Supplementary Materials

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1 Supplementary Materials 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*, Ryuhei Nakamura* This file includes: Experimental Section Figs. S1 to S6 S1

2 Experimental Section Synthesis of oxo-mos x. The oxo-mos x was synthesized using the same procedure as described in our previous report. 1 Briefly, 3 mmol of sodium molybdate (Na 2 MoO 4, Sigma Aldrich) and 3 mmol of L-cysteine (C 3 H 7 NO 2 S, Wako) were separately dissolved in 30 ml of ultrapure water (18.2 MΩ, Millipore Ltd., USA). The two solutions were mixed under stirring for 20 min and were subsequently transferred to a Teflon-lined, 100 ml stainless steel autoclave reaction vessel. After hydrothermal treatment at 200 C for 24 h, the reaction vessel was allowed to cool down naturally to room temperature. The formed black precipitates were collected by filtration and washed with ultrapure water and ethanol alternatively for three times. The as-obtained powder products were dried under vacuum (4 Pa) at 60 C for 3 h. Crystalline MoS 2. The oxo-free crystalline MoS 2 was commercially available hexagonal MoS 2, purchased from Wako Pure Chemical Industries, Ltd., Japan (see Figure S5). Preparation of working electrodes. For the preparation of the working electrode, a diluted Nafion solution (0.13 wt%) was first prepared by dissolving 50 μl of 10 wt% Nafion solution (Sigma Aldrich) into 3 ml of H 2 O and 1 ml of ethanol solution. The synthesized powder samples (3 mg) were dispersed in 200 μl of diluted Nafion solution and the mixture was sonicated for about 1 h to generate a homogeneous ink. 10 μl of the ink suspension was transferred onto a carbon paper substrate (EC TP1 060T, TOYO Corporation) with a geometrical area of cm 2 and dried naturally at room temperature for use in cyclic voltammetry (CV) measurements. For preparing working electrode for electrolysis, 100 μl of the ink suspension was coated onto a carbon paper substrate with a geometrical area of 1.5 cm 2. S2

3 Operando EPR spectroscopy. Electron paramagnetic resonance (EPR) measurement was recorded on a Bruker EMX/Plus spectrometer equipped with a dual mode cavity (ER 4116DM). The preparation procedures of EPR samples are as follows. 3 mg of catalyst powder was added into 200 μl of a buffer solution (Buffer solution was prepared using either 0.2 M phosphate (Wako, Japan) for ph 6 and 7, or 0.2 M citric acid (Wako, Japan) for ph 4 and 5, respectively) and the mixture was sonicated for 5 min to generate a homogeneous suspension. After removing the dissolved oxygen in the suspension by N 2 purging, 8.3 μl of 0.5 M dithionite (Sigma Aldrich) solution was implemented into the suspension to reduce the catalyst under N 2 bubbling. For activating nitrite reduction, a mixture of 8.3 μl dithionite solution (0.5 M) and 10 μl of nitrite (Wako) solution (2 M) was added. After reacting for 5 min under N 2 bubbling, 20 μl of glycerol (Sigma Aldrich) was added and 200 μl of the suspension was transferred to an EPR tube and frozen in liquid nitrogen immediately. The measuring temperature was maintained at 4 K by using a liquid Helium quartz cryostat (Oxford Instruments ESR900) with a controller of temperature and gas flow (Oxford Instruments ITC503). Operando Raman spectroscopy. Raman spectra of oxo-mos x were collected on a Bruker Raman microscopy system (Senterra, Karlsruhe, Germany) using a 532 nm excitation laser. For operando measurements, the surface of an oxo-mos x electrode was immersed in a degassed buffer solution with and without 20 mm dithionite under N 2 atmosphere. The coadditions of the set of 20 spectra were recorded with integration times of 10 s. A low power of 2 mw was employed to avoid the damage induced by laser radiation. S3

4 Cyclic voltammetry (CV) measurements. Electrochemical nitrite reduction was performed in a one-compartment, three-electrode system connected to a commercial potentiostat and potential programmer (HZ 5000, Hokuto Denko, Tokyo, Japan) with a scan rate of 2 mv s 1 at room temperature. For CV measurements, an Ag/AgCl (in a saturated KCl solution) and platinum wire were employed as the reference and counter electrodes, respectively. All the potentials were converted to a reversible hydrogen electrode (RHE) by the equation: U RHE = U Ag/AgCl (KCl sat.) ph. The electrolyte consisted of either 0.2 M citric acid (Wako) or 0.2 M phosphate (Wako) as the buffering compounds at different ph range and the concentration of nitrite was 0.1 M. Prior to the CV measurements, the dissolved oxygen was removed by bubbling the solution with argon ( %) for 30 min. During the measurements, the solution was stirred at a rate of 500 rpm by using a stirring bar. Electrolysis experiments and product analysis. For electrochemical nitrite reduction under potentiostatic conditions, a two-compartment cell separated by a proton-exchange Nafion perfluorinated membrane (Nafion 117, Sigma Aldrich) was used. The working electrode was prepared as described above. For electrolysis, the electrolyte consisted of 0.1 M nitrite in either 0.2 M citric acid (at ph 4, 5) or 0.2 M phosphate buffer (at ph 6, 7) solutions; 28 ml and 20 ml of electrolyte was implemented into the working and counter electrode chambers, respectively. Before electrolysis, the solution in both chambers was bubbled for 30 min with argon ( %) to remove dissolved oxygen. After 4 h of electrolysis, the gas products NO and N 2 O were analyzed by gas chromatography (GC) equipped with thermal conductivity detector (GC 8APT, Shimadzu, Japan). Argon ( %) was used as the carrier gas. S4

5 In the experiments using isotope-labeled nitrite (Na 15 NO 2, 98 atom % 15 N, Sigma Aldrich), gas chromatography equipped with mass spectrometer detector (GCMS QP2010 Ultra, Shimadzu, Japan) was used for identification and quantification of. Helium ( %) was used as the carrier gas. Prior to electrolysis, the solution was bubbled with helium ( %) for 30 min to remove dissolved oxygen. Ammonium was detected and quantified by using commercially available colorimetric titration kits (HACH, Düsseldorf, Germany). The concentration absorbance curves were calibrated using standard ammonia hydrocarbonate solutions, which contained the same concentrations of buffer as used in the electrolysis experiments as blanks. The Faraday efficiency is calculated as follows: FE=zn/(Q/F), where z is the stoichiometric number of electrons consumed for generating one mole product, n is the molar amount of the products determined by gas chromatography, Q is the total charge passing through the electrochemical cell and F is the Faraday constant (96485 C mol 1 ). S5

6 a b 0.67 nm 100 nm (Mo=O) c g=2.05 Mo (V)-S g=2.004 sulfur radical g =1.94 Mo (V)=O g =1.88 d (Mo-O) B 1g (Mo-O) B 2g (Mo-S) E 1 2g (Mo 3 -S a ) (Mo-S) A 1g (S-S) bridge (Mo-O) B 3g g-factor Raman shift (cm -1 ) Figure S1. (a) SEM image, (b) TEM image, (c) EPR spectrum and (d) Raman spectrum of synthesized oxo-mos x catalyst powders. The synthesized oxo-mos x contained trigonal prismatic [MoS 6 ] units with a mixed valence state (Mo 6+,5+,4+ ). 1 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figure S1a, b) show that oxo-mos x is composed of ultrathin two-dimensional nanosheets with a interlayer spacing of 0.67 nm assigned to the (002) plane. The active sites of nitrite reductase feature a mononuclear Mo center coordinated by dithiolene sulfur groups, oxo ligand, and other ligands (cysteine, OH, etc.). 2 3 The existence of Mo oxo species, which is the characteristic feature of molybdopterin enzymes, was confirmed by EPR and Raman spectroscopy (Figure S1c, d). S6

7 E p vs. RHE / V j (ma cm -2 ) j p (ma cm -2 ) a mv s -1 5 mv s mv s U (V vs. RHE) b ( / mv s -1 ) log ( / mv s -1 ) Figure S2. (a) Cyclic voltammograms of oxo-mos x in presence of 0.1 M NO 2 at ph 5 at various scan rates (ν = 2, 5, 10 mv s 1 ). The valley-like reduction peaks observed for NO 2 reduction were indicated by arrows. (b) Plot of the peak potential of NO 2 reduction (E p ) vs. log(ν) (black) and the current density at peak potential of NO 2 reduction (i p ) vs. ν 1/2 (blue) with a linear fitting (Dash line: Positive Scan; Solid line: Negative Scan. R 2 > 0.99). As expected for irreversible processes, the peak potential shifts to more negative values with increasing scan rates during the negative-going sweep and more positive values during the positive-going sweep, following a linear relationship with log (ν). A linear relationship between peak current density j p and ν 1/2 applies throughout the scan rate range investigated, indicating the participation of an adsorption process according to the equation (j p = ( )α 1/2 n 3/2 D o 1/2 C o ν 1/2 ). 4 S7

8 Potential (V vs. RHE) Adding dithionite ph-4 ph-5 ph-6 ph Time (s) Figure S3. Time courses of open circuit potential of an oxo-mos x electrode immersed in anaerobic buffered solutions at different ph values. At different ph, the solution was buffered by either 0.2 M citric acid (at ph 4, 5) or 0.2 M phosphate (at ph 6, 7). After adding dithionite (20 mm) (as shown by green arrow), the open circuit potentials were dropped to ca. 50 mv (vs. RHE) at all ph values investigated. S8

9 S-H Intensity (a.u., normalized by I A1g ) Intensity (a.u.) a 180 b buffer buffer+dithionite 600 ph 5 D 2 O +Dithionite ph 5 H 2 O +Dithionite (S-D) ph 400 (S-H) Raman shift (cm -1 ) Figure S4. (a) Raman band intensity of S H vibration at 2534 cm 1 for an oxo-mos x catalyst before (black line) and after (green line) adding dithionite (20 mm) at different ph values. The intensity of S H vibration was normalized by the intensity of A 1g vibration at 406 cm 1 assignable to the out-of-plane S phonon mode in hexagonal MoS 2. (b) Raman spectra of S D(H) bonds generated by the reduction of an oxo-mos x catalyst by dithionite (20 mm) in D 2 O (red line) or H 2 O (green line) at ph 5. The isotopic shift of the S H (2534 cm 1 ) to S D (1835 cm 1 ) band is consistent with both reported and theoretical isotopic shift factor (ν S H /ν S D ) of S9

10 j partial (ma cm -2 ) Intensity (a.u.) FE (%) a 2 buffer buffer + dithionite b 80 ph 4 g=2.004 sulfur radical N 2 O NH 4 + NO 20 ph 5 ph g-factor c ph N 2 O NH 4 + NO ph Figure S5. (a) EPR spectra of crystalline MoS 2 in buffered solutions at different ph conditions collected before (black lines) and after adding dithionite reductant (red lines). No signals assigned to Mo(V) oxo species were detected for crystalline MoS 2 ; only a radical signal was detected (g = 2.003) (b) Faraday efficiency (FE) and (c) reaction rate for NO, N 2 O and NH 4 + formation with a crystalline MoS 2 electrocatalyst at 0.1 V vs. RHE for 4 h at different ph values. Inhibiting the formation of Mo(V) oxo species was associated with ph-independent FEs and reaction rates for NO, N 2 O, and NH 4 + formation between ph 5 and 7, whereas NO production was accelerated at ph 4 due to the solution chemistry of HNO 2 (2HNO 2 (aq) NO (aq) + NO 2 (aq) + H 2 O). 4 S10

11 Absolute Intensity Absolute Intensity a 6x N 2 w / 0.1M 15 NO 2 - b 2.0x10 7 w / 0.5 M 15 NO 2-5x10 6 O O 4x x N 2 15 NO 3x NO 1.0x10 7 2x10 6 1x x10 6 c e 0 8.0x x x x Retention Time x10 6 w/o isotope 15 NO 2-1.0x x x x x x10 6 w/ 0.5 M 15 NO 2-14 N 2 14 N d 8.0x x x x10 6 f x x x x x10 5 Retention Time x10 6 w/ 0.1 M 15 NO 2-15 NO 14 N x x x x x x x x x x g x10 6 O x x x Figure S6. Mass spectrometry data for, 15 NO and O generated after electrolysis at 0.1 V (vs. RHE) with an initial 15 NO 2 concentration of 0.1 M (a) and 0.5 M (b) in ph 5 buffer solution for 4 h. (c e) Mass spectrometry for generated without isotope, with 0.1 M and 0.5 M 15 NO 2. (f g) Mass spectrometry for 15 NO and O. S11

12 Reference (1) Li, Y.; Yamaguchi, A.; Yamamoto, M.; Takai, K.; Nakamura, R., J. Phys. Chem. C 2017, 121, (2) Maia, L. B.; Moura, J. J. G., Chem. Rev. 2014, 114, (3) Yang, J.; Giles, L. J.; Ruppelt, C.; Mendel, R. R.; Bittner, F.; Kirk, M. L., J. Am. Chem. Soc. 2015, 137, (4) Duca, M.; van der Klugt, B.; Koper, M. T. M., Electrochim. Acta 2012, 68, 32. (5) Deng, Y.; Ting, L. R. L.; Neo, P. H. L.; Zhang, Y. J.; Peterson, A. A.; Yeo, B. S., ACS Catal. 2016, 6, S12