Construction And Mechanism Of The Sacrificial-Agent-Free System For Photocatalytic Reduction Of NO₃^-

Nitrate pollution in water still needs to be solved urgently at present.And photocatalytic reduction of nitrate has gradually attracted attention in nitrate pollution control.However,in the conventional photocatalytic reduction of nitrate system,the reduction process NO₃^-must be driven by external sacrificial agents such as formic acid The reason is that NO₃^-reduction is a chain reaction,and the energy barrier of one elementary reaction[NO₃^-+e-?·NO32-,E0?NO₃^-/·NO32-?=-0.89 V versus SHE]is too high for common photocatalysts[for example,the conduction band minimum?CBM?of TiO2 is about-0.4 V],making it impossible to thermodynamically realize this elementary reaction directly relying on photogenerated electrons.What's more,the carbon dioxide free radicals,generated by the reaction between holes and sacrificial agents like formic acid?HCOO-+hVB+?H++·CO2-?,have a strong reducing ability[E0?CO2/·CO2-?=-1.81V versus SHE],which can overcome the high energy barrier to reduce NO₃^-to·NO32-,thus ensuring the subsequent reaction.Therefore,sacrificial agents play roles not only in promoting the separation of photogenerated carriers and reducing the reoxidation of intermediate products,but more importantly in making the reduction of NO₃^-thermodynamically feasible.But the disadvantage of adding sacrificial agents is that it will increase the treatment cost and produce secondary pollution.Therefore,exploring new photocatalytic processes and new catalysts that can effectively reduce NO₃^-without secondary pollution is of great significanceFirstly,lithiothermic reduction method was used to synthesize R-P25?0,5,10%Li?with different degrees of oxygen vacancies?OV?,then chemical method was used to load plasma bimetals onto the R-P25,and therefore 9 kinds of catalysts,0,5,10%Li R-P25,R-P25@Ag/Cu and R-P25@Pd/Cu,were obtained.The performance of these catalysts in sacrificial-agent-free system indicated that bimetallic modification promoted the reaction or even was crucial,and appropriate amount of Ov would significantly promote conversion while excessive inhibit.These results can be attributed to that the effect of bimetallic modification and Ov defective treatment can change the optical performance of photocatalysts and affect the separation of photogenerated carriers as well as the efficiency of photocatalysis.Taking factors such as NO₃^-conversion ratio and N2 selectivity into account,the comprehensive performance of 5%Li R-P25@Ag/Cu is optimal.Mechanism of nitrate reduction in the sacrificial-agent-free system was explored with 5%Li R-P25@Ag/Cu as a typical catalyst.Since the conduction band electrons of R-P25 obtained by Ov defect treatment could not reduce NO₃^-directly,the plasma bimetals on the surface of R-P25 may play a crucial role.According to the comparison experiment results using SiO2@Ag/Cu?inert substrate?and R-P25@Ag/Cu as catalysts,it could be inferred that under the effect of localized surface plasmon resonance?LSPR?,bimetals could produce sufficient hot electrons overcoming the high energy barrier of the elementary step of NO₃^-reduction.In summary,a sacrificial-agent-free,highly efficient and selective system for photocatalytic reduction of nitrate as well as R-P25@Bimetals photocatalysts with surface Ov defects and bimetal modification were constructed,which have certain guiding significance to NO₃^-reduction technologies.