Fabricating Noble-Metal-Free Co-Catalyst To Enhance G-C₃N₄ Photocatalytic Performance For Carbon Dioxide Reduction And Hydrogen Production

Zero-carbon energy production and conversion of CO₂ into renewable hydrocarbons using solar energy based photocatalysis,is one of the most challenging and promising ways to improve the energy structure and reach carbon dioxide peak-emission by 2030 and be carbon-neutral by 2060.That photocatalysis technology can resolve the problems of environmental pollution,greenhouse effect,energy shortage simultaneously owing to its mild reaction conditions and environmental friendliness.Graphitic carbon nitride（g-C₃N₄）as a semiconductor material with suitable band gap has been extensively investigated due to its visible light responsive,low-cost,easy availability and good chemical stability.However,the photocatalytic performance of pure g-C₃N₄ still challenges with high recombination rate of electrons and holes,poor electroconductibility,inefficient solar utilization,poor CO₂ absorption and as well as ambiguous mechanism for selective CO₂ conversion.Coupling with suitable noble metal co-catalyst such as Pt and Pd with g-C₃N₄ can tackle these challenges but the high-cost and rarity of them has hindered the large-scale application.Therefore,we constructed efficient non-noble metal co-catalyst substitutes in this paper to improve the performance of g-C₃N₄ and reveals the corresponding reaction mechanism through the physicochemical and photoelectric characterizations,in situ FTIR and DFT calculations.These studies will make an instructive exploration for the development of economic and efficient photocatalysts.The detailed research works and main results are summarized as follows.（1）Rare earth metal oxide modified g-C₃N₄ nanosheets was developed by in-situ calcination method and was achieved efficient CO₂ reduction to CH₄ and water splitting to hydrogen.The research work has shown that the existed Eu³⁺/Eu²⁺pairs existed in Eu_2-xO₃ can rapidly capture and transfer photoelectrons generated by g-C₃N₄ and can act as alkaline active sites to promote CO₂adsorption,activation and selective transformation to CH₄.As such,optimized Eu_2-xO₃/g-C₃N₄ catalyst achieved a maximum CH₄ production amount of 4.92μmol h^-1g^-1 in CO₂ reduction experiment and a continuous H₂ production with the evolution rate of 78.1μmol h^-1g^-1 in water splitting reaction,which was 19.7 times and 7.3 times as high as than that of pure g-C₃N₄,respectively.（2）Transition metal boride Co₂B modified g-C₃N₄ nanosheets were prepared by in situ chemical reduction method and achieved highly efficient water splitting to hydrogen.It has been proved that the metallic,amorphous,and full-spectrum responded properties of Co₂B,as well as its SPR effect in near infrared（NIR）region made the g-C₃N₄ capable of efficient carrier separation and transfer,as well as full spectrum light response（>800 nm）.The optimized Co₂B/g-C₃N₄ composites achieved efficient water splitting activities with H₂ evolution rate of 1137.2μmol h^-1g^-1,which was 96 times higher that of pure g-C₃N₄,even outperform than that of 0.5%Pt/CN(1099.6μmol h^-1g^-1).However,Co₂B could not be qualified for photocatalytic CO₂ reduction since it is unstable in weak acid media.（3）Transition metal phosphide MoP modified g-C₃N₄ nanosheets were prepared by temperature programmed reaction（TPR）combined ultrasonic agitation method and achieved efficient CO₂ reduction and water splitting to hydrogen.Studies have shown that MoP not only acted as the electron trap to facilitate electrons separation and transfer,and provided abundant active sites,but also reduced the reaction barrier of COOH*intermediate to promote the generation of CO.As a result,optimized MoP/g-C₃N₄composites achieved a maximum CO yields with the value of 18.4μmol h^-1 g^-1 in CO₂reduction and high water-splitting activities with H₂ evolution rate 807.6μmol h^-1 g^-1,which was 5.5 times and 75.5 times as high as than that of pure g-C₃N₄,respectively.（4）Bifunctional co-catalysts Mg&MoP modified g-C₃N₄ nanosheets were prepared by in-situ phosphating pyrolysis combined with in-situ hydrothermal method and achieved highly selective CO₂ reduction to CH₄.Theoretical and photophysical experimental studies synergistically confirm that MoP in Mg&MoP acted as an electron trap to inhibit the electron-hole pair recombination and reduce COOH*reaction energy barrier while Mg（a mixture of basic magnesium nitrate and magnesium hydroxide）provides weak alkaline adsorption sites to enhance the chemisorption of CO₂,regulate the binding energy of CO*and promote the protonation of CO*to CHO*intermediate.The optimized Mg-MoP/g-C₃N₄ composites have achieved CH₄and CO yields up to24.45μmol h^-1 g^-1 and 4.82μmol h^-1 g^-1.That production activity was respectively 97.8times and 1.6 times higher than that of pure g-C₃N₄ and the selectivity also increased from 12.0%to 83.3%.In addition,the hydrogen production performance(1162.2μmol h^-1 g^-1)over Mg-MoP/g-C₃N₄ also outperformed than that of 0.5%Pt/CN(1153.2μmol h^-1 g^-1).The reaction mechanism for CO₂ selective activation and the conversion path of CH₄ on Mg-MoP/g-C₃N₄ composites system was also proposed assisted with in-situ FTIR and DFT calculation.In conclusion,this paper shows the great potential of non-noble metal co-catalyst modified g-C₃N₄ as efficient photocatalysts to achieve high-efficient water splitting to hydrogen and selectively catalytic reduction of CO₂ to CH₄.The research fruits could provide theoretical support and technical reference for producing zero-carbon energy and CO₂ resource recovery.