Study On Mechanism Of CO₂ Adsorption From Coal-fired Flue Gas By Alkali Metal Salt Doped Li₄SiO₄

Due to the rapid increase of CO₂ emissions from coal-fired flue gas due to the extensive use of fossil fuels,it has become the main factor causing global warming.Limited by the development level of clean energy,fossil fuels will still be the main energy source for human activities in the future.Therefore,the development of efficient CO₂adsorbents is very important for improving global climate problems.Li₄SiO₄has attracted much attention due to its high theoretical CO₂adsorption capacity（36.7 wt.%）,good stability and wide sources.It is very important to study the CO₂ adsorption mechanism of Li₄SiO₄for the development of efficient Li₄SiO₄based adsorbents.At present,the mechanism of improving CO₂adsorption performance of Li₄SiO₄at low temperature（<500℃）by doping alkali metal carbonate is not clear.In order to solve this problem,this paper researches the CO₂adsorption mechanism of Li₄SiO₄doped with Na₂CO₃by DFT（Density functional theory）calculation and combining with the experimental results.Characterization and DFT calculation of pure Li₄SiO₄and Na₂CO₃doped Li₄SiO₄show that there are two kinds of active sites on the surface of Li₄SiO₄,Li_slabis physical adsorption,O_slabis chemical adsorption,the adsorption energy is-0.101e V and-1.121e V respectively;Na₂CO₃can react with Li₄SiO₄to form Li₂O,the electronic structure of Li_slaband O_slabchanges,XPS of Li_slaband O_slabshifted to the direction of high energy,enhanced the activity and increased the number of surface basic sites,which improved the CO₂adsorption performance of Li₄SiO₄at low temperature.The adsorption energy of Na₂CO₃doped Li₄SiO₄was-1.357e V.The adsorption properties of Li₄SiO₄ doped with different alkali metal salts（carbonate and halogenated salt）were tested and calculated by DFT,it is found that the peak shifts of XPS and DOS of Li_slaband O_slabin Na F doped Li₄SiO₄are more obvious than those of Na₂CO₃doped Li₄SiO₄,and the CO₂adsorption performance of Na F doped Li₄SiO₄is higher than that of Na-doped Li₄SiO₄.F doping is similar to Na doping,which can also activate the reactivity of Li_slabsite and O_slabsite,the adsorption energy is-1.813ev.The diffusion of Li⁺and O^2-in pure Li₄SiO₄and alkali metal ion/halogen ion doped Li₄SiO₄was calculated by DFT.The diffusion barrier of Li⁺and O^2-from the inside to the surface in pure Li₄SiO₄was 0.96e V and 2.35e V respectively.Li⁺diffusion was exothermic reaction and O^2-diffusion was endothermic reaction,O^2-diffusion was the rate control step of diffusion process.Doping Na and f could effectively reduce the diffusion barrier of Li⁺and O^2-.