The Research Of CO₂ Hydrogenation And Photocatalytic Splitting Water Reaction Based On Structure-Controlled Nickel Species

In this thesis, based on the research of the surface Ni species catalysts with structure-controlled in the CO₂ methanation reaction and photocatalytic H₂ evolution from water splitting, based on the metal-organic frameworks（MOFs） materials, we have successfully designed and constructed high dispersion and high-efficient noble-metal-free Ni-based catalyst. In addition, we systematically investigated the relationship between catalytic performance and surface nickel species structure for CO₂ methanation reaction and photocatalytic H₂ evolution reaction. The main and new results are obtained and presented as following:1. Enhancing Catalytic Activity and Stability for CO₂ Methanation on Ni-Ru/γ-Al₂O₃ via Modulating Impregnation Sequence and Controlling Surface Active SpeciesThe catalysts with different impregnation method were systematically investigated for CO₂ methanation. It was found that the segregation phenomenon of Ru occurred on the catalyst surface in the co-impregnation preparation process, by which more active Ni and Ru specises（metallic Ru） could be provided on the surface of 10Ni-1.0Ru catalyst. The 10Ni-1.0Ru showed more stable and highly active properties during the long-term reaction. Based on the characterizations, it was proposed that CO₂ was dissociated and activated to form carbon species（COads） on Ru species surface, then reacted with activated H species on Ni surface to form methane.2. Enhancing catalytic activity and stability for CO₂ methanation on Ni@MOF-5 via controlling active species dispersionThe high active catalysts Ni@MOF-5 for CO₂ methanation had been prepared by in-situ impregnation-reduction methods. The 10Ni@MOF-5 showed unexpected higher activity for CO₂ methanation than reference catalyst Ni/SiO₂ under the low temperature. Based on the characterization studies of BET, TEM and chemistry adsorption techniques, the high specific surface area（²961 m2/g） and large pore volume lead to high dispersion of Ni（⁴1.8%） over 10Ni@MOF-5. At 320°C, CO₂ conversion was 75.09% and CH4 selectivity was 100% over 10Ni@MOF-5. In addition, this 10Ni@MOF-5 catalyst performed higher stability and selectivity, which showed almost no deactivation in long term stability tests up to 100 h.3. Small-sized Ni（111） particles in metal-organic frameworks with low over-potential for visible photocatalytic hydrogen generationBased on the MOF with high specific surface area, we fabricated high dispersion, small-sized, and high active co-catalyst Ni@MOF-5 for visible photocatalytic hydrogen evolution, The Ni@MOF-5 showed a low over-potential of-0.37 V, which have the similar over-potential as Pt@MOF-5, and exhibited the excellent photocatalytic activity, stability, larger transient photocurrent and longer fluorescence lifetime. The apparent quantum efficiency（AQE） of 16.7% over EY-Ni@MOF-5 was achieved under 430 nm illuminations. In addition, Ni NPs with exposed（111） facets was more benefited the electrons transfer from MOF-5 to Ni than that of（200） facets.4. Fabrication of Low Adsorption Energy Ni-Mo Clusters co-catalyst in Metal-Organic Frameworks for Visible Photocatalytic Hydrogen EvolutionWith the help of density-functional theory（DFT） and front molecular orbital（FMO） theory calculations, the hydrogen adsorption free energies（?GH） of MoNi4-Hads（458 kJ·mol-1） is lower than Ni-Hads（537 kJ·mol-1）. We prepared high active and stability noble-metal-free co-catalyst for HER, Ni-Mo nanocluster anchored on the frameworks of MIL-101, using double solvents methods（DSM）. Compared with the monometallic-based（Ni and Mo） photocatalysts, NiMo@MIL-101 as a co-catalyst exhibited the highest photocatalytic activity（the amount of H₂ was 1480.4 μmol in 2h） and high AQEs（75.7%） under 520 nm illuminations at pH 7. Moreover, the NiMo@MIL-101 material showed a larger transient photocurrent, lower over-potential（-0.51V） and longer fluorescence lifetime（1.57ns）. These results further uncover that the high photocatalytic activity of HER depends on the low ?GH of MoNi4 nanocluster.