Tin-palladium supported on alumina as a highly active and selective catalyst for hydrogenation of nitrate in actual groundwater polluted with nitrate

Abstract

We developed Sn_0.5Pd/Al₂O₃ showing high activity and high selectivity to gaseous products towards the hydrogenation of NO₃⁻ with H₂ in aqueous NO₃⁻ solutions via precise control of the Sn/Pd molar ratio. For the catalyst with the optimum Sn/Pd molar ratio, the surface concentration of adsorbed nitrogen on the Pd sites is thought to be high and that of the adsorbed hydrogen on the Pd sites is thought to be low. This is due to the abundant supply of NO₂⁻ for the Pd sites and the prompt consumption of adsorbed H on the Pd sites for reduction of the oxidized Sn sites formed by the reduction of NO₃⁻, respectively, leading to the high selectivity to gaseous products. Although the catalytic performance of Sn_0.5Pd/Al₂O₃, like that of Cu_0.5Pd/Al₂O₃, was lower in groundwater, the decrease for Sn_0.5Pd/Al₂O₃ was less than that for Cu_0.5Pd/Al₂O₃. Nitrate in the groundwater polluted with 0.4 mmol dm⁻³ (250 cm³) of NO₃⁻ was completely reduced at 298 K in 24 h with a selectivity for gaseous products of around 90% in the presence of 10 mg of Sn_0.5Pd/Al₂O₃, whereas it took 60 h in the presence of Cu_0.5Pd/Al₂O₃, and the selectivity for gaseous products was around 75%. However, both catalysts showed comparable high activity and high selectivity in aqueous NO₃⁻ solutions. When reactions were performed in aqueous NO₃⁻ solutions containing other anions (Cl⁻, SO₄²⁻, and SiO_xⁿ⁻) present in the groundwater, Cl⁻ had the largest negative impact but it had a smaller impact in the presence of Sn_0.5Pd/Al₂O₃ than it did in the presence of Cu_0.5Pd/Al₂O₃. On the basis of adsorption isotherms for Cl⁻ and kinetics analysis of the hydrogenation of NO₃⁻, it was concluded that Sn_0.5Pd/Al₂O₃ had less affinity for Cl⁻ and a strong affinity for NO₃⁻ on the Sn sites, leading to its superior catalytic performance in groundwater.