Photodegradation of 4-chlorophenol to carbon dioxide and HCl using high surface area titanium dioxide anodes

Chemosphere, Vol. 26, No. 7, pp. 1301-1309, 1993 Printed in Great Britain

0045-6535/93 $6.00 + 0.00 Pergamon Press Ltd.

Photodegradation of 4-chlorophenol to carbon dioxide and HCl using high surface area titanium dioxide anodes

Inam U1 Haque~ and James F. Rusling* Department of Chemistry U-60, University of Connecticut, Storrs, CT 06269-3060, USA

(Received in Germany 4 January 1993; accepted 2 F ~ r u a ~

1993)

ABSTRACT

High surface area titanium dioxide films coated on conductive glass were evaluated for light-assisted electrochemical oxidation of the model aryl halide pollutant 4-chlorophenol. Operation of the electrochemical cell with the anode at 0.8 V vs SCE and illuminated with light of i>320 nm. gave complete conversion of micromolar concentrations of 4-chlorophenol to carbon dioxide, HCI, and water in oxygen s a t u r a t e d solutions. Experiments at open circuit in a cell with separate anode and cathode compartments suggest the possibility of generating hydrogen gas at the platinum cathode in the cell.

INTRODUCTION

Over the past decade numerous reports dioxide

either

decomposing

as

suspended

a variety

of organic

called photomineralization. physical

separation

from

(i-i0) have documented the use of titanium

particles

or

as

pollutants

coatings

to carbon

on

glass

dioxide

( i 0 ) for

and water,

so

The use of a particulate catalyst necessitates its the

solutions at

the

end

of

first

cycle

of

use,

followed by resuspension in an active form for subsequent cycles of pollutant degradation.

On

the other hand,

gave good efficiency

titanium dioxide attached

for pollutant degradation,

reuse of the catalyst

(i0). Recently,

photoelectrodes

used

unprecedented

were

high

by

efficiency

to a solid support

with the advantage of ease of

very high surface area titanium dioxide

Gratzel

and

coworkers

for energy production

to

(ii).

make In

devices

this

paper,

of we

evaluate high surface area titanium dioxide photoelectrodes in electrochemical cells for decomposition of an aryl halide pollutant. The use of titanium dioxide in a photoelectrochemical

cell presents

the possibility

of mineralization of

organic pollutants at the anode along with generation of useful materials such as hydrogen at the cathode. We

chose

chlorophenols widespread

4-chlorophenol as

fungicides

presence

in

as

(12,13)

the

the

test

pollutant

environment.

Also,

~Permanent

titanium

address:

dioxide.

Direct

Department

of

(17,18)

1301

(14,15)

and mediated

Chemistry,

Technology, Lahore 54890, Pakistan

has

the led

use

University

of

and supported

(19) degradations

of

of

to their

photomineralization

chlorophenols has been studied extensively at dispersed (5,10,16)

because

in industrial processes

Engineering

of

and

1302

these compounds Titanium pigment

with ultraviolet dioxide

in w h i t e

titanium

paint.

dioxide of

conduction

band

TiO 2

extremely

illuminated,

holes

(h+)

in

of

the

(20,215

high

reactions

"OH

h+

+

OH-(ads)

-->

"OH

reactive

led

cell.

dioxide by

In

surface

in

+3

the

V

(e-)

NHE

the

band

(21).

radicals

of to

in

valence

vs.

to h y d r o x y l

leads

a

of

When

('OH) by

H+

(i) (2)

radicals

can

oxidize

organic

molecules

organic

molecules

can be o x i d i z e d

directly

reaction

employing

titanium dioxide

particles,

half-reactions

redox p r o d u c t s

ti t a n i u m

about

nm

electrons

hole

as

Irradiation

310-420

and

The

eV.

used

in

the

by holes

on

surface.

In a p h o t o c h e m i c a l

be

+

hydroxyl

Alternatively,

reduction

Thus,

its

3.0-3.3

range

band

power,

material

(1,3): -->

the c a t a l y s t ' s

valence

at

of

wavelength

semiconductor.

water

H20(ads)

solution.

the

semiconducting

gap energy

the

oxidizing

+

highly

a band in

h+

These

light have also been explored.

inexpensive

light

TiO 2 o x i d i z e s

two p o s s i b l e

and

an

It has

with

separation has

is

an

from these

is used

external

this

occur

way,

at

reactions

as an anode,

wire

to

desirable

different

the

are obtained

the

electrons

cathode

reduction

sites

on

the

as mixtures.

in the

can

particle.

However,

conduction

in a d i f f e r e n t

products

oxidation

same

band

can

of

the

compartment

be p r e p a r e d

and

if

collected

separately.

EXPERIMENTAL

4-Chlorophenol,

Chemicals.

from

absolute

obtained

upon

ethanol high

[m.p.

acid,

Baker

Frederick

Smith

Co.,

distilled, Reagent

Titanium with

a

thin

resistance of the

strip

these

area

ethoxide

fractal

to

concentrations

strip half

so that cm.

30 min

The

0.5

cm

above

the drop titanium

in a c l o s e d

of

anhydrous

Ti(IV)

strip

layer was

~

25 or the

in

Analyzed

G.

doubly PHOTOREX

(99.9%5

was

from

grade. cm

tin

x

2.5

cm

dioxide

coated

reported

with

was

(23).

cm -I. L e a v i n g the

then

was

solution.

perchlorate,

chloride

0.5

doped

once

peak

4-CP

solutions

Baker

reagent

0.1-2

coated Typical

upper

0.5 cm

fourteen

layers

procedure

for producing

(115. The key steps are as follows:

prepared

ethanol

spread

glass

was

single

sodium

electrolyte

to a previously

was

alkoxide

constant

and

methanol

from

electrodes

absolute

of

antimony

Sb/SnO2

solution

titanium

about

the

A

(HPLC)

make

A

of

varied

according

chloride

placed

i0 mL

layer

crystallized

(22)].

from AAPER.

electrodes.

uncovered,

43.5

was

oc

to

was

99+%)

chromatography

were ACS analytical

coatings

dioxide,

high surface titanium

dioxide

, lit.

Absolute

ethanol

(Aldrich,

Reagent,

used

chemicals

conducting

of

of t i t a n i u m

Analyzed

water.

absolute

All other

oc

liquid

were

deionized

and

Aldrich.

44

pressure

Perchloric

4-CP,

by

carefully

and diluting

50 mg/mL.

bottom

edge

with

A drop of

absolute

of

doped

over most of the glass

25 m g / m L

chamber

2.3

Ti

dioxide

strip

except

containing

mL

methanol

tin

layer was then hydrolyzed

humidity

adding

Ti(IV5 to give

solution coated

was

glass

for the upper

at room temperature a saturated

solution

for of

1303

potassium

thiocyanate

(22).

The

electrode was

subsequently

heated

in air

in a

tubular oven fitted w i t h a quartz tube at 450 oc. Preheating at the entrance of the oven for 5 min was followed by 15 min of heating in the interior. layers

were

produced

deposited

each

procedure

as

layer

by

using

for the

lasted

in

for

the

same

the

50 mg/mL

first

30

way.

Subsequently, titanium

min.

The

final

geometric

up to 550 oC w h i c h was m a i n t a i n e d

solution.

of

the

were

The

same

of the last

electrode

was

in Ar in the tube oven.

the tube was twice evacuated and purged with

Ar. The e l e c t r o d e was then heated under Ar flux cooling.

size

Three more

layers

that h e a t i n g

then done by heating

After i n s e r t i o n of the electrodes,

thicker

alcoholate

layers was applied except

a p p r o x i m a t e l y 0.5 x 1.5 cm. D o p i n g of the TiO 2 film was

using

i0

(2.5 L/h)

at a rate of 500 °C/h

for 35 min. The electrodes were removed after

A lead was attached to the top uncoated part of the u n d e r l y i n g Sb/Sn02

conducting

silver

epoxy.

This

contact

and

a

small

part

of

adjoining

titanium dioxide were subsequently thus f a b r i c a t e d s h o w e d resistances

covered w i t h i n s u l a t i n g epoxy. Electrodes in the dark from 1200-3500 ~ cm -I These

electrodes

a microscopic

times by

have

larger

been

shown

to have

than the geometric area of the electrode

scanning

electron

micrographs,

which

had

that the crystal

Electrochemical

cells using

medium porosity

saturated

calomel

salt b r i d g e was

several

cracked,

hundred

confirmed

fractal-like

(Ii). Raman spectroscopy showed

form was anatase.

Photoelectrolyses. cells,

area

(ii). This was

rough,

appearance identical to that reported previously

compartment

surface

glass

reference

placed

close

quartz w i n d o w a l l o w i n g

experiments were done

a three electrode potentiostat.

in single or two

In the two compartment

frits separated anode and cathode electrode

(SCE)

connected

to the anode.

All cells

to

compartments.

the

solution

featured

with

A a

a flat glass or

for focussing of the light on the anodes.

All

solutions

in the anodic compartment were thoroughly saturated with oxygen by bubbling with water-vapor-saturated

oxygen

for

at

least

1 hr.

A

slow

stream

of

oxygen

was

maintained in the anode solution during controlled potential electrolyses. Typical

e l e c t r o l y s e s with the light-on were done at 0.8 V vs.

SCE.

Cyclic

v o l t a m m e t r y was done at the scan rate of 25 mV/s using BAS 100A Electrochemical Analyzer.

Most

of the cell resistance was compensated.

All p o t e n t i a l values are

vs. SCE. Illumination

was

s p e c t r u m u s i n g Model Light was each

35

mm

Analytical

p r o v i d e d with a i000 W Xe LPS 255 HR Power Supply

focussed

onto

thick,

using

Instruments).

the electrode the

lens

Light

was

through

fixture

that

simulates

Analytical

two w a t e r - f i l l e d

in LH

focussed

lamp

(Kratos,

onto

151N/2 the

Lamp

anode

the solar

Instruments). glass

Housing from

the

filters, (Kratos, rear,

so

that it p a s s e d through the glass and the Sb/SnO2 layers before striking the TiO2 layer.

Light-on

experiments

employed

500

w

source

power,

except

when

noted

otherwise. Analysis

of

the

products.

Gaseous

products

from

electrolysis

experiments w e r e swept out of the anode compartment of the t w o - c o m p a r t m e n t cell

1304

with

a slow

saturated hydroxide

was

The (Orion

stream

barium

of oxygen

hydroxide

t i t r a t e d with standard

concentration

Research

(9417-B)

coupled

electrode. was

detector

280

GVWP

Type U L T R E M E X

of

All The

were

phase was

acetic

chloride

SP8810

filtered

solutions

and excess

of

barium

acid. potentiometrically,

ion

junction

75/25

selective

electrode

silver-silver

pump

and

a

with

0.22

chloride

SP8450

UV-VIS

micron

acetonitrile/water

from electrolyses

acid

to

give

ca.

for analysis

retention

system

consecutive

collected

filters

column was 2.0 x 250 mm 5 m i c r o n C18 PHENOMENEX

products

The

a

Physics

solutions

at 0.15 m L / m i n

for a similar

three

determined

double

Spectra

Mobile

cells.

was

using

90-02

HPLC

enough

ions

EA920)

a

solution

compartment

reported

using

nm.

(15)

through

0.0100 N hydrochloric

chloride

Model

047).

containing

acetonitrile two

a

PP/5815B.

for a n a l y s i s water

with

done

at

(Millipore

of

IonAnalyzer

reference HPLC

and bubbled

(7). These solutions were

at 0.i mL/min

in u n d i v i d e d pH

3,

35%

of solutions

time of 21 min

for

cells,

methanol

or

55%

and

10%

from electrolyses

4-CP was

close

in

to that

(18).

RESULTS

Preliminary of

bromide

at

greater w i t h

work

an

showed

applied

the light

anode

indicate

a much

the

In

potassium

current

1 mM

increases

upon

titanium

dioxide

Prussian

blue-type

ferrocyanide illuminated

rate

substance

V

a

blue

at

have

upon

in

of

about

20-fold

illumination dioxide

addition

was

material

for oxidation

was

titanium

KBr,

reaction

(24).

electrodes

SCE

material

blue

from

complex

current

vs.

increases

+ 0.i M

This

resulting

parent

solution, 0.8

current

ferrocyanide

surface.

dioxide

of

of oxidation

illumination,

the

titanium

potential

higher

photoanode

with

in 0.I M KBr

on than off. These

the anode dark.

that

to

results

oxidizing

power

on

attributed

extensively

These

the to

a

oxidized

showed similar

in

20-fold

deposited

was

of

than

that

the

to that of

anatase powders. Current-potential current

of

the

characterize measured cycling

cell

the

while pack

scanning

current

the

M

at

were

slightly

concentration electrolyte

0.80

V

M

of

were

upon

HCIO4

is about

mM

the

in

linearly

rate

titanium first

(4-CP)

for

this

range.

dependant

was

to more

0.002-0.012

for electrolyte

alone,

Also,

the

the

dioxide

solution

dark

to

4-CP

little in

on the power of the light

in in

la).

the

same

supporting that of dark

solutions

but showed

anodes

at

this

photocurrents

and then

(Fig.

current

added

To was

increases

than 200 times

mM

anode.

current

Large

titanium

electrolyte of

at

negative

direction

potential. of

the

electrodes,

in a p o s i t i v e

that

at 0.8 V increased

of

of o x i d a t i o n

illumination

times

magnitude

dioxide

starting

4-chlorophenol

than

4-CP

The

supporting 90

Photocurrents

larger

of

the

negative

observed

0.i

ib).

of

anode potential,

photocurrent

(Fig.

indicator

original

were

When

electrolyte, current

the

NaCI04/0.001

Photocurrent potential.

is an

performance

to

ne g a t i v e 0.i

characteristics.

(Fig.

dependence

the

source

ic) on

supporting (Fig.

2).

1305

Dark

current

and

photocurrent

for

hrs

of

been u s e d

obtained w i t h

>12

for a t i t a n i u m

electrolysis

the same anode before

fact, p h o t o c u r r e n t

(Fig.

id)

dioxide

are

not

electrode

very

that

different

it was used for e l e c t r o l y s i s

(Fig.

had

those

la).

In

is slightly larger on the anode after use. This suggests that

p h o t o e l e c t r o d e p e r f o r m a n c e did not degrade during usage. When

freshly p r e p a r e d

titanium dioxide

electrodes w e r e

illuminated

in the

presence of 0.i mM 4-CP in supporting electrolyte,

copious evolution of gas was

observed

the

at

the p l a t i n u m

cathode

load in a t w o - c o m p a r t m e n t the

pH

of

the

suggesting

that

gas

connecting

two

electrodes

across

a

cell. During such an experiment of less than 30 min.,

solution the

upon

in

the

cathode

is h y d r o g e n

compartment

produced

by

changed

from

electrochemical

to

5,

reduction

3

of

h y d r o g e n ions at the Pt cathode. The pH change c o r r e s p o n d s to g e n e r a t i o n of a p p r o x i m a t e l y 60 ~L of hydrogen gas. At an applied potential of 0.8 V, much less gas e v o l u t i o n was o b s e r v e d at the platinum cathode.

0.1!

0.10

~.~
Previously used TiO2 anodes

b

J

dark

O.m

0.00

-0.28

-0.10

y

light

800

' $00

-0.20

_.-0.65 -200

-0.08

._-

-0.22 -0.40

$00

-200

800

300

-200

0.10

0.15 ~

800

C

,.,i

c~

light ' 300

800

.:

0.00 -0.10 -0.20

-200

E, mV vs SCE

E, mV v8 SCE

Figure 1. Cyclic c u r r e n t - p o t e n t i a l curves on titanium dioxide anodes at 25 mV s -I in dark and w i t h anode illuminated with 500 W source in 0.1 M NaCiO4/0.001 M HCIO4:

(a) no 4-CP;

(b) 0.i mM 4-CP;

previously u s e d for 12 hrs electrolysis.

(c) 2 ~M 4-CP;

and

(4) no 4-CP on anode

Scans begin at -200 my.

1306

500

E

400

0

"E

300

200

J

i

,

I

200

,

Influence

often b e c a m e

1000

observed w h e n

types 2.6

of

dioxide

of cells.

~M

4-CP

hydrochloric and

HCI

gave

complete

Table

followed

solution acid

to

of

by

[4-CP]/~M found

2.60 a

0.0

product

and a half gave

that

of

the

a more

0.800 were

V vs

done

of electrolysis conversion

methods

from

used.

the

A

4-CP

four

in

to

solution

at

carbon M~ss

dioxide balance

was

electrolysis

(Table

in a quartz

I).

t/hr

0.8

charge (Coul.)

V

on

cuvette

theory

found

19.2

17.16

12.4

10.8

7.5

3.67

2.86

2.6

3.08 b

1.2

4

1.94

2.0

1.85

0.25 c

trace

2

0.42

0.22

0.20

a25 mL

in single

compartment

anode c o m p a r t m e n t . C 2 . 5

cell.

cuvette.

cell,

theory

20 mL

two hour

showed

. . . .

bTwo-compartment

mL in a 1 cm quartz

the

in a divided

[C02]/~/~

found

and

of C02

within

A similar

TiO2 a n o d e s a

[CI-]/~M

different

I) of 25 mL of a

of 4-CP.

at

was

illuminated

several

(Table

oxidation

hour

of 4-chlorophenol 4-CP

evolution

SCE

cell with a glass window.

expected

dilute

or no h y d r o g e n

in the cell.

at

analyses

hours

at 0.8 V.

Little

quantitative

Photoelectrolyses

init.

edge.

Photoelectrolyses

60% d e g r a d a t i o n

degradation

I.

the upper

in an u n d i v i d e d

error

about

electrolysis

at

4-CP.

Seven

compared

experimental cell

blue

these electrode were used as anodes

Photoelectrolysis

titanium

Watts

of lamp power on photocurrent

light

i

600 Lamp Power,

Figure 2.

I

in

nearly

1307

DISCUSSION Results dioxide

of

the

anodes

for

solution v o l u m e larger cells, relatively

photoelectrolyses

demonstrate

photomineralization

of

4-CP.

to anode geometric surface area

the

utility

Given

that

the

titanium ratios

of

(i cm 2) are fairly large in the

Photocurrents that were

times of electrolysis in not unreasonable.

of 4-CP concentration at m i c r o m o l a r

independent

of

levels and dependent

on light power

in the supporting electrolyte suggest that the p r i m a r y oxidation

involves w a t e r

and produces hydroxyl radicals.

(16,25) .

Further

degradation

of

the

These radicals

organic

radicals

then

thus

attack 4-CP

formed

can

envisaged as being facilitated by dissolved oxygen, as recently p r o p o s e d

be

(26).

O2 4-CP At

0.i

+

mM

"OH -->[4-CPOH]" --> --> -->

4-CP,

the

increased

current

o x i d a t i o n via e l e c t r o n transfer

CO 2 + H20 suggests

(3)

contribution

a possible

from 4-CP directly to holes

from

in the valence band

of the photoanode. Decomposition likely

absent

of

in

4-CP

the

by

direct,

present

system,

radiation with wavelengths intermediate typically

of

may have

low,

precluded

for chlorate

and

its

because

photolysis

filters

<300 nm. We did not detect

photochemical

quite

uncatalyzed

degradation

it is oxidized

detection.

of

4-CP

rather

Also,

hydroquinone,

(15).

Its

rapidly.

electrolysis

(16,17)

effectively

a reported

concentration

Our

long

mixtures

(27), which was detected with platinum loaded

is most

eliminate

is

electrolyses

tested

negative

(28) titanium dioxide

particles. Experiments hydrogen This

at

the

depended

edges upon formed

as

without

somewhat

a result

potential

in a cell

showed

of

This

color

trapped

interface

the

possibility

in which p h o t o o x i d a t i o n

on the age of the anodes,

e x t e n d e d use.

anode s o l u t i o n

applied

Pt cathode

which

is p r e s u m a b l y

electrons

from

making

turn light blue at top

due to Ti(III)

resulting

of

is taking place.

the

in the

oxidations

films at the

(29). Thus, all the electrons injected into TiO2 during

p h o t o e l e c t r o l y s i s may not reach the external circuit. Experimental optimized

in

mineralization

conditions,

order

to

electrode performance,

achieve

of p o l l u t a n t s

a

practical

and efficient

and cell design need to be

system

capable

of

hydrogen production.

simultaneous R e s e a r c h along

these lines is u n d e r w a y in our laboratory.

Acknowledgements. of

This w o r k was supported

Environmental

assistance National

Protection.

during

Science

a

high

Foundation.

spectroscopic

analyses

and

International

Exchange

of

Fellowship

(1991-1992)

travel grant.

and

The

school

authors

J.

Neth

Scholars, the

thank

summer

The authors E.

financially by the C o n n e c t i c u t

U.S.

Jeffrey

research also

for

Meli

program

I.U.H

Bradley

thanks

Washington,

D.C.

for

Educational

Foundation

experimental

sponsored

thank M i c h a e l

SEM.

for

a

the

by

the

for Raman

Council

Senior in

Dept.

for

Fulbright

Pakistan

for a

1308

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