st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Development of photocatalytic reactor having light source
inside by electrical discharge
S. Kimura, S. Kodama and H. Sekiguchi
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
Abstract: A new photocatalytic reactor was developed in which light sources emitting UV
light by electrical discharge were placed and coated with TiO2, photocatalyst. The activity of
the reactor was evaluated by the degradation of 2-propanol. The experimental result showed
that 2-propanol was successfully decomposed, suggesting the proposed reactor utilized
photocatalyst effectively.
Keywords: Photocatalysis, Titanium dioxide (TiO2), High-frequency plasma
1.
INTRODUCTION
2.
Titanium dioxide (TiO2) shows catalytic activity by
irradiating ultraviolet (UV) light onto its surface and is
the most common photocatalyst because of its low cost,
high stability and environmental friendly features [1].
Photocatalysts are useful for environmental issues and
possible to decompose organic compounds into harmless
substances such as carbon dioxide and water [2].
Generally, photocatalytic reactor consists of TiO2
supported pellets packed in a transparent tube [3]. The
catalyst is activated by irradiating UV light from the outside of the tube. When the catalyst is packed densely,
most of catalysts are not irradiated by UV light, resulting
in the insufficient utilization of the catalysts. To solve the
problem, we proposed a new reactor having UV light
source inside. By coating the surface of light source with
TiO2, effective irradiation to TiO2 was expected in the
reactor. Usually UV light is generated by electrical discharge, however, normal electrical discharge with electrodes has several problems such as the limitation of size
and shape of light source due to the electrodes and the
necessity of replacement and maintenance due to the wear
of the electrodes. When we use electrodeless lamp, these
disadvantages are eliminated. It is reported that the
electrodeless lamp can be miniaturized and shows significantly longer life time compared with normal lamp with
electrodes [4].
In this study, a new photocatalytic reactor was developed. The reactor had light sources its inside generated by
electrical high-frequency discharge. The surface of the
light source was coated with TiO2. 2-propanol was used as
the model decomposition substance for evaluating the
activity of photocatalyst in the reactor. 2-propanol is decomposed into acetone by the photocatalysis as shown in
equation (1)
(CH3)2CHOH + 1/2O2 → (CH3)2CO + H2O
(1)
EXPERIMENTAL
Experimental apparatus used in this study is shown in
Fig.1. It consists of reactor, gas cylinders of argon and air,
pressure gauge, vacuum pump, stabilized DC power supply (TEXIO, PA120-0.6B), neon transformer (LECIP SLP
CORPORATION, αNEON M-5), voltage probe and oscilloscope (Tektronix, TDS1012B).
Fig.2 shows the detailed scheme of the reactor. This is
a double tube reactor. In order to increase the surface area,
three quartz glass tubes coated with TiO2 (inner tubes)
were placed in an acrylic tube (outer tube). The inner
tubes were vacuumed, and argon gas was introduced. The
outer tube had a sampling port for injecting 2-propanol
and collecting the sample for gas analysis. High voltage
electrodes and ground electrodes were put on the outer
tube in alternating to generate the discharge widely along
the tubes.
Table 1 shows the experimental conditions. Inside
diameter of inner tube was 1.0mm and the distance of
each electrode was 15mm. The effect of applied voltage,
inner tube pressure, amount of supported TiO2 and initial
concentration of 2-propanol were evaluated. The values of
standard conditions are indicated by bold font and underlined.
After enough air was introduced in the outer tube, the
valves located upstream and downstream of the reactor
were closed, thus the reactor was kept in a sealed. Then
2-propanol was injected through the sampling port. After
waiting for 150 minutes to reach the adsorption equilibrium, high voltage was applied. As a result, plasma was
generated in the inner tube and photocatalytic degradation
of 2-propanol started.
The sample was analyzed by a gas chromatograph
(SHIMADZU GC-14B) with flame ionization detection
(FID) equipped with a capillary column, TC-WAX(GL
Sciences Inc.). Emission spectrum from the plasma was
measured by spectroscope (HR2000, Ocean Optics Inc.).
Commercial TiO2 photocatalyst powder (Degussa
P25) was used as photocatalyst. TiO2 was coated on the
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
quartz tubes by the following dip coating technique:
a. preparing a dip solution by adding 21g of TiO2 in
420mL of ethanol.
b. quartz tubes were washed by ethanol and then dried.
c. the tubes whose both ends sealed were immersed in
the dip solution and pulled up.
d. dry the tubes at 60°C for 10 minutes in a dryer.
e. the processes c and d were repeated until the amount
of TiO2 on the tube reached the predetermined
amount. finally, the tubes were dried at 150 °C for 1
hour.
Fig.1
3.1 Effect of initial concentration of 2-propanol
Fig.3 shows the effect of initial concentration of
2-propanol. The black plots of Fig.3 show the time courses of concentrations of 2-propanol and acetone, in the
standard conditions. On the surface of photocatalyst, the
following mechanisms can be considered: first,
2-propanol was adsorbed followed by the photocatalytic
decomposition of 2-propanol to acetone. Acetone produced was desorbed from the catalyst surface. The decomposition reaction of 2-propanol can be approximated
with first-order reaction and the concentration of
2-propanol decays exponentially as shown in Fig.3. Acetone concentration showed a peak, indicating acetone was
also decomposed by the catalyst. Because 2-propanol was
initially adsorbed on the catalyst surface before the plasma ignition, the maximum concentration of acetone exceeds the initial concentration of 2-propanol.
When the experiment was performed by raising the
initial concentration to 900ppm, it was observed that the
generation rate of acetone became faster as shown in
Fig.3. In general, the generation rate of acetone is proportional to the initial concentration. However despite the
doubled of initial concentration, the generation rate increased a little, implying that the number of active sites
were limited.
Experimental apparatus
2-propanol(480ppm)
2-propanol(900ppm)
Acetone(480ppm)
Acetone(900ppm)
Concentration [ppm]
1000
Fig.2
500
Detailed scheme of reactor
0
Table 1 Experimental conditions
Inside diameter of inner tube [mm]
1.0
Electrode distance [mm]
15
Frequency [kHz]
16.2
Voltage [kV]
3.0, 4.0, 5.0
Inner tube pressure [Torr]
4.0, 12.0
Amount of TiO2 [mg]
2.0, 8.3
Initial concentration [ppm]
480, 900
3. RESULT AND DISCUSSION
50
100
Irradiation time [min]
Fig.3
Effect of initial concentration on
photocatalytic activity
(TiO2: 2.0 mg , Voltage: 5.0 kV ,
Inner tube pressure: 4.0 Torr)
3.2 Effect of the amount of TiO2
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Effect of applied voltage on the change in the concentration is shown in Fig.5. The degradation rate of
2-propanol and the generation rate of acetone became
faster as the applied voltage was increased. Fig.6 shows
the effect of inner tube pressure. From this figure, the
reaction rates increased when the pressure decreased.
It was observed that increasing applied voltage and reducing pressure of inner tubes brightened the plasma.
1.2
1
C/C0 [-]
Fig.4 shows the effect of amount of catalyst. The vertical axis shows the mole concentration normalized by the
initial concentration of 2-propanol. It was observed that
both the degradation rate of 2-propanol and the generation
rate of acetone were decreased when the amount of catalyst increased. It was because the light which reached the
reaction surface of catalyst became weaker due to increasing the thickness of the catalyst. When the amount of
catalyst increased, the initial amount of 2-propanol adsorbed increased, leading to higher peak of acetone concentration.
In general, if the amount of catalyst increases, reaction
sites increase, resulting in the acceleration of reaction rate.
However, the reaction occurred on the outside surface of
catalyst while the catalyst was irradiated from the inside
of the catalyst. Hence when the catalyst was too thick, the
activity of the catalyst became weak. Experiments with
two different amounts of catalyst were only done in this
study, however considering the balance between increasing reaction sites and decreasing brightness at the surface,
an optimum value of the amount of catalyst will exist for
enhancing the reaction.
0.8
0.6
0.4
0.2
0
50
100
150
Irradiation time [min]
1.2
Fig.5 Effect of Voltage on concentration-changes
(TiO2: 2.0 mg , Inner tube pressure: 4.0 Torr ,
Initial concentration of 2-propanol: 480 ppm)
1
C/C0 [-]
2-propanol(5.0kV)
2-propanol(4.0kV)
2-propanol(3.0kV)
Acetone(5.0kV)
Acetone(4.0kV)
Acetone(3.0kV)
0.8
2-propanol(TiO 2:2.0mg)
2-propanol(TiO 2:8.3mg)
Acetone(TiO2:2.0mg)
Acetone(TiO2:8.3mg)
0.6
0.4
1.2
0.2
1
50
100
150
C/C0 [-]
0
Irradiation time [min]
Fig.4
Effect of amount of TiO2 on
photocatalytic activity
(Voltage: 5.0 kV , Inner tube pressure: 4.0 Torr ,
Initial concentration of 2-propanol: 480 ppm)
0.8
2-propanol(4.0Torr)
2-propanol(12.0Torr)
Acetone(4.0Torr)
Acetone(12.0Torr)
0.6
0.4
0.2
0
50
100
Irradiation time [min]
Fig.6
3.3
Effect of applied voltage and inner tube pressure
3.4
Effect of inner tube pressure on
photocatalytic activity
(TiO2: 2.0 mg , Voltage: 5.0 kV ,
Initial concentration of 2-propanol: 480 ppm)
Emission intensity of discharge
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
In order to verify the relationship between brightness
of plasma and reaction rate as mentioned before, the
emission intensity of discharge at each condition was
measured (Fig.7). Graphs on the leftmost were the intensity of discharge in the standard conditions. The conditions changed from the standard conditions were shown as
bold numbers. It showed the intensities decreased when
the catalyst was coated on the inner tube.
In the conditions of higher applied voltage or lower
inner tube pressure, the intensities of discharge became
higher. Furthermore, when the amount of catalyst increased, intensity at the surface became weak because
light was blocked by the catalyst owing to the thickness of
catalyst.
did not show the same trend; the apparent reaction rate
constant became smaller. It may be because that more
catalyst coated increases initial amount of 2-propanol
adsorbed.
In this study, three thin tubes were used as light
sources. It is possible to increase the reaction rate by increasing the surface area of light sources and
photocatalyst. Hence it is expected to enhance the reaction by miniaturizing the light sources such as beads
packed in the reactor.
4.
CONCLUSION
Photocatalytic reactor having light source inside generated by electrical discharge was developed and
2-propanol was successfully decomposed in this reactor. It
was observed that the degradation rate of 2-propanol became faster as the emission intensity of discharge was
high. As the catalyst became thick, the light was blocked
while the active sites increased. It implied that the optimum amount of photocatalyst for using light effectively
exists. If the surface area of light sources and catalyst
increase by miniaturizing the light sources such as glass
beads, it is expected to obtain better performance of
photocatalysis.
REFERENCES
Fig.7 Emission intensity at the surface of
glass tube with and without TiO2 coating
Apparent reaction rate constant [min-1]
Fig.8 shows the effects of emission intensity and the
amount of catalyst on apparent reaction rate constants of
2-propanol degradation. When the amount of catalyst was
2.0 mg, the apparent reaction rate constants increased
with increasing intensities. Therefore it was clear that
higher intensity promoted photocatalytic reaction. However when the amount of catalyst increased to 8.3 mg, it
0.04
TiO 2:2.0mg
TiO 2:8.3mg
0.03
0.02
0.01
0
20
40
60
80
100
Intensity [a.u.]
Fig.8
Effect of intensity on the apparent reaction rate
constant
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