Enhanced photo-degradation of bisphenol pollutants onto gold-modified photocatalysts

Article abstract of DOI:10.1016/j.cattod.2016.11.009

Here we report a comparative study of Au supported on different types of TiO₂in the photocatalytic degradation of BPA. The beneficial presence of a small amount of rutile was proven once again, the TiO₂anatase showing a lower activity despite a higher surface area. Also, the role of gold plasmon was evidenced by the use of an inert silica support.

Full text of DOI:10.1016/j.cattod.2016.11.009

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Enhanced photo-degradation of bisphenol pollutants onto
gold-modified photocatalysts
Bogdan Cojocaru , Veronica Andrei , Madalina Tudorache , Feng Linb,c, Chris Cadiganb,
a
a
a
b,∗
a,∗
Ryan Richards , Vasile I. Parvulescu
Department of Organic Chemistry, Biochemistry and Catalysis Faculty of Chemistry, University of Bucharest, Bdul Regina Elisabeta, 4-12, Bucharest
30016, Romania
Department of Chemistry and Geochemistry, Materials Science, Colorado School of Mines, Golden, CO 80401 USA
Department of Chemistry, Virginia Tech, Blacksburg, VA24061, USA
a
0
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2016
Received in revised form 3 October 2016
Accepted 6 November 2016
Available online xxx
Here we report a comparative study of Au supported on different types of TiO2 in the photocatalytic
degradation of BPA. The beneficial presence of a small amount of rutile was proven once again, the
TiO2 anatase showing a lower activity despite a higher surface area. Also, the role of gold plasmon was
evidenced by the use of an inert silica support.
©
2016 Elsevier B.V. All rights reserved.
Keywords:
TiO2 photocatalyst
Gold
Plasmon
Bisphenol A
1
. Introduction
cancer-causing agents. However, the human health risks that may
be associated with these low-level yet constant exposures are still
largely unknown and highly controversial. Since the turn of the
century, manufacture and use of synthetic chemicals has rapidly
increased. Many of the environmental estrogens, which are man-
made chemicals, are persistent and ubiquitous, leading to a higher
risk of environmental pollution.
Bisphenol A (BPA) is an endocrine disruptor (ED), i.e. an exoge-
nous substance that alters function(s) of the endocrine system
and consequently causes adverse health effects in intact organism,
or their progeny, or (sub)populations [1–4]. Other chemicals that
are known EDs include diethylstilbestrol (the synthetic estrogen),
dioxins, polychlorinated biphenols (PCBs), phthalates, methoxy-
chlor, phenolic derivatives, linear alkylphenols, DDT and some
other pesticides. All these chemicals are found in our daily lives (in
many of the products we encounter or use regularly such as plastics,
liners of metal food cans, detergents, flame-retardants, toys, cos-
metics, and pesticides). In some cases, these chemicals imitate the
estrogens in the body [5–7]. Exposure to these substances occurs
throughout our lives from food, air, water, soil, and household prod-
ucts that can reach a developing fetus and additionally be passed
on from a mother via breast milk [3,7]. Furthermore, since they can
sometimes leach out of the plastics or other materials and migrate
into the food, especially after heating or when the plastic is old
or scratched, these compounds have aroused concerns as possible
Bisphenol A (BPA) is a synthetic resin widely used in food pack-
aging, dental sealants and polycarbonate plastic products, which
range from CDs and eyeglass lenses to tableware and food and
beverage containers, including baby bottles [8]. This compound
has now been found to disrupt important effects of estrogens in
the developing brain, according to a report from Medical Research
News, 2 December 2005 [9]. While high doses cause little effect,
analysis of cellular and molecular markers of estrogens signaling
revealed that near-maximal effects of BPA on rat brain neurons not
only occurred at surprisingly low doses of 0.23 parts per trillion,
but they also happened in a matter of minutes. From the litera-
ture, it’s clear that these low concentrations are in line with human
fetal exposures, and at levels one might even see in the water sup-
ply. This “low-dose” effect of BPA is troubling since its maximal
effects occur at the level typical of human exposure. This means
that the harmful effects of BPA could easily be missed using stan-
dard approaches for determining the risks of chemical exposure.
The scientific research in the field of EDs involves mainly the study
∗ Corresponding authors.
E-mail addresses: rrichard@mines.edu (R. Richards),
vasile.parvulescu@chimie.unibuc.ro (V.I. Parvulescu).
http://dx.doi.org/10.1016/j.cattod.2016.11.009
0
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synthesized by the quick hydrolysis of titanium butoxide (reagent
grade, Sigma-Aldrich) by adding drops of DI water into a solution
of titanium butoxide and butanol followed by centrifugation, dry-
◦
ing and calcination at 500 C for 6 h. Then SBA-15, TiO2 NA and
TiO2 were used as the supports for the deposition of Au nanopar-
ticles. To deposit Au nanoparticles, supports were dispersed in
2
00 mL of 6 mM trisodium citrate (premium quality level, Sigma-
Aldrich) aqueous solution and stirred vigorously for 1 h. Then,
0 mL of 3 mMHAuCl4 (99.9% metals basis, Aldrich) aqueous solu-
5
tion was added to the above suspension dropwise and the mixture
was allowed to react overnight under vigorous magnetic stirring.
Finally, the product was centrifuged and cleaned with DI water
◦
Scheme 1. Interfacial charge transfer in Au-TiO2 NPs with Fermi level equilibration.
several times and dried at 100 C for 24 h. Au/meso-TiO2 was syn-
thesized using a one-pot method. In detail, a designated amount
of HAuCl4 was dissolved in 40 mL of butanol and stirred at room
temperature to complete dissolution, then 4 g of poly(ethylene
glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
on endocrine disruptors’ toxicology as well as the development of
analytical methods for detection and quantification of EDs con-
taminants at low concentration levels [10–12]. Currently, a new
aspect of the EDs issues is considered such as the environmental
cleanup of ED hazard compounds. A brief search on the literature
shows the scientific interest on EDs remediation, especially cat-
alytic remediation (e.g. photo- remediation) that is continuously
growing.
Among “classical” routes to degrade BPA, the photocatalytic
route is gaining much interest as a green method requiring a
cheap (practically free) energy source and limited only by the
light absorbing capacity of the catalysts. Besides the reference TiO2
(EO20PO70EO20, P123, MW = 5800, Aldrich) was added and stirred
for 30 min. Titanium butoxide solution (20 g of titanium butoxide
in 25 mL of butanol) was added dropwise to the above solution.
◦
Finally, the mixture was aged at 40 C overnight, filtered, dried
◦
and calcined at 500 C for 6 h. The deposition of Au onto Degussa
P25 was carried out following the precipitation-deposition method.
Thus, 1 g of TiO2 Degussa P25 was added to the corresponding vol-
ume of an aqueous solution of HAuCl ·3H O (0.2 M). The pH of the
4
2
solution was stabilized at 8–9 by addition of a 0.2 M NaOH solution.
◦
The mixture was stirred at 80 C for 12 h. After that, the suspension
[
13], “improved” photocatalysts like Ti/TiO2 [14], Pr,N-TiO2 [15],
−
was filtered, washed with water till free of Cl . Finally, the solid
was dried at 80 C for 48 h. Using this procedure samples with 0.1,
H PW12O40/TiO [16], Ag-TiO2 [17], Pt-TiO2 [18] or Photo-Fenton
3
2
◦
systems [19,20] showed a promising behavior for the removal of
BPA.
0
.3, 0.7 and 1 wt% Au, respectively, were obtained.
Gold catalysis has attracted great interest in the past period
[
21]. In addition to the catalytic oxidative reactions, gold nanopar-
2
.2. Catalyst characterization
ticles were found to promote the catalytic activity of TiO2 [22–26].
It has also been observed that the visible light absorbed by gold
nanoparticles due to the surface plasmon resonance is leading to
the generation of photo-excited states of gold nanoparticles fol-
Textural characteristics of the investigated catalysts were deter-
mined from the adsorption-desorption isotherms of nitrogen at
◦
−
196 C using a Micromeritics ASAP 2020 Surface Area and Porosity
[
27]
lowed by the transfer of the electrons to the TiO2
. Thus, the
Analyzer. DR-UV–vis spectra were collected under ambient con-
ditions with a Specord 250 (Analytic Jena) with an integrating
sphere and MgO as reference. The registered spectra were trans-
formed using the Kubelka-Munk F(R) function. The band gap energy
surface plasmon resonance originating from the collective oscil-
lations of the electrons on the surface of the gold nanoparticles
was suggested to be the essential factor of the promoting effect.
The enhanced interfacial charge transfer between TiO2 and gold
resulting from the accumulation of more electrons can be achieved
by a negative shift in the Fermi level of the Au–TiO2 composite
2
was calculated from the dependence of [F(R) • hꢀ] as a func-
tion of hꢀ, were hꢀ is the energy of the incident photons. Raman
spectra were taken with a Horiba Jobin Yvon − Labram HR UV–vis-
NIR (200–1600 nm) Raman Microscope Spectrometer, using a laser
with the wavelength of 632 nm. Powder X-ray Diffraction patterns
were collected at room temperature using a Shimadzu XRD-7000
with Cu K␣ monochromatic radiation (␭ = 1.5406 Å, 40 kV, 40 mA)
[
22–28]. Metal or metal ion doped semiconductor composites typ-
ically exhibit a shift in the Fermi level to more negative potentials.
This improves the energetics of the composite system and enhances
the efficiency of the interfacial charge-transfer process [22]. For
the particular case of Au/TiO , previous reports already indicated
2
◦
−1
with a scanning rate of 0.1 min , in the 2ꢁ range of 5–80.
XPS spectra were recorded at room temperature using a SSX-100
spectrometer, Model 206 from Surface Science Instrument. The
pressure in the analysis chamber during the analysis was 1.33
mPa. Monochromatized Al-K␣ radiation (hꢀ = 1486.6 eV) generated
by bombarding the Al anode with an electron gun operated with
a beam current of 12 mA and acceleration voltage of 10 kV has
been used. The spectrometer energy scale was calibrated using the
Au 4f7/2 peak centered at 83.98 eV. Charge correction was made
with the C 1 s signal of adventitious carbon (C C or C H bonds)
located at 284.8 eV. The atomic surface compositions were cal-
culated using the sensitivity factors provided with the ESCA 8.3
D software, applied to the surface below the corresponding fit-
ted XPS signals. An estimated error of ± 0.1 eV can be assumed for
all measurements. Specimens for electron microscopy were pre-
pared by suspending them in acetone and transferring to a copper
grid coated with an amorphous carbon support. TEM images were
recorded on a JEOL JEM ARM 200 F electron microscope operated
the accumulation of more electrons in system [25] having as an
ultimate effect an enhanced photo-catalytic activity (Scheme 1).
The resonances of the surface plasmons are directly affected by the
wavelength, particle size, shape, and local dielectric environment.
Starting from this state of the art, the aim of this study was to
investigate the photo-activity of a series of catalysts containing gold
deposited on titania in the decontamination of water solutions con-
taining BPA and to elucidate the effect of the gold plasmon and of
the nature of the support.
2
. Experimental
2.1. Catalyst preparation
SBA-15 was synthesized according to the procedure developed
in a previous work [29]. TiO2 NA was obtained from NanoScale
Corporation (NanoActive TiO ) and used as received. TiO2 was
2
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Table 1
Surface area, pore size for the main pores, pore volume and calculated band-gaps for the Au/TiO2 photo-catalysts.
2
3
Sample
Surface area BET m /g
Main pore size nm
Pore volume, cm /g (BJH desorption)
Band-gap (eV)
Degussa P25
52
56
53
51
39
398
7
23, 460
493
493
463
486
32
19
61
62
0.32
0.51
0.55
0.54
0.57
0.34
0.06
0.15
0.93
3.06
3.07
3.05
3.08
2.99
3.25
3.07
3.09
–
0
0
0
1
1
1
1
1
.1% Au/P25
.3% Au/P25
.7% Au/P25
% Au/P25
% Au/TiO2 NA
% Au/TiO2
% Au/mesoTiO2
% Au/SBA15
76
446
Table 2
Average particle size (dm) of Au determined by TEM.
at 300 keV. The gold crystallite sizes were established from the
measurement of 500–1000 particles for each sample.
Sample
dm Au [nm]
2
.3. Preparation of BPA stock solution
0
0
0
1
1
1
.1% Au/P25
.3% Au/P25
.7% Au/P25
% Au/P25
% Au/TiO2 NA
% Au/TiO2
8.2
11.2
14.3
17.1
19.9
16.7
19.1
14.8
1
6.7 mg of BPA was dissolved in 0.5 mL of ethanol. The resulting
solution was moved in a 250 mL flask, which was filled to the
mark with distilled water. The solutions of BPA derivatives (4,4 -
sulfonyldiphenol,
4
ꢀ
ꢀ
4,4 -(1,4-phenylen-diisopropylen)bisphenol,
,4 -(1-phenyletiliden)bisphenol, bis(2-hydroxyphenyl)methane,
1% Au/mesoTiO2
1% Au/SBA15
ꢀ
andbis(4-hydroxyphenyl)methane) were prepared in a similar
way.
For the supports synthesized in this study, 1% Au/TiO2 showed
a very small surface. The other three catalysts (1% Au/meso-TiO2,
2.4. Photocatalytic tests
1
% Au/TiO2 NA and 1% Au/SBA-15) showed larger surface area
and quite narrow pore size distribution. The adsorption-desorption
isotherms of all samples are presented in Figure SI1.
TEM measurements (Figure SI2) indicated for the P25 series
average gold particle sizes that varied from 8 nm (0.1% Au/P25) to
Photocatalytic tests were carried out in closed Pyrex test tubes
at room temperature under continuous stirring. For experiments
carried under UV, a Vilber Lourmat lamp centered at 365 nm (VL-
3
40.BL) and 120 W was used, while a 150 W Philips Master Colour
1
7 nm (1% Au/P25) (Table 2). Along with the increase of the Au
CDM-T 150W/830lamp was used for Vis experiments. The photore-
actors were fitted with electric fans to remove the heat produced
by the lamps and to keep the samples at room temperature.
In a typical BPA oxidation experiment, 10 mg of photocatalyst
was added to a 10 mL aqueous solution of bisphenol. Samples were
irradiated under continuous stirring. Samples were centrifuged at
concentration, the density of Au particles is increasing. The mor-
phology of Au particles is generally spherical, but with the increase
of the size, the particles become irregular exhibiting flat facets. In
the case of 1% Au loading, many Au particles were found to sit at
the interface between TiO2 particles, whereas for the low loadings,
most of the Au particles were supported on the surface of TiO2
particles. As expected, the TiO2 morphology was not influenced by
6
00 rpm for 15 min to separate the solid and then were analyzed
by high performance liquid chromatography (HPCL, Agilent Tech-
nologies 1260 Infinity with DAD detector. Column Eurosphere C18,
the deposition of gold. For the 1% Au/TiO2 NA, 1% Au/TiO , and 1%
2
Au/mesoTiO samples, the size of Au particles was in the 16–20 nm
2
−1
flow rate 1 mL min , ACN: H O = 75: 25, ␭ = 227 nm, V = 10 ␮L).
2
inj
range, while for the 1% Au/SBA15 the gold particle size was about
Control experiments were carried out testing substrates irradiated
without catalysts and with catalysts stirred under darkness. In all
these cases the conversion of substrates was close to zero.
The reaction efficiency (conversion) was calculated from the
area of peaks from the chromatograms (which is proportional with
the moles of transformed BPA):
1
4.8 nm.
The XP spectra of the photocatalysts in the region of the Au 4f7/2
level display a band in the region 82.6–83.3 eV indicating the pres-
ence of the Au species (Table 3). Cationic Au species usually exhibit
a band at values higher than 84 eV [29]. The XPS Ti2 band was
located at 458 eV which corresponds to Ti from the TiO2 frame-
0
p
4+
work, while the framework oxygen exhibits the O1s band at around
A − A
i
f
C% =
× 100
(1)
5
29 eV (spectra not shown). XPS Au/Ti ratios very well correlate
A
i
with the TEM results (Table 3).
where: A = area of the peak of the substrate subjected to reaction;
i
DR-UV–vis spectra of 1% Au/TiO , 1% Au/meso-TiO , 1% Au/TiO
2
2
2
Af = area of the peak of the substrate in the reaction mixture.
NA and Au/P25 series (Fig. 1) exhibits absorption thresholds around
15 nm (3–3.1 eV) (Table 1) (typical for the transitions between
The carbon content of the reaction product mixture was deter-
mined using a TOC system (1200–Thermo Analyzer equipement).
4
valence band and conduction band of TiO2 and 550 nm due to the
vibrations of superficial plasmons of gold nanoparticles and which
gives the pink-violet color to the samples. 1% Au/SBA-15 exhibits
only the band associated with the resonance of the superficial plas-
mons. 1% Au/mesoTiO2 exhibits a supplementary band at about
3
. Results and discussion
3
.1. Characterization
4
50 nm (Fig. 1).
All samples Au/P25 show a macroporous texture, having large
The intensity of the band associated with the vibration of the
pore size distribution. Low loadings of gold did not change the sur-
face area and pore volume (Table 1). However, loadings of 1% Au
caused an important decrease of both properties. It is also impor-
tant to notice that the addition of Au, even in the low loadings was
blocking the pores with an average around 23 nm of P25 support.
superficial plasmon of the Au/P25 series is increasing with the
increase of the Au loading that represents an indication of the
increase of the size of Au nano-particles [30]. The slight asym-
metric shape was associated with an Au-anatase interaction [30]
confirming the composition of Degussa P25.
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Table 3
The Au/Ti ratio determined from XPS measurements.
Sample
Ti 2p3/2 energy level (eV)
Au 4f energy level (eV) 4f5/2/4f7/2
O 1 s energy (eV)
XPS Au/Ti ratio (%)
0
0
0
1
1
1
1
1
.1% Au/P25
.3% Au/P25
.7% Au/P25
% Au/P25
% Au/TiO2 NA
% Au/TiO2
457.9
458.0
458.2
458.5
458.1
458.0
458.1
–
87.1/83.0
87.1/83.1
87.1/83.3
86.4/82.6
86.4/82.6
86.5/82.7
86.4/82.7
86.2/82.4
529.6
529.7
529.5
529.9
529.9
529.9
529.8
529.9
0.11
0.31
0.62
0.97
0.96
0.98
0.99
% Au/mesoTiO2
% Au/SBA15
a
0.98
a
Au/Si ratio.
Fig. 1. DR-UV–vis spectra for: (A). a) 1% Au/SBA-15, b) 1% Au/mesoTiO2, c) 1%
Au/TiO2, d) 1% Au/TiO2 NA; (B). a) Degussa P25; b) 0.1% Au/P25; c) 0.3% Au/P25;
d) 0.7% Au/P25; e) 1% Au/P25.
Fig. 2. Raman spectra for: (A). a) 1% Au/mesoTiO2, b) 1% Au/TiO2 NA, c) 1% Au/TiO2;
B). a) Degussa P25; b) 0.1% Au/P25; c) 0.3% Au/P25; d) 0.7% Au/P25; e) 1% Au/P25.
(
No Raman bands were observed for 1% Au/SBA-15 (figure not shown).
In the case of the series of the photocatalysts with non-
commercial support, the band associated with the vibration of
the superficial plasmon presents a red-shift for the samples
having TiO2 as support. Compared to SBA-15, the peak posi-
tion for 1% Au/mesoTiO2 was shifted about 50 nm to higher
wavelengths. Also, its intensity increased in the order 1%
Au/SBA–15 < 1% Au/mesoTiO < 1% Au/TiO < 1% Au/TiO NA. This
(Fig. 2B). This phenomenon was associated with a slight perturba-
tion of the anatase phase by the interaction of gold ions with the
TiO2 surface [31] and it was suggested to indicate the migration of
gold from the exterior layer of TiO into the bulk. The Raman active
2
modes with A1g + B1g + B2g + Eg symmetries specific for TiO -rutile
2
−
1
start to be visible with the increase of the gold loading (∼820 cm
,
2
2
2
− −1
1
represents another confirmation of the increase of the size of
Au nano-particles [30]. The shift to higher wavelengths for 1%
Au/mesoTiO2 compared with 1% Au/TiO2 and 1% Au/TiO2 NA is a
consequence of the interaction Au/rutile for this sample [30].
440 cm , 600 cm ).
The XRD patterns indicate the preservation of the crystalline
state of TiO P25 for all samples based on commercial TiO (Fig. 3B).
2
2
In accordance with the small gold loading, Au lines cannot be
observed. They can also be overlapped by the more intense lines of
−
1
Three bands at 400, 515 and 640 cm were observed in the
Raman spectra of the samples containing TiO (Fig. 2). These corre-
TiO . The diffraction lines from 2␪ = 25.3, 36.9, 48.1, 53.9, 55.1, 68.9,
75.1 are attributed to (101), (004), (200), (105), (211), (116) and
2
2
◦
spond to A1g + 2B1g + 3Eg symmetries of TiO -anatase. Fig. 2A shows
2
the Raman spectra of 1% Au/mesoTiO , 1% Au/TiO NA, and 1%
(215) reflections of TiO -anatase, while the diffraction lines from
2␪ = 27.4, 36.1, 38.5, 41.2, 53.7, 56.6, 62.6 and 70.3 are attributed to
(110), (101), (200), (111), (211), (220), (200) and (112) reflections
2
2
2
◦
Au/TiO . Spectra of 1% Au/mesoTiO and 1% Au/TiO2 presented a
2
2
−
1
shoulder at ∼820 cm associated to the B2g mode of rutile. Differ-
ently, for 1% Au/TiO NA (ie with a high anatase/rutile ratio) the
of TiO -rutile phase.
2
2
intensity of this line is very low. For the Au/P25 series the intensi-
ties of the Raman modes arising from the extension vibration of the
anatase structure were drastically reduced by the presence of gold
Mesoporous-TiO supported photocatalysts presented different
XRD patterns. 1% Au/TiO2 NA (Fig. 3B) shows a structure in incip-
ient crystallization, which can be identified as anatase (PDF card
2
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3.2. Photocatalytic behavior
The results of the degradation efficiency under UV irradiation
are presented in Fig. 4A. Deposition of Au on Degussa P25 increased
the activity from 62%, disregarding the Au content. The maximum
of about 76% was achieved for 1% Au/P25 and 0.1% Au/P25. This is a
clear confirmation that Au is acting as a trap for the electrons gen-
erated in TiO2 by the charge separation after UV light absorption.
This effect is beneficial, preventing the charge recombination and
allowing the generated holes to create hydroxyl radicals that are
generally assumed to be oxidative agents in a photocatalytic reac-
tion. The samples consisting of different types of TiO2, as support,
exhibit lower activity than the P25 series, despite higher surface
areas. The higher activities were achieved for 1% Au/TiO2 (∼57%)
and 1% Au/TiO2 NA (∼56%), while 1% Au/meso-TiO2 exhibited a
conversion of about 37%. The explanation of this behavior is the
co-existence of these structures of pure anatase promoted by rutile
in Degussa P25. The beneficial role of a small amount of rutile has
been already proved in the literature [34] as a sink effect for the
electrons generated in anatase. This enhances the physical separa-
tion of the electron and the hole pairs and it lowers the rate of their
recombination [35,36]. The synergetic effect between anatase and
rutile was found to be relevant at an anatase content of 40–80%,
with an optimum of 60% anatase and 40% rutile [37]. The content
of anatase in the Au/P25 series fits in the optimal range.
1
% Au/P25 and 0.1% Au/P25 exhibited similar activity after 24 h
of irradiation. This represents a good indication of the fact that the
size of gold particles (and, accordingly, the intensity of the absorp-
tion of the vibration of the superficial plasmon) is not so important
under UV irradiation.
The addition of gold induces photocatalytic activity for the pho-
tocatalytically inert support SBA-15. Thus, after 24 h of irradiation
the conversion was of about 33%. A similar behavior was previously
observed for the doped SBA-15 [38], where the doping effect has
been explained by an oxidation state change of the dopant during
the photocatalytic reaction. A similar behavior is also valuable for
Fig. 3. X-ray diffractograms for: (A). a) Degussa P25, b) 0.1% Au/P25; c) 0.3%
Au/P25; d) 0.7% Au/P25; e) 1% Au/P25; (B) a) 1% Au/TiO2 NA, b) 1% Au/TiO2, c) 1%
Au/mesoTiO2, d) 1% Au/SBA-15.
0
21-1272). Also, 1% Au/mesoTiO exhibits a well crystalline anatase
2
structure. Differently, 1% Au/SBA-15 exhibits an amorphous struc-
ture. Au is also XRD visible in these samples even the patterns
indicate only very small reflections at 2␪ of 38, 44, 64 and 77,
respectively (PDF card 004-0784).
According to literature [32], Degussa P25 consists of a physical
mixture of 73–85% anatase, 14–17% rutile. XRD checking experi-
ments carried out in this study confirmed these reports indicating
a composition of around 81% anatase and 18% rutile [33]. For the
mesoporous-oxide series we prepared, the content in rutile was
1
% Au/SBA-15 photocatalyst, where the photocatalytic activity is
0
+
attributable to the Au -Au redox process.
The activity under visible irradiation followed the same trends
(Fig. 4B). The deposition of Au on P25 increased the catalytic photo-
activity with a maximum of 80–83% for the Au loadings in the
range 0.3–0.7%, which underlines the importance of an optimal
Au particle size (11–14 nm). Further increase of the gold loading
to 1% Au/P25 corresponded to a decrease to almost half of the
activity (38%). The samples prepared from different types of TiO2
even smaller (around 7% in 1% Au/TiO ).
2
A
B
90
90
80
70
60
50
40
30
20
10
0
8
7
6
5
4
3
0
0
0
0
0
0
20
10
0
4
8
12
16
Time (h)
20
24
4
8
12
16
Time (h)
20
24
Fig. 4. The activity of the prepared materials under (A) UV irradiation, (B) Vis irradiation. (᭿ = Degusa P25; ᭹ = 0.1% Au/P25; ꢀ = 0.3% Au/P25; ꢁ = 0.7% Au/p25; ꢂ = 1% Au/P25;
ꢃ = 1% Au/meso-TiO2; ꢄ = 1 % Au/TiO2; ᮀ = 1 % Au/ TiO2 NA; ꢁ = 1 % Au/SBA-15).
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B. Cojocaru et al. / Catalysis Today xxx (2016) xxx–xxx
1
00
100
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
10
20
30
BPA
1
2
3
4
5
Quantity of catalyst, mg
Substrate
Fig. 5. The degradation of BPA with 0.1% Au/TiO2 after 24 h of UV irradiation as a
Fig. 7. The degradation of BPA and its derivatives with 0.1% Au/TiO2 after 24 h of UV
function of catalyst loading.
irradiation.
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
BPA
1
00
4
5
9
0
80
70
6
5
4
3
2
1
0
0
0
0
0
0
0
1
2.5
5
10
1
1
2.5
5
Substrate solution concentration, mg/L
BPA Solution concentration, mg/L
Fig. 8. The degradation of BPA and its derivatives with 0.1% Au/TiO2 after 24 h of UV
irradiation as function of substrate solution concentration.
Fig. 6. The degradation of BPA with 0.1% Au/TiO2 after 24 h of UV irradiation as a
function of BPA concentration.
support exhibited a lower activity than the P25 series. The higher
activity was achieved for 1% Au/meso-TiO2 (61%). Noteworthy, 1%
Au/SBA15 led to a 55% conversion of BPA, that was higher than that
under UV irradiation, and even higher than of titania supported Au
photocatalyst. As Fig. 7 shows the catalyst is active in the degra-
dation of all these compounds. More than 75% of the bis-phenol
derivatives were eliminated during the 24 h (reaction time).
(
1% Au/TiO –48% and 1% Au/TiO2 NA − 42%). This provides clear
2
ꢀ
ꢀ
Evenmore, for 4,4 -(1,4-phenylendiisopropylene)bisphenol, 4,4 -
1-phenyltilidene)bisphenol, and bis(4-hidroxyphenil)methane,
the conversion of the degradation process was closed to 100%.
Three of the pollutants (e.g. BPA, (4) bis(2-
evidence on the role of Au nano-particles in the Vis assisted pho-
tocatalytic reactions, and the importance of the presence of rutile.
For determination of other factors governing activity, 0.1% Au-
TiO2 was selected for further UV investigations, as this sample
presented a good activity both under UV and Vis irradiation. Also,
from the economical point of view such a loading is more inter-
esting since it contains the smallest amount of expensive gold. The
degradation efficiency of BPA decreased to almost half with the
increase of catalyst quantity, from 10 to 30 mg, with a minimum
for 20 mg (Fig. 5).
(
hidroxyphenil)methane and (5) bis(4-hidroxyphenil)methane)
were investigated for different concentrations. The results are
presented in Fig. 8. BPA exhibited an expected behavior (Fig. 6) for
which the efficiency of the degradation process was simultane-
ously enhanced with the increase of the pollutant concentration.
For the pollutants 4 and 5, the system performance was influenced
by the pollutant concentration. Accordingly, the catalyst’s activity
was inhibited by pollutant concentrations higher than 2.5 mg/L.
The continuous evaluation of the photocatalytic degradation by
HPLC-DAD indicated it mainly occurred via two intermediates. A
relevant chromatogram is presented in the SI section (Figure SI5).
TOC analysis also confirmed the mineralization of BPA. Over 24 h
around 50% of the carbon content has been removed that corre-
The activity of this photocatalyst in 24 h of irradiation under
UV, increased with the increase of BPA concentration (Fig. 6). The
activity for 10 mg/L of BPA was very close to that for 5 mg/L (87.6%
vs. 86.6%).
To further asses the activity of these photocata-
lysts, different substrates related to BPA were selected
ꢀ
for
photocatalytic
experiments:
4,4 -
sulfonildifenol;
ꢀ
ꢀ
sponded in fact to the total oxidation of BPA into CO2 and H O. The
4
,4 -(1,4-phenylendiisopropylene)bisphenol;
4,4 -(1-
bis(2-hidroxyphenil)methane;
and
respectively) taking as reference the behavior of 0.1% Au/TiO2
2
discrepancy between 80% BPA degradation and only 50% BPA min-
eralization may result from the emergence of BPA’s intermediates
whose presence is indicated in Figure SI4.
phenyltilidene)bisphenol;
bis(4-hidroxyphenil)methane (noted as 1, 2, 3,
4
5
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7
3
.3. General assessments
materials for the degradation of contaminants of environmental
concern. VIP also wants to thank project PN-II-PT-PCCA-2011-3.2-
0083 for financial support.
The collected data confirmed the role of Au and it’s interaction
with TiO in enhancing the photocatalytic behavior in the degrada-
2
tion of the organic substrates. Under UV irradiation the metal can
acts as an electron trap. Then, the trapped electron is transferred
to molecular O2 with production of OOH radicals. Trapping of elec-
trons allows the formation of positive holes that further react with
adsorbed OH by generating hydroxyl radicals exhibiting strong
oxidative properties [39]. These species attack the BPA molecule
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.11.
009.
[
13] affording the mineralization of BPA and its derivatives having
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Acknowledgements
[
[
The authors wish to express their deepest and sincerest recogni-
tion of Prof. András Dombi a key figure in the topic of photocatalytic
Please cite this article in press as: B. Cojocaru, et al., Enhanced photo-degradation of bisphenol pollutants onto gold-modified photo-
catalysts, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.009