| With the development of science and technology,people’s demand for energy is increasing.Fossil fuels are the main means for people to obtain energy.In the process of burning fossil fuels such as coal and oil,a large number of harmful gases are produced,which will cause environmental pollution and greenhouse effect.The main component of natural gas is methane（CH4）,which is also an important part of fossil fuels.Although it is not a renewable fuel,it has become a potential transitional fuel for low-carbon energy in the future.CH4 also contributes to the increasing greenhouse effect,accounting for about 20 percent of global warming.Therefore,the adsorption and storage of CH4 are of great significance for the development and utilization of energy and environmental protection.Based on the first principles of density functional theory,the adsorption properties of different metal atom modified graphene and porous graphene with different pore sizes for CH4 molecule were systematically studied by CASTEP software.In this paper,the adsorption properties of CH4 by graphene（GR）system and the influence of metal atom modification on the adsorption of CH4 by graphene system were studied.The results show that the adsorption of CH4 molecule on the intrinsic graphene is weak,the most stable adsorption position of a single CH4 molecule is the graphene pore site,and the adsorption energy is-0.359 eV;By calculating the adsorption energy of CH4 molecules in the graphene system modified with different metal atoms,it is found that the modified Mn atom can significantly improve the adsorption performance of CH4 molecules in the GR system.The most stable modification position of single Mn atom is the center of carbon ring,and the binding energy is-3.282 ev.The system can adsorb up to 4 CH4 molecules on one side,and the average adsorption energy is-0.510 eV.The optimal position of two Mn atoms is the space hole on the opposite side.The system can adsorb 10 CH4 molecules on both sides,the adsorption capacity can reach 32.93 wt.%,and the average adsorption energy is-0.402 eV.After the adsorption of CH4 molecules,charge transfer occurs between Mn atom and CH4,resulting in electrostatic interaction,which enhances the adsorption performance of CH4 molecules.Based on the above research,two different types of porous graphene,C6H3 and C6H2,were selected to study the effect of pore size on the adsorption capacity of CH4 molecules,and the adsorption properties of CH4 molecules in the porous graphene system modified with different metal atoms was tested.The results showed that C6H3 porous graphene（PG-C6H3）has the best adsorption properties of CH4 molecules when modified with Mn atom.The best modification position of a single Mn atom is the central hole of the carbon ring,and 4 CH4 molecules can be adsorbed on one side with an average adsorption energy of-0.510eV.The optimal position of the two Mn atoms was the different carbon annular holes on the different sides of PG-C6H3.The system could adsorb 12 CH4 molecules on both sides with an average adsorption energy of-0.338 eV and an adsorption capacity of 38.43 wt.%.C6H2 porous graphene（PG-C6H2）has the best adsorption performance for CH4 molecules when modified with Ti atoms.The best modification position for a single Ti atom is the central pore of the substrate,with binding energy of-3.603 eV.It can adsorb 12 CH4 molecules on one side,and the average adsorption energy is-0.264 eV.When PG-C6H2 system was modified by two Ti atoms,24 CH4 molecules could be adsorbed on both sides,with an average adsorption energy of-0.207 eV and an adsorption capacity of 54.7 wt.%.CH4 molecules can be adsorbed by electrostatic interaction with modified metal atoms,porous graphene substrate and van der Waals force due to the polarity of CH4 molecules after adsorption.In addition,the surface positive charge of CH4molecules decreases and the repulsion between molecules decreases after adsorption,which is also conducive to the increase of adsorption capacity.The results show that metal atom modification can significantly improve the adsorption energy of graphene materials to CH4 molecules,and can also increase the number of CH4molecules adsorption;The introduction of pore defects can change the size of pores,reduce the mass,and improve the specific surface area of the system.The existence of pore structure can significantly improve the CH4 molecular adsorption capacity of the system. |
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