Confining and Highly Dispersing Single Polyoxometalate Clusters in Covalent Organic Frameworks by Covalent Linkages for CO2 Photoreduction.

Single clusters have attracted extensive research interest in the field of catalysis. However, achieving a highly uniform dispersion of a single-cluster catalyst is challenging. In this work, for the first time, we present a versatile strategy for uniformly dispersed polyoxometalates (POMs) in covalent organic frameworks (COFs) through confining POM cluster into the regular nanopores of COF by a covalent linkage. These COF-POM composites combine the properties of light absorption, electron transfer, and suitable catalytic active sites; as a result, they exhibit outstanding catalytic activity in artificial photosynthesis: that is, CO2 photoreduction with H2O as the electron donor. Among them, TCOF-MnMo6 achieved the highest CO yield (37.25 µmol g-1 h-1 with ca. 100% selectivity) in a gas-solid reaction system. Furthermore, a mechanism study based on density functional theory (DFT) calculations demonstrated that the photoinduced electron transfer (PET) process occurs from the COF to the POM, and then CO2 reduction and H2O oxidation occur on the POM and COF, respectively. This work developed a method for a uniform dispersion of POM single clusters into a COF, which also shows the potential of using COF-POM functional materials in the field of photocatalysis.

Stable Dioxin-Linked Metallophthalocyanine Covalent Organic Frameworks (COFs) as Photo-Coupled Electrocatalysts for CO2 Reduction.

In this work, we rationally designed a series of crystalline and stable dioxin-linked metallophthalocyanine covalent organic frameworks (COFs; MPc-TFPN COF, M=Ni, Co, Zn) under the guidance of reticular chemistry. As a novel single-site catalysts (SSCs), NiPc/CoPc-TFPN COF exhibited outstanding activity and selectivity for electrocatalytic CO2 reduction (ECR; Faradaic efficiency of CO (FECO )=99.8(±1.24) %/ 96.1(±1.25) % for NiPc/CoPc-TFPN COF). More importantly, when coupled with light, the FECO and current density (jCO ) were further improved across the applied potential range (-0.6 to -1.2 V vs. RHE) compared to the dark environment for NiPc-TFPN COF (jCO increased from 14.1 to 17.5 A g-1 at -0.9 V; FECO reached up to ca. 100 % at -0.8 to -0.9 V). Furthermore, an in-depth mechanism study was established by density functional theory (DFT) simulation and experimental characterization. For the first time, this work explored the application of COFs as photo-coupled electrocatalysts to improve ECR efficiency, which showed the potential of using light-sensitive COFs in the field of electrocatalysis.

Semiconductor/Covalent-Organic-Framework Z-Scheme Heterojunctions for Artificial Photosynthesis.

A strategy to covalently connect crystalline covalent organic frameworks (COFs) with semiconductors to create stable organic-inorganic Z-scheme heterojunctions for artificial photosynthesis is presented. A series of COF-semiconductor Z-scheme photocatalysts combining water-oxidation semiconductors (TiO2 , Bi2 WO6 , and α-Fe2 O3 ) with CO2 reduction COFs (COF-316/318) was synthesized and exhibited high photocatalytic CO2 -to-CO conversion efficiencies (up to 69.67 µmol g-1 h-1 ), with H2 O as the electron donor in the gas-solid CO2 reduction, without additional photosensitizers and sacrificial agents. This is the first report of covalently bonded COF/inorganic-semiconductor systems utilizing the Z-scheme applied for artificial photosynthesis. Experiments and calculations confirmed efficient semiconductor-to-COF electron transfer by covalent coupling, resulting in electron accumulation in the cyano/pyridine moieties of the COF for CO2 reduction and holes in the semiconductor for H2 O oxidation, thus mimicking natural photosynthesis.