Design And Synthesis Of Polyoxometalate-based Photo/Electro-catalysts For Carbon Dioxide Reduction Performance

Energy shortage and environmental pollution are the most serious problems facing the development of human society.At present,the world’s energy consumption is still dominated by fossil energy.The increasing human activities not only accelerate the consumption of fossil fuels,but also cause the greenhouse gas,mainly CO₂,to increase year by year,breaking the carbon balance in nature.The use of renewable clean energy（such as sunlight,electricity）to reduce CO₂ to prepare high value-added chemicals provides a very promising means to solve environmental problems and enrich energy supply.At the same time,this is also the only way for my country to carry out energy transformation and achieve the"dual carbon"goal.However,due to the high structural stability of CO₂ molecules and the multiple reduction reaction pathways,CO₂ reduction reaction（CO₂RR）has low reaction efficiency and poor product selectivity,which restricts its practical application.Therefore,to realize efficient CO₂value-added conversion,the construction of high-efficient and well-defined heterogeneous CO₂RR model catalysts is an important means to elucidate the reaction mechanism of CO₂RR and achieve effective CO₂ conversion.Based on the above scientific problems,in this thesis,a series of novel polyoxometalate（POM）-based heterogeneous catalysts were designed and synthesized,which were used as model catalysts for photocatalytic and electrocatalytic CO₂RR.The specific work is as follows:1.A series of POM-carbonyl manganese compounds（abbreviated as POM-Mn L）with formula of[Mn^I（bipy）（CO）₃（CH₃CN）]₄(Si W₁₂O₄₀)·5CH₃CN(1,Si W₁₂-Mn L),[Mn^I（bipy）（CO）₃（CH₃CN）]₃(PW₁₂O₄₀)·2CH₃CN(2,PW₁₂-Mn L)and[Mn^I（bipy）（CO）₃（CH₃CN）]₃(PMo₁₂O₄₀)·2CH₃CN(3,PMo₁₂-Mn L),were prepared by integrating the electron sponge feature of POMs and the CO₂RR activity of carbonyl manganese species,which were applied as heterogeneous electrocatalysts for CO₂RR.A series of electrochemical,photoluminescence spectra,transient photovoltage experiments,and DFT calculations indicate that electron transfer from POM to Mn L exists in the POM-Mn L composite catalyst.By changing the types of POMs in the POM-Mn L composite catalyst,the electron transfer pathways can be regulated,and the Faradaic efficiency of CO₂RR can be improved.Among them,Si W₁₂-Mn L exhibits excellent electron transfer behavior and efficient electrocatalytic CO₂ reduction activity,with particularly high CO selectivity,and the Faradaic efficiency of 95%at an overpotential of 0.61 V vs.RHE.This work demonstrates an electron transfer modulation strategy in electrocatalytic CO₂ reduction and provides a new perspective for designing efficient CO₂RR electrocatalysts.2.Using the highly reduced{P₄Mo₆}clusters and ruthenium pyridine complexes as precursors,a class of photocatalytic systems（{P₄Mo₆}/Ru（bpy））with tunable CO₂RR activity was constructed.The molecular formula of the reduced{P₄Mo₆}compounds are:Na₆[Co（H₂O）₂（H₂tib）]₂{Co[Mo₆O₁₅（HPO₄）₄]₂}·5H₂O（4）,Na₃[Co（H₂O）₃][Co₂（bib）]（H₂-bib）_2.5{HCo[Mo₆O₁₄（OH）（HPO₄）₄]₂}·4H₂O（5）and（H₂bpp）₃{HNa[Mo₆O₁₂（OH）₃（HPO₄）₃（H₂PO₄）]₂}·6H₂O（6）.In these compounds,different numbers and types of coordinated metal ions(Co²⁺and Na⁺)can change the electron structure of{P₄Mo₆}compounds.The photocatalytic systems composed of 4/Ru（bpy）and 5/Ru（bpy）both exhibited good photocatalytic performance,with CO yields of 322.7 nmol and 281.81 nmol within 10 hours,and selectivity of 96.3%and 96.4%,respectively.The photocatalytic system composed of compound6/Ru（bpy）exhibited poor photocatalytic performance,with a CO yield of only 7.58 nmol and a selectivity of 83.6%.The photocatalytic mechanism study shows that the reduced{P₄Mo₆}-based compound in the{P₄Mo₆}/Ru（bpy）photocatalytic system is the active center of the oxidation sacrificial agent,and the Ru（bpy）component is the active center for CO₂RR.In addition,the modified metal ions on the{P₄Mo₆}clusters can tune their energy level structure and promote the transfer of photogenerated electrons on{P₄Mo₆}to Ru（bpy）,thereby achieving efficient photocatalytic CO₂ reduction.3.To investigate the effect of the electron transfer process in the heterogeneous photocatalytic CO₂RR,a series of well-defined POM-encapsulated metal-organic frameworks（POM@Cd MOF）with formula of{H₄[Cd₆L₇]₄[PW₁₂O₄₀]₈}_n·n H₂O(7,PW₁₂@Cd MOF),{H₄[Cd₆L₆Cl₇]₄[PMo₁₂O₄₀]₈}_n·n H₂O(8,PMo₁₂@Cd MOF)and{[Cd₆Cl₈L₆]₄[H₂Si W₁₂O₄₀]₈}_n·n H₂O(9,Si W₁₂@Cd MOF)were synthesized as model photocatalysts.In POM@Cd MOF,the POM guests can induce different electron transfer pathways and photogenerated charge separation behaviors between POM and Cd MOF,which have a huge impact on their photocatalytic activity.Among them,Si W₁₂@Cd MOF and PW₁₂@Cd MOF exhibited excellent photocatalytic CO₂RR performance,while PMo₁₂@Cd MOF showed poor photocatalytic performance.A series of in situ transient photovoltage（TPV）measurements,femtosecond transient absorption（TA）spectroscopy and DFT calculations indicate that the different electron transfer in POM@Cd MOF directly affect the adsorption behavior of CO₂molecules,which modulates the adsorption free energy of reaction intermediates,which in turn affects the activity and selectivity of photocatalytic CO₂RR.4.By employing electrochemical deposition technology,a kind of highly-dispersed palladium polyoxometalate/reduced graphene oxide composite electrocatalyst（POP/r GO）was prepared with[Cu Pd₁₂O₈（As O₄）₈]^14-（POP）and r GO as precursor.POP/r GO as heterogeneous electrocatalyst exhibits excellent electrocatalytic activity and can efficiently co-reduce CO₂and N₂ to synthesize urea with a yield of 1.8 mmol·g^-1·h^-1 and a Faradaic efficiency of 4%.DFT calculations show that the unique Pd-O bond in POP not only contributes to the adsorption and activation of CO₂ and N₂,but also has suitable adsorption free energy for the intermediates in the N₂ activation process,thus promoting the C-N bond coupling to synthesize urea.This work provides insights into the preparation of efficient electrocatalysts for electrochemical CO₂ and N₂ co-reduction to synthesize urea.In general,this thesis aims to develop novel and well-defined POM-based heterogeneous CO₂RR catalysts,to study the electron transfer mechanism and elucidate the intrinsic structure-performance relationship in photo-or electrocatalysis,providing a feasible solution for realizing efficient CO₂ value-added conversion.