Theoretical Study On Mechanism Of Water Oxidation And Nitrogen Reduction Catalyzed By Keggin-type Polyoxometalate-based Derivatives

Polyoxometalates（POMs）represent a diverse family of anionic metal-oxygen species,which are condensed with MO₆（M = Mo,W,V,Nb,Ta）as the basic building unit through the corner-and edge-sharing.There are six basic structures of Keggin,Silverton,Lindqvist,Waugh,Anderson and Wells-Dawson in POM.POM is strong Br?nsted acid,and its molecular structure is excellently stable under the harsh reaction conditions.Therefore,POM and its derivatives are promising as catalysts.At present,the application of POM-based derivatives in water oxidation catalysis（2H₂O → O₂ + 4H⁺ + 4e^-）is one of the most concerned research fields.A large number of POM-based water oxidation catalysts（WOCs）have been synthesized.In POM-based WOCs,the monovacant,divacant and trivacant Keggin-type POMs act as monodentate or multidentate ligand to chelate transition metal（Ru,Co,Ni,Mn,Fe,Cu）.However,it is a challenge to experimentally reveal the catalytic mechanism of POM-based WOC due to the presence of metal（tungsten）oxo clusters in these catalysts.With the development of the research on POMs,chemists began to expand and develop the potential application of POMs.Yan group in National University of Singapore synthesized a series of single-atom catalysts（SAC）by dispersing Pt as a single atom on the surface of Keggin-type POMs.However,the application of POM-supported Pt SAC has been limited to several hydrogenation reactions so far.In addition,whether other metals can exist in the form of single atom on the surface of POM remains to be further studied.Therefore,exploring the existing forms of other metals on the surface of POM and the performance of POM-supported SAC in catalytic reactions,such as the great concern electrocatalytic nitrogen reduction reaction（N₂ + 6H⁺ + 6e^-→ 2NH₃）is more significant.Electrocatalytic nitrogen reduction reaction not only provides chemical raw material NH₃ for human life,but also proceeds under mild reaction condition（normal remperature and pressure）,which is particularly important in the increasingly serious situation of environmental pollution and energy shortage.In this thesis,the catalytic mechanism of POM-based WOCs and the performance of POM-supported SAC for nitrogen reduction reaction were investigated by density functional theory（DFT）methods.The present works are expected to provide reliable theoretical guidance for the sythesis of excellent WOCs and exploring the potential applications of POMs.The thesis consists of six chapters.The first chapter is "Introduction" in which the POM-based WOCs,catalytic mechanism of water oxidation and nitrogen reduction reaction are included.The second chapter is theoretical basis.The main contents of the thesis are from the third to the sixth chapters:1.The performance of Mn^II/III-monosubstituted heteropolytungstates [Mn^III（H₂O）GeW₁₁O₃₉]^5-([GT-Mn^III–OH₂]^5-)and [Mn^II（H₂O）GeW₁₁O₃₉]^6-([GT-Mn^II–OH₂]^6-)as WOC was theoretically explored.The counterion effect was fully considered.Calculated analysis indicates that the electron transfer（ET）and proton transfer（PT）proceed via the ways of synergism（PCET）or first PT and then ET（PT-ET）.The Mn^IV-O? species is proposed to be responsible for O?O bond formation via water nucleophilic attack（WNA）or oxo-oxo coupling mechanisms.In the last stage,the O₂ molecule could be readily evolved from the peroxo or dinuclear species and the catalyst returns to the ground state after the coordination of a water molecule（s）.This report provides effective information for further exploring the catalytic mechanism of Mn-containing POM-based WOCs.2.The key O?O bond formation step in water oxidation catalyzed by a POM-based complex [Mn^II₃（H₂O）₃(SbW₉O₃₃)₂]^12-was theoretically investigated.The Mn^V-oxo species rather than Mn^IV-oxo species,is computed to be responsible for O?O bond formation via WNA mechanism.During O?O bond formation based on Mn^V-oxo species,two-state reactivity is found and the energy profile switches from the 13 et state to the 15 et state,which significantly reduces the activation barrier.The observed spin inversion phenomenon in O?O bond formation is attributed to the exchange-enhanced reactivity for 15 et state,but not for 13 et state.3.The mechanism of water oxidation catalyzed by a tricopper-containing POM-based complex [Cu^II₃（H₂O）₃(SbW₉O₃₃)₂]^12-([AT-Cu^II₂Cu^II?OH₂]^12-)was theoretically investigated for the first time.Calculations suggest that the Cu center is redox-inert and remains bivalence throughout the catalytic reaction.In oxidative activation stage,two hydrogen atoms are removed from one water ligand.Therefore,an unusual metal-oxyl-radical species [AT-Cu^II₂Cu^II?O??]^12-is obtained,in which two unpaired electrons are assigned to the active O center.A monocopper-containing Keggin-type POM [Cu^II（O??）SbW₁₁O₃₉]^5-was selected as the model to explore the O?O bond formation mediated by [AT-Cu^II₂Cu^II?O??]^12-.The O?O bond formation proceeds via the single electron transfer-WNA（SET-WNA）mechanism.4.Using first principles calculations,the performance of silicomolybdic acid(H₄SiMo₁₂O₄₀（H₄SM）)supported single-atom catalysts（SACs）for nitrogen reduction reaction was theoretically investigated.The counterion effect was fully considered.The calculations suggest that the metals Sc,Ti,V,Zn,Mn,Y,Cd and Zr dominantly exist as single atoms on the surface of H₄SM,thus forming a series of SACs.Among the studied POM-supported SACs,the H₄SM supported V SAC was demonstrated to be the promising catalyst for nitrogen reduction reaction in which the optimal reaction path is via distal and alternating mechanisms with an overpotential of 0.68 V.