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ISSN 2041-6539

EDGE ARTICLE

Bo Huang, Hiroshi Kitagawa et al.
Coreduction methodology for immiscible alloys of CuRu
solid-solution nanoparticles with high thermal stability andversatile exhaust puriﬁcation ability

Volume 11
Number 42
14 November 2020
Pages 11355–11634

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EDGE ARTICLE

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Coreduction methodology for immiscible alloys of
CuRu solid-solution nanoparticles with highthermal stability and versatile exhaust puriﬁcation
ability†
Bo Huang, ‡*a Hirokazu Kobayashi,ab Tomokazu Yamamoto,cd Syo Matsumura,dce
Yoshihide Nishida,f Katsutoshi Sato, fg Katsutoshi Nagaoka, f Masaaki Haneda,hiShogo Kawaguchi,j Yoshiki Kubotak and Hiroshi Kitagawa *ael
This study provides a coreduction methodology for solid solution formation in immiscible systems, with an
example of a whole-region immiscible Cu–Ru system. Although the binary Cu–Ru alloy system is very
unstable in the bulk state, the atomic-level well-mixed CuRu solid solution nanoparticles were found to

Received 18th June 2020
Accepted 18th September 2020

have high thermal stability up to at least 773 K in a vacuum. The exhaust puriﬁcation activity of the CuRu
solid solution was comparable to that of face-centred cubic Ru nanoparticles. According to in situ
infrared measurements, stronger NO adsorption and higher intrinsic reactivity of the Ru site on the CuRu
surface than that of a pure Ru surface were found, aﬀected by atomic-level Cu substitution.

DOI: 10.1039/d0sc03373a

Furthermore, CuRu solid solution was a versatile catalyst for puriﬁcation of all exhaust gases at

rsc.li/chemical-sciencea stoichiometric oxygen concentration.

IntroductionaDivision of Chemistry, Graduate School of Science, Kyoto University,
Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan. E-mail: kitagawa@kuchem.kyoto-u.ac.jp; bohuang@xjtu.edu.cn

bJST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japanc

Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,
Motooka 744, Nishi-ku, Fukuoka 819-0395, Japand

The Ultramicroscopy Research Centre, Kyushu University, Motooka 744, Nishi-ku,
Fukuoka 819-0395, Japane

INAMORI Frontier Research Centre, Kyushu University, Motooka 744, Nishi-ku,
Fukuoka 819-0395, Japanf

Department of Chemical Systems Engineering, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japang
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, 1-30 GoryoOhara, Nishikyo-ku, Kyoto 615-8245, JapanhAdvanced Ceramics Research Centre, Nagoya Institute of Technology, 10-6-29
Asahigaoka, Tajimi, Gifu 507-0071, Japani

Frontier Research Institute for Materials Science, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya 465-8555, JapanjJapan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayocho, Sayo-gun, Hyogo 679-5198, Japank
Department of Physical Science, Graduate School of Science, Osaka Prefecture
University, Sakai, Osaka 599-8531, Japanl

Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida,
Sakyo-ku, Kyoto 606-8501, Japan† Electronic supplementary
10.1039/d0sc03373a

information(ESI)

available.

See

DOI:

‡ Current addresses: School of Chemical Engineering and Technology, Xi'an
Jiaotong University, Xi'an 710049, P. R. China.

This journal is © The Royal Society of Chemistry 2020

Platinum group metals (PGMs) play a crucial role in bothindustrial and scientic areas, including use in fuel cells,1,2
water splitting3,4 and exhaust purication.5,6 Due to theirextremely low reserves in the Earth's crust, decreasing thedemand for PGMs is extremely important. Alloying PGMs withearth-abundant metals (EAMs) has attracted much attention to
decrease the usage of PGMs, and to simultaneously generatesynergic performance in specic systems.7,8 Of all the alloy
types, solid solutions are the most eﬀective at controlling theelectronic structure, which leads to better performance inapplications such as catalysis,9,10 optics11,12 and magnetism.13,14
However, for the most famous catalytic reaction of PGMs –exhaust purication – there has been little research on the
catalytic ability of solid solutions of PGMs alloyed with EAMs as
exhaust purication catalysts. This may be due to the decreased
exhaust purication activity or poor thermal stability of PGMsEAMs solid solutions prepared to date. In fact, the number ofavailable PGMs-EAMs solid solutions has been highly restricted
due to the solubility of diﬀerent metals in binary systems; inmany binary systems two diﬀerent metal elements cannot bemixed (known as immiscible systems). Among these immiscible
binary systems, Cu–Ru is one of the most challenging systems
since Cu and Ru elements are completely immiscible at any
temperature and composition in the bulk state, including theliquid phase.15 Up to now, much eﬀort has been made forpreparation of CuRu nanoparticles in colloidal chemistry or

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theoretical calculations.16–19 Breaking through this limitation insolid solution formation for immiscible systems is highly
important, especially for the development of novel exhaust
purication catalysts with PGMs and EAMs. Until now, there
has been no practical methodology reported for immiscible
binary systems, although there are a few successful syntheses ofbimetallic solid solutions.20–25
In this work, the rst methodology for solid solution
formation in an immiscible binary system is reported, with theexample of a whole-region immiscible Cu–Ru system. Thismethodology was proved to be very practical for investigating
the coreduction conditions and other parameters; the coreduction synthesized CuRu solid-solution nanoparticles (NPs)
had an atomic-level randomly mixed alloying state, and wereconrmed to have high thermal stability up to 773 K, suitable
for low-temperature exhaust purication reactions.26

Results and discussion
Coreduction methodology
Coreduction is considered necessary for solid-solution structureformation in immiscible systems, where two diﬀerent metallic
precursors are reduced simultaneously to isolated atoms, followed by aggregation to clusters and ending up with a solid
solution (Fig. 1a). Unlike the rapid synthetic strategy achievedby using strong reductants,27 this methodology involved
searching for coreduction conditions by measuring the reaction
times of several potential precursors to nd a combination that
had similar reaction times to each other (Fig. 1b).

Fig. 1 Schematic images for the coreduction mechanism and coreduction methodology. Schematic of the (a) coreduction mechanism
for solid solution structure formation and the (b) methodology toachieve coreduction conditions.

11414 | Chem. Sci., 2020, 11, 11413–11418

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The Ru precursor was determined to be ruthenium acetylacetonate, Ru(acac)3, since it can form face-centred cubic (fcc)-Ru
NPs28 and is easier to mix with fcc-structured Cu according to
the Hume-Rothery rules.29 The reaction time was measured forRu(acac)3 by sampling the reaction solution at several times
during a typical polyol reduction reaction (see the ESI and
Fig. S1†). The aliquots were analysed by transmission electron
microscopy (TEM), as shown in Fig. 2a. A continuous increase in
the mean diameter of fcc-Ru NPs was observed from 1–5 min,and aer 5 min the mean diameter remained constant; therefore, the reaction time of Ru(acac)3 was considered to be ca. 5
min.
For Cu, three potential precursors were considered: copper
acetylacetonate, Cu(acac)2, copper acetate monohydrate
Cu(OAc)2$H2O and copper formate tetrahydrate Cu(HCOO)2$4H2O. The reaction times of these Cu precursors were
measured by the same method as for Ru(acac)3. The reaction
time of Cu(acac)2 was determined by TEM analysis to be ca. 1
min (Fig. S2†). The reaction time of Cu(OAc)2$H2O was determined by colorimetry to be ca. 5 min (Fig. 2b), which was thesame as that of Ru(acac)3. The reaction time of Cu(HCOO)2$4H2O was ca. 2 min (Fig. 2c). It was proposed that the reductionof metallic precursors to metallic nanoparticles by polyol
molecules involved a reaction intermediate, in which the bulky
ligands sterically impeded the attack of polyol molecules on theligand-protected metallic ion centre, thereby blocking theformation of oxygen–metal bonds (Fig. 2d). As shown in
Fig. S3,† Ru(III) in Ru(acac)3 is at the centre of a hexa-coordinate
arrangement with large ligands, which slowed down the stericattack process. This was consistent with the relatively longreaction time of ca. 5 min shown in Fig. 2a.
Similarly, Cu(II) is in a penta-coordinate dimeric structure in
Cu(OAc)2$H2O, which is very similar to a hexa-coordinate
structure with large hindrance. Considering the similar redox
potentials of Cu2+ and Ru3+ (Table S1†), the redox potential
eﬀect of this Cu–Ru system contributed little to the velocity
diﬀerence of the Cu2+ and Ru3+ ion centres. Thus, a similar

Fig. 2 Reaction time measurements and reaction mechanism. Reaction time measurement results of (a) Ru(acac)3 by TEM measurements,and (b) Cu(OAc)2$H2O and (c) Cu(HCOO)2$4H2O by colorimetry. All
scale bars in (a) are 10 nm. (d) Proposed reaction intermediate showing
attack on the ligand-protected metallic ion centre by a polyol
molecule.

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reaction time of Cu(OAc)2$H2O and Ru(acac)3 resulted, ca. 5
min. On the other hand, Cu(II) at the centre of tetra-coordinated
Cu(HCOO)2$4H2O and tetra-coordinated Cu(acac)2 with small
hindrance gave relatively short reaction times of ca. 2 min andca. 1 min, respectively. The faster reaction velocity of Cu(acac)2
than Cu(HCOO)2$4H2O may have arisen from electron donationby the methyl group in the acetylacetonate ligand to the Cu(II)ion centre via a conjugation structure.
Aer measuring the reaction times of each precursor, thecoreduction conditions for CuRu alloy synthesis were experimentally tested by the reaction of each combination using
Ru(acac)3 with each of Cu(acac)2, Cu(HCOO)2$4H2O andCu(OAc)2$H2O. For the combination of Ru(acac)3 and Cu(acac)2,
the reaction product showed an asymmetric powder X-ray
diﬀraction (PXRD) pattern containing a large amount of the Cu
pattern (Fig. S4†). Core-shell structured NPs accompanying Ru
NPs were also found by scanning transmission electron
microscopy-energy-dispersive X-ray spectroscopy (STEM-EDX)
mapping (Fig. S5†). This core–shell structure formation
demonstrated a high reaction velocity diﬀerence between
Ru(acac)3 and Cu(acac)2 (Fig. S6†), which was consistent withthe reaction time measurement results. For the combination ofRu(acac)3 and Cu(HCOO)2$4H2O, the PXRD pattern of the
product was much better than that of Cu(acac)2, but the byproduct was still included as indicated by the asymmetric broad
main peak at a 2q of 42 (Fig. S7 and Table S2†). The multiplecomponent product observed by TEM (Fig. S8†) was a result of
the diﬀerent reaction times of Ru(acac)3 and Cu(HCOO)2$4H2O.
For the combination of Ru(acac)3 and Cu(OAc)2$H2O,
a symmetrical PXRD pattern and single-size distribution in the
TEM image were obtained, which demonstrated the formationof a single component (Figs. S9, S10 and Table S3†). Furthermore, the random distribution of Cu and Ru elements was
directly observed by conducting STEM-EDX mapping measurements (Fig. S11†). However, a lower Cu content was found: ca.
25 at% compared to the nominal ratio of 50 at% (Fig. S12†).
This random alloy structure showed good consistency withcoreduction conditions in which the reaction times of Ru(acac)3and Cu(OAc)2$H2O are the same. Notably, the coreduction
conditions were interrupted at exactly the same condition
except that anhydrous solvent was used, and the product wasestimated to be a mixture of fcc-Ru and Cu components(Fig. S13, S14 and Table S4†). The increased reaction velocity
from Cu(II) to Cu(0) by using the anhydrous solvent is considered to be due to the removal of coordinating H2O in
Cu(OAc)2$H2O dimers, resulting in its change from pentacoordinate to tetra-coordinate.
To further optimize the coreduction conditions, based on
conditions for the combination of Ru(acac)3 and Cu(OAc)2$H2O,
the following factors were also investigated: temperature (Figs.
S15, S16 and Table S5†); atmosphere (Fig. S17, S18 and Table
S6†); reducing agents and solvent (Fig. S19–S23, Scheme S1,Tables S7 and S8†); solvent oxygen content (Fig. S24–S26 andTable S9†), by 3 days of degassing with liquid N2; and stirring
speed (Fig. S27–S30 and Table S10†). Based on their impact on
the reaction, the relative importance of these factors for the Cu–
Ru system was estimated as follows: precursor > reducing agent

This journal is © The Royal Society of Chemistry 2020

Chemical Scienceand solvent > solvent oxygen content > stirring speed > atmosphere > temperature. Specically, for the Cu(OAc)2$H2O
precursor, the anhydrous solvent was the most importantfactor. Finally, the coreduction conditions were determined tobe the same as those of a previously reported protocol16 andnally achieved 46.3 at% Cu content close to the nominal ratio,
determined by X-ray uorescence (XRF) measurements (Table
S11†). The CuRu NPs of this study were obtained under thesame synthetic conditions.
In addition to PXRD, TEM, STEM-EDX mapping and XRF
results, high resolution STEM and X-ray photoelectron spectroscopy (XPS) measurements were performed to further characterize the obtained CuRu NPs. From the high resolution
STEM image of the CuRu NPs, the lattice fringe distance wasdetermined to be 2.16 Å, which is consistent with the {111}lattice plane calculated for an fcc-Cu0.5Ru0.5 alloy (Fig. S31†).
The electronic state change due to the homogeneous alloy
formation by XPS measurements is shown in Fig. S32 and Table
S12.† The Ru 3p binding energy of the CuRu NPs shied tohigher energy with Cu alloying, which is in agreement with the
Ru electron deciency. Meanwhile, the Cu 2p binding energy of
the CuRu NPs shied to lower energy with Ru alloying, which isin agreement with electron lling-in of Cu. Therefore, the XPS
spectra indicate the electronic state diﬀerence for CuRu NPs
compared with those of Cu and Ru metals, which provides
further proof for the atomic-level surface homogeneity of Cu
and Ru by the coreduction methodology. Combining the above
characterization, the CuRu solid solution NPs were successfully
synthesized where Cu and Ru were homogeneously mixed at theatomic level. Since Cu and Ru are totally immiscible in any
region of the bulk-state phase diagram,15 even in the liquid
phase, the successful atomic-level mixing of this challenging
Cu–Ru combination strongly suggested that the methodology
employed in this study for the determination of coreduction
conditions for solid solution formation could be universal for
immiscible systems.

Thermal stability
It is well known that the solid-solution state of immiscible
systems is thermodynamically unstable in the bulk state due tothe tremendous mixing enthalpy from the large local latticestrain caused by the large diﬀerence of atomic radii, known as
the Hume-Rothery rules (Fig. S33A†).29 Although the unstable
solid-solution state of immiscible systems can be stabilized byincreasing mixing entropy, through the formation of highentropy alloys,30 it can only be stabilized when the number of
elements is more than four. Recently, some new ndings havesuggested that the surface of nanoscale materials has a much
more exible structure.31 The possible mechanism for stabilization of the solid-solution state on the nanoscale is considered
to be relaxation of local strain by the more exible structure ofnanoparticles, as compared to with the rigidity of bulk material
structures (Fig. S33B†). This prompted us to investigate thethermal stability of CuRu solid-solution NPs, even though thebulk state solid-solution Cu–Ru system is very unstable.15,32 The
thermal stability of CuRu solid-solution NPs was determined by

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Chemical Sciencein situ synchrotron XRD in capillary under vacuum. Surprisingly, no phase separation was observed from 298 K up to 773 K(Fig. 3a). Rietveld renement results also suggested the solidsolution structure of each component at 773 K, with only a small
ratio of fcc-to-hcp (hexagonal close-packed) phase transition(Fig. 3b and Table S13†). Furthermore, STEM-EDX mapping
measurements were performed on the sample aer the 773 K
thermal stability, revealing homogenous distribution of Cu andRu elements and providing evidence of the high thermal
stability of CuRu solid-solution NPs up to at least 773 K ina vacuum (Fig. 3c and Fig. S34†). It was proposed that an
atomic-level well mixed solid solution may have higher stability
than that of a domain mixed alloy, since domain mixing alloy is
the intermediate state before conversion to the phase separation alloy, by suﬃcient heating (Fig. S35A†). It is believed thatthe high thermal stability of CuRu solid-solution NPs originatedfrom the well optimized coreduction conditions, which led toa solid-solution structure that was homogeneously mixed at theatomic level (Fig. S35B†).

Exhaust purication activity
PGMs are well known as practical catalysts for exhaust purication reactions in the automotive industry, and replacement of
PGMs to reduce the cost has long been a key issue. Cu is anearth-abundant metal with a much lower price and higher
natural reserves; The high thermal stability of the CuRu solidsolution NPs formed in this study inspired the application ofFig. 3 Thermal stability of CuRu solid-solution nanoparticles. (a) In situ
synchrotron PXRD measurements under vacuum from 298–773 K with
0.5787 Å wavelength. (b) The synchrotron XRD pattern of CuRu
nanoparticles (black dots) at 773 K and the calculated pattern (red line).
The bottom lines show the diﬀerence proﬁle (grey) and the ﬁtted
curves of the fcc component (blue) and hcp component (green). (c)High-angle annular dark-ﬁeld (HAADF)-STEM, Cu–K, Ru–L andreconstructed overlay STEM-EDX maps obtained from a group of
CuRu NPs after heating at 773 K. All scale bars are 50 nm.

11416 | Chem. Sci., 2020, 11, 11413–11418

Edge Article
the solid solution to an exhaust purication test. Fcc-Ru NPs
were also synthesized as a comparison.
The extra polyvinylpyrrolidone (PVP) of CuRu and fcc-Ru NPs
was removed by repeated ethanol washing down to 34.4 and38.8 wt%, respectively; the remaining PVP will be decomposed
during the catalytic test.33 CuRu solid-solution and fcc-Ru NPs
were supported on g-Al2O3 at 1 wt% via the wet impregnation
method (see the ESI†). The HAADF-STEM image of a g-Al2O3
supported nanoalloy exhibits good dispersion of CuRu NPs(Fig. S36†). The exhaust purication tests for each catalyst werecarried out from r.t. to 773 K without further pre-treatment, at
a stoichiometric O2 concentration of l ¼ 1 (see the ESI†). The
catalytic activities expressed as T50 of CuRu solid-solution NPs
and fcc-Ru NPs were almost identical for NOx reduction and CO
oxidation, even though 50 at% of the PGM Ru element was
replaced by inexpensive Cu (Fig. 4a and b). Meanwhile, thecatalytic activity of CuRu solid-solution NPs for C3H6 oxidation
was close to that of fcc-Ru NPs (Fig. 4c).
The l dependence of each catalyst at a xed temperature of
400  C was also investigated. The activity of CuRu solid-solution
NPs for CO oxidation was slightly higher than that of fcc-Ru NPs
within l < 1 (Fig. 4d), while no obvious diﬀerence was foundbetween CuRu solid-solution and fcc-Ru NP activities for NOx
reduction within a wide l range, except for l ¼ 1 (Fig. 4e). CuRu
solid-solution NPs performed moderately in C3H6 oxidationcompared with fcc-Ru NPs, within l < 1 (Fig. 4f). Interestingly, inthe range near l ¼ 1, all the CO, NO and C3H6 conversions of
CuRu solid-solution NPs reached 100%, and performed better
than fcc-Ru NPs. The versatile CuRu solid solution catalyst has
great potential to be applied as a single-component exhaust

Exhaust puriﬁcation performance of CuRu solid-solution
nanoparticles. Temperature dependence of (a) CO, (b) NO, and (c)C3H6 conversion in exhaust puriﬁcation reactions for CuRu/g-Al2O3(red), and fcc-Ru/g-Al2O3 (black) at l ¼ 1. l dependence of (d) CO, (e)NO, and (f) C3H6 conversion for CuRu/g-Al2O3 (red), and fcc-Ru/gAl2O3 (black) at 400  C.
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purication catalyst. Even Rh, the most famous exhaust purication catalyst, cannot simultaneously purify CO, NOx andhydrocarbon species up to 100%, it also requires Pt and Pd as
co-catalysts for complete CO conversion.34 Notably, under theconditions of the similar loading amount and the same alumina
support, the CuRu solid solution catalyst exhibited close
conversion with Pd/Rh and Pt/Rh catalysts in NO reduction andhydrocarbon oxidation, but did not match with Pd/Rh and Pt/
Rh catalysts in CO conversion;35 this may be due to a relatively
large mean diameter for CuRu NPs (Fig. S37, S38†). Interestingly, N2O (damaging the central nervous system36 and ozonosphere37) was not detected from the CuRu catalysed reaction bygas chromatography (GC); however, Pt/Rh and Pt/Rh catalysts
generated much N2O in a lower temperature range.35 Furthermore, considering that the mean particle size of CuRu solid
solution is larger than that of fcc-Ru NPs (Fig. S37†), theintrinsic exhaust purication activity of CuRu solid solution ishigher than that of fcc-Ru NPs. The mass and heat transfer
could be neglected according to the calculations (see theExperimental details in the ESI†). Finally, repetitive tests
showed that the NOx reduction ability decreased especially aer
the second test (Fig. S39†).

Chemical Science
To understand the origin of the higher intrinsic exhaust
purication activity of CuRu solid solution, in situ Fouriertransform infrared spectroscopy (FTIR) was performed (Fig. 5).
In Fig. 5a and b, under a NO ow, a much stronger NO
adsorption onto Ru sites (at 1864 cm1) was found for CuRu
compared with fcc-Ru, and disabled CO adsorption during a NO
+ CO ow. In Fig. 5c and d, under a CO ow, an intense IR bandat 2043 cm1 due to CO adsorbed on Ru was detected in CuRu.
This proved its better adsorption ability for CO than what was
found for fcc-Ru. With the addition of NO, the Ru–CO species
was quickly consumed, with the formation of N]C]O species
with a shoulder peak at 2247 cm1,38 which may act as theintermediate of NOx reduction with CO and then nally be
decomposed to N2 and CO2.39 Thus, the higher intrinsic exhaust
purication activity of CuRu solid solution may originate fromthe intensive interaction between reaction molecules and theisolated highly active Ru sites under Cu atomic-level substitution. Meanwhile, considering that the Cu element is easily
oxidized under reaction condition Cu sites were found to be
intrinsically inert to NOx reduction in CuRu. In addition, the insitu FTIR results revealed that no N2O species (sharp double
peaks at 2237 and 2210 cm1)40 was detected for the CuRu
catalyst, consistent with GC analyses of catalytic products.

Conclusions
This study provides a methodology for achieving the optimal
coreduction conditions of the metallic precursors, investigatedfrom the viewpoint of the steric hindrance eﬀect on the reaction
times of the metallic precursors. By applying this methodology,
atomic-level homogeneous mixing was successful even in thewhole-region immiscible Cu–Ru system. This well-mixed CuRu
solid solution demonstrated high thermal stability up to at least
773 K in a vacuum. For this CuRu solid-solution three-way
catalyst, even though 50% of the PGM Ru was replaced by EAMCu, its exhaust purication activity was still comparable to thatof fcc-Ru NPs. Furthermore, the CuRu solid solution performed
well with versatile exhaust purication ability and zero N2Oemission, thus promising to be an alternative exhaust purication catalyst for further application in the automotive
industry. Most importantly, the coreduction methodology presented in this report will greatly shorten the synthetic trial anderror period for new solid solutions in other immiscible
systems, and give much valuable insight into functional materials developments.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
In situ FTIR spectra under a NO and CO ﬂow. In situ FTIR
spectra of (a) CuRu/g-Al2O3 and (b) fcc-Ru/g-Al2O3 under a NO ﬂow
followed with a NO + CO ﬂow. In situ FTIR spectra of (c) CuRu/g-Al2O3
and (d) fcc-Ru/g-Al2O3 under a CO ﬂow followed with a CO + NO
ﬂow. All the spectra were measured at 200  C with He balance.
Fig. 5This journal is © The Royal Society of Chemistry 2020

This research was supported by the ACCEL program, Japan
Science and Technology Agency (JST), JPMJAC1501, and theFundamental Research Fund of Xi'an Jiaotong University,xxj032019005. STEM observations were performed as part ofa program conducted by the Advanced Characterization

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Nanotechnology Platform sponsored by the MEXT of the Japanese Government. Synchrotron XRD measurements werecarried out at SPring-8 under proposal No. 2015B1467 and2017A11189. The activities of the INAMORI Frontier ResearchCentre, Kyushu University are supported by the KYOCERA
Corporation. Acknowledgment to my colleagues Dr Quan Zhang
and Dr Dongshuang Wu for their kind help on reaction timemeasurements and manuscript polishing. Acknowledgment to
my student Ms. Zhe Tan for her suggestion and discussion on
mass and heat transfer calculations. Special acknowledgment toProf. F. Fiévet of Université Paris Diderot for his kind advice onreference for the viscosity factor.

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