Research On High-efficiency Hydrogen Purification By Two-dimensional Porous Nanomaterials

With the rapid development of social economy,people’s demand for energy is increasing.Common non-renewable energy reserves are very limited and will pollute the environment.Therefore,it is very important to find efficient and clean energy.Hydrogen has the advantages of high combustion calorific value,no pollution to the environment,simple transportation and storage,etc.It is included in the ranks of clean energy.The production of large amounts of high-purity hydrogen has become an important research topic at the moment.In the existing production methods,hydrogen will inevitably be mixed with methane,carbon monoxide,carbon dioxide,nitrogen,water vapor and other impurities.Therefore,it is of great significance to study how to efficiently purify hydrogen.Two-dimensional porous nanomaterials as separation membranes for hydrogen purification have received a lot of attention because of their low cost,convenient operation,and high separation efficiency.At present,researchers mainly focus on constructing suitable molecular-scale pores in the two-dimensional membrane to separate gases,but the stability and membrane performance of the two-dimensional separation membrane constructed by this method often change under high temperature conditions.Therefore,it is necessary to find two-dimensional nanomaterials with natural pores of appropriate size as gas separation membranes.In this paper theoretically predicts the feasibility of several natural pores of two-dimensional nanomaterials as hydrogen separation membranes through first-principles methods which is based on density functional theory.The main research contents are as follows:（1）The stability of geometric structure,electronic structure and other characteristics of two-dimensional technetium trichloride（TcCl₃）monolayer film with natural pores were studied.It was found that there was nanopore with an effective radius of 3.2（?）in the structure of two-dimensional TcCl₃ nanomaterial.Then the diffusion barriers of H₂,CO,CO₂,N₂ and CH₄through the TcCl₃ nanopore were calculated.The diffusion energy barrier of hydrogen is only0.09e V,which indicates that 2-D TcCl₃ nanomaterials have high permeability to hydrogen at room temperature.Considering both selectivity and permeability,the calculated selectivity and permeability results show that the two-dimensional Tc Cl3 nanomaterials have higher selectivity for CO₂/CH₄(S_H2/CH4=4×10⁸,S_CO2/CH4=3×10⁵)and extremely high permeability for H₂/CH₄(₂=4×10^-2 mol·m^-2·s^-1·pa^-1,₂=2.5×10^-5 mol·m^-2·s^-1·pa^-1).This proves that TcCl₃ is a good membrane for purifying hydrogen and methane.The feasibility of using two-dimensional nano-material iron trichloride（FeCl₃）as a separation membrane to purify hydrogen from a mixture of H₂,N₂,O₂,CO,CO₂,CH₄,and H₂O was studied.Firstly,the stability of the geometric structure,electronic structure,charge density map and other characteristics of the two-dimensional nanomaterial FeCl₃ are calculated.We found that there is a nanopore with an effective radius of 2.8（?）in the structure of the two-dimensional nanomaterial FeCl₃,which indicates that it may be suitable as a separation membrane for purifying hydrogen.We calculated the adsorption energy of gases on the nanopores and the diffusion energy barriers of various gases passing through the nanopores.The energy barrier of hydrogen passing through nanopores is much smaller than that of other gases.This shows that when the mixed gas passes through the nanopore,the resistance of the impurity gas will be much greater than the resistance of hydrogen,and the hydrogen will pass through the nanopores more easily.Finally,we calculated the selectivity of the two-dimensional FeCl₃ nanomaterials to hydrogen relative to other gases,and the permeability of various gases through the nanopores.The calculation results show that at a temperature of 400K,all selectivities are higher than 1×10¹⁰.Hydrogen has an ultra-high selectivity relative to other gases.Regarding the permeability,only the permeability of hydrogen has reached the industry’s lowest permeability standard(6.7×10^-9 mol·m^-2·s^-1·pa^-1).The permeability of other gases has not reached the industry’s lowest standard.Therefore,it shows that the two-dimensional FeCl₃ nanomaterial meets the standard as a hydrogen separation membrane.（2）The feasibility of two-dimensional nano-material chromium trichloride（CrI₃）as a hydrogen separation membrane was discussed.We found that there is a nanopore with an effective radius of 2.9（?）in the structure of the two-dimensional nanomaterial CrI₃,which is similar to the molecular dynamic diameter of hydrogen（2.89（?））,through the analysis of the geometric structure and electronic structure of CrI₃.It indicates that CrI₃is suitable as a separation membrane for purifying hydrogen.Subsequently,the adsorption energy of two-dimensional CrI₃ nanomaterials for mixed gases,the diffusion energy barrier of gas mixtures through two-dimensional CrI₃ nanomaterials,the selectivity of hydrogen relative to other gases,and the permeability of various gases to two-dimensional CrI₃ nanomaterials research and calculation.The diffusion energy barriers of hydrogen and impurity gases（N₂,CO,CO₂,H₂O,CH₄）through the two-dimensional CrI₃ nanomaterial nanopores are 0.27 e V,1.49 e V,1.31 e V,1.54 e V,0.87 e V and 3.88 e V,respectively.The energy barrier for hydrogen passing through nanopores is much smaller than that of other gases.This shows that when the mixed gas passes through the nanopore,the resistance of the impurity gas will be much greater than that of hydrogen,and the hydrogen will pass through the nanopores more easily.At a temperature of300K,the selectivity of hydrogen relative to other gases is higher than 1×10¹⁶,only hydrogen reaches the lowest industrial permeability(6.7×10^-9 mol·m^-2·s^-1·pa^-1),the permeability of other gases is less than the industry’s lowest standard.Therefore,theoretically proved the superiority of the two-dimensional CrI₃ nanomaterial membrane as a separation membrane for hydrogen purification.