Local Environment Regulation Of Single-atom Catalysts And Their Catalytic Properties

Developing high-efficiency catalyst is an effective approach to aim at global problems including energy crisis and ecological environment pollution.Researches have revealed that downsizing metal particle from nanoscale to sub-nanometer to single-atom shows different catalytic behavior for target reaction,usually achieving a trend of enhanced catalytic activity,selectivity and stability.Single-atom catalyst（SAC）is one special of supported catalyst,which can not only maximize the utilization efficiency of metal atoms to greatly expose the catalytic sites,but also exhibit unique electronic structure properties and strong-interaction between metal and substrates due to unusual coordination environment.These specificities promote the catalytic performance of SAC,and thus attract intensive attention in many fields.The novel SAC can be deemed as a bridge-link between homogeneous and heterogeneous catalysis,and precise regulation of local environment of the metal center is a feasible path to promote its catalytic property.Designing novel SAC with fine-tuned geometric structure and deepening the scientific understanding of structure-activity relationship are still formidable challenge.This thesis specializes in the precise regulation of local environment of SACs,and to explore their catalytic mechanism.The detailed researches are generalized in the following sections:A thermal replacement strategy was developed to fine-tune the local coordination environment of single-atom Co center,which achieves the efficient transformation of carbon dioxide electroreduction reaction（CO₂RR）.Due to the strong affinity between S atom and transition metal as well as the high temperature instability of Co–N₄ structure,precise substitution of coordination atoms can be achieved by introducing sulfur-containing ligands during pyrolysis.By controlling the pyrolysis temperature,the coordination structure of Co single-atom site was continuously controlled,and thereby a series of Co–S_xN_4-x（x=0,1,2,3）SACs were successfully synthesized.X-ray absorption spectra confirmed that the chemical environment and geometric structure of single-atom Co center were continuously regulated.The CO₂RR results showed that the catalytic activity presented a volcanic trend,and Co–S₁N₃ is at the peak of the volcano.The CO Faraday efficiency(FE_CO)reached 98%and the turnover frequency（TOF）value reached 4886 h^-1 at an overpotential of410 m V on Co–S₁N₃SAC,indicating the highest catalytic properties of CO₂ to CO,which were better than most of the reported atomic catalysts.Furthermore,DFT simulations uncovered the precise regulation of local structure of Co–S_xN_4-x（x=0,1,2,3）SACs enhanced the adsorption capacity for CO₂ reduction intermediates,and thus showed different catalytic activities.An advanced N-bridged Co–N–Ni diatomic site catalyst was constructed for efficient promoting CO₂RR.Aberration correction electron microscopy combined with X-ray near-edge absorption fine structure spectroscopy investigated the local environment of Co–N–Ni diatomic sites with the advanced N bridge structure.The N bridge not only modulated the electronic structure of Co and Ni metal centers,but also retained partially independent Co/Ni–N_x coordination structure to achieve synergistic effect.The N-bridged Co–N–Ni diatomic site catalyst showed a higher TOF of 2049 h^-1 at a low overpotential of 370 m V and FE_CO of96.4%compared to those of Co–N₄ and Ni–N₄ single-atom site catalysts.The high catalytic activity of Co–N–Ni diatomic site is derived from the unique N bridge structure.In situ XAS and synchrotron radiation Fourier transform infrared spectra techniques as well as DFT calculations were employed to further elucidate the catalytic behavior of accelerating the formation of COOH*species at advanced N-bridged bimetallic sites.Aiming at the difficulty of poor activity and selectivity of catalytic degradation catalyst for 1,2-dichloroethane（1,2-DCE）,one of chlorine-containing environmental pollutants,a high-efficiency Ru SAC was synthesized.The high activity and selectivity of Ru SAC was fabricated on mesoporous alumina,featuring a stable and partially positive charged Ru₁–O₅ site,and its electronic structure was obviously different from that of loaded Ru nanoparticles（NPs）.The temperature programmed chemical adsorption and desorption technique substantiated that Ru SAC had more suitable redox property and rich acidity,which exhibited high reaction kinetics and selectivities in the catalytic degradation of 1,2-DCE and realized almost complete degradation of 1,2-DCE into CO₂ and HCl.This work fully reflected the advantages of SAC.The selectivity difference between Ru single-atom sites and Ru NPs in dechlorination step was further illustrated by DFT calculations.This work showed the potential application value of SAC in the field of industrial catalysis.In this thesis,the local environment of metal center in SACs is finely regulated,and a series of single-atom and double-atom catalytic sites with controllable coordination environment were synthesized,which showed excellent performance in electrocatalysis and thermal catalysis related to environmental pollutants,as well as their structure-activity relationship also were explored.These results indicate that SACs with fine-tuned and controllable chemical environment bestow broad application prospects in many fields,and are expected to contribute to the beauty of mountains and rivers.