JOURNAL OF CATALYSIS 175, 236–244 (1998)
ARTICLE NO. CA981998

Ethanol Synthesis from Carbon Dioxide on [Rh10Se]/TiO2 Catalyst
Characterized by X-Ray Absorption Fine Structure Spectroscopy
Yasuo Izumi,1 Hiroshi Kurakata, and Ken-ichi Aika
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Instituteof Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Received August 21, 1997; revised November 26, 1997; accepted November 29, 1997

The active site structures and electronic states were studied
for the [Rh10Se]/TiO2 catalyst by means of edge spectra and
EXAFS. The rate and selectivity of ethanol synthesis from CO2 on[Rh10Se]/TiO2 had strong dependence on the heating temperature
in vacuum (Tevac) (1). Corresponding to the maximum of rate andselectivity when the Tevac was 623 K, the distance rSe–Rh reached aminimum (2.41 Å) by Se K- and Rh K-edge EXAFS analyses. The
contracted [Rh10Se] cluster was found to have an electronic state
similar to that of Rh3Se8 or RhSe2 rather than of metallic Rh. A
new reaction path control is proposed by regulating the distance
between the interstitial Se atom and the metal framework [Rh10].
A strong peak due to 1s → np transition was observed around the
Se K absorption edge. The peak intensity did not exhibit significant change when the Tevac was varied for [Rh10Se]/TiO2. On theother hand, the area of peak observed around 23,230 eV in Rh
K-edge spectra gradually decreased when the Tevac was elevatedfor [Rh10Se]/TiO2. Hence, the Se atom surrounded by [Rh10] framework was always kept in anionic state, while the electronic state of
Rh atoms gradually changed by the interaction with TiO2 surface.
The change for Rh atoms was also supported by the gradual increaseof rRh–Rh obtained by EXAFS with the temperature increase. In thecase of Rh sites with lower total coordination number around Rh([Rh10Se]/SiO2, [Rh10Se]/Al2O3, and [Rh10Se]/MgO), the reaction
CO2 → CO(a) + O(a) occurred predominantly and formed CO(a)poisoned the catalysis. °c 1998 Academic Press

INTRODUCTION

The increased emission of carbon dioxide is one of
the major environmental problems. It is desirable to convert CO2 into useful chemicals effectively. New catalyst[Rh10Se]/TiO2 was reported which synthesized ethanolfrom CO2 + H2 (1). The activity was high (≈8.0 mol ·cluster−1 min−1 ) and the conversion reached to 80% within
15 min (47 kPa of reaction gas; 0.3 g of catalyst) compared1
Correspondence should be addressed. E-mail: yizumi@chemenv.
titech.ac.jp.

to the other supported Rh catalysts. The selectivity was also
very good (≈83%). Promoting effect of Se was implied similar to reported bimetallic catalysts (2, 3).
Detailed characterizations were difficult because (morethan) two elements coexist in bimetallic catalysts. XAFS
(X-ray absorption fine structure) spectroscopy is one of thebest tools to study bimetallic catalysts owing to the tunability of absorption edge of elements (3). Lee et al. (4) reported
the selective benzene synthesis from acetylene on Au/Pd
colloid catalyst. The synthesis rate increased as the heating
temperature of the colloid was elevated. According to thetemperature increase, Pd skin formation around Au core
was suggested by EXAFS and also the decrease of Pd 4dvacancy was suggested by monitoring the 1s → 4d transition
around Pd K-edge. The [Rh10Se] cluster consisted of elevenatoms has an advantage to escape from the statistical uncertainty which bimetallic particles of larger size (20–100 Å)have in XAFS analysis (4–6). A wide variety of metal clusters was supported on inorganic oxides, where the metalframework was stabilized through the metal-O(s) (O(s): oxygen atom at surface) bondings (7). The promoting effects ofanionic Se were reported by modifying the electronic stateof active site of Rh/MgO, Rh/ZrO2, or Rh/SiO2 or by the
direct coordination of Se to surface intermediate species inethene hydroformylation (8–10). Simple promoting mechanism is expected when the Se is incorporated inside themetal cluster framework [Rh10].
In this paper, the origin of ethanol synthesis from CO2 isdiscussed in terms of the effects of Se by means of XAFS
spectroscopy. The active site structures of [Rh10Se] on TiO2
for ethanol synthesis were characterized by Rh K-edge and
Se K-edge EXAFS. The change of distance between interstitial Se and framework [Rh10] was monitored when the
catalyst preparation conditions were varied. The strengthof the interaction between Se and [Rh10] can be monitoredby the distance rSe–Rh besides the each Rh K-edge and Se
K-edge structure changes. On the basis of electronic state
and the cluster structure, the reaction mechanism of ethanol
synthesis is proposed.

236
0021-9517/98 $25.00
c 1998 by Academic Press
Copyright °All rights of reproduction in any form reserved.

ETHANOL SYNTHESIS FROM CO2

EXPERIMENTAL

Sample Preparation and Catalysis
The supported cluster was prepared from [PPN]2[Rh10Se(CO)22] (1) crystal (PPN = N(PPh3)2) (Fig. 1). The
cluster was synthesized from Rh4(CO)12 and [PPN][SeCN](11, 12). The TiO2(P25), Al2O3(alon C), and SiO2(Davison952) were treated at 573 K under vacuum for 2 h prior to use
as supports. MgO was prepared from Mg(OH)2 by heating
at 773 K for 2 h in vacuum. The (1) was reacted with eachoxide at 290 K for 2 h in purified THF in Ar atmosphere,with subsequent removal of THF in vacuum. In order to obtain enough large edge jump (>0.25) at Se K-edge and Rh
K-edge (transmission mode), the loadings of Rh were fixed
to 1.3 (TiO2 and Al2O3), 2.8 (SiO2), and 1.6 wt% (MgO)
in consideration of the BET surface area (50 for TiO2, 100
for Al2O3, 300 for SiO2, and 200 m2 g−1 for MgO). Thusprepared catalysts are denoted [Rh10Se]/oxide. The incipient cluster catalysts were totally decarbonylated by heatingin vacuum at 593–813 K, followed by the treatment in H2 at
623 K. When the catalysts were heated in vacuum at 523 K,
they were treated in H2 at 523 K.
Conventional Rh/TiO2 catalyst was impregnated fromwater solution of Rh(NO3)3. After the evaporation of water, the powder was treated in O2(1 h) and then in H2(1 h) at
523 K. The loading of Rh was 0.5 wt%. Rh3Se8 powder was
synthesized from elemental Rh and Se in the reported procedure (13). The reaction was carried out under 47 kPa of
CO2 + H2 (CO2 : H2 = 1 : 2) in the temperature range 523–
623 K in a closed circulation system (dead volume 210 cm3).
The products were analyzed by gas chromatography andmass spectroscopy (1).
XAFS Measurements
The Rh K-edge and Se K-edge XAFS spectra were measured at room temperature at the beamlines 10B and 6Bof the Photon Factory in the National Laboratory for HighEnergy Physics (KEK-PF) (Proposal No. 95G226). The accumulation ring energy was 2.5 GeV and the ring current

237

was 360–290 mA. All the spectra were measured at 10B
except for Se K-edge of [Rh10Se]/TiO2 (beamline 6B).
The beamline 10B utilized the channel-cut double crystal
monochrometer of Si(311). The incident and transmitted
X-ray were monitored by ion chambers filled with Ar forI0 and Kr for I. The beamline 6B utilized double crystal
monochrometer of Si(111). The beam was focused by thesagittal focus of second crystal. The incident and fluorescence X-ray were monitored by ion chambers filled withthe mixture of Ar(15%) + N2(85%) for I0 and Ar for I. A
30 µm-filter of As (K-edge ≈ 11,875 eV) was located between the sample and the ion chamber for I. The XAFS
spectrum for Se powder was measured also at 6B in thesame conditions as for [Rh10Se]/TiO2 as comparison.
Edge Spectra Analysis
The photon energy was calibrated by equalizing thederivative maximum of the absorption edge of Se powder
12,654.5 eV for Se K-edge and that of Rh foil 23,219.8 eV
for Rh K-edge. The background was subtracted by fitting
the polynomial to the preedge region and extrapolating it
to postedge region. The edge jump of subtracted spectrum
was normalized at 12,670.0 eV (Se K-edge) or 23,270.0 eV(Rh K-edge).
A strong peak was observed around Se K absorption
edge. In order to evaluate the peak area, the normalizededge spectra were analyzed by non-linear least square curve
fitting (14). The arctangent function was used as the absorption edge. The mixture of Gaussian (G) and Lorentzian (L)was used as the strong peak around the edge. Generally,
the Gaussian line width represents the energy resolution of
apparatus (14, 15). On the basis of the preliminary curve
fitting for more than ten spectra, the FWHM (full width at
half maximum) of G was fixed: 3.6 and 4.9 eV for spectra
measured at 10B and 6B, respectively. The Lorentzian line
width represents the natural line width, which was fixed to
2.1 eV (16). The intensity ratio of G : L was fixed to 1 : 1. The
broader feature in the range 12,662–12,680 eV was easily
reproduced by two L functions. The peak deconvolution in
the range 12,650–12,680 eV was performed with these fivefunctions (arctangent, G + L, and two L). Alternative peakdeconvolution in the region 12,650–12,660 eV was also performed with the arctangent function and the mixture of G
and L functions (three functions).
EXAFS Analysis

FIG. 1. The structure of [Rh10Se(CO)22]2− inferred from the structureof [Rh10S(CO)22]2− determined by X-ray crystallography (12). Carbonyl
ligands are not shown for clarity.

The EXAFS spectra were analyzed by the program
EXAFSH (17). The background subtraction was performedby calculating the three-block cubic spline, followed bynormalization by using the Victoreen parameter. The
Fourier transform of k3-weighted χ function was carriedout over the range kmin = 3 and kmax = 12.5–15.0 Å−1. The
filtered inverse Fourier transform was performed in the238

IZUMI, KURAKATA, AND AIKA

range rmin = 1.50–1.60 and rmax = 2.65–2.75 Å for Se K-edge
EXAFS and rmin = 1.30–1.60 and rmax = 2.75–2.90 Å for Rh
K-edge EXAFS.
The curve fitting analysis was performed in the range
kmin = 4 and kmax = 12–14 Å−1 by utilizing the empiricalphase shift and amplitude functions extracted from the
EXAFS of model compounds: Rh foil for Rh–Rh bond
(2.689 Å), Rh3Se8 powder for Rh–Se bond (2.498 Å) (13),
Rh2O3 powder for Rh–O bond (2.05 Å) (18), [PPN][Rh3Se2(CO)6] (2) crystal for Se–Rh bond (2.458 Å) (11),and SeO2 powder for Se–O bond (1.76 Å) (19). The goodness of the fit was estimated by the R factor.
ÁZ
Z
3
3
2
|k 3 χobs |2 dk.
Rf = |k χobs − k χcalc | dk
RESULTS

Ethanol Synthesis from CO2 on [Rh10Se]/TiO2
The ethanol synthesis rates, turnover numbers per surface Rh, and ethanol selectivities are summarized inTable 1 in relation to the effects of Se and support on
the ethanol synthesis. Ethanol synthesis was observed only
on [Rh10Se]/TiO2. The [Rh10Se] clusters on the other supports had very poor (or no) activities; the [Rh10Se]/Al2O3
and [Rh10Se]/MgO produced methane and/or CO, and the[Rh10Se]/SiO2 had entirely no activity. The conventional
Rh/TiO2 had the intermediate activity compared to supported [Rh10Se] catalysts, but did not produce ethanol.
The initial rate and selectivity of ethanol synthesis exhibited strong dependence on the heating temperature (523–813 K) in vacuum (Tevac) (Table 1). Both the initial rate andselectivity were best when the Tevac was 623 K. The production rate of CH4 and/or CO did not show significant change
when the Tevac was varied.
Se Absorption Spectra
The Se K-edge spectra are shown in Fig. 2A for supported[Rh10Se] clusters and reference compounds. A strong peak

was observed around absorption edge in all the spectra.
The peak intensity was relatively weak for (1) (b) and[Rh10Se]/TiO2 (e). Only the spectra of Se powder (c, d)
did not have an evident feature within the range of 12663–
8 eV, different from the others.
The dependence on the Tevac was examined for [Rh10Se]/
TiO2 (Fig. 2B). The shape was very similar to each otherfor the three (i–k). The edge position shifted by heating: by+ 0.2 eV from 523 to 623 K and by −0.6 eV from 623 to813 K. As a result, the edge energy had the maximum when
the sample was heated at 623 K.
As the peak is very near the edge, the area was evaluatedby peak deconvolution (Fig. 2C). When the curve fitting
was tried with arctangent (as edge) and G + L (as strong
peak around edge) in the range 12,650–12,660 eV, it wasdifficult to determine the height of arctangent. Besides, thefitting was not good when the FWHM for G was fixed (theunfixed FWHM varied in the range 3.4–4.7 for spectra obtained at beamline 10B). Therefore, the least square fitting
calculation was performed with five functions: arctangent,
G + L, and two L functions in the range 12,650–12,680 eV.
The results are listed in Table 2. The normalized peak area
exhibited almost no change (4.2–4.3) for the [Rh10Se]/TiO2
heated at 523–813 K. The normalized peak area was in thefollowing order [Rh10 Se]/TiO2 < (1) < [Rh10 Se]/MgO <[Rh10 Se]/SiO2 ≈ [Rh10 Se]/Al2 O3 < Se ≈ Rh3 Se8 . The normalized peak area for the Se powder had a difference of
23% when the spectra were observed at different beamline.
The above order of peak area should be discussed taking
the difference of beamline into account. The difference wasmainly due to the difference of monochrometer face used.
Rh Absorption Spectra
The Rh K-edge spectra was observed for supported[Rh10Se] catalysts and (1) (Fig. 3). The edge positions
for [Rh10Se]/TiO2, [Rh10Se]/SiO2, [Rh10Se]/Al2O3, and[Rh10Se]/MgO fell within the range 23,211.9–23,212.4 eV
when the sample was heated at 623–653 K in vacuum. When
the Tevac was varied (523, 623, and 813 K) for [Rh10Se]/TiO2,

TABLE 1The CO2–H2 Reactions on Supported [Rh10Se] Catalysts and Conventional Rh/TiO2
TOF/min−1

Initial rate/10−3 mol h−1 g−1cataCatalyst

Rh wt%

Tevac/K

Treact/K

Ethanol

Methane + CO

Ethanol

Methane + CO

Ethanolselectivity/mol%[Rh10Se]/TiO2

1.3[Rh10Se]/Al2O3[Rh10Se]/MgO[Rh10Se]/SiO2
Rh/TiO2

1.3
1.6
2.8
0.5

523
623
813
653
623
623
523

523
523
523
623
623
623
623

0.09
1.9
0.9
0
0
0
0

0.6
0.4
0.4
0.2
0.04
0
1.8

0.012
0.25
0.12
0
0
0
0

0.079
0.053
0.053
0.026
0.004
0
(0.62)

13
83
71
0
0
0
0

Note. Total pressure 47 kPa. PCO2 : PH2 = 1 : 2.
aTurnover per surface Rh assuming 100% exposure of Rh to catalyst surface.

ETHANOL SYNTHESIS FROM CO2

239

FIG. 2. Se K-edge spectra. (A) Rh3Se8 powder (a), crystal (1) (b), Se powder at beamline 10B (c), and at 6B (d), [Rh10Se]/TiO2 (e), [Rh10Se]/MgO (f),[Rh10Se]/Al2O3 (g), and [Rh10Se]/SiO2 (h). (e)–(h), heated in vacuum at 623–653 K. (B) [Rh10Se]/TiO2 heated in vacuum at 523 (i), 623 (j), and 813 K (k).
(C) Peak deconvolution analysis for crystal (1) and [Rh10Se]/TiO2 (623 K).

the edge positions were 23,214.1, 23,211.9, and 23,212.3 eV,
respectively. It reached a minimum when the Tevac was
623 K.
The peak A in Fig. 3 gradually decreased as the Tevac waselevated for [Rh10Se]/TiO2 (a–d). The change looks small,but evident when the derivative spectra are plotted (notshown). The intensities of peak A for [Rh10Se]/SiO2 (e),[Rh10Se]/Al2O3 (f), and [Rh10Se]/MgO (g) were similar to
those for [Rh10Se]/TiO2 of Tevac = 813 K (d), 623 K (c), and
523 K (b), respectively.

240

IZUMI, KURAKATA, AND AIKATABLE 2The Positions (E/eV = 12650 + α) of Se K Absorption Edge
and First Peak Area
Sample[Rh10Se]/TiO2[Rh10Se]/SiO2[Rh10Se]/Al2O3[Rh10Se]/MgO[Rh10Se(CO)22]2−
Rh3Se8Se powder

Tevac/K

Edge
α/eVa

First peak
areab

523
623
813
623
653
623

4.4
4.6
4.0
5.0
5.1
5.0

4.3
4.2
4.3
6.9
6.9
5.7

4.5
6.1
4.5

4.9
8.0
8.4

a

The maximum point of derivative for arctangent function.
The peak area of the mixture function normalized by the height ofarctangent function.
b

Se EXAFS
In the k3-weighted Fourier transform of χ function, astrong peak was observed around 2.1 Å (phase shift uncorrected) for [Rh10Se]/TiO2 of Tevac = 623 K (Fig. 4c). The Se
K edge jump coincided with the calculated value from theimpregnated amount of crystal (1) on TiO2, and the edge
jump did not change when the heating temperature waselevated up to 813 K. Hence, Se atom remained in catalyst in the whole procedure, and one-wave fitting of Se–Rh

was tested. The result was rSe–Rh = 2.41 Å and coordination number NSe–Rh was 8.2 (Rf = 4.9%). The fitting was
not satisfactory, especially in lower wave number region. A
close inspection of Fig. 4c indicates a shoulder on the lowerdistance side than the main Se–Rh peak. Therefore, twowave fitting was tried with Se–Rh and Se–O as a possibility
that the bonding of Se with the O(s) of TiO2 was formed.
The best fit results were summarized in Table 3. The Rf was
improved to 3.0%. The obtained rSe−O(s) (1.97 Å) did notcontradict with attached [Rh10Se] structure model on TiO2
with the obtained distances rSe–Rh, rRh–Rh, and rRh−O(s) , andtheir coordination numbers (Fig. 5, discussed later).
In a similar procedure, curve fitting results were obtained for [Rh10Se]/TiO2 of Tevac = 813 K (rSe–Rh = 2.42 Å
and rSe−O(s) = 1.98 Å) and [Rh10Se]/SiO2 (rSe–Rh = 2.42 Å
and rSe−O(s) = 1.95 Å). As the EXAFS for [Rh10Se]/TiO2 ofTevac = 523 K was not fit with the two waves, the result of
one-wave fitting was listed in Table 3. In summary, when the
Tevac was varied for [Rh10Se]/TiO2, the rSe–Rh value reachedthe minimum (2.41 Å) at 623 K. The Se–O bonding appeared when the Tevac was higher than 623 K.
Rh EXAFS
The temperature-programmed gas desorption was monitored for incipient supported [Rh10Se] clusters. Because
almost all the gas desorption (CO + CO2) occurred below 523–573 K originating from the carbonyl ligands of(1), curve fitting analysis was performed with two waves

FIG. 3. Rh K-edge spectra. Crystal (1) (a), [Rh10Se]/TiO2 heated in vacuum at 523 (b), 623 (c), and 813 K (d), [Rh10Se]/SiO2 (e), [Rh10Se]/Al2O3 (f),and [Rh10Se]/MgO (g).

241

ETHANOL SYNTHESIS FROM CO2

FIG. 4. Se K-edge EXAFS for [Rh10Se]/TiO2 heated in vacuum at 623 K: (a) χ function; (b) k3-weighted χ function; (c) its associated Fouriertransform; and (d) inversely Fourier-transformed k3χ(k) function and curve fitting.

(Rh–Rh and Rh–Se) or three waves (Rh–Rh, Rh–Se, andRh–O(s)). As the NSe–Rh obtained from Se K-edge EXAFS
did not significantly change from ten for crystal (1) considering the errors (Table 3), the NRh–Se was fixed at 1.0 in Rh
K-edge curve fitting procedure. In all the cases of supported[Rh10Se] clusters, the Rf was always significantly better withthree waves than with two waves. The obtained rRh−O(s)and NRh−O(s) did not contradict attached [Rh10Se] structure
model on TiO2 with the obtained distances rSe–Rh and rRh–Rh,
and their coordination numbers (Fig. 5, discussed later).
The best fit results are summarized in Table 4. The NRh–Rh
was 4.8 for [Rh10Se]/TiO2 of Tevac = 523 K. The anion of (1)has average NRh–Rh of 4.8 (Fig. 1). According to the increase of heating temperature, the rRh–Rh and NRh–O(s) val-ues gradually increased, while the NRh–Rh value gradually
decreased. The obtained rRh–Se values coincided with corresponding rSe–Rh values (Table 3) within the error of 0.01 Å. It
should be noted that the total coordination number around
Rh atom (= NRh–Rh + NRh–Se + NRh−O(s) ) was classified into
two ranges: 6.4 ± 0.7 for [Rh10Se]/TiO2 and 5.5 ± 0.2 for[Rh10Se]/SiO2, [Rh10Se]/Al2O3, and [Rh10Se]/MgO.
DISCUSSION

The ethanol synthesis from CO2 was observed exclusively on [Rh10Se]/TiO2 (Table 1). The synthesis rate wasbest when the evacuation temperature was 623 K. The total

TABLE 3The Results of Curve Fitting Analysis of Se K-Edge EXAFS for [Rh10Se]/TiO2 and [Rh10Se]/SiO2
Se–RhaSe–Ob

SampleTevac/KN

r/Å

1(σ 2)/10−3 Å2

Nc

r/Å

1(σ 2)/10−3 Å2

Rf/%[Rh10Se]/TiO2

523
623

10
8.6
(±1.2)
9.5
9.1

2.44
2.41
(±0.007)
2.42
2.42

3.8
4.5

—
(1.0)

—
−1.6

3.1d
3.0

3.3
4.6

(1.0)
(1.0)

—
1.97
(±0.019)
1.98
1.95

−1.4
−0.3

0.6
1.3

[Rh10Se]/SiO2
a, b

813
623

Empirical model parameters extracted from crystal (2)a and SeO2 powderb. Debye–Waller factors are relative values compared to these references.
Fixed value.
d
Not fitted with two waves.
c242

IZUMI, KURAKATA, AND AIKATABLE 4The Results of Curve Fitting Analysis of Rh K-Edge EXAFS for Supported [Rh10Se] Clusters
Rh–Rha

Rh–Seb

Rh–Oc

SampleTevac/KN

r/Å

1(σ 2)/10−3 Å2

Nd

r/Å

1(σ 2)/10−3 Å2

N

r/Å

1(σ 2)/10−3 Å2

Rf/%[Rh10Se]/ TiO2

523
623

2.69
2.72
(±0.001)
2.75

5.3
6.0

(1.0)
(1.0)

−0.7

2.10
2.09
(±0.008)
2.13

0.10
0.12

(1.0)

1.2
1.5
(±0.6)
1.7

1.3
1.9

2.0

2.45
2.41
(±0.012)
2.43

4.4
1.6

813

4.8
4.0
(±0.6)
3.0

6.9

0.41

623

2.9

2.74

3.0

(1.0)

2.42

1.3

1.4

2.13

1.5

0.17

[Rh10Se]/SiO2[Rh10Se]/Al2O3

653

3.7

2.72

5.5

(1.0)

2.41

0.8

0.9

2.05

−0.7

0.33

[Rh10Se]/MgO

623

2.7

2.70

5.7

(1.0)

2.39

2.1

2.0

2.18

5.0

0.69

a, b, cEmpirical model parameters extracted from Rh foil,a Rh3Se8,b and Rh2O3.c Debye–Waller factors are relative values compared to these references.
dFixed value.

conversion rate was in the order [Rh10Se]/TiO2 > conventional Rh/TiO2 > [Rh10Se]/Al2O3 > [Rh10Se]/MgO >[Rh10Se]/SiO2. The [Rh10Se]/TiO2 had remarkable dependence on the Tevac in terms of rate and selectivity.
The [Rh10Se] Structure on Oxides and Its Transformation
The curve fitting results of Rh K-edge EXAFS were summarized in Table 4. With regard to [Rh10Se]/TiO2, the coordination number of Rh–Rh bond was 4.8 when heatedat 523 K. It is the same value as (1). Hence, the totallydecarbonylated (1) by the evacuation at 523 K was attachedon TiO2 without significant structure change of the [Rh10Se]
framework (Fig. 1). One of the [Rh3] faces of polyhedral[Rh10Se] may interact with O(s) atoms (Fig. 5a). The unchanging edge jump of Se K-edge for all the samples when
the crystal was impregnated on oxides also supported theretention of stable [Rh10Se] unit. By heating to 623 K, theNRh–Rh decreased to 4.0, while the NRh−O(s) increased to 1.5.
One of the explanations for the change is that the Rh–Rh

bonds around the [Rh3] triangle face were cleaved by theadditional formation of Rh–O(s) bondings at 623 K (Fig. 5b).
By further heating to 813 K, the NRh–Rh further decreased(3.0) and the NRh−O(s) further increased (1.7). The treatmentin H2 (523–623 K) had negligible effect on the EXAFS results in Tables 3 and 4.
The aggregation of metal clusters is plausible on support
in these or more severe conditions (7). The fragmentationof [Rh10Se] into smaller portions is unlikely based on theunchanging NSe–Rh values (≈10, Table 3). The gradual decrease of NRh–Rh according to the increase of temperature
contradicts the aggregation of [Rh10Se] (Table 4), supporting the unimodal [Rh10Se] cluster model given in Fig. 5,
owing to the stabilization effect of [Rh10] framework by Se.
Although EXAFS has a weak point to measure the averageof bulk, the majority of clusters in this case should remain
integral and increase the interaction with TiO2 as the temperature increases.
The rRh–Se value obtained from Rh K-edge EXAFS
reached a minimum (2.41 Å) when the Tevac was 623 K. The

FIG. 5. Proposed model structures of [Rh10Se] on TiO2(110) surface. Heated in vacuum at 523 (a), 623 (b), and 813 K (c). Only the interface partof [Rh10Se] framework is drawn for clarity.

243

ETHANOL SYNTHESIS FROM CO2

rSe–Rh values obtained from Se K-edge EXAFS (Table 3)
coincided with the rRh–Se values within the error of 0.01 Å.
The EXAFS analysis was very reliable by the curve fitting
analysis from both edges.
The dependence of ethanol synthesis rate on the Tevac
for [Rh10Se]/TiO2 is explained by the EXAFS results asfollows. The key should be the change of rRh–Se. The synthesis rate had the maximum and the rRh–Se value reachedthe minimum when the Tevac was 623 K. The minimum rRh–Se
was shorter by 0.02–0.09 Å than the rRh–Se of Rh3Se8 alloy(2.498 Å) (13), crystal (2) (2.458 Å) (11), or chemical-vapordeposited Se on [Rh6]/MgO (2.43–2.44 Å) (10). On the basis of the shortest distance of Se–Rh, the electronic state of[Rh10Se] should become similar to Rh selenide (Rh3Se8 or
RhSe2). It was reported that the cationic transition metal
catalyzed the CO associative reactions rather than the CO
dissociative reactions (20).
As the Tevac was elevated, the interaction between[Rh10Se] and TiO2 surface monotonously increased on thebasis of the increase of rRh–Rh and NRh−O(s) , and the decreaseof NRh–Rh (Table 4). On the other hand, the interaction between Se and [Rh10] was strongest when the Tevac was 623 K
on the basis of rRh–Se. It is interesting to note that the interaction of Se with the O(s) atom of TiO2 was suggested when
the Tevac was above 623 K (Table 3).
Proposed model structures of [Rh10Se] on TiO2 surface
are illustrated in Fig. 5. As the obtained NRh−O(s) values wererelatively large (1.2–1.7), the exposure of surface-oxygenrich TiO2(110) face was assumed (21). The [Rhn] (n = 3–5)
face was assumed to be in parallel to the TiO2 face. As
suggested above, the [Rh10Se] should interact with TiO2
surface through the [Rh3] face when the Tevac was 523 K(Fig. 5a). If each of the three interface Rh atoms bonded
to four O atoms, the NRh−O(s) should be 1.2 in average (thecluster has 10 Rh atoms), equal to the value obtained by
EXAFS. If the rRh−O(s) was fixed to 2.10 Å (Table 4) inaverage, the Se–O(s) distance was calculated 2.62 Å. This
longer coordination compared to Se–Rh (2.44 Å) may not
be detected by curve fitting analysis in Table 3.
Figure 5b illustrates the structure of [Rh10Se]/TiO2
heated at 623 K. In a similar procedure to a, the model
was constructed. It reproduced well the EXAFS data. The
metrical parameters of model b was rRh–Rh of 2.72 Å, rRh–Se
of 2.41 Å, rRh−O(s) of 2.09 Å, NRh−O(s) of 1.3, and rSe−O(s) of
1.97 Å, compared to the values in Tables 3 and 4 (2.72,
2.41, 2.09, 1.5, and 1.97 Å, respectively). The interface
model of Fig. 5c corresponds to the [Rh10Se]/TiO2 heated at
813 K. The model c was with rRh–Rh of 2.75 Å, rRh–Se of
2.43 Å, rRh−O(s) of 2.14 Å, NRh−O(s) of 1.4, and rSe−O(s) of
1.98 Å in good coincidence with values in Tables 3 and 4(2.75, 2.42–2.43, 2.13, 1.7, 1.98 Å, respectively). The interface model of [Rh10Se]/SiO2 with rSe−O(s) of 1.95 Å (Table 3)
could be constructed on the relatively stable SiO2(111) face
(22).

Electronic State of [Rh10Se] on Oxides
The edge positions of Se K-edge and Rh K-edge exhibited
shifts within 0.6 eV for Se and within 2.2 eV for Rh. The Seedge position was highest at Tevac = 623 K, implying largest
charge donation to Rh. The Rh K-edge position reachedminimum at the temperature, supporting the change donation from Se.
The peak A intensity in Rh K-edge spectra (Fig. 3) was in
the order (1) > [Rh10Se]/TiO2 (Tevac = 523 K) ≈ [Rh10Se]/
MgO > [Rh10Se]/TiO2 (Tevac = 623 K) > [Rh10Se]/Al2O3 >[Rh10Se]/TiO2 (Tevac = 813 K) ≈ [Rh10Se]/SiO2, in accordance with the smallness of rRh–Rh values except for the[Rh10Se]/MgO (Table 4); the larger the rRh–Rh, the weaker
the peak A. The decrease of 1s → 4d peak intensity as afunction of heating temperature was reported at Pd K-edge
for Au/Pd bimetallic colloid catalyst (4). In summary, peak
A and rRh–Rh demonstrated monotonous change of electronic state of Rh as the increase of Tevac.
The 1s → np transition intensity can be expressed in the
dipole approximation (23)I (1s → np) ∝ |hφf |M|φi i|2 ,where M is dipole operator and φ i and φ f are the initialand final states, respectively. The peak area varied muchin the series of samples of Table 2, suggesting that thetransition was 1s → 4p rather than 1s → 5p. Hence, by asimple approximation on the basis of the peak area inTable 2 (4.2–4.3 for [Rh10Se]/TiO2 and 8.0–8.4 for Se powder and Rh3Se8), the valence state of Se was suggested to be≈−1 for [Rh10Se]/TiO2. The peak area exhibited negligible
changes for [Rh10Se]/TiO2 when the heating temperature
was varied. Therefore, the Se atom should be always anionic, and the extent of interaction between Se and [Rh10]
controlled the electronic state of Rh. The chemical shiftof Se for crystal (1) and [Rh10Se]/TiO2 was measured by
XPS. The Se 3d5/2 peak was centered at 54.1 and 53.7 eV,
respectively, suggesting the valence states of Se of −2–−1.
Reaction Mechanism
The reaction mechanism of ethanol synthesis on[Rh10Se]/TiO2 was studied by in-situ FT-IR in CO2 + H2
(24). The intensity of two peaks at 1614 and 1243 cm−1 coupled in the course of reactions. They were assigned to bidentate carbonate species (25). Two peaks were observed in thesame region for unimpregnated TiO2 at 1570 and 1230 cm−1
in CO2 + H2. Two unresolved peaks between the two carbonate peaks were also observed on [Rh10Se]/TiO2. The
two peaks may be assigned to acetate species (26). In thecase of the reaction of CO2 compared to of CO, predominant carbonate absorption was too strong to quantitatively
measure the acetate absorption. The carbonate species for[Rh10Se]/TiO2 should be affected by [Rh10Se], either beingnear [Rh10Se] or on Rh of [Rh10Se].

244

IZUMI, KURAKATA, AND AIKA

The increase of the peaks at 2965, 2883, 2919, and2853 cm−1 was proportional to the ethanol synthesis rate.
They were assigned to CH3 ν as, ν s, CH2 ν as, and ν s, respectively. The dissociation of C–O bonds of carbon dioxide
occurs on Rh and the four peaks rapidly disappeared when
the supply of reaction gas (CO2 + H2) stopped. Therefore,
methyl and methylene species should be mainly on Rh.
Compared to the cases of [Rh10Se] on the other supports(Al2O3, SiO2, and MgO), the stretching peaks of CO at
2174 and 2108 cm−1 were weak. The reason why CO(a)was not stabilized on [Rh10Se]/TiO2 may be due to the difference of coordination number around Rh (6.4 ± 0.7) for[Rh10Se]/TiO2 compared to [Rh10Se]/Al2O3, [Rh10Se]/SiO2,and [Rh10Se]/MgO (5.5 ± 0.2). These two wavenumbers
were relatively high and suggesting CO on positive Rh sites
in accordance with XAFS study.
The CH3(a) or CH2(a) on Rh reacted with CO3(a) or
CO2 to form acetate, followed by the hydrogenation intoethanol on Rh sites. The reaction of CO + H2 was very
slow and of entirely different product distribution from thereaction of CO2 + H2 on [Rh10Se]/TiO2 (1), demonstrating the ethanol synthesis was not via CO in the case of[Rh10Se]/TiO2. The stability of CHx(ads) on Rh should be
pertinent. The “rhodium selenide”-like electronic state poisoned methane formation, and tuned the reaction path toethanol synthesis by promoting the C–C bond formation ofCHx with carbonyl derivative to form acetate species.
The [Rh10Se] clusters on SiO2, Al2O3, and MgO could
not catalyze effectively probably because the surface waspoisoned by CO(a) due to the very fast process CO2 →CO(a) + O(a) (FT-IR), supported by their lower coordination numbers by EXAFS (5.5 ± 0.2).
CONCLUSIONS(1) The edge spectra indicated the electronic states of Se
and Rh: interstitial Se atom was always anionic while thestate of Rh atoms around Se gradually changed.
(2) The EXAFS indicated the minimum rSe–Rh (2.41 Å)for [Rh10Se]/TiO2 when heated at 623 K. The Se-edge andRh-edge positions supported the strongest interaction between Se and [Rh10] at 623 K.
(3) Ethanol synthesis from CO2 was observed only when
TiO2 was utilized as support. The higher coordination

around active site should be one reason to tune the reaction path to CHx(a) + COy(a) → acetate(a) → ethanol bysuppressing the CO(a) and CH4 formations.
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