Ambient electrosynthesis of urea with nitrate and carbon dioxide over iron-based dual-sites

Urea (CO(NH₂)₂) has been applied both in agricultural and pharmaceutical field. The widely used Bosch-Meiser process has high energy consumption and CO₂ emission. Therefore, it is imperative to explore energy-saving and economical routes for urea synthesis under mild conditions.

The electrosynthesis of urea with CO₂ and NO₃- under ambient conditions is an efficient way, but it is far from application. This is because the key step needs an efficient electrocatalyst enabling adsorption and activation of NO₃- and CO₂ to accomplish the C-N coupling.

Researchers from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences have now developed a liquid-phase laser irradiation route to fabricate symbiotic carbon encapsulated amorphous iron (Fe(a)@C) and iron oxide nanoparticles (Fe₃O₄ NPs) on carbon nanotubes (denoted as Fe(a)@C-Fe₃O₄/CNTs).

The as-fabricated Fe(a)@C-Fe₃O₄/CNTs contained two Fe-based active components, namely, Fe@C NPs with the particle sizes of 10~20 nm and Fe₃O₄ NPs with the particle sizes of 1~5 nm.

The presence of two different structural units in Fe(a)@C-Fe₃O₄/CNTs made it possible to synergistically electrocatalytic activate CO₂ and NO₃- to realize the C-N coupling for urea synthesis.

As expected, Fe(a)@C-Fe₃O₄/CNTs exhibited superior activity toward the electrocatalytic coupling of CO₂ and NO₃- for urea synthesis, affording a urea yield of 1341.3±112.6 μg h-1 mgcat-1 and a faradic efficiency of 16.5±6.1% at -0.65 V (vs. RHE) in 0.1 M KNO3 electrolyte.

Both experimental and theoretical results unveiled that Fe(a)@C was mainly responsible for the electrocatalytic reduction of NO₃- to form *NH₂ intermediates, while Fe₃O₄ was more beneficial for the electrocatalytic reduction of CO₂ to form *CO intermediates.

The synergistically catalytic effect contributes to the excellent electrocatalytic performance of urea synthesis at ambient conditions.

The research was published in Angewandte Chemie International Edition.