Electrosynthesis of urea by using Fe2O3 nanoparticles encapsulated in a conductive metal–organic framework

Abstract

The synthesis of urea by the electrochemical co-reduction of CO2 and nitrate is a crucial and challenging task. Catalysts typically suffer from either low Faradaic efficiency (FE) or inadequate current density, leading to a restricted yield rate of urea. Here we report ultrasmall γ-Fe2O3 nanoparticles (<2 nm) encapsulated in the pores of a conductive (40 S cm−1) metal–organic framework Ni-HITP (HITP = 2,3,6,7,10,11-hexaaminotriphenylene), resulting in a composite material, γ-Fe2O3@Ni-HITP. Under neutral conditions, γ-Fe2O3@Ni-HITP exhibited a state-of-art electrocatalytic performance for urea synthesis through the co-reduction of CO2 and nitrate in CO2-saturated 1 M KHCO3 and 0.1 M KNO3 aqueous solutions, achieving a FEurea of 67.2(6)%, a current density of −90 mA cm−2 and an high yield rate of $$20.4(2)\,{\mathrm{g}}\,{\mathrm{h}}^{-1}\,{\mathrm{g}}_{\mathrm{cat}}^{-1}$$ (7.7(1) mg h−1 cm−2), which is about five times higher than the rates of previously reported catalysts. No degradation was observed over 150 h of continuous operation at such a high yield rate. Enlarging the electrode area by 125 times yielded about 1.05(4) g of high-purity urea over 8 h. A mechanistic study revealed that Fe(III) ions in the γ-Fe2O3 nanoparticles exhibit high activity, generating the key intermediates *NH2 and *COOH. Furthermore, pairs of adjacent Fe(III) ions in the γ-Fe2O3 nanoparticles can act as highly active catalytic sites for catalysing the C–N coupling between *NH2 and *COOH, resulting in the formation of the subsequent key intermediate *CONH2, thereby contributing to the exceptionally high performance of γ-Fe2O3@Ni-HITP for urea production. Small γ-Fe2O3 nanoparticles (<2 nm) encapsulated in the pores of a conductive metal–organic framework enable the efficient electrosynthesis of urea through the co-reduction of CO2 and nitrate under neutral conditions.