Synergy of Ru and Ir in the Electrohydrogenation of Toluene to Methylcyclohexane on a Ketjenblack-Supported Ru-Ir Alloy Cathode

Article abstract of DOI:10.1021/acscatal.8b03610

An organic hydride system based on hydrogenation/dehydrogenation of toluene (TL)/methylcyclohexane (MCH) has been studied as a hydrogen storage technology. Electrohydrogenation of TL to MCH using a proton exchange membrane (PEM) electrolyzer is proposed as a candidate for the hydrogenation of TL in the organic hydride system. Recently, we reported that a Ketjenblack-supported Ru-Ir alloy (Ru-Ir/KB) cathode was effective for the reaction; however, electrohydrogenation mechanisms and catalyses of Ru and Ir in the electrohydrogenation have been unclear. In this paper, detailed characterization studies using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and electrochemical studies using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) for hydrogen evolution and kinetic studies for catalytic hydrogenation of TL by Ru-Ir/KB catalysts were conducted. On the basis of the experimental results, the electrohydrogenation mechanisms and synergy of Ru and Ir were proposed.

Full text of DOI:10.1021/acscatal.8b03610

Research Article
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2019, 9, 2448−2457
Synergy of Ru and Ir in the Electrohydrogenation of Toluene to
Methylcyclohexane on a Ketjenblack-Supported Ru-Ir Alloy Cathode
Yuta Inami,† Hitoshi Ogihara,† Shinichi Nagamatsu,‡ Kiyotaka Asakura,‡ and Ichiro Yamanaka*,†
†Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology,
Tokyo, 152-8552, Japan
‡Institute for Catalysis, Hokkaido University, Hokkaido 001-0021, Japan
S
* Supporting Information
ABSTRACT: An organic hydride system based on hydrogenation/
dehydrogenation of toluene (TL)/methylcyclohexane (MCH) has been
studied as a hydrogen storage technology. Electrohydrogenation of TL
to MCH using a proton exchange membrane (PEM) electrolyzer is
proposed as a candidate for the hydrogenation of TL in the organic
hydride system. Recently, we reported that a Ketjenblack-supported Ru-
Ir alloy (Ru-Ir/KB) cathode was effective for the reaction; however,
electrohydrogenation mechanisms and catalyses of Ru and Ir in the
electrohydrogenation have been unclear. In this paper, detailed
characterization studies using transmission electron microscopy
(TEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS),
X-ray photoelectron spectroscopy (XPS), and electrochemical studies using cyclic voltammetry (CV) and linear sweep
voltammetry (LSV) for hydrogen evolution and kinetic studies for catalytic hydrogenation of TL by Ru-Ir/KB catalysts were
conducted. On the basis of the experimental results, the electrohydrogenation mechanisms and synergy of Ru and Ir were
proposed.
KEYWORDS: electrohydrogenation, Ru and Ir synergy, spillover, PEM electrolyzer
process for the catalytic dehydrogenation of MCH to TL using
a Pt/Al2O3-based catalyst has been developed,3,5 there is room
for improvement for the hydrogenation process. As mentioned
before, a catalytic process for hydrogenation of TL to MCH
was proposed as shown below.3,5 The first step is water
electrolysis using renewable energy to produce H2 (eq 1), and
INTRODUCTION
■
Hydrogen energy has attracted attention as an alternative
energy source to fossil fuels. If hydrogen is produced from
water and renewable energy, hydrogen will be the ultimate
clean secondary energy. One of the most critical obstacles for
practical utilization of hydrogen energy is the difficulty in
storage and transportation of gaseous H2.1 A high-pressure
technology of H2 is well-established; however, the technology
is not appropriate for large-scale storage and long-term
transportation of H2, because of a low volumetric mass density
(39 kg of H2 m−3) even at very high pressure (70 MPa) and a
high volatility. In addition, the handling of gaseous H2 is not
easy because it is an explosive and leaky gas. Therefore,
candidate technologies, such as cryogenic liquid hydrogen,
liquid ammonia, absorption in metal compounds, and liquid
organic hydrides are proposed for hydrogen storage and
transportation.1
The organic hydride technology is based on the hydro-
genation/dehydrogenation of organic compounds couple.2−4 It
is considered that the toluene (TL)/methylcyclohexane
(MCH) couple is the most suitable for large-scale storage for
the following reasons.5 (i) Handling of TL and MCH is easy
because their toxicities are not too high and they are stable as
liquids under ambient conditions. (ii) The volumetric mass
density is fairly high (47.4 kg of H2 m−3). (iii) The current
petroleum infrastructure can be used. (iv) A large amount of
TL is available from petroleum resources. Though an industrial
the second step is the catalytic hydrogenation of TL at higher
temperature and under pressure conditions (eq 2), though the
second step is an exothermic reaction. We cannot avoid a heat
loss in the hydrogenation process, because a perfect heat
recovery cannot be achieved in the chemical process.
If MCH could be directly produced from TL and water by
an electrochemical method, we would avoid the heat loss in eq
2. In other words, the efficiency of energy conversion defined
as a quantity of MCH production against the energy
consumption would increase in the direct production process.
Thus, electrohydrogenation of TL to MCH coupled with
Received: September 7, 2018
Revised: January 8, 2019
© XXXX American Chemical Society
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electro-oxidation of water using a proton exchange membrane
(PEM) electrolysis cell6,7 is proposed.8−10 In the system, water
is oxidized to oxygen, protons, and electrons on an anode (eq
3) and TL is reduced with protons and electrons to MCH on a
EXPERIMENTAL SECTION
■
All reagents used in this work are listed in the Supporting
Information.
Preparation of Electrocatalysts. Ru/KB, Ir/KB, and Ru-
Ir/KB electrocatalysts were prepared by chemical reduction
using NaBH4. The KB powder (90 mg) and deionized water
(100 mL) were mixed in a 200 mL PTFE beaker with vigorous
stirring. To disperse KB in deionized water, ultrasonication was
performed by using an ultrasonic homogenizer (UD-200,
TOMY SEIKO Co, Ltd.). Certain amounts of 20 mM RuCl3·
nH2O aqueous solutions and 13 mM IrCl3·nH2O aqueous
solutions were added to the KB dispersion, and the mixture
was heated to 353 K in a bead bath with stirring by a magnetic
stirrer. After the mixture was stirred at 353 K for 1 h, NaOH
aqueous solutions were added to adjust the pH to 12. NaBH4
(10 equiv against the amount of total metal) dissolved in
NaOH aqueous solution (pH 12, 20 mL) was added dropwise.
After overnight stirring, the mixture was filtered, washed with
deionized water (100 mL) and 2-propanol (20 mL), and dried
under vacuum at 343 K for 1 h. The obtained powders are
denoted as Ru(x)-Ir(y)/KB electrocatalysts, where x and y are
the Ru and Ir loadings (wt %), respectively. The total metal
loadings (x + y) were fixed to 10 wt % for all cathode
electrocatalysts prepared in this study.
A 50 wt % Pt/KB electrocatalyst for anode was prepared by
the conventional impregnation method. Briefly, 0.1 M H2PtCl6
aqueous solutions were impregnated to KB and dried. The
obtained powder was reduced with H2 at 573 K for 2 h.
Fabrication of Electrode and Membrane−Electrode
Assembly (MEA). An electrocatalyst ink was prepared by
mixing 4.0 mg of the electrocatalyst, 20 μL of 10 wt % Nafion
dispersion, and 250 μL of acetone and ultrasonicating for 10
min. The entire ink was cast on GDL (SIGRACET GDL-
25BC, 2.0 cm2). Subsequently, the electrocatalyst/GDL was
dried under vacuum at 323 K for 10 min. The electrocatalyst/
GDL was used as a cathode. The catalyst loading was 2.0
mgcatal cm−2, which corresponds to a total metal loading of 0.20
mgmetal cm−2. A 50 wt % Pt/KB/GDL anode (5.0 mgcatal cm−2)
was prepared by a similar method. A Nafion117 membrane was
sandwiched by the cathode and anode and hot-pressed for 1
min at 50 MPa and 413 K. The MEA electrocatalyst/GDL|
Nafion117|Pt/KB/GDL was fabricated.
Electrohydrogenation of TL to MCH. The MEA was set
in a batch type two-compartment electrolysis cell.13,14 Pure TL
(25 mL) was added to the cathode compartment and stirred
using a magnetic stir bar. Ar (101 kPa, 20 mL min−1) was
bubbled into TL solution. Deionized water (15 mL) was added
to the anode compartment to humidify the Nafion membrane,
and H2 (101 kPa, 20 mL min−1) was flowed as a H+ source.
Hydrogen oxidation proceeded at the anode. A reference
electrode of Ag/AgCl saturated KCl (+0.199 V vs standard
hydrogen electrode (SHE)) was connected to the Nafion
membrane via a 1 M H2SO4 aqueous solution. After Ar and H2
were flowed for 20 min, galvanostatic electrolysis with 0.10−
0.40 A cm−2 was conducted using a IviumStat.XR apparatus
(Ivium Technology) for 2 h at 298 K. Products of MCH, 1-
methyl-1-cyclohexene (MCH-1-ene) and H2, were analyzed by
gas chromatography techniques (GC-2025-FID (Shimadzu,
HP-1 capillary column (0.25 mm i.d. × 100 m, Agilent Co.))
and GC-8A-TCD (Shimadzu, active carbon column (3 mm i.d.
× 2.5 m))). The cathode potentials were monitored by the
reference electrode of Ag/AgCl saturated KCl; however, all
cathode (eq 4). The total reaction is indicated in eq 5. The
theoretical electrolysis voltage of the reaction is 1.07 V, which
is smaller than the 1.23 V for the water electrolysis (eq 1). The
efficiency of energy conversion for the MCH production would
increase by using the electrohydrogenation system. In addition,
the electrochemical formation of H2 (eq 6) is a competitive
reaction with the MCH formation. To achieve an efficient
electrohydrogenation of TL to MCH, selective and active
electrocatalysis of the cathode is essential. Suppression of the
hydrogen formation is the key technology.
It has been reported that cathodes with a high loading of Pt
(50 wt %) supported on carbon (Pt/C) and a high loading of
Pt-Ru alloy (54 wt %) supported on carbon (Pt-Ru/C) are
effective for the selective electrohydrogenation of TL to
MCH.9,11 However, studies focusing on non-Pt cathodes are
very limited.12−14 Recently, we widely evaluated the electro-
hydrogenation activities of carbon-supported precious-metal
cathodes and reported that Ru/Ketjenblack (KB) and Rh/KB
cathodes showed good Faradaic efficiencies of MCH formation
(FEs(MCH)) even at low metal loadings. The Ru/KB cathode
was especially active at a lower loading of Ru, but a drawback is
the large overpotential for the electrohydrogenation of TL to
MCH.13,14 Furthermore, we have found that the overpotential
for the electrohydrogenation of TL drastically decreased by
addition of Ir to Ru/KB.13 In the case of a 5 wt % Ru−5 wt %
Ir/KB cathode (0.20 mgmetal cm−2), it showed superior
performance for the electrohydrogenation in comparison to
the 50 wt % Pt/KB cathode (1.0 mgPt cm−2) under
galvanostatic electrolysis of 0.20 A cm−2. A brief reaction
mechanism of the electrohydrogenation of TL to MCH was
proposed as (i) adsorbed hydrogen species (Had) formed on
the metal surface by the electrochemical reduction of H+ and
(ii) Had on the metals attacked TL adsorbed on metals to form
MCH. A strong synergy of Ru and Ir was observed; however,
the synergy mechanism for the electrohydrogenation of TL has
not been clear.
The purposes of this paper are to identify the active site of
the Ru-Ir/KB electrocatalyst by detailed characterization
studies and to clarify the synergy mechanism of the
electrohydrogenation by Ru and Ir on KB by voltammetric
studies and kinetic studies for the catalytic hydrogenation of
TL.
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electrode potentials in this paper are denoted against the SHE
without iR correction. Electrocatalytic activities were evaluated
by the FE(MCH) (Faradaic efficiency of MCH formation) and
cathode potential during the electrolysis. The FE is defined as
shown in eq 7.
another compartment. Before the CV measurements, electro-
chemical cleaning of the working electrode was applied
between 0.0 and +0.8 V (SHE) at 500 mV s−1 for 100 cycles.
We confirmed that hydrogen adsorption/oxidation currents
did not decrease on the cleaning. Thereafter, CVs were
measured between 0.0 and +0.8 V (SHE) at 50 mV s−1 for 3
cycles. The CV data were normalized by the geometric area of
the GC electrode (0.20 cm2). The CVs at the third cycle were
reported.
FE = (yield × F × n/Q) × 100%
(7)
where F is the Faraday constant (96485 C mol−1), n is the
number of electrons, and Q is the charge passed.
Linear sweep voltammetry (LSV) measurement: Electro-
catalytic activities for the hydrogen evolution reaction in 0.5 M
aqueous H2SO4 were evaluated by LSV in Ar. The working
electrode was made by the same method as for the CV
measurement. The counter electrode and reference electrode
were the same as for the CV measurement. LSV measurements
were carried out at 1 mV s−1 from +0.050 V (SHE) to negative
potential for 3 cycles. The LSV data were normalized by the
geometric area of the GC electrode (0.20 cm2, jgeo) and mass
loading of Ir (jIr/A mgIr−1). The LSVs at the third cycle were
reported.
Evaluation of Heterogeneous Catalytic Activity for
the Hydrogenation of TL. The catalytic hydrogenation
under pressure of H2 was conducted in an autoclave (volume
30 mL, i.d. 20 mm) with an inner tube of Pyrex glass (o.d. 20
mm, i.d. 17.6 mm, height 110 mm). A 10 mL portion of TL
(contained 0.5 vol % n-heptane as internal standard), 1.0 mg of
the catalyst, and a magnetic spin bar were placed in the inner
tube. Air was purged by introduction of 0.7 MPa N2 and
evacuation for several times. Thereafter, H2 was introduced to
the autoclave under 6.0 MPa at 298 K and catalytic
hydrogenation continued for 1 h. The reaction was stopped
by evacuation of H2 at 1 h, and the reaction mixture was
filtered. The filtrate was analyzed using a GC-2025-FID gas
chromatograph. The products were MCH and MCH-1-ene,
and other products were not detected. The consumption rate
of H2 (−r(H2)) and the turnover frequency of Ru (TON(Ru))
calculated using the dispersion of metal particles (D) obtained
from TEM images were calculated as
Characterization Studies. Transmission electron micro-
scope (TEM) measurement: TEM images were measured with
a JEM-2010F electron microscope (JEOL, 200 kV) equipped
with an EDX spectrometer (Genesis).
X-ray diffraction (XRD) measurement: XRD patterns were
measured with a MiniFlex-600 X-ray diffraction instrument
(Rigaku) with Cu Kα radiation (40 kV, 15 mA).
X-ray absorption fine structure (XAFS) measurement: XAFS
spectra of Ru K edge and Ir LIII edge were measured in
transmission mode at the beamlines NW10A of Photon
Factory Advanced Ring (PF-AR) and BL9C of Photon Factory
(PF) of the Institute for Materials Structures Science, High
Energy Accelerator Research Organization (KEK-IMSS),
Tsukuba, Japan, respectively. The X-ray beam was mono-
chromated using a Si(311) and Si(111) double-crystal
monochromator for Ru K edge and Ir LIII edge, respectively.
The X-ray energy was calibrated using Ru metal (E0 = 22119.3
eV) and Ir metal (E0 = 11212 eV). XAFS samples were
prepared by mixing the electrocatalyst and BN powders and
pressing into a pellet (10 mm diameter). XAFS of the used
MEA was measured in fluorescence mode. The MEA sample
was fabricated by a method similar to that for the
electrohydrogenation reaction except for the use of a 20 wt
% Pd/KB/GDL anode instead of a 50 wt % Pt/KB/GDL
anode because of overlap of the Pt LIII edge with the Ir LIII
edge. Experimental procedures in detail are described as
follows: after galvanostatic electrohydrogenation at 0.2 A cm−2
for 2 h, the MEA was removed from the electrolysis cell in an
N2-filled glovebox and enclosed in a plastic bag, which was
used as a sample for XAFS measurement. The X-ray absorption
near-edge structure (XANES) and extended X-ray absorption
fine structure (EXAFS) spectra were analyzed using REX2000
software (Rigaku). The backscattering amplitudes and the
phase shifts of Ru-Ru, Ru-Ir, Ru-O, Ir-Ir, Ir-Ru, and Ir-O were
calculated using the FEFF program.15 More details on the
method of data reduction of EXAFS spectra are given in the
Supporting Information.
X-ray photoelectron spectroscopy (XPS) measurement: XPS
spectra were measured using a JPS-9010MC instrument
(JEOL) with Al Kα radiation (12 kV, 25 mA) and an analyzer
pass energy of 30 eV. XPS spectra were calibrated by using the
Au 4f7/2 peak of mechanically mixed Au powder at 84.0 eV and
normalized by the maximum intensity of respective spectra.
Electrochemical Studies. Cyclic voltammetry (CV)
measurement: CVs of electrocatalysts were measured using a
two-compartment cell divided by a glass filter. A working
electrode was prepared by casting 4.0 μL of electrocatalyst ink
on a glassy-carbon electrode (i.d. 5.0 mm, 0.19 mgcatal cm−2)
and dried in air. The ink was prepared by ultrasonication of 2.0
mg of the electrocatalyst, 20 μL of a 10 wt % Nafion dispersion
and 200 μL of acetone. The working electrode was set into 0.5
M H2SO4 aqueous electrolyte in one compartment, and a
counter electrode of Pt black/Pt wire and a reference electrode
of Ag/AgCl saturated KCl were set into the same electrolyte in
−r(H2) = 2r(MCH‐1‐ene) + 3r(MCH)
(8)
TOF(Ru) = − r(H2) × awRu/(Dmetal × mcatal
× wtRu/100)
(9)
where r is the reaction rate of H2, awRu is the atomic weight of
Ru (101.07 g mol−1), mcatal is the amount of catalyst (1.0 mg),
wtRu is the mass loading of Ru in the catalyst, and Dmetal is the
metal dispersion obtained from TEM images; details are given
in the Supporting Information.
In addition, a similar TOF normalized by whole amount of
Ru in catalyst (app-TOF(Ru)) was calculated as
app‐TOF(Ru) = − r(H2) × awRu/(mcatal × wtRu/100)
(10)
RESULTS AND DISCUSSION
■
Electrohydrogenation of TL to MCH at Ru(x)-Ir(y)/KB
Cathodes. Figure 1 shows the electrocatalytic activities of the
Ru(10)/KB, Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, Ru(2)-Ir(8)/
KB, and Ir(10)/KB cathodes for the electrohydrogenation of
TL to MCH under galvanostatic electrolysis at 0.20 A cm−2
and 298 K. Figure 1a shows the time courses of the cathode
potential during hydrogenation by the five cathodes. All
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Figure 1. Electrohydrogenation of TL in galvanostatic electrolysis at
0.2 A cm−2 at Ru(x)-Ir(y)/KB cathodes: (a) time profile of cathode
potential during electrolysis; (b) Faradaic efficiency of MCH, MCH-
1-ene, and H2 formation. Legend to cathodes: (i) Ru(10)/KB; (ii)
Ru(8)-Ir(2)/KB; (iii) Ru(5)-Ir(5)/KB; (iv) Ru(2)-Ir(8)/KB; (v)
Ir(10)/KB.
Figure 2. Effects of electrolysis current densities (j) on (a) FE(MCH)
and (b) cathode potential during the electrohydrogenation of TL at
Ru(x)-Ir(y)/KB cathodes. Legend to cathodes: (i) Ru(10)/KB; (ii)
Ru(8)-Ir(2)/KB; (iii) Ru(5)-Ir(5)/KB; (iv) Ru(2)-Ir(8)/KB; (v)
Ir(10)/KB.
cathode potentials were very stable after 10 min. The order of
cathode potential was Ru(10)/KB < Ru(8)-Ir(2)/KB < Ru(5)-
Ir(5)/KB < Ru(2)-Ir(8)/KB < Ir(10)/KB < +0.16 V (standard
potential) at 0.20 A cm−2 electrolysis. Figure 1b shows FEs of
the products MCH, MCH-1-ene, and H2. The sum of
FE(MCH), FE(MCH-1-ene), and FE(H2) was 100% for all
cathodes. At the Ru(10)/KB cathode, a very high FE(MCH)
of 94%, the same as the selectivity to MCH, was observed;
however, the working potential of the Ru(10)/KB cathode at
−0.330 V was far to the negative side from the standard
potential of +0.16 V (eq 4). A large overpotential of 0.49 V at
the Ru(10)/KB cathode is a disadvantage. In contrast to the
Ru(10)/KB cathode, the hydrogen evolution reaction
dominantly proceeded at the Ir(10)/KB cathode, though the
working potential was relatively positive (−0.171 V). When 2
wt % of Ir was loaded with a 8 wt % of Ru on KB (Ru(8)-
Ir(2)/KB cathode), the working potential shifted to positive
potential at −0.253 V while a high FE(MCH) of 90% was
retained. A synergy of Ru and Ir was observed on the working
potential and FE(MCH). Therefore, the ratios of Ru and Ir
were changed to 5:5 and 2:8; on the Ru(5)-Ir(5)/KB cathode,
the working potential shifted to being more positive at −0.183
V with 86% for FE(MCH), and on the Ru(2)-Ir(8)/KB
cathode, the FE(MCH) slightly decreased to 78% at −0.182 V,
similar to that for the Ru(5)-Ir(5)/KB cathode. As described
above, we found that the working potential shifted positively
without a significant decrease in FE(MCH) by mixing Ir and
Ru on KB.
Ir(5)/KB, and Ru(2)-Ir(8)/KB cathodes, FE(MCH) values
were almost constant between 0.10 and 0.20 A cm−2 and
slightly decreased at 0.40 A cm−2. The order of FE(MCH)
values was Ru(10)/KB > Ru(8)-Ir(2)/KB > Ru(5)-Ir(5)/KB
> Ru(2)-Ir(8)/KB > Ir(10)/KB without dependence on j
value. The cathodes containing more Ru show higher
FE(MCH) values. For working cathode potentials, the order
of the cathodes was reverse at 0.10 and 0.20 A cm−2. The
cathodes containing more Ir show more positive working
potentials. The cathode potentials at 0.40 A cm−2 were
extremely negative for all cathodes and should contain different
overvoltages of activation, resistance, and concentration, which
may be the reason a simple rule cannot be applied to relations
between Ru/(Ru + Ir) and working potentials.
Characterization of Ru(x)-Ir(y)/KB Electrocatalyst.
Figure 3 shows XRD patterns of the Ru(x)-Ir(y)/KB
electrocatalysts. In the XRD patterns of the Ru(10)/KB and
Ir(10)/KB electrocatalysts, we observed diffraction patterns
assigned to hcp Ru and fcc Ir, respectively. Crystallite
diameters of Ru and Ir were estimated as 3−4 nm from
Ru(100), Ru(102), and Ru(110) and 2.5−3 nm from Ir(111)
and Ir(220), respectively. The XRD patterns of the Ru(8)-
In order to confirm the above synergy of Ru and Ir, the
influence of the Pt/KB anode on the cathode reaction was
studied. A 20 wt % Pd/KB anode instead of the Pt/KB anode
was applied for electrohydrogenation, and the same results
were obtained. This indicated no influence of the Pt/KB anode
on the synergy of Ru and Ir. The synergy of Ru and Ir on the
electrohydrogenation is attractive; therefore, studies have been
done to reveal the mechanism of the synergy and electro-
catalysis.
Figure 2 shows the effects of current densities (j) on the
galvanostatic electrohydrogenation at the Ru(x)-Ir(y)/KB
cathodes at 298 K. FEs of MCH-1-ene and H2 are indicated
in Figure S1 in the Supporting Information. At the Ru(10)/KB
cathode, FE(MCH) values were always high without depend-
ence on j values. A high FE(MCH) of 93% was observed at
0.40 A cm−2. In contrast, the FE(MCH) value was only 48%
even at 0.10 A cm−2 at the Ir(10)/KB cathode, and it
decreased with an increase in j. At the Ru(8)-Ir(2)/KB, Ru(5)-
Figure 3. XRD patterns of (a) Ru(10)/KB, (b) Ru(8)-Ir(2)/KB, (c)
Ru(5)-Ir(5)/KB, (d) Ru(2)-Ir(8)/KB, and (e) Ir(10)/KB electro-
catalysts and (f) KB material.
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Figure 4. TEM images and distribution of particle diameters of (a) Ru(10)/KB, (b) Ru(8)-Ir(2)/KB, (c) Ru(5)-Ir(5)/KB, (d) Ru(2)-Ir(8)/KB,
and (e) Ir(10)/KB electrocatalysts.
Ir(2)/KB and Ru(5)-Ir(5)/KB electrocatalysts were similar to
that of Ru(10)/KB electrocatalyst. However, the diffraction
peaks slightly shifted to a lower angle to compare with
Ru(10)/KB electrocatalyst (see Figure S2 in the Supporting
Information), which suggested expansion of lattice parameters
of hcp structure and formation of a hcp-structured solid-
solution alloy of Ru and Ir.16 The XRD pattern of the Ru(2)-
Ir(8)/KB electrocatalyst showed diffraction patterns assigned
to fcc and hcp structures. This suggested that there were Ru-Ir
alloy and monometallic Ir (fcc) on the Ru(2)-Ir(8)/KB
electrocatalyst. The formation of Ru-Ir alloy was also proposed
by the results of TEM-EDS and EXAFS analysis, as discussed
later.
Figure 4 shows TEM images of Ru(x)-Ir(y)/KB electro-
catalysts and distributions of particle diameters. For all
samples, distributions of nanoparticle diameter were relatively
wide: 1.5−4 nm. The average particle diameters of Ru(10)/
KB, Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, Ru(2)-Ir(8)/KB, and
Ir(10)/KB electrocatalysts were 2.3, 2.1, 2.5, 2.1, and 2.3 nm,
respectively, and were very similar values. The mass ratios of
Ru and Ir on the Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB and
Ru(2)-Ir(8)/KB samples were determined using EDS analysis
(Figure S3, Supporting Information). The mass ratio of Ru:Ir
at particles on the Ru(8)-Ir(2)/KB was 70:30 to 80:20. That
on the Ru(5)-Ir(5)/KB was 50:50. These ratios were very
similar to the ratios in the preparation conditions, indicating
that Ru and Ir formed a uniform Ru-Ir alloy on the Ru(8)-
Ir(2)/KB and Ru(5)-Ir(5)/KB electrocatalysts. In the Ru(2)-
Ir(8)/KB sample, the mass ratios at particles were from 35:65
to 0:100. This wide distributions of the mass ratio of Ru and Ir
indicates that nanoparticles of Ru-Ir alloy and monometallic Ir
are on the Ru(2)-Ir(8)/KB sample.
Figure 5. Ru K-edge (a) XANES spectra and (b) Fourier transform of
k3-weighted EXAFS spectra (solid line, observed spectra; dashed line,
fitting) of (i) Ru(10)/KB, (ii) Ru(8)-Ir(2)/KB, (iii) Ru(5)-Ir(5)/KB,
and (iv) Ru(2)-Ir(8)/KB electrocatalysts, (v) Ru(5)-Ir(5)/KB-used,
and (vi) Ru metal, (vii) RuO2, and (viii) RuO2·nH2O reference
materials.
RuO2·nH2O, Ir metal, and IrO2 were measured as reference
materials. As shown in Figure 5a(i),a(vi), the shape of the
XANES spectrum of the Ru(10)/KB electrocatalyst was similar
to that of Ru metal. The XANES spectra of the Ru(8)-Ir(2)/
KB, Ru(5)-Ir(5)/KB, and Ru(2)-Ir(8)/KB electrocatalysts
were also similar to that of Ru metal. However, the intensities
around 22133 eV of the electrocatalysts were stronger than
that of Ru metal, which indicated partial oxidation of Ru for
the electrocatalysts. No major differences in Ru K-edge
XANES spectra were observed among Ru(10)/KB, Ru(8)-
Ir(2)/KB, Ru(5)-Ir(5)/KB, and Ru(2)-Ir(8)/KB electro-
catalysts regardless of Ru/(Ru+Ir). In the case of Ir (Figure
6a), the white-line intensities of the electrocatalysts were
stronger than that of Ir metal and weaker than that of IrO2,
which indicated partial oxidation of Ir for the electrocatalysts.
There were no major differences in the Ir LIII-edge XANES
Parts a and b of Figure 5 and Figure S5 show Ru K-edge
XANES spectra, Fourier transform of EXAFS spectra, and
EXAFS oscillations, respectively. Parts a and b of Figure 6 and
Figure S6 show Ir LIII-edge XANES spectra, Fourier transform
of EXAFS spectra, and EXAFS oscillations, respectively. In
addition to Ru(x)-Ir(y)/KB electrocatalysts, Ru metal, RuO2,
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clearly indicated that Ru and Ir formed an alloy structure in the
Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, and Ru(2)-Ir(8)/KB elec-
trocatalysts and that the Ru/Ir composition of Ru-Ir alloy
changed according to Ru/Ir ratios under the preparation
conditions.
XPS spectra were measured to obtain knowledge about the
electronic states of Ru and Ir in detail. Bulk compositions of
Ru and Ir corresponded to those obtained form XPS data
(Figure S4). Parts a and b of Figure 7 show XPS spectra of the
Figure 6. Ir LIII-edge (a) XANES spectra and (b) Fourier transform of
k3-weighted EXAFS spectra (solid line, observed spectra; dashed line,
fitting) of (i) Ru(8)-Ir(2)/KB, (ii) Ru(5)-Ir(5)/KB, (iii) Ru(2)-
Ir(8)/KB, and (iv) Ir(10)/KB electrocatalysts, (v) Ru(5)-Ir(5)/KB-
used, and (vi) Ir metal and (vii) IrO2 reference materials.
spectra of all electrocatalysts. In the case of Ru K-edge EXAFS
spectra (Figure 5b), a peak at 2.4 Å observed for Ru metal (vi)
was assigned to the first neighboring atom of Ru. Peaks at 1.6
Å observed for RuO2 (vii) and RuO2·nH2O (viii) were
assigned to the first neighboring atom of O. In the spectrum of
Ru(10)/KB electrocatalyst, peaks at 2.4 and 1.6 Å were
observed, which indicated a mixture of Ru metal and oxidized
Ru species on the Ru(10)/KB electrocatalyst. Peaks at 1.6 and
2.1−2.4 Å were observed in the spectra of Ru(8)-Ir(2)/KB,
Ru(5)-Ir(5)/KB, and Ru(2)-Ir(8)/KB electrocatalysts, and the
shape of spectra at 2−3 Å changed depending on Ru/(Ru+Ir),
which indicated a change in the coordination environment
around Ru. Results of a curve fitting analysis of the EXAFS
spectra are given in Table 1. More detailed fitting results (edge
shifts and Debye−Waller factors) are shown in Tables S1 and
S2 in the Supporting Information. The coordination number of
Ru around Ru (CN(Ru−Ru)) decreased and the CN(Ru-Ir)
increased as Ru/(Ru+Ir) decreased. CN(Ru-O) values were
almost constant. In the case of Ir (Figure 6b), a peak at 2.6 Å
observed at Ir metal was assigned to the first neighboring atom
of Ir, and a peak at 1.6 Å observed at IrO2 was assigned to the
first neighboring atom of O. In the spectrum of Ir(10)/KB
electrocatalyst, peaks at 2.6 and 1.6 Å were observed, which
indicated a mixture of Ir metal and oxidized Ir species on the
Ir(10)/KB electrocatalyst. Peaks at 1.6 and 2−3 Å were
observed in the spectra of Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB,
and Ru(2)-Ir(8)/KB electrocatalysts. As Ru/(Ru+Ir) in-
creased, the CN(Ir-Ir) value decreased and the CN(Ir−Ru)
value increased as shown in Table 1. These EXAFS analyses
Figure 7. XPS spectra of (a) Ru 3p3/2 and (b) Ir 4f regions: (i)
Ru(10)/KB, (ii) Ru(8)-Ir(2)/KB, (iii) Ru(5)-Ir(5)/KB, (iv) Ru(2)-
Ir(8)/KB, and (v) Ir(10)/KB electrocatalysts.
Ru 3p3/2 and Ir 4f regions of the Ru(x)-Ir(y)/KB electro-
catalysts, respectively. For all electrocatalysts, metallic Ru and
oxidized Ru species were detected. The XPS spectra were
deconvoluted to three Gaussian−Lorentzian functions, which
are assigned to Ru(0) (461.6 eV), RuO2 (462.4 eV). and
hydrous Ru oxide species (RuOxHy).17,18 Though ratios of
metallic Ru slightly increased with a decrease in Ru content of
Ru-Ir/KB electrocatalysts, the binding energy of electrons in
Ru was constant. Major differences in electronic states of Ru
were not observed for various Ru-Ir/KB electrocatalysts. In the
case of the Ir 4f region, metallic Ir and oxidized Ir species were
observed. The spectra of the Ir 4f region were analyzed using a
linear combination fitting (LCF) of reference spectra of Ir
metal and IrO2 because they are difficult to deconvolute using
theoretical functions due to complex satellite peaks of the Ir 4f
region.19 The binding energy of electrons in Ir was constant.
No major differences in XPS spectra were observed for all Ru-
Ir/KB electrocatalysts. XPS results indicated that there were no
strong electronic interactions between Ru and Ir in the Ru-Ir
Table 1. Curve-Fitting Analysis of Ru K-Edge and Ir LIII-Edge EXAFS Spectra of Ru(x)-Ir(y)/KB Electrocatalysts
Ru K edge
Ir
Ir LIII edge
Ir
Ru
O
Ru
O
R/Å
CN
R/Å
CN
R/Å
CN
R/Å
CN
R/Å
CN
R/Å
CN
Ru(10)/KB
2.67
2.67
2.67
2.67
5.5
4.2
3.7
3.4
1.97
2.03
1.97
1.94
1.4
1.7
1.0
0.3
Ru(8)-Ir(2)/KB
Ru(5)-Ir(5)/KB
Ru(2)-Ir(8)/KB
Ir(10)/KB
2.65
2.66
2.67
0.6
2.3
5.2
2.66
2.66
2.67
2.4
2.3
1.3
2.64
2.66
2.70
2.69
2.65
1.3
3.1
5.3
7.1
4.5
2.00
2.00
2.00
2.00
2.03
2.9
2.0
1.5
1.3
1.1
Ru(5)-Ir(5)/KB-used
2.66
3.6
2.66
2.4
2.66
3.3
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alloy. This weak interaction between Ru and Ir may be due to
the similar electronegativities of Ru and Ir.20
The XPS and XAFS results revealed the contents of oxidized
Ru and oxidized Ir species in all electrocatalysts. These
oxidized species should form by exposure in air after the
catalyst preparations. Therefore, the observed oxidation states
of Ru and Ir differ from the real character of the catalyst during
the electrohydrogenation at negative potentials. The electro-
hydrogenation reactions are conducted at potentials lower than
the standard redox potentials of Ru/RuO2 and Ir/IrO2,21
which indicates the reduction states of the catalyst. To confirm
the oxidation states of Ru and Ir during the electro-
hydrogenation of TL, XAFS measurements of used Ru(5)-
Ir(5)/KB/GDL|Nafion117|Pd/KB/GDL (Ru(5)-Ir(5)/KB-
used) was conducted without exposure to air.
Figure 5a(v),b(v) and Figure S5(v) show Ru K-edge
XANES spectra, Fourier transform of EXAFS spectra, and
EXAFS oscillations, respectively. Figure 6a(v),b(v) and Figure
S6(v) show Ir LIII-edge XANES spectra, Fourier transform of
EXAFS spectra, and EXAFS oscillations, respectively. The
shape of Ru K-edge XANES spectra of the Ru(5)-Ir(5)/KB-
used (Figure 5a(v)) was very similar to that of Ru metal, which
indicated electrochemical reduction of oxidized Ru species to
metallic Ru. In the case of Ir LIII-edge XANES spectra (Figure
6a), the white-line intensity of the Ru(5)-Ir(5)/KB-used was
smaller than that of Ru(5)-Ir(5)/KB electrocatalysts though it
was slightly stronger than that of Ir metal, which indicated
electrochemical reduction of oxidized Ir species. The shape of
Ru K-edge and Ir LIII-edge EXAFS spectra of the Ru(5)-Ir(5)/
KB-used differed from that of the Ru(5)-Ir(5)/KB electro-
catalysts. In the curve-fitting analysis of the EXAFS spectra of
Ru(5)-Ir(5)/KB-used (Table 1), coordination of Ru and Ir
around Ru and coordination of Ru and Ir around Ir were
observed, which indicated that Ru and Ir kept the Ru-Ir alloy
after the electrohydrogenation. Furthermore, no coordination
of oxygen around Ru was observed at the EXAFS spectrum of
the Ru(5)-Ir(5)/KB-used. Though the CN(Ir-O) value of the
Ru(5)-Ir(5)/KB-used was not zero, it was smaller than that of
Ru(5)-Ir(5)/KB electrocatalyst. These EXAFS results corre-
spond to results of XANES spectra. A small amount of oxidized
Ir species may remain after the electrohydrogenation or may
reoxidize with water after the electrohydrogenation. Though
the origin of oxidized Ir species was not clear, these XAFS
results indicated that the majority of Ru and Ir was in a
metallic state during the electrohydrogenation.
Electrochemical Studies of Ru(x)-Ir(y)/KB Electro-
catalysts. To obtain knowledge of the surface structure of
Ru(x)-Ir(y)/KB electrocatalysts, CVs of hydrogen adsorption/
desorption feature were obtained. Figure 8 shows CVs of the
Ru(x)-Ir(y)/KB electrocatalysts coated on glassy-carbon
electrodes in 0.5 M H2SO4 aqueous solution. At the
Ru(10)/KB electrocatalyst (Figure 8a), a weak oxidation/
reduction couple of Ru was observed between 0.2 and 0.6 V
(SHE).22 At the Ir(10)/KB electrocatalyst (Figure 8e), redox
couples assigned to electrochemical adsorption/desorption of
hydrogen species were observed between 0.0−0.3 V (SHE). At
the Ru(8)-Ir(2)/KB (b), Ru(5)-Ir(5)/KB (c) and Ru(2)-
Ir(8)/KB (d) electrocatalysts, redox couples of Ru and
hydrogen were overserved. Because no redox couple of
hydrogen was observed at the Ru(10)/KB electrocatalyst, the
redox couples of hydrogen on the Ru(8)-Ir(2)/KB, Ru(5)-
Ir(5)/KB, and Ru(2)-Ir(8)/KB electrocatalysts should be due
to electrocatalysis of the Ir surface. Therefore, a coulomb for
Figure 8. CVs (50 mV s−1) of (a) Ru(10)/KB, (b) Ru(8)-Ir(2)/KB,
(c) Ru(5)-Ir(5)/KB, (d) Ru(2)-Ir(8)/KB, and (e) Ir(10)/KB
electrocatalysts coated on a glassy-carbon electrode in 0.5 M
H2SO4. Origins of respective CVs are denoted with “+”.
the hydrogen desorption (QH, shaded area in Figure 8) is
proportional to the electrochemical active surface area (ECSA)
of Ir. The QH values of Figure 8 were plotted as a function of
Ru/(Ru+Ir) in Figure 9. The QH increased with a decrease in
Figure 9. Charge passed for hydrogen desorption (QH) observed in
CVs in 0.5 M H2SO4.
Ru/(Ru+Ir). In other words, the ECSA of Ir increased with a
decrease in Ru/(Ru+Ir). The QH curve was a weak sigmoid but
was almost straight. This indicated that the Ru/Ir composition
at the surface of the Ru-Ir alloy is similar to that of the bulk of
the Ru-Ir alloy. In addition, three peaks of Had were observed
from the shape of CV of Ir(10)/KB (Figure 8e). They are
denoted as HA, HB, and HC from negative potentials. At the
Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, and Ru(2)-Ir(8)/KB elec-
trocatalysts, current peaks of HA in CVs Figure 8b−d seemed
to be constant and independent of Ru/(Ru+Ir). In contrast,
current peaks of HB and HC decreased with and increase in
Ru/(Ru+Ir). From previous reports, we assigned HA as Had
adsorbed at an on-top site of Ir and HB and HC as Had
adsorbed at bridge and hollow sites of Ir, respectively.23,24 On
a large Ir ensemble of the Ir(10)/KB, the three hydrogen
species HA, HB, and HC were observed in the CV of Figure 8e.
In a mixture of Ir and Ru, HB and HC decreased because the Ir
ensemble decreased and bridge and hollow sites decreased.
When the Ir ensemble significantly decreased, Ir species were
isolated but HA species at the on-top site of Ir remained. The
above observations indicated that Ir and Ru were well mixed in
all components.
The hydrogen evolution reaction was evaluated by LSV in
0.5 M H2SO4, as shown in Figure 10. Figure 10a shows LSVs
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On the basis of the above mechanism, FE(MCH) should be
determined by the ratio of the chemical reaction rates of the
hydrogenation at the surface (iii) and the coupling of Had (iv).
It is known that the catalytic hydrogenation of aromatics under
pressure of H2 proceeds via a catalytic surface reaction with
dissociative adsorption species of H2 and adsorbed aro-
matics.28−30 Therefore, the catalytic hydrogenation activity is
closely related to the electrohydrogenation activity. To obtain
information about the synergy of Ru and Ir in the
electrohydrogenation of TL to MCH, catalytic hydrogenation
activities of Ru(x)-Ir(y)/KB catalysts were evaluated at 298 K
under 6.0 MPa of H2 (Table 2). Under high-pressure
Figure 10. LSVs of Ru(x)-Ir(y)/KB electrocatalysts at 0.5 M H2SO4;.
Currents are normalized by (a) geometric area and (b) mass loading
of Ir. Legend for electrocatalysts: (i) Ru(10)/KB; (ii) Ru(8)-Ir(2)/
KB; (iii) Ru(5)-Ir(5)/KB; (iv) Ru(2)-Ir(8)/KB; (v) Ir(10)/KB.
Table 2. Catalytic Activities of Ru(x)-Ir(y)/KB Catalysts for
the Catalytic Hydrogenation of TL in Liquid Phase under
a
6.0 MPa of H2
yield/mmol
MCH-
normalized by the geometric area of electrode (jgeo). Ru(10)/
KB showed a large overpotential for the hydrogen evolution
reaction. As Ru/(Ru+Ir) decreased, the overpotentials for the
hydrogen evolution drastically decreased. The Ru(5)-Ir(5)/KB
and Ru(2)-Ir(8)/KB electrocatalysts were as active as the
Ir(10)/KB electrocatalyst. Figure 10b shows LSVs of Ru(8)-
Ir(2)/KB, Ru(5)-Ir(5)/KB, Ru(2)-Ir(8)/KB, and Ir(10)/KB
electrocatalysts normalized by the mass loading of Ir (jIr). The
jIr values of the Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, Ru(2)-
Ir(8)/KB, and Ir(10)/KB electrocatalysts were almost the
same, which indicated that hydrogen evolution reaction
proceeded on the Ir site of the Ru-Ir alloy. Furthermore, the
similar jIr values of Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, Ru(2)-
Ir(8)/KB, and Ir(10)/KB suggest that the specific activity of Ir
for hydrogen evolution is very similar to those of the Ir particle
and Ru-Ir alloy particle. In other words, the nature of Ir at the
surface of the Ir particle and that at the surface of the Ru-Ir
alloy particle were very similar.
Catalytic Hydrogenation Activity of Ru(x)-Ir(y)/KB
Catalysts and Reaction Mechanisms of the Electro-
hydrogenation of TL to MCH. In our previous work,13 we
proposed a primitive reaction mechanism for the electro-
hydrogenation of TL to MCH as follows: (i) formation of Had
on the metal surface by the electrochemical reduction of H+
(H+ + e− + * → Had), (ii) TL was adsorbed on the catalyst
surface (TL + * → TLad), and (iii) TLad and Had catalytically
reacted to form MCH (TLad + 6Had → MCH + *). H2
formation proceeds via (iv) a coupling of Had (2Had → H2 +
*). On the basis of the mechanism, we considered that the
overpotential for the electrohydrogenation is determined by
the overpotential for Had formation (i). It is well-known that
the hydrogen evolution reaction also proceeds via the
electrochemical formation of Had.25−27 Therefore, electro-
catalytic activities for the hydrogen evolution reaction should
be closely related to the electrohydrogenation activity. As
mentioned above, Ru(10)/KB showed a large overpotential for
the electrochemical hydrogen evolution, which indicated a
large overpotential for formation of Had on Ru. This is the
reason Ru(10)/KB showed a large overpotential for the
electrohydrogenation of TL. The Ru(5)-Ir(5)/KB and Ru(2)-
Ir(8)/KB electrocatalysts showed as small an overpotential as
the Ir(10)/KB electrocatalyst, which is the reason Ru(5)-
Ir(5)/KB and Ru(2)-Ir(8)/KB cathodes showed smaller
overpotentials for the electrohydrogenation of TL similar to
the overpotential for H2 evolution at the Ir(10)/KB cathode.
b
catalyst
MCH
1-ene
−r(H2)/mmol h−1
TOF(Ru)/104 h−1
Ru(10)/KB
Ru(8)-
Ir(2)/KB
Ru(5)-
Ir(5)/KB
Ru(2)-
Ir(8)/KB
6.8
4.6
1.0
0.6
23
15
4.5
3.2
3.4
0.32
0.1
11
4.6
2.9
1.0
3.1
Ir(10)/KB
0.014
trace
0.042
a
Conditions: catalyst, 1.0 mg; reactant, toluene 10 mL (contains 0.5
vol % n-heptane as internal standard); T = 298 K; p(H2) = 6.0 MPa;
reaction time 1 h. consumption rate of H2
b
conditions, the catalytic hydrogenation activity is determined
by the rate of the surface reaction between Had and TLad.
Ru(10)/KB showed the highest catalytic activity for the
catalytic hydrogenation of TL. As shown in Figure S7 of the
Supporting Information, a short induction period of about 10
min was observed and the yield of MCH linearly increased
until 4 h. This catalytic hydrogenation indicated a fast surface
reaction between Had and TLad on the Ru surface. Adsorption
energies of Had and TLad on Ru should be appropriate for the
hydrogenation reaction. This is the reason the Ru(10)/KB
cathode showed a high FE(MCH) in the electrohydrogenation
of TL. As Ru/(Ru+Ir) decreased, catalytic hydrogenation
activities decreased. Ir(10)/KB showed the lowest catalytic
activity, which corresponds to a low FE(MCH) of Ir(10)/KB
in the electrohydrogenation of TL. The TOF(Ru) values for
the hydrogenation for Ru(10)/KB, Ru(8)-Ir(2)/KB, Ru(5)-
Ir(5)/KB, and Ru(2)-Ir(8)/KB catalysts were similar ((2.9−
4.6) × 104 h−1) and were not strongly dependent on Ru/(Ru
+Ir). In addition, the app-TON(Ru) values were almost the
same, ∼2 × 104 h−1 (Supporting Information). These indicated
that catalytic hydrogenation proceeded on the Ru surface at
the Ru(10)/KB, Ru(8)-Ir(2)/KB, Ru(5)-Ir(5)/KB, and
Ru(2)-Ir(8)/KB catalysts and the surface nature of Ru on
the Ru particle and that of Ru on the Ru-Ir alloy particle were
similar. These facts strongly suggested that MCH should be
formed by the reaction between Had on Ru and TLad on Ru in
the electrohydrogenation reaction on the Ru(10)/KB, Ru(8)-
Ir(2)/KB, Ru(5)-Ir(5)/KB, and Ru(2)-Ir(8)/KB cathodes.
On the basis of the above results, we conclude that
electrohydrogenation of TL at the Ru-Ir alloy particle
proceeded as follows (Figure 11): (i) Had on Ir formed by
the electrochemical reduction of H+ (H+ + e− + Ir → Had-Ir),
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ACKNOWLEDGMENTS
■
This work was financially supported by the Cross-ministerial
Strategic Innovation Promotion Program (SIP), “energy-
carrier” (funding agency: Council for Science, Technology
and Innovation (CSTI)), and New Energy and Industrial
Technology Development Organization (NEDO). XAFS
measurements were performed at the Photon Factory and
Photon Factory Advanced Ring of Institute for Materials
Structures Science, High Energy Accelerator Research
Organization (KEK-IMSS), Tsukuba, Japan, with the proposal
number 2016G546. TEM-EDS measurements were supported
by Ookayama Materials Analysis Division, Tokyo Institute of
Technology.
Figure 11. Reaction model for the electrohydrogenation of TL to
MCH on a KB-supported Ru-Ir alloy cathode.
(ii) Had migrates from the Ir surface to the Ru surface (Had-Ir +
Ru → Ir + Had-Ru), and (iii) Had on Ru reacted with TLad on
Ru and as a result MCH is formed (6Had-Ru + TLad-Ru →
MCH + nRu). Ir and Ru work as an Had formation
electrocatalyst and a hydrogenation catalyst, respectively. The
overpotential for the electrohydrogenation of TL at the Ru-Ir/
KB cathode remarkably decreased to compare with that of
Ru(10)/KB because formation of Had on Ru was accelerated
by the addition of Ir. The slight decrease in FE(MCH) by the
addition of Ir was due to formation of H2 by coupling of Had-Ir
on a small Ir ensemble. A key factor of the unique nature of Ru
and Ir would be a weak interaction between Ru and Ir in the
Ru-Ir alloy, as observed in XPS spectra.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.8b03610.
■
S
Experimental details, XRD analysis, reagents and
materials, TEM-EDS analysis, and EXAFS oscillations
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for I.Y.: yamanaka.i.aa@m.titech.ac.jp.
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Kohno, Y.; Matsuzawa, K.; Awaludin, Z.; Kato, A.; Nishiki, Y.
Membrane Electrolysis of Toluene Hydrogenation with Water
ORCID
Kiyotaka Asakura: 0000-0003-1077-5996
Ichiro Yamanaka: 0000-0002-9734-5659
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acscatal.8b03610
ACS Catal. 2019, 9, 2448−2457