Nitrogenase-mimic iron-containing chalcogels for photochemical reduction of dinitrogen to ammonia

Abstract

A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (SPh) 3 -containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (SPh) 3 is capable of N 2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N 2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15 N 2 isotope experiments confirm that the generated NH 3 derives from N 2 . Density functional theory (DFT) electronic structure calculations suggest that the N 2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N 2 under ambient conditions. nitrogenase mimics | chalcogel | N 2 fixation | ammonia synthesis | photocatalytic T he reduction of atmospheric nitrogen to ammonia is one of the most essential processes for sustaining life. Currently, roughly half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the industrial Haber-Bosch process, which operates under high temperature (400-500°C) and high pressure (200-250 bar) in the presence of a metallic iron catalyst (1). Nitrogenase, a two-component protein system comprising a MoFe protein and an associated Fe protein, carries out this "fixation" in nature under ambient temperature and pressure (2-4). N 2 substrate binding and activation take place at the ironmolybdenum-sulfur cofactor (FeMoco), and in some cases, Mofree iron-sulfur cofactor FeFeco and iron-vanadium-sulfur cofactor FeVco cofactors. Electron transfer during this catalytic process is believed to proceed from a [4Fe:4S] cluster located in the Fe protein to another Fe/S cluster (the P cluster) buried in the MoFe protein and finally to the FeMoco (Fig. 1A) (2, 5, 6). Whereas the role of Mo in the reactivity of nitrogenase has been the subject of long debate, iron is now well recognized as the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data point to iron as the site of N 2 binding in the FeMoco (7-9). Naturally, understanding and mimicking how the nitrogenas...

Improving the Reliability and Accuracy of Ammonia Quantification in Electro‐ and Photochemical Synthesis

“…Finally, the use of isotopically labeled nitrogen ( 15 N 2 ) is an excellent option to define photocatalytic and/or electrocatalytic formation of ammonia and the contamination from different sources. NMR can be employed to identify the 15 NH 4 + ions when 15 N 2 is used as the feed gas . Owing to the high sensitivity, 1 H NMR was always used to distinguish 15 NH 4 + and 14 NH 4 + with low ammonium concentration .…”

“…Owing to the high sensitivity, 1 H NMR was always used to distinguish 15 NH 4 + and 14 NH 4 + with low ammonium concentration . Specifically, 14 NH 4 + has three 1 H resonance symmetric signals with a spacing of 52 Hz in the region around 6.8–7.05 ppm in 1 H NMR spectrum, whereas 15 NH 4 + has two symmetric signals with a spacing of 73 Hz in the same region . However, the impurities, such as 15 NH 3 and 15 NO x species, should be removed before the isotope‐labeled gas was used .…”

“…Common quantification methods, including the Nessler's reagent and the indophenol blue methods, ion chromatography (IC), and some other sophisticated methods (e.g., 1 H NMR spectroscopy and ammonia ion selective electrode) have been employed to quantify the generated ammonia (i.e., ammonium in acidic and neutral solutions). However, the accurate quantification of generated ammonia has been a technical challenge, primarily because of the extremely low concentration of ammonium, interferences from coexisting materials, and contaminations from materials and surroundings, including reagent water, catalysts, nitrogen feed gas, air, and human breath .…”

Ammonia Synthesis from Electrocatalytic N2 Reduction under Ambient Conditions by Fe2O3 Nanorods

“…Numerous efforts have been devoted toward artificial NH 3 synthesis, including plasma‐induced, biological, photocatalytic, and electrochemical methods . In particular, electrochemical synthesis of NH 3 from N 2 and H 2 O as the hydrogen source is considered as an energy‐efficient N 2 reduction process.…”

Recent Advanced Materials for Electrochemical and Photoelectrochemical Synthesis of Ammonia from Dinitrogen: One Step Closer to a Sustainable Energy Future

“…The investigation on these molecular catalysts also facilitates the understanding of the NRR mechanism in more complex biological systems such as nitrogenase . However, due to their instability, the reported molecular NRR catalysts usually need to operate under extremely low temperatures, typically below −70 °C, limiting their potential practical applications until Kanatzidis' group developed the Fe‐containing chalcogel systems that can serve as photochemical catalysts under ambient conditions . Photochemical systems are considered to be clean and renewable due to the use of light, potentially solar, as the energy source.…”