Comparative study on the photocatalytic decomposition of methanol on TiO2 modified by N and promoted by metals

Article abstract of DOI:10.1016/j.jcat.2012.07.020

The photo-induced vapor-phase reaction of methanol was investigated on Pt metals deposited on pure and N-doped TiO₂. Infrared spectroscopic measurements revealed that illumination of the CH₃OH-TiO₂ and CH₃OH-M/TiO₂ systems led to the conversion of adsorbed methoxy species into adsorbed formate. In the case of metal-promoted TiO ₂ catalysts CO bonded to the metals was also detected. Pure titania exhibited very little photoactivity, its efficiency, however, increased with the narrowing of its bandgap by N-doping, a feature attributed to the prevention of electron-hole recombination. Deposition of Pt metals on pure and N-doped TiO₂ dramatically enhanced the extent of photoreaction of methanol even in visible light: hydrogen and methyl formate with selectivities of 83-90% were the major products. The most active metal was Pt followed by Pd, Ir, Rh, and Ru. The effect of metal was explained by a better separation of charge carriers induced by illumination and by enhanced electronic interaction between metal nanoparticles and TiO₂.

Full text of DOI:10.1016/j.jcat.2012.07.020

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Journal of Catalysis 294 (2012) 199–206
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Comparative study on the photocatalytic decomposition of methanol on TiO2
modified by N and promoted by metals
⇑
Gyula Halasi, Gábor Schubert, Frigyes Solymosi
Reaction Kinetics and Surface Chemistry Research Group of the Hungarian Academy of Sciences, Department of Physical Chemistry and Materials Science, University of Szeged,
P.O. Box 168, H-6701 Szeged, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
The photo-induced vapor-phase reaction of methanol was investigated on Pt metals deposited on pure
and N-doped TiO2. Infrared spectroscopic measurements revealed that illumination of the CH3OH–TiO2
and CH3OH-M/TiO2 systems led to the conversion of adsorbed methoxy species into adsorbed formate.
In the case of metal-promoted TiO2 catalysts CO bonded to the metals was also detected. Pure titania
exhibited very little photoactivity, its efficiency, however, increased with the narrowing of its bandgap
by N-doping, a feature attributed to the prevention of electron–hole recombination. Deposition of Pt met-
als on pure and N-doped TiO2 dramatically enhanced the extent of photoreaction of methanol even in vis-
ible light: hydrogen and methyl formate with selectivities of 83–90% were the major products. The most
active metal was Pt followed by Pd, Ir, Rh, and Ru. The effect of metal was explained by a better separation
of charge carriers induced by illumination and by enhanced electronic interaction between metal nano-
particles and TiO2.
Received 14 June 2012
Revised 18 July 2012
Accepted 18 July 2012
Available online 20 August 2012
Keywords:
Photodecomposition of methanol
TiO2 photocatalyst
N-doped TiO2
Pt metals deposited on TiO2
Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction
performed in liquid phase. Mainly TiO2 was used as a photocatalyst
promoting by Cu, Pt, Pd, Au, and Ag. All these metals enhanced the
In the last two decades an extensive research was devoted to
the generation of hydrogen by the thermal decomposition of oxy-
genated organic compounds [1–3]. Supported Pt metals were
found to be the most active catalysts [4–7], although Mo2C pre-
pared by the reaction of MoO3 with multiwall carbon and carbon
Norit exhibited also high activity [8,9]. Taking into account the
requirement of fuel cell, a growing attention is also paid to the pro-
duction of hydrogen free of CO. For this purpose the thermal
decomposition of formic acid on supported metals seemed to be
the most appropriate reaction [10–14]. Further step in this area
is to generate hydrogen by photocatalytic reactions at room tem-
perature. Recently we examined the photolysis of ethanol [15]
and formic acid [16] on TiO2-based materials with the aim to select
the most effective catalysts and work out more suitable experi-
mental conditions. It was reported that in the presence of water
hydrogen free of CO can be produced in the photolysis of formic
acid on Rh/TiO2 at room temperature [16].
extent of the decomposition of methanol in ultraviolet light. In the
visible light only few experiments were executed [19,30,33]. An
interesting feature of the thermal reaction of methanol is that be-
sides the well known degradation into CO and H2
CH3OH ¼ CO þ 2H2
ð1Þ
by certain catalysts it is converted into methyl formate and H2
[38–46,48,49]
2CH3OH ¼ HCOOCH3 þ 2H2
ð2Þ
Methyl formate has been considered as a precursor in the syn-
thesis of formamide, dimethyl formamide, acetic acid, propionic
acid, cyanhydric acid, and several other materials [45], therefore
its efficient production represents technological importance.
Methyl formate is mainly synthesized by the dehydrogenation of
methanol over Cu-based catalyst at higher temperatures. However,
recent works showed that it is also formed in the photo-oxidation
of methanol at room temperature [31,34–37]. In the present study
we examine the photodecomposition of methanol on pure and me-
tal-promoted TiO2. Attempts will be also made to convert metha-
nol into other compounds in the visible light by narrowing the
bandgap of TiO2 by N-doping.
In the continuation of our research program in this work we
deal with the photocatalytic reaction of methanol. This alcohol is
one of the perfect and ideal resources for the generation of hydro-
gen and other organic compounds. Its photodecomposition has
been the subject of several studies [17–37]. Most of them were
⇑
Corresponding author. Fax: +36 62 544 106.
E-mail address: fsolym@chem.u-szeged.hu (F. Solymosi).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.07.020

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G. Halasi et al. / Journal of Catalysis 294 (2012) 199–206
gas-mixing unit serving for the adjustment of the composition of
the gas or vapor mixtures to be photolyzed in situ. We used a
15 W germicide lamp (type GCL 307T5L/CELL, Lighttech Ltd.,
Hungary), which emits predominantly in the wavelength range of
250–440 nm, its maximum intensity is at 254 nm. For the visible
photocatalytic experiments another type of lamp was used (Light-
tech GCL 307T5L/GOLD) with 400–640 nm wavelength range and
two maximum intensities at 453 and 545 nm. The approximate
light intensities at the catalyst films are 3.9 mW/cm2 for the germi-
cide lamp and 2.1 mW/cm2 for the other lamp. Methanol (ꢂ9.0%,
2. Experimental
2.1. Materials
Two types of TiO2 were used: Hombikat, UV 100 (300 m2/g), and
Degussa, P 25 (50 m2/g). For the preparation of N-doped TiO2 we
applied the description of Beranek and Kisch [47], who reacted
TiO2 with urea and calcined the catalyst at different temperatures.
This sample is noted with ‘‘SK’’. N-modified TiO2 sample (named
‘‘SX’’) was also produced following the description of Xu et al.
[48]. Titanium tetrachloride was used as a precursor. After several
steps the NH3-treated TiO2 slurry was vacuum dried at 353 K for
12 h, followed by calcination at 723 K in flowing air for 3 h. Me-
tal-promoted TiO2 samples were prepared by impregnating pure
or doped TiO2 with the solution of metal compounds to yield a
nominal 2 wt% metal. The following salts of Pt metals were used:
H2PtCl6ꢀ6H2O, Pd(NO3)2, RhCl3ꢀ3H2O, H2IrCl6, and RuCl3ꢀ3H2O.
The suspension was dried at 373 K and annealed at 573 K for 1 h.
For IR studies the dried samples were pressed in self-supporting
wafers (30 ꢁ 10 mm ꢂ10 mg/cm2). For photocatalytic measure-
ments the sample (70–80 mg) was sprayed onto the outer side of
the inner tube from aqueous suspension. The surface of the catalyst
film was 168 cm2. The catalysts were oxidized at 573 K and re-
duced at 573 K in the IR cell or in the catalytic reactor for 1 h.
Methanol was the product of Scharlau with a purity of 99.98%.
2160 lmol) was introduced in the reactor through an externally
heated tube avoiding condensation. The carrier gas was Ar, which
was bubbled through methanol at room temperature. The gas-
mixture was circulated by a pump. The reaction products were
analyzed with a HP 5890 gas chromatograph equipped with
PORAPAK Q and PORAPAK S packed columns. The sampling loop
of the GC was 500
to this loop.
ll. The amounts of all products were related
3. Results
3.1. Characterization of the catalysts
In our previous paper we determined the bandgap energy of
several TiO2 + N catalysts [15] following the method described by
Beranek and Kisch [47]. In the present work this characteristic
was measured only for the new samples. The Kubelka–Munk func-
tion F(R1) vs. wavelength curves were obtained from diffuse reflec-
2.2. Methods
ꢃ Eg)n/h was used in the
For FTIR studies a mobile IR cell housed in a metal chamber was
used. The sample can be heated and cooled at 150 K. The IR cell can
be evacuated to 10ꢃ5 Torr using a turbo molecular pumping sys-
tem. The samples were illuminated by the full arc of a Hg lamp
(LPS-220, PTI) outside the IR sample compartment. The IR range
of the light was filtered by a quartz tube (10 cm length) filled with
triple distilled water applied at the exit of the lamp. The filtered
light passed through a high-purity CaF2 window into the cell. The
light of the lamp was focused onto the sample. The output pro-
duced by this setting was 300 mW cmꢃ2 at a focus of 35 cm. The
maximum photon energy at the sample is ca. 5.4 eV. After illumi-
nation, the IR cell was moved to its regular position in the IR beam.
Infrared spectra were recorded with a Biorad (Digilab. Div. FTS
155) instrument with wavenumber accuracy of 4 cmꢃ1. All the
spectra presented in this study are difference spectra.
tance data, and the equation
a
= A(h
m
m
calculation, where
a
is the absorption coefficient, A is a constant,
h is the energy of light and n is a constant depending on the nature
m
of the electron transition. Assuming an indirect bandgap (n = 2) for
TiO2, with
a
proportional to F(R ), the bandgap energy can be ob-
1
1/2
tained from the plots of [F(R1)h
m
]
vs. hm, as the intercept at
[F(R )h ]1/2 = 0 of the extrapolated linear part of the plot. The val-
1
m
ues obtained for pure and doped TiO2 are collected in Table 1. This
shows that depending on the preparation the incorporation of N
into TiO2 considerably lowers its bandgap. In contrast, no or only
slight decrease occurred by doping TiO2 with fluorine.
3.2. FTIR study of photolysis of methanol
The primary aim of the IR study is to ascertain the development
of adsorbed complexes on the effect of illumination on TiO2, and to
establish the influence of metal deposition on these features. As
observed before [41] prior to the addition of methanol the IR spec-
trum of TiO2 showed the usual mOH absorption at 3720, 3671, and
3648 cmꢃ1. The adsorption of methanol resulted in a very broad
absorption region between 3500 and 2700 cmꢃ1 due to the high
concentration of OH groups. In the C–H stretching region absorp-
tion bands of different intensities were traced at 2948, 2935–
2938, 2923, 2914, 2891, 2873, 2844, 2842, and ꢂ2810 cmꢃ1 (not
shown). In the low frequency range weak bands appeared at
1561, 1445, 1442, 1380, 1361, 1157, and ꢂ1080 cmꢃ1 (Fig. 1A).
Illumination of the CH3OH vapor–TiO2 system caused no observa-
ble change in the high frequency range, but led to the significant
intensification of the bands in the low frequency region (Fig. 1A).
We experienced similar changes on the IR spectra of metal/TiO2
samples (Fig. 1B and C). A new feature was the sudden appearance
of a strong band between 2000 and 2045 cmꢃ1 due to adsorbed CO,
which grew only slightly with the progress of illumination. This is
illustrated in Fig. 1D.
For the determination of bandgap of solids, diffuse reflectance
spectra of TiO2 samples were obtained using an UV/Vis spectro-
photometer (OCEAN OPTICS, Typ.USB 2000) equipped with a dif-
fuse reflectance accessory. The surface area of the catalysts was
determined by BET method with N2 adsorption at ꢂ100 K. The dis-
persion of metals was determined by the adsorption of H2 at room
temperature. Data are listed in Table 1.
Photocatalytic reaction was followed in the same way as de-
scribed in our previous paper [15]. The photoreactor (volume:
970 ml) consists of two concentric Pyrex glass tubes fitted one into
the other and a centrally positioned lamp. It is connected to a
Table 1
Some characteristic data for pure and doped TiO2.
Sample
Pretreatment
temperature (K)
Surface area
(m2/g)
Bandgap
(eV)
TiO2
(Hombikat)
As received
ꢂ300
3.17, 3.21
TiO2 + N (SK)
TiO2 + N (SK)
TiO2 + N (SK)
TiO2 (SX)
450
573
773
723
723
260
115
81
265
79
3.04
3.00
2.17
3.02
ꢂ1.96
When adsorbed methanol was irradiated the first change was
the instant appearance of the CO band at 2000–2045 cmꢃ1, which
became larger as a result of continuous illumination (not shown).
TiO2 + N (SX)

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G. Halasi et al. / Journal of Catalysis 294 (2012) 199–206
201
Fig. 1. Effects of illumination time on the FTIR spectra of TiO2 (A), Pt/TiO2 (B), and Ir/TiO2 (C) obtained in the presence of CH3OH vapor and on the intensity of CO band at
2000–2060 cmꢃ1 (D). a. 0 min, b. 1 min, c. 3 min, d. 6 min, e. 9 min, f. 30 min.
In the high frequency range a pair of strong absorption bands at
2936–2939 and 2835–2839 cmꢃ1 of almost same intensity became
the dominant spectral features for all M/TiO2 catalysts. With the
increase of illumination time both bands slowly attenuated.
In the low frequency range absorption bands appeared at
ꢂ1558–1578, ꢂ1458, ꢂ1442, 1359–1380, 1158–1164, and
1026–1030 cmꢃ1
. The intensity of the absorption bands at
1558–1578 cmꢃ1 increased, while the other ones slightly attenu-
ated with the time of irradiation. In Fig. 2 spectra of 2750–3000
and 1100–1650 cmꢃ1 regions are shown for three M/TiO2 catalysts.
IR bands observed and their possible assignment are presented in
Table 2.
3.3. Photocatalytic decomposition of methanol on pure and N-doped
TiO2
Illumination initiated the decomposition of methanol on TiO2 at
300 K, but the conversion was very low: ꢂ1.5% in 210 min (Fig. 3A).
The main products are methyl formate and H2 with very small
amounts of CO and CO2. The selectivity of the formation of methyl
formate is very high, ꢂ90%. The photoactivity of TiO2 was signifi-
cantly enhanced by N doping, and the promoting effect of N in-
creased with the modification temperature of TiO2 + N sample.
This feature is more pronounced when the amount of methyl for-
mate formed is related to the surface area of the TiO2 samples
(Fig. 3B). When the photolysis was performed in the visible light,
the extent of photodecomposition markedly decreased, but the ef-
fect of N-doping was appreciable.
It was a general observation that the amount of H2 formed was
much less than that of methyl formate. We experienced a similar
lack in H2 in the photodecomposition of ethanol and formic acid
on TiO2 [15,16]. We assume that hydrogen may reduce the TiO2
Fig. 2. Effects of illumination time on the FTIR spectra of adsorbed CH3OH on
Rh/TiO2 (A), Pt/TiO2 (B), Ir/TiO2 (C) 300 K. a. 0 min, b. 1 min, c. 3 min, d. 9 min, e. 30 min.

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G. Halasi et al. / Journal of Catalysis 294 (2012) 199–206
Table 2
This is illustrated by the results obtained on Rh/TiO2 (SX sample)
in Fig. 6.
IR vibrational frequencies of methoxy species on TiO2 and those observed following
the illumination of adsorbed methanol on M/TiO2 catalysts at 300 K.
Although the catalyst sample was cooled during the photolysis,
the possibility cannot be excluded that the illumination caused a
temperature rise of the catalyst. However we measured not more
than 1–2° increase in the sample temperature as a result of irradi-
ation. We also examined the thermal reaction on the Pt/TiO2 and
Pd/TiO2 layer used for photolysis without illumination. No decom-
position was observed on either catalyst at 300–423 K in 60 min.
The decomposition started at 473 K, it reached ꢂ1–2% in 60 min.
The results of these control experiments lead us to exclude the
contribution of thermal effects to the decomposition of methanol
induced by photolysis. An interesting feature of these experiments
is that only very small amounts of methyl formate, less than 5–8%
was formed in the thermal decomposition at higher temperature,
523 K, when the conversion of methanol reached 5–10%.
Detailed experiments were performed in the visible light using
a lamp emitting at 400–640 nm. Whereas the photoactivity of pure
TiO2 is hardly detectable in this wavelength range, the reaction
was well measurable on TiO2 doped with N. This was observed
both for SK and SX samples. The positive influence of N-doping
was more appreciable in the case of metal-promoted TiO2 samples.
Fig. 7 depicts the photocatalytic effects of Rh, Ir, and Pd deposited
on pure and N-modified TiO2 (SX). The comparison of the data
immediately reveals that the photoactivity of the N-modified cata-
lysts is markedly higher than that of M/TiO2 free of nitrogen. This is
reflected in the conversion of methanol and in the amounts of the
products formed. In the case of Ir/TiO2 the incorporation of N into
TiO2 increased the amount of hydrogen by a factor of 10.
Vibrational
mode
TiO2
[40]
Ru/TiO2 [present
work]
Pd/TiO2 [present
work]
m
m
as(CH3)
s(CH3)
(2965)
2930
2830
1462
1436
1151
1125
2933
2835
1457
1442
1158
1029
2939
2839
1456
1443
1114
1023
2ds(CH3)
das(CH3)
ds(CH3)
r(CH3)
m
(CO)
surface or combine with surface oxygen to yield OH groups. In the
case of N-doped TiO2 a fraction of H2 can also react with the surface
species produced by the preparation of TiO2 + N samples [15].
3.4. Effects of Pt metals
In the next experiments, we examined the effects of the deposi-
tion of various Pt metals on TiO2. All metals exerted a dramatic
influence on the photodecomposition of methanol. The conversion
attained a value of 30–70% in 210 min. In Fig. 4 the results obtained
on various M/TiO2 catalysts are displayed. On the basis of these
data, the activity order is Pt, Pd, Ir, Rh, and Ru. The major product
is H2 followed by methyl formate (C2H4O2). Much less formalde-
hyde, CO, CO2, and CH4 were evolved. The formation of minor prod-
ucts obtained over 2% Pd/TiO2 is shown in Fig. 5A. The selectivity of
methyl formate was very great on all catalysts. Highest values
were measured at the beginning of the reaction (87–97%), which
lowered to 83–90% at the termination of the experiments, at
210 min. The CO/H2 ratio was very low, 0.01–0.02 (Table 3). In or-
der to judge the importance of TiO2 support and that of metal/TiO2
contact, the photolysis of methanol was also carried out on 2%
Pd/SiO2 catalyst. As can be seen in Fig. 5B, we measured only very
slight decomposition; the conversion approached 1% in 210 min.
When metals were deposited on N-doped TiO2 an appreciable
enhancement was observed in the photoactivity of the catalysts.
4. Discussion
4.1. IR studies
FTIR study of the adsorption of methanol on titania has been the
subject of extensive research [23,34,40,41]. Based on the results of
previous studies the bands registered at 2948 and 2844 cmꢃ1 can
be assigned to the asymmetric and symmetric CH3 stretching fre-
quencies of adsorbed CH3OH, and those at 2933–2939 and
2835–2839 cmꢃ1 to adsorbed CH3O. The spectral features in
Fig. 3. Effects of the modification temperature of N-doping of TiO2 (SK) on the conversion of CH3OH (A) and on the formation of methyl formate (B) related to the surface area
of the TiO2 samples.