Journal of Materials Science

, Volume 47, Issue 23, pp 8134–8144

The preparation and characterization of nano-sized Pt–Pd/C catalysts and comparison of their superior catalytic activities for methanol and ethanol oxidation

Article

DOI: 10.1007/s10853-012-6709-3

Cite this article as:
Ozturk, Z., Sen, F., Sen, S. et al. J Mater Sci (2012) 47: 8134. doi:10.1007/s10853-012-6709-3

Abstract

In this study, two groups of carbon supported PtPd samples with different percentages of metals were prepared to examine the effects of Pd and stabilizing agents on the catalytic activity towards methanol and ethanol oxidation reactions. As a stabilizing agent, 1-hexanethiol and 1,1-dimethyl hexanethiol were used for group “a” and “b” catalysts, respectively. Cyclic voltammetry, chronoamperometry, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to understand the nature of the prepared catalysts. TEM and XRD results indicated a similar size distribution of the metal nanoparticles with a narrow average crystallite size of 3.0–3.7 nm. XPS data revealed the presence of two different oxidation states for both platinum and palladium, being Pt(0), Pt(IV), Pd(0), and Pd(II). Electrochemical studies indicated that the group “b” type catalysts have a higher catalytic activity than group “a”. The most active catalyst was found to be a carbon supported 88 %Pt/12 %Pd prepared with 1,1-dimethyl hexanethiol, which has an activity of ~5 times (~0.450 A/mg Pt at 0.57 V for methanol) and ~14 times (~0.350 A/g Pt at 0.56 V for ethanol) greater than the commercial E-TEK catalyst.

Introduction

The harmful products, such as sulfur dioxide (SO2), carbon monoxide (CO), and nitrogen oxides (NOx), formed during power generation from the combustion of fossil fuels and the rapid consumption rate of limited fossil fuel sources led to an interest in the development of fuel cells [1, 2]. Among the many kinds of fuel cells available, direct alcohol fuel cells are a promising future technology, e.g., direct methanol fuel cells (DMFCs) (a subcategory of direct alcohol fuel cells) have been receiving a great attention owing to high energy content per unit mass, ease of storage, transportation, handling, availability, low pollutant emission [3, 4, 5], and low cost of methanol [6, 7]. Ethanol can also be used as a fuel in direct alcohol fuel cells due to the same advantages stated above and the non-toxic feature of ethanol (usually called a green fuel) [8, 9, 10, 11]. Although methanol and ethanol are hopeful candidates as fuels for direct alcohol fuel cells, further developments are required, in terms of enhancement of catalyst performance, because these alcohols have no electrochemical activity in acidic solutions.

It is known that the improvement of more active catalysts is a key point for direct alcohol fuel cells and for that reason this is the focus of this study. Even though Pt-based catalysts are known as the best electrocatalysts for these types of fuel cells [12, 13, 14], methanol and ethanol oxidation reactions require a significant overpotential. In addition to this, high cost and limited platinum resources restrict the uses of these catalysts in practical applications [15, 16]. In order to overcome these problems, one of the most common approaches is to use another metal, such as Ru [17, 18, 19], Sn [20], Rh [21], and Mo [22], along with platinum and some of them have produced quite encouraging results. For this reason, in this study palladium was utilized as a second metal and PtPd nanoparticles were dispersed on a carbon support.

It is well known that the small size of nanoparticles increases the surface area of precious metals in the catalysts and consequently the activity of catalysts; ultimately decreasing the cost of the fuel cells. A problem with this approach is the agglomeration of PtPd nanoparticles. In order to prevent this in these nanoparticles [5, 23, 24, 25, 26], 1-hexanethiol (group “a” in our study) and 1,1-dimethyl hexanethiol (group “b”) were employed as stabilizing agents for the first time for this aim. Cyclic voltammetry (CV), chronoamperometry (CA), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were used for physical and electrochemical characterization of the prepared catalysts.

Experimental

Chemicals

PtCl4 (99 %) and PdCl2 (59 %) were purchased from Alfa, tetrahydrofuran (THF, 99.5 %), methanol (≥99.5), ethanol (99.9 %), and HClO4 (60 %) were obtained from Merck, lithium triethylborohydride (superhydride, 1.0 M dissolved in THF) was bought from Aldrich, 1,1-dimethyl hexanethiol (C8H18S) (85 %), and hexanethiol (95 %) were acquired from ABCR, and carbon XC-72 was purchased from Cabot Europa Ltd.

Water was purified using a Millipore water purification system (18 MΩ) analytical grade.

Synthesis of catalyst Ia

A 0.24 mmol of PtCl4 and 0.050 mmol PdCl2 were dissolved in 25 ml of anhydrous tetrahydrofuran for 1 h, then 0.27 mmol of 1-hexanethiol was added to this solution and the mixture was stirred vigorously for 2.5 h. The platinum and palladium species were reduced by the drop wise addition of lithium triethylborohydride. All these steps were carried out under a pure argon atmosphere. An observed color change from yellowish to black was the indication of formation of Pt–Pd nanoparticles, therefore addition of superhydride was maintained till this color change was observed [24]. The resulting solution was washed with dry ethanol in an ultrasonic bath, just before centrifugation, in order to remove excess thiols [25]. This process was repeated until a clear filtrate solution was obtained. Afterwards, the solid precipitate was dried under vacuum at room temperature. A 1:10 ratio of metal nanoparticles to carbon XC-72 were mixed in ethanol for 2 days and the solvent removed under vacuum at room temperature.

Synthesis of catalysts IIa, IIIa, Ib, IIb, and IIIb

An appropriate amount of PtCl4 and PdCl2 were used in order to prepare the catalysts IIa, IIIa, Ib, IIb, IIIb, Pt(a) [26], and (b) [26], using the same procedure as described in the synthesis of catalyst Ia. Table 1 gives the atomic percentages of platinum and palladium added to the reaction medium (a) and obtained from ICP results (b). Although the same amount of Pt and Pd were added to the reaction medium for catalysts Ia and Ib, catalysts IIa and IIb, and catalysts IIIa and IIIb, ICP results indicated that the amount of Pd present in group “a” is always larger than group “b”. The only difference between the group “a” and “b” is the stabilizing agents. These experiments were repeated several times and similar results were obtained. The electrode preparation method was given in a previous paper [27].
Table 1

The name of catalysts, surfactants, and atomic percentage of platinum and palladium which are added to the reaction medium (A) and obtained from ICP results (B)

Catalysts

Surfactants

(A)

(B)

Pt/C(a)

1-Hexanethiol

  

Catalyst Ia

1-Hexanethiol

90 %Pt/10 %Pd

87.4 %Pt/12.6 %Pd

Catalyst IIa

1-Hexanethiol

80 %Pt/20 %Pd

76.8 %Pt/23.2 %Pd

Catalyst IIIa

1-Hexanethiol

70 %Pt/30 %Pd

65.1 %Pt/34.9 %Pd

Pt/C(b)

1,1-Dimethyl hexanethiol

  

Catalyst Ib

1,1-Dimethyl hexanethiol

90 %Pt/10 %Pd

88.2 %Pt/11.8 %Pd

Catalyst IIb

1,1-Dimethyl hexanethiol

80 %Pt/20 %Pd

81.3 %Pt/18.7 %Pd

Catalyst IIIb

1,1-Dimethyl hexanethiol

70 %Pt/30 %Pd

72.1 %Pt/27.9 %Pd

Physical techniques

Transmission electron microscopy

A JEOL 200 kV TEM instrument was used to acquire TEM images. Sample preparation was performed through the suspension of ~0.5 mg catalyst in 3 ml of ethanol in an ultrasonic bath and this was followed by the placement of a drop of this solution on to a carbon covered 400-mesh copper grid. Finally, the solvent was evaporated at room temperature.

X-ray photoelectron spectroscopy

A Specs spectrometer was used for XPS analysis and Kα lines of Mg (1253.6 eV, 10 mA) were utilized as an X-ray source. All XPS peaks have been fitted using Gaussian function, and the C 1s line at 284.6 eV was considered as the reference line.

X-ray diffraction

A Rigaku diffractometer with Ultima + theta–theta high resolution goniometer, the X-ray generator (Cu Kα radiation, λ = 1.54056 Å) with an operation conditions at 40 kV and 40 mA were employed for the XRD analysis.

Electrochemistry

A microcomputer-controlled potentiostat/galvanostat Solartron 1285 was used for CV and CA measurements. The saturated calomel electrode (SCE), glassy carbon and prepared catalysts on glassy carbon were employed as reference, counter and working electrodes, respectively.

Inductively coupled plasma spectroscopy

The amount of platinum and palladium in each catalyst was investigated by a Leeman Lab inductively coupled plasma spectroscopy (ICP).

Results and discussion

TEM was used to observe individual and average particle size of metals and their distribution on a carbon support. About 300 metal nanoparticles were examined and the results are given in Table 2 and Fig. 1. The transmission electron micrographs indicates the presence of a uniform and narrow particle size distribution in the range of 2.8 and 3.6 nm, Fig. 2, and which is similar to syntheses carried out by other workers [28, 29]. There is a small difference in size between group “a” and “b” and agglomeration was not observed for any of the catalysts.
Table 2

The size of average metal particles in nm

Pt(a)

2.00 ± 0.40 [26]

Catalyst Ia

2.80 ± 0.52

Catalyst IIa

3.30 ± 0.47

Catalyst IIIa

3.50 ± 0.46

Pt(b)

3.00 ± 0.40 [26]

Catalyst Ib

3.00 ± 0.44

Catalyst IIb

3.30 ± 0.56

Catalyst IIIb

3.60 ± 0.51

Fig. 1

TEM histograms of catalysts Ia (a), IIa (b), IIIa (c), Ib (d), IIb (e), and IIIb (f)

Fig. 2

Transmission electron micrograph of catalyst Ib

Besides transmission electron micrographs, the EDS analyses were also performed on TEM to define the composition of the nanoparticles. For example, both point and region analyses indicated that the atomic ratio of Pt is ~88 % and Pd is ~12 % for catalyst Ib (Fig. 3), which are very close to the elemental analyses results. Similar results were obtained for catalyst Ia.
Fig. 3

a Transmission electron micrograph and b EDS results for the spectrum 1 region of catalyst Ib

Some regions of the other catalysts, however, revealed different percentages of Pt and Pd, in addition to the expected ones. For instance catalyst IIIb, most of the regions surveyed indicated ~70 %Pt and ~30 %Pd which is very close to the expected value. In addition to this, it is also possible to observe some regions which contain excess Pd (~85 %). The same phenomenon was observed for other catalysts. Two possibilities might be considered to explain the presence of excess Pd in some regions of the catalysts. The first possibility could be the variable composition of PtPd alloy formed (XRD data indicates alloy formation and will be discussed later). In the synthesis of the nanoparticles, stabilizing agents can form micelles and Pt and Pd complexes take their places in these micelles. These Pt4+ and Pd2+ species are then reduced to Pt0 and Pd0 as much as possible to prepare metal nanoparticles. It is difficult to predict the exact composition, in terms of Pt and Pd, in each micelle. However, the interaction between the different metal species and the stabilizing agents should be the same, consequently we do not favor this explanation of the variability. The second possibility is the formation of a PtPd alloy to a certain extent with the existence of unalloyed Pd. The EDS data were unable to prove this possibility, consequently XRD techniques were employed and these data will be presented later. In addition to these TEM measurements, high resolution TEM images were also recorded to determine the crystal structure of metal nanoparticles. It is known that the lattice fringes for Pt(111) and Pt(200) are 0.227 and 0.196 nm [30], and for Pd(111) and Pt(200) are 0.225 and 0.195 nm [31], respectively. The lattice fringes were found be 0.220 and 0.194 nm for catalysts Ib, and 0.216 and 0.192 nm for catalysts IIIa, Fig. 4, very close to what would be expected.
Fig. 4

High resolution transmission electron micrograph of catalyst IIIa

XRD was used to define the crystal structure and average crystallite size of the metal particles of all prepared catalysts. XRD data revealed that the platinum has crystallized in the face centered cubic (fcc) structure [30], and the Pt(220) [28] peak position was used to calculate the d spacing, lattice parameters, percentage of alloying and the crystallite size of the metal nanoparticles. The XRD pattern for Pt(220) plane is given in Fig. 5 and the peak positions of Pt(220), d values, lattice parameters, atomic fraction of Pd, percent palladium incorporated into PtPd alloy and unmixed Pd percent are summarized in Table 3. As can be seen from the data, the diffraction peaks for the catalysts containing palladium were slightly shifted to higher 2θ values compared to those of Pt(a) and (b) [26], which indicates the formation of PtPd alloy for all prepared catalysts. The d value for pure Pt(220) is 0.1387 nm [30] which is very closed to Pt(a) and (b). For Pd(220), it is 0.1375 nm [31], i.e., the Pd’s d value is smaller than the pure Pt one. We can use this fact to indicate the insertion of Pd into Pt crystal structure. For instance, as the Pd inserted into Pt, the d value decreases from 0.1388 nm (Pt(a)) to 0.1384 nm (catalyst Ia) to 0.1381 nm (catalyst IIa) to 0.1377 nm (catalyst IIIa) and the same trend is observed for group “b”.
Fig. 5

XRD diffraction pattern of Pt/C(a) (a), catalysts Ia (b), IIa (c), IIIa (d), Pt/C(b) (e), catalysts Ib (f), IIb (g), and IIIb (h)

Table 3

Peak positions, d, lattice parameters, atomic percentages of platinum and palladium, atomic fraction of Pd, percent palladium incorporated into PtPd alloy, and unmixed Pd %

Catalysts

a

b

c

d (Pt)

d (Pd)

e (xPd)

f

g

Pt/C(a)

67.40

0.1388

3.927 ± 0.014

     

Catalyst Ia

67.65

0.1384

3.914 ± 0.012

87.4

12.6

10.5

83.3

2.1

Catalyst IIa

67.80

0.1381

3.906 ± 0.009

76.8

23.2

16.9

72.8

6.3

Catalyst IIIa

68.00

0.1377

3.896 ± 0.007

65.1

34.9

25.0

71.6

9.9

Pt/C(b)

67.50

0.1387

3.925 ± 0.012

     

Catalyst Ib

67.70

0.1383

3.912 ± 0.010

88.2

11.8

10.5

89.0

1.3

Catalyst IIb

67.85

0.1381

3.906 ± 0.012

81.3

18.7

15.3

81.8

3.4

Catalyst IIIb

67.95

0.1378

3.900 ± 0.011

72.1

27.9

20.1

72.0

7.8

a The peak positions of (hkl) (220) in 2θ, bd (in nm) calculated from Pt (220) peak according to Bragg formula, c lattice parameters, a (in Ǻ) ± standard errors, d atomic percent of platinum and palladium obtained from ICP, e atomic fraction of Pd inserted into Pt calculated by XRD data, f % of Pd in the reaction mixture inserted into Pt, and g %Pd not inserted into Pt (or %Pd unalloyed)

The PtPd alloy formation can also be supported by lattice parameters which were calculated from Eq. 1 [8, 32] and the results are given in Table 3
$$ a = \lambda (h^{2} + k^{2} + l^{2} )^{1/2} /2{\text{Sin}}\theta \,({\text{for}}\;{\text{a}}\;{\text{cubic}}\;{\text{structure}}) $$
(1)
where a, λ, θ, and (hkl) are the lattice parameters, the wavelength of X-ray (1.54056 Ǻ), the peak position in θ and the planes of atoms, respectively. It is well known that Pt and Pd both crystallize in fcc structure and the lattice parameter of Pt (0.392 nm) is slightly larger than that of Pd (0.389 nm). To determine the extent of alloy formation, namely the amount of Pd incorporated into Pt fcc structure, Vegard’s law (Eq. 2) [33] was utilized and the results are again summarized in Table 3
$$ a_{{ ( {\text{Pt}} - {\text{Pd}})}} = a_{{ ( {\text{Pt)}}}} - kx_{{ ( {\text{Pd)}}}} $$
(2)
where a(Pt–Pd), a(Pt), k, and xPd are the lattice parameters of the PtPd alloy, and Pt, lattice constant (0.124 Å) and atomic fraction of Pd, respectively. Calculations revealed that most of the palladium was incorporated into the Pt fcc structure in catalysts Ia and Ib, ~83.3 and 89.0 %, respectively, only small percentage of Pd was left unalloyed; 2.1 % for catalyst Ia, and 1.3 % for catalysts Ib. This is not the same for all catalysts. As can be seen from Table 3, our calculations seem to suggest that ~72 % is the limit for Pd insertion into Pt, i.e., as Pd percentage present in the samples increases from 23 to 35 %, the amount of Pd insertion into Pt does not change much; being ~72.8 and 71.6 % for catalysts IIa and IIIa, respectively. Other researchers have prepared PtPd alloys using different experimental methods and found 10 and 50 % Pd insertion into Pt [34, 35]. Therefore, we believe that it is our preparation method which causes this and studies continue to try and fully understand why we have this ~72 % limit.
The average crystallite sizes of the metal particles were also calculated by Debye–Scherrer equation (Eq. 3) [36] using the half width maximum of the Pt(220) reflection peaks in XRD. They were found to be 3.00 ± 0.05, 3.20 ± 0.07, 3.50 ± 0.09, 3.20 ± 0.08, 3.40 ± 0.11, and 3.70 ± 0.14 nm for catalysts Ia, IIa, IIIa, Ib, IIb, and IIIb, respectively
$$ d({\text{\AA}}) = k\lambda /\beta \cos \theta $$
(3)
where k is the coefficient (0.9), λ is the wavelength of X-ray (1.54056 Ǻ), β is the full width half-maximum of respective diffraction peak (rad), and θ is the angle at the position of peak maximum (in rad). These results are in good agreement with our TEM measurements. The particle sizes are similar to those produced with polyol methods [29, 37], but our size distributions are narrower than those studies produced. The oxidation state and chemical composition of the metal nanoparticles were examined by XPS. A Gaussian–Lorentzian method was used for the fitting of Pt 4f and Pd 3d regions, and the peaks were analyzed in terms of relative peak area and chemical shifts of Pd and Pt peaks. The curve fitting analysis indicated that Pt 4f regions can be separated into two doublets with the spin orbit splitting of 3.33 eV. The first doublet of the platinum species (Pt 4f7/2 at ~71.0 and 74.2 eV) can be attributed to Pt(0) while the second doublet of the platinum specie (Pt 4f5/2 ~74.6 and 77.8 eV) can be assigned to the Pt(IV) on the surface, such as platinum oxides or hydroxides (Fig. 6). The Pd 3d region could also be separated into two components with spin orbit splitting of 5.33 eV. The first component with the binding energy of ~335.2 eV can be assigned as metallic Pd and the second component with the binding energy of ~337.9 eV can be attributed as Pd(II) species, such as palladium oxides or hydroxides. The peak positions and the relative integrated intensities of the peaks are given in Table 4. The observed binding energy Pt(0) is very closed to bulk platinum or platinised carbon electrode [38, 39]. A small shift towards lower binding energies, corresponding to an increase in the electronic charge density on Pt, occurs as the PtPd catalyst compositions change from Ia to IIIa and from Ib to IIIb. This may indicate that the Pd atoms behave like an electron donor in the PtPd alloy crystallite. While this explanation is favored, it is surprising that a opposite trend (shifting of the peak position to higher binding energies) was not observed for Pd 3d peak.
Fig. 6

Pt 4f electron spectra of catalyst Ia (a), IIa (b), IIIa (c), Ib (d), IIb (e), and IIIb (f)

Table 4

Pt 4f7/2 and Pd 3d3/2 core binding energies, eV, for carbon supported PtPd catalysts (the numbers in parentheses are the relative percent intensities of the species)

Catalysts

Pt(0)

Pt(IV)

Pt(0)/Pt(IV)

Pd(0)

Pd(II)

Pd(0)/Pd(II)

Pt/Pda

Pt/C(a)

71.2 (71)

74.4 (29)

2.45

    

Ia

71.1 (85.1)

74.5 (14.9)

5.67

335.3 (88.1)

338.1 (11.9)

7.40

8.6

IIa

71.0 (72.4)

74.2 (27.6)

2.62

335.1 (78.2)

338.1 (21.8)

3.59

4.1

IIIa

70.9 (67.2)

73.8 (32.8)

2.05

335.0 (68.4)

337.8 (31.6)

2.16

2.5

Pt/C(b)

71.0 (76)

74.4 (24)

3.16

    

Ib

71.0 (87.4)

74.2 (12.6)

6.94

335.2 (93.2)

338.1 (6.8)

13.7

8.9

IIb

70.8 (83.9)

74.0 (16.1)

5.25

335.1 (84.4)

337.7 (15.6)

5.41

4.3

IIIb

70.7 (70.7)

73.8 (29.3)

2.41

335.0 (73.6)

337.6 (26.4)

2.79

2.9

aIntegrated peak area ratios of Pt and Pd in XPS

Consequently, it is possible that the absolute numbers given here may incorporate some error, since absolute quantification in XPS involves a number of approximations, the relative amounts, however, should be unaffected. The main species is undoubtedly Pt(0) (between 85 and 67 %) and Pd(0) (between 93 and 74 %) for all catalysts, and the Pt(0)/Pt(IV) and Pd(0)/Pd(II) ratios are at a maximum for Catalysts Ia and Ib. The Pt(0)/Pt(IV) ratio decreases going from catalyst Ia (5.67) to IIa (2.62) to IIIa (2.05) and for catalyst Ib (6.94) to IIb (5.25) to IIIb (2.41) (Table 4).

The cyclic voltammograms of all catalysts in 0.1 M HClO4 electrolyte solution at room temperature showed typical hydrogen and oxygen adsorption/desorption regions and the cyclic voltammogram of catalyst Ib is given as an example in Fig. 7a. Although small shifts were observed in the positions of the peaks, the cyclic voltammograms of the all prepared catalysts are similar. Immense changes were observed after addition of methanol or ethanol to the electrolyte solution and the response was the classical alcohol oxidation. Fig. 7b indicates the best activity performance toward methanol and ethanol oxidation reactions as expected.
Fig. 7

a Cyclic voltammogram of catalyst Ib in 0.1 M HClO4 at room temperature with a scan rate of 50 mV/s. b Cyclic voltammogram of catalyst Ib in 0.1 M HClO4 + 0.5 M CH3OH at room temperature with a scan rate of 50 mV/s

To compare the performance of the catalysts only the anodic part of the voltammograms were considered and are shown in Fig. 8, 9, 10, and 11. These figures indicate that group “b” catalysts, which were stabilized by a branched surfactant, have ~1.2 times higher catalytic activity than that of group “a” catalyst, stabilized by a linear surfactant, for both methanol and ethanol oxidation reactions. Among the catalysts, both alcohol oxidation reactions commenced at the lowest potential for catalyst Ib, it is the most active catalyst with a catalytic activity of ~0.450 A/mg Pt at 0.57 V and ~0.350 A/mg Pt at 0.56 V for methanol and ethanol oxidation reactions, respectively. The catalytic activity of catalyst Ib was found to be ~1.8 times greater than the catalytic activity of Pt/C(b) [26]; with catalytic activities of ~0.260 A/mg Pt at 0.65 V and ~0.190 A/mg Pt at 0.73 V for methanol and ethanol oxidation reactions, respectively, and the results are summarized in Table 5. Furthermore, to examine the enhancement in the catalytic performance, catalyst Ib was also compared with the commercial E-TEK catalyst (40 % Pt/Vulcan XC-72, with catalytic activities of ~90 A/g Pt and 25 A/g Pt for methanol and ethanol oxidation, respectively) and it was found to be ~5.0 and 14.0 times greater enhancement towards methanol and ethanol oxidation, respectively.
Fig. 8

Anodic part of the cyclic voltammogram of catalysts Ia, IIa, IIIa, and Pt/C in 0.1 M HClO4 + 0.5 M CH3OH at room temperature. Scan rate is 50 mV/s

Fig. 9

Anodic part of the cyclic voltammogram of catalysts Ia, IIa, IIIa, and Pt/C in 0.1 M HClO4 + 0.5 M CH3CH2OH at room temperature. Scan rate is 50 mV/s

Fig. 10

Anodic part of the cyclic voltammogram of catalysts Ib, IIb, IIIb, and Pt/C in 0.1 M HClO4 + 0.5 M CH3OH at room temperature. Scan rate is 50 mV/s

Fig. 11

Anodic part of the cyclic voltammogram of catalysts Ib, IIb, IIIb, and Pt/C in 0.1 M HClO4 + 0.5 M CH3CH2OH at room temperature. Scan rate is 50 mV/s

Table 5

The If/Ib ratio, onset and anodic peak potentials for all catalysts in MeOH (a) and EtOH (b) electrolyte solution

 

(a)

(b)

Onset (V)

Max current (A/mg Pt)

Anodic peak (V)

If/Ib

Onset (V)

Max current (A/mg Pt)

Anodic peak (V)

If/Ib

Catalyst Ia

0.26

0.40

0.55

7.54

0.32

0.34

0.56

3.39

Catalyst IIa

0.26

0.28

0.56

4.97

0.34

0.22

0.57

2.25

Catalyst IIIa

0.30

0.19

0.59

3.18

0.48

0.13

0.66

2.87

Pt/C(a)

0.30

0.16

0.62

2.30

0.50

0.10

0.69

2.19

Catalyst Ib

0.19

0.45

0.57

18.8

0.33

0.35

0.56

3.67

Catalyst IIb

0.21

0.33

0.57

5.63

0.34

0.28

0.58

2.32

Catalyst IIIb

0.22

0.27

0.58

4.09

0.42

0.21

0.55

3.03

Pt/C(b)

0.28

0.26

0.65

3.80

0.50

0.19

0.73

1.06

It is well known that If/Ib ratio, where If and Ib are the currents in the forward and reverse scan, respectively, can be used to define the performance of the catalysts. The higher the If/Ib ratio, the better the alcohol oxidation to the final product would be owing to the low accumulation of intermediate species on the surface of the catalysts for methanol and ethanol oxidation reactions. The If/Ib ratios are summarized in Table 5 and found that all PtPd catalysts have higher If/Ib ratios than Pt/C(a and b) and catalyst Ib has the highest ratio which indicates the best activity performance toward methanol and ethanol oxidation reactions as expected.

CA was used to examine the long-term stability of the catalysts and group “b” catalyst results for methanol oxidation reaction are given as an example in Fig. 12. A double-layer charge and a great number of active points on the catalysts can be held responsible for observing high initial current for methanol oxidation reaction.
Fig. 12

Chronoamperometric curves of methanol oxidation on catalyst Ib (a), IIb (b), IIIb (c), and Pt/C(b) (d) at 0.5 V in 0.1 M HClO4 + 0.5 M CH3OH

The most active catalyst, Ib, was found to have a current that is ~4.4 times greater than the catalyst, Pt/C(b), after a decay of 3,600 s. The results are consistent with those obtained from CV study. In addition to these outcomes, it is possible to state that the group “b” catalysts not only have higher stability but also higher poisoning tolerance when compared with group “a” catalysts for both methanol and ethanol oxidation reactions.

In summary, electrochemical studies indicated that catalysts Ia and Ib are the most active catalysts within each group. TEM and XRD data revealed that they have similar metal particle sizes and possess the highest percent alloy formation with respect to the amount of Pd that exists in the sample, between 87.3 and 89.0 %. The insertion of Pd into the Pt fcc structure changes between 72 and 82 % for catalysts IIa, IIb, IIIa, and IIIb and the rest of Pd remains unmixed, Table 3. As it is known that methanol and ethanol oxidation take place on platinum not on palladium, therefore it is clear we should observe a decrease in the performance of these catalysts compared to catalysts Ia and Ib. Besides TEM and XRD data, XPS and electrochemical results were also examined to find the reason for the high activity performance of these two catalysts. According to these data, catalysts Ia and Ib have an extraordinary high Pt(0)/Pt(IV) (6.94, 5.67), Pd(0)/Pd(II) (13.7, 7.4), Pt/Pd (8.9, 8.6), and If/Ib (18.8, 7.54 for methanol and 3.67, 3.39 for ethanol) ratios. It is also known that although alcohol oxidation reactions occur on platinum, second metal or metal oxide or hydroxide aid this reaction. Consequently, it is obvious that these two catalysts have very appropriate reaction media to form alcohol oxidation reaction which causes to have high performance of these catalysts.

In general, the enhancement in the performance of the PtPd catalysts compared to platinum only containing catalysts towards alcohol oxidation reaction can be attributed to the higher CO tolerance of these catalysts, which can be explained by the bifunctional mechanism suggesting the oxidation of adsorbed CO species by OH species. Even though bulk Pt and Pd have similar characteristics such as the coverage by OH species in the same potential range, oxidation of Pd takes place slightly easier than that of Pt [37]. As mentioned earlier, palladium was totally inactive for methanol and ethanol electrooxidation reactions in an acidic medium. However, the presence of palladium has a synergistic effect which causes formation of CO2 with a high rate at lower potential by decreasing the electrode poisoning, Pt–CO, on the surface of catalysts as given below
$$ \begin{aligned} & {\text{Pt}} + {\text{CO}} \to {\text{Pt}}-{\text{CO}}_{\text{ad}} \\ & {\text{Pd}} + {\text{H}}_{ 2} {\text{O}} \to {\text{Pd}}-{\text{OH}}_{\text{ad}} + {\text{H}}^{ + } + e^{-} \\ & {\text{Pt}}-{\text{CO}}_{\text{ad}} + {\text{Pd}}-{\text{OH}}_{\text{ad}} \to {\text{Pt}} - {\text{Pd}} + {\text{CO}}_{2} + {\text{H}}^{ + } + e^{-} . \\ \end{aligned} $$

Conclusions

In this article, two groups of carbon supported PtPd nanoparticle catalysts were synthesized by superhydride reduction method using 1-hexanethiol (group “a” catalysts) and 1,1-dimethyl hexanethiol (group “b” catalysts) as surfactants for the first time for this goal. XRD, XPS, TEM, CV, and CA were employed for physical and electrochemical characterization of the catalysts. TEM and XRD results revealed the uniform distribution of the PtPd alloy nanoparticles on carbon support with a narrow size ranging between 2.8 and 3.6 nm. Electrochemical studies indicated that group “b” catalysts have higher catalytic activities than that of group “a”, and that catalyst Ib is the most active one. Its performance is 5.0 and 14.0 times higher than that of commercially available Pt catalyst (E-TEK) for methanol and ethanol oxidation reactions, respectively. The reason for having a good performance could be due to the higher extent of alloy formation, CO tolerance, Pt/Pd, Pt(0)/Pt(IV), and Pd(0)/Pd(II) ratios.

Acknowledgements

The authors gratefully acknowledge Türkiye Bilimsel ve Teknik Araştırma Kurumu (TUBITAK), Grant 108T065 for the financial support. F. Şen and S. Şen thank the Middle East Technical University (METU) for Grant BAP-08-11-DPT2002K120510 and TUBITAK 2211 scholarships. The authors also thank Dr. W. Michael Pitcher for the proofreading of this manuscript.

Copyright information

© Springer Science+Business Media, LLC 2012

Authors and Affiliations

  1. 1.Department of ChemistryMiddle East Technical UniversityAnkaraTurkey
  2. 2.Department of ChemistryYuzuncu Yil UniversityVanTurkey

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