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Urea (CO(NH2)2) is the most common nitrogen fertilizer that is promoting food production worldwide due to its high nitrogen content. However, the conventional urea synthesis involving hydrogenolysis of nitrogen and C–N bond coupling requires harsh conditions and a massive carbon footprint. Herein, we report a promising technology for the green synthesis of urea using nitrate (NO3 –) and carbon dioxide (CO2) under ambient conditions over Ru/Pt/Pd-modified three-dimensional copper foam (Ru/Pt/Pd-Cu CF) electrode. The Ru-Cu CF catalyst delivers a high urea yield of 151.6 μg h–1 cm–2 at a low onset potential of −0.3 V vs Ag/AgCl (0.13 V vs RHE) as well as an FE of 25.4%, surpassing that of Pt/Pd-Cu CF. Moreover, it is confirmed by operando electrocatalytic Raman spectroscopy and theoretical calculations that the *COOH intermediate is the rate-determining step of C–N bond coupling. Benefiting from the partial 3d states of surface Ru sites, the Ru-Cu CF electrode possesses the lowest formation energy of the *COOH intermediate to accelerate the urea synthesis. This work provides an extraordinarily sustainable strategy for green urea synthesis.

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Mesoporous polydopamine/titanium dioxide (PDA/TiO2) composite nanospheres with different polydopamine content were successfully synthesized by a facile organic–inorganic self-assembly method. The synthesized composite materials were characterized by N2 adsorption–desorption isotherm, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy (TEM), energy dispersive spectrometry, field emission scanning electron microscopy, Fourier transform infrared spectroscopy,  ζ--potential, and cyclic voltammetry. The results showed that the composite material with the most exposure active site exhibits a spherical morphology. The composite material could also be used as a reducing agent, and spheres of the composite material were used as adsorbents for removing hexavalent chromium from aqueous solutions by reducing Cr­(VI) to Cr­(III). The influence of factors, including the initial solution pH, contact time, initial ion concentration, temperature, coexisting ions, and dose of adsorbent, was also studied. The hexavalent chromium adsorption data fit well to a Langmuir isotherm model, and the maximum adsorption capacity was shown to be 244.5 mg/g. The kinetics data follow a pseudo-second-order kinetic model. The study of thermodynamics indicated that the adsorption on the composite nanospheres was a spontaneous and endothermic process. Moreover, a mechanism of adsorption and reduction on the composite spheres was proposed.