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Ammonia oxidizing archaea (AOA) are among the most abundant microorganisms on earth and are known to be a major source of nitrous oxide (N₂O) emissions, although biochemical origins of this N₂O remain unknown. Enzymological details of AOA nitrogen metabolism are broadly unavailable. We report the recombinant expression, purification, and characterization of a multicopper oxidase, Nmar_1354, from the AOANitrosopumilus maritimus. We show that Nmar_1354 selectively produces nitroxyl (HNO) by coupling the oxidation of the obligate nitrification intermediate hydroxylamine (NH₂OH) to dioxygen (O₂) reduction. This HNO undergoes several downstream reactions, although the major fates are production of N₂via reaction with NH₂OH and dimerization with itself to yield N₂O. These results afford one plausible enzymatic origin for N₂O release by AOA. Moreover, these results reveal a physiologically relevant enzymatic reaction for producing HNO, an enigmatic nitrogen oxide speculated to be operative in cellular signaling and in energy transduction. 

We have previously shown that Pt–Ni alloy nano-octahedra with {111} facets exhibit outstanding electrochemical performance in the oxygen reduction reaction (ORR) in acidic media when their surfaces are finely tailored at the atomic level. In this investigation, we further refine the surface structure of Pt2.2Ni octahedral nanocatalysts to improve ORR performance in a 0.1 M KOH solution using diverse surface manipulation techniques. Through systematic analysis using electrochemical CO stripping, cyclic voltammetry, and X-ray photoelectron spectroscopy, we examined the surfaces of Pt2.2Ni octahedral nanocatalysts pretreated with various methods, including etching in acetic acid or perchloric acid, and subsequent electrochemical activation in an alkaline solution or an acidic solution. Among these treatments, those involving acidic media, particularly electrochemical cycling in acidic electrolytes, demonstrated significantly enhanced ORR activity in 0.1 M KOH. The latter exhibited a mass activity of 2.95 A/mgpt and a specific activity of 8.71 mA/cm2 at 0.90 V, surpassing state-of-the-art Pt/C by 12-fold and 34-fold, respectively. Furthermore, this identified nanocatalyst displayed robust stability, with negligible activity decay observed after 10,000 cycles. This study suggests that the improved ORR activity can be attributed to the Pt-rich surfaces with well-preserved {111} lattices on the surface-modified Pt–Ni nano-octahedra.