Enhanced oxygen reduction activity of platinum subnanocluster catalysts through charge redistribution

Article abstract of DOI:10.1039/c9cc06327g

Single-size platinum Pt₆ subnanoclusters exhibit superior mass-specific and surface-specific activities for the oxygen reduction reaction. The enhanced activity is attributed to polarized electron distributions based on rigorous structure chara

Full text of DOI:10.1039/c9cc06327g

Volume 55 Number 84 28 October 2019 Pages 12583–12724
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H. Tsunoyama, A. Ohnuma et al.
Enhanced oxygen reduction activity of platinum
subnanocluster catalysts through charge redistribution

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Enhanced oxygen reduction activity of platinum
subnanocluster catalysts through charge
redistribution†
Cite this: Chem. Commun., 2019,
55, 12603
Received 15th August 2019,
Accepted 17th September 2019
Hironori Tsunoyama, ‡a Akira Ohnuma, ‡b Koki Takahashi,a Archana Velloth,c
c
d
Masahiro Ehara,
Atsushi Nakajima
Nobuyuki Ichikuni,
*
Masao Tabuchie and
a
DOI: 10.1039/c9cc06327g
rsc.li/chemcomm
Single-size platinum Pt6 subnanoclusters exhibit superior mass- reduction reaction (ORR), is lower than that for the anodic
specific and surface-specific activities for the oxygen reduction hydrogen oxidation reaction, improvements in mass specific
reaction. The enhanced activity is attributed to polarized electron activity (MA/A gPtÀ1) for the ORR are essential. To this end,
distributions based on rigorous structure characterization by X-ray various active catalysts have been developed so far, using e.g.,
absorption fine structure spectroscopy and density functional theory. core–shell, alloy, and unique nanostructures,9,10,12–14 by tuning
the reactivity through ligand, strain, and ensemble effects.15,16
The development of durable and active catalysts with high
For platinum (Pt)-based nanoparticle (NP) catalysts, one of
atomic efficiency has been a key issue for a sustainable world, the convincing arguments for catalyst development is to
where size-selected, sub-nanoscale clusters (sub-NCs) consisting increase the surface-to-volume ratio through downsizing the
of a few to tens of atoms have been strong potential candidates catalytic NPs; MA increases with Àth1e14surface ratio, and reaches a
attributable to their high surface area and tunability of activity by constant value above 80 m2 gPt
.
In terms of high surface
‘‘atomicity’’. A wide variety of catalytic properties of sub-NCs ratio, NCs with less than ten atoms are good candidates to
have been extensively reported,1–8 and among the various enhance the activity. Although the atomicity is a unique parameter
uses for the catalytic transformations, fuel cell (FC) technology for the activity in monometallic Pt NCs, size specificity is still
appears to be a promising way to reduce environmental loads controversial;2,17–24 the difficulty arises from the complex
based on the potential of a principally carbon-free energy nature of catalysts, e.g., non-systematic variation of structures
source.9–11 Among the various types of FCs, proton-exchange with the size and effect of surrounding materials (supports and
membrane FCs have attracted much attention due to their low- ligated molecules). For a systematic understanding of NC
temperature operation, which makes them particularly suitable catalysis, a dry fabrication method which reduces the complexity
for mobile vehicles. Although various technological advancements through the entirely clean nature of the synthetic procedure
are needed to upgrade FCs, the development of more active carried out under high vacuum conditions, facilitates the
catalysts is required for the cost reduction and the downsizing catalysis preparation of naked NCs;2,3,25–30 high surface specific
of FC units. Since the activity for the cathodic reaction, the oxygen activity (SA) of Pt46 NCs has been reported at high coverage.2
Despite numerous efforts, the correlation between activity and
structure has not been elucidated so far, which is crucial to
establish a reliable guiding principle for NC catalysts.
a Department of Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan.
We have demonstrated herein that the ORR activity of Pt6
sub-NCs supported on a glassy carbon (GC) electrode shows
superior activity compared to a standard Pt/C catalyst, the origin
of which is discussed in terms of the geometric and electronic
structures investigated via X-ray absorption fine structure (XAFS)
spectroscopy and density functional theory (DFT) calculations.
Pt NC catalysts were prepared by soft-landing of size-selected
NCs using a high-flux single-size NC generation system, nano-
jimas-NAP01,31,32 combined with a quadrupole mass filter
(Fig. S2, ESI†), where the collision energy of the NCs was kept
under 1 eV per atom to avoid fragmentation during the deposition.
In order to suppress agglomerations of NCs on the GC surface,
E-mail: nakajima@chem.keio.ac.jp
b New Field Pioneering Division, Toyota Boshoku Corp., 1-1 Toyoda-cho, Kariya,
Aichi 448-8651, Japan
c Department of Theoretical and Computational Molecular Science,
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
d Department of Applied Chemistry and Biotechnology, Graduate School of
Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
e Synchrotron Radiation Research Center, Nagoya University, Furo-cho, Chikusa,
Nagoya 464-8603, Japan
† Electronic supplementary information (ESI) available: Mass spectrum of Pt NC,
STEM image, XANES spectra, EXAFS fitting results, optimized structures of free
and adsorbed Pt NC by DFT calculations, and simulations for XAFS. See DOI:
10.1039/c9cc06327g
‡ Equal contribution.
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with decreasing ECSA for larger NPs.14 In this system, both the
MA and SA of Pt6 are higher than those for Pt NPs, which is in
sharp contrast to conventional NP catalysts.
We now discuss the origin of the superior activity of Pt6/GC
catalysts in terms of their geometric and electronic structures
through rigorous characterizations using extended XAFS (EXAFS)
and DFT calculations. The charge state for deposited Pt6 sub-NCs
is almost neutral based on analysis of the X-ray absorption near
edge structure (XANES) as shown in Fig. 2a, where the white-line
for the Pt L3 edge is almost identical with that for bulk Pt foil.
Surprisingly the Pt6/GC catalyst is robust against oxidation under
ambient conditions (Fig. S4, ESI†), suggesting that the structure
of Pt6 sub-NCs remains unchanged in EC experiments under an
oxygen atmosphere from the EXAFS measurements under argon.
The XAFS w(R) profile (Fig. 2b) clearly exhibits a radial distribution
peak for Pt–Pt bonds; Pt–Pt bond distance and its coordination
number (CN) for Pt6/GC are slightly smaller and much smaller
than those for the bulk Pt, respectively (Table S2, ESI†). In order to
characterize the Pt6 structure in detail, we have compared between
(1) experimental w(k) spectra (wobs(k)) extracted with the Athena
program35 and (2) those simulated for the calculated equilibrium
geometries by the FEFF code36 (wsim(k)). As a result of DFT
calculations using the VASP program,37,38 where the GC surface
was modeled with a graphene (Gr) structure, seven isomers
were found for Pt6/Gr (Fig. S5, ESI†) within a total energy
difference of 0.5 eV. Details of geometry optimization are
explained in the ESI.† Based on the matching of the phases
between wobs(k) and wsim(k) spectra for Pt6/GC, we can safely exclude
planar isomers (Pt6-1–3) due to the deviation at the higher k regime
(k 4 6 ÅÀ1) (Fig. S6, ESI†). Bent triangular isomers (Pt6-4–6) can
Fig. 1 A cyclic voltammogram under argon (a) and a hydrodynamic
voltammogram under oxygen (b) for Pt6/GC. The red area in (a) corre-
sponds to the desorption of hydrogen. Experimental conditions: concen-
tration of HClO4; 0.1 M and sweep rate; 50 mV sÀ1
.
the density of deposited Pt6 sub-NCs was kept under 0.5 ML
equivalents based on the density of a Pt(111) surface by assum-
ing that the GC electrode has a flat surface. In addition,
prepared catalysts were kept under an oxygen-free argon atmo-
sphere until just before the electrochemical (EC) measurements
to avoid oxidation of tiny NCs and contamination by organic
substances. Successful immobilization of Pt NCs was con-
firmed by high-resolution scanning transmission electron
microscopy as shown in Fig. S3 (ESI†).
Fig. 1a shows a representative cyclic voltammogram under
an argon atmosphere. Current for underpotential adsorption/
desorption of hydrogen is clearly observed around 0.05–0.3 VRHE
.
The electrochemically active surface area (ECSA) was evaluated
based on the reported procedure33 by integrating the hydrogen
desorption current using a factor of 210 mC cmPtÀ2. Surprisingly
the ECSA for Pt6 (Table 1) is ca. 0.8 times that of a commercial
standard Pt/C catalyst (TEC10E50E, TKK) with a crystallite size of
3 nm, which is a large deviation from the hypothetical surface
ratio for such tiny Pt6 sub-NCs. The origin of the decrease in the
ECSA is discussed based on its structure in a later section.
A hydrodynamic voltammogram under oxygen (Fig. 1b) represents
clear ORR activity despite the ultra-low loading of NC catalysts
using the reported procedure.34 Since high sensitivity measure-
ments for the present ultra-low loading catalyst are susceptible to
impurities, it is difficult to evaluate kinetic limiting current
density ( Jkin) from the Koutecky–Levich plot, and so the ORR Jkin
has been estimated using the Koutecky–Levich equation based
procedure reported by Yamamoto and co-workers.17,18 Despite the
relatively lower ECSA for Pt6, the MA at 0.9 VRHE for Pt6/GC is 74%
higher than that of the standard Pt/C catalyst. Furthermore, the
Pt6 SA at 0.9 VRHE is 2.3 times higher than that for the Pt/C
catalyst. For the typical Pt NP catalyst with a diameter larger than
3 nm, the MA increases with increasing ECSA, while SA increases
also be excluded due to a slight phase deviation at k 4 8 ÅÀ1
.
By considering the structure scaling (À2% to +2%) for the
Table 1 Catalytic properties for Pt6/GCa
ECSAb (m2 gPt
)
MAc (A mgPt
)
SAd (mA cmPt
)
À1
À1
À2
Fig. 2 Results of EXAFS spectroscopy for Pt6/GC. (a) XANES spectra that
have been offset vertically for clarity of presentation, (b) w(R) profile
Pt6/GC
Pt/Ce
54 (8)
71 (2)
0.52 (8)
0.30 (0)
0.96 (7)
0.42 (1)
obtained by Fourier transform of w(k) spectra in the range k = 5–12 ÅÀ1
,
a
and (c) observed w(k) spectra along with simulation for the most probable
isomer of Pt6-7. Simulation conditions: energy shift: +5.0 eV, distance
shift: +0.075 Å, Debye–Waller factor: 0.008. (d) Optimized structure for
Uncertainties are given in parentheses, referred to the corresponding
b
c
last digit. Evaluated from desorption current of hydrogen. Obtained
from kinetic limiting current at 0.9 VRHE
ing current at 0.9 VRHE and ECSA. Standard Pt/C catalyst (TEC10E50E). Pt6-7 on graphene from DFT calculations.
d
.
Obtained from kinetic limit-
e
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cuboctahedron or icosahedron. On the other hand, the Pt6-7
sub-NC has four outermost Pt atoms and two Pt atoms ligating
to carbon, producing a Psurf of 66%, which is almost compar-
able or at most 20% higher than that of the standard Pt/C
catalyst (d = 3 nm). Therefore, the effect of surface atom
exposure probably plays a minor role in the enhanced activity.
In the electronic structure of larger NPs, such as Pt309 (d = 2.43 nm),
the density of states and d-band-center energy look similar to those
for a bulk Pt surface based on DFT.43 In addition, atomic charge
distribution in Pt309 NC seems less polarized even after adsorption
on Gr, excluding the peripheral atoms.43 As a result, the ORR
activity of NPs is mainly influenced by Psurf values as summarized
in the previous literature,14 which has been believed to be a key
principle in the catalyst development. On the other hand, Pt6 is
Fig. 3 Atomic charge distribution of Pt6/Gr (isomer 7). (a) Bader charge
and (b) electron density difference iso-surface; charge depletion and
accumulation regions are shown in red and green, respectively.
optimized Pt6/Gr geometry, the above conclusion is thoroughly highly polarized (À0.15 to +0.13 e) after adsorption on the Gr
supported (Fig. S7, ESI†). Since both the phase and amplitude (Fig. 3a). As a result of charge redistribution in Pt6, the average of
are quantitatively matched in Fig. 2c, the most probable the d-orbital energy, corresponding to the d-band-center in the
structure for Pt6/GC is concluded to be the isomer Pt6-7 bulk, for negatively charged Pt sites is À1.47 eV from the Fermi
(Fig. 2d). Although the isomer Pt6-7 is not the most stable energy (eF), which is higher in energy than other Pt sites
one in the DFT calculations, the discrepancy may be explained (À1.81 to À1.66 eV). The d-orbital-center energy is much higher
by the occurrence of a different structure on the GC surface39 than the d-band center for larger NPs and the bulk (111)
from that on Gr.
surface.44 Based on our knowledge of the correlation between
The origin of the size-specific activity is discussed in terms the d-band-center and ORR activity,41 the enhanced MA for
of their electronic structure and the reported reaction mecha- Pt6/GC can be attributed to its electronic structure changed by a
nism for the ORR.40,41 A Bader charge analysis (Fig. 3a) shows large degree of charge separation in the tiny NCs. On the other
that Pt6 sub-NC is polarized on the Gr surface; two Pt sites hand, ECSA determined from hydrogen adsorption/desorption
with a low CN (CN = 3, Pt(6) and Pt(5)) are negatively charged is smaller than the larger NP of Pt/C. This probably originates
(q = À0.15) while the other Pt atoms are positively charged; two from site specific adsorption44 of hydrogen on the smaller NCs,
Pt not coordinated to Gr (q = +0.04) and two Pt coordinated to which also supports the idea of a large change in the surface
Gr (q = +0.09, +0.13). Charge redistribution upon adsorption chemical nature for Pt6 catalysts; Pt sites charged neutral
can be also identified from the electron-density-difference presumably act as adsorption sites for hydrogen. The nature
isosurface shown in Fig. 3b; charge accumulation regions are of site-specificity for sub-NCs, originating from a greater
spread over the Pt(5) atom and Pt(1)–Pt(5) bond. In the ORR flexibility of charge redistribution, will enable much higher
mechanism,40,41 dissociative adsorption of molecular O2 on activity by further alloying and strong support interactions
a Pt site is an important step to 4eÀ reduction for direct through electronic interaction (ligand and strain effects)16 in
formation of water, which results in higher ORR activity rather addition to the geometric ensemble effect.15
than 2eÀ reduction to form hydrogen peroxide through mole-
In conclusion, we have demonstrated superior mass-specific
cular O2 adsorption. Electron density tends to accumulate at and surface-specific activities of Pt6 subnanoclusters for oxygen
the apex site in NCs and the negatively charged site is electroreduction, which are 1.7 and 2.3 times higher than the
reactive.42 In a kinetic picture, the first electron transfer step conventional standard Pt/C catalysts with a crystallite diameter
to an O2 molecule is an important step, where cleaving the O–O of 3 nm. Based on rigorous structure analyses by X-ray absorption
bond forms adsorbed atomic oxygen (O*), although the rate fine structure spectroscopy and density functional theory calcula-
determining step for the 4eÀ reduction pathway in a thermo- tions, we have identified the geometric structure of Pt6. The origin
dynamic picture is reduction of adsorbed hydroxy radical of simultaneous enhancements of SA and MA for ORR, which is
species (OH*).40 To activate the adsorbed molecular O2 through unusual in pure metallic Pt catalysts, is attributed to larger charge
electron transfer, the d-band center level for a Pt adsorption site redistribution in the tiny sub-NC, leading to pushing the d-band-
plays a crucial role, because a relatively higher d-band-center center energy up compared to the NP catalysts. The present study
with respect to bulk Pt(111) enhances the activity.41 In fact, the provides an important guideline for catalyst developments and
d-band-center closely correlates with a partial charge on the opens ‘‘precise NC catalysis chemistry’’ for a future FC technology.
site; a partial negative charge pushes the d-band-center up.
The authors thank Dr Tomoyuki Nagai for the advice on
Two possibilities can be considered as the reasons for the electrochemical measurements, Dr Kouta Iwasaki for discussion
high activity of Pt6/GC; (i) the high surface area of Pt6 and and comments on the manuscript, Mr Motoki Kondo for the
(ii) the site-specific activation of oxygen molecules as described experimental support, and Prof. Dr Hidehiro Yasuda and
above. The ratio of surface atoms (Psurf) in NPs depends on the Prof. Dr Kazuhisa Sato for STEM measurements. Development
diameter; Psurf values are 45% for Pt561 (d = 2.97 nm) and 63% on nanojimas apparatus was supported by Ayabo Corp. The
for Pt147 (d = 1.89 nm) based on a spherical geometry of computations were partly performed using Research Center for
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ments were supported by ‘‘Advanced Characterization Nano-
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H. T. and A. N. are inventors on JAPAN patent JP 5493139,
submitted by JST agency and Ayabo Corp., which covers the
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