Photoelectrochemistry on TiO2/Ti anodes as a tool to increase the knowledge about some photo-oxidation mechanisms in CH3CN

Article abstract of DOI:10.1002/poc.1303

Through current efficiency measurements, obtained from the photoelectrochemical oxidation at TiO₂ (rutile)/Ti anodes, further mechanistic information has been obtained regarding the TiO₂ photosensitized oxidation of benzylic alcohols, ethers and 1,2-diols in CH ₃CN. In deaerated medium, two electrons are captured by the semiconductor from all the considered substrates (one from the substrate, the second from the intermediate benzylic radical). In contrast, in aerated CH ₃CN, the number of TiO₂-captured electrons can be reduced to one because, depending on its oxidizability, the benzylic radical can be competitively captured by oxygen. Copyright

Full text of DOI:10.1002/poc.1303

Research Article
Received: 23 July 2007,
Revised: 25 October 2007,
Accepted: 29 October 2007,
Published online in Wiley InterScience: 17 January 2008
(
www.interscience.wiley.com) DOI 10.1002/poc.1303
Photoelectrochemistry on TiO /Ti anodes
2
as a tool to increase the knowledge
about some photo-oxidation
mechanisms in CH CN
Marta Bettoni , Cesare Rol * and Giovanni V. Sebastiani **
3
a
b
a
2
Through current efficiency measurements, obtained from the photoelectrochemical oxidation at TiO (rutile)/Ti
anodes, further mechanistic information has been obtained regarding the TiO2 photosensitized oxidation of
benzylic alcohols, ethers and 1,2-diols in CH CN. In deaerated medium, two electrons are captured by the
3
semiconductor from all the considered substrates (one from the substrate, the second from the intermediate benzylic
radical). In contrast, in aerated CH CN, the number of TiO -captured electrons can be reduced to one because,
3
2
depending on its oxidizability, the benzylic radical can be competitively captured by oxygen. Copyright ß 2008 John
Wiley & Sons, Ltd.
2
Keywords: TiO ; photoelectrochemistry; current efficiency; oxidation; radical cation
Generally, when m
with different involved electrons (n
i
moles of different i products are formed
INTRODUCTION
i
), Eqn (2) can be considered
Semiconductors, such as TiO
degradation of organic pollutants in waste water. Generally, TiO
is used as a dispersed powder. Alternatively, in the photoelec-
trochemical technology, a film of semiconductor supported on Ti,
such as the anode of an electrolytic cell, can be used. This
technology is more complex but has the following advantages:
2
, can sensitize the photo-oxidative
h
i
ni  mi
2
c:e: ¼ S
i
 100
(2)
[
1]
f
In this way, after all the m and f values are determined, a
suitable combination of all the n values that corresponds to a
quantitative c.e. can be obtained.
i
[
2]
i
(
(
i) the semiconductor is immobilized at the electrode surface,
ii) the process efficiency can be improved because the anode,
The organic solvent considered in this paper, CH CN, does not
significantly compete, in contrast to water, with the substrate
3
when polarized, reduces the rate of the (unproductive)
recombination of the hole, (TiO hþ, with the photogenerated
oxidation; therefore, whereas c.e. is always ꢀ 100% in water, a
2
)
quantitative c.e. can be observed in CH CN, making the process
3
electron and (iii) the anodic and cathodic reactions occur in
physically separated compartments (therefore, the correspond-
ing intermediates do not interact).
more useful for mechanistic purposes.
In this context, recently, we have obtained mechanistic
information on the TiO2 sensitized photo-oxidation of some
benzylic derivatives by the photoelectrochemical technique with
On principle, the photoelectrochemical technology should
allow an experimentally observed electrical parameter, as the
current efficiency (c.e.), to be used to acquire mechanistic
information relative to previously investigated oxidative pro-
cesses; in particular, the kinetically significant steps of the primary
oxidation products and of the reaction stoichiometry (the
number of electrons exchanged per molecule of formed product)
can be confirmed. The c.e. can be defined as the amount of
current passed ( f) in relation to the number of electrons
exchanged per molecule of the formed product (n) and to the
product amount (m moles), provided the quantitative material
balance is observed. From Eqn (1), the corrected n value is that
giving a quantitative c.e. (100 Æ 20%).
[
3]
TiO /Ti anodes in CH CN. This information could be useful for
2
3
*
Dipartimento di Chimica, Laboratorio di Chimica Organica, Via Elce di Sotto 8,
6123 Perugia, Italy.
E-mail: rol@unipg.it
0
** Dipartimento di Ingegneria Civile ed Ambientale, Facolt a` di Ingegneria,
Universit a` di Perugia, Via G. Duranti 93, 06125, Perugia, Italy.
E-mail: gseb@tech.ing.unipg.it
a
M. Bettoni, G. V. Sebastiani
Dipartimento di Ingegneria Civile ed Ambientale, Universit a` di Perugia, Via G.
Duranti, 06125 Perugia, Italy
ꢀ
ꢁ
b
C. Rol
n  m
`
di Perugia, Via Elce di Sotto, 06123
c:e: ¼
 100
ð1Þ
Dipartimento di Chimica, Universita
f
Perugia, Italy
J. Phys. Org. Chem. 2008, 21 219–224
Copyright ß 2008 John Wiley & Sons, Ltd.

M. BETTONI, C. ROL AND G. V. SEBASTIANI
applicative purposes because we have observed that, at least
with respect to the primary oxidation steps of these substrates,
[
4]
the mechanism should be the same as in the aqueous medium.
In this paper, we report the data of c.e. obtained in the
photoelectrochemical oxidation of other benzylic substrates
(
alcohol 1, ethers 2a–2b and 1,2-diols 3a–3f).
Scheme 1.
Regarding the known mechanistic aspects of the above
[
3,5g]
reaction,
give the corresponding cation radical; this intermediate, after
(TiO
2
)
h
þ captures an electron from the alcohol to
[
5c,5f,6]
C—H deprotonation,
gives an a-hydroxy-4-methoxybenzyl
radical that, after oxidation, finally leads to 4-methoxybenzal-
dehyde (Scheme 1).
At the same time, the electron in the conduction band, which is
þ
transferred through the external circuit, is captured by Li
(
4
furnished by LiClO as the support electrolyte) to give Li (as a
metallic layer deposited on the Pt cathode).
Previously, we observed that, after the same time of thermal
treatment (2 h), 7008C is the temperature at which the highest
[
3a]
current density is measured.
In this paper, we report that the current density increases (at
2
These results verify and complete the previously suggested
this temperature) on going from 2 h (0.15 mA/cm ) to 4 h
2
mechanisms of the TiO sensitized photo-oxidation of the above
(0.23 mA/cm ) of thermal treatment. This behaviour can be
2
[
5]
substrates, that is determining: (i) the involvement of an
electron transfer to the benzylic radical intermediate within the
justified on the basis of the increased film thickness from 1.8 mm
to 2.3 mm (Fig. 1); this phenomenon can be attributed to a better
(TiO ) þ/(TiO ) À separation (larger space charge layer) as the
initial steps, (ii) if, in deaerated medium (under N
indicated as ), a second electron transfer is operative in the
steps from the benzylic radical intermediate to the final product
and (iii) if, in aerated medium (atmospheric O , indicated as ‘O ’),
2
bubbling,
2
h
2 e
TiO film becomes thicker.
2
2
2
the radical intermediate is trapped by oxygen and/or is oxidized
to the corresponding cation.
Photoelectrochemical oxidation of
1
-(4-methoxyphenyl) ethanol (1)
It must be observed that the confirmed ET mechanisms for 1,
[
5b,e,f]
With the more efficient photoanode (7008C of thermal treatment
for 4 h) it has been possible to perform the photo-oxidation of
2
a and 2b in this heterogeneous medium
those reported in ET homogeneous oxidations. On the contrary
we showed that, owing to adsorption phenomena, the electron is
are very similar to
[
6]
alcohol 1 on TiO /Ti. As previously observed in the photo-
2
[
5d]
[5f]
[
5a]
oxidation sensitized by TiO as powder and as colloid, the
reaction product is the corresponding acetophenone, in both
extracted from oxygen of one OH group of diols 3a–3f and not
2
from the aromatic ring, as happens in homogeneous oxi-
[
7]
dations; consequently, this phenomenon involves a different
[
5a]
C—C fragmentation path of the cation radical.
Finally, it should be noted that the photoelectrochemical
[
3a]
technology previously reported by us
has been improved in
(rutile) film at the Ti
this paper by preparing a more efficient TiO
2
surface. In this way, it has been possible to extend the study to the
photoelectrochemical oxidation of substrates (1, 2a and 2b) that
did not significantly react under the previous experimental
conditions.
RESULTS AND DISCUSSION
Improvement of the photoelectrochemical technology
[
3a]
As previously reported, the photoelectrochemical oxidation of
-methoxybenzyl alcohol in CH CN is considered as the reference
4
3
reaction to evaluate, through current density measurements, the
photocatalytic efficiency of TiO /Ti anodes, in which the TiO2
2
(
rutile) film is prepared by thermal treatment of a Ti plate under
Figure 1. Field emission scanning electron micrography (FESEM) of cross
different experimental conditions.
2
section of the TiO film after 4 h of thermal treatment at 7008C
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 219–224

2 3
TiO /Ti PHOTOANODES FOR PHOTO-OXIDATION MECHANISMS IN CH CN
Table 1. Current efficiency and conversion in
-methoxyacetophenone for the photoelectrochemical oxi-
dation of 1-(4-methoxyphenyl) ethanol (1) on TiO (rutile)/Ti
4
2
3
in CH CN
Scheme 3.
Unreacted
substrate
(%)
4-Methoxy
acetophenone
(%)
Current
efficiency
a
b
Substrate
(%)
n
methyl ethers 2a and 2b gives (Scheme 4) the corresponding
3
carbonyl compound (C) in deaerated CH CN accompanied, in
6
0
30
30
110
85
2
2
1
1
(
)
aerated medium, by methyl 4-methoxybenzoate (E). As reported
in Table 2, the material recovery is quantitative (ꢁ90%).
(O
2
)
58
On the basis of the previous results obtained with TiO
[5b,d]
2
a
Error ¼ Æ20.
powder
and from c.e. determinations, our complete mechan-
b
Electrons exchanged per molecule of formed acetophenone.
istic suggestion is reported in Scheme 5, both in deaerated and in
aerated medium.
In the first medium (aldehyde formation from 2a) the c.e.
aerated and deaerated medium (Eqn (3)).
(
Table 2) is nearly quantitative considering that two electrons
are involved per molecule of formed product that is, the first
electron is captured from the substrate by (TiO
2
)
h
þ and a second
electron is captured from the benzylic radical 5 (via a in
Scheme 5).
ð3Þ
In aerated medium, the current efficiency is nearly quantitative
considering that two electrons per molecule of carbonyl com-
pound (as observed above in deaerated CH CN) and one electron
3
for the methyl benzoate formation are necessary (Table 2).
According to Scheme 5, the benzylic radical can give the
carbonylic compound through the oxidation of the free radical
As reported in Table 1, material recovery is ca. 90%.
On the basis of the previous results obtained from competitive
and from the c.e. deter-
minations with TiO photoanode in an electrolytic cell (this work),
[
5f]
and quantum yield measurements
2
our definitive hypothesis about the mechanism, valid for both the
(
(
via a) and the ester through the competitive capture by oxygen
via b).
deaerated and aerated medium, is reported in Scheme 2.
In deaerated CH CN, the c.e. is nearly quantitative considering
two electrons involved per molecule of formed product
4-methoxyacetophenone) (Table 1), one electron is captured
3
It must be observed that, in aerated CH CN, the radical 5
3
competitively undergoes both oxidation to cation and capture by
oxygen, whereas the radical 4 (from alcohol 1) is only oxidized.
Therefore, under the experimental conditions of this work, the
(
by (TiO ) þ from the substrate and the second is captured from
2
h
the benzylic radical by a second hole (or by the conduction band
.
.
oxidizability order should be 4-CH
R)OH. Accordingly to this behaviour, the reported half-wave
oxidation potentials of two structurally related radicals as
3 3 3
OPhC (R)OCH < 4-CH OPhC
[
8]
2
of TiO , if the phenomenon of ‘current doubling’ is involved).
(
In aerated medium, the results obtained in the photoelectro-
chemical experiments, that is a quantitative c.e. for two electrons
exchanged (Table 1), exclude a competitive path (Scheme 3) that
involves the capture of the benzylic radical intermediate by
oxygen where a second hole does not participate. This exclusion
can be explained on the basis of the low oxidation potential of
the benzylic radical, due to the presence of an a-OH group (the
.
.
PhC (CH
3
)OCH
3
and PhC (CH
3
)OH are À0.11 and À0.24 V (vs.
[9]
SCE), respectively.
Photoelectrochemical oxidation of 1,2-diols (3)
[
5a]
[
9]
In a previous work, we have reported the reaction mechanism
oxidation to cation is favoured).
of photo-oxidation of diols 3a–f, sensitized by TiO (as powder or
2
As a confirmation, the c.e. measurements performed in the
photoelectrochemical reaction of 4-methoxybenzyltrimethyl-
colloid) in aerated CH
3
CN, to give carbonylic compounds
[
3b]
as C —C fragmentation products. On the basis of the reaction
a
b
silane to 4-methoxybenzaldehyde
showed that the corre-
products, adsorption equilibrium constants, competitive kinetic
experiments and quantum yields, the mechanism in Scheme 6
has been suggested.
sponding benzylic radical intermediate (less oxidizable due to the
a-OH group absence) is captured by oxygen instead of being
oxidized to cation.
The photoelectrochemical reaction of the considered diols has
been performed (3a–f), both in deaerated and in aerated CH CN,
always obtaining the same products observed in the presence of
3
Photoelectrochemical oxidation of benzylic ethers (2)
As previously reported in the photo-oxidation sensitized by TiO
2
TiO
2
as powder or colloid in aerated medium. This study has
[5b,d]
as powder,
the photoelectrochemical oxidation of 4-methoxybenzyl
confirmed the steps reported in Scheme 6 and has allowed the
Scheme 2.
J. Phys. Org. Chem. 2008, 21 219–224
Copyright ß 2008 John Wiley & Sons, Ltd.
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M. BETTONI, C. ROL AND G. V. SEBASTIANI
Scheme 4.
Table 2. Product yield and current efficiency in the photoelectrochemical oxidation of 4-methoxybenzyl methyl ethers 2a and 2b
on TiO (rutile)/Ti in CH CN
2
3
Products (%)
Unreacted
substrate (%)
Current
efficiency (%)
a
b
c
Substrate
R
C
E
n
C
n
E
H
H
65
25
—
110
2
—
2
a ( )
2
2
a (O
2
)
78
76
13
9
6
6
105
110
2
2
1
1
b (O2)
CH3
a
b
c
Error ¼ Æ20.
Electrons exchanged per molecule of carbonyl compound.
Electrons exchanged per molecule of methyl benzoate.
Scheme 5.
paths from the radical 6 to the corresponding carbonyl
compound to be defined.
(Eqn (4)).
ð4Þ
As reported in Table 3, the material recovery (substrate þ
PhCRO) is nearly quantitative (ꢁ90%).
The c.e. determinations in Table 3 for the deaerated reactions
of the diols 3a and 3b are quantitative for two electrons
exchanged per molecule of aromatic carbonyl compound.
Therefore, the first electron is captured from the substrate by
the hole and a second electron must be involved for the oxidation
Scheme 6.
In particular, 1,2-diols 3a and 3b give C —C fragmentation
products, the corresponding benzaldehyde or acetophenone,
respectively, that were always accompanied by formaldehyde
a
b
0
00
of the a-hydroxyl radical intermediate 6 with R ¼ H (Scheme 6
and via a in Scheme 7).
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 219–224

TiO
2
/Ti PHOTOANODES FOR PHOTO-OXIDATION MECHANISMS IN CH
3
CN
Table 3. Product yield and current efficiency in the photo-
electrochemical oxidation on TiO
Table 4. Product yield and current efficiency in the photo-
electrochemical oxidation of 1,2-diaryl-1,2-ethanediols 3c–f
on TiO (rutile)/Ti in CH CN
2
(rutile)/Ti of 1,2-diols 3a,b
in CH CN
3
2
3
PhCRO
(%)
Unreacted
substrate (%)
Current
efficiency (%)
XPhCHO
a
(%)
Unreacted
substrate (%) efficiency (%)
Current
a
b
b
c
Substrate
n
Substrate
n
1
8
6
78
60
81
93
2
2
28
13
8
70
75
94
71
90
82
94
85
2
3
3
a (
)
3c (
3d (
)
2
2
2
2
b (
)
)
3
3
a (O
2
)
27
20
80
70
80
95
1
1
3
e (
f (
)
2
b (O )
2
0
3
)
a
Error ¼ Æ20.
3c (O2)
3d (O2)
3e (O2)
15
16
21
16
70
73
71
75
83
91
85
70 (140)
2
2
2
2 (1)
b
Electrons exchanged per molecule of aromatic carbonyl
compound.
3
f (O
2
)
a
According to the XPhCHO/substrate stoichiometry (2/1) the
reported yield is half of the measured one.
In contrast, the c.e. determined in aerated medium for the diols
a and 3b (Table 3) are quantitative considering the exchange of
only one electron. This means that one hole is involved
in catching the electron from the substrate to give the cation
radical, whereas the hydroxymethyl radical reacts with oxygen
b
Error ¼ Æ20.
3
c
Exchanged electrons with respect to the half of molecules of
benzaldehyde.
(
Scheme 7, via b) and is not oxidized by the semiconductor (via a).
Regarding 1,2-diaryl-1,2-ethanediols 3c–f, the corresponding
benzaldehyde was obtained, both in deaerated and in aerated
medium (Eqn (5)).
corresponding cation (Scheme 7, via a) instead of being captured
by oxygen (via b).
In this aerated medium, the current yield for 3f is not
quantitative when either two electrons (c.e. ¼ 140%) or one are
involved (70%). This result suggests that, in the case of benzylic
0
00
radical 6 with R ¼ 4-CF
3
Ph, the oxidation to the corresponding
cation by the hole (path a in Scheme 7) and the capture by
oxygen (path b) are in competition. The different behaviour of the
radicals 6 from 3c–e with respect to that from 3f can be ascribed
to the lower oxidizability of the latter radical due to the presence
of the electron-withdrawing ring substituent.
ð5Þ
In all the photoelectrochemical experiments the material
recovery was quantitative considering two moles of carbonyl
product per mole of reacted substrate (Table 4).
Finally, the c.e. data show that it is possible to evaluate, in these
As observed for 3a and 3b, the c.e. determinations in
deaerated medium showed that two electrons are involved in
the reaction. This is in line with Scheme 6 and via a in Scheme 7
experimental conditions, the relative oxidizability of different
.
a-hydroxy radicals 6 derived from diols 3a–f that is: CH
OH <
2
.
.
CH(4-CF
Ph)OH < CH(XPh)OH (X ¼ 4-OCH
, 3-OCH , H).
3 3
3
(
where the corresponding benzaldehyde is formed through the
0
00
oxidation of the a-hydroxybenzylic radical 6, with R ¼ Ph,
-CH OPh, 3-CH OPh and 4-CF Ph, to the corresponding cation).
The c.e. measurements in the photo-oxidation of the diols 3c–e
EXPERIMENTAL
4
3
3
3
1
H-NMR spectra were run on a Bruker AC 200 (200 MHz)
with TMS as internal
standard. GC-MS analyses were performed on a Hewlett Packard
890A gas-chromatograph (HP-Innovax capillary column, 15 m)
in aerated medium showed that two electrons are again
involved (Table 4); the corresponding ring-substituted benzylic
spectrometer, with solutions in CDCl
3
000
radicals 6 (R ¼ Ph, 4-CH
3 3
OPh, 3-CH OPh) are oxidized to the
6
coupled with a MSD-HP 5973 mass selective detector (70 eV). GC
analyses were carried out on a HP Agilent Technologies 6850
gas-chromatograph using a HP capillary column, 30 m. HPLC
analyses were performed with a liquid chromatograph HP 1100.
Starting materials
Titanium plates (CpG2 from Titania S.p.A. Terni Italy 2.5 Â 8.0 cm,
thickness ¼ 0.5 mm containing N < 0.007%, C < 0.004%, H < 0.012%,
Fe < 0.024% and O < 0.09%) were cleaned in acetone, then
etched for 30 s in dilute Kroll’s acid (4% w/w HF, 30% w/w HCl),
[
3a]
rinsed in distilled water and then in acetone.
2 4
H SO ,
Scheme 7.
LiClO , Na CO and CH CN (99.9%, HPLC grade, containing
4
2
3
3
J. Phys. Org. Chem. 2008, 21 219–224
Copyright ß 2008 John Wiley & Sons, Ltd.
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M. BETTONI, C. ROL AND G. V. SEBASTIANI
0
.02% water from Karl Fisher analysis) were analytical grade
Formaldehyde was recognized as dimedone adduct [MS m/z (rel
þ
commercial products. 1(4-Methoxyphenyl)ethanol, 1-phenyl-1,2-
ethanediol, 2-phenyl-1,2-propanediol, 1,2-diphenyl-1,2-ethanediol
intensity) 292 M , 191, 165(100), 124, 97, 83, 69, 55].
[
10]
were commercial samples. 4-Methoxybenzyl methyl ether
11]
,
Acknowledgements
[
4-methoxy-a-methylbenzyl methyl ether
,
1,2-bis(3-meto-
[
12]
This work was carried out with the financial support of
the Ministero dell’Istruzione, dell’Universit a` e della Ricerca
contract/grant number: PRIN 2006. Thanks to Dr Luca Valentini
of the Dipartimento of Ingegneria Civile ed Ambientale and Dr
Federica Meloni of NIPLAB – INSTM (Perugia) for the FESEM
analyses.
xyphenyl)-1,2-ethanediol , 1,2-bis(4-metoxyphenyl)- 1,2-ethan-
[12]
ediol
[13]
,
1,2-bis[4-(trifluoromethyl)phenyl]-1,2-ethanediol
prepared and characterized as described in the literature.
were
Preparation and characterization of TiO /Ti electrodes
2
The titanium plates were subjected to constant current
À2
(
25 mA cm ) mild anodic oxidation (6443B DC HP power supply)
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positioned in front of Ti/TiO electrode and the apparatus was
covered by a closed aluminium cylinder. Current flowed only
1
63, 481–487; d) T. Del Giacco, C. Rol, G. V. Sebastiani, J. Phys. Org.
0
under irradiation and after 10–30 (depending of the substrate) it
Chem. 2003, 16, 127–132; e) M. Bettoni, T. Del Giacco, C. Rol, G. V.
Sebastiani, J Chem. Res. (S) 2003, 415–417; f) T. Del Giacco, M.
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was practically constant. After 15–20 coulombs passed, the
mixture was concentrated at room temperature, poured into a
double volume of water and repeatedly extracted with diethyl
ether. The organic layer was washed with NaCl-saturated water,
745–751; g) L. Amori, T. Del Giacco, C. Rol, G. V. Sebastiani,
J. Chem. Res. (S) 1998, 644–645.
[
[
[
[
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2 4
dried on Na SO and concentrated. The crude product was
1
analysed by H-NMR, HPLC and GC in the presence of suitable
internal standards, and by GCMS.
1
18, 5952–5960.
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Reaction product analysis
The reaction products were identified directly from the crude by
comparison of H-NMR and GC analysis data with those of the
commercial samples (acetophenone, 4-methoxyacetophenone,
methyl 4-methoxybenzoate, 4-methoxybenzaldehyde, 3-methoxy-
benzaldehyde, benzaldehyde and 4-trifluoromethylbenzaldehyde).
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1
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