Applied Catalysis A: General 325 (2007) 276–282
www.elsevier.com/locate/apcata

Photo-oxidation of ethanol on mesoporous vanadium–titaniumoxide catalysts and the relation to vanadium(IV) and (V) sites
Dilshad Masih a, Hideaki Yoshitake b, Yasuo Izumi a,*aInterdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16,
Midori-ku, Yokohama 226-8502, Japanb
Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai,
Hodogaya-ku, Yokohama 240-8501, Japan
Received 21 July 2006; accepted 7 February 2007
Available online 3 March 2007

Abstract
Ethanol photo-oxidation was investigated over mesoporous, amorphous V + TiO2 and V + TiO2(anatase) catalysts. Under the UV + visible
light, mesoporous V + TiO2 generally exhibited faster photo-oxidation rates than V + TiO2(anatase) catalysts did. VIV doping directed preferable
formation of acetic acid rather than predominant acetaldehyde formation. Under the visible light only, mesoporous VIV–TiO2 catalyst exhibitedbest reactivity among all V + TiO2 catalysts. Ethanol dehydration reaction was preferred. Initial quicker water evolution may suggest greateroxidation capability compared to V + TiO2(anatase) catalysts.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Photocatalyst; Photocatalysis; Oxidation; Vanadium; Titania; Mesoporous; UV–vis

1. Introduction
Titanium dioxide works as photocatalysts based on the semiconducting property [1,2]. The light absorption by TiO2
predominantly lies in UV range. Only 3% of solar energy isutilized using the anatase TiO2 phase at the surface of earth [1].
The modification of TiO2 improved the efficiency of solar
radiation utilization up to 20–30% by adding different
elements, e.g. chromium, vanadium, platinum, or nitrogen to
TiO2 [3–9]. In contrast, chloride, sulfate, or phosphateexhibited detrimental effects on the photocatalysis [10–13].
To improve the photocatalysis of TiO2-based materials,
another approach is to synthesize mesoporous TiO2 with highspecific surface area. The applications of nano-crystalline andmesoporous TiO2 to the photodecomposition of 2,4,6-trichloropheonol and other organic compounds were suggested
utilizing the availability of larger number of active sites[14,15]. Syntheses of mesoporous and nano-crystalline TiO2
were reported via different routes [15–22]. In this paper,
wormhole-like, amorphous mesoporous materials with specificsurface area as much as 1200 m2 g1 were used [23,24]. The
doping effect on the red shift of UV–vis absorption wasreported to follow the order V > Cr > Mn > Fe > Ni to TiO2[3]. Therefore, series of mesoporous V + TiO2 samples wereprepared and the performance of ethanol photo-oxidation
reaction was compared to conventional V + TiO2(anatase)catalysts with the illumination of UV + visible light or visible
light only.
The V site structures on/in TiO2 have been intensively
studied by means of Raman, UV–vis, 51V nuclear magnetic
resonance, and X-ray absorption fine structure spectroscopies[25–28] and we recently reported the V structure transformation in on-site conditions and also the V structure formesoporous V + TiO2 catalysts [24,29]. The ethanol photooxidation reactivity over various mesoporous and conventional
V + TiO2 catalysts was compared to the V site structure(geometric and electronic) information.
2. Experimental
2.1. Syntheses of V + TiO2 catalysts* Corresponding author. Tel.: +81 45 924 5569; fax: +81 45 924 5569.
E-mail address: izumi.y.ac@m.titech.ac.jp (Y. Izumi).
0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2007.02.037

TiO2 (P-25, Degussa) with a specific surface area of
60 m2 g1 was impregnated with V triisopropoxide oxide (1) inD. Masih et al. / Applied Catalysis A: General 325 (2007) 276–282

277

2-propanol solution (impregnated V/TiO2). Major phase of
TiO2 (P-25) was anatase being the ratio of anatase/rutile 95/5.
Mesoporous V–TiO2 samples were prepared from compound 1,
Ti tetraisopropoxide (2), and dodecylamine (3). An aqueous
solution of the reactants was maintained at 333 K for 6 days andthen filtered. The obtained powder was heated at 453 K for 10days, and then washed with p-toluenesulfonic acid in ethanol.
Based on X-ray diffraction patterns, the basal spacing was 30 Å
and a wormhole-like structure prevailed rather than highlyordered periodic mesostructure [23,24]. Following similarroute, mesoporous TiO2 was synthesized from compounds 2
and 3. Mesoporous TiO2 was impregnated with compound 1 in
2-propanol (V/mesoporous TiO2). The V contents in thesesamples were 3.0 or 1.0 wt% on the V metal basis. All the dried
powders were calcined in air at 523 K.
2.2. Ethanol photo-oxidation measurements on V + TiO2
catalysts
The reaction was performed in a closed circulating glass
system (total volume 132 ml). Hundred milligrams of catalyst
was homogeneously spread in a quartz reaction cell (bottom platearea 23.8 cm2) and illuminated with UV–vis light from xenon arc
lamp operated at 500 W (UXL-500D, Ushio). The catalyst was
set at 2 mm apart from the exit window of the light. The distance
between catalyst and the mirror set in the light path was 65 mm.
Kenko UV-cut filter L-42 was set on the light exit window forphotocatalysis measurements under visible light only.
Before the photo-oxidation reaction measurements, all thecatalysts were evacuated (106 Pa) for 2 h at 290 K. Photocatalytic oxidation of 55 mmol of gas phase ethanol was carriedout at 290 K [30–34]. One hundred and ten micromoles ofoxygen was introduced as an oxidant [30]. Products and reactants
were analyzed using online gas chromatograph equipped withthermal conductivity detector (Shimazu GC-8A) connected tothe closed circulating system. All the reaction products andreactants were analyzed using Porapak-Q column (GL Sciences).
2.3. UV–vis absorption spectrum measurements for
V + TiO2 catalysts
Optical spectra were recorded on UV–vis spectrometer V550 (Jasco) equipped with an integrating sphere attachment
ISV-470 (Jasco) for diffuse-reflectance measurements. Measurements were made at 290 K in the wavelength range
between 280 and 650 nm. UV–vis absorbance for all thecompounds was obtained by the transformation based on the
Kubelka–Munk equation.
3. Results
3.1. Ethanol photo-oxidation kinetics with UV–vis
illumination
The time course of photo-oxidation reaction for ethanol(initial pressure 1.33 kPa) was depicted in Fig. 1 on mesoporous
V + TiO2 catalysts. Major products were acetaldehyde, water,

Fig. 1. Ethanol oxidation reaction as a function of time under the illuminationof UV–vis light on mesoporous V + TiO2 catalysts. Measured at 290 K. Ethanol(1.33 kPa) and O2 (2.67 kPa) were introduced in closed circulating glass system(132 ml). (A) Mesoporous TiO2. (B) Mesoporous V–TiO2 (3.0 wt% V). (C)Impregnated V/mesoporous TiO2 (3.0 wt% V).

carbon dioxide, and acetic acid on mesoporous TiO2 (Fig. 1A)[34]. Because the ratio of formation rates for acetaldehyde andwater was 2.2 (Table 1A), dehydration and dehydrogenation
reactions for ethanol proceeded with comparable rates. Further
photo-oxidized products acetic acid and carbon dioxide wereminor. The formations of ethene and carbon monoxide were
negligible.
In the photo-oxidation over mesoporous V–TiO2 (3.0 wt% V),
acetaldehyde and water were produced with essentially the samerates (Fig. 1B). Thus, ethanol dehydration proceeded predominantly in the presence of vanadium in the TiO2 matrix [29]. Acetic

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D. Masih et al. / Applied Catalysis A: General 325 (2007) 276–282

Table 1
Products formation rates in the ethanol photo-oxidation over various V + TiO2 catalysts illuminated with UV + visible light (A) and visible light only (B)aFormation rates (mmol h1 gcat1)
MeCHO

H2 O

MeCO2H

CO

CO2

C2H4

Sf

(A) UV + visible
Mesoporous TiO2
Mesoporous V–TiO2 b
V/mesoporous TiO2 b
TiO2 (P-25)
V/TiO2 b

72
42
43
61
28

33 c
44
24
50 d
0

11c
35c
0
0
0

0.85
0
0
0.3
0

14 c
8.8 c
2.1 c
6.4
0.1

1.3
0.2
0.3
0
0

92
82
44
64
28

(B) Visible only
Mesoporous TiO2
Mesoporous V–TiO2 b
V/mesoporous TiO2 b
TiO2 (P-25)
V/TiO2 b

2.3
23
11
19
18

16 c
212(16e)141(15e)
4.9 c
0

0
0
0
0
0

0
0
0
0
0

0.3 c
0.3 c
0.2 c
0.2 c
0.9 c

0
0
0
0
0

2.5
23
11
19
18

a

Initial reactants: CH3CH2OH (55 mmol) and O2 (110 mmol).
3.0 wt% V.
cConstant rates later than the induction period.
dSerious deactivation observed.
eConstant rate later than initial faster rate.
f
The summation of formation rates on the basis of carbon. The formation rates of CO and CO2 were multiplied with a half in the summation.
bacid and carbon dioxide produced later than the induction periodof 1–2 h because they were secondary or multiple-step products.
For the impregnated V/mesoporous TiO2 catalyst, the ratio ofacetaldehyde and water formation rates was 1.8 (Fig. 1C and
Table 1A). Thus, ethanol dehydration and dehydrogenation
reactions proceeded with the rate ratio 1.3. Minor products werecarbon dioxide and ethene. Compared to reactions on mesoporous TiO2 and mesoporous V–TiO2 (Fig. 1A and B), no acetic acidwas found over impregnated V/mesoporous TiO2.
The kinetics of ethanol photo-oxidation on V + TiO2 (P-25)
catalysts were summarized in Fig. 2 and Table 1A. On TiO2 (P25), major products were acetaldehyde and water, however, theformation rates were not constant (Fig. 2A) compared to thekinetics on mesoporous V + TiO2 catalysts. The time coursechange for water formation was not monotonous. It was first
deactivated and reactivated at 3 h. When vanadium was
impregnated on TiO2 (P-25) (Fig. 2B), the catalysis was
suppressed compared to pure TiO2 (P-25). Acetaldehyde was
essentially the only one product via the ethanol dehydrogenation. The ethanol amount even increased in first 1 h in Fig. 2B.
On vanadium-doped TiO2 formic acid formation was reported[34], and in our Porapak-Q column formic acid and ethanol
were not separated. Thus, the initial apparent increase ofethanol may be catalytic formation of formic acid.
In summary, under the illumination of UV–vis light, theethanol dehydration rates followed the order (Table 1A)
TiO2  mesoporous VTiO2 > mesoporous TiO2
> V=mesoporous TiO2  V=TiO2(1)

Ethanol dehydrogenation rates assumed based on the differenceof acetaldehyde and water formation rates followed the ordermesoporous TiO2 > V=TiO2 > V=mesoporous TiO2
> TiO2  mesoporous VTiO2(2)

Fig. 2. Ethanol oxidation reaction as a function of time under the illuminationof UV–vis light on conventional V + TiO2 (P-25) catalysts. Reaction conditions
were the same as noted in the caption for Fig. 1. (A) TiO2 (P-25). (B)Impregnated V/TiO2 (3.0 wt% V).

D. Masih et al. / Applied Catalysis A: General 325 (2007) 276–282

Acetic acid (and carbon dioxide) formation rates followed theordermesoporous VTiO2 > mesoporous TiO2
 V=mesoporous TiO2  TiO2  V=TiO2

279

No acetic acid and negligible carbon dioxide were formed on all
V + TiO2 catalysts under the illumination of visible light only.
Under dark conditions at room temperature, no reaction proceeded in ethanol and O2 over Pt-doped TiO2 catalyst [7].

(3)Ethanol photo-oxidation was reported on anatase TiO2 andplatinum-modified one [7]. The addition of Pt enabled catalytic
formation of acetic acid similar to over mesoporous [V–]TiO2(Table 1A).
3.2. Ethanol photo-oxidation kinetics with visible light
only illumination

3.3. Diffuse-reflectance UV–vis absorption spectra for
V–TiO2 catalysts
UV–vis spectra were measured for V + TiO2 catalysts in
diffuse-reflectance mode. In comparison to the absorption data
for TiO2 (P-25), the absorption was extended to the higher

The ethanol photo-oxidation results on mesoporous
V + TiO2 with visible light only illumination were depicted
in Fig. 3 and the formation rates were summarized in Table 1B.
The mesoporous TiO2 catalyst was fairly inactive compared tothe case illuminated with UV + visible light (Figs. 3A and 1A).
Water was formed in addition to negligible acetaldehyde andcarbon dioxide. Because the water evolving rate was greaterthan that of acetaldehyde by 7.0 times and ethanol in gas phasesignificantly decreased in the first 1 h, formed acetaldehyde
may be trapped in mesopores of the TiO2.
For mesoporous V–TiO2 catalyst (3.0 wt% V), water wasformed in first 1 h faster than for mesoporous TiO2 by 13 times(Fig. 3B), even faster than in the measurement with
UV + visible light illumination (Fig. 1B) by 4.8 times. Laterthan 1 h, the formation rates of water and acetaldehyde became
constant and comparable (16 and 23 mmol h1 gcat1, respectively, Table 1B). Minor product was carbon dioxide. The
kinetic result for impregnated V/mesoporous TiO2 (Fig. 3C)
was qualitatively similar to that for mesoporous V–TiO2(Fig. 3B). Initial faster evolution of water was again observed infirst 1 h and then water and acetaldehyde were constantly
produced (15 and 11 mmol h1 gcat1, respectively). The
constant formation rates decreased to 94 and 48%, respectively,of corresponding rates for mesoporous V–TiO2 (Table 1B).
Illuminated with visible light only, ethanol dehydrogenation
proceeded predominantly at the rate between 18 and
19 mmol h1 gcat1 both on TiO2 (P-25) and impregnated V/
TiO2 (Fig. 4A and B). The time course results were
quantitatively the same on the two catalysts. Negligible carbon
dioxide and water (at 5 h) formations were observed on
impregnated V/TiO2 and TiO2 (P-25), respectively. As
mentioned above under UV–vis light (Fig. 2B), the apparent
increase of ethanol in first 1 h may be the contribution of
produced formic acid [34].
In summary, under the illumination of visible light only, theethanol dehydration rates followed the order (Table 1B)
mesoporous VTiO2 > V=mesoporous TiO2
 mesoporous TiO2 > TiO2 > V=TiO2(4)

Ethanol dehydrogenation rates followed the order
V=TiO2 > TiO2  mesoporous VTiO2 > mesoporous
TiO2  V=mesoporous  TiO2

(5)

Fig. 3. Ethanol oxidation reaction as a function of time under the illuminationof visible light only on mesoporous V + TiO2 catalysts. Reaction conditions
were the same as noted in the caption for Fig. 1. (A) Mesoporous TiO2. (B)Mesoporous V–TiO2 (3.0 wt% V). (C) Impregnated V/mesoporous TiO2
(3.0 wt% V).

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D. Masih et al. / Applied Catalysis A: General 325 (2007) 276–282

Fig. 5. Diffuse-reflectance UV–vis absorption spectra for TiO2 (P-25) (a),impregnated V/TiO2 (P-25) (1.0 and 3.0 wt% V) (b and c, respectively),
mesoporous TiO2 (d), mesoporous V–TiO2 (1.0 and 3.0 wt% V) (e and f,
respectively), and impregnated V/mesoporous TiO2 (1.0 and 3.0 wt% V) (g andh, respectively). (Inset) Expanded data in the region between 280 and 400 nm
for TiO2 (P-25) (a) and V/mesoporous TiO2 (3.0 wt% V) (h).

Fig. 4. Ethanol oxidation reaction as a function of time under the illuminationof visible light only on conventional V + TiO2 (P-25) catalysts. Reaction
conditions were the same as noted in the caption for Fig. 1. (A) TiO2 (P25). (B) Impregnated V/TiO2 (3.0 wt% V).

wavelength side when 1.0–3.0 wt% of V was impregnated(Fig. 5a–c) [4,8]. The extension toward visible light side wasmore enhanced for mesoporous TiO2-based catalysts. Similar
to the vanadium impregnation with TiO2 (P-25), the V
impregnation with mesoporous TiO2 progressively extended
the light absorption toward visible light region (Fig. 5g and h).
In contrast, the extent of extension was independent to the V
contents in catalysts for mesoporous V–TiO2 between 1.0 and
3.0 wt% of V (Fig. 5e and f, respectively). The extent ofextension toward visible light region was in the order
V=mesoporous TiO2 ð3:0%Þ > mesoporous V
TiO2 ð1:0  3:0%Þ > V=mesoporous TiO2 ð1:0%Þ
> V=TiO2 ð3:0%Þ > mesoporous TiO2 > V=TiO2 ð1:0%Þ
> TiO2 ðP-25Þ

(6)

4. Discussion
Under the illumination of UV + visible light, total formation
rates on carbon basis were in the order (Table 1)
mesoporous TiO2 > mesoporous VTiO2 > TiO2 >V=mesoporous TiO2 > V=TiO2(7)

As a general trend, mesoporous TiO2-based catalysts weresuperior to anatase TiO2-based catalysts (Table 1A). Various
kinds of specific photocatalysis under the illumination oflight > 320 nm was reported using mesoporous TiO2 [35].
Acetic acid that was further oxidized from acetaldehyde
was exclusively found in the mesoporous TiO2-based catalysis. The doping of vanadium did not always work positively. Typical trend by the doping of vanadium cannot befound either in mesoporous (amorphous) TiO2 or anatase
TiO2.
Under the illumination of visible light only, total formation
rates on carbon basis followed the order (Table 1B)
mesoporous VTiO2 > TiO2  V=TiO2> V=mesoporous TiO2  mesoporous TiO2

(8)

Only mesoporous V–TiO2 was superior to anatase TiO2-based
catalysts. Other mesoporous TiO2-based catalysts were evenworse than anatase TiO2-based ones. Two groups of V + TiO2
catalysts showed clear contrast. Ethanol dehydration proceededon mesoporous TiO2-based catalysts whereas exclusive dehydrogenation proceeded on anatase-TiO2 based catalysts. For the
2-propanol decomposition, product switching from acetone(dehydrogenation) to propene (dehydration) was reported as
the increase of vanadium content in V/TiO2(anatase) catalysts
at 473 K [36].
It is contradictory that the initial water formations on
mesoporous V–TiO2 and V/mesoporous TiO2 were even faster
when illuminated with visible light only than under theillumination both UV and visible light (Table 1). This may berationalized by assuming the balance between acetaldehyde/
acetic acid desorption and further consecutive oxidation
reaction steps finally to form H2O and CO2, CO, orD. Masih et al. / Applied Catalysis A: General 325 (2007) 276–282

carbonaceous species adsorbed
C2 H5 OH þ ðx þ 1ÞO2 ! 3H2 O þ 2COx

ðx ¼ 2; 1; or0Þ
(9)

The consumption rates ratio of O2 and ethanol in the first 1 h
were 1.8 and 1.1 for mesoporous V–TiO2 and V/mesoporous
TiO2, respectively (Fig. 3B and C). In addition to constantdehydration reaction to form acetaldehyde and water, further
breakdown reaction(s) via equation (9) may have proceeded infirst 1 h on the two mesoporous TiO2-based catalysts to form
CO2 and/or carbonaceous species (Table 1B). When water was
mixed in the reactants ethanol and O2 for the photo-oxidation
reaction on Pd- and Cu-modified TiO2(anatase) catalysts,
catalytic formation of acetaldehyde was active and constantalong with the minor formation of ethyleneglycol [33]. On theother hand, photocatalytic activity of pure TiO2(anatase) and
one doped with Fe became deactivated during the time course[33]. Thus, the possibility cannot be excluded in this study that
initially formed water modified the mesoporous V + TiO2
catalysts in Fig. 3B and C within 1 h of reaction.
The effects of vanadium doping were remarkable inmesoporous TiO2-based catalysts whereas no effects of V
doping were detected in anatase TiO2-based catalysts(Table 1B).
The vanadium local structure was reported for theseV + TiO2 catalysts [29]. Common vanadium(V) surface
dispersed species (Fig. 6A) [25,27,29] was suggested for
impregnated V/mesoporous TiO2, impregnated V/TiO2 (P-25),
and sol–gel V–TiO2 whereas V(IV) sites substituted on the Ti
sites of mesoporous TiO2 matrix for mesoporous V–TiO2(Fig. 6B). The relevance to ethanol photo-oxidation is firstconsidered for data with the illumination of UV + visible light
listed in Table 1A. The phase of support TiO2, amorphous(mesoporous) or predominant anatase (P-25), was the primaryfactor to control the catalysis. The doping of V(V) deactivatedthe catalysis to form acetaldehyde, water, or acetic acid in both
environments [V/mesoporous TiO2 and V/TiO2 (P-25)]. The
doping of V(IV) maintained the total activity of mesoporous
TiO2 and directed the formation of acetic acid rather than
predominant acetaldehyde over mesoporous V–TiO2 (Fig. 6B
and Table 1A). The photocatalytic activity of sol–gel V–TiO2
catalysts in which V(IV) sites substituted on the Ti sites of TiO2
was reported for the decompositions of methylene blue andacetaldehyde either under UV light or under visible light [6].
For the ethanol photo-oxidation with the illumination ofvisible light only, the phase of TiO2 primarily controlled the281

catalysis again (Table 1B). The doping of both V(IV) and V(V)promoted the catalysis, however, only to mesoporous TiO2. The
total formation rates increased more by the doping of V(IV)(9.2 times, mesoporous V–TiO2) than by V(V) doping (4.4times, V/mesoporous TiO2). These catalytic trends wereconsistent with extension of optical absorption spectra toward
visible light wavelength region for mesoporous V–TiO2 and V/
mesoporous TiO2 (Fig. 5).
5. Conclusions(1) Under the illumination of UV + visible light, mesoporous
V + TiO2 catalysts generally showed faster ethanol oxidation reaction than anatase V + TiO2 catalysts did. Major
products were acetaldehyde, water, acetic acid, and carbon
dioxide. Deeply oxidized acetic acid and carbon dioxide
were preferably formed over the mesoporous V + TiO2
catalysts.
(2) Under the illumination of visible light only, mesoporous V–
TiO2 catalyst was best and superior to anatase V + TiO2
catalysts. The phase of TiO2 controlled the productselectivity. Ethanol dehydration and dehydrogenation
proceeded on mesoporous V + TiO2 and V + TiO2(anatase)catalysts, respectively. VIV doping was effective than Vv
doping to mesoporous TiO2, however, vanadium had noeffects to anatase TiO2.
(3) Initial water evolution from mesoporous V–TiO2 and V/
mesoporous TiO2 catalysts under the illumination of visible
light only suggested specific catalysis utilizing themesopore environment. Consecutive oxidation reactions
to H2O and COx (x = 0, 1, and 2) were suggested.
(4) The improved photo-oxidation performance of mesoporous
V–TiO2 (and V/mesoporous TiO2) catalyst(s) was correlated with the extension of optical absorption spectra
toward visible light wavelength region.

Acknowledgements
The authors are thankful for financial support from theGrant-in-Aid for Scientific Research (YI, C-17550073, B13555230) and that on the Priority Area ‘‘Molecular Nanodynamics’’ (YI, 432-17034013) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The
XAFS experiments were performed under the approval of theSPring-8 Program Review Committee (2003A0146-NX-np and
2002B0739-NX-np) and that of the Photon Factory Proposal
Review Committee (2001G308 and 2002G285).
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Fig. 6. Vanadium site models suggested by V K-edge XAFS study common for
V/mesoporous TiO2 and impregnated V/TiO2 (P-25) (A) and for mesoporous
V–TiO2 (B).

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