Article pubs.acs.org/JPCC
Molybdenum Sulfide: A Bioinspired Electrocatalyst for Dissimilatory Ammonia Synthesis with Geoelectrical Current Yamei Li,† Akira Yamaguchi,†,‡ Masahiro Yamamoto,§ Ken Takai,§ and Ryuhei Nakamura*,† †
Biofunctional Catalyst Research Team, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 273-0061, Japan S Supporting Information *
ABSTRACT: Abiotic reduction of nitrate/nitrite to ammonia in ancient ocean environments is essential for prebiotic chemical evolution and the early evolution of cellular metabolisms. Although Fe-bearing minerals have been extensively studied as reductants, the incompatibility of these reactions with the paleo-oceanic chemical conditions would have contributed only to low conversion rates. A recent model of prebiotic synthesis driven by geoelectricity suggests that less-abundant transition metals might have played a role in ammonia synthesis. Herein, we report that the electrochemical reduction of nitrate/nitrite to ammonia is effectively catalyzed by molybdenum sulfide over a wide pH range (pH 3−11). The pH dependence of the onset potential of the denitrification current indicates that this superior activity is due to the ability to induce concerted proton−electron transfer (CPET), contributing to a turnover frequency comparable to those of the extant Modependent nitrate reductases. In a manner similar to biological dissimilatory denitrification, molybdenum sulfide catalyzes nitrite reduction to ammonia through NO and N2O intermediates, as revealed by online differential electrochemical mass spectroscopy. These findings demonstrate that molybdenum sulfide functions as a new family of bioinspired electrochemical denitrification catalysts and suggest that molybdenum, the indispensable element in extant denitrification enzymes, likely played an essential role in prebiotic ammonia synthesis with the continuous supply of geoelectrical current.
1. INTRODUCTION The conversion of the abundant, but inert, dinitrogen (N2) molecule into reduced nitrogen compounds, such as ammonia (NH3), was essential for chemical evolution in the prebiotic ocean and the early evolution of energy metabolism pathways and cellular functions on the ancient Earth.1−3 Reduced nitrogen compounds participate in the synthesis of amides, amino acids, and nucleotides and are therefore postulated as prerequisites for the “RNA world”4−6 and other scenarios for the origin of life.7−9 However, it is widely accepted that the early atmosphere in the Hadean Eon and the early Archean Eon did not contain even a tiny amount of ammonia (NH3)10,11 and that the electrical discharge in the early atmosphere, which consisted of N2 as one of the most abundant atmospheric gas components, would have produced nitric oxide (NO).11,12 The generated NO was likely converted into nitric (HNO3) or nitrous (HNO2) acid by photochemical and aqueous-phase reactions and eventually deposited in the ocean through rain.11−13 Therefore, in addition to electrical discharge events, prebiotic ammonia synthesis is considered to require geochemical processes in the early ocean in which nitrite and nitrate were reduced by abundant chemical reductants. Fe-bearing compounds are speculated to be the most plausible chemical reductants for the ammonia synthesis in © XXXX American Chemical Society
the early ocean. This hypothetical pathway has been extensively examined using Fe-bearing compounds, particularly Fe(II) ions,2,14 mackinawite (FeS),1,15 pyrite (FeS2),3,16 and green rust [FeII4FeIII2(OH)12SO4·yH2O].17,18 For example, the reaction of Fe(II) ions with nitrite under slightly alkaline conditions (pH > 819 or 7.32) yields ammonia. In addition, although Fe(II) ions cannot reduce nitrate, the addition of a trace amount of Cu(II) ions enables nitrate to be converted to ammonia at pH 8.19 Ammonia synthesis from nitrate has also been demonstrated with green rust and pyrite in anion-free aqueous solutions at pH 8−8.25.17,18 Notably, however, the reduction potentials of Fe(II) ions and green rust are sharply decreased under neutral or slightly acidic pH conditions.2 Moreover, the presence of chloride, phosphate, sulfate, and dissolved CO2 was found to suppress the nitrate reduction reaction through competing adsorption and to inhibit the formation of ammonia.1 As interpretations from the geological and geochemical records in the present Earth indicate that the early ocean was relatively acidic (pH ∼5.5) as a result of saturation with CO2,10,20,21 the combined effects of inhibiting anions, dissolved CO2, and low Received: August 18, 2016 Revised: November 14, 2016 Published: November 15, 2016 A
DOI: 10.1021/acs.jpcc.6b08343 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
synthesis of MoS2, 3 mmol of sodium molybdate (Na2MoO4, Sigma-Aldrich) and 3 mmol of L-cysteine (C3H7NO2S, Wako) were separately dissolved in 30 mL of deionized water. The two solutions were mixed under stirring for 20 min and were subsequently transferred to a Teflon-lined, 100 mL autoclave reaction vessel. After hydrothermal treatment at 200 °C for 24 h, the reaction vessel was allowed to cool naturally to room temperature. The formed black precipitates were collected by filtration and washed three times with deionized water followed by ethanol. The as-obtained powder products were dried under vacuum for 3 h at 60 °C. For the synthesis of greigite (Fe3S4), 3 mmol of iron(II) sulfate heptahydrate (FeSO4·7H2O, SigmaAldrich) was used in place of the Mo source. For the synthesis of greigite with the incorporation of 5 atom % Mo, sodium molybdate (Na2MoO4) with a nominal atomic ratio of 5 atom % was added to the above precursor suspension, which was then subjected to the same hydrothermal treatment. All chemicals were of the highest grade available and were used as received from the manufacturer. 2.2. Material Characterization. X-ray diffraction (XRD) analyses were conducted on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.54059 Å) using a voltage and current of 45 kV and 200 mA, respectively. Unless stated otherwise, all of the samples were measured at a scanning rate of 3°/min in steps of 0.02°. The microscopic morphologies of the samples were assessed by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Tokyo, Japan) at an acceleration voltage of 10 kV. High-resolution transmittance electron microscopy (HRTEM) images were collected using a JEM-2100F/SP microscope (JEOL, Ltd., Tokyo, Japan). The Raman spectra of the catalyst film before and after reactions were collected using a Raman spectrometer (Senterra, Bruker, Karlsruhe, Germany). X-ray photoelectron spectroscopy (XPS) spectra were collected using a photoelectron spectrometer (AXIS Ultra DLD, Kratos Analytical, Ltd., Kyoto, Japan) with Al Kα radiation. 2.3. Electrode Preparation. For the preparation of the working electrode, a dilute Nafion solution (0.123 wt %) was first prepared using 3 mL of H2O, 1 mL of ethanol, and 50 μL of 10 wt % Nafion solution (Sigma-Aldrich). The as-obtained powder samples (1.5 mg) were dispersed in 202.5 μL of dilute Nafion solution and then sonicated for 1 h to form a homogeneous ink. The ink suspension (5 μL) was then coated onto a freshly cleaned carbon paper or a glassy carbon surface (area = 0.07 cm2), after which the electrode was dried at room temperature under vacuum for use in cyclic voltammetry (CV) measurements. For CV measurements over a wide pH range, a porous carbon paper substrate was used to provide a more mechanically stable working electrode. The intact carbon paper and glassy carbon were confirmed to have negligible nitrate/ nitrite reduction activities. 2.4. Cyclic Voltammetry (CV) and Online Differential Electrochemical Mass Spectroscopy (DEMS) Measurements. A one-compartment, three-electrode system was employed for the electrochemical nitrite/nitrate reduction experiments. The electrolyte consisted of a 0.2 M Na2SO4 aqueous solution buffered with 0.1 M sodium phosphate (mixture of 0.04 M NaH2PO4 and 0.06 M Na2HPO4) and was adjusted to pH ∼7 using dilute sulfuric acid and sodium hydroxide solutions. NaNO2 and NaNO3 were added to the electrolyte at a final concentration of 0.1 M. Electrochemical measurements were conducted using a commercial potentiostat and potential programmer (HZ-5000, Hokuto-Denko, Tokyo,
pH might have limited ammonia synthesis using Fe-based compounds in the early ocean. This implies that another mechanism was responsible for enhancing the nitrate and nitrite reduction kinetics. Herein, we report that geoelectrical currents driven by electrochemical gradients across deep-sea hydrothermal vent chimneys and deposits22−27 could serve as an alternative to Febearing minerals for the reduction of nitrate and nitrite to ammonia. A high abundance of Fe-bearing minerals would likely contribute to their significant role as reductants of nitrate and nitrite because the continuous synthesis of ammonia is considered to require a constant supply of reductive chemicals in the environment. However, a newly established model of prebiotic chemical synthesis triggered by deep-sea hydrothermal geoelectricity22−27 circumvents this requirement. In this model, an electrochemical potential gradient that exists between the H2- and H2S-rich reducing hydrothermal fluid and oxidative seawater serves as the driving force for triggering redox chemical conversions. This model is supported not only by the finding of on-site electricity generation in a modern hydrothermal vent field,26 but also by the unique electrochemical properties of naturally occurring iron sulfide-bearing chimney minerals, such as an excellent conductivity,24 a thermoelectrical conversion ability,28 and an ability to oxidize hydrogen sulfide.26 Geoelectrical currents can also be triggered by the self-potential that is established in all water columns with natural potential gradients.29 Under conditions in which a geoelectrical current is sustained by a disequilibrium of pH, redox, and temperature between the hydrothermal fluid and the surrounding seawater, the less abundant but indispensable transition-metal elements Mo, W, Ni, and so on, which are extensively utilized in extant enzymatic reactions, likely played an essential role in prebiotic chemical evolution. Because the Mo−S2 (pterin) cofactor-containing nitrate reductase is the dominant component of biological dissimilatory denitrification reactions30 and because members of the Mo−S2 (pterin) xanthine oxidase family can also catalyze nitrite reduction,31 we hypothesized that sulfidic forms of Mo ions can function as catalysts for ammonia synthesis with the aid of geoelectrical current. In addition, several studies have suggested that early oceanic sediments and sulfide deposits were rich in sulfidic forms of Mo ions32,33 and that these thiomolybdate ions could be stabilized by forming Fe2+−[MoS4]2− complexes in sulfidic environments.34 Accordingly, in the present study, we examined the potential of Mo-bearing sulfide minerals to induce ammonia formation from both nitrite and nitrate under CO2-saturated conditions to mimic the early ocean. Our results show that molybdenum-bearing sulfides represent a new family of bioinspired denitrification electrocatalysts that has been explored as a hydrogen evolution catalyst inspired by Fe−S− Mo-bearing nitrogenase.35−37 We also demonstrate the ability of molybdenum sulfide to regulate proton-coupled electron transfer (PCET) reactions under geologically relevant conditions, as PCET is a key mechanism for efficient and stable redox chemical transformations mediated by nearly all extant enzymes, including Mo-dependent nitrate and nitrite reductases.31,38
2. EXPERIMENTAL METHODS 2.1. Material Synthesis. All materials were synthesized using hydrothermal methods. MoS2 and greigite (Fe3S4) powders were prepared in aqueous solution using standard hydrothermal techniques reported in the literature.39 For the B
DOI: 10.1021/acs.jpcc.6b08343 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Phase and microstructure of as-synthesized molybdenum disulfide (MoS2) powders. (a) XRD pattern of hydrothermally synthesized MoS2 powders. XRD peaks of hexagonal MoS2 (standard JCPDS card no. 37-1492) shown as a reference. (b) Microstructure analysis by high-resolution TEM. The edge region shows that each nanosheet was composed of several layers. The 3D lattice structure is shown in the inset, with the basal and edge planes indicated. (c) XPS spectra of S 2p and Mo 3d orbitals. The full-range spectra were deconvoluted using a Lorentzian−Gaussian function with reference to the literature.41 A bridging disulfide coordinated Mo−S2 edge site is indicated in the inset. (d) Raman spectrum of the assynthesized MoS2 powders, with two vibrational modes corresponding to E2g1 and A1g symmetries shown in the insets.
solution. The pD value was corrected according to pD = measured pH + 0.4. The [D]/[H] ratio was sufficiently high (111:0.13) to ensure the correct mass numbers of the examined volatile species, which were detected simultaneously during the catalytic reaction. The specific mass/charge (m/z) values for 18 species {H2, H2O, N2, O2, CO2, N2H4, NH3, NO, N2O, NO2, NH2OH, CH4, CO, CH3OH, HCOOH, urea [CO(NH2)2], acetonitrile (CH3−CN), and Ar} were monitored with a collection time of 200 ms per signal. During CV scanning, the Ar or CO2 gas was continuously flowed into the headspace to avoid possible air contamination. 2.5. Electrolysis Experiments. For bulk electrolysis, a twocompartment, three-electrode system was applied for electrochemical nitrite/nitrate reduction under potentiostatic conditions. The cathodic and anodic chambers were separated by a proton-exchangeable Nafion membrane. For the preparation of the working electrode, 1.5 mg of the as-obtained powder samples was dispersed in 202.5 μL of dilute Nafion solution (0.123 wt %) and then sonicated for 1 h to form a homogeneous ink. The prepared ink suspension (100 μL) was coated onto a clean carbon paper (diameter =14 mm) substrate, and the as-coated electrode was allowed to dry naturally at room temperature under vacuum. For electrolysis, a Pt wire and a Ag/AgCl (KCl-saturated) electrode were utilized as the counter and reference electrodes, respectively. The electrolyte consisted of a 0.1 M Na2SO4 aqueous solution buffered with 0.05 M sodium phosphate to maintain the pH at
Japan). For CV measurements, Pt wire and a Ag/AgCl (KClsaturated) electrode were utilized as counter and reference electrodes, respectively. Potential values versus the Ag/AgCl (KCl-saturated) standard electrode were converted into reversible hydrogen electrode (RHE) values using the equation: potential (V vs RHE) = potential [V vs Ag/AgCl (KClsaturated)] + 0.197 + 0.05916 × pH. All measurements were conducted at 30 °C, which was maintained using a watercirculating system. Before the reaction, Ar or CO2 gas was bubbled into the electrolyte solution for 15 min, and the same gas was flowed over the headspace of the electrochemical chamber for a 30-min period. After bubbling, the solution pH was 7 (Ar-saturated) or 6.34 (CO2-saturated). CV measurements were performed by scanning from the resting potential at a scanning rate of 2 mV/s. The CV scans were conducted three times to examine the repeatability of the electrochemical properties. For detection of the evolved gas products, a mass spectrometer tube detector was inserted into the electrolyte using a water-resistant Teflon membrane as a separator. The detection surface was adjusted to a depth of less than ∼1 mm below the catalyst surface. In these experiments, isotope-labeled nitrite (Na15NO2), nitrate (Na15NO3), and water (D2O) were used as the substrates and the solvent, respectively, to differentiate different products generated during nitrite reduction, and the pH of the electrolyte was adjusted to 7 using 0.04 M NaOH and 0.03 M H3PO4 prepared in D2O C
DOI: 10.1021/acs.jpcc.6b08343 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C 7 or 6.4 after saturation with Ar and CO2, respectively. NaNO2 or NaNO3 were added to the working electrode chamber to give a final concentration of 0.05 M. Before the reaction, either Ar or CO2 gas was bubbled into the electrolyte for 30 min, and the headspace of the chamber was then purged with the same gas over a 30-min period. The potential was held at a fixed value between ∼0 and −0.2 V vs RHE for 2−4 h. All measurements were conducted at 30 °C with a watercirculation temperature-control system. After electrolysis, analysis of the reduction products was performed using a gas chromatography (GC) system (Shimadzu, Kyoto, Japan) equipped with flame ionization and thermal conductivity detectors for the detection of hydrogen, carbonaceous gases, and organics. In addition, ammonia and nitrite were detected and quantified using commercially available colorimetric titration kits (HACH, Düsseldorf, Germany). The concentration−absorbance curves were calibrated using standard ammonia hydrocarbonate and sodium nitrite solutions, which contained the same concentrations of sodium sulfate and phosphate as used in the electrolysis experiments, as blanks.
Figure 2. Redox transition of as-synthesized molybdenum disulfide. The potential dependence of the current density for the MoS2 electrode was measured in an Ar-saturated, phosphate-buffered aqueous solution (pH 7, scanning rate = 2 mV/s). Two reversible redox couples were observed and tentatively assigned to MoVI/V and MoV/IV valence-state transitions. For clarity, the two redox transition peaks were expanded by subtracting the charging current (gray lines).
3. RESULTS AND DISCUSSION 3.1. Characterization and Redox Transitions of Molybdenum Disulfide. MoS2 powder synthesized by a hydrothermal method using sodium molybdate and L-cysteine as the sources cof Mo and sulfur, respectively, exhibited a hexagonal lattice structure with a hierarchically assembled nanosheet morphology resembling the two-dimensional habit that was found in naturally occurring molybdenite minerals40 (XRD and TEM, Figure 1a,b; SEM image in Figure S1). The two-dimensional nanosheet structure imparted an edge [Mo− S] with a cluster-like nature that was attributable to weak interlayer interactions. XPS inspection revealed that the MoS2 was mainly composed of Mo4+, as was determined from the split peak at 229.0 and 232.2 eV (Mo 3d5/2 and Mo 3d3/2), although small amounts of Mo5+ (230.3 and 233.5 eV) and Mo6+ (233.0 and 236.1 eV) were also detected (Figure 1c).41 The deconvoluted spectra in the S 2p region indicate the existence of bridging S22− species (163.4 eV, S 2p3/2) that were mainly present on the (1̅010) edge plane of the single layers of MoS2,42,43 whereas the intense peak at 161.9 eV was assigned to lattice-bridging S2−. Raman spectral analysis of the synthesized MoS2 powder revealed the presence of two peaks at 380 and 404 cm−1, corresponding to the E2g1 and A1g vibrational modes, respectively, representing the in-plane Mo−S phonon mode and the out-of-plane S phonon mode, respectively (Figure 1d).44 The ratio of the edge and basal plane is typically reflected by the integral intensity ratio between the E2g1 and A1g vibration bands (E2g1/A1g). For MoS2, ratio values of 0.4 and 0.68 were previously reported for the edge and basal planes, respectively.44 In the present analyses, the E2g1/A1g ratio was estimated to be 0.47 ± 0.07, revealing that the edge portion in the as-formed MoS2 was relatively high owing to the nanoscale particle size. The cyclic voltammetry (CV) analysis of as-synthesized MoS2 at pH 7 showed that two well-resolved, quasireversible redox waves were present at the midpoint potentials of 0.42 and 0.16 V (vs RHE) (Figure 2). The number of electrons (n) involved in the redox reaction was estimated from the halfwidth potential of the redox wave, ΔEp/2, using the equation ΔEp/2 = 3.53RT/nF (= 92.2/n at 30 °C).45 The ΔEp/2 value was determined to be ∼130 mV, corresponding to an n value of 0.7,
implying that MoS2 was able to carry out two sequential and quasireversible one-electron transfer steps; the deviation from unity might be due to the electrostatic interactions between centers with different valence states. Valence changes on Mo centers among the 6+, 5+, and 4+ states were also observed in amorphous molybdenum sulfide by XPS46 and extended X-ray absorption fine structure (EXAFS)47 studies, and a change in oxidation states from S22− to S2− in sulfur ligands was also proposed to occur.47,48 Notably, the sequential redox transition mediated by MoS2 was similar to that reported for Mo−S2 (pterin)-based nitrate reductase enzymes,49,50 where valence changes corresponding to Mo6+/Mo5+ and Mo5+/Mo4+ were detected by means of EPR spectroscopy.50 The number of redox active sites in MoS2 was estimated to correspond to 2.62 nmol based on an analysis of the Coulomb numbers of the redox waves. This value is equivalent to approximately 0.57% of the total number of Mo atoms deposited on the electrode (see Supporting Information). The Mo atomic ratio on the edge plane {1̅010} was estimated using nanosheet geometric calculations to be in the range from ∼0.56 to 1.1 atom % (see Supporting Information), which was close to that determined for the electroactive Mo sites but much lower than that estimated for the basal plane {0001} (7.7 atom %). The findings are consistent with a speculation that the {1̅010} edge planes of MoS2 with unsaturated sulfur coordination are responsible for the observed sequential redox transition, whereas the basal plane is well-known to be inactive.44,51 Although further spectroscopic study is required to clarify the exact chemical conversion during the redox transition, the results imply the possibility that trace amounts of [Mo−S] clusters, rather than bulk MoS2, function as catalytic centers for dissimilatory ammonia synthesis, in which the bridging or terminal S22− ligand of the edge cluster might provide coordination sites for binding of nitrate or nitrite.47,48 3.2. Electrochemical Ammonia Synthesis from Nitrite. To investigate whether dissimilatory ammonia synthesis is mediated by redox-active MoS2 under a supply of geoelectrical current, the electrocatalytic reduction of nitrite by MoS2 was investigated in an Ar-saturated aqueous solution containing sulfate and phosphate ions. Figure 3a shows potential- (U-) dependent current density (j) curves for the synthesized MoS2 electrocatalyst, which consisted of MoS2 nanoparticles D
DOI: 10.1021/acs.jpcc.6b08343 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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NO (m/z = 31) was formed at a potential of 0.6 V and then decreased with a corresponding increase in 15N2O (m/z = 46) (Figure 4a). When the potential was scanned to more negative
Figure 3. Specificity of Mo−S over Fe−S for nitrite reduction. Potential dependences of the current density for (a) molybdenum disulfide (MoS2), (b) greigite (Fe3S4), and (c) 5 atom % Mo-doped greigite in the presence (red lines) and absence (black lines) of nitrite (0.1 M) in neutral media (pH 7) at a scanning rate of 2 mV/s. The same loading amount of the powders was used (0.523 mg/cm2).
Figure 4. Evolution of nitrogen oxides and ammonia during nitrite reduction. Potential-dependent evolution of volatile products from (a) MoS2 and (b) Fe3S4 as catalysts at the same loading amount (0.523 mg/cm2) in Ar-saturated neutral (pH 7) solution in the presence of nitrite (15NO2−, 0.1 M). j−U curves were collected simultaneously with mass spectroscopic measurements, at a scanning rate of 2 mV/s with the scanning direction denoted by arrows. D2, 15ND3, 15N2O, and 15 NO were detected by monitoring the specific mass/charge (m/z) values shown in parentheses. The units of the scale bars are mass ionic current (pA). During the measurements, isotope-labeled Na15NO2 and D2O were utilized.
deposited onto an inactive carbon paper substrate. All potential values reported here were converted to values versus a reversible hydrogen electrode (RHE). The measurements were performed in electrochemical cells operated at pH 7 with Ar-saturated aqueous solution supplemented with and without nitrite. The iron sulfide mineral greigite (Fe3S4) was used as a reference electrocatalyst. In the absence of nitrite, MoS2 generated a cathodic current that was assigned to the reduction of protons to form H2 at potentials more negative than −0.18 V vs RHE (Figure 3a, black line). Upon the addition of nitrite to the electrolyte, the cathodic current was greatly enhanced, and the onset potential shifted from −0.18 to 0.07 V (Figure 3a, red line). Consistent with the previous observations that iron sulfide minerals impair the catalytic potential for nitrite reduction in the presence of phosphate and sulfate,1 addition of nitrite to the system containing Fe3S4 produced a much lower current (∼2.1% of the current value for MoS2 at −0.1 V) (Figure 3b). It should be noted that, although pristine Fe3S4 showed a low activity to reduce nitrite, the doping of a small amount of Mo ions into the lattice of Fe3S4 (5 atom %) enabled it to facilitate the reactions (Figure 3c). Considering the fact that the density of active redox sites in MoS2 was estimated to be 0.57 mol/(mol of catalyst), the observed effect of Mo doping further affirmed the importance of a redox-active [Mo−S] cluster in the catalytic turnover and thus implies that even a trace amount of precipitated [Mo−S] cluster in the sulfidic environments could overcome the inability of iron sulfide minerals to promote denitrification in the early ocean and that a local structure with a high density of active centers will support much higher activity. To monitor the reaction products generated during the electrocatalysis of MoS2, the online differential electrochemical mass spectroscopy (DEMS) method was used. As the mass spectrometer sampling port was placed in close vicinity to the electrode surface (
