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...

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