Hydrogenation of Nitrate in Water to Nitrogen over Pd–Cu Supported on Active Carbon

Catalytic removal of nitrate in water by hydrogenation was investigated by using supported Pd–Cu catalysts at 333 K in a flow system. Active carbon (AC) was superior in conversion, selectivity, and insolubility to other supports. Pd and Cu were not dissolved when supported on AC, even at low pH reaction conditions, while they were considerably released from the surface of γ-Al2O3, SiO2, and ZrO2. The contact time dependence of the reactant and products suggested that the hydrogenation of NO3− is a consecutive reaction with NO2− as an intermediate product and that the hydrogenation of NO2− is a key step in determining selectivity to N2 and NH3. The conversion of NO3− was enhanced greatly by the addition of a small amount of Cu to 5 wt% Pd/AC or of Pd to 3 wt% Cu/AC, indicating that the Pd–Cu site was indispensable to activate the nitrate ion. The yield of N2 went through a maximum at 0.6 wt% Cu in the 5 wt% Pd–Cu/AC. A similar dependence of the N2 yield on Cu content for NO2− hydrogenation strongly supports the proposed reaction pathways. The enhancement of N2 selectivity by the addition of small amounts of Cu can be explained by the selective poisoning of the edge or corner sites of the Pd particles by Cu atoms.