Structure-Activity Relationships Of Li⁺-doped NiO Solid Solution,NiO/CeO₂ And CeO₂/NiO Catalysts

Exploring the relationship between structure and catalytic performance is important for the rational design of high performance catalysts.In this work,structure-activity relationships of Li⁺-doped NiO solid solution,NiO/CeO₂ and CeO₂/NiO catalysts have been investigated and elucidated for CO catalytic reaction.The main results are summarized here:1.To investigate the Li⁺doping effect on the structure and reactivity of NiO,a series of NiO catalysts modified by Li⁺cations have been prepared and used for CO oxidation.With the combination of experimental methods and DFT calculations,it has been revealed for the first time that the Li⁺cations are preferentially replace the lattice Ni²⁺cations instead of directly refilling the Ni²⁺vacancies in the cubic NiO lattice matrix to form a solid solution structure below the lattice capacity.For samples possessing a pure solid solution phase,the Ni³⁺cation amount increases with the increasing of lattice Li⁺cation content,which induces the formation of larger quantities of surface mobile oxygen species.In addition,surface reducibility,CO adsorption and activation ability can be enhanced,leading to the easier formation of surface oxygen vacancies and the extraction of surface active oxygen species.Therefore,the intrinsic CO oxidation activity of the catalysts can be remarkably enhanced.In contrast,by the addition of excess Li⁺cations above the lattice capacity,Li⁺is present as an additional surface Li₂CO₃ phase,which degrades the activity of the catalysts evidently due to the loss of lattice Ni³⁺cations and active oxygen species.It is concluded that the best catalyst can be tailored at atomic level by engineering the maximum amount of Ni³⁺cations in the NiO lattice matrix with a pure solid solution phase by Li⁺addition.2.The monolayer dispersion capacity of NiO supported on the CeO₂ support（NiO/CeO₂）and CeO₂ supported on the NiO support（CeO₂/NiO）has been measured by XRD extrapolation method.The obtained capacity is 0.526 mmol NiO/（100 m²CeO₂）for the former,corresponding to 1.62 wt%NiO loading,and is 0.0638 mmol CeO₂/（100 m² NiO）for the latter,equaling 0.42 wt%CeO₂ loading.Then,the Ni-O-Ce interface（1.62 wt%NiO/CeO₂）and Ce-O-Ni interface（0.42 wt%CeO₂/NiO）were constructed based on monolayer dispersion capacity of NiO/CeO₂ and CeO₂/NiO.The CO oxidation activity results indicate that the activity of 1.62 wt%NiO/CeO₂ catalysts is much higher than 0.42 wt%CeO₂/NiO.The formation of Ni-O-Ce interface is proved to be more favorable for CO adsorption and the formation of more mobile oxygen species than the formation of Ce-O-Ni interface,accounting for higher CO catalytic activity on the Ni-O-Ce interface.In addition,the CO oxidation activity of 1.62 wt%NiO/CeO₂ is higher than 1.62wt%NiO+CeO₂,implying the activity of Ni-O-Ce interface at monolayer dispersion state is higher than that of the interface between NiO and CeO₂ crystalline states,which is attributed to the formation of Ni-O-Ce interface leading to the enhanced CO adsorption ability and the formation of more active oxygen species.In contrast,the CO oxidation activity of 0.42 wt%CeO₂/NiO is lower than 0.42 wt%CeO₂+NiO,suggesting the activity of Ce-O-Ni interface at monolayer dispersion state is lower than that of the crystalline state interface between CeO₂ and NiO,which is responsible for that the formation of Ce-O-Ni interface is not benefical for CO adsorption and the formation of active oxygen species.