Defect Engineering And Catalytic Performance Of Tungsten Oxides

The rapid development of nanocatalysts in recent years has provided an appealing approach to alleviate severe global energy consumption and environmental problems.The quantum confinement effect driven by nanoscale size leads to the distinct physical and chemical characters,and thus offers unique catalytic properties.Among various catalyst candidates,metal oxides have been paid much attention,as they cannot only act as catalysts to carry out the reactions,but also serve as substrates for metal catalysts to enable strong metal-support interactions.However,the inefficient charge transfer and lack of catalytically active sites for metal oxides severely impede their catalytic applications.Defect engineering provides a promising strategy for modulating catalysts towards enhanced catalytic performance.In addition to the construction of catalytically active sites,defect engineering can also offer the bridge for electron and energy transfer from catalyst to adsorbed molecules.In this dissertation,we mainly focused on the construction and modulation of defects on metal oxide nanocatalysts to improve their catalytic performace.We chose tungsten oxides as the research candidates and strive to understand the effect of defects on both molecular adsorption/activation and electron/energy transfer at molecular/atomic level.Moreover,we intended to explore the relationship between defect and catalytic activity.Our research findings may provide fresh insights into the influence of defects on catalysts and the rational designs of highly active sites.In summary,the main results are listed as follows:1.We chose tungsten oxide?WO₃?as the catalyst candidate and introduce the oxygen vacancies via defect engineering to construct coordinately unsaturated W atoms.We investigated the relationship between the defect and molecular adsorption/activation.Taking oxygen activation as the model reaction,our investigations demonstrated that coordinately unsaturated W atoms on the surface can serve as highly active sites for oxygen chemisorption while facilitating electron transfer from catalytic sites to molecules.Under light irradiation,photoexcited electrons can be accumulated at defect sites and transferred into oxygen molecules,activating the molecules into superoxide radicals.Finally,we applied the defective WO₃ into light-driven aerobic coupling of amines to imines,achieving excellent catalytic activity.The concept demonstrated here highlights the importance of chemisorption to utilization of excitons in solar-driven chemical transformation,and calls for future efforts on tailoring catalyst structures at atomic precision.2.Based on the results in Section 1,we attempted to modulate the adsorption of amine molecule on catalyst surface to further improve the activity in aerobic coupling.In this section,we employed defective tungsten oxide hydrate?WO₃·H₂O?as a catalyst to carry out thermal-based catalytic reaction.The investigation indicated that oxygen vacancies derived from surface defects supplied coordinatively unsaturated sites to adsorb and activate oxygen molecules,producing superoxide radicals.More importantly,the Br???nsted acid sites from lattice water can contribute to enhancing the adsorption and activation of alkaline amine molecules.The synergistic effect of oxygen vacancies and Br???nsted acid sites eventually boosted the catalytic activity and lowered the apparent activation energy.This work provides a different angle for metal oxide catalyst design by maneuvering subtle structural features,and highlights the importance of synergistic effects to heterogeneous catalysts.3.We attempted to modulate defect state to confront the dismerits drived from defect engineering,in which doping an exotic element was considered as an appealing approach to refine the defect state.Taking Mo-doped W₁₈O₄₉ as a model material,we invesitigated the performance for photocatalytic N₂ fixation to NH3.The investigation indicated that the coordinatively unsaturated metal atoms with oxygen defects served as the sites for N₂ chemisorption and electron transfer.The doped low-valence Mo species played a multi-role in facilitating N₂ activation and dissociation by refining the defect states of W₁₈O₄₉:?1?polarizing the chemisorbed N₂ molecules with larger charge difference and facilitating the electron transfer from active sites to N₂ adsorbates,which enabled N=N bond more feasible for dissociation through proton coupling;?2?elevating defect-band center towards the Fermi level,which preserved the energy of photoexcited electrons and endows more driving force for N₂ reduction.This work provides fresh insights into the design of photocatalyst lattice for N₂ fixation,and reaffirms the versatility of subtle electronic structure modulation in tuning catalytic activity.4.Common metal catalysts usually suffer from the low selectivity in hydrogenation of ?,?-unsaturated aldehyde/ketone,escpecially for the selective hydrogenation of C=O.Taking Mo-doped W₁₈O₄₉ as a catalyst,we developed a light-driven hydrogen transfer process for selective hydrogenation of cinnamyl aldehyde using alcohol as a hydrogen source.The surface defects served as active sites for the adsorption and activation of C=O,while C=C could remain unreacted.Meanwhile,the doped Mo species at defect sites improved catalytic acivity.