Mesoporous Electrocatalysts with p–n Heterojunctions for Efficient Electroreduction of CO₂ and N₂ to Urea

Abstract

The electrocatalytic synthesis of high-value-added urea by activating N₂ and CO₂ is a green synthesis technology that has achieved carbon neutrality. However, the chemical adsorption and C-N coupling ability of N₂ and CO₂ on the surface of the catalyst are generally poor, greatly limiting the improvement of electrocatalytic activity and selectivity in electrocatalytic urea synthesis. Herein, novel hierarchical mesoporous CeO₂/Co₃O₄ heterostructures are fabricated, and at an ultralow applied voltage of -0.2 V, the urea yield rate reaches 5.81 mmol g^-1 h^-1, with a corresponding Faraday efficiency of 30.05%. The hierarchical mesoporous material effectively reduces the mass transfer resistance of reactants and intermediates, making it easier for them to access active centers. The emerging space-charge regions at the heterointerface generate local electrophilic and nucleophilic regions, facilitating CO₂ targeted adsorption in the electrophilic region and activation to produce *CO intermediates and N₂ targeted adsorption in the nucleophilic region and activation to generate *N ═ N* intermediates. Then, the electrons in the σ orbitals of *N ═ N* intermediates can be easily accepted by the empty e_g orbitals of Co³⁺ in CeO₂/Co₃O₄, which presents a low-spin state (LS: t_2g⁶e_g⁰). Subsequently, *CO couples with *N ═ N* to produce the key intermediate *NCON*. Interestingly, it was discovered through <i>in situ</i> Raman spectroscopy that the CeO₂/Co₃O₄ catalyst has a reversible spinel structure before and after the electrocatalytic reaction, which is due to the surface reconstruction of the catalyst during the electrocatalytic reaction process, producing amorphous active cobalt oxides, which is beneficial for improving electrocatalytic activity.