A Stable and Conductive Covalent Organic Framework with Isolated Active Sites for Highly Selective Electroreduction of Carbon Dioxide to Acetate

Abstract

Electroreduction of CO2 to acetate provides a promising strategy to reduce CO2 emissions and store renewable energy, but acetate is usually a by‐product. Here, we show a stable and conductive two‐dimensional phthalocyanine‐based covalent‐organic framework (COF) as an electrocatalyst for reduction of CO2 to acetate with a single‐product Faradaic efficiency (FE) of 90.3(2)% at −0.8 V (vs. RHE) and a current density of 12.5 mA cm−2 in 0.1 M KHCO3 solution. No obvious degradation was observed over 80 hours of continuous operation. Combined with the comparison of the properties of other catalysts with isolated metal active sites, theoretical calculations and in situ infrared spectroscopy revealed that the isolated copper‐phthalocyanine active site with high electron density is conducive to the key step of C−C coupling of *CH3 with CO2 to produce acetate, and can avoid the coupling of *CO with *CO or *CHO to produce ethylene and ethanol.

Reusable Copper Catechol‐based Porous Polymers for the Highly Efficient Heterogeneous Catalytic Oxidation of Secondary Alcohols

“…[53] In Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) data, a small increase of intensity of the first-shell contribution is visible (Figure 2b) for the reacted catalyst, which corresponds to the change in the number of oxygen neighbors, N Cu-O from 3.6 to 3.9 (see Table S4). In summary, the combination of XANES, EXAFS and XPS spectra demonstrate that there are no Cu containing clusters or nanoparticles (Cu 0 , Cu 2 O, CuO), the Cu remains in the oxidation state + II and remains atomically dispersed in a square planar coordination by four ligand atoms, [55] even after 10 catalytic cycles. [c] Toluene (0.5 mL) was added.…”

Rational Design of Metal–Organic Frameworks for Electroreduction of CO2 to Hydrocarbons and Carbon Oxygenates

“…Therefore, the tandem pathway on dual copper sites can also result in an excellent C 2+ selectivity. Different with the dual copper site, in a PcCu-TFPN covalent-organic framework (COF) with identical isolated copper sites for ECR, the active site allows the generation of a *CH 3 intermediate and the asymmetrical C–C coupling between a *CH 3 species and a CO 2 molecule, resulting in the formation of acetate . The detailed mechanism will be discussed in the next section.…”

“…Different with the dual copper site, in a PcCu-TFPN covalent-organic framework (COF) with identical isolated copper sites for ECR, the active site allows the generation of a *CH 3 intermediate and the asymmetrical C–C coupling between a *CH 3 species and a CO 2 molecule, resulting in the formation of acetate. 56 The detailed mechanism will be discussed in the next section. These facts indicate that controlling the hydrogenation of *CO intermediates and subsequent C–C coupling of *CO–*CHO or *CO–*COH are of great importance for tuning the MOF-catalyzed ECR selectivity toward C 1 /C 2 products, and MOFs with two or more closely located metal sites are conducive for the C–C coupling to yield C 2 or C 2+ products.…”

“…Unpredictably, as an isolated active site with a more enhanced Lewis basicity, the Cu-phthalocyanine site in a COF PcCu-TFPN ( Figure 5 d) exhibits a high selectivity for the C 2 product acetate in ECR. 56 Compared with a classical single-atom copper catalyst ( CuSAC ) as well as a copper-porphyrin-based COF ( Cu-porphyrin ), PcCu-TFPN has a stronger electron delocalization and higher electron density around its Cu active sites, which should be attributed to the high abundance of electron-rich nitrogen atoms in the phthalocyanine units. Therefore, unlike CuSAC , which generates CO as the main product, PcCu-TFPN can form a stronger interaction with *CO species, thus suppressing release of CO as the product.…”

“…Adapted with permission from refs , , and . Copyright 2021 and 2022 American Chemical Society, and from ref . Copyright 2022 Wiley-VCH GmbH.…”