Reduction of NO with Fe(II) and subsequent regeneration of Fe(II) in a fuel cell

Abstract

We report complete reduction reaction of NO into N₂ by oxidizing Fe²⁺ into Fe³⁺. To regenerate Fe²⁺ as a NO absorbent from Fe³⁺, the H₂–Fe³⁺ fuel cell supplied by Fe³⁺-containing solution at the cathode is utilized producing maximum power density of 110 mW cm⁻² at 70 °C.

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Nitrogen oxides (NO_x), produced by the combustion of fuels, are major air pollutants responsible for photochemical smog, acid rain, ozone depletion, and the greenhouse effect. Sources of NO_x emissions include both stationary commercial and industrial sources, as well as mobile sources. In other words, NO_x emissions come from all types of combustion sources, including power plants, furnaces and boilers, and automobiles.¹

Several NO_x removal approaches from fuel gas, such as selective catalytic reduction (SCR), selective non-catalytic reduction, adsorption and absorption, have been proposed for a few decades. The SCR of NO_x using reducing agents such as NH₃, urea, hydrocarbons, and H₂ is used as a de-NO_x technology. However, NH₃ and urea require storage tanks and replenishment and hydrocarbons need operating temperatures above 200 °C because of their low reactivity. The NO_x reduction utilizing H₂ as a reducing agent requires Pt-based catalysts for active reactions.²

Nitrogen oxide gas phases can be easily absorbed in the iron complex solutions such as FeSO₄ in H₂SO₄, Fe-thiochelate and Fe(II)-ethylenediaminetetraacetate (EDTA).³ Iron complexes for NO_x scrubbing has considerable advantages such as low operation cost and relatively low temperature. In particular, the biological process using Fe-EDTA for NO reduction into N₂ is regenerated as the following equation:

However, the biological process for NO reduction is relatively slow and complicated in comparison with the de-NO process by reducing NO adsorbed on FeSO₄.⁴ The NO-adsorbed FeSO₄ (eqn (2)) may be oxidized into Fe³⁺ reducing nitrogen oxides into nitrogen (eqn (3)) at low temperature (<80 °C).

2FeSO₄ + 2NO(g) ↔ 2FeSO₄–NO (2)

2FeSO₄–NO + 2Fe²⁺ + 4H⁺ ↔ 4Fe³⁺ + 2SO₄²⁻ + N₂ + 2H₂O (3)

The oxidized Fe³⁺ may be reduced into Fe²⁺ (eqn (4)) and then the reduced Fe²⁺ can reabsorb NO.

Fe³⁺ + e⁻ ↔ Fe²⁺, E° = 0.77 V (4)

It has been reported that the iron couple (Fe³⁺/Fe²⁺) is employed in a redox fuel cell as a reductant or oxidant, in which Fe³⁺ may be reduced into Fe²⁺ generating an electrochemical power. In the redox fuel cell using Fe³⁺ as a reductant, Pt catalyzes the oxidation of CH₄ by Fe³⁺ in acidic solution to generate mixtures of Fe²⁺ and CO₂. The anodic oxidation of the Fe²⁺ ions is coupled to the reduction of VO₂⁺ to VO²⁺ at the cathode through an external circuit.⁵

Herein, we report a NO reduction reaction combined with H₂–Fe³⁺ fuel cell with a non-precious cathode catalyst. The NO reduction reaction in NO-adsorbed FeSO₄ was characterized by voltammetry and UV/Vis absorbance spectroscopy. The polarization curves were obtained using a H₂–Fe³⁺ fuel cell, in which H₂ gas as a fuel was supplied at the anode and Fe³⁺-containing solution as an oxidant was flowed at the cathode.

In the NO-absorbed FeSO₄ solution, NO can be reduced into N₂ by oxidizing Fe²⁺ into Fe³⁺ (eqn (5)), which can be supplied as an oxidant at the cathode in the H₂–Fe³⁺ fuel cell (eqn (6)). As shown in Scheme 1, the Fe²⁺ reduced by the electrochemical cell can continuously absorb NO gas phase. The electrochemical cell, supplied by H₂ gas at anode (eqn (7)) and the solution containing Fe³⁺ at cathode, may be utilized as an electrochemical power source (eqn (8)).

NO reduction: 4Fe²⁺(aq) + 2NO(g) + 4H⁺(aq) ↔ 4Fe³⁺(aq) + N₂(g) + 2H₂O (5)

Cathode: 4Fe³⁺(aq) + 4e⁻ ↔ 4Fe²⁺(aq), E° = 0.77 V (6)

Anode: 2H₂(g) ↔ 4H⁺(aq) + 4e⁻, E° = 0 V (7)

Overall reaction: 2H₂(g) + 2NO(g) ↔ N₂(g) + 2H₂O (8)

Scheme 1 NO reduction process with Fe²⁺ and subsequent regeneration of Fe²⁺ using H₂–Fe³⁺ fuel cell.

The as-prepared Fe(II)SO₄ (Fe-as, blue) is saturated with NO gas to absorb NO in the solution. At 30 °C, the Fe²⁺ in the NO-absorbed solution (Fe-NO, dark brown) is oxidized into Fe³⁺. After the complete oxidation of Fe²⁺ (eqn (5)), the solution containing Fe³⁺ (Fe-N₂) becomes yellow from dark brown. At relatively low temperature, the nitrogen oxide can oxidize Fe²⁺ into Fe³⁺ simultaneously reducing NO into N₂ because nitrogen oxide is a strong oxidant. Fig. 1(a) shows cyclic voltammograms of Fe-as, Fe-NO, and Fe-N₂ using catalyst-coated glassy carbon, Ag/AgCl, and Pt wire as a working, reference, and counter electrodes, respectively. The oxidation peaks at ∼0.5 V in Fe-as and Fe-NO solutions correspond to oxidation of Fe²⁺ to Fe³⁺, whereas the reduction peaks at ∼0.4 V correspond to reduction of Fe³⁺ into Fe²⁺. Furthermore, the oxidation peak at ∼0.5 V in Fe-N₂ solution corresponds to oxidation of Fe²⁺ into Fe³⁺. However, Fe-N₂ solution exhibits a limiting reduction current density at ∼0.4 V. To further characterize reduction properties of Fe-as, Fe-NO, and Fe-N₂ solution, linear sweep voltammetric curves were obtained using a catalyst-coated rotating disc electrode as indicated in Fig. 1(b). The limiting reduction current densities of Fe³⁺ to Fe²⁺ in Fe-N₂ is much higher than those in Fe-as and Fe-NO. The high limiting current density in Fe-N₂ may be mainly due to the saturated concentration of Fe³⁺ produced by completely reducing NO into N₂.⁶

Fig. 1 Electrochemical reduction reactions of N-doped carbon catalysts in Fe-as, Fe-NO, and Fe-N₂. (a) Cyclic and (b) linear sweep voltammograms for Fe(III) reduction on the catalysts.

As already shown in the Fig. 1, the Fe-NO solution kept at 30 °C for 24 h was completely transformed into the Fe-N₂. Fig. 2 shows absorbance intensities of the Fe-NO solutions maintained at 50 and 80 °C (referred to as Fe-NO-50 and Fe-NO-80, respectively) as a function of time. The absorbance spectra intensities of Fe-NO-50 after 1, 3, 5, and 7 h gradually decrease (Fig. 2(a)). As indicated in the inset of the Fig. 2(a), the color of Fe-NO-50 is changed from dark brown to yellow by oxidizing Fe²⁺ into Fe³⁺ with complete reduction of NO into N₂. Furthermore, the absorbance intensities of Fe-NO-80 after 1, 2, 3, and 4 h abruptly decrease (Fig. 2(b)). With increasing duration, Fe-NO-80 turns yellow from dark brown by simultaneously oxidizing Fe²⁺ into Fe³⁺ with transformation of NO into N₂, as indicated in the inset of Fig. 2(b).⁷Fig. 2(c) shows the reduction rate of NO with oxidize Fe²⁺ into Fe³⁺ at 50 and 80 °C. At relatively high temperature of 80 °C, Fe²⁺ can be oxidized into Fe³⁺ with almost complete NO conversion in the range of 95–100%. Especially, the NO in the Fe-NO-80 after 4 h is mainly converted into N₂(98.43%) with a slight amount of N₂O(1.57%). However, the residual N₂O during the reduction of NO could be a serious limitation because of its very high global warming potential and ozone depleting potential. As shown in Fig. S1, ESI,† it is found that the Fe²⁺ in the N₂O-bubbled solution can be oxidized into Fe³⁺ by forming N₂. After the oxidation of Fe²⁺ (eqn (9)), the solution containing Fe³⁺ becomes yellow representing transformation of N₂O into N₂.

N₂O reduction : 2Fe²⁺(aq) + N₂O(g) + 2H⁺(aq) ↔ 2Fe³⁺(aq) + N₂(g) + H₂O (9)

Fig. 2 UV-Vis absorbance spectra of the Fe-NO solutions maintained at 50 and 80 °C as a function of time. UV-Vis absorbance spectra of (a) Fe-NO-50 and (b) Fe-NO-80. (c) Comparison of NO concentration in Fe-NO-50 and Fe-NO-80.

Fig. 3 shows characteristic curves of a H₂–Fe³⁺ fuel cell measured at 70 °C using Pt/C and N-doped carbon as anode and cathode catalyst, respectively.⁸ The H₂ gas humidified at 70 °C was supplied at anode with a flow rate of 27.8 mL min⁻¹ and Fe-NO-80 after 4 h was flowed with a flow rate of 2.0 mL min⁻¹ at cathode without backpressure regulators of the cell. In the polarization curves at 70 °C, open circuit voltage and maximum power density are 0.71 V and 110 mW cm⁻², respectively. The H₂–Fe³⁺ electrochemical cell exhibits an improved activation polarization representing highly efficient catalytic activity for the reduction of Fe³⁺ to Fe²⁺ at the cathode. At 0.3 A cm⁻², the percentage of conversion from Fe(III) to Fe(II) is about 37.3%. However, to systematically characterize H₂–Fe³⁺ electrochemical cell, the effect of concentration and flow rate on the cell performance will be considered in further work.

Fig. 3 Polarization curves of H₂–Fe³⁺ fuel cell at 70 °C with H₂ supplied at anode and Fe-NO-80 flowed at cathode.

The chemical reduction of a contamination source such as NO by oxidizing Fe²⁺ into Fe³⁺ may be also utilized in the reduction of Cl₂ as in the following equation:

2Fe²⁺ + Cl₂ ↔ 2Fe³⁺ + 2Cl⁻ (10)

The Fe²⁺ in the Cl₂-absorbed solution (Fe–Cl₂) is oxidized into Fe³⁺ by forming HCl. After the complete oxidation of Fe²⁺ (eqn (10)), the solution (Fe–HCl) containing Fe³⁺ becomes brown yellow from blue. The oxidation peaks at ∼0.90 V in Fe–Cl₂ and Fe–HCl solution correspond to oxidation of Fe²⁺ to Fe³⁺, whereas the reduction peaks at ∼0.05 V correspond to reduction of Fe³⁺ to Fe²⁺ (Fig. S2(a), ESI†). The reduction current density of Fe³⁺ to Fe²⁺ in Fe–HCl is much higher than that in Fe–Cl₂ (Fig. S2(b), ESI†), suggesting an excellent reduction reaction of Cl₂ into HCl by oxidizing Fe²⁺ into Fe³⁺.

In summary, we have demonstrated complete reduction reaction of NO into N₂ by oxidizing Fe²⁺ contained in a NO-absorbed solution into Fe³⁺. To regenerate Fe²⁺ as an absorbent of NO from Fe³⁺, the electrochemical cell, supplied by H₂ at anode and Fe³⁺-containing solution at cathode, has been combined producing electricity. The H₂–Fe³⁺ electrochemical cell at 80 °C exhibited open circuit voltage of 0.71 V and maximum power density of 110 mW cm⁻², representing highly efficient catalytic activity for the Fe³⁺ reduction reaction at the cathode.

Acknowledgements

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2011-0030335).

References

1. (a) C. Johnson, J. Henshaw and G. Mclnnes, Nature, 1992, 355, 69–72; (b) V. I. Pârvulescu, P. Grange and B. Delmon, Catal. Today, 1998, 46, 233–316; (c) R. W. Allen, O. Amram, A. J. Wheeler and M. Brauer, Atmos. Environ., 2011, 45, 369–378; (d) M. A. Gómez-García, V. Pitchon and A. Kiennemann, Environ. Int., 2005, 31, 445–467.
2. (a) G. Carja, G. Delahay, C. Signorile and B. Coq, Chem. Commun., 2004, 1404–1405; (b) P. Leung, A. Tsolakis, J. Rodríguez-Fernández and S. Golunski, Energy Environ. Sci., 2010, 3, 780–788; (c) Gongshin Qi and T. Yang Ralph, Chem. Commun., 2003, 848–849; (d) H. Jiang, L. Xing, O. Czuprat, H. Wang, S. Schirrmeister, T. Schiesteld and J. Caro, Chem. Commun., 2009, 6738–6740.
3. (a) Y. Shi, D. Littlejohn, P. B. Kettler and S.-G. Chang, Environ. Prog., 1996, 15, 153–158; (b) E. K. Pham and S.-G. Chang, Nature, 1994, 369, 139–141; (c) X.-H. Mi, L. Gao, S.-H. Zhang, L.-L. Cai and W. Li, Bioresour. Technol., 2009, 100, 2940–2944; (d) L. Gao, X.-H. Mi, Y. Zhou and W. Li, Bioresour. Technol., 2011, 102, 2605–2609; (e) S. Bosio, A. Ravella, G. B. Saracco and G. Genon, Ind. Eng. Chem. Process Des. Dev., 1985, 24, 149–152; (f) S. Uchlda, T. Kobayashl and S. Kageyama, Ind. Eng. Chem. Process Des. Dev., 1983, 22, 323–329.
4. (a) V. Blagojevic, M. J. Y. Jarvis, E. Flaim, G. K. Koyanagi, V. V. Lavrov and D. K. Bohme, Angew. Chem., Int. Ed., 2003, 42, 4923–4927; (b) N. Macleod, F. J. Williams, M. S. Tikhov and R. M. Lambert, Angew. Chem., Int. Ed., 2005, 44, 3730–3732.
5. (a) S. H. Bergens, C. B. Gorman, G. Tayha, R. Plamore and G. M. Whitesides, Science, 1994, 265, 1418–1420; (b) D. B. Weibel, R. Boulatov, A. Lee, R. Ferrigno and G. M. Whitesides, Angew. Chem., Int. Ed., 2005, 44, 5682–5686; (c) J. T. Kummer and D-G. Oei, J. Appl. Electrochem., 1985, 15, 619–629.
6. (a) G. C. C. Yang and H.-L. Lee, Water Res., 2005, 39, 884–894; (b) K. B. Holt, D. J. Caruana and E. J. Millán-Barrios, J. Am. Chem. Soc., 2009, 131, 11272–11273.
7. (a) Y. G. Zhang, H. S. Wang, G. Somesfalean, Z. Y. Wang, X. T. Lou, S. H. Wu, Z. G. Zhang and Y. K. Qin, Atmos. Environ., 2010, 44, 4266–4271; (b) M. Jaworska and Z. Stasicka, New J. Chem., 2005, 29, 604–612.
8. S.-B. Han, Y.-J. Song, Y.-W. Lee, A-R. Ko, J.-K. Oh and K.-W. Park, Chem. Commun., 2011, 47, 3496–3498.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra21750c

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