Electrochemical oxidation of sugars at moderate potentials catalyzed by Rh porphyrins

Article abstract of DOI:10.1039/c003026k

In this communication, we demonstrate that certain kinds of Rh porphyrins on carbon black can electrochemically oxidize aldose at low potentials. The onset potential was much lower than those with the other complex-based catalysts. A product analysis suggested that this reaction involves 2-electron oxidation of the aldehyde group.

Full text of DOI:10.1039/c003026k

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Electrochemical oxidation of sugars at moderate potentials catalyzed
by Rh porphyrinsw
Shin-ichi Yamazaki,*a Naoko Fujiwara,a Sahori Takedab and Kazuaki Yasudaa
Received 12th February 2010, Accepted 24th March 2010
First published as an Advance Article on the web 9th April 2010
DOI: 10.1039/c003026k
In this communication, we demonstrate that certain kinds of Rh
porphyrins on carbon black can electrochemically oxidize aldose at
low potentials. The onset potential was much lower than those with
the other complex-based catalysts. A product analysis suggested that
this reaction involves 2-electron oxidation of the aldehyde group.
complexes have been demonstrated to exhibit high activity
toward the electrochemical oxidation of CO and HOOC–COOH
in low-potential regions in acidic solution.25–27 The high activities
toward these carbonyl compounds encouraged us to examine
the electrochemical reactivities of Rh porphyrins toward low-
potential sugar (especially aldose) oxidation, since sugar electro-
oxidation would occur via carbonyl activation.
The significance of the electrochemical oxidation of sugars has
been emphasized in the fields of analytical chemistry and fuel
cell technology. This reaction is useful for amperometric
sensors for sugars1–4 and HPLC detectors for sugars.5 The
low-potential electro-oxidation of sugars is important for fuel
cells which use them as a fuel.6–9
In this work, we found that certain kinds of Rh porphyrins
can act as good catalysts for oxidizing aldose in low-potential
regions. These catalysts exhibited higher activity toward
aldose than ketose. The overpotential for glucose oxidation
was the lowest among those of the complex-based catalysts
reported so far. A high catalytic current for glucose oxidation
was observed in moderate potential regions.
The importance of this reaction has inspired many researchers
to develop a new catalyst that promotes the electrochemical
oxidation of sugars. Metal electrodes7–15 and enzyme-modified
electrodes1,2,6 have been investigated with regard to their activity
to catalyze the electro-oxidation of sugars. Although these
catalysts have good electrocatalytic activity for the electro-
chemical oxidation of sugars, some problems have been pointed
out. In the case of Pt and Au, poisoning by adsorbed inter-
mediates has been shown to be a significant problem.16 The
amount of Pt required for glucose oxidation is also a bottleneck.
It is somewhat difficult to increase the surface area of Au by
preparing stable nano-particles due to its low melting point.
Hence, the catalytic current by Au electrocatalysts remains low.
Enzyme-modified electrodes can be used only under limited
conditions of pH and temperature. Enzyme-modified electrodes
sometimes require mediators for electron transfer and the
immobilization of enzymes, which can result in complex structures.
These drawbacks of noble-metal and enzyme-modified electrodes
have stimulated the study of new electrocatalysts. Complex-
immobilized chemically-modified electrodes have attracted much
interest as new catalysts.17 The electrochemical oxidation of
sugar by Co phthalocyanins,5,18–20 Ru complexes,21,22 and Ni
complexes23,24 has also been examined. However, the over-
potentials are much higher than those with metal electrodes. Such
high overpotentials reduce their sensitivity in the determination of
sugars and the efficiency of fuel cells that use sugars as a fuel.
In this study, we focused on Rh porphyrins adsorbed on a
conductive support as an electrocatalyst. Rh N4-macrocycle
Rh porphyrins were prepared by reflux of Rh2Cl2(CO)4 and
the corresponding porphyrin ligands (see ESIw). The
abbreviation for the complexes are as follows: [RhIII(DPDE)]+
(DPDE = deuteroporphinate dimethyl ester), [RhIII(OEP)]+
(OEP
=
octaethylporphinate), [RhIII(T(–COOCH3)PP)]+
(T(–COOCH3)PP = 5,10,15,20-tetrakis(4-methylcarboxyphenyl)-
porphinate) and [RhIII(TBPP)]+ (TBPP = 5,10,15,20-tetrakis-
(4-bromophenyl)porphinate). Rh porphyrins were adsorbed on
carbon black with an evaporation-to-dryness method, and the
powder was used as an electrocatalyst. The amount of Rh
porphyrins adsorbed was fixed at 30 mmol gÀ1. Carbon-supported
platinum (Pt/C) was purchased from Johnson-Matthey Co. Ltd.
Powders of these catalysts (0.02 mg) were trapped on a glassy
carbon electrode (A = 0.07065 cm2).
Fig. 1A shows voltammograms of a carbon-supported
[RhIII(DPDE)]+ (Rh(DPDE)/C) in the absence/presence of
glucose. Upon the addition of glucose, the oxidation current
drastically increased. The anodic current started to flow
at ca. À0.75 V (vs. Ag|AgCl|KCl(sat.)). This onset potential and
those with previous catalysts are shown in Table S1 (ESIw).
The onset potential with Rh(DPDE)/C is extremely low
compared to those with other complex-based electrocatalysts
such as Co macrocycles, Ru bipyridine complexes, and
polymerized Ni porphyrins.5,18–24 The potential is comparable
to those with Au-based catalysts, although it is higher than
those with Pt-based catalysts.
Fig. 1B shows a comparison of the activities of Rh porphyrins.
A significant difference in current was observed between Rh
porphyrins. The Rh(DPDE)/C (line a) and Rh(OEP)/C (line b)
exhibited high activities, while Rh(T(–COOCH3)PP)/C (line c)
and Rh(TBPP)/C (line d) gave relatively low current.
a Research Institute for Ubiquitous Energy Devices, National Institute
of Advanced Industrial Science and Technology (AIST),
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan.
E-mail: s-yamazaki@aist.go.jp; Fax: +81-72-751-9629
b Research Institute for Innovation in Sustainable Chemistry,
National Institute of Advanced Industrial Science and Technology
(AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
w Electronic supporting information (ESI) available: Experimental details
for the synthesis of Rh porphyrins, catalyst preparation, electrochemical
measurements, and product analysis. See DOI: 10.1039/c003026k
Potential-step chronoamperometry was also performed for the
analysis of glucose electro-oxidation. The potential was stepped
from À1.0 V to À0.6 V. Fig. 2A shows chronoamperograms of
Rh(DPDE)/C. In the absence of glucose, the current approached
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Fig.
3
Normalized potential-step chronoamperograms of (a)
Rh(DPDE)/C, (b) Rh(OEP)/C, (c) RhT(–COOCH3)PP/C, and (d)
Rh(TBPP)/C in the presence of 0.5 M glucose. Other conditions were
the same as those in Fig. 2. If and Ic denotes an observed Faradaic
current and a charging current, respectively.
Fig. 1 (A) Cyclic voltammograms of Rh(DPDE)/C at a scan rate of
10 mV sÀ1 in the presence of (a) 0 mM, (b) 10 mM, and (c) 0.5 M
glucose. (B) Cyclic voltammograms of (a) Rh(DPDE)/C, (b)
Rh(OEP)/C, (c) RhT(–COOCH3)PP/C, and (d) Rh(TBPP)/C in the
presence of 0.5 M glucose. Measurements were performed in 1 M
NaOH solution under a nitrogen atmosphere at 25 1C.
Fig. 4 (A) Cyclic voltammograms of (a) Rh(DPDE)/C, (b) Pt/C, and
(c) Au electrodes in the presence of 0.5 M glucose. (B) Potential-step
chronoamperograms of (a) Rh(DPDE)/C, (b) Pt/C, and (c) Au
electrodes. Other conditions were the same as those in Fig. 1 or 2.
Fig. 4A shows a comparison of electrocatalytic activities for
glucose oxidation with several electrodes. Pt/C shows a lower
onset potential than Rh(DPDE)/C. However, the current
increase in regions of higher potential is sluggish compared to
that with Rh(DPDE)/C. The peak current per 1 g catalyst is
significantly higher with Rh(DPDE)/C (105 Æ 9.5 A gcatÀ1) than
with Pt/C (33.5 Æ 6.4 A gcatÀ1). Since the amount of Rh metal on
carbon support is low (in Rh porphyrin catalysts, Rh atoms are
highly dispersed at an atomic level), the difference in activity per
noble metal is much greater (3.4 Â 104 Æ 3.1 Â 103 A gRhÀ1 for
Rh(DPDE)/C, 84 Æ 15.9 A gPtÀ1 for Pt/C). Rh(DPDE)/C gave
high catalytic current even though a very small amount of Rh
was used. Under electrode rotation conditions, Rh(DPDE) also
exhibited higher current than Pt/C above À0.65 V (Fig. S1,
ESIw). The results of potential-step chronoamperometry
(Fig. 4B) also indicate that Rh(DPDE)/C has higher activity
than Pt/C. The onset potential with Au electrode was also low.
However, the catalytic current was much lower than
Rh(DPDE)/C in chronoamperometry (Fig. 4B) as well as
voltammetry (Fig. 4A). The low current is attributed to the
low surface area of gold electrode. Rh(DPDE)/C is a good
catalyst in terms of both the overpotential and current.
Interestingly, in constant-potential amperometry at À0.6 V,
Pt/C gave much higher current than Rh(DPDE)/C. The
application of more negative potential (À1.0 V) seems to be
favourable to increase the activity of Rh(DPDE)/C
Fig. 2 (A) Potential-step chronoamperograms of Rh(DPDE)/C in
the presence of (a) 0 mM, (b) 10 mM, and (c) 0.5 M glucose. (B)
Potential-step chronoamperograms of (a) Rh(DPDE)/C, (b)
Rh(OEP)/C, (c) RhT(–COOCH3)PP/C, and (d) Rh(TBPP)/C in the
presence of 0.5 M glucose. The potential was stepped from À1.0 V to
À0.6 V. Other conditions were the same as those in Fig. 1.
virtually zero immediately. In the presence of glucose, the
significant currents flowed, indicating that a Faradaic process
occurred. The current increased with the increase in the
concentration of glucose. Chronomaperograms of other carbon-
supported Rh porphyrins are shown in Fig. 2B. The activities of
Rh(DPDE)/C and Rh(OEP)/C were much higher than those of
Rh(T(–COOCH3)PP)/C and Rh(TBPP)/C. Rh(DPDE)/C, which
exhibits the highest activity, was used as an electrocatalyst for
further experiments.
The currents in Fig. 2B were normalized by charging
currents to eliminate the effect of the surface area. Charging
currents reflect electrochemically active areas. Hence, the
normalized catalytic currents correspond to the activities per
surface area. The charging current was determined from cyclic
voltammograms in the absence of glucose. The results are
shown in Fig 3. The difference in the activity between active
catalysts (Rh(DPDE)/C and Rh(OEP)/C) and relatively
inactive catalysts (Rh(T(–COOCH3)PP)/C and Rh(TBPP)/C)
is not attributed to the difference in the surface area but
to that in the intrinsic activity. The lower activity of
Rh(T(–COOCH3)PP)/C and Rh(TBPP)/C might be related
with the electron transfer rate. The bulkiness of meso-phenyl
groups prevents the access of these porphyrins to carbon
support and makes the electron transfer to electrode slow.
The reactivity of Rh(DPDE)/C toward other sugars (Scheme S1,
ESIw) was examined, and the results are summarized in Fig. 5.
The catalyst oxidized aldose, such as galactose (line a), glucose
(line b), maltose (a-D-glucopyranosyl-(1 - 4)-D-glucose) (line c),
and lactose (b-D-galactopyranosyl-(1 - 4)-D-glucose) (line d).
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Fig. 5 Cyclic voltammograms of Rh(DPDE)/C at a scan rate of
10 mV sÀ1 in the presence of 0.5 M (a) galactose, (b) glucose, (c) maltose,
(d) lactose, (e) fructose, and (f) sucrose. Other conditions were the
same as those in Fig. 1.
Fig. 6 Cyclic voltammograms of Rh(DPDE)/C at a scan rate of
10 mV sÀ1 in the presence of (a) 0 mM, (b) 10 mM, and (c) 0.5 M
sodium gluconate. Other conditions were the same as those in Fig. 1.
aldose and product analysis suggested that the reaction mainly
proceeds via 2-electron oxidation of the aldehyde group.
This work was supported by KAKENHI (19750131). We
are grateful to researchers of Technical Service Center (AIST)
for elemental analysis.
The catalyst exhibited little activity toward ketose, such as
fructose (line e) and sucrose (a-D-glucopyranosyl-(1 - 2)-b-D-
fructofuranoside) (line f). The activity toward monosaccharides
(glucose and galactose) was greater than that toward
disaccharides (maltose and lactose), but this difference was less
significant than that between aldose (a–d) and ketose (e and f).
These data suggest that glucose oxidation proceeds via aldehyde
oxidation. The reactivity toward a variety of aldoses is favorable
for the application in HPLC detection.
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the generation of carboxylic acids. A product analysis of glucose
oxidation at À0.5 V was performed to verify the reaction path.
Detailed procedures and results are shown in ESIw. An i–t curve
at À0.5 V (vs. Ag|AgCl|KCl(sat.)) is shown in Fig. S2w. The
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In conclusion, we found that Rh porphyrins on carbon
black can catalyze the electro-oxidation of glucose at low
overpotentials. The onset potential for the electro-oxidation
of aldose is the lowest among the complex-based catalysts, and
the potential regions are extremely lower than those with
conventional complex-based catalysts. A high current density
was obtained with this catalyst, even though only a small
amount of rhodium was used. These properties make the
catalyst a promising candidate for use as an anode catalyst
in fuel cells and as an electrocatalyst for sensing sugars.
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porphyrins. The Rh porphyrin-based catalysts gave higher
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Chem. Commun., 2010, 46, 3607–3609 | 3609