In situ infrared study of photoreaction of ethanol on Au and Ag/TiO2

Article abstract of DOI:10.1016/j.cattod.2015.07.038

An in situ IR technique was used to study the role of Au and Ag additives on photocatalytic reaction of ethanol on TiO₂ at 300 K. Au and Ag additives increased water/ethanol coverage and decreased the rate of ethanol's C-H scission, a step involving in scavenging photogenerated holes. Au and Ag promoted adsorption of ethanol as monodentate ethoxy, accelerated its conversion to formate (HCOO^-_ad) and acetate (CH₃COO^-_ad). In contrast, adsorbed ethanol on TiO₂ did not produce IR-observable products and exhibited a Stark effect with a decreased C-H intensity upon accumulation of photogenerated electrons.

Full text of DOI:10.1016/j.cattod.2015.07.038

Catalysis Today 264 (2016) 16–22
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
In situ infrared study of photoreaction of ethanol on Au and Ag/TiO2
Azadeh Rismanchian , Yu-Wen Chenb,c, Steven S.C. Chuanga,∗
a
a
Department of Polymer Science, The University of Akron, Akron, OH 44325-3909, USA
Department of Chemical Engineering, National Central University, Chung-Li 32054, Taiwan
Department of Chemistry, Tomsk State University, Tomsk 634050, Russia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 May 2015
Received in revised form 23 July 2015
Accepted 24 July 2015
Available online 9 September 2015
An in situ IR technique was used to study the role of Au and Ag additives on photocatalytic reaction
of ethanol on TiO2 at 300 K. Au and Ag additives increased water/ethanol coverage and decreased the
rate of ethanol’s C H scission, a step involving in scavenging photogenerated holes. Au and Ag pro-
−
moted adsorption of ethanol as monodentate ethoxy, accelerated its conversion to formate (HCOO ad)
−
and acetate (CH3COO ad). In contrast, adsorbed ethanol on TiO2 did not produce IR-observable prod-
ucts and exhibited a Stark effect with a decreased C H intensity upon accumulation of photogenerated
electrons.
Keywords:
In situ IR
Au/TiO2
©
2015 Elsevier B.V. All rights reserved.
Ag/TiO2
Surface hydroxyl
Water/ethanol coverage
Photo-oxidation
1
. Introduction
Au and Ag nanoparticles have been extensively used as co-catalysts
for TiO2.
The reaction mechanism of photocatalytic reactions has been
The catalytic activity of gold depends on its size, dispersion,
morphology, support, and preparation method [12]. Table 1 sum-
extensively studied by various techniques [1]. It has been rec-
ognized that electron–hole recombination is a very fast process,
occurring in a time scale of picoseconds. Slowing down the
electron–hole recombination should increase the reaction rate [2].
Although adding precious metals as a co-catalyst has been exten-
sively reported [3–7], the role of metal on TiO2 photocatalysis has
been largely identified from the rate of reactant disappearance and
product selectivity. The role of metal on the specific reaction steps
remains to be highly speculated.
marizes the preparation techniques for Ag and Au/TiO reported in
2
some literatures. Gold particles deposited with hemisphere shape
and particle size smaller than 5 nm, shows high activities for many
oxidation reactions due to the gold–metal oxide perimeter [13]. The
dispersion of nanoparticles and the interaction between the dopant
and TiO2 are dependent on the preparation process. Impregnation
method for dispersing gold on a support does not create active gold
catalyst and also does not lead to highly dispersed catalyst. Four
methods have been studies for deposition of gold with small par-
ticle size as low as 5 nm on metal oxide support: co-precipitation,
deposition–precipitation, co-sputtering, and chemical vapor depo-
sition among which the deposition–precipitation has shown the
highest activity in literatures. This high activity is attributed to
the hemisphere shape of the particle rather than the size of
the nanoparticle [13]. The deposition–precipitation creates hemi-
sphere particles but the other two method result in spherical
particles with poor dispersion [14]. A variety of methods have
also been carried out to prepare silver-deposited titania materials.
The widely used methods include the photo-deposition, chemical
deposition, and conventional impregnation. However, synthesis of
Ag/TiO2 by chemical deposition method has showed the highest
catalytic activity and dispersion [6].
We have previously employed an in situ IR spectroscopy tech-
nique to study photocatalytic oxidation of ethanol on TiO2 and
Pt/TiO2 [8–10]. Pt has been shown to provide the sites for oxygen
absorption, accelerating the formation rate of oxygen containing
•
intermediates. The simultaneous occurrence of OH generation,
photoelectron generation, and presence of Pt sites play a syner-
getic role in enhancing C H bond scission and formation of C
and COO species. Pt has also been shown to decrease the phenol
O
−
photocatalytic oxidation on P-25 TiO due to decreasing charge sep-
2
aration efficiency [11]. The conclusion drawn on Pt/TiO2 may not
be directly applicable to other metals such as Au and Ag. Especially,
In this study, Ag and Au nanoparticles were deposited on
TiO2 surface by chemical deposition and deposition–precipitation
∗ Corresponding author.
E-mail address: chuang@uakron.edu (S.S.C. Chuang).
http://dx.doi.org/10.1016/j.cattod.2015.07.038
0
920-5861/© 2015 Elsevier B.V. All rights reserved.

A. Rismanchian et al. / Catalysis Today 264 (2016) 16–22
17
Table 1
Preparation techniques for Au/TiO2 and Ag/TiO2 catalyst.
Catalyst
Preparation method
Particle size
Loading
Ref.
Au/TiO2
Au/TiO2
Deposition–precipitation
Deposition–precipitation
2–3 nm
1.96 nm
8 wt%
1 wt%
[15]
[16]
12.9 nm
5 wt%
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Deposition–precipitation
Photo deposition
Photo deposition
Impregnation
Vapor deposition
Vapor deposition
Impregnation
<5 nm
5 nm
N/A
[17]
[18]
[19]
[20]
[21]
[22]
[5]
0.99 wt%
0.37 wt%
0.8 wt%
7 wt%
N/A
1 wt%
7–12 nm
N/A
2.6 nm
2.6 nm
N/A
Photo deposition
Chemical deposition
Photo reduction
Photo reduction
Deposition-
4.6 ± 1.5 nm
3.5 ± 1.1 nm
7–8 nm
2–20 nm
10 nm
1–3.6 wt%
0.5–3.1 wt%
N/A
1.08 wt%
1.5 wt%
Ag/TiO2
Ag/TiO2
Ag/TiO2
[23]
[18]
[7]
precipitation
Ag/TiO2
Ag/TiO2
Ag/TiO2
Sol-gel
Sol-gel
Impregnation
Photo deposition
Chemical deposition
3–5 nm
5 nm
5–7 nm
3–5 nm
0.24 nm
N/A
[24]
[25]
[6]
7.4 wt%
0.1 wt%
2.1 wt%
0.6 wt%
methods, respectively. In situ IR was employed to monitor the
dynamics of adsorbed ethanol and its intermediate products on
TiO , Au/TiO , and Ag/TiO . The results of this study allowed elu-
Prob spectrometer) was used to characterize the electronic states
of Au/TiO2 and Ag/TiO2 materials.
2
2
2
cidation of Au and Ag role on the electron–hole transfer and
recombination during ethanol photocatalytic reaction on TiO2.
2.3. In situ IR study
The DRIFT cell sample holder was filled with 0.08 g calcium
fluoride (Alfa Aesar, reagent grade, 97%) and a layer of 0.02 g of cat-
alyst on the top without any prior treatments. The catalyst in the
DRIFT cell was exposed to a saturated ethanol/Ar flow at 20 ml/min
for 10 min. The saturated ethanol/Ar was obtained by flowing Ar
through a bottle with half-filled ethanol. Gaseous ethanol in the
DRIFT cell was purged by flowing Ar at flow rate of 20 ml/min for
2
. Experimental
2.1. Catalysts preparation
Reagents used in this study were all analytical grade. TiO (P-25;
2
Degussa, 80% anatase and 20% rutile) was used as support. HAuCl4
Strem Chemicals, 99.0%) was used as the precursor for loading
2
0 min. The DRIFT cell was then switched to a batch mode by closing
(
2
the inlet and outlet valves. UV light with intensity of 0.013 W/cm
was irradiated on the sample through an optic fiber which guided
the UV light from a 350 W mercury UV lamp (Oriel 6286) to the
DRIFT cell equipped with CaF2 glass. The adsorbed ethanol on the
catalysts was irradiated with UV light for 5 min and then kept in
dark for 5 min. The UV light was on–off for 3 consecutive cycles.
IR spectra were recorded by an FTIR (Digilab FTS 4000) during the
experiment every 3.3 s.
gold. NH OH (Tedia, +95%) was used as neutralizer to adjust the
pH value. AgNO3 (Merck) was used as the detecting agent to check
the remaining chloride.
HAuCl4 solution (1 g in 1 l distilled water) was added at a rate
of 10 ml/min into a solution containing suspended support under
vigorous stirring and the precipitation temperature of solution was
maintained at 65 C. Ammonia solution was used to adjust the pH
value at 7. After aging for 2 h, the precipitate was filtered and
washed with hot water (65 C) until no Cl was detected with
AgNO3 solution, and dried at 80 C for 12 h. The cake was calcined
4
◦
◦
−
◦
3. Results and discussion
◦
at 180 C for 4 h to obtain Au/TiO catalysts. The amount of gold
2
loaded on the support catalyst corresponds to 1 wt%.
XPS analysis is carried out to analyze the surface states of
Ag/TiO2 and Au/TiO2 samples. Fig. 1 shows the XPS for Ti, Ag, and
Au in Ag/TiO2 and Au/TiO2. The XPS spectra presented peaks of Ag
3d centered at 373.3 eV and 367.3 eV. The low binding energy of
these Ag species compared to that of metallic silver at 374.2 eV and
368.2 eV indicate the presence of significant interaction between
Ag and TiO2 [6,18,26]. The Au 4f peaks appeared at 87.9 eV and
84.1 eV which is close to the binding energy of Au 4f bulk metallic
gold. The peaks for Au+ 4f 7/2 and Au 4f 7/2 at 84.6 and 87.0 eV
were not observed [27].
Ag/TiO2 was prepared by chemical deposition method using
P-25 titanium dioxide, silver nitrate, cetyltrimethylammonium
bromide (CTAB, TCI Company), and sodium borohydride (NaBH4,
Lancaster Company) as materials. Suitable amount of AgNO3 was
dissolved in water and then added to TiO powder. The sample was
2
reduced by NaBH in aqueous solution. It is then filtered and dried
4
◦
◦
at 80 C overnight and finally heated at 180 C for 4 h. The loading
of Ag was 0.89 wt.% as determined by ICP-MS.
3+
Fig. 2 shows the TEM images of TiO , Ag/TiO , and Au/TiO .
2
2
2
2
.2. Characterization
HRTEM was used to investigate the particle size. Based on the sev-
eral HRTEM images, the majority of the Au and Ag particles had
a size distribution in the range of 1.5–3 nm and 5–10 nm, respec-
tively. The characterization results are summarized in Table 2.
Inductively coupled plasma mass spectrometry (ICP-MS) (PE-
SCIEX ELAN 6100 DRC) was used to determine the contents of Au
and Ag in the samples. The morphology and particle size of the
samples were determined by TEM on a JEM-2000 EX II operated
at 160 kV and HRTEM on a JEOL JEM-2010 operated at 160 kV. X-
ray photoelectron spectroscopy (XPS) (Thermo VG Scientific Sigma
Fig. 3 shows the UV–Vis spectra of Au/TiO2 and Ag/TiO . Both
2
exhibited absorbance within visible light region with the summit
peak at 550 nm. The absorbance in visible region is attributed to
localized surface plasma resonance effect of the metal particles

1
8
A. Rismanchian et al. / Catalysis Today 264 (2016) 16–22
5
00 cps Ag/TiO2
Ag 3d
4000 cps Ag/TiO2
Ti 2p
2
p
3
/2
58.2 eV
4
3
d
5
/2
67.3 eV
3
d
2p
1/2
464.0 eV
3
3/2
3
73.3 eV
360
365
370
375
380
450
455
460
465
470
Binding Energy (eV)
Binding Energy (eV)
Au/TiO2
Au 4f
Au/TiO2
Ti 2p
4000 cps
500 cps
2
p
3
/2
58.8 eV
4
2
p
4
f
1/2
7
/2
4
f
5
/2
8
4.1 eV
464.2 eV
87.9 eV
80
82
84
86
88
90
450
455
460
465
470
Binding Energy (eV)
Binding Energy (eV)
Fig. 1. XPS spectra for Ti, Ag, and Au in Ag/TiO2 and Au/TiO2.
Fig. 2. TEM images of Au/TiO2 and Ag/TiO2. The inset is the HR-TEM image.
forms, which showed the C H bands at 2971, 2930 and 2870 cm 1
−
.
[
23,28]. The peak typically appears around 520 nm. The red shift
to 550 nm could be due to the encapsulation of the particles and
their size. The broadness of the peak can be attributed to the con-
finement of the electrons within the dispersed metal nanoparticles
The dissociative form of adsorbed ethanol exhibited ethoxy bands
−
1
−1
at 1378 cm for CH2 and 1147 cm for C O group [8,10].
Ethoxy, the dissociative form of adsorbed ethanol, is formed by
the acid–base interaction. Ethanol could release the H atom from
[
29].
Fig. 4a shows the IR absorbance spectra taken in the sequence:
its hydroxyl; Ti OH on the TiO surface could release OH, allowing
2
4+
starting from the spectrum of fresh catalyst labeled as A, after
ethanol adsorption, At0, and followed by 5 min UV irradiation on
the ethanol adsorbed sample and 5 min dark period. Fig. 4b shows
the IR difference spectra obtained by subtracting the fresh TiO2
spectrum from subsequent spectra taken during on and off of UV
irradiation. Ethanol adsorbed in both molecularly and dissociative
the formation of ethoxy, (CH CH O)ad–Ti [30]. The involvement
3 2
of Ti OH is evidenced by the negative Ti OH bands (linear OH
−
1
−1
at 3694 cm , bridge OH at 3664 cm , triply bonded –OH at
−
1
3632 cm ) [9,10,31], shown in At0–A in Fig. 4b. The negative water
bending band at 1625 cm indicated that adsorbed ethanol dis-
−
1
placed adsorbed H O on the TiO2 surface.
2
Table 2
Characterization results of Au and Ag species in Au/TiO2 and Ag/TiO2 catalysts.
Sample
XPS binding energy (eV)
Ti
ICP-MS (Load)
TEM Particle size
Ag
Au
2p1/2
2p3/2
3d3/2
3d5/2
4f5/2
4f7/2
Au/TiO2
Ag/TiO2
464.2
464.0
458.8
458.2
–
–
87.9
–
84.1
–
1 wt% (0.74 wt%)
1 wt% (0.89 wt%)
1.5–3 nm
5–10 nm
373.3
367.3

A. Rismanchian et al. / Catalysis Today 264 (2016) 16–22
19
2.0
1.5
1.0
0.5
0.0
(c)
1.4
UVon UVoff UVon UVoff UVon UVoff
1.2
1.0
0.8
A
A
A
A
A
A
A
t0
t5
t10
t15
t20
t25
t30
A
5
Au/TiO2
Ag/TiO2
TiO2
t
0.6
0.4
A
t2
A
t0.5
0
.2
.0
0
3
00
400
500
600
700
800
Time( min)
Wavelength(nm)
Fig. 3. UV–Vis spectra of TiO2 compared with Au/TiO2 and Ag/TiO2.
(b)
A
-A
-A
t30
0.5
The ethoxy could bind on the TiO surface in a monodentate
2
form on one Ti and/or bidentate form on two Ti4+. Monoden-
4
+
A
−
1
t25
tate ethoxy exhibits the C O stretching above 1100 cm
while
bidentate ethoxy shows the C O below 1100 cm−1. The rate of the
photo-oxidation of monodentate form has been reported to be 1.5
times faster than that of bidentate form [32]. Ethoxy’s C O exhib-
A
A
-A
-A
t20
t15
−
1
ited the band wavenumbers above 1100 cm in Fig. 4b, indicating
adsorbed ethanol was mostly adsorbed in monodentate form on
A
-A
t10
A
A
2-A
TiO . In contrast to ethoxy, OH in molecularly adsorbed ethanol
t
t
2
A -A
t5
−1
appeared at 3424 cm which could have a weak hydrogen bond-
ing interaction with the surface hydroxyl (OH) groups if the surface
OH remained on the surface [33].
0.5-A
A -A
t0
UV irradiation on ethanol adsorbed TiO2 resulted in the rise of
−
1
the difference spectra from 3500 to 1000 cm in a nearly linear
and structureless form. This structureless band is a manifestation
of the accumulation of the photo-induced electrons within the
(a)
0.5
A
t30
conduction band of TiO . The extent of photo-induced electron
accumulation can be increased by adsorbed methanol, a hole scav-
enger, and decreased by adsorbed electron scavengers such as gas
A
25
2
t
A
t20
phase O2 or adsorbed –OH or H O [34–36].
A
2
t15
The intensity of this structureless band at 2000 cm−1, which has
been served as an index of the extent of accumulation of photogen-
erated electrons, is plotted as a function of time during UV radiation
on and off in Fig. 4c. The rising intensity reflects the fact that the
rate of photoelectron generation and trapping is higher than that of
electron–hole recombination and vice versa for decreasing inten-
sity. The gradual decay after turning off UV indicates that electrons
in the conduction band are slowly recombined with holes. The gen-
eration of the photo-induced charge carriers (i.e., electrons and
holes) occurs in femtoseconds, but their recombination process can
be slow on the TiO2 under high vacuum [34–36]. It is interesting to
note that the electron–hole recombination process is exceptionally
slow in the presence of adsorbed ethanol. Our previous studies indi-
A
t
10
A
t
5
A
t2
A
t0.5
A
A
t0
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Fig. 4. IR Absorbance spectra of TiO2 (A), ethanol adsorbed TiO2 before UV irradia-
tion (At 0), and successive UV irradiation on ethanol adsorbed TiO2 (a), IR absorbance
difference spectra obtained by the ratio of each spectrum to TiO2 (b), and the
absorbance intensity at 2000 cm vs. time (c). (For interpretation of the references
to color in the text, the reader is referred to the web version of this article.)
−
1
cated that displacement of majority of adsorbed H O by adsorbed
2
ethanol is needed for slowing down the process of electron–hole
recombination [10].
Fig. 5a compares the IR absorbance spectra of TiO , Ag/TiO ,
to ethanol adsorption were determined by comparing the inten-
2
2
and Au/TiO2 before and after ethanol adsorption. The spectra
sity of H O bending with those with a calibrated intensity [38] and
listed in Table 3. Comparison of water coverage with the wavenum-
2
−
1
−1
.
are enlarged in the region between 3550 cm
and 3750 cm
The peaks in this region was assigned to the bands of hydroxyl
ber of OH revealed that increasing H O coverage shifted the linear
OH band upward.
The intensity of the OH bands is related to the density of defects
on the surface [1,31,39]. High intensity of the OH bands on Au/TiO2
and Ag/TiO2 suggest that the addition of Au and Ag co-catalysts
created defect sites on TiO2 surface due to variation in pH during
2
−
1
−1
groups: linear OH at 3694 cm , bridged OH at 3664 cm , and
−1
triply bonded OH at 3632 cm [37]. The contour of these bands
is expected to be governed by the density of these OH sites and the
quantity of H O adsorbed since H O adsorbed on the top of these OH
2
2
through hydrogen bonding. The quantities of water adsorbed prior

2
0
A. Rismanchian et al. / Catalysis Today 264 (2016) 16–22
0.5
Au/TiO2
Ag/TiO2
TiO2
3
750 3700 3650 3600 3550
-1
Wavenumber (cm )
(b)
(a)
1.0
0.2
A -A
t0
Au/TiO +EtOH(A )
2
t0
Au/TiO (A)
2
A -A
Ag/TiO +EtOH (A )
t0
2
t0
Ag/TiO (A)
2
A -A
t0
TiO +EtOH (A )
2
t0
TiO (A)
2
4000
3000
Wavenumber (cm )
2000
1000
4000
3000
2000
1000
-
1
-1
Wavenumber cm
Fig. 5. IR absorbance spectra (a) and IR difference spectra (b) of fresh and ethanol adsorbed Au/TiO2, Ag/TiO2, and TiO2.
the preparation [40]. The increase in surface hydroxyl due to the
presence of Ag on TiO2 has been observed previously in literature
Au/TiO and Ag/TiO compared to TiO . Ethanol adsorption on TiO
2 2 2 2
was mainly in molecularly type which may be associated with triply
bonded OH since the intensity of triply bonded OH was suppressed
more than that of linear and bridged OH. Molecularly adsorbed
ethanol has been reported to produce acetaldehyde and ethoxy
adsorbed species will produce acetate, formate and water [32,46].
[
41].
The addition of excess metal atoms could block the OH sites
42–44]. High metal loading will result in elimination of surface
[
hydroxyl groups. An increase from 0.42 wt% to 5 wt% of Au on TiO2
has been reported to decrease the hydroxyl groups [17]. Since the
hydroxyl groups can participate in the reaction, blocking of the
hydroxyl groups results in decrease in photocatalytic rate [17]. A
previous study on Pt/TiO2 in our laboratory has shown that high Pt
loading (3 wt. %) resulted in elimination of surface hydroxyl ( OH)
−
1
Fig. 6a compares the IR absorbance intensity at 2000 cm as
a function of time during the 3 cycles of UV on–off experiment
on TiO , Au/TiO , and Ag/TiO . As discussed in Fig. 4c, the slope of
2
2
2
these rising or decaying intensities reflects the difference in the rate
of photoelectron generation/trapping and that of electron–hole
•
−1
groups and blocking all the OH generation sites. Instead, 0.5 wt%
recombination. Au/TiO2 showed sudden change in the 2000 cm
Pt loading did not block the OH groups [9]. One can conclude that
the metal loading in this study was not sufficient to block the OH
sites.
Surface hydroxyls are the adsorption sites for ethanol and the
charge separation sites [35,45]. The presence of more hydroxyl
intensity immediately after UV light irradiation and switching back
to dark. The noble metals could accelerate the transport of the pho-
togenerated electron to surface and its interactions/reactions with
organic compound [20,47]. The difference in the work function of
TiO and metal nanoparticles provides the driving force for the elec-
2
groups on metal/TiO is expected to lead to the adsorption of more
ethanol on the surface [6], as shown in Table 3. Difference spectra
for adsorbed ethanol in Fig. 5b showed a higher intensity of ethoxy
tron transfer from the conduction band of TiO2 to the metal. The
work function of Au and Ag are 5.1 eV and 4.6 eV, respectively, both
higher than that of TiO2 (4.2 eV). The higher work function could
2
on Ag and Au/TiO than TiO ; higher negative intensities of the lin-
give a higher rate of electron transfer [47]. Au/TiO and Ag/TiO cat-
alysts have a higher intensity of adsorbed –OH or H O and a lower
2
2
2
1
2 2
−
−1
ear –OH band at 3694 cm , bridge –OH band at 3664 cm , triply
−
1
−1
bonded –OH band at 3632 cm , and H O band at 1625 cm on Ag
intensity of the structureless band than TiO , further showing that
2
2
and Au/TiO2 than TiO . Ethanol adsorption displaced most of the
adsorbed H O serves as electron scavengers [35].
2
2
−1
−
triple –OH band at 3632 cm . The difference spectra for ethanol
adsorbed Au/TiO and Ag/TiO showed a more intense ethoxy peak
The observed high rate of formation of formate (HCOO ad) at
−
1
−
1558, 1420 and 1361 cm and acetate (CH COO ad) at 1405 and
2
2
3
−
1
−1
−1
at 1113 cm and 1147 cm than TiO . The results revealed that
1344 cm in Fig. 7 indicated that Au accelerate the photocatalytic
2
ethanol was adsorbed more in the form of monodentate ethoxy on
reaction. Since the formation of acetate involves with the following
steps:
TiO + hv → h + e−
+
(1)
(2)
Table 3
2
Water and ethanol coverage on TiO2, Au/TiO2, and Ag/TiO2 samples.
CH CH O + h+ + O(lattice) → CH COO + 2H+
−
3
2
ad
3
Water coverage (␮mol
H2O/g TiO2)
Ethanol coverage
(␮mol/g TiO2)
The rapid formation of acetate suggests that Au accelerates the
reaction at step (2) involving both hole and lattice oxygen. In the
absence of gas phase O2 in the reaction chamber, the photo oxida-
tion occurs via lattice oxygen atoms [48]. Since most of the adsorbed
water has been displaced by the adsorbed ethanol, a direct hole
Initial
Ethanol adsorbed
Au/TiO2
Ag/TiO2
TiO2
52
53
25
8
20
∼0
620
780
540

A. Rismanchian et al. / Catalysis Today 264 (2016) 16–22
21
oxidation pathway is more probable than oxidation via OH radi-
cal [10]. The participation of the lattice oxygen produces oxygen
defects [49]. The oxygen defects can be further replenished by the
remaining initial adsorbed H O and the H O produced during the
photocatalytic reaction as evidenced by increasing of 1620 cm
band in Fig. 7 [10].
UV on
t0
UV on
(b)
A
A
A
A
t15
t5
t10
2
2
−
1
TiO2
Au/TiO2 produced a higher intensity for adsorbed formate
−
(HCOO
) than acetate. In contrast, Ag produced mainly adsorbed
ad
Au/TiO2
−
acetate (CH3COO ad). The rate of electron transfer in Au/TiO2 was
higher than that in Ag/TiO as evidenced by the rapid rise of the
2
−
1
intensity at 2000 cm for Au/TiO2 in Fig. 6a. The higher rate of
electron transfer resulted in higher rate of oxidation in Au/TiO2
than in Ag/TiO . The high of electron transfer in Au/TiO2 could be
related to its high work function since the work function of Au is
higher than that of Ag.
Ag/TiO2
2
The accumulation of the species such as formate and acetate
on the surface indicates the species cannot be further converted
Time (min)
to final products of the CO . The products concentration has not
2
UV on
UV on
UV on
been quantified in this study. The rate of acetate formation has been
reported to be 300 ␮mol/g TiO2 during 90 min UV irradiation in pre-
vious studies [8]. The addition of gaseous O2 is needed to increase
the rate and productivity and to facilitate the further conversion to
CO2 [50,51].
(a)
1.0
0.8
0.6
0.4
0.2
0.0
A
A
A
A
A
A
A
t0
t5
t10
t15
t20
t25 t30
Since H on CH3 of alcohols can serve as hole scavenger and
cause accumulation of photogenerated electron in the conduction
band [9], we further plotted the intensity of C H along with the
TiO2
−
1
2000 cm
intensity in Fig. 6b. The lack of correlation between
curves in Fig. 6b indicated that ethoxy on these TiO2 did not play
a significant role in scavenging photogenerated holes. The initial
Au/TiO2
Ag/TiO2
decay of C H intensity on TiO can be attributed to the Stark effect,
2
a result of accumulation of photogenerated electrons [49]. The rapid
−
1
decay of the 2000 cm
intensity on Au/TiO2 and Ag/TiO2 upon
turning off UV further revealed that Au and Ag can facilitate not
only the electron and hole transfer for ethoxy reaction but also
electron–hole recombination.
Time (min)
−
1
4. Conclusion
Fig. 6. The change in the IR absorbance intensity at 2000 cm (a) and C H band
−
1
at 2971 cm (b) vs. time during the UV illumination on ethanol adsorbed TiO2,
Au/TiO2, and Ag/TiO2.
In situ IR spectroscopic studies of photocatalytic reaction of
ethanol on TiO and TiO with Ag and Au co-catalysts revealed that
2
2
Au and Ag increased the surface hydroxyl group, allowed ethanol
to adsorb as monodentate ethoxy, and accelerated the formation of
formate and acetate. Au and Ag enhanced the rate of electron–hole
transfer for promoting the product formation. Au and Ag also accel-
erated the rate of electron–hole recombination. The results of this
study further confirmed that displacement of H O by adsorbed
ethanol allowed the photogenerated electrons to be accumulated
in the conduction band of TiO2.
A
A
-A
2
t30 t0
0.5
-A
t20 t0
A
-A
Au/TiO2
Ag/TiO2
TiO2
t10 t0
Acknowledgment
A
-A
t30 t0
A
A
-A
The authors acknowledge the financial support received for this
research from the University of Akron faculty initiation fund.
t20 t0
-A
t10 t0
A
-A
t30 t0
References
A
A
-A
t20 t0
[1] T.L. Thompson, J.T. Yates, Chem. Rev. 106 (2006) 4428–4453.
[2] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)
-A
t10 t0
6
9–96.
3] C.-H. Li, Y.-H. Hsieh, W.-T. Chiu, C.-C. Liu, C.-L. Kao, Sep. Purif. Technol. 58
2007) 148–151.
[
(
2500
2000
1500
1000
[4] I. Arabatzis, T. Stergiopoulos, M. Bernard, D. Labou, S. Neophytides, P. Falaras,
Appl. Catal. B: Environ. 42 (2003) 187–201.
-
1
Wavenumber (cm )
[
5] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, Catal. Lett. 44 (1997)
3–87.
8
Fig. 7. IR absorbance spectra of 15 min UV illumination on ethanol adsorbed
[
[
[
6] J. Li, J. Xu, W.-L. Dai, K. Fan, J. Phys. Chem. C 113 (2009) 8343–8349.
7] X. You, F. Chen, J. Zhang, M. Anpo, Catal. Lett. 102 (2005) 247–250.
8] F. Guzman, S.S. Chuang, J. Am. Chem. Soc. 132 (2010) 1502–1503.
Au/TiO2, Ag/TiO2, and TiO2.

2
2
A. Rismanchian et al. / Catalysis Today 264 (2016) 16–22
[9] Z. Yu, S.S. Chuang, Appl. Catal. B: Environ. 83 (2008) 277–285. [32] W.-C. Wu, C.-C. Chuang, J.-L. Lin, J. Phys. Chem. B 104 (2000) 8719–8724.
[
[
[
[
[
[
[
[
[
10] Z. Yu, S.S. Chuang, J. Catal. 246 (2007) 118–126.
[33] M. Takeuchi, J. Deguchi, S. Sakai, M. Anpo, Appl. Catal. B: Environ. 96 (2010)
218–223.
[34] T. Berger, M. Sterrer, O. Diwald, E. Knözinger, D. Panayotov, T.L. Thompson, J.T.
Yates, J. Phys. Chem. B 109 (2005) 6061–6068.
[35] D.A. Panayotov, J.T. Yates Jr., Chem. Phys. Lett. 410 (2005) 11–17.
[36] D.A. Panayotov, S.P. Burrows, J.R. Morris, J. Phys. Chem. C 116 (2012)
4535–4544.
[37] K.S. Finnie, D.J. Cassidy, J.R. Bartlett, J.L. Woolfrey, Langmuir 17 (2001)
816–820.
11] B. Sun, A.V. Vorontsov, P.G. Smirniotis, Langmuir 19 (2003) 3151–3156.
12] H. Chen, X. Jia, Y. Li, C. Liu, Y. Yang, Catal. Today (2015).
13] M. Haruta, Catal. Today 36 (1997) 153–166.
14] M. Haruta, Catal. Surv. Asia 1 (1997) 61–73.
15] D.A. Panayotov, J.T. Yates, J. Phys. Chem. C 111 (2007) 2959–2964.
16] T. Kecskés, J. Raskó, J. Kiss, Appl. Catal. A: Gen. 273 (2004) 55–62.
17] A. Orlov, D.A. Jefferson, N. Macleod, R.M. Lambert, Catal. Lett. 92 (2004) 41–47.
18] S.F. Chen, J.P. Li, K. Qian, W.P. Xu, Y. Lu, W.X. Huang, S.H. Yu, Nano Res. 3
(
2010) 244–255.
[38] M. Lohrasbi, In Situ FTIR Study of the Photocatalytic Properties of TiO2 and
Conductivity of Polyaniline, Dissertation, University of Akron, 2014.
[39] T. Berger, O. Diwald, Defects at Oxide Surfaces, Springer, 2015, pp. 273–301.
[40] F. Moreau, G.C. Bond, A.O. Taylor, J. Catal. 231 (2005) 105–114.
[41] B. Xin, L. Jing, Z. Ren, B. Wang, H. Fu, J. Phys. Chem. B 109 (2005)
2805–2809.
[42] M. Primet, P. Pichat, M.V. Mathieu, J. Phys. Chem. 75 (1971) 1216–1220.
[43] A. Zwijnenburg, A. Goossens, W.G. Sloof, M.W. Crajé, A.M. van der Kraan, L. Jos
de Jongh, M. Makkee, J.A. Moulijn, J. Phys. Chem. B 106 (2002) 9853–9862.
[44] R. Zanella, L. Delannoy, C. Louis, Appl. Catal. A: Gen. 291 (2005) 62–72.
[45] H. Lin, J. Long, Q. Gu, W. Zhang, R. Ruan, Z. Li, X. Wang, Phys. Chem. Chem.
Phys. 14 (2012) 9468–9474.
[
[
[
19] J. Murcia, M. Hidalgo, J. Navío, J. Arana, J. Do n˜ a-Rodríguez, Appl. Catal. B:
Environ. 142 (2013) 205–213.
20] S. Sakthivel, M. Shankar, M. Palanichamy, B. Arabindoo, D. Bahnemann, V.
Murugesan, Water Res. 38 (2004) 3001–3008.
21] J.A. Rodriguez, G. Liu, T. Jirsak, J. Hrbek, Z. Chang, J. Dvorak, A. Maiti, J. Am.
Chem. Soc. 124 (2002) 5242–5250.
22] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647–1650.
23] L. Yang, X. Jiang, W. Ruan, J. Yang, B. Zhao, W. Xu, J.R. Lombardi, J. Phys. Chem.
C 113 (2009) 16226–16231.
[
[
[
24] G. Krylova, Y.I. Gnatyuk, N. Smirnova, A. Eremenko, V. Gun’ko, J. Sol-Gel Sci.
Technol. 50 (2009) 216–228.
[
[
25] H. Zhang, G. Chen, Environ. Sci. Technol. 43 (2009) 2905–2910.
26] Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei, J. Zhu, J. Phys. Chem. C 112
[46] L.-F. Liao, W.-C. Wu, C.-Y. Chen, J.-L. Lin, J. Phys. Chem. B 105 (2001)
7678–7685.
(
2008) 10773–10777.
27] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, J. Am. Chem. Soc. 129 (2007)
538–4539.
28] X. Zhang, Y. Li, J. Zhao, S. Wang, Y. Li, H. Dai, X. Sun, J. Power Sources 269
2014) 466–472.
[47] P. Sangpour, F. Hashemi, A.Z. Moshfegh, J. Phys. Chem. C 114 (2010)
13955–13961.
[48] M. El-Maazawi, A. Finken, A. Nair, V. Grassian, J. Catal. 191 (2000) 138–146.
[49] S.H. Szczepankiewicz, J.A. Moss, M.R. Hoffmann, J. Phys. Chem. B 106 (2002)
7654–7658.
[
[
[
[
[
4
(
29] R.T. Tom, A.S. Nair, N. Singh, M. Aslam, C. Nagendra, R. Philip, K.
Vijayamohanan, T. Pradeep, Langmuir 19 (2003) 3439–3445.
30] A. Nadeem, J. Muir, K. Connelly, B. Adamson, B. Metson, H. Idriss, Phys. Chem.
Chem. Phys. 13 (2011) 7637–7643.
[50] S. Kundu, A. Vidal, M. Nadeem, S. Senanayake, H. Idriss, P. Liu, J. Rodriguez, D.
Stacchiola, J. Phys. Chem. C 117 (2013) 11149–11158.
[51] J.M. Coronado, S. Kataoka, I. Tejedor-Tejedor, M.A. Anderson, J. Catal. 219
(2003) 219–230.
31] C. Deiana, E. Fois, S. Coluccia, G. Martra, J. Phys. Chem. C 114 (2010)
2
1531–21538.