Preparation Of Urea By Photocatalytic Coupling Of Nitrogen And Carbon Dioxide With Mesoporous CeO₂-based Nanomaterials

With the population explosion and the rapid development of industry,urea will face greater demand in the whole country and even in the whole world.At the same time,due to the increasing greenhouse gas emissions,accelerating the green and low-carbon transformation and development of energy and high-pollution industries,promoting the progress and innovation of energy science and technology are all important measures to achieve the goal of"double carbon".Using environmentally friendly,simple and low-cost photocatalytic technology to couple N₂ and CO₂ into urea has become an important research topic to alleviate the energy crisis and environmental problems.The weak adsorption of inert gas on the surface of photocatalyst,the high stability and slow cracking of C=O bond and N≡N bond and the existence of parallel reduction reaction competing with C-N coupling reaction further inhibit the catalytic performance of photocatalytic preparation of urea.Therefore,the reasonable design and preparation of photocatalyst with high catalytic activity has always been a research hotspot and difficulty in this field.Based on this,mesoporous CeO₂ nanomaterials with adjustable oxygen vacancies were synthesized by hydrothermal method in this paper.The relationship between the concentration of oxygen vacancies and the performance and internal mechanism of photocatalytic preparation of urea was mainly explored,co-conversion of CO₂and N₂ into urea was realized by photocatalytic reaction under mild environmental conditions.At the same time,the transition metal Pd was loaded on the surface of CeO₂ nanomaterials by deposition-precipitation method,and the reaction performance of photocatalytic preparation of urea by semiconductor photocatalyst was further improved by strengthening charge transfer.The specific research contents are as follows:1.In this work,CeO₂nanomaterials with mesoporous structure were developed and calcined at different temperatures in Ar/H₂（90%:10%）atmosphere to obtain CeO₂ nanorods with adjustable oxygen vacancy concentration,aiming at enhancing the targeted adsorption and activation of inert N₂ and CO₂ molecules by introducing oxygen vacancies.Among them,CeO₂-500 NPs calcined at 500℃showed high photocatalytic activity for the synthesis of urea by C-N coupling reaction and its urea yield reached 6.45μmol g^-1h^-1.The results show that the oxygen vacancy on the surface of mesoporous CeO_2-xcatalyst does promote the chemical adsorption of CO₂ and N₂,thus significantly improving the catalytic activity of photocatalytic C-N coupling to produce urea.In addition,we have established a series of characterization methods,including X-ray photoelectron spectroscopy（XPS）,electron paramagnetic resonance spectroscopy（EPR）and electron energy loss spectroscopy（EELS）to determine the existence and distribution of oxygen vacancies,and further reveal the role of oxygen vacancy defects in photocatalytic preparation of urea.At the same time,the mechanism of photocatalytic C-N coupling reaction was studied by density functional theory.2.In this work,CeO₂ nanorods modified by metal Pd were prepared by deposition-precipitation method（and used as photocatalyst）,and the photocatalytic effect of their C-N coupling reaction to synthesize urea under mild conditions was studied.The experimental results showed that 7%Pd-CeO₂ NPs showed the highest photocatalytic activity for C-N coupling to urea,and the yield of urea reached 9.20μmol g^-1 h^-1（3.7 times that of CeO₂ NPs）.At the same time,we also investigated the photocatalytic performance of CeO₂-based materials loaded with other metals as photocatalysts.Among them,the synthetic urea performance of 7%Ag-CeO₂ NPs,7%Au-CeO₂ NPs and 7%Pt-CeO₂ NPs are all lower than that of 7%Pd-CeO₂NPs(5.50μmol g^-1 h^-1,3.33μmol g^-1h^-1 and 2.55μmol g^-1 h^-1 respectively).The comprehensive study further confirmed that the space charge region at CeO₂（111）/Pd（111）interface not only effectively promoted the targeted capture and activation of inert CO₂ and N₂,but also stabilized the formation of key intermediate（*NCON）.In addition,the charge transfer mechanism of Pd-CeO₂ NPs photocatalyst was further understood by photocurrent test,ultraviolet-visible diffuse reflectance spectrum,fluorescence spectrum and electrochemical impedance.At the same time,the calculation results of density functional theory（DFT）show that the adsorbed N₂ molecules can promote the conversion of CO₂ to CO,and the photocatalytic urea synthesis process then goes through a thermodynamically feasible alternate path rather than a terminal path.