Tandem Catalysts Enabling Efficient C-N Coupling toward the Electrosynthesis of Urea

Abstract

The development of a methodology for synthesizing value-added urea (CO(NH₂)₂) via a renewable electricity-driven C-N coupling reaction under mild conditions is highly anticipated. However, the complex catalytic active sites that act on the carbon and nitrogen species make the reaction mechanism unclear, resulting in a low efficiency of C-N coupling from the co-reduction of carbon dioxide (CO₂) and nitrate (NO₃ ^-). Herein, we propose a novel tandem catalyst of Mo-PCN-222(Co), in which the Mo sites serve to facilitate nitrate reduction to the *NH₂ intermediate, while the Co sites enhance CO₂ reduction to carbonic oxide (CO), thus synergistically promoting C-N coupling. The synthesized Mo-PCN-222(Co) catalyst exhibited a noteworthy urea yield rate of 844.11 mg h^-1 g^-1, alongside a corresponding Faradaic efficiency of 33.90 % at -0.4 V vs. reversible hydrogen electrode (RHE). By combining in situ spectroscopic techniques with density functional theory calculations, we demonstrate that efficient C-N coupling is attributed to a tandem system in which the *NH₂ and *CO intermediates produced by the Mo and Co active sites of Mo-PCN-222(Co) stabilize the formation of the *CONH₂ intermediate. This study provides an effective avenue for the design and synthesis of tandem catalysts for electrocatalytic urea synthesis.

Keywords: C−N coupling; Electrocatalysis; Tandem catalysts; Urea synthesis.