and simple operation,while graphene-like materials among 2D nanomaterials have become a hot research topic due to their large surface area,more active sites,convenient surface functionalization,etc.The ability of graphene to capture CO₂ can be improved by constructing defects and doping metal or non-metal atoms.In order to explore its adsorption mechanism,we discussed and predicted the CO₂ adsorption performance of 2D nanomaterials from the theoretical aspect through simulation technology,which can provide theoretical guidance for the preparation of CO₂ adsorbent with high selectivity and adsorption performance in the laboratory and reduce the experimental cost.This thesis focuses on the adsorption behavior of pristine graphene（PG）,defective graphene（SW,VG）,N/P-doped mono-vacancy graphene（N/P-M-VG）,N-MgO co-doped mono-vacancy graphene（MgO-N-VG）on CO₂ gas molecules using density functional theory,and explores the forces of N/2N/3N-doped mono-vacancy graphene（N/2N/3N-M-VG）and metalloporphyrin nanosheets（M-N-C,M=Fe,Co and Ni）CO₂ adsorption under applied electric field conditions.The calculated results are as follows:（1）The adsorption characteristics,geometric changes and charge transfer of CO₂ molecules on PG,SW and VG were calculated by DFT.The results show that the different configurations of CO₂ adsorption on PG,SW and VG are all physical adsorption through van der Waals forces with less charge transfer.Moreover,the presence of defects can improve the adsorption capacity of the adsorbent for CO₂.When N/P doped VG adsorbed CO₂,P/N doping enhanced the adsorption and charge transfer ability,but N doping had a stronger interaction with CO₂ than P doping.The adsorption between MgO-N-VG and CO₂ molecules is a chemical adsorption process to form a new chemical bond.And MgO-N-VG can separate CO₂ gas molecules from CO₂/H₂/CH₄ mixture.（2）CO₂ capture by N/2N/3N-M-VG nanosheets in 0.000-0.040 a.u.electric field was studied.With the increase of the applied electric field strength,the interaction between CO₂ and 3N-M-VG is enhanced and the adsorption of CO₂ by N/2N/3N-M-VG changes from physisorption to chemisorption at an electric field strength of 0.020-0.030 a.u..In addition,the above materials can capture and release CO₂ by turning the applied electric field on and off.The thermodynamic properties of the above materials for CO₂ adsorption under an applied electric field are also explored.It is worth noting that the applied electric field can significantly improve the adsorption and selectivity of N/3N-M-VG for CO₂ gas in CH₄/H₂/CO₂ mixture.（3）Since metals can enhance electron transport,the trapping of CO₂ by M-N-C nanosheets in an electric field of 0.000-0.040 a.u.was investigated in this thesis.The results show that the interaction between M-N-C nanosheets and CO₂ is enhanced as the applied electric field increases,and the adsorption of CO₂ by Fe-N-C and Co-N-C nanosheets changes from physical to chemical adsorption in the range of 0.020-0.025 a.u.Fe-N-C and Co-N-C are better materials for electric field modulated CO₂ capture.Therefore,the processes of CO₂ capture and release by electric field switching and the thermodynamic properties of adsorbed CO₂ by Fe-N-C and Co-N-C nanosheets were discussed.More interestingly,the Fe-N-C and Co-N-C nanosheets can effectively separate CO₂ from the CH₄/H₂/CO₂ mixture at 0.030 a.u..