Photothermal Catalytic CO₂ Hydrogenation Over MOFs-derived Non-noble Metal Nanostructures

Due to the increasing CO₂ emissions caused by the excessive consumption of fossil fuels,the sustainable development of human society is facing global challenges in energy and environment.Alternatively,photothermal catalysis provides an effective way to convert greenhouse gas（e.g.CO₂）into valuable chemical fuels,which in the long run can not only mitigate the greenhouse effect and achieve a carbon-neutral cycle,but also convert solar energy into chemical energy and solve the problem of renewable energy conversion and storage.Currently,non-noble metal catalysts are widely studied due to their low price and abundant yield,which are expected to be used in large-scale practical production of heterogeneous catalysis.However,the existing non-noble metal photothermal catalysts have limited light absorption capacity in the full solar spectrum range.Meanwhile,non-noble metal catalysts often require higher reduction temperatures compared to noble metals,resulting in poor dispersity and low intrinsic catalytic activity.Therefore,it remains a central scientific issue in this field to improve the light absorption performance and metal particle dispersion of non-noble metal photothermal catalysts.Thus,in this thesis,a series of highly light-absorbing and highly dispersed powder composite nanostructures were rationally designed and prepared using non-noble metal atomic-level dispersed MOFs as precursor materials.Through the construction of the catalyst structures,on the one hand,the light absorption performance of the materials is improved and the sunlight utilization rate is enhanced.On the other hand,the dispersion of the non-noble metal catalysts is increased and the catalytic activity is enhanced to realize the efficient conversion of light energy to chemical energy.The specific research work carried out is described as follows:（1）In chapter 2,we developed a silica-protected metal-organic framework pyrolysis strategy for array-based photothermal catalysts with low active components per unit light area to obtain highly loaded and highly light-absorbing Co-Si O₂ plasma superstructured powder catalysts.It is shown that the Co-Si O₂ catalyst can achieve 90%light absorption in the full solar spectrum.Compared with the array-based plasmonic superstructure catalyst,the metal loading per unit irradiation area of the Co-Si O₂ catalyst was increased from 0.3 mg·cm^-2 to 1.4 mg·cm^-2,and the CO₂ conversion efficiency increased from 0.9%to 26.2%under the same catalytic conditions.Meanwhile,the Co-Si O₂ demonstrated excellent catalytic stability under long time test conditions.This study confirms that the construction of powder plasma ultra-structured catalysts can substantially improve the light absorption capacity and the loading of active metal catalysts per unit illumination area,laying the foundation for the practical application of the catalysts.（2）In chapter 3,in order to solve the problems of large metal Co particle size（27.0nm）and relatively low catalytic activity,we propose the strategy of changing the geometry of MOFs precursors to obtain highly loaded and dispersed photothermal catalysts with smaller size.In this work,the Ni@Si O₂ plasmonic superstructure catalyst was synthesized by using 2D MOFs instead of 3D MOFs to reduce the size of the precursors,which not only has the ultra-high stability due to the domain-limiting effect of Si O₂ and the high light absorption capacity due to the plasma hybridization effect of metal particles,but also further reduced the size of metal particles（3.7 nm）.Specifically,the activity of the Ni@Si O₂ reached 2.6 mmol·g _Ni^-1·h^-1 under 3200 m W·cm^-2 illumination in photothermal catalytic CO₂ hydrogenation.This study overcomes the mutual constraint of loading and dispersion of metal catalysts,and provides a new idea for constructing highly loaded and dispersed small-size catalysts.（3）Considering the limited light absorption of small-sized metal particles,in this work,we prepared an egg yolk-shell structured metal catalyst（Co@Si O₂）by the MOFs domain-limited template method to achieve high light absorption of small-size metal catalysts.The Co@Si O₂ catalyst exhibits excellent light absorption performance（90%）in the full solar spectrum and photothermal conversion efficiency thanks to the strong light absorption performance of large-sized cobalt particles.Specifically,compared to the sample（Co-SNP）without large-sized metal particles to assist in light absorption,the Co@Si O₂ sample is 28 K higher under 2000 m W·cm^-2 illumination.And the catalytic activity of the Co@Si O₂sample was about 14 times higher.Furthermore,the versatility of the MOFs domain-limited template method for the preparation of catalysts with yolk-shell structure was verified by preparation of Cube Co@Si O₂ catalysts.The study provides a new approach for the preparation of catalysts with excellent light absorbing capacity and high dispersity,and also lays the foundation for their large-scale applications.