200
T. Soltani, B.-K. Lee / Journal of Molecular Catalysis A: Chemical 425 (2016) 199–207
even at a low concentration of Li doping [18]. It has been reported
that the substitution of Ca and Sr for Bi induced oxygen vacancies
and mixed Fe valences, which can play an important role in the
photocatalytic property of BFO MNPs [19,20].
peak at 205 nm using a UV–vis spectrophotometer. The conversion
of toluene was defined as the following Eq. (1):
ꢀ
ꢁ
C0 − C
Conversion =
× 100
(1)
C0
In this study, we prepared Bi1-xBaxFeO3 MNPs with differ-
ent Ba2+ ion-dopant levels (x = 0.03, 0.08, 0.12) via a rapid sol
gel procedure to improve heterogeneous photo-Fenton catalytic
degradation of toluene under visible light irradiation. For the first
time, we found that Ba substitution in BFO MNPs greatly improved
the photo-Fenton catalytic degradation of toluene at a very weak
acidic medium, pH = 5.5, due to greatly affect in iron redox cycling
and oxygens vacancies as compared to pure BFO MNPs. The iron
redox cycling and existence of oxygen on the surface of Ba doped
BFO had been affected after photo Fenton process. Finally, we found
that, singlet oxygen was partly exhibited roles in the photo-Fenton
catalysis process of toluene in addition to •OH, and relevant radical
reaction mechanism was also proposed.
Where Co is the initial concentration of the toluene solution and C
is the concentration of toluene remaining in the solution after the
photocatalytic reaction.
Furthermore, a blank solution (without the photocatalyst) was
used in all experiments to evaluate the amount of toluene evapo-
ration. Toluene evaporation during the photo degradation process
was negligible when a cap was used
2.4. Characterization equipment
The crystalline structure was identified by using X-ray diffrac-
tion (XRD) (BrukerD8Advance) with monochromatic Cu K␣
radiation ( = 1.5406 Å). The samples’ morphology and size were
examined by transmission electron microscopy [(TEM, JEOL TEM
Model 2100)]. To analyze the chemical states of the constituents,
X-ray photoelectron spectroscopy (XPS) was conducted with a
Thermo Scientific Sigma Probe spectrometer with a monochro-
matic AlK␣ source (photon energy 1486.6 eV), spot size 400 m,
pass energy 200 eV and energy step size 1.0 eV. A Genesis 10S
UV–vis spectrophotometer was used to obtain the absorbance of
toluene at max of 205 nm. The chemical oxygen demand (COD) test
was used to express the COD of toluene solution. The total organic
carbon (TOC) of the toluene solution was determined by TOC 500 A
(Shimazu, Japan).
2. Materials and methods
2.1. Chemicals
Bismuth nitrate (Bi(NO3)3 5H2O), iron nitrate (Fe(NO3)3·9H2O),
barium nitrate (Ba (NO3)2), and H2O2 from Fluka, ethylene glycol
(EG), isopropanol (IP), sodium azide (NaN3) and 1–4 benzoquinone
(BQ) from Merck, and toluene and ethanol from Aldrich were used
as received.
3. Results and discussion
2.2. Catalyst preparation
3.1. Characterization of the catalyst
To synthesize pure BFO MNPs, 25 mmol of Bi (NO3)3·5H2O and Fe
(NO3)3·9H2O were dissolved in 65 mL EG. The mixture was stirred
for 40 min at 30 ◦C to obtain a dark red sol. For the preparation of
Bi1-xBaxFeO3 (x = 0.03, 0.08, 0.12), Bi (NO3)3·5H2O, Fe (NO3)3·9H2O
and Ba (NO3)2 were added to the EG at a molar ratio of 1-x: 1: x;
where x = 0.03, 0.08 and 0.12 for 3%, 8%, and 12% Ba doped sam-
ples, respectively. Then, samples irradiated for 40 min at 30 ◦C to
obtain other brownish red sols. Then, all samples were incubated
at 80 ◦C to 20 h to remove the organic pollutants and to form xero-
gels. Finally, the powders were calcined at 500 ◦C for 1 h at a heating
rate of 6 ◦C/min to afford pure BFO and Ba doped BFO MNPs, respec-
tively.
3.1.1. XRD results
XRD patterns of as-prepared pure BFO and Ba doped BFO sam-
ples with varying content of Ba are shown in Fig. 1. All XRD peaks
corresponding to BFO perovskite structure with R3c space group
(JCPDS No. 86-1518), demonstrating that single crystalline BFO
MNPs phase. The rhombohedral structure of BFO MNPs does not
change appreciably with Ba doping, except increase in Bi25FeO39
impurity phases. From a small magnified section of XRD patterns in
the 2 range of 31–33, it is clear that the (104) and (110) peaks split
for pure BFO MNPs and it was merged into a single (110) peak, aris-
ing from the substitution of larger ionic radius of Ba2+(1.36 Å) than
that of Bi3+(1.17 Å). The calculated crystalline sizes using Scherer’s
formula were found to be 33. 23 nm, 31.7 nm and 30.1 nm for pure
BFO, Ba 8% −BFO and Ba 12%-BFO, respectively. Compared with
the pure BFO MNPs, with increasing the Ba doping content, the
crystalline size of Ba doped BFO slightly decreased, revealing the
structural distortion of BFO induced by the Ba doping.
2.3. Degradation experiments
This study focuses on the application of the photo-Fenton
catalytic degradation of toluene from aqueous solutions using
undoped and Ba doped BFO MNPs. Before irradiation, the suspen-
sion was magnetically stirred for 45 min in a dark place and then
irradiated for different time intervals at a fixed temperature of
25 ◦C by using a water bath. The photo-Fenton catalytic degrada-
tion of toluene was carried out under the irradiation of a 55 W
fluorescent lamp at a distance of 15 cm with an emission peak
at 550 nm under continuous magnetic stirring. For photo-Fenton
catalytic degradation of toluene under visible light irradiation, pho-
tocatalyst powder (40- mg) and 0.6–65 mM H2O2 were added into
100 mL of toluene solution (100 mg L−1) in a 250 mL Erlenmeyer
flask with an appropriate cap to stop evaporation of toluene solu-
tion during photodegradation process. After reaction completion,
the photocatalyst was easily separated from the solution by using
an external magnetic field. The concentration of the toluene solu-
tion was evaluated by measuring the intensity of the absorption
3.1.2. XPS results
Further evidence for the quality and composition of the BFO
and Ba doped BFO MNPs was obtained from XPS studies. The sur-
vey spectra from 0 to 1000 eV for pure and Ba doped BFO MNPs as
shown in Fig. 2 confirm the presence of Bi, Fe, O and minor quanti-
ties of Ba for Bi1−xBaxFeO3 (x = 0.03, 0.08 and 0.12), without traces
of any other impurities and with the exception of a small amount of
adsorbed carbon peak C 1 s at 285 eV to calibrate the system. Fur-
thermore, in the pure BFO MNPs, the molar ratio of bismuth, iron
and oxygen is almost 1:1:3. However, in the Ba doped BFO MNPs
samples, the amounts of Ba identified by the XPS analysis were a
little less than those actually added in the synthesis. The detected
atomic fractions of Ba in Ba doped BFO MNPs were 2.8, 7.5 and 11.6