Electro-oxidation of methanol on platinum–organic metal complex
mixed catalysts in acidic media†
Tatsuhiro Okada,*a Yoshifumi Suzuki,b Takuji Hirose,c Takako Todad and Takeo Ozawab
a National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Central 5, Tsukuba,
Ibaraki 305-8565, Japan. E-mail: okada.t@aist.go.jp
b Department of Electrical Engineering, Chiba Institute of Technology, Tsudanuma, Narashino, Chiba
275-0016, Japan
c
Department of Applied Chemistry, Faculty of Engineering, Saitama University, 255 Shimo-Ohkubo,
Urawa, Saitama 338-8570, Japan
d Japan Science and Technology Corporation, Honmachi 4-1-8, Kawaguchi, Saitama 332-0012, Japan
Received (in Cambridge, UK) 6th August 2001, Accepted 25th October 2001
First published as an Advance Article on the web 15th November 2001
Novel mixed catalysts systems have been developed using a
platinum tetraammine complex with a cobalt or nickel
quinolyldiamine complex supported on graphite powder and
heat treated at 600 °C in argon atmosphere, for the methanol
oxidation reaction in direct methanol fuel cells.
phenylenediamine diacetate [Co(mqph)] as a claret or reddish
purple powder.
Mixed catalysts were prepared from the platinum precursor,
platinum tetraammine chloride Pt(NH3)4Cl2·xH2O, and the
cobalt complex Co(mqph) in various mixing ratios. Mixing
ratios used were Pt(NH3)4Cl2·xH2O/Co(mqph) = 100/0, 80/20,
60/40, 50/50, 40/60, 20/80 and 0/100 (on a weight basis). 10 mg
of the mixed catalyst and 40 mg of graphite powder (1–2 mm,
Aldrich) were mixed in 0.5 cm3 of ethanol in a mortar, dried in
air at 80 °C for 60 min and then heat-treated in Ar atmosphere
at 600 °C for 2 h in a furnace. 5 mg of the powder thus obtained
(denoted Pt–Co(mqph)/C] was mixed with 100 mg of 5%
Nafion solution (Aldrich) together with 0.1 cm3 of ethanol, to
obtain an ink of the mixture. 0.01 cm3 of this mixture was
transferred to the disk electrode [6 mm diameter, basal plane of
high-density pyrolytic graphite (BHPG)]. The overall loading
of the mixed catalyst was 1.8 3 1024 g cm22, for the apparent
electrode area of BHPG.
Electrochemical measurements were conducted in a glass cell
consisting of a three-electrode system in 1 mol dm23 CH3OH–
0.05 mol dm23 H2SO4 at 25 °C, in deaerated conditions under
N2. The working electrode was the catalyst supported BHPG
disk, the counter electrode was a platinum plate and a saturated
calomel electrode (SCE) was used as the reference electrode.
The electrode was pretreated by cyclic potential scanning
between 2300 and 1200 mV vs. SCE (+316 mV on the RHE
scale) at 100 mV s21. Potentials were scanned in the anodic
direction at a scan rate of 1 mV s21 for the polarization
measurement in the potential range 2200 to 1000 mV.
Fig. 1 shows the polarization curves of MOR for mixed
catalysts Pt–Co(mqph)/C of various mixing ratios. The total
catalyst amount corresponds to 12% Pt/C in the case of 100/0
mixing ratio. The polarization curves were reproducible to
within 10%. Compared with Pt/C or Co(mqph)/C alone, 40/60
? 60/40 mixtures revealed extraordinarily high MOR current
density indicating cooperative phenomena between two com-
ponents.
On the other hand, using the organic complex Co(NH3)6Cl3
in place of Co(mqph) as a precursor material led to no MOR
activity. In this system, the activity did not exceed that of pure
Pt. The initial structure of the complex thus clearly affects the
catalytic function after the heat treatment.
XRD showed evidence of inter-metallic compounds between
Pt and Co for the mixed system of Pt and Co(NH3)6Cl3 after
heat treatment, but not for the mixed system of Pt and
Co(mqph). According to X-ray photoelectron spectroscopy
(XPS), Co 2p and N 1s peaks for Pt–Co(mqph)/C mixed
catalysts were identified as Co coordinated by N ligands. As
anticipated from the temperature of heat treatment of the
catalyst, only a small extent of demetallation of the Co complex
was observed after the heat treatment, but major peaks indicated
coordinated states between Co and N. The particle size of the
The methanol oxidation reaction (MOR) is of great techno-
logical interest because of its potential application to direct
methanol fuel cells (DMFCs).1 DMFCs have an advantage over
polymer electrolyte fuel cells utilizing hydrogen gas generated
through a methanol reformer, because the system can be made
much simpler. However, in order to accomplish the DMFC
reaction, the methanol fuel should undergo the six-electron
oxidation process on the electrode catalyst given by eqn. (1).
CH3OH + H2O ? CO2 + 6H+ + 6e2
(1)
This is extremely difficult to control, and therefore leads to a
high overpotential. Moreover, the reaction is strongly hindered
by adsorbed CO, which occurs on the catalyst surface when Pt
based catalysts are used.2,3 Electrode catalysts free from CO
poisoning are thus desired in order to realize high conversion
efficiency in the DMFC.
Several types of MOR electrocatalysts have been proposed
including platinum based alloy catalysts4,5 or platinum finely
dispersed on oxides such as TiO2, MoO2, WO3, etc.6 New base
catalysts have also been proposed, e.g. mixtures of nickel–
tungsten alloy and WC prepared from nickel tungstate.7 The
best catalyst known so far is Pt–Ru alloy (50/50), the function
of Ru being as sites of oxygen containing species, and capable
of CO elimination by oxidation.8–11 However, a low natural
abundance of Ru is a drawback of this catalyst for practical
uses.
Here, the possibility of constituting a new class of catalysts is
pursued using new components that may possess novel
functions. We select organic metal complexes as candidates,
because these could provide the possibility to generate new
functions such as CO removal by selective oxidation or strong
coordination, compared to metal alloy catalysts etc., by
manipulating molecular structures.
An aqueous mixture of 8-hydroxyquinoline (2.90 g, 0.02
mol), o-phenylenediamine (1.08 g, 0.01 mol) and sodium
disulfate (3.08 g, 0.02 mol) was refluxed for a week at 110 °C.
Recrystalization from methanol gave N-8-quinolyl-o-pheny-
lenediamine (mqph) as an amber colored crystals (ESI†).
Equimolar amounts of mqph and cobalt acetate tetrahydrate
were added at room temperature in ethanol under nitrogen
atmosphere in a glass flask, and the resulting solution was
concentrated and refrigerated to obtain cobalt N-8-quinolyl-o-
† Electronic supplementary information (ESI) available: reaction scheme
suppdata/cc/b1/b10/b107132g/
2492
Chem. Commun., 2001, 2492–2493
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b107132g
Okada
