| Oxygen,which plays an important role in air and earth’s crust,is closely related to people’s life and industrial production.It exists in different forms,such as O,O-,O2,O2-,O2-,O22-,O32-.These oxygen species exhibit different characteristics in the catalytic reaction,because their structure and electronic properties are significantly different,which will affect the activity and selectivity of the reaction.Therefore,it is important in theory and application to explore the conversion of oxygen species for the understanding of related reaction catalysts.In this paper,density functional theory(DFT)calculation and cluster model are used to study the energetics of oxygen species conversion on ceria and doping ceria with different metal cations,in order to understand the stability,transformation mechanism and catalysis mechanism of oxygen species,etc.Relevant research content and results are mainly as follows:1.The activation mechanism of oxygen was explored by using(CeO2)m(m=1～3)clusters as a model.When singlet oxygen is activated by(CeO2)1～3 clusters,it follows the O2→O2-→O32-→2O22-conversion path.However,the activation and conversion of triplet oxygen varies with cluster size,that is,it has a connecting channel for O2 and O2-on CeO2 clusters,while existing physical adsorption on the Ce2O4 and Ce3O6 clusters onlyThe mechanism of oxygen species conversion on clusters is nucleophilic activation,so electrons flow from clusters to oxygen species.As the clusters increase,the activation energy barrier of oxygen molecules increases,the stability of superoxide species decreases,but the stability of peroxidic species increases.And the bridging oxygen of the clusters behaved higher activity than the terminal oxygen in O2 activation.2.We also explored the Pd-doped clusters(PdCeO42-and PdCe2O62-)and oxygen molecule activation on them.The results show that there are three stable configurations for the two doped clusters respectively,and there are significant differences in the effects of oxygen species on these clusters.The conversion of oxygen species at the Ce-O site is O2-→O2-→O32-→2O22-,while the Pd-O site also has another conversion pathway that does not requires ozone species(O2-→2O22-).The oxygen species on doped clusters are all chemically adsorbed,and the stability of oxygen species is higher than that of undoped clusters.What’s more,the stability of peroxygen species is generally higher than that of superoxide species.The free energy barrier of oxygen species conversion on doped clusters is much lower than that of undoped clusters,indicating that the introduction of Pd significantly increases the ability of CeO2 to activate oxygen molecules.Frontier orbital and charge population analysis showed that the electron transfer direction between the PdCeO42-and PdCe2O62-clusters and oxygen was from the cluster to the oxygen species,which is nucleophilic activation.3.The results of oxygen activation on doping CeO2 clusters with metal cations(La3+,Zr4+,V5+)shown that the oxygen species conversion path on La-doped clusters is O2-→O2-→O32-→2O22-,while it is O2→O32-→2O22-on Zr and V-doped clusters.The free energy barrier of the optimal oxygen activation pathway for doping clusters is lower than that of undoped clusters,so metal cation doping is beneficial to increase the activity of the CeO2 catalyst.The doping site(M-O)is the active site of oxygen activation.When the La3+ and Zr4+ dcped clusters activate oxygen,electrons flow from the cluster to oxygen.While the V5+ doped cluster activates oxygen,there is a case where electrons flow from oxygen to the cluster on theV-O(Tl)site during the conversion between oxygen and ozone.Therefore,the valence of the doped metal cations leads to different charges of the clusters,so the effects on the degree of oxygen activation and the activation mechanism are different as well. |
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