(Invited) Ligands in Electroreduction of CO2: N-Heterocyclic Carbene-Capped Polymers to Promote Catalytic Efficiency

Abstract

N-heterocyclic carbenes (NHCs) are excellent ligands that bind to a variety of transition metals with extremely high binding strength via σ donation. In my talk, I will highlight the use of two polymeric N-heterocyclic carbenes (NHC) (polydentate and monodentate) to stabilize metal nanocatalysts (Au and Pd) for efficient CO2 electroreduction in aqueous solution. Compared with other conventional ligands including thiols and amines, metal-carbene bonds are stable under reductive potentials prevent the nanoclustering of nanoparticles, which is of key importance for any practical application. In particular, Au nanocatalysts modified by polymeric NHC ligands show an activity retention of 86% after CO2 reduction at -0.9 V for 11 h, while it is less than 10% for unmodified Au. We demonstrate that the hydrophobicity of polymer ligands and the enriched surface electron density of metal NPs through σ-donation of NHCs substantially improve the selectivity for CO2 reduction over proton reduction. As a passive coating on the surface of metal nanocatalysts, carbene-terminated polymers can control the accessibility of substrates, i.e., CO2 vs. H2O, as a means to improve the Faradic efficiency toward CO2 reduction. Our results provide guideline to improve catalytic efficiency and stability of metal nanocatalysts for CO2 electroreduction and, more generally, as cathodic materials for other electroreduction reactions.