Ligands Regulated Photocatalytic CO₂ Reduction Of Metal-Organic Frameworks

Environmental pollutions and climate changes caused by combustion of fossil fuels have restricted social developments.Since 1750,the concentration of CO₂ has been increasing at a rate of 2 ppm per year and exceeded 400 ppm until now,which becomes one of the main factors for global warming.By mimicing photosynthesis,light-driven CO₂ conversion into value-added chemicals has been regarded as one of the effective ways to reduce CO₂ concentration.CO₂ molecule is so stable that The reduction reaction of CO₂ needs to overcome high energy barrier due to its high chemical stability.Therefore,the development of high-efficiency photocatalytic CO₂ reduction catalysts to reduce the reaction energy barrier and improve the catalytic performance is in great demand.Metal-organic frameworks（MOFs）with flexible chemical and physical properties have been widely used as a new type photocatalysts for CO₂reduction.Herein,we demonstrate that the photocataytic performances can be substantially improved by functionalizing the organic linkers of MOF catalysts,（1）Two metal ions(Co²⁺,Re⁺)were incorporated into the bipyridine ligand of UiO-67 to synthesize Co-UiO-67 and Re-UiO-67 by a post-metallization method for photocatalytic reduction of CO₂.The photocatalysis experimental results,reveal that the metal ions doping has strong influences on the catalytic activity of MOFs.The Co-UiO-67 catalyst exhibit superior photocatalytic performance and long-term stability than that of Re-UiO-67 catalyst as regards to the reduction of CO₂ to CO.The generation rate of CO is eight-fold higher than that of Re-UiO-67.Density-functional theory（DFT）calculations reveal that the energy barrier of Co-UiO-67（0.86 eV）for catalytic CO₂ reduction to CO is lower than that of Re-UiO-67（0.92 eV）.Experimental results including photoluminescence,electrochemistry in combination with DFT calculations reveal that the doped Co ions into UiO-67 lower the photocatalytic reaction energy barrier and contribute to the enhancement of charge transportability,further lead to its excellent photocatalytic performance.（2）Due to the relatively poor CO selectivity of the Co-based MOFs Ni-based MOFs have been synthesized.In this work,Ni-BDC nanosheets（NS）and their derivatives（NH2-Ni-BDC NS）were prepared and their photocatalytic performance towards CO₂ reduction have been investigated.The results show that Ni-BDC NS exhibitss excellent CO2 reduction performance,with CO product selectivity up to 92%.In addition,amino-functionalized MOFs（NH₂-Ni-BDC NS）can further improve the photocatalytic performance due to to the presence of amino groups,which leads to the enhancement of CO₂ adsorption capacity and electron migration.