Two birds with one stone: A bifunctional Fe3O4- decorated 3D honeycomb-like N-doped carbon catalyst for cascade upcycling of nitric oxide to ammonia

The direct conversion of pollutant nitrogen oxides (NOx) into valuable ammonia (NH3) represents an ideal route toward closing the nitrogen cycle and achieving sustainable development, which remains challenging due to limited catalyst functionality. Herein, we develop a Fe3O4 nanoparticle-loaded 3D honeycomb-like carbon–nitrogen material (Fe3O4@3D HS), and construct an efficient “NO oxidation–nitrate electroreduction” cascade system. In addition, during the NO oxidation stage, a micro-nano-bubble method is adopted, which can cooperate with Fe3O4@3D HS to activate H2O2 via a Fenton-like reaction, significantly enhancing the NO-to-NO3- conversion efficiency from 15% to 83%. Interestingly, during the electrocatalytic reduction stage, the same catalyst exhibits outstanding nitrate reduction reaction (NO3--RR) performance at –0.6 V (vs. RHE), achieving a Faradaic efficiency of 91.6% and an NH3 production rate of 32 mg h-1 mgcat-1. Finally, by directly utilizing the nitrate solution produced from oxidation step for electrocatalysis, a high Faradaic efficiency of 90.3% is maintained even at 134 mM nitrate, successfully demonstrating the complete pathway from NO to NH3. Electrochemical impedance spectroscopy (EIS) and ultraviolet photoelectron spectroscopy (UPS) reveal that the hierarchical micro-meso-macroporous framework, coupled with high graphitization shows a reduced work function (4.39 eV), significantly enhancing the mass transport and interfacial charge transfer. Mechanistic investigations employing in-situ differential electrochemical mass spectrometry (DEMS) and density functional theory (DFT) substantiate a "deoxygenation-first, subsequent-hydrogenation" pathway, identifying the hydrogenation of *NO to *HNO as the rate-determining step. This work not only provides a high-performance bifunctional catalyst, but also offers a new paradigm for resource-oriented treatment of nitrogen pollutants.