Dispersing Pt and Pd atoms on Au nanoparticles deposited on n-GaN substrates for formic acid oxidation

Article abstract of DOI:10.1039/c5ra16807d

Au nanoparticles, with an ultra low-loading of Pt and Pd atoms, supported on n-GaN were prepared using electrochemical atomic layer deposition and surface-limited redox replacement. The as-prepared electrocatalysts showed an enhanced performance for formi

Full text of DOI:10.1039/c5ra16807d

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Dispersing Pt and Pd atoms on Au nanoparticles
deposited on n-GaN substrates for formic acid
oxidation†
Cite this: RSC Adv., 2015, 5, 92757
Received 20th August 2015
Accepted 18th October 2015
ab
b
b
b
b
S.-J. Qin, Y. Zhao, F. Peng, X.-Q. Chen and G.-B. Pan*
DOI: 10.1039/c5ra16807d
www.rsc.org/advances
Au nanoparticles, with an ultra low-loading of Pt and Pd atoms, sup- signicantly higher stabilities and better activities than Pt or Pd
14
ported on n-GaN were prepared using electrochemical atomic layer alone. Previous studies showed that during the oxidation of FA
deposition and surface-limited redox replacement. The as-prepared on both Pt and Pt–Ru the catalysts are easily poisoned by
1
5
electrocatalysts showed an enhanced performance for formic acid adsorbed CO, while the Pt–Pd catalyst is not greatly inhibited.
PtPdꢀ
1
oxidation, and the mass activity reached 3.5 mA mg
Chem-O
. The active Theoretical work reveals that the d-band electronegativity or
on the Pt
sites. On the other hand, the greatest challenge is to reach
00% utilization of the catalytically active species while main-
sites of the Pt
acid oxidation.
species were found to play a key role in formic occupancy affects the adsorption energy of CO and H
2
16
1
taining good enough durability against over oxidation and
poisoning. Previous attempts have been made to grow
monolayer-type Pt shells onto Pd cores with an enhanced mass
Proton exchange membrane fuel cells (PEMFCs) are considered
to be a promising technology for clean and efficient power
1,2
generation. Direct formic acid fuel cells (DFAFCs) are one of
the most important types of PEMFCs due to their high elec-
tromotive force, limited fuel crossover, and reasonable power
catalytic activity toward the oxygen reduction reaction
17,18
(ORR).
Although, such structures could approach close to
100% Pt utilization, they are not favourable for formic acid
3–5
density. Pt and Pd are the preferred anode catalysts for formic
acid (FA) oxidation in acidic media, however, they should be
drastically optimized in both the anode and the cathode
oxidation because the continuous Pt atoms would facilitate the
formation of poisoning CO intermediates via the indirect
pathway. Meanwhile, dispersed Pt atoms have also been
deposited on conductive substrates through a molecular self-
assembly/electrodeposition process and atomic layer deposi-
6,7
because of their high costs. Meanwhile, they also have some
shortcomings. Pd possesses a high initial activity and produces
less carbon monoxide (CO) for this reaction, but it is much less
11
tion process. It showed that single-atom catalysts exhibited
8
stable. Pt shows greater resistance to corrosion but it is easily
superior electrocatalytic activity (up to 10 times) over that of
a commercial Pt/C catalyst due to the highly desired direct
reaction pathway. Actually, alloying isolated Pt atoms with
transition metals such as Pd atoms would develop a better
solution for optimizing the performance and suppressing the
indirect pathways.
8,9
poisoned by CO produced from the dehydration of FA. It is
generally agreed that the indirect oxidation pathway (dehydra-
tion of FA) requires continuous surface Pt lattices or dimers or
10
trimers of Pt atoms. An isolated Pt atom favours the direct
11
oxidation pathway (dehydrogenation of FA) on its surface. This
12
has been proven using DFT calculations and demonstrated by
increasing the coverage of Pt in the topmost mixed PtPd alloy
layer which could change the electrochemical formic oxidation
Herein, we report a facile and one-cell conguration to form
dispersed Pt atoms surrounded by Pd atoms on Au nano-
particles (Au–PtPd NPs), which are electrodeposited onto n-type
gallium nitride (n-GaN). Our previous studies have shown that
GaN might be a promising support for Pt, Au and Pd due to its
high chemical stability, electric conductivity, and strong metal–
13
mechanism from the direct pathway to the indirect pathway.
Alloying Pt with Pd is one of the promising solutions to
accomplish a portion of the goals for DFAFCs and results in
19–21
semiconductor interactions.
The catalysts show an
a
enhanced performance for FA oxidation, and their mass activity
Department of Chemistry, College of Sciences, Shanghai University, 99 Shangda Road,
ꢀ1
Chem-O
2
b
00444 Shanghai, China
reaches 3.5 mA mgPtPd . The active sites of the Pt
species
Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, play key roles in FA oxidation.
2
15123 Suzhou, China. E-mail: gbpan2008@sinano.ac.cn; Fax: +86 512 62872663
Electrochemical atomic layer deposition (E-ALD) and
surface-limited redox replacement (SLRR) are a highly efficient
†
Electronic supplementary information (ESI) available: Experimental detail in
Fig. S1–S3. See DOI: 10.1039/c5ra16807d
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92757–92761 | 92757

View Article Online
RSC Advances
Communication
22–24
route to grow an atomic layer on gold substrates.
In a typical atoms embedded into Pt atom spaces, which caused discon-
process, an atomic layer of sacricial metal was formed by tinuous surface Pt lattices and strengthened the self-stability. In
underpotential deposition (UPD) and subsequently replaced by comparison, electrocatalysts with a Pt or Pd monolayer on the
noble metals in the corresponding precursor ions. The redox Au NPs (Au–PtML or Au–PdML) were produced and the mono-
replacement is limited by the coverage of the UPD metal, and layers were deposited by similar routes.
therefore monolayer lms are produced. For instance, a Pt and
Pd monolayer can be deposited on single-crystal metallic elec- NPs are investigated using transmission electron microscopy
The morphology and structure of the as-fabricated Au–PtPd
25,26
trodes of Au(111) and Rh(111).
However, previous reports (TEM). Fig. 2a shows a typical TEM image, revealing sphere- or
focused on depositing a Pt or Pd monolayer for the ORR, rather peanut-like NPs with an average diameter of ꢂ7 nm. This is
than dispersing Pt and Pd atoms which present discontinuous much smaller than those of the NPs (ꢂ50 nm) grown by
surface lattices and is more suitable for FA oxidation.
constant current electrodeposition and this is due to diffusion-
Fig. 1 shows a scheme of the preparation of Au–PtPd on GaN limited nucleation. Fig. 2b shows a high resolution TEM (HR-
substrates. The Au NPs are prepared by pulse-current electro- TEM) image, and the inset is a corresponding fast Fourier
deposition in 0.5 M NaCl + 1 mM HAuCl . The electrodeposition transform (FFT) pattern. The bright diffraction spots are
4
ꢀ
2
ꢀ2
current densities are set at ꢀ20 mA cm (1.5 s) and 0 mA cm
4.5 s) (Fig. S1†). The stabilized potential could be renewed aer polycrystalline nature of the Au NPs. The lattice spacing of Au is
every pulse, indicating that the depletion zones would be clearly seen, however, the structure of the PtPd cluster could not
ascribed to the (220), (200) and (111) facets of Au, indicating the
(
3
+
repopulated with Au during the time-off. Therefore, new be clearly certied due to ultra low-loading. Fig. 2c shows the
nuclei are formed, rather than existing nuclei growth. A energy-dispersive spectrum (EDS) for the nanoparticles from
previous study had reported that an ideal monolayer of Cu a large area signal collection. Although the Pt and Pd content
cannot be formed on Au NPs by a simple UPD, which is affected was too low to be determined quantitatively, their peaks are
27
markedly by the deposition potential and time. The Cu UPD is seen clearly in the spectrum with a relatively low intensity.
carried out at 0.2 V (before the OPD) for 10 s to form sub- Furthermore, for a single nanoparticle, the line prole analysis
monolayer absorbed Cu atoms in 0.1 M NaClO
4
+ 10 mM CuSO
4
by STEM-EDS shows the distribution of the Pt and Pd compo-
ꢀ
(Fig. S2†). On the other hand, a Cl adlayer on the Au surface nents (Fig. 2f). It is clear that the Pt and Pd atoms fully cover the
lattices, such as a c(2 ꢁ 2) structure, would affect the Cu adatom Au NPs. The identity of the Pt and Pd components along the
ꢀ
structure. When in the presence of Cl , the (2 ꢁ 2) surface centre of Au NPs undulates slightly due to discontinuous
structure of Cu /Au(111) would transfer to a (5 ꢁ 5) structure, surface distribution.
UPD
28
˚
and the interatomic spacing of CuUPD would increase to 3.67 A.
Such a route favours the formation of discontinuous surface
lattices. Cu UPD adatoms were subsequently exchanged with Pt
in 0.1 M HClO
4 2 6
+ 0.4 mM H PtCl at an open circuit potential
(Fig. S3†). The Pt coverage (qPt) was calculated as the ratio of the
charge from Cu adlayer oxidation. The calculated qPt obtained
via surface-limited redox replacement was approximately 64%
28
(
monolayer) aer two growths in this situation. Then, the Cu
UPD adatoms were re-deposited and exchanged in 0.1 M
HClO + 0.4 mM PdCl . With this method, the deposited Pt and
4
2
Pd adatoms were fully exposed on the Au surface and formed
interactive Au–Pt, Au–Pd and Pt–Pd bonds. Moreover, the Pd
Fig. 2 The characterization of the morphology and structure of the
Fig. 1 Schematic illustration of the preparation of Au–PtPd on n- Au–PtPd NPs, TEM (a), HR-TEM (b), EDS (c), size distribution (d), HR-
GaN(0001). STEM (e), and STEM-EDS (f).
92758 | RSC Adv., 2015, 5, 92757–92761
This journal is © The Royal Society of Chemistry 2015

View Article Online
Communication
RSC Advances
In addition to morphological characterization, the electro- loss of the ECSA for individual PtML and PdML on Au NPs was
chemical properties of the electrocatalysts are explored with found aer hundreds of cycles. It has been noted that a clear
cyclic voltammograms (CVs, Fig. 3a) using 0.5 M H SO and at relaxation phenomenon occurs during the initial 600 cycles,
2
4
ꢀ
1
a scan rate of 20 mV s . Characteristic redox peaks (C /A ) of Au and then the ECSA reduces rapidly to 75%, which reects the
3
3
at ꢂ0.8 and 1.0 V disappear with the deposition of Pt and Pd weakness and instability of such catalysts during the initial
atoms, and new redox peaks appear at ꢂ0.75 and 0.3 V. These stage of catalytic reaction. A similar phenomenon was also
27
are due to the loss of the Au surface by masking with Pd and Pt observed for a 3% Pt@Au/C catalyst in the literature. This is
atoms. Notably, with the cover of Pt and Pd atoms, the typical possibly because of the SLRR process only producing weakly
hydrogen adsorption/desorption peaks in the negative and adsorbed Pt and Pd atoms on the Au NPs, i.e., they do not form
positive scans become sharper in the potential range of ꢀ0.3 to a stable alloy. During the potential scan, surface atoms will
0
V. The desorption peaks (A ¼ ꢀ0.256 V and A ¼ ꢀ0.201 V) rearrange into a more stable structure. The catalysts display an
2
3
are ascribed to H desorption on the Pt and Pd atoms, and there activation stage in the initial scan. The active surface area is lost
was a little negative shi when compared to Fig. S4.† The aer 600 cycles, probably caused by the strong segregation of
2
7
enhanced peak current (C
4
/A
4
) at ꢀ0.09/ꢀ0.06 V was attributed the Pt/Au system or the dissolution of Pd.
to H species in and out of the lattice interstices of PtPd
Fig. 3b shows the CVs of Au–PtPd, Au–PdML, and Au–PtML
+
10
ꢀ1
accompanied by H electroreduction and H electrooxidation.
NPs in 0.5 M H
2
SO
4
+ 0.2 M HCOOH at a scan rate of 50 mV s
.
Meanwhile, the electrochemically active surface area (ECSA) was Compared with the Pt and Pd monolayer, the positive-scanning
calculated by the charges associated with H adsorption/ current shows a broad peak, consisting of two sub-peaks at
ꢀ
2
desorption, assuming 210 mC cm for oxidizing a monolayer around 0.1 and 0.3 V, which are due to the direct oxidation of FA
2
9
of hydrogen on the catalyst. The ECSA of the Au–PtPd NPs to CO on the Pd and Pt atoms, respectively. Although, the
(
2
2
ꢀ1
100.15 m gPtPd ) is two times higher than a commercial Pt/C positive shis of the oxidation peaks are 138 and 53 mV,
catalyst and little higher than Pd@Pt2–3L octahedral nano- respectively. During the negative scanning, the oxidation
17
catalysts. The stability of the electrocatalyst was also evaluated current increases until a maximum current is obtained at about
by seeing how the ECSA changed during prolonged CV tests 0.02 V, which is attributed to the direct oxidation of FA on the
(shown in the inset). The relative percentage of the ECSA restored Pd atoms. Meanwhile, the oxidation current is effec-
reduces to ꢂ50% aer 8000 cycles from ꢀ0.4 to 1.2 V. A higher tively inhibited in a high potential range, indicating that the
indirect oxidation pathway is diminished. The presence of Au
and Pd hinders CO formation on the Pt atoms and shis the
oxidation onset potential to a much lower value. This is because
Pd atoms block the aggregation of Pt atoms, and a single Pt
atom surrounded by Pd atoms favours the direct oxidation of FA
13
on its surface. On the other hand, Au atoms also have a great
inuence on the reaction pathways of the Pt atoms. When the
Pt : Au ratios are less than 1 : 2 or qPt is less than 34%, the
30,31
indirect pathway becomes diminished.
Simultaneously, the
Au–PtPd NPs have an enhanced catalytic performance, and
ꢀ
1
reach a mass activity of 3.5 mA mgPtPd at 0 V. The inset shows
the durability towards FA oxidation versus cycles. Half of the
catalytic performance remains aer 460 cycles, and a similar
relaxation phenomenon of increasing rst and then decreasing
has also been observed.
The oxidation states of Pt and Pd have been investigated by
X-ray Photoelectron Spectroscopy (XPS). The Pt 4f spectra
exhibit a spin-orbit-split doublet, with the two peaks (4f7/2, 5/2
)
exhibiting a spacing of 3.3 eV. Before FA oxidation (Fig. 4a), two
pairs of peaks, located at 71.22/74.52 and 72.19/75.49 eV, are
0
Chem-O
29,32
Chem-O
assigned to Pt and Pt
, respectively.
The Pt
surface core level shi shis positively from 0.7 to 1.0 eV when
GaN substrates are used. Meanwhile, a very strong binding of
oxygen to Pt atoms could be found. There still is 24.47% of
Chem-O
Pt
even in the fresh catalysts. It is mainly because of the
interfacial charge transfer process between Pt and n-GaN
32
substrates. It has been noted that when subjected to FA
Chem-O
oxidation (Fig. 4b), the relative percentage of Pt
decreases
Fig. 3 CVs of Au, Au–PtML, Au–PdML and Au–PtPd NPs in 0.5 M H
2
SO
4
0
2+
ꢀ1
to 13.06%, the Pt percentage increases 8.61%, and Pt (72.46
and 75.76 eV) is detected with a percentage of 2.8%. Similarly,
the Pd 3d spectra (Fig. 4c) display a doublet signal with binding
at a scan rate of 20 mV s (a), 0.5 M H SO + 0.2 M HCOOH at a scan
2 4
ꢀ1
rate of 50 mV s (b). The insets in (a) and (b) are the curves of the ECSA
and mass activity versus cycles, respectively.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92757–92761 | 92759

View Article Online
RSC Advances
Communication
Notes and references
1
2
R. Taherian, J. Power Sources, 2014, 265, 370.
S. J. Peighambardoust, S. Rowshanzamir and M. Amjadi, Int.
J. Hydrogen Energy, 2010, 35, 9349.
3
4
5
X. W. Yu and P. G. Pickup, J. Power Sources, 2008, 182, 124.
U. B. Demirci, J. Power Sources, 2007, 169, 239.
K. J. Jeong, C. A. Miesse, J. H. Choi, J. Lee, J. Han, S. P. Yoon,
S. W. Nam, T. H. Lim and T. G. Lee, J. Power Sources, 2007,
168, 119.
6
7
8
9
C. Rice, S. Ha, R. I. Masel and A. Wieckowski, J. Power
Sources, 2003, 115, 229.
Z. L. Liu, L. Hong, M. P. Tham, T. H. Lim and H. X. Jiang, J.
Power Sources, 2006, 161, 831.
Fig. 4 XPS spectra of Pt 4f (a and b) and Pd 3d (c and d) before and
after FA oxidation.
C. X. Xu, Q. Hao and H. M. Duan, J. Mater. Chem. A, 2014, 2,
8875.
M. N. Cao, D. S. Wu and R. Cao, ChemCatChem, 2014, 6, 26.
energies of 335.3 and 340.6 eV for 3d5/2 and 3d3/2, respectively.
Aer FA oxidation, another two small doublets (Fig. 4d) at 336.8
and 342.1 eV can be assigned to the Pd 3d and Pd 3d3/2 peaks
1
1
1
1
0 H. X. Zhang, C. Wang, J. Y. Wang, J. J. Zhai and W. B. Cai, J.
Phys. Chem. C, 2010, 114, 6446.
5
/2
1 R. Y. Wang, J. G. Liu, P. Liu, X. X. Bi, X. L. Yan, W. X. Wang,
X. B. Ge, M. W. Chen and Y. Ding, Chem. Sci., 2014, 5, 403.
2 M. Neurock, M. Janik and A. Wieckowski, Faraday Discuss.,
33
of the PdO species. With the weaker oxygen absorption, the Pd
atoms have a zero value in the fresh catalysts, and aer the
catalytic oxidation of FA, PdO species form with a percentage of
2008, 140, 363.
2
1.49%.
The Pt
3 C. H. Cui, H. H. Li, H. P. Cong, S. H. Yu and F. Tao, Chem.
Commun., 2012, 48, 12062.
Chem-O
species may play a role as an active site,
resulting in the greatest decrement in composition. The
chemisorbed oxygen on the Pt atoms presents a lower reaction
1
1
4 X. G. Li and I. M. Hsing, Electrochim. Acta, 2006, 51, 3477.
5 L. G. Feng, F. Z. Si, S. K. Yao, W. W. Cai, W. Xing and
C. P. Liu, Catal. Commun., 2011, 12, 772.
Chem-
barrier, the intermediates are absorbed preferentially on Pt
O
and they undergo an oxidation process to form CO and the
Pt–O bond breaks, forming either Pt or Pt . The kinetics of the
2
16 E. Christoffersen, P. Liu, A. Ruban, H. L. Skriver and
J. K. Norskov, J. Catal., 2001, 199, 123.
0
2+
CO oxidation reaction on the Pt(111) surface with a preadsorbed
1
7 J. Park, L. Zhang, S. I. Choi, L. T. Roling, N. Lu, J. A. Herron,
S. F. Xie, J. G. Wang, M. J. Kim, M. Mavrikakis and Y. N. Xia,
ACS Nano, 2015, 9, 2635.
3
4
p(2 ꢁ 2) layer of atomic oxygen was investigated by HR-XPS.
The reaction of CO with the preadsorbed O to CO occurred on
2
the top-sites or bridge-sites. The formation of a Pt oxide state
from chemisorbed oxygen has also been claimed on a Pt single-
1
8 X. Wang, S. I. Choi, L. T. Roling, M. Luo, C. Ma, L. Zhang,
M. Chi, J. Liu, Z. Xie, J. A. Herron, M. Mavrikakis and
Y. Xia, Nat. Commun., 2015, 6, 7594.
35,36
crystal surface.
Simultaneously, the interaction of Pd and Pt
would give rise to some charge redistribution through the
hybridization of the states from each atom, so that the forma-
1
2
2
2
2
9 Y. Zhao, S. J. Qin, Y. Li, F. X. Deng, Y. Q. Liu and G. B. Pan,
Electrochim. Acta, 2014, 145, 148.
37
tion of PdOH prevails and PtOH is suppressed. The nal
0 Y. Li, Y. Zhao, G. B. Pan, Z. H. Liu, G. L. Xu and K. Xu,
Electrochim. Acta, 2013, 114, 352.
1 F. X. Deng, Y. Zhao, L. F. Hu, T. Xu, Y. Q. Liu and G. B. Pan,
RSC Adv., 2014, 4, 42240.
2 N. Jayaraju, D. Vairavapandian, Y. G. Kim, D. Banga and
J. L. Stickney, J. Electrochem. Soc., 2012, 159, D616.
3 K. Sasaki, H. Naohara, Y. Cai, Y. M. Choi, P. Liu,
M. B. Vukmirovic, J. X. Wang and R. R. Adzic, Angew.
Chem., Int. Ed., 2010, 49, 8602.
results are the formation of the PdO species, and the percentage
0
of Pt increasing. This corroborates the above proposition of the
role of PdOH in suppressing PtOH formation.
In summary, Au nanoparticles, with an ultra low-loading of
Pt and Pd atoms, supported on n-GaN substrates are fabricated
using electrochemical atomic layer deposition and surface-
limited redox replacement, and they showed an enhanced
Chem-O
electrocatalytic performance. The Pt
species in such n-
GaN-supported monolayer catalysts act as the active sites for
formic acid oxidation, and the charge redistribution through
the hybridization of the states from the Pt and Pd atoms could
produce a positive effect in the integral catalytic performance.
2
4 R. R. Adzic, J. Zhang, K. Sasaki, M. B. Vukmirovic, M. Shao,
J. X. Wang, A. U. Nilekar, M. Mavrikakis, J. A. Valerio and
F. Uribe, Top. Catal., 2007, 46, 249.
2
2
2
5 Y. G. Kim, J. Y. Kim, D. Vairavapandian and J. L. Stickney, J.
Phys. Chem. B, 2006, 110, 17998.
6 R. Hoyer, L. A. Kibler and D. M. Kolb, Surf. Sci., 2004, 562,
275.
Acknowledgements
7 Y. L. Yu, Y. P. Hu and X. Wang, Electrochim. Acta, 2009, 54,
This work was nancially supported by the National Natural
3092.
Science Foundation of China (No. 21273272).
92760 | RSC Adv., 2015, 5, 92757–92761
This journal is © The Royal Society of Chemistry 2015

View Article Online
Communication
RSC Advances
2
2
3
3
3
8 D. Gokcen, S. E. Bae and S. R. Brankovic, J. Electrochem. Soc., 34 M. Kinne, T. Fuhrmann, J. F. Zhu, C. M. Whelan, R. Denecke
010, 157, D582. and H. P. Steinruck, J. Chem. Phys., 2004, 120, 7113.
9 F. J. Vidal-Iglesias, J. Solla-Gullon, E. Herrero, A. Aldaz and 35 D. R. Butcher, M. E. Grass, Z. H. Zeng, F. Aksoy, H. Bluhm,
2
J. M. Feliu, Angew. Chem., Int. Ed., 2010, 49, 6998.
0 N. Kristain, Y. L. Yu and X. Wang, Electrochim. Acta, 2009, 54,
W. X. Li, B. S. Mun, G. A. Somorjai and Z. Liu, J. Am. Chem.
Soc., 2011, 133, 20319.
4
916.
1 N. Kristain, Y. S. Yan and X. Wang, Chem. Commun., 2008,
53.
2 S. Schafer, S. A. Wyrzgol, R. Caterino, A. Jentys, S. J. Schoell,
36 J. G. Wang, W. X. Li, M. Borg, J. Gustafson, A. Mikkelsen,
T. M. Pedersen, E. Lundgren, J. Weissenrieder, J. Klikovits,
M. Schmid, B. Hammer and J. N. Andersen, Phys. Rev.
Lett., 2005, 95, 256102.
3
M. Havecker, A. Knop-Gericke, J. A. Lercher, I. D. Sharp and 37 J. Zhang, Y. Mo, M. B. Vukmirovic, R. Klie, K. Sasaki and
M. Stutzmann, J. Am. Chem. Soc., 2012, 134, 12528.
3 M. Brun, A. Berthet and J. C. Bertolini, J. Electron Spectrosc.
Relat. Phenom., 1999, 104, 55.
R. R. Adzic, J. Phys. Chem. B, 2004, 108, 10955.
3
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 92757–92761 | 92761