Preparation Of Cobalt,nickel Based Nanomaterials And Their Application In Photocatalytic CO₂ Reduction

A large amount of CO₂ emitted from the consumption of fossil fuels is a major cause for global warming.Driven by clean and sustainable solar energy,the catalytic reduction of CO₂into high value-added chemical products could alleviate environmental problems and energy crisis simultaneously.However,due to the high thermodynamic stability of CO₂ molecules,the photocatalytic CO₂ reduction process is very challenging.So,it is necessary to choose a suitable catalyst to promote the photocatalytic CO₂reduction reaction.Cobalt and nickel-based transition metal materials are widely used in photocatalytic CO₂ reduction researches owing to their merits of rich crust content,low cost,and high catalytic activity.But,traditional cobalt and nickel-based nanocatalysts often suffer from high electron recombination rate,poor CO₂adsorption and activation capabilities,which limit the optimization of their catalytic reduction efficiency.Therefore,our thesis is aimed at cobalt and nickel-based nanocatalysts,through a series of regulation strategies to designed and modified newly and efficient cobalt,nickel-based catalysts,for which enhance the photocatalytic CO₂ reduction efficiency and achieve highly selective photoreduction of low concentration CO₂.The main research contents of this thesis are as follows:(1)We prepared metal-organic frameworks(MOFs)derived ultrathin Co₃O₄ nanosheets by ion-assisted solvothermal method combined with calcination under air.A photocatalytic CO₂ reduction system with acetonitrile/water/triethanolamine as the reaction medium,[Ru(bpy)₃]Cl₂ as the photosensitizer,and ultrathin Co₃O₄ nanosheets as the catalyst was constructed.The ultrathin Co₃O₄ nanosheets calcined at 350°C have the most excellent photocatalytic activity and selectivity of CO₂-to-CO conversion(4.52?mol·h^-1,70.1%),which is significantly better than the Co₃O₄ bulk catalysts(2.48?mol·h^-1,58.4%).The corresponding analysis showed that catalysts with ultrathin two-dimensional structure has faster electron transport ability and more exposed catalytically active sites than bulk structure.Density functional theory(DFT)calculations indicated that Co₃O₄ monolayer has stronger adsorption energy for CO₂ and weaker adsorption energy for H protons.These advantages are conducive to the improvement of photocatalytic CO₂ reduction activity and selectivity.This work provided a new perspective for the design of efficient photocatalytic CO₂reduction catalysts.(2)Through in-situ growth of cobalt-cobalt hydrotalcite(Co-Co LDH)nanosheets on conductive two-dimensional Ti₃C₂T_X(TNS)substrates,we delicate obtained 3D Co-Co LDH/TNS nanoarrays.On the basis of the above reaction system,a photoreduction CO₂ system was constructed using Co-Co LDH/TNS nanoarrays as catalysts.Our study showed that the photocatalytic CO₂ reduction performance of Co-Co LDH/TNS nanoarrays is significantly better than that of pristine Co-Co LDH nanosheets.As the TNS content increased,the photocatalytic CO₂ reduction performance of the composites increased first and then decreased.Among them,15 mg is the optimal loading amounts of TNS,and its photocatalytic activity of CO₂-to-CO conversion is 6.25?mol·h^-1,which is 2.2 times that of pristine Co-Co LDH nanosheets.The photoelectrochemical test showed that the introduction of conductive Ti₃C₂T_Xspecies could accelerate the transfer of surface charge above the catalysts and improve the utilization rate of electrons during the photocatalytic CO₂reduction process.In addition,the“Co-Co LDH+TNS-15”two-dimensional/two-dimensional composite was prepared by mechanical stirring for comparison,and its photocatalytic activity of CO₂-to-CO conversion(4.11?mol·h^-1)is significantly lower than that of the Co-Co LDH/TNS nanoarray.This indicated that the excellent photocatalytic CO₂ reduction activity of the Co-Co LDH/TNS composite is attributed to the synergy between the conductive Ti₃C₂T_X species and the hierarchical nanoarray structure.This work provided valuable ideas for the designation of photocatalytic CO₂ reduction composite catalysts.(3)NiO nanosheets with rich oxygen vacancies(r-NiO)were prepared by calcination under the protection of Ar.On the basis of the above reaction system,CO₂/Ar(1/9)mixed gas was used instead of high purity CO₂ gas,and low concentration CO₂ photocatalytic reduction system was constructed using r-NiO as catalysts.The results showed that the photocatalytic CO₂ reduction performance of r-NiO sample is significantly higher than that of the sample with no oxygen defects(n-NiO).Under low concentration CO₂environment,r-NiO sample could maintain more than 70%CO yield,and its photocatalytic activity of CO₂-to-CO conversion is3.14?mol·h^-1,the corresponding selectivity is 82.11%.However,as for n-NiO sample,the corresponding activity and selectivity are only 1.97?mol·h^-1 and 65.26%,respectively.The related analysis displayed that the r-NiO sample has a higher photo-generated carrier separation rate and a larger specific surface area than n-NiO.Moreover,DFT calculations disclosed that the existence of oxygen vacancies can not only increase the adsorption energy of CO₂ molecules,but also weaken the adsorption of H protons,so that CO₂ molecules hold advantages during the competitive adsorption process in the reaction system,thus achieving highly efficient photocatalytic reduction in diluted CO₂ environment.The apparent quantum efficiency of r-NiO catalyzed diluted CO₂ photoreduction is 0.46%,which is equivalent to many pure CO₂photocatalytic reduction systems.This work provided a great reference for the direct and efficient reduction of diluted CO₂.