Design And Investigation On Carbon-Nitride-Based Metal Single-Atom Catalysts Toward Photocatalytic CO₂ Reduction

The emerging solar-driven technology for photocatalytic CO₂ reduction（CO₂RR）into hydrocarbon products boosts the high-value-added solar-fuel transformation potential for applications.Design and preparation of such a desirable photocatalytic system along with high-efficient evolution of hydrocarbon-based solar-fuels represent a formidable status,which is a key foundation for expanding CO₂RR application.However,essential accessibility of photocatalysts with high activity and stability in CO₂RR remains challenging due to the obstacles such as rapid recombination of photogenerated carriers and low-utilization rate of active sites.Besides,since CO₂RR reaction involves the interconnection of multiple factors including photoexcitation of catalysts,charge transfer,adsorption activation of reactant molecules,and intermediates conversion,comprehensive insights into CO₂RR towards highly-selective generation of target products is still at an initial stage.Single-atom catalysts（SACs）,constructing by isolated atoms deposited on suitable support materials through a well-defined coordinated-configuration,have be regarded as prominent candidates in structure-properties manipulation,which can not only expand almost 100%effective atomic utilization but also a platform for microstructure modulation from the atomic insight.Therefore,Study on structure-activity relationship of SACs has advantages in improving the kinetic transformation for CO₂RR.Owing to two dimensional nitrogen-rich periodic structures,carbon nitride can serve as one of the ideal supports for isolated atoms deposition.By using carbon nitride as supports,this dissertation fouces on design and investigation on carbon-nitride-based metal SACs toward CO₂RR.With the fine-tuning properties of single metal-atom-sites,the effects of local electron configuration,metal-carrier coupling,surface interface regulation and multifunctional integration of SACs on the activity,stability and product selectivity of CO₂RR were systematically investigated.Meanwhile,a series of advanced characterization techniques,such as X-ray absorption spectroscopy,in-situ X-ray photoelectron spectroscopy,in-situ diffuse Fourier-transform infrared spectroscopy,femtosecond transient absorption spectroscopy,were used to track photocatalytic transformation mechanism during CO₂RR process.The main concents and results are as follows:1.Single Ni atoms anchored on porous few-layer g-C₃N₄ for CO₂RR.The porous few-layer g-C₃N₄ was prepared through a bottom-up synthetic method and used as supports to anchor single Ni atoms.Effects on local electronic structure and coordination environment of the as-prepared Ni-based heterogeneous systems with different Ni-loading were investigated in detail towards CO₂RR.The results confirm that single Ni atoms favorably coordinate at the edge of g-C₃N₄ supports based on the unsaturated edge confinement.Moreover,with Ni-loading increased to a certain extent,the Ni-based heterogeneous systems presents well-defined Ni clusters converged on the edge of g-C₃N₄,forming an inhomogeneous coordination pattern with Ni species clustering at the edge of the vector but isolating in the plane.The results show that this well-defined inhomogeneous atomic coordination effectively immobilize the stability of Ni atoms in the plane,along with high atom-utilization efficiency for Ni-loading up to 7.95 wt%.2.Single Co atoms driven CeO₂/C₃N₄ S-scheme heterojunctions for highly selective photocatalytic reduction of CO₂ to CH₄ generation.Based on the unsaturated coordination and electron configuration of isolated Co atomic sites,a Co-deposited CeO₂/C₃N₄ S-scheme heterojunctions composite（Ce Co-PTI）was designed with charged defects of CeO₂ as medium by using an atom-specific tailoring strategy.The results demonstrate that the anchored single-atomic Co species introduced charge separation driving force strongly associating with the directional migration of photogenerated charge carrier across the heterojunction interface,effectively promoting the photogenerated electronic transferability and thus boosting the multielectron-involved CO₂-to-CH₄ photoreduction.Particularly,through direct gas-solid reaction without adding any sacrificial agent,the as-prepared Ce Co-PTI exhibits the remarkable photocatalytic activity and selectivity towards CO₂-to-CH₄ generation,of which the selectivity of CH₄ yield up 88.3%,along with high turnover number up to 411.4.3.Co Ru dual-single-atoms system induces synergistic atom-collaboration towards CO₂RR.Based on the study that single Co atoms can effectively drive photogenerated charge separation and transfer,this section introduces a self-seeded Co Ru dual-single-atom heterogeneous catalyst（Co Ru-HCNp）through using disordered carbon nitride polymer as supports.By combining advanced experimental characterizations with theoretical calculation,the atomic Co and Ru species with atom-specific function identification are of exploration,along with the targeted two-role-defined synergistic configuration through atomic precision.Results show that single Co species as transport medium promoted the photogenerated electronic transferability,whereas the single Ru species benefited the adsorption and activation of CO₂ molecules,and the latter remarkably accelerated the formation of COOH*active intermediates as well as CO*dissociation and desorption.Owing to synergistic effects of Co Ru dual-single-atoms with bifunctional properties integration,the photocatalytic yield of CO₂-to-CO conversion of the as-prepared Co Ru-HCNp reaches up to 27.3μmol g^–1 h^–1,which is nearly 2-,and 5-fold higher than that of the isolated Co-and Ru-deposited systems,and the apparent quantum efficiency of Co Ru-HCNp reaches up to 2.8%.4.Cu Pt dual-single-atoms supported on crystalline g-C₃N₄ towards CO₂RR.Based on the study of bimetallic single atoms anchored into disordered carbon nitride polymer supports,this section demonstrates a uniformly Pt-and Cu-loaded single atoms with using N-vacancy-rich high-crystalline g-C₃N₄ as the deposited material.The main focus is placed on the synergistic contribution across Cu and Pt leading to the effects on dynamic catalytic mechanism to CO₂RR.The detailed analysis involves that atomically dispersed Pt and earth-abundant transition metal Cu species guide synergistic collaboration,which regulates the photoelectric properties of crystallized g-C₃N₄.Meanwhile,such functional synergistic coupling of Cu Pt dual-single-atoms promotes hydrogen activation and overflow during CO₂RR process.The results demonstrate that Cu Pt dual-single-atoms system exhibits effecient photocatalytic activity and selectivity with maximizing effective mass activity,of which the Pt loading significantly decreases to 0.32 wt%.This dissertation focus on SACs exploration in regulating CO₂RR kinetic transformation on the way towards the internal relationship of local stability of Ni atoms,interfacial charge drive of Co atoms,bifunctional coupling of Co Ru atoms,and surface modification of Cu Pt atoms.