Surface-Dependent Phenomena in Cobalt-Based Electrocatalysts for Improved Nitrogen Oxyanion Reduction

Abstract

More than half of the world is fed and fueled through the Haber-Bosch process, and ammonia consumption and global population continue to grow in tandem. However, resulting nitrogen oxyanion (NOx) waste threatens aquatic ecosystems by feeding harmful algal blooms. Molecular transition metal electrocatalysts are poised to selectively reduce these NOx waste products: macrocyclic complexes are particularly robust to electrocatalytic N–O bond activations. Here, two cobalt-based macrocycles with redox-active ligand frameworks prove capable of reducing nitrate and nitrite, with an intriguing dependence on surface interactions. With an applied potential under monochromatic irradiation, a cobalt(III)-based diiminomethyl macrocycle, [Co(DIM)Br2]+, can more efficiently reduce NO2 – to ammonia by bypassing an auto-destructive ligand-based radical. Similarly, an intermediate with ligand-radical character is active toward NO3 – reduction in cobalt(II) tetraphenylporphyrin (CoTPP) in bulk systems, but can be stabilized in aqueous conditions for more affordable onset potentials once immobilized. Inspired by surface influence, homogeneous and heterogeneous systems are blended into molecular catalyst-support assemblies: physisorbing CoTPP on nanosupports imparts catalyst durability and nitrate electroreduction in coordinating buffers. These hybrid assemblies remarkably improve onset potentials in previously inaccessible conditions, ultimately overcoming many compromises between overpotential and turnover in purely homogeneous designs. Figure 1