Letter Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9662-9666
pubs.acs.org/journal/ascecg
Electrochemical Ammonia Synthesis Mediated by Titanocene Dichloride in Aqueous Electrolytes under Ambient Conditions Eun-Young Jeong,†,‡ Chung-Yul Yoo,† Chan Hee Jung,† Jong Hyun Park,† Young Choon Park,§ Jong-Nam Kim,† Seong-Geun Oh,‡ Youngmin Woo,† and Hyung Chul Yoon*,† †
Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305343, South Korea Department of Chemical Engineering, Hanyang University, 222, Wangsimni-ro, Seongdong-gu, Seoul 04763, South Korea § Quantum Theory Project, University of Florida, Gainesville, Florida 32611, United States ‡
S Supporting Information *
ABSTRACT: Under ambient conditions, the catalytic and electrocatalytic syntheses of ammonia from nitrogen and various proton sources including wet tetrahydrofuran (THF) and the protic solvents methanol and water were performed using titanocene dichloride ((η5-C5H5)2TiCl2, commonly abbreviated to CP2TiCl2) in a two-electrode cell containing 1.0 M LiCl as the electrolyte. The highest rate of ammonia synthesis, 9.5 × 10−10 mol·cm−2·sec−1·M CP2TiCl2−1, was achieved at −1 V in water, whereas the highest faradaic efficiency (0.95%) was achieved at −2 V in THF. On account of its lower Gibbs free energy, density functional theory calculations suggest that the nitrogen-reduction reaction catalyzed by CP2TiCl2 in the presence of THF, methanol, or water preferably occurs via the Cp2TiClN2 intermediate rather than Cp2TiN2N2. Future strategies to improve both the rate of ammonia synthesis and its faradaic efficiency must consider ways of maximizing nitrogen selectivity to the catalytic active sites by controlling the transfer rates of protons and/or nitrogen. KEYWORDS: Nitrogen fixation, Ammonia synthesis, Electrocatalyst, Titanocene dichloride, DFT calculations
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INTRODUCTION Renewable energy production and supply rates are rising worldwide as serious attempts to combat greenhouse gas emissions caused by the depletion of fossil fuels are pursued to mitigate catastrophic climate change; concomitantly, relevant research and development are actively being explored.1 Renewable energy requires energy carriers or storage systems because of regional and temporal variabilities. Recently, the use of ammonia (17.6 wt % H2) as a renewable-energy carrier has drawn considerable research interest in terms of storing and converting renewable energy, the so-called “power-to-gas technology”.2 As a hydrogen reservoir containing 17.6 wt % H2, ammonia is a noncarbon fuel that releases only water and nitrogen during combustion. Ammonia has a higher energy density per volume (NH3 HHV: 13.6 GJ·m−3) than that of hydrogen and is much easier to store and transport than hydrogen3,4 because it is liquid below 10 bar at room temperature. Furthermore, more than 150 million tons per year of ammonia are currently consumed globally;5 thus, infrastructure to support ammonia-based technologies already exists.2 Synthesized ammonia can be applied directly as a fuel for existing energy systems such as automobiles6 and fuel cells.2,7 Ammonia is synthesized through the Haber−Bosch process using Fe, Co, and Ru catalysts, with hydrogen produced from natural gas or coal and atmospheric nitrogen as the raw materials, as indicated in eq 1.8 However, this process consumes considerable amounts of energy (>30 GJ/ton NH3) because it involves high temperatures (300−500 °C) © 2017 American Chemical Society
and high pressures (200−350 bar), demonstrates a low ammonia conversion rate of 10−15% due to thermodynamic limits,2 and is associated with the emission of large amounts of carbon dioxide (1.8 ton CO2/ton NH3) because natural gas or coal is used as the source of hydrogen.9 N2(g) + 3H 2(g) → 2NH3(g), ΔH = −92kJ mol−1
(1)
Presently, significant levels of research into the chemical or electrochemical synthesis of ammonia as a carrier of renewable energy are being conducted, and syntheses using water and atmospheric nitrogen as the raw materials at ambient pressure and low temperature ( V > Cr; Zr > Nb > Mo). Among them, Ti exhibits high activity for nitrogen.18 Judicious choice of coordinating ligand in these catalysts can lead to enhanced capacity of the metal to bind to nitrogen, thereby affecting the activity the metal toward nitrogen. These ligands include cyclopentadienyl,20 acetylacetonates,21 and phosphine complexes.19 Bayer and Schurig studied the chemical synthesis of ammonia using titanium compounds such as titanocene dichloride (Cp2TiCl2), cyclopentadienyltitanium(IV) trichloride (Cp2TiCl3), zirconocene dichloride (Cp2ZnCl2), and titanium tetrachloride (TiCl4), as well as various alkali and alkaline earth metals (Li, Na, K, Mg, Ca, La, Cs, and Na).22 These studies included tetrahydrofuran (THF) and other hydrogen-bond-donating solvents as reducing agents at room temperature and atmospheric pressure.22 This study revealed that large amounts of ammonia could be produced, especially when Cp2TiCl2 and Na (2.15 × 10−3 mol·s−1·M Cp2TiCl2−1) or Li (1.48 × 10−3 mol·s−1·M Cp2TiCl2−1) are used.22 Table 1 summarizes previously reported nitrogen-fixation results using a variety of catalytic systems.
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Experimental Approach. Figure 1(A) depicts the electrochemical synthesis of ammonia involving a proton and nitrogen in the presence of Cp2TiCl2 at the cathode. As shown, ammonia can be synthesized electrochemically from the reaction of H+ generated from the decomposition of water or other hydrogen-containing solvents at the anode. Nitrogen is activated by Cp2TiCl2 at the cathode. Figure 1(B) shows a schematic diagram of the two-electrode cell (ECC-DEMS, ELCELL GmbH) used to perform the key ammonia synthesis reactions at the cathode. We note that similar experiments using ECC-DEMS to measure gas evolution during electrochemical reactions in a Li battery have been reported.23 In the present study, the protic solvents methanol and water and the aprotic solvent THF were used to determine the effect solvent on ammonia synthesis. As shown in Figure 1(B), nitrogen is supplied through the gas inlet in (a), (d) is the glass fiber that serves as the separator (fixed with Pt/C), and synthesized ammonia exits through (e). A photographic image of the cell is shown in Figure S1. When aprotic THF was used, wet nitrogen, generated by bubbling nitrogen through water, was supplied through the gas inlet. Mixtures were stirred at room temperature for 1 h to completely dissolve 1.0 M LiCl and 0.03 M Cp2TiCl2 in the solvent. After soaking glass wool (0.5 cm3), which was used as the separator, in the solvent for 3 min, Pt/C electrodes (2.54 cm2) were fixed to each side. Ammonia was synthesized for 1 h in each reaction at controlled potentials of 0, − 1, − 1.5, and −2 V using an AutoLab potentiostat (Autolab PGSTAT302N, Metrohm Autolab B.V, Filderstadt, Germany) at a flow rate of 10 cm3 N2/min and was collected in 30 mL of aqueous sulfuric acid (0.001 M). Nitrogen flow was regulated by a mass-flow controller (Brooks, Model 5850E). Prior to each experiment, the cell was washed with 0.001 M H2SO4 and distilled water and dried in an oven at 80 °C in order to remove any remaining ammonia. All experiments were repeated at least three times. Prior to the quantitative analysis of the products, a calibration curve for the indophenol blue method, the details of which have been reported previously,24 was generated through the use of 0.1−1.5 mg of a NH4+/ L reagent, which was prepared by dissolving ammonium sulfate in a 0.001 M sulfuric acid solution. After reacting the synthesized ammonia following the indophenol blue method, it was quantitatively analyzed using UV−vis spectrophotometry (UV-1800 spectrometer, Shimadzu, Japan).
Table 1. Catalytic Nitrogen Fixation under Ambient Conditions (25 °C, 1 atm)18,22 System
Molar ratio of reagents
Cp2TiCl2 + Li Cp2TiCl2 + Na Cp2TiCl2 + K Cp2TiCl2 + Mg Cp2TiCl2 + Ca Cp2TiCl2 + La Cp2TiCl2 + Ce CpTiCl3 + Na CpTiCl3 + Mg TiCl4 Cp2ZnCl2 + Li
excess Li 1/4 excess K 1/2 excess Ca excess La excess Ce 1/4 1/2 excess Na excess Li
Ammonia formation rate (mol·s−1·M Ti−1) 1.48 2.15 1.24 2.65 2.52 4.42 1.11 5.73 1.17 1.61 6.96
× × × × × × × × × × ×
EXPERIMENTAL SECTION
10−3 10−3 10−4 10−4 10−4 10−4 10−3 10−3 10−3 10−4 10−4
Figure 1. (A) Electrochemical ammonia synthesis reaction involving a proton and nitrogen. (B) Schematic diagram of the ammonia synthesis cell: (a) gas inlet, (b) gasket, (c) plunger, (d) Pt/C·glass-fiber separator·Pt/C, and (e) gas outlet. 9663
DOI: 10.1021/acssuschemeng.7b02908 ACS Sustainable Chem. Eng. 2017, 5, 9662−9666
Letter
ACS Sustainable Chemistry & Engineering
Figure 2. (a)−(c) Cyclic voltammograms of Cp2TiCl2 with LiCl in (a) THF, (b) methanol, and (c) H2O. Working electrode: glassy carbon (Ø 3 mm); counter electrode: Pt wire; reference electrode: Pt wire; and scan rate: 100 mV/s. (d) Rate of ammonia synthesis and faradaic efficiency with different solvents from 0 to −2 V vs RHE at room temperature and atmospheric pressure. DFT Calculations. All density functional theory (DFT) calculations in this study were performed using the Gaussian 09 software package utilizing hardware at the Korea Institute of Science and Technology Information.25 Full geometry optimizations for all molecules were performed without symmetry restrictions using the unrestricted B3LYP functional in combination with the LANL2DZ basis set for Ti and the 6-31G basis set for all other atoms. The polarizable continuum model was used to include solvent effects (i.e., THF and MeOH) on the molecular structures and their corresponding energies. Analytical frequency calculations were also performed on all converged structures in order to obtain Gibbs free energies.
where ṅ denotes the amount of electrochemical ammonia synthesized, I denotes the current, and F is the Faraday constant. In the absence of Cp2TiCl2, no chemical or electrochemical ammonia syntheses were observed across the applied voltage range under ambient conditions in THF, methanol, or H2O with 1.0 M LiCl. These results show that Li ions alone were unable to fix nitrogen in the given voltage range under these conditions. When 0.03 M Cp2TiCl2 was used, however, the rate of ammonia synthesis in THF was 1.48 × 10−11, 6.43 × 10−11, 8.10 × 10−11, and 2.37 × 10−10 mol·s−1· cm−2·M Cp2TiCl2−1 at 0, − 1, − 1.5, and −2 V, respectively. The highest ammonia synthesis rate was observed at −2 V. No ammonia synthesis was observed in methanol at 0 V, but rates of 7.07 × 10−10, 3.47 × 10−10, and 3.21 × 10−10 mol·s−1·cm−2·M Cp2TiCl2−1 were observed at −1, − 1.5, and −2 V, respectively. The highest ammonia synthesis rate was observed at −1 V in H2O, with values of 1.94 × 10−10, 9.50 × 10−10, 7.07 × 10−10, and 6.17 × 10−10 mol·s−1·cm−2·M Cp2TiCl2−1 at 0, − 1, − 1.5, and −2 V, respectively. The faradaic efficiencies by solvent were 0.48% at −1 V, 0.57% at −1.5 V, and 0.95% at −2 V in THF; 0.03% at −1 V, 0.04% at −1.5 V, and 0.04% at −2 V in methanol; and 0.23% at −1 V, 0.04% at −1.5 V, and 0.02% at −2 V in H2O. These faradaic efficiencies are similar to those reported previously (
