Hydrogenation of nitrate in water was carried out in the presence of Ni-based solid catalysts. When the hydrogenation was performed on 200 ppm (mg dm(-3)) of nitrate, our results showed that, among various base metals, porous Ni was most active forming predominantly NH3, while Ni rapidly deactivated during the reaction. Remarkably, subsequent studies revealed that addition of a small amount of Zr greatly enhanced the stationary activity of porous Ni. To the best of our knowledge, this is the first example of a noble metal-free catalyst exhibiting high catalytic activities for the hydrogenation of nitrate in water. It was found that the loading amount of Zr greatly influenced the activity, in which the addition of 0.5% Zr (w/w) exhibiting the maximum activity. XPS measurements revealed that Zr played an important role in suppressing the oxidation of the Ni-0 atoms, thus maintaining the active Ni surface. Furthermore, the addition of Zr prevented the blockage of pores of Ni, thus preserving the mesopore structures. These effects of Zr serve to increase the stationary activity of porous Ni. Since it was confirmed that Ni does not dissolve during the reaction, the novel catalytic system can be applicable to the treatment of industrial wastewater, combining the recycle system of NH3. (C) 2003 Elsevier B.V. All rights reserved.