Reduction of Nitrate Ions at Rh-Modified Ni Foam Electrodes

Commercial Ni foams were modified by spontaneous deposition of Rh nanoparticles, achieved by immersion of foam samples in acid, deaerated Na3RhCl6 solutions, at open circuit. The surface area of the Rh deposits was estimated, for different Rh loadings, by measuring the H adsorption/desorption charge. The surface area per unit Rh mass was found to exceed 50 m2 g-1, for loading values below 2 mg cm-3. The Rh-modified Ni foam electrodes were used as cathodes for the reduction of nitrate ions, and showed good catalytic activity, increasing with the Rh loading in a sublinear way; thus, the mass activity of the electrodes was higher at low loading. Prolonged electrolyses showed that the Rh-modified Ni foam electrodes underwent only moderate poisoning. Ion chromatography was used to assess the reaction products: irrespective of the Rh loading, ammonia was the main product, and nitrite accounted only for a few percent of the reduced nitrate. The beneficial effect of forcing a solution flow through the foam pores on the nitrate reduction current was shown.