Unraveling photocatalytic electron transfer mechanism in polyoxometalate-encapsulated metal-organic frameworks for high-efficient CO2 reduction reaction

Unraveling the fundamental electron transfer mechanism is vital to deeply understand the heterogeneous photocatalysis process so as to develop new efficient photocatalysts. Herein, we present a deep investigation on the electron-transfer mechanism in heterogeneous photocatalytic CO2 reduction reaction by adopting a new type of structurally clear polyoxometalate-encapsulated metal-organic frameworks {H-n[Cd6L6Cl7](4)[POM](8)} (L=1,4-di (4H-1,2,4-triazol-4-yl)benzene, POM={SiW12}, {PW12} and {PMo12}, abbr. POM@CdMOF) as model photo-catalysts. Transient photovoltage measurements indicate that various POM guests in photoactive CdMOF hosts show different electron transfer behaviors and charge separation pathways, inducing different photocatalytic activities. Thereinto, SiW12@CdMOF and PW12@CdMOF exhibit high photocatalytic CO2 reduction activities with CO yields of 4.35 mmol(CO).mol(Cd)(-1).h(-1) and 3.60 mmol(CO).mol(Cd)(-1).h(-1) comparably to most reported photo-catalysts, whereas the performance of PMo12@CdMOF is poor. The in-situ transient photovoltage and DFT calculations revealed the key regulation role of different POM units on the interfacial electron transfer and the adsorption behavior of the important intermediates, thus describing a feasible route to efficiently perform the photocatalytic CO2 reduction. This work may provide a new and significant guideline for exploring high-performance photocatalysts.