Electrocatalytic oxidation of alcohols by a carbon-supported Rh porphyrin

Article abstract of DOI:10.1039/c2cc30888f

A Rh porphyrin on carbon black was shown to catalyze the electro-oxidation of several aliphatic alcohols (ethanol, 1-propanol, and 2-propanol) and benzyl alcohols. The overpotentials for alcohol oxidation were very low. The reaction mechanism and substrate specificity are discussed. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30888f

View Article Online / Journal Homepage / Table of Contents for this issue
This article is part of the
Porphyrins &
Phthalocyanines
web themed issue
Guest editors: Jonathan Sessler, Penny Brothers and
Chang-Hee Lee
All articles in this issue will be gathered together
online at
www.rsc.org/porphyrins

Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 4353–4355
www.rsc.org/chemcomm
COMMUNICATION
Electrocatalytic oxidation of alcohols by a carbon-supported
Rh porphyrinwz
Shin-ichi Yamazaki,* Masaru Yao, Naoko Fujiwara, Zyun Siroma, Kazuaki Yasuda and
Tsutomu Ioroi
Received 8th February 2012, Accepted 9th March 2012
DOI: 10.1039/c2cc30888f
A Rh porphyrin on carbon black was shown to catalyze the
electro-oxidation of several aliphatic alcohols (ethanol, 1-propanol,
and 2-propanol) and benzyl alcohols. The overpotentials for alcohol
oxidation were very low. The reaction mechanism and substrate
specificity are discussed.
Rh porphyrins have good catalytic activity for the electro-
9
10
11
4 2 4
oxidation of inorganic compounds (CO, NaBH , and N H )
1
2
13
and organic compounds (oxalic acid and glucose ). In addition
to electrochemical reactions, the reaction of Rh porphyrins and
14–18
alcohols has been extensively studied.
Recently, Liu et al.
reported that a Rh porphyrin can catalyze the oxidation of
1
4
2
various alcohols by O . It is reasonable to expect that alcohols
The electrochemical oxidation of alcohols is an important
process with regard to direct alcohol fuel cells, alcohol sensing,
and the electrochemical conversion of alcohols to aldehydes
and ketones. While Pt or Pt alloys have mainly been studied
might undergo electro-oxidation by Rh porphyrins.
In this report, we show that Rh porphyrins can catalyze the
electro-oxidation of aliphatic and benzyl alcohols at much
lower overpotentials than with other metallocomplex-based
catalysts. Several reaction properties such as substrate specificity
and pH-dependence were examined, and a reaction mechanism
is discussed based on these properties.
1
as electrocatalysts for this electrochemical reaction, some
metallocomplex-based catalysts have also been examined for
–5
alcohol oxidation.
2
Metallocomplexes are widely used as homogenous catalysts
for oxidation. These homogenous oxidation catalysts offer
several advantages: (1) only a small amount of metal is used,
2
,3,7,8,12,13,17,18-Octaethylporphyrinato rhodium(III) chloride
III
[Rh (OEP)(Cl)]) was synthesized as described in the literature
19
(
with some modifications. The Rh porphyrins were adsorbed on
carbon black by an evaporation-to-dryness method. The amount
(
2) the reaction mechanism is simpler than with heterogenous
catalysts, and (3) the activity can be controlled to some extent
by manipulating the ligand structure. For alcohol oxidation,
Ru complex- or Ni complex-based electrocatalysts have been
ꢀ
1
of Rh porphyrin was fixed at 30 mmol gcarbon black . The activity
of electrochemical alcohol oxidation was evaluated by hydro-
dynamic voltammetry using a rotating disk electrode modified
2–5
investigated. Although metallocomplex-based electrocatalysts
III
III
20
with a carbon-supported [Rh (OEP)(Cl)] ([Rh (OEP)(Cl)]/C ).
Electrochemical measurements were performed using this
electrode in 1 M NaOH. The products of this reaction
were examined by HPLC. Experimental details are described
in ESI.z
for the electro-reduction of O (cathode catalysts) have been
2
substantially investigated for the development of non-platinum
–8
6
cathode catalysts in fuel cells,
there are few examples of
catalysts for electro-oxidation (anode catalysts) that include
2
–5
metallocomplexes, since the overpotentials are high and the
maximum reaction rate is slow.
III
Fig. 1A shows voltammograms of [Rh (OEP)(Cl)]/C,
which exhibit ethanol oxidation in a 1 M NaOH solution.
Line (a) is a voltammogram of this catalyst under an argon
atmosphere without ethanol. A clear Faradaic current was
not observed. Upon the addition of ethanol (0.48 M), the
oxidation current significantly increased (line b). The results
Against this background, we have tried to develop
metallocomplex-based electro-oxidation catalysts that oxidize
alcohols at low overpotentials. In this study, we paid special
attention to Rh porphyrins as a potential component of
metallocomplex-based electrocatalysts. It has been shown that
III
clearly show that [Rh (OEP)(Cl)]/C catalyzes ethanol oxidation
at low overpotentials. When the electrode was rotated at
3
600 rpm, the current increased due to enhanced mass transfer
Research Institute for Ubiquitous Energy Devices,
National Institute of Advanced Industrial Science and Technology (AIST),
of ethanol (line c). A further increase in the electrode rotation
rate from 3600 rpm to 6400 rpm did not increase the oxidation
current (line d). Above at least 3600 rpm, the current was
limited by the kinetics of ethanol oxidation. The current
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
E-mail: s-yamazaki@aist.go.jp; Fax: +81-72-751-9629
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
ꢀ
2
z Electronic supplementary information (ESI) available: pH-dependence
controlled by the kinetic process reached 3.5 mA cm at
(Fig. S1), inhibitory effects of aldehydes and ketones (Fig. S2), a kinetic
0
.55 V (vs. a reversible hydrogen electrode (RHE)). The results
III
isotope effect (Fig. S3), and experimental details for the synthesis of
Rh porphyrins, catalyst preparation, electrochemical measurements,
and product analysis. See DOI: 10.1039/c2cc30888f
clearly indicate that carbon-supported [Rh (OEP)(Cl)] catalyzes
ethanol oxidation.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4353–4355 4353

Fig. 1 (A) Voltammograms of Rh-OEP/C (a) in the absence of ethanol,
Fig. 2 Voltammograms of Rh-OEP/C in the presence of alcohol
10 mM) with electrode rotation (6400 rpm); (a) 2-propanol, (b) ethanol,
c) 1-propanol, (d) methanol, (e) benzylalcohol, (f) 4-fluorobenzylalcohol,
(
(
b) in the presence of ethanol (0.48 M), (c) in the presence of ethanol
0.48 M) with electrode rotation (3600 rpm), and (d) in the presence of
(
(
ethanol (0.48 M) with electrode rotation (6400 rpm). (B) Voltammograms
of Rh-OEP/C in the presence of alcohol (0.48 M) with electrode rotation
and (g) 4-methoxybenzylalcohol. Measurements were performed in a
ꢀ1
1
M NaOH solution at 25 1C (scan rate = 10 mV s ) under an argon
atmosphere.
(6400 rpm); (a) 2-propanol, (b) ethanol, (c) 1-propanol, and (d) methanol.
Measurements were performed in a 1 M NaOH solution at 25 1C
ꢀ1
(
scan rate = 10 mV s ) under an argon atmosphere.
The onset potential (ca. 0.28 V vs. RHE) was much lower
than those with previous Ru complex- or Ni complex-based
4
,5
electrocatalysts (above 1.0 V vs. RHE). The onset potential
was also much lower than those for O electro-reduction by
Pt catalysts (ca. 0.7 V–1.0 V vs. RHE). Thus, a combination of
2
III
[
Rh (OEP)(Cl)]/C-catalyzed alcohol oxidation and Pt-catalyzed
O reduction can generate electric power. This corresponds to the
2
result that the oxidation of alcohols by O proceeds rapidly when
2
1
4
catalyzed by Rh porphyrin.
III
Ethanol oxidation by [Rh (OEP)(Cl)]/C was also examined
in neutral solutions as well as basic solutions by voltammetry.
Ethanol oxidation sharply decreased with a decrease in pH.
Fig. 3 A plot of the oxidation current at 0.6 V vs. concentrations of
(a) 2-propanol, (b) ethanol, and (c) 1-propanol. The conditions are the
same as those in Fig. 2.
Below pH 12, oxidation current was hardly observed (Fig. S1,
ESIz). This strongly suggests that OHꢀ is required for ethanol
oxidation.
Fig. 1B shows linear sweep voltammograms of the electro-
III
oxidation of several aliphatic alcohols by [Rh (OEP)(Cl)]/C.
2-propanol oxidation by a possible product. The results clearly
showed that propionaldehyde strongly inhibits 2-propanol
oxidation even at a low concentration (1 mM), while acetone
and acetaldehyde have mild effects (Fig. S2, ESIz). The low
activity of 1-propanol at high concentrations can be explained
by the inhibitory effect of the product.
The results showed that all of the alcohols tested here undergo
III
electro-oxidation by [Rh (OEP)(Cl)]/C at low overpotentials.
For all of these alcohols, an increase in the rate of electrode
rotation from 3600 rpm to 6400 rpm did not affect alcohol
oxidation, as observed in ethanol oxidation. The intrinsic
activity of alcohol oxidation controls the current under these
conditions. This activity significantly depends on the alcohol
used. The current increased in the order 2-propanol (line a) >
ethanol (line b) > 1-propanol (line c) c methanol (line d).
While the oxidation of ethanol and 2-propanol gave high
current, the oxidation of methanol gave only slight current.
To investigate the difference in activity according to alcohol
used in detail, the oxidation of low-concentration (10 mM)
At low concentrations, the current reflects the intrinsic
activity without inhibition by products. The oxidation current
of methanol was also very low at 10 mM (Fig. 2A). The
III
intrinsic activity of [Rh (OEP)(Cl)]/C toward methanol was
low. The current increased in the order 1-propanol (line c) >
2-propanol (line a) > ethanol (line b) c methanol (line d).
This order suggests two possibilities: one is that electron
donation to a-carbon is favorable for this reaction and the
other is that the presence of a hydrogen atom on b-carbon is
key for this reaction.
III
alcohols by [Rh (OEP)(Cl)]/C was also examined. Fig. 2A
III
shows the results. Interestingly, [Rh (OEP)(Cl)]/C exhibited
Fig. S3 (ESIz) shows a comparison of 2-propanol and
significantly high activity toward 1-propanol (line c), while
1
2-propanol-2-d . A large kinetic isotope effect is apparent.
1
-propanol was much less reactive than 2-propanol at a higher
The rate-determining step involves the extraction of a hydrogen
atom on the a-carbon.
Here, we discuss a possible reaction mechanism for alcohol
concentration (0.48 M, Fig. 1B).
Fig. 3 shows the dependence of the oxidation current at
0
1
.6 V on the concentrations of alcohols. The profile of
-propanol is totally different from those of 2-propanol and
oxidation. It has been reported that alcohol coordinates on a
14,17
water-soluble Rh porphyrin.
Alcohols should also coordinate
ethanol. It exhibited a bell-shaped curve with a peak at 20 mM.
This strongly suggests that the product of the reaction had
a kind of inhibitory effect. Next, we examined inhibition of
on Rh porphyrins during the electro-oxidation of alcohols.
The coordinated alcohol (alkoxide in basic solutions) undergoes
electrochemical oxidation. Since ethanol oxidation occurs only
4
354 Chem. Commun., 2012, 48, 4353–4355
This journal is c The Royal Society of Chemistry 2012

in basic solution (Fig. S1, ESIz), the extraction of a hydrogen
metallocomplexes at such low overpotentials might lead to the
development of an anode catalyst for not only direct alcohol
fuel cells, but also the electrochemical conversion of alcohols
to ketones and aldehydes.
ꢀ
atom by OH is needed for alcohol oxidation. The kinetic
isotope effect observed in Fig. S3 (ESIz) indicates that the
elimination of an a-hydrogen atom occurs in the rate-determining
step. As well as the extraction of a hydrogen atom, electron
transfer to the electrode via a Rh porphyrin molecule occurs to
generate ketones or aldehydes. The generation of Rh(I) or
Rh–H species has been observed in the oxidation of alcohols
We thank Dr S. Takeda (AIST) for the measurement of
ESI-MS, and Center for Organic Elemental Microanalysis
(Kyoto University) for elemental analysis.
1
4
by O
2
.
It is unknown whether these species participate in
Notes and references
the electro-oxidation of alcohols, since they would be easily
oxidized by the electrode.
1
N. Fujiwara, Z. Siroma, S. Yamazaki, T. Ioroi, H. Senoh and
K. Yasuda, J. Power Sources, 2008, 185, 621.
We also examined the electro-oxidation of benzyl alcohol
derivatives. Since benzyl alcohols exhibit low solubility, the
reactivity was examined at a low concentration (10 mM). The
2 K. Cheung, W. Wong, D. Ma, T. Lai and K. Wong, Coord. Chem.
Rev., 2007, 251, 2367 and references therein.
3
B. A. Moyer, M. S. Thompson and T. J. Meyer, J. Am. Chem. Soc.,
980, 102, 2310.
1
III
results are shown in Fig. 2B. Carbon-supported [Rh (OEP)(Cl)]
4 (a) A. L. B. Marques, W. Li, E. P. Marques and J. Zhang,
Electrochim. Acta, 2004, 49, 879; (b) N. Chanda, B. Mondal,
V. G. Puranik and G. K. Lahiri, Polyhedron, 2002, 21, 2033.
also oxidized benzyl alcohols at low overpotentials. The reactivity
significantly depended on the p-substituent. An electron-donating
substituent was favorable. Liu et al. reported that the aerobic
oxidation of benzyl alcohols containing electron-donating groups
proceeds faster than that of benzyl alcohols containing electron-
5
A. N. Golikand, S. Shahrokhian, M. Asgari, M. G. Maragheh,
L. Irannejad and A. Khanchi, J. Power Sources, 2005, 144, 21.
6 H. Liu, L. Zhang, J. Zhang, D. Ghosh, J. Jung, B. W. Downing
and E. Whittemore, J. Power Sources, 2006, 161, 743.
7
S. Yamazaki, Y. Yamada, T. Ioroi, N. Fujiwara, Z. Siroma,
K. Yasuda and Y. Miyazaki, J. Electroanal. Chem., 2005, 576, 253.
8 R. Jasinski, Nature, 1964, 201, 1212.
1
4
withdrawing groups. The order of overpotentials observed in
electrochemical oxidation coincides with the order observed in
9
(a) J. F. van Baar, J. A. R. van Veen and N. de Wit, Electrochim.
Acta, 1982, 27, 57; (b) S. Yamazaki, T. Ioroi, Y. Yamada,
K. Yasuda and T. Kobayashi, Angew. Chem., Int. Ed., 2006,
2
alcohol oxidation by O .
We determined the product of the oxidation of benzyl
alcohol. The oxidation of benzyl alcohol gave benzoic acid
4
5, 3120; (c) S. Yamazaki, Y. Yamada, S. Takeda, M. Goto,
T. Ioroi, Z. Siroma and K. Yasuda, Phys. Chem. Chem. Phys.,
010, 12, 8968; (d) S. Yamazaki, M. Yao, Z. Siroma, T. Ioroi and
(
3.9 mmol) as well as benzaldehyde (2.2 mmol). The amount of
2
electrons (20 mmol) calculated from the amounts of benzaldehyde
and benzoic acid almost coincides with the amount of electrons
K. Yasuda, J. Phys. Chem. C, 2010, 114, 21856; (e) S. Yamazaki,
M. Yao, S. Takeda, Z. Siroma, T. Ioroi and K. Yasuda,
Electrochem. Solid-State Lett., 2011, 14, B23; (f) S. Yamazaki,
M. Yao, N. Fujiwara, Z. Siroma, T. Ioroi and K. Yasuda,
J. Electroanal. Chem., 2012, 668, 60.
21
(23 mmol) calculated from coulometry. The generation of
carboxy acid is favorable in terms of fuel cell applications, but
is undesirable with respect to the chemical conversion of alcohol
to aldehyde. Regulation of the product is now being studied.
1
0 (a) S. Yamazaki, H. Senoh and K. Yasuda, Electrochem. Commun.,
009, 11, 1109; (b) S. Yamazaki, M. Yao, H. Senoh, Z. Siroma,
2
In conclusion, our results showed that carbon-supported
III
Rh (OEP)(Cl)] catalyzed the oxidation of aliphatic and
N. Fujiwara, T. Ioroi and K. Yasuda, Catal. Today, 2011,
170, 141.
[
1
1
1
1
1
1
1 S. Yamazaki, T. Ioroi, K. Tanimoto, K. Yasuda, K. Asazawa,
S. Yamaguchi and H. Tanaka, J. Power Sources, 2012, 204, 79.
2 S. Yamazaki, Y. Yamada, N. Fujiwara, T. Ioroi, Z. Siroma,
H. Senoh and K. Yasuda, J. Electroanal. Chem., 2007, 602, 96.
3 S. Yamazaki, N. Fujiwara, S. Takeda and K. Yasuda, Chem. Commun.,
benzyl alcohols at low overpotentials. The potential regions
are much lower than those for alcohol oxidation by other
metallocomplexes. The rate of alcohol oxidation significantly
depends on the alcohol used. Among aliphatic alcohols,
2010, 46, 3607.
2-propanol exhibited the highest activity at a high concentration
4 L. Liu, M. Yu, B. B. Wayland and X. Fu, Chem. Commun., 2010,
46, 6353.
5 H. S. Fung, Y. W. Chan, C. W. Cheung, K. S. Choi, S. Y. Lee,
Y. Y. Qian and K. S. Chan, Organometallics, 2009, 28, 3981.
6 S. Sarkar, S. Li and B. B. Wayland, J. Am. Chem. Soc., 2010,
(
0.48 M), and methanol gave the lowest activity. 1-Propanol
gave the highest activity at a low concentration (10 mM), while
the activity was low at a high concentration. The unusual
concentration-dependence of 1-propanol can be explained by
the inhibition by the product. For the oxidation of benzyl
alcohols, substrate specificity is correlated with electronic
properties of substituents. The results of experiments on
pH-dependence and a kinetic isotope effect suggest that alcohols
coordinated on a Rh atom undergo the extraction of a hydrogen
atom on the a-carbon atom that is attached to a hydroxy group.
The oxidation of benzyl alcohols gave benzoic acid as well
as benzaldehyde. Thus, the electro-oxidation of alcohols by
1
32, 13569.
17 S. Bhagan, S. Sarkar and B. B. Wayland, Inorg. Chem., 2010,
9, 6734.
4
1
1
8 R. Irie, X. Li and Y. Saito, J. Mol. Catal., 1983, 18, 263.
9 H. Ogoshi, J. Setsune, T. Omura and Z. Yoshida, J. Am. Chem. Soc.,
1975, 97, 6461.
20 It should be noted that this denotation is the initial state, since the
axial ligand (Cl) and the oxidation number of the central metal
during the catalytic cycle are unclear.
2
1 The difference should be attributed to the instability of benzaldehyde
in the basic solution.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4353–4355 4355