Efficient electrocatalytic acetylene semihydrogenation by electron-rich metal sites in N-heterocyclic carbene metal complexes.

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.