Improvement of catalyst durability in benzene photooxidation by rhodium deposition on TiO2

Article abstract of DOI:10.1246/cl.2001.582

Photocatalytic oxidation of benzene in air was carried out over TiO₂ and metal-loaded TiO₂ catalysts at room temperature. Deposition of Rh on TiO₂ improved the catalyst durability by inhibiting the formation of carbon depo

Full text of DOI:10.1246/cl.2001.582

582
Chemistry Letters 2001
Improvement of Catalyst Durability in Benzene Photooxidation by Rhodium Deposition on TiO2
Hisahiro Einaga,* Shigeru Futamura, and Takashi Ibusuki
National Institute of Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569
(Received March 21, 2001; CL-010247)
Photocatalytic oxidation of benzene in air was carried out
over TiO2 and metal-loaded TiO2 catalysts at room temperature.
Deposition of Rh on TiO2 improved the catalyst durability by
inhibiting the formation of carbon deposits on the catalyst sur-
face and accelerating their decomposition.
slurry by the washcoat techniques and the rod was then dried at
383 K. Concentrations of CO2 and CO were determined with a
gas chromatograph equipped with a TCD, a FID and a methane
converter. Benzene was determined with a GC. As a pretreat-
ment, the catalyst was irradiated in humid air.
Figure 1 shows the time courses for the benzene photooxi-
dation over TiO2 and Rh/TiO2 catalysts (benzene 250 ppm in
air, relative humidity of 50%). No reaction took place in the
dark. The reaction profiles were taken after the adsorption equi-
librium was achieved between the gas phase and the catalyst
surface in the photoreactor. In the initial period, benzene con-
version for pure TiO2 was about 70%. However, it rapidly
decreased with time on stream, and reached a stationary value
(13%) after about 180 min.
When Rh-deposited TiO2 catalyst was used for the reac-
tion, the conversion in the initial period was comparable with
that for pure TiO2. Although the conversion decreased with
time, the decrement level was much smaller than that for the
pure TiO2. After about 10 h on stream, the conversion reached
the stationary state and was about 3 times larger than that for
pure TiO2. Here, total amount of benzene reacted was estimated
to be 3.0 × 10–4 mol. Turnover number for the benzene decom-
position was estimated by taking the total amount of benzene
reacted and normalized by the Ti density of the single crystal
Air pollution with volatile organic compounds (VOCs) is
one of the serious environmental problems. Hitherto, heteroge-
neous photocatalytic oxidation systems using TiO2 catalysts
have been extensively studied for the VOC removal in gas
phase.1 In this system, irradiation of semiconductors with UV or
near-UV light generates highly reactive electron-hole pairs and
initiates redox reactions decomposing the VOCs. The photocat-
alytic system has an advantage over other systems such as the
thermal oxidation and catalytic incineration in that it can effi-
ciently decompose the low concentration of VOCs under mild
conditions without additive fuel. It has been reported that so
many kinds of organic compounds including hydrocarbons,
alcohols, halocarbons and amines decompose on the illuminat-
ed TiO2.2–6 In many cases, however, deactivation of TiO2 has
been generally observed during the photoreaction.6 Therefore,
the mechanism for the catalyst deactivation should be elucidat-
ed to practically utilize photocatalytic systems.
Benzene is an important feedstock and widely used.
However, it is required to remove benzene from the flue gases
from various sources, due to its carcinogenicity. We have car-
ried out the photooxidation of dilute benzene with TiO2 in air
(80 – 250 ppm).7,8 We showed that benzene decomposition was
greatly suppressed with increasing the benzene concentration,
due to the increasing amount of carbon deposits on the used cat-
alyst.7 We herein report that deposition of Rh on TiO2 depress-
es the catalyst deactivation in the benzene photooxidation, by
inhibiting the formation of carbon deposits on the catalyst and
accelerating their decomposition.
TiO2 P-25 was used as the catalyst and the precursor. The
surface area of TiO2 was 43 (m2/g). Rhodium-deposited TiO2
was prepared by the photodeposition method. TiO2 powder was
dispersed into an ethanol–water solution containing RhCl3·3H2O
in a Pyrex vessel with vigorously stirring. N2 gas was bubbled
into the suspension to remove the dissolved O2. The suspension
was irradiated with a 500-W high-pressure Hg lamp for 2 h.
After the irradiation, the resultant Rh-deposited TiO2 powder
was filtered off, washed with water, and dried at 110 °C.
Reactions were carried out with a flow reactor. The reactor
used in this study was composed of an inner rod (8 mm outer
diameter, 500 mm length) and an outer tube (13 mm inner
diameter), which were fabricated from Pyrex glass. Catalysts
were coated on the surface of the inner rod by wash coat tech-
niques. It was surrounded by four 20-W black light bulbs. The
reaction temperature was maintained approximately at 303 K.
The catalyst was coated onto the inner rod from an aqueous
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
583
surfaces of anatase (1.90–7.00 atoms/nm2)9 and the amount of
Rh loaded (0.5 wt%). This led to 2.5–9.2 molecules per a Ti site
and 25 molecules per a Rh site, showing that the reaction cat-
alytically proceeded.
Benzene was oxidized to CO2 and CO. Figure 1(b) shows
the time courses for the selectivities to CO2 and CO, and the
carbon balance in the benzene photooxidation with Rh/TiO2.
The selectivities to CO2 and CO were 94 and 6%, respectively.
They were almost unchanged with time. The carbon balance
was imperfect at the initial time. However, it reached the steady
value in the range of 97–103% after about 4 h.
Table 1 summarizes the results of benzene photooxidation
over metal loaded TiO2 catalysts. The conversions were meas-
ured at the stationary state. Among these catalysts, Rh/TiO2
was most effective for the reaction. The rate with Rh/TiO2 was
about 3 times higher than that with TiO2. Deposition of Ir on
TiO2 had the negative effect on the reaction. On the other hand,
deposition of Pt had almost no effect.
surface. TiO2 was significantly browned by their formation dur-
ing the reaction. The brownish carbon deposits decomposed to
COx and the catalyst color was recovered to white by the irradi-
ation in humid air without benzene. Figure 2 shows the forma-
tion behavior of COx in decomposing the carbon deposits on the
used TiO2 catalyst. There was an induction period for the COx
formation, and it decreased with time. According to the time
course plot, the amount of carbon deposits was estimated to be
241 µmol per 1.0 g catalyst. This corresponds to 2.0–7.4 mole-
cules of C atom per a surface Ti site.
Figure 2 also shows the formation behavior of COx from
the carbon deposits on the used Rh/TiO2. The amount of carbon
deposits was estimated to be 134 µmol per 1.0 g catalyst, which
was approximately 0.5 times smaller than that for the pure
TiO2. In addition, induction period was not observed for the
COx formation and the initial rate of the COx formation for
Rh/TiO2 was higher than that for TiO2. These findings imply
that deposition of Rh on TiO2 inhibited the formation of carbon
deposits on the catalyst surface and accelerated their decompo-
sition, which improved the catalyst durability of TiO2 in the
benzene photooxidation.
Deactivation of TiO2 during the benzene photooxidation
was ascribed to the formation of carbon deposits on the catalyst
This work was partly supported by Industrial Technology
Research Grant Program from the New Energy and Industrial
Technology Development Organization of Japan.
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