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Selective Electrocatalytic Reduction of Nitrite to Dinitrogen based on Decoupled Proton-Electron Transfer Daoping He, Yamei Li, Hideshi Ooka, Yoo Kyung Go, Fangming Jin, Sun Hee Kim, and Ryuhei Nakamura J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12774 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018
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Selective Electrocatalytic Reduction of Nitrite to Dinitrogen based on Decoupled Proton-Electron Transfer Daoping He,a,b Yamei Li,b Hideshi Ooka,b,c Yoo Kyung Go,d Fangming Jin*,a Sun Hee Kim*,d Ryuhei Nakamura*b,e a)
School of Environmental Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P.R. China b) Biofunctional Catalyst Research Team, RIKEN Center for Sustainable Resource Science (CSRS), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan c) Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan d) Western Seoul Center, Korea Basic Science Institute (KBSI), 150, Bukahyeon-ro, Seodaemun-gu, Seoul 120-140, Korea e) Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, 2-12-1-I7E Ookayama, Meguro-ku, Tokyo, 1528550, Japan Supporting Information Placeholder
which can reduce nitrate and nitrite to dinitrogen is critical for sustaining the nitrogen cycle. However, regulating the selectivity has proven to be a challenge, due to the difficulty of controlling complex multi-electron/proton reactions. Here we report that utilizing sequential proton-electron transfer (SPET) pathways is a viable strategy to enhance the selectivity of electrochemical reactions. The selectivity of an oxo-molybdenum sulfide electrocatalyst towards nitrite reduction to dinitrogen exhibited a volcanotype pH dependence with a maximum at pH 5. The pH-dependent formation of the intermediate species (distorted Mo(V) oxo species) identified using operando electron paramagnetic resonance (EPR) and Raman spectroscopy was in accord with a mathematical prediction that the pKa of the reaction intermediates determines the pH-dependence of the SPET-derived product. By utilizing this acute pH dependence, we achieved a Faradaic efficiency of 13.5% for nitrite reduction to dinitrogen, which is the highest value reported to date under neutral conditions.
Selective denitrification by the reduction of nitrogen oxyanions (NO3-/NO2-) to harmless N2 is critical for restoring the balance of the nitrogen cycle.1-5 In nature, microorganisms perform the sequential conversion of NO3- to N2 under mild pH conditions using a series of reductase enzymes (Figure 1a).6 However, anthropological nitrogen fixation through the Haber-Bosch process combined with fossil fuel combustion exceeds terrestrial nitrogen fixation by twofold and has resulted in extensive eutrophication of marine ecosystems and accumulation of toxic nitrogen oxyanions in groundwater environments.1-3 To help balance the natural nitrogen cycle, extensive research efforts have been directed towards replicating biological denitrification processes.3-4, 7-13 However, increasing the selectivity towards N2 production with respect to undesirable byproducts such as NO, N2O, or NH4+ remains a challenge due to the difficulty of controlling multi-electron/proton transfer processes (Figure 1a).4, 14 Although many transition metal complexes have the ability to reduce nitrate and nitrite, the production of N2 is limited to strong-
ly acidic (pH 2) solutions.14-16 Similarly, heterogeneous electrocatalysts, which also operate optimally at pH > 13 or < 1, have drastically decreased selectivity for N2 production at neutral pH.4, 17-18 As nitrogen oxyanions found in nature often exist in pHneutral aquatic environments, it is critical to identify materials that can selectively produce N2 under neutral-pH conditions.3 a
NH4+
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ABSTRACT: The development of denitrification catalysts
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N2
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Figure 1. (a) Typical reaction pathways for NO3- and NO2- reduction. (b) Relationship between pH and the driving force for proton and electron transfer for an elementary proton-coupled electron transfer step. (c) The rate of two competing reactions as a function of the pH at a constant potential on the RHE scale. The relative reaction rate of two CPET pathways is pH independent (solid and broken black lines), whereas the relative reaction rate of two SPET pathways shows a strong pH dependence (solid and broken green lines). Herein, using an oxo-molybdenum sulfide (oxo-MoSx), we demonstrate the benefits of decoupled proton-electron transfer to regulate the selectivity. In contrast to the extensively studied concerted proton-electron transfer (CPET) pathway,19-22 sequential
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Figure 2. (a) CVs for an oxo-MoSx catalyst in the absence (upper panel) and presence (bottom panel) of NO2- (0.1 M) at different pH values. (b) Upper panel: Faradaic efficiency (FE) for NO, NH4+ and N2O formation during NO2- reduction (0.1 M) at different pH values at 0.1 V for 4 h; Bottom panel: Partial current densities for NO, NH4+ and N2O formation at 0.1 V calculated by multiplying the current density (j) with the FE values.
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To confirm whether an SPET pathway is indeed dictating the reaction selectivity, the pKa of the reaction intermediate was probed using operando spectroscopy. At all pH values investigated, 20 mM dithionite was used as a reductant to activate the catalyst. The electrochemical potential thus generated was +50 mV, which approximately corresponds to the electrolysis conditions in Figure 2b (0.1 V) (Figure S3). The addition of dithionite increased the intensity of EPR signals with g values of 1.85~1.98 (Figure 3a, red lines). These signals are assignable to axial signals from Mo(V) oxo species,29-31 and because no signals were detected in the absence of dithionite (Figure 3a, gray lines), the Mo(V) oxo species were likely generated in-situ by the reduction of Mo(VI) oxo species. The consumption of Mo(V) oxo species upon addition of 0.1 M nitrite (Figure 3a, blue lines) further supports the involvement of this intermediate in nitrite reduction at all examined pH values. It is noted here that upon increasing the pH from 4.0 to 7.0, the EPR spectrum of the Mo(V) oxo species exhibits a clear transition at pH 5.5. Specifically, in the pH region from 4.0 to 5.5, Mo(V) oxo species exhibited splitting in the high-field region of 1.91~1.94, indicating that distorted species were formed by the orbital motion of molybdenum ion.32 However, from pH 5.5 to 7.0, the splitting of the EPR spectra was diminished, indicating that the Mo(V) oxo species undergoes a transition from a distorted to isotropic structure at pH > 5.5.
EPR Intensity (a.u.)
(decoupled) proton-electron transfer (SPET) pathways possess an acute pH dependence as shown in Figure 1b.22-24 When the total free energy change of the reaction is kept constant, the driving force for proton transfer and electron transfer exhibits a trade-off relationship upon changing the pH. Based on a mathematical model by Koper,25 this leads to a pH-dependent reaction rate which exhibits a maximum at the pH corresponding to the pKa of the reaction intermediate (Figure 1c). By taking advantage of this unique pH dependence, we demonstrate the highest selectivity for N2 production in neutral pH (13.5 % Faradaic efficiency). An oxo-MoSx catalyst that structurally mimics the active sites of biological nitrite reductases was synthesized hydrothermally (Figure S1). Figure 2a shows cyclic voltammograms (CVs) for the synthesized oxo-MoSx electrocatalyst. All potentials are reported in the RHE scale to compensate any change in the thermodynamic driving force associated with pH.22 In the absence of nitrite, no notable differences in the CVs were observed, showing that the efficiency of H2 evolution was independent of the solution pH. In contrast, the cathodic current was markedly enhanced upon the addition of nitrite to the electrolyte, and a current maximum in the cathodic direction appeared for both the anodic and cathodic scans at ca. 0.4 V. The generation of this valley-like feature is characteristic for electrochemical processes with inhibitory interactions. Possible origins may be adsorbed forms of NO (NOads), although further experiments will be necessary to confirm the chemical origin (see Figure S2 for kinetics analysis). The reduction of NOads is the selectivity-determining step of electrochemical denitrification (Figure 1a).26-27 To investigate the pH dependence of product selectivity, we conducted electrolysis experiments at 0.1 V, as optimal selectivity towards N2O was previously observed at this potential.28 Each product exhibited a unique pH dependence (Figure 2b), as the FE of NO increased monotonically upon lowering the pH from 7 to 4, whereas the reverse was observed for NH4+. Notably, the FE of N2O showed a volcano-type pH dependence with a local maximum at pH 5 (FE: 40%). The formation rate of each product is also pH dependent (Figure 2b). A deviation of only one pH unit from the optimal pH of 5 dramatically decreases the formation rate of N2O. The observed volcano-type relationship between pH and reaction rates suggests a strong influence of an SPET mechanism for N2O formation.25
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Distorted (pKa~5.5)
Figure 3. (a) EPR spectra of oxo-MoSx and (b) Raman spectra of S-H bonds generated after reduction by dithionite at different pH conditions. (c) EPR signal intensity of distorted Mo(V) oxo species and Raman signal intensity of S-H vibration as a function of pH. The EPR intensity was deduced from peak amplitude defined by the intensity difference of peak-to-trough in the axial Mo(V) oxo signals in panel (a). (d) The proposed catalytic reaction route for the formation of distorted and isotropic Mo(V) oxo species. Consistent with the observed pH dependence for Mo(V) oxo species in the EPR spectra, a pH-dependent formation of a thiol
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moiety was detected using operando Raman spectroscopy (Figure 3b, Figure S4a). Upon addition of dithionite at pH 4.0~5.5, a new species was observed at 2534 cm-1, which corresponds to the frequency region of S-H vibration.33-34 The shift of the Raman band to 1835 cm-1 upon changing the solvent from H2O to D2O (Figure S4b), as well as the isotopic shift factor of 1.38 (frequency ratio of νS-H/νS-D)33-34 further corroborates the assignment of the Raman band to a thiol moiety. No S-H bands other than that at 2534 cm-1 were resolved in the Raman spectra, demonstrating that the protonation and deprotonation reactions over the catalyst surface are controlled by a single pKa derived from one S-H group at ∼ 5.5. The pH dependence of each spectral feature of the reaction intermediate is summarized in Figure 3c. Most notably, the Raman band intensity of S-H (green line) exhibited the same pH dependence as that of the EPR signal corresponding to distorted Mo(V) oxo species (red line). The synchronized changes in the EPR and Raman spectra indicate that the transition from isotropic to distorted Mo(V) oxo species is a consequence of protonation at the thiol ligand of the Mo(V) oxo species through a SPET mechanism (red arrows in Figure 3d). At pH values above 5.5, the S-H band disappears due to the deprotonation of the -SH moiety, leading to an increase of isotropic Mo(V) at the expense of distorted Mo(V). On the other hand, at pH values below 5.5, the Raman band assignable to Mo(V)-SH decreases despite protonation equilibrium favoring the protonated state, because increasing the driving force for proton transfer diminishes the electron transfer driving force (Figure 1b and c). This would inhibit the reductive formation of Mo(V) from Mo(VI), as evidenced by the decrease in EPR intensity at pH < 5.5. a
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dence for this rationalization, the pH-dependent distribution of nitrite reduction products was examined using oxo-free crystalline MoS2 (see SI). The absence of EPR signals of 1.85~1.98 (Figure S5a) indicates that crystalline MoS2 lacks the ability to generate Mo(V) oxo species through redox processes. As expected, inhibiting the formation of Mo(V) oxo species was associated with pHindependent FEs for NO, N2O, and NH4+ formation between pH 5 and 7, whereas NO production was accelerated at pH 4 due to the solution chemistry of HNO2 (Figure S5b).15 Further, the partial current densities for each product were also constant from pH 5 to 7 (Figure S5c) and were distinct from the volcano-type relation observed for the oxo-MoSx catalyst. Thus, the observed pH dependence of the surface intermediates (Figure 3c), together with the results of the control experiments using an oxo-free catalyst, provide strong evidence that oxo-MoSx catalysts are uniquely able to decouple electron and proton transfer during the selectivitydetermining step of denitrification. The desired product of denitrification is harmless N2, which is expected to be formed via N2O.3-4 Here, by taking advantage of the strong pH dependence of N2O production on oxo-MoSx (Figure 2), we demonstrate unambiguously that N2 is produced using 15 NO2-. Upon analyzing the gas products using GC-MS (Figure S6) both the FE and the net formation rate of 15N2 showed a clear pH dependence (Figure 4a), with a maximum at pH 5 (FE: 3.5%), as would be expected if N2 is formed via N2O. Furthermore, the FE of 15N2 was found to be significantly improved to 13.5% by increasing the concentration of 15NO2- from 0.1 to 0.5 M (Figure 4b). Not only is this the highest FE value for complete denitrification in neutral media reported to date, this is the first report which explicitly shows that decoupled proton-electron transfer can be a viable strategy to manipulate the selectivity of electrochemical reactions. The saturation at 72 hours is due to the increase of electrolyte pH from pH 5 to pH 6.88, which is inactive for N2 production, thus lending further support to the importance of pH to control the selectivity of SPET reactions. In conclusion, the sequential conversion of NO2NON2ON2 during microbial denitrification (Figure 1a) was successfully replicated in an artificial system through an SPET mechanism. By utilizing the acute pH dependence of SPET reactions, here, we have achieved the highest FE for denitrification under neutral conditions to date through simple pH optimization. The involvement of SPET during the selectivity-determining step of denitrification on oxo-MoSx is supported by electrochemical, operando EPR, and operando Raman spectroscopy measurements. The pH-independent reaction selectivity observed in crystalline MoS2 also supports the conclusion that the SPET pathway is a prerequisite for the reaction selectivity to be influenced by the pH conditions. Although the SPET pathway has previously been overshadowed by the kinetically-favorable CPET pathway,22, 25 we anticipate that utilization of SPET pathways will become an important concept for electrocatalyst design, particularly for systems with many competing reaction pathways such as CO2 reduction.
Time (h)
Figure 4. (a) Plot of the Faradaic efficiency (FE) and the reaction rate for 15N2 formation during 15NO2- reduction (0.1 M) at 0.1 V for 4 h as a function of pH. (b) Plot of FE and molar amount for 15 N2 generated after electrolysis at different time intervals at 0.1 V and an initial 15NO2- concentration of 0.5 M in pH 5 buffer solution.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental Section (PDF)
AUTHOR INFORMATION The results of the operando spectroscopy support the mathematical model25 that the optimal pH for N2O production coincides with the pKa of the reaction intermediate (distorted Mo(V) oxo species) responsible for the SPET reaction. To obtain further evi-
Corresponding Author
[email protected] [email protected] ACS Paragon Plus Environment
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[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by a JSPS Grant-in-Aid for Scientific Research (no. 26288092) to R.N and NRF-2017M3D1A1039380 to S.H.K. D.H is supported by a PhD fellowship from “International Program Associate” of RIKEN, Japan.
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