Efficient electrochemical conversion of carbon monoxide by rhodium octaethylporphyrin adsorbed on carbon black

Article abstract of DOI:10.1021/ic050911f

We have found efficient electrocatalytic removal of CO by rhodium octaethylporphyrin on carbon black at a wide potential range. Using carbon-supported rhodium octaethylporphyrin, we have separated the Rh(II) state participating reaction and the Rh(III) state participating reaction with CO. We have clearly demonstrated electrocatalytic CO oxidation by rhodium(III) porphyrin. The onset potential for CO oxidation is much lower than that for CO oxidation by conventional Pt/Ru catalysts and cobalt porphyrin.

Full text of DOI:10.1021/ic050911f

Inorg. Chem. 2005, 44, 6512−6514
Efficient Electrochemical Conversion of Carbon Monoxide by Rhodium
Octaethylporphyrin Adsorbed on Carbon Black
Shin-ichi Yamazaki,* Yusuke Yamada, and Kazuaki Yasuda
Research Institute for Ubiquitous Energy DeVices, National Institute of AdVanced Industrial
Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
Received June 6, 2005
We have found efficient electrocatalytic removal of CO by rhodium
octaethylporphyrin on carbon black at a wide potential range. Using
carbon-supported rhodium octaethylporphyrin, we have separated
the Rh(II) state participating reaction and the Rh(III) state
participating reaction with CO. We have clearly demonstrated
electrocatalytic CO oxidation by rhodium(III) porphyrin. The onset
potential for CO oxidation is much lower than that for CO oxidation
by conventional Pt/Ru catalysts and cobalt porphyrin.
rhodium tetraphenylporphyrin can perform electrocatalytic
CO oxidation.4 However, little is known about this interesting
catalysis. It is attributed to the complexity of rhodium
porphyrin catalysts.
Rhodium porphyrins can catalyze various useful catalytic
reactions,5-9 not only CO molecule activation6,7 but also
C-H bond activation,8 etc. As the reactive species in these
catalytic reactions, rhodium(II) and rhodium(III) porphyrins
are emphasized.5-9 For the development of rhodium por-
phyrin catalysis, it is important to understand and regulate
the reactive species during the catalytic reactions. However,
the reactive species in electrocatalytic CO oxidation have
not been clearly identified and regulated,4 despite the fact
that this reaction has attracted much interest for the above-
mentioned reasons.
For the development of a fuel cell, a critical problem is
CO poisoning of the anode catalyst. CO adsorption on the
platinum anode catalyst causes a drastic decrease in the cell
performance.1 From this viewpoint, CO-tolerant anode
catalysts such as Pt/Ru alloy and Pt/metal oxide have been
extensively investigated.1,2 It has been proposed that elec-
trocatalytic CO oxidation would play an important role in
CO tolerance.2b,d On the other hand, the removal of CO from
the anode gas by electrochemical CO oxidation has been
performed using a Pt/Ru alloy catalyst.3 From this context,
electrochemical CO oxidation catalysts attract increasing
amounts of interest. However, unfortunately, conventional
catalysts require a large overpotential for CO oxidation;1,2
the exploration of the electrochemical CO removal catalyst
at lower potential is strongly requested for the solution of
CO poisoning.
Most of the difficulties could be attributed to the inter-
molecular reactions between rhodium(II) porphyrins.5,10,11
Rhodium(II) porphyrin can undergo dimerization to produce
(4) (a) Van Baar, J. F.; van Veen, J. A. R.; de Wit, N. Electrochim. Acta
1982, 27, 57-59. (b) Van Baar, J. F.; van Veen, J. A. R.; van der
Eijk, J. M.; Peters, Th. J.; de Wit, N. Electrochim. Acta 1982, 27,
1315-1319.
(5) Collman, J. P.; Boulatov, R. J. Am. Chem. Soc. 2000, 122, 11812-
11821 and references therein.
(6) (a) Wayland, B. B.; Woods, B. A.; Pierce, R. J. Am. Chem. Soc. 1982,
104, 302-303. (b) Miller, R. G.; Kyle, J. A.; Coates, G. W.; Anderson,
D. J.; Fanwick, P. E. Organometallics 1993, 12, 1161-1166.
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Chem. Soc. 1992, 114, 1673-1681. (b) Sherry, A. E.; Wayland, B.
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Chem. Soc. 2000, 122, 8569-8570. (d) Del Rossi, K. J.; Wayland, B.
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Am. Chem. Soc. 1977, 99, 3869-3870.
Metalloporphyrins might be one of the promising catalysts
of electrochemical CO removal. van Veen et al. reported that
* To whom correspondence should be addressed. E-mail: s-yamazaki@
aist.go.jp.
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Electrochem. Soc. 2003, 150, A1225-A1230 and references therein.
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Miyazaki, Y.; Kobayashi, T. Catal. Today 2003, 84, 223-229. (b)
Oetjen, H.-F.; Schmidt, V. M.; Stimming, U.; Trila, F. J. Electrochem.
Soc. 1996, 143, 3838-3842. (c) Niedrach, L.; McKee, D.; Paynter,
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6512 Inorganic Chemistry, Vol. 44, No. 19, 2005
10.1021/ic050911f CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/17/2005

COMMUNICATION
the rhodium(II)-rhodium(II) porphyrin dimer in both organic
and aqueous solutions.10 Furthermore, Save´ant and co-
workers clearly demonstrated that the disproportionation of
rhodium(II) porphyrin can occur in solution so as to produce
rhodium(I) and rhodium(III) porphyrins during the electro-
chemical reduction of the rhodium(III) porphyrins.11 These
intermolecular reactions cause irreversible and complex
cyclic voltammograms (CVs) of Rh macrocycles including
porphyrins.11-13 To understand the reactive species, one
should suppress these intermolecular reactions of the rhod-
ium(II) porphyrins and separate the rhodium(II) and rhod-
ium(III) porphyrin participating reactions.
In our previous study, we intended to suppress the
intermolecular reactions of a rhodium porphyrin by adsorbing
it on carbon black.14 We succeeded in obtaining clear
reversible CVs of rhodium octaethylporphyrin ([RhIII(OEP)]-
Cl) on carbon black. A clear Gaussian-shaped voltammo-
gram, which is derived from the molecule strongly adsorbed
on an electrode surface, suggests that the possible intermo-
lecular reactions be successfully suppressed.14 In such a
heterogeneous system, the number of electrons transferred
to electrodes (n) is calculated from the width of the peak at
half-height. The peak width of the CV clearly indicates that
the electrode reaction is a one-electron reaction; hence, the
wave should be ascribed to the reaction between Rh(II) and
Rh(III) states because we used carbon-supported [RhIII(OEP)-
Cl].14 The observed one-electron reaction discards the
possibility of the participation of the Rh(I) states in adsorbed
rhodium porphyrin, in contrast to rhodium porphyrin in
solution.11 Clear and simple voltammograms can allow us
to distinguish between the [RhII(OEP)] and [RhIII(OEP)]+
participating reactions.
Figure 1. (A) CV and LSV of [Rh(OEP)] at scan rate ) 50 mV s-1 (a)
under an argon atmosphere without a rotating electrode, (b) under a CO
atmosphere without a rotating electrode, and (c) under a CO atmosphere
with a rotating electrode (3600 rpm). Inset: an expanded figure of curve a.
(B) CV and LSV of [Rh(OEP)] at scan rate ) 10 mV s-1 (a) under an
argon atmosphere without a rotating electrode, (b) under a CO atmosphere
without a rotating electrode, and (c) under a CO atmosphere with a rotating
electrode (6400 rpm). The measurements were performed in a 0.1 M H2-
SO4 aqueous solution at 25 °C. To prevent contamination of the coordinating
ion or ligands, ultrapure water was used in the experiments.
XC 72R using an equilibrium adsorption method. The
prepared Vulcan XC 72R supported [RhIII(OEP)Cl] was fixed
at the electrode surface with Nafion.
Figure 1A (inset curve a) is a CV of Vulcan XC 72R
supported [RhIII(OEP)Cl] in an argon-saturated atmosphere
at a scan rate of 50 mV s-1 in an aqueous solution. The
peak separation of this CV is also virtually zero, indicating
that the CV is derived from the molecule strongly adsorbed
on an electrode surface. From the peak width, the clear
reversible voltammogram (curve a) in Figure 1A should be
ascribed to the conversion between [RhIII(OEP)]+ and [RhII-
(OEP)]. In this experiment, the strong adsorption of [Rh-
(OEP)] on carbon black prevents undesirable reactions so
as to stabilize [RhII(OEP)] on the surface, as demonstrated
in our previous paper.14
The introduction of CO gas into the test solution produced
a drastic change in the CV of [Rh(OEP)] (Figure 1A, curve
b). The drastic increase in the anodic current clearly indicates
electrocatalytic CO oxidation by [RhIII(OEP)]+. A sharp
anodic peak in Figure 1A does not mean that the catalytic
active species decompose above the peak potential. In fact,
after the measurement of CVs in the presence of CO, the
properties of the CV of [Rh(OEP)] under an argon atmo-
sphere did not change (data not shown), indicating that [RhIII-
(OEP)]+ is preserved after the CO oxidation. Such a peak
generation occurs when the rate of catalytic reactions exceeds
that of mass transfer. This phenomenon is clearly analyzed
from theoretical viewpoints by Save´ant and co-workers.16
The reaction is so fast that the mass transfer of CO is the
rate-determining step. A linear sweep voltammogram (LSV)
under rotating-electrode conditions provided a steady-state
anodic current even above the peak potential (Figure 1A,
curve c). The enhancement of the mass transfer by electrode
rotation results in the emergence of the steady-state current.
These features are much more prominent in the LSV at a
slower scan rate (10 mV s-1) and higher electrode rotation
rate (6400 rpm), as shown in Figure 1B (curve c).
The separation and regulation of the Rh(II) and Rh(III)
states in the rhodium porphyrin in our system can provide a
clue for the elucidation of the electrocatalytic reaction
between CO and rhodium porphyrin. Using carbon-supported
[RhIII(OEP)Cl], we have clearly separated the two reactions
(CO with [RhII(OEP)] and CO with [RhIII(OEP)]+). We have
also demonstrated CO oxidation by [RhIII(OEP)]+ and found
the CO participating reduction by [RhII(OEP)].
The detailed experimental procedures are described in ref
14 and the Supporting Information. [RhIII(OEP)Cl] was
synthesized according to the method of Ogoshi et al. with a
slight modification.15 [RhIII(OEP)Cl] was adsorbed on Vulcan
(10) (a) Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2623-
2631. (b) Fu, X.; Basickes, L.; Wayland, B. B. Chem. Commun. 2003,
520-521. (c) Setsune, J.; Yoshida, Z.; Ogoshi, H. J. Chem. Soc.,
Perkin Trans. 1982, 1, 983-987. (d) Setsune, J.; Yoshida, Z.; Ogoshi,
H. J. Am. Chem. Soc. 1977, 99, 3869-3870. (e) Wayland, B. B.;
Woods, B. A. J. Chem. Soc., Chem. Commun. 1981, 475-476.
(11) (a) Grass, V.; Lexa, D.; Momenteau, M.; Save´ant, J.-M. J. Am. Chem.
Soc. 1997, 119, 3536-3542. (b) Grass, V.; Lexa, D.; Save´ant, J.-M.
J. Am. Chem. Soc. 1997, 119, 7526-7532.
(12) (a) Kadish, K. M.; Yao, C.-L.; Anderson, J. E.; Cocolios, P. Inorg.
Chem. 1985, 24, 4515-4520. (b) Kadish, K. M.; Hu, Y.; Boschi, T.;
Tagliatesta, P. Inorg. Chem. 1993, 32, 2996-3002.
(13) Tse, Y.; Seymour, P.; Kobayashi, N.; Lam, H.; Leznoff, C. C.; Lever,
A. B. P. Inorg. Chem. 1991, 30, 4453-4459.
(14) Yamazaki, S.; Yamada, Y.; Yasuda, K. Inorg. Chem. 2004, 43, 7263-
7265.
(15) Ogoshi, H.; Setsune, J.; Omura, T.; Yoshida, Z. J. Am. Chem. Soc.
1975, 97, 6461-6466.
(16) Limoges, B.; Moiroux, J.; Save´ant, J.-M. J. Electroanal. Chem. 2002,
521, 8-15.
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The steady-state current above the redox potential of
[RhIII/II(OEP)] indicates that [RhIII(OEP)]+ would catalyze
the CO oxidation. Our results clearly show that [RhIII(OEP)]+
catalyzes the CO oxidation. From the magnitude of the
steady-state current and the amount of adsorbed species, the
second-order rate constant of this reaction is calculated to
be (9.1 ( 0.1) × 103 M-1 s-1.
1, curve c). This cathodic current was not observed without
CO under rotating-electrode conditions (data not shown).
[RhII(OEP)] exists as the predominant species in this potential
region. The cathodic current should be ascribed to the CO
participating reduction by [RhII(OEP)]. This reaction was not
observed without rotating the electrode. The efficient supply
of CO, the removal of the product, or both might be required
for driving this reaction.18
The product analysis of the test solution after potentiostat
electrolysis at -0.2 V revealed a slight generation of
formaldehyde (0.036 µmol). However, the amount of form-
aldehyde was much lower than the total amount of product
(0.66 µmol) calculated from the number of electrons. Almost
all products would be gaseous compounds. It is well-known
that a CO molecule is activated upon coordination to [RhII-
(OEP)].5 It is possible that the activated CO molecule
participates in the cathodic reaction. The details of the
cathodic reaction remain to be clarified.
The proposed reaction mechanism can be written as
follows:
[RhIII(OEP)]+ + CO / [RhIII(OEP)(CO)]+
[RhIII(OEP)(CO)]+ + H2O / [RhIII(OEP)(COOH)] + H+
(1)
(2)
[RhIII(OEP)(COOH)] f CO2 + [RhIII(OEP)]+ + H+ + 2e-
(3)
This reaction mechanism was proposed by Anson and co-
workers concerning the electrocatalytic CO oxidation by [Co-
(OEP)].17 The onset potential for the CO oxidation (ca. -0.1
V vs Ag/AgCl/KCl(sat.)) by [RhIII(OEP)]+ is much lower
than those for the CO oxidation by [Co(OEP)] and noble
metal catalysts such as Pt and Pt/Ru alloys. The property
would be favorable for the electrochemical removal of CO.
For the electrocatalytic CO oxidation by cobalt and
rhodium porphyrins, the onset potential is closely correlated
with the redox potential of the central metal, because trivalent
species participate in CO oxidation for both porphyrins. The
redox potential of [Rh(OEP)] is much lower to that of [Co-
(OEP)], resulting in the low onset potential for the CO
oxidation. Thus, the regulation of the Rh(II) and Rh(III) states
in the rhodium porphyrin in our system provides mechanistic
insight for the onset potential of electrocatalytic CO oxidation
by rhodium porphyrin.
In conclusion, we have succeeded in clearly separating
the Rh(II) and Rh(III) states in [Rh(OEP)] on the surface of
carbon black and distinguishing between the [RhII(OEP)] and
[RhIII(OEP)]+ participating reactions with CO. We have
clearly demonstrated the CO oxidation by [RhIII(OEP)]+. The
high reactivity of [Rh(OEP)] toward CO at a wide potential
range would be attractive in terms of the electrochemical
removal of CO from a fuel cell anode catalyst.
Acknowledgment. The authors thank researchers in
Advanced Fuel Cell Research Group (National Institute of
Advanced Industrial Science and Technology) for their
fruitful discussions.
Supporting Information Available: Experimental details in-
cluding the synthesis of [Rh(OEP)] and the adsorption of [Rh(OEP)]
on carbon black (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Interestingly, below the redox potential of [Rh(OEP)], the
cathodic current also increased, and a steady-state current
was observed at the higher electrode rotation rates (Figure
IC050911F
(18) Without a rotating electrode, electrocatalytic CO oxidation proceeds
by the slight amount of [RhIII(OEP)]+ even below the redox potential
of [Rh(OEP)] (Figure 1, curve b).
(17) Shi, C.; Anson, F. C. Inorg. Chem. 2001, 40, 5829-5833.
6514 Inorganic Chemistry, Vol. 44, No. 19, 2005