Contributions of photocatalytic/catalytic activities of TiO2 and γ-Al2O3 in nonthermal plasma on oxidation of acetaldehyde and CO

Article abstract of DOI:10.1016/j.molcata.2005.10.002

The effects of photocatalysis and thermocatalysis in surface discharge type plasma reactors combined with TiO₂- and Al₂O ₃-based catalysts were analyzed. The photocatalytic decomposition of acetaldehyde was not observed although the TiO₂ absorbed the UV light (300 < λ < 380 nm) emitted from N₂ plasma. The intensity of UV emission from plasma decreased with increasing O₂ concentration. In atmospheric air, the photocatalytic decomposition rate by UV light of plasma is lower than 0.2% of the decomposition rate by the plasma itself. The coating of γ-Al₂O₃ on the inside wall of plasma reactor improved the oxidation rate for CO by a factor of 3.5, while TiO₂ improved by only a factor of 1.8, compared with the bare plasma reactor. The high activity of Al₂O₃ seems to be due to the active oxygen species produced by the O₃ decomposition.

Full text of DOI:10.1016/j.molcata.2005.10.002

Journal of Molecular Catalysis A: Chemical 245 (2006) 235–241
Contributions of photocatalytic/catalytic activities of TiO and ␥-Al2O3
2
in nonthermal plasma on oxidation of acetaldehyde and CO
∗
Taizo Sano , Nobuaki Negishi, Emiko Sakai, Sadao Matsuzawa
Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST),
AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
Received 18 August 2005; received in revised form 18 August 2005; accepted 2 October 2005
Available online 10 November 2005
Abstract
The effects of photocatalysis and thermocatalysis in surface discharge type plasma reactors combined with TiO
2
2 3
- and Al O -based catalysts
were analyzed. The photocatalytic decomposition of acetaldehyde was not observed although the TiO absorbed the UV light (300 < λ < 380 nm)
emitted from N plasma. The intensity of UV emission from plasma decreased with increasing O concentration. In atmospheric air, the pho-
tocatalytic decomposition rate by UV light of plasma is lower than 0.2% of the decomposition rate by the plasma itself. The coating of
-Al on the inside wall of plasma reactor improved the oxidation rate for CO by a factor of 3.5, while TiO improved by only a factor
seems to be due to the active oxygen species produced by the O
2
2
2
␥
2
O
3
2
of 1.8, compared with the bare plasma reactor. The high activity of Al
decomposition.
2
O
3
3
©
2005 Elsevier B.V. All rights reserved.
Keywords: Plasma; Photocatalyst; TiO2; Acetaldehyde; CO; Ozone; Surface discharge
1
. Introduction
combustion method, nonthermal plasma method, photocatalytic
method, ozone oxidation method, etc. [6–22]. Since the cat-
alytic combustion method requires a heat source, it is highly
energy-waste process when the VOCs concentration is low.
Photocatalytic method using UV-illuminated titanium dioxide
(TiO2) is expected as a safety method with low-energy con-
sumption, since TiO2 is less toxic powder and the photocatalytic
reaction proceeds in air, without any additives, and at ambient
pressure [8–10]. Only UV light is required for causing degra-
dation of VOCs. However, the performance of photocatalytic
method is too low in the application for VOCs emission sources
[11–14]. Alternatively, the photocatalytic method is attractive
for the degradation of VOCs with lower concentration such as
sick-building syndrome.
VOCs (volatile organic compounds) are hazardous pollutants
emitted from paints, solvents, preservatives, automobile exhaust
gas, industrial facilities, etc. [1–5]. Recently, VOCs are also rec-
ognized as causative agents of the sick-building syndrome. In
addition, emissions of VOCs can contribute to the formation of
urban smog and ozone, the stratospheric ozone depletion and
the greenhouse effect. Therefore, it is considered that effluent
controls at the VOCs emission sources will be severe all over
the world. Now, most of the small emission sources of VOCs,
such as dry cleaning shop, small printing factory, and construc-
tion site using paint, take no effective environmental measures
because of cost.
The conventional technique to remove VOCs is activated-
carbon adsorption method, which needs to recycle and dispose
the adsorbent used. The recycle of adsorbent requires energy,
and the disposal will cause the next environmental problem. The
alternative methods are presently being studied widely; catalytic
The nonthermal plasma (NTP) have been investigated by
many researchers for the decomposition of fluorocarbons and
VOCs, and found to have excellent performance [6,7,15–22].
Additionally, recently, it is revealed that catalysts in NTP
improve the specific energy efficiency and reduce byproduct
formation in the VOCs degradation [6,18–22]. In this plasma-
enhanced catalyst (PEC) method, many kinds of heterogeneous
∗
3 2 3 2 2 2
catalyst, such as BaTiO , Al O , SiO , TiO , MnO and their
Corresponding author. Tel.: +81 29 861 8166; fax: +81 29 861 8258.
E-mail address: sano-t@aist.go.jp (T. Sano).
derivatives, were evaluated. Among them, TiO2 often revealed
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.10.002

2
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T. Sano et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 235–241
the higher enhancement effect on the energy efficiency. Lee
et al. reported that the decomposition of benzene by using a
discharge plasma-photocatalyst hybrid system using dielectric
barrier discharge (DBD) plasma [18]. They described that the
photocatalytic reaction by TiO2 increases the benzene conver-
sion and the CO2 selectivity. Li et al. reported that a combination
of dc streamer corona plasma and TiO2 pellet improved the
toluene decomposition [19]. In these reports, the emission of
UV light was confirmed but the UV intensities were not ana-
lyzed. Additionally, net photocatalytic reaction rates were not
evaluated. Ogata et al. reported that the removal rate of fluo-
rocarbons in Ar atmosphere increased when the plasma reactor
was filled with TiO2 pellets as a catalyst [21]. Because signifi-
cant UV emission and temperature increase were not observed
during the plasma operation, they concluded that the catalytic
effect seemed to be derived from the direct activation of the TiO2
surface by the plasma discharge. Thus, photocatalytic effects of
TiO2 in NTP were not well understood. If the photocatalytic
effects can improve the specific energy efficiency for the decom-
position of VOCs by NTP, the contribution and the theoretical
limitation of photocatalytic effects should be discussed. For the
purpose, since UV absorption by TiO2 is essential for photocat-
alytic reactions, the UV intensity emitted from plasma discharge
should be analyzed.
The surface areas of ST-01, P25 and TA200I were 300, 50
and 200 m /g, respectively. Since ␥-Al2O3 has no photocatalytic
activity while it has enough catalytic activities, the effect of ␥-
Al2O3 in plasma was compared with that of TiO2. Pt-deposited
TiO2 (Pt–TiO2) and Pt-deposited ␥-Al2O3 (Pt–Al2O3) were also
used. Pt–TiO2 was prepared by photodeposition method with
2
P25 and H2PtCl as precursors [23]. Pt–Al2O3 was prepared
6
by chemical reduction method with H2PtCl and NaBH4. The
6
amounts of Pt deposited were 1.0 wt.% against TiO2 and Al2O3.
These catalysts were used without further heat-treatment or H2-
reduction.
The catalysts were coated on the reactor walls by the fol-
lowing procedure. A small portion of the catalyst powder was
dispersed into distilled water (20 mg-cat/ml-water), and the sus-
pension was painted on the reactor wall while sending a warm
current of air to fix the catalyst, and finally the reactor was dried
◦
at 110 C for 12 h. The coated lengths were same as the ground
electrode described later.
2.2. Plasma system
The configuration of the basic reactor that generates surface
discharge plasma is illustrated in Fig. 1a. A quartz tube (length
180 mm, outer diameter 12 mm, inner diameter 9 mm) was used
as the dielectric barrier that played a role of the reactor. A spiral
stainless steel wire with a 0.4 mm diameter was set in contact
with the inner wall of the barrier tube (15 turns in 100 mm). A
copper foil was wrapped on the outside of barrier tube as ground
electrode (100 mm length). A high-voltage ac was applied across
the electrodes using a 24 kHz neon transformer (S-light Co. Ltd.:
model alpha-NEON M-5). The input power (plug-in power) was
measured with a digital power meter (Yokogawa Analytical Sys-
tems: model WT210). The surface temperatures of the plasma
reactor were analyzed by an infrared thermography (Nippon
Avionics Co.: TVS-700).
In the present paper, we assembled three types of surface dis-
charge plasma reactors equipped with oxide catalysts, and eval-
uated the contributions of photocatalytic and catalytic effects
on the decomposition of VOCs. The intensity of UV emission
from plasma was analyzed, and the photocatalytic oxidation rate
for acetaldehyde was evaluated. The roles of catalysts, deposited
metals, ozone, and heat were discussed using CO oxidation reac-
tion.
2
. Experimental
2
.1. Preparation of catalysts
Fig. 1b shows the double tube photocatalytic reactor, where
the UV light emitted from plasma was irradiated to the pho-
tocatalyst existing out of plasma region. The three windows
(6 mm × 90 mm) were made on the ground electrode, through
Anatase type of TiO2 (Ishihara Co.: ST-01 and Degssa: P25)
and ␥-Al2O3 (Sumitomo Chemical Co.: TA200I) were used.
Fig. 1. Schematic illustrations of plasma reactors: (a) basic configuration, (b) double tube photocatalytic reactor, (c) plasma enhanced catalytic reactor, and (d)
combined reactors.

T. Sano et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 235–241
237
which the UV light passed. Emission spectra from plasma dis-
charge were measured with an optical spectrograph (Eagle Engi-
neering Co.: LEO/CDI 200–800). TiO2 powder (P25, 36 mg)
2
was coated on the inner wall of the envelope (60 cm ) and any
catalyst was not coated in the barrier tube. In this case, the effect
of photocatalyst would be emphasized since the reactant gas did
not contact with plasma. The reactant gas (100 ppm of acetalde-
hyde diluted in the air) was passed through the space between
3
the inner tube and the envelope at the flow rate of 100 cm /min,
and the concentration of acetaldehyde in the effluent was ana-
lyzed by a gas chromatograph with FID detector (Shimadzu:
GC-14B).
In the analyses of the effects of catalyst in plasma, the catalyst
was coated on the inner wall of the barrier tube together with
the inner electrode (Fig. 1c). The amount of catalyst was 50 mg.
To remove organic impurities on the catalyst surface, plasma
was generated at 5 W while clean air was passed through the
reactor until the evolution of CO2 stopped. The reactant gas
Fig. 2. UV–vis emission spectra of surface discharge plasma observed from
the outside of envelope without catalyst (a) and with TiO2 (b), and the diffuse
reflectance spectrum of TiO2. Air was passed through the barrier tube at a flow
3
rate of 100 cm /min. The input power was 8 W.
(
800 ppm of CO diluted in Ar/O2 stream) was passed through
However, the photocatalytic degradation of AcH by TiO2 was
not observed when 100 ppm of AcH gas was introduced to the
reaction space between the barrier tube and the envelope. Fig. 3a
shows the relation between the AcH conversion and the input
power. AcH conversion was defined as follows:
3
the barrier tube at a flow rate of 100 cm /min. The concentration
of CO and CO2 were analyzed by Fourier transform infrared
spectroscopy (FT-IR, Nicolet: Magna650) with a multireflec-
tion gas cell (path length 2.4 m). The partial pressures of gases
were calculated from the absorbance areas, which were cali-
brated with standard gases. The concentration of evolved O3
was analyzed by an ozone analyzer (Ebara Co.: HARE model
ꢀ
ꢁ
AcH concentration at the reactor exit
supply concentration of AcH
AcH conversion = 1 −
6
20).
Contributions of thermal catalysis in the presence of O3 were
×100
(2)
evaluated with the combined reactors shown in Fig. 1d. The
reactant gas was same as Fig. 1c. O3 was formed in the plasma
reactor, and was passed through the catalytic reactor, whose
temperature was controlled by a thermocouple and an electric
furnace. The dimension of catalytic reactor tube was same as
the barrier tube.
When the input power was 0–3 W, the AcH conversions
were zero. By increasing supplied voltage, the AcH conversions
were increased. The increases in AcH conversions are due to
the plasma and ozone generated at the ground electrode side
(Fig. 3b). It seemed that parts of the edges of windows opened
on the ground electrode acted as discharge electrodes. However,
plasma was mainly generated in the inner electrode side since the
concentration of ozone formed on the ground electrode side was
much lower than that possibly formed in the inner electrode side.
Therefore, it is more suitable to use the ground electrode side for
evaluating photocatalytic activity. The AcH conversions with the
TiO2 coating were identical to that without catalyst at the input
power range analyzed, and no enhancement effect was observed.
This result suggests that the UV intensity of plasma is quite weak
to decompose 100 ppm-level of AcH photocatalytically.
3
. Results and discussion
3
.1. Photocatalytic decomposition rate by UV light
originated with plasma
The photocatalytic decomposition of acetaldehyde (AcH) by
UV radiation from surface discharge plasma was analyzed with
thesetup(b)inFig. 1. N2 gaswaspassedthroughthebarriertube,
and then high voltage was applied to the electrodes. The input
power was controlled to be 5 W. Fig. 2 shows the UV–vis emis-
sion spectra observed from the outside of envelope tube. The
light emissions with the wavelengths between 290 and 400 nm
were observed without TiO2 coating. This UV-emission range
lies within the absorption range of TiO2. When the envelope
with TiO2 coating was used, the UV light was not detected.
This indicates the UV light from plasma was absorbed by TiO2.
Since TiO2 absorbing UV light generally degrades AcH as Eq.
2
When UV light (70 ␮W/cm ) of black lamp bulb was irra-
diated to the TiO2 from the outside of the envelope, 43% of
supplied AcH was decomposed. Since the wavelengths of UV
light irradiated were longer than 300 nm, O3 was not produced.
Alsothetemperatureofthephotocatalystwassimilartotheroom
temperature. Therefore, the decomposition of AcH was due to
only the photocatalytic reaction. The quantum efficiency (inci-
dent photon-to-reaction efficiency) was estimated to 28%. In
this case, the total UV energy irradiated to TiO2 was only 4 mW.
Considering the result of Fig. 3a, the UV intensities derived from
plasma with 3–8 W is estimated to be much smaller than 4 mW.
The intensity of UV light emitted from the surface discharge
plasma was measured by GaN type UV sensor (Fuji Xerox Co.:
(1) [24], the TiO2 coated in the envelope can also degrade AcH
photocatalytically apart from the problem of its efficiency
CH3CHO + 5/2O2 → 2CO2 + 2H2O
(1)

2
38
T. Sano et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 235–241
Fig. 4. Intensities of UV emission from surface discharge plasma. The total flow
3
rate was 100 cm /min.
decomposition were quite high. The energies to decompose ben-
7
7
zene and fluorocarbons are 1 × 10 to 5 × 10 J/mol-VOC in the
surface discharge plasma [15,20]. The required energy for AcH
decomposition may be less. The potential decomposition rate for
AcH by plasma is larger than 20 ␮mol/min at the input power
of 5 W. From these values, the contribution of photocatalysis
induced by UV light of the plasma is estimated to be less than
0.2% against the AcH decomposition by the plasma. Since this
contribution was obtained by assuming the quantum efficiency
of photocatalytic reaction is 100%, the only way to increase
the contribution is to increase the UV emission efficiency from
plasma dramatically.
3.2. Effects of catalysts existing in plasma
The effects of catalysts, ultrafine TiO2 and ␥-Al2O3, in non-
Fig. 3. Relation between the input power and AcH conversion (a), and O3 con-
centration (b), obtained with the double tube photocatalytic reactor. The filled
squares (ꢀ) indicate the values for the experiment without catalyst, and the open
circles (ꢀ) indicate the values with TiO2 (P25) coated in the envelope.
thermal plasma on oxidation reaction were investigated. As a
model reaction, the oxidation of CO into CO2 by surface dis-
charge plasma was analyzed. Effects of deposition of Pt on
TiO2 and Al2O3 were also analyzed since Pt deposited on TiO2
enhances the photocatalytic oxidation of CO [24,25]. The reac-
tion atmosphere was Ar/O2 mixture (50%) containing trace
amount of N2 as illuminant, because plasma in the atmospheric
air (N2/O2) produces HNO3, which could deactivate the oxida-
tion sites of TiO2 [26].
When the surface discharge plasma was generated, the con-
centration of CO decreased and the concentration of CO2
increased. Since the decrease in CO concentration was nearly
same as the increase in CO2 concentration, it is considered that
CO was oxidized into CO2 as Eq. (3)
UV care mate Pro) placed inside of the envelope (Fig. 1b), and
was shown in Fig. 4. The diameter of GaN detector was 1.5 mm.
The UV intensities were analyzed while controlling the input
powerandtheO2 concentrationinthegascurrentthatwaspassed
throughthebarriertube. ThetotalUVintensitywascalculatedby
considering the shapes of the reactor and the window of ground
electrode. UV emission was not detected below 2 W and the
UV intensity increased with the input power above ca. 3 W. The
UV intensity was decreased by increasing the O2 concentration.
This suggests that oxygen molecules act as quenching agent for
N2 excitation state. The total UV intensity in the atmospheric
CO + 1/2O2 → CO2
(3)
2
condition (O2 20%) was only 0.15 mW (ca. 2.5 ␮W/cm on an
average) at the input power of 5 W. The photocatalytic reac-
tion rate at the UV irradiation of 0.15 mW is estimated to be
The oxidation rate was 29 ␮mol/h at the input power of 5 W
without catalyst, and the CO conversion was 23%. The CO con-
version was improved by all of the catalysts analyzed (Fig. 5a).
The enhancement effects by the Al2O3 group were larger than
−2
3
× 10 ␮mol/min, assuming the quantum efficiency is 100%.
On the other hand, the energy efficiencies of plasma for VOCs

T. Sano et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 235–241
239
In the photocatalytic oxidation of CO, the deposition of
Pt particles on TiO2 increased the CO oxidation rate. It was
explained that Pt on TiO2 acted as the active sites on which CO
was chemically adsorbed and was oxidized to CO2 [25]. In the
present paper, the deposition of Pt on TiO2 did not increase the
CO oxidation rate significantly. Therefore, the enhancement of
CO oxidation by TiO2 was not a photocatalytic effect. Also,
the increase in the CO oxidation rate by Al2O3 is not due to
photocatalytic effect because the UV adsorption of Al2O3 is
weak. The conventional catalytic effect of Al2O3 surface or the
direct excitation of Al2O3 by plasma may be responsible for the
enhancement of CO oxidation.
The roles of catalyst and ozone on CO oxidation were fur-
ther analyzed with the combined reactor shown in Fig. 1d. The
thermal reactor composed of a quarts tube and the catalyst was
connected at the downstream of the plasma reactor. No catalyst
was contained in the plasma reactor. The reaction gas contain-
ing CO was passed through the system, and the temperature
of the reactor was controlled by a thermoregulator. When the
plasma was not generated, Al2O3 and TiO2 did not oxidize
◦
CO at the temperatures between 40 and 180 C (Fig. 6a). Only
Fig. 5. CO conversions (a) and O3 concentrations (b) obtained by the plasma
enhanced catalytic reactor.
those by the TiO2 group. Compared with two type of TiO2, ST-
2
0
1 (relative surface area = 300 m /g) showed the higher activity
2
than P25 (50 m /g). This suggests that the surface of catalysts
are responsible for the CO oxidation. Also, since the activity of
Al2O3 (200 m /g) was higher than that of ST-01, the surface of
2
␥
-Al2O3 seems to be effective on the CO oxidation rather than
that of TiO2. The deposition of Pt on TiO2 or Al2O3 did not
change their activity significantly.
TiO2 and Al2O3 coated in the barrier tube suppressed the
concentration of O3 (Fig. 5b). This is due to (i) the inhibition of
O3 production or (ii) the decomposition of produced O3. Since
O3 was formed in the plasma without surface, the later rea-
son seems important. The suppression effects of Al2O3 group
were stronger than those of TiO2 group. Approximately, the
catalyst with stronger O3-suppression effect showed the higher
CO-oxidation activity. The relation between O3 decomposition
and the CO oxidation is discussed later.
Fig. 6. CO conversions by the combined reactors (a) and O3 concentrations in
the outlet gas (b) plotted against the temperature of catalytic reactor. The plasma
reactor was operated at 3 W.

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T. Sano et al. / Journal of Molecular Catalysis A: Chemical 245 (2006) 235–241
plasma discharge and is released from the solid phase [21]. If
the activity of surface oxygen of ␥-Al2O3 is also increased by
plasma, the oxidation of CO and the decomposition of O3 are
possibly enhanced by the activated surface oxygen. To clarify the
behavior of the surface oxygen in plasma, the precise analysis
of temperature and isotope techniques are effective.
4. Conclusion
•
The contribution of photocatalysis induced by UV light from
the surface discharge plasma on the AcH decomposition by
the plasma is estimated to be less than 0.2%.
Fig. 7. Proposed mechanism of the CO oxidation in the presence of Al2O3 and
plasma.
•
The coating of ␥-Al2O3 on the inside wall of plasma reactor
improved the oxidation rate dramatically. The high activity
of Al2O3 is due to the catalytic activity that utilizes O3 and
heat. The direct activation of the surface oxygen by plasma
was also suggested.
To improve energy efficiency of the VOCs decomposition
by plasma method, improvements of catalysts that utilize O3
and lattice oxygen are more important than improvements of
photocatalyst activated by weak UV light derived from the
plasma.
◦
Pt–Al2O3 oxidized CO at the temperatures above 80 C. This
can be ascribed to the catalytic oxidation activity of Pt with
O2. When the plasma was generated at 3 W and O3 was sup-
plied to the catalytic reactor, Pt–Al2O3 and Al2O3 oxidized CO
even at the room temperature. The oxidation activity of TiO2
with O3 was smaller, and CO was not oxidized without catalyst.
These results suggest that CO was oxidized by active species
formed on surface of Al2O3 by the decomposition of O3. In
the experiment using the setup c in Fig. 1, the surface temper-
•
◦
ature of plasma reactor became 48–83 C at the input power
of 3–8 W. These temperatures are high enough to take advan-
tage of the catalytic activity of Al2O3-based catalyst for CO
oxidation.
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