Reduction Reaction Kinetics And Carbon Deposit Behavior Of Low-content Copper And Lanthanum Doped Fe-based Oxygen Carrier In Chemical Looping

Low-content doping as an efficient oxygen carrier modification method has been wildely studied recently,however,there are relatively few researches on its reaction kinetics.In this work,combining macroscopic kinetic modeling and density functional theory(DFT)calculations,the effect of low content copper doping on reduction kinetics and carbon formation of iron-based oxygen carriers was manifested for chemical looping process using methane as a reduction agent,and the kinetic effect of low content lanthanum doping was also primarily evaluated.Compared with the undoped Fe-Zr oxygen carrier,a slight red shift of all diffraction peaks is found in the Cu-doped oxygen carriers,which indicates that the incorporation of Cu atoms into Fe₂O₃ causes lattice distortion.Increasing copper content leads to the decrement of surface area and the increment of the particle size.2 mol%Cu doped oxygen carrier exhibits the formation of spinel CuFe₂O₄.As for the comparison of the reduction reaction rate,1 mol%Cu dopant enhances the reduction rates of the first two reactions.The improved reaction rate derives from that Cu doping reduces the energy barriers in the first two reactions(i.e.R₁:90.6vs.56.5 k J?mol^-1,R₂:225.9 vs.192.2 k J?mol^-1);however,it hinders the progress of the last step(R₃:164.1 vs.208.5 k J?mol^-1).Meanwhile,the grain model indicates that Cu dopant promotes oxygen ion migration from the interface to the grain surface and improves reduction reaction rate constant in R₁ and R₂.DFT calculations confirm that after adding Cu dopant,the oxygen vacancy formation becomes harder with the progress of reduction reaction,which leads to the higher activation energy and slower reaction rate in the third stage.Moreover,the decrement of carbon deposit from Cu-doped Fe-Zr oxygen carrier is attributed to that Cu atom increases the energy barriers to form carbon-carbon chain on the surface of the reduced oxygen carrier.The difference in ion radius causes the slight blue shift of the La doped samples,and the higher doping concentration,the more obvious shift.When the La doping content ups to 3mol%,the new phase with perovskite structure LaFeO₃ appears.And the 1 mol%La dopant improves the reaction rate of the entire process,while the 2 and 3 mol%La dopants only played a positive role in the first two reactions and an inert role in the last reaction.In terms of activation energy,1 mol%La dopant can reduce the activation energy of the entire reaction(the E_a of the R₁,R₂ and R₃ reactions were reduced from 90.6 to 60.6 k J·mol^-1,from 225.9 to164.9 k J·mol^-1,and from 225.9 152.2 k J·mol^-1,respectively);2 mol%La dopant can also reduce the activation energy of the first two reactions(R₁ and R₂ are respectively reduced from 90.6 to 68.3 k J·mol^-1,from 225.9 to 157.4 k J·mol^-1),but increased the R₃ reaction energy barrier(from 164.1 to 175.2 k J·mol^-1).Although 2%Cu has the lowest reaction rates and the highest activation energy of R₁,it has the lower E_a,2 value and a E_a,3 value similar to that of the Fe-Zr oxygen carrier.Moreover,the presence of LaFeO₃ did not make the 3%La oxygen carrier appear anomalous,and its function is very close to the 2%La oxygen carrier.