Reaction Mechanism Between Spinel Ferrites And CO Or H₂S Based On Density Functional Theory

Chemical-looping combustion is a novel combustion technology,which has unique advantages in inherent separation of CO2 with less cost and low energy penalties as well as reducing NOx generation.The key issue in CLC system performance is the reactivity of oxygen carrier.Using mixed metal oxides can potentially overcome the shortcomings of single metal oxides.In many cases the improvement of performance were ascribed to the formation of spinel ferrites.Despite extensive experiments have been carried out to test the redox properties of spinel ferrites,comprehensive understanding of the fundamental reaction mechanism and the influence mechanism of H2S are desirable.Therefore,it is of great theoretical and practical significance to investigate the surface reactivity of NiFe2O4 and CoFe2O4 oxygen carrier at the molecular lever,which is conducive to design and evaluate appropriate ferrite-based oxygen carriers.The periodic slab models of NiFe2O4 surface were constructed to represent the NiFe2O4 oxygen carriers.In this study,the possible adsorption and reaction mechanisms between CO and NiFe2O4 were investigated by the density functional theory calculations.Oxygen defect and the coverage of the adsorbed molecules were also involved to clarify the influences on CO adsorption and oxidation progress.The results showed that NiFe2O4 exhibited a high surface reactivity,because CO was capable to bind with oxygen atoms on the surface and generate CO2 molecule directly.Increasing the coverage of CO would decrease the adsorption energy.In the cases of higher coverage,CO no longer adsorbed on the top of Ooct atom to generate CO2 molecule directly.Instead,it migrated to the neighboring bridge site and formed a CO2 precursor on the surface.The desorption of CO2 precursor was an important route for CO2 generation,because the energy barrier was pretty low.Oxygen defect on the surface enhanced CO adsorption on the neighboring nickel atoms,which may hinder the direct interaction between CO and the oxygen atom of the surface.Experiments showed that reactivity of the combined CoFe2O4 oxygen carrier is superior to that of Fe-based oxygen carriers.However,few researches focused on the microscopic mechanism of this phenomenon.Thus DFT calculations were performed to clarify the fundamental reaction mechanism between CoFe2O4 oxygen carrier and CO molecule.The results showed that cobalt atoms on the surface provide active sites for CO adsorption.Although CO adsorption on most sites were weakened by the formation of oxygen defect,the effect on cobalt atom site was negligible.The adsorption behavior revealed that CO oxidation process included two reaction stages,bent-CO2 formation and CO2 precursor desorption.Energy profile showed that bent-CO2 formation process was a rate-limiting step during CO oxidation.Cobalt atom and Co-Ooct bridge site played crucial role in formation of bent-CO2 intermediate,suggesting that cobalt atom on the surface can promote CO oxidation performance of CoFe2O4 oxygen carrier.Furthermore,CoFe2O4 showed good lattice transfer capacity because the energy barriers for lattice oxygen diffusion were lower than those in Fe-based oxygen carrier.The effects of H2S in gaseous fuel on the reactivity of spinel ferrites have not been studied sufficiently.Therefore,we investigated the adsorption and dissociation mechanisms on NiFe2O4(001)and CoFe2O4(001)separately,aiming to reveal the influence mechanism of H2S on the reaction performance of both oxygen carriers.The results showed H2S primarily adsorbed on the top of nickel and cobalt atoms.Oxygen defect on the surface promoted the adsorption on NiFe2O4(001)surface while the adsorption on CoFe2O4(001)surface weakened.For NiFe2O4 oxygen carrier,H2S adsorbed on the surface may hinder CO adsorption on the oxygen atom site.The presence of H2S on the surface may adversely affect the CO oxidation reaction.Meanwhile,adsorbed H2S was easy to dissociate into SH and S species and oxygen defect promoted the process by decreasing energy barrier.Sulfur atoms deposited on NiFe2O4 surface increased the energy barrier of O anion migration,suggesting that H2S limits the rate of lattice oxygen diffusion.Thermodynamic simulations indicated that the nickel sulfides was easier to generate than iron sulfides under equilibrium conditions.As for CoFe2O4 oxygen carrier,the adsorption ability of CO was stronger than that of H2S.CO would preferentially adsorbed on the top of cobalt atom.Energy barriers of H2S dissociation on CoFe2O4(001)surface were higher than those on NiFe2O4(001)surface,suggesting that CoFe2O4 oxygen carrier has good resistance to H2S compared to the NiFe2O4 oxygen carrier.