Abstract
Electrocatalytic acetylene semihydrogenation is a promising alternative to thermocatalytic acetylene hydrogenation due to its environmental benignity and economic efficiency, but its performance is far below that of the thermocatalytic reaction because of strong competition from side reactions, including hydrogen evolution, overhydrogenation and carbon–carbon coupling reactions. We develop N–heterocyclic carbene–metal complexes, with electron–rich metal centers owing to the strongly σ–donating N–heterocyclic carbene ligands, as electrocatalysts for selective acetylene semihydrogenation. Experimental and theoretical investigations reveal that the copper sites in N–heterocyclic carbene–copper facilitate the absorption of electrophilic acetylene and the desorption of nucleophilic ethylene, ultimately suppressing the side reactions during electrocatalytic acetylene semihydrogenation, and exhibit superior semihydrogenation performance, with faradaic efficiencies of ≥98 % under pure acetylene flow. Even in a crude ethylene feed containing 1 % acetylene (1 × 104 ppm), N–heterocyclic carbene–copper affords a specific selectivity of >99 % during a 100–h stability test, continuous ethylene production with only ~30 ppm acetylene, a large space velocity of up to 9.6 × 105 mL·gcat−1·h−1, and a turnover frequency of 2.1 × 10−2 s−1, dramatically outperforming currently reported thermocatalysts.
Highlights
Electrocatalytic acetylene semihydrogenation is a promising alternative to thermocatalytic acetylene hydrogenation due to its environmental benignity and economic efficiency, but its performance is far below that of the thermocatalytic reaction because of strong competition from side reactions, including hydrogen evolution, overhydrogenation and carbon–carbon coupling reactions
Thermocatalytic acetylene semihydrogenation (TAH) is universally applied for hydrogenating acetylene impurities[7,8,9,10,11], but several major challenges remain: (i) the catalysts are rare and expensive Pd-based materials; (ii) relatively high temperature is imperative for improving the sluggish acetylene hydrogenation kinetics; (iii) excessive hydrogen gas (H2) is involved as a hydrogen source; and (iv) side reactions such as overhydrogenation and carbon–carbon coupling occur concomitantly
In a crude ethylene flow, NHC–Cu exhibits an space velocity (SV) of 9.6 × 105 mL/gcat/h and a turnover frequency (TOF) of 2.1 × 10−2 s–1 and continuously outputs ethylene feedstock containing only ~30 p.p.m. acetylene, outperforming state-of-the-art thermocatalysts. In addition to these experimental results, in situ electrochemical Raman analyses and theoretical simulations reveal that the electron-rich Cu sites in NHC–Cu are beneficial for acetylene adsorption and ethylene desorption, improving the acetylene semihydrogenation kinetics
Summary
Electrocatalytic acetylene semihydrogenation is a promising alternative to thermocatalytic acetylene hydrogenation due to its environmental benignity and economic efficiency, but its performance is far below that of the thermocatalytic reaction because of strong competition from side reactions, including hydrogen evolution, overhydrogenation and carbon–carbon coupling reactions. We demonstrate NHC–metal complexes featuring electron-rich metal centers as electrocatalysts for selective acetylene hydrogenation (Supplementary Fig. 1; IUPAC names are given in the “Materials” subsection).
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