Preparation of Pd-Co-based nanocatalysts and their superior applications in formic acid decomposition and methanol oxidation

Article abstract of DOI:10.1002/cssc.201402926

Formic acid (FA) and methanol, as convenient hydrogen-containing materials, are most widely used for fuel cells. However, using suitable and low-cost catalysts to further improve their energy performance still is a matter of great significance. Herein, PdCo and PdCo@Pd nanocatalysts (NCs) are successfully prepared by the facile method. Pd 3d binding energy decreases due to the presence of Co. Consequently, PdCo@Pd NCs exhibit high catalytic activity and selectivity toward FA dehydrogenation at room temperature. The gas-generation rate at 30 min is 65.4 Lh-1 g-1. PdCo/C has the worst catalytic performance in this reaction, despite the fact that it has a high gas-generation rate in the initial 30 min. Furthermore, both PdCo and PdCo@Pd NCs have enhanced electrocatalytic performance toward methanol oxidation. Their maximum currents are 966 and 1205 mAmg-1, respectively, which is much higher than monometallic Pd/C.

Full text of DOI:10.1002/cssc.201402926

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DOI: 10.1002/cssc.201402926
Preparation of Pd–Co-Based Nanocatalysts and Their
Superior Applications in Formic Acid Decomposition and
Methanol Oxidation
Yu-ling Qin,[a, b] Ya-cheng Liu,[a, b] Fei Liang,[a, c] and Li-min Wang*[a, c]
Formic acid (FA) and methanol, as convenient hydrogen-con-
taining materials, are most widely used for fuel cells. However,
using suitable and low-cost catalysts to further improve their
energy performance still is a matter of great significance.
Herein, PdCo and PdCo@Pd nanocatalysts (NCs) are successful-
ly prepared by the facile method. Pd 3d binding energy de-
creases due to the presence of Co. Consequently, PdCo@Pd
NCs exhibit high catalytic activity and selectivity toward FA de-
hydrogenation at room temperature. The gas-generation rate
at 30 min is 65.4 LhÀ1 gÀ1. PdCo/C has the worst catalytic per-
formance in this reaction, despite the fact that it has a high
gas-generation rate in the initial 30 min. Furthermore, both
PdCo and PdCo@Pd NCs have enhanced electrocatalytic per-
formance toward methanol oxidation. Their maximum currents
are 966 and 1205 mAmgÀ1, respectively, which is much higher
than monometallic Pd/C.
terms of catalytic activity and selectivity,[5] the development of
suitable Pd-based nanocatalysts (NCs) for FA dehydrogenation
based on transition-metal doping still is highly desirable due
to the limited availability and high cost of precious metals.[6]
On the other hand, methanol is regarded as a highly impor-
tant fuel for future renewable energy applications (direct
methanol fuel cells) thanks to its high availability from biomass
production, and high energy density.[7] In recent decades, Pd
NCs have been considered as versatile catalysts in methanol
oxidation owing to their excellent electrochemical activity.[8]
Unfortunately, monometallic Pd is still prone to deactivation
due to the adsorption of poisoning carbon monoxide inter-
mediates during methanol oxidation.[9] Therefore, it is impera-
tive to further increase the CO tolerance and electrocatalytic
performance of Pd NCs.
Herein, carbon black-supported Pd–Co-based NCs are pre-
pared using facile methods. Simply, using carbon black as carri-
2À
er, PdCo/C could be obtained from the coreduction of PdCl4
With increasing levels of energy shortage and environmental
pollution, hydrogen is thought to be an alternative energy
vector because of its high energy density and efficiency with
a low environmental impact.[1] Especially, it can be efficiently
used to generate electricity in fuel cells without pollution (e.g.,
in proton-exchange membrane fuel cells).[2] However, the
search for potential hydrogen-storage materials remains one of
the most significant barriers in the implementation of a ‘hydro-
gen-economy’’ society.[3]
Formic acid (FA) has attracted increasing attention as a con-
venient source of hydrogen for sustainable chemical synthesis
and renewable energy storage because it is a benign energy
source and can be safely handled in aqueous solutions.[4]
Although heterogeneous catalysts (e.g., Pd-based nanocata-
lysts) for FA decomposition have significantly improved in
and Co2+ precursors by NaBH4. After that, PdCo@Pd/C NCs are
2À
produced by a Pd replacement between PdCl4 and PdCo
nanoparticles. Naturally, the cost of NCs is reduced because of
Co. As a result, Pd–Co-based nanoparticles are proven to be
multifunctional and efficient catalysts for FA decomposition
and methanol oxidation. On one hand, both PdCo and
PdCo@Pd NCs exhibit enhanced catalytic activity toward FA
dehydrogenation at room temperature. On the other hand,
and more importantly, 4 mmol FA decomposes completely on
PdCo@Pd at room temperature in only 90 min. Pd–Co-based
NCs both display higher electrocatalytic performance than Pd/
C NCs toward methanol oxidation in alkaline media.
Figure 1 shows the variation of the volume of generated gas
(CO2+H2) versus reaction time during dehydrogenation of
aqueous FA solutions catalyzed by Pd-based NCs. Clearly, FA
dehydrogenation over Co/C NCs produces no gas, which can
be explained by the fact that Co scarcely shows catalytic activi-
ty toward FA decomposition in these conditions. Interestingly,
PdCo catalysts exhibit higher activity than Pd/C in FA decom-
position, indicating that Co plays a critical role in improving
the catalytic activity of Pd. However, because of weak corro-
sion-resistance ability, PdCo has lower catalytic activity in sub-
sequent dehydrogenation reactions, despite the fact that it has
a high gas generation rate (47.6 LhÀ1 gÀ1) in 30 min. The turn-
over ratio of FA on PdCo/C at 90 min is 55.2%. Surprisingly,
PdCo@Pd NCs have high catalytic activity toward FA decompo-
sition: 4 mmol FA decomposes completely on PdCo@Pd at
room temperature in only 90 min, which is more than five
times higher relative to Pd/C. The gas-generation rate in the in-
[a] Dr. Y.-l. Qin, Y.-c. Liu, F. Liang, Prof. L.-m. Wang
State Key Laboratory of Rare Earth Resource Utilization
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun, 130022, Jilin (PR China)
Fax: (+86)431-85262836
E-mail: lmwang@ciac.ac.cn
[b] Dr. Y.-l. Qin, Y.-c. Liu
University of Chinese Academy of Sciences
Beijing, 100049, (PR China)
[c] F. Liang, Prof. L.-m. Wang
Changzhou Institute of Energy Storage Materials and Devices
Changzhou, 213000, Jiangsu (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402926.
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Figure 2. Stable CV curves obtained for the Pd/C, PdCo/C, and PdCo@Pd/C
catalysts in NaOH (0.5m) and CH3OH (0.5m) at a scan rate of 50 mVsÀ1 at
room temperature.
Figure 1. Plots of volume of CO2/H2 gas liberation over time from a stirred
tank reactor containing aq. FA (4 mL, 1m HCOOH and 0.1m HCOONa) and
carbon-supported Pd, PdCo, PdCo@Pd, and Co NCs; mcat =80 mg. Inset: rate
of gas production (volume of CO2+H2 produced per hour per gram of Pd) in
30 min.
PdCo/C NCs, methanol oxidation on PdCo/C starts at about
0.58 V. The maximum current is reached at about 1.04 V, which
is similar to that on PdCo@Pd/C. The maximum current on
PdCo is about 240 mAmgÀ1 lower than that on PdCo@Pd
(Figure 2). Given all that, the current densities from CV meas-
urements follow the order: PdCo@Pd>PdCo>Pd. Thus, it is
concluded that Pd–Co-based catalysts have higher catalytic ac-
tivity toward the electro-oxidation of methanol in alkaline solu-
tions.
itial 30 min is as high as 65.4 LhÀ1 gÀ1. After reaction of the gas
mixture (H2+CO2) with aq. NaOH, the volume of CO2 and H2
was measured; the gas mixture consisted of equimolar
amounts of CO2 and H2 (see the Supporting Information, Fig-
ure S1A), indicating that PdCo@Pd NCs promote the complete
conversion of FA into CO2 and H2. Notably, the H2:CO2 ratio
from other catalysts (see the Supporting Information, Fig-
ure S1B) is also about 1:1. Furthermore, the molar H2:CO2 ratio
obtained by MS analysis (see the Supporting Information,
Table S2) shows similar results. Moreover, MS profiles (see the
Supporting Information, Figure S2) show no detectable
amount of CO (<10 ppm) in the gas mixture generated from
the dehydrogenation of FA over NCs. Therefore, we can con-
clude that PdCo@Pd NCs show high selectivity toward FA de-
composition at room temperature.
Stability is of great importance for the application of cata-
lysts in fuel cells. There is a <5% decline in activity for PdCo
and PdCo@Pd NCs in the electro-oxidation current after 400
potential scans, whereas Pd/C suffers a >10% decline (see the
Supporting Information, Figure S5). It was thus evident that
PdCo and PdCo@Pd NCs have higher stability toward methanol
oxidation in alkaline media.
In view of the high catalytic and electrocatalytic per-
formance of Pd–Co-based NCs, TEM and XRD analyses were
performed to investigate the morphology, dispersity, and struc-
ture of the NCs: As is shown in Figure 3A–C, well-dispersed
NCs could be found. Particle-size distribution (Figure 3, inset)
derived from TEM images reveals that additional Co decreases
the size of the nanoparticles prepared in this way. In XRD pat-
terns (Figure 3D), the peaks of Pd/C exhibit the characteristics
of a single face-centered-cubic (fcc) crystallographic structure
of Pd (PDF#46-1043). It also should be noted that the charac-
teristic (111) peak of Pd in PdCo/C catalysts is shifted to higher
2q values relative to corresponding peaks in Pd/C, indicating
the formation of an alloy between Pd and Co. In addition,
from high-resolution TEM (HRTEM) images (Figure 3E), the
(111) d-spacing of PdCo nanoparticles is calculated to be
2.137 ꢁ, which is smaller than that of pure fcc-Pd (2.245 ꢁ),
confirming the alloy structure again.
Inspired by its high catalytic activity, we further tested the
catalytic stability of PdCo@Pd/C by performing FA dehydrogen-
ation in aqueous solutions at room temperature. Repeated
testing was achieved by adding pure FA to the reaction vessel
after completion of the previous run; PdCo@Pd NCs have high
catalytic activity after the fourth run (see the Supporting Infor-
mation, Figure S3). The finding reveals that there is a possibility
to use PdCo@Pd NCs for multiple rounds of FA decomposition
in those conditions.
The electrocatalytic performance of the three catalysts on
methanol oxidation was also tested. The electrochemically
active surface areas of the three catalysts (see the Supporting
Information, Figure S4) was measured using the hydrogen de-
sorption area from cyclic voltammetry (CV) spectra in aq.
H2SO4 (0.5m). Figure 2 exhibits the CV spectra of Pd-based NCs
in a mixture of methanol (0.5m) and NaOH (0.5m). Apparently,
methanol oxidation on PdCo@Pd/C starts at about 0.55 V
versus the reversible hydrogen electrode (RHE) and maximum
current is reached at 1.04 V (Figure 2). Moreover, the maximum
current on PdCo@Pd/C in the positive potential scan is 264%
higher than that on Pd/C. The secondary oxidation peak at
0.89 V in the negative scan is far lower than that on Pd/C. For
X-ray photoelectron spectrometry (XPS) and inductively cou-
pled plasma atomic emission spectroscopy (ICP-AES) analyses
were performed to further explore the exact framework of NCs
(see the Supporting Information, Table S1). PdCo NCs consist-
ing of Co and Pd atoms were verified by ICP data. However,
from XPS spectra of Co2p (see the Supporting Information,
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optimal catalytic performance toward FA decomposition at
room temperature with HCOONa (0.1m) as promoter. As evi-
dence of their multifunctionality, NCs were further employed
as electrocatalysts for methanol oxidation, in which they also
exhibited good electrocatalytic performance. This facile
method could be easily extended to other noble-metal–transi-
tion-metal materials, which may shed light on the synthesis of
carbon-supported materials for future applications.
Experimental Section
Palladium chloride (60%), cobalt chloridehexahydrate (CoCl2·6H2O,
99%), sodium borohydride (NaBH4, 98%), FA (ꢀ98%), sodium for-
mate (HCOONa, AR), and methanol (CH3OH, >99%) were pur-
chased from Aladdin Reagent. All chemicals were used without
any further purification. Ar was bubbled through distilled water for
1 h; Na2PdCl4 solutions were prepared at 330 K by dissolving PdCl2
(0.89 g) and NaCl (0.7 g) in ultrapure water (40 mL) under vigorous
stirring, and then diluting to 100 mL at room temperature with ul-
trapure water. Preparation of PdCo/C: XC-72 carbon powder
(180 mg) was ultrasonically dispersed in water (15 mL); then,
1.12 mL of aq. Na2PdCl4 (0.05m) and 0.29 mL of aq. CoCl2 (0.193m)
were added to the mixture, followed by addition of aq. NaBH4
(1 mL, 200 mgmLÀ1) under an Ar atmosphere. The mixture was
stirred for another 4 h at room temperature. Finally, the desired
catalyst was isolated from the resulting suspension by centrifuga-
tion, and was subsequently washed with distilled water and dried
under vacuum at 608C overnight. Preparation of PdCo@Pd/C:
PdCo/C was washed with distilled water and then diluted with
20 mL water. Na2PdCl4 (1 mL) was added to the mixture with slow
stirring under an Ar atmosphere. After 8 h, the product was centri-
fuged, washed, and then dried under vacuum at 608C overnight.
The methodology for the preparation of Pd/C and Co/C was similar
to that followed for PdCo/C.
Figure 3. TEM images of: (A) Pd/C; (B) PdCo/C; and (C) PdCo@Pd/C NCs.
Inset in (A–C): particle-size distribution derived from TEM images. (D) XRD
patterns of Pd/C, PdCo/C, and PdCo@Pd/C NCs; (E) HRTEM image of PdCo
nanoparticles.
Figure S6B) it was evident that CoOx and Co(OH)2 still existed
in the PdCo catalyst besides monoplasmatic Co. Furthermore,
the changes in XPS-signal intensity on Pd3d (see the Support-
ing Information, Figure S6A) and Co 2p (see the Supporting In-
formation, Figure S6B) were pronounced after adding Na2PdCl4
to aq. PdCo/C, indicating the replacement reaction between
Na2PdCl4 and Co (surface of PdCo nanoparticles). Therefore, it
might be concluded that a PdCo@Pd core–shell structure with
a PdCo core and a Pd shell was obtained.
Acknowledgements
Previously, the Pd d-band center shifted downwards when
Pd was alloyed with Ni, which suggested that Pd–adsorbate
bonds were weaker.[10] In addition, the lowering of Pd3d bind-
ing energy influenced the chemisorption energy of adsorbate,
which enhanced the CO antipoisoning ability of the catalyst.[11]
Herein, XPS analysis was employed to probe the change in the
electronic structure of Pd centers: compared to Pd/C (see the
Supporting Information, Figure S6C), the peak of Pd3d5/2 shift-
ed from 335.70 to 335.27 eV in PdCo/C, and to 335.30 eV in
PdCo@Pd/C. The shift of binding energy is due to the slight
electron transfer from Co to Pd, which could be explained by
the fact that Pd is more electronegative than Co. From preview
reports, the lowering of the Pd3d binding energy promoted
the antipoisoning ability of Pd active sites, which in turn was
propitious to FA decomposition and methanol oxidation.[11]
Especially in FA dehydrogenation, PdCo@Pd had higher activity
than PdCo because of the higher antiacid erosion in FA solu-
tions.
This work is supported by the National Nature Science Founda-
tion of China (Grant No. 20111061).
Keywords: dehydrogenation · green chemistry · hydrogen ·
oxidation · palladium
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Published online on && &&, 0000
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Twin alloy—twin performance:
Y.-l. Qin, Y.-c. Liu, F. Liang, L.-m. Wang*
A simple method is employed to pre-
pare Pd–Co-based nanoparticles that ex-
hibit high catalytic performance toward
formic acid decomposition and metha-
nol oxidation. The Pd3d binding energy
decreases owing to the presence of Co,
which gives rise to the improved cata-
lytic activity and selectivity toward FA
dehydrogenation at room temperature
and electrocatalytic performance in
methanol oxidation.
&& – &&
Preparation of Pd–Co-Based
Nanocatalysts and Their Superior
Applications in Formic Acid
Decomposition and Methanol
Oxidation
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 4
&
5
&
ÞÞ
These are not the final page numbers!