Combining Noble Metals and Enzymes for Relay Cascade Electrocatalysis of Nitrate Reduction to Ammonia at Neutral pH

Electrocatalytic reduction of nitrate at noble metals is particularly sluggish at neutral pH, while nitrate reductase enzymes are highly active for selective reduction of nitrate to nitrite under these conditions. However, noble metal electrodes can effectively reduce nitrite, even at millimolar concentrations in neutral media. In this study, we demonstrate tandem electrochemical reduction of nitrate to ammonia by combining Escherichia (E.) coli nitrate reductase (NarGHI) on an electrode with Pt or Rh nanoparticles. Under potential control, the enzyme first reduces nitrate to nitrite, which is further reduced by the metal at a slightly more negative potential. This creates a novel electrocatalytic relay cascade where the enzyme overcomes an inherent limitation of the metal. This proof-of-concept study, using biological and metal catalysts supported on commercial carbon beads, demonstrates a simple approach to the assembly of hybrid' electrocatalysts.