Structure of catalytically active Rh-In bimetallic phase for amination of alcohols

Article abstract of DOI:10.1039/c4ra03984j

The structure of Rh-In bimetallic catalysts supported on carbon for amination of alcohols was determined by XRD, TEM-EDX, XPS, CO adsorption and EXAFS. At low In/Rh ratio (In/Rh ≤ 0.2), Rh metal particles with sizes of <3 nm were observed, and the particles were partially covered with indium oxide species. With addition of more In, a tetragonal RhIn alloy with a particle size of ～20 nm was formed. This tetragonal alloy has a structure with a = 0.315 nm and c = 0.328 nm where metal atoms are located at (0, 0, 0) and (0.5, 0.5, 0.5). The catalytic activity of the tetragonal RhIn alloy is much higher than that of Rh metal particles with or without indium oxide species. With an excess amount of In (In/Rh > 1) on the high Rh loading (20 wt%) catalyst, the cubic RhIn phase with a CsCl structure was observed instead of the tetragonal RhIn phase, and the catalytic activity was much decreased.

Full text of DOI:10.1039/c4ra03984j

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Structure of catalytically active Rh–In bimetallic
phase for amination of alcohols†
Cite this: RSC Adv., 2014, 4, 28664
Tsukasa Takanashi, Masazumi Tamura, Yoshinao Nakagawa* and Keiichi Tomishige*
The structure of Rh–In bimetallic catalysts supported on carbon for amination of alcohols was determined
by XRD, TEM-EDX, XPS, CO adsorption and EXAFS. At low In/Rh ratio (In/Rh # 0.2), Rh metal particles with
sizes of <3 nm were observed, and the particles were partially covered with indium oxide species. With
addition of more In, a tetragonal RhIn alloy with a particle size of ꢀ20 nm was formed. This tetragonal
alloy has a structure with a ¼ 0.315 nm and c ¼ 0.328 nm where metal atoms are located at (0, 0, 0) and
Received 1st May 2014
Accepted 17th June 2014
(0.5, 0.5, 0.5). The catalytic activity of the tetragonal RhIn alloy is much higher than that of Rh metal
particles with or without indium oxide species. With an excess amount of In (In/Rh > 1) on the high Rh
loading (20 wt%) catalyst, the cubic RhIn phase with a CsCl structure was observed instead of the
tetragonal RhIn phase, and the catalytic activity was much decreased.
DOI: 10.1039/c4ra03984j
www.rsc.org/advances
28
has been suggested; however not identied. In the previous
1
. Introduction
paper, we characterized Rh–In/C catalyst with 5 wt% Rh and In/
4
Supported bimetallic catalysts can sometimes promote reactions Rh ¼ 1 with XRD, TEM and XPS. The data suggested the
that do not proceed over monometallic catalysts, and are an formation of RhIn alloy; however, because of the small loading
1–3
important class of heterogeneous catalysts. The structure of amount and broad XRD signals it is difficult to determine the
bimetallic particles generally much affects the catalytic perfor- structure of catalytically active species. In this study, we inves-
mance, and therefore characterization of the catalysts is essen- tigated the structure of carbon-supported Rh–In catalysts with
tial. We have very recently discovered that Rh–In supported on large Rh loadings and different In/Rh ratio in order to clarify the
carbon can catalyze the amination of alcohols in ammonia water, relationship between the structure and catalytic activity. We
4
while monometallic Rh/C and In/C are almost inactive. Amina- found a catalytically active RhIn phase with tetragonal structure
tion of alcohols (eqn (1)) is one method of amine production, and that has not been reported in the literature.
the advantages over conventional amine production methods
such as reductive amination of carbonyl compounds include
inexpensive starting material and high atom efficiency.
2
. Experimental section
5–7
2.1. Catalyst preparation
R–OH + NH / R–NH + H
3
2
2
O
(1)
Rh/C and In/C catalysts were prepared by impregnating the
carbon (Shirasagi FAC-10, BET surface area 851 m g ; Japan
EnviroChemicals, Ltd.) with an aqueous solution of
2
ꢁ1
Some Rh-based bimetallic catalysts such as those using Sn,
Mo, and Re have been reported to show unique activity and/or
selectivity in reductive transformations of organic molecules,
RhCl
Wako Pure Chemical Industries, Ltd.), respectively. The Rh–In/
C catalysts were prepared by a co-impregnation method using
the carbon support and mixed aqueous solution of RhCl $3H
and In(NO $3H O. The loading amount of Rh was 5 or 20 wt%,
3 2 3 3 2
$3H O (Soekawa Chemical Co., Ltd.) and In(NO ) $3H O
(
8–17
and the structures have been intensively investigated.
On the
other hand, the combination of noble metal and In has been
3
2
O
18,19
20–24
rarely studied for catalyst materials
except Pt–In system.
3
)
3
2
Even for bulk material, the structure or use of Rh–In alloy
system has not been so intensively studied.
and that of In was 0.1–2 by molar ratio of In to Rh. Aer
impregnation, the catalysts were dried at 383 K for half a day
25,26
The known
phases of Rh–In alloy are RhIn with cubic CsCl structure (a ¼
and reduced under H owing at 773 K for 3 h. For Rh 5 wt%
2
0
0
.320 nm) and RhIn
.701 nm, c ¼ 0.715 nm). The existence of Rh-rich Rh–In phase
3
with tetragonal CoGa
3
structure (a ¼
catalysts, they were heated under N
aer drying followed by reduction under H
2
owing at 773 K for 3 h
owing at 773 K for
27
2
4
3
h, as reported in the previous paper.
Department of Applied Chemistry, School of Engineering, Tohoku University, 6-6-07,
Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. E-mail: tomi@erec.che.tohoku.ac.
jp; yoshinao@erec.che.tohoku.ac.jp; Fax: +81-22-795-7214; Tel: +81-22-795-7214
2.2. Characterization of the catalysts
The concentration of indium in the catalysts was determined by
X-ray uorescence spectroscopy (XRF) using a Bruker AXS K.K.,
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra03984j
1
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S8 Tiger spectrometer with the wavelength dispersive system. transmission mode. For extended XAFS (EXAFS) analysis, the
The pretreated catalyst powders were mixed with cellulose and oscillation was rst extracted from the EXAFS data using a
Sn as binder and internal standard, respectively, and then spline smoothing method. Oscillation was normalized by the
3
compressed to 10 f pellets with about 2 mm thickness. The edge height around 50 eV. Fourier transformation of the k -
weight concentration of indium was determined by calibration weighted EXAFS oscillation from the k space to the r space was
curve method.
performed to obtain a radial distribution function. The
The XRD patterns of the reduced catalysts were recorded on a inversely Fourier ltered data were analyzed using a usual curve
Rigaku MiniFlex600 diffraction-meter using Cu Ka (l ¼ tting method. For curve tting analysis, the empirical phase
0.154 nm) generated at 40 kV and 15 mA. The catalysts with shi and amplitude functions for the In–O bonds and metallic
various In/Rh molar ratios were reduced under H owing at bonds such as Rh–Rh, In–In, Rh–In and In–Rh were extracted
2
7
73 K for 3 h. The reduced catalyst powders were transferred to from data for Rh O and Rh foil, respectively, because of the
2 3
the sample holder lled with N without exposing air in the similar behavior of Rh and In as backscattering atoms in EXAFS
2
glove bag. The average particle size of metal was estimated and the complex structures of In metal and In
using the Scherrer equation. Dispersion of Rh particles (the XAFS data were performed using computer program
ratio of surface atoms to total atoms; Rh /Rh) was calculated by (REX2000, ver. 2.5.9; Rigaku Corp.) for both EXAFS and XANES
the equation: Rh
2 3
O . Analyses of
a
s
2
9
s
/Rh ¼ 1.098 nm/(particle size). XRD pattern (X-ray adsorption near edge structure).
of model structure was calculated with CrystalDiffract for
Windows ver. 1.4 soware (CrystalMaker Soware Ltd.).
The amount of CO chemisorption was measured in a high-
vacuum system using a volumetric method. Before adsorption
2.3. Procedure for the activity test in the amination of
alcohol
Activity tests of 1,2-propanediol (Wako Pure Chemical Indus-
tries, Ltd.) were performed in a 190 ml autoclave with an
inserted glass vessel. Substrate (10 mmol), aqueous ammonia
supplied by Wako Pure Chemical Industries, Ltd. (28 wt%,
0.1 mol), catalyst (50 mg), a spinner and water were placed into
the autoclave. The total volume of the reaction solution was
adjusted to 20 ml by addition of water. Aer sealing the reactor,
measurements, the catalysts were treated with H
2
at 773 K for
3
h. Subsequently, the adsorption was performed at room
temperature. The gas pressure at adsorption equilibrium was
about 1.1 kPa. The sample weight was about 0.15 g. The dead
3
volume of the apparatus was about 80 cm . The adsorption
amount of CO is represented as the molar ratio to Rh.
The X-ray photoelectron spectra (XPS) were recorded using a
Shimadzu AXIS-ULTRA with monochromatic Al Ka irradiation
the N
2
content was purged by ushing three times with 1 MPa
at room temperature
H
2
. The reactor was lled with 5 MPa H
2
(
3
3
hn ¼ 1486.6 eV). The spot size of the X-ray was ca. 700 ꢂ
2
and then heated to the reaction temperature (453 K). The
temperature was monitored using a thermocouple inserted in
the reactor. During the experiment, the stirring rate was xed at
00 mm . The catalysts were reduced in owing H
h, and then, the reduced catalysts were transported to the
atmosphere. The spectra were analyzed
2
at 773 K for
analysis chamber in N
2
500 rpm. Aer 24 h, the reactor was cooled down and the gases
with the program CasaXPS soware, version 2.3.16. A binding
energy of 284.5 eV of the C 1s level was used as an internal
standard. The curve tting of the In 3d level spectra was con-
ducted with Voigt lineshapes and a Shirley background
function.
were collected in a gas bag. The reactor contents were trans-
ferred to a vial, and the catalyst was separated by ltration. Then
reaction solution was diluted by water to analysis. The products
in both liquid and gas phases were analyzed by a gas chro-
matograph (Shimadzu GC-2014) equipped with InertCap for
amines column (GL Sciences Inc.). The selectivity was calcu-
lated in carbon basis.
Field emission-scanning transmission electron microscope
(FE-STEM) images and EDX analysis were obtained on a Hitachi
spherical aberration corrected STEM/SEM HD-2700 instrument
operated at 200 kV. The reduced catalyst was dispersed in
ethanol in the glove bag. Then dispersed sample was deposited
on a Cu grid in air. The measurement was begun within 10
minutes aer deposition.
3
. Results and discussion
3.1. Sample preparation and loading amount
The X-ray absorption ne structure (XAFS) spectra were We prepared Rh–In/C catalysts with 20 wt% Rh loading amount,
measured at the BL01B1 station at SPring-8 with the approval of in addition to those with 5 wt% Rh as reported in the previous
4
the Japan Synchrotron Radiation Research Institute (JASRI; study, in order to characterize the bimetallic phase more
proposal no. 2013B1067). The storage ring was operated at 8 clearly. We applied co-impregnation method to the catalyst
GeV. A Si (1 1 1) single crystal was used to obtain a mono- preparation, and the method could bring all the used precur-
chromatic X-ray beam. Two ion chambers for I and I were lled sors on to the support. However, aer calcination and reduction
0
with Ar for Rh and In K-edge measurement. We measured the of the sample with larger Rh and In loadings, some amount of
XAFS spectra of the catalysts aer reduction with owing H at yellow–orange solid was deposited at the end of reactor tube.
2
7
73 K for 3 h. The reduced catalyst powders were transferred to The solid was probably In O considering the low melting point
2 3
30
the measurement cell without exposing air in the glove bag. The of In metal (430 K). We therefore analyzed the actual loading
thickness of the cell lled with the powder was 1 cm to give an amount of In on the reduced sample with XRF. The XRF data are
edge jump of 0.8–4.0 and 0.1–1.0 for Rh K-edge and In K-edge shown in Table 1. Although signicant loss of In was observed
measurement, respectively. The XAFS data were collected in a for the sample with very large In loading (In/Rh ¼ 2, nominally),
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Table 1 XRF analysis of Rh–In/C (Rh 20 wt%)
ratio. We calculated the turnover frequency (TOF) as dened in
eqn (2) to assess the catalytic activity, and the values are also
listed in Fig. 1.
In/Rh in preparation
In/Rh from XRF
In loss/%
1
1
2
.0
.5
.0
1.0
1.4
1.6
2
7
18
ꢁ
1
TOF (h ) ¼ (converted substrate (mol))/
{(amount of CO adsorption (mol)) ꢂ (reaction time (h))} (2)
The amount of CO adsorption corresponds the amount of
surface metallic Rh atoms, since In metal or metal oxides do not
adsorb CO. The CO adsorption amount was gradually decreased
from In/Rh ¼ 0 to 0.5 and was rapidly decreased by further
addition of In. The TOF value was much increased with
increasing In amount from In/Rh ¼ 0.2 to 1, and the catalyst
the loss of In from the sample with smaller In loading (In/Rh ¼
) was negligible. We use the nominal value of In/Rh ratio of the
samples in the following sections.
1
3
.2. Catalytic performance of Rh–In/C (20 wt%) with
different In loading amount, and the difference between Rh
0 wt% and 5 wt% catalysts
ꢁ
1
with In/Rh ¼ 1 showed the highest TOF value (14 h ). However,
2
further addition of In much decreased the catalytic activity:
ꢁ
1
when In/Rh ¼ 2, the catalyst showed no activity (TOF < 2.3 h ).
The catalytic performances of Rh–In/C catalysts with smaller Rh
amount (5 wt%) in the same reaction conditions as those in
The results of activity tests of Rh–In/C (Rh 20 wt%) for 1,2-
propanediol amination are shown in Fig. 1. The products were
2-amino-1-propanol, 1-amino-2-propanol, and the dimerization
4
Fig. 1 have been reported in the previous paper. Both series of
products (2,5- and 2,6-dimethylpiperazines) (Scheme 1). The
catalyst without In (Rh/C) showed very low activity. The
substrate conversion as a function of In amount showed a
volcano curve, and the catalyst with In/Rh ¼ 0.5 gave the highest
conversion. The selectivity to aminopropanols was increased
with increasing In amount. The ratio of 1-amino-2-propanol/2-
amino-1-propanol was almost 1 and was independent of In/Rh
catalysts (Rh 20 wt% and 5 wt%) have similar trends of
performance when In/Rh ratio is 1 or less: the activity of Rh/C
without In is very low; the catalysts with low In/Rh have low
activity; the TOF value is much increased by In addition from In/
Rh ¼ 0.2 to 1; the selectivity to amino alcohols is increased with
increasing In/Rh, and the ratio of 1-amino-2-propanol/2-amino-
1-propanol is almost constant. It should be noted that both
series of catalysts with Rh 20 wt% and 5 wt% showed similar
ꢁ
1
TOF values for low In/Rh ratio (In/Rh ¼ 0.1 and 0.2; TOF ꢀ3 h
)
ꢁ1
and similar highest TOF values (14 and 15 h for Rh 20 wt%
and 5 wt%, respectively). The highest TOF values were obtained
when In/Rh ¼ 1 for both series. However, when In/Rh ¼ 2, the
catalysts with different Rh loadings showed different behavior:
the catalyst with Rh 20 wt% (Fig. 1) showed almost no activity,
while the catalyst with Rh 5 wt% has moderate activity (TOF
ꢁ
1
1
0 h ). Overall, these data suggest that the structures of sup-
ported phases are similar for both series of catalysts (Rh 20 wt%
and 5 wt%) when In/Rh ¼ 1 or less.
3.3. Characterization of Rh–In/C: XRD
Fig. 2 shows the XRD patterns of reduced Rh–In/C (Rh 20 wt%)
catalysts under inert atmosphere. The patterns can be classied
Fig. 1 Catalytic performance of Rh–In/C (Rh 20 wt%) for amination of into three groups based on In/Rh ratio: (i) In/Rh # 0.2, (ii) 0.2 <
,2-propanediol. Reaction conditions: water 13.2 g, 1,2-propanediol In/Rh # 1, and (iii) In/Rh > 1. The pattern of group (i) had the
1
0
.76 g, NH
3 2
aq 6.08 g (total 20 ml), catalyst 50 mg, H 5 MPa (at r.t.), 453
peaks of fcc Rh. The pattern of group (iii) had the intense peaks
of cubic RhIn (PDF 59516). The pattern of group (ii) showed the
peaks different from those of fcc Rh and cubic RhIn. The
pattern was composed of pairs of peaks with intensity ratio of
K, 24 h.
2
: 1. The peak positions were almost unaffected by the differ-
ence of In/Rh ratio and were similar to those of cubic RhIn.
Each pair corresponded to a peak of cubic RhIn. We conducted

tting of the calculated peak patterns to the pattern of group (ii)
by distortion of cubic RhIn structure. The distortion along the z-
axis well tted the peak split, and the tetragonal structure with
a ¼ 0.315 nm and c ¼ 0.328 nm gave the best t (Fig. 3).
However, since the atomic number of Rh and In is similar, the
distribution of Rh/In atoms cannot be determined based on the
Scheme 1 Amination of 1,2-propanediol.
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previous paper. Therefore, the structure of reduced catalyst
surely reects the catalytic performance.
Comparison with the results of activity tests (Fig. 1) shows
that the catalytic activity is in the following order: group (ii)
>
group (i) [group (iii) ꢀ0. The very low activity of group (iii)
(Rh–In/C (Rh 20 wt%, In/Rh > 1)) indicates that the cubic RhIn
species is not catalytically active. The high activity of group (ii)
suggests the activity of tetragonal alloy phase; however this idea
has not been fully proven because another active phase may
exist which does not appear in XRD (amorphous or very small
size). We further characterized each group of catalysts.
3.4. Characterization of Rh–In/C (Rh 20 wt%, In/Rh # 0.2)
(group (i))
The XRD peak positions (Fig. 2) were not changed by different In
amount, indicating that alloy was not formed. Table 2 shows the
Rh particle size determined by the width of XRD peak. The CO
adsorption amount is also shown in Table 2. The particle size was
decreased with increasing In amount, which means that more
atoms in the particles are located on the surface. On the other
hand, the CO adsorption amount, which means the amount of
surface Rh atoms, was slightly decreased with increasing In
amount. This type of discrepancy has been frequently observed
Fig. 2 XRD patterns of Rh–In/C (Rh 20 wt%) with different indium
amount. (a) Rh/C, (b–i) Rh–In/C; (b) In/Rh ¼ 0.1, (c) In/Rh ¼ 0.2, (d) In/ for supported bimetallic catalysts that consist of noble metal +
9
,10,31–33
Rh ¼ 0.5, (e) In/Rh ¼ 0.75, (f) In/Rh ¼ 1, (g) In/Rh ¼ 1.2, (h) In/Rh ¼ 1.5, (i) non-noble metal,
and it can be explained by the partial
In/Rh ¼ 2. : fcc Rh metal, : tetragonal alloy, : cubic RhIn.
covering of noble metal particle (Rh) with non-noble metal (In)
species. We further pay attention to the difference between
surface Rh atoms calculated by XRD and those determined by CO
adsorption (Rh /Rh ꢁ CO/Rh). The (Rh /Rh ꢁ CO/Rh) value was
s
s
increased by In addition, and the increase was almost the same
as the amount of In addition. This agreement suggested that all
added In atoms interacted with Rh surface assuming that
covering with one In atom blocks one adsorption site.
The In 3d XPS data are shown in Fig. 4. Each peak can be

tted by one single function, and all the peak positions (444.5
and 444.6 eV for 3d5/2; 452.0 and 452.1 eV for 3d3/2) are within
the range for In(III) species (3d5/2: 444.26–444.84 eV, 3d3/2
:
34
4
51.80–452.38 eV). The absence of In(0) signal supported the
absence of Rh–In alloy. The summary of XPS data for all
samples is shown in ESI (Table S1).†
Based on the data, we propose the model structure of this
group of catalyst (Fig. 5). The Rh metal particles are formed on
the carbon support, and all the added In atoms are located on
Fig. 3 Calculated and observed XRD patterns of Rh–In/C (In/Rh ¼ 1). the Rh particle surface as In(III) oxide. The comparison between
the structure (Fig. 5) and catalytic activity (Fig. 1) shows that the
modication of Rh metal with indium oxide generates the
catalytic activity in the amination.
XRD patterns at this stage; the distributions will be discussed in
Section 3.5. The XRD patterns of Rh–In/C (Rh 5 wt%) are shown
in Fig. S1 (ESI),† where similar structure change is evident from
fcc Rh phase to alloy phase with CsCl (or CsCl-like) structure
Table 2 Rh particle size of Rh–In/C (Rh 20 wt%; In/Rh # 0.2)
between In/Rh ¼ 0.2 and 0.5. However, the XRD peaks in In/Rh
$
0.5 samples were too broad to distinguish tetragonal (group
Particle size/nm
from XRD
Rh /Rh
from XRD
Rh /Rh
ꢁ CO/Rh
s
s
(ii)) and cubic (group (iii)) phases.
In/Rh
0
CO/Rh
The XRD of Rh–In/C (Rh 20 wt%, In/Rh ¼ 1) aer reaction
3.7
2.8
2.3
0.30
0.39
0.48
0.21
0.19
0.18
0.09
0.20
0.30
was also measured, and the pattern was almost unchanged
from that of the fresh reduced sample (Fig. S2, ESI†). The
reusability of Rh–In/C catalyst has been conrmed in the
0
0
.1
.2
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smaller, the phase of fcc Rh metal was also evident. These
pattern changes and the characterization results in the above
section suggest that the addition of In above In/Rh ¼ 0.2 causes
the phase change from indium-oxide-modied Rh metal parti-
cles into tetragonal Rh–In alloy. The In 3d XPS data of Rh–In/C
(
Rh 20 wt%, In/Rh ¼ 0.5, 1) are shown in Fig. 7. Each peak
can be tted by two functions. The peak positions of the func-
tions with higher binding energy were within the range of In(III),
and those at lower binding energy were almost the same as In
metal (443.9 and 451.4 eV (ref. 34)). Therefore the samples
contained both In(0) and In(III) species. The ratio of In(0) to
(In(0) + In(III)) within the effective escape depth was determined
to be 50% and 65% for the sample of In/Rh ¼ 0.5 and 1,
respectively. The increasing amount of In(0) supports the
formation of Rh–In alloy.
Fig. 4 In 3d XPS of Rh–In/C (Rh 20 wt%, In/Rh ¼ 0.1 (a) and 0.2 (b)). [ ]:
area ratio.
The TEM images and EDX analyses of Rh–In/C (Rh 20 wt%,
In/Rh ¼ 1) are shown in Fig. 8 and 9, respectively. Surprisingly,
large dendritic structure was present as well as carbon-sup-
ported particles. The dendritic structure was composed of
domains with the size of 10–30 nm. The particles on the support
had various sizes from <5 nm to ꢀ20 nm. The EDX analysis was
35
conducted with either Cliff–Lorimer method, which gives the
average element ratio in the total eld of image, or conventional
line analysis. The results showed that both dendritic structure
and carbon-supported particles had the In/Rh ratio of 1.0. We
also measured the mapping of elemental distributions (Fig. S5,
ESI†), which also conrmed the bimetallic structure. From XRD,
the particle size of tetragonal Rh–In alloy in Rh–In/C (Rh 20 wt%,
In/Rh ¼ 1) was calculated to be 20 nm. In this calculation, we
assumed that the broadening by the heterogeneity of alloy
composition did not affect the peak width since the change of In/
Rh ratio did not affect the XRD peak positions. Based on TEM-
EDX and XRD, we determined the phase of both the dendritic
structure and the larger particles on the support as tetragonal
RhIn alloy with the atomic ratio of 1 : 1. The analogy with cubic
RhIn alloy with CsCl structure suggests that the Rh and In atoms
in tetragonal RhIn alloy are located at (0, 0, 0) and (0.5, 0.5, 0.5),
respectively, in the unit cell as similar to CsCl structure. It is not
known why tetragonal RhIn alloy was formed on this catalyst
rather than well-known cubic RhIn alloy. One factor may be the
shortage of In. In the literature, cubic RhIn phase has been
reported to exist in slightly indium-rich conditions of In + Rh
Fig. 5 Structure model of Rh–In/C (Rh 20 wt%, In/Rh # 0.2).
The state of Rh and In was also investigated with EXAFS. The
results of Fourier transform and curve tting are shown in Fig. 6
and Table 3, respectively. The EXAFS oscillation and Fourier

ltered EXAFS data are shown in ESI (Fig. S3 and S4).† The Rh K-
edge EXAFS data were well tted by single Rh–Rh shell with
coordination number (CN) ꢀ11, supporting that all Rh atoms
were in the metallic state. The In K-edge EXAFS data can be

tted by In–O shell and In–Rh shell as expected by the model
structure as shown in Fig. 5.
3
1
.5. Characterization of Rh–In/C (Rh 20 wt%, 0.2 < In/Rh #
) (group (ii))
The XRD data (Fig. 2) showed that the main species of this
group was tetragonal Rh–In alloy. When the In amount was
28
system: from In/(In + Rh) ¼ 51% to 56% at 873 K.
Fig. 10 and Table 4 show the Fourier transform and the curve

tting results of EXAFS, respectively. The EXAFS oscillation and
Fourier ltered EXAFS data are shown in ESI (Fig. S3 and S4).†
The Rh K-edge EXAFS data can be tted by three Rh–Rh (or In)
shells, which consists of one shell for fcc Rh structure (R ¼
0
0
.268 nm) and two shells for bcc-like structure (R ¼ 0.28 and
.32 nm; ratio of CN ¼ 8/6). The latter shells corresponded to
the tetragonal RhIn alloy. The CN of Rh–Rh shell for fcc Rh
structure was decreased and the CNs for RhIn alloy were
increased with increasing In amount. This behavior supports
the transformation of fcc Rh particles, probably modied with
indium species, into RhIn alloy by addition of indium. The
In(III) species detected by XPS analysis (Fig. 7) can be assigned to
the In species covering the fcc Rh metal particles. The In K-edge
3
Fig. 6 Results of Fourier transform of k -weighted EXAFS oscillation of
Rh–In/C (Rh 20 wt%, In/Rh # 0.2). (a) Rh K-edge, FT range: 30–150
nm ; (b) In K-edge, FT range: 30–120 nm
ꢁ1
ꢁ1
.
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Table 3 EXAFS curve fitting results of Rh–In/C (Rh 20 wt%; In/Rh # 0.2)
Sample
Fourier ltering
range/nm
a
b
c
d
0
e
f
(In/Rh)
Edge
Shell
CN
R nm
s /nm
DE
/eV
R
/%
0
0
Rh K
Rh K
In K
Rh–Rh
Rh–Rh (or –In)
In–O
11.1
10.8
2.1
2.4
1.2
10.5
1.4
1.6
2.3
12
0.268
0.268
0.211
0.225
0.271
0.268
0.211
0.225
0.271
0.268
0.206
0.0079
0.0084
0.0060
0.0060
0.0083
0.0077
0.0076
0.0078
0.0083
0.0060
0.0060
3.0
5.3
ꢁ9.6
ꢁ0.3
5.5
0.6
1.2
2.4
0.166–0.292
0.166–0.292
0.126–0.279
.1
.2
In–O
In–Rh (or –In)
Rh–Rh (or –In)
In–O
In–O
In–Rh (or –In)
Rh–Rh
0
Rh K
In K
3.4
2.4
1.0
2.2
0.166–0.292
0.126–0.279
ꢁ4.6
0.4
Rh foil
Rh
Rh K
Rh K
0
0
—
—
—
—
2
O
3
Rh–O
6
a
b
c
d
Coordination number. Bond distance. Debye–Waller factor. Difference in the origin of photoelectron energy between the reference and the
e
sample. Residual factor.
3
Fig. 10 Results of Fourier transform of k -weighted EXAFS oscillation
of Rh–In/C (Rh 20 wt%, In/Rh ¼ 0.5 and 1). (a) Rh K-edge, FT range:
ꢁ1
ꢁ1
3
0–150 nm ; (b) In K-edge, FT range: 30–120 nm .
Fig. 7 In 3d XPS of Rh–In/C (Rh 20 wt%, In/Rh ¼ 0.5 (a) and 1 (b)). [ ]:
EXAFS data can be tted by two In–metal (metal ¼ Rh or In)
shells and In–O shell. The two In–metal shells had almost the
same distances (R) as those of Rh–metal shells for the bcc-like
structure. In addition, the number of In–metal bond calculated
by (CN of In–metal shell) ꢂ (number of In atoms in the sample)
was almost the same as the number of Rh–metal bond (¼ (CN of
Rh–metal shell) ꢂ (number of Rh atoms in the sample)) with
similar distance. These similarities further support the forma-
tion of bcc-like RhIn alloy where Rh and In are structurally
equal.
area ratio.
The molar ratio of tetragonal RhIn alloy to total Rh can be
calculated from the ratio of the CN of the shells of bcc-like
tetragonal phase to the theoretical values (8 and 6 for R ¼ 0.28
Fig. 8 TEM images of Rh–In/C (Rh 20 wt%, In/Rh ¼ 1).
Fig. 9 TEM-EDX analysis of Rh–In/C (Rh 20 wt%, In/Rh ¼ 1). (a) and (b): Cliff–Lorimer method for the elemental analysis in the whole area. (c)
Line analysis.
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Table 4 EXAFS curve fitting results of Rh–In/C (Rh 20 wt%; In/Rh ¼ 0.5 and 1)
Sample
Fourier ltering
range/nm
a
b
c
d
e
(In/Rh)
Edge
Rh K
Shell
CN
R nm
s /nm
DE
0
/eV
R
f
/%
0
.5
Rh–Rh (or –In)
Rh–Rh (or –In)
Rh–Rh (or –In)
In–O
In–Rh (or –In)
In–Rh (or –In)
Rh–Rh (or –In)
Rh–Rh (or –In)
Rh–Rh (or –In)
In–O
6.0
2.8
2.2
0.7
5.4
4.0
1.5
5.4
4.0
0.5
5.5
4.1
0.268
0.277
0.320
0.223
0.277
0.319
0.268
0.277
0.319
0.223
0.277
0.320
0.268
0.206
0.0076
0.0068
0.0088
0.0073
0.0077
0.0087
0.0082
0.0070
0.0090
0.0076
0.0071
0.0088
0.0060
0.0060
6.6
3.4
8.8
9.9
6.4
ꢁ8.2
9.9
5.5
9.8
9.9
6.4
ꢁ4.0
0
1.0
2.2
2.5
0.7
0.166–0.292
0.126–0.279
0.166-0.292
0.126–0.279
In K
Rh K
In K
1
In–Rh (or –In)
In–Rh (or –In)
Rh–Rh
Rh foil
Rh K
Rh K
12
6
—
—
—
—
Rh
2
O
3
Rh–O
0
a
b
c
d
Coordination number. Bond distance. Debye–Waller factor. Difference in the origin of photoelectron energy between the reference and the
e
sample. Residual factor.
and 0.32 nm shells, respectively). We did not use the calculation RhIn phase. Therefore, we conclude that the catalytic activity of
from the CN of the shell of fcc Rh phase because the small the different phases is in the following order: tetragonal RhIn
particle size of fcc Rh phase could decrease the CN, while alloy > Rh metal modied with indium oxide [ unmodied Rh
tetragonal RhIn alloy particles were large enough. The calcu- metal.
lated ratio was 36% and 68% for In/Rh ¼ 0.5 and 1, respectively.
The amount of tetragonal RhIn phase was also calculated by In 3.6. Characterization of Rh–In/C (Rh 20 wt%, In/Rh > 1)
XPS: the amount of In(0) corresponded to that of tetragonal (group (iii))
RhIn; however the amount might be underestimated because
not all atoms in larger RhIn alloy particles were detected by XPS
The main phase in this group of catalyst was cubic RhIn. The
width of the XRD peaks in the catalyst with In/Rh ¼ 2 corre-
analysis. The calculated values were 25% and 65% for In/Rh ¼
sponded to the particle size of 20 nm. The TEM image of Rh–In/
0.5 and 1, respectively, and in fact these values were smaller
C (Rh 20 wt%, In/Rh ¼ 2) is shown in Fig. 12a. There were
spherical particles with various sizes from 10 nm to 100 nm. The
EDX analysis of the particles (Fig. 12b) conrmed that the
particles had the compositions of Rh : In ¼ 1 : 1. As shown in
Section 3.1, the In/Rh ratio in the net sample was 1.6. Therefore
In species or In-rich domains were present in the sample,
probably on carbon support. It should be noted that the CO
adsorption amount of Rh–In/C (Rh 20 wt%, In/Rh ¼ 2) was very
low (CO/Rh ¼ 0.006; Fig. 1). The cubic RhIn particles might be
covered with In species.
than those obtained from EXAFS.
Based on the above characterizations, we propose the model
structure of Rh–In/C catalyst in this group (0.2 < In/Rh # 1)
(Fig. 11). Small Rh metal particles (3 nm or less) partially
covered with indium oxide species and large tetragonal RhIn
alloy particles (ꢀ20 nm) are present on the support carbon. In
addition, the large tetragonal RhIn particles are attached
together, and dendritic structure is formed. The formation of
large RhIn particles may be due to the low melting point of
In(0). During the reduction process, In species that did not
interact with Rh particle might be reduced to liquid In metal.
The drops of liquid In metal moved on the support to collect
many Rh particles, and large alloy particles were formed.
From the catalysis data (Fig. 1), the catalytic activity (TOF
value) was sharply increased with the formation of tetragonal
Fig. 12 TEM-EDX result of Rh–In/C (Rh 20 wt%, In/Rh ¼ 2). (a) TEM
image of particles on the support. (b) TEM image of the dendritic
structure and the EDX analysis with Cliff–Lorimer method for the
whole area.
Fig. 11 Structure model of Rh–In/C (Rh 20 wt%, In/Rh ¼ 0.5, 1).
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.7. Mechanism of activity promotion by In addition
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lower energy and slight decrease of white line intensity. The
changes agreed with the shis in XPS signal.
Now we comment the nature of the activity of Rh–In bimetallic
catalysts. It has been accepted that metal-catalyzed amination
of alcohols proceeds via three steps: dehydrogenation of
The low activity of In/Rh ¼ 2 catalyst (cubic RhIn alloy) is not
explained by this electronic effect. Cubic RhIn alloy was formed
only when excess amount of In was present, and in fact the CO
adsorption amount on Rh–In/C (20 wt% Rh, In/Rh ¼ 2) was very
low. Therefore, a number of In atoms were present on the cubic
RhIn alloy surface. The surface In species might block the
access of substrate molecule that is much larger than CO on the
surface Rh atom.
substrate alcohol to carbonyl compound (R
C]O + H ), reaction of carbonyl compounds with
ammonia to imines and water (R C]O + NH / R C]NH
H O), and hydrogenation of imine to amine product (R R C]
1 2
R CHOH /
R
1
R
2
2
1
R
2
3
1 2
R
+
2
1 2
6
NH + H / R R CHNH ). In the previous paper, we showed
2
1
2
2
that the rate-determining step of Rh–In/C-catalyzed amination
is the dehydrogenation step based on the data of amination of
optically pure substrate: rapid dehydrogenation before rate-
determining step would induce racemization of substrate via
dehydrogenation–hydrogenation, but in fact racemization of
We also mention the leaching of Rh and In during the
reaction. As reported in the previous paper, leaching of both Rh
4
and In from Rh–In/C (In/Rh ¼ 1) was observed during the
amination reaction, especially when carbon black support was
used instead of activated carbon. In fact, the leaching accom-
panied the change of reaction medium into turbid colloidal
solution even aer the ltration. Increasing the loading amount
from 5 wt% Rh to 20 wt%, the solution aer reaction became
much more turbid: the leaching problem was more evident.
Based on the results in Section 3.5, especially TEM result, a part
of Rh–In alloy phase might well be detached from the support
and might become colloid. In other words, the cause of leaching
can be the formation of Rh–In alloy colloid. The observed
leaching of Rh, which as metallic state is insoluble in water, also
supports this idea.
4
substrate was not observed at all. We also showed that the
addition of In to Rh increases the resistance to the inhibition
effect of amine/ammonia in the dehydrogenation step: while Rh
metal has some activity in dehydrogenation of alcohols, the
activity was almost totally suppressed by the presence of amines
4
when indium was not added. It should be noted that the
dehydrogenation activity of Rh–In/C was also higher than that
of Rh/C even in the absence of amine/ammonia: the yield of
hydroxyacetone, which is the dehydrogenation product from
1
,2-propanediol, over Rh–In/C (5 wt% Rh, In/Rh ¼ 1) was 3.1
4
times higher than that over Rh/C (5 wt% Rh). The difference of
activity was even larger when the number of surface Rh atoms
was taken into consideration. One explanation of activity 4. Conclusions
increase by the presence of In is that the adsorption of amine,
The structure of Rh–In/C with 20 wt% Rh changes with
ammonia or carbonyl compound on surface Rh atom is weak-
ened by the presence of In. The direct In–Rh bond is present in
the two catalytically active structures (Rh metal particles
partially covered with indium oxide and tetragonal RhIn alloy),
and the number of Rh–In bond per Rh atom is larger in the
more active phase (tetragonal RhIn alloy). The electronic effect
of In addition to Rh center might induce the activity. The peak-
top positions of XPS signal are listed in Table 5. The positions of
Rh peaks were gradually shied to lower binding energy by
addition of In, and large shi was observed when alloy phase
became dominant (In/Rh ¼ 0.5 / 1). It is difficult to conduct as
detailed analysis of Rh bands as In (Fig. 4 and 7) because of the
overlap of these Rh bands and C 1s band (Fig. S6, ESI†). The
change of electronic structure may be also detected by XANES.
From the Rh K-edge XANES data (Fig. S7, ESI†), addition of In
up to In/Rh $ 1 led a slight shi of the absolute edge position to
increasing In amount. At low In/Rh ratio (#0.2), there are fcc Rh
metal particles with #3 nm size on the carbon support, and the
particles were partially covered with indium oxide species.
Almost all In species were located on the Rh metal surface. More
addition of In leads to the transformation of Rh particles to
tetragonal RhIn alloy. The tetragonal RhIn alloy, which has not
been reported in the literature, is the distorted structure (a ¼
0
.315 nm and c ¼ 0.328 nm) of cubic RhIn phase with CsCl
structure (a ¼ 0.320 nm). The tetragonal RhIn alloy forms large
particles (ꢀ20 nm), and the particles are attached together to
form dendritic solid. The transformation lasts until In/Rh ¼ 1,
at which the catalyst shows the highest turnover frequency for
amination of alcohols. Above In/Rh ¼ 1, the structure further
changes into cubic RhIn alloy covered with indium species. The
catalytic activity of phases is decreased in the following order:
tetragonal RhIn alloy > Rh particles partially covered with In >
Rh particles without In ꢀ 0.
Acknowledgements
Table 5 XPS peak-top positions of Rh–In/C (Rh 20 wt%)
This work was supported by the Cabinet Office, Government of
Japan through its “Funding Program for Next Generation
Binding energy/eV
In/Rh
Rh 3d5/2
Rh 3d3/2 World-Leading Researchers”.
0
0
0
1
2
.1
.2
.5
307.40
307.35
307.32
306.81
306.69
312.15
312.10
312.03
311.44
311.31
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