Hydrogenation of nitrate (200 ppm) in water with H-2 over Cu-Pd clusters supported on active carbon (AC) was investigated at 333 K using a gas-liquid co-current flow system. Two types of Cu-Pd bimetallic clusters, stabilized with either poly(vinylpyrrolidone) (PVP) or sodium citrate (SC), revealed that the catalysts possessed similar activity (per unit weight of Pd) and high selectivity toward nitrite when pH was 10.5 at the outlet of the reactor. The high selectivity toward nitrite on PVP-stabilized cluster/AC was minimally influenced by the atomic ratio of Cu/Pd (=0.5-4.0); activity was maximal at a ratio of 1:1. Increasing pH to 12.4 by addition of NaOH enhanced the selectivity toward nitrite to 93% over SC-stabilized Co-0.63-Pd cluster/AC, but caused a decrease in the reaction rate. Over Cu-0.63-Pd cluster/AC, hydrogenation of nitrite as an intermediate occurred much more slowly than that of nitrate at pH 10.5, suggesting that high selectivity toward nitrite is attained by OH- inhibiting adsorption of nitrite. XRD and STEM gave the size of the Cu-0.63-Pd cluster on AC as 4 nm; the structure of the cluster remained almost unchanged during the reaction. The activity and selectivity of the Cu-0.63-Pd cluster/AC was superior to those of the Cu-0.63-Pd cluster on oxides such as TiO2, Al2O3, and ZrO2. In addition, the Cu-0.63-Pd cluster/AC was more active and selective than conventionally prepared Cu-0.63-Pd/AC, indicating that the Cu-Pd cluster is an excellent precursor for selective catalysts in the hydrogenation of nitrate to nitrite. (c) 2006 Elsevier B.V. All rights reserved.