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In both the pristine and palladized TiO2 catalytic systems, all
three nona-BDE intermediates were found to further undergo
reductive debromination, which was concomitant with the
dominant accumulation of the octa-BDEs (Figure 3). There are
12 potential congeners for the octa-BDEs. Because not all of
these octa-BDE standards are available, some intermediates
had to be assigned according to the relative retention times
from the literature.[32] As shown in Figure S9 in the Supporting
Information, in the pristine TiO2 systems, seven peaks of octa-
BDEs were observed after 10 min of irradiation. With well-iso-
lated GC peaks, the congeners of BDE195 (2,3,4,4’,5,5’,6,6’-
BDE), BDE194 (3,3’,4,4’,5,5’,6,6’-BDE), BDE196 (2,3’,4,4’,5,5’,6,6’-
BDE), BDE197 (2,2’,4,4’,5,5’,6,6’-BDE), BDE204 (2,2’,3,4,4’,5,6,6’-
BDE), and BDE201 (2,2’,3,4’,5,5’,6,6’-BDE) were confidently iden-
tified. Two congeners of BDE198 (2,3,3’,4,5,5’,6,6’-BDE) and
BDE199 (2,3,3’,4’,5,5’,6,6’-BDE) with retention time at 24.44 min,
which were generated from the debromination of ortho- and
meta-Br from one ring and from two rings, respectively, were
actually inseparable under GC conditions.[32] Hence, at most
eight congeners of octa-BDEs might be detected throughout
the photoreaction on pristine TiO2. Further irradiation for
60 min led to the disappearance of BDE195, which indicated
that it can be further debrominated, whereas another six
peaks remained and their relative intensities did not change
much during all the testing time (300 min). A common charac-
teristic for the detected 8Br products, except BDE204, in the
pristine TiO2 system is that the two bromine atoms are re-
moved from the different benzene rings of BDE209. To satisfy
this rule, it is plausible that the BDE199, formed by the loss of
two Br atoms of BDE209 from different rings, should be domi-
nant in the peak at 24.44 min. On the Pd–TiO2, however, many
more peaks in the octa-BDE region were detected. The most
remarkable difference is the appearance of a strong peak with
retention time of 24.51 min, which is absent in the pristine
TiO2 systems. This peak was assigned to BDE200
(2,3,4,4’,5,5’,6,6’-BDE) and/or BDE203 (2,3,3’,4,4’,5,6,6’-BDE),
which are inseparable under the GC conditions. It is notable
that these two congeners are formed by debromination of two
Br atoms on the same aromatic ring of BDE209. Two minor in-
termediates with retention times of 23.62 and 25.04 min were
also observed on Pd–TiO2, and assigned to BDE202
(2,2’,3,3’,5,5’,6,6’-BDE) and BDE205 (2,3,3’,4,4’,5,5’,6-BDE). Thus,
all the 12 congeners of octa-BDEs during the BDE209 reduction
on Pd–TiO2 can be detected, which indicates that the bromides
at all the positions of PBDEs could be removed. However, the
formation of only partial 8Br intermediates (seven of 12) on
the pristine TiO2 means that the debromination reaction is dif-
ficult for the Br atom in some positions of PBDEs. In this sense,
the photocatalytic debromination reaction is less selective on
Pd–TiO2 than on the pristine TiO2 surface. Evidently, the lack of
position selectivity on the Pd–TiO2 is attributable to the cocata-
lytic effect of Pd, which makes the cleavage of the CÀBr bond
at every position of the PBDEs occur at comparable rates, in
agreement with the rapid debromination of low PBDEs on Pd–
TiO2 (see below). Moreover, on the Pd–TiO2, the octa-BDEs
formed by loss of one or both the meta-Br atoms (seven in
total, i.e., BDE195, 196, 197, 200, 201, 203, and 204) generally
appeared as main intermediates, whereas those in which all
the meta-Br are preserved (five in total, i.e., BDE194, 198, 199,
202, and 205,) were always detected in minor amounts (Fig-
ure 4b). This is consistent with the observation in Figure 4a
that the meta-Br is preferential to leave on Pd–TiO2.
The further debromination of the octa-BDE intermediates on
the pristine TiO2 surface was rather slow. Only a small amount
of 7Br intermediates appeared after irradiation for 5 h. In the
Pd–TiO2 system, in contrast, plenty of intermediates with fewer
than 8Br were detected. However, only a few of them were
identified by EO5113 standard solution (with 39 known PBDE
congeners), including BDE183, 154,138, 116, and 33/28
(Figure 5). Most of the main intermediates were not among
the 39 congeners of EO5113. Significantly, BDE47, which is the
Figure 5. Formation of the photodegradation intermediates of BDE209 in
Pd–TiO2 dispersions at different irradiation times. The chromatogram of the
standard sample of PBDEs (EO5113) is displayed in the top panel for com-
parison. The corresponding peaks of the solid lines are assigned to BDE33,
BDE28, BDE47, BDE116, BDE154, and BDE183, respectively, but the peaks on
the dotted lines are not identified.
most commonly detected PBDE in the environment and in
samples collected from organisms,[33,34] was hardly detected
throughout the whole debromination process.
The accumulation of octa-BDEs on pristine TiO2 during the
photocatalytic degradation of BDE209 demonstrates its ineffec-
tiveness on the low-brominated PBDEs. The remarkable charac-
teristic of the photocatalytic reductive debromination on the
Pd–TiO2 system is the emergence of the deep-debrominated
congeners even at the very early stage of the reaction, which
implies that the Pd–TiO2 is able to efficiently debrominate the
low-bromo DEs. This assertion is in agreement with the obser-
vation in the ZVI–Pd system in which the less brominated
BDEs exhibited a more significant enhancement in the reduc-
tion rates by palladization.[35] To further support this perspec-
tive, the photocatalytic reduction of a lower-bromo DE, 4,4’-di-
bromodiphenyl ether (BDE15), was examined. As shown in
Figure 6, the photocatalytic reduction reaction for BDE15 was
not detected under UV irradiation on the pristine TiO2. Howev-
er, in the Pd–TiO2 photocatalytic system, BDE15 was degraded
quickly within 20 min of irradiation, and concomitantly the 4-
bromodiphenyl ether intermediate was formed. Further irradia-
tion resulted in the disappearance of 4-bromodiphenyl ether
and the accumulation of diphenyl ether. The final yield of di-
Chem. Eur. J. 2014, 20, 11163 – 11170
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