Modern Research in Catalysis, 2012, 1, 1-9
http://dx.doi.org/10.4236/mrc.2012.11001 Published Online April 2012 (http://www.SciRP.org/journal/mrc)
Influence of H-Type and L-Type Activated Carbon in the
Photodegradation of Methylene Blue and Phenol
under UV and Visible Light Irradiated TiO2
Juan Matos*, Karina Quintana, Andreina García
Engineering of Materials and Nanotechnology Centre, Venezuelan Institute for Scientific
Research (I.V.I.C.), Caracas, Venezuela
Received March 2, 2012; revised April 3, 2012; accepted April 13, 2012
ABSTRACT
Photodegradation of methylene blue (MB) and phenol (Ph) on TiO2 in presence of H-type and L-type activated carbons
(AC) was studied. Photodegradation of MB and Ph were studied under two different lamps and results were compared
against those obtained on a commercial TiO2. Apparent first order rate constant for the degradation of MB was higher in
presence of any AC in comparison of TiO2 alone while only in presence of ACCO2-800 phenol was photodegradated in
shorter irradiation time than that required by TiO2. It can be concluded that TiO2 enhances its photoactivity by a factor
up to 8.7 in the degradation of MB in presence of AC and this effect is associated to the specific surface properties of
AC.
Keywords: Photocatalysis; TiO2; Activated Carbon; Methylene Blue; Phenol
1. Introduction
An important quantity of the total world production of
azo-dyes is released in textile effluents [1]. Different
technologies for the removal of dyes are adsorption, bio-
and chemical degradation methods including advanced
oxidation technologies as heterogeneous photocatalysis.
Since heterogeneous photocatalysis with TiO2 emerged
as an efficient method for purifying water and air [2,3]
several attends such as ion doping or metal depositions
have been used [4] to increase its photoefficiency. An-
other way to possibly increase the photoefficiency of
TiO2 consists of adding an inert co-adsorbent such as
activated carbon (AC) [5,6]. A synergy effect between
both solids has been observed in the photocatalytic deg-
radation of model pollutants [7,8]. This has been ascribed
to a contact interface that promotes an appropriated dif-
fusion of pollutants from AC to photoactive titania and
introduce changes in the semiconductor properties [5-8].
Photocatalysis and adsorption with activated carbon (AC)
have received an increase attention for the degradation of
different dyes [9-11] and halo phenol molecules [7]
where recently, we have showed that surface function-
alization of AC play an important role on TiO2 photoac-
tivity on 4-chlorophenol degradations [7]. The objective
of this work is to study the photodegradation of methyl-
ene blue (MB) as a model dye and phenol (Ph) as a
model aromatic molecule on UV- and visible light irradi-
ated TiO2 in presence of H-type and L-type AC which
are characterized by different texture and surface func-
tionalities.
2. Experimental
Methylene blue (MB) and phenol (Ph) were analytical
grade and purchased from Aldrich. For comparative
purpose, photocatalyst was TiO2 P25 (Degussa). H-type
AC were prepared by physical activation of a soft wood
under CO2 flow at 800˚C () or by pyrolysis
under N2 flow at 1000˚C (2
N-1000 ) while L-type AC
were prepared by impregnation with 5% (w/w) of ZnCl2
(2
ZnCl -5%
AC ) and H3PO4 (34
HPO -5%
AC ) following activa-
tion under N2 flow at 450˚C. Samples were characterized
by adsorption-desorption N2 isotherms, infrared spectros-
copy (FTIR) and surface pH (pHPZC). The experimental
set-up [11] consists in an open to air batch photoreactor
of 200 mL made of Pyrex. Irradiation was provided with
two different lamps [11] with different UV proportions.
One a Hg lamp (82.9 W·m–2 of UV and 362.6 W·m–2 for
visible light) and metal halide (MH) lamp (70.2 W·m–2 of
UV and 452.5 W·m–2 of visible) and a last one, a sodium
(Na) lamp, 99% visible light (8.4 W·m–2 of UV and 831.6
W·m–2 for visible light). Photocatalytic tests were per-
2
CO -800
AC
AC
*Corresponding author.
C
opyright © 2012 SciRes. MRC

J. MATOS ET AL.
2
formed at 25˚C with 62.5 mg TiO2 and 6.2 mg AC under
stirring in 125 mL of MB, 25 ppm (78.2 μmol·L–1) initial
concentration or in 125 mL of phenol, 50 ppm (0.5 × 10–3
mol· L –1) initial concentration. Samples were maintained
in the dark by 60 min to complete adsorption at equilib-
rium before irradiation. After centrifugation of MB ali-
quots at some selected reaction times, samples were ana-
lyzed by UV-spectrophotometer at 664 nm. For the case
of phenol and their main intermediate products, hydro-
quinone (HQ) and benzoquinone (BQ), Millipore disks
(0.45 µm) were used to remove particulate matter before
HPLC analysis. Although non-agglomerate solid parti-
cles may pass through these membranes, our experience
showed that the performance of the chromatographic
column was not impaired for a long period of use. The
HPLC system adjusted at 270 nm for the detection of
phenol and of the main intermediate products was used.
A reverse-phase column with a mobile phase composed
of acetonitrile and deionized doubly distilled water was
used. The v/v ratio CH3CN/H2O was 10/90 and the flow
rate was 1 ml/min.
3. Results and Discussion
3.1. Characterization
Table 1 shows textural properties and pHPZC of photo-
catalysts. AC developed high surface areas BET (SBET)
and the main pore width in the the microporous range.
For the case of mixed system TiO2-AC, SBET decreases
one order magnitude with respect to activated carbon.
Table 1. BET surface area (SBET), mean pore diameter (D)
and surface pH (pHPZC).
Sample SBET (m2·g–1) D(Å) pHPZC
TiO2 P25 45.17 ± 0.16 577.86 6.5
2
CO -800
AC 942.86 ± 1.41 6.29 8.5
TiO2–
2
CO -800
AC 86.46 ± 0.48 974.01 6.7
2
N -1000
AC 644.27 ± 0.62 5.90 8.9
TiO2–
2
N -1000
AC 60.40 ± 0.39 1051.78 6.7
2
ZnCl -5%
AC 689.39 ± 0.61 5.89 6.0
TiO2–
2
ZnCl -5%
AC 92.51 ± 0.50 979.03 6.4
34
HPO -5%
AC 246.66 ± 0.44 5.94 4.0
34
2-H PO-5%
TiO 63.38 ± 0.39 1034.43 6.3
This fact can be attributed to a strong interaction between
both solids [12]. It can be seen from Table 1 that H-type
AC presented basic pHPZC while L-type AC showed acid
pHPZC which suggest the presence of basic and acid oxy-
genated functional groups on the surface of H- and
L-type AC, respectively. This inference can be verified
by FTIR analysis which is shown in Figure 1. It can be
seen that functional surface groups principally are basic
as cyclic ethers (-C-O-C-) and quinones (C=O) [7,13].
For the case L-type AC, these showed acid pHPZC and by
FTIR can be observed that the main functional surface
group was carboxylic acid (C=O). Furthemore, cyclic
ethers were also detected (-C-O-C-). Finally, it should
0
50
100
5001000150020002500300035004000
% T
Wavenumber (cm
-1
)
TiO
2
0
50
100
5001000150020002500300035004000
%T
Wavenumber (cm-1)
AC
CO2 800
0
50
100
5001000150020002500300035004000
%T
Wavenumber (cm
-1
)
TiO
2
-A C
CO2 800
Ti-O
H
2
O
Ad sorb ed
Ti-OH
C=O
C-O-C
C=C
0
50
100
5001000150020002500300035004000
% T
Wavenumber (cm
-1
)
0
50
100
50
0
100015002000250030 0035004000
% T
Wavenumber (cm
-1
)
AC
H3PO4-5%
0
50
100
5001000150020002500300035004000
% T
Wavenumber (cm
-1
)
TiO
2
-AC
H3PO4-5%
TiO
2
Ti-O
H
2
O
Adsorbed
Ti-OH
P-O-C
C-O-C
C=O
34
HPO -5%
AC
34
2-H PO-5%
TiO AC
2
CO -800
AC
2
2-CO -800
TiO AC
Figure 1. FTIR spectra of TiO2, , , , and .
2
CO -800
AC 34
HPO-5%
AC 2
2-CO -800
TiO AC34
2-H PO-5%
TiO AC

J. MATOS ET AL. 3
be remarked the presence of phosphates in 34
HPO
[7,8]. Figure 1 shows that TiO2 presented a broader peak
in the region of bulk Titania in presence of AC. Also, the
corresponding peaks in the AC clearly decreased in the
binary materials probably by the coordination from car-
bon to the metallic centre in TiO2 [7]. A similar behavior
in the FTIR spectra for the other AC and the binary ma-
terials was found [7]. Figure 2 shows the XRD patterns
of TiO2, AC and the binary materials TiO2-AC. It can be
seen that no changes in the corresponding XRD patterns
AC for the case of TiO2-AC in comparison than that obtained
for TiO2 alone. The only change detected in the XRD
pattern of the binary materials was a remarkable decrease
in the main peaks attributed to a dilution effect by means
of the presence of AC.
3.2. Adsorption in the Dark of MB and
Photodegradation
Figure 3 shows the kinetics of adsorption in the dark of

2
ZnCl -5%
AC
2
2-ZnCl -5%
TiO AC
2
N
-1000
AC
34
HPO -5%
AC
2
2-N -1000
TiO AC
34
2-HPO-5%
TiO AC
2
CO -800
AC
2
2-CO -800
TiO AC
Figure 2. XRD patterns of TiO2, AC and binary materials TiO2-AC.

J. MATOS ET AL.
4
0
1
2
3
4
5
0 20406080
MB adsorbed (μmol)
Time (min)100
AC CO2 800
TiO2-AC CO2 800
TiO2
AC CO2 800
TiO2– AC CO2 800
TiO2
Figure 3. Micromoles adsorbed in presence of some selected solids.
MB on AC and TiO2-AC. In all cases, adsorption oc-
curred within 30 min but to ensure the equilibrium of
adsorption, a period of 60 min of adsorption in the dark
was selected prior to the photodegradation experiments.
The results indicated that there are no additive effects in
the adsorption capacities of both solids after they are
mixed. It can be ascribed to a strong interaction between
TiO2 particles and AC [7]. Kinetics of photocatalytic
disappearance of MB in presence of TiO2-AC under each
lamp was performed. Figure 4 shows an example of the
kinetic of MB photodegradation under UV irradiated
TiO2, TiO2-2
CO -800 , and TiO2-2 N -1000 samples.
Assuming a first-order reaction rate [7], linear transfor-
mations (figure inset Figure 4) from the kinetic data
were performed to estimate the apparent first-order rate
constant (kapp). Table 2 contains a summary of the ki-
netic results obtained for the MB photodegradation. The
apparent first-order rate constant permits to estimated the
photoactivity relative to TiO2 defined as
AC AC
2
relapp-i app-TiO
kk 



and the synergistic effect in the
photoactivity between TiO2 and AC materials defined by
the expression: 2
Fi app-TiOapp-AC

It can be seen from kapp values in Table 2 that for both
lamps used the binary materials TiO2-AC have higher
photoactivity than that obtained on TiO2 alone and this
enhancement in the photoactivity was clearly higher with
the MH lamp which has higher proportion of visible light
with respect to the Hg lamp with an enhancement in the
photoactivity up to 8.7 and 6.0 times higher than TiO2 on
TiO2-and TiO2-2
N -1000. Both 2 CO-800
and 2
N-1000 can be classified as H-type [5] AC be-
cause its surface oxygenated functional groups are basic
in nature as suggest the FTIR spectra from Figure 1 and
the basic pHPZC in Table 1. In addition, it should be
pointed out that the photocatalytic activity of activated
carbons is lower than that of TiO2 alone, however, a clear
synergistic effect between both solids was estimated
(Table 2) being clearly higher under visible light irradia-
tion.
app-
Ikk k

 .
2
CO -800
AC
AC
AC AC
On the other hand, Table 2 shows that photoactivity of
the binary materials TiO2-2
ZnCl -5%
A
C and TiO2-34
HPO -5%
AC
were only about 3 times higher than that on TiO2 alone in
any of cases of lamps studied. This fact has been attrib-
uted to a more acidic surface pH and to a lower surface
area of these L-type AC (Table 2) [6,8]. In previous
works [5,7] we have showed that oxygenated functional
groups in the surface of AC play a double role in photo-
catalytic reactions. First, these AC can play the role of
electron carriers that could inhibit the recombination of
photoelectrons to improve the photoactivity of TiO2 and
secondly, under visible light irradiation several functional
groups on carbon’s surface are able to excited electrons
from π to π* orbital to then be injected into the conduc-
tion band of TiO2 [11]. This phenomena has been de-
scribed by our group as a photo-assisting process [12,14].
3.3. Adsorption in the Dark of Phenol and
Photodegradation
Kinetics of adsorption in the dark of phenol on AC and
TiO2-AC was performed before irradiation tests. Figure
5 shows that phenol adsorption occurred within 30 min
but 60 min of adsorption in the dark was selected prior to
the photodegradation experiments to ensure the equilib-
rium of adsorption. The results indicated that there are no
additive effects in the adsorption capacities of both solids
after they were mixed indcating a strong interaction i

J. MATOS ET AL. 5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0306090120 150 180 210 240 270 300 330 360
Ct/Co
Time (min)
Photolysis Lamp Hg
TiO2 Lamp Hg
TiO2-AC N2 1000 Lamp Hg
TiO2-AC CO2 800 Lamp Hg
Photolysis Lamp Hg
TiO2 Lamp Hg
TiO2– AC N2 1000 Lamp Hg
TiO2– AC CO2 800 Lamp Hg
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
TiO
2
Lamp Hg
0 30 60 90 120
Time
(
min
)
a14.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Ln (Co/Ct)
Figure 4. Kinetic of disappearance of MB on TiO2-AC under Hg Lamp (A1) and lineal regression of kinetic data (a1).
Table 2. Summary of kinetics parameters obtained in the photodegradation of MB.
Sample Adsa(%) kapp-UV ×10–3 (min–1) IF-UVb φrel-UV c kapp-Vis × 10–3(min–1) IF-Visb φrel-Vis c
TiO2 P25 25 12.10 1.0 1.0 4.60 1.0 1.0
2
CO -800
AC 33 4.04 - 0.3 2.59 - 0.6
TiO2–
2
CO -800
AC 31 59.12 3.7 4.9 39.89 5.5 8.7
2
N -1000
AC 23 3.33 - 0.3 2.24 - 0.5
TiO2–
2
N -1000
AC 28 34.48 2.2 2.8 27.80 4.1 6.0
2
ZnCl -5%
AC 27 1.10 - 0.1 2.54 - 0.6
TiO2–
2
ZnCl -5%
AC 26 28.85 2.2 2.4 14.63 2.0 3.2
34
HPO -5%
AC 14 0.71 - 0.1 1.01 - 0.2
34
2-H PO-5%
TiO 23 39.41 3.1 3.3 13.39 2.4 2.9
aAfter 60 min of adsorption in the dark. bSynergy defined as IF =


2
app-iapp-TiO app-AC
kk k
. cRelative photoact ivity defined as φrel =

2
app-i app-TiO
kk .
between TiO2 and AC [6,7].
Figure 6(a) shows the kinetics of disappearance of
phenol in absence of solids (direct photolysis) and under
some selected solids irradiated with UV light (Hg lamp).
Linear transformations from the kinetic data were per-
formed assuming a first-order reaction rate (Figure 6(b)).
Apparent rate constant of first-order (kapp) and photo-
catalytic activity relative to TiO2 alone (Aphoto) defined as
2
app-iapp-TiO , were estimated. These values are compiled
in Table 3. It can be seen in Figure 6 that disappearance
of phenol by direct photolysis without solids and under
irradiated AC were negligible. Table 3 shows that kapp
was higher under the Hg Lamp with respect to MH Lamp,
for all systems studied. TiO2-2
CO-800 presented higher
photoactivity that TiO2 alone. The other binary materials
TiO2-AC showed moderate photoactivity under Hg lamp
and inhibition of the photoactivity under MH lamp. The
enhancement in the photoactivity of TiO2 can be due to
the presence of a common contact interface between both
solids as reported elsewhere for the case of 4-chloro-
phenol [7,8].
In spite of phenol adsorbed in the dark (Table 3) on
TiO2- is lightly higher than that adsorbed on
TiO2-2
CO , it can be seen from Figure 6(a) that photo-
catalytic activity of the TiO2-2
CO binary material is
higher than that of TiO2-2
N. This behavior can be
attributed to two main reasons. First, TiO2-2 has a
lower BET surface area than that of TiO2-2
CO (Table
1) with a concomitant less capability to adsorb both
phenol as the main intermedroducts. In addition,
2
N
AC
AC
AC
AC
N
AC
AC
k k
AC
iate p

J. MATOS ET AL.
6
0
5
10
15
20
25
30
0 20406080100
Adsorption
%
Time (min)
120
TiO2 P25
TiO2-AC CO2 800
TiO2-AC N2 1000
AC CO2 800
AC N2 1000
TiO
2
P25
TiO
2–
AC CO
2
800
TiO
2–
AC N
2
1000
AC CO
2
800
AC N
2
1000
Figure 5. Phenol adsorbed in the dark on some selected solids.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0100 200 300 400 500
Ct/Co
Time (min)
TiO2 P25 Lamp Hg
TiO2-AC CO2 800 Lamp Hg
TiO2-AC N2 1000 Lamp Hg
AC CO2 800 Lamp Hg
AC N2 1000 Lamp Hg
Photolysis Lamp Hg
TiO
2
P25 Lamp Hg
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
AC CO
2
800 Lamp Hg
AC N
2
1000 Lamp Hg
Photolysis Lamp Hg
(a)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
030 60 90120150180
Ln( Co/Ct)
Time (min)
TiO2 P25 Lamp Hg
TiO2-AC CO2 800 Lamp H
g
TiO2-AC N2 1000 Lamp Hg
TiO
2
P25 Lamp Hg
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
(b)
Figure 6. (a) Kinetic of disappearance of phenol on TiO2–AC under Hg Lamp; (b) Lineal regression of the kinetic data.

J. MATOS ET AL. 7
Table 3. Kinetic parameters in the degradation of phenol.
UV irradiation (Hg lamp) Visible irradiation (MH lamp)
Sample Ads % kapp×10–3 (min–1) R2 a Aphotob k
app×10–3 (min–1) R2 a Aphotob
TiO2 6.8 5.02 0.9973 1.00 4.41 0.9980 1.00
TiO2–
2
CO -800
AC 14.8 6.83 0.9936 1.4 5.02 0.9866 1.2
TiO2–
2
ZnCl -5%
AC 13.1 5.22 0.9878 1.1 3.45 0.9826 0.8
TiO2–
2
N -1000
AC 15.8 4.82 0.9894 1.0 3.16 0.9764 0.7
TiO2–
34
HPO -5%
AC 10.3 5.53 0.9883 1.1 4.14 0.9782 0.9
aR is the square factor of the lineal regression. bPhotocatalytic activity relative to TiO2 defined as Aphoto =


2
app-i app-TiO
kk .
our group has been previously reported for the case of
the 4-chlorophenol photodegradation [5,7] that 2
CO
has a more intimated interaction than 2
N with the Ti
atoms in TiO2. We have shown that this interaction oc-
curs by means of a common contact interface [7] spon-
taneously created during reaction between both solids by
the coordination the oxygenated functional groups on
2
CO , mainly cyclic ethers and carboxylate anions
(Figure 1). This interaction is clearly lower for the case
of TiO2 and 2
N than with 2 CO because
has lower oxygen surface composition than that of ,
about 7% against 12 wt%, respectively [5].
AC
2
N
AC
2
CO
AC
AC
AC
AC AC
Figure 7 shows the kinetic of hydroquinone (HQ) and
benzoquinone (BQ) appearance and disappearance dur-
ing phenol photodegradation under UV-irradiated some
selected solids. These two molecules were the main in-
termediates products observed in all samples studied and
under both types of irradiation. The maximum time of
appearance and time require for the total disappearance
of intermediates are lower for the binary materials
TiO2-AC with respect to TiO2 alone, only in presence of
mixed system that showed higher photocatalytic activity.
This fact is an indicative that intermediates products are
also photodegradated in shorter irradiation time than that
on TiO2 reported by our group for the case of 4-chloro-
phenol [7,8]. As we appointed above, an explanation for
the apparent synergy effect can be based on the conven-
tional Langmuir-Hinshelnwood mechanism with the rate
being proportional to the surface coverage θ varying as:

ads eqads eq
rkkK C1K CKiCi 

 
 
. (1)
being: Kads and Ki correspond to the adsorption constants
of phenol and the intermediate i, Ceq and Ci is the phenol
and intermediate concentration in solution after achieve
the equilibrium adsorption in the dark. Owing to the
similarity of the reactants and of the main initial aromatic
intermediates formed, the term ΣKi·Ci can be estimated
as constant, thus explaining the apparent first order:

ads eqads eq
rkK C1KiCiK C



(2)
The nature of the intermediate main products (HQ and
BQ) is the same for TiO2-AC as for neat TiO2. This con-
firms that reaction mechanism has not been altered nor
changed by the addition of AC, or at least, for these car-
bons [15]. UV photons create electron hole pairs in Tita-
nia
2
TiO hvep


 (3)
which separate because of electron transfer reactions:

22
OOae  ds
H
(4)
2
OOp

. (5)
As we have already appointed in previous works [15]
radicals created by Equation (5) react with phenolic
compounds to produce hydroxylated aromatic compounds,
mainly hydroquinone in equilibrium with benzoquinone
(Figure 7), and then aliphatic fragments resulting from
the opening before producing CO2 such as picric acid,
oxalic acids, and humic acids, are difficult to well quan-
tified by HPLC. Thus, synergy effect can also be pointed
out in the kinetics of intermediate products appearance
and disappearance. For hydroquinone, its kinetics can be
summarized as:
OH
65 2
123
CH-OHHQBQCO
kkkkn
 (6)
4. Conclusions
For the case of MB photodegradation, the binary materi-
als TiO2-AC showed a clear increase in the photocata-
lytic activity with respect to TiO2 alone, under the two
lamps studied. Under the MH lamp which has a higher
proportion of visible light, TiO2 in presence of H-type
AC showed higher photocatalytic activity with respect to
TiO2 in presence of L-type AC. This beneficial effect has
been attributed to the specific properties of H-type AC
with a high surface area and basic pHPZC. By contrast, for
he case of phenol photodegradation, only TiO2- t
2
CO -800
AC

J. MATOS ET AL.
8
0
1
2
3
4
5
6
7
8
9
10
0100 200 300 400 500 600
HQ (μmo l)
Time (min)
TiO2 P25 Lamp Hg
TiO2-AC CO2 800 Lam
p
H
g
TiO2-AC N2 1000 Lamp Hg
TiO
2
P25 Lamp Hg
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
0
1
2
3
4
5
6
7
8
9
10
0100 200 300 400 500 600
BQ (μmol)
Time (min)
TiO2 P25 Lamp H
g
TiO2-AC CO2 800 Lamp H
g
TiO2-AC N2 1000 Lamp Hg
TiO
2
P25 Lamp H
g
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
Figure 7. Hydroquinone (HQ) and benzoquinone (BQ) appearance and disappearance during phenol photodegradation with
some selected solids under Hg Lamp.
presented higher photocatalytic activity than TiO2.
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