10942

J. Phys. Chem. 19!32,96, 10942-10948

SeleniumDoped Rhe Carbonyl Clusters on MgO: Structures and Promoting Effects In
Ethene Hydroformylation
Yasuo Izumi and Yasuhiro Iwasawa*
Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan(Received: June 17, 1992; In Final Form: August 17, 1992)

Selenium-modified Rh6(CO)16/Mg0catalysts were prepared by the reaction of Rh6(CO)16/Mg0with (CH3)2Sein order
to examine the promoting effect of an electronegative additive on Rh catalysis for ethene hydroformylation. The deposited
Se (Se/Rh6 = 0.6) enhanced the rate of propanal formation 1.9times as compared with the case of an undepited catalyst.
The selectivity of the hydroformylation was improved from 20 to 50% by the Se addition. To the contrary, the monotonous
suppression of ethane formation by increasing the Se amount was observed. Further doping of Se reduced the hydroformylation
activity. TPD, IR, and Se K-edge and Rh K-edge EXAFS revealed that (CH3)2Sereacted with the Rh atoms of partially-decarbonylated Rh, species on the MgO surface, forming Se-Rh bonds at a distance of 0.244nm. XPS data suggested
that the oxidation state of Se in the catalyst is Se-, while Rh is in a nearly metallic state. CO adsorbs on the rhodium atoms
bonded to Se, which is contrasted to blocking of the neighboring Rh sites by Se atom observed with usual impregnated Rh
catalysts. The structures of the Se-undoped and -doped Rh6 clusters on MgO are presented in relation to the active siteof the cluster catalyst. The data indicate an advantage of molecular clusters over impregnated particles in adsorption capability
and catalysis.

Introduction
Metal clusters have often been used as precursors of supported
metal catalysts. The most prevailing advantage for using metalclusters is to prepare dispersed, fine metal particles on inorganicoxides.] It is possible to control the molecular size of cluster
particles on supports by changing the number of nuclear metal
atoms. Kirlin et al. reported the dependency of catalytic activity
on cluster sizes for supported rhenium carbonyl clusters (HRe(CO), and H3Re3(C0)12)on MgOn2
Another advantage in using metal clusters is that metal clusters
are considered as simulated structures of active particular ensembles like step sites and kink sites on supported metal particles.
Better understanding of essential factors of solid catalysis may
be obtained in these systems rather than in conventional catalysts.
Furthermore, new catalytic sites or ensembles can be created byusing metal cluster precursors supported on inorganic oxides.
Nevertheless, precursor clusters with well-defined structures areoften converted to illdefined metal particles after treatments or
during catalytic reactions, even if they were dispersed as fineparticles less than 1.0 nm.
The promoting effect of the second element added on metal
catalysts is a subject to examine in metal cluster systems when
the structure of the second-elementdoped metal cluster is definedon a molecular level. Rhodium is an industrially important element
for synthesis gas reaction, hydroformylation, etc., and cationic
Rh sites rather than metallic Rho have been supposed to be active
for CO insertion elementary step.3 In fact, doping of sulfur as
an anionic element to Rh or Ni catalysts resulted in the augmentation of selectivity for the formation of oxo compounds?Vs
Selenium also behaves as an electronegativeadditive, but the ionicradius is 1.08 times larger than that of sulfur (1.06times for atomic
radius). Sedoped Rh/Zr02 or Rh/Si02 catalysts prepared from
(CH3),Se or Se02showed an increase in selectivity for oxygenatesin ethene hydroformylation at an optimum Se/(surface Rh) rati^.^,' The examples of the promoting effects by electronegative
elements are still much fewer than those by electropositive elements
and transition metal elements.
In the present report we studied the structures of Se-undoped
and -doped Rh6(C0)16xclusters on MgO by EXAFS, FT-IR,
TPD, and adsorption. The enhancement of the reaction rate andselectivity of ethene hydroformylation by Se addition was found.
The cluster metal framework on MgO was maintained, and(CH3)2Sein gas-phase selectively reacted with the partially-decarbonylated & clusters on MgO at 398 K to form direct S e R hbonds, by which the nearest and next-nearest neighboring Rh sites*
for the attached Se atom were created.

,l-K
m- 4

(d) n=- 8(e) n= 13

Experimental Seetion
Se-doped catalysts were prepared by the following procedure(Scheme I). Rh6(C0),6 (0.3 g) (Aldrich Chemical Co.) was
dissolved in 500 mL of chloroform which had been purified byreflux over 5A molecular sieves and distillation. MgO (Soekawa,
99.999%) pretreated at 773 K for 2 h in vacuum was immersedwith the solution, followed by evacuation to remove the solvent.
The Rh loading was chosen in the range 1.3-5.0 wt % as theconvenience of each experiment. All procedures of preparation
were conducted in a flow of Ar (99.9999%) or under vacuum.
&(CO),,/MgO was heated to 398 K under vacuum to remove
about onethird of ligand CO. Then the partiallydccarbonylated
Rh clusters were contacted with the vapor of (CH3)*Se at 398
K for 30 min in a closed circulating system c o ~ e ~ t etoda vacuum
line. After reaction for 30 min, the system was evacuated for 1
min. In the range of 0 < Se/Rh6 < 4,all the (CH3)2Sereactedwith the MgO-supported clusters because no (CH3)3c remainedin the gas phase after reaction for 30 min. (CH3)2Seadsorbed
physically on the MgO, and the physisorbed (CH3),Se easily
desorbed by evacuation at 298 K.
Se K-edge and Rh K-edge EXAFS spectra were measured ina transmission mode at a beam line 10B of the Photon Factoryin the National Laboratory for High Energy Physics (Proposal
No. 87013). The EXAFS spectra were taken in an EXAFS cellwithout contacting air by using a transfer technique from theclosed circulating system for sample preparation to the cell. The
EXAFS analysis was camed out using the theoretical phase shift
and amplitude functions,on the basis of the formula of plane-wave
single scattering theory: and also the empirical ones extractingfrom the measurements of Rh foil, Rh&XQI6,and (CH3)$e as

0022-3654/92/2096-10942S03.00/00 1992 American Chemical Society

Se-Doped Rh6 Carbonyl Clusters on MgO

The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10943

TABLE I: Results of the Cune Fitting Analyses for Se K-Edge
EXAFS Spectra of !&-Doped R U M P
S t R_
h _sampleSe/Rhn
Nb
r/nm
d
1.o
1.3
0.244
dc
1 .o
1.4
0.244
e
2.0
1.4
0.243
ec
2.0
1.3
0.243
~

Rf
0.043
0.047
0.050
0.037

"The notation of the samples is same as in Scheme I. bThe reduction factor is not considered. 'After ethene hydroformylation (60min).

Temperature/K
Figure 1. TPD spectrum for Rh6(C0),6/Mg0.

model compounds for Rh-Rh, Rh-C,, Rh-Cb, Rh(-C-)0, and
Se-C bonds, respectively. The Fourier transform of the k3weighted EXAFS oscillation was carried out over the ranges of30-140 and 30-150 nm-' for Se and Rh K-edge EXAFS, respectively. The filtering ranges for inverse Fourier transform tok space were 0.16-0.25 and 0.12-0.30 nm for Se and Rh K-edge
EXAFS, respectively. In the curvefitting analysis for Rh K-edge
EXAFS, the Debye-Waller factors ui for each coordination shellwere fixed (=0.005 nm), and the coordination numbers Ni for
Rh-Rh, Rh-C,, Rh-Cb, and Rh(-C-)O were determined bycomparison with the Ni obtained from the analysis of Rh6(C0)16
or Rh foil at the same fixed uivalue. In the case of Se K-edge
EXAFS the ui)s were floated, but the N,'s were not corrected by
the amplitude reduction factor Sibecause of the lack of modelcompounds for S e R h bonding to estimate the Si(also for Rh-Se
bond in the Rh K-edge EXAFS). The errors in the present
analysis for coordination number and bond distance are regarded
as 15-20 and 0.5-1.0%, respectively. The degree of the curve
fitting to experimental data was estimated by a reliability factor(Rf):
Rf = SIk3xoh(k)- k3~"1C(k)12
dk/ Slk3xoh(k)l2dk
XPS spectra for Se 3dSIZ,Rh 3d5,2 and 3d3/2 were recorded
on a Shimadzu ESCA750 spectrometer. The samples weretransferred into the XPS chamber by a glovebox technique without
contacting air. The binding energy of C 1s was referred to thevalue of the 4f level (83.8 eV) of deposited Au and the XPS
binding energies were normalized by the C 1s energy.
Ethene hydroformylation was carried out at 398 K and 40.0
kPa (C2H4:COH2= 1:l:l) in a closed circulating system (deadvolume: 120 cm3)with a U-shaped liquid N2-acetone trap (179K). The reaction products were analyzed by a gas chromatographusing a 2-m column of VZ-10 at 353 K for ethene and ethane
and a 4-m column of DOS at 353 K for propanal and propanol.
Methane was also analyzed by a 2-m column of 5A molecularsieves at 353 K.
IR spectra were measured under in situ reaction conditions by
an FT-IR spectrometer (JASCOFTIR7000). MgO (0.03 g) waspressed into a self-supportingdisk of 20 mm in diameter which
was placed in an IR cell with two NaCl windows which was
combined with a c l a d circulating system. A chloroform solutionof &(CO),, was added dropwise to the MgO in Ar (99.9999%)
atmosphere. The spectra were recorded by subtracting the gasphase spectra.
Results

TPD Spectra for &(CO)16-Derlved Catalysts. The TPD
spectrum for Rh6(CO)l6/MgO in vacuum is shown in Figure 1.
The ligand CO desorbed around 400 K, and the desorption wascompleted until 430 K. About one-third of ligand CO was desorbed before 398 K. No C 0 2 desorbed. When the Rh6/Mg0
were reacted with (CH3),Se at 398 K, 0.9 mol of CO per Rh6(CO),,were evolved and also three IR peaks (1632, 1524, 1388cm-I) which can be assigned to formates were observed.

EXAFS Observation for Selenium-Deposited &/M@ Catalysts. The structures of R&/MgO and Se-deposited Rb/MgO
catalysts in the preparation procaw of catalysts shown in Scheme
I were examined by the EXAFS analysis of Se K-edge and Rh
K-edge absorption data. We varied the ratio of Se/R& (=O, 1.O,and 2.0).
The Fourier transform of the Se K-edge EXAFS spectrum for
the Se-modified Rh6/Mg0 with Se/Rh6 = 1.0 in Figure 2b
showed only one peak around 0.22 nm (phase shift uncorrected).
The oscillation in the higher wavenumber region (Figure 2a)suggests the presence of heavy atom scatterers around the Se atom.
To analyze the peak around 0.22 nm, the peak was inverselytransformed to the k-space oscillation, followed by the w e f i t t i n ganalysis in Figure 2c which confirms the peak to be due to the
Se-Rh bond. The determined bond distance (r) and coordination
number (N) for Se-Rh are shown in Table I. Next, we tried thetwo-wave fitting (Se-Rh and S d ) because of the probability
that a part of the two methyl ligands of (CH3)2Semight remain,
but the Rf was worse than that for one-wave fitting (Se-Rh).
These results suggest that (CH3)$3ereacted with the partiallydecarbonylated R h 6 cluster at 398 K, releasing the CH3 ligands.
The N and r after ethene hydroformylation for 60 min were 1.4(the reduction factor S, is not considered) and 0.244 nm, respectively, which were almost the same as those for the catalyst
before catalytic reaction (1.3, 0.244 nm) (Table I).
The analysis for the catalyst with Se/Rh6 = 2.0 showed N =1.4 and r = 0.243 nm. In the case of this catalyst, it was observedthat a small amount of acetaldehyde was formed at the initial stage(0-15 min) of ethene hydroformylation at 398 K. In the catalyst
with Se/Rh6 = 2.0, a small portion of the methyl ligand bonded
to Se may have remained after the reaction of (CH3)2Se with thecluster.
Figures 3 and 4 showed the Rh K-edge EXAFS data for the
Se-undoped and Se-doped Rh6/Mg0 catalysts. On the basis of
the analysis for Rh6(CO)16as a model compound, the NRh-Rhforthe catalyst (Se/Rh6 = 2.0) was obtained to be 4.0 (Table 11),which implies that the incipient Rh6 framework remained unchanged after the reaction between Rh6/Mg0 and (CH&Se at
398 K.
Catalytic Ethene Hydroformylation. As is shown in Figure 5,
ethane (go%), propanal (17%), and propanol (3.3%) were producedon Rb/MgO without Se in ethene hydroformylation at 398 K.
With the increase of deposited Se the activity (the reaction rate
per total Rh atoms) for propanal increased and reached a maximum around Se/& = 0.6. The maximum activity was 1.9 times
higher than the activity of the Se-undoped catalyst. This iscontrasted to a monotonous depression for the formations of ethaneand propanol. The selectivity for hydroformylation (propanal andpropanol) showed a maximum at Se/Rh6 = 1.0, where the selectivity was 50%. Further doping of Se (Se/Rh6 > 1.O) reduced
the rates of both hydroformylation and ethane formation. The
decrease in the rate of ethane formation was more drastic.
For the evaluation of unblocked Rh sites after the doping ofSe, we observed CO uptake on the Rh6/Mg0 catalysts withdifferent Se/& ratios (Figure 6). The amount of CO adsorbed
decreased with Se deposition. The sum of this CO uptake andthe remaining ligand CO of the Rh cluster after the reaction with
(CH,)& at 398 K is also shown in Figure 6,which also decreased
with an increase of Se.

10944 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
8.80 I

5.28

Izumi and Iwasawa
19.0

.

1"

15.2

P

11.4

0
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L

9
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1 ' ' 1 1
5 6 7 8 9 1 0 1 1 121314
Wave Number k (10 nm-I)
I

.

'

'

0

0

1

2
3
4
Distance r ( 10-1 nm)

5

6

Wave Number k (10 nm-')
Figure 2. EXAFS spectra for Se K-edge of M o p e d &/MgO (Se/& = 1.O): (a) k'-weighted EXAFS oscillation, (b) its associated Fourier transform,and (c) inversely Fourier transformed curve-fitting analysis (-, observed; - -,calculated).
TABLE II: Results of the Curve Fitting A ~ l y m
for Rb K-Edge EXAFS Spectra of RhJMgO rad Se-Doped RbJMgCP
Rh-Rh
Rh-Se
Rh-C,
RhCb
Rh(C-)O
Se/Rh6
N
rlnm
Nb
r/nm
N
rlnm
N
r/nm
N
r/nm
a
(4)
0.277
(2)
0.187
(2)
0.219
(4)
0.306
b
0
3.8
0.276
1.8
0.185
1.9
0.215
4.0
0.307
C

d
d'
e
ad

0
1.o
1.o
2.0

3.8
3.7
3.9
4.0

0.275
0.276
0.275
0.273

4

0.2776

0.4
0.4

0.7

0.244
0.244
0.243

1.3
1.2
1.5

0.9

0.185
0.187
0.189
0.187

1.6
1.5
1.7
1.1

0.219
0.207
0.206
0.207

3.0
2.4
3.1
1.2

0.306
0.310
0.310
0.309

2

0.18642

0.216840.306'

Rf
0.035
0.054
0.034
0.021
0.041
0.017

"The notation of the samoles is same as in Scheme I. bThe reduction factor is not considered. 'After ethene hydroformylation (60 min).
dDetermined by XRD. CTheaverage for terminal and bridged CO.

TABLE 111: Wavenumbers of IR Perks for R4(CO)16, RUM&,
a d Se-Doped R U M &wavenumbcr/cm-'
twin, terminal
bridged2105 w, 2070 s, 2047 w, 2040
1833 w, 1793 s
RhdC0)16'
w,2022mw,2020mw
Rh6/Mg02090 w (sh), 2071 s, 2055 m,
1800 s(298 K)
2046 w, 2027 w, 2022 m
Rh6/Mg02091 s, 2072 s, 2055 w, 2046
1800 s(398 K)vw, 2022 m
Se/Rh,/MeO
2092 m, 2067 s, 2055 m, 2046
1800 s(Se/Rh6 = 1.0)
VW, 2022 m
Se/Rh,/MgO
2094 s, 2047 w, 2023 s
1805 m
(Se/Rh6 = 2.0)
substance"In ref 10.

FI'-IR Obsemtion for Selenium-DepOsitedRh&f@.
Figure7b shows the spectrum for the incipient Rh6/Mg0. For comparison the peaks for Rh6(co)16 in nujol are also shown in Table
III.'O After the sample was heated at 398 K in vacuum, the IR

TABLE IV: Wavenumbers of IR Perks for W M g O and &Doped
R U M & in Ethene Hydroformylation
wavenumbcr./cm-l
substance
twin. terminal
bridged
2092
w
(sh),
2071
s,
2055
w,
1802 s
Rh6/MgoSe/Rh6/Mg0(Se/Rh, 3 1.0)2046 vw, 2021 s
2094 s, 2071 w (sh), 2055 w,
2046 VW, 2022 S

1797 speaks were not significantly changed, though their intensities
reduced (Figure 7c). The spectrum in Figure 8a shows the IR
spectrum for the Se-deposited Rh,/MgO (Se/Rh6 = 1.O) after
the reaction of the supported cluster with (CH&Se at 398 K.
Further CO ligands desorbed upon Se deposition, but the peak
pattern was similar to thosefor Rh,/MgO (298 K)and Rh,/MgO(398 K), as shown in Figure 7b or c and Table 111. For the catalyst
with Se/& = 2.0 the twin CO peaks at 2094 and 2023 cm-l anda broad bridge-CO peak at 1805 cm-l were observed (Table 111).
In situ IR spectra in ethene hydroformylation on Se-undoped
Rh,/MgO and Se/Rh6/Mg0 with Se/Rh6 = 1.0 are shown in

%Doped Rh6 Carbonyl Clusters on MgO

The Journal of Physical Chemistry, Vol. 96,No. 26, 1992 10945
14.0
11.2

5 0.4

n

x

3
s
5.6
LL
2.0
0

3 4

4

5 6 7 8 9 10 11 12 13 14 15
Wave Number k (10 nm-')

0

1

2

3

4

5

6

Distance r ( lo" nm)

7 8 9 10 11 12 13 14
Wave Number k (10 nm-l)
Flgwe 3, EXAFS spectra for Rh K-edge of Se-undopedRh,/MgO treated at 298 K (a) k3-weightedEXAFS oscillation, (b) its associated Fouriertransform, and (c) inversely Fourier transformed curve-fitting analysis (-, observad, - -,calculated).

5

6

Figures 7 and 8 and Table IV. For Rh6/Mg0 the wavenumber
and intensity of each peak were almost the same as those beforehydroformylation. For the Se-doped catalyst (Figure 8b) thegrowth of two peaks at 2094 and 2022 cm-' in ethene hydroformylation was observed (Table IV). We tried to find somecorrelation of these two peaks with ethene hydroformylation byusing I3CO,but the faster exchange of CO ligands with gas-phase"CO than the rate of ethene hydroformylation prevented us fromfurther experiment under reaction conditions.

Discussion
Rh6(C0)16 reacted with MgO Surface to form Rh6(CO)lb,/
MgO (incipient cluster b in Scheme I), releasing about one (1-1 S)
CO ligand per &,as proved by the analysis of CO evolved during
the TPD spectrum and the amount of CO,fanned in the oxidationof the TPDtreated spacies with 02.The incipient Rh6 carbonylcluster was further decarbonylated to form the species Rh6(CO),, (c) in Scheme I by evacuation at 398 K,where m wasabout 6.5 by the analysis of CO evolved. The intensity of the IR
peaks of the CO ligands in F i i 7 also suggests the loes of about
one-third of the CO ligands in cluster c, which is compatible with
the TPD data. Both terminal and bridged CO ligands werereduced in intensity. These ligands are exchangeable with eachother at 398 K. When the partially-dccarbonylated &(CO),,(c) on MgO was quantitively interacted with (CH3),Se (1:lstoichiometry) at 398 K,about one CO was evolved. Besides, theformate peaks at 1632, 1524, and 1388 cm-I were observed by
I R the formates are formed by the reaction of CO with surface
OH group of MgO. The amount of the surface formates wasestimated to be 1 molecule per &,judging from the peak intensity. Thus, the composition of the M o p e d & cluster d on
MgO is suggested to be [SeRh,(CO),]/MgO, where one CO
ligand was replaced by a Se atom and another CO ligand wasdesorbed at 398 K.

We examined the active structure of Se&/MgO by meansof EXAFS and IR. The coordination numbers of Rh-Rh for theincipient supported cluster b and the 398 K-treated cluster c were
3.8, as shown in Table 11, which is close to the expected value (4)for the original cluster, suggesting that the Rh, framework was
retained on the MgO surface. The Rh-Rh coordination number
was not changed after the reaction with (CH3)& at 398 K,where
it was 3.7 (species d) or 4.0 (species e), as shown in Table 11. This
demonstrates that (CH&Se reacted with cluster c without destroying the six rhodium atom cluster framework. The Rh-Rh
bond distance is almost the same as samples a 4 The directbonding of Se-Rh was observed at 0.244 MI with the sample of
Se/& = 1.O. For the sample of Se/& = 2.0 the bond distancesof Rh-Rh and Se-Rh decreased to 0.273 and 0.243 nm, respectively. The cluster framework may have shrank a little by
the addition of two Se atoms to the Rh, framework.
The typical p3-C0 peak around 1800 cm-I for the & carbonylcluster was always observed in the range of 0 ISe/& 5 2(Figures 7 and 8), which also confvms the retention of the Rh,
framework.
The binding energy of Se 3d5,, for the catalyst with Se/&
= 1.0 was 54.7 eV. The value ranges between 53.5 eV for Se2(ZnSe and PbSe) and 55.9 eV for Seo(Se metal)." The XPS
data revealed that the Se atom bonded to the Rh, cluster isnegatively charged, possibly to be Se" (n Y 1). The binding
energies of Rh 3dy2 and 3d3/2for Se/&/MgO were 307.1 and
311.7 eV, respectively, which correspond to those for Rho species.
Se is a more electronegative atom than Rh. If the positive charge
induced by Se attachment is distributed over the cluster framework, the slightly charged Rh could not be discriminated fromthe Rh metallic state by XPS.'2
Sulfur and selenium are preferably located at the p3 site inchalcogen-coordinated metalthough our results of
EXAFS in Tables I and I1 cannot tell the exact value of the10946 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992

Izumi and Iwasawa

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9.30 I

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Wave Number k (10 nm-I)
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Distance r ( 10.' nm)

567.70 ,

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0

A

a
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2

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7 8 9 10 1 1 12
Wave Number k (10 nm-')13

14

Figure 4. EXAFS spectra for Rh K-edge of Se-doped Rh,/MgO (Se/Rh, = 2.0): (a) k3-weighted EXAFS oscillation, (b) its associated Fouriertransform, and (c) inversely Fourier transformed curve-fitting analysis (-, observed; - calculated).

-.

I

I

I

-O

Sel R bFigure 5. Catalytic activities and selectivitiesof ethene hydroformylationon the Se-doped Rh,/MgO as a function of Se content: reaction temperature, 398 K total pressure, 40.0Wa; C2H4:COH2= 1:l:l; Rh, 1.3
wt %. Key: 0,ethane; A, propanal; 0,propanol.

coordination number of S e R h because of the lack of an appre
priate model oompound with the S t R h bond. There are some
Rh complexes or clusters with the coordination of Se,but the bondlength of S t R h and the electronic state are too different fromthose for our Se-deposited catalysts and cannot be used as modelcompounds in the curve-fitting procedure. The coordination
number of S t R h remained unchanged after the hydroformylation,
which suggests the stable structure with p3-Se on the Rh6framework. The intensity of the peaks of the pn-COligands andthe terminal CO ligands of the Rh6 carbonyl cluster decreasedby the interaction with MgO and the reaction with (CH3),Se at
398 K in Figures 7 and 8. We propose supported structures for

\

llo
0

I

m

lo

Sel R%
Figure 6. CO uptake for Se-doped W / M @ (CO 12 Wa, 298 K) andtotal amount of CO (CO uptake + ligand CO).

the Se-undoped and -doped Rh6 cluster in the CO atmospherein Figure 9. These structures are based on (1) the retention of
the cluster framework shown by the detailed EXAFS analysis,(2)the number of CO ligands on the Q unit in the samples after
the interaction of the incipient Rh6 cluster b with MgO surface
at 398 K and after the reaction of species c (Scheme I) with
(CH3),Se, (3) the amount of CO uptake on species c and d(Scheme I) at 298 K, (4) the decrease in the intensity of theterminal and triply-bridged CO peah by the treatments, and (5)the preferable location of Se atom at the p3 site. It is suggested
by FT-IR that the face with the prCO preferentially interacts
with the MgO surface at 398 K,releasing one p3-C0 and fiveor six (due to heterogeneous environment of the surface) terminal
CO's. When thus obtained partially-decarbonylated & clusterc was exposed to CO at 298 K, two or three (average value: 2.5,
as shown in Figure 6) CO molecules, corresponding to the caseof the five or six terminal carbonyls removed, respectively, adsorbed

Se-Doped Rh6 Carbonyl Clusters on MgO

The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10947

d co
0

d 'co

0

Figure 9. Proposed structures for Rh6(C0)12and StRh6(CO)s (Se/Rh,= 1.0) clusters on MgO in CO atmosphere; *, removed at 398 K in
vacuum; **, partially removed at 398 K in vacuum. The arrows imply
the interaction of the face with MgO surface.

Q

,O

0
a3

00

ZR

7

Wavenumber lcm'
Figure 7. FT-IR spectra for Rh6/Mg0 catalyst: (a) MgO, (b) incipient
Rh,/MgO, (c) sample evacuated at 398 K, and (d) sample after ethenehydroformylation for 60 min.
I t

I

I

I

n

\ c.

R8

8
s?

Wavenumber /cm'
Fm 8. FT-IR spectra for Se/Rh,/MgO catalysts (Se/Rh,

1.0 (a,b) and 2.0 (c)): (a) after reaction with (CH3)$3e for 30 min at 398 K,(b) condition a after ethene hydroformylation for 60 min, and (c) afterreaction with (CHJ2Se for 30 min at 398 K.

to recover half the twin carbonyls at the corners of the faceinteracted with the surface, as illustrated in Figure 9. Uponreaction with (CH3)2Seat 398 K, ca. three CO ligands were
evolved as shown by the gas-phase analysis and the amount offormate, which corresponds to the decarbonylation of one p3-C0
and two terminal CO ligands, as shown in Figures 7 and 8, accompanied with the formation of the Se-Rh bond at 0.244 nm
in Table I and Figure 2. When the Se-modified Rh6 cluster dwas exposed to CO at 298 K,one CO adsorbed, as shown in figure

6, to form [Se&(CO)*] on MgO, as illustrated in Figure 9. The
comprehensive data of FT-IR, EXAFS,and composition analysis
demonstrate the structures in Figure 9 for the Rh6 clusters s u p
ported on MgO. If the face of the cluster without the p&O was
attached to the MgO surface, all of the experimental results could
not be explained.
In the octahedral Rh6 structure with p 3 coordination of Se(Se/Rh6 = 1.0) in Figure 9,three Rh atoms are directly bonded
to Se, which correspond to the nearest-neighbor sites (Rh,) in
the Se/Rh( 111) system, and the other three Rh atoms may
correspond to the next-nearest-neighborsites (RhB). In Figure
5 the selectivity of propanal formation increased by Se doping
and showed a maximum at Se/Rh6 = 1.0.
It is suggested that ethene hydrogenation needs more sites (13)than ethene hydroformylati~n.~~~*~The selective site blocking by
Se atom may be a reason for the increase of the selectivity to
propanal. But the prominent increase of the catalytic hydroformylation rate per total Rh atoms by the Se addition in Figure
5 indicates the creation of a new active site for propanal formation.
It has been demonstrated that the next-nearest-neighbor site (RhB)
is an active site for the promotion of hydroformylation reactionin the Rh/Zr02 and Rh/Si02 ~ a t a l y s t s . ~The
* ~ RhBcorner siteof the S e w cluster in Figure 9 may be suggested to be the activesite. When another Se atom was coordinated to the remainingp 3 site of the Se-modified R h 6 cluster, furthermore, the activityof the hydroformylation disappeared, as shown in Figure 5 . This
fact suggests that the RhBsite in the cluster is the active site forthe hydroformylation. In the Rh/Zr02 and Rh/Si02 catalysts
the selectivity and turnover frequency (TOF) (rate per surface
unblocked Rh) increased by Se, but the reaction rate per total
Rh did not increase. The promotion of catalytic hydroformylation
on a supported Rh catalyst by S addition has been reported,16
where S atoms were assumed to adsorb on the p 3 site of the flat
Rh(ll1) plane, without blocking of step sites. On Ru(l,l,lO)
single crystal S atoms at first selectively adsorb on the step sites,
followed by the adsorption on the flat plane after the completionof the step site adsorption.17
The RhAsites of the Semodified & cluster in Figure 9 should
be most electronically effected by Se. Alternative explanationof the promotion of hydroformylation by the Se addition may be
the creation of the RhAsites as active sites. This possibility CaMOt
be excluded at the present.
The selective structure for propanal formation is proposed tobe the structure in Figure 9. However, the maximum reaction
rate was observed at Se/Rh6 = 0.6 in Figure 5 , where a part of
the Rh6 clusters are not modified by Se. The reason for thedifferent optimum Se content between the rate and the selectivity
in the cluster system is not completely clear, but hydrogen spillover
might occur from the Se-undoped cluster to the Sedoped clusteron MgO surfaces.
Conclusion(1) The Se-doped Rh6 cluster catalysts were prepared by thecontrolled reaction between the partially-decarbonylated Rh6(CO)16-,,,( m H 6.5) on MgO and (CH3)2Seat 398 K.
(2)Selenium increased the reaction rate for propanal formationin catalytic ethene hydroformylation.

1094%

J. Phys. Chem. 1992,96, 10948-10953(3) The selectivity of hydroformylation increased from 20 to50%.

(4) Selenium was selectively bound to Rh at the bond distanceof 0.2434.244 nm by Se K-edge EXAFS.
( 5 ) The Rh6 cluster framework was maintained after the reaction with (CH&Se as suggested by the coordination numberof Rh-Rh in the Rh K-edge EXAFS and the peak at 1800 cm-’for the r 3 - C 0 ligands.
(6)XPS showed that selenium on the Rh6 cluster was situatedin an oxidation state of ca. -1, while no shift of Rh 3d binding
energies from the metallic level was observed.
(7) The structures of the partially-decarbonylatedRh, cluster
and the Se-modified Rhs cluster on MgO were proposed on thebasis of the number of CO ligands, its change by the treatments,
FT-IR, and EXAFS.
(8) The active sites for the catalytic hydroformylation reaction
were suggested to be RhAor Rhs on the Rh6 framework.
R@3W NO. Rh6(C0)16, 28407-51-4; MgO, 1309-48-4; Se, 778249-2; CO, 630-08-0.

Refereoces and Notes(1) (a) Gates, B. C., Guczi, L., Knbzinger, H., Eds. Metal Cluster inCatalysis; Elsevier: Amsterdam, 1986. (b) Iwasawa, Y., Ed. Tailored Metal

Catalysts; Reidel: Dordrecht, The Netherlands, 1986.
(2) Kirlin, P. S.; KnBzinger, H.; Gates, B. C. J . Phys. Chem. 1990, 91,
8451.
(3) Lee, G.; Ponec, V. Catal. Rev.-Sci. Eng. 1987, 29, 183.
(4) Jackson, S. D.; Brandreth, B. .J.; Winstanley, D.J. Catal. 1991,129,
540.
(5) Chuang, S. S.; Pien, S. I.; Sze, C. J . Catal. 1990, 126, 187.
(6) Izumi, Y.; Asakura, K.; Iwasawa, Y. J. Catal. 1991, 127. 631.
(7) Izumi, Y.; Asakura, K.; Iwasawa, Y. J . Catal. 1991, 132, 566.
(8) MacLaren, J. M.; Pendry, J. 8.;Joyner, R. W. Surf. Sci. 1986, 178,856.
(9) Teo, B. K. EXAFS Basic Principles and Data Analysis; Springer:
Berlin, 1986.
(IO) Chini, P. J . Chem. Soc., Chem. Commun. 1967, 440.
(1 1) Ley, L.; Pollak, R. A.; McFeely, F. R.; Kowalczyk, S.P.; Shirley, D.
A. Phys. Rev. B 1974, 9, 600.
(12) Baltanas, M.A.; Onuferko, J. H.; McMillan, S.T.; Katzer, J. R. J .
Phys. Chem. 1987, 91, 3772.
(1 3) Galli, D.; Garlaschelli, L.; Ciani, G.; Fumagalli, A,; Martinengo, S.;
Sironi, A. J . Chem. Soc., Dalton Trans. 1984, 55.
(14) Sachtler, W. M. H. Faraday Discuss. Chem. Soc. 1981, 72, 7.
(1 5) Siegel, S. J . Catal. 1973, 30, 139.
(16) Konishi, Y.; Ichikawa, M.; Sachtler, W. M. H. J. Phys. Chem. 1987,
91, 6186.
(17) Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 198, L329.

Ellipsometrlc Study of Surfactants Comprlslng Linear and Branched Hydrocarbon Chainsat the Air-Water Interface
L. Chicha, R. M. Leblanc,
Centre de Recherche en Photobiophysique, UniversitP du QuPbec d Trois RiviPres, CP.500 Trois RiviPres,
QuPbec G9A 5H7,Canada

and A. K . Chattopadhyay*
ICI Explosives Group Technical Centre, McMasterville Site, 801 Richelieu Blvd., McMasterville,
Quebec J3G 1 T9, Canada (Received: June 23, 1992; In Final Form: September 14, 1992)
The ellipsometric isotherms of diethanolaminederivatives of n-alkylsuccinic anhydride (where the alkyl group comprised
C12, C14, and C16 hydrocarbon chains) and polyisobutylene succinic anhydride (where the average backbone carbon numbersof polyisobutylene chains were 14, 22, and 34) at the air-water interface were compared with a view to the developmentof an understanding of the structural organisation of their monomolecular films. The ellipsometric angle, Ahs, has provento be very sensitive to the nature of the physical state of the film that occurs during film compression. At 20 f 1 OC,thetwo series of surfactants exhibited very different behavior upon compression. A rodlike model was used to describe the different
molecular behaviors exhibited by the surfactants comprising linear and branched hydrocarbon chains. Equations relating
variations of refractive indexes, film thicknesses, and molecular areas were used for the determination of the molecular propertiesof the films during monolayer compression. Calculated refractive indexes and film thicknesses were obtained from experimental
hsA values. The results indicated that the change in hsA values during film compression is more sensitive to the refractive
indexes than to the film thicknesses. For the surfactantswith n-alkyl chains the refractive index component inmasad Linearly,
whereas a reverse trend was observed with the surfactant molecules comprising polyisobutylene chains. Such an anomaly
in refractive index can be explained by the effect of substituents in the hydrocarbon chains and subsequent differences inpacking behavior of monolayers. The l6Al of n-alkyl derivatives increased upon compression and reached a steady state atcollapse pressure, whereas a sharper rise in l6Al was observed with the derivatives comprising polyisobutylene chains. Suchphenomena indicated multilayer formation by the molecules with polyisobutylene chains in the collapse region.

riItrodllction
The role of molecular structure, polar head-group charge, andthe influence of counterions on the polar head groups of surfactants
has great signifcance in determining their interfacial pr0Perties.l”
These phenomena have bcm studied in detail in the past with little
attention having been paid to the nonpolar components of surfactants.
However, more recently, attention has been turned toward thenonpolar group of the amphiphilic
Biologists, forinstance, arc intmsted in the componentsof surfactant molecules
which possess unsaturation or substitution in their hydrophobic
To whom correspondence should be addressed.

0022-3654/92/2096-10948$03.00/0

region. It has been found that in some membranes fatty acids
possess unsaturation or substitution in their hydrophobic chaii?
The properties of such fatty acids adsorbed at the interface inbiological membranes are dependent partly on the lateral interactions between adjacent alkyl chains. This leads to greater
fluidity and permeability of surface films in membranes compoeadof such fatty acids.
The behavior of certain fatty acids and lecithin has beem studid
at the air-water interface by surface pmrsurearea “ents.
It was reported that the unsaturated or branched hydrocarbon
chains of such molecules lead to an expansion of their moleculararea when compared to the corresponding saturated or unsub
stituted ~ o ~ c c u ~ c s . * J ~ ~ ~0 1992 American Chemical Society