Reduction of nitrate ions at Rh-modified Ni Foam 3D electrodes

In recent years, our group has investigated the modification of metal foams (both Ni and Fe-Cr-Al alloy) by spontaneous deposition of noble metals (Pd, Pt, Rh) as a method for the preparation of electrocatalysts [1,2] and catalysts for high-temperature gas-phase reactions [3,4]. In the present communication we describe the preparation and characterization of electrocatalysts for nitrate reduction [5], a process of interest in environmental electrochemistry. 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 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. Used as cathodes for the reduction of nitrate ions, the Rh-modified Ni foam electrodes showed good catalytic activity, increasing with the Rh loading in a sub-linear way. 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 effect of mass transport on nitrate reduction was tested using a cell which allowed to impose an electrolyte flow through the Rh-modified Ni foam electrode pores, in a flow-by configuration. The reduction current increased with the flow rate, but the process was far from pure mass transfer control.