Photocatalytic oxidation of ethylene to CO2 and H2O on ultrafine powdered TiO2 photocatalysts in the presence of O2 and H2O

Article abstract of DOI:10.1006/jcat.1999.2472

The complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O has been achieved on ultrafine powdered TiO₂ photocatalysts and the addition of H₂O was found to enhance the reaction. The details of the photocatalytic reaction have been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C₂H₄ with O₂ into CO₂, CO, and H₂O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C₂H₄. The photo-formed OH species as well as O₂^- and O₃^- anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O.

Full text of DOI:10.1006/jcat.1999.2472

Journal of Catalysis 185, 114–119 (1999)
Article ID jcat.1999.2472, available online at http://www.idealibrary.com on
Photocatalytic Oxidation of Ethylene to CO and H O on Ultrafine
2
2
Powdered TiO Photocatalysts in the Presence of O and H O
2
2
2
1
2
Dal-Rung Park,† Jinlong Zhang, Keita Ikeue, Hiromi Yamashita, and Masakazu Anpo
Department of Applied Chemistry, College of Engineering, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599-8531, Japan;
and †Division of Gas Utilization, KOGAS R&Center, Ansan City, Kyunggi-Do 425-150, Korea
Received October 29, 1998; revised March 5, 1999; accepted March 8, 1999
lytic effectiveness of the fine particles varies greatly with
The complete photocatalytic oxidation of C2H4 with O2 into CO2 their chemical and physical properties as well as their chem-
and H2O has been achieved on ultrafine powdered TiO2 photocata- ical and physical molecular environments.
lysts and the addition of H2O was found to enhance the reaction.
The details of the photocatalytic reaction have been studied by IR,
ESR, and analysis of the reaction products. UV irradiation of the
On the other hand, because C2H4 is harmful to all like
forms, in order to keep enough food fresh in the CELSS
Controlled Ecological Life Support Systems), it is neces-
(
photocatalysts at 275 K led to the photocatalytic oxidation of C2H4
with O2 into CO2, CO, and H2O. The large surface area of the photo-
catalyst is one of the most important factors in achieving a high ef-
ficiency in the photocatalytic oxidation of C2H4. The photo-formed
sary to remove ethylene, which is released from plants. In
this regard, photocatalysts can be considered as they are
generally referred to as an advanced oxidation process.
Along these lines, in the present paper, we investigated
the photocatalytic oxidation of C2H4 with O2 into CO2 and
H2O on ultrafine powdered TiO2 photocatalysts. Special
attention is focused on the effects of the particle size on
the reaction and the effects of the addition of H2O on the
reaction rates by means of in situ ESR and FT-IR measure-
ments. Analyses of the reaction products as functions of
the UV irradiation time were also carried out. Although
we investigated the detailed mechanisms of the photocata-
lytic oxidation of the olefins with O2 on the well-evacuated
dried TiO2 photocatalysts in the past (8), the effect of the
coexistence of H2O on the photocatalytic oxidation has not
been explored until now.
�
�
OH species as well as O2 and O3 anion radicals play a significant
role as a key active species in the complete photocatalytic oxidation
of C2H4 with O2 into CO2 and H2O.
�c 1999 Academic Press
Key Words: photocatalyst; oxidation; ethylene.
INTRODUCTION
Photocatalysis of titanium oxide semiconductors has re-
cently received considerable attention in relation to a va-
riety of applications for environmental issues such as the
purification of polluted air and water (1–3). Especially, the
efficient photocatalytic degradation of hazardous wastes
is one of the most desirable and challenging goals in the
research of the development of environmentally friendly
catalysts. As one of the most popular photoactive catalysts,
TiO2 photocatalysts have long been investigated in their
different forms as bulk materials, colloidal suspensions, and
dispersed into SiO2 matrices and on porous supports such
as zeolites and porous vycor glass (4, 5). Although the uti-
lization of extremely small powdered TiO2 particles has at-
tracted a great deal of attention as photocatalysts due to
their high photocatalytic reactivities, the actual factors con-
trolling the photocatalytic activity of TiO2 particles are still
unknown, except for a few important factors such as the
magnitude of the band gap, surface area, and crystallinity
In the present paper, we report that ethylene can be com-
pletely oxidized into CO2 on the powdered titanium oxide
photocatalysts, the rate depending on the nature of the cata-
lysts, and that the photocatalytic oxidation of C2H4 can be
enhanced by the coexistence of H2O vapor in the reaction
system.
EXPERIMENTAL
Five different types of ultrafine powdered TiO2 catalysts
which were supplied by the Catalysis Society of Japan as ref-
erence TiO2 catalysts (JRC-TIO-1, 2, 3, 4, 5) were used as
photocatalysts (grain size, 0.02� 1 mm). Detailed informa-
tion on these reference TiO2 catalysts is available from the
Catalysis Society of Japan. Three types of photocatalysts
referred to as F-2, F-4, and F-6 were supplied by Shyowa
Denko Company. Photocatalytic reactions were carried
(
6–9). Such difficulties mainly arise because the photocata-
1
Permanent address: Institute of Fine Chemicals, East China Univer-
sity of Science and Technology, Shanghai 200237, P. R. China.
2
3
To whom correspondence should be addressed.
out in a quartz cell (volume: 100 cm ) with a flat bottom
114
0021-9517/99 $30.00
Copyright �c 1999 by Academic Press
All rights of reproduction in any form reserved.

PHOTOCATALYTIC OXIDATION OF C2H4
115
FIG. 1. Product distribution of the photocatalytic oxidation of ethylene on TiO2 photocatalysts: JRC-TIO-1, -TIO-2, -TIO-3, -TIO-4, -TIO-5, F-2,
F-4, and F-6.
connected to a conventional vacuum system (10 6 Torr was calculated from the intensities of the IR peak of the
range). The catalysts were degassed at 723 K for 2 h and samples assigned to the surface OH group on TiO2. The
calcined in O2 at 723 K for 3 h. Prior to the photoreaction, acid concentration was calculated from the peak intensi-
�
�
6
the catalysts were degassed at 473 K for 2 h until 10 Torr. ties observed in the temperature-programmed desorption
The pretreated catalysts were spread out on the flat bottom (TPD) patterns of preadsorbed NH3. The surface area of
of the quartz cell, and then ethylene and oxygen were intro- the titanium oxide powders, F-2, F-4, F-6, are 27, 54, and
2
duced onto the catalyst at 275 K. The catalyst was irradiated 102 m /g, respectively. It was observed that the surface area
from the flat bottom side. UV irradiation of the catalysts in of F-6 is the largest in all of the catalysts.
the presence of ethylene and oxygen was carried out using
UV irradiation of several standard TiO2 catalysts in the
a Toshiba SHL 100 W high-pressure Hg lamp (�ꢀ 280 nm) presence of C2H4 and O2 led to the oxidation of C2H4 to
at 275 K. The reaction time is 2 h. The reaction products produce CO2, CO, and H2O as the main products, but with
were analyzed by gas chromatography. The ESR spectra relative reactivity and selectivity, depending on the specific
were recorded with a JES-RE-2X (X-band) spectrometer properties of the catalysts. Figure 1 shows the product dis-
at 77 K and the IR spectra were recorded on a Shimadzu tribution in the photocatalytic oxidation of ethylene on var-
IR-460 spectrometer at 295 K. Experimental details can be ious TiO2 catalysts.
found in previous literature (6, 8).
The observed photocatalytic reactivities were in the or-
der of JRC-TIO-4 ꢀ -5 ꢀ -1 ꢀ -2 ꢀ -3. So, these results are
in good agreement with those of the reactions investigated,
i.e., the hydrogenolysis of methyl acetylene with H2O, the
RESULTS AND DISCUSSION
The typical physical properties of these TIO-1 � 5 cata- isomerization of 2-butene, and the reduction of CO2 with
lysts are shown in Table 1. The relative OH concentration H2O (9). From Table 1 and Fig. 1, it can be seen that anatase
TiO2 with a large surface area, large band gap, and nu-
merous OH groups is more efficient for the oxidation of
ethylene. Especially, for the TiO2 photocatalyst, F-6, hav-
TABLE 1
Physical Properties of TiO2 Powdered Photocatalysts
ing a large surface area, shows the highest photocatalytic
reactivity. From these results, anatase TiO2, with a large
surface area, a large band, and numerous OH groups, is
preferable for efficient photocatalytic reactions to proceed.
The increased band gap is accompanied by a shift in the
conduction band edge to higher energies. This moves the
reductive potential to more negative values and enhances
the photocatalytic reactivity. It is also often suggested that
the surface OH groups and/or physisorbed H2O plays a
Catalysts
JRC-TIO-)
Surface
area (m /g)
Relative
OH conc.
Band
gap (eV)
2
(
1
. Anatase
. Anatase
. Rutile
. Anatase
. Rutile
73
16
51
49
3
1.3
1.0
1.6
3.0
3.1
3.54
3.47
3.32
3.50
3.09
2
3
4
5

1
16
PARK ET AL.
FIG. 2. Product distribution of the photocatalytic oxidation of ethylene with oxygen TiO2 powder (F-6), (ethylene 7 �mol, oxygen 7 �mol) (a),
ethylene 3 �mol, oxygen 12 �mol) (b), and (ethylene 3 �mol, oxygen 24 �mol) (c).
(
�
+
significant role in the photocatalytic reactions through the irradiated, e –h pairs were formed. In the absence of the
formation of OH radicals (6). electron and hole scavengers, most of them recombined
Figure 2 shows the product distiribution of the photocata- with each other within a few nanoseconds. If the scavengers
lytic oxidation of C2H4 with different amounts of oxygen on or surface defects are present to trap the electron or hole,
�
+
the anatase TiO2 powder (F-6). The yield of CO2 increases an e –h recombinations can be prevented and the sub-
with the amount of added oxygen; however, the yield of sequent reactions caused by the electrons and holes may
CO decreases with the amount of added oxygen. So, the be dramatically enhanced. In this case, ethylene adsorbed
oxygen added is responsible for the product distribution on the surface plays a significant role in the reaction with
�
of CO and CO2. Only when there is excessive oxygen in holes. The surface OH groups may also play an important
the reaction system, it is possible that ethylene is oxidized role in the trapping of the holes. The contribution of hole
�
completely to CO2 and does not produce CO. It is sug- trapping by C2H4 or surface OH strongly depends on their
gested that initially ethylene is oxidized to produce CO and surface concentration. On the other hand, the electron can
then CO continues to oxidize to CO2 by the excessive oxy- be trapped by Ti 4( to give an isolated Ti ion. Details on
+
3+
l)
gen. The mechanism of the reaction can be expressed as the chemical nature of such electrons and holes have been
follows:
noted in previous papers (8, 12). 3
+
In the presence of O2, the Ti sites easily react with
�
�
�
O
+ C2H4(a) → (C2H4O) → CO → CO2.
O2 leading to the formation of O , as shown in stage (c),
(
a)
2
�
while the hole trapping center (O ) reacts with O2 to form
1
�
�
�
Figures 3a and 3b show the ESR signals obtained under UV
O
anion radicals. Therefore, such O2 and O3 and also
3
irradiation of anatase TiO2 (JRC-TIO-4) in the presence of OH radicals may play an important role in the oxidation of
ethylene. These signals are attributed to the characteristic C2H4. Investigations on the detailed mechanisms are now
photogenerated Ti3 ions (g⊥ = 1.975 and gk = 1.963) and in progress.
+
the signals of g = 2.198 and g = 1.856 were assigned to the
In order to understand the effect of H2O adsorption on
H radicals. When the powdered TiO2 photocatalysts were the catalysts on the product yields in the photocatalytic

PHOTOCATALYTIC OXIDATION OF C2H4
117
FIG. 3. ESR spectra obtained by UV irradiation of the JRC-TIO-4 catalysts in the presence of ethylene and oxygen at 77 K (recorded at 77 K).
oxidation of C2H4, different amounts of water vapor were
From Fig. 4, it can be seen that the photocatalytic
added into the well-evacuated catalysts, and their results activities are high with an increase in the amount of water
are shown in Fig. 4. From Fig. 4, it was found that oxidation initially added, while when the amount of the water vapor
reactions producing CO2 are promoted by adding 1 Torr introduced is more than 5 Torr, the photcatalytic activity
water vapor, and when 5 Torr water vapor was added, the gradually decreases. To clarify the mechanism of this
oxidation reactivity was the highest. As the water vapor reaction further, the effect of the addition of water vapor
was added onto the well-evacuated catalysts, the amount of on the photo-induced uptake of oxygen was investigated.
CO was not changed, while the amount of CO2 increased The results are shown in Fig. 5. The addition of water vapor
greatly. This indicates that the addition of water vapor onto onto the photocatalyst leads to an enhancement of the
the catalysts is of benefit in realizing efficient photoreac- photo-induced uptake of oxygen. But, when the amount
tions for the complete oxidation of C2H4. The adsorbing of water vapor introduced exceeds 5 Torr, the uptake of
water is activated on the irradiated TiO2 surface to form oxygen decreases. These results clearly indicate that when
OH species as precursors of radicals. As reported previ- a small amount of water vapor was introduced onto the
ously (11), the OH radicals are reactive for the complete photocatalyst, water and oxygen were adsorbed on the sur-
oxidation of C2H4.
face ofthe photocatalyst and produced reactive OH radicals

1
18
PARK ET AL.
under irradiation. This was shown to enhance the oxidation
reaction of ethylene and the uptake of oxygen, while when
water vapor was introduced in excess the effective adsorp-
tion sites were occupied completely by water molecules.
This decreases the photo-induced uptake of oxygen, and
the OH radicals decrease, also.
A comparative study of Fig. 4 and Fig. 5 shows that the
photocatalytic reactivity of JRC-TIO-4 at 5, 9, and 16 Torr
of water is almost similar, and in the case of 1 Torr (Fig. 4),
it is higher, but the photoreactivity for oxygen uptake is re-
markablydifferent amongthese systems(Fig. 5). It isknown
that the adsorption of water on the TiO2 surface causes a
decrease in the upward bending of the band (12), resulting
in the efficient recombination of the photo-generated elec-
trons and holes. On the other hand, the adsorption of H2O
results in the formation of surface OH groups by the reac-
tion of H2O with the bridging-oxygen atoms (13). These
two factors may play an important role in determining
the photocatalytic reactivity of TiO2 and the contribution
of these factors strongly depends on the amount of H2O
added. When 1 Torr of water was introduced, the oxygen
FIG. 4. The effect of the addition of H2O on the yields of photoprod-
ucts in the photocatalytic oxidation of ethylene on JRC-TIO-4 catalysts.
FIG. 5. The effect of the addition of water vapor on the photo-induced uptake of oxygen at 275 K. (After 4 Torr of oxygen was introduced, 2 Torr
of ethylene was added, and then varying amounts of water were added.)

PHOTOCATALYTIC OXIDATION OF C2H4
119
time. These results clearly suggest that adsorbing water on
the surface of TiO2 does indeed produce OH species as well
as OH radicals via the OH species under UV irradiation.
CONCLUSIONS
Evidently, the presence of C2H4 and oxygen makes it pos-
sible to produce CO2 and CO when the TiO2 photocatalysts
are irradiated by UV light (�ꢀ 280 nm). It has been clar-
ified that ethylene can be completely oxidized to produce
only CO2 if titanium oxide catalysts of large surface area
are used and oxygen is introduced onto photocatalysts in
excess. The addition of a constant amount of water vapor
leads to an increase in the yield of CO2, while it has no ef-
fect on the yield of CO. It was confirmed that the adsorbed
H2O and O2 on the photocatalyst activate to generate OH
�
�
radicals and O2 and O , respectively, when the photocata-
3
lysts are irradiated. It can be seen that the OH radicals and
�
�
O
and O3 play significant roles as the key active species
2
in the complete photocatalytic oxidation of ethylene.
FIG. 6. Infrared spectra of the TiO2 photocatalyst at 295 K: (a) after
pretreatment at 450 C in O2 and introduction of C2H4 onto the catalyst,
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�
(
b) after UV irradiation of the catalyst (a) for 30 min.
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2
3
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�
at 450 C in O2 and the introduction of C2H4 onto the cata-
1
1
1
1
�
1
lyst. The peak at 3640 � 3680 cm was attributed to H2O.
Figure 6b shows the IR spectrum after UV irradiation of
the catalysts. From Fig. 6, it can be seen that the peak at
�
1
3
738 cm attributed to the adsorbed OH species increases
in intensity with increasing irradiation time, while the peak
of water decreases in intensity with increasing irradiation
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