Structural Design Of Carbon Nitrides Loaded With Uranium, Tungsten And Scandium Metals And Theoretical Study Of Carbon Dioxide Reductio

Over the last few hundred million years,the concentration of carbon dioxide has remained relatively constant at around 180 ppm.However,with the onset of the Industrial Revolution,CO2 emissions have rapidly increased,resulting in its current concentration exceeding 410 ppm.The large-scale emission of carbon dioxide has had an impact on human being.To reduce dependence on fossil fuels,researches have been worked on including carbon dioxide reduction reaction(CO2RR),nitrogen reduction reaction(NRR)and water splitting(HER and OER),to produce clean energy that is recyclable.Carbon dioxide is a renewable C1 energy source,and its reduction has become a cutting-edge topic in the fields of environment and energy.However,carbon dioxide itself has two extremely strong carbon-oxygen double bonds,and breaking or activating them requires to overcome high activation energy barriers.Therefore,it is very crucial to search out suitable catalysts.In addition to extensively explored transition metal catalysts,complexes containing actinide elements have also shown good catalytic reactivity in the field of energy.In this thesis,the structures,bonding properties as well as reaction pathways and mechanism of tungsten-group,scandium-group and uranium complexes were designed and studied for the activation and reduction of CO2 using relativistic density functional theory(DFT).We have calculated a series of tetravalent tungsten-group and uranium complexes supported by the carbon nitride.Structural optimization shows that the carbon nitride structural unit renders metal ions in the MN6 coordination mode,which stabilizes the newly-formed complex.Calculations demonstrate that these single-atom catalysts are capable of catalyzing CO2RR.The catalytic ability increases in going from W to Mo to Cr to U.Along the CO2RR path,structural parameters(such as M-O/C bonds)of catalystCO2 complexes and derivates change regularly.Electron-spin density and electronicstructure analyses show that the metal and ligand together provide electrons to synergistically achieve the catalytic reduction of carbon dioxide.A series of scandium-group and actinium complexes of carbon nitride were designed,and their cyclic catalytic reduction of carbon dioxide was addressed and compared with their divalent metal analogues.Structural and energetic calculations show that the complexes can capture CO2 through either metal site or peripheral N-H bond(insertion).The former is more thermodynamically stable.Associated with geometric parameters,charges/electron spin density,and thermodynamic reactions as well as topological analyses of the metal-CO2 bonding,it is found that all the designed catalytic reaction can achieve the conversion of carbon monoxide,where the Sc complex exhibits the excellent catalytic performance.The structures and adsorption properties of uranium nitride carbon complexes were further explored for the activation of carbon dioxide.The metal oxidation state ranges from+Ⅰ to +Ⅳ.Electron-spin density calculations indicate that more electron transfer occurs between ligand and lower-valent metal center in the complex.Among the various complexes,the tetravalent uranium one exhibits unique geometric structures and electronic properties,offering good carbon dioxide activation performance.Therefore,the study on structures,bondings and catalytic CO2RR provides theoretical support for developing novel catalytic materials.Meanwhile,it presents the basis for protecting/remedying environment,reducing CO2 greenhouse effect and producing clean fuels.