Research On The Reaction Characteristics And Mechanism Of Fe-based Composite Oxygen Carriers With CO During Chemical-looping Combustion

Chemical-looping combustion（CLC）is regareded as a promising technology for CO₂capture with low energy consumption.Oxygen carriers with high performance is the key to its practical application.Due to the complementary and synergistic effects between active components,Fe-based composite oxygen carriers show better comprehensive performance.Understanding the reaction mechanism between oxygen carriers and fuel molecules is very important for the rational design and performance control of oxygen carriers.Based on the combination of reaction thermodynamics,TGA experiments,and density functional theory calculations,this paper systematically studied the reaction characteristics and microscopic reaction mechanism between the CO and Fe-based composite oxygen carriers,clarifies the relationship between the structure and performance of Fe-based composite oxygen carriers and the nature of synergistic effect between different metal atoms.These results can offer a guidance for the application of Fe-based composite oxygen carriers in CLC process.Four representative Fe-based composite oxygen carriers were synthetised by using the sol-gel combustion method,including spinel phase NiFe₂O₄,CoFe₂O₄ and CuFe₂O₄,as well as non-spinel phase(Mn_0.33Fe_0.67)₂O₃.The reaction thermodynamic analysis and TGA tests were conducted to study the reaction characteristics of Fe-based composite oxygen carriers.The results of reaction thermodynamic analysis show that the reduction of Cu²⁺,Co²⁺and Mn³⁺are prior to Fe³⁺while Fe³⁺is prior to Ni²⁺.The spinel MnFe₂O₄ can be formed during the reduction process of(Mn_0.33Fe_0.67)₂O₃.TGA results suggest that Ni and Co in NiFe₂O₄and CoFe₂O₄ evidently accelerate the reaction rate ofFe₂O₃ into Fe.(Mn_0.33Fe_0.67)₂O₃ and CuFe₂O₄ show oxygen releasing properties.Mn in Mn-Fe composite oxygen carrier mainly improved the reduction ofFe₂O₃ to Fe O,and Cu in CuFe₂O₄ promoted the low temperature reactivity.Overall,the activity and oxygen carrying capacity of Fe-based composite oxygen carriers are improved because of the synergistic effects between different components as compared to the singleFe₂O₃ oxygen carrier.The reaction conditions have an important influence on the reaction characteristics and reaction rate of oxygen carriers.Hence,the key influencing factors in reaction process on Fe-based composite oxygen carriers are further studied.The results reveal that the reaction temperature and CO concentration have certain effects on the reduction degree and reaction rate of Fe-based composite oxygen carriers.The increasing of reaction temperature and CO concentration can accelerate the reaction rate and deepen the reduction degree of Fe-based composite oxygen carriers.However,higher reaction temperature will cause the sintering of oxygen carriers.The Mn-Fe composite oxygen carrier has no sintering at 700-900℃,which owns best sintering resistance.CuFe₂O₄ only shows slightly sintering at 900℃.When CO concentration is lower than 5%,it is unfavourable for deep reduction of Fe-based composite oxygen carriers.Increasing the heating rate,the Fe-based composite oxygen carriers exhibit various thermal hysteresis,which follows the order:NiFe₂O₄<CuFe₂O₄≈(Mn_0.33Fe_0.67)₂O₃<CoFe₂O₄.Smaller thermal hysteresis is more benefit to use the lattice oxygen efficiently.The above experiments imply that the presence of Ni and Co in NiFe₂O₄ and CoFe₂O₄can significantly improve the reactivity of oxygen carriers,however the specific mechanism is unclear.Based on DFT calculations,the adsorption behavior and oxidation mechanism of CO on these surfaces of NiFe₂O₄ and CoFe₂O₄ were systematically investigated.The results suggest that the Ni and Co in NiFe₂O₄ and CoFe₂O₄ can improve the bonding properties of oxygen carriers,and their reactivity and stability are also enhanced.The CO adsorption and oxidation activity of the Ni region on NiFe₂O₄ surface are higher than that of the Fe region.The CO adsorption activity of the Co region over the CoFe₂O₄ surface is higher than that of the Fe region at the corresponding position,the CO oxidation in the Fe region is easier than in the Co region.The oxidation of CO by the three-coordinate oxygen is a two-step reaction including CO adsorption and CO₂ desorption,and the latter si the rate-determining step.CO oxidation by the two-coordinate oxygen is a three-step reaction including CO adsorption,CO₂ formation and CO₂ desorption,and the rate-determining step is CO₂ formation.The energy barriers for these four reaction routes on each surface of NiFe₂O₄ and CoFe₂O₄ are almost the same,and can be occured continuously in a certain temperature range,which agrees well with the result of only one weight loss peak observed in the TGA experiments.As compared with the Ni-Fe and Co-Fe composite oxygen carriers,Mn-Fe and Cu-Fe composite oxygen carriers exhibit the ability to release oxygen at high temperatures,but the mechanism of their oxygen release and its reaction with CO has not been reported.Based on the DFT calculations,the mechanism of CO adsorption and oxidation over MnFe₂O₄ and CuFe₂O₄ surfaces were studied.The results indicate that the Fe atom is the active center for CO adsorption.CO oxidation on the surface of MnFe₂O₄ includes three elementary reaction steps:CO adsorption,CO₂ formation and CO₂ desorption.On the Fe-terminated surface,CO₂ formation is the rate-determining step.On the Mn-terminated surface,CO₂ desorption becomes the rate-determining step owing to the steric hindrance of the Mn atoms.There are two reaction channels for CO oxidation on CuFe₂O₄ surface.In one-step reaction channel,CO can directly react with the most surface-active oxygen atom to form CO₂.This reaction process does not require an energy barrier.The two-step reaction channel primarily includes CO adsorption and CO₂ desorption,and the latter is the rate-determining step.The energy barriers in this reaction channel can be as low as 3.87 k J/mol.The existence of Cu atoms in CuFe₂O₄ can significantly reduce the energy barrier for CO oxidation,which indicates a good low temperature reactivity.The oxygen release property is related to the activity of the metal atom coordinated by lattice oxygen,and Cu²⁺is easier to release oxygen than Mn²⁺.The reactivity as well as structure-activity relationship of the four Fe-based composite oxygen carriers are compared and analyzed.Overall,the order of lattice oxygen activity of four Fe-based composite oxygen carriers follows CuFe₂O₄>NiFe₂O₄>CoFe₂O₄>MnFe₂O₄.CuFe₂O₄ possesses the highest lattice oxygen activity among the four Fe-based composite oxygen carriers.However,CuFe₂O₄ appears slight sintering phenomenon at 900℃,which can be improved by using the modification methods such as inert doping or supporting.By comparing and analyzing the coordination environment of active oxygen on the surface of Fe-based composite oxygen carriers,it is found that the synergistic effect can be attributed to the formation of coordination environment by different metal atoms.The surface oxygen atom coordinated with tetrahedral metal atom is the lowest,thus the tetrahedral metal atom is an optimal doping position for inert atom.According to this,a targeted regulation method is proposed where tetrahedral metal atoms were replaced with inert metal atoms to increase the sintering resistance of this type of oxygen carriers.