Catalytic Hydrogenation Of Nitrate And Nitrite On Pd-Cu/TiO₂

High concentration of nitrates in groundwater is a potential threat to human health. Due to economical and ecological advantages, catalytic hydrogenation of nitrate has been considered to be one of the most promising technologies for the remedy of groundwater pollution. Much attention has been concentrated on developing technologies for catalysts preparation and optimizing the reaction conditions, because high activity and nitrogen selectivity are needed for practical use.In this work, different semiconductors are used as supports and different methods are explored to prepare Pd-Cu bimetallic catalysts which are then tested in the reaction of catalytic hydrogenation of nitrate. Finally the TiO2 (P25) based catalyst when prepared by photo-deposition reveals the highest activity and selectivity.The influence of pH value and supported metal components on the reaction of nitrate and nitrite hydrogenation is systematically investigated. For nitrite hydrogenation, high activity and selectivity are achieved only in acidic condition. Different buffers are tested and acetate acid seems to be more suitable for the reaction. While basic condition is beneficial to nitrate hydrogenation, nitrite hydrogenation is retarded because of large amount of hydroxide ions formed during the reaction.A two-stage method for nitrate elimination from water is proposed in the end. Firstly, nitrate is reduced into nitrite in basic condition. Secondly, the solution is adjusted to acidic condition ensuring the reduction of nitrite into nitrogen. By applying this strategy, excellent results are achieved in our experiments:during 60 min reaction,91.6% of nitrate is eliminated with 96.8% nitrogen selectivity, while no nitrite can be detected in the final products.The catalyst is characterized by TEM,H2-TPR and CO-FTIR. Well dispersion of the supported metals and the formation of Pd-Cu bimetallic structure are detected, which insure the excellent performance in the reaction. The change in the redox states of copper implies it participating in the reaction. It is demonstrated that nitrate could only be reduced on the bimetallic sites and is firstly reduced on the copper sites through a direct redox mechanism, while nitrite could only be reduced on the palladium sites by a catalytic reaction. Moderate oxidation of copper surface by OH- enhances nitrate adsorption, while nitrite adsorption is retarded.