A continuous process for the catalytic reduction of water nitrate

The catalytic reduction of nitrate in drinking water was studied, using bimetallic Pd-Sn catalysts, supported on a macroporous anionic resin (WA30- Mitsubishi), with different Pd and Sn contents. The catalysts were prepared by the controlled surface reduction method, in which the promoter metal (Sn) is deposited on the surface of the previously reduced noble metal (Pd). The solids characterization by XRF, XPS and H-2 chemisorption confirmed the effectiveness of the preparation technique. The catalytic evaluation was carried out both in a fixed-bed reactor with hydrogen bubbling and in a batch reactor. In both cases, the most active catalyst was the Pd 2%-Sn 0.5%, and it was observed that the decrease in the Sn content below this ratio hinders activity. Catalysts containing 1% Pd showed the better N-2 selectivities. Catalysts did not show deactivation after treating more than 300 water bed volumes, even in the presence of competing anions. The effect of different reaction conditions was investigated, and it was found that a pH decrease does not modify the conversion but strongly improves N-2 selectivity. A reaction mechanism was proposed for the different species involved in the reduction on the continuous reactor with H-2 bubbling.