Coordination complex synthesis of noble metals in the preparation of nanoparticles supported on MWCNTs used as electrocatalysts

Article abstract of DOI:10.1016/j.ica.2013.06.042

Coordination complex salts were synthesized in an aqueous solution and used as precursors to prepare metallic and bimetallic nanoparticles supported on multiwalled carbon nanotubes (MWCNTs) to be applied as electrocatalysts on methanol electrooxidation. The precursors were obtained by the reaction between (NH₄)₂PtCl₆, (NH₄) ₂PdCl₆, (NH₄)₃RuCl₆ and HAuCl₄ with tetraoctylammonium bromide (TOAB). These were obtained as (TOA)nMCl_y salts (where M = Pt, Pd, Ru or Au) and were characterized by FT-IR and TGA analysis. The precursors were used to prepare metallic and bimetallic nanoparticles of Pt, Pt-Pd, Pt-Ru, as well as Pt-Au coated on MWCNT (Pt-M/MWCNTs, where M = Pd, Ru or Au). Materials were characterized by TEM, XRD and TGA. The electrocatalytic properties of the Pt-M/MWCNTs electrodes for the methanol oxidation were determined by cyclic voltammetry (CV) and chronoamperometry (CA). The bimetallic nanoparticles presented a higher electrocatalytic activity and stability than the Pt/MWCNTs electrocatalyst, attributed to the addition of M which not only led to a smaller average particle size and higher dispersion of Pt of the metallic nanoparticles on MWCNTs but also promoted an elevated electronic transfer between bimetallic nanoparticles.

Full text of DOI:10.1016/j.ica.2013.06.042

Inorganica Chimica Acta 406 (2013) 138–145
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Coordination complex synthesis of noble metals in the preparation
of nanoparticles supported on MWCNTs used as electrocatalysts
b
b
a
a
J.R. Rodriguez a, , R.M. Félix , E.A. Reynoso , S. Fuentes Moyado , G. Alonso-Núñez
⇑
a Universidad Nacional Autónoma de México, Centro de Nanociencias y Nanotecnología, Ensenada, B.C. C.P. 22860, Mexico
b Instituto Tecnológico de Tijuana, Centro de Graduados e Investigación, Tijuana, B.C. C.P. 22000, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Coordination complex salts were synthesized in an aqueous solution and used as precursors to prepare
metallic and bimetallic nanoparticles supported on multiwalled carbon nanotubes (MWCNTs) to be
applied as electrocatalysts on methanol electrooxidation. The precursors were obtained by the reaction
between (NH4)2PtCl6, (NH4)2PdCl6, (NH4)3RuCl6 and HAuCl4 with tetraoctylammonium bromide (TOAB).
These were obtained as (TOA)nMCly salts (where ‘‘M’’ = Pt, Pd, Ru or Au) and were characterized by FT-IR
and TGA analysis. The precursors were used to prepare metallic and bimetallic nanoparticles of Pt, Pt–Pd,
Pt–Ru, as well as Pt–Au coated on MWCNT (Pt–M/MWCNTs, where M = Pd, Ru or Au). Materials were
characterized by TEM, XRD and TGA. The electrocatalytic properties of the Pt–M/MWCNTs electrodes
for the methanol oxidation were determined by cyclic voltammetry (CV) and chronoamperometry
(CA). The bimetallic nanoparticles presented a higher electrocatalytic activity and stability than the Pt/
MWCNTs electrocatalyst, attributed to the addition of ‘‘M’’ which not only led to a smaller average par-
ticle size and higher dispersion of Pt of the metallic nanoparticles on MWCNTs but also promoted an ele-
vated electronic transfer between bimetallic nanoparticles.
Received 20 April 2013
Received in revised form 21 June 2013
Accepted 24 June 2013
Available online 9 July 2013
Keywords:
Coordination complex salts
Nanoparticles
MWCNTs
Electrocatalysts
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
The use of a capping agent improves the control of the size and
shape of the nanoparticles, according to the Brust–Schiffrin meth-
The synthesis of nanostructure materials with controlled size
and composition is very important for numerous applications,
because of their novel and attractive physical and chemical proper-
ties [1], which differ from those properties that exhibited for bulk
[2,3]. For example, the transition metal nanoparticles of Pt, Pd, Ru
and Au have been studied as catalysts in organic reactions [4–6], as
electrocatalysts in fuel cells [7], or as materials with novel elec-
tronic [8], chemical sensor [9] and magnetic properties [10]. The
catalytic and electronic properties are strongly influenced by the
size of the particles, especially in the range 1–10 nm, evidencing
the need for size-selective synthesis techniques. The synthesis of
metallic and bimetallic nanoparticles is mainly achieved by the
chemical reduction of coordination complex salts using different
reduction agents such as hydrogen, borohydrides, hydrazine or
aldehyde [11–14]. The nanoparticles synthesized by reduction of
the coordination complex salts using sodium borohydride as a
reducing agent, generally allows the obtaining a very small and
uniform particle size because sodium borohydride enables a rapid
reduction to afford very small nuclei whose growth and agglomer-
ation is hindered [15].
od [16,17], where the TOAB is used as a capping agent, which per-
mits the preparation of the nanoparticle of 2–3 nm, also this
precursor allow to control the morphology of the nanoparticles,
which can be spherical or cubic [18–20].
We are interested in the preparation of Pt nanoparticles follow-
ing the Brust–Schiffrin method, with the aim of controlling their
size and surface properties [21–23]. Beside catalysis, the synthesis
of coordination complex salts as agent precursors for the prepara-
tion of nanoparticles has allowed to acquire nanostructured mate-
rials with attractive catalytic properties [24]. In particular, Pt–M
nanoparticles have been investigated for their high electrocatalytic
activity for methanol oxidation and CO tolerance oxidation related
to fuel cell application [25,26].
The present report describes a new promising approach based
on the synthesis of new organometallic salts which were charac-
terized by FT-IR and TGA analysis. These salts precursors were used
for the preparation of nanoparticles of noble metals to be deposited
on MWCNTs and used as electrocatalysts. The materials were char-
acterized by TEM, XRD and TGA. Moreover the Pt–M nanoparticles
deposited on MWCNTs (Pt–M/MWCNTs) were studied as electro-
catalysts, by cyclic voltammetry (CV) and choronoamperommetry
(CA) in the methanol oxidation reaction (MOR).
⇑
Corresponding author. Tel.: +52 646 1744602.
E-mail address: jrodrig@cnyn.unam.mx (J.R. Rodriguez).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.06.042

J.R. Rodriguez et al. / Inorganica Chimica Acta 406 (2013) 138–145
139
starting solution was composed of ferrocene (1.0 g, Alfa Aesar
98.0%) as catalyst which is dissolved in 25 mL of toluene (Fulka
99.5%). This solution was atomized and carried by a flow of argon
into a Vycor tube that is heated in a cylindrical furnace equipped
with a high precision temperature controller ( 1 °C). The furnace
temperature was set at 900 °C and the flow rate was regulated at
75 mL/s. The solution was fed for 30 min. A black film of MWCNTs
was then formed at the inner surface of the Vycor tube and was
mechanically removed and collected. The crude MWCNTs (0.5 g)
were added to 20 mL of a HNO3 solution (Aldrich, 70%). The mix-
ture was placed in an ultrasonic bath for 1 h and then stirred for
12 h while it refluxed. The product was filtered under vacuum,
washed with deionized water, and dried at 70 °C for 8 h.
2. Experimental
The chemicals used in this study, (NH4)2PtCl6 and (NH4)2PdCl6,
were acquired from Alfa Aesar and HAuCl4, (NH4)2RuCl6, NaBH4,
Tetraoctylammonium bromide (TOAB), and 2-propanol were pur-
chased from Aldrich Co. All chemicals were used without further
purification. Triply distilled water was used as dissolvent of the
metallic salts.
2.1. Organometallic salts synthesis
The synthesis of the different organometallic salt precursors in
this work were obtained by the exchange reaction in an aqueous
solution between the ammonium group [NH4]+ (in the case of
(NH4)2PtCl6, (NH4)2PdCl6 and (NH4)2RuCl6) or H+ (in the case of
HAuCl4) and [TOA]+, following a similar method reported previ-
ously [27]. Eq. (1) shows the general reaction:
2.3. NPs/MWCNTs
The nanoparticles of Pt, Pt–Pd, Pt–Au and Pt–Ru (with atomic
ratio Pt:M, 1:1) in situ synthesized on the MWCNTs surface from
the metallic precursors (TOA)nMCly, is described as follows. In
the first step, MWCNTs (25 mg) were added to 25 mL of 2-propanol
and dispersed in an ultrasonic bath for 1 h. In a second step,
3.13 ꢁ 10ꢀ5 mol of the metal precursor in 5 mL of 2-propanol solu-
tion was added to the MWCNTs suspension and stirred for 1 h.
Finally, in the third step, 10 mL of an aqueous solution of NaBH4
in excess (stoichiometric metal:NaBH4 ratio = 1:10) was added by
drip during 5 min to the suspension of (TOA)nMCly/MWCNTs,
which was stirred at room temperature for 12 h. The mixture
obtained was then filtered, washed with acetone and water, and
finally dried at 70 °C for 4 h. In this step the reaction reduction
occurred to obtain NPs/MWCNTs as shown in Eq. (2).
XnMClnꢀ þ TOAB ! ðTOAÞ MCly þ XBr
ð1Þ
y
n
ðwhere M ¼ Pt; Pd; Ru; Au; and X ¼ NHþ4 ; HþÞ
2.1.1. (TOA)2PtCl6 synthesis
Ammonium hexachloroplatinate(IV), (NH4)2PtCl6, (0.3 g,
6.76 ꢁ 10ꢀ4 mol) was dissolved into 20 mL of triply distilled water.
This aqueous solution was then added to 25 mL of a TOAB in 2-pro-
panol solution (0.739 g, 1.35 ꢁ 10ꢀ3 mol) at room temperature. The
resulting solution was left under stirring until the complete precip-
itation of (TOA)2PtCl6. The product was filtered under vacuum,
washed with deionized water, and dried at 70 °C for 8 h. The yield
of the (TOA)2PtCl6 was of 86%.
ðTOAÞnMCly þ NaBH4 þ MWCNTs
! M=MWCNTs þ ðSolution : ðTOAÞþCl; NaCl; BðOHÞ3Þ
ðWhere M ¼ Pt; Pd; Ru; AuÞ
ð2Þ
2.1.2. (TOA)2PdCl6 synthesis
Ammonium hexachloropalladate(IV), (NH4)2PdCl6, (0.3 g,
8.45 ꢁ 10ꢀ4 mol) was dissolved into 20 mL of triply distilled water.
This aqueous solution was then added to 25 mL of a TOAB in 2-pro-
panol solution (0.924 g, 1.69 ꢁ 10ꢀ3 mol) at room temperature. The
resulting solution was left under stirring until the complete precip-
itation of (TOA)2PdCl6. The product was filtered under vacuum,
washed with deionized water, and dried at 70 °C for 8 h. The yield
of the (TOA)2PdCl6 was of 91%.
2.4. Electrode preparation
The catalysts’ electrodes were prepared with 2 mg of Pt/
MWCNTs or bimetallic Pt–M/MWCNTs, 150 L of Nafion (Aldrich,
5%) and 550 L of ethanol (Aldrich, 99.5%) and mixed ultrasoni-
cally. A measured volume (5 L) of this ink was deposited using
l
l
l
2.1.3. (TOA)AuCl4 synthesis
a micro-pipette onto a glassy carbon (GC) electrode (BAS, 3 mm
in diameter) which was first polished with alumina powder, fol-
lowed by 2 min sonication in water. The catalyst ink was then
dried at 40 °C in an oven. Each electrode contained about
4.97 ꢁ 10ꢀ4 mmol of the metal catalyst.
Gold(III) chloride hydrate, HAuCl4ꢂ3HO, (0.3 g, 7.62 ꢁ 10ꢀ4 mol)
was dissolved into 20 mL of triply distilled water. This aqueous
solution was then added to 25 mL of a TOAB in 2-propanol solution
(0.417 g, 7.62 ꢁ 10ꢀ4 mol) at room temperature. The resulting solu-
tion was stirred until the formation of a (TOA)AuCl4 precipitate. The
product was filtered under vacuum, washed with deionized water,
and dried at 70 °C for 8 h. The yield of the (TOA)AuCl4 was of 77%.
2.5. Characterization
Organometallic salts were characterized by infrared spectros-
copy (FT-IR) using a Perkin-Elmer FTIR 1605 spectrometer and
the thermogravimetric analysis (TGA) was performed using a TA-
Instruments SDT 2920. Samples were heated in a platinum-pan un-
der dry air flux from room temperature up to 700 °C (heating rate:
2.1.4. (TOA)2RuCl6 synthesis
Ammonium hexachloruthenate(IV), (NH4)2RuCl6,
(0.3 g,
8.57 ꢁ 10ꢀ4 mol) was dissolved into 20 mL of triply distilled water.
This aqueous solution was then added to 25 mL of a TOAB in 2-pro-
panol solution (0.937 g, 1.71 ꢁ 10ꢀ3 mol) at room temperature. The
resulting solution was left under stirring until the complete precip-
itation of (TOA)2RuCl6. The product was filtered under vacuum,
washed with deionized water, and dried at 70 °C for 8 h. The yield
of the (TOA)2RuCl6 was of 78%.
5 °C/min). The NPs/MWCNTs, X-ray diffraction (XRD) studies were
´
performed using a Phillips XPert Diffractometer, with a Cu K
a
(k = 1.5405 Å) radiation source operated at 45 kV and 40 mA. All
samples were evaluated in a range from 10 at 70 of 2h, and the
crystalline phases were identified using a database JCPDS-ICDD
2003. The morphology of metallic NPs supported on MWCNTs
was studied by transmission electron microscopy (TEM) using a
JEOL 2010 microscope. Samples were dispersed ultrasonically in
ethanol and a drop of suspension was placed onto a holey
carbon-coated Cu grid and it was allowed to dry. The images were
2.2. MWCNTs synthesis
The MWCNTs used in this work were obtained by spray pyroly-
sis following a similar method reported elsewhere [28]. The

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J.R. Rodriguez et al. / Inorganica Chimica Acta 406 (2013) 138–145
Table 1
FT-IR frequencies observed for the different organometallic salts.
Compound
IR frequencies (cmꢀ1
)
vas (CH2)
vs (CH2)
d(C–H) of N–(CH2)4
b-(CH2)
(TOA)2PtCl6
(TOA)2PdCl6
(TOA)AuCl4
(TOA)2RuCl6
TOAB[29]
2924
2921
2920
2924
2925
2853
2852
2854
2853
2859
1481
1471
1476
1468
1458
725
723
721
724
714
(TOA)
2RuCl6
500
oxygen atmosphere; the final residue is assumed to be in the oxide
form. The detailed analysis of the thermal decomposition mecha-
nisms and the resulting stoichiometry for each organometallic salt
is discussed below.
4000 3500 3000 2500 2000 1500 1000
-1
Wavenumber(cm )
The thermal decomposition profile of the ruthenium salt is
shown in Fig. 2. Up to two different decomposition steps can be
recognized in the first-derivative curve of the weight loss at aver-
age temperatures of 180–280 °C and 280–375 °C. Table 2 summa-
rizes the different weight losses observed for each step and the
possible molecules eliminated during the treatment under oxygen.
In the first step, the experimental weight loss observed corre-
sponds to the removal of 8(CH3(CH2)5CH@CH2), 2NH4Cl and Cl2
that result from the disproportionation of the radicals. The second
step corresponds to the elimination of Cl2 with a final Ru2O3
Fig. 1. FT-IR spectrum of the (TOA)2RuCl6 compound.
obtained in a transmission electron microscopy (TEM) mode using
a bright field (BF). The Pt–M/MWCNTs, TGA studies were per-
formed in a dry air flux atmosphere to oxidize them. In this case,
MWCNTs supported samples were heated at 5 °C/min from room
temperature to 700 °C. The electrochemical measurement were
performed using an EC Epsilon BAS potentiostat/galvanostat and
a conventional three electrode electrochemical cell, using a glassy
carbon electrode as a working electrode (BAS, 3 mm diameter), a Pt
electrode as a counter electrode and a Ag/AgCl electrode as a
reference electrode. All the solutions were prepared using H2SO4
(Fermont, 98%), methanol (Aldrich, 99.5%) and ultrapure water
residue suggesting that
a reduction process occurs around
280–375 °C. There is a cumulative weight loss of 90.3% and the
residual weight corresponds to a Ru2O3 formula.
The thermal decomposition of the platinum salt shows two
steps of weight loss that can be distinguished at average tempera-
tures of 240–350 °C and 350–480 °C (Supplementary material
Fig. S1). A detailed analysis of weight loss percentages and possible
molecules that were eliminated in each step are shown in Table S1
(supplementary material). The first step corresponds to the loss of
the main parts of the hydrocarbons 8(CH3(CH2)5CH@CH2) that re-
sults from the disproportionation of radicals with a weight loss
of 66%. The second step is related to the removal of the remnant
of the two ammonium groups (2NH4Cl) and of the two chloride
molecules (2Cl2) leading to a theoretical weight loss of 18%. The
chemical formula of the residue is PtO2 suggesting that Pt is still
present at an oxidation state of IV.
(Millipore MilliQ, 18 MX cm).
3. Results and discussion
3.1. Characterization of the organometallic salts
3.1.1. FT-IR study
Fig. 1 shows the infrared spectrum corresponding to the (TOA)2-
RuCl6 structure, whereas Table 1 summarizes the infrared frequen-
cies detected on the different organometallic precursors with their
corresponding modes of vibration. The IR spectrum of (TOA)2RuCl6
presents four main groups of vibration corresponding to the TOA
group at 724, 1468, 2853, and 2924 cmꢀ1. Close frequency values
were generally acquired on the other organometallic precursors
(Table 1) confirming a relative similar arrangement in all cases.
The bands at 2924 and 2853 cmꢀ1 are assigned to the asymmetric
and symmetric C–H respectively, due to stretching vibrations of
the methylene (CH2) group of the TOA. A small shoulder around
2924 cmꢀ1 is due to the asymmetric C–H stretch of the methyl
(CH3) group. The band observed at 1468 cmꢀ1 is attributed to the
asymmetric C–H scissoring vibration of the CH2–N+ whereas the
band at 724 cmꢀ1 corresponds to a rocking mode vibration of the
CH2 chain. If these bands are compared to those obtained from
the TOAB molecule [29], the C–H stretching vibration bands of
the methylene chain are similar to those shown in Table 1. At
the same time very similar band positions were found for (TOA)2-
PtCl6, (TOA)2PdCl6 and (TOA)AuCl4. These unchanged frequencies
show that the conformation of the methylene chain is maintained
in the as-formed organic salt.
The thermal decomposition profile of the palladium salt is
shown in Fig. S2 (supplementary material). Up to two different
decomposition steps can be recognized in the first-derivative curve
of the weight loss at average temperatures of 170–330 °C and
330–470 °C. Table S2 (supplementary information), summarizes
100
2.5
2.0
1.5
1.0
0.5
0.0
(TOA)2RuCl6
80
60
40
20
0
3.1.2. TGA study
0
100
200
300
400
500
(°C)
600
700
TGA was performed to confirm the stoichiometry of the as-pre-
pared different organometallic salts and to investigate their
respective thermal decomposition mechanisms. For this purpose
it was assumed that organometallic salts presented a similar
degree of decomposition, as the decomposition took place in an
Temperature
Fig. 2. TGA analysis under a dry air flux atmosphere of the (TOA)2RuCl6 compound.
Sample was heated at 5 °C/min from room temperature to 700 °C. Weight loss and
its first derivative curves.

J.R. Rodriguez et al. / Inorganica Chimica Acta 406 (2013) 138–145
141
Table 2
TEM images of the MWCNTs supported nanoparticles obtained
from Pt, Pt–Pd, Pt–Au, and Pt–Ru catalysts. It can be seen that for
all samples, the Pt–M nanoparticles were uniformly formed on
the external wall of the MWCNTs; these nanoparticles showed
spherical morphologies in all materials. Well-dispersed Pt–M nano-
particles can be found in any microregion of the sample viewed
from the TEM grid. Their sizes were obtained by measuring the
nanoparticles from the TEM images using an imaging analysis soft-
ware digital micrograph (Gatan, Inc.). More than 300 nanoparticles
were measured to ensure statistically significant representation of
the nanoparticles sizes (Table 3). The Pt–M had a mean size of less
than 2 nm, except for Pt–Ru which had a mean size of less than
3 nm. The size distributions of these Pt–M nanoparticles (Fig. 4)
are fairly consistent with a Gaussian distribution, obtaining a
geometric mean of 1.7, 1.4, 1.8 and 2.3 for the samples Pt, Pt–Pd,
Pt–Au and Pt–Ru, respectively, showing that the nanoparticles of
the Pt–Ru had the largest particles and broadest size distribution.
Thus, these values of the mean particle size are different to those
obtained by XRD analysis, but the behavior of the particle size is
the same, because the TEM study relies on a statistical analysis from
values of mean particle size deposited on MWCNTs and the particle
size obtained by XRD characterization is calculated with Scherrer
equation which is not as accurate for nanoparticles.
TGA results for the (TOA)2RuCl6 compound decomposition under
atmosphere.
a dry air flux
(TOA)2RuCl6
T1 (°C)
180–280
83.5
83.4
D
w1, wt.% (exp)
D
w1, wt.% (theor)
Assuming loss
T2 (°C)
8(CH3(CH2)5CH@CH2) + 2NH4Cl + Cl2
280–375
6.8
6.9
Cl2
90.3
9.7
D
w2, wt.% (exp)
D
w2, wt.% (theor)
Assuming loss
P
D
w, wt.% (exp)
Residual, wt.% (exp)
Residual, wt.% (theor)
Assuming residual as Ru2O3
9.7
the different weight losses observed for each step and the possible
molecules eliminated during the treatment under oxygen. In the
first step, the experimental weight loss observed corresponds to
the removal of 6(CH3(CH2)4CH@CH2) and 2(CH3(CH2)5CH@CH2)
that results from the disproportionation of the radicals. As for
the preceding cases, the second step corresponds to the elimina-
tion of the 2(CH3CH2)3N and 3Cl2 with a final PdO2 residue suggest-
ing that
a reduction process occurs around 330–470 °C. A
3.2.3. TGA study
cumulative weight loss of 87.3% corresponds to the elimination
of all the organic part of the organometallic salt to obtain PdO2 sug-
gesting that Pd is still present at the oxidation state of IV. The dis-
crepancies between the theoretical and experimental weight losses
were ꢀ1.0% and ꢀ1.3% for the precursor salts of Pt and Pd,
respectively.
The TGA curve and its first-derivate signal for the gold salt are
presented in Fig. S3 (supplementary material). Two decomposition
steps can be noticed at 200–335 °C and 335–490 °C. The analysis of
the weight loss and the possible products eliminated are summa-
rized in Table S3 (supplementary material section). The first step
corresponds to the removal of the main parts of the hydrocarbons
3(CH3(CH2)4CH@CH2) and (CH3(CH2)5CH@CH2) that result from
the disproportionation of radicals, corresponding a weight loss of
49%. In the second step, the weight loss is the ratio between the
amount removed to the remnant of (CH3)3N and 2Cl2 being
24.2%. The chemical formula of the residue is Au2O3 suggesting
that Au is still present at an oxidation state of III.
TGA were performed under a dry air flux, for the different Pt–M/
MWCNTs metallic systems to quantify the amount of metal depos-
ited on the support. Fig. 5 reports the different TGA curves for the
Pt–M/MWCNTs systems, whereas quantification results are listed
in Table 4. The onset and final temperatures for the oxidation pro-
cess are also reported in Table 4. For Pt/MWCNTs (solid line), the
beginning of the oxidation of the support is observed at 426 °C.
The oxidation is completed at 655 °C. Only a residual mass
(14 wt.%) of the initially loaded Pt/MWCNTs sample is found at
the end of the oxidative treatment. This residue corresponds to
PtO2 coming from the (TOA)2PtCl6. For Pt–Pd/MWCNTs (dash line),
the beginning of the oxidation of the support is observed at 435 °C
and the oxidation is completed at 645 °C. Only a residual mass
(19 wt.%) of the initially loaded Pt–Pd/MWCNTs sample is found
C (002)
Pt (220)
Pt (111)
Pt (200)
Pt-Au/MWCNTs
3.2. Nanoparticles on MWCNTs
3.2.1. XRD study,
To obtain the crystallographic information of the electrocata-
lysts, the powder X-ray diffraction technique was used. The XRD
patterns (Fig. 3) for Pt–M/MWCNTs catalysts show three diffrac-
tion peaks at the Bragg angles of 39.8, 45.7 and 67.7 corresponding
to the (111), (200) and (220) planes of Pt face-centered cubic (fcc)
crystal structure. The diffraction peaks in the samples are slightly
shifted to higher 2h values with respect to the corresponding peaks
reported in the database JCPDS file No. 04-0802. This was due to
the fact that the diffraction patterns were obtained with the pow-
der mode and the samples were characterized in form of stacked
sheets. The diffraction peak at 26.4 corresponds to the (002) plane
of C support, reported in the database JCPDS file No. 08-0415. Ta-
ble 3 lists the calculated average sizes of nanoparticles supported
on MWCNTs which were calculated from the FWHM of (111) peak
using the Scherrer equation [30,31].
Pt-Ru/MWCNTs
Pt-Pd/MWCNTs
Pt/MWCNTs
10
20
30
40
50
60
70
2θ (Degree)
3.2.2. TEM study
Micrographs of the Pt–M/MWCNTs were obtained from the TEM
mode using a BF image detector. Fig. 4 shows low magnification
Fig. 3. Powder XRD patterns of the electrocatalysts supported on MWCNTs: Pt, Pt–
Pd, Pt–Au and Pt–Ru, recorded from 10 at 70 of 2h.

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J.R. Rodriguez et al. / Inorganica Chimica Acta 406 (2013) 138–145
Table 3
Average particle size of catalysts from XRD and TEM data of Figs. 3 and 4, respectively.
Compound
Average particle size XRD (nm)
Surface area (cm2/g) [31]
Average particle size TEM (nm)
Standard deviation,
r
(nm)
Pt/MWCNTs
3.7
2.5
4.4
4.2
76
112
64
1.7
1.4
1.8
2.3
0.4
0.2
0.2
0.5
Pt–Pd/MWCNTs
Pt–Au/MWCNTs
Pt–Ru/MWCNTs
67
b
a
5 nm
10 nm
100 nm
100 nm
d
c
10 nm
20 nm
200 nm
100 nm
Fig. 4. Transmission electron micrographs of (a) Pt/MWCNTs, (b) Pt–Pd/MWCNTs, (c) Pt–Au/MWCNTs, (d) Pt–Ru/MWCNTs.
at the end of the oxidative treatment. This residue corresponds to
PtO2 and PdO2 coming from the precursor salts. For Pt–Au/
MWCNTs (dot line) and Pt–Ru/MWCNTs (dash-dot line), the begin-
ning of the oxidation of the support is observed at 402 °C for both
samples. The oxidation is completed at 633 °C for the sample that
contains Au and 634 °C for the one that contains Ru. The residual
mass (11 wt.%) of the initially loaded Pt–Au/MWCNTs sample is
found at the end of the oxidative treatment; a similar procedure
was followed by Pt–Ru/MWCNTs obtaining a residual mass of
17 wt.%. For sample Pt–Au/MWCNTs the residue corresponds to
PtO2 and Au2O3 and for Pt–Ru/MWCNTs the residues correspond
to PtO2 and Ru2O3. In both cases, the samples residues correspond
to metal oxides coming from the precursor salts.
For Pt–Au/MWCNTs and Pt–Ru/ MWCNTs, the onset of the oxi-
dation process starts at lower temperatures than for the Pt/
MWCNTs and Pt–Pd/MWCNTs catalysts. Contrary to Pt and Pt–Pd
containing solids, Pt–Au and Pt–Ru effectively catalyzes the
oxidation of the MWCNTs. This effect is due to the oxygen species
formed on Pt–Au and Pt–Ru which are transferred to the carbon
surface thanks to an intimate contact between Pt–M and C
100
80
60
40
20
0
Pt/MWCNTs
Pt-Pd/MWCNTs
Pt-Au/MWCNTs
Pt-Ru/MWCNTs
100
200
300
400
500
600
700
Temperature (°C)
Fig. 5. TGA analyses under
MWCNTs systems. Sample was heated at 5 °C/min from room temperature to
700 °C. Weight loss curves.
a dry air flux atmosphere of the different Pt–M/

J.R. Rodriguez et al. / Inorganica Chimica Acta 406 (2013) 138–145
143
Table 4
TGA results for the Pt–M/MWCNTs systems decomposition under a dry air flux atmosphere.
Compound
Onset Toxid (°C)
Final Toxid (°C)
Residual metal oxide amount (wt.% exp)
Residual metal oxide amount (wt.% theor)
Pt/MWCNTs
426
435
402
402
655
645
633
634
14 (PtO2)
22 (PtO2)
Pt–Pd/MWCNTs
Pt–Au/MWCNTs
Pt–Ru/MWCNTs
19 (PtO2, PdO2)
11 (PtO2, Au2O3)
17 (PtO2, Ru2O3)
19 (PtO2, PdO2)
22 (PtO2, Au2O3)
19 (PtO2, Ru2O3)
resulting in the formation of CO2. Pt–M is definitely oxidized into
PtO–MOx only after the complete oxidation of the carbon support
[32]. The final oxide residue corresponds to a 10 wt.% of Pt theoret-
ical loading on bimetallic systems and of 20 wt.% of Pt theoretical
loading on a metallic system. This leads to a relatively low yield
of deposited Pt–M (Table 4) during the preparation of the Pt–M/
MWCNTs solid. Therefore, the very high achieved dispersion of
Pt–M nanoparticles with an average size of 1.7 nm observed by
TEM is compensated by the difficulties to deposit selectively Pt–
material is more electropositive that Pt polycrystalline, thus mod-
ifying the electronic environment and therefore the adsorption
strength of protons. Moreover Fig. 6 shows that Pt, Pt–Ru and
Pt–Au do not exhibit an oxidation peak between 0.4 and 0.45 V
in contrast to Pt–Pd, which is possibly due to the OH-adsorbed spe-
cies on Pd present on the surface of the nanoparticles. At the same
time in the cathodic scan it can be observed a shift in the peak of
the OH-desorbed species of the Pt–Pd surface at 0.1 V (0.3 V) com-
pared to Pt, Pt–Ru and Pt–Au which appeared at 0.37, 0.37 and
0.4 V. This is because the Pt–Pd material is more electropositive
and the OH-adsorbed species is stronger for Pd [35].
nꢀ
M on MWCNTs using the (TOA)nMxCly salts.
It should be underlined that metal deposition on the MWCNTs
support is more or less directly related to the stability of the differ-
ent organometallic salts used and particularly to the strength of
the interaction between the polar headgroup of the TOA moiety
and the metal chloride anions. A stronger interaction between
the hydrophobic TOA group and the metallic anions seems to de-
crease the anchoring of the organometallic salt on the MWCNTs
support after reduction by NaBH4 and before loading of the NP’s.
This is the case for Pd and Ru showing a low stability of their
respective precursors compared to Pt and Au (Table 4). The selec-
tive deposition of the different metals on the MWCNTs support will
therefore partly depend on the respective stabilities of the organo-
metallic salts [23].
3.3.1. Methanol oxidation
To evaluate the activity of Pt/MWCNTs and Pt–M/MWCNTs cat-
alysts toward the methanol electrooxidation, cyclic voltammogram
experiments were performed in a 1 M MeOH solution in 0.5 M
H2SO4 (potential range between 0.2 and 1.3 V) [36,37]. The cycling
was repeated until a reproducible CV curve was obtained before
the measurement curves were recorded. As seen in Fig. 7, in the
forward scan, methanol oxidation produced a prominent symmet-
ric anodic peak for Pt/MWCNTs at 1.239 V (versus Ag/AgCl) with
56.2 mA mgꢀ1Pt (Table 5). The incorporation of the second metal
in the Pt–M/MWCNTs systems induced a catalytic effect on the
methanol electrooxidation: Pt–Pd/MWCNTs catalyst oxidized
methanol at 0.960 V, Pt–Au/MWCNTs at 0.807 V and Pt–Ru/
MWCNTs at 0.809 V. In the reverse scan, an anodic peak was de-
tected with a current density (jb), attributing this anodic peak in
the reverse scan to the removal of the incompletely oxidized carbo-
naceous species formed on the forward scan [38]. Hence the ratio
of the forward anodic peak with current density jf to the reverse
anodic peak with current density jb, (jf/jb) can be used to describe
the catalyst tolerance to carbonaceous species accumulation. A
low jf/jb ratio indicates poor oxidation of methanol to carbon diox-
ide during the anodic scan and an excessive accumulation of carbo-
naceous residues on the catalyst surface. On the other hand, a high
jf/jb ratio shows the inverse case. Pt–Pd/MWCNTs and Pt–Au/
MWCNTs catalysts were the systems with higher jf/jb ratios.
3.3. Electrochemical evaluation
Before the evaluation of the electrochemical activity of Pt–M/
MWCNTs catalysts, 30 scans were performed to clean the surface
of the Pt–M/MWCNTs electrodes within a potential range from
ꢀ0.2 to 1.3 V at a scan rate of 100 mV sꢀ1 in a 0.5 M H2SO4 solution
saturated with N2 [33,34]. The CV curves looked similar to those of
pure Pt polycrystalline (Fig. 6). However, the hydrogen adsorption/
desorption region presented a shift to higher potentials for bime-
tallic Pt–M catalysts. This was caused by a change of the lattice
parameters of the crystal structure of Pt, due to the incorporation
of the second metal, which promoted changes in the adsorption
strength of the hydrogen species on the Pt because the Pt–M
70
30
Pt/MWCNTs
Pt-Pd/MWCNTs
Pt-Au/MWCNTs
Pt/MWCNTs
60
50
40
30
20
10
0
Pt-Pd/MWCNTs
Pt-Au/MWCNTs
Pt-Ru/MWCNTs
20
Pt-Ru/MWCNTs
10
0
-10
-20
-30
-10
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E / V (Ag/AgCl)
E / V (Ag/AgCl)
Fig. 6. Cyclic voltammograms of Pt–M/MWCNTs catalysts in 0.5 M H2SO4 solution
saturated by N2 at 25 °C, recorded at 100 mV sꢀ1
Fig. 7. Cyclic voltammograms of Pt–M/MWCNTs catalysts performed with 1 M
methanol in 0.5 M sulfuric acid at 25 °C. Scan rate: 100 mV sꢀ1
.
.

144
J.R. Rodriguez et al. / Inorganica Chimica Acta 406 (2013) 138–145
Table 5
catalyst type (Pt–M/MWCNTs). An effect is seen upon adding a sec-
ond metal to the Pt surface, with a decrease in the poisoning rate
from d = 0.030% sꢀ1 for Pt/MWCNTs catalyst to d = 0.028% sꢀ1 for
Pt–Au/MWCNTs. Pt–Pd/MWCNTs and Pt–Ru/MWCNTs catalysts
were poisoned much less than the catalyst Pt–Au/MWCNTs, with
a poisoning rate of 0.017 and 0.016% sꢀ1, respectively.
Oxidation peaks of cyclic voltammograms of the Pt–M/MWCNTs systems.
Compound
Ef
jf
Eb
jb
jf/jb
Pt/MWCNTs
1239
960
807
809
56.2
53.7
30.0
27.4
676
508
370
437
38.8
30.9
16.6
21.6
1.45
1.74
1.81
1.27
Pt–Pd/MWCNTs
Pt–Au/MWCNTs
Pt–Ru/MWCNTs
4. Conclusions
The catalytic stability of the Pt/MWCNTs and Pt–M/MWCNTs
were examined using chronoamperometry. Fig. 8a) shows the
chronoamperometric curves of 1 M MeOH in a 0.5 M H2SO4 solu-
tion for Pt/MWCNTs and Pt–M/MWCNTs catalyst electrodes at a
fixed potential of Ef. For all catalysts the potentiostatic current
decreases rapidly at the initial stage, which might be due to the
formation of intermediate species, such as COads and CHOads etc.,
during the methanol oxidation reaction [39]. Gradually, the current
decayed and a pseudo-steady state was achieved. The current den-
sity of Pt–Pd/MWCNTs was higher than that of Pt–Ru/MWCNTs
and this in turn greater than that of Pt/MWCNTs and of Pt–Au/
MWCNTs catalyst electrodes during the whole testing time indicat-
ing that the electrocatalytic stability of Pt–Pd/MWCNTs catalyst for
the methanol oxidation was also higher than that of the other cat-
alysts probed.
New organometallic salts: (TOA)2PtCl6, (TOA)2PdCl6, (TOA)2-
RuCl6, (TOA)AuCl4 were successfully synthesized and characterized
by FT-IR and TGA studies. As shown by TEM characterization, these
precursors were used in a second step to prepare Pt–M/MWCNTs
electrocatalysts presenting very high dispersion and small average
particle sizes (particularly Pt–Pd) with similar metal loading.
Pt-based binary electrocatalysts consisting of Pd, Au and Ru were
prepared by the co-precipitation method at room temperature.
The binary electrocatalysts showed higher activities and enhanced
CO relative tolerance than Pt/MWCNTs. The best performance was
developed by the binary Pt–Pd/MWCNTs electrocatalyst. The sec-
ond metal (in this case Pd) promoted an increase in the adsorption
of OHꢀ species because Pd is more hydrophilic that platinum; cou-
pled with this, Pd had an electronegativity different from that of Pt,
which also modified the bond strength between Pt and CO species
due to electronic effects.
The linear decay of the current at times greater than 500 s may
be characterized by the long time poisoning rate [40], d:
Acknowledgements
d ¼ ð100=i0Þðdi=dtÞt>500 s=%sꢀ1
ð3Þ
We gratefully acknowledge for their technical support to A. Elo-
isa, Fco. Ruiz, C. Belman, M. Saenz and J. Peralta from CNyN-UNAM.
We also thank CONACyT (project 155388) for their financial
support.
where (di/dt)t > 500 s is the slope of the linear portion of the current
decay, and io is the current at the start of the polarization. In Fig. 8b)
the variation of this parameter (d) is plotted as a function of the
Pt/MWCNTs
Pt-Pd/MWCNTs
Appendix A. Supplementary data
(a) 14
6
5
4
3
2
Pt-Au/MWCNTs
Pt-Ru/MWCNTs
12
10
8
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2013.06.042.
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