CO₂ Photoreduction And Catalytic Mechanism On LaTiO₂N With Surface-Interface Regulation

Energy shortage and environmental pollution have become main factors restricting the sustainable society development.Photocatalytically converting CO2 and H2O into high value-added hydrocarbon fuels over semiconductor materials,achieving the conversion and storage of solar energy to chemical energy,provides an ideal pathway to adjust energy structure and reduce the environmental negative effect caused by CO2 emissions,thus having a promising prospect.To achieve the efficient photocatalytic CO2 reduction is a challenging subject in the fields of materials,chemistry and energy.The conversion efficiency of photocatalytic CO2 is mainly affected by the following factors:1)Photogenerated electrons and holes are easy to recombine.2)Both CO2 and H2O are inert in thermodynamics,and the H2O oxidation and CO2 reduction involve multi-electron transfer processes with the slow reaction kinetics.3)The mechanism of CO2 reduction is still unclear,resulting in lack of effective theoretical guidance in the development of new photocatalytic materials.4)The surface-interface properties of catalysts are not well known.LaTiO2N,with visible-light absorption ability due to its small band gap(2.1 eV),and with valance and conduction band positions stretching across the reaction potentials of H2O oxidation and CO2 reduction,is regarded as one promising semiconductor for high photocatalytic performance.Therefore,in this thesis,LaTiO2N is selected as the photocatalyst.Starting from aspects of molecular activation,charge transport,and crystal surface adsorption,surface and interface regulation are the main strategies that aiming to improve the performance of photocatalytic CO2 reduction,and to reveal the factors determining the selectivity of CO2 reduction products.The main research contents and conclusions are as follows:La2O3-modified LaTiO2N with oxygen vacancy enhances CO2 reduction into CH4.In-situ La2O3 modified LaTiO2N photocatalyst has been prepared by nitriding La2TiO5 under flowing NH3.Oxygen vacancy defects on LaTiO2N surface can be released after partially removing La2O3 by acid washing.The spatially separated active sites effectively improves performance of reducing CO2 into CH4 over LaTiO2N.The results show that La2O3,serving as a solid base,can effectively activate CO2 due to the strong alkalinity of oxygen ions(O2-)in La2O3,thus generating carbonate(CO32-)species and decreasing reaction activation energy.Oxygen vacancy can dissociate H2O into hydroxyl groups(-OH),thus promoting the release of protons.Activation of CO2 and H2O molecules improves the sluggish reaction kinetics of CO2 reduction and H2O oxidation,helping to match reaction rates of oxidation-reduction reaction.On the other hand,the separated activation sites induced the separation of the redox reactions,thus helping to suppress the reverse reaction.KOH modified Ni/LaTiO2N Schottky junction improves CO2 reduction into CH4.LaTiO2N was synthesized by nitridation of La2Ti2O7,and then Ni nanoparticles were loaded on as-prepared surface of LaTiO2N by thermal reduction of Ni(NO3)2 under NH3,and KOH modification was achieved by immersing Ni/LaTiO2N into KOH solution.Ni/LaTiO2N Schottky junction could be formed due to the higher work function of Ni(5.2 eV)than LaTiO2N(4.7 eV).The resulting spatial charge layer formed at LaTiO2N and Ni interface is conducive to accelerate the separation of electron-hole pairs.KOH can not only chemically adsorb and convert CO2 into CO32-,but also accelerate consumption of holes,boosting the reaction kinetics of H2O oxidation.The improved charge separation efficiency and reduced reaction barrier efficiently promote the reduction of CO2 into CH4.Ta3N5/LaTiO2N heterojunction improves the conversion of CO2 into CO.Photocatalysts of LaTiO2N with dominant(002)facet and Ta3N5 with dominant(002)facet are prepared by nitriding La2TiY2O7 and KTaO3,respectively.And,heterojunction of Ta3N5/LaTiO2N is prepared by one-step nitridation of La2Ti2O7 and KTaO3 mixtures.The results show that CO2 could be reduced into CH4 and CO over(002)LaTiO2N and(020)Ta3N5,respectively.The Ta3N5/LaTiO2N heterojunction can induce the photogenerated electrons of LaTiO2N migrating to Ta3N5,causing CO2 reduction on Ta3N5 surface.The increased electrons on Ta3N5 surface improves the CO2 reduction,but CO is the main reduction product still.On basis of the detected intermediate species,the CO2 reduction on the surface over LaTiO2N and Ta3N5 are followed the reaction pathway via CO2 ?COOH*? CO? CHx*?CH4.Theoretical calculation and experimental results show that the kinetics of converting CO2 into-COOH is slow during the reaction process,and N-rich Ta3N5(020)facet presents weak adsorption ability of CO,thus causing the CO selectivity.On the contrary,the La atoms in LaTiO2N(002)facet are active sites for adsorbing and activating CO2 and CO.Meanwhile,strong adsorption of CO on LaTiO2N favors the subsequent hydrogenation reaction to generate CH4.This proves that the product distribution of photocatalytic CO2 reduction over LaTiO2N and Ta3N5 depends on the chemical properties of catalyst surface,and changes in density of charge carriers on surface play little effects on the product selectivity.