Host–guest molecular interaction promoted urea electrosynthesis over a precisely designed conductive metal–organic framework†
Abstract
The highly selective electrocatalytic activation of N2 and CO2 to synthesize value-added urea via a C–N coupling reaction is an extremely challenging reaction that is largely inhibited by the poor chemisorption and coupling abilities of the reactant molecules. Herein, the novel conductive MOF Co–PMDA–2-mbIM (PMDA = pyromellitic dianhydride; 2-mbIM = 2-methyl benzimidazole) is designed, attaining a record-high urea yield rate of 14.47 mmol h−1 g−1 with a FE of 48.97% at −0.5 V vs. RHE. The host–guest interactions involved not only generate desirable local electrophilic and nucleophilic regions but they also allow evolution from high-spin state Co3+ (HS: t42ge2g) to intermediate-spin state Co4+ (IS: t42ge1g) in CoO6 octahedrons. Thus, N2 and CO2 can be adsorbed in a targeted fashion and activated to produce the desired *NN* and *CO intermediates. Subsequently, the low eg orbital occupied Co4+ (t42ge1g) easily accepts electrons from σ orbital of *NN* and effectively triggers the C–N coupling reaction to emerge *NCON* urea precursor.