CO₂ Adsorption Properties Of Modified Graphene Nanosheets And Its Mechanism

CO₂ is not only the main greenhouse gas but a kind of significant industrial materials.If CO₂ is efficiently separated from industrial waste gases and converted into industrial products,it could solve both environmental and energy issues.Therefore,it is significant to efficiently capture and make the most of CO₂ for solving environmental and energy problems.Recently,porous carbon materials have been widely used in capturing CO₂ because of their wide variety,stable properties and controllable pore structure.Based on simulation technology,CO₂ adsorption properties of porous carbons could be theoretically explored and predicted,which could provide guidance for experimental preparation of CO₂ adsorbents,shorten the experimental period and reduce the experimental cost.Therefore,exploring the relationship between structure,composition and surface properties of porous carbon and its CO₂ adsorption capacity is extremely important to design CO₂ adsorbents with high CO₂ adsorption capacity.In this thesis,CO₂ adsorption properties of Graphite Slit-Pore and carbon nanoscroll(CNS)are investigated by using MD,GCMC and DFT methods.Then,CO₂ adsorption properties on charged C₃N and M-N-C surfaces are also explored using DFT method.The calculated results are as follows:(1)CO₂ adsorption properties of Modified Graphite Slit-PoresCO₂ adsorption properties of N,S and N/S codoped Graphite Slit-Pores are explored using GCMC and DFT methods.The sulfur instead of nitrogen doping could significantly improve the CO₂ adsorption ability of Graphite Slit-Pores in humid environment(the adsorption amount increases by more than 30%)due to the increased electrostatic interactions between CO₂ molecule and the adsorbent.This investigation demonstrates that the increase of N/S ratio is a good method to increase CO₂ capture in humid environment,which could solve the issue that the adsorbent has low CO₂adsorption capacity in the humid condition.(2)CO₂ adsorption and separation over C₂H₂ and CO on F-CNSCNS and F-CNS are constructed using MD method.Then,their CO₂adsorption properties and CO₂/C₂H₂,CO₂/CO selectivity are investigated using GCMC and DFT methods.It is found that the CO₂ uptake of F-CNS is 92.85 mmol/mol at 300 K and 1bar,which is 4 times higher than that of CNS.Additionally,its CO₂/C₂H₂ and CO₂/CO selectivities are twice of these of CNS.It is found that F modification could not only enlarge the interlayer spacing of CNS but also enhance the interaction between CO₂ and CNS,resulting in its high CO₂ adsorption capacity and CO₂/C₂H₂,CO₂/CO selectivity.(3)The effects of charge modulation on CO₂ adsorption properties of modified graphene nanosheetsThe CO₂ adsorption capacity on neutral,negatively and positively charged C₃N and M-N-C(M=Fe,Co,Cu)surfaces are studied using DFT method to explore the possibility of C₃N and M-N-C as charged-modulated CO₂ adsorption materials.It is demonstrated that the negatively charged C₃N and M-N-C exhibit high CO₂ adsorption capacity:C₃N with the negative charge density of 18.551×10¹³e^-/cm² has a CO₂adsorption capacity of 10.64×10¹³cm^-2.The CO₂ adsorption capacity of M-N-C is related to metal doping density.Additionally,it should be noted that gas molecules adsorption and desorption processes on C₃N and M-N-C surfaces could be simply controllable by tuning negative charge density.The adsorption energy of gas molecule on doped graphene could be greatly increased by adding extra charge,for example,the CO₂adsorption energy on negatively charged C₃N surface with 3 e^-is-4.122 e V,which is 20 times higher than that on neutral C₃N surface.When extra charges are added to doped graphene surface,CO₂ is easily adsorbed on doped graphene surfaces due to high adsorption energy;when extra charges are removed from doped graphene,CO₂is desorbed from doped graphene surfaces because of low adsorption energy.