( Invited ) EC-STM Investigation of Oxygen and Nitrate Reduction Reaction on Iron-Octaethylporphyrin Adsorbed on Au(111)

Nitrate reduction reaction (NO 3 RR) is acquiring attention as an effective method to remove nitrate from drinking water and convert it to ammonia. The complete reduction to ammonia is recognized as a complex reaction, involving an 8e - transfer and a variety of reaction pathways, each leading to different products. A promising candidate catalyst for this reaction is the M-N 4 single site, already known for its ability to catalyse oxygen reduction. Among 3 d metals, iron exhibits good activity and selectivity towards ammonia [1] and has several benefits such as sustainability and low cost. In this study, iron-octaethylporphyrins (FeOEP) adsorbed on an Au(111) single crystal are used to simulate the Fe-N 4 system. Their reactivity towards the reduction of various nitrogen oxides, specifically nitrate, nitrite, and nitric oxide, is investigated by EC-STM. A statistical method was then used to assess the differences in height and profile shape among the molecules. A bright, off-centre spot is observed on top of the molecules when nitrate is inserted into an acidic environment (fig. 1a), indicating a potential interaction with the active sites. However, in a neutral medium, no significant changes were revealed. In the case of nitrite, porphyrins were observed to adopt a "donut-like" structure when sufficient reductive potentials were applied (fig.1b); this behavior contrasts with the typical tunnelling shape of free-base octaethylporphyrins [3]. Furthermore, in the case of nitric oxide, a reduction peak was observed during cyclic voltammetry tests (fig. 1c), which differed from the electrochemical response of the previous species and allowed for direct observation of the active site activity. References [1] E. Murphy, Nature Communications 2023 , 14 , 4554 [2] A. Facchin, ACS Catalysis 2024 , 14 , 14373-14386 [3] F. Cazzadori, Small Science 2025 , 5 , 2400294 Figure 1