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A simple route for the preparation of Pmodified TiO2: Effect of phosphorus on
thermal stability and photocatalytic activity
Article in Journal of the Taiwan Institute of Chemical Engineers · January 2012
DOI: 10.1016/j.jtice.2011.06.011
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Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers
journal homepage: www.elsevier.com/locate/jtice
A simple route for the preparation of P-modified TiO2: Effect of phosphorus
on thermal stability and photocatalytic activity
Kais Elghniji a,*, Julien Soro b, Sylvie Rossignol b, Mohamed Ksibi a
a
b
Laboratoire Eau, Energie et Environnement (LR3E), Ecole Nationale d’Ingénieurs de Sfax, BP W 3038, Sfax, Tunisia
Groupe d’Etude des Matériaux Hétérogènes (GEMH-ENSCI) Ecole Nationale Supérieure de Céramiques Industrielles de Limoges, 47-73 Avenue Albert Thomas, 87065 Limoges, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 28 February 2011
Received in revised form 30 April 2011
Accepted 5 June 2011
Available online 15 August 2011
Phosphorus-modified dioxide nanoparticles were prepared by sol–gel method. The effect of phosphorus
precursor and calcination temperatures on phase transformation, grain growth and surface area were
investigated using various spectroscopic and basic techniques (ICP-AES, XRD, BET, 31P MAS NMR, FT-IR,
and UV–vis methods). It was found that the phosphorus existed as amorphous titanium phosphate in
TiO2 framework after calcination at temperature of 500–700 8C. As results, slows down the particle
growth of anatase and increases the anatase-to-rutile phase transformation. The average crystallite size
of P-modified TiO2 increased dramatically from 8 to 59 nm when the temperature increased from 500 to
900 8C. This change was associated with the formation of pyrophosphate TiP2O7 species through
condensation of the concentrated phosphate species. The BET surface area of modified samples was 3.4fold higher than that of unmodified TiO2 and was 70% higher than that of commercial Degussa P-25. The
photocatalytic activity of P-modified TiO2 was 1.5-fold higher than that of commercial Degussa P-25 and
was 49% higher than that of unmodified TiO2 under UV irradiation. Phytotoxicity was assessed before
and after irradiation against seed germination of tomato (Lycopersicon esculentum).
ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords:
TiO2
Phosphorus content
Particle growth
TiP2O7
Thermal stability
1. Introduction
TiO2 is considered to be the most promising heterogeneous
photocatalyst, because of its high photocatalytic activity, nontoxicity, inexpensiveness, chemical stability and favourable
optoelectronic properties. Titania exists in three different forms:
anatase, rutile, and brookite, of which anatase generally show the
best photocatalytic activity performance [1,2]. Without any
modification, transformation of anatase to rutile usually occurs
at 500–700 8C, which may result in the decrease in photocatalytic
ability of TiO2 [3–5].
In addition to the crystallite transformation obtained by hightemperature calcination, crystallite growing and serious sintering
are observed with increasing the temperature of calcinations leading
to the drastic decrease in surface area and photocatalytic activity.
Hence, the inhibition of the growth of the anatase crystals could
maintain the anatase metastability at high temperature and control
crystallite size. To improve the thermal stability and photocatalytic
activity, titania has been modified with La2O3 [6], ZrO2, SiO2 [7], or
doped by inorganic nonmetal such as sulphur [8] and fluor [9,10]. Lv
et al. [10] developed a simple and novel synthetic method for the
fabrication of TiOF2 via a microwave-assisted hydrothermal route
using tetrabutyl titanate and hydrofluoric acid as raw materials. The
* Corresponding author. Tel.: +216 25 511 432; fax: +216 74 665 190.
E-mail address: [email protected] (K. Elghniji).
prepared anatase TiO2 from TiOF2 shows very high thermal stability
and the phase transformation temperature from anatase to rutile is
up to 1000 8C. The high thermal stability and the photocatalytic
enhancement of catalysts were attributed to the adsorbed Fluoride
ion on the surface of anatase after calcination at 700 8C. Among the
various methods, modification and doping of TiO2 by phosphorus
seems to be a another promising approach [11–16]. Kõrösi and
Dekany [13] prepared a series of phosphate modified-TiO2 samples
by the sol–gel method. They argued that the surface bound
phosphate have delayed the formation of the anatase phase,
crystallite growth and inhibited the anatase–rutile phase transformation. Lin et al. [14] demonstrated that the doping of phosphorus
could efficiently inhibit the grain growth and enhance the surface
area of TiO2 nanoparticles. The above researches in literature seem
discrepant and complicated. This is probably due to the variety of
synthetic methods adopted to prepare the solid and the different
phosphorus contents. The present work aims at complementing
such investigations with P-modified TiO2 nanoparticles which are
synthesized through an easy procedure. The thermal stability, the
control of crystalline structure and the effect of phosphorous content
on the photocatalytic degradation of 4-chlorophenol were extensively investigated. To the best of our knowledge, there is no report
on the impact of the 4-CP solution before and after photocatalytic
treatment to environment using P-modified TiO2. In fact, chlorophenols constitute a group of serious environmental pollutants
that must be eliminated [17–21]. As a result of their widespread use
in mothproofing, miticides, pesticides, herbicides, germicides and
1876-1070/$ – see front matter ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jtice.2011.06.011
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
133
atomic emission (ICP-AES) spectrometry. The intensity of the
spectral lines of 213.6 and 336.1 nm were measured.
fungicides, chlorophenols pose a serious threat to the environment.
The US Environmental Protection Agency [22] and the European
Union directive [23] have labelled chlorophenols as ‘‘priority
pollutants’’, which means that they need to be constantly monitored
in the aquatic environment. Considering the potential effect of this
compound on the ecosystem, a phytotoxicity test using tomato
(Lycopersicon esculentum) seeds was successfully applied in
ecological risk assessment of 4-CP phototreated solutions.
2.3.2. Second method
The concentration of phosphate in the above samples was
measured using UV-Vis Double Beam PC, Scanning spectrophotometer (UVD-2950) following the ascorbic acid reduction method
[24].
2. Experimental
2.4. Surface acidity measurement
2.1. Chemical and catalyst preparation
Surface acidity was determined volumetrically by the adsorption of sodium hydroxide NaOH from solutions of different
concentrations. 0.2 g of the solid oxide was mixed with 30 ml of
the base solutions for 10 h, with continuous stirring then left 24 h.
For each sample, a blank run was carried out first on the base alone
and the difference between the blank run and that with the sample
gave the amount adsorbed.
Titanium (IV) isopropoxide (Ti(OC2H5)4, 97%, Aldrich) and
isopropylic alcohol were obtained from Fluka ((CH3)2CHOH, 99.8%,
Riedel de Haei
#n), phosphoric acid (H3PO4, 85%, Aldrich,), the
commercially available TiO2 (Degussa P25) was obtained from
Degussa Chemical and UHQ (Milli-Q 18.2 MV) water.
The catalyst P-modified TiO2, were prepared by the sol–gel
method. In a typical preparation procedure, titanium (IV)
isopropoxide was diluted in isopropylic alcohol (molar ratio
1:4). Deionized water was added dropwise to Ti(OPri)4 solution
during stirring (molar ratio between water and alcohol 1:5). White
precipitate starts appearing indicating the hydrolysis process.
After being aged for an hour, 100 ml of a phosphoric acid aqueous
solution with concentration 34.5 mM was added to the resulting
titania suspension to prepare TiO2 sample of phosphorus content.
For comparison, unmodified TiO2 was also prepared by the same
procedure without the addition of H3PO4. The dried materials were
calcined at 500, 700, 900 and 1000 8C in air for 3 h and with heating
rate 108/min and the calcined powders are labelled according to its
phosphorus content and calcination temperature. TPt, and Tt,
where t means the calcination temperature.
2.2. Structural characterization
The powder X-ray diffraction patterns were recorded at room
temperature on a (advanced D8, Bruker, Germany). The experimental
conditions for refinement of XRD data of TP1000 are the following: Xray tube operating at 40 kV and 40 mA, 0.6 mm fixed divergence slits,
diffracted beam curved graphite monochromator (Cu Ka+1 radiation,
l1 = 1.540600 Å, l2 = 1.544390 Å) and 0.1 mm fixed slit in front of the
scintillation detector. The data were collected in the 2u range 2–708
with a step size of 0.028 and a counting time of 5 s/step. Infrared
absorption spectra were measured on a (Nicolet 380 ATR/FT-IR,
International Equipment Trading Ltd., USA) spectrometer by the
transmission method using the KBr pellet technique with 4 cm1
resolution. The Brunauer–Emmett–Teller (BET) surface area was
measured by a fully automated surface area analyzer (ASAP 2020
Accelerated Surface Area and Porosimetry, Micromeritics, USA). The
samples were degassed in vacuum overnight at 180 8C prior to
adsorption measurements. The UV–Vis diffuse reflectance (DR UV–
Vis) spectra were recorded by a Varian Cary 5/UV–Vis-N.I.R.spectrometer. The 31P and (coupling polarization) 1H–31P CP NMR/MAS
spectra of the prepared TiO2 solids were recorded on a (300 ultra
shield, Bruker, USA) spectrometer at 300 MHz, resonance frequency
operating at 121.5 MHz and the external magnetic field was 9.4 T. The
pulse repetition is 10 s, pulse width of 7 ms and a spinning speed of
8.0 kHz. Chemical shifts were indicated using an external H3PO4
(85%) reference (0 ppm).
2.3. Determination of phosphorus content
2.3.1. First method
The Ti and P content of the samples was determined by all argon
sequential (Thermo Jarrell ASH, USA) inductively coupled plasma
2.5. Photocatalytic reaction experiment
Photodegradation of 4-chlorophenol (4-CP) (98%) was conducted in a laboratory-scale photoreactor. It is a 120 cm3
cylindrical photoreactor, operating in a closed recirculating circuit
driven by a centrifugal pump and with a stirred reservoir tank
equipped with a device for withdrawal of samples. Illumination
was carried out using a lamp (11 W low-pressure mercury lamp,
Philips, Holland) with a wavelength (lmax 254 nm). At given time
intervals, about 4 ml aliquots were sampled, centrifuged and
filtered with a cellulose acetate membrane filter membrane
(0.45 mm pore size, 25 mm diameter) to remove all solid particles.
The 4-CP concentration was estimated by measuring their
absorbance at 225 and 280 nm using a UV-Vis Double Beam PC,
Scanning spectrophotometer UVD-2950.
Free chloride ions are quantified directly after each irradiation
period with an ion chromatograph (HIC-6A Shimadzu, Japan)
equipped with a conductivity detector and a Shim-pack column.
The separation was achieved using an isocratic elution at a flow
rate of 1.5 ml/min. A mobile phase of 1 mM of tris(hydroxymethyl)aminomethane and 1 g/l of sodium chloride was used as
standards solution. Chemical oxygen demand (COD) was measured
according to standard methods described in the Japanese
International Standard handbook [25].
2.6. Phytotoxicity (germination tests)
4-Chlorphenol phytotoxicity was assessed before and after
irradiation against seed germination of tomato (L. esculentum).
Phytotoxicity was determined using a modified Zucconi test [26] by
measuring seed germination. Twenty seeds were placed on filter
papers in 9 cm Petri dishes and 6 ml of treated solution was then
uniformly added to each dish. Dishes were incubated in the dark at
26 2 8C for 5 days. Distilled water was used as control. All samples,
including controls, were triplicated. A germination index (GI) was
calculated by counting the number of germinated seeds and the average
root length observed in each sample compared to control treatments
[27]. Results finally expressed according to the following formula:
GI ¼
number of germinated seeds in sample
number of germinated seeds in control
average of root lengths in sample
100
average of root lengths in control
A seed was considered germinated when the root length
exceeded 5 mm. For root lengths below 5 mm, it was considered
3.3. Lattice parameters
From Rietveld analysis of P-modified and unmodified TiO2,
lattice parameters and anatase molar fraction were obtained and
summarized in Table 2. As clearly shown, calcination at temperature in the range from 500 to 900 8C produces a progressive
increasing on the tetragonality (c/a) of anatase structure of Pmodified TiO2 catalyst (TP). This is similar to the value reported in
the (JPCDS 84-1286) for well-crystallized anatase (c/a = 2.510).
Hence, the observed expansion is not likely related to a crystal size
effect. Even though small changes are expected in the lattice
parameters (c/a) upon changing the titania anatase crystal size.
Regarding the unmodified TiO2 (T), the increase of particle size is in
inverse proportion to the tetragonality. That means 47 nm is
critical size to undergo anatase-to-rutile transformation at 700 8C
with (c/a = 0.644) for well-crystallized rutile.
Fig. 2(a) shows a typical XRD data of TP1000 analyzed using the
program FULLPROF [30]. The quality of the agreement between
observed and calculated patterns for each phase is measured by a set
of factors given by the FULLPROF program. These results confirmed
Table 1
Phosphorus content in P-modified TiO2 samples obtained from ICP-AES and
ascorbic acid reduction method.
Sample
Weight calculated
(ppm)
ICP-AES weight
(ppm)
A.R. method
weight (ppm)
TP500
3.97
3.84
4.04
r [101]
r [211]
r [220]
r [111]
a [004]
r [200]
a [101]
3.2. XRD studies
T700
TP700
a [105]
a [211]
T500
a [200]
a [103]
a [004]
a [112]
a [101]
XRD was used to investigate the changes of phase structure of
the as-prepared catalysts after heat treatment at different
temperatures. Fig. 1 shows the XRD patterns of unmodified and
P-modified TiO2 calcined at different temperatures. With increasing calcination temperature, the peak intensity of anatase
increases, and the width of the (1 0 1) plane diffraction peak of
anatase becomes narrower, indicating the enhancement of
crystallization. For unmodified TiO2, the rutile phase starts to
appear at 700 8C, and it becomes the only phase at 900 8C (JPCDS
83-2242). In contrast, P-modified TiO2 retains a pure anatase phase
(JPCDS 84-1286) in the calcination temperature range of 500–
900 8C, and no obvious XRD peaks corresponded to titanium
phosphate are observed after phosphorus modification. The
titanium phosphate should be amorphous also in nature, mixed
to anatase TiO2. When calcined at 900 8C, two peaks at 2u = 22.55
and 27.708 appeared in spectrum of TP900 correspond to the (6 0 0)
and (7 2 1) XRD diffraction peak of crystalline TiP2O7 (JPCDS 381468) [28,29]. This phase with cubic lattice phase comes into being
at the same time with anatase phase. At temperature extremely
higher, the anatase TP900 transformed to the rutile TP1000 structure
and the TiP2O7 phase still remained in the solid composition.
r [210]
p [ 721]
r [211]
p [ 600]
r [110]
TP900
a [211]
r [220]
TP1000
a [105]
Table 1 summarizes the P content of P-modified TiO2
determined by ICP-AES and the ascorbic acid reduction method.
As shown, the Phosphorus content is identical with this of
calculated one, indicating the added phosphoric acid reacts
completely with titania at the applied preparation conditions.
p [ 630]
a [200]
T900
r [111]
3.1. Determination of phosphate content
r [210]
3. Results and discussion
r [200]
a [101]
equal to 0 and the seed was not considered germinated. The
average sum of root lengths comprised the sum of the lengths of all
germinated seeds in a Petri dish.
r [101]
r [110]
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
134
TP500
10
20
30
40
50
60
2θ
Fig. 1. The XRD patterns of T and TP calcined at various temperatures. a, r and p
denote anatase, rutile and TiP2O7 respectively.
the presence of rutile TiO2 and TiP2O7 phases at 1000 8C. Fig. 2(b)
shows the 31P MAS NMR spectrum of the TP1000 spun at 8 kHz and at
room temperature. The existence of lines at 38.6, 40.5, 42.9,
44.5, 46.15, 49.48 and 52.35 ppm should be associated with
the presence of several crystallographic sites for P atoms in a
3 3 3 crystal structure of TiP2O7 with space group (Pa-3) [31,32].
3.4. Average crystalline sizes and surface area
The average crystalline size anatase and rutile can be calculated
from the broadening of the (1 0 1) peak and the (1 1 0) peak, as
shown in Fig. 3. For the T500, the average crystalline size of anatase
Table 2
Crystallographic Parameters and phase contents of unmodified and P-modified
TiO2.
Sample
Phase contentsa
Lattice parameters
V (Å3)
A (%)
R (%)
a (Å)
b (Å)
c (Å)
T500
T700
T900
TP500
TP700
TP900
TP1000
100
26
–
100
100
100 + P
–
–
74
100
–
–
–
100 + P
3.783
4.589
4.598
3.788
3.789
3.784
4.594
3.783
4.589
4.598
3.788
3.789
3.784
4.594
9.472
2.960
2.963
9.480
9.507
9.531
2.958
135.77
62.38
62.37
135.98
136.30
136.48
62.44
a
The phase content of TiO2 was obtained from the following formulas: WR = 1/
[1 + 0.884(AA/AR)], WA = 1 WR, where WA and WR are the content of anatase and
rutile, respectively, AA and AR are the diffraction intensity of anatase (1 0 1) and
rutile (1 1 0). A, R and P denote anatase, rutile and TiP2O7 respectively. The cell
volume and lattice parameters obtained from the Rietveld refinement of X-ray
diffraction data.
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
135
140
200
120
TP
T
TP
T
Crystallite size (nm)
2
SBET(m /g)
160
100
80
60
120
80
40
0
500
600
700
800
900
1000
Temperature(°C)
40
20
0
500
600
700
800
900
Temperature (°C)
Fig. 3. Average crystallite sizes and specific surface areas (inserted) of the samples
calcined at various temperatures. The average crystallite sizes are calculated from
the broadening degree of the (1 0 1) XRD peak of anatase phase.
Fig. 2. (a) Rietveld plot for the TP calcined 1000 8C, r and p are denote rutile TiO2 and
TiP2O7, respectively. (b) 31P MAS-NMR spectra of TiP2O7 obtained at 1000 8C.
Chemical shifts are referenced to that of phosphoric acid.
(21 nm) quickly increases at temperatures above 500 8C, indicating
the sintering of TiO2 particles and the rutile phase becomes the
major phase at 700 8C. At this temperature, the mass fraction of
rutile is 74%, and its crystallite size is 45 nm. At 900 8C, anatase
completely turns into rutile phase and the crystallite size of rutile
rapidly increases to more than 100 nm. The similar result is also
reported by Yu and Wang [33]. They prepared TiO2 nanotube
arrays by electrochemical anodization of titanium foil. With the aid
of XRD and SEM morphology of the 600–700 8C-calcined samples,
they found a stable tubular structure with anatase/rutile particles
size of 35–40 nm. At 800 8C, the nanotube arrays are destroyed and
only dense rutile crystallites with size of over 180 nm are observed.
They suggested that the high temperature and phase-transformation heat cause the growth of rutile crystallites, on the other hand
the diffusion of oxygen in air into the nanotube–support interface
region oxidize the titanium in that region and directly transform
titanium into rutile phase.
However, for TP500, the crystallite size of anatase is 8.5 nm at
500 8C. That means the P species markedly slows down the crystalgrowth rate of anatase and maintains the anatase metastability at
high temperature. Beyond this temperature an increase in anatase
particle size is observed, becoming significant at 900 8C, reflecting
increased sintering. Reidy et al. [34] reported that the critical size
for 25% anatase-to-rutile conversion was 45.1 nm, respectively. In
the current work, the particle size of TP800 catalyst is 34 nm, i.e., it
is small to undergo anatase-to-rutile transformation.
The BET surface areas of the samples calcined at various
temperatures are shown in Fig. 3 inset. With increase of calcination
temperature, the measured specific surface areas oppose to the
calculated crystallite size, indicating the crystallite growth of
anatase during calcination. Upon calcination at 500–700 8C, the
surface area of T700 is quickly decreased to 5 m2/g, reflecting the
loss of coordinated water (as revealed by FT-IR analysis) and
sintering of rutile T700 particles [35,36]. In contrast, the surface
area of TP500 is larger than that of the anatase T500 one. This finding
indicates that P-modified TiO2 surface has more hydroxyl groups
than the pure TiO2 which may be attributed to the less loss of water
during calcination temperature. Above 700 8C, a decrease in
surface area is observed for modified TiO2, becoming significant
at 900 8C. This is a reflection of sintering of anatase particles at
higher temperatures (as revealed by FT-IR and XRD analysis).
3.5. FT-IR studies
FT-IR spectra of the unmodified and P-modified TiO2 samples
calcined at various temperatures are shown in Fig. 4. For all
samples, the broad peak at 1630 cm1 and the peak at 3420 cm1
corresponds to bending and asymmetric stretching modes of
molecular water, respectively. Obviously, the P-modified TiO2 has
more surface-adsorbed water and hydroxyl groups than the
unmodified TiO2, and these absorption bands gradually decrease
as the temperature increases.
In addition, three absorption bands at 1100, 1135 and
1035 cm1 are observed in the IR spectra of TP500 but absent for
T500 (Fig. 4(a) inset). The shifts of these bands suggest that different
chemical environments existed around the phosphorus. The
former band is possibly the 1082 cm1 band, characteristic of
the n3 vibration of the phosphate ions coordinated to TiO2, shifted
to lower wavelength as a result of hydrogen bond interactions in
the host metal oxide lattice (hydrogen phosphate states) [37]. The
shoulder peak at 1135 cm1 appears gradually, characteristic of
the n2 vibration of the phosphate in a bidentate state (bridging
bidentate surface) [38]. The band at 1036 cm1 is assigned peak to
the vibrations of Ti–O–P bond [39–42]. Therefore, the phosphorus
may exist as the surface (1100 and 1135 cm1) and the form Ti–O–
P (1036 cm1) bulk of TiO2 anatase, with P replacing part of Ti4+
ions.
Furthermore, the TP700 catalyst shows a gradual appearance of
bands at 1047 cm1, 968 cm1 and a shoulder broad band at
1184 cm1 (Fig. 4(b) inset). These bands can be assigned to the
phosphate probably in a bidentate state [43].
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
136
(a)
(a)
Absorbance (a.u)
TP 500
T 500
103 5
1082
1135
2
1
1400
130 0
120 0
1100
cm
TP 500
100 0
90 0
80 0
-1
-100
-50
0
50
100
ppm
T 500
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
(b)
x 10
(b)
TP 700
Absorbance (a.u)
460
1047
968
1184
-100
Fig. 5.
1300 120 0 11 00 100 0
-1
cm
T P 700
90 0
800
3.6.
T 700
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Absorbance (a.u)
(c)
T P 1000
T P 900
1130
467
1107
1 0 50 9 7 8
523
1184
1214
TP 1000
1 4 00
1300
1 2 00
1100
cm
1 0 00
900
80 0
-1
TP 900
T 900
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 4. The FT-IR spectra of TP and T calcined at (a) 500 8C, (b) 700 8C, and (c) 900 8C
and 1000 8C.
-50
0
50
100
31
P MAS-NMR (a) and 1H–31P CP NMR (b) spectra of TP calcined at 500 8C.
31
P MAS-NMR studies
MAS-NMR measurements were carried out to study the
phosphorus microenvironments. As shown in Fig. 5(a), the
broadening of 31P NMR spectra for TP500 stems from the wide
distribution of phosphorus among two sites with slightly different
environments. Two chemical shifts are observed after computer
simulation. The first one at 4.49 ppm (deconvolution-peak 1),
which in good argument with chemical shift values for phosphates
coordinated via one oxygen to one titanium atoms 1P–O–Ti bound,
i.e. Ti(H2PO4)4. The second signal at 9.5 ppm (deconvolutionpeak 2) and may connect to phosphate coordinated via two
oxygens to two titanium atoms P–O–Ti bounds in bridging
bidentate structure, i.e. Ti(HPO4)2 [44,45]. Similar results have
been reported by Fan et al. [46]. They argued that the use of H3PO4
as phosphorus source leads to three possible chemical states of
amorphous titanium phosphate, including Ti3(PO4)4, Ti(HPO4)2
and Ti(H2PO4)4 after calcination temperature (<600 8C).
As mentioned in XRD section, the titanium phosphate should be
amorphous in nature, mixed to anatase TiO2. Evidently, the 1H–31P
(coupling polarization) NMR spectrum, Fig. 5(b) shows the
detection of remaining P–OH groups on the TP500 catalyst. On
the basis of the results of XRD and 31P NMR, phosphorus exists as
amorphous titanium phosphate that embedded in the TiO2
crystalline grain at 500–700 8C. While the FT-IR spectra support
a bulk/surface Ti–O–P structure in the titanium phosphate
samples.
3.7. Surface acidity
Evidently, the TP900 displays sharp peaks at 978, 1110 and
1214 cm1 attributable to frequencies of PO43 co-ordinated to
metals such as Ti4+ on the solid surface. These bands somewhat
shifted to lower wave number and we can observe a significant
change in the trend of the FT-IR spectrum (at 1107, 1184, 1050 and
523 cm1) at 1000 8C (Fig. 4(c) inset). Considering the formation of
the bands at 523 cm1 (formation of rutile TiO2), it is possible to
infer that all of these changes at 1000 8C consequently caused the
anatase–TiO2 to completely transform to the rutile structure as
revealed by DRX analysis.
Surface acidity values, expressed in mmol/g for catalysts T and
TP calcined at different temperatures, are presented in Fig. 6. As
calcination process is carried out, the acidity of the T sample is seen
to decrease fastly. This was believed to be due to a reflection of the
fast loss of surface-adsorbed water and hydroxyl groups and the
increases of sintering due to the formation of rutile. Moreover, it
has been established that the amount of the surface-adsorbed
water and hydroxyl groups is related to the crystallite form and
surface area. As a result, anatase is more active than rutile in
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
4.0
3.5
shows that the UV–vis spectrum of TP500 is blue-shifted by several
nanometers in comparison with that of T500. This indicates that the
band gap of the TP500 catalyst is larger than that of the T500. To
determine the band gap of the powders, the Kubelka–Munk
method based on the diffuse reflectance spectra was employed
[50]. The (F(R)hn)1/2 versus hn plots of the samples calcined at
various temperature are presented in Fig. 7 (inset). The red shift
observed during the calcination at higher temperature may be due
TP
T
3.0
2.5
3
2.0
1.5
1.0
TP 500
(a) 250
T
P25
Photolysis
-0.5
500
600
700
800
900
Temperature (°C)
Fig. 6. Evolution of surface acidity of TP and T as function of calcination
temperature.
adsorbing water and hydroxy groups and this is probably the main
reason of higher photocatalytic efficiency of anatase compared to
rutile phase [47].
Regarding the P-modified TiO2 calcined at 500 8C, the surface
acidity is higher than that of T500 because of the additional
hydroxyl groups resulting from phosphorus modification. This is
corroborated by the infrared spectra, where different chemical
environments can exist around the phosphorus (bidentate
structure) as revealed by 31P NMR analysis.
Similar results reported by Ramadan et al. [48]. Who argued that
the observed acidity between 400 and 500 8C would be arising from
the acidic hydrogen in the structure (bidentate ligation). After
calcination at 800 8C, who observed a drastic decrease in acidity
associated with the phosphate groups devoid of the acidic hydrogen.
However, the decrease in the surface acidity at high temperature up
to 900 and 1000 8C is probably due to formation of polyphosphate
TiP2O7 and rutile phase, which decreases the number of Brönsted
acid sites and consequently the total number of acid sites.
140
120
100
150
80
60
100
40
20
50
0
-20
0
0
50
100
150
200
(b)
TP
P25
T
70
60
50
40
30
20
10
0
Because of the quantum-size effect, the absorption band edge is
a strong function of titania size for diameters <10 nm [49]. Fig. 7
150
210
8
T
TP
P25
7
3.15
6
3.10
kapp(10-3min-1)
Eg (eV)
270
Irradiation Time (min)
TP
T
3.20
90
0
(c)
3.25
Absorbance (a.u)
250
Irradiation Time (min)
3.8. UV–vis diffuse reflectance spectroscopy
3.05
3.00
2.95
2.90
5
4
3
2
2.85
800
900
1000
1100
1200
1
Temperature (°c)
T
TP
0
500
300
180
160
200
COD (mg/L)
0.0
Concentration of 4-CP (uM)
0.5
Concentration of CL-(uM)
Surface acidity (10 umol/L)
137
400
500
600
700
800
λ (nm)
Fig. 7. The diffuse reflectance spectra of catalysts calcined at 500 8C, the inset shows
the band gaps energy of TP and T as function of calcination temperature.
600
700
800
900
Calcination Temperature (°C)
Fig. 8. (a) Typical curves of the degradation of 4-CP (white symbols), release
concentration of chloride ions (black symbols) under UV irradiation. 4-CP; (b)
chemical oxygen demand (COD); (c) the dependence of the apparent rate constants
kapp at various temperatures under UV: 0.23 mM, TiO2: 1.5 g, natural pHs.
138
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
partly to increasing particle size of anatase and partly to the
presence of rutile phase at temperature extremely higher.
3.9. Photocatalytic degradation of 4-CP
Fig. 8(a) shows the photocatalytic properties of 4-chlorophenol
(4-CP) on the TiO2 photocatalysts under UV-irradiation times,
using lmax 254 nm ultraviolet light (11 W). A control experiment
was performed by an identical aqueous solution without catalyst.
Control experiment showed that degradation of 4-CP was less than
8% within 270 min in the direct photolysis. The photocatalytic
degradation, dechloration rates were enhanced after the addition
of phosphorus species. Nearly 85, 43 and 72% of 4-CP was degraded
and about 74, 38 and 55% of chlorine was removed by using
respectively TP500, T500 catalysts and Degussa P25 within the same
irradiation time (270 min). The release of chloride ions is easily
understood because the breaking of C–Cl bond is energetically
more favourable than that of C–OH bond (the bond dissociated
energy for a C–Cl bond is approximately 10 kcal/mol less than that
for a C–OH bond) [51].
The temporal changes of chemical oxygen demand (COD) in the
degradation process of 4-CP are shown in Fig. 8(b). It can be
observed that the removal rates of COD became a faster in the Pmodified TiO2 systems than that in P25 and unmodified-TiO2
system. For the P-modified TiO2 case, the COD reduction is slower
up to 87% and 13% COD remained in solution within 270 min.
This could be related to organic intermediates generated during
the oxidation of 4-CP, as reported in previous work both from our
laboratory and from other research group [52,53]. Hence, the
complete mineralization of 4-chlorophenol by photocatalytic
degradation is rather difficult.
As shown in Fig. 8(c), the apparent rate constant kapp value of
TP500 under UV irradiation is 7 103 min1, which is 3.2-fold
higher than that of T500 (2.0 103 min1) and is 1.6-fold higher
than that of commercial Degussa P-25 (4.3 103 min1). The
high activity of P-modified TiO2 after calcination at different
temperatures may be attributed to the following reasons.
First, according to FT-IR spectra, the P-modified TiO2 has more
surface-adsorbed water and hydroxyl groups than the unmodified
TiO2. These species are considered to play a key role in the
photocatalytic reaction because the photoinduced holes can attack
those surface hydroxyl groups and yield surface hydroxyl radicals
with high oxidation capability [54,55].
Second, the P-modified TiO2 calcined at 500 8C has a higher
surface area and size-quantized particle (8.5 nm) than that of
unmodified titania. It is well known that the photocatalytic activity
of UV illuminated semiconductors is due to the production of excited
electrons in the conduction band, along with corresponding positive
holes in the valence band. Apparently, the small size of P-modified
TiO2 expedites the surface charge carrier transfer, consequently
decreasing the chance of recombination of photoinduced electron–
hole pairs, hence increasing the photocatalytic activity [56].
Third, according to the Scherrer equation from the (1 0 1) plane,
the average crystallite size of TP700 is 15 nm. Indeed, it has been
established the optimal size of anatase crystal for the excellent
photocatalytically active is 15 nm [57]. Therefore, the reason why
TP700 have the highest photocatalytic activity could be related to
their good crystallization (anatase crystal) and appropriate particle
size. In contrast, the T700 shows poor activity under UV irradiation,
which can be attributed to its smaller surface area and large
crystalline size. Concerning the P25 Degussa, it is also known that
its optimal photocatalytic efficiency is due to the mixture of
anatase and rutile (70:30%) which meant a low density of
recombination centers. The decrease in photocatalytic abilities
of the TP900 should be mainly ascribed to the formation of
nonphotocatalytic phase TiP2O7 and rutile.
Fig. 9. Germination index as a function as a function of reaction time under UV
illumination, using TP500 catalyst.
3.10. Phytotoxicity
Phytotoxicity tests were conducted to assess the impact of the
release of the irradiated solution to the environment as well as to
evaluate the possible use of the pre-treated aqueous solution in the
irrigation field. Indeed, this practice can alleviate the burden on
underground water overexploitation and promote the practice of
using treated water to irrigate golf courses, parks and gardens.
Phytotoxicity of raw and treated 4-chlorophenol solution was tested
using a seed germination assay and are depicted in Fig. 9. As clearly
shown the seed germination in the tomato (L. esculentum) was
strongly inhibited by raw solution or with a photocatalysis duration
(45 min) reflecting the recalcitrant nature of 4-CP. However, as
photocatalysis progressed, a fast increase of germination index was
observed. At 270 min; the germination index reaches a maximum
(75%). the toxicity of the treated solutions fell within the non-toxic
range [58]. Hence, our data indicated that the photocatalytic
oxidation technique could be useful as a pretreatment technique for
reducing toxicity of toxic/hazardous wastewaters.
4. Conclusions
On the basis of the described results, a number of conclusions
can be proposed:
- Preparation method adopted in this work leads to P-modified
TiO2 showing a stability of anatase structure at relatively high
temperatures and new crystalline TiP2O7 phase comes into being
at the same time with anatase TiO2 for TP900.
- The BET surface area of P-modified samples largely exceeds
which can offer more active sites and surface acidity (hydroxyl
groups) for photocatalytic reaction.
- The quantum efficiency of P-modified TiO2 photocatalysts is
higher than that of unmodified TiO2, as a result of the inhibition
of charge carrier recombination.
- The photocatalytic activity of 4-chlorophenol was significantly
faster on P-modified than it is on unmodified TiO2 and
commercial Degussa P-25. Phytotoxicity to tomato (L. esculentum) seeds was successfully applied to assess the feasibility of the
treated solution in the environment.
Acknowledgement
The author would like to thank Miss. Najwa Mlaik for the help in
phytotoxicity test.
K. Elghniji et al. / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 132–139
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