Reduction of NO with Fe(II) and subsequent regeneration of Fe(II) in afuel cell{

Sang-Boem Han, Young-Woo Lee, Si-Jin Kim, Do-Young Kim, Je-Suk Moon, Ah-Reum Park and

Kyung-Won Park*

Received 9th August 2012, Accepted 19th October 2012

DOI: 10.1039/c2ra21750c

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 cm22 at 70 uC.

Nitrogen oxides (NOx), produced by the combustion of fuels, are

major air pollutants responsible for photochemical smog, acid rain,

ozone depletion, and the greenhouse effect. Sources of NOx

emissions include both stationary commercial and industrial sources,

as well as mobile sources. In other words, NOx emissions come from

all types of combustion sources, including power plants, furnaces and

boilers, and automobiles.1

Several NOx removal approaches from fuel gas, such as selective

catalytic reduction (SCR), selective non-catalytic reduction, adsorp-

tion and absorption, have been proposed for a few decades. The

SCR of NOx using reducing agents such as NH3, urea, hydro-

carbons, and H2 is used as a de-NOx technology. However, NH3 and

urea require storage tanks and replenishment and hydrocarbons need

operating temperatures above 200 uC because of their low reactivity.

The NOx 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

NOx 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:

Fe(II)complex�NOzelectron donor

/CC?microorganisms

2Fe(II)complexzN2

(1)

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 uC).

2FeSO4 + 2NO(g) « 2FeSO4–NO (2)

2FeSO4–NO + 2Fe2+ + 4H+

« 4Fe3+ + 2SO422 + N2 + 2H2O (3)

The oxidized Fe3+ may be reduced into Fe2+ (eqn (4)) and then

the reduced Fe2+ can reabsorb NO.

Fe3+ + e2 « Fe2+, Eu = 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

Soongsil University, Chemical and Enviromental Engineering, 511 Sangdo-dong, Dongjak-gu, Seoul 156-743, Republic of Korea{ Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra21750c

Scheme 1 NO reduction process with Fe2+ and subsequent regeneration of

Fe2+ using H2–Fe3+ fuel cell.

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12628 | RSC Adv., 2012, 2, 12628–12630 This journal is � The Royal Society of Chemistry 2012

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Scheme 1, the Fe2+ reduced by the electrochemical cell can conti-

nuously 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) + 4e2 « 4Fe2+(aq), Eu = 0.77 V (6)

Anode: 2H2(g) « 4H+(aq) + 4e2, Eu = 0 V (7)

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

The as-prepared Fe(II)SO4 (Fe-as, blue) is saturated with NO gas

to absorb NO in the solution. At 30 uC, 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 y0.5 V in Fe-as and Fe-NO solutions

correspond to oxidation of Fe2+ to Fe3+, whereas the reduction

peaks at y0.4 V correspond to reduction of Fe3+ into Fe2+.

Furthermore, the oxidation peak at y0.5 V in Fe-N2 solution corres-

ponds to oxidation of Fe2+ into Fe3+. However, Fe-N2 solution

exhibits a limiting reduction current density at y0.4 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

As already shown in the Fig. 1, the Fe-NO solution kept at 30 uCfor 24 h was completely transformed into the Fe-N2. Fig. 2 shows

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.

Fig. 2 UV-Vis absorbance spectra of the Fe-NO solutions maintained at 50

and 80 uC 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.

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 12628–12630 | 12629

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absorbance intensities of the Fe-NO solutions maintained at 50 and

80 uC (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).7 Fig. 2(c) shows the reduction rate

of NO with oxidize Fe2+ into Fe3+ at 50 and 80 uC. At relatively high

temperature of 80 uC, 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. 3 shows characteristic curves of a H2–Fe3+ fuel cell measured

at 70 uC using Pt/C and N-doped carbon as anode and cathode

catalyst, respectively.8 The H2 gas humidified at 70 uC was supplied

at anode with a flow rate of 27.8 mL min21 and Fe-NO-80 after 4 h

was flowed with a flow rate of 2.0 mL min21 at cathode without

backpressure regulators of the cell. In the polarization curves at

70 uC, open circuit voltage and maximum power density are 0.71 V

and 110 mW cm22, 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 cm22, 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.

The chemical reduction of a contamination source such as NO by

oxidizing Fe2+ into Fe3+ may be also utilized in the reduction of Cl2as in the following equation:

2Fe2+ + Cl2 « 2Fe3+ + 2Cl2 (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 y0.90 V in Fe–Cl2 and

Fe–HCl solution correspond to oxidation of Fe2+ to Fe3+,

whereas the reduction peaks at y0.05 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 uC exhibited open

circuit voltage of 0.71 V and maximum power density of 110 mW

cm22, 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. Parvulescu, 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. Gomez-Garcıa, 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.Rodrıguez-FernandezandS.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. Zhouand 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. Lavrovand 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. Millan-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.

Fig. 3 Polarization curves of H2–Fe3+ fuel cell at 70 uC with H2 supplied at

anode and Fe-NO-80 flowed at cathode.

12630 | RSC Adv., 2012, 2, 12628–12630 This journal is � The Royal Society of Chemistry 2012

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