Photocatalytic degradation of gaseous trichloroethylene on porous titanium dioxide pellets modified with copper(II) under visible light irradiation

Article abstract of DOI:10.1016/j.jphotochem.2019.03.037

Porous titanium dioxide pellets modified with copper(II) ion (Cu-TiO₂) were synthesized by sol-gel method with dialysis for photocatalytic degradation of gaseous trichloroethylene (TCE) under visible light (VL) irradiation. TCE was completely degraded by passing the gas stream (mole fractions of oxygen and TCE were 0.2 and 1.75 × 10^?4, respectively) at the flow rate of 25 mL min^?1 through 0.2 g of the Cu-TiO₂ pellets (Cu content: 0.1 atom%) calcined at 200 °C. TCE was converted mainly to carbon dioxide, dichloroacetic acid (DCAA), and inorganic chlorine species. Relatively small quantities of pentachloroethane (PCA) and trichloroacetaldehyde (TCAH) were detected as products on the Cu-TiO₂ surface. Comparison with porous TiO₂ pellets under ultraviolet irradiation revealed that more chlorinated products and less carbon dioxide were formed on Cu-TiO₂ under VL irradiation. The mineralization of TCE to carbon dioxide was calculated to be only ca. 30.0%. It is noted that DCAA, PCA and TCAH were accumulated on the surface and were extracted with ethyl acetate. The porous Cu-TiO₂ pellets show promise as the photocatalyst acting under VL irradiation for converting TCE gas to chlorinated compounds which can be used in industries.

Full text of DOI:10.1016/j.jphotochem.2019.03.037

Accepted Manuscript
Title: Photocatalytic Degradation of Gaseous
Trichloroethylene on Porous Titanium Dioxide Pellets
Modified with Copper(II) under Visible Light Irradiation
Authors: Keigo Tashiro, Toshifumi Tanimura, Suzuko
Yamazaki
PII:
S1010-6030(19)30189-3
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.03.037
JPC 11774
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
31 January 2019
13 March 2019
24 March 2019
Please cite this article as: Tashiro K, Tanimura T, Yamazaki S,
Photocatalytic Degradation of Gaseous Trichloroethylene on Porous Titanium
Dioxide Pellets Modified with Copper(II) under Visible Light Irradiation,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.03.037
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Photocatalytic
Degradation
of
Gaseous
Trichloroethylene on Porous Titanium Dioxide
Pellets Modified with Copper(II) under Visible Light
Irradiation
Keigo Tashiro, Toshifumi Tanimura, and Suzuko Yamazaki*
Division of Earth Science, Biology, and Chemistry, Graduate School of Sciences and Technology
for Innovation, Yamaguchi University, Yamaguchi 753-8512, Japan
Corresponding Author
*E-mail: yamazaki@yamaguchi-u.ac.jp.
Tel. +81 83-933-5763.
ORCID
Suzuko Yamazaki: 0000-0002-9440-1213
1

Graphical Abstract
Highlights
 Photocatalyst pellets suitable for the treatment of gaseous pollutants are synthesized.
 Porous titanium dioxide pellets modified with Cu(II) ion can degrade gaseous
trichloroethylene under visible light irradiation.
 Mineralization of trichloroethylene to carbon dioxide is suppressed and chlorinated by-
products accumulated on the surface can be extracted with ethyl acetate.
 The synthesized photocatalyst can be used for converting gaseous organic pollutants to
useful materials under visible light irradiation.
2

Abstract
Porous titanium dioxide pellets modified with copper(II) ion (Cu-TiO2) were synthesized by
sol-gel method with dialysis for photocatalytic degradation of gaseous trichloroethylene (TCE)
under visible light (VL) irradiation. TCE was completely degraded by passing the gas stream (mole
fractions of oxygen and TCE were 0.2 and 1.75 × 10−4, respectively) at the flow rate of 25 mL
min−1 through 0.2 g of the Cu-TiO2 pellets (Cu content: 0.1 atom%) calcined at 200oC. TCE was
converted mainly to carbon dioxide, dichloroacetic acid (DCAA), and inorganic chlorine species.
Relatively small quantities of pentachloroethane (PCA) and trichloroacetaldehyde (TCAH) were
detected as products on the Cu-TiO2 surface. Comparison with porous TiO2 pellets under
ultraviolet irradiation revealed that more chlorinated products and less carbon dioxide were formed
on Cu-TiO2 under VL irradiation. The mineralization of TCE to carbon dioxide was calculated to
be only ca. 30.0%. It is noted that DCAA, PCA and TCAH were accumulated on the surface and
were extracted with ethyl acetate. The porous Cu-TiO2 pellets show promise as the photocatalyst
acting under VL irradiation for converting TCE gas to chlorinated compounds which can be used
in industries.
Keywords: photocatalyst, TiO2 pellets modified with Cu2+, visible light, trichloroethylene, gas
phase
3

1. Introduction
Volatile chlorinated organic compounds (VCOC) such as trichloroethylene (TCE) have been
widely used as solvent in industries, especially as degreaser for metal before the 1990s [1,2]. Many
soil and groundwater supplies have been contaminated with VCOC as a result of leaking
underground storage tanks and improper disposal practices. They are toxic or carcinogenic, e.g.
TCE has been classified as Group 1, carcinogenic to humans, based on the re-evaluation by the
International Agency for Research on Cancer in 2012 [2]. Dibble and Raupp first reported the
degradation of TCE on anatase TiO2 powder in the gas phase under ultraviolet (UV) irradiation in
1990 [3]. Since then, many studies have been conducted to understand the reaction mechanism
and to improve the photocatalytic activity [4-15]. Because the powder is not appropriate to treat
with gaseous pollutants, Anderson and his research group synthesized porous TiO2 pellets by sol-
gel method and constructed annular reactor packed with them for the field demonstration to
degrade gaseous VCOC collected from the contaminated soil [7]. However, the drawback of the
remediation technology based on TiO2 photocatalyst is the use of high-power UV source to
degrade high concentration of VCOC and the formation of undesirable chlorinated byproducts.
We demonstrated that the chlorinated byproducts such as chloroform, phosgene, carbon
tetrachloride, and chloroacetates were formed by Cl radical initiated reaction during the
degradation of chloroethylene, TCE, and tetrachloroethylene on the TiO2 pellets in the continuous
flow reactor [8]. The formation of chloroform and phosgene from the degradation of the gaseous
TCE was suppressed by mixing Cu or Ca(OH)2 powder with TiO2 powder [9]. Even now, the
degradation of gaseous TCE under UV irradiation has been performed in batch or continuous flow
photoreactor by using hybrid composites of TiO2 with zeolites and TiO2 coating films doped with
4

Cu(II) or Fe2O3 [10-13]. Ndong et al. synthesized Cu doped anatase TiO2 nanosheets by a
hydrothermal method for the degradation of VCOC in water [14]. On the other hand, visible light
(VL) responsive TiO2 has attracted much attention from the viewpoint of utilizing solar energy
[16-29]. Previously, we synthesized TiO2 doped with Pt(IV) or Cr(III) ion by the sol-gel method
and demonstrated its high photocatalytic activity for the degradation of 4-chlorophenol in aqueous
solution under VL irradiation [30-32]. Moreover, we used TiO2 doped with Pt(IV) for the
degradation of TCE in the gas phase under VL irradiation to study the reaction mechanism [33].
However, these VL-responsive TiO2 were powder. In this study, porous TiO2 pellets modified with
Cu(II) (Cu-TiO2) were synthesized by the sol-gel method and the photocatalytic degradation of
gaseous TCE was carried out in the gas flowing system under VL irradiation.
2. Experimental
2.1 Synthesis of Cu-TiO2 pellets
Cu-TiO2 pellets were synthesized by the sol-gel method. Fifteen mL of titanium
tetraisopropoxide (TTIP) was added dropwise into 181.3 mL of water containing 1.3 mL of nitric
acid under stirring at 700 rpm. The mixture was peptized for 6 days in the dark and the obtained
TiO2 sol was dialyzed in 2750 mL of water using dialysis tubing (molecular weight cut off: 3500)
for 2 days. Water was changed once a day. Appropriate quantities of copper(II) nitrate trihydrate
(Cu(NO3)2 ∙ 3H2O) were added to the TiO2 sol and the dialysis was performed for 1 day. The
prepared Cu-TiO2 sol was dried at 40oC and the obtained solid was cut into pellets whose diameter
was 0.5 - 1.0 mm. Cu-TiO2 pellets were calcined by using the ramping rate of 3oC min−1 and
keeping at 100 - 600oC for 2 hours. Concentration of Cu(II) ion which came out of the TiO2 sol
5

into water during the dialysis was measured by inductively coupled plasma optical emission
spectroscopy (Varian, ICP-OES Liberty Series II) and was used for the calculation of Cu(II)
content in the finally obtained pellets. TiO2 pellets were also synthesized by the same method
without addition of Cu precursor. Water used in this study was prepared with Milli-Q water system.
For comparison, Cu-grafted TiO2 powder was prepared according to the method reported by Irie
et al. [34]. One gram of TiO2 powder (TAYCA MT-150A) was suspended in aqueous solution (20
mL) containing 0.0054 g of CuCl2 ∙ 2H2O at ca. 90oC under stirring for 1 h. After filtration, the
obtained powder was washed with water thoroughly and dried at 40oC. The Cu(II) amounts grafted
on TiO2 were estimated by the ICP-OES measurements of the Cu(II) concentration in the filtrate.
2.2 Characterization
X-ray diffraction (XRD, Rigaku miniflex 600) analysis was carried out with Cu Kα radiation
(40 kV, 15 mA) at 2 theta angles from 10o to 90o with scan speed of 10o min−1. The Brunauer-
Emmett-Teller (BET) surface area was measured by nitrogen adsorption-desorption method using
automatic surface area analyzer (Shimadzu, Trister II 3020). The surface of Cu-TiO2 was observed
by scanning electron microscope (SEM, JSM-6360LA, JEOL) with energy-dispersive X-ray
spectroscopy (EDS). Diffuse reflectance spectra of synthesized Cu-TiO2 pellets were measured
with UV-vis spectrophotometer (JASCO, V-670).
2.3 Photocatalytic degradation of TCE
Photocatalytic degradation experiments were performed in packed bed tubular reactor (Pyrex,
11.5 cm long, 0.24 cm of inner diameter, 0.32 cm of outer diameter) in gas flowing system.
6

Reactant gas stream was prepared by mixing gases provided from compressed gas cylinders, TCE
(500 ppmv, balance nitrogen, Sumitomo Seika Chemical Co.), nitrogen (> 99.9 %, Taiyo Nissan
Co.) and oxygen (> 99.9 %, Iwatani Industrial Co.). Humid air was prepared by bubbling the mixed
gas of nitrogen and oxygen through the saturator containing water. All experiments except for
those examining the effect of mole fraction of water vapor were performed with dry air. The
photocatalyst pellets were packed in the reactor and the both ends were plugged with glass wool
to hold them in place. After the reactant gas was passed through the pellets at the flow rate of 25 -
150 mL min−1 for 180 min to achieve adsorption-desorption equilibrium in the dark, the pellets
were irradiated by VL for 300 min using 300 W Xenon lamp (Asahi Bunko, MAX-303) equipped
with 422 nm cut-off filter. The light intensity was measured to be 72 mW cm−2 by using a power
meter (Satoh Shouji, SPM-SD). Concentrations of TCE and CO2 in the gas stream were analyzed
by gas chromatography (Hewlett-Packard 5890 Series II equipped with a Porapak R column using
thermal conductivity and flame ionization detectors). Conversion of TCE was calculated by the
following equation.
[TCE]0 − [TCE]t
( )
Conversion % =
×100
[TCE]0
where [TCE]0 and [TCE]t denoted the TCE concentration just before and after the irradiation for t
min, respectively. To identify the gaseous products, gas chromatography/mass spectrometry
(GC/MS, Hewlett Packard 5890/5971) analysis was performed by using Aquatic column (60 m ×
0.25 mm). The products accumulated on the pellets were extracted with ethyl acetate and then
were analyzed by GC/MS (Agilent Technologies, 7890A/5975C) using DB-5MS column (30 m ×
0.25 mm). The ionic species which were dissolved into water from the pellets used for the
experiments were analyzed by ion chromatography (Shimadzu, PIA-1000). For comparison of the
7

reaction products, some experiments were conducted under UV irradiation by using four 4 W
fluorescent black light bulbs (Toshiba, FL 4BLB).
2.4 Detection of hydroxyl radical
Fluorescence method using coumarin was carried out to detect OH radical [35]. The
photocatalyst powder (1.0 g) which was obtained by grinding the synthesized pellets was
suspended in 500 mL of 1.0 × 10−5 mol dm−3 coumarin aqueous solution and was purged with air.
The suspension was illuminated by VL using 300 W xenon lamp with 422 nm cut-off filter or UV
light using eight 4 W fluorescent black light bulbs. The suspension (5 mL) was taken at appropriate
times and then centrifuged at 1200 rpm for 15 min. Fluorescence spectra of the supernatant were
measured by using spectrofluorometer (JASCO, EP-8200) by excitation at 322 nm.
3. Results and Discussion
3.1. Preparation of Cu-TiO2 pellets
During dialysis, we have found that 40 - 65% of Cu(II) ion used for the synthesis is diffused out
of the dialysis tubing containing TiO2 sol. Table S1 lists moles of Cu(NO3)2 added and those of
Cu(II) ion lost during the dialysis to synthesize 0.1 - 3.5 atom% Cu-TiO2. It is worthy to note that
Pt(IV), Cr(III) and Fe(III) were hardly lost whereas Cu(II) and Co(II) were easily lost during
dialysis [31]. The former three have similar ionic size as Ti(IV) (0.605 Å), i.e. 0.625 (Pt(IV)),
0.615 (Cr(III)), and 0.645 Å (Fe(III)). Choi et al. reported that a metal ion having an ionic radius
similar to that of Ti(IV) was doped by being substituted in the Ti(IV) site whereas a larger metal
ion was most likely located in the interstitial positions of the TiO2 lattice [18]. Since ionic radius
8

of Cu(II) (0.73 Å) is larger than that of Ti(IV), Cu(II) ion is expected to exist in the interstitial
positions of the TiO2 lattice or on TiO2 surface. The presence of Cu(II) on the Cu-TiO2 pellets was
confirmed by SEM-EDS measurements (Fig. S1). N2 adsorption/desorption isotherms of the Cu-
TiO2 pellets indicated hysteresis and were classified as type IV (IUPAC), indicating that the
prepared Cu-TiO2 pellets were porous. Table 1 lists crystallite size, BET specific surface area
(SSA), and pore volume of the Cu-TiO2 pellets which contained various Cu(II) amounts and
calcined at 200oC. The SSA and the pore volume tends to decrease with increasing the Cu(II)
content more than 0.5 atom%. These findings suggest that Cu(II) exists near the surface. Fig. 1
shows diffuse reflectance spectra of the Cu-TiO2 with 0 – 3.5 atom% Cu(II) ion content.
Absorption edge was shifted toward longer wavelength and absorption at ca. 450 nm increases
with increasing amounts of Cu(II) ion. Similar spectra were reported for Cu-grafted TiO2 on which
the absorption at ca. 450 nm was ascribed to the direct interfacial charge transfer (IFCT) from the
valence band of TiO2 to Cu(II) on the surface whereas the absorption at 700 – 800 nm was assigned
to the d-d transition of Cu(II) [36]. Previously, we reported that XPS spectra of 0.7 atom% Cu-
TiO2 powder which was prepared by crushing the pellets after the sintering at 200oC showed a
peak at ca. 932 eV which was coincident with that observed with 0.5 atom% Cu-grafted TiO2 [32].
Therefore, it is likely that the amorphous CuO cluster forms near or on the Cu-TiO2 surface, just
like Cu-grafted TiO2. The XRD pattern of Cu-TiO2 with 0.1 – 3.5 atom% Cu(II) ion content
showed no peaks except for those attributable to anatase phase, suggesting that CuO may exist
homogeneously as small cluster on the Cu-TiO2 pellets (Fig.S2).
Fig. 2a shows the time profiles of the TCE conversion when the gas stream containing TCE
(mole fraction of TCE: Ftce = 1.75 × 10−4) and oxygen (mole fraction of oxygen: Foxy = 0.21) is
passed through 0.1 g of Cu-TiO2 pellets sintered at 200oC at the flow rate of 50 mL min−1 under
9

VL irradiation. The TCE conversion increases gradually with an increase in the VL irradiation
time and then decreases. Based on Fig. 2a, the mole of TCE degraded under VL irradiation for 300
min ([TCE]degraded) was calculated and plotted against the Cu(II) content in Fig. 2b, indicating that
[TCE]degraded decreases as the Cu(II) content increases. Hereafter the experiments were performed
with 0.1 atom% Cu-TiO2 pellets.
3.2 Effect of reaction conditions
Dependence of [TCE]degraded on sintering temperature of Cu-TiO2 is shown in Fig. 3a. The
optimal sintering temperature is 200 - 300oC and the degradation of TCE hardly occurs on Cu-
TiO2 sintered at 600oC. The XRD pattern and SSA of Cu-TiO2 calcined at different temperature
are shown in Fig. 4 and Table 2, respectively. Only anatase peak is obtained at the sintering
temperature lower than 300oC, small rutile peak is appeared at 400oC, and anatase completely
transforms to rutile at 600oC. Hereafter, Cu-TiO2 calcined at 200oC was used for the experiments.
Figs. 3b-d show the effects of mole fraction of water vapor (Fwv), Foxy, and Ftce on [TCE]degraded
,
respectively. In the absence of water vapor, TCE is degraded and [TCE]degraded decreases with an
increase in Fwv. On the other hand, TCE is not degraded without oxygen and [TCE]degraded increases
with an increase in Foxy or Ftce and approaches a constant value. These behaviors are the same as
reported for the degradation of TCE on TiO2 powder and that of chlorinated ethylenes on porous
TiO2 pellets under UV irradiation [8]. Therefore, the degradation of TCE follows the Langmuir-
Hinshelwood kinetic model where adsorption of the reactant gas is assumed prior to reaction. The
decrease in [TCE]degraded observed with high Fwv is attributable to the inhibitory effect of water on
the adsorption of TCE, i.e. competitive adsorption between water vapor and TCE, on the Cu-TiO2
surface.
10

It is worthy to note that the color of the Cu-TiO2 pellets was changed from green to brown in
the reactant gas stream containing no oxygen under VL irradiation (Fig. S3). The resulting brown
Cu-TiO2 pellets were returned to green when the air was passed through them in the dark. This
finding indicates that electron is transferred from valence band in TiO2 to Cu2+ to form Cu+ under
VL irradiation. Such a color change was not observed for the degradation of TCE in the presence
of oxygen, suggesting that Cu+ is easily oxidized to Cu2+ by oxygen. Similar electron transfer was
reported for Cu-grafted TiO2. The reduction of grafted Cu(II) ions and the alternative reduction of
O2 by the resultant Cu(I) ions under VL irradiation were demonstrated by ESR measurements [37].
Fig. 5 shows effect of space time (ST) which is varied by changing the flow rate in the range of
25 - 150 mL min−1 on 0.2 g of Cu-TiO2 pellets and is calculated by the following equation.
Weight of catalyst (g)
ST (g s mol -1)=
Molar flow rate of TCE (mol s-1)
The TCE conversion of 100% is achieved at ST of 7.7 × 107 g s mol−1.
3.3 Identification of products
When the reactant gas stream containing oxygen (Foxy = 0.21) and TCE (Ftce = 1.75 × 10−4) was
passed through 0.2 g of Cu-TiO2 pellets at the flow rate of 25 mL min−1, the product gas stream
under VL irradiation for 240 - 300 min was collected in the sampling bag and analyzed by GC/MS,
indicating the formation of carbon dioxide and chloroform (Fig. 6a). The products accumulated on
the Cu-TiO2 pellets were identified as trichloroacetaldehyde (TCAH), dichloroacetic acid
(DCAA), and pentachloroethane (PCA) (Figs. 6b, 6c). For the comparison, product analysis was
performed for the degradation of TCE on TiO2 or Cu-TiO2 pellets under UV irradiation (Figs. S4,
S5). It is noted that TCAH is detected only on Cu-TiO2 irrespective of whether the light source is
11

UV or VL. Table 3 summarizes the amounts of products formed under UV or VL irradiation for
300 min, except for phosgene and chloroform. These two compounds were detected under UV or
VL irradiation for 240 - 300 min and only phosgene was not determined quantitatively but
indicated as the peak area at 7.3 min (m/z = 63). Table 3 indicates that the main products are CO2,
DCAA, and Cl− and that the mineralization percent ([CO2]/[TCE]degraded × 50) is calculated to be
ca. 30.0, 46.7 and 43.7% for Cu-TiO2 under VL or UV and TiO2, respectively. Chloroform and
phosgene on Cu-TiO2 under VL were less than that under UV by one-order of magnitude. These
facts suggest that the mineralization of TCE to CO2 and the formation of gaseous products as
chloroform and phosgene are suppressed on the Cu-TiO2 pellets under VL irradiation. Since the
same products were detected, the reaction mechanism of the TCE degradation on Cu-TiO2 pellets
under VL irradiation seems to be essentially the same as that on TiO2 under UV irradiation.
3.4. Detection of OH radical
In generally, OH radical (•OH) is known as a reaction initiator for the photocatalytic degradation
on TiO2 under UV irradiation. Although we performed experiments in the absence of water vapor
•
in the reactant gas, water adsorbed on the Cu-TiO2 surface may generate OH. It is well known
that coumarin reacts with •OH and forms 7-hydroxycoumarin (7-HC) which exhibits fluorescence
peak at 454 nm under the excitation at 322 nm (Scheme S1) [35]. As shown in Fig. 7, no
fluorescence is observed in coumarin aqueous solution suspended with Cu-TiO2 powder under VL
irradiation whereas fluorescence attributable to 7-HC increases with an increase in the UV
irradiation time. Therefore, •OH is not generated on Cu-TiO2 under VL irradiation, suggesting that
TCE is degraded by the photogenerated holes. This is reasonable because TCE is degraded most
easily in the absence of water vapor in the reactant gas stream.
12

3.5. The repeated experiments
Previously, we reported the degradation mechanism of the chlorinated ethylenes on TiO2 pellets
under UV irradiation as follows: at the initial step, TCE was degraded by OH radical produced by
the oxidation of water vapor through photogenerated holes, one of the reaction products, i.e., Cl−
was accumulated, oxidized to Cl radical on the TiO2 surface and then Cl radical initiated reaction
occurs to form DCAA, chloroform and phosgene [8,9]. Fig. 8 shows time profiles of the TCE
conversion by repeating the experiments three times on Cu-TiO2 pellets. The second and third runs
did not exhibit the gradual increase in the TCE conversion which was observed at the initial 120
min in the first run. This fact suggests that it takes ca. 120 min to attain the steady state of the Cl
radical initiated reaction. It is worthy to note that the TCE conversion at the second run is almost
the same as that obtained under VL irradiation for 300 min in the first run. Both in the second and
third runs, the TCE conversion decreases gradually. These findings indicate that the decrease in
the TCE conversion observed at the longer irradiation time might be due to the accumulation of
the products on the Cu-TiO2 pellets.
3.6. Reaction mechanism
Under VL irradiation, the photogenerated electron moves from the valence band of TiO2 to
Cu(II) ion. The resultant Cu(I) returns to Cu(II) by reducing O2 to form superoxide anion radical.
TCE is directly degraded by the photogenerated holes at the initial step because no water vapor is
included in the reactant gas stream. As the TCE degradation proceeds in the gas flow system,
inorganic chlorine species such as HCl are accumulated on the Cu-TiO2 surface and generate Cl
radicals by the reaction with the photogenerated holes. Scheme 1 shows a possible degradation
mechanism to explain all products detected. The addition of Cl radical to TCE generates
13

CCl2CHCl2 or CCl3CHCl radical. These carbon-centered radicals subsequently react with oxygen
to form peroxyradicals which are converted to CCl2(O)CHCl2 or CCl3CHCl(O). The subsequent
elimination of Cl radical gives DCAA or TCAH. The C-C bond scission of CCl2(O)CHCl2 radical
yields phosgene and chloroform. As shown in Table 3, the amounts of PCA and TCAH on Cu-
TiO2 was more than that on TiO2. Therefore, the formation of these two products are affected by
the presence of Cu(II) species on the surface. Mosallanejad et al. reported that Cl atom of HCl is
bound to the Cu cations in Cu-modified zeolite [38]. Lomnicki and Dellinger reported that CuO
catalyzed the formation of polychlorinated compounds from 2-chloropropanol [39]. Therefore, it
is likely that Cl− or HCl adsorbs on Cu2+ sites and induces polychlorination of TCE to PCA and
TCAH. Superoxide anion radical produced by the photogenerated electrons can participate for the
mineralization of TCE. In general, reducing O2 forms superoxide anion radical, HO2 radical, H2O2
and OH radicals [40] but further studies are necessary to clarify their roles for the TCE degradation.
Reduction of TCE is ruled out on Cu-TiO2 pellets under VL irradiation because no TCE was
degraded even on TiO2 pellets without O2 under UV irradiation where the photogenerated electron
was excited to the conduction band of TiO2.
3.7. Comparison with Cu-grafted TiO2 powder
0.1 atom% Cu-TiO2 pellets were ground into powders and their photocatalytic activity was
compared with 0.1 atom% Cu-grafted TiO2 powders which was synthesized by using rutile TiO2
powder as reported by Irie et al. [34]. The calculated [TCE]degraded during VL irradiation for 300
min is almost the same ((2.0 ± 0.6) × 10−5 mol) although the TCE conversion is different at the
initial period as shown in Fig. 9(a). The Cu-grafted TiO2 can be synthesized through simple
14

process, i.e. immersion and dryness by using commercially available TiO2 powder [34] and thus,
it is superior VL responsive photocatalyst. However, the powder form is not appropriated for the
practical use in the packed bed reactor to treat gaseous pollutants. Therefore, we tried to graft
Cu(II) ion on porous TiO2 pellets synthesized by the sol-gel method. The synthesized TiO2 pellets
were calcined at 200oC and then 0.1 atom% Cu(II) ion was grafted on them to compare the
photocatalytic activity of Cu-TiO2 pellets calcined at 200oC. Figs. 9(b) and (c) shows time profiles
of the TCE conversion on the Cu-TiO2 pellets and the Cu-grafted TiO2 pellets at the ST of 1.7 ×
107 or 7.7 × 107 g s mol−1, indicating that the photocatalytic activity of Cu-TiO2 pellets are higher
than the Cu-grafted TiO2 pellets. This finding suggests that conducting the sol-gel synthesis in the
presence of Cu(II) ion is more beneficial than the post-treatment of the TiO2 pellets with Cu(II) to
obtain VL-responsive TiO2 modified with Cu(II). It is worthy to note that Cu-grafted TiO2 acts as
the VL responsive photocatalyst only when rutile TiO2 is grafted with Cu(II) ion [41]. Although
our Cu-grafted TiO2 pellets consist of only anatase phase, they shows VL activity. Porous TiO2
pellets with high specific surface area might affect the interaction of CuO nanoclusters and TiO2
surface. Our research will be continued to clarify the VL photoactivity of the Cu-grafted TiO2
pellets.
4. Conclusion
In this paper, we synthesized porous Cu-TiO2 pellets for photocatalytic degradation of gaseous
TCE under VL irradiation. TCE was efficiently degraded on 0.1 atom% Cu-TiO2 calcined at 200oC
in the reactant gas stream containing oxygen and no water vapor and 100% of the TCE conversion
was achieved at the space time of 7.7 × 107 g s mol−1. Various chlorinated products were detected
15

and the mineralization percent of TCE to CO2 was lower than that on Cu-TiO2 or TiO2 pellets
under UV irradiation. We proposed the degradation mechanism of TCE on the basis of the
quantitative analysis of the products.
Recently, low cost LED lamp emitting VL has been developed and thus, the synthesized Cu-
TiO2 pellets are promise as new photocatalysts for converting gaseous harmful chlorinated
compounds to useful chemicals under VL irradiation.
Supplementary Information
Supplementary information related to this article can be found in the online version, at doi:
Acknowledgement
The measurements of XRD and ICP-OES were performed at the Center for Instrumental Analysis,
Yamaguchi University. We thank Associate Prof. I. Fujiwara for the measurement of the
fluorescence spectra.
Reference
[1] W. A. Chiu, J. Jinot, C. S. Scott, S. L. Makris, G. S. Cooper, R. C. Dzubow, A. S. Bale, M. V.
Evans, K. Z. Guyton, N. Keshava, J. C. Lipscomb, S. Barone Jr., J. F. Fox, M. R. Gwinn, J.
Schaum,
J.
C.
Caldwell,
Environ.
Health
Perspect.
121(2013)
303-311.
https://doi.org/10.1289/ehp.1205879
16

[2] International Agency for Research on Center Trichloroethylene, Tetrachloroethylene, and
Some Other Chlorinated Agent; Wiley: IARC Monographs on the Evaluation of Carcinogenic
Risks to Humans, 2014, Vol.106.
[3] L. A. Dibble, G. B. Raupp, Catal. Lett. 4 (1990) 345-354. https://doi.org/10.1007/BF00765320
[4] J. Fan, J. T. Yates, J. Am. Chem. Soc. 118(19) (1996) 4686-4892.
https://doi.org/10.1021/ja952155q
[5] L. B. B. Ndong, M. P. Ibondou, X. Gu, M. Xu, S. Lu, Z. Qiu, Q. Sui, S. M. Mbadinga, Water,
Air, Soil Pollut. 225(5) (2014) 1951-1959. https://doi.org/10.1007/s11270-014-1951-8
[6] L. B. B. Ndong, X. Gu, S. Lu, M. P. Ibondou, Z. Qiu, Q. Sui, S. M. Mbadinga, B. Mu, Chem.
Eng. Sci. 123 (2015) 367-375. https://doi.org/10.1016/j.ces.2014.11.034
[7] S. Yamazaki-Nishida, H. W. Read, J. K. Nagano, T. Jarosch, C. Eddy, S. Cervera-March, M.
A.
Anderson,
Journal
of
Soil
Contamination
3(4)
(1994)
363-378.
https://doi.org/10.1080/15320389409383476
[8] S. Yamazaki, T. Tanimura, A. Yoshida, K. Hori, J. Phys. Chem. A 108 (2004) 5183-5188.
https://doi.org/10.1021/jp0311310
[9] T. Tanimura, A. Yoshida, S. Yamazaki, Appl. Catal. B:Environ. 61 (2005) 346-351.
https://doi.org/10.1016/j.apcatb.2005.05.015
[10] I. Jansson, K. Kobayashi, H. Hori, B. Sánchez, B. Ohtani, S. Suárez, Catal. Today 287 (2017)
22-29. https://doi.org/10.1016/j.cattod.2016.11.041
17

[11] I. Jansson, S. Suárez, F. J. Garcia-Garcia, B. Sánchez, Appl. Catal. B:Environ. 178 (2015)
100-107. https://doi.org/10.1016/j.apcatb.2014.10.022
[12] K. W. Nam, H. R. Jeong, S. H. Ahn, International Journal of Chemical Engineering and
Application 7(6) (2016) 359-364. https://doi.org/10.18178/ijcea.2016.7.6.605
[13] A. Banisharif, A. A. Khodadadi, Y. Mortazavi, A. A. Firooz, J. Beheshtian, S. Agah, S.
Menbari,
Appl.
Catal.
B:
Environ.
165
(2015)
209-221.
https://doi.org/10.1016/j.apcatb.2014.10.023
[14] L. B. B. Ndong, M. P. Ibondou, X. Gu, S. Lu, Z. Qiu, S. Zhaofu, Q. Sui, S. M. Mbadinga,
Ind. Eng. Chem. Res. 53(4) (2014) 1368-1376. https://doi.org/10.1021/ie403405z
[15] D. X. M. Vergas, J. R. D. L. Rosa, C. J. Lucio-Ortiz, A. Hernández-Ramirez, G. A. Flores-
Escamilla, C. D. Garcia, Appl. Catal. B:Environ. 179 (2015), 249-261.
https://doi.org/10.1016/j.apcatb.2015.05.019
[16] L. G. Devi, B. N. Murthy, S. G. Kumar, Mater. Sci. Eng. B 166 (2010) 1-6.
https://doi.org/10.1016/j.mseb.2009.09.008
[17] K. B. Jaimy, S. Ghosh, S. Sankar, K. G. K. Warrier, Mater. Res. Bull. 46 (2011) 914-921.
https://doi.org/10.1016/j.materresbull.2011.02.030
[18] J. Choi, H. Park, M. R. Hoffmann, J. Phys. Chem. C 114, (2010) 783-792.
https://doi.org/10.1021/jp908088x
[19] M. Miyauchi, H. Irie, M. Liu, X. Qiu, H. Yu, K. Sunada, K. Hashimoto, J. Phys. Chem. Lett.
7 (2016) 75-84. https://doi.org/10.1021/acs.jpclett.5b02041
18

[20] A. Hernández-Gordillo, V. R. González, Chem. Eng. J. 261 (2015) 53-59.
https://doi.org/10.1016/j.cej.2014.05.148
[21] Y. Park, W. Kim, H. Park, T. Tachikawa, T. Majima, W. Choi, Appl. Catal. B: Environ. 91
(2009) 355-361. https://doi.org/10.1016/j.apcatb.2009.06.001
[22] H. Yu, J. Phys. Chem. Solids 68 (2007) 600-607. https://doi.org/10.1016/j.jpcs.2007.01.050
[23] Y. Wang, Y. L. Meng, H. M. Ding, Y. K. Shan, X. Zhao, X. Tang, J. Phys. Chem. C 112
(2008) 6620-6626. https//doi.org/10.1021/jp7110007
[24] X. Zhang, K. Udagawa, Z. Liu, S. Nishimoto, C. Xu, Y. Liu, H. Sakai, M. Abe, T. Murakami,
A. Fujishima, J. Photochem. Photobiol. A: Chem. 202(1) (2009) 39-47.
https://doi.org/10.1016/j.jphotochem.2008.11.007
[25] K. Nishijima, N. Murakami, T. Ohno, Appl. Catal. B: Environ. 84 (2008) 584-590.
https://doi.org/10.1016/j.apcatb.2008.05.014
[26] B. Choudhury, S. Bayan, A. Choudhury, P. Chakraborty, J. Colloid Interface Sci. 465 (2016)
1-10. https://doi.org/10.1016/j.jcis.2015.11.050
[27] Y. Fang, Z. Li, B. Yang, S. Xu, X. Hu, Q. Liu, D. Han, D. Lu, J. Phys. Chem. C 118 (2014)
16113-16125. https://doi.org/10.1021/jp502208y
[28] T. M. Wandre, P. N. Gaikwad, A. S. Tapase, K. M. Garadkar, S. A. Vanalakar, P. D.
Lokhande, R. Sasikala, P. P. Hankare, J. Mater. Sci.: Mater. Electron 27 (2016) 825-833.
https://doi.org/10.1007/s10854-015-3823-4
19

[29] X. Xiong, H. Chen, Y. Xu, J. Phys. Chem. C 119 (2015) 5946-5953.
https://doi.org/10.1021/jp510974f
[30] S. Yamazaki, Y. Fujiwara, S. Yabuno, K. Adachi, K. Honda, Appl. Catal. B:Environ. 121-
122 (2012) 148-153. https://doi.org/10.1016/j.apcatb.2012.03.026
[31] N. Nishiyama, Y. Fujiwara, K. Adachi, K. Inumaru, S. Yamazaki, Appl. Catal. B:Environ.
176-177 (2015) 347-353. https://doi.org/10.1016/j.apcatb.2015.04.015
[32] N. Nishiyama, K. Kozasa, S. Yamazaki, Appl. Catal. A:General 527 (2016) 109-115.
https://doi.org/10.1016/j.apcata.2016.09.001
[33] T. Seyama, T. Tanimura, K. Tashiro, S. Yamazaki, Res. Chem. Intermed. 43 (2017) 5025-
5039. https://doi.org/10.1007/s11164-017-3051-z
[34] H. Irie, S. Miura, K. Kamiya, K. Hashimoto, Chem. Phys. Lett. 457 (2008) 202-205.
https://doi.org/10.1016/j.cplett.2008.04.006
[35] K. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Electrochem. Commun. 2 (2000)
207-210. https://doi.org/10.1016/S1388-2481(00)00006-0
[36] H. Irie, K. Kamiya, T. Shinbanuma, S. Miura, D. A. Tryk, T. Yokoyama, K. Hashimoto J.
Phys. Chem. C 113 (2009) 10761-10766. https://doi.org/10.1021/jp903063z
[37] Y. Nosaka, S. Takahashi, H. Sakamoto, A. Y. Nosaka J. Phys. Chem. C 115 (2011), 21283-
21290. https://doi.org/ 10.1021/jp2070634
[38] S. Mosallanejad, B. Z. Dlugogorski, E. M. Kennedy, M. Stockenhuber, J. Phys.Chem. C
117(38) (2013) 19365-19372. https://doi.org/10.1021/jp3122243
20

[39] S. Lomnicki, B. Dellinger, J. Phys. Chem. A. 107 (2003) 4387-4395.
https://doi.org/10.1021/jp026045z
[40] Y. Nosaka, A. Nosaka, Introduction to Photocatalysis From Basic Science to Applications,
Royal Society of Chemistry (2016).
[41] Y. Nosaka, A. Y. Nosaka, J. Phys. Chem. Lett.
7
(2016) 431-434.
https://doi.org/10.1021/acs.jpclett.5b02804
21

Table 1. Physicochemical properties of Cu-TiO2.
BET surface
area
[Cu2+] incorporated Crystallite size
Pore volume
(atom%)
(nm)
(10−3 cm3 g−1)
(m2 g−1)
260
0.1
0.2
0.5
1.1
3.5
3.6
3.6
3.6
3.8
3.8
261
261
264
223
219
265
264
216
246
Table 2. Crystallite size and BET surface area.
Sintering
Crystallite size
temperature
(nm)
BET surface area
(m2 g−1)
(oC)
None
100
200
300
400
500
600
3.7
3.7
4.1
5.1
5.4
223
234
257
213
144
59.7
0.32
13.5 26.8*
58.6*
22

Amount of materials (10−5 mol)
Cu-TiO2
VL
Cu-TiO2
UV
TiO2
UV
Reactant
[TCE]degraded
CO2
4.64
4.73
4.97
4.34
3.2
Products in gas phase
2.78
4.42
Phosgene (106 area)
Chloroform
0.2
1.4
0.00054
5.99
0.0067
9.04
0.0072
9.76
2.36
0.083
0.00
Products accumulated Chloride ion
on catalyst surface
DCAA
2.83
2.08
PCA
0.15
0.20
TCAH
0.0065
0.0026
Crystallite size was estimated from anatase (101) facet.
Symbol * denoted crystallite size estimated from rutile (110) facet.
Table 3. Amount of the degraded TCE and all products.
23

FIGURE CAPTIONS
Fig 1. Diffuse reflectance spectra of Cu-TiO2 pellets with 0 - 3.5 atom% Cu(II) content.
Fig. 2. (a) Time profiles of the TCE conversion on 0.1 g of Cu-TiO2 under VL irradiation. (b)
Effect of the Cu(II) content on the [TCE]degraded under VL irradiation for 5 h.
Fig. 3. Effects of (a) sintering temperature of Cu-TiO2 pellets and mole fractions of (b) water
vapor, (c) oxygen, and (d) TCE in the reactant gas stream on [TCE]degraded under VL irradiation for
5 h.
Fig. 4. XRD patterns of 0.1 atom% Cu-TiO2 calcined at 100-600oC.
Fig. 5. Time profiles of the TCE conversion at various space times under VL irradiation.
Fig. 6. GC/MS chromatograms of (a) the products in the gas phase and (b and c) those accumulated
on the Cu-TiO2 surface which were extracted with ethyl acetate after photocatalytic degradation
of TCE under VL irradiation.
Fig. 7. Time profiles of the fluorescence spectra of coumarin aqueous solution suspended with Cu-
TiO2 powder under (a) VL or (b) UV irradiation.
Fig. 8. Time profiles of the TCE conversion by repeating experiment three times under VL
irradiation.
Fig. 9. Time profiles of the TCE conversion on (a) 0.1 atom% Cu-TiO2 powder or Cu-grafted TiO2
synthesized by using commercial TiO2 powder and (b, c) 0.1 atom% Cu-TiO2 pellets and TiO2
pellets grafted with 0.1 atom% Cu(II). Space time of (b) or (c) is 1.7 × 107 or 7.7 × 107 g s mol−1,
respectively.
24

FIGURES
Fig. 1
Cu(II)
content
(atom%)
100
80
60
40
20
0
3.5
1.4
0.5
0.2
0.1
0
300
500
700
900
1100
Wavelength (nm)
Fig. 2
100
(a)
10
8
Cu(II)
(b)
content
80
60
40
20
(atom %)
0.1
6
0.2
0.5
1.1
3.5
4
2
Light off
0
0
0
100
200
300
400
0
1
2
3
4
VL irradiation time (min)
Cu(II) content (atom%)
25

Fig. 3
10
10
8
(b)
(a)
8
6
4
2
0
6
4
2
0
0
200
400
600
800
0
2
4
6
8
Sintering temperature (oC)
Mole fraction of water vapor (10-3)
10
8
10
8
(c)
(d)
6
6
4
4
2
2
0
0
0
0.2
0.4
0.6
0.8
0
1
2
3
4
5
Mole fraction of TCE (10-4)
Mole fraction of oxygen
26

Fig. 4
600oC
500oC
400oC
300oC
200oC
100oC
None
10
30
50
70
90
2θ (degree)
Fig. 5
(g s mol-1)
100
80
60
40
20
0
7.7 × 107
3.6 × 107
1.5 × 107
1.1 × 107
Light off
0
100
200
300
400
VL irradiation time (min)
27

Fig. 6
ｱﾊﾞﾝﾀﾞﾝｽ
TIC: 160715_5.D
1000000
a
900000
800000
700000
600000
500000
400000
300000
200000
100000
0
6.00
8.00
10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00
Time-->
Retention time (min)
20
15
b
c
20
15
CHCl2COOH
CHCl2CCl3
CCl3CHO
10
10
5
0
5
0
3.6
3.7
3.8
3.9
5
15
25
35
Retention time (min)
Retention time (min)
28

Fig. 7
30
25
20
15
10
5
30
(b)
UV irradiation time (min)
120
90
60
50
40
30
20
10
0
(a) VL irradiation time (min)
120
90
60
50
40
30
20
10
0
25
20
15
10
5
0
0
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 8
100
80
First
Second
Third
60
40
Light off
20
0
0
100
200
300
400
VL irradiation time (min)
29