| Regarded as the "gold gas",helium(He)has been widely exploited in cryogenic scientific research,medical treatment,industrial manufacturing,aerospace and other advanced technologies.With the rapid development of these technologies,the rising demand for He has led to an acute shortage of this species.Currently,the only commercially available supply of He is helium-rich natural gas,which makes it urgent to explore effective means to purify He from natural gas.As a renewable,ecologically clean energy,hydrogen(H2)is an important raw material in many chemical industries such as fuel cells,chemical hydrogenation,semiconductor processing,and other processes.Due to the depletion of fossil energy,it is considered as one of the most promising alternatives to fossil fuels in the future.In industry,it is common practice to produce hydrogen by steam recombination cracking of methane,electrolysis of water or to recover hydrogen from industrial waste gas.However,the H2 obtained by these methods usually contains other impurity gases that need to be further treated to collect pure H2.At present,the gas recovery methods used in industry are mainly to separate the target gas from the mixed gas(such as natural gas or industrial waste gas)by cryogenic distillation and pressure swing adsorption.However,these mothds are not only energy-intensive and complicated to operate,but also produce by-products with a high risk of pollution.In recent years,the emerging membrane separation technology has the advantages of low energy cost,eco-friendliness,easy operation,and no pollution,and is considered to be a promising gas purification technology.Membrane material itself plays a crucial role in membrane separation technology,which directly determines the selection barrier of each molecule in the mixed gas.In recent years,with the development of membrane technology and nanotechnology,a large number of new two-dimensional nanomaterials have been synthesized one after another.These two-dimensional nanomaterials have the potential to become high-quality membrane materials due to their atomic-layer-level thickness and wide pore size distribution,which promotes the rapid development of membrane separation technology.In this thesis,through first-principles calculations and molecular dynamics simulations,we explored the application possibility of nano-pore size and nano-channel of phosphorene in gas separation,providing a new possibility for industrial applications and experimental research.The details of research process and results are shown as following:Phosphorene has good mechanical stability,and the introduction of vacancy defects can form self-passivating porous phosphorene,which lays the foundation for its application in gas separation.In practical separation process,the pores of membranes will be deformed under the ambient pressure.The young’s modulus of phosphorene is small,so the deformation under the action of external pressure is obvious.Changes in membrane pore size or shape are critical to the separation efficiency,thus it is important to study the effect of strain on porous phosphorene during gas separation.Through first-principles calculations and molecular dynamics simulations,our results show that self-passivating porous phosphorene under both strained and unstrained conditions can be used to efficiently get He from natural gas(CH4,CO2,CO,N2).The selectivity of porous phosphorene for He to other impure molecules is about 104～1018,and the permeance of He at 300 K is 10-3 mol m-2 s-1 Pa-1,both far exceeding industry standards.More importantly,under the control of microstrain,the porous phosphorene not only maintains high selectivity to He,but also the permeability can be effectively regulated.Therefore,our study demonstrates that porous phosphorene is a highly robust,mechanically tunable and efficient semipermeable membrane for He purification with long-term industrial application prospects.The vertically stacked layered phosphorene can form specific nanochannels of different sizes due to its naturally buckled structure,which can be used for gas separation.Our results show that at room temperature of 300 K,the phosphorene channel with the interlayer spacing of 0.45～0.55 nm can achieve separation of He from impurity molecules of natural gas;when the interlayer spacing is 0.45 nm,the separation of He from the noble gas Ne can be achieved.In addition,the phosphorene nanochannel achieves high selectivity while maintaining the high permeance of He as high as～103 mol m-2 s-1 Pa-1.Further calculation shows that H2 can be efficiently separated from the mixed gas(CH4,CO2,CO)by adjusting the interlayer distance of the phosphorene nanochannel(0.5～0.55 nm),and the permeance of H2 can also reach～103 mol m-2 s-1 Pa-1 to achieve hydrogen puriftcation process.When the phosphorene channels are stacked in an AB manner,the gas permeance can be significantly improved for 5～10 times than AA manner.Furthermore,as for the relationship between gas separation efficiency and channel length,we found that the change of the length of channel has less effect on gas selectivity.While the length of channel is longer than 3nm and the minimum layer spacing is greater than 0.45 nm,the permeance of He and H2 varies little.It can be seen that the phosphorene channel for gas separation has excellent controllability and thus has good industrial application value. |
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