Electroreduction of nitrate to ammonia in alkalinesolutions using hydrogen storage alloy cathodes

Chun Lu, Shigang Lu *, Weihua Qiu, Qingguo Liu

Department of Physical Chemistry, University of Science and Technology, Beijing 100083, PR China

Received 16 March 1998; received in revised form 23 July 1998

Abstract

The galvanostatic and potentiostatic electrolysis of nitrate was carried out in alkaline solution using three types of

hydrogen storage alloy electrodes. Ammonia was produced and its current e�ciency was in¯uenced by a series offactors, such as nitrate concentration, type of electrode and its initial state, applied current and potentials. Theelectrolysis on the fully hydrided electrode in the 0.5 M KOH+0.05 M KNO3 solution produced ammonia with acurrent e�ciency of 90% at ÿ1.2 V versus SCE. It was found that the dissolved hydrogen atom in the alloy

electrode could directly participate in the reduction process and enhance the catalytic activity of the electrode.# 1999 Published by Elsevier Science Ltd. All rights reserved.

Keywords: Hydrogen storage alloy electrode; Nitrate reduction; Current e�ciency; Ammonia; Electrolysis

1. Introduction

Due to the extensive use of nitrate in detergents and

fertilizers and an increasing number of harmful nuclear

wastes, the contamination of water by nitrate becomes

a serious environmental concern. Consequently, e�-

cient reduction of nitrate to harmless products is an

important process in the treatment process of harmful

wastes. Many methods, including biological,

photocatalytic [1, 2] and electrochemical methods, have

been proposed. In general, nitrate was expected to be

®nally reduced to ammonia which could be removed

from the waste easily. Otherwise, intermediate pro-

ducts, such as nitrite, could cause secondary environ-

mental pollution. Because of the ease of controlling

the reaction process by changing the applied potential

and simplifying the reactor design, the electrochemical

method is now extensively used, especially for liquid

waste treatment. Thus, the electrochemical reduction

of nitrate to ammonia has attracted more attention.

Various electrode materials have been studied for the

process of the reduction of nitrate in alkaline solutions.

Because of its chemical inertness, the Pt electrode has

been investigated for a long time [3, 4] and thin-®lm B-

doped diamond cathodes were found to obtain an

ideal Faradaic e�ciency for the production of

ammonia [5±7]. On a copper cathode, nitrate can be

reduced to nitrite at ÿ1.1 V versus SCE and to ammo-

nia with high yields at ÿ1.4 V versus SCE [8]. Lead,

zinc and nickel are also able to be used as cathodes.

They can also reduce nitrate to nitrite and ammonia

which is con®ned by the applied potential, electrode

material and other factors [9, 10].

The hydrogen storage alloy is a new type of func-

tional material. It has been used as catalyst in the syn-

thesis of ammonia, the conversion of syngas and liquid

hydrogenation of unsaturated organic compounds, in

which it showed many advantages, such as excellent

activity and selectivity. The main feature of a hydrogen

storage alloy in liquid hydrogenation is that it can pro-

vide a large amount of reactive hydrogen atom [11].

Although there are some reports in which hydrogen

storage alloy is used as catalytic electrode [12], relevant

Electrochimica Acta 44 (1999) 2193±2197

0013-4686/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved.

PII: S0013-4686(98 )00290-4

PERGAMON

* Corresponding author. Present address: General Research

Institute for Non-Ferrous Metals, 2 XinJie Kou Wai Dajie,

Beijing 100088, PR China.

publications about the use of the material in the elec-trolysis of nitrate have not yet been found. Therefore,

the exploration on the performance of the hydrogenstorage alloy electrode in the electrochemical reductionprocedure is very intriguing and promising. In this

work, we are trying to carry out the electrochemical re-duction of nitrate by using the mixed mischmetalhydrogen storage alloys as catalytic electrodes.

2. Experimental details

Three kinds of alloys used in our experiments have ageneral form of Mm(NiAlMnCo)5 and are indicated

by MH. The Mm represents the mixed mischmetalcontaining La, Ce and Pr elements which are presentin the alloys in di�erent proportions. One of the

samples was rich in Ce and was symbolized by MHCe

in the following. Two other samples, rich in La, dif-fered in surface coating. MHCu represents one alloy

coated with 8% Cu, while MHNi symbolizes the otheralloy coated with 10% Ni [13, 14]. The alloy particles,conductive materials and Te¯on bonder were mixed inthe proportion of 90:2:8 (by weight), rolled into a thin

®lm and then pressed onto both sides of a nickel foamplate to obtain hydrogen storage electrodes. Thedimension of the working electrode is 10�20� 0.2

mm and the weight of the electrodes is about 0.3 g.Before electrolysis, the working electrode was acti-

vated to enhance its catalytic activity [15]. This process

was performed in an ordinary glass beaker in whichthe hydrogen storage alloy was the working electrodeand a sintered nickel plate having more capacity was

used as counter electrode. The electrolyte was 6 MKOH solution. The hydrogen storage alloy electrodewas charged and discharged more than 20 cycles withconstant current density of 40 mA cmÿ2. A fully

charged hydrogen-storage alloy electrode served as theworking electrode for the nitrate reduction unlessnoted otherwise. The electrodes were discharged after

electrolysis to calculate the capacity loss [12].Electrolysis was carried out in a lab-made cell of

three-electrode con®guration using an EG&G

Potentiostat/Galvanostat Model 273. The counter elec-trode was a Pt plate and separated from the hydrogenstorage electrode by the Na®on1 117 membrane. Thesaturated Hg/Hg2Cl2 was used as a reference electrode

and installed close to the working electrode through aLuggin capillary. 0.5 M KOH served as a supportingelectrolyte. After the hydrogen storage alloy electrode

was fully charged, the electrolyte in the cathodic com-partment was changed to the solution containing 0.05M KNO3. The electrochemical reduction of nitrate was

then started galvanostatically or potentiostatically. Argas passed the con®guration to stir the solution duringthe electrolysis and went through 10 ml 1 M H2SO4

solution which acted as adsorbent for the productionof ammonia. No ammonia was found in the gas

released from this collective solution. After electrolysis,the electrolyte and the adsorbent of 1 M H2SO4 weremixed thoroughly for analysis. A UV 3000 spec-

trometer was used to analyze the amount of ammonia.The ammonia concentration was determined using theindophenol method and careful spectroscopic analysis

was carried out in the wavelength range 500±700nm [16]. The sample for determining ammonia wasprepared by putting no more than 40 mg ammonia into

a 25-ml standard ¯ask. Then 2.5 ml sodium phenolatesolution and 1 ml sodium hypochlorite solution wereadded. Finally the solution was diluted to the markwith water. After 30 min, the absorbance at 625 nm

against water was measured. The standard curve wascalibrated by standard ammonia chloride solution.All reagents, such as KNO3, KOH and KCl were

certi®ed A.C.S grade. The water was twice distilled.

3. Results and discussion

3.1. The participation of absorbed hydrogen atom in thenitrate reduction

In the hydrogenation of nitrate to ammonia, thehydrogen atom was an intermediate reactant. Whenthe hydrogen storage alloy was used as catalyst, it

could release the absorbed hydrogen atom and act ashydrogen a source. This suggestion has been con®rmedby the preliminary experiment in which the fullycharged hydrogen storage alloy electrode was

immersed in the 0.5 M KOH solution containing 0.05M KNO3 for a few hours. The results were a decreasein the electrode capacity of hydrogen atom and the

production of ammonia detected in the solution by theindophenol method, which was a strong evidence ofthe participation of a hydrogen atom in the process of

reduction of nitrate.In addition, we have studied the discharge behavior

of MHCu electrodes in the 1 M KOH with the presenceand absence of nitrate. The galvanostatical discharge

curves are shown in Fig. 1. Curve (a) and (b) representthe discharge curves in the 1 M KOH solution withoutand with the presence of KNO3, respectively. The elec-

trode lost its capacity in shorter time and its plateaupotential shifted to a more positive value in the sol-ution containing KNO3 (curve b). Analysis showed

that ammonia existed in the electrolyte, which indi-cated that the reactive hydrogen atom released fromthe electrode reacted with nitrate.

C. Lu et al. / Electrochimica Acta 44 (1999) 2193±21972194

3.2. Constant current electrolysis

The current e�ciency (Z) of ammonia production

was calculated using the following expression:

Z � nxF=Q �1�in which n (=8) is the number of electrons involved in

the following electrode reaction:

NOÿ3 � 7H2O� 8e � NH4OH� 9OH ÿ �2�x, F and Q are the mole of nitrate produced by the re-

duction of nitrate in the electrolysis process, the

Faraday constant (96500 C molÿ1) and the electrolysis

charge (C), respectively.

In Table 1, the charge for the NH3 formation was

calculated from the determination of NH3 production.

The current e�ciency of the electrolysis of 0.05 M

KNO3 was presented in Table 1 at a constant current

density of 5 mA cmÿ2 by using discharge electrodes

without preliminarily absorbing any hydrogen atom.

Experimental results showed that a better current e�-

ciency was obtained from the electrolysis using MHCu

and MHNi cathodes. During electrolysis, the electrode

capacity of the hydrogen atom increased, which means

that the electrolysis charges were partially used to

charge the electrodes. In comparison, the capacity

increase of the MHCe electrode was more obvious than

that of the other two electrodes, which can explain

why the current e�ciency of the MHCe electrode is the

smallest.

It was observed in Table 1 that the sum of the

charge which was consumed in the reduction of nitrate

to ammonia and that which was used to load the elec-

trode was not equal to the total charge. Some side

reactions, such as hydrogen evolution and the re-

duction of nitrate to nitrite, might take place during

the electrolysis.

The results of constant current electrolysis (I=5

mA cmÿ2) in the 0.5 M KOH solution with di�erent

concentrations of nitrate are surveyed in Table 2. In

this series of experiments the working electrode was

fully charged MHCu. The dissolved hydrogen atom

would be extracted to react with nitrate during electro-

lysis. As shown from Table 2, the capacity decreased

and current e�ciency calculated by Eq. (1) was more

than 100%, which was di�erent to the result in Table 1.

In Table 2 the results exhibited that the reduction

reaction of nitrate to ammonia was a�ected by the

concentration of KNO3. When the concentration of

KNO3 was smaller than 0.002 mol/l, the current e�-

ciency was low. With the increase of concentration of

KNO3, the electrode lost its capacity obviously. The

current e�ciency became larger than 100%, which

could be attributed to the absorbed hydrogen atom

reacted with nitrate. When the KNO3 concentration

was 0.25 M, the electrode lost its capacity completely.

In order to include the contribution of the absorbed

hydrogen atom to the nitrate electrolysis, we de®ned

the total e�ciency (Zt) as following:

Zt � QA=�Q�QL� �3�Here, QA and QL are the charge for the formation of

ammonia and the capacity loss, respectively, and Q is

the electrolysis charge. As shown in Table 2, the total

e�ciency increased with the increase of the KNO3 con-

centration. Although the electrolysis in 0.25 M KNO3

obtained a larger current e�ciency than that in 0.05 M

KNO3 solution, the total e�ciency increased slightly.

It seems that the hydrogen storage alloy released more

Fig. 1. The hydrogen storage alloy electrode discharge curves

in the 1 M KOH solution with absence of 0.05 M KNO3 (a)

and presence of KNO3 (b).

Table 1. Constant current electrolysis results using di�erent discharged hydrogen storage alloy electrodes and the electrolyte 0.05

M KNO3+0.5 M KOH. I=5 mA cmÿ2

Type of HSA

electrode

Electrolysis charge

(C)

Capacity increase

(C)

Charge for the formation of NH3

(C)

Current e�ciency

(%)

MHCu 144 4.82 58.3 40.5

MHNi 144 10.62 61.76 42.9

MHCe 144 57.96 48.04 33.4

Before the electrolysis, the discharge electrodes did not absorb any hydrogen atom.

C. Lu et al. / Electrochimica Acta 44 (1999) 2193±2197 2195

absorbed hydrogen atom reacting with nitrate in the

concentrated KNO3 solution.

If all hydrogen atoms originating from the electrode

capacity loss reacted with nitrate, the part of electroly-

sis charge, `real applied charge', consumed for the for-

mation of NH3, can be obtained by subtracting the

capacity loss from the charge for the formation of

NH3. `Real applied charges' were 21, 49.5, 56.5, 42.1

in a solution of 0.002, 0.01, 0.05, 0.25 M KNO3, re-

spectively, which indicated that the electrolysis charge

was increasingly consumed for the reduction of nitrate

to ammonia when the concentration of KNO3 became

larger. Consequently, the reduction of nitrate on the

hydrogen storage alloy electrode extracted more

absorbed hydrogen atom and consumed more applied

charge in the concentrated KNO3 solution.

Accordingly, during electrolysis not only nitrate was

reduced on the hydrogen storage electrode, but nitrate

reacted with absorbed hydrogen atom in the alloy elec-

trode. In addition, when the electrolyte was an aqu-

eous solution, there is an additional side process,

hydrogen evolution reaction. These processes deter-

mined the current e�ciency for the formation of NH3

and would determine the electrode potential in the pro-

cess of electrolysis, which behaved quiet di�erently in

the solution containing a di�erent amount of KNO3.

When the electrolysis was carried out in the dilute

KNO3 solution, the electrode potential was aboutÿ1.08 V versus SCE and hardly changed over the reac-tion time, as shown in Fig. 2(curve a). In the concen-trated NO3

ÿ solution the electrode potential sharply

decreased and then almost stabilized at ÿ0.8 V versusSCE, Fig. 2(curve b).For hydrogen storage alloy electrodes, when the po-

tential is lowered to less than ÿ0.85 V versus SCE inthe KOH solution, they certainly release all the hydro-gen atoms. Therefore the electrode potential of the

electrolysis in the 0.25 M KNO3 might be largelydetermined by the hydrogen desorption. In the dilutenitrate solution the electrolysis showed a much lowercurrent e�ciency and the electrode potential seems to

be determined by the hydrogen evolution reaction andthe reduction of nitrate also.

3.3. Potentiostatic electrolysis

As described in Section 3.2, the potential changedduring the galvanostatic electrolysis of nitrate. In thefollowing we show the results of potentiostatic electro-

lysis. As the electrode potential varied, the initial stateof hydrogen storage alloy and the electrode processwould change consequently. The potentiostatic electro-lysis method provided more critical detail about the

nitrate reduction.In these experiments, the hydrogen storage alloy

electrode was ®rst charged to the potential for electro-

lysis. Three potentials, ÿ0.8, ÿ1.06 and ÿ1.20 V versusSCE were chosen so that alloys were in the dehy-drided, partially dehydrided and fully hydrided states.

After the electrode potential was stabilized, the nitratewas added and the electrolysis started. The current e�-ciency and total e�ciency was surveyed in the Table 3.When the electrode was in the dehydrided and fully

hydrided states, the current e�ciency was the same asthe total e�ciency and the capacity loss was notobserved. In contrast, the electrolysis at ÿ1.06 V

involved the capacity loss and resulted in the currente�ciency larger than 100%. Similar to the calculationin the galvanostatic electrolysis, the `real applied

charge' for electrolysis at ÿ1.06 V was 207.6 C. Thiswas 90.7% of the total applied charge (228 C), whichwas close to the 89.4% current e�ciency for the elec-

Table 2. The results of constant current electrolysis in 0.5 M KOH and di�erent concentration of KNO3 solution at room tempera-

ture using charged MHCu electrodes. I=5 mA cmÿ2

Concentration

of KNO3(mol/l)

Electrolysis

charge (C)

Capacity

losses (C)

The charge for the

formation of NH3 (C)

Current

e�ciency (%)

Total

e�ciency (%)

0.002 72 3.6 24.6 34.2 32.5

0.01 72 28.8 78.3 108.7 72.0

0.05 72 111.6 168.1 233.4 91.6

0.25 72 309.6 351.7 488.5 92.2

Fig. 2. Variation of electrolysis potential during electrolysis of

0.002 M KNO3 (a) and 0.25 M KNO3 (b) solutions using

MHCu electrodes. Electrolysis current density=5 mA cmÿ2.

C. Lu et al. / Electrochimica Acta 44 (1999) 2193±21972196

trolysis at ÿ1.20 V. This result suggested that theincrease of the total e�ciency on the partially hydridedalloy electrode was due to the participation of the

absorbed hydrogen atom in the reaction. When the po-tential was lower to ÿ0.80 V, the current e�ciencydecreased. The electrolysis at ÿ1.20 V obtained a muchhigher current e�ciency, although there is a side pro-

cess, hydrogen evolution, carried out on the electrode.The alloy containing hydride metal seemed to exhibitmore favorable activity to the nitrate reduction.

In summary, we have shown that the nitrate couldbe reduced to ammonia electrochemically in an alka-line solution by using hydrogen storage alloy electro-

des. Although copper and nickel metal electrodes canalso be used as catalytic electrodes in this process, onlyat a potential as high as ÿ1.4 V versus SCE can these

electrodes obtain a good current e�ciency, but nitriteis the main product when the potential is ÿ1.1 V ver-sus SCE [8, 9]. Because the hydrogen storage alloy waschemically stable in the alkaline solution, it was more

favorable than the conventional cathodes, such as Znand Fe metal electrodes which were corroded in the al-kaline solution [10]. Therefore we can draw the con-

clusion that the hydrogen storage alloy electrodeexhibited many advantages, including high selectivity,excellent current e�ciency and good stability, in the

process of reduction of nitrate to ammonia.

4. Conclusions

The electroreduction of nitrate in the alkaline sol-ution was found to produce ammonia on the hydrogen

storage alloy electrode. The dissolved hydrogen atomin the alloy may directly participate in the reduction.The current e�ciency for the NH3 production was

in¯uenced by a series of factors, such as nitrate con-centration, alloy type and its initial state and theapplied current or potentials. With the increase of

nitrate concentration, more dissolved hydrogen atomswere extracted and also a greater applied charge wasconsumed in the reduction. The current e�ciency waslower for electrolysis on the electrode without absorb-

ing any hydrogen atom, while the catalytic activity wasgreatly improved when the electrode was in the par-tially and fully hydride form.

Acknowledgements

This work was ®nancially supported by the ChineseNational Scienti®c Foundation.

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Table 3. The results of potentiostatic electrolysis using MHCu electrodes in 0.5 M KOH+0.05 M KNO3 solutions at ambient tem-

perature

Applied potential

(V) versus SCE

The states of

electrodes

Electrolysis

charge (C)

Charge for the

formation of NH3 (C)

Capacity

loss (C)

Current

e�ciency (%)

Total

e�ciency (%)

ÿ0.80 dehydrided alloy 256 98.6 0 38.5 38.5

ÿ1.06 partially hydrided alloy 228 250.8 43.2 110 92.4

ÿ1.20 fully hydrided alloy 265 236.9 0 89.4 89.4

C. Lu et al. / Electrochimica Acta 44 (1999) 2193±2197 2197