Platinum-Group-Metal High-Entropy-Alloy Nanoparticles

Platinum-Group-Metal High-Entropy-Alloy Nanoparticles

Article

Dongshuang Wu,* Kohei Kusada,* Tomokazu Yamamoto, Takaaki Toriyama, Syo Matsumura,

Shogo Kawaguchi, Yoshiki Kubota, and Hiroshi Kitagawa*

Cite This: https://dx.doi.org/10.1021/jacs.0c04807

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sı Supporting Information

ABSTRACT: The platinum-group metals (PGMs) are six neighboring elements

in the periodic table of the elements. Each PGM can eﬃciently promote unique

reactions, and therefore, alloying PGMs would create ideal catalysts for complex

or multistep reactions that involve several reactants and intermediates. Thus,

high-entropy-alloy (HEA) nanoparticles (NPs) of all six PGMs (denoted as

PGM-HEA) having a great variety of adsorption sites on their surfaces could be

ideal candidates to catalyze complex reactions. Here, we report for the ﬁrst time

PGM-HEA and demonstrate that PGM-HEA eﬃciently promotes the ethanol

oxidation reaction (EOR) with complex 12-electron/12-proton transfer

processes. PGM-HEA shows 2.5 (3.2), 6.1 (9.7), and 12.8 (3.4) times higher

activity than the commercial Pd/C, Pd black and Pt/C catalysts in terms of

intrinsic (mass) activity, respectively. Remarkably, it records more than 1.5 times higher mass activity than the most active catalyst to

date. Our ﬁndings pave the way for promoting complex or multistep reactions that are seldom realized by mono- or bimetallic

catalysts.

INTRODUCTION

entropy eﬀect, which can crucially enhance material stability.

■

(

Very recently, HEA NPs were prepared under ambient

pressure by either the carbon-thermal-shock method or

Platinum-group metals (PGMs), i.e., ruthenium (Ru), rhodium

Rh), palladium (Pd), osmium (Os), iridium (Ir), and

spark ablation, both of which require special experimental

apparatuses. Here, we report the ﬁrst example of HEA NPs of

all six PGMs (denoted as PGM-HEA) using a facile wet

chemical synthesis, hypothesizing that HEA NPs will be

promising catalysts for complex reactions because HEA NPs

intrinsically possess various active sites and high stability. We

investigated the electrocatalytic performance of PGM-HEA for

the ethanol oxidation reaction (EOR) with a complex 12-

electron/12-proton transfer process as a ﬁrst demonstration

and found that PGM-HEA can eﬀectively catalyze the EOR

with record-high activity and high stability.

platinum (Pt), are crucial to our daily lives, from household

items to industrial catalysis. One in four of the goods

manufactured today either contain PGMs or are produced

through processes promoted by PGM catalysts. PGMs are

located next to each other in the periodic table of the elements,

and each PGM can promote unique reactions. For example, Ru

can eﬃciently catalyze carbon−heteroatom (such as N, O, P,

S) bond formation, Rh is famous for asymmetric catalysis, Pd

can promote C−C formation through Suzuki−Heck coupling,

Os shows advantages in (de)hydrogenation reactions, Ir is the

most eﬃcient catalyst for water oxidation, and Pt can

eﬃciently promote the oxygen reduction reaction. Simple

reactions that include a few proton/electron transfer processes

can be eﬃciently catalyzed on mono- or bimetallic surfaces.

However, these surfaces usually do not work for complex

reactions that contain multiple proton/electron transfer

processes involving various reactants and intermediate species.

The most favorable adsorption sites for these species should be

diﬀerent, in general, and therefore it is assumed that a catalyst

with a variety of adsorption sites could eﬃciently promote

complex reactions.

RESULTS AND DISCUSSION

■

PGM-HEA was obtained by adding a mixed aqueous solution

of six metal precursors in an equimolar ratio (approximately

16.7%) into a preheated triethylene glycol solution (TEG)

containing poly(N-vinyl-2-pyrrolidone) (PVP, K30, M_w

≈

0000) as a protecting agent at a temperature of 230 °C. The

TEG plays the role of both reducing agent and solvent. The

Received: May 2, 2020

High-entropy alloys (HEAs), which are deﬁned as at least

ﬁve elements in near-equiatomic concentrations (recently

reported as 5−35 atom % for each constituent), boost the

discovery of new materials and phenomena, especially in

structural bulk materials. The conﬁgurational entropy increases

as the number of elements increases, known as the high-

XXXX American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

a and Figure S2 ). The atomic-resolution scanning TEM

Article

key point of this method is a concurrent reduction of all the

1,12

metal precursors to form solid−solution alloys.

The

transmission electron microscopy (TEM) image revealed that

PGM-HEA NPs were homogeneously dispersed with a narrow

size distribution (3.1 ± 0.6 nm, Figure S1a). The Rietveld

reﬁnement of a synchrotron X-ray diﬀraction pattern shows

that PGM-HEA has a single-phase face-centered-cubic (fcc)

structure with a lattice parameter of a = 3.8473(3) Å (Figure

Figure 1. (a) Rietveld reﬁnement of the synchrotron XRD pattern of

PGM-HEA in the range of 2θ = 10−70°. The raw data, ﬁtting proﬁle,

diﬀerences, and background are shown as black open circles and red,

light purple, and gray solid lines, respectively. Wavelength: λ =

.62938(6) Å. (b) HAADF-STEM image of the as-prepared PGM-

HEA and the corresponding EDX maps.

Figure 2. Core-level XPS spectra showing the existence of six

elements of PGM-HEA in the regions of (a) 273−350 eV and (b)

(

STEM) images show an interplanar spacing of 0.23 nm for

5−88 eV. (c) Composition analysis results of PGM-HEA based on

the {111} facets and twinned structures (Figure S3), which

further supports an fcc structure. The high-angle annular dark-

ﬁeld STEM (HAADF-STEM) images and their corresponding

energy-dispersive X-ray spectroscopy (EDX) maps in both low

diﬀerent techniques, XRF (blue) and XPS (orange).

Figure S4) and high magniﬁcation (Figure 1b) regions show

Os, Ir, and Pt are clearly seen (Figure 2b, blue dashed lines).

The XPS core-level spectrum of each element was ﬁtted with

one component having a binding energy in line with its bulk

that each of the elements is homogeneously and randomly

distributed throughout the entire nanoparticle: i.e., they form a

single-phase solid−solution alloy.

The local structure and composition of PGM-HEA were

further characterized by a combination of electron microscopy,

X-ray ﬂuorescence (XRF), and X-ray photoelectron spectros-

copy (XPS). The XRF result reveals that the atomic percentage

of each element in PGM-HEA is approximately 16.7 atom %

metal (Figure S5 and Table S2); i.e., all of the elements are

in the metallic state. Here, XPS has a probe depth of

approximately 1 nm estimated from the beam energy of Mg Kα

(photon energy 1253.6 eV). Therefore, we can estimate the

surface composition of each element by integrating the

corresponding XPS peaks. As shown in Figure 2c, the atomic

percentage of each element at the surface given by XPS is quite

similar to the bulk composition given by XRF. This veriﬁes the

homogeneity of the alloy and excludes surface segregation and

phase separation, which is consistent with the EDX analysis.

Moreover, we note that the presence of each element on the

surface can provide a variety of adsorption sites for catalysis.

Monometallic and equimolar ternary, quaternary, and

quinary alloy NPs were also prepared as control groups for

(Table S1). Next, XPS as a surface-sensitive probe was

employed to reveal the homogeneity of each of the elements

on the surface. In the binding energy (BE) region of 280−350

eV (Figure 2a), the core-level spectra of 4d metals including

Ru 3d, Rh 3d and Pd 3d doublets were observed as the main

peaks with high intensities (blue dashed lines) along with the

low-intensity Pt 4d and Ir 4d spectra (black dashed lines). In

the BE range of 60−80 eV, the characteristic 4f doublets from

Journal of the American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 3. Comparison of the EOR j_s_p_e_c_i_f_i_cvalues of PGM-HEA with those of (a) the as-synthesized monometallic NPs and (b) the commercial

catalysts. The arrows show the scan directions of CVs. Comparison of the (c) j_s_p_e_c_i_f_i_cand (d) j_m_a_s_s₁ values of PGM-HEA at potentials of 0.45 and 0.6

V in the forward scan with those of the monometallic catalysts and the reported Au@PtIr/C, respectively. The current densities of Pt/C are

magniﬁed 10 times for easy comparison. (e) Comparison of the initial CV curve for EOR and the CV curve after 50 cycles of PGM-HEA. The

current is normalized by the geometric area of the electrode. (f) CV curves in the positive scan of PGM-HEA and other monometallic NPs showing

diﬀerent electrochemical CO stripping behaviors. The peak potentials are given. Note that the peaks of Ru and Os might come from dissolution

rather than CO stripping away.

catalytic tests (Figures S1 and S2). All of them have a size of

approximately 2−5 nm.

on the EOR suggested that C−C splitting, which is the key

step of the 12e process, occurred at a more negative potential

19,21

The EOR is the anodic reaction for direct ethanol fuel cells

that have a high energy density (theoretically 26.8 MJ/kg) for

portable power applications such as mobile phones, laptop

in comparison to that of the 4e process.

The onset

potential (here deﬁned as the potential where the anodic

current density is 1/20 of the anodic peak current density) of

PGM-HEA (around 0.28 V at 50 mV/s and 0.31 V at 20 mV/

s) is approximately 200 and 240 mV more negative than those

of Au@PtIr/C (around 0.51 V at 20 mV/s) and Pd NPs

(approximately 0.52 V at 50 mV/s), respectively. This strongly

suggests that a preadsorbed species such as ethanol can quickly

react on the PGM-HEA surface. We further compared the

j_s_p_e_c_i_f_i_cand j_m_a_s_svalues at 0.45 and 0.60 V in the forward scan.

PGM-HEA shows 2.5−30.1 times higher current density in

comparison to all of the synthesized and commercial

monometallic catalysts measured (Figure 3c and Figure S7).

For example, PGM-HEA is 2.5 (3.2), 6.1 (9.7), and 12.8 (3.4)

times more active than the commercial Pd/C, Pd black, and

Pt/C catalysts in terms of j_s_p_e_c_i_f_i_c(j_m_a_s_s) at 0.45 V, respectively.

Signiﬁcantly, PGM-HEA shows current density comparable

to and 1.5 times higher than the most active Au@PtIr/C

computers, and forklifts. From a fundamental research point

of view, the EOR is of great interest because it is a multistep

reaction, and the reaction path is highly dependent on the

metal (oxide) species formed and their surface structures.

Complete ethanol oxidation is a 12-electron/12-proton

−

reaction (CH CH OH + 3H O = 2CO + 12H + 12e )

that involves cleavage of the C−C bond and multiple

dehydrogenation and oxidation steps. However, most catalysts,

such as monometallic Pd and Pt and Pd/Pt-based bimetallic

alloys, can only catalyze processes with 4e transfer, producing

16−18

acetic acid or acetate.

Quite recently, to our knowledge,

the trimetallic nanocatalyst Au@PtIr/C recorded the best EOR

activity, showing a high selectivity for the 12e process. We

tested the performance of PGM-HEA and compared it with

that of the reported Au@PtIr/C under similar conditionsa

.0 M KOH solution with 1.0 M ethanol. The as-prepared

catalyst at 0.45 and 0.6 V, respectively, which was conﬁrmed

to catalyze the EOR in a 12e process (Figure 3d). In

comparison with previous studies (see details in the summary

table in ref 19), PGM-HEA shows record-high activity,

particularly at low potentials. To further conﬁrm that the

high activity of PGM-HEA comes from the 12e process, a CO₂

detector (detection limit of 0.0005 mL/min in the air) and CO

detector (detection limit of 5 ppm) were used to identify the

monometallic NPs and commercial catalysts Pt/C (20 wt % in

Pt, Alfa), Pd/C (10 wt % in Pd, Sigma-Aldrich), and Pd black

(

Sigma-Aldrich) were also tested for comparison.

We compared the intrinsic EOR activities by two metrics:

the speciﬁc activity (j_s_p_e_c_i_f_i_c, where the current is normalized by

the electrochemical active surface area (ECSA)) and the mass

activity (j_m_a_s_s, where the current is normalized by the mass of

PGMs). The ECSA is obtained by a Cu underpotential

generated gas product (CO or CO) at 0.45 V (Video S1). In

deposition method (Figure S6). As shown in Figure 3a,b,

PGM-HEA showed the highest j_s_p_e_c_i_f_i_cvalue among the tested

catalysts over the whole potential range. Spectroscopic studies

the case of PGM-HEA, a signiﬁcant amount of CO was

detected and (the red signal and buzzing sound, Video S1 ) and

gas bubbles were clearly observed on the anode (Figure S8a,b).

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

). The CV of PGM-HEA after 50 EOR scans and

Article

In contrast for Pd, there was a negligible amount of CO (no

on Pt catalysts by a low-volume coverage of Os. Moreover,

PGM-HEA shows the highest current density in the full

potential range, which suggests that PGM-HEA is capable of

being able to most eﬃciently catalyze the oxidation of multiple

intermediates among these catalysts. On the basis of these

results, combining all six elements can provide various

adsorption sites that greatly improve the EOR eﬃciency and

promote the 12e process. That is, the coordination environ-

ment of the surface metal sites can be continuously tuned by

changing the composition; therefore, a balance of the various

adsorption sites and optimal adsorption energies for the

reactants and intermediates can be realized to obtain high

eﬃciency in complex reactions.

buzzing sound, Video S2 , no bubbles, Figure S8c,d). All of

these results suggest that PGM-HEA promotes the 12e process

of the EOR.

To understand the high EOR performance on PGM-HEA

NPs, we compared the cyclic voltammetry (CV) curve of each

monometallic catalyst in the presence of ethanol with the CV

curve in the absence of ethanol. As shown in Figure S9c,f , Pd

and Pt catalysts are very active for the EOR, similar to the

16,17

results in previous studies.

However, they show a much

more positive oxidation peak potential in comparison to the

other metals because of the poisoning eﬀect caused by

carbonyl functional groups such as −CO and −COOH

(

denoted as (CO) ) or CH generated during the

The stability of PGM-HEA in the EOR was examined by

continuous CV scans in the electrolyte with ethanol. As shown

in Figure 3e, after 50 cycles, there is no change in the CV

curves. In contrast, after 50 cycles, Pd sharply decreased in

activity by 3.5 times, which might be ascribed to the strongly

adsorbed CO or the formation of stable Pd oxides causing

deactivation of the catalyst (Figure S12). In addition,

chronoamperometry was also adopted as another method to

evaluate the stability of PGM-HEA. We ﬁxed the electrode

potential at 0.45 V and monitored the current change with

time (Figure S13). The steady current density of PGM-HEA

was 27.6 times higher than that of Pd NPs. CO electrochemical

oxidation was conducted to provide further insight into the

stability. As shown in Figure 3f, the most active Pd catalyst

has the most positive CO oxidation peak located at 1.05 V,

which suggests strong adsorption of CO on the Pd surface. In

contrast, the PGM-HEA catalyst shows a CO oxidation peak at

0.62 V, which is an extraordinarily negative shift of 0.16−0.46

V in comparison to the monometallic PGMs, excluding Ru.

The HAADF-STEM image and its EDX maps show that the

alloy structure is maintained after multiple CV scans (Figure

6,17,21

reaction.

The Rh catalyst has quite a complicated

EOR behavior, showing a total of four peaks (Figure S9b). In

particular, Rh shows the ﬁrst oxidation peak at low potential,

but with low current density. The Ir catalyst shows a broad

plateau from 0.2 V_R_H_Ebut with a small current density in the

presence of ethanol. These results are consistent with previous

experimental and theoretical studies indicating that Ir and Rh

catalysts can promote dehydrogenation or C−C cleavage at a

19,22−24

low potential but that further oxidation is diﬃcult.

they show almost overlapping CVs with/without ethanol, Ru

and Os alone are not active for the EOR (Figure S9a,d ).

However, the adsorption of OH, which is preferred on Os and

Ru sites at a negative potential, could greatly help facilitate the

oxidation of (CO) on adjacent metal sites, especially in

ads

4−26

bases.

Moreover, we found that the physical mixture of all

six elements shows much lower activity in comparison to

monometallic Pd, Pt, and Rh and is inactive in comparison to

PGM-HEA (Figure S10). This suggests that eﬁcient EOR

comes from the atomic conﬁgurations obtained by all six

elements mixing as a solid−solution alloy.

In addition, we compared the EOR activity of PGM-HEA

with those of equimolar ternary PdPtRh (because these three

are the most active metals), quaternary IrPdPtRh (because Ir is

the fourth most active), and quinary IrPdPtRhRu (to

understand the role of Ru and Os). The EOR current density

in the higher potential range is mainly a result of the oxidation

of adsorbed C (minor) and C (major) species, which are the

main products of monometallic catalysts (such as CO, CH_x_,

CH COOH, CH CHO, etc.), while C−C breaking happens at

14,15

a lower potential range.

First, PdPtRh shows enhanced

activity at potentials similar to those of Pd and a much lower

oxidation peak in the backward scan (Figure S11a); thus,

alloying Pd with Pt and Rh would reduce the coverage of

poisoning species. Next, IrPdRtRh has current densities

comparable to those of PdPtRh but at lower potentials,

which implies that the Ir acts to tune the adsorption energies of

intermediates on the active sites by the formation of a solid

solution. On the other hand, if we look into the activity in a

lower potential range from 0.26 to 0.4 V (Figure S11b), it is

found that the current density increased in the order PGM-

HEA ≫ IrPdPtRhRu > IrPdPtRh ≈ PdPtRh ≥ Pd. These

control groups suggest that, although Ru and Os do not have

higher EOR activity alone, they might greatly facilitate the

Figure 4. (a) HAADF-STEM image of PGM-HEA after multiple

EOR scans and the corresponding EDX maps showing each element:

(b) Ru; (c) Rh; (d) Pd; (e) Os; (f) Ir; (g) Pt.

chronoamperometry testing was quite similar to that of the

freshly prepared sample (Figure S14 ). This result suggests that

there is little to no change on the PGM-HEA surface during

the reaction. These results suggest that the surface of PGM-

HEA is highly stable, showing excellent tolerance to CO.

cleavage of C−C bond and C species oxidation at low

CONCLUSION

potentials. The C species (especially (CO) ) oxidation can

■

ads

be accelerated by the adsorption of −OH, while Ru is

reported to be a highly eﬃcient metal for −OH adsorption at a

We have demonstrated the ﬁrst example of an HEA containing

all PGMs. X-ray-based techniques and electron microscopy

studies veriﬁed the homogeneous fcc-phase alloy structure.

Futhermore, PGM-HEA shows record-high activity for the

4,26

low potential.

Although Os is seldom used for electro-

chemistry, there is a report showing improved C−C breaking

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.0c04807.

Article

EOR, implying a 12e transfer process. The six elements

together on the surface might provide an optimal surface

conﬁguration that has ideal adsorption/desorption sites for the

EOR. Given the facile wet-chemistry method employed, it is

possible to easily tune the compositions and structure of HEAs

for various complex reactions.

ogy of the Japanese government. Synchrotron XRD measure-

ments were performed at SPring-8 under proposal No.

2018B1480.

ABBREVIATIONS

■

platinum-group metal,PGM; high-entropy alloy,HEA; nano-

particles,NPs; ethanol oxidation reaction,EOR; cyclic voltam-

metry,CV

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

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Dongshuang Wu − Division of Chemistry, Graduate School of

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Kohei Kusada − Division of Chemistry, Graduate School of

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Hiroshi Kitagawa − Division of Chemistry, Graduate School of

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Tomokazu Yamamoto − Department of Applied Quantum

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Takaaki Toriyama − The Ultramicroscopy Research Center,

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Syo Matsumura − Department of Applied Quantum Physics and

Nuclear Engineering and The Ultramicroscopy Research Center,

Kyushu University, Fukuoka 819-0395, Japan

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