Flexible Cuprous Triazolate Frameworks as Highly Stable and Efficient Electrocatalysts for CO₂ Reduction with Tunable C₂H₄/CH₄ Selectivity

Abstract

Cu-based metal-organic frameworks have attracted much attention for electrocatalytic CO 2 reduction, but they are generally instable and difficult to control the product selectivity. We report flexible Cu(I) triazolate frameworks as efficient, stable, and tunable electrocatalysts for CO 2 reduction to C 2 H 4 /CH 4 . By changing the size of ligand side groups, the C 2 H 4 /CH 4 selectivity ratio can be gradually tuned and inversed from 11.8 : 1 to 1 : 2.6, giving C 2 H 4 , CH 4 , and hydrocarbon selectivities up to 51 %, 56 %, and 77 %, respectively. After long-term electrocatalysis, they can retain the structures/morphologies without formation of Cu-based inorganic species. Computational simulations showed that the coordination geometry of Cu(I) changed from triangular to tetrahedral to bind the reaction intermediates, and two adjacent Cu(I) cooperated for CÀ C coupling to form C 2 H 4 . Importantly, the ligand side groups controlled the catalyst flexibility by the steric hindrance mechanism, and the C 2 H 4 pathway is more sensitive than the CH 4 one. Electrocatalytic carbon dioxide reduction reaction(CO 2 RR) is promising to reduce the use of fossil fuels and achieve global carbon neutrality. [1] Among various catalytic active centers, only copper has demonstrated high selectivity to the valuable hydrocarbons. [2] Copper-based catalysts can also show selectivity to aldehydes, ketones, carboxylic acids and alcohol. [3] Cu-based inorganic catalysts have been extensively studied, but elucidation of the structure-performance relationship remains a great challenge because of the lack of well-defined structures of the active sites.As molecule-based crystalline materials with diversified and well-defined pore-surface structures, metal-organic frameworks (MOFs) have been widely studied in various fields including catalysis. [4] Many MOFs, including the classic ones consisting of Cu(II), have been studied for CO 2 RR. [5] However, Cu(II)-based MOFs usually serve as the precursors of inorganic catalysts such as Cu and Cu 2 O. [6] In the few Cu(II)-based MOF catalysts stable in CO 2 RR, the Cu(II) ions are stabilized by chelating ligands. [7] Considering that Cu(II) needs to be reduced to Cu(I) during the CO 2 RR processes, and the common coordination geometries of Cu(I) and Cu(II) are quite different, Cu(I)-based MOFs should be more suitable to serve as stable CO 2 RR catalysts. [8] Metal azolate frameworks (MAFs) are a unique kind of MOFs with outstanding thermal and chemical stabilities. [9] Compared with other types of ligands, azolates are particularly useful for linking Cu(I) ions to form stable MOFs, but have been scarcely used for CO 2 RR. [10] [Cu(detz)] (MAF-2 or MAF-2E, Hdetz = 3,5-diethyl-1,2,4-triazole) is a classic Cu(I)-based MOF, in which trigonal Cu(I) ions are bridged to form dimers (Cu•••Cu 3.4 Å) with both faces exposed on the pore surface (Figure 1). [11] It is well known that the bicopper active sites are essential to CÀ C coupling in CO 2 RR for the valuable C 2 products. [12] The coordination mic...

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

“… 23 , 53 , 54 Lan’s group reported that a series of one-dimensional (1D) coordination polymers, Cu-PzX (X = H, Cl, Br, I), with dicopper sites ( Figure 4 b) can allow the C–C coupling of *CO–*COH to yield C 2 H 4 . 54 Besides, dicopper sites in CuBtz ( 55 ) and MAF-2E ( 21 ) have also been reported, which led to C 2+ compounds as the main products and will be discussed in subsequent sections. We further revealed by periodic density functional theory (PDFT) calculations that a 3D MOF, [Cu 3 (μ 3 –OH)(μ 3 -trz) 3 (OH) 2 (H 2 O) 4 ]· x H 2 O ( Cutrz , Htrz = 1 H ,1,2,4-triazole), 23 can bind three C 1 intermediates at its tricopper active site prior the formation of *CO.…”

“… 70 , 71 The flexibility of MOFs can even affect the microenvironment of catalytic active sites, hence the selectivity of products as very recently documented by Zhang and co-workers that three isoreticular MAFs, [Cu(detz)] ( MAF-2 or MAF-2E , Hdetz = 3,5-diethyl-1,2,4-triazole), [Cu(dmtz) 0.33 (detz) 0.67 ] ( MAF-2ME , Hdmtz = 3,5-dimethyl-1,2,4-triazole), and [Cu(dptz)] ( MAF-2P , Hdptz = 3,5-dipropyl-1,2,4-triazole) with different triazolate ligands or different ratios of triazolate ligands ( Figure 8 ) give different selectivities of C 2 H 4 /CH 4 in ECR. 21 These MAFs possess dicopper sites ( Figure 4 b, Cu···Cu distance = 3.4 Å) exposed on the pore surfaces and have different sizes of triazolate side groups (methyl, ethyl, and propyl) in the frameworks. Very interestingly, as the size of ligand side group increases, the product ratio of C 2 H 4 /CH 4 can be gradually tuned and even inversed from 11.8:1 to 1:2.6.…”

“…Color codes: carbon (gray), copper (orange), hydrogen (white), nitrogen (blue), oxygen (red). Reproduced with permission from ref ( 21 ). Copyright 2022 Wiley-VCH GmbH.…”

“… 10 MOFs were used as catalysts for ECR in 2012 for the first time. 19 , 20 Up to now, many MOFs, especially metal-azolate frameworks (MAFs), 21 − 23 have been proven to be robust and highly efficient for the ECR process ( Figure 1 ). Despite the many advantages showcased by MOF electrocatalysts, their industrial applications are still restricted by several non-negligible shortcomings such as the low current density and stability.…”

Electrocatalytic Reduction of Carbon Dioxide to High-Value Multicarbon Products with Metal–Organic Frameworks and Their Derived Materials

“…Metal–organic frameworks (MOFs), as a unique type of porous materials, are typically constructed from metal nodes and organic ligands/linkers. − Because of the ultrahigh surface area, structural diversity, and chemical tunability, MOFs have drawn considerable attention in various applications, including gas adsorption and separation, , catalysis, − magnetism, , chemical sensing, − and biomedicine. − In particular, MOFs have also shown promising applications in the electrochemical CO 2 RR as a result of the following advantages. − First of all, the porous structure of MOFs can promote the CO 2 adsorption/activation and shorten the transport distance between CO 2 molecules and metal active sites. , Second, metal clusters in MOFs contribute to enhancing the catalytic activity and turnover frequency. , Third, the atomic-level periodicity of metal nodes in MOF structures endows the precise control of metal active sites for electrocatalysis. , Fourthly, perturbation of coordination microenvironment of metal centers can affect the charge density distribution of catalytic active sites, making it possible to delicately regulate the adsorption and/or desorption energy of various key reaction intermediates. − Fifthly, modification of organic ligands with various functional groups can adjust the free energy of different critical intermediates that are adsorbed on catalytic active sites. , Last but not the least, the well-defined and tunable crystallographic structure of MOFs is conducive to build theoretical calculation models to study the structure–activity relationship. ,,, …”