Heterogeneous rhodium-tin nanoparticles: Highly active and durable electrocatalysts for the oxidation of ethanol

Article abstract of DOI:10.1039/c5ta02991k

Facile synthesis of Rh-Sn catalysts for the electrocatalytic oxidation of ethanol is carried out via a surfactant-free microwave-assisted method. The bifunctional mechanism and electronic modification with C-C bond splitting enable this electrocatalyst to be remarkably active and durable at high fuel concentrations, which allows for a significant reduction in the volume and weight of the fuel cell system.

Full text of DOI:10.1039/c5ta02991k

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Heterogeneous rhodium–tin nanoparticles: highly
active and durable electrocatalysts for the
oxidation of ethanol†
Cite this: DOI: 10.1039/c5ta02991k
a
b
a
b
*
Minjeh Ahn, In Young Cha, Joong Kee Lee, Sung Jong Yoo
cd
*
and Yung-Eun Sung
Received 24th April 2015
Accepted 17th July 2015
Facile synthesis of Rh–Sn catalysts for the electrocatalytic oxidation of ethanol is carried out via a
surfactant-free microwave-assisted method. The bifunctional mechanism and electronic modification
with C–C bond splitting enable this electrocatalyst to be remarkably active and durable at high fuel
concentrations, which allows for a significant reduction in the volume and weight of the fuel cell system.
DOI: 10.1039/c5ta02991k
www.rsc.org/MaterialsA
Ethanol is an attractive fuel in fuel-cell technology because it is functional theory calculations, cleavage of the C–C bond of
more easily stored and transported than gaseous fuels, and ethanol is the step with the highest energy barrier, therefore,
because it is less toxic and more energy dense than other liquid CO2, formic acid, and formaldehyde are minor products in the
fuels.1 Moreover, ethanol is abundant, and can be obtained oxidation of ethanol.10 To address the issue of low fuel effi-
from sugar- and cellulose-containing raw materials and from ciency, ternary nanoparticles additionally alloyed with Rh and Ir
inedible plants.2 However, catalytic rates obtained in the elec- were recently employed, and it was found that Rh and Ir can
trocatalytic oxidation of ethanol are generally sluggish; it is of cleave the C–C bond.11–14 IR spectra revealed that bridge-bonded
importance that they are increased. Various Pt-transition-metal CO species are mainly formed on Rh during ethanol oxidation,
alloy nanoparticles have been researched with regard to their suggesting that Rh is more effective than Ir in breaking the C–C
performance in ethanol oxidation, and Pt–Sn catalysts have bond.15 Nonetheless, fuel efficiency and kinetics had improved;
been widely known.3,4 Carbon monoxide (CO) is one of the Pt-based ternary catalysts are expensive and also require
several intermediates that are generated when ethanol is complicated processes. Binary catalysts that avoid the use of Pt
oxidized. CO is strongly adsorbed on the Pt atoms due to the have been proposed in an alkaline system where the over-
overlap considerations between the Pt and CO orbitals, result- potentials are much smaller, however, these materials that
ing in the inhibition of further oxidation reactions.5–7 In the Pt– promote the fuel efficiency via C–C bond splitting have not been
Sn system, Sn plays a critical role in facilitating the oxidation of reported.15,16 Furthermore, both Rh and Sn have been only
CO species by transferring Sn-adsorbed oxygen-containing focused on the promotional effect toward ethanol oxidation,
species to the poisoned Pt surface.5 Although this catalytic because even phase-controlled nanoparticles with high-index
system provides higher reaction rates and is more cost-effective, facets exhibited too poor activities.13,16–22
the fuel efficiency is still low. Most of the ethanol molecules are
Herein, we introduce that electronic modication of the Rh–
converted into acetic acid and acetaldehyde by partial oxidation Sn nanoparticles remarkably improves the kinetics toward
(2 to 4 electrons produced per molecule of ethanol), and only electrocatalytic oxidation of ethanol in an alkaline electrolyte
small amounts of CO2 are formed by complete oxidation (12 and that the catalyst remains active at high concentrations of
electrons produced per molecule of ethanol), which signies ethanol. These binary catalysts achieve outstanding catalytic
the deterioration of fuel utilization.8,9 According to density properties and durability with ethanol C–C bond splitting.
Uniform Rh–Sn nanoparticles are readily synthesized using a
microwave-assisted reaction; no surfactant is required, which
inhibits the access of reactants owing to the adsorption of
aGreen City Technology Institute, Korea Institute of Science and Technology, Seoul,
136-791, Korea
bFuel Cell Research Center, Korea Institute of Science and Technology, Seoul, 136-791, organic molecules onto the active sites, thus the synthesis and
Korea. E-mail: ysj@kist.re.kr; Fax: +82-2-958-5199; Tel: +82-2-958-5260
cSchool of Chemical and Biological Engineering, Seoul National University, Seoul, 151-
post-treatment procedure can be simplied.23
Polarization curves for the oxidation of ethanol on various
742, Korea. E-mail: ysung@snu.ac.kr; Fax: +82-2-888-1604; Tel: +82-2-880-1889
electrocatalysts were acquired in 0.5 M and 6.0 M ethanol
dCenter for Nanoparticle Research, Institute for Basic Science, Seoul, 151-742, Korea
electrolytes (Fig. 1a). Carbon-supported Sn nanoparticles
† Electronic supplementary information (ESI) available: Experimental details,
display negligible reactivity, which means that Sn on its own
cyclic voltammetry, chronoamperometry, electrochemical analysis, HR-TEM
cannot effect the oxidation of ethanol. Likewise, Rh in the
images, particle size distribution and in situ ATR-FTIR data. See DOI:
10.1039/c5ta02991k
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absence of Sn also shows a poor activity. Interestingly, the Rh– according to the Butler–Volmer equation.29 The slopes of Rh
Sn nanoparticles are highly active for the electrocatalytic catalysts decrease sharply when the concentration of ethanol
oxidation of ethanol. The electrocatalytic activities increased increases above 0.1 M, whereas those of the Rh–Sn catalysts
and the onset potentials shied to lower values with increasing increase with an increase in the concentration of ethanol. In
Sn content in Rh–Sn catalysts. The negative shi in onset particular, Rh3Sn7/C maintains a positive slope even at high
potentials is the result of the bifunctional mechanism.4,24 Since concentrations of ethanol, which indicates that the reaction
the hydroxyl groups are easily formed on the Sn surface at lower rates of the intermediates have enhanced relative to other
potentials, an increased Sn surface leads to increased amounts catalysts.
of adsorbed hydroxyl groups which were more oen transferred
The morphology of the electrochemically most active catalyst
to the poisoned Rh active sites. More surprising results were Rh3Sn7/C was investigated (Fig. 2a) using high-resolution
encountered in 6.0 M ethanol solution, a concentration at transmission electron microscopy (HR-TEM) combined with
which most catalysts are deactivated because of the excessive energy-dispersive X-ray spectroscopy (EDS). A uniform distri-
adsorption of CO intermediates and insufficient access of bution of metallic nanoparticles on the surface of the carbon
hydroxyl groups on the catalytic sites. Conventional catalysts support was accomplished using a surfactant-free microwave-
usually suffer from a decrease in activity when the concentra- assisted method. Since a lot of defects and functional groups
tion of ethanol increases above 2 M,25,26 while the Rh3Sn7/C adsorbed on the surface of the carbon support serve as stabi-
catalyst exhibits a signicantly high current density even in 6.0 lizing agents, it could be possible to attain highly uniform
M fuel electrolyte. This is very attractive from a commercial nanoparticles without any organic molecules. Furthermore,
perspective, because highly active and endurable catalysts can microwave irradiation enables the swi and simultaneous
lead to a signicant reduction in the volume and weight of the nucleation of reactive precursors by a subsequent heating
system. The ratios of the forward and backward peak current mechanism.23 It could be also benecial for obtaining the
densities (Fig. S1†) provide information on the CO tolerance of uniformity of nanoparticles so that ethylene glycol as a solvent
the catalysts; higher ratios correspond to greater tolerance to can efficiently absorb microwave energy and convert into heat,
CO intermediates.27 From the calculation, the ratios increase which has the highest loss tangent value among various
with increasing Sn content. The ratios of monometallic catalysts solvents. The loss tangent is expressed as the ratio of dielectric
were lower than 1.0, while those of Rh3Sn7/C were considerably loss and dielectric constant, which implies the ability of a
enhanced to 8.42 and 6.13 in 0.5 M and 6.0 M ethanol electro- solvent to convert electromagnetic energy into heat. The average
lytes, respectively. Electrochemical surface areas calculated particle size of Rh3Sn7/C was approximately 3.0 nm, and
from the carbon monoxide displacement method of commer- nanoparticles had slightly faded relative to monometallic cata-
cial Pt/C, Rh/C, Rh9Sn1/C, Rh6Sn4/C and Rh3Sn7/C were 40.3, lysts due to the deformation of the crystal structure between the
28.9, 37.2, 49.0 and 37.2 m2 gÀ1
,
respectively.28 Chro- face-centered cubic structure (Rh) and tetragonal structure (Sn)
noamperometry (Fig. S3†) was conducted at a constant potential
of 0.3 V for 1 hour to assess the durability of the catalysts, and a
substantially high steady-state current density was observed for
Rh3Sn7/C. Higher steady-state current density can be also
observed in comparison with the fresh Pt/C catalyst, although
the activity was diminished aer the accelerated stability test
(Fig. S4†). Initial and steady-state activities were summarized
and compared with those of Pt/C (Fig. 1b). Not only did we
obtain high initial current densities, but they are also main-
tained at the steady state. It represents that Rh3Sn7/C exhibits a
highly active and durable electrochemical performance for the
oxidation of ethanol even at high concentrations of fuel. These
characteristics are attributed to faster charge-transfer kinetics
and lower overpotentials. For both ethanol solutions, over-
potentials (Tafel slopes, Fig. S5†) calculated from the quasi-
steady state curves were lower for the Rh–Sn catalysts than for
the monometallic catalysts, indicating that the charge transfer
kinetics were faster on bimetallic catalysts.17 Tafel slopes of Rh–
Sn catalysts slightly increased at high concentrations of ethanol,
while that of the monometallic catalyst increased up to 40%,
Fig. 1 (a) Voltammetric profiles for the electrocatalytic oxidation of
ethanol in an oxygen-free solution containing 0.1 M potassium
which means that the adsorption and transfer of hydroxyl hydroxide and (left) 0.5 M or (right) 6.0 M ethanol at a scan rate of 20
mV sÀ1. (b) Current densities of commercial Pt/C and Rh3Sn7/C in 0.5
M and 6.0 M ethanol electrolytes, at the initial stage and after appli-
groups are enough to oxidize adsorbed CO intermediates on
bimetallic nanoparticles. This can be attributed to abundant Sn
atoms where the hydroxyl groups are prone to adsorb. The
reaction rates (Fig. 1c) were also determined from the rela-
cation of
a constant potential of 0.3 V for 1 hour (chro-
noamperometry). (c) Plot of log(current density) versus log(ethanol
concentration) of Rh–Sn catalysts and Pt catalyst in a 0.5 M ethanol
tionship between the concentration and the current density electrolyte at 0.4 V with a scan rate of 1 mV sÀ1
.
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and defects of the oxidized surface (Fig. S6†). The EDS results interactions between metals. The Rh 3d core-level peak of Rh–
demonstrated that Rh and Sn atoms co-exist in the nano- Sn/C shis to higher energies as the Rh content increases,
particles (Rh, 22.3%; Sn, 77.7%). With regard to the X-ray which is attributed to changes in the electron density of the
diffraction (XRD) experiments, the diffraction peaks of Rh metal and in the bonding energy between metal and adsorbed
negatively shied without separation as the Rh content molecules, as an indicator of alloying.34 According to the
decreases (Fig. 2b). This is due to the formation of an alloy with effective-medium theory, the upshi in binding energy can be
Sn; the lattice expands because of the larger lattice distance of induced by a decrease in the number of neighboring Rh atoms
Sn.30,31 Since Sn is reluctant to form wide solid solutions with around a single Rh atom.35–37
Rh, heterogeneous Rh–Sn nanoparticles are formed via a
microwave-assisted reaction without any surfactants.
Rh K-edge of X-ray absorption near-edge spectroscopy
(XANES) on the Rh–Sn catalysts was performed to indirectly
In order to elucidate the composition and the chemical state estimate the shis in the electron density of metal. Peak
of the Rh–Sn nanoparticles in detail, high-resolution X-ray intensities at the white line increase as the Rh content
photoelectron spectroscopy (HR-XPS) employing two incident decreases, which implies a concomitant decrease in the elec-
photon energies was used. Based on the universal curve, the tron density at the Rh atom (Fig. 3). This means that the
surface-sensitive information and bulk-dominant properties tendency in interactions between Rh and the adsorbates
can be attained separately by controlling the incident photon strengthens on the nanoparticle, resulting in easier adsorption
energies.32 The XPS peaks of Rh 3d (Fig. 2c) and Sn 3d (Fig. 2d) of ethanol molecules (relative to pure Rh, for which poor
were deconvoluted to the metallic states (Rh0 and Sn0) and the ethanol oxidation reactivity was observed in the polarization
oxidized states (RhIII and SnIV). The ratios of Rh0 in Rh/C, curve).38,39 And these adsorbed CO species on Rh can be readily
Rh6Sn4/C, and Rh3Sn7/C were found to be 77.6, 98.9, and 84.9%, oxidized by abundant hydroxyl groups on nearby Sn atoms.
respectively, when an incident photon energy of 630 eV was Although the spectra of Sn-abundant samples resemble those of
used, whereas they were found to be 99.3, 97.1, and 95.2%, oxides due to the increased oxidation state of Rh evaluated from
respectively, at 1000 eV. The Rh surface region of Rh/C is further the XPS depth-prole previously, up-shis of binding energy in
oxidized compared to that of other catalysts, which is a result of XPS and negative shis in XRD demonstrate that electronic
the absence of lateral repulsion by hydroxyl groups adsorbed on modication occurs at the Rh atom coordinated by Sn atoms.40
Sn nearby Rh atoms.33 With regard to Sn, the SnIV in Rh6Sn4/C, For the Rh–Sn electrocatalysts, therefore, the reinforced
Rh3Sn7/C, and Sn/C was found to be 95.3, 99.8, and 99.9%, adsorption ability between metal and molecules (electronic
respectively, when an incident photon energy of 630 eV was modication) and enhanced oxidation probability on the Rh
used (82.2, 90.8, and 92.9%, respectively, at 1000 eV). The XPS surface with adjacent SnO2 (bifunctional effect) may lead to a
results demonstrate that metallic Rh and oxidized Sn dominate superior ethanol oxidation reactivity.
in the heterogeneous nanoparticles and that the metallic
To evaluate the yield of the complete oxidation pathway,
properties are maintained well at the core part of a nanoparticle qualitative information about the nal product as CO2 was
relative to the surface, which contribute to the electronic obtained using in situ attenuated total reection-Fourier trans-
form infrared (ATR-FTIR) measurements under electrochemical
test conditions (Fig. 4a).41 The normalized CO2 band intensity is
very low for the Pt/C catalyst, which means that most of the
ethanol is only partially oxidized (Fig. 4b, Fig. S7†).8 However,
intense peaks appear for the Rh3Sn7/C catalyst, even at low
potentials, in comparison with the commercial catalyst. This
indicates that the ability of the Rh3Sn7/C catalyst to split the
ethanol C–C bond is signicantly enhanced, and that the fuel
Fig. 2 (a) High-magnification TEM images of heterogeneous Rh3Sn7
nanoparticles supported on carbon, and line-scanning results of an
nanoparticle. (b) HR-XRD patterns of Rh/C, Sn/C, and Rh–Sn catalysts
with indicators for crystal structures. (c) Rh 3d and (d) Sn 3d HR-XPS
spectra with depth profiles.
Fig. 3 Rh K-edge of XANES spectra of Rh–Sn catalysts.
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been achieved; more of the ethanol is completely oxidized to
form CO2. Thus the Rh–Sn catalyst has not only superior elec-
trochemical activity and durability but also an increased fuel
efficiency for the oxidation of ethanol through the combination
effects of the bifunctional mechanism, electronic modication,
and C–C bond splitting. We expect that these results will relieve
the dependence on noble metals, leading to the distribution of
inexpensive catalysts, and future work may focus on the modi-
cation of the nanoparticle surface and its application in fuel
cell systems.
Acknowledgements
This research was supported by the Institute for Basic Science (IBS)
in Korea, and supported nancially by Global Frontier Program
on Center for Multiscale Energy System (2012M3A6A7054283),
the NRF grant funded by MSIP (2014R1A2A2A04003865), the KIST
Institutional Program of KIST, and the New and Renewable Energy
Core Technology Program of KETEP grant funded by MOTIE
(20143030031340) in Korea.
Fig. 4 (a) In situ ATR-FTIR measurements and (b) normalized IR inten-
sities (2343 cmÀ1, asymmetric stretch of CO2) on Pt/C and Rh3Sn7/C
during electrocatalytic oxidation of ethanol, employing a potential
sweep in 0.5 M ethanol electrolyte at a scan rate of 20 mV sÀ1
.
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