Mechanically Constrained Catalytic Mn(CO)₃Br Single Sites in a Two-Dimensional Covalent Organic Framework for CO₂ Electroreduction in H₂O

Abstract

The development of CO2 electroreduction (CO2RR) catalysts based on covalent organic frameworks (COFs) is an emerging strategy to produce synthetic fuels. However, our understanding on catalytic mechanisms and structure–activity relationships for COFs is still limited but essential to the rational design of these catalysts. Herein, we report a newly devised CO2 reduction catalyst by loading single-atom centers, {fac-Mn­(CO)3S}, (S = Br, CH3CN, H2O), within a bipyridyl-based COF (COFbpyMn ). COFbpyMn shows a low CO2RR onset potential (η = 190 mV) and high current densities (>12 mA·cm–2, at 550 mV overpotential) in water. TOFCO and TONCO values are as high as 1100 h–1 and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent manganese-based molecular catalyst. Furthermore, we accessed key catalytic intermediates within a COF matrix by combining experimental and computational (DFT) techniques. The COF imposes mechanical constraints on the {fac-Mn­(CO)3S} centers, offering a strategy to avoid forming the detrimental dimeric Mn0–Mn0, which is a resting state typically observed for the homologous molecular complex. The absence of dimeric species correlates to the catalytic enhancement. These findings can guide the rational development of isolated single-atom sites and the improvement of the catalytic performance of reticular materials.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

“…In 2021, Lloret-Fillol et al prepared a Mn( i )-metalated COF as a CO 2 RR electrocatalyst by grafting Mn(CO) 5 Br on COF bpy via CO–bipyridine ligand exchange. 246 The COF bpyMn |NT composite, composed of COF bpyMn and multiwalled carbon nanotubes, exhibits higher CO 2 RR activity than the corresponding molecular electrocatalyst composite fac -Mn(2,2′-bipyridine)(CO) 3 Br|NT. In 0.5 M NaHCO 3 solution, the CO faradaic efficiency of COF bpyMn |NT was 62% under optimized conditions, while that of fac -Mn(2,2′-bipyridine)(CO) 3 Br|NT was only 13%, and the TOF of COF bpyMn |NT was 627 h −1 , which was 15.7 times higher than that of fac -Mn(2,2′-bipyridine)(CO) 3 Br|NT.…”

“…Briefly, post-synthetic metalation is a general and versatile method for 2,2′-bipyridine-COF metalation that has been widely used for the synthesis of metalated 2,2′-bipyridine-COFs, as summarized in Table 2. 136,141,146,229–254…”

Single-atom catalysts: stimulating electrochemical CO2 reduction reaction in the industrial era

“…In summary, the strategies for the synthesis of carbon-supported SACs can be classified into the four above-mentioned categories. The molecular skeleton encapsulation method is commonly used in specific mesoporous carbon materials (MOFs, 65 COFs, 66 HOFs, etc. ).…”

Recent progress on covalent organic framework materials as CO2 reduction electrocatalysts

“…In addition to these, several other COFs for the highly efficient electrocatalytic CO 2 reduction to CO in recent years were also worth mentioning. These unique structures include a single Mn atom loaded on bipyridyl-based COFs (COF bpyMn ) via post-modification (Dubed Bandomo et al, 2021), a crown ether B18C6 embedded Co(TAPP)-COF (An et al, 2021), and a zwitterionic ultrathin cobalt tetraamino phthalocyaninesquaraine based COFs (COP-SA) (Song et al, 2021). All these ingenuous works guide the rational design of novel COF electrocatalysts for CO production from CO 2 .…”