The electro-oxidation of formic acid (an essential reaction in direct formic acid fuel cells) is a challenging process because of the deactivation of anodes by the adsorption of the poisoning intermediate carbon monoxide (CO). Pt electrodes in two geometries (planar and nanohole-array) were modified by the electrodeposition of manganese oxide nanorods (nano-MnOx). The modified Pt electrodes were then tested for their electrocatalytic activity through the electro-oxidation of formic acid in a solution of pH 3.45. Two oxidation peaks (I d p and I ind p ) were observed at 0.2 and 0.55 V, respectively; these were assigned to the direct and indirect oxidative pathways. A significant enhancement of the direct oxidation of formic acid to CO2 was observed at the modified electrodes, while the formation of the poisoning intermediate CO was suppressed. I d p increases with surface coverage (h) of nano-MnOx with a concurrent depression of I ind p . An increase in the ratio I d p/m 1/2 with decreasing potential scan rate (m) indicates that the oxidation process proceeds via a catalytic mechanism. The modification of Pt anodes with manganese oxide nanorods results in a significant improvement of the electrocatalytic activity along with a higher tolerance to CO. Thus nano-MnOx plays a crucial role as a catalytic mediator which facilitates the charge transfer during the direct oxidation of formic acid to CO2.
Trang 1ORIGINAL ARTICLE
Electrocatalysis by nanoparticles: Oxidation of formic
acid at manganese oxide nanorods-modified Pt planar
and nanohole-arrays
Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt
KEYWORDS
Nanostructures;
Nanohole-arrays;
Manganese oxide nanorods;
Modified surfaces;
Electrocatalysis
Abstract The electro-oxidation of formic acid (an essential reaction in direct formic acid fuel cells)
is a challenging process because of the deactivation of anodes by the adsorption of the poisoning intermediate carbon monoxide (CO) Pt electrodes in two geometries (planar and nanohole-array) were modified by the electrodeposition of manganese oxide nanorods (nano-MnOx) The modified
Pt electrodes were then tested for their electrocatalytic activity through the electro-oxidation of for-mic acid in a solution of pH 3.45 Two oxidation peaks (Idand Iindp ) were observed at 0.2 and 0.55 V, respectively; these were assigned to the direct and indirect oxidative pathways A significant enhancement of the direct oxidation of formic acid to CO2was observed at the modified electrodes, while the formation of the poisoning intermediate CO was suppressed Idincreases with surface cov-erage (h) of nano-MnOx with a concurrent depression of Iind
p An increase in the ratio Id
/m1/2with decreasing potential scan rate (m) indicates that the oxidation process proceeds via a catalytic mech-anism The modification of Pt anodes with manganese oxide nanorods results in a significant improvement of the electrocatalytic activity along with a higher tolerance to CO Thus nano-MnOx plays a crucial role as a catalytic mediator which facilitates the charge transfer during the direct oxi-dation of formic acid to CO2
ª 2009 University of Cairo All rights reserved.
Introduction The development of durable and effective electrocatalysts is of prime importance for fuel cells Among the various types of fuel cells, the direct formic acid fuel cell has several advantages over direct methanol fuel cell, including its high theoretical cell voltage as well as low fuel crossover[1,2] A major drawback
of such fuel cells however is the generation of poisonous reac-tion intermediates, particularly carbon monoxide (CO) The latter species get strongly adsorbed on the surface of the cata-lyst and lead eventually to its deactivation It is well-known
* Tel.: +20 2 3567 6603.
E-mail addresses: msaada68@yahoo.com ,
mohamed.el-deab@uni-ulm.de
1 Present address: Institute of Electrochemistry, University of Ulm,
89069 Ulm, Germany Tel.: +49 731 50 25413; fax: +49 731 50 25409.
2090-1232 ª 2009 University of Cairo All rights reserved Peer review
under responsibility of University of Cairo.
Production and hosting by Elsevier
University of Cairo
Journal of Advanced Research
doi:10.1016/j.jare.2010.01.001
Trang 2that the electro-oxidation of formic acid on Pt proceeds via
various pathways [3,4], including (i) direct oxidation to CO2
(i.e., dehydrogenation pathway) via a reactive intermediate
(presumably the formate radical HCOOads[5]) and (ii)
forma-tion of a poisoning CO intermediate (i.e., dehydraforma-tion
path-way) In this context, several bimetallic Pt–M (M = Ru, Pd,
Au, and Pb) catalysts have been studied extensively in order
to enhance the activity of the direct oxidation of formic acid
on the one hand and to reduce the Pt content on the other
[6–9] For instance, the deposition of Pt into Au
nanorods-modified carbon black electrodes has shown significantly
high-er activity towards the direct oxidation of formic acid [10]
Similarly, the modification of Pt electrodes with Fe-macrocycle
compounds has been found to promote the direct oxidation
pathway for formic acid [11] Furthermore, bimetallic PtPb
electrodes have been found to be catalytically more active than
pure Pt towards the electro-oxidation of formic acid[12]
Over the last decade, catalysis and electrocatalysis at
nano-particles-based electrodes have attracted significant attention
due to the unusual and fascinating properties of nanoparticles
compared to bulk materials These properties include high
effective surface area, catalytic activity and quantum
confine-ment[13,14] The stimulus for the growing interest in
nanopar-ticles can be traced to new and improved abilities to make,
assemble, position, connect, image and measure the properties
of nanometer-scale materials with controlled size, geometry,
shape, composition, surface topography, charge and
function-ality for prospective use in the macroscopic real world[15] In
addition to the extraordinary catalytic activity with regards to
oxygen reduction[16], Au nanoparticle-based substrates have
been efficiently utilised for the hydrogenation of unsaturated
organic compounds[17]as well as for the low temperature
oxi-dation of CO[18] The catalytic activity of nanoparticle-based
electrodes is inherently related to particle size, shape,
geome-try, crystallographic orientation, nature of the support and
the method of preparation[16,18] These parameters are
cru-cial for the kinetics of electrode reactions involving adsorption
of intermediates [19] Some metal oxide modified electrodes,
such as Ni, Co and Mn oxide modified electrodes, have been
reported to catalyse several electrochemical reactions e.g.,
oxy-gen evolution and reduction reactions The extent of catalysis
is based on the synthesis method of the oxide and the nature of
the dopant such as Mo or W[20–22] For instance, anodically
deposited MnO2has been utilised for the electrolytic evolution
of oxygen gas from seawater[22] Furthermore, MnOx
modi-fied Ru and Pt electrodes have been efficiently utilised to
oxi-dise methanol[23,24]
The Pt nanohole-array is a typical example of a nano-scale
electrode This type of electrode brings several operational
advantages over the ordinary planar electrode, including:
(i) enhanced mass transport (due to the dominance of the
radial diffusion),
(ii) decreased charging currents and
(iii) decreased deleterious effects of solution resistance
These properties render the Pt nanohole-array electrode a
promising model catalyst for studying electrocatalytic
reac-tions, such as formic acid oxidation
In this context, the present study addresses the modification
of Pt surfaces (in planar and nanohole-array geometries) with
manganese oxide nanorods (nano-MnOx) for obtaining high
electrocatalytic activity towards formic acid oxidation Accordingly, crystallographically oriented nano-MnOx (in the manganite phase, c-MnOOH [25]) was electrodeposited onto Pt substrates and were subsequently shown to enhance the direct oxidation of formic acid (to CO2) via facilitating the charge transfer, while suppressing the formation of the poi-soning CO intermediate
Material and methods
Pt electrodes of two geometries were used as working elec-trodes: (i) a planar Pt disk electrode sealed in a Teflon jacket (2.0 mm in diameter and having an exposed geometric surface area of 0.031 cm2) and (ii) a Pt nanohole-array supported on a glass substrate (4.0 mm in width, 5.0 mm in length and 0.5 mm
in thickness) The nanohole-array Pt electrode had the follow-ing characteristics: an average of 200 Pt holes perlm2
, each of which was 20–25 nm in diameter and 30 nm in depth A satu-rated calomel electrode (SCE) and a spiral Pt wire were the ref-erence and counter electrodes, respectively The planar Pt electrode was mechanically polished with aqueous slurries of successively finer alumina powders (down to 0.05lm), soni-cated for 10 min in Milli-Q water, then electrochemically pre-treated in 0.1 M H2SO4 solution by cycling in the potential range of0.3 to 1.25 V vs SCE at 50 mV s1for 10 min or un-til a reproducible cyclic voltammogram (CV) characteristic for
a clean Pt electrode was obtained (cf curve a inFig 2A) The
Pt nanohole-array electrode was subjected to the same proce-dure of the electrochemical treatment without mechanical pol-ishing (cf curve a inFig 2B)
MnOx was electrodeposited on the surface of Pt (planar and nanohole-arrays) from an aqueous solution of 0.1 M
Na2SO4 containing 0.1 M Mn(CH3COO)2 by cycling the po-tential at 20 mV s1 between 0.05 and 0.35 V vs SCE
[25,26] The surface coverage h of the nano-MnOx on the Pt electrode was controlled by the number of potential cycles em-ployed during the electrodeposition step
The values of h are listed inTable 1for various numbers of cycles Morphological characterisation of the prepared nano-MnOx was carried out by scanning electron microscopy (SEM) using a JSM-T220 (JEOL, Optical Laboratory, Japan)
at an acceleration voltage of 12–30 kV and a working distance
of 8 mm
The electrocatalytic activity of the nano-MnOx modified Pt electrodes towards formic acid oxidation was examined in a solution of 0.3 M formic acid of pH 3.45 adjusted by adding NaOH The CVs were performed in a conventional three-elec-trode glass cell All chemicals were of analytical grade and were used without further purification; all measurements were performed at room temperature; the solutions were de-oxygen-ated by N2bubbling Current densities were calculated on the basis of the geometric surface area of the Pt working electrodes
Results and discussion Characterisation of nano-MnOx/Pt electrodes
Fig 1 shows typical SEM micrographs obtained for (a) unmodified and (b) nano-MnOx modified (A) planar and (B) nanohole-array Pt electrodes Inspection of the two images
Trang 3(marked b) reveals that MnOx was electrodeposited onto the
Pt substrates in a porous texture composed of intersected
nanorods with an average thickness of about 20 nm This
porous texture enables the access of the solution species to the underlying substrate XRD and the high resolution TEM patterns (data shown elsewhere[27]) show these nanorods were
Figure 1 SEM micrographs of (a) unmodified and (b) MnOx modified (A) planar and (B) nanohole-array Pt electrodes The nano-MnOx was electrodeposited as described in the experimental section by applying 50 potential cycles between50 and 350 mV vs SCE at
20 mV s1
Table 1 Variation of Idand Iind
p for formic acid oxidation with surface coverage (h) of the nano-MnOx electrodeposited onto planar Pt electrode The values of the real surface area (S) of the unmodified and modified Pt electrodes are also listed
No of potential cycles employed for
nano-MnOx deposition
Real surface area
of Pt (S) a /cm 2
Surface coverage (h) b /%
I d /mA cm2 I ind
p /mA cm2
a
As estimated from the charge consumed during the reduction of the surface oxide monolayer (at ca 0.4 V, Fig 2 A) using a reported value of
420 lC cm 2 [28]
b
The values of surface coverage (h = 1 – S modified /S unmodified ) were calculated for the various nano-MnOx/Pt electrodes S modified and
S unmodified refer to the real surface area of the modified and the unmodified Pt electrodes, respectively.
Trang 4identified to electrodeposit exclusively in the manganite phase
(c-MnOOH)
Fig 2 shows CVs of (a) unmodified and (b) nano-MnOx
modified (A) planar and (B) nanohole-array Pt electrodes in
0.1 M H2SO4 The decrease in the intensity of the cathodic
peak current around 0.4 V indicates a decrease in the accessible
surface area of the underlying Pt planar substrate as a result of
the successful electrodeposition of the nano-MnOx on the Pt
surface The coverage of the electrodeposited nano-MnOx
was estimated from the decrease of charge due to the reduction
of the Pt-surface oxide using a reported value of 420lC cm2
for a surface oxide monolayer[28].Table 1lists the real surface
area (S) of the underlying Pt substrate and the corresponding
surface coverage h of nano-MnOx electrodeposited by
apply-ing various numbers of potential cycles
Oxidation of formic acid at nano-MnOx/Pt electrodes
The thus-prepared nano-MnOx/Pt electrodes were tested for
their electrocatalytic activity for formic acid oxidation
Fig 3A and B show typical CVs measured in 0.3 M formic
acid (pH 3.45) at (a) unmodified and (b) nano-MnOx modified (A) planar (h 30%) and (B) nanohole-array Pt electrodes (h 15%) Two oxidation peaks (marked as Idand Iind
p ) were observed at ca 0.2 and 0.55 V in the forward (positive-going) scan for the unmodified Pt electrode (Fig 3A, curve a) These two peaks were assigned to the direct oxidation of formic acid
to CO2and to the oxidation of the intermediate CO generated
by the dissociative (non-faradaic) adsorption step[4] The ratio
of the two oxidation peaks (Id/Iind
p ) reflects the preferential oxidation pathway of formic acid at a particular electrode The appearance of peak Ibin the reverse scan was attributed
to the oxidative removal of the incompletely oxidised carbonaceous species formed during the forward scan The modification of Pt (planar and nanohole-array) electrodes with nano-MnOx resulted in several significant changes (curves marked b inFig 3A and B):
(i) Observation of a significant increase of the first peak,Id, concurrently with a noticeable depression of the second oxidation peak,Iind
p This indicates that the direct oxida-tion of formic acid becomes more favourable, while less poisoning intermediate (CO) is produced
A
B
-0.4
-0.2
0
b
E / V vs SCE
E / V vs SCE
.
-40
-20
0
20
a b
Figure 2 CVs for (a) unmodified and (b) nano-MnOx modified
(A) planar (h 20%) and (B) nanohole-array (h 15%) Pt
electrodes measured in 0.1 M HSO at 50 mV s1
0 0.2 0.4 0.6 0.8 1 1.2
E / V vs SCE
a b
0 5 10 15
E / V vs SCE
a
b
Ip ind
Ib
Ip
Ip ind
Ib
Ip
B A
Figure 3 CVs for formic acid oxidation at (a) unmodified and (b) nano-MnOx modified (A) planar (h 30%) and (B) nanohole-array (h 15%) Pt electrodes at 50 mV s1 measured in 0.3 M HCOOH (pH 3.45)
Trang 5(ii) Nano-MnOx/Pt oxidises the adsorbed carbonaceous
intermediates (formed during the forward scan) at lower
anodic potentials (at ca 0.13 V) than the unmodified Pt
electrode (at ca 0.3 V) This might indicate that the
modified electrode maintains its high surface activity
In other words, the modified electrode has a high
toler-ance against the poisoning effect of CO
(iii) TheId
p/Ibratio probes the catalytic tolerance of the
elec-trode against the formation of carbonaceous species
for-mation[29] A lowId/Ibratio indicates poor oxidation
of formic acid to CO2and excess accumulation of
carbo-naceous species at the electrode surface A value ofId/Ib
of 0.85 was obtained at nano-MnOx/Pt (curve b,
Fig 3A), which is about 10 times higher than that
observed at the unmodified Pt (curve a,Fig 3A) This
indicates that less intermediate carbonaceous species
are produced in the forward scan at the modified Pt
elec-trode surface than at the unmodified Pt surface It also
indicates a higher reversibility of the reaction at the
modified electrode
Similar enhancement was observed at the Pt
nanohole-ar-ray electrode following modification with a smaller coverage
of nano-MnOx (h 15%) That is, the nano-MnOx modified
Pt nanohole-array electrode (curve b,Fig 3B) showed a
signif-icantly enhanced oxidation of formic acid at relatively lower
anodic potential (ca 0.2 V vs SCE) with a 7-times higher peak
current compared to the unmodified electrode (curve a,
Fig 3B) It is worth mentioning here that under the described
experimental conditions the diffusion layers at individual Pt
nanoholes overlap each other to form a linearly expanding
dif-fusion region as can be expected by comparison of the
diffu-sion layer thickness (d =p
(pDt) 0.0336 cm; D is the diffusion coefficient of formic acid in aqueous medium
(105cm2s1) and t is the electrolysis time (36 s)) and the
average distance between Pt nanoholes (about 100 nm, see
im-age a inFig 1B) As can be readily seen, d is much larger than
the average distance between the Pt nanoholes and thus the CV
pattern for formic acid oxidation (curve a,Fig 3B) is similar to
that observed at the planar Pt electrode with the same geomet-ric surface area
Table 1shows the dependence of Idand Iind
p of formic acid
on surface coverage h of nano-MnOx on planar Pt electrodes
It shows that Idpincreases with h.Fig 4shows the variation of
Id/Iind
p ratio with h This shows that the rate of direct oxidation pathway increases strongly with h and reaches about 14-times higher than the poison formation pathway at h of about 30%
In other words, the amount of CO (produced as a poisonous reaction intermediate and remaining at the Pt active sites) is markedly reduced upon the deposition of nano-MnOx onto the Pt surface This indicates that formic acid oxidation at the nano-MnOx/Pt electrode shifts towards the dehydrogena-tion pathway with increasing h
Long-term stability tests of nano-MnOx/Pt (planar and nanohole-array) electrodes were conducted by recording repet-itive CVs up to 100 cycles Reproducible CVs were obtained (data are shown inFig 5A and B) This demonstrates the high stability of the nano-MnOx/Pt and its high electrocatalytic performance towards the direct oxidation of formic acid It demonstrates also that nano-MnOx/Pt has a high tolerance against CO formation
0
10
20
Ip
Ip
Figure 4 Variation of Id/Iind
p ratio with surface coverage h of nano-MnOx at planar Pt electrodes for formic acid oxidation in
0.3 M HCOOH (pH 3.45)
0 5 10 15
E / V vs SCE
0 0.2 0.4 0.6 0.8 1 1.2
E / V vs SCE
A
B
Figure 5 First (solid lines) and 100th (dashed lines) CV for formic acid oxidation at nano-MnOx modified (A) planar (h 30%) and (B) nanohole-array (h 15%) Pt electrodes measured in 0.3 M formic acid (pH 3.45)
Trang 6Mechanistic approach of the electrocatalytic enhancement
Fig 6A shows CVs recorded for formic acid oxidation at a
nano-MnOx/Pt electrode at various potential scan rates (m)
This figure shows that Id
increases with m in a rather non-linear fashion.Fig 6B shows the variation of Idp/m1/2ratio with m As
can be seen, Id
/m1/2decreases with increasing m, which is a
char-acteristic feature of catalytic reactions[30] Thus the
electrode-posited nano-MnOx plays a crucial catalytic role during the
course of this reaction A plausible involvement of the single
crystalline MnOOH in the mechanistic pathway of formic acid
oxidation might proceed in the following sequence It has been
reported that the non-catalysed oxidative pathway of formic
acid proceeds at Pt surfaces according to[4]:
where the subscript ‘ads’ refers to the surface adsorbed species
Note that the adsorbed formate radicals (HCOOads) can be
di-rectly formed by the oxidation of formate anions co-existing at this pH (i.e., HCOOfi HCOOads+ e) The porous texture
of nano-MnOx allows for Reactions(1)–(4)to proceed at the modified Pt surface The presence of nano-MnOx would en-hance the direct pathway (Eq.(3)) via a reversible oxidative transformation of MnOOH into MnO2 through a reversible proton removal process as[31]:
The produced MnO2 is involved in one (or more) of the above steps according to[32]:
The sequential coupling of Reactions (6) and (7) results, effectively, in the generation of CO2 Thus the total reaction can be written as:
i.e., the presence of manganese oxide favours the dehydrogena-tion pathway of formic acid oxidadehydrogena-tion That is, one formic acid molecule requires two molecules of MnO2 to achieve a com-plete oxidation to CO2 This might account for the increase
of Idpwith increasing h (seeTable 1)
Another possibility for the catalytic role of manganese oxide nanorods might be assigned to the mediated oxidation
of COads(produced in Reaction(4)) to CO2with MnO2as: COadsþ MnO2þ H2O! CO2þ MnOOH þ Hþþ e ð9Þ This reaction shows that: (i) a re-activation of the Pt surface active sites is achieved by the oxidative removal of adsorbed
CO and thus leads eventually to the high observed current (Id) at +0.2 V and (ii) the regeneration of MnOOH species, which are believed to participate in catalytic cycles through Reactions (5)–(9), facilitating the direct oxidation of formic acid
Conclusion This paper addresses the development of a novel nano-MnOx modified Pt electrode for the electrocatalytic oxidation of for-mic acid with enhanced activity Modification of Pt (planar and nanohole-array) electrodes with nano-MnOx resulted in
a significant enhancement of the direct oxidation pathway for formic acid, at a rate up to 14-times higher than the indirect pathway (i.e., suppressing the generation of poisoning CO) The nano-MnOx (in the manganite phase, c-MnOOH) is be-lieved to facilitate the oxidation of the reaction intermediates (formate radical and/or CO) into CO2 via a series of redox reactions
Acknowledgement The author is grateful for a fellowship at University of Ulm from the Alexander von Humboldt Foundation
References [1] Demirci UB Direct liquid-feed fuel cells: thermodynamic and environmental concerns J Power Sources 2007;169(2):239–46.
0
0.5
1
1.5
.
0
10
20
30
40
E / V vs SCE
a b c d e f
.
Ip
d / ν
A
B
Figure 6 (A) CVs for formic acid oxidation at nano-MnOx/Pt
planar electrode (h 30%) in 0.3 M formic acid (pH 3.45) at
potential scan rate, m, of (a) 10, (b) 20, (c) 50, (d) 100, (e) 200, (f)
500 mV s1 (B) Variation of Id
/m1/2with m
Trang 7[2] Wang X, Hu JM, Hsing IM Electrochemical investigation of
formic acid electro-oxidation and its crossover through a
Nafion membrane J Electroanal Chem 2004;562(1):73–80.
[3] Lovic´ JD, Tripkovic´ AV, Gojkovic´ SL, Popovic´ KD, Tripkovic´
DV, Olszewski P, et al Kinetic study of formic acid oxidation
on carbon-supported platinum electrocatalyst J Electroanal
Chem 2005;581(2):294–302.
[4] Feliu JM, Herrero E In: Vielstich W, Lamm A, Gasteiger HA,
editors Handbook of fuel cells – fundamentals, technology and
applications Hoboken, NJ: John Wiley & Sons; 2003 p 625.
[5] Miyake H, Okada T, Samjeske´ G, Osawa M Formic acid
electro-oxidation on Pd in acidic solutions studied by
surface-enhanced infrared absorption spectroscopy Phys Chem Chem
Phys 2008;10(25):3662–9.
[6] Rice C, Ha S, Masel RI, Wieckowski A Catalysts for direct
formic acid fuel cells J Power Sources 2003;115(2):229–335.
[7] Choi JH, Jeong KJ, Dong Y, Han J, Lim TH, Lee JS, et al.
Electro-oxidation of methanol and formic acid on PtRu and
PtAu for direct liquid fuel cells J Power Sources
2006;163(1):71–5.
[8] Zhou X, Xing W, Liu C, Lu T Platinum-macrocycle co-catalyst
for electro-oxidation of formic acid Electrochem Commun
2007;9(7):1469–73.
[9] Uhm S, Chung ST, Lee J Activity of Pt anode catalyst modified
by underpotential deposited Pb in a direct formic acid fuel cell.
Electrochem Commun 2007;9(8):2027–31.
[10] Wang S, Kristian N, Jiang S, Wang X Controlled deposition of
Pt on Au nanorods and their catalytic activity towards formic
acid oxidation Electrochem Commun 2008;10(7):961–4.
[11] Zhou X, Liu C, Liao J, Lu T, Xing W Platinum-macrocycle
co-catalysts for electro-oxidation of formic acid J Power Sources
2008;179(2):481–8.
[12] Zhang LJ, Wang ZY, Xia DG Bimetallic PtPb for formic acid
electro-oxidation J Alloys Compounds 2006;426(1–2):268–71.
[13] Alivisatos AP Semiconductor clusters, nanocrystals and
quantum dots Science 1996;271(5251):933–7.
[14] Brust M, Kiely CJ Some recent advances in nanostructure
preparation from gold and silver particles: a short topical
review Colloids Surf A: Physicochem Eng Aspects 2002;202(2–
3):175–86.
[15] Ozin GA, Arsenault AC Nanochemistry: a chemical approach
to nanomaterials New York, LLC: Springer-Verlag; 2005.
[16] El-Deab MS, Sotomura T, Ohsaka T Oxygen reduction at
electrochemically deposited crystallographically oriented
Au(1 0 0)-like gold nanoparticles Electrochem Commun
2005;7(1):29–34.
[17] Schimpf S, Lucas M, Mohr C, Rodemerck U, Bru¨ckner A,
Radnik J, et al Supported gold nanoparticles: in-depth catalyst
characterization and application in hydrogenation and
oxidation reactions Catal Today 2002;72(1–2):63–78.
[18] Haruta M Size- and support-dependency in the catalysis of gold Catal Today 1997;36(1):153–66.
[19] El-Aziz AM, Kibler LA Influence of steps on the electrochemical oxidation of CO adlayers on Pd(1 1 1) and on
Pd films electrodeposited onto Au(1 1 1) J Electroanal Chem 2002;534(2):107–14.
[20] Fujimura K, Matsui T, Izumiya K, Kumagai N, Akiyama E, Habazaki H, et al Oxygen evolution on manganese– molybdenum oxide anodes in seawater electrolysis Mater Sci Eng A 1999;267(2):254–9.
[21] Fujimura K, Matsui T, Habazaki H, Kawashima A, Kumagai
N, Hashimoto K The durability of manganese–molybdenum oxide anodes for oxygen evolution in seawater electrolysis Electrochim Acta 2000;45(14):2297–303.
[22] Abdel Ghany NA, Kumagai N, Meguro S, Asami K, Hashimoto K Oxygen evolution anodes composed of anodically deposited Mn–Mo–Fe oxides for seawater electrolysis Electrochim Acta 2002;48(1):21–8.
[23] Rebello JS, Samant PV, Figueiredo JL, Fernandes JB Enhanced electrocatalytic activity of carbon-supported MnOx/Ru catalysts for methanol oxidation in fuel cells J Power Sources 2006;153(1):36–40.
[24] Zhao GY, Li HL Electrochemical oxidation of methanol on Pt nanoparticles composited MnO 2 nanowire arrayed electrode Appl Surf Sci 2008;254(10):3232–5.
[25] El-Deab MS, Ohsaka T Manganese oxide nanoparticles electrodeposited on platinum are superior to platinum for oxygen reduction Angew Chem Int Ed 2006;45(36): 5963–6.
[26] Wu MS, Chiang PCJ Fabrication of nanostructured manganese oxide electrodes for electrochemical capacitors Electrochem Solid-State Lett 2004;7:A123–6.
[27] El-Deab MS, Ohsaka T Electrosynthesis of single-crystalline MnOOH nanorods onto Pt electrodes: electrocatalytic activity toward reduction of oxygen J Electrochem Soc 2008;155(1):D14–21.
[28] Trasatti S, Petrii OA Real surface area measurements in electrochemistry J Pure Appl Chem 1991;63:711–34.
[29] Liu Z, Ling XY, Su X, Lee JY Carbon-supported Pt and PtRu nanoparticles as catalysts for a direct methanol fuel cell J Phys Chem B 2004;108(24):8234–40.
[30] Greef R, Peat R, Peter LM, Pletcher D, Robinson J Instrumental methods in electrochemistry Chichester, UK: Ellis Horwood; 1990.
[31] Poubaix M Atlas of electrochemical equilibria in aqueous solutions Oxford: Pergamon Press; 1966.
[32] El-Deab MS, Kibler LA, Kolb DM Enhanced electro-oxidation
of formic acid at manganese oxide single crystalline nanorod-modified Pt electrodes Electrochem Commun 2009;11(4): 776–8.