Electrocatalytic nitrogen cycle regulation based on stepwise proton electron transfer

On September 12, 2022, Professor Ho Do-ping of the Institute of Earth Life At Tokyo Institute of Technology, Professor Ryuhei Nakamura of the Institute of Rikyo and his collaborators published an article in the journal Nature Catalysis titled “Regulation of the electrocatalytic nitrogen cycle based on sequential proton-electron transfer.” new research.

The results report how to regulate the thermodynamic drivers of proton and electron transfer to control nitrite electrochemical reduction networks, and verify them through microscopic kinetic calculations, providing a rational solution for the regulation of complex reaction networks. The corresponding authors of the paper are He Daoping and Nakamura Longping; The first author is He Daoping.

The mutual conversion of nitrogen compounds constitutes the nitrogen cycle of the chemical industry and natural ecosystems. Based on the natural enzyme catalyzed nitrogen redox conversion, humans have developed important industrial nitrogen chemistries such as the Haber method of ammonia and the Oosterwald process for the synthesis of nitrate. However, these industrial processes rely heavily on fossil fuels, causing large accumulations of nitrogen oxides in the natural environment and leading to eutrophication of ecosystems.

Figure 1: Nitrogen cycle networks in nature (orange) and industrial processes (blue). The reactions in this study are shown in red, with labels indicating the number of protons and electrons transferred in the reaction.

The use of renewable electricity to electrocatalytically convert nitrogen oxides is a current research hotspot and future development direction. But there are still significant challenges in properly directing the reaction path to the target product. Nitrite reduction is widely recognized as a selective decision process for the entire nitrogen cycle, with products including NO, N2O, NH4+ and N2. The pH of the electrolyte and the electrode potential (E) simultaneously affect the primitive steps of multiple reactions, making it difficult to maximize the efficiency of a single-target reaction.

Figure 2: Proton-electron transfer model and its application in selective nitrite reduction. a, schematic diagram of proton-coupled electron transfer. b, free energy of electron transfer (ET) and proton transfer (PT) steps. cd, which changes the effect of the reaction intermediate pKa(c) or electrode potential E(d) on the reaction rate.

Preliminary exploration of electrochemical reduction regulation of nitrite (Proc. Natl. Acad. Sci. U.S.A., 2020, 117(50): 31631; J. Am. Chem. Soc., 2018, 140(6): 2012) Shows that the N2O generation rate catalyzed by molybdenum sulfide in the metallic phase exhibits volcanic pH dependence. Based on the step-by-step proton electron transfer theory, changing the pKa or electrode potential E of the reaction intermediate is expected to regulate the rate distribution of N2O and promote or inhibit the formation of NO or NH4+ at the same time.

Figure 3: Product distribution (a-c) of nitrite electrochemical reduction under different pH-E conditions and corresponding numerical simulations of microscopic kinetics (d-f).

To this end, the experiment first synthesized metallographic molybdenum sulfide materials with different pKa, and characterized and identified by XRD, Raman, UV-vis, XPS, XAFS, CW-EPR and ENDOR. Electrolytic experiments showed that faraday efficiencies of NO, N2O and NH4+ showed significant pH-E dependence on metallic molybdenum sulfide. Low pH and positive E values favor no generation, neutral pH and negative E values promote N2O generation, while NH4+ dominates in regions with high pH and negative E values. Differences in pH-E dependence of the three competing reactions allow for an optimal reaction space for each product. Microscopic dynamic numerical simulation reproduces the experimental results, reveals that the stepwise proton electron transfer mechanism is the pH-E dependent origin of the selective catalytic nitrite reduction of molybdenum sulfide in the metal phase, and demonstrates that the complex nitrite reduction network can be reasonably regulated by independent optimization of proton electron transfer driving force under this mechanism.

Figure 4: N-N coupling mechanism (a-b) and catalytic performance of complete denitrification (c-d).

Finally, the mechanism of N-N coupling to generate N2O was studied by gas chromatography isotope ratio mass spectrometer. The dominant isotope analysis of the 15N site showed that the N-N coupling occurred between the no in the adsorbed state and the NO in the free state, that is, through the Eley-Rideal type mechanism. Based on the pH-E-pKa optimization formed by the precursor N2O, the highest N2-generated Faraday efficiency was obtained experimentally. Compared with a single pH optimization, the Faraday efficiency and reaction rate of N2 after pH-E-pKa optimization increased by 2 and 7 times, respectively. So faraday efficiency of the four nitrite reduction products exceeds or is comparable to the highest reported in molecular catalysts or heterogeneous electrocatalysts optimized for a particular product.

This work shows that a single catalyst can also regulate the selectivity of complex reaction networks by inducing stepwise proton electron transfer, and it is expected to inspire more in-depth theoretical and experimental research in the future to establish complex systems of multiproton electron transfer reactions with controllable pathways, such as CO2 reduction, N2 reduction and O2 reduction. (Source: Science Network)

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