Deciphering Nickel-catalyzed Electrochemical Ammonia Synthesis from Nitric Oxide

doi:10.21203/rs.3.rs-1313118/v1

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Deciphering Nickel-catalyzed Electrochemical Ammonia Synthesis from Nitric Oxide

https://doi.org/10.21203/rs.3.rs-1313118/v1

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Electrochemical conversion of nitric oxide (NO) to ammonia (NH₃) is a highly attractive route to achieve artificial nitrogen fixation and rebalance the global nitrogen-circle. Yet today’s poor mechanistic understanding and unsatisfied selectivity towards NH₃ curtails NOR ecologic prospects. Here, we screened a library of transition metals as electrocatalysts, in which non-noble Ni outperformed others at the top of a volcano-shaped plot. To further elucidate the active motifs, we rationally designed and fabricated five single-crystal Ni foils with various surface orientations as electrocatalysts, and observed higher conversion efficiencies on high-index facets. In particular, Ni(210) demonstrates a unique selectivity towards NH₃ with a yield rate nearly two-folds higher than those of low-index facets. Under the guidance of density functional theory (DFT) calculation, the energy required for key intermediates switching from hollow to bridge sites in the rate-determining hydrogenation step, was established as a descriptor for the capability towards NH₃ production. Our work guides the rational design of NOR electrocatalysts, and more importantly, establishes a paradigm to understand the structure-function correlation in catalysis.

Transformations between various nitrogen species are essential for maintaining the global biogeochemical nitrogen cycle ^[1–3]. Human activities have led to an anthropogenically induced increasing concentration of atmospheric nitrogen oxides (NO_x), which is harmful to human health and assumed the major responsibility for acid rain formation ^[4]. Nitric oxide (NO) is a major component of NO_x in flue gases, and the uptake of NO will further result in an in vivo conversion to nitrite ^[5], a known carcinogen and one of the main causes for methemoglobinemia. Towards this end, closing the NO-nitrogen cycle is therefore of interest: it is highly desirable to transform NO to harmless or better yet, value-added products.

As an irreplaceable nitrogenous fertilizer, ammonia (NH₃) has been added to agricultural soils to drive global increases in crop yields. Despite of the high efficiency and scalability, the industrial Haber-Bosch process for NH₃ synthesis operates at harsh conditions (300-500 ℃, pressure >200 atm), unsustainably employs natural gas as a H₂ feedstock and yields greenhouse gas a byproduct ^[6–7]. Alternatively, electrochemical NH₃ synthesis driven by renewable electricity is a sustainable way at ambient conditions and has a great market potential. Dating back to 1960s, electrochemical NH₃ synthesis from N₂ has garnered significant research interest ^[8–10], mimicking the biological nitrogen fixation. Major challenges are related to the low activity and selectivity towards NH₃, considering the high energy input required to cleavage N≡N triple bond (941 kJ/mol). Distinguished with the kinetically unfavorable N₂ reduction, electrochemical dissociation of NO is much easier to perform and will potentially address the challenge of low efficiency in nitrogen fixation ^[11]. In this regard, electrochemical conversion of NO offers an appealing One-Stone-Two-Birds direction in alleviating NO accumulation and simultaneously providing valuable chemicals such as NH₃.

Beyond the practical interest, NO is also a key intermediate in nitrogen fixation starting from nitrate ^[12–14], in this case, electrochemical NO reduction (NOR) provides an intriguing platform to mechanistically understand and elucidate the intrinsic catalyst structural parameters determining the nature of further reduced products. In contrast to proton reaction, NOR is confirmed to possess multiple reaction pathways, leading to a broad range of reduced nitrogen species, such as nitrous oxide (N₂O), N₂, hydroxylamine (NH₂OH) and NH₃ ^[15]. Such complexity mainly originates from the multiple electrons transfer steps involved and the strong surface structure sensitivity of the process. To date, electrochemical NOR has been realized on several model systems with variable product distribution ^[16–23]. Although N₂ is the thermodynamically favorable product, NH₃ and NH₂OH are more than usually observed, particularly in an acidic electrolyte. Recently, Xiao et al ^[18] and Jiao et al ^[19] separately demonstrated NO electrochemical conversion on Cu metal with a high selectivity over 90% towards NH₃. Single atom catalysts (SACs) decorated on carbon supports such as Nb-SACs ^[20] and Fe-SACs ^[21] were reported to majorly produce NH₃ and NH₂OH, respectively. However, enzymatic-inspired catalysts ^[22–23], such as heme proteins myoglobin and haemoglobin, primarily catalyze NO to N₂O, although they have structurally similar active sites with Fe-SACs, i.e., Fe-N₄ core. Despite of the initial progresses, searching for a more active and selective catalytic system for NH₃, and more importantly, identifying the intrinsic parameters that govern the product distribution and fixation pathways remain grand challenges in this emerging area.

Herein, we firstly screened a library of transition metals (Ti, V, Cu, Ni, Pd, Pt, Ag and Au) to understand how the electronic structure affects and modulates the product distribution in NOR. Remarkably, we observed an interesting volcano-shaped correlation between calculated nitrogen adsorption free energy and NH₃ production on various transition metals, among which, non-noble metal Ni outperformed others with a 94% selectivity and 9.48 µmol cm^-2 h^-1 yield rate towards NH₃. Furthermore, we rationally designed and synthesized five large-area single-crystal Ni foils with various surface orientations to elucidate the structure-function correlation, and observed that the high-index Ni facets exhibit higher selectivities towards NH₃. In particular, Ni(210) achieved 100% selectivity towards NH₃ with a yield rate of 12.02 µmol cm^-2 h^-1. Confirmed by density functional theory (DFT) calculations, the abundant steps on high-index facets facilitate to reduce the energy required for the hollow-to-bridge location switch of the key intermediates associated in the rate-determining hydrogenation step, thereby resulting in a higher NH₃ production capability. This work established a benchmark system to study NOR electrocatalysis and setup a paradigm to mechanistically understand the correlation between atomic configuration and catalysis.

Comparison of NOR on various transition metals. A suite of transition metals including Ti, V, Cu, Ag, Au, Ni, Pd and Pt were prepared as electrodes (See details in Methods), and the catalytic activity was evaluated in a sulfuric acid-diluted K₂SO₄ (0.5 M, pH=3) aqueous solution under 1 atm of NO in a two-compartment gastight H-cell. Each transition-metal electrode was tested using chronoamperometry at multiple potentials. The gas products were quantified via an on-line gas chromatography (GC), and liquid products were confirmed off-line by three techniques, UV-vis, ion chromatography (IC) and nuclear magnetic resonance (NMR) spectroscopy, for accuracy (see Methods, Supplementary Figures S1-4). Here we should emphasize that the removal of oxygen in electrochemical system is essential to ensure the stability of NO before getting electrochemically reduced on cathode. Otherwise, the residual oxygen will chemically oxidize NO to NO₂, NO₂^- and finally to NO₃^-, leading to an inflated current density and complex reaction kinetics (Supplementary Figure S5). Beyond product distribution (represented by Faradaic efficiency, FE), we also take emphasis on quantifying product yield rates. To accurately determine the intrinsic activity, the topological surface area (TSA) of electrode was quantified by measuring the topological surface roughness factor (see Methods, Supplementary Figure S6 and Table S1). Figure 1a describes the FEs and yield rates of NH₃ at -0.6 V vs. RHE on different transition metals. Notably, Ni exhibited the highest FE (~94%) and highest yield rate (9.48 µmol cm^-2_TSA h^-1) towards NH₃ simultaneously. Cu shows a lower FE of 80% with a yield rate of 6.61 µmol cm^-2 _TSA h^-1. Instead, the VIII group metal Pd and Pt are more favorable to generate H₂, having a remarkable decreased NH₃ selectivity of 59% and 51%, respectively. As another competing reaction pathway, NH₂OH was observed with a decent selectivity on some metals, such as Ag, Ti and V. The comparison of activity and selectivity at multiple potentials was summarized in Supplementary Figure S7.

Concurrently, we employed DFT calculations to understand electrochemical NOR. It is confirmed that the adsorption free energy of N (∆G_N*) is a good descriptor for NH₃ production. Figure 1b demonstrates a volcano-shaped plot between calculated ΔG_N* and NH₃ production (shown by both FE and yield rate) on various transition metals. The ΔG_N* of Ni is close to the optimal value, neither too strong nor too weak. Accordingly, Ni exhibited simultaneously the highest FE and highest yield rate towards NH₃. As the adjacent transition-metal to Ni, Cu exhibits a weaker N adsorption, thus a lower FE and production rate. The poor performance of metal Pd and Pt is due to their remarkable capability to generate H₂ ^[24–25], which is a major competing reaction to produce NH₃. Moreover, Ti, V, Ag and Au show much lower selectivities and production rates because of large deviations from the optimal value of ΔG_N*.

To further understand the activities and selectivities of these transition metal electrodes, the electrochemical process of NOR were theoretically investigated on the most stable facets of various transition metals (see Methods, Supplementary Figure S8). After screening possible NOR pathways, two feasible ones, O* and N* pathways, were proposed as listed in Supplementary Table S2. Rather than directly dissociating into atomic O* and N*, the NO molecule forms adsorbed NHO* (O* pathway) or NOH* (N* pathway) intermediate at the hollow site of metal atoms through an associative pathway ^[16]. Regardless of each pathway, the adsorption energies of intermediates involved in the rate-determining steps (RDS), NO→NHO* for O* pathway and NH*→NH₂*for N* pathway, depend almost linearly on ΔG_N* due to the scaling relation (Supplementary Figure S9), which can well explain the remarkable correlation between ΔG_N* and NH₃ production.

Design and fabrication of single-crystal Ni foils. To understand the active site motifs of Ni towards NOR, we rationally designed monocrystalline Ni with well-defined lattices as electrocatalysts. According to the Miller indices {hkl}, Ni facets can be classified into low-index facets including {100}, {111} and {110}, and high-index facets which constitute terraces, steps and/or kinks with variable orientations. A general microfacet notation for cubic structure has been proposed by Somorjai in 1980 ^[26], in which each high-index facet can be described by a combination of low-index facets with different constitution ratios. For example, Ni(210) is denoted as Ni(S)-[1₁(110)+1₁(100)], indicating that one-atom-wide (100) terraces are separated by one-atom-wide (110) steps. Likewise, Ni(310), denoted as Ni(S)-[1₁(110)+2₂(100)], can be visualized as a kinked surface with a repetitive unit having two-atom-wide (100) terraces and one-atom-wide (110) steps. The presence of low-index terraces/steps suggests a high degree of site heterogeneity on high-index facets, which can be clearly observed in the side view of Ni(210), where the kinks are shown to have atomic sites with six-(pink), nine-(yellow) and eleven-(blue) fold coordination (Figure 2a). To elucidate the structure-function correlation, we rationally prepared five single-crystal Ni facets with distinct coordination environments and site motifs, including two low-index facets, e.g., Ni(100) and Ni(111), and three high-index facets, e.g., Ni(210), Ni(310) and Ni(520). Those high-index facets can be denoted as different combinations of low-index facets, and are located on the inverse pole figure as shown in Supplementary Figure S10.

The growth of single-crystal Ni follows our recently developed “seeded abnormal grain growth” technique ^[27]. Figure 2b shows an optical image of a large-format single-crystal Ni foil with (111) facet, the crystal orientation of which is corroborated by the low energy electron diffraction (LEED) (Figure 2c). The uniform color contrast of five single-crystal Ni foils shown in electron-back scattered diffraction (EBSD) at the micro-meter scale strongly suggests the absence of in-plane misorientation on surface (Figure 2d). X-ray diffraction was engaged to track the crystallinity of bulk Ni foils. As shown in Figure 2e, XRD symmetric scans confirm the single crystallinity nature of fcc Ni foils textured in five out-of-plane directions, which are in good agreement with EBSD mappings. On the whole, these characterizations proved that the synthesis of single-crystal Ni is atomically controllable.

Understanding NO-to-NH ₃ conversion on Ni. The electrochemical performance of single-crystal Ni was evaluated under the identical condition as we described above. Selectivity and yield rate on five single-crystal Ni facets were plotted as a function of applied potential, as summarized in Figure 3a and Supplementary Figure S11. Remarkably, NH₃ is the major product on monocrystalline Ni at a more positive bias. The hydrogen evolution reaction (HER), as a major competing reaction, was gradually dominant as the applied potential increased negatively from -0.8 V to -1.0 V vs. RHE, due to the mass transfer limitation of NO (solubility ~1.94 mM at 25 ℃) ^[28] under a large electrochemical polarization. In the medium potential region, NOR kinetically dominates the reaction. To exclude the influence from mass transport, we focused on the medium potential region to understand NOR. As shown in Figure 3a, all high-index Ni facets explicitly demonstrate higher selectivity towards NH₃. In particular, Ni(210) achieved almost a unique selectivity with a yield rate nearly two-folds higher than those of Ni(100) and Ni(111) (Supplementary Figure S11). It is worth to note that the facet orientation of all selected single-crystal Ni foils remained stable after electrochemical NOR, confirmed by EBSD characterization (Supplementary Figure S12).

We performed DFT calculations to understand the activities on various Ni facets, including both N* pathway (NO→NOH*→N*+H₂O, N*→NH*→NH₂*→NH₃) and O* pathway (NO→NHO*→NH₂O*→NH₃+O*, O*→OH*→H₂O)^[18]. DFT results suggest that N* pathway with NH*→NH₂* as the RDS dominants NOR on Ni(111), Ni(100), Ni(310) and Ni(520) facets (see Supplementary Figures S13-14 and Table S3 for adsorption energies of all intermediates), and the energy increase associated with RDS are 0.16, 0.33, 0.10, and 0.09 eV, respectively (Figure 3b). Since the stepped surface Ni(310) and Ni(520) demand less energy than that of flat surface Ni(100) and Ni(111), they would exhibit better NH₃ production performances, which is in-line with the experimental results in Figure 3a. For Ni(210), both N* and O* pathway were plotted in Figure 3c. Although the N* pathway exhibits a fairly smaller barrier of 0.07 eV, the O* pathway is energetically preferred on Ni(210) as it shows a continuous downhill conversion, which can well explain its best performance towards NH₃ production in experiment.

We also observed a switch of adsorption sites for intermediates in the RDS of N* pathway on all involved facets. DFT calculations show that the first three generated species, NOH*, N* and NH*, prefer to occupy a hollow active site, the NH₂* intermediate, instead, is only stabilized at a bridge site. Therefore, the adsorption site has to switch from a hollow to a bridge site at the step of NH*→NH₂*. It is presumable that the free energy demanded for such switch, denoted as ∆G_s, may determine the activity towards NH₃ production. Figure 3d plotted the NH₃ yield rate in log scale as a function of ∆G_s on Ni(111), Ni(310), Ni(210) and Ni(520), which demonstrates an roughly exponential correlation. As for Ni(210), although it favors O* pathway, the hydrogenation of O* to produce OH* similarly associates with a location switch from hollow to bridge site, which is exothermic considering the energetic downhill nature of O* pathway on Ni(210). In this regard, ∆G_s can be regarded as 0 eV on Ni(210). Suprisingly, the ∆G_s of O* pathway on Ni(210) appears along the trend line in Figure 3d, suggesting ∆G_s is a reasonable theoretical parameter to describe the yield of NH₃. Such correlation can be well explained from a view of geometric structure. Ni(100) displays the largest ∆G_s as the coordination number of NH* has to decrease from four (hollow site) to two (bridge site) associated with its hydrogenation. Instead, the NH* coordination only need to drop from three (hollow site) to two (bridge site) at the closely-packed Ni(111) surface, which exhibits a smaller ∆G_s in relative to Ni(100). For high-index Ni facets, the adsorption capability of bridge site will be strongly elevated because of the abundant undercoordinated step sites, which effectively reduce the required ∆G_s and ultimately accelerate the yield of NH₃.

To conclude, we demonstrate a volcano-shaped correlation between calculated nitrogen adsorption free energy and NH₃ production on various transition metals, and screened out Ni to be the most active catalyst for electrochemical NO-to-NH₃ conversion. To gain deep insight in such conversion, we rationally designed and fabricated large-area single-crystal Ni foils with various surface orientations, and achieved ~100% selectivity of NH₃ on Ni(210) facet. DFT calculation revealed the RDS of NOR on Ni associated with a hollow-to-bridge location switch of intermediate, and the energy required in this step is demonstrated as a descriptor for the yield of NH₃. Our results demonstrate a benchmark system for NOR electrocatalysis, and provide an important mechanistic insight that could further guide the rational design and optimization of Ni-based NOR catalysts.

Supporting Information. Methods, Supplementary figures and tables are included in Supporting Information. The quantification of liquid and gas products, calculation energy diagrams and adsorption energies of all intermediates, high- and low-index Ni facets and their electrochemical NOR results.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]; [email protected]

Author Contributions

L.Z. designed and conceived the experiment. S.Z., C.Z., C.Y., J.W. G.S. and M.C. characterized the electrodes and performed electrochemical characterization as well as data analysis. J.L. and S.L. worked on DFT calculations. Z.Z, Q.W. and K.L. carried out the synthesis of monocrystalline Ni electrodes, and performed EBSD, XRD and LEED characterizations. All authors discussed the results and participated in writing the manuscript.

NOTES

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This research was supported by National Natural Science Foundation of China (Grants 21872039, 22072030 and 52025023), Science and Technology Commission of Shanghai Municipality (Grants 18JC1411700 and 19DZ2270100), The Key R&D Program of Guangdong Province (Grant 2020B010189001). L.Z. thanks funding support from the original personalized project of Fudan University. S.L. thanks funding support from Singapore Academic Research Fund Tier 1 (RG8/20 and RG10/21). C.Z thanks funding support from China Postdoctoral Science Foundation (2021M700810).

Bolle, H. J. et al. The natural environment and the biogeochemical cycles. Eds. Springer. Berlin Heidelberg: Berlin, Heidelberg. pp 61-81 (1982).
Canfield, D. E., Glazer, A. N. & Falkowski, P. G. The evolution and future of earth's nitrogen cycle. Science 330, 192–196 (2010).
Chen, J. G. et al. Beyond fossil fuel-driven nitrogen transformations. Science 360, eaar6611 (2018).
Gerhardsson, L. et al. Chapter 15 - Effects of acid precipitation on the environment and on human health. In advances in environmental control technology: health and toxicology. Eds. Gulf Professional Publishing: Burlington. pp 355-364 (1997).
Liou, G.Y. & Storz, P. Reactive oxygen species in cancer. Free Radic. Res. 44, 479–496 (2010).
Lehnert, N. et al. Reversing nitrogen fixation. Nat. Rev. Chem. 2, 278–289 (2018).
Kandemir, T. et al. The Haber–Bosch process revisited: on the real structure and stability of “ammonia iron” under working conditions. Angew. Chem. Int. Ed. 52, 12723–12726 (2013).
Licht, S. et al. Ammonia synthesis by N₂ and steam electrolysis in molten hydroxide suspensions of nanoscale Fe₂O₃. Science 345, 637–640 (2014).
Foster, S. L. et al. Catalysts for nitrogen reduction to ammonia. Nat. Catal. 1, 490–500 (2018).
Bao, D. et al. Electrochemical reduction of N₂ under ambient conditions for artificial N₂ fixation and renewable energy storage using N₂/NH₃ Cycle. Adv. Mater. 29, 1604799 (2017).
Hara, K. et al. Electrocatalytic reduction of NO on metal electrodes and gas diffusion electrodes in an aqueous electrolyte. J. Electroanal. Chem 451, 181–186 (1998).
Duca, M. & Koper, M. T. M. Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 5, 9726–9742 (2012).
Chen, G. F. et al. Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper–molecular solid catalyst. Nat. Energy. 5, 605–163 (2020).
Li, J. et al. Efficient ammonia electrosynthesis from nitrate on strained ruthenium nanoclusters. J. Am. Chem. Soc. 142, 7036–7046 (2020).
Katsounaros, I. et al. Structure- and coverage-sensitive mechanism of NO reduction on platinum electrodes. ACS Catal. 7, 4660–4667 (2017).
Clayborne, A. et al. Elucidation of pathways for NO electroreduction on Pt(111) from first principles. Angew. Chem. Int. Ed. 54, 8255–8258 (2015).
Chun, H. J. et al. Atomistic insights into nitrogen-cycle electrochemistry: A combined DFT and kinetic Monte Carlo analysis of NO electrochemical reduction on Pt(100). ACS Catal. 7, 3869–3882 (2017).
Long, J. et al. Direct electrochemical ammonia synthesis from nitric oxide. Angew. Chem. Int. Ed. 59, 9711–9718 (2020).
Ko, B. H. et al. Electrochemical reduction of gaseous nitrogen oxides on transition metals at ambient conditions. J. Am. Chem. Soc. 144, 1258–1266 (2022).
Peng, X.et al. Ambient electrosynthesis of ammonia with efficient denitration. Nano Energy 78, 105321 (2020).
Kim, D. H. et al. Selective electrochemical reduction of nitric oxide to hydroxylamine by atomically dispersed iron catalyst. Nat. Commun. 12, 1856 (2021).
Pan, K.C. et al. Electrocatalytic reactions of nitric oxide on prussian blue film modified electrodes. J. Electroanal. Chem. 501, 160–165 (2001).
Bedioui, Z. F. Electrocatalysis of nitric oxide reduction by hemoglobin entrapped in surfactant films. Electrochem Commun. 3, 435–438 (2001).
Janssen, L. J. J., Pieterse, M. M. J. & Barendrecht, E. Reduction of nitric oxide at a platinum cathode in an acidic solution. Electrochim. Acta. 22, 27–30 (1977).
de Vooys, A. C. A. et al. Mechanistic study on the electrocatalytic reduction of nitric oxide on transition-metal electrodes. J. Catal. 202, 387–394 (2001).
Van Hove, M. A. & Somorjai, G. A. A new microfacet notation for high-Miller-index surfaces of cubic materials with terrace, step and kink structures. Surf. Sci. 92, 489–518 (1980).
Wu, M. et al. Seeded growth of large single-crystal copper foils with high-index facets. Nature 581, 406–410 (2020).
Kim, D. et al. Unveiling electrode-electrolyte design-based NO reduction for NH₃ synthesis. ACS Energy Lett. 5, 3647–3656 (2020).

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Deciphering Nickel-catalyzed Electrochemical Ammonia Synthesis from Nitric Oxide

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