Amorphization Induces High-Density Undercoordinated Indium Sites for Enhanced Electrocatalytic Urea Synthesis

Abstract

Undercoordinated sites on metal catalysts are pivotal for enhancing electrocatalytic reactions, particularly in processes like coreduction, where multiple intermediates must be generated and coupled. Traditional synthesis methods, however, are limited in their ability to produce these low-coordination sites. In this study, we developed an amorphous indium catalyst (A-In@BO_x) using a boron oxide-assisted method that achieves a uniquely low coordination number (CN = 3.6) with a high density of 67.2 wt %. This structural characteristic significantly enhances the catalytic efficiency for urea synthesis, achieving a yield rate of 2317.58 μg h^–1 mg_cat^–1 and a Faradaic efficiency of 51.43% at −0.45 V versus RHE. The undercoordinated indium sites (UC–In) on A-In@BO_x improve the conversion of NO₃^– to NO₂^–, effectively generating *NO₂ as a crucial nitrogen intermediate for carbon–nitrogen coupling, while the inherently limited activity for CO₂ reduction maintains *CO₂ as the primary carbon intermediate. Our integrated in situ spectroscopy and theoretical simulations show that electron transfer from UC–In to *NO₂ markedly reduces the free energy barrier for CO₂ protonation from 1.77 to 0.04 eV, thus promoting the formation of the key *COOH–NO₂ intermediate. This breakthrough not only offers a fresh pathway for optimizing urea synthesis but also elucidates the coreduction mechanisms at undercoordinated metal sites, paving the way for the design of highly selective catalysts.