Photocatalytic degradation of dichloroacetyl chloride adsorbed on TiO 2

Article abstract of DOI:10.1007/s11164-010-0207-5

Dichloroacetyl chloride (DCAC) attracted our attention as an intermediate product of the photocatalytic degradation of trichloroethylene (TCE). The adsorption and photocatalytic reaction of DCAC on TiO₂ have been investigated by FTIR spectroscopy. The influence of the surface structure of several TiO₂s on the reaction mechanism was discussed in order to understand the complete degradation mechanism of TCE as well as DCAC. DCAC was transformed into dichloroacetic acid (DCAA) on the relatively hydrophobic TiO₂ surface by the small amount of the water molecules weakly adsorbed on the surface. This DCAA was degraded to phosgene, CO₂, and CO during UV irradiation. For the hydrophilic TiO₂, DCAC was mainly transformed into the dichloroacetate anion. UV irradiation allowed this species to produce chloroform in addition to phosgene, CO₂, and CO. It is suggested that DCAC easily reacts with the Ti-OH group on the hydrophilic TiO₂ and forms the bidentate titanium chelate of dichloroacetate, which efficiently degrades into chloroform.

Full text of DOI:10.1007/s11164-010-0207-5

Res Chem Intermed (2010) 36:947–957
DOI 10.1007/s11164-010-0207-5
Photocatalytic degradation of dichloroacetyl chloride
adsorbed on TiO2
•
Hiromasa Nishikiori Makoto Tagahara
•
•
Leo Mukoyama Tsuneo Fujii
Received: 2 July 2010 / Accepted: 27 October 2010 / Published online: 20 November 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Dichloroacetyl chloride (DCAC) attracted our attention as an intermediate
product of the photocatalytic degradation of trichloroethylene (TCE). The adsorption
and photocatalytic reaction of DCAC on TiO2 have been investigated by FTIR
spectroscopy. The influence of the surface structure of several TiO2s on the reaction
mechanism was discussed in order to understand the complete degradation mechanism
of TCE as well as DCAC. DCAC was transformed into dichloroacetic acid (DCAA) on
the relatively hydrophobic TiO2 surface by the small amount of the water molecules
weakly adsorbed on the surface. This DCAA was degraded to phosgene, CO2, and CO
during UV irradiation. For the hydrophilic TiO2, DCAC was mainly transformed into
the dichloroacetate anion. UV irradiation allowed this species to produce chloroform
in addition to phosgene, CO2, and CO. It is suggested that DCAC easily reacts with the
Ti–OH group on the hydrophilic TiO2 and forms the bidentate titanium chelate of
dichloroacetate, which efficiently degrades into chloroform.
Keywords Titanium dioxide Á Photocatalyst Á Dichloroacetyl chloride Á
Adsorption Á Degradation
Introduction
There is a serious problem of groundwater and soil pollutions due to trichloroethylene
(TCE) that isused as an effective organic solventor a cleaning solvent in many locations
all over the world. Photolysis is one of the effective purification methods to alleviate
such pollution. This method, however, produces some harmful intermediate com-
pounds, such as dichloroacetyl chloride(DCAC) and phosgene, during its degradation
H. Nishikiori (&) Á M. Tagahara Á L. Mukoyama Á T. Fujii
Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University,
4-17-1 Wakasato, Nagano 380-8553, Japan
e-mail: nishiki@shinshu-u.ac.jp
123

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H. Nishikiori et al.
process [1, 2]. Although phosgene is degraded to HCl and CO2 in the presence of water
during the photoreaction process, further light irradiation is required for the degradation
of DCAC. It is very important to perform the complete purification of such pollutants
without producing any harmful compounds. The photocatalytic degradation of TCE
usingTiO2 duringlowerenergyirradiationisalsoamoreefficientpurificationmethodof
thispollution[1, 2]. However, thisalsoproduceschlorinatedcompounds, suchasDCAC
and phosgene [1–22]. Many researchers have suggested the relevant adsorption and
degradation mechanisms based on a physicochemical analysis. They reported that the
degradation of TCE proceeded by active species, such as the hydroxyl radical, chlorine
radicals [3, 5–7, 10–16], and adsorbed oxygen species [8, 9]. Although it is well known
that DCAC is degraded on TiO2 to produce phosgene, CO2, and HCl during the
photocatalytic process, the details of its degradation mechanism including the
adsorption structure of such chemicals have not yet been totally clarified. One requires
a more detailed mechanism related to the adsorption and photocatalytic degradation of
TCE and the products on the catalyst. Fan et al. [8] and Hwang et al. [10] observed the
DCACandphosgeneadsorbedonTiO2 duringthe photocatalyticdegradationofTCEby
FTIR and solid-stateNMR analyses, respectively. Driessenetal. reportedtheadsorption
of the dichloroacetate produced from TCE on TiO2 by FTIR spectroscopy [9]. Joung
et al. published the FTIR spectra of the carboxylate species of DCAC and phosgene
adsorbed on TiO2 [20–22]. On the other hand, there are a few studies that have reported
the adsorption form and reaction mechanism of the intermediate species, such as DCAC
and phosgene [20–23]. It is important for developing the preparation of photocatalysts
and control of the photocatalytic reaction to elucidate the relationship between the
adsorption form of the species and the reactivity.
In this study, the adsorption and photocatalytic reaction of DCAC on TiO2 as an
important process of the TCE degradation was investigated by FTIR spectroscopy.
The influence of the surface structure of several TiO2s on the reaction mechanism
was discussed in order to understand the complete degradation mechanism of TCE
as well as DCAC.
Experimental
Materials
Titanium tetraisopropoxide, ethanol, and nitric acid, (Wako Pure Chemicals, S or a
reagent grade) were used without further purification. Water was ion-exchanged and
distilled. The titanium dioxide (Wako Pure Chemicals, Anatase-type) was used
without further purification. DCAC of reagent grade purchased from Tokyo Kasei
and the dry air gas (ca. nitrogen 79% ? oxygen 21%) purchased from Okaya Sanso
were used without further purification.
Preparation of photocatalysts
The sol–gel reaction system was prepared in a nitrogen atmosphere by mixing
10.0 cm3 of titanium tetraisopropoxide, 50.0 cm3 of ethanol, 0.42 cm3 of water, and
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Photocatalytic degradation of dichloroacetyl chloride
949
0.42 cm3 of concentrated nitric acid as the catalyst of the sol–gel reaction. The sol–
gel reaction system was maintained at 60°C for 2 days to attain its gelation. The gel
was ground to a powder using an agate mortar and pestle, and then the powder was
soaked in water and evaporated to dryness in order to remove some organic
compounds. The resultant TiO2 powder was heated at 300, 400, or 500°C for
180 min. The samples heated at 300, 400, and 500°C were labeled Cat300, Cat400,
and Cat500, respectively, and the commercial anatase TiO2 was labeled CatA.
These TiO2 samples were used as the photocatalysts to degrade the DCAC.
Characterization of photocatalysts
The surface structures of the TiO2 samples were observed using a field emission
scanning electron microscope (Hitachi S-4100). The crystalline phase was
determined using a X-ray diffractometer (Rigaku RINT-2200V). The specific
surface area was measured by the volumetric gas adsorption method using nitrogen
gas (BEL Japan BELSORP-mini).
Adsorption and photocatalytic degradation of DCAC
Cat300, Cat400, Cat500, and CatA, 0.030 g, were separately immersed in the
undiluted liquid of DCAC, 1.0 dm3, for 24 h in the dark in order to adsorb the DCAC
molecules. These TiO2 samples were dried under vacuum for 240 h in the dark. Each
TiO2 sample was pressed in KBr pellets for the FTIR analysis. The IR spectra of the
samples were measured by a FTIR spectrophotometer (Shimadzu FTIR-8300) in
order to determine the species adsorbed on the TiO2 surface. The IR measurement of
the samples was carried out as a function of time during the near-UV light irradiation
from a 4-W black light bulb (Toshiba FL4BLB) at ambient temperature. The amount
of DCAC adsorbed on the samples was estimated by gas chromatography–mass
spectrometry of the spices desorbed at 100°C using a Shimadzu GCMS-QP5000.
Each DCAC-adsorbing TiO2, 0.10 g, was placed in an infrared cell filled with
dried air. The infrared cell consisted of a cylinder made of Pyrex glass and two
plates of KBr single crystals as the infrared windows, which are sealed by Teflon
O-rings. The degradation reaction of the DCAC was carried out in the cell by the
near-UV light irradiation at ambient temperature. The IR spectra of the gas phase
in the cell were observed as a function of time during the UV irradiation.
The changes in the concentrations of the species in the gas phase were determined
by the IR analysis.
Results and discussion
Characterization of photocatalysts
Figure 1 is a comparison among the SEM images of the prepared and commercial
TiO2 samples, i.e., Cat300, Cat400, Cat500, and CatA. The particles in Cat300,
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H. Nishikiori et al.
Fig. 1 SEM images of the prepared and commercial TiO2 samples; a Cat300, b Cat400, c Cat500,
and d CatA
Fig. 2 XRD patterns of the
prepared and commercial TiO2
samples; Cat300, Cat400,
Cat500, and CatA
0.5
CatA
Cat500
Cat400
Cat300
50
20
30
40
60
2θ / degree
Cat400, Cat500, and CatA look almost spherical and their sizes were estimated to be
5–40, 10–50, 10–60, and 90–250 nm, respectively. The particles grow by heating
the titania gel powder at a higher temperature.
Figure 2 shows the XRD patterns of the TiO2 samples. The peaks at around 25,
38, 48, 54, and 55° appeared in the XRD pattern of Cat500 and Cat A, and are
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Photocatalytic degradation of dichloroacetyl chloride
951
assigned to the anatase-type TiO2 crystal structure. The peak at around 25° was also
observed in Cat300 and Cat400, indicating the formation of the same crystal
structure. The XRD peaks in the sample heated at the higher temperature are
sharper, and the crystallite sizes in Cat300, Cat400, and Cat500 were estimated to be
6.2, 8.6, and 19 nm from their full-width at half-maximum of the 25°-peak using
Sherrer’s equation, D = 0.9k/bcosh. These values are much lower than those
estimated from the SEM images, indicating that the apparent particles consist of the
smaller primary crystalline particles. The primary particle size of CatA was not
estimated because the peak width of the XRD pattern was sharper than the
resolution limit of the instrument.
The measured specific surface areas of Cat300, Cat400, Cat500, and CatA were
139, 107, 35.8, and 8.46 m2 g-1, respectively. It is reasonable that the larger-sized
particles have lower specific surface areas. The characteristics of the samples, i.e.,
their particle sizes, crystallite sizes, and specific surface areas, are summarized in
Table 1.
Figure 3 shows the FTIR spectra of the TiO2 samples. The bands at around
560 cm-1 in the spectra of all the samples were assigned to an octahedral Ti–O
stretching band [24–27]. The spectra of the prepared TiO2 mainly have two bands
originating from the OH-stretching of the Ti–OH and the adsorbed water
(3,500–2,800 cm-1), and the OH-bending of the adsorbed water (1,630 cm-1).
The bands due to the Ti–OH and the adsorbed water are smaller by heating at the
Table 1 Characteristics
Cat300 Cat400 Cat500 CatA
of Cat300, Cat40, Cat500,
and CatA
Particle size/nm
5–40
6.2
10–50
8.6
10–60
19
90–250
–
Crystallite size/nm
Specific surface area/m2g-1 139
107
35.8
8.46
Fig. 3 FTIR spectra of the
prepared and commercial TiO2
samples; Cat300, Cat400,
Cat500, and CatA
CatA
Cat500
Cat400
Cat300
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber / cm-1
123

952
H. Nishikiori et al.
higher temperatures. The sample prepared at the lower temperature has a more
hydrophilic and reactive surface, which is favorable for adsorbing polar molecules.
Reaction analysis of DCAC adsorbed on TiO2
Figure 4 shows the FTIR spectra of the DCAC adsorbed on the TiO2 samples in the
spectral range of 1,900–1,100 cm-1. Gaseous DCAC normally exhibits a peak at
around 1,800 cm-1 assigned to the C=O stretching vibration for the COCl group. On
the other hand, the spectrum of CatA has a peak at 1,740 cm-1 assigned to the C=O
of the COOH group and peaks at around 1,300 and 1,200 cm-1 assigned to the O–H
and C–O. In addition to these, the 1,600 and 1,390 cm-1 peaks assigned to the COO-
antisymmetric and symmetric stretching vibrations, respectively, were also observed
in the spectrum of CatA [9, 22]. These results indicate that DCAC was adsorbed on
the surface of TiO2 and formed the neutral or anion of dichloroacetic acid (DCAA).
The neutral DCAA and DCAA anion bands were also observed in the spectra of
Cat300, Cat400, and Cat500. The relative intensity of the DCAA anion band is
stronger in the order of Cat300, Cat400, Cat500, and CatA. DCAC easily formed
the DCAA anion on the hydrophilic surface of the TiO2 having the OH groups and
the large amount of water molecules, whereas it formed the neutral DCAA on the
relatively hydrophobic surface due to the small amount of the water molecules
weakly adsorbed on the surface. According to the dependence of the adsorption
structure of DCAA on the hydrophilicity of the TiO2 surface, DCAC more
preferentially reacts with the Ti–OH group to form the bidentate titanium chelate of
the DCAA anion than with the water weakly adsorbed on the TiO2.
Analysis of gas produced from TiO2 adsorbing DCAC
The gaseous products released from the surface of the DCAC-absorbing TiO2 were
analyzed by measuring the IR spectra as a function of time during the UV
Fig. 4 FTIR spectra of the
prepared and commercial TiO2
samples adsorbing DCAC;
Cat300, Cat400, Cat500,
and CatA
CatA
Cat500
Cat400
Cat300
1800
1600
1400
1200
Wavenumber / cm-1
123

Photocatalytic degradation of dichloroacetyl chloride
953
irradiation, although they are not shown here. The peaks at 1,830 and 856 cm-1 for
phosgene were observed during the UV irradiation from all the samples [17–19],
whereas the peaks for DCAA were scarcely observed. The peaks assigned to
chloroform were detected at 1,212 and 770 cm-1 from Cat300 and Cat400 having
hydrophilic surfaces. The peaks at 3,050–2,700 cm-1 for HCl, at 2,400–2,250 cm-1
for CO2, and at 2,250–2,050 cm-1 for CO were observed from all the samples.
These products were obtained by degradation of the adsorbed DCAA during the UV
irradiation.
Figure 5 shows the change in the concentration of each gaseous product released
from the surface of the DCAC-absorbing Cat300, Cat400, Cat500, and CatA during
1.4
1.2
1
1
0.8
0.6
0.4
0.2
0
Cat300
Cat400
(d)
(a)
(b)
(c)
Cat300
Cat400
0.8
0.6
0.4
0.2
0
Cat500
CatA
CatA
Cat500
0
50
100
150
0
0
0
50
100
150
Irradiation time / min
Irradiation time / min
2.5
2
0.8
0.6
0.4
0.2
0
Cat300
(e)
Cat300
1.5
1
Cat400
Cat500
Cat400
Cat500
CatA
0.5
0
CatA
50
100
150
0
50
100
150
Irradiation time / min
Irradiation time / min
3.5
3
Cat300
2.5
2
1.5
1
Cat400
CatA Cat500
0.5
0
50
100
150
Irradiation time / min
Fig. 5 Change in absorbance of each gaseous product released from the surface of the DCAC-absorbing
Cat300, Cat400, Cat500, and CatA during UV irradiation. The absorbances of a phosgene, b HCl,
c chloroform, d CO2, and e CO are plotted versus irradiation time
123

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H. Nishikiori et al.
the UV irradiation. The desorption of DCAA was scarcely observed from all the
samples. The concentration of phosgene gradually increased during the UV
irradiation of all the samples. The concentrations of phosgene produced from
Cat300, Cat400, Cat500, and CatA after a 150-min irradiation were 9.3, 6.6, 2.3,
and 1.9 9 10-4 mol dm-3, respectively. The adsorption amounts of Cat300, Cat40,
Cat500, and CatA for DCAC are summarized in Table 2. As all the DCAC adsorbed
on these samples are transformed into phosgene, their concentrations in the gas
phase are 9.3, 7.4, 2.4, and 2.4 9 10-4 mol dm-3, respectively. Therefore, almost
all of the DCAA species adsorbed on the TiO2 should be transformed into phosgene.
HCl was observed during the UV irradiation of all the samples. Its concentration
was 1/10–1/4 of that of the produced phosgene. HCl is expected to be mainly
produced by the formation of DCAA on the TiO2 surface and its amount depends on
that of the DCAC reacting with the surface Ti–OH group or water. In the case of the
relatively hydrophobic TiO2, some amount of HCl was probably desorbed during
the dry process before the IR measurement due to its low HCl adsorption ability. On
the other hand, the hydrophilic TiO2 can still adsorb some amount of HCl. The
adsorbed HCl can be desorbed during the UV irradiation.
Certain amounts of CO2 and CO were also produced from all the samples
although chloroform was observed using Cat300 and Cat400 having hydrophilic
surfaces. The amounts of these species are ca. 1/100 of that of phosgene produced
from each sample. CO2 and CO should be produced by the decomposition of DCAA
species and phosgene on the TiO2. There are two types of adsorbed DCAAs, i.e., the
neutral DCAA weakly adsorbed on the relatively hydrophobic surface and the
DCAA anion strongly adsorbed on the hydrophilic surface. Chloroform is expected
to be produced from the DCAA anion because the C–C bond of the DCAA anion is
weakened by the strong interaction between its carboxylate group and the surface Ti
species. The resultant CHCl2 radical reacts with Cl radical in the gas phase and
forms chloroform. A larger amount of chloroform was observed on the more
hydrophilic TiO2. It is suggested that the reaction easily progressed due to the
bidentate chelate form of the DCAA anion. For the hydrophobic TiO2, the b-carbon
of the adsorbed DCAA is oxidized by the activated oxygen species, and then forms
phosgene. Almost all of the carbon oxide species produced by the decomposition of
the DCAA species can be adsorbed on the TiO2 surface. Figure 6 shows the FTIR
spectra of Cat300 and the DCAC-adsorbing Cat300 before and after the UV
irradiation. The band intensity of the peaks at 1,600 and 1,390 cm-1 assigned to the
COO- decreased during the UV irradiation while that of the peaks at 1,640 and
2-
around 1,450 cm-1 assigned to the CO3
species increased [28–30]. The
1,640 cm-1 band is clearly different from the adsorbed water band observed at
1,630 cm-1 in the spectrum for Cat300. The CO32- is the species resulting from the
adsorption of the carbon oxide species on the TiO2 after the DCAA decomposition.
Table 2 Adsorption amounts of Cat300, Cat40, Cat500, and CatA for DCAC (mol g-1
)
Cat300
Cat400
Cat500
CatA
3.1 9 10-1
2.5 9 10-1
7.9 9 10-2
7.9 9 10-2
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Photocatalytic degradation of dichloroacetyl chloride
955
Fig. 6 FTIR spectra of (1)
Cat300 and the DCAC-
absorbing Cat300 (2) before
and after (3) 180-min and
(4) 360-min UV irradiation
4
3
2
1
1800
1600
1400
1200
Wavenumber / cm-1
In many of the studies of the photocatalytic degradation of TCE, DCAA was not
observed in the gas phase. In this study, DCAA was also not observed in the gas
phase even using the TiO2 saturated with the adsorbed DCAA.
Examination of degradation mechanism of TCE on TiO2 surface
It was suggested that the TiO2 adsorbed TCE and oxygen molecules formed the
charge transfer species ([Ti3?–O-]*) with these molecules by light irradiation [17, 18,
31, 32], and caused the photocatalytic reaction of TCE. HCl, CO2, and CO are
produced via DCAC, phosgene, and chloroform during the TCE degradation process.
In previous papers, it was reported that the produced DCAC was desorbed from the
TiO2 surface, so that it was temporally observed in the gas phase [17–19]. The
released DCAC should be again adsorbed on the TiO2 surface in order to further react.
The present study indicated that DCAC was adsorbed on the TiO2 surface and formed
the DCAA species. TCE can also be transformed into DCAC and degraded without its
desorption. It should be an important process that TCE, once adsorbed on the TiO2
surface is transformed into DCAC, and then desorbed. Once TiO2 adsorbed the DCAC
and transformed it into the DCAA species, the DCAA species were not desorbed.
Phosgene and chloroform were produced from the DCAA species and then desorbed
from TiO2.
Conclusions
The adsorption and photocatalytic reaction of DCAC on TiO2 as an intermediate
product of TCE was investigated by FTIR spectroscopy in order to understand the
photocatalytic degradation mechanism of not only DCAC, but also TCE. DCAC
was transformed into the neutral DCAA on the relatively hydrophobic surface by
the small amount of water molecules weakly adsorbed on the surface. This neutral
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H. Nishikiori et al.
DCAA was degraded into phosgene, CO2, and CO during the UV irradiation. For
the hydrophilic TiO2, DCAC was mainly transformed into the DCAA anion. UV
irradiation allowed this species to produce chloroform in addition to phosgene, CO2,
and CO. We strongly suggest that the following process is important during the
photocatalytic degradation of TCE: the DCAC produced from TCE on TiO2 was
first desorbed, then adsorbed on the TiO2 again, and degraded to produce phosgene,
chloroform, CO2, and CO in addition to the process to directly produce these
compounds. The IR spectroscopic analysis indicated that DCAC easily reacts with
the Ti–OH group and forms the bidentate titanium chelate of the DCAA anion, and
transforms into chloroform.
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