Preparation Of Metal-doped LiNbO₃ For Efficient Photocatalytic Reduction Of Nitrate

Pure and metal?Cu,Pr,Fe?-doped LiNbO₃ catalysts were synthesized by hydrothermal method for photocatalytic reduction of nitrate.The crystal structure,morphology,elemental composition and valence state,light absorption of the catalyst before and after doping of LiNbO₃ catalyst were characterized via XRD,SEM,XPS,TEM etc.The effects of conditional factors on the reduction effect was studied.Moreover,the influence of doping modification on the structure of LiNbO₃ catalyst and the performance of photocatalytic reduction of nitrate was investigated.The mechanism of photocatalytic reduction of nitrate by LiNbO₃ catalyst and the process of nitrate conversion were discussed.The catalysts obtained via adding surfactant and ethylenediamine showed similar effects on the photocatalytic degradation of nitrate and the selectivity to nitrogen?pH=7.0,60.5%and 62.4%?.The results showed that acidic conditions contributed to the reduction of NO₃^-,formic acid was the best hole scavenger.In the unadjusted pH system,the addition of Na⁺and Cl^-did not affect the rate of NO₃^-degradation?73.3%and 71.2%?,the conversion rate of NO₃^--N would increase?78.5%?after adding CO₃^2-.The results showed that the doping modification was beneficial to the regulation of the microstructure of LiNbO₃ catalyst and the performance of photocatalytic reduction of nitrate.Taking Fe element as an example,the morphology of the catalyst did not change before and after metal-doped.With the gradual increase of the Fe element concentration,the diffraction peak of the crystal first increased,then moved to a low angle.The absorption intensity of the semiconductor catalyst samples to the light source gradually increased,and a significant red shift occurred.Fe element was attributed to prolonging the lifetime of the charge carrier via separating the electrodes and holes at the interface.The 1%Fe-LiNbO₃ catalyst sample exhibited better conductivity in the photocatalytic reduction system.Typically,200 mL NO₃^-solution(0.8 mmol·L^-1)could be 86.7%NO₃^--N degradation with doping amount x=1%catalyst dosage of 0.4 g·L^-1and the HCOO^-concentration of 1 mmol·L^-1,under the condition of pH=7.0.After 3 cycles of use,the catalyst still had higher activity?nitrate removal rate of84.9%,nitrogen selectivity of 82.7%?.The mechanism of photocatalytic reduction of nitrate by LiNbO₃ was investigated by photocatalytic reduction of ion concentration in the nitrate system and hole scavenger substitution experiments.The E_CB value of LiNbO₃ was sufficient to reduce NO₃^-and NO₂^-directly by negative electrons.The Lewis acid site on the surface of LiNbO₃ was the active site for adsorption and photocatalytic reaction.Based on DFT calculation,it could be found that the appearance of impurity energy level within the band gap of Fe doped LiNbO₃ crystal was conductive to reduce the energy for electronic transitions.