Rational Design of Electrocatalytic Interfaces: The Multielectron Reduction of Nitrate in Aqueous Electrolytes

An electrode incorporating two distinct heterogeneous electrocatalysts acting in series was specifically designed to promote the reduction of nitrate beyond the nitrite stage in weakly buffered aqueous solutions (pH = 3) containing cadmium ions (Cd2+). This novel interface consists of gold (Au) nanoparticles on which underpotentially deposited cadmium reduces nitrate predominantly to nitrite, dispersed on a hemin-modified glassy carbon surface. In this arrangement, nitrite is further reduced to yield hydroxylamine as the only detectable product using a rotating Au(ring)|Hm|Au(np)|GC disk electrode. Additional evidence supporting this series mechanism was obtained from numerical simulations, modeling the bifunctional electrode as a hexagonal, closed-packed array of concentric Cd|Au(np) disks and Hm|GC rings without insulating gaps, utilizing rate constants determined independently from rotating Au and Hm|GC disk electrodes in solutions with either nitrate or nitrite. These findings highlight the effectiveness of the designed electrocatalytic interface in facilitating multistep redox reactions under mild conditions.