A First Principles Study On The Competitive Adsorption Mechanism Of Calcium-based Sorbrent Surface

Calcium looping?CaL?is one of the most promising carbon capture technologies due to its abundant natural limestone reserves,low price,and high theoretical CO₂ uptake capacity.To solve the problems of carbonation performance deactivation after long cycles and high system cost,several new CaL methods such as steam-enhanced carbonation,combining sulfur capture with carbon capture,and calcium-copper united cycles are developed in recent years.A series of experimental investigations have been conducted.However,these studies are merely describing phenomenon and analyzing characterization,and in-depth mechanism analysis is deficient.In this study,the adsorption stages of new CaL methods are calculated using CASTEP by Density Functional Theory?DFT?through first principles,aiming at explaining previously reported results from a mechanism perspective.To reveal the effect of steam on carbonation performance of calcium sorbents,single H₂O adsorption,single CO₂ adsorption,co-adsorption of H₂O and CO₂ on CaO?001?surface,CO₂adsorption on H₂O-CaO?001?surface and H₂O adsorption on CO₂-CaO?001?surface are calculated,the smallest adsorption energy,smallest distance between adsorbent and adsorbate,larger charge transfer and bond population show that O-top site is the favorable site for single H₂O adsorption or single CO₂ adsorption.The result of co-adsorption suggests that H₂O adsorption on CaO?001?surface is easier to proceed than CO₂ adsorption.When the steam exists in carbonation,H₂O adsorbs on CaO?001?surface first and O activates O site nearby.H₂O adsorption promotes CO₂ adsorption,and the mechanism of H₂O-enhanced CO₂adsorption is E-R mechanism rather than L-H mechanism.H₂O adsorbs on CaO?001?surface as an active material,forming H₂O-CaO?001?surface at first,then CO₂ adsorption happens.To explain steam-enhanced carbonation and steam-declined sulfation mechanism,SO₂adsorption and desorption,CO₂ desorption on CaO?001?surface,CO₂ adsorption and desorption on SO₂-CaO?001?surface,SO₂ adsorption,and desorption,CO₂ adsorption and desorption on H₂O-CaO?001?surface are calculated.According to the calculation results,Bridge site is the favorable site for SO₂ adsorption,and SO₂ adsorption inhibits CO₂ adsorption on CaO?001?surface as the barrier energy of CO₂ desorption on SO₂-CaO?001?surface is less than that on CaO?001?surface.H₂O adsorption inhibits SO₂ adsorption on CaO?001?surface because of the smaller barrier energy of SO₂ desorption on H₂O-CaO?001?surface and SO₂tends to interacts with OH-of adsorbed H₂O molecule.What's more,CO₂ adsorption is promoted by H₂O adsorption as the barrier energy of CO₂ desorption on H₂O-CaO?001?surface is less than that on CaO?001?surface.To illuminate the mechanism of the effect of copper-based oxygen carrier on calcium sorbents carbonation performance,CO₂ adsorptions on CaO-Cuon?001?surface and CaO-Cuin?001?surface are calculated.The results suggest that CO₂ adsorption of NearO-top site and FarO-top site on CaO-Cuon?001?surface are similar because of the approximate adsorption energy,charge transfer,bond population and distance between C atom and O_s atom,and on CaO-Cuin?001?surface,CO₂ adsorption of NearO-top site is stronger for its lower adsorption energy,lower energy density of states,larger charge transfer,as well as shorter distance between C atom and O_s atom.Beside Cu-top site,the adsorption energies of CO₂ adsorption of other sites are lower than the adsorption energy on CaO?001?surface,which shows that CO₂adsorption is enhanced by the addition of Cu atom.The results of this paper are expected to provide reference for better explaining predecessers'experiments studies as well as the further design optiomization of the calcium based sorbent.