Study Of Reactivity And Mechanism Of Nickel- And Iron- Based Oxygen Carriers In Chemical Looping Reforming

Chemical looping reforming based on chemical looping combustion can not only greatly reduce the cost of CO₂ capture,but also can achieve higher energy conversion efficiency.Hence,this technology has broad development prospects.The oxygen carrier with excellent performance and economic feasibility is the bottleneck that limits the success of the technology to industrialization.The Ni-and Fe-based oxygen carriers are the most promising due to their unique advantages.For Ni-based oxygen carrier,due to the limitations of experimental techniques and the complexity of reaction process,there is a lack of detailed understanding of the reaction mechanism between fuel and oxygen carrier.In addition,the influence mechanism of H₂S and supports on the reactivity of oxygen carrier is still unclear.For Fe-based oxygen carrier,the most important problem is its low reactivity performance.It is an effective strategy to introduce the dopant into the oxygen carrier to improve its reactivity.However,the understanding of the doping modification mechanism is not deep enough.And there is an urgent need to propose new screening strategy for dopants.Based on this,the density functional theory（DFT）calculations combined with experiments were performed to systematically study the reactivity and mechanism of Ni-and Fe-based oxygen carriers.The main contents are as follows:For chemical looping reforming,CH₄ is the most widely used fuel.Firstly,the reaction mechanism of CH₄ and Ni-based oxygen carrier was explained by using DFT calculations.The CH₄ dehydrogenation characteristics and H₂ formation mechanism on the surface of oxygen carrier,and oxygen migration mechanism of oxygen carrier were studied from the atomic level.It was found that CH₄ undergoes sequential dehydrogenation to form H atom,where CH₃→CH₂+H is the rate-limiting step.There are two possible paths for the H₂formation on the surface of oxygen carrier.It is the main route that the H atom from O-top site to Ni-top site bonds with another H atom on O-top site to form H₂ molecule.The energy barrier required for oxygen migration process from inner layer to surface is higher.This process significantly limits the reaction rate between the CH₄ and Ni-based oxygen carrier compared to the CH₄ dehydrogenation and H₂ formation processes.Secondly,two structural models of supported NiO oxygen carriers were constructed based on the cluster theory.The influence mechanism of two supports,including MgAl₂O₄and ZrO₂,on the CH₄ dehydrogenation,and H₂ and CO formation on the surface of Ni-based oxygen carrier were investigated.The influence mechanism of the support on active component NiO was revealed.The results showed that the NiO cluster is easy to adhere to the surface of two supports.The two supports can significantly reduce the energy barrier of CH₄ dehydrogenation process.The MgAl₂O₄ support is more favorable for CH₄dehydrogenation and H₂ formation,while the ZrO₂ support makes CO formation easier.The ZrO₂ can weaken the Ni-O bond of NiO more than MgAl₂O₄,so that the active component NiO of NiO/ZrO₂ oxygen carrier has higher oxygen activity and then shows better reactivity performance.The supports mainly affect the reactivity performance of Ni-based oxygen carrier by regulating the oxygen activity of active component NiO.The H₂S in the fuel can significantly reduce the reactivity performance of Ni-based oxygen carrier.A series of studies on its influence mechanism were performed based on DFT calculations.The adsorption behaviors of H₂S and CO on NiO（001）perfect surface were compared and analyzed.The adsorption dissociation mechanism of H₂S on NiO（001）defect surface was studied.The effect of pre-adsorbed H₂S on the CO adsorption on NiO（001）perfect and defect surfaces was investigated.By analyzing the role of H₂S in the oxygen migration process,the influence mechanism of H₂S on the reactivity performance of Ni-based oxygen carrier was revealed.It was found that H₂S preferentially adsorbed on NiO（001）perfect surface before CO.The H₂S molecule will generate a two-step adsorption dissociation reaction on NiO（001）defect surface with the H-S bond cleavage:H₂S→HS+H and HS→S+H,where the first dissociation is the rate-limiting step.The presence of H₂S is not conducive to the CO adsorption on NiO（001）perfect and defect surfaces,hindering the CO further reaction.The H₂S can significantly increase the energy barrier of oxygen migration process,which in turn affects the reactivity of Ni-based oxygen carrier.Alkali dopants have been widely used to modify Fe-based oxygen carrier.In order to reveal the mechanism of alkali doping modification of oxygen carrier,the effect of alkali doping on the surface microstructure of Fe₂O₃ was studied by DFT calculations.The effect of alkali doping on surface oxygen activity of Fe₂O₃ surface was analyzed by calculating the surface oxygen vacancy formation energy.Our calculation results showed that Li and Na dopants have little effect on the surface structure of Fe₂O₃,while the K,Rb and Cs dopants cause surface oxygen atoms to move outward,which helps to enhance surface oxygen activity.The Li,Na and K dopants can significantly increase the activity of surface oxygen near and away from the dopant.Although the Rb and Cs dopants can increase the activity of surface oxygen close to the dopant,they significantly reduce the activity of surface oxygen away from the dopant.The Li,Na and K dopants have a better synergistic effect on the reactivity of Fe₂O₃ oxygen carrier.The alkali dopants can improve the reactivity performance by regulating the microstructure and surface oxygen activity of Fe-based oxygen carrier.Finally,the screening strategy of modified Fe-based oxygen carrier was studied in combination with DFT calculations and experimental studies.Based on the DFT calculations,the distribution of 27 dopants in the surface structure of Fe₂O₃ was investigated.The surface oxygen vacancy formation energy was employed as a descriptor to preliminarily screen effective dopants for the modification of Fe₂O₃ oxygen carrier.According to DFT screening results,five kinds of dopants were selected for experimental research to verify the reliability of DFT calculation.The results showed that Li,Na,K,Rb,Cs,Mg,Ca,Sr,Ba,Co,Ni,Cu,Zn,Y,Ag,Au,La,Ce and Pb are more likely to replace the surface Fe atoms of Fe₂O₃ oxygen carrier,while the remaining Sc,Ti,V,Cr,Mn,Zr,Al and Sn preferentially replace the bulk Fe atoms（B sites）.The Li,Na,K,Rb,Cs,Mg,Ca,Ni,Cu,Zn,La,Ce,Al,Ti,Cr,Mn and Zr are screened as effective dopants,which can be used to modify Fe₂O₃ oxygen carrier and enhance its reduction performance.The H₂-TPR test results showed that the initial reduction temperature of unmodified Fe₂O₃ is about 350°C,while Mn,Cu,K,Zr and Ce-modified Fe₂O₃ start to be reduced by H₂ at lower temperature（<350°C）and the initial reduction temperature of Cu-modified Fe₂O₃ is the lowest.This well verified our DFT calculation results.