The anodic polarization of the sn02 ti electrode in the koh solution

This type film of has good stability for ano-dic dissolving in medium contained high concentration of CI- ions and good possibility of electron exchange.. Sn02/Ti electrodes have been us

THE ANODIC POLARIZATION OF THE SN02/Ti

ELECTRODE IN THE KOH SOLUTION

N.T C a m H a ,T X u a n S e n ,

V, N g o c B a n

Department of Chemistry, Hanoi National University,

19 Le Thanh Tong, Hanoi, Vietnam ABSTRACT

Sn02 Thin films on titanium metal subotrates have been produced

by spray pyrolysis method This type film of has good stability for ano-dic dissolving in medium contained high concentration of CI- ions and good possibility of electron exchange Sn02/Ti electrodes have been used in the electrochemical oxidation of phenol in alkaline medium contained CI- ions with high efficiency Therefore, it has made them the possibility of new application in the wastewater treatment by electro-chemical method

1 INTRODUCTION

In electrochemical process, two types of anode are used usually: dissolving and inert one We expect to prepare anodes, which are inert, stable and durable in some conditions of process such as the change of temperature, the corrosion of medium Nowadays, there is the trend to make the coating with a thin film of oxide metal on the surface of inert material that becomes more durable and h a s higher catalyst activation Titanium is a metal, which shows the good anti-corrosion and its passive oxide film on the surface has the semiconduc-tive nature and can do ionic conductivity So it is possible to cover a thin film of oxide metal on the surface of titanium to increase

conduc-tivity and improve the catalyst activation Meanwhile thin film of Sn02 has been known as inert electrode in several mediums [1.4] Moreover, due to its high overpotential in liberation of oxygen and chlorine, S n 0 2 electrode is used as anode in synthesis of organic sub-stances, in treatment of wastewater In this paper, it is reported the preparation of thin film Sn02 on the surface of titanium and exami-nation its anodic dissolving stability in medium

2 EXPERIMENTAL

The experiments were carried out on a potentioganvanostat PGSHH2 with three-electrodes cell: platinium sheet was used as a counter electrode, a saturated calomel electrode (SCE) was used as reference one and the working electrode were Sn02/Ti samples

3 RESULTS AND DISCUSSIONS

3.1 P r o d u c t i o n of S n 0 2 t h i n film on t h e surface

of t i t a n i u m

The samples were titanium foils of 1 cm^ geometric area These samples were polished by emery papers, cleaned with solution contai-ned oxalic acid, hydrochloric acid and distilled water [2] Titanium electrodes were coated with Sn02 film by spray pyrolysis method Sn02 film obtained by spraying the solution of mixture SnCl4, NH4F and CH3OH on hot titanium substrates in time of 2 minutes

3.2 I n f l u e n c e of NaOH solution on the a n o d i c d i s s o l v i n g stability of Sn02/Ti electrode

In order to determine electrochemical characteristics of SnO^/Ti

in NaOH medium, it is measured the anodic current-voltage curves of Sn02/Ti electrode in alkaline medium in potential range of (1) -IV/SCE with sweep rate 0.05 V/s Results showed t h a t Sn02 films were stable in alkaline medium The increase of concentration of NaOH changes lightly the passive region in potential range of (0.5) -0.5 V/SCE and passive current density The higher increase of the

con-c e n t r a t i o n of N a O H , t h e e a r l i e r t h e l i b e r a t i o n of o x y g e n w a s (see Table 1 a n d Fig 1)

Table 1 D e p e n d e n c e of p o t e n t i a l of O2 l i b e r a t i o n a n d p a s s i v e

r e g i o n o n c o n c e n t r a t i o n of N a O H s o l u t i o n

C o n c e n t r a t i o n

of N a O H

s o l u t i o n (M)

P o t e n t i a l of O2

l i b e r a t i o n

(V/SCE)

P a s s i v e r e g i o n

(V/SCE)

P a s s i v e c u r r e n t

d e n s i t y (mAycm^)

0.01

0.698

(0.500) -0.698 3.48.10-^'

0.1

0.644

(0.502) -0.604 3.89.10"'

0.2

0.591

(0.482) -0.591 4.09 1 0 '

1

0.532

(0.464) -0.532 4.39 1 0 '

r

1

3

2

L

0

J

4

n

-0 -"

o

0

-0

0

0

-\

i

'/

ill I

IL

• 1 / /

/

r - » ' t - * - A a-«K — ' w - ^ !

It" .- ',~.r^ f, -4„ ( , , , I

J O 1J fj o *» — o *^ \") K - f-» r > o s ^ ^^

« < < o > x o '

Fig, 1 Anodic polarization of SnOg /Ti in alkaline m e d i u m

(1,1):0.01M (2,2'):0.1M (3,3'): 0.2M (4,4'): IM

Fig 1 showed that Sn02 was not dissolved in alkaline medium because of the absence dissolving peak of Sn02 or of Ti did and the Og liberation can be used for oxidation of organic substances in waste water by using of Sn02/Ti electrode

3.3 The a n o d i c d i s s o l v i n g stability of Sn02/Ti e l e c t r o d e

in NaCI s o l u t i o n

By using potentiogalvanostat PGSHH2, it was measured the ano-dic polarazation of Sn02/Ti electrodes in NaCl solutions and found that Sn02/Ti electrodes being passive in potential range of (-0.3) - 0.9 V/SCE When concentration of NaCl is more 10 g/1, the peak of CI2 liberation appeared at 1.23 - 1.26 V/SCE The higher increase the con-centration of NaCl solution, the earlier CI2 liberation was and the pas-sive potential has moved forward to negative, the paspas-sive region

beca-me narrower This proved that the anodic dissolving stability of Sn02/Ti reduced However the passive current of this electrode was very small (about 10-5 mA/cm-) So this film could be used in electro-lytic technology (See Table 2)

Table 2 Dependence of passive region, potential of CI2 liberation

on concentration of NaCl

C o n c e n t r a t

i o n of N a C l

s o l u t i o n

(gA)

P a s s i v e

r e g i o n

(V/SCE)

P a s s i v e

c u r r e n t

d e n s i t y

( m A / c m ^ )

P o t e n t i a l

of CI2

l i b e r a t i o n

(V/SCE)

0.8

(-0.34) ^ 0.94 9.44 10-'

10

(-0.34)-0.94 9.85.10*^

1.25

20

(0.31) -0.93 11.21 10^^

1.26

35

(0.33) -0,83 19.02 10-^

1.23

NaCl

s a t u r a t e d

(0.43) -0.80 21.04 lO*^

1.24

3.4 The anodic dissolving stability of SnOg/Ti e l e c t r o d e

in alkaline m e d i u m contained CI- ions

Results obtained from anodic polarization of SnOg/Ti electrodes in medium contained 5 g/1 NaCl with pH in range of 10 - 12 showed the

competition between CI2 liberation and O2 one At pHlO, there was

not CI2 liberation, only Og one but at higher pH, CI2 liberation appea-red more early This electrode can used and profit CI2 liberation in weak alkaline medium to disinfect wastewater

Fig 2 are anodic current - voltage curves of Sn02/Ti electrodes in medium pH12 with and without CI' ions This shows that at

potenti-al in range of 0.76 - 0.79 V/SCE, CI2 liberation appeared before O2 one when present CI' ions and Og liberation appeared at this potential range when absence of CI' ions

M f t / ' c M ' ^ a J J.O o

.a

/ ' > , / ;

1

?

1

/

/ / / /

I /

y

i

/

/

1

S

t

* K w > i n

Fig 2 Anodic current • voltage curves of Sn02/Ti electrodesin

medium pHl2 with and without CI" ions

(1): Solution NaOH pH12 (2): Solution NaOH pH12 contained 5 g/1 NaCl

Table 3 Potential of O^ and Clg liberations in alkaline medium

pH12 with and without CI" ions Concentration of NaCl (g/1)

Potential of CI2 liberation (V/SCE)

Potential of O2 liberation (V/SCE)

5

0.76

1.5

0 -0.79 3.5 P o s s i b i l i t y o f e l e c t r o n e x c h a n g e o f S n 0 2 / T i e l e c t r o d e

i n o x i d a t i o n - r e d u c t i o n s y s t e m : O.IM K4[Fe(CN)g ] + O.IM K 3 [ F e ( C N ) 6 ] + O.IM N a O H

Determination of exchange current iO of direct oxidation - reduc-tion system allows to estimate possibility of electron exchange of elec-trode material by means of oxidation - reduction reaction happening

on the surface of electrode Oxidation - reduction system has been

used : [Fe(CN)6 l""'^ + le [Fe(CN)6 l"'^ (*)

i ^ n n / ' c n - ^ a ) X O -L

i s

-T l |

— ~ "

/ /

J i

1 i

/

i

r - ^ \ / / ^-7 / y ^ /

y /

y"

,?<,,_

/

t

>

1

/ •

F W 2 H ^ • ' F • 3 + U i n

•»->•• • 1 • ! »

• 0 4 - 0 2 - 0 0 o a 0 4 0 6 o e 1.0 1.2 1.4 o

ii<v> 1 0

I

Fig 3 Cyclic polarization of Ti and SnOg/Ti electrodes in solution

O.IM (K4[Fe(CN)g ] + K3tFe(CN)6 ] + NaOH)

If surface of electrode has possibility of electron exchange to oxy-dation - reduction system according to reaction (*), the cyclic polariza-tion curve has the symmetryf oxidapolariza-tion - reducpolariza-tion currents Results

of cyclic polarization measured of SnOg/Ti electrode in solution O.lM -O.IM - -O.IM shows that the surface of titanium has not possibihty of electron exchange while Sn02 film has a good conductivity and high possibility of electron exchange

3.6 E s t i m a t i o n of e l e c t r o c h e m i c a l o x i d a t i o n o f p h e n o l b y

S n 0 2 / T i e l e c t r o d e in a l k a l i n e m e d i u m c o n t a i n e d CI" i o n s

Because SnOg film has high overpotentials of O2 liberation and of CI2 one, SnOg/Ti anode can be used in electrochemical oxidation of organic substances in wastewater In this paper, phenol is used as organic pollutant (23mmol/l) in NaOH solution (pHl2) contained 5g/l NaCl In order to estimate efficiency of electrochemical oxidation, che-mical oxygen demand (COD) index has been determined throughout electrolytic process [14] Electrolysis performed with current density 8 mA/cm2 and after every 30 minutes, we determined COD index of sample and results combined are in table 4

Table 4 COD index of sample throughout electrolytic process

T i m e

( m i n u t e )

COD

i n d e x

(mgOaA)

0

787.07

30

657.25

60

562.74

90

505.9

120

441.06

150

363.6

180

324.11

240

79.05

After 4 hours of electrolysis, COD index of sample reduced 10 times, so efficiency of phenol ehmination of Sn02/Ti electrode was 90% For this reason, we can beheve that Sn02/Ti electrode can treat organic pollutants by electrochemical method

4 CONCLUSIONS

S o m e conclusions c a n be m a d e :

• The electrodes of Sn02 films on titanium substrates were prepared by spray pyrolysis method

• Sn02/Ti electrode shows high anodic dissolving stability in solution contained high concentration of CI" ions This electrode can be used in elec-trolytic technology

• Sn02/Ti electrode has the good possibility of electron exchange, so it can be used as an electrode substituted for platinium in electrochemical technology

• Anodic polarization of Sn02/Ti electrode in alkaline medium contained CI" ions caused CI2 liberation prior O2 one, so it can be used to yield CI2 for wastewater treatment

• Sn02/Ti electrode was used in electrochemical oxidation of phenol in alkaline medium contained CI" ions with high efficiency

REFERENCES

[1] H Cachet, F Zenia, M Froment Journal of Electrochemical Society

Vol 146(1999), p.977-983

[2] Xir ly be mat va an mon dien cue Ti/Ti02, Tap chi khoa hoc va cong nghe 4/2001.52-56'

[3] V6 Thanh Vinh, Nguydn Diic Hiing, Tiiyen tap bdo cdo hoi nghi dien

hod vd img dung 2001 129-133

[41 Trinh Xuan Sen, D6 Thi Anh Thu, Nguy6n Ngpc Toan Proceeding of

the 7"' national conference of fundamental research projects on physical and theoretical chemistry Ha noi 2003, 122-125

[5] Trinh Xuan Sen, Nguyin Thi Cam Ha Proi

Pacific corrosion control conference 1999, vol 2, 752-759

[5] Trinh Xuan Sen, Nguydn Thi Cam Ha Proceeding of the 11^" Asian