Anchoring Single Fe Site In Vacant Polyoxometalate For Biomimetic CH₄ Photocatalysis

Direct conversion of CH₄ into high value-added chemicals（such as methanol,formic acid,ethanol etc.）have always been a huge challenge in the field of catalysis.However,the inertness of C-H bond in CH₄ results in complex and harsh reaction conditions for the conversion of CH₄.Hence,the direct conversion of CH₄ has always been considered a"holy grail"reaction.In nature,the monooxygenase can oxidize CH₄with oxygen through Fe or Cu sites to directly obtain a series of metabolic products.Inspired by the metal catalytic centers of methane monooxygenase,a series of photocatalysts with transition metal sites（TM=Fe,Co,Ni）based on vacant polyoxometalate were explored,and the effects of different transition metal types and coordination environments on the catalytic conversion of CH₄ to formic acid were explored.1.Inspired by methane monooxygenase,a Fe single catalytic site was precisely incorporatedintodimericpolyoxometalatesK₉Na₂[Fe（H₂O）₂{γ-Si W₉O₃₄Fe（H₂O）}₂]·25H₂O（Fe₁-POM）to clarify the key role of Fe species towards CH₄ photooxidation.Further,[N（C₄H₉）₄]₅[Fe（H₂O）Si W₁₁O₄₀](Fe Si W₁₁),K₁₂Na₂[Fe₂Si W₁₀O₃₈（OH）]₂·32H₂O（Fe₄-POM）and Li₄K₁₆[P₈W₄₈O₁₈₄Fe₁₆（OH）₂₈（H₂O）₄]·66H₂O·2KCl(Fe₁₆-POM)with different Fe-coordination enviroment were constructed to investigate the microenvironment for CH₄conversion.Under visible light irradiation and combient condtions,Fe₁-POM exhibits the highest CH₄-to-formate conversion performance among these POMs,with a yield of 1570.0μmol g_cat^-1 and 95.8%selectivity.Meanwhile,taking advantage of the clear structure of Fe₁-POM,the mechanism of methane catalytic conversion can be fully explored that well-defined（WO）₄-Fe_a-（H₂O）₂ active center in Fe₁-POM can efficiently lower the C-H activation barrier via forming（WO）₄-Fe_a-O···CH₄ intermediate to promote the CH₄ conversion..2.The single-dispersion of TM₁-POM on the polymeric carbon nitride（PCN）matrix was facilely achieved by electrostatic assemble interaction,named as TM₁-POM@PCN.As a result,the resulting Fe₁-POM@PCN-7 exhibits the highest formic acid yield of 5296.1 umol g_POM^-1.This well-defiend single sites dispersed method overcomes the difficulty in characterizing the structure of single-atom catalysts.Moreover,the single metal site immobilization in the vacant polyoxometalates solves the issues of the aggregation of single catalytic sites.