Metal Oxides For Electrocatalytic Urea Synthesis By Coupling Of Nitrate And Carbon Dioxide

The tremendous food demand imposed by the ever-increasing human population has placed a demand for more efficient and greener nitrogen production.However,the traditional synthesis of urea is a two-step synthesis process with high energy consumption and high pollution,which first produces ammonia via the Haber-Bosch process and then reacts with carbon dioxide at high temperature and pressure.Currently,the Ha ber-Bosch process alone consumes 2%of the global energy and about 80%of the produced NH ₃ is used for urea synthesis.Meanwhile,with the excessive use of fossil fuels and the development of industrial activities,human society is facing serious environme ntal problems and an energy crisis.Electrocatalytic technology,which can utilize renewable clean electricity,reduce the reaction energy barrier,and realize C-N coupling to prepare urea under mild conditions,is a green and highly efficient alternative to traditional urea synthesis.In this paper,through catalyst design,using greenhouse gas carbon dioxide and water pollutant nitrate as raw materials,explore the electrocatalytic C-N coupling method and mechanism of urea.The specific research contents are as follows:（ⅰ）The introduction of oxygen vacancies,which can increase active sites,improve adsorption of feedstock and enhance stabilization of intermediate species,is often applied in electrocatalyst design.The introduction of oxygen vacancies tailors the common catalyst carrier into an efficient electrocatalyst with a high urea yield rate of 15.7 mmol·h^-1·g^-1,superior to that of partial noble-metal based electrocatalysts.The oxygen vacancy enriched Ce O ₂ was demonstrated as the efficient electrocatalyst with stabilization of the crucial intermediate of*NO via inserting into vacant sites,which was conducive to th e subsequent C-N coupling process rather than protonation.Whereas the poor selectivity of C-N coupling with protonation was observed on the vacancy-deficient catalyst.The oxygen vacancy mediated selective C-N coupling was distinguished and validated by t he in-situ sum-frequency generation spectroscopy.This work provides novel insights into catalyst design and developments of coupling systems.（ⅱ）Copper-based catalysts,with unique C-C coupling performance and strong NO₃^-reduction performance,can reali ze the coupling of low concentrations of nitrate with CO₂ to synthesize urea.Based on the different energy barriers of adsorption and coupling between the raw materials and the reaction intermediates,cubic Cu₂O NPs with{100}facets,octahedral Cu ₂O NPs with{111}facets,and truncated-octahedral Cu₂O NPs with both{111}and{100}facets were synthesized.The results showed that the Cu ₂O{111}crystal planes were favorable for the adsorption and activation of CO ₂,which matched the excellent NO ₃RR performance of Cu₂O and were favorable for C-N coupling.The urea yield of octahedral Cu₂O has reached 21.3 mmol·h^-1·g^-1 at-1.3 V（vs.RHE）,while the Faradaic efficiency reache d 5.94%.（ⅲ）The active sites of Cu⁺in cu-based catalysts can promote CO₂ activation and C-C coupling through synergistic interaction with Cu⁰,however,Cu⁺from Cu₂O is unstable under CO ₂ reduction conditions.The multihollow Cu₂O catalyst with nanocavities was synthesized by using the nanodomain limiting strategy.The reaction intermediates formed in situ were enriched by‘confinement effect’to promote C-N coupling.Meanwhile,the enriched intermediates in turn cover ed the local catalyst surface and thereby stabilized Cu⁺species,which further improved and retained the catal ytic performance of the catalyst.The urea yield of multihollow Cu₂O catalyst was up to 29.2 mmol·h^-1·g^-1 at-1.3 V（vs.RHE）,and the Faradaic efficiency was up to 9.43%.The preservation of Cu⁺species was confirmed by phase analysis（Raman,TEM and EDX）after electrochemical reconstruction.