Electro-catalysis of carbon black or titanium sub-oxide supported Pd-Gd towards formic acid electro-oxidation

Article abstract of DOI:10.1039/c6ra13097f

Carbon black supported Pd-Gd catalysts (Pd-xGd/C, x is weight percent in catalyst) with different amounts of Gd were prepared by a simultaneous reduction reaction with sodium borohydride in aqueous solution. The structure, morphology and element valence s

Full text of DOI:10.1039/c6ra13097f

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Electro-catalysis of carbon black or titanium sub-oxide supported
Pd-Gd towards formic acid electro-oxidation
a,b
b
b
a
a,*
a
Nan He , Chuanguang Qin , Rumin Wang , Shuhui Ma , Yi Wang , Tao Qi
a
National Engineering Laboratory for Hydrometallurgical Cleaner Production
Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing
100190, China
b
School of Natural and Applied Sciences, Northwestern Polytechnical University,
Xi’an 710072, China
*
Corresponding author: Y. Wang, Fax: (+86) 10 82544848-802, Tel: (+86) 10
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82544967, E-mail: wangyi@ipe.ac.cn
Abstract
The carbon black supported PdꢀGd catalysts (PdꢀxGd/C, x is weight percent in
catalyst) with different amounts of Gd were prepared by a simultaneous reduction
reaction with sodium borohydride in aqueous solution. The structure, morphology and
element valence state of these catalysts were characterized by Xꢀray diffraction
(XRD), transmission electron microscopy (TEM) and Xꢀray photoelectron
spectroscopy (XPS), respectively. The electroꢀcatalytic performance of these catalysts
for formic acid oxidation was investigated using cyclic voltammetry (CV),
chronoamperometry (CA) and CO stripping experiments. It is found that the
Pdꢀ2.5Gd/C catalyst has a better electroꢀcatalytic activity than the Pd/C catalyst,
which can be explained by a biꢀfunctional mechanism. In addition, the higher content
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of metallic Pd caused by the addition of Gd also contributes to the better catalytic
activity of Pdꢀ2.5Gd/C. Based on the good electroꢀcatalytic performance of the
Pdꢀ2.5Gd/C, the PdꢀxGd/Ti
4 7
O (x stands for weight percentage in whole catalyst)
catalysts were also prepared and characterized in the same way. The Pdꢀ2.5Gd/Ti O7
4
and Pdꢀ4Gd/Ti
catalyst, which mainly results from the further increase of metallic Pd caused by
Ti
4 7
O catalysts exhibited better catalytic activities than the Pdꢀ2.5Gd/C
4
7
O .
4 7
Keywords: PdꢀGd/C; Ti O ; Electroꢀcatalysts; Formic acid electroꢀoxidation; fuel cell
Introduction
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As human attaches much importance to energy and environment, direct formic
acid fuel cells (DFAFCs) have gained increasing attention as a promising clean energy
conversion device, due to their merits, such as relatively low fuel toxicity, high energy
[1ꢀ3]
density and low penetration efficiency through Nafion membrane . It is well known
that Pt based catalysts are effective to the electroꢀoxidation of formic acid. However,
with Pt as anode catalyst, the surface is usually heavily poisoned by the strong
adsorption of CO intermediates produced during the oxidation of formic acid, leading
[4]
to the lowering of catalytic performance . Moreover, the high cost of Pt is also an
obstacle to its use for commercial applications. As compared with Pt, Pd based
[
5,6]
catalysts not only possess good electrochemical activity , but also have much better
tolerance to CO poisoning and relatively cheaper price.
Although palladium has many advantages, the electroꢀoxidation of formic acid on
Pd catalysts in general requires a substantially high overpotential (~0.3 V), which
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significantly impedes the largeꢀscale commercialization of DFAFCs . Additionally,
the stability of Pd in acidic environment still needs to be further enhanced to meet
practical application. A great many efforts have been made to improve the stability
and catalytic activity of palladium by means of adding another metal into Pd based
[
8]
[9]
[10]
[11]
[12]
[13,14]
[15]
[16]
[17]
[18]
catalyst, such as Co , Fe , Ni , Bi , Mn ,Cu
, Cr , Au , Pb , Pt
and so on. The main reason for enhancement in Pd based catalyst can be attributed to
[
19,20]
a biꢀfunctional mechanism
. In recent years, researches on Pt and Pd catalysts
doped with rareꢀearth metals demonstrated that the rareꢀearth metal doping could also
enhance the electroꢀcatalytic activity of these Pt or Pd catalyst, which may not be only
[
21,22]
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due to the biꢀfunctional mechanism
. In our previous work, it was found that
[
23]
terbium can promote the electroꢀoxidation of formic acid on Pt based catalyst. XPS
analyses showed that the addition of Tb results in the increase of metallic Pt content,
which is the other reason of the promotive effect besides the biꢀfunctional mechanism.
This research inspires our interest in the promotive effect of other rare earth elements.
Gadolinium is adjacent to terbium in the periodic table of elements. Moreover,
gadolinium has a more abundant terrestrial reserve than terbium. Thus, in this study,
Pd/C electroꢀcatalysts containing different amounts of Gd were prepared, and their
activities towards the electroꢀoxidation of formic acid were evaluated.
It is well known that catalyst supports have a great impact on catalytic
performance. Although carbon black is a common support material for Pt or Pd based
[
24]
catalysts due to its good electron conductivity and high surface area ,the corrosion
of the carbon support would cause an adverse impact on working efficiency and the
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performance of fuel cells .Therefore, the nonꢀcarbon materials have attracted much
[
26.27]
[28]
greater attention
. In our previous work , it is found that Pt catalyst supported on
titanium subꢀoxide (Ti
4
O
7
) possesses better catalytic performance than Pt catalyst
supported on carbon black for formic acid electroꢀoxidation. The performance
improvement was mainly attributed to the higher content of metallic Pt caused by
Ti
4
O
7
, as well as the high electrical conductivity of the Ti
4
O
7
support material. To
was also
further enhance the performance of the PdꢀGd bimetallic catalysts, Ti
4 7
O
used as a support material in this study, and the activities of these catalysts supported
on Ti O towards the electroꢀoxidation of formic acid were investigated.
4
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Experimental
The chemicals used in this work were carbon black (Vulcan XCꢀ72), PdCl
·2H O, NaBH , H SO , HCOOH, Gd(NO , Nafion solution (5 wt% in
was purchased from ShanDong
2
,
Na
3
C
6
H
5
O
7
2
4
2
4
3 3
)
isopropanol and water) and deꢀionized water. Ti
Lianmeng Chemical Group Co. LTD.
4 7
O
The catalyst ink was prepared by ultrasonically mixing 4 mg electroꢀcatalyst
sample in 2 mL of ethanol for about 20 min. The electroꢀcatalyst was made as follows.
The carbon black or Ti
4
O
7
was dispersed into 75 mL water under ultrasonication.
were added into the above
Then 0.01 M PdCl aqueous solution and Gd(NO )
2
3 3
suspension under magnetic stirring. Subsequently, sodium citrate, whose amount of
substance is 1.5 times of that of Pd and Gd, was added into the above suspension.
4
Afterwards, NaBH aqueous solution (freshly prepared) was added dropwise. About 6
hours later, the suspension was filtered, washed and dried under 60 ℃ overnight. Then
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the catalysts were obtained and they were denoted as Pdꢀ1Gd/C, Pdꢀ2Gd/C,
Pdꢀ2.5Gd/C, Pdꢀ10Gd/C, Pdꢀ2.5Gd/Ti Pdꢀ4Gd/Ti and Pdꢀ5Gd/Ti
respectively. The theoretical content of Pd is 20 wt%.
Structure and morphology of the catalysts were investigated using Xꢀray
diffraction (XRD, X’PertꢀPRO MPD, Cu K ) and transmission electron microscopy
4
O
7
,
4
O
7
4 7
O ,
α
(TEM, JEMꢀ2100, 200KV). Element valences state analyses were performed by Xꢀray
photoelectron spectroscopy (XPS, ESCALAB 250Xi (Thermo Fisher Scientific Inc.)).
The electrochemical measurements were conducted on an electrochemical
workstation system (CHI760D, Chenhua, Shanghai) with a threeꢀelectrode cell using
Pt foil and saturated calomel electrode (SCE) as the counter and reference electrodes,
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2
2
respectively. Cyclic voltammetry (CV) was performed in a 0.5 M H
2 4
SO + 0.5 M
HCOOH solution, where oxygen was removed by purging N for 15 min. The
2
working electrode was prepared by dropping 5 uL of the electroꢀcatalyst ink onto
glassy carbon electrode (GCE). Then 1 uL of Nafion solution was dropped on top to
fix the electroꢀcatalysts. All potentials in this work were given versus SCE electrode
(+0.241V vs. NHE). The CV experiments were conducted at a sweep rate of 50 mV
ꢀ1
s , with the potential ranging from ꢀ0.2 V to 1.0 V. CO striping was performed as
follows. After purging the solution with N 20 min, gaseous CO was bubbled for 15
min to form CO adlayer on catalysts while maintaining potential at 0.1 V. Excess CO
in solution was purged with N for 20 min, and CO stripping voltammetry was
recorded in 0.5 M H SO at 20 mVs . To investigate the stabilities of electrodes, the
2
2
ꢀ1
2
4
chronoamperometry (CA) experiments were conducted at 0.2 V for 4000 s with the
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same potential range. All the measurements were performed three times in this study.
Results and discussion
Fig. 1 displays the XRD patterns of the asꢀprepared PdꢀGd/C catalyst as well as
the Pd/C catalyst for comparison Pd/C. All of them were supported on the carbon
black. Thus, the peak appeared at about 25° in all the XRD patterns is recognized as
the (002) plane of support material Vulcan XCꢀ72 carbon support. The other four
peaks around 40°, 46°, 67° and 81° represent the diffraction of Pd crystal faces (111),
(200), (220) and (311), respectively, which shows the characteristics of faceꢀcentered
cubic (fcc) structure of palladium, but no Gd peaks can be found. However, when
compared with the Peak of Pd for Pd/C, the addition of Gd causes slight shifts of the
Pd peaks with 0.2°, 0.3°, 0.3° and 0.4° for Pdꢀ1Gd/C, Pdꢀ2Gd/C, Pdꢀ2.5Gd/C and
Pdꢀ10Gd/C, respectively, which indicates that the Gd had been alloyed with the Pd. In
other words, a solid solution of Pd and Gd formed.
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Fig. 2 displays the TEM images and particle size distribution of the Pd /C and
Pdꢀ2.5Gd/C catalysts. It can be observed that the catalyst particles in Pd/C are well
dispersed on carbon supports. After the addition of Gd, the PdꢀGd particles have a
little aggregation. The average sizes of particles in Pd/C and Pdꢀ2.5Gd/C could be
estimated to be about 5.1 nm and 3.45 nm from from Fig. 2(c) and (d), respectively.
Fig. 3(a) presents the CVs of Pd/C, Pdꢀ1Gd/C, Pdꢀ2Gd/C, Pdꢀ2.5Gd/C and
2 4
Pdꢀ10Gd/C catalysts in 0.5 M H SO solution. The peaks ranging from ꢀ0.2 to 0.1 is
in accordance with the absorption/desorption of hydrogen, while those above 0.2 V
are ascribed to the oxidation and reduction of surface metal. The electrochemical
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active surface areas (ECSAs) can be calculated using oxide reduction voltammetry by
following equation:
S
ECSAs =
v × Q × m
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Where S is the Pd oxide reduction peak area in voltammtry; v is the scan rate; Q is the
charge needed for the reduction of single layer of oxide on Pd, which is 0.000405
2
[29]
2
ꢀ1
C/cm and m is the loading amount of catalyst . The values are 22.1 m g for Pd/C
2
ꢀ1
and 47.2 m g for Pdꢀ2.5Gd/C, respectively. It is apparently seen that the ECSAs of
PdꢀGd/C catalysts mainly increase with the addition of Gd. Fig. 3(b) shows the CVs
of formic acid oxidation on the prepared catalysts, and the results were normalized by
the ECSA. It can be found that the current density on the Pdꢀ2.5Gd/C catalyst is much
higher than that on the Pd/C catalyst, indicating that formic acid electroꢀoxidation is
more active on the former than on the latter. This suggests that the addition of an
appropriate amount of Gd has a significantly promotive effect on the electroꢀcatalytic
activity of Pd/C catalyst for formic acid oxidation. Therefore, in the remaining session,
attention is focused on the bestꢀperforming catalyst, namely, Pdꢀ2.5Gd/C.
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It is well known that the most commonly accepted mechanism of formic acid
[30]
oxidation is the soꢀcalled parallel or dual pathway mechanism . Direct oxidation on
Pd catalysts occurs via a dehydrogenation reaction without forming CO as a reaction
intermediate:
+
ꢀ
Pathway 1
The second reaction pathway forms adsorbed CO as a reaction intermediate by
dehydration:
Pd + HCOOH → Pd + CO
2
+ 2H + 2e
(1)
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Pathway 2
Pd + HCOOH → PdꢀCO + H
2
O
(2)
(3)
(4)
+
ꢀ
Pd + H O → PdꢀOH + H + e
2
+
ꢀ
2
PdꢀCO + PdꢀOH → Pd + CO + H + e
Pathway 2 is similar to the wellꢀknown biꢀfunctional catalysis of methanol
[19]
electroꢀoxidation on Ptꢀalloy catalysts , which involves the successive oxidation of
the functional group (ꢀOH group) with adsorbed CO on Pd surface. Although Pathway
1 is dominant for formic acid oxidation on Pd, a partial formic acid still oxidizes on
Pd via Pathway 2. Thus, a high resistance to CO poisoning is also important for Pd
based catalysts towards formic acid electroꢀoxidation. To evaluate the resistance to the
CO poisoning of the Pd/C and PdꢀGd/C catalysts, CO stripping were recorded, as
shown in Fig. 4. Although Pdꢀ2.5Gd/C and Pd/C have similar peak potentials, the
onset potential of CO oxidation on Pdꢀ2.5Gd/C at 0.45 V is slight more negative than
that on Pd/C at 0.55 V, indicating that the addition of Gd contributes to the removal of
CO poisoning intermediate out of the surface of the Pdꢀ2.5Gd/C catalyst. In other
1
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2
2
2
[19,20]
words, according to the biꢀfunctional mechanism
, the Gd activated water at lower
potentials than Pd and the activated water could oxidize the adsorbed CO and
therefore liberated Pd active sites. This result helps to explain the higher activity of
the Pdꢀ2.5Gd/C catalyst for the oxidation of formic acid.
To further investigate the function of Gd, Pd 3d core level spectra of the Pd/C and
Pdꢀ2.5Gd/C catalysts were recorded in Fig. 5. Both of the profiles were fitted by two
pairs of overlapping Lorentzian curves. For Pd/C, the more intensive peaks (336.3 and
341.6 eV) are attributed to metallic palladium, Pd(0). The other pair of peaks (337.5
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and 342.8 eV) are ascribed to the Pd(℃ ) chemical state on Pd oxides or hydroxides.
For Pdꢀ2.5Gd/C, they are similar to those of Pd/C. However, the relative intensity of
metallic Pd(0) in Pdꢀ2.5Gd/C with 53.5% is higher than that in Pd/C with 49.4%.
Table 1 lists the relative intensity of Pd(0) and Pd(II) of these two catalysts evaluated
from XPS analyses. This suggests that the addition of Gd can enhance the content of
Pd(0) while reduces the content of Pd(II). That is to say that Gd has a “metallization”
effect on Pd. This may ascribe from the difference of valence electron configuration
7
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between Gd and Pd. The valence electron configuration of Gd is 4f 5d 6s , electrons
outside which are easy to lose. On the contrary, the Pd has a valence electron
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configuration of 4d , which contains unoccupied orbits. Therefore, some electrons of
Gd could have a trend to transfer to Pd, resulting in an increase of metallic palladium.
As is well known, the higher the content of metallic state in catalysts, the better the
[31,32]
catalytic performance
. Hence, the higher content of Pd(0) caused by the addition
of Gd also contributes to the better catalytic activity of the Pdꢀ2.5Gd/C catalyst.
Additionally, the durability of different catalysts towards formic acid
electroꢀoxidation was examined by CA experiments. Fig.6 shows the CA plots of
Pdꢀ2.5Gd/C and Pd/C at 0.2 V vs. SCE in 0.5M HCOOH + 0.5 M H
2 4
SO solution.
The currents on both catalysts drop rapidly at first, and then become relatively stable.
[
33]
The initial surge of current is caused by a charging current . The current for formic
acid oxidation on the Pdꢀ2.5Gd/C catalyst is significantly larger than that on the Pd/C
catalyst over the whole time range. This indicates that the stability of Pdꢀ2.5Gd/C is
fair besides better catalytic activity.
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Based on the good electroꢀcatalytic performance of the Pdꢀ2.5Gd/C catalyst,
PdꢀGd catalysts supported on Ti were prepared and their catalytic performance for
4
O
7
formic acid electroꢀoxidation was investigated.
Fig. 7 shows the XRD patterns of PdꢀxGd/Ti O (x: 2.5, 4 and 5 at percentage)
4
7
catalysts. All of them have a similar pattern due to approximate content of Gd. The
peaks around 40°, 46°, 67° and 82° represent the (111), (200), (220) and (311) planes
of Pd, respectively. Other peaks except Pd peaks were well matched with the
4 7
characteristic peaks of Ti O (PDF (The Powder diffraction File): 50ꢀ0787). Although
no peak of Gd can be seen, it is still observed that the diffraction peaks at ca. 40° shift
a little to lower 2θ value with the increase of Gd content, which indicates that the
alloying of Gd and Pd had occurred.
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Fig. 8 presents the TEM images of the Pdꢀ2.5Gd/Ti
4 7 4 7
O and Pdꢀ4Gd/Ti O
catalysts. It can be seen that PdꢀGd nanoparticles gather together in these two
[28]
catalysts, which is similar to the dispersion state of Pt nanoparticles on Ti O7 . This
4
4 7
results from the hydrophobicity and higher mass density of Ti O as compared with
[28]
carbon black according to our previous analyses
The CVs of these PdꢀGd catalysts supported on Ti
SO solution are shown in Fig. 9. For comparison, the CVs of Pd/C and
Pdꢀ2.5Gd/C are also shown in this figure. It can be seen that with the same amount of
Gd, the peak current density on Pdꢀ2.5Gd/Ti is higher than that on Pdꢀ2.5Gd/C.
Furthermore, with the increase of Gd content, the peak current density firstly
increases and then decreases. The Pdꢀ4Gd/Ti catalyst exhibits best catalytic
.
4
7
O in 0.5 M HCOOH + 0.5 M
H
2
4
4
O
7
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activity.
To investigate the effect of the supports on electronic properties of catalysts, Pd
3d core level spectra of the Pdꢀ2.5Gd/Ti and Pdꢀ4Gd/Ti catalysts were recorded
4
O
7
4 7
O
in Fig. 10. An analysis similar to Fig. 5 was carried out, and the results were
summarized in Table 2. It can also be found that the relative intensity of metallic Pd(0)
in Pdꢀ2.5Gd/Ti
4 7 4 7
O and Pdꢀ4Gd/Ti O is 56.4 and 60.3%, respectively, higher than
53.5% obtained for Pdꢀ2.5Gd/C. In addition, the Gd 3d core level spectra of
3+
Pdꢀ4Gd/Ti
4 7
O were also recorded in Fig. 10(c), which suggested the existence of Gd
and thus prove its trend to lose electrons. The higher metallic content of Pd on Ti O7
4
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compared to that on carbon black suggests that the Ti
4
O
7
support also has a
catalyst
“metallization” effect for the PdꢀGd catalyst, which is similar to the Pt/Ti
4
O
7
[
26]
in our previous work . Specifically, the effect may be due to the electrostatic
interaction between metal particles and supports. Because the valence electron
2
2
configuration of Ti is 3d 4s , Ti O is an oxygen deficit species, namely betatopic
4
7
4 7
configuration. Therefore, some electrons of Ti O may have a tendency to transfer to
Pd, resulting in an increase of metallic palladium content. As mentioned before, a
higher content of metallic palladium in catalysts leads to a better catalytic activity.
This is the main reason for the better performance of the Pdꢀ2.5Gd/Ti
Pdꢀ4Gd/Ti O catalysts.
4 7
O and
4
7
Conclusions
The carbon supported PdꢀGd alloy catalysts were prepared, and the catalytic
performance towards formic acid electroꢀoxidation was investigated. As compared
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with the Pd/C catalyst, the Pdꢀ2.5Gd/C catalyst has a better electroꢀcatalytic activity
and higher resistance to CO poisoning, which can be explained by a biꢀfunctional
mechanism. The analyses for XPS spectra indicate that the higher content of metallic
Pd caused by the addition of Gd also contributes to the better catalytic activity of
Pdꢀ2.5Gd/C. Based on the good electroꢀcatalytic performance of the Pdꢀ2.5Gd/C, the
Ti
4
O
7
ꢀsupported PdꢀGd catalysts with different amounts of Gd were synthesized, and
and Pdꢀ4Gd/Ti catalysts displayed better electroꢀcatalytic
the Pdꢀ2.5Gd/Ti
4
O
7
4 7
O
activities. The improvement is mainly attributed to the further increase of metallic Pd
caused by the Ti O support.
4
7
1
1
1
1
1
1
1
1
1
1
2
2
2
Acknowledgments
The authors are grateful for the financial support by One Hundred Talent
Program of Chinese Academy of Sciences, as well as by the 973 Program (Grant No.
2015CB251303).
References
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1
Table 1 Binding energy (B.E.) and relative intensity of species from curveꢀfitted XPS
spectra of Pd 3d.
Samples
B.E. of Pd
B.E. of Pd
3d3/2(eV)
341.6
Species
Relative intensity
3
d
5/2(eV)
(%)
49.4
50.6
53.5
46.5
Pd/C
336.3
37.5
335.6
36.7
Pd(0)
Pd(℃ )
Pd(0)
Pd(℃ )
3
342.8
Pdꢀ2.5Gd/C
340.8
3
342.1
1
1
1
1
3
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5
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7
8
9
0
1
2
1
1
1
Table 2 Binding energy (B.E.) and relative intensity of species from curveꢀfitted XPS
spectra of Pd 3d.
Samples
B.E. of Pd
B.E. of Pd
3d3/2(eV)
340.7
Species
Relative intensity
3d
5/2(eV)
(%)
56.4
43.6
60.3
39.7
Pdꢀ2.5Gd/Ti O
335.3
36.5
335.4
36.5
Pd(0)
Pd(℃ )
Pd(0)
Pd(℃ )
4
7
3
342.3
Pdꢀ4Gd/Ti
4
O
7
340.6
3
342.6
1
1
1
3
4
5
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1
Figure captions
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Fig. 1 XRD patterns of (a) Pd/C, (b) Pdꢀ1Gd/C, (c) Pdꢀ2Gd/C, (d) Pdꢀ2.5Gd/C and (e)
Pdꢀ10Gd/C catalysts.
Fig. 2 TEM images and particle size distribution of the Pd/C (a, c) and Pdꢀ2.5Gd/C(b,
d) catalysts.
Fig. 3 CVs of Pd/C, Pdꢀ1Gd/C, Pdꢀ2Gd/C, Pdꢀ2.5Gd/C and Pdꢀ10Gd/C catalysts in (a)
2 4 2 4
0.5 M H SO and (b) 0.5 M HCOOH + 0.5 M H SO solutions at a scan rate of 50
mV/s.
Fig. 4 CO stripping curves on the Pd/C (a) and Pdꢀ2.5Gd/C (b) catalysts recorded in
0.5 M H SO solution.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
4
Fig. 5 XPS spectra of Pd (3d) for (a) Pd/C and (b) Pdꢀ2.5Gd/C catalysts.
Fig. 6 CA curves of the Pd/C and Pdꢀ2.5Gd/C catalysts in 0.5 M HCOOH + 0.5 M
2 4
H SO solution at 0.2 V.
Fig. 7 XRD patterns of PdꢀxGd/Ti O (x: 2.5, 4 and 5 at percentage).
4
7
Fig. 8 TEM images of Pdꢀ2.5Gd/Ti
4
7 4 7
O (a) and Pdꢀ4Gd/Ti O (b).
Fig. 9 CVs of formic acid electroꢀoxidation on the Pd/C, Pdꢀ2.5Gd/C, Pdꢀ2.5Gd/Ti
Pdꢀ4Gd/Ti and Pdꢀ5Gd/Ti catalysts in 0.5 M HCOOH + 0.5 M H SO solution
at a scan rate of 50 mV/s.
Fig. 10 XPS spectra of Pd (3d) for Pdꢀ2.5Gd/Ti O (a) and Pd (3d) and Gd (3d) for
4 7
O ,
4
O
7
4
O
7
2
4
4
7
4 7
Pdꢀ4Gd/Ti O (b, c).
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1
2
3
a
b
c
d
e
20
30
40
50
60
70
80
90
2
ꢀtheta/degree
4
5
6
7
8
9
0
1
2
3
4
5
Fig. 1
1
1
1
1
1
1
2
0

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1
2
3
4
5
a
b
6
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
mean diametre: 3.45nm
d
c
mean diametre: 5.1nm
0.20
0.15
0.10
0.05
0.00
2
ꢀ2.7 2.7ꢀ3.4 3.4ꢀ4.1 4.1ꢀ4.8 4.8ꢀ5.5 5.5ꢀ6.2 6.2ꢀ6.9 6.9ꢀ7.6 7.6ꢀ8.3
1ꢀ1.6 1.6ꢀ2.2 2.2ꢀ2.8 2.8ꢀ3.4 3.4ꢀ4 4ꢀ4.6 4.6ꢀ5.2 5.2ꢀ5.8 5.8ꢀ6.4
diametre(nm)
diametre(nm)
7
8
9
0
1
2
3
4
5
Fig. 2
1
1
1
1
1
1
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0.00008
0.00006
0.00004
0.00002
0.00000
a
ꢀ0.00002
ꢀ0.00004
ꢀ0.00006
ꢀ0.00008
ꢀ0.00010
ꢀ0.00012
Pd/C
Pdꢀ1Gd/C
Pdꢀ2Gd/C
Pdꢀ2.5Gd/C
Pdꢀ10Gd/C
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential/V
1
2
0.0020
0.0018
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
b
Pd/C
Pdꢀ1Gd/C
Pdꢀ2Gd/C
Pdꢀ2.5Gd/C
Pdꢀ10Gd/C
ꢀ0.0002
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential/V
3
4
5
6
7
8
9
Fig. 3
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2

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1
2
st
0.20
0.15
0.10
0.05
0.00
1
Scan
nd
a
2
Scan
ꢀ0.05
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential/V
3
st
1
0
.25
.20
.15
.10
.05
.00
Scan
b
nd
2
Scan
0
0
0
0
0
ꢀ0.05
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential/V
4
5
6
7
8
Fig. 4
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3

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1
2
a
0
Pd
2+
Pd
350 348 346 344 342 340 338 336 334 332 330
B.E.(eV)
3
b
0
Pd
2+
Pd
350 348 346 344 342 340 338 336 334 332 330
B.E.(eV)
4
5
6
7
8
Fig. 5
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4

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1
2
3
4
5
6
0.0020
0.0018
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
Pd/C
Pdꢀ2.5Gd/C
0
1000
2000
3000
4000
Time/s
7
8
Fig. 6
9
0
1
2
3
4
5
1
1
1
1
1
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2
3
4
Pdꢀ2.5Gd/Ti4O7
Pdꢀ4Gd/Ti
Pdꢀ5Gd/Ti
4
O
7
7
4O
∇
Ti4O7
Pd
∇
♣
∇
∇
♣
∇
∇
∇
∇
∇
∇
♣
∇
♣
♣
10
20
30
40
50
60
70
80
90
2
ꢀtheta/degree
ꢀ
5
6
7
8
9
0
1
2
3
4
5
6
Fig. 7
1
1
1
1
1
1
1
2
6

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1
2
3
4
5
6
a
b
7
8
9
0
1
2
3
4
5
6
7
8
9
Fig. 8
1
1
1
1
1
1
1
1
1
1
27

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1
2
3
4
5
0
0
0
0
0
0
0
0
0
0
0
0
0
.0024
.0022
.0020
.0018
.0016
.0014
.0012
.0010
.0008
.0006
.0004
.0002
.0000
Pd/C
Pdꢀ2.5Gd/Ti
4
O
7
Pdꢀ4Gd/Ti
4O
7
7
Pdꢀ5Gd/Ti
4O
Pdꢀ2.5Gd/C
ꢀ0.0002
ꢀ
0.0004
ꢀ0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential/V
6
7
8
9
0
1
2
3
Fig. 9
1
1
1
1
2
8

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a
0
Pd
2
+
Pd
350 348 346 344 342 340 338 336 334 332 330
B.E.(eV)
1
b
0
Pd
2+
Pd
350 348 346 344 342 340 338 336 334 332 330
B.E.(eV)
2
2
9