Selective Ethanol Synthesis
from Carbon Dioxide
ROLES OF RHODIUM CATALYTIC SITESBy Yasuo Izumi
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology,
Yokohama, JapanWork on the synthesis of ethanol from carbon dioxide over a rhodiumselenium catalyst i s reported, and related reactions and characterisation
studies are briefly reviewed. I n order to inhibit theformation of methane (complete reduction of carbon dioxide) and simultaneously activate carbon-carbon
bond formation by the reaction of CH, with carbonyl derivatives, it is necessary
to control the active rhodium sites. Based on a study of single crystal rhodium
surfaces it is proposed that acetyl and acetate intermediates are formed. Recently
it has been discovered that supported RhlTi02, promoted by selenium frominside the rhodium metal sites, is apotential catalyst for ethanol synthesis fromcarbon dioxide. The action of this catalyst is compared to related studies.
Emitted carbon dioxide has been implicatedas one of the major sources of global warming.
Therefore the conversion of carbon dioxide into
alcohols is an environmentallydesirable process,making use of an abundant, cheap resource.
The synthesis of methanol from synthesis gas(CO + H2)is a well known process (1-3). The
dissociationenergy for carbon monoxide is large(107 1.8 kJ mol-I). It chemisorbs on metal surfaces via the carbon atom through the 5 0 orbital,
which is the HOMO (highest occupied molecular orbital). When adsorbed CO, CO(ads),
is completely hydrogenated, the product will
either be methanol or methane, depending onwhether the C - 0 bond is retained or cleaved,
respectively.
The C - 0 dissociation energy for carbon dioxide (526.1 kJ mol-') is smaller than for carbon
monoxide. The HOMO, In,, is on the two oxygen atoms, which suggests that the reactionsof carbon dioxide occur in a direction perpendicular to the 0-C-0 bond. Due to these bonding differences, carbon dioxide can undergo amuch wider range of chemical transformations
on a heterogeneous catalyst surface. For example, protonation forms formate species,

Platinum Metals Rev., 1997,41, (4), 166-170

HCOz(ads), while reaction with a surfacehydroxyl group or oxygen should produce carbonate species, COs(ads). Cleavage of one of
the C-0 bonds often occurs on a transition metal
surface, making the reaction more complex, due
to the formation of CO as an intermediate
species. (In this case the selectivity reflects thereaction path.)
Under reducing conditions, methane and lower
hydrocarbons (2 I C 5 = 10) are predominantly
produced from C 0 2+ H2on most platinumgroup metal catalysts, and here we examine somestudies for controllingthe conversion of carbon
dioxide to ethanol. In relation to our new RhloSe
catalyst, a proposed reaction intermediate,whichforms during ethanol synthesis, is described,
and the controlled reaction path over our
new ethanol synthesis catalyst is reported. The
structure and electronic state of the rhodium
catalyst sites are studied.

Proposed Mechanism Basedon an Acetate Intermediate
Model studies of ethanol decomposition andoxidation on Rh( 1 10) have been reviewed (4).
Stable acetate species are formed on the surface

166

(a 1
CHI

\cH z x

/”\

iH’

0

I

/c\o

\

/

(5)). Propionate species formed from the reactionof propanoyl with O(ads) were also detected onthese catalysts; however, they were mainly stabilised on the surface of the support, and werein equilibrium with propanoyl species on therhodium surface.

Lithium or Iron-Promoted
Rh/SiOz Catalysts(b)

SchemeThe stabilisatione of:(a) acyl species by adsorbed oxygen and(b) propanoyl speeies by promoter seleniumof pure rhodium. Acyl species, formed from thedecomposition of adsorbed ethanol, interact
with surface oxygen atoms to form the more stable acetate, see Scheme (a). Indeed, it has been
proposed that the role of promoters, which
preferably exist as metal oxides, is to effect thisstabilisation of acetyl species (evident in temperature programmed desorption) by forming
surface acetates.
Findings similar to this study of a model
rhodium surface have been reported for selenium-doped Rh/ZrOz(5) and FWSiOz catalysts(6).During ethene hydroformylation ( C Z H+~
CO + Hz) propanoyl species (C2H5CO)were
detected predominantlyon rhodium metal sites,
by FT-IR (Fourier Transform Infrared) spectroscopy, at 1770 and 1740
for Se-Rh/Zr02
and Se-Rh/SiOz, respectively. Propanoyl, corresponding to the acetyl species formed fromethanol decomposition on the Rh( 1 10) surface,
was formed by the migration of C2H5(ads)toCO(ads). The doped selenium had an electrical charge of -2 and -1 for Se-Rh/ZrOzand
Se-Rh/SiOz, respectively, and is suggested tofunction as a base to stabilise the propanoyl
intermediate by “locking” the carbonyl carbon,
see Scheme (b) (based on temperature-programmed desorption and in-situ FT-IR studies-

Platinum Metals Rev., 1997, 41, (4)’Extensive surveys of carbon dioxide hydrogenation have been reported on promoted
Rh/Si02 catalysts (7, 8). On unpromoted
Rh/SiOzmethane was the major product. Amongpromoter atoms that have been examined, it was
found that co-impregnation with lithium, strontium, iron and silver promoted ethanol formation (7).With lithium, ethanol selectivity was
15.5 per cent at 5 MPa and 513 K, and withiron (at a molar ratio iron:rhodium = 2) the
ethanol selectivity was 16.0 per cent at 5 MPa
and 533 K (8).

Poisoning and Promotion Effects ofSelenium &omInside Rh,, Cluster Sites
As described above, in general, supported
rhodium metal sites easily dissociate the C - 0
bonds of COz leading to the formation of
methane and lower hydrocarbons. For example, when the iron:rhodium ratio was varied iniron-promoted WSiOZ,the selectivity and conversion into ethanol reached a maximum aroundiron:rhodium = 2 (8). (The dependence of selectivity and conversion is often observed having
a maximum as a function of the amount of promoter.) However, the reason for the dependence
is not completely clear because an excessive
amount of doping by promoter can affect theresult by steric (geometric) poisoning andor byelectronic induction, and it is hard to distinguish between the two.
The synthesis of ethanol from carbon dioxideand hydrogen over [RhloSe]/TiOzis examinedin the Table (9). At the lower temperature (523K), the initial rate of formation of ethanol was
1.9 x
mol h-’gc$ and the selectivity toethanol was 83 per cent. The synthesis rateincreased with reaction temperature, to reach
6.0 x
mol h-’g,;’ at 723 K. However, the167

Ethanol Synthesis on Supported [RhloSe] and Conventional Rh/Ti02*Catalysts(A) Using carbon dioxide and hydrogen
Catalyst

Initial rate, lo” mol h - ’ g i lT,,,,,,

K[R hloSe]/Ti02[R hroSe]/A1203[Rh&e]/MgO[R hloSe]/Si02[R heC]ITi02[R h&Ti02
RhITiOn

523
623
723
623
623
623
623
623
623

Ethanol

Methane + CO

Ethane

1.9
3.7
6.0

0.4
1.4

0

0.15

0
0
0.4
0
0

0.04
0

0
0
0
0
0
0
2.4
0
0

5.8

1.2
6.2

1.8

Ethanolselectivity,rnol %
83
71

51
0
0
0

10
0

0

(B)Using carbon monoxide and hydrogen
Catalyst

Treact.

Initial rate,

mol h ‘gG1

Ethanol

EthaneEthanolselectivity,rnol %

0

0.03

0

K

[Rh,oSeITTiOz

623

Total pressure 47 kPa: PCO?(or PCO)’ P H =~ 1 : 2: Rh 1.3 for Ti02and A1203.2.8 for Si02and 1.6 w.%for MgO
As pretreatment, incipient catalyst was in vacuum at 593473 K. then in hydrogen at 623 K for 1 h.
RhTTiO, was in vacuum at 523 K, then in OdH, at 523 K

selectivity gradually decreased with increasing
reaction temperature due to the larger activation energy for methane + CO.
The model catalyst was prepared from theorganometallic cluster [RhloSe(CO)22]2and thestarting supported catalyst was totally decarbonylated by heating in vacuum (9-1 2). The
structure of the active sites was studied by
EXAFS (Extended X-ray Absorption Fine
Structure) spectroscopy, which is tunable to theabsorption edge of each element. Both selenium
K- and rhodium K-edge EXAFS spectra were
measured for [RhloSe]/Ti02catalyst (12). By
curve fitting analysis of selenium K-edge
EXAFS, based on model EXAFS parametersof [Rh3Se2(CO)6](Se-Rh bond) and Se02(Se0 bond), the Se-Rh bond distance was foundto be 2.41 A. The fit was dramatically improvedby two-wave fitting, suggesting the existence ofa surface oxygen atom at a distance of 1.97 A
from selenium. The model parameters forrhodium K-edge curve fitting were extracted
from rhodium foil (Rh-Rh bond), Rh3Ses(Rh-Platinum Metals Rev., 1997, 41, (4)Se bond) and Rh203(Rh-0 bond). The best
fit for rhodium K-edge EXAFS showed thebonding distances for Rh-Rh,Rh-Se and Rh-0
to be 2.72,2.41 and 2.09 A, with co-ordination
numbers 4.0, 1.O and 1.5, respectively. In comparison with [RhloSe(CO)22]2(4.8), the co-ordination number, Nm.,, (4.0) became smaller,
due to partial decomposition of the [RhloSe]
core by the formation of Rh-O(surf) bonds.
Based on all the bond distances and co-ordination numbers obtained by EXAFS, a surface
cluster model in which four rhodium atoms areinteracting with O(surf) of Ti02was proposed(12).
It is interesting to compare this model cluster consisting of ten rhodium atoms and one
selenium atom (atomic ratio Se:Rh = 0.1) toother reports. When dimethyl selenide (gas) was
reacted with [Rh,]/MgO, prepared from
Rh6(CO)16, the ethene hydroformylation reaction rate to propanal + propanol reached a maximum at around Se:[Rhh] = 0.6 (10). The atomic
Se:Rh ratio was 0.1, equivalent to that for168

[RhloSe]/TiOz.However, this may be coinci- carbonate species on the [RhloSe]/TiOzshould
dental, due to the complex effects of the sele- be affected by the [RhloSe]cluster, as they arenium promoter. When the amount of selenium either near the [RhloSe]or on the Rh of [RhloSe].
= 2, the sele- A peak at 3356 c d was also observed for unimincreased to the ratio Se: [a6]nium-doped [Rhb]/MgOlost all its catalytic activ- pregnated TiO, which shifted to 2460 cm-' inity. It appears that geometric hindrance of the deuterium gas, showing that the peak was due
reactants was significantly affected by poison- to hydroxyl species on Ti02.
The increases in intensity of the peaks at 2965,
ing of [Rhs]/MgO(doped onto the rhodium surface). Selenium in the [Rhlo]framework should 2883, 2919 and 2853 cm-' were proportional
to the ethanol synthesis rates, and were assigned
affect the poisoning only electronically.
Different supported rhodium clusters have to CH3vas,vs, CH, vs., and vE,respectively. The
been prepared and compared with [RhloSe]for dissociation of the C-0 bonds of carbon dioxthe COz + Hz reaction, see the Table (9).The ide occurred on rhodium and the four peaks[Rh6]/TiOzwas the most active, but only pro- rapidly disappeared when the supply of reacduced methane and/or CO. Rh/TiOz, prepared tion gas (COz+ Hz)was discontinued. This sugby conventional impregnation from the Rh salt, gests that the methyl and methylene species arealso only formed methane + COYbut at a lower formed mainly on rhodium. The intensities ofrate resulting from the larger rhodium particle the CO stretchingpeaks (2174and 2108 cm-')size, as observed by EXAFS. [RhsC]/Ti0~gave were weak compared to those for [RhloSe]supethanol, but significant amounts of methane (+ ported on Alz03,MgO and SOz, which exhibCO) and ethane were also produced, suggest- ited lower total activities and no activities to
ing that inhibition of the total carbon dioxide ethanol.
The reason why CO(ads) was not stabilised
reduction was not as successful on [Rh6C]/TiOz
on[RhloSe]/TiOzis unclear, but the different(ethanol selectivity was 10 per cent) compared
co-ordination
number around rhodium, comto [RhloSe]/TiOz(71per cent). Special features
pared
with
that
around conventional rhodiumof using [RhloSe]for ethanol synthesis should[RhloSe]/SiO~orbe the retention of the [RhloSe]cluster under or supported [RhLoSe]/Al~03,
the reaction conditions (stabilisation by sele- [RhloSe]/MgOcatalysts, may be important (12).
nium) and an effective electronic modification These two wavenumbers, 2 174 and 2108 ern-',by selenium (anionic selenium contacts all the were relatively high, suggesting that CO wasbound to relatively positive rhodium sites.
ten rhodium atoms in the cluster).
Based on these assignments and on the peakintensity changes, a mechanism for ethanol synReaction Mechanism of Ethanol
thesis on [RhloSe]/TiOzis proposed, see Figure
Synthesis on [RhloSe]/TiOt Catalyst
The reaction mechanism for the formation of 1.The CH3(ads)or CHz(ads) on rhodium may
followed byethanol from COz + Hz on [RhloSe]/TiOzwas first react with C03(ads) or COZY
studied by in-situ FT-IR ( 1 1). The intensity of hydrogenation to ethanol on the rhodium sites.
two peaks (1 6 14 and 1243 mi') increased dur- The reaction of CO + H2was very slow anding the course of the reaction. They were totally different from the reaction of COZ+ HZ
assigned to bidentate carbonate species, based on [RhloSe]/TiOz,suggesting that ethanol synon the frequencies for [Co(NH3)4(CO3)]' thesis does not proceed via CO for this catalyst.
On many rhodium catalysts, the order of reac(1635-1593 and 1292-1265 cm-') (13). Twopeaks were observed in the same region (1570 tion with respect to CO is negative, thus highand 1230 cm-') for unimpregnated TiOzin COZ CO concentrations would poison the reaction.
+ H2.Two unresolved peaks between the two Hence, it is possible in this case that COZprocarbonate peaks were also measured. On vides a small, steady state amount of CO for thisthe basis of several references in (4), these two reaction (14).The observed acetate species may
peaks may be assigned to acetate species. The support the reaction mechanism, not via CO as

Platinum Metak Rev., 1997, 41, (4)169

Fig. 1 The proposed reaction
mechanism of ethanol synthesis
from COZ + H1on [RhlaSe]fl’i02
catalyst. The reaction between
CO(ads) and CH,(ads) to form
ethanol is also possible,o 1243cm-’
H 3356cm’0/////TiO//////////-Rhodium selenide’ - like dectronic structure (rRh-SC= 2.41A)

shown in Figure 1. The stability of the Cl%(ads)on rhodium should be a key point, the rhodiumsite being affected by the interior negatively
charged selenium (-1 -2 by XPS) (12). The“rhodium se1enide”-like electronic state inhibited methane formation, but tuned the reaction
path to ethanol by promoting the C-C bondformation of CH, with the carbonyl derivative.

-

Summary
Controlling the reaction path of COz+ Hzto
give ethanol was found to be possible due to thepresence of carbon, oxygen, selenium, lithium
and iron around rhodium sites. Electronic control by selenium for ethanol formation was investigated by several techniques, and electronic
modification of the platinum metal cluster fromwithin should be applicable to other systems.

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Hexagonal Nanostructured Platinum
Platinum particles can be prepared as nanostructures or monolayers, in polymers and in
templates of ceramic materials by the reductionof platinum acids or salts. Researchers fiom the
U.K. and Germanynow report the use of a liquidcrystalline phase template to form nanostructured
platinum (G. S Attard, C. G. Goltner, J. M.
Corker, S. Henke and R. H. Templer, Angew.
Chem.Znt.Ed End., 1997,36, (12), 13151317).
Hexachloroplatinicacid and ammonium tetrachloroplatinate were added to the surfactant,octaethyleneglycol monohexadecyl ether, which
was used to prepare the lyotropic liquid-crystalline phases because it forms a wide hexagonal mesophase. The platinum salts were reduced,
forming platinum powder of particle size 90 to
500 nm of hexa onal nanostructure with cylindrical pores 30 in diameter, separated by 30 A
thick platinum walls. The lyotropic phase may
act as a structure-directing medium. Platinumis stabilised at the hydrophobidhydrophilicinterface as small colloidal particles which agglomerate and coalesce to a stable wall thickness.
Only fast reductions allow this structure to form
before the liquid-crystal phase rearranges.
This nanostructured platinum is mesoporous
and of large surface area - fea,tures useful forcatalytic, fuel cell and sensor applications.

x170