Coupled Electrocatalytic-Enzymatic Tandem Reduction for Selective Nitrate-to-Ammonia Conversion

Abstract

As a promising strategy for green synthesis, tandem electroenzymatic catalysis has recently emerged. This work designs a tandem electroenzymatic catalysis platform by directionally immobilizing ionic liquid modified chloroperoxidase (CPO-IL_EMB) onto silver-loaded reduced graphene oxide (Ag/rGO) modified with polyethyleneimine (PEI), thereby constructing the CPO-IL_EMB/Ag/rGO-PEI bioconjugate for NO₃^– reduction reaction (NO₃^–RR). Firstly, Ag/rGO-PEI electrocatalytically reduces NO₃^– to NO₂^–. Subsequently, CPO-IL_EMB converts NO₂^– to NH₄⁺ with high substrate selectivity and catalytic specificity. PEI covalently immobilizes enzyme molecules via glutaraldehyde, enhancing their stability, and its protonated form provides positive charges to enrich NO₃^– and the intermediate NO₂^–. In situ FTIR analysis reveals that the NO₂^– intermediate is rapidly consumed at the CPO-IL_EMB/Ag/rGO-PEI interface, accompanied by the accumulation of NH₄⁺ species, verifying the successful construction of this tandem electroenzymatic system. As a result, the CPO-IL_EMB/Ag/rGO-PEI bioconjugate achieves a Faradaic efficiency of 94.97% and an NH₄⁺ yield rate of 4.94 mg h^–1 mg_cat^–1 at –0.56 V vs. RHE, representing an NH₄⁺ yield rate of 19.8-fold increase over Ag/rGO-PEI, and maintains stable performance during 216 h of continuous operation.