Preparation Of Halide Perovskites And Metal Organic Frameworks For Studying Their Photocatalytic CO₂ Reduction Performance

With the expansion of human population and high dependence on fossil fuels,global environmental problems and energy crises are becoming serious challenges.In order to alleviate the above two issues,it is urgent to develop a“green technology”to convert CO₂ into high value-added low-carbon fuels,such as CO,HCOOH and methanol,with an efficient and stable catalyst.Because linear CO₂ molecules are very thermodynamically stable,the efficient catalysts and sufficient energy input are essential.The CO₂ photocatalytic reduction is considered to be one of the best strategies to address global climate change and potential energy shortages since the energy source is renewable sunlight.Nowadays,scientific researchers have devoted themselves to exploring the CO₂ photocatalytic reduction,and various catalysts have been developed and reported.Although the remarkable achievements have been obtained,it is still in the primary stage of artificial photosynthesis.There are many basic problems for CO₂ photoreduction,such as the need for photosensitizers in reaction systems,poor adsorption and activation of CO₂ molecules,lack of combination of theoretical calculations and experimental results,and so on.Based on the above challenges,we first explore the catalytic performance of the non-porous semiconductor catalyst based on halide perovskite nanocrystals in a CO₂ photocatalytic system without photosensitizer;Next,in order to improve the adsorption and activation capacity of CO₂ in the photocatalytic system,we explore the catalytic activity of porous metal-organic framework?MOF?micromaterials in the CO₂ photoreduction,and provide the theoretical basis for experimental results through density functional theory?DFT?calculations.The study mainly focuses on the following three parts:1.In order to reduce the cost and avoid secondary environmental pollution during CO₂ photoreduction,we explore non-porous inorganic halide perovskite nanocrystals as a catalyst which do not need noble-metal photosensitizers.Under simulated sunlight,the activity of all-inorganic perovskites in CO₂photocatalytic reduction was improved sharply by adjusting the proportion of halides.We prepared a series of mixed halides perovskite nanocrystals CsPb(Br_x/Cl_1-x)₃?x=0.7,0.5,0.3?with cubic phase by the hot injection method.With these mixed-halide perovskites as catalysts,the experimental results show that under simulated sunlight CO₂ can be efficiently and selectively converted CO₂ into CO and CH₄.When CsPb(Br_0.5/Cl_0.5)₃ is used as the catalyst,the total output of CO and CH₄ reaches to 875?mol/g?99%selectivity?,which is 4.5and 9.1 times higher than the catalytic performance of CsPbBr₃ and CsPbCl₃,respectively.In mixed halides perovskites,the efficient charge separation and moderate stability make the CO₂ catalytic performance improved.This topic provides a new strategy for designing high-performance and inexpensive halide perovskite catalysts.2.Due to the excellent optical and electronic structural properties of the halide perovskite nanocrystals,they have been successfully employed as catalysts in the CO₂ photocatalytic reduction.However,the catalytic performance still needs to be improved.On the basis of the first part of the work,to further enhance the CO₂ photocatalytic performance,the strategy of doping manganese ion(Mn²⁺)into the inorganic perovskite nanocrystals was proposed.We first synthesized a mixed halides perovskite nanomaterials CsPb?Br/Cl?₃:x%Mn²⁺?x=20,33,60,70?,and named them samples 1,2,3and 4,respectively.The catalytic results show that these catalysts can reduce CO₂ to CO and CH₄ under visible-light condition without photosensitizer.Especially,with sample 2 as the catalyst,the yields of CO and CH₄ are up to1917?mol/g and 82?mol/g,respectively.Meanwhile,the yields of these two products are 14.2 and 1.4 folds than that of CsPbBr₃,respectively.To some extent,the improved CO₂ photocatalytic performance may be ascribed to the enhanced charge separation efficiency and visible-light absorption capacity of the mixed-halide perovskite doped with Mn.3.In the CO₂ photoreduction,the adsorption and activation of CO₂ on the catalyst surface is of great significance for the catalytic reaction.To improve CO₂ adsorption and activation performance in simulated flue gas,we select the bimetallic Ni/Mg-MOF-74 containing coordination unsaturated metal sites as a catalyst,and use DFT calculations to reveal the underlying reasons of CO₂ photocatalytic performance.Under the irradiation of visible light,with Ni_0.75Mg_0.25-MOF-74 as a catalyst,the generation rate of HCOO^-in a pure CO₂system is 0.64 mmol.h^-1g^-1_MOF,which is higher than Ni-MOF-74(0.29mmol.h^-1g^-1_MOF)and Ni_0.87Mg_0.13-MOF-74(0.54 mmol.h^-1g^-1_MOF),while Mg-MOF-74 has almost no catalytic activity.The results indicate that the catalytic activity depends on the metal node.Subsequently,in the simulated flue gas,the HCOO^-produced by Ni_0.75Mg_0.25-MOF-74 reached 0.52mmol.h^-1g^-1_MOF,and the rate of this formate was about 80%in the pure CO₂reaction system.This demonstrates that Ni/Mg-MOF-74 not only overcomes the limitation of CO₂ concentration,but also has good resistance to other gas components in the flue gas.DFT calculations reveal two key factors for the high yield of HCOO^-:the strong CO₂ binding energy at the Mg site,and the synergistic effect of Mg and Ni leading to the stabilization of the key^*OCOH intermediates with appropriate energy barriers.