Research Journal of 
Recent 
Sciences 
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___
 
ISSN 2277
-
2502
 
 
Vol. 
2
(
ISC
-
2012
), 
82
-
89
 
(201
3
)
 
Res.
 
J.
 
Recent
.
 
Sci.
 
 
 
International Science Congress Association
 
 
 
 
    
82
 
Influence of Reaction Intermediates on the Oscillation in the Concentration 
of insitu formed Hydrogen peroxide during the Photocatalytic Degradation 
of Phenol Pollutant in Water on Semiconductor Oxides
 
 
Sindhu Joseph, Jyothi K.
P
.
, Suja P. Devipriya, Suguna
 
Yesodharan
 
and
 
Yesodharan E.P
.
 
School of Environmental Studies, Cochin University of Science and Technology, Kochi 682022, INDIA
 
 
Available online at: 
www.isca.in
 
Received
 
31
th
 
J
uly
 
2012, revised
 
29
th
 
December
 
2012, accept
ed
 
22
nd
 
January
 
201
3
 
 
 
 
Abstract
 
 
Phenols are common pollutants in many petrochemical industry wastewaters. Due to the stability of the aromatic ring their 
destruction requires extreme conditions. Photocatalysis using semiconductor oxides as catalysts is 
found to be an effective 
Advanced Oxidation Process (AOP) for the mineralisation of phenol.  The degradation proceeds through the formation of various
 
intermediates which eventually get mineralized to yield CO
2
 
and H
2
O. The intermediates identified are hyd
roquinone, catechol, and 
benzoquinone which are formed by the interaction of photogenerated OH radicals with phenol. These intermediates do not 
accumulate beyond a particular concentration even though the phenol degradation continues unabated. The insitu f
ormed H
2
O
2
 
concentration increases and decreases periodically in a wave like fashion indicating concurrent formation and decomposition. 
Externally added H
2
O
2
 
enhances the degradation rate of phenol initially due to the generation of more reactive OH radica
ls by 
inhibiting the recombination of photogenerated electrons and holes as well as by its own self decomposition. Externally added
 
catechol and hydroquinone inhibit the degradation of phenol initially.  However their influence on the fate of H
2
O
2
 
is not q
uite 
significant. The study also shows that the formation/decomposition of H
2
O
2
 
is concentration dependent and after the initial build 
up, the formation or decomposition takes precedence depending on the concentration and composition of the reaction system
. 
Possible reasons for the observed phenomenon are analysed and a mechanism is proposed. 
 
 
Keywords:
 
Photocatalysis, 
zinc 
oxide, 
titanium 
dioxide, 
hydrogen 
peroxide, 
phenol
, 
oscillation
.
 
 
Introduction
 
Heterogeneous photocatalysis is an Advanced Oxidation
 
Process (AOP), widely investigated for the degradation of 
organic pollutants in water and air
1
-
6
. The basic mechanism of 
photocatalysis involving hydroxyl radicals has also been well 
established
6
. Some of the major industrial pollutants in water 
degraded 
by photocatalysis include dyes, pesticides, 
petrochemicals etc. Semiconductor oxides such as TiO
2
 
and 
ZnO and their modifications by doping, immobilizing, metal 
deposition etc. are the most widely tested photocatalysts
7
-
9
. The 
process involves illumination
 
of the catalyst particles either 
dispersed as slurry in the contaminated aqueous solution or in 
immobilized form. Combination of sonolysis and 
photocatalysis, referred to as sonophotocatalysis is emerging as 
another AOP in water purification
10
. It is also
 
reported that the 
sonophotocatalytic degradation of phenol pollutant in water is 
more than the sum of individual sono and photocatalysed 
degradation, implying synergy in the combined process
11
.
 
 
Phenols are common pollutants in many petrochemical industry
 
wastewaters. Photocatalytic degradation of phenols in water has 
been investigated extensively
2,12
-
14
. The degradation mechanism 
involves complexation of OH radicals with the pi
 
(Ï€) system of the 
aromatic ring forming a pi complex in which the OH groups do not 
have a specific position in the molecule. The second step 
corresponds to the formation of a sigma complex between the 
carbon atom of the aromatic ring and the radical OH. T
he 
formation of the sigma complex is usually the rate determining step 
of the reaction
2,13
.
 
 
One of the reaction products in all these processes is H
2
O
2
.  
However, the concentration of H
2
O
2
 
does not increase beyond a 
limit, even though the degradation of t
he pollutant proceeds 
unhindered. It has been reported from our laboratories that the 
concentration of insitu formed H
2
O
2
 
undergoes an oscillatory type 
of behavior resulting in periodic increase and decrease, possibly 
due to concurrent formation and decomp
osition
15,16
. Externally 
added H
2
O
2
 
enhances the degradation rate significantly by 
inhibiting the rate of recombination of photogenerated electrons 
and holes, thereby increasing formation of reactive oxygen 
species and the degradation rate of pollutants
17,
18
. However, the 
attention of researchers was always focused on achieving 
optimum mineralisation of the pollutant and consequently the fate 
of concurrently formed H
2
O
2
 
has received much less attention. In 
the case of phenol, the eventual mineralisation pro
ceeds through a 
number of intermediates, of which some are fairly stable. These 
intermediates can influence the overall photocatalytic degradation 
rate of phenol including the fate of insitu formed H
2
O
2
. In this 
paper, we are reporting our findings on the 
influence of reaction 
intermediates on the photocatalytic degradation of phenol and the 
oscillatory behavior of insitu formed and externally added H
2
O
2
. 
 
Research Journal of 
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ISSN 2277
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Vol. 
2
(
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2012
), 
82
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89
 
(201
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Res.
 
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International Science Congress Association
 
          
 
83
 
 
a
 
 
 
 
 
 
 
 
b
 
Figure
-
1
 
Typical SEM image of (a) TiO
2
 
and
 
(b) ZnO
 
 
Material
 
and Methods 
 
ZnO and TiO
2
 
used in the study were supplied by Merck India 
Limited. In both cases the purity was over 99%.  The surface 
areas of TiO
2
 
and ZnO, as determined by the BET method were 
15 and 12 m
2
/g respectively. The average particle size of both 
ZnO and TiO
2 
was approx.1
0 µm, as determined by Scanning 
Electron Microscopy (SEM). Typical SEM of ZnO and TiO
2
 
are 
shown in figure 1.
 
 
Phenol AnalaR Grade (99% purity) from Qualigen (India) was 
used as such without further purification. All other chemicals 
were of AnalaR Grade or
 
equivalent.  The catalytic reactions 
were performed as reported earlier
19
. The concentration of 
phenol left behind was analyzed periodically by 
Spectrophotometry at 500 nm. The intermediates catechol (CC), 
hydroquinone (HQ) and benzoquinone (BQ) were moni
tored by 
HPLC using UV detector. H
2
O
2 
was determined by iodometry
16
.  
Mineralization was identified by the evolution of CO
2
.   
 
 
Results and Discussion
 
Investigations on the photocatalytic degradation of phenol using 
ZnO and TiO
2
 
catalysts under identical 
conditions showed that 
no significant degradation took place in the absence of UV light 
or the catalyst suggesting that both catalyst and light are 
essential to effect degradation.  ZnO is slightly more efficient as 
a photocatalyst compared to TiO
2
. The ul
timate reaction 
products are CO
2
 
and H
2
O formed by the mineralisation of 
phenol. H
2
O
2
 
is an end
-
 
product as well as intermediate which 
participate in further reactions with phenol and reactive oxygen 
species resulting in the formation of H
2
O and O
2
. The re
action 
intermediates detected are CC, HQ and traces of BQ. The 
concentration of the intermediates is comparatively small 
probably because they may be getting transformed into other 
products and eventually mineralized at the same rate or faster 
compared to 
parent phenol (figure 2).
 
 
 
 
Figure
-
2
 
Photocatalytic degradation of phenol and formation of 
intermediates on ZnO
 
 
While there is a clear increase and later stabilization in the 
degradation of phenol, the concentration of intermediates does 
not increase co
rrespondingly. The intermediates detected are the 
same in the case of both ZnO and TiO
2
 
even though the 
quantities are slightly different. BQ is not detected in the case of 
TiO
2
 
while a very small quantity is detected in one instance in 
the case of ZnO.  I
ntermediates such as pyrogallol, 2
-
hydroxy 
benzoquinone and 1,2,4 benzene triol reported by other authors
2 
are not detected in our experiments. The H
2
O
2
 
formed insitu 
increases initially. However, after reaching a maximum its 
concentration decreases and re
aches a minimum where it starts 
rising again. This periodic increase and decrease in the 
concentration of H
2
O
2
 
in a wavelike fashion indicates 
concurrent formation and decomposition (fig
ure
-
3).
 
 
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Figure
-
3
 
Formation and decomposition of H
2
O
2
 
during the 
pho
tocatalytic degradation of Phenol
 
 
One of the most important operating parameters in the 
photocatalytic degradation of phenol in presence of 
semiconductors is the pH of the medium because it affects the 
surface charge or the isoelectric point of the cataly
st particles, 
size of the catalyst aggregates and the positions of the valence 
and conduction bands. In the case of ZnO and TiO
2
, the effect of 
pH is often correlated with their respective Point of Zero Charge 
(PZC). When the pH of the medium is less than 
the PZC of the 
catalyst the surface of the catalyst becomes positively charged 
and the neutral or negatively charged contaminant ions can get 
adsorbed onto the activated catalyst surface leading to enhanced 
degradation. In the case o both ZnO and TiO
2
 
acid
ic pH favours 
such a situation. The natural pH of water containing small 
quantities of phenol contaminant is in the acidic range, approx. 
5.5 and 6 in ZnO and TiO
2
 
suspension respectively. Hence all 
studies in presence of ZnO and TiO
2
 
were conducted in the
 
natural pH of phenol solution which also happens to be the 
optimum pH for degradation of phenol
11
. 
 
 
Similarly, the optimum catalyst loading of ZnO and TiO
2
 
for 
phenol degradation has been determined experimentally and the 
values are100 and 250 mg/L respe
ctively. Beyond the optimum 
loading the light photon absorption coefficient decreases 
radially. However such a light attenuation over the radial 
distance does not obey the Beer
-
Lambert law due to the strong 
absorption and scattering of light photons by the
 
catalyst 
particles
20
. Excess catalyst particles lead to screening of light 
which reduces the effective surface area of the semiconductor 
being exposed to illumination leading to decreased 
photocatalytic efficiency. Investigations on the effect of 
concentr
ation of phenol on its photocatalytic degradation show 
that the rate of degradation is optimum at 30 mg/L in presence 
of both ZnO and TiO
2
 
(
f
igure 4)
 
 
Figure
-
4
 
Effect of concentration of Phenol on its photocatalytic 
degradation on ZnO 
and
 
TiO
2
 
 
All furthe
r investigations in this report were carried out using 
the optimized parameters as above, unless mentioned otherwise. 
 
 
The effect of addition of major intermediates catechol and 
hydroquinone at different concentrations on the rate of phenol 
degradation in
 
presence of ZnO are shown i
n figures 5 and 6 
respectively.
 
 
 
Figure
-
5
 
Effect of externally added intermediate Catechol (CC) on 
the 
photocatalytic degradation of phenol 
on ZnO
 
 
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85
 
 
Figure
-
6
 
Effect of externally added intermediate Hydroquinone (HQ) 
on the 
ph
otocatalytic degradation of phenol 
on ZnO
 
 
The rate of degradation of phenol is retarded slightly, probably 
because the intermediates are also getting adsorbed on the 
surface, thereby reducing the surface concentration and 
consequent activation of phenol m
olecules. The intermediates 
are also getting degraded at comparable or even faster rate. 
Once they are completely removed from the surface, majority of 
the vacated sites will be occupied by phenol and its degradation 
proceeds smoothly without any hindrance
. The behavior is 
identical in the case of TiO
2
 
also. Evaluation of comparative 
adsorption of phenol, catechol and hydroquinone in equimolar 
concentrations on ZnO and TiO
2
 
show that the extent of 
adsorption is approximately the same. The inhibition of phen
ol 
degradation by CC as well as HQ is increasing with increase in 
their concentration. At higher concentrations the intermediates 
occupy more surface sites and because of the concentration 
advantage, at least part of the surface sites vacated by 
mineralize
d intermediates will be occupied by fresh molecules 
of the same. In the absence of externally added intermediates, 
the concentration of insitu formed intermediates is very small 
and because they degrade concurrently, the relative 
concentration of phenol wi
ll always be more.  Hence the surface 
will always be preferentially occupied by phenol and influence 
of the intermediates on the degradation will be negligible. 
 
 
The effect of the intermediates on the fate of H
2
O
2
 
is plotted in 
figure 7. 
 
 
In presence of 
the added intermediates also the oscillation is 
sustained. However, the H
2
O
2
 
at the maxima as well as the 
minima in the oscillation curve is more in presence of the added 
intermediate. This implies that the rate of formation is enhanced 
by the intermediate
 
while the rate of decomposition is inhibited.  
This can be attributed to the relatively faster rate of degradation 
of catechol or hydroquinone which produces more OH radicals 
compared to phenol only. This is further verified by 
investigating the effect of
 
concentration of the intermediates on 
the oscillation in the concentration of H
2
O
2 
(figures 8 a,b) in 
presence of ZnO.  The results show that the maxima and minima 
of H
2
O
2
 
in the oscillation curve are increasing with increase in 
the concentration of the i
ntermediate.
 
 
 
Figure
-
7
 
Effect of added intermediate CC 
and
 
HQ on H
2
O
2
 
formed 
during the 
photocatalytic degradation of phenol
 
 
 
Figure
-
8 (a)
 
Effect of intermediate CC at different concentration on H
2
O
2
 
the photocatalytic degradation of phenol in presence
 
of ZnO
 
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Figure
-
8 (b)
 
Effect of added intermediate HQ on H
2
O
2
 
formed during the 
photocatalytic degradation of phenol 
in presence of ZnO
 
 
The influence of concentration of the intermediate is less 
pronounced in the case of TiO
2
 
catalysts (not shown here). 
 
The 
surface composition as well as the composition of the catalyst
-
bulk interface is more or less the same in presence of the 
intermediate.
 
 
The basic mechanism of the photocatalytic degradation of 
phenol involves the formation of OH radicals initiated by
 
the 
activated catalyst surface followed by their interaction with the 
substrate as well as various intermediates such as catechol, 
hydroquinone and p
-
benzoquinone as follows:
 
2
.  
 
Semiconductor + h ν → h
+
 
+ e
-
 
 
 
 
         
(1)
 
h
+
 
+ e
-
 
  
→  Heat
 
 
 
 
 
         
 
(2)
 
h
+
 
+ H
2
O
 
  
→  H
+
 
+ 
.
OH
 
 
 
 
         
 
(3)
 
OH + 
.
OH→ H
2
O
2
 
 
 
 
         
 
(4)
 
O
2 
+ e
-
 
  
→  O
2
-
.
 
 
 
 
 
         
 
(5)
 
O
2
-
. 
 
+ H
2
O → HO
2
.
 
+ 
-
OH
 
 
 
 
         
 
(6)
 
h
+
 
+ 
-
OH → 
.
OH
 
 
 
 
 
     
    
 
(7)
 
HO
2
.
 
+ HO
2
.
 
→ H
2
O
2
 
 
 
 
         
 
(8)
 
Various reactive oxygen species formed as above will in
teract 
with phenol as shown in 
scheme 
1.
 
Reactive oxygen species + phenol → Intermediates  → H
2
O + 
CO
2
 
 
 
 
 
 
          
(9)
 
 
 
Sch
eme
-
1
 
Mechanism of the photocatalytic mineralisation of phenol on ZnO
 
 
OH
 
 
  
O
 
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
  
OH
 
 
  
 
O
 
 
OH
 
Further
-
oxidised
 
Products
 
 
CO
2
 
+ H
2
O
 
 
     
CC
 
    
HQ
 
    
B
Q
 
          
PG
 
HHQ
 
H
B
Q
 
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It is also possible for the photogenerated holes to react with 
adsorbed phenol to yield dihydroxycyclohexadienyl radicals via 
phenol radical cation
2,21
. The energy of holes in the valen
ce band 
of TiO
2
 
is approx. 
-
7.5eV which is high enough for the holes to 
react with H
2
O and organic compounds.  This would happen if 
phenol is strongly adsorbed onto the surface.  Adsorption studies 
of very dilute solution of phenol in water ( 30 mg/L) on 
ZnO 
and TiO
2
 
shows that phenol does get adsorbed on to the surface 
under our reaction conditions, though not very significantly. 
Hence the possibility of scheme 2 cannot be ruled out.
 
 
Based on the observations from the direct photosonochemical 
degradation
 
of phenol in water, Wu etal
13 
suggested that reaction 
of hydroxyl radical with phenol in presence of oxygen leads to the 
formation of peroxyl radicals, which form hydroquinone and 
catechol after eliminating superoxide radicals and rearranging the 
aromatic
 
system. These compounds degrade to carboxylic acids 
and eventually to CO
2
. However, in the present study carboxylic 
acid intermediate is not detected probably because of its shorter 
lifetime or a different mechanism of degradation altogether.
 
 
Under mild 
oxidizing conditions provided by H
2
O, direct 
conversion of phenol to CO
2 
and H
2
O is not probable.  But 
water can take part in the reaction along with the dissolved 
oxygen present in the medium to yield hydroxylated 
intermediates and then the final product
2
2
. The intermediates 
formed during the degradation depend on the reaction 
conditions and the reagents used. Velasco etal
23
 
reported the 
presence of hydroquinone and benzoquinone during the 
photocatalytic degradation of phenol in presence of 
carbon/titania 
composite. In the presence of Fenton’s reagent, 
more intermediates such as catechol resorcinol, hydroquinone, 
p
-
benzoquinone and o
-
benzoquinone have been detected
24
. 
When the degradation was carried out with Fe(II)
-
Fe(III) green 
rust, only non
-
aromatic int
ermediates were detected
25
. 
Sivalingam etal
14
 
reported formation of catechol and 
hydroquinone when the degradation was carried out in presence 
of Degussa P
-
25.  Pyrogallol also has been detected during the 
photocatalytic degradation of phenol by base metal
-
substituted 
vanadates
22
. In the present study we could detect the presence of 
only three intermediates; catechol, hydroquinone and traces of 
benzoquinone which also disappeared eventually. This indicates 
that the intermediates also get degraded fast and a
s time 
progresses, as the phenol concentration decreases, the rate of 
degradation of the intermediate becomes even faster. The rates 
of degradation of phenol, catechol and hydroquinone under 
identical conditions on ZnO and TiO
2
 
are given in figure 9 
which 
shows
 
that the rates are comparable.
 
 
 
Scheme
-
2
 
Reaction of photogenerated hole with H
2
O and phenol
 
 
 
Figure
-
9
 
Comparative initial rate of degradation of Phenol, Catechol and Hydroquinone
 
  
OH
 
  
OH
 
 
OH
 
OH
 
+ h
+
 
H
2
O
 
-
 
H
+
 
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-
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88
 
The present study was carried out at 
the natural pH of the 
phenol solution
-
catalyst suspension, ie 5.6
-
5.7. Serpone etal 
26
 
have reported that the intermediates depend on the pH 
conditions in the case of ultrasonic removal of phenol. They 
reported that the principal intermediates at pH 3 are 
catechol, 
hydroquinone and p
-
benzoquinone while only catechol and 
hydroquinone were detected at pH 5.4
-
5.7. No intermediates 
were observed at pH 12. In the current study, the intermediates 
are the same in case of both ZnO and TiO
2 
indicating that in the 
ca
se of photocatalysis, pH is the prime factor that determines 
the mechanism of the reaction. 
 
 
The periodic change in the concentration of H
2
O
2
 
and the 
oscillatory behavior suggest that its formation and 
decomposition takes place simultaneously. The decreas
e in 
concentration can be either due to its parallel decomposition 
into water and oxygen and/or its participation in the degradation 
of phenol
15,27,28
.  At the same time the removal of phenol 
continues. H
2
O
2
 
formed consequent to reactions (1) to (8) 
decomp
oses simultaneously as follows: 
 
H
2
O
2
 
+ hν (λ<380 nm) → 2 
.
OH                     
                    
(10)
 
2 h
vb
+ + H
2
O
2
 
→   O
2
 
+ 2 H
+ 
                          
                    
(11)
 
2 e
cb
-
 
+ 2 H
+
 
+ H
2
O
2
 
→ 2 H
2
O                         
                 
   
(12)
 
H
2
O
2
 
+ 
.
OH→ H
2
O +HO
2
.                                              
 
                    
(13)
 
H
2
O
2
 
+ HO
2
.
 
→ H
2
O + 
.
OH + O
2                                 
                             
 
(14)
 
 
Being a complex free radical system, other interactions le
ading 
to the formation and decomposition of H
2
O
2
 
are also possible. 
 
 
Since both the formation and decomposition are initiated by free 
radicals and reactive oxygen species, the surface plays an 
important role.  Phenol and various intermediates formed have 
similarity in the adsorption and photocatalytic behavior. Hence 
the intermediates are  not expected to influence the oscillatory 
behavior of H
2
O
2
 
significantly, at least in the concentration  
range of insitu formation.  However, the concentrations of H
2
O
2
 
at the maxima and minima of the curve increase in the presence 
of the added intermediates.  This indicates that the intermediates 
enhance the formation of H
2
O
2
 
and inhibits its decomposition, 
though slightly. The formation and decomposition of H
2
O
2
 
proceed
s in parallel after build up of a critical initial 
concentration. Depending on the concentration of the substrate 
and H
2
O
2
 
at any point of time, either o
f the process will 
predominate.
 
 
Conclusion
 
Photocatalytic degradation of phenol proceeds through two 
m
ajor intermediates; catechol and hydroquinone. The 
intermediates do not influence the rate of degradation 
significantly as they themselves undergo degradation and 
eventual mineralisation under the experimental conditions.   
H
2
O
2 
formed in the process under
goes simultaneous formation 
and decomposition resulting in oscillation in its concentration. 
Depending on the relative concentration of the substrate, stable 
intermediates and H
2
O
2
, the formation or decomposition will 
predominate. The degradation of phenol
 
as well as the formation 
and decomposition of H
2
O
2 
is initiated by 
.
OH radicals 
generated at the photo
-
activated semiconductor oxide surface. 
The degradation ultimately leads to complete mineralization of 
phenol. The study provides convincing experimental
 
evidence 
for the concurrent formation and decomposition of H
2
O
2 
in 
photocatalytic systems. Further, it demonstrates that 
semiconductor mediated photocatalysis can be developed into a 
viable route for the removal of organic water pollutants.  
 
 
Acknowledge
ment
 
The authors gratefully acknowledge the financial support from 
the University Grants Commission (UGC)/Dept of Science and 
Technology (DST), Government of India to  SJ (UGC JRF), 
JKP (UGC JRF) and  SPD (DST WOS
-
A).  
 
 
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1.
 
Herrmann J
.
M
.
, Heterogen
eous Photocatalysis: 
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Okamoto K
.
, Yamamoto Y
.
, Tanaka H
.
, Tanaka M
.
 
and 
Itaya A
.
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TiO
2
 
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-
2022 
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.
 
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.
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, Arabindoo
 
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.
 
and  Murugesan V
.
, Solar photocatalytic 
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, 
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, 
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Uchida H
.
, Itoh S
.
 
and Yoneyama H
.
, Photocatalytic 
dec
omposition of propyzamide using TiO
2
supported on 
activated carbon, 
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Turchi C.S
.
 
and Ollis D.F
.
, Photocatalytic degradation of 
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hydroxyl radical attacks, 
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Wu C.G
.
, Chao C.C
. and 
Kuo F.T
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