Preparation And Catalytic Performance Of ZnNiAl-LDHs Catalysts For Light-driven 5-HMF Oxidation And CO₂ Reduction Reaction

With the continuous advancement of the industrialization process around the world,two urgent problems of energy crisis and environmental pollution need to be solved.Traditional fossil fuel reserves are facing depletion,and at the same time,the burning of fossil fuels produces too many CO₂,which leads to an increasingly serious greenhouse effect.Countries around the world are looking for feasible carbon balance and carbon neutrality solutions.Since the20^th century,related research on artificial photosynthesis has become a hot topic in the research field,and its essence is the photocatalytic CO₂ reduction reaction.In the related research on the traditional photocatalytic CO₂ reduction reaction,most of them choose to use H₂O as the oxidation half-reaction to couple with it.However,the H₂O oxidation half-reaction has the problems of low catalytic rate and low product added value,which severely limits the CO₂reduction half-reaction performance.To solve this problem,organics that are more prone to oxidation can be selected to replace H₂O and reduced with CO₂ to construct a new photocatalytic reaction system.As a biomass platform compound molecule,5-Hydroxymethylfurfural（5-HMF）has aldehyde groups and hydroxyl functional groups.It is not only prone to oxidation,but its oxidation products also have extremely high added value to meet the above reaction requirements.Therefore,this dissertation focuses on the photocatalytic CO₂ reduction and 5-HMF oxidation coupled reaction system to carry out related research.The photocatalytic CO₂ reduction and 5-HMF oxidation coupling reaction catalysts need to meet several requirements at the same time:1)pocess the ability to absorb light to generate electron-hole pairs;2)have a suitable energy band structure to meet the requirements of chemical thermodynamics;3)It can activate both CO₂ and 5-HMF reactant molecules at the same time.Layered double hydroxides（LDHs）are two-dimensional layered inorganic functional materials.With the help of the tunable chemical composition of LDHs,reasonable selection of metal elements with semiconductor properties can achieve light response.Besides,the cationic and hydroxyl sites of LDHs can simultaneously adsorb and activate CO₂ and 5-HMF molecules.Therefore,ZnNiAl-LDHs photocatalysts have been designed and synthesized to successfully realize the coupling of photocatalytic CO₂ reduction and biomass oxidation,focusing on revealing the mechanism of the two defect structures of oxygen and metal vacancies in the photocatalytic coupling reaction,that is,suppressing electron-hole separation and improving the adsorption of reactants on the catalyst surface.This dissertation provides feasible methods and ideas for the related researches of photocatalytic coupling reactions.First of all,memory effect is a characteristic that LDHs lose water and trun into Mixed Metal Oxides（MMO）under calcination,and then MMO reconstruct into LDHs.Based on this property,ZnNiAl-LDHs photocatalysts with light-absorbing ability were prepared by calcination-reconstruction method.Comparing the ZnNiAl-X-R series materials（X is the calcination temperature,200,300,400,450 and 500 ^oC,respectively）with precursor,it is found that although MMO can recover the layered structure through the restoration process after being calcinated at different temperatures.However,there is a loss of metal contents after recovery,with a lightly change in the average thickness and pore structure of the laminate.In addition,the calcination process in H₂ atmosphere also introduced the oxygen vacancy defect structure into the catalyst surface.Therefore,ZnNiAl-X-R catalysts have two defect structures at the same time,and ZnNiAl-450-R has the largest number of defect structures.The 300 W Xe lamp light source was selected to provide light in the ultraviolet-visible range,and the light intensity was controlled to be 550 m W/cm^-2.The performance of the ZnNiAl-X-R catalyst was evaluated for 4 h.It can be found that all of the catalysts calcined and restored at different temperatures can achieve CO₂reduction and 5-HMF oxidation.Among them,the ZnNiAl-450-R sample had the highest catalytic performance（the yields of CO and 5-formyl-2-furancarboxylic acid reached 4.04 and 78.23μmol/g,respectively）.ZnNiAl-450-R can still maintain more than 90%of the initial activity after 5 rounds of repeatability tests.Combined with the results of PL spectra and electrochemical tests,it can be confirmed that defective structures enhanced the photoelectric conversion efficiency.It was further confirmed by FT-IR that the introduction of defective structures promoted the adsorption and activation of CO₂ and 5-HMF molecules on the surface of catalysts.To further explore the different mechanisms of oxygen vacancies and metal vacancies,the MMO samples calcined at 450 ^oC were selected for reconstruction in Na OH solution,resulted in ZnNiAl-OH-Y photocatalyst（Y is the recovery time of 2,4 and 6 hours respectively）having the same number of oxygen vacancies but different numbers of metal vacancies.The ZnNiAl-OH-Y catalysts have good LDHs characteristic structure,which proves the successful progress of the reconstruction process in Na OH soloution.According to ICP-AES,EPR and XPS results,with the increase of recovery time in strong alkaline solution,the amount of metal contents loss increased,but there was no significant difference in the number of oxygen vacancies.All three groups of catalysts realized the coupling of CO₂ reduction and 5-HMF oxidation,among which ZnNiAl-OH-4h had the best catalytic activity（CO yield of 4.01μmol/g and FFCA yield of 261.15μmol/g）.Combined the catalytic performances,PL spectra and electrochemical tests results,it is confirmed that the oxygen vacancies have two promoting effects of inhibiting the recombination of electron-hole and optimizing the activation process of CO₂,while metal vacancies promote the adsorption and activation process of 5-HMF.