Photocatalytic decomposition of gaseous acetone using TiO2 and Pt/TiO2 catalysts

Article abstract of DOI:10.1002/kin.20303

Metallic platinum was photodeposited on TiO₂ particles, and morphological characteristics of the Pt/TiO₂ catalyst were determined. The dispersion of metallic platinum was uniform and did not alter the morphology of the TiO₂ particles. However, absorbance of the Pt/TiO₂ catalyst for light with wavelength more than 400 nm was significantly improved by the addition of metallic platinum. Gaseous acetone was decomposed in an annular photoreactor coated with TiO₂ or Pt/TiO₂ catalysts using a UV or a fluorescent lamp as light source. The decomposition of acetone with the application of a UV lamp was obviously enhanced for experiments conducted with Pt/TiO₂ catalyst. Decomposition of acetone was promoted considerably with increasing oxygen concentration for experiments conducted with oxygen less than 50,000 ppmv, yet the decomposition of acetone was kept relatively constant for experiments conducted with oxygen above 50,000 ppmv. On the basis of the mass balance for carbon species, the amount of organic intermediates formed for experiments conducted under various conditions was found to be minimal.

Full text of DOI:10.1002/kin.20303

Photocatalytic
Decomposition of Gaseous
Acetone Using TiO and
2
Pt /TiO Catalysts
2
1
1
2
YOUNG KU, KUN-YU TSENG, CHIH-MING MA
1Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4,
Taipei 106, Taiwan
2
Department of Cosmetic Application & Management, St. Mary’s Medicine Nursing and Management College,
100, Lane 265, San-Shing Road, Section 2, San-shing Shiang, Yi-lan 266, Taiwan
Received 31 March 2007; revised 24 October 2007; accepted 4 November 2007
DOI 10.1002/kin.20303
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Metallic platinum was photodeposited on TiO2 particles, and morphological char-
acteristics of the Pt/TiO2 catalyst were determined. The dispersion of metallic platinum was
uniform and did not alter the morphology of the TiO2 particles. However, absorbance of the
Pt/TiO2 catalyst for light with wavelength more than 400 nm was significantly improved by the
addition of metallic platinum. Gaseous acetone was decomposed in an annular photoreactor
coated with TiO2 or Pt/TiO2 catalysts using a UV or a fluorescent lamp as light source. The
decomposition of acetone with the application of a UV lamp was obviously enhanced for exper-
iments conducted with Pt/TiO2 catalyst. Decomposition of acetone was promoted considerably
with increasing oxygen concentration for experiments conducted with oxygen less than 50,000
ppmv, yet the decomposition of acetone was kept relatively constant for experiments con-
ducted with oxygen above 50,000 ppmv. On the basis of the mass balance for carbon species,
the amount of organic intermediates formed for experiments conducted under various condi-
tions was found to be minimal. ꢀC 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 209–216,
2008
INTRODUCTION
organic pollutants merely relocate the pollutants from
one gas phase to another phase. Consequently, a
distinct need exists for developing innovative treatment
methods that are extensively applicable to most volatile
organic compounds and can be employed with mini-
mum construction and operational requirements. The-
oretically, the decomposition of volatile organic pollu-
tants by oxidation processes is capable of converting
these organic compounds completely to carbon diox-
ide, water, and several simple species. Therefore, var-
ious oxidation processes are considered to be effective
approaches to decompose volatile organic pollutants.
Volatile organic compounds are typical gaseous emis-
sions from numerous industries and pose various
hazards for human health and environment. Many tech-
nologies currently used for the treatment of volatile
Correspondence to: Young Ku; e-mail: ku508@mail.ntust.
edu.tw.
Contract grant sponsor: National Scientific Council.
Contract grant number: NSC 92-2211-E-011-031.
ꢀ
c 2008 Wiley Periodicals, Inc.

2
10
KU, TSENG, AND MA
Photocatalytic technologies are reported to be ef-
alyst under various operating conditions. The tempo-
ral variation of acetone and carbon dioxide concentra-
tions during the photocatalytic treatment of gaseous
acetone under various operating conditions was exam-
ined and described in terms of a dual-site Langmuir-
Hinshelwood–type model.
fective on oxidizing several volatile organic pollutants
at considerably mild conditions and have been consid-
ered to be promising alternatives. Various photocata-
lysts have been developed for the treatment of volatile
organic pollutants by photocatalytic processes. How-
ever, the practical application of photocatalytic pro-
cesses for the treatment of volatile organic pollutants
in gaseous streams is hindered by the development
of effective photocatalysts. The anatase form of tita-
nium dioxide (TiO2) is the most frequently studied
photocatalyst for various environmental applications.
Although the quantum yields of several applications of
the UV/TiO2 process were demonstrated to be higher
than most other photocatalysts; however, the quan-
tum efficiency of TiO2 catalyst is yet hindered pri-
marily by the recombination of electrons and electrical
holes generated on the surface of TiO2 under UV light
irradiation.
Numerous technologies are developed to modify the
structural and morphological properties of TiO2 cata-
lyst by doping numerous metallic species, such as Ag,
Ni, Cu, Pd, and Pt, to enhance the photocatalytic activ-
ity of TiO2 [1–3]. However, experimental results with
the addition of various metals were found to present
controversial results on the photocatalytic activity of
TiO2, depending mainly on the dispersion and char-
acteristics of metallic species doped on the catalyst.
Usually, the deposition of metallic species onto TiO2
catalyst was accomplished by chemical [4,5], wet im-
pregnation [6], thermal [7], or photo [8–10] methods.
Doping TiO2 with a certain kind of metal and metal
ion has been frequently attempted not only to retard
the fast charge pair recombination but also to enable
visible light absorption by providing defect states in
the band gap [11,12]. In the former case, the modifica-
tions of the TiO2 surface with noble metal and its ion
are the successful ways to improve photocatalytic ac-
tivity of TiO2 and increase the quantum yield due to the
increase of the rate of electron transfer to the oxidant
EXPERIMENTAL
Chemicals used in this study were analytical grade
without further purification. A half liter of aqueous so-
lution containing 160 g Degussa P-25 TiO2 was held in
a 1-L beaker stirred by a magnetic stirrer; the solution
was then purged with nitrogen gas to remove dissolved
−1
oxygen. About 0.02 g L dioctyl sulfosuccinate was
subsequently put into the mixed solution as a dispers-
ing agent. The solution was then stirred by a sonicator
for about 24 h, a 23-cm quartz tube was subsequently
impregnated in the mixed solution for about 1 min be-
fore it was taken out and air-dried. For the preparation
of Pt/TiO2 catalyst, 0.16 g of H2PtCl6 · 6H2O was dis-
solved in 20 mL methanol, the methanol solution was
then mixed with the aqueous solution containing TiO2
and dioctyl sulfosuccinate. The mixed solution was
−2
then irradiated for 8 h with 1.33 mW cm UV light us-
ing a GTE F15T8/BLB lamp to photoreduce platinum
ions into metallic platinum to be deposited on TiO2
particles. A quartz tube was subsequently impregnated
in the mixed solution and took out and air-dried, as
described for the coating of TiO2.
The amount of catalyst coated onthe quartztube was
determined by measuring the weight of the quartz tube
before and after coating. The morphology and the spe-
cific BET surface area of the TiO2 coating was deter-
mined by a Siemens D-8 X-ray diffractometer (XRD)
and a Micromeritics ASAP 2000 analyzer, respectively.
The light absorbance of the TiO2 coatings was deter-
mined by a Cray-300 UV–vis spectrophotometer. The
impregnation process could be repeated several times
to increase the amount of catalyst coated on the quartz
tube. The distribution of catalyst coated on the sur-
face of the quartz tube was observed by a JOEL JSM-
6500F field emission scanning electron microscope
(FESEM). The coated quartz tube was then included
in a photoreactor to house a UV lamp or a fluorescent
lamp. The plug-flow annular photoreactor was made
exclusively of stainless steel with an effective volume
of 4.1 L. The output of the GTE F15T8/BLB UV lamp
used in this study was primarily at 365-nm wavelength,
while the output of the FL15D-T25 fluorescent lamp
used ranged from 400 to 700 nm. The light intensity
of both lamps was adjusted by a variable voltage trans-
former and was detected by a Spectroline model DRC-
100X digital radiometer with a radiation sensor.
[
2,11]. In the latter case, the electronic transitions from
valence band to defect states or from defect states to
conduction band can be allowed under sub-band-gap
illumination to show visible activity.
Several studies focused on the optical properties
of TiO2 film and dosage of Pt and Pd impurities in
the TiO2 film to absorb visible light [1–3,11,12]. In
this study, metallic platinum was deposited on TiO2
particles by photoreduction [12]. The structural and
morphological characteristics of the prepared Pt/TiO2
catalyst were examined. To investigate the photocat-
alytic activity of the Pt/TiO2 catalyst, gaseous acetone
was chosen and decomposed by photocatalysis in an
annular photoreactor coated with TiO2 or Pt/TiO2 cat-
International Journal of Chemical Kinetics DOI 10.1002/kin

PHOTOCATALYTIC DECOMPOSITION OF GASEOUS ACETONE
211
The acetone-laden air stream was prepared by ex-
tracting acetone vapor by dry air from an isother-
mal bottle containing acetone. The acetone-laden gas
stream was subsequently mixed in a gas mixer with hu-
midity containing air stream and oxygen gas to main-
tain constant levels of humidity and oxygen concen-
tration. The mixed gas stream was then directed into
the prewarmed photocatalytic reactor for the study of
acetone decomposition. The temperature of the pho-
◦
toreactor system was maintained constant at 37 C. Gas
samples were taken for further analyses after the tem-
perature, flow rate, pressure, and humidity of the re-
actor system reached steady state. The operating time
required to reach the steady state usually was about
45 min. A China chromatography model 8700F GC
equipped with a flame ionization detector was used to
analyze the inlet and effluent acetone concentrations
in the air streams. The concentration of carbon diox-
ide (CO2) present in the effluent streams was deter-
mined with a nondispersed infrared (NDIR) detector.
The photocatalytic ability of the Pt/TiO2 was examined
for several duplicate runs and found to be relatively
steady to avoid the consideration of possible activity
decay of catalyst during the experiments.
RESULTS AND DISCUSSION
The composition of the TiO2 particles coated on the
photoreactor was determined by XRD to contain about
80% of anatase and 20% of rutile, similar to reports in
our previous studies [13,14]. The addition of 0.04 wt%
of metallic platinum did not alter the composition of
the TiO2 particles to any noticeable extent, as shown
in Fig. 1. The specific BET surface area of the TiO2
coating containing platinum (Pt/TiO2) was measured
2
−1
to be 57.0 m g , comparable to the surface area of
5
2
−1
6.4 m g determined for the TiO2 coating without
platinum, similar to the results reported by Vorontsov
et al. [4]. The particle size, roughness, and coverage of
the TiO2 coating were observed by the FESEM micro-
graph under the magnification of 60,000×. The particle
sizes of the TiO2 and Pt/TiO2 were both observed to
be between 50 and 200 nm without noticeable differ-
ence. With the addition of dispersant into the aqueous
solution containing TiO2slurry, the surface of quartz
tube coated with TiO2 or Pt/TiO2 layers was observed
to be reasonably smooth. The dispersion of metallic
platinum coated on the Pt/TiO2 layer was examined to
be relatively homogeneous by the X-ray energy disper-
sive spectrometer affiliated to the FESEM, as shown in
Fig. 2. The light absorbance by the TiO2 and Pt/TiO2
layer was detected by a UV–vis spectrophotometer and
is illustrated in Fig. 3. The absorbance for UV light with
Figure 1 X-ray diffraction patterns of different catalysts:
(a) TiO2 (P25) and (b) 0.04% Pt/TiO2.
a wavelength less than 400 nm was not enhanced by the
addition of platinum; however, the light absorbance in
the visible region (wavelength more than 400 nm) was
found to be significantly improved by the addition of
platinum, similar to the results that have been reported
in several studies [11,12,15–17].
The concentrations of acetone, organic interme-
diates, and CO2 were monitored during the reaction
course to establish the elemental mass balances of car-
bon during the reaction in this work [2,13]. The mass
balance of carbon is
[acetone]C,o=[acetone]C+[Interme]C+[CO2]C (1)
International Journal of Chemical Kinetics DOI 10.1002/kin

2
12
KU, TSENG, AND MA
Figure 3 The UV–vis spectra absorbance of the photocat-
alysts in the range of 300–800 nm.
Figure 2 SEM images of (a) 0.04 wt% Pt-deposited TiO2
coating and (b) Ti elemental mapping image of 0.04 wt%
Pt-deposited TiO2 coating by EDAX from SEM analysis.
Figure 4 Effect of light intensity on the acetone decompo-
sition by photocatalytic oxidation using a UV lamp at various
photocatalysts: acetone 80 ppmv, 200 mL min , and 10%
humidity.
where [acetone]C,o is the total initial amount of acetone
as carbon (ppmv), [acetone]C the amount of residual
acetone as carbon (ppmv), [Interme]C the amount of
organic intermediates as carbon (ppmv), and [CO2]C
the amount of carbon dioxide as carbon (ppmv).
−1
The effect of UV light intensity on the decomposi-
tion and minealization of acetone with Pt/TiO2 (con-
taining 0.04 wt% Pt) coating on the surface of the tubu-
lar photoreactor using a UV lamp at a residence time
of 20.3 min is shown in Figs. 4 and 5. Both the decom-
position and minealization of acetone were increased
with increasing UV light intensity for experiments con-
ducted in the photoreactor coated with Pt/TiO2 with
UV light intensity less than 2.0 mW cm , similar
to the results reported by several previous researchers
−2
International Journal of Chemical Kinetics DOI 10.1002/kin

PHOTOCATALYTIC DECOMPOSITION OF GASEOUS ACETONE
213
form carbon dioxide for experiments conducted with
−2
UV light intensity greater than 2.0 mW cm in the
photoreactor with Pt/TiO2 coating, compared to only
about 40% of acetone decomposition for experiments
conducted under similar conditions in the photoreactor
with TiO2 coating.
On the basis of the mass balance for carbon species,
the amount of organic intermediates formed for exper-
iments conducted with Pt/TiO2 under various condi-
tions was found to be minimal. The enhancement of
the activity of TiO2 photocatalyst in the presence of
platinum was frequently contributed to the prevention
of recombination electrons and electrical holes. Be-
cause the work function (ϕ, defined as the minimal
energy an electron must contain to be loosened from
specific molecular structure and become a free elec-
tron) of metallic platinum (5.6 eV) is higher than that
of TiO2 (4.2 eV), the electrons induced by light irra-
diation were flown from TiO2 to platinum. Therefore,
the Pt/TiO2 interface is electron depleted and accumu-
lates positive charges to prevent the recombination of
electrons and electrical holes [10,22,23].
Various photocatalysts, e.g., titanium dioxide, are
now under intensive investigation for practical appli-
cation in air and water purifications. But it is active
only under UV irradiation (λ < 388 nm), which ac-
counts for less than 5% of solar light energy. There
have been various attempts to utilize visible light in
TiO2 photocatalysis, which include dye sensitization,
semiconductor coupling, and impurity doping [12]. In
this study, more than 95% of acetone was decomposed
by photocatalytic process for experiments conducted
in the Pt/TiO2-coated photoreactor using a fluorescent
lamp, as shown in Fig. 6. As discussed in the previous
Figure 5 Effect of light intensity on the CO2 generation
ratio by photocatalytic oxidation using a UV lamp at various
−1
photocatalysts: acetone 80 ppmv, 200 mL min , and 10%
humidity.
[18–20]. Almost complete decomposition and mineal-
ization of gaseous acetone were accomplished for ex-
periments conducted with higher UV light intensi-
ties. Compared to the experimental results reported
by the authors on the photocatalysis of acetone by the
UV/TiO2 process without platinum [21], both the de-
composition and minealization of acetone were found
to be considerably increased with the presence of plat-
inum. More than 90% of acetone was decomposed to
Figure 6 Photocatalytic oxidation acetone using a fluorescent lamp: acetone 80 ppmv, 200 mL min 1, and 10% humidity.
−
International Journal of Chemical Kinetics DOI 10.1002/kin

2
14
KU, TSENG, AND MA
Figure 8 Behavior of molecule adsorption for acetone and
oxygen on different surface sites.
sition for experiments conducted with TiO2 under sim-
ilar experimental conditions [21]. As shown in Fig. 6,
the presence of excessive oxygen molecules (more than
50,000 ppmv) in gas stream neither promotes nor hin-
ders the photocatalytic decomposition of acetone [25].
A possible explanation is that the active sites for oxy-
gen molecules on the surface of Pt/TiO2 are not identi-
cal to the sites for acetone molecules; therefore, there is
no apparent competition between oxygen and acetone
molecules for active sites to precede the photocatalytic
reaction as shown in Fig. 8. The available active sites
for oxygen molecules on the surface of Pt/TiO2 were
limited and may be readily saturated when sufficient
oxygen molecules were present in gas stream.
Assuming that the reactor used is an ideal plug-
flow reactor and operated at steady state, the results
discussed in the previous sections for experiments con-
ducted at constant relative humidity can be correlated
by a dual-site Langmuir-Hinshelwood–type rate equa-
tion taking into consideration the effects of acetone and
oxygen concentrations, and light intensity:
Figure 7 The temporal distribution of acetone and carbon
dioxide in gas steams by the UV/Pt–TiO2 process for exper-
iment conducted at a retention time of 20.3 min with various
initial O2 concentrations ranging from 0 to 600,000 ppmv:
−1
acetone 80 ppmv, 200 mL min , and 10% humidity.
section and shown in Fig. 3, the light absorbance for
a wavelength more than 400 nm was found to be sig-
nificantly improved by the addition of platinum. The
enhancing of the acetone decomposition was supposed
to be due to the charge transfer between metallic de-
fect site and TiO2 surface. With the presence of metallic
platinum, the application of fluorescent lamps provides
a more inexpensive and efficient light source for the
photocatalysis of gaseous pollutants using TiO2.
Figure 7 depicts the results for experiments using a
UV lamp with the presence of various initial O2 con-
centrations in gas stream ranging from 0 to 600,000
ppmv. Roughly 20% of acetone was decomposed for
the experiment conducted without the presence of oxy-
gen in gaseous stream. The photocatalytic decomposi-
tion of acetone was found to be increased drastically
with increasing initial O2 concentration in gas stream
for experiments conducted with Pt/TiO2 in the pres-
ence of oxygen less than 50,000 ppmv. The presence of
oxygen molecules is believed to promote the photocat-
alytic reaction by reducing the recombination of elec-
tric holes and electrons [24]. The presence of metal-
lic platinum on TiO2 obviously reduced the oxygen
molecules required for the photocatalytic decomposi-
tion of acetone. For instance, nearly complete decom-
position of acetone was achieved for experiments con-
ducted with Pt/TiO2 in the presence of oxygen above
ꢀ
ꢁꢀ
ꢁ
∂
CA
KACA
KO CO
2 2
n
−
= −RA = k0I
∂
τ
1 + KACA 1 + KO CO
2
2
(
2)
where k0 is the rate constant, I is the illuminated light
intensity, and τ is the retention time of gas flow in
the photoreactor. CA and CO are the concentrations
of gaseous acetone and oxygen in the gas flow, re-
spectively. KA and KO are the equilibrium adsorption
constants of gaseous acetone and oxygen molecules on
TiO2, respectively. The above rate equation was solved
to describe the decomposition behavior of acetone in
the annular photoreactor and expressed as
2
2
0
.00683 · CA
0.747
−
rA = 0.2905 · I
· 1
+ 0.00683 · CA
−4
1
.91 × 10 · CO
50,000 ppmv, whereas more than 200,000 ppmv O2
2
×
−
4
was required for achieving complete acetone decompo-
1 + 1.91 × 10 · CO2
International Journal of Chemical Kinetics DOI 10.1002/kin

PHOTOCATALYTIC DECOMPOSITION OF GASEOUS ACETONE
215
Figure 9 The calculated results for describing the variations of acetone concentration by photocatalytic oxidation using a UV
−2
lamp with different retention times: I = 3.09 mW cm , O2 concentration is 22,000 ppmv, and 10% humidity.
As shown above, the determined reaction order with
respect to light intensity is similar to the results re-
ported by our previous study at the lower light-intensity
levels [21] indicating a Langmuir-Hinshelwood–type
rate equation is adequate to evaluate the performance
of an annular photoreactor. Calculated results using
the developed rate equation for acetone decomposition
by photocatalytic oxidation under specific operational
conditions are shown in Fig. 9. More than 99% of
the acetone was photocatalytically decomposed within
about 30 min of retention time.
tion and minealization of gaseous acetone were ac-
complished for experiments conducted with UV light
−2
intensities higher than 2.0 mW cm . On the basis of
mass balance for carbon species, the amount of organic
intermediates formed for experiments conducted with
Pt/TiO2 under various conditions was found to be min-
imal. The activity enhancement of TiO2 photocatalyst
by the presence of platinum, due to the increase of elec-
tron transfer rate to the oxidant, enables visible light
absorption by providing defect states in the band gap.
With the presence of metallic platinum, the application
of fluorescent lamps provides a more inexpensive and
efficient light source for the photocatalysis of gaseous
acetone using TiO2. The photocatalytic decomposi-
tion of acetone was increased drastically with increas-
ing initial O2 concentration for experiments conducted
CONCLUSION
The photodeposited platinum did not alter the morphol-
ogy, specific surface area, and particle size of the TiO2
particles to any evident extent. The light absorbance of
the TiO2 particles in the visible region and the decom-
position of acetone by UV/TiO2 process were found
to be significantly enhanced by the addition of plat-
inum. Both the decomposition and minealization of
acetone were increased with increasing UV light in-
tensity for experiments conducted in the photoreactor
coated with Pt/TiO2 for experiments conducted with
lower light intensities. Almost complete decomposi-
with Pt/TiO in the presence of oxygen less than 50,000
ppmv. Experimental results obtained in this study can
be satisfactorily described by the developed dual-site
Langmuir-Hinshelwood kinetic model.
2
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International Journal of Chemical Kinetics DOI 10.1002/kin