Photo-catalytic selectivity of anthranilic acid over iron oxide incorporated titania nanoparticles: Influence of the Fe2+/Fe3+ ratio of iron oxide

Article abstract of DOI:10.1016/j.molcata.2015.12.001

Iron oxide incorporated titania nanoparticles (FIT) with various atomic ratios of Fe/Ti, i.e. 3.53% (FIT-1), 8.3% (FIT-2), and 20.79% (FIT-3), are prepared to monitor the selectivity of photo-degradation for anthranilic acid (AA) in molecular level. In the FIT-1 route, the products obtained from the photo-degradation of AA are aniline, 4-aminophenol, and benzene. Since the surface of iron oxide domain in FIT-1 is fully reduced to FeO. The FeO, by trapping the photo-generated (e^-), acts as a Lewis base to facilitate the de-carboxylation reaction for AA to create aniline. It is noteworthy that the highly alkaline surface of FIT-1 executes the hydroxylation addition for AA to produce 4-aminophenol. In FIT-2, and FIT-3 routes, the surface of incorporated iron oxide is only partially reduced to FeO. After the photo-degradation of AA, the collected products are 2-aminobenzaldehyde, and benzene. In this route, the low spin (LS) Fe³⁺ ion in the Fe₂O₃ region adopts the photo-generated (e^-). The LS Fe³⁺ is reduced to stable LS Fe²⁺, i.e. FeO. At this stage, the Fe₂O₃ domain acts as a Lewis acid to push forward the carbonyl reduction of AA to obtain 2-aminobenzaldehyde. In addition, the de-amination is contributed by the nucleorphilic attack of water molecule to the carbon atom of aromatic ring bonded to NH₂ group. After the de-amination reaction, the intermediate product is phenol. The following de-hydroxyl reaction by FeO domain proceeds immediately. The final product is benzene that is observed in all FIT involved catalytic routes.

Full text of DOI:10.1016/j.molcata.2015.12.001

Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Photo-catalytic selectivity of anthranilic acid over iron oxide
2
+
3+
incorporated titania nanoparticles: Influence of the Fe /Fe ratio of
iron oxide
∗
Ya-Hui Chang, Chun-Chang Ou, Hui-Wen Yeh, Chung-Sung Yang
Department of Applied Chemistry, National Chia Yi University, Chiayi 60004, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2015
Received in revised form
Iron oxide incorporated titania nanoparticles (FIT) with various atomic ratios of Fe/Ti, i.e. 3.53% (FIT-
1
), 8.3% (FIT-2), and 20.79% (FIT-3), are prepared to monitor the selectivity of photo-degradation for
anthranilic acid (AA) in molecular level. In the FIT-1 route, the products obtained from the photo-
degradation of AA are aniline, 4-aminophenol, and benzene. Since the surface of iron oxide domain
2
8 November 2015
Accepted 1 December 2015
Available online 3 December 2015
−
in FIT-1 is fully reduced to FeO. The FeO, by trapping the photo-generated (e ), acts as a Lewis base to
facilitate the de-carboxylation reaction for AA to create aniline. It is noteworthy that the highly alkaline
surface of FIT-1 executes the hydroxylation addition for AA to produce 4-aminophenol. In FIT-2, and FIT-3
routes, the surface of incorporated iron oxide is only partially reduced to FeO. After the photo-degradation
Keywords:
Photocatalysis
TiO2
Fe2O3
Selectivity
3
+
of AA, the collected products are 2-aminobenzaldehyde, and benzene. In this route, the low spin (LS) Fe
−
3+
2+
ion in the Fe2O3 region adopts the photo-generated (e ). The LS Fe is reduced to stable LS Fe , i.e. FeO.
At this stage, the Fe2O3 domain acts as a Lewis acid to push forward the carbonyl reduction of AA to
obtain 2-aminobenzaldehyde. In addition, the de-amination is contributed by the nucleorphilic attack of
water molecule to the carbon atom of aromatic ring bonded to NH2 group. After the de-amination reac-
tion, the intermediate product is phenol. The following de-hydroxyl reaction by FeO domain proceeds
immediately. The final product is benzene that is observed in all FIT involved catalytic routes.
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
DNA damage mediated by the active forms of hydroxyl radicals
7,8].
In the previous literature, most effort focuses on the synthesis
of Fe/TiO nanoparticles, i.e. Fe TixO nanoparticles. The prepara-
[
Solar application of photochemical oxidation methods is a con-
venient method for the treatment of highly contaminated effluents
1,2]. Among the available photochemical oxidation methods, the
2
1−x
2
[
tion of iron oxide incorporated TiO2 nanoparticles is few [9–11]. In
this study, a novel nano-composite photocatalyst, iron oxide incor-
porated titania (FIT) nanoparticles is developed via a non-aqueous
route with a tunable atomic ratio of Fe/Ti from 3.53% to 20.79%
via a non-aqueous route as a solution for the dilemma. The tunable
incorporating level of iron oxide not only flexibly adjusts the surface
Lewis acidity of FIT, but also effectively avoids the agglomeration of
composite nanoparticles. As mentioned in the previous paragraph,
kinetic peculiarity of hydrogen peroxide in the reaction system is
that iron catalyzes the univalent reduction of hydrogen peroxide to
generate the neutral hydrogen oxide radicals and hydrogen oxide
2
+
photo Fenton reaction (Fe /H O /UV–vis) is the most economi-
cal way to treat the environmental or industry waste water [1–3].
Generally, the efficiency of the reaction depends mainly on the
2
2
2+
concentration of H O , molar ratio of [Fe /H O ], pH value, and
2
2
2
2
reaction time [2,3]. The initial concentration of the pollutant and
its character as well as the temperature, also have a substan-
tial influence on the final efficiency [3]. The another censure is
the yield of too many hydroxyl radicals besides the reaction,
2+
3+
−
•
Fe + H O → Fe + HO + OH [4–6]. It is a paradox that the reac-
2
2
tion is successfully used in environmental protection, but causes
the damages to biomolecules and a variety of diseases, such as the
−
+
ions [5,6]. By use of the photo-generated (e –h ) pair in TiO region,
2
the newly prepared FIT photocatalyst can perform the redox reac-
−
•−
•−
+
−
•
tion: O + e → O2 , and 2 O2 + 2 H → OH + OH + O2 to obtain
2
the required hydrogen oxide radicals and hydrogen oxide ions for
the reaction [12–15]. In this photocatalytic reaction, the hydrogen
peroxide is excluded. Moreover, the photocatalytic selectivity of FIT
∗ Corresponding author. Fax: +886 5 271 7901.
E-mail address: csyang@mail.ncyu.edu.tw (C.-S. Yang).
http://dx.doi.org/10.1016/j.molcata.2015.12.001
1
381-1169/© 2015 Elsevier B.V. All rights reserved.

6
8
Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
is tunable via adjusting the ratio of Fe2+/Fe3+ in the incorporated
iron oxide. By elimination of hydrogen peroxide in the photocat-
alytic reaction and control of the selectivity in photocatalytic route,
the novel prepared FIT is a practical solution for the treatment of
environmental organic pollutants.
FIT-2–24 h, Fig. 1(b), the heterogeneous iron oxide domains are
clearly incorporated on the surface of TiO2 nanoparticle. The
selected area electron diffraction (SAED) of FIT-2–24 h is a ring
pattern, meaning that FIT-2–24 h has a short-range crystalline
structure (on the nano-scale) [11,12]. In the energy dispersive X-
ray spectrum (EDX) of TEM for Fig. 1(a), two Ti peaks, (4.51 keV
(
7
K␣), 4.93 keV (K␤)), and three Fe peaks (0.7 keV (L␣), 6.39 keV (K␣),
.06 keV (K␤)) are identified in FIT-1–24 h [14,15]. The gross atomic
2
. Experimental
ratio of Fe/Ti is < 5% in FIT-1–24 h. The same characterization pro-
cedure is employed for FIT-2 (Fe/Ti < 10%) and FIT-3 (Fe/Ti < 25%).
(
a) Chemicals
Iron(III) acetylacetonate (Fe(C5H7O ) , IAA) was obtained from
2
3
(
II) Powder XRD and Raman spectra
Sigma–Aldrich, Titanium(IV) butoxide (TBO) were purchased from
Fluka. Tetra-n-propylammonium bromide (TPAB) was got from Alfa
Aesar. Reagent-grade ethylene glycol (EG, 99% Showa) was used
without any further purification. HPLC grade methanol and reagent
grade ethanol were obtained from Sigma–Aldrich, and were used
as supplied.
The gradual changes in the composition and the variations in
crystallinity of FIT are monitored by powder X-ray diffraction (P-
XRD). The FIT samples used for P-XRD are collected at the 24th h.
These samples are denoted as FIT-1, FIT-2, and FIT-3 for clarity
purpose.
◦
The P-XRD of FIT-1 is given in Fig. 2(a). In the region of 10–80
(
b) Synthesis and characterization of FIT
(
(
two theta), nine peaks match the (1 0 1), (0 0 4), (2 0 0), (1 0 5),
2 1 1), (2 0 4), (1 1 6), (2 2 0), and (2 1 5) of anatase TiO2 [16a]. In
The FIT photocatalyst was synthesized by the reaction of IAA,
addition, one peak matches (1 2 1) of brookite TiO2 [16b]. The
diffraction pattern shows that the main domain of FIT-1–24 h
remains anatase TiO2 structure. On the other hand, the iron oxide
TBO, and template TPAB in EG. A typical synthesis route for mole
ratio of IAA:TBO = 0.1:1 is described as follows. TPAB 2.0 g, a lim-
iting reagent, was mixed with 30 mL EG in a tri-neck bottle. The
mixture was stirred for 20 min to dissolve the template TPAB com-
pletely in EG. The starting reagents IAA (0.207 g) was added into the
EG/TPAB solvent. The temperature of solution was raised slowly
(Fe O ) matrix is not evident in the P-XRD of FIT-1. A similar result
3 4
of P-XRD pattern is observed in the P-XRD of FIT-2, and FIT-3.
The Raman spectrum is employed to investigate the incorpora-
tion behavior of iron oxide in TiO . Two samples, FIT-1 and FIT-3,
are selected for Raman study. In FIT-1, six peaks, 144 cm (Eg),
1
6
amount of iron oxide in FIT-1 is too little to be detected in Raman
spectrum. In FIT-3, one new peak, 281 cm is observed, which is
assigned to E1g of ␣-Fe O , Fig. 2(c) [18]. The six TiO anatase peaks
do not show any obvious shift in locations. The TiO anatase domain
2
◦
from room temperature to refluxing temperature (180–190 C)
−
1
with a standard stirring speed of 250 rpm. After 20 min, the color
of solution turned to clear. TBO (2.0 mL) was added into the clear
solution. The color of the mixture was slowly changed from clear
to orange–brown after 20 min refluxing. The orange–brown color
remained for more than 48 h. The first aliquot withdrawal (3 mL)
was taken at the 4th h. Each withdraw was washed twice with
0 mL of ethanol, and then transferred in 10 mL DI water to remove
template residue. The final orange–brown precipitate was dried as
a powder for further investigation.
− −1 −1 −1
1
97 cm (Eg), 399 cm (B1g), 515 cm (A1g), 519 cm (B1g), and
−
1
39 cm (Eg), match the TiO2 anatase structure, Fig. 2(b) [17]. The
−
1
2
3
2
2
1
obviously remains in FIT. The results agree with the data of P-XRD.
(III) X-ray photo-emission spectra (XPS)
3
. Results
The quantitative atomic ratios of Fe/Ti for FIT-1, FIT-2, and
FIT-3 composite photocatalysts are measured by X-ray photo-
emission spectra ( XPS). The region-wise XPS scan of FIT-1 is
provided in Fig. 3, from 750 eV to 440 eV. Following a correction
for sample charging, the binding energy (BE) values for TiO2 are Ti
3
.1. Characterization of FIT nanoparticles
The different amount of atomic ration of Fe/Ti from FIT-1
(
Fe/Ti < 5%), FIT-2, (5% < Fe/Ti < 15%), to (FIT-3, 15% < Fe/Ti < 30%)
2
3
s = 566.0 eV, Ti 2p1/2 = 465 eV, Ti 2p3/2 = 458.8 eV, Ti 3s = 63.1 eV, Ti
p = 37.8 eV, and O 1s = 529.9 eV, O 2s = 23.0 eV. The BE values for
were prepared for the study. The rate of growth of FIT nanocrystal-
lites is highly dependent on the competition between titania and
iron oxide crystalline seeds [12–15]. The partial positive charge on
titanium (+0.63) in TBO is higher than that of iron (+0.46) in IAA
iron oxide are Fe 2p1/2 = 724 eV, and Fe 2p3/2 = 708.8 eV. These data
are in agreement with the values reported in the literature [19]. The
elemental microanalysis data show that the atomic ratio (Fe/Ti) of
Fe (1.13%)/Ti (31.97%) in FIT-1 is 3.53%. In FIT-2 samples, the Fe/Ti
ratio of Fe (2.57%)/Ti (31.18%) is 8.24%. In the FIT-3 samples, the
atomic ratio of Fe (6.18%)/Ti (29.76%) is 20.79%.
The commercial software, XPSPEAK 4.0, was acquired for the
peak deconvolution. The selected peak was Fe 2p3/2 of XPS mea-
sured for FIT. The Shirley baseline subtraction method was applied
[
11,12]. Therefore, the nucleophilic attack between EG and TBO
is earlier than that of EG and IAA. In addition, because the TBO
has the advantage in concentration, the growing speed of titania
crystallites is much larger than that of iron clusters.
(
I) TEM and EDX of TEM
before the peak deconvolution. The blow-up section for Fe 2p3/2
,
The TEM and high-resolution TEM (HR-TEM) images of FIT sam-
from 705 eV to 718 eV, has a broad band of interest. Theoretically,
each component peak corresponds to a typical binding energy.
ples were used to study the crystallinity and detailed composition
of these nanoparticles. A typical TEM image of FIT-1 collected at
the 24th h, denoted as FIT-1–24 h, is given in Fig. 1(a). The prod-
After the deconvolution work, an iron 2p3 peak was judged to
/2
have two component peaks, P1 (Fe3+), P2 (Fe ) and two satellite
2+
◦
peaks, S1 and S2, to reach the minimum X2 value, given in Fig. 4
uct is calcinated at 400 C for 1 h to remove EG. The size of iron
3+
oxide observed in this micrograph is smaller than 0.6 nm, marked
by short arrows at upper left corner of Fig. 1(a). The titania part is
well grown to be crystalline and the diameter is larger than 5 nm.
The d-spacing of lattice fringes (0.305 nm), marked by A, matches
[13–15,21,22]. The binding energy assigned for peak P1 (Fe ) is
around 710.33–710.70 eV. For peak P2 (Fe2+), the binding energy
is in the range of 708.47–708.80 eV. The atomic ratio of Fe2+/Fe
can be calculated precisely. The detail deconvoluted data and theX2
values are provided in Table 1.
3+
the (1 0 1) crystal phases of anatase TiO . In the TEM image for
2

Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
69
Fig 1. (a) A typical TEM image of FIT-1–24 h. The size of iron oxide is smaller than 0.5 nm, marked by short arrows. The d-spacing of lattice fringes (0.305 nm), marked
by A, matches the (1 0 1) of anatase TiO2. (b) A TEM image of FIT-2–24 h. The size of iron oxide is ∼0.6–0.8 nm. (c) EDX spectra of Fig. 1(a). Two Ti peaks, (4.51 keV (K␣),
4
.93 keV (K␤)), and three Fe peaks (0.71 keV (L␣), 6.39 keV (K␣), 7.06 keV (K␤)) are identified.
Table 1
The deconvoluted peak area, peak position (eV) and ꢀX 2 value.
FIT-1 (IAA:TBO = 0.05:1)
FIT-2 (IAA:TBO = 0.1:1)
FIT-3 (IAA:TBO = 0.25:1)
P1 (Fe3
P2 (Fe2
S1
+
)
)
2089.4 (710.70 eV)
4152.9 (708.80 eV)
318.24 (714.60 eV)
823.28 (713.20 eV)
0.125
2747.4 (710.33 eV)
4556.6 (708.47 eV)
224.62 (714.87 eV)
763.80 (712.70 eV)
0.0346
3962.9 (710.52 eV)
4630.9 (708.50 eV)
216.24 (715.20 eV)
997.3 (713.50 eV)
0.109
+
S2
ꢀ
X2
Background: Shirly method; smoothing: FFT 20%.
2
exp
fit
]
/ꢁ ), d = exp. data; d = fitted data; ꢁ2 = square exp. uncertainties.
2
2
exp
fit
(X
= ꢀ[d − d
Basically, the Fe3+ ions and Fe2+ ions co-exist in all samples. One
photo-degraded by photochemically-produced hydroxyl radicals
in air.
typical tendency of peak areas observed in the curve-fitting data is
that the atomic ratio of Fe2+/Fe is inversely proportional to the
3+
2+
3+
amount of iron oxide in FIT. The ratio of peak area for Fe :Fe
(I) Diffuse reflectance spectrum (DRS) of FIT
decreases from 1.99 (FIT-1), to 1.35 (FIT-2), and finally to1.17 (FIT-
+
3
). The oxidation state of divalent ion (Fe2 ) is the dominate valence
state between Fe/Ti = 3.53% and Fe/Ti = 20.79%.
The selection of a suitable irradiation wavelength is essential
to prove that the degradation of target compound is authentic by
the photocatalyst. The 365 nm irradiation wavelength satisfies the
cut-off band gap of diffuse reflectance spectrum (DRS) for FIT-3
3.2. Photocatalytic degradation of anthranilic acid
(
480 nm), FIT-2 (445 nm), and FIT-1 (410 nm), shown in Fig. 5. Since
The photoluminescence (PL) emission spectra were used to
the band gap of ␣-Fe O3 is 2.1 eV, the increase of the incorporated
amount of iron oxide may lead to the red-shift of band gap observed
in FIT samples. A similar result is also observed in previous research
2
explore the adsorption ability and the photocatalytic mechanism
of the organic anthranilic acid (AA) over FIT. The FIT samples were
◦
calcinated at 400 C for 1 h before used in the photo-degradation
[
17,18].
reaction. The organic acid AA is an amphoteric species with com-
plicate metabolism intermediates in organism beings. If released
to air, AA will exist solely as a vapor in the atmosphere until
(II) Photo-degradation of adsorbed AA on FIT

7
0
Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
(
a)
1
0
20
30
40
Two theta
50
60
Eg
70
80
750
700
650
600
550
500
450
Binding energy (eV)
(
b)
Fig. 3. The XPS spectrum of FIT-1, ranging from 750 eV to 440 eV.
B1g
A1g
B1g
Intensity
Background
S1
Eg
S2
Eg
P1 (Fe 3+)
P2 (Fe 2+)
1
00 200 300 400 500 600 700 800
-
1
Raman Shift (cm )
7
16
714
712
710
708
706
(
c)
Binding energy (eV)
Fig. 4. The blow-up section of XPS for FIT-1, from 705 eV to 718 eV assigned to Fe
3+
Fe O3
2p3/2. An iron 2p3/2 peak was judged to have two component peaks, P1 (Fe ), P2
2
2
+
2
(
Fe ) and two satellite peaks, S1 and S2, to reach the minimum X value.
E1
g
FIT-3
FIT-2
FIT-1
1
00 200 300 400 500 600 700 800
-1
Raman Shift (cm
)
Fig. 2. (a) The P-XRD of FIT-1. Nine peaks match the (1 0 1), (0 0 4), (2 0 0), (1 0 5),
2 1 1), (2 0 4), (1 1 6), (2 2 0), and (2 1 5) of anatase TiO2. Raman spectra for (b)
(
−
1
−1
−1
FIT-1 and (c) FIT-3. In (b), six peaks, 144 cm (Eg), 197 cm (Eg), 399 cm (B1g),
5
In (c), one new peak, 281 cm is recorded, assigned to E1g of ␣-Fe2O3.
− −1 −1
1
15 cm (A1g), 519 cm (B1g), and 639 cm (Eg), match the TiO2 anatase structure.
−1
3
00
350
400
450
500
550
600
Wavelength (nm)
The self-degradation rate of AA is measured by the photolumi-
nescence (PL) emission spectrum. During the 50 min irradiation, the
intensity of PL decreases 1.23% (10th min), 2.18% (30th min), and
Fig. 5. The cut-off band gap of diffuse reflectance spectrum (DRS) for FIT-3 (480 nm),
FIT-2 (445 nm), and FIT-1 (410 nm).
3
.03% (50th min), respectively. The self-degradation rate constant
−
5
(
kself), calculated from the plot of time-involved photo-reaction,
2 = 1 × 10 M). The immersed photocatalysts were bubbled with
nitrogen before the UV irradiation (18 W, ꢂ = 365 nm). The sam-
ples irradiated for 10 min were marked as series “a”, i.e. FIT-1-a,
FIT-2-a, and FIT-3-a. The other samples, irradiated for 20, 30, 40,
and 50 min, were marked as series “b”, “c”, “d”, and “e”, respec-
tively. The collected solutions were filtrated and the filtrates were
analyzed by PL emission spectra to determine the concentration
of non-degraded AA remained in the filtrate. A typical PL spec-
trum for photo-degradation of AA over FIT is provided in Fig. 6(a)
−
4
−1
is (−) 6 × 10 min . By deduction of the kself, the rate constants
(
kcat) derived from the plot will be the real rate constant.
The photocatalytic degradation routes of absorbed AA were
explored by two routes for experimental purpose. One route is with
high [AA]/[cat.] ratio, (AA:cat. = 1:0.67, denoted as route 1). The
other route is with low [AA]/[cat.] ratio, (AA:cat. = 1:100, denoted as
route 2). Each photocatalyst (route 1 = 10 mg, route 2 = 30 mg) was
immersed in 30 mL AA stock solution (route 1 = 5 × 10− M, route
4

Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
71
for route, and Fig. 6(b) for route 2. One major peak between 340
and 520 nm, ꢂmax = 395 nm, was observed for all resulting solu-
tions collected from 0 to 50 min. The intensity of PL was normalized
before the calculation of photocatalytic degradation rate constant
(kcat). Theoretically, the slope value (R) obtained from the plot of
(
a)
0
1
2
3
4
5
min
0 min
0 min
0 min
0 min
0 min
ln(c/c ) vs. time (min) will be equal to the photocatalytic degrada-
0
tion rate constant (kcat). The kcat values for the two degradation
routes are given as follows. In route 1, the kcat values for FIT-1
−
1
−1
−1
(
0.252 min ), FIT-2 (0.154 min ), FIT-3 (0.113 min ) are in the
ratio of 1.00:0.61:0.45, (Fig. 6(c)). In route 2, the kcat values of FIT-
− −1 −1
1
1
(0.0196 min ), FIT-2 (0.0129 min ), FIT-3 (0.0101 min ) are in
the ratio of 1.00:0.67:0.51, (Fig. 6(d)). The kcat value of route 1 is ∼30
360
400
440
480
520
times higher than that of route 2. The ratio of k is very similar in
cat
Wavelength (nm)
the two routes. The Michaelis–Menten kinetic equation show that
−
1
the turn-over number, TON, equals to kcat (min ) in a high con-
centration of substrate [23,24]. The TON of route 1 is in the range of
0.225 and 0.113, which satisfy the requirement of industrial stan-
(
b)
0
min
10 min
20 min
30 min
40 min
50 min
−
1
2
dard, 10 < TON < 10 [20]. Furthermore, in the FIT-1 experiments,
the highest catalytic efficiency and maximum photo-degradation
rate were recorded. It is clear that increase of the iron oxide amount
in FIT cannot improve the photocatalytic activity of AA over FIT.
(III) Spectrum study for photcatalytic products
In theory, the final mineralized product of organic acid species
is regarded as CO2 and H O. The final mineralization product CO2
2
is detectable, if the initial concentration of adsorbed organic acid
360
400
440
480
520
−
4
species can reach 1 × 10 M or higher [15]. Unfortunately, in this
Wavelength (nm)
−
6
case, the adsorbed concentration of AA is less than 6.6 × 10
M.
(
c)
Therefore, we employed 13C NMR, FT-IR, and LC–mass spectra to
2
0
2
4
6
8
clarify the structures of compounds after the 60 min UV irradiation,
-1
instead of measuring the final products, CO2 and H O.
2
R FIT-3=(-) 0.1125 min
V
-
-
-
-
(
a) The 13C NMR spectra and FT-IR spectra
The samples for 13C NMR spectra measurement were prepared in
DMSO. In comparison with the stock sample, two phenomena were
observed. First, the peak intensity of carboxylic acid, located at
V
-
1
R FIT-2=(-) 0.1543 min
∼
170 ppm, decreases about ∼10% (FIT-3), −12% (FIT-2), and ∼70%
-
-
-
10
12
14
V
FIT-1
FIT-2
FIT-3
(
FIT-1), respectively, for the samples irradiated by UV lights. Sec-
-1
ond, a new peak located at ∼205 ppm, assigned for CHO group
of benzaldehyde, is recorded in the FIT-2 and FIT-3. The 13C NMR
spectra show that the de-carboxylation reaction mainly occurred
in FIT-1 route, and the carbonyl reduction reaction is initialized in
R FIT-1=(-) 0.2521 min
0
10
20
30
40
50
60
Time (min)
(
d)
the FIT-2, and FIT-3 routes.
0
.2
0
Beside 13C NMR measurement, the samples were characterized
-1
by FT-IR simultaneously. Fig. 7(a) shows the FT-IR spectrum of the
product isolated from FIT-1 route. Twelve peaks match the charac-
teristic modes of aniline. On the other hand, the FT-IR spectrum for
the product isolated from FIT-2/FIT-3 route, (Fig. 7(b)), shows seven
recorded characteristic modes of 2-aminobenzaldehyde, (Fig. 7(b)).
R FIT-3 = (-) 0.0101 min
V
-
-
-
-
0.2
0.4
0.6
V
-1
0.8 R FIT-2 = (-) 0.0129 min
(b) LC–mass spectra
-
1
V
FIT-1
FIT-2
FIT-3
-1
-
1.2
1.4
R FIT-1= (-) 0.0196 min
In the LC–mass spectra for the photo-catalytic products collected at
the 60th min, the m/e data reveal that polymerization of products is
generally occurred in all samples. However, the photo-degradation
mechanisms still can be derived from the collected m/e data. In
the route of FIT-1, the m/e data suggest that the de-carboxylation
reaction prevails. Nevertheless, the alkaline environment favors
the hydroxylation addition when the de-carboxylation reaction has
conducted. The resulting products are aniline, and 4-aminophenol.
In the routes of FIT-2, and FIT-3, the recorded product is 2-
aminobenzaldehyde. The carbonyl reduction is believed to be the
main reaction in these routes. Since benzene is collected in all cat-
alytic routes, the de-amination reaction should be initialized in all
-
0
10
20
30
40
50
60
Time (min -1)
Fig. 6. PL spectra for photo-degradation of AA over FIT-1, (a) for route 1 (b) for route
. One major peak between 340 and 520 nm, ꢂmax = 395 nm, was observed from 0
to 50 min under UV irradiation. (c) In route 1, the kcat values of FIT-1 (0.252 min ),
FIT-2 (0.154 min ), FIT-3 (0.113 min ) are in the ratio 1.00:0.61:0.45. (d) In route 2,
the kcat values of FIT-1 (0.00914 min ), FIT-2 (0.0065 min ), FIT-3 (0.0046 min )
2
−
1
−1
−1
−1
−1
−1
are in the ratio 1.00:0.67:0.51.

7
2
Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
(
a) 100
FIT involved reactions. The proposed polymerization mechanisms
based on the m/e data observed in LC–mass spectra are given in the
following paragraph.
9
9
8
8
7
7
6
6
5
0
5
0
5
0
5
0
(
IV) The proposed photo-degradation mechanisms
a) In FIT-1 spectrum
(
In the route of FIT-1, four peaks, m/e = 1 3 6, 1 7 3, 2 8 3, and
2
9 5, shown in Fig. 9(a), are recorded in the LC/mass spectrum.
The m/e = 136 peak is attributed to un-reacted AA. The peaks for
m/e = 1 7 3, 2 8 3, and 2 9 5 are contributed by the polymerization
of photo-degraded products, as shown in Eqs. (a-1), and (a-2).
The m/e values recorded in Fig. 8(a) suggest that the polymeriza-
tion between benzene, aniline, and 4-aminophenol are the main
reactions in this route. Therefore, mechanisms for products are clas-
sified into two portions, i.e. photo-degradation and polymerization.
3
500 3000 2500 2000 1500 1000 500
-1
Wavenumber (cm
)
(
b) 100
9
8
7
6
5
0
0
0
0
0
3
500 3000 2500 2000 1500 1000 500
Wavenumber (cm -1)
Fig. 7. FT-IR spectra for product isolated from (a) AA over FIT-1 (b) AA over FIT-
/FIT-3. In (a), 7 characteristic modes of aniline are recorded. In (b), 12 peaks match
the characteristic modes of 2-aminobenzaldehyde.
2
Fig. 8. LC–mass spectra. (a) FIT-1 spectrum: four peaks, m/e = 1 3 6, 1 7 3, 2 8 3, and 2 9 5, are shown. (b) FIT-2 spectrum, and (c) FIT-3 spectrum, five peaks, m/e = 1 3 8, 1 9 7,
3 9, 3 2 5, and 4 3 7, are observed.
2

Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
73
Products from photo-degradation

7
4
Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
(
b) In FIT-2 and FIT-3 spectra
4. Discussion
4.1. The effect of TPAB in the oxidation state of iron
Five peaks, m/e = 1 3 8, 1 9 7, 2 3 9, 3 2 5, and 4 3 7, are recorded
In previous study, we reported that TPAB [N(n-Pr) +Br ] is able
−
in LC–mass spectrum for FIT-2, Fig. 8(b), and FIT-3, Fig. 8(c),
respectively. The peak of m/e = 138 stands for the un-reacted
AA. The m/e = 1 9 7, 2 3 9, 3 2 5 and 4 3 7 are contributed by the
polymerization of photo-degraded products. In this route, the car-
bonyl reduction of AA is preferential. The observed product is
4
to dissociate the vanadyl bond of V O5 monomer on the surface of
TiO2 nanoparticle [15]. In this study, the XPS data show that TPAB
still plays a reducing reagent in the reduction of Fe O3 monomers.
The suggested mechanism is given in Scheme 1. The detail expla-
nation is in the following paragraphs.
2
2
2
-aminobenzaldehyde. Nevertheless, the de-carboxylation reac-
tion and the de-amination are also occurred in this route. The
observed product is benzene. The polymerization between benzene
and 2-aminobenzaldehyde is the major reaction in FIT-2 and FIT-3
routes.
(
I) At the beginning of reduction, (N(n-Pr) +
)
initializes an
4
electronphilic attack to the terminal oxygen of Fe O ,
2
3
3+
O ) O], to form a 6-member ring structure
Fe3+
[
O
(Fe
Products from photo-degradation

Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
75
Fig. 9. The illustrated mechanism for the growth of FeO in Fe3O4 (FeO/Fe2O3) matrix of (a) FIT-1, (b)FIT-2, and FIT-3.
Scheme 1. Mechanism of dissociation of iron oxide on FIT nanoparticles.
on TiO -(0 0 1). The hydrogen of ␤ carbon of propyl group
in N(n-Pr)4 start a nucleophilic attack to the iron atom of
Since TPAB is a limiting reagent, the amount of FeO monomer
created by TPAB is restricted. Therefore, in the low ratio of Fe/Ti
2
+
terminal Fe O group. The terminal iron atom adopts the hydro-
sample, the Fe O3 monomers could be mostly convert to FeO.
Therefore, the ratio of Fe /Fe is high. On the contrary, in the
2
2+
3+
gen in ␤-carbon of propyl group, and the terminal oxygen of
3
+
3+
[
O
(Fe
O
Fe
)
O] is removed by N(n-Pr) , to form a good
high atomic ratio of Fe/Ti sample, the Fe O3 monomers are only
3
2
2+
3+
leaving group O N(n-Pr) , simultaneously [15]. The resulting
partially converted to FeO. The atomic ratio of Fe /Fe is low.
The XPS deconvolution data confirms this fact. The atomic ratio of
Fe /Fe in FIT-1 is up to 1.99. The surface of Fe O matrix in FIT-1
3
Fe
product is [H (Fe2+
O
3+
)
O], denoted as compound (A),
2+
3+
and the co-product is C H (g), shown in Scheme 1, step 1.
3
6
3 4
The co-product C H (g) is unable collected, but the O N(n-Pr)
is reduced to FeO completely. On the other hand, in FIT-2 (1.35) and
FIT-3 (1.17), the surface of Fe O matrix is partially reduced to FeO.
The evolution of iron oxide domain is illustrated in Fig. 9.
3
6
product can be detected from the solvent. The IR spectrum for
the collected solvent shows the characteristic O N stretching
3
4
−1
at 1592 cm , which confirms the existence of O N(n-Pr) in the
solvent [15,21].
4.2. The mechanism of photo-selectivity of AA over FIT
(
II) The compound (A), [H (Fe2+
O
Fe3+
)
O], is easily oxidized by
−
+
O molecule from atmosphere. The O molecule attacks the ter-
(I) The capture of photogenerated electron (e ) and hole (h )
2
2
minal H of compound (A) to form a 4-member ring. In the acidic
situation, the terminal hydrogen is rapidly removed to form
Metal oxide doped TiO2 is a photo-sensitive catalyst. When an
electron–hole (e–h) pair is created by 365 nm UV (3.39 eV) irra-
diation, the photogenerated (e ) from the top level of TiO2 valence
band (VB, +3.0 eV) is activated to reach the bottom level of TiO2
conduction band (CB, −0.2 eV), band gap ∼3.2 eV [14,24]. When
the relaxation of excited electrons starts, the excited electrons
are released from TiO2 CB, (-0.2 eV) and are temporarily trapped
•
2+
3+
[
( Fe
O Fe ) O] radical, denoted as compound (B), shown
in Scheme 1, step 2. The compound (B) is dissociated to two FeO
monomers by kinetic equilibrium, because the concentration
of compound (B) grows. The FeO monomers incorporate with
−
neighboring Fe O to form a stable Fe O4 matrix. The reaction
2
3
3
keeps going until the equilibrium lost [22,23].

7
6
Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
in Fe O matrix, either in eg of FeO (+0.3 eV), (FIT-1) or t2g of
In FIT-2, and FIT-3 routes, the FeO moiety of FeO/Fe O still pro-
2 3
3
4
3+
Fe O (+2.5 eV) (FIT-2, FIT-3), before reach the TiO VB (+3.0 eV)
ceeds the de-protonation reaction. If the LS Fe ion of Fe O adopts
2
3
2
2 3
−
+
−
the photo-generated (e ) by t orbital, the LS Fe is reduced to sta-
2g
3+
[
14,15,25]. The photogenerated (e ) and (h ) pair is thus delayed.
ble LS Fe2+. The lone pair electrons of (:FeO) attract the proton ion of
(s)(FIT-1)(s) → (e−) (s) (FIT-1) (s) (h+)
−
H O to form (H:FeO) and OH ion. The H atom of (H:FeO) attack the
2
C atom of COOH group to form the C H bond. At this moment, LS
2+
Fe acts as a Lewis acid to attack the other oxygen atom of COOH
group. This electrophilic attack leads to the bond cleavage between
(
II) The de-carboxylation reaction
−
•
the anion oxygen and C atom. Then, the OH leaves and (H)C
O
The hydroxyl radicals ( OH) and hydroxyl ions (OH ) play an
is formed. The mechanism is given in the follows.
influential role in the initialization of decarboxylation of AA. In FIT-
route, the low spin (LS) state FeO site of FeO/Fe O is able to adopt
the photo-generated (e ) by empty eg orbital. The stable LS Fe is
temporarily reduced to unstable Fe . At this moment, Fe acts as an
electron-rich Lewis base, and is able to donate the adopted electron
quickly, Scheme 1, step 3. The reduction of the oxygen molecules
adsorbed on the surface of FIT-1 with the available trapped elec-
1
2
3
−
2+
+
+
−
−
•−
•−
trons (e ) starts: O + e → O2 . The O2
ions then react with
2
proton ion of COOH group of adsorbed AA molecules to create
−
•
•
•−
+
−
−
•
−
HO ions and ( OH) radicals: 2 O
The unstable ( OH) radicals are quickly reduced to OH groups.
At this step, the abundant OH ions are able to consume all
+ 2 H + e → OH + OH + O
2
2.
−
available proton ions of COOH group of AA. Therefore, a follow-
ing chain-reaction of bond cleavage makes the COO group as
a leaving group. The photo-initialized de-carbonation reaction of
(
V) The de-amination reaction
−
AA over FIT-1 keeps going until OH ions is no longer available:
−
−
The de-amination reaction is induced by the reduction of the nitro-
gen atom of aniline adsorbed on the surface of FIT. The oxygen atom
of water molecule performs a nucleorphilic attack to C atom bonded
to N atom of NH group. The following reaction is H atom of H O to
HOOC (C H ) NH + OH → H (C H ) NH + OH + CO2 [15,26].
6
4
2
6
4
2
2
2
attack the N atom of NH2 group. The NH2 group is reduced to NH3
as a good leaving group. The H O lost one H atom and substitutes
2
the position of NH2 at the benzene ring. After the SN2 substi-
tution, the temperate product is phenol. The FeO domain of FIT
performs an electrophilic attack to oxygen atom of OH group. The
OH group is replaced by H atom, and the FeO is oxidized to Fe O .
2
3
The de-hydroxylation reaction has finished. The final product is
benzene.
5. Summary
In this paper, we report a newly developed FIT photocatalyst
with tunable photocatalytic selectivity in photo-degradation of
AA. The photo-degradation selectivity is controlled by the atomic
2+
3+
ratio of Fe /Fe in FIT. In low atomic ratio of Fe/Ti route (i.e.
high Fe2+/Fe3+ ratio), the de-carboxylation reaction is the dominate
selectivity to perform the photo-degradation. On the other hand, a
high Fe/Ti ratio with low Fe2+/Fe ratio is necessary for the ini-
3+
(
III) The hydroxylation addition
The hydroxylation addition is the other possible reaction in
−
+
tialization of carbonyl reduction. The photo-induced (e –h ) pair
−
and (FeO/Fe2O3) moiety in FIT can substitute the roles of [HO ]
•
an alkaline environment. The para-addition of OH group to ani-
line, i.e. hydroxylation reaction, has the priority to be performed,
when the nitrogen atom of aniline adsorbed on the surface of FIT-1
with the Lewis base (FeO/Fe O ) moiety, as shown in the following
and [ OH] in the photo catalytic reaction. The yield of too many
hydroxyl radicals in the photo Fenton reaction is able to be avoided
in the treatment of environmental organic pollutants or biological
application.
2
3
equation.
(
IV) The carbonyl reduction reaction

Y.-H. Chang et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 67–77
[10] Y. Li, J.S. Wu, D.W. Qi, X.Q. Xu, C.H. Deng, P.Y. Yang, X.M. Zhang, Chem.
77
Acknowledgments
Commun. 5 (2008) 564–566.
[
[
11] W.S. Tung, W.A. Daoud, ACS Appl. Mater. Interfaces 1 (2009) 2453–2461.
12] C.-S. Yang, C.-J. Chen, Appl. Catal. A 294 (2005) 40–48.
The research is partially supported by Ministry of Science
and Technology, Taiwan, ROC under the contract number NSC
[13] C.-S. Yang, Y.-J. Wang, M.-S. Shih, Y.-T. Chang, C.-C. Hon, Appl. Catal. A 364
2009) 182–190.
14] C.-C. Ou, C.-S. Yang, S.-H. Lin, Catal. Sci. Technol. 1 (2011) 295–307.
(
1
02-2622-M-415-001-CC3 and MOST-103-2622- M-415-001-CC3.
[
[
[
Authors thank Mr. Ren-Xing Rai for the help in Chem Draw.
15] S.-H. Lin, C.-C. Ou, M.D. Su, C.-S. Yang, Catal. Sci. Technol. 3 (2013) 2081–2091.
16] (a) JCPDS, 21–1272.;
(
b) JCPDS, 29–1360.
References
[
[
17] D. Gazzoli, G. Mattei, M. Valigi, J. Raman Spectrosc. 38 (2007) 824–831.
18] C. Baratto, P. Lottici, D. Bersani, G. Antonioli, J. Sol–Gel Sci. Technol. 13 (1998)
[
[
[
[
1] R. Bauer, G. Waldner, H. Fallmann, S. Hager, M. Klare, T. Krutzler, S. Malato, P.
Maletzky, Catal. Today 53 (1999) 131–144.
2] J. Herney-Ramireza, M.A. Vicenteb, L.M. Madeirac, Appl. Catal. B 98 (2010)
6
67–671.
[
19] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray
Photoelectron Spectroscopy, PerkinElmer Corporation, Norwalk, CT, 1992.
20] L. Michaelis, M.L. Menten, Z. Biochem, (1913) 333–369.
21] G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York and
London, 1980.
1
0–26.
[
[
3] M. Pera-Titus, V. Garc ı´ -Molina, M.A. Ba n˜ os, J. Giménez, S. Esplugas, Appl.
Catal. B 47 (2004) 219–256.
4] M. Pelaeza, N.T. Nolanb, S.C. Pillai, M.K. Seeryc, P. Falarasd, A.G. Kontosd,
P.S.M. Dunlope, J.A. Hamiltone, K. O’sheaf, M.H. Entezarig, D.D. Dionysioua,
Appl. Catal. B 125 (2012) 331–349.
[
[
22] M.J. Tiernan, P.A. Barnes, G.M.B. Parkes, J. Phys. Chem. B. 105 (2001) 220–228.
23] E.R. Monazam, R.W. Breault, R. Siriwardane, Energy Fuels 28 (2014)
5
406–5414.
[
[
5] H.J.H. Fenton, J. Chem. Soc. Trans. 65 (1894) 899–911.
6] S. Goldstein, D. Meyerstein, G. Czapski, Free Rad. Biol. Med. 53 (1993)
[
[
[
24] A.G. Thomas, W.R. Flavell, A.K. Mallick, A.R. Kumarasinghe, D. Tsoutsou, N.
Khan, C. Chatwin, S. Rayner, G.C. Smith, Phys. Rev. B 75 (2007) 035105.
25] J. Tang, M. Myers, K.A. Bosnick, L.E. Brus, J. Phys. Chem. B 107 (2003)
4
35–445.
[
[
[
7] Y. Luo, E.S. Henle, S. Linn, J. Biol. Chem. 271 (1996) 21167–21176.
8] J.A. Imlay, S.M. Chin, S. Linn, Science 240 (1988) 640–642.
9] W. Wu, X. Xiao, S. Zhang, F. Ren, C. Jiang, Nanoscale Res. Lett. 6 (2011)
7
501–7506.
26] A.R. Khataee, M.B. Kasiri, J. Mol. Catal. A: Chem. 328 (2010) 8–26.
5
33–547.