Infrared study of CO adsorbed on Co/c-Al O based
2 3
Fischer–Tropsch catalysts ; semi-empirical calculations as a tool for
vibrational assignments
L. E. S. Rygh,* O. H. Ellestad, P. Kl~boe and C. J. Nielsen
Department of Chemistry, University of Oslo, P.O. Box 1033, N-0315 Oslo, Norway.
E-mail : l.e.s.rygh=kjemi.uio.no
Received 6th January 2000, Accepted 14th February 2000
The adsorption of CO(g) on a Co/c-Al O catalyst has been studied by di†use reÑectance infrared
2 3
spectroscopy. The e†ects of pressure, temperature and the addition of O (g) and H (g) have been investigated.
2
2
Vibrational bands can be assigned to speciÐc carbonyl species on the reduced Co catalyst, at low surface
concentration, via a theoretical model. The basis for the model is an empirical relation between the number of
valence electrons in transition metal carbonyl complexes and the corresponding carbonyl stretching frequency.
This relation has been extended to metallic surfaces and combined with results from extended HuŽckel
calculations.
Introduction
Experimental
Infrared spectroscopy can be used to study CO which is
adsorbed on various metal surfaces and catalysts. The spectral
interpretations are mostly limited to the classiÐcation of
““ linear ÏÏ and ““ bridged ÏÏ adsorptions and are often based on
the assumptions drawn from comparisons with data published
on other similar systems. Little attention has been directed
towards an interpretation based on established calculation
methods. Infrared spectroscopy is a frequently employed
method to study ““ non-ideal ÏÏ systems like small metal particles on a support, where the application of more exact structural methods is limited ; vibrational assignments that are
more speciÐc would therefore be of particular interest. Vibrational spectroscopic studies of various Co/Al O catalysts,1h10
2 3
cobalt Ðlms11h20 and of the cobalt single crystals,
Co(0001)21h23 and Co(10-10)24 have been presented. Cobalt
single crystals have also been studied by LEED, Auger spectroscopy and thermal desorption methods.25h31
In this work, we report our studies of a 12% Co/c-Al O
2 3
catalyst. To interpret the spectra, we have combined an
empirical method to predict the carbonyl stretching frequencies in transition metal carbonyl complexes with semiempirical ASED-MO (Atom Superposition and Electron
Delocalisation Molecular Orbital ; extended HuŽckel) calculations on metal clusters.32,33 Essentially, the empirical
method is based on a correlation which assumes that the force
constants for metal carbonyl complexes are related to the
number of available valence electrons of the metal. The
COÈCO interaction constants are assumed to depend on the
bonding angles alone. Expansion of this model to include carbonyl species adsorbed on surfaces requires primarily a knowledge of the actual number of valence electrons on the various
surface metal atoms. Semi-empirical calculations can, once
veriÐed, be considered as an additional tool for assigning the
vibrational bands observed in real catalysts. Information from
various studies of single crystals21h24 is especially interesting
in this connection. The infrared studies of Co O ,34
3 4
Co (CO) ,35h37 Co(CO) ,38 CoAl O ,39 Co Ðlms or
2
8
n
2 3
foils11h20 provide relevant reference material.
DRIFTS (Di†use ReÑectance Infrared Fourier Transform
Spectroscopy) spectra were recorded with a Perkin Elmer
model 2000 FTIR instrument using a spectral resolution of 8
cm~1. The DRIFT unit including a vacuum chamber was
delivered from Harrick Sci. Corp. For information about
DRIFTS for heterogeneous catalyst studies, see refs. 40 and
41. All spectra presented here are transformed into KubelkaÈ
Munk units.
The reactants were introduced via a gas-handling manifold
including a mixing chamber of glass. The pressure was measured by an absolute pressure transducer from Balzers. The
purities of the gases used in the experiments were 99.997%
(CO), 99.9997% (H ), 99.998% (O ) and 99.9995% (N ). Fur2
2
2
thermore, the gases were cleaned prior to use by passing them
through the following gas puriÐers from Supelco : an OMI-1
Indicating PuriÐer (H and N ), a Supelpure O-trap (CO) and
2
2
a drying tubeÈMolecular Sieve 5A (CO, O ). Contamination
2
from oxygen should be reduced to less than 0.5 ppm by this
procedure.
The 12% Co/c-Al O catalyst was made by impregnating
2 3
c-Al O (Akzo, Alumina 000È1 1/2 E) with an aqueous solu2 3
tion of Co(NO ) É 6H O, using the incipient wetness tech32
2
nique. Subsequently, the sample was dried at 100 ¡C and
calcinated at 300 ¡C.
The catalyst was reduced in-situ for 16È20 h under atmospheric pressure by a stream of H at 400 ¡C. After the
2
reduction procedure the system was cooled to the desired temperature before evacuation and subsequent introduction of
CO(g). The CO pressure was typically 1È150 mbar, and the
CO adsorption was studied as a function of temperature (30È
400 ¡C) and time before and after exposing the catalyst surface
to additional H or O . Temperature changes were performed
2
2
with a rate of 1È2 deg min~1.
DOI : 10.1039/b000188k
Theoretical model for spectral assignments at low surface
coverage
An empirical relation between the number of valence electrons
and the carbonyl stretching frequency has been found for carPhys. Chem. Chem. Phys., 2000, 2, 1835È1846
This journal is ( The Owner Societies 2000
1835
bonyl complexes of transition metals.42 (See Appendix A.) In
this relation the force constants for metal carbonyl complexes
are expressed in terms of the number of valence electrons of
the metal atom, the ionic charge of the complex and of the
various ligands. Essentially, the model is a parametrization of
the number of electrons available for bonding. The Co(CO)
x
(x \ 1È4) species found on Co surfaces are very ““ complexlike ÏÏ, as they are rather loosely attached to the neighbouring
cobalt atoms and are able to migrate along the metal surface.
Consequently, the only modiÐcation we have introduced to
the model presented by Timney,42 is to replace the combined
contribution from ionic charge and ligand e†ects, by a calculated number of valence electrons for the individual cobalt
atoms.
The number of available electrons on the various surfaces,
on edges and corners was obtained by ASED-MO (extended
HuŽckel) calculations.32,33 The gross atomic charges obtained
in this way do not necessarily represent the number of electrons available for bonding. The number of available electrons
may be either lower, as s-electrons are occupied in metalÈ
metal bonding, or higher, as surface atoms may draw on electrons originally ““ belonging ÏÏ to the underlying layers during
the formation of the metal carbonyl bond. It is therefore
essential that the results be compared with the experimental
data in a critical way.
The ASED-MO calculations were performed using a
quantum chemistry program ICONC&INPUTC developed
by G. Calzaferri and M. BraŽndle43 and obtained through
Quantum Chemistry Program Exchange (QCPE). However, it
is necessary to have data for the atomic valence orbital ionization potentials and the Slater orbital exponents. Using this
program, we have not experienced any problems with metal
cluster models consisting of as much as 160È200 atoms
depending on the symmetry of the actual cluster. The cluster
models employed are shown in Appendix B. Primarily, we
aimed to Ðnd the number of valence electrons typical of Co
atoms situated well within the various surface planes, that is,
as far away from the edges/corners as possible. This number is
best approximated by the theoretical values calculated for the
Co atoms situated in the middle of the model surfaces. The
models are such that 2.4 to 4 atoms in all directions surround
various central surface atoms. The number of valence electrons for Co atoms situated at edges/corners was estimated in
a similar manner.
perature. The various CO stretching modes arising from CO
adsorbed on Co (and Al) atoms, all lead to absorptions in the
2300È1700 cm~1 region. The e†ects of exposure time, temperature, CO pressure, oxidation, and hydrogenation are illustrated in Figs. 2È7, respectively. The inÑuence of the reduction
temperature is presented in Fig. 8.
Bands above 2100 cm—1. When CO(g) is introduced to the
Co catalyst at 200È250 ¡C, an increased absorbance in the
region 2400È2300 cm~1 is observed after some time (Fig. 9).
This indicates the increase in the amount of both adsorbed
and gaseous CO present. (The latter increase is not an arti2
fact due to the variations of the CO concentration outside
2
the cell.) The amount of adsorbed CO decreases simultaneously in the same period.
In the region 2300È2100 cm~1 we observe bands at
2250(sh), 2230 and 2180 cm~1 (Figs. 1 and 3È9). The inten-
Fig. 2 FTIR spectra of CO on 12% Co/c-Al O after exposure to
2 3
P \ 1 mbar at room temperature : ÈÈ immediately
after introCO
duction, É É É É after 2 min, È È È after 50 min, È É È É after 6 h, ÈÈ after
20 h.
Results
The Co/c-Al O catalyst
2 3
A typical DRIFTS spectrum of CO(g) adsorbed on a 12 wt.%
Co/c-Al O catalyst is given in Fig. 1. This shows a reduced
2 3
catalyst exposed to 1 mbar of CO(g) for 5 min at room tem-
Fig. 1 FTIR spectrum of CO on 12% Co/c-Al O 5 min after expo2 3
sure to P \ 1 mbar at room temperature.
CO
1836
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
Fig. 3 FTIR spectra of CO on 12% Co/c-Al O as a function of
2 3 (b) 125 ¡C ; (c)
temperature, P \ 1 mbar : (a) room temperature,
CO
200 ¡C, (d) 250 ¡C.
Fig. 4 FTIR spectra of increasing h
on 12% Co/c-Al O after
CO
2 3
exposure to CO(g) at 200 ¡C : (a) P \ 0.3 mbar, (b) P \ 1 mbar,
CO
CO
(c) P \ 5 mbar, (d) P \ 20 mbar, (e) P \ 20 mbar (after 8 min),
CO
CO
CO
(f ) P \ 60 mbar, (g) P \ 150 mbar.
CO
CO
sities of the bands increase with both time and temperature.
However, the increase as a function of time is small at room
temperature. The band intensities are to some extent a†ected
by the pressure of CO(g), but the bands are still visible after 50
min of evacuation at room temperature. In contrast, all bands
disappear when the sample is exposed to H (g) at 200È250 ¡C
2
(Figs. 6 and 7). The 2180 cm~1 band is somewhat difficult to
study since it is of low intensity and therefore often hidden by
neighboring bands. At higher CO pressures, the 2180 cm~1
band is concealed by the R-branch of the CO(g) band centered
at 2143 cm~1, and at higher temperatures, it is obscured by
the 2230 cm~1 band. However, we observed that the 2180
cm~1 band is relatively invariant with respect to evacuation
or oxidation, (Fig. 5). Traces of oxygen may increase its band
intensity. After some time it disappears on exposure of high
concentrations of O . Introduction of H may also in some
2
2
Fig. 5 IR spectrum of CO on 12% Co/c-Al O catalyst before and
2 3exposure to P \ 1
after oxidation at room temperature : (a) 1 h after
CO of
mbar followed by rapid evacuation, (b) 5 min after introduction
P \ 0.2 mbar to the system in (a).
O2
Fig. 6 FTIR spectra of CO on 12% Co/c-Al O before and during
2 3
FischerÈTropsch activity at 200 ¡C : (a) after 10 min with P \ 150
mbar, (b) after 3 h with P \ 150 mbar, P \ 300 mbar, (c)COafter 20
CO
H2
h with P \ 150 mbar, P \ 300 mbar, (d) after additional 6 h with
CO
H2
P \ 150 mbar, P \ 600 mbar followed by evacuation to P \ 10
CO
H2
tot
mbar. The addition of extra hydrogen did not result in any intensity
changes, only a decrease in l from 2023 cm~1 to 2015 cm~1.
max
cases result in an immediate increase in the intensity. A closer
study of the absorption reveals a varying degree of structure,
with band maxima at 2210(sh), 2200(sh), 2182, 2175(sh) and
2155(sh) cm~1. There also seems to be a weak, broad feature
near 2120 cm~1.
Under FischerÈTropsch conditions, CÈH stretching bands
are observed around 3000 cm~1, indicating reaction products.
In our experiments, we observed mainly an absorption due to
CH (g) at 3017 cm~1. This may obscure other neighboring
4
bands. Nevertheless, we were able to distinguish an initial
band at 2908 cm~1 which developed prior to the band at 3017
cm~1.
Fig. 7 FTIR spectra of CO on 12% Co/c-Al O before and during
FischerÈTropsch activity at 250 ¡C (P \ 21503 mbar, P \ 300
CO
H2 after
mbar) : (a) 50 min after introduction of CO(g),
(b) immediately
introduction of H (g), (c) 15 min after introduction of H (g), (d) spec2
2
trum (b) minus spectrum
(a).
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
1837
Fig. 8 IR spectrum of CO on 12% Co/c-Al O catalyst 20 min after
2 3
exposure to P \ 1 mbar at room temperature : È È È catalyst
CO
reduced at 400 ¡C, ÈÈ catalyst reduced at 500 ¡C.
Bands in the region 2100–1700 cm—1. At CO(g) pressures of
less than 1 mbar, a broad absorption band was observed at
ca. 1990 cm~1. When the pressure was increased, it was
shifted towards 2060 cm~1 and/or was overlapped by absorptions at higher wavenumbers. Variations in the band shape at
di†erent pressures or di†erent temperatures indicated that, at
high wavenumbers, it consisted of at least two bands located
around 2060 and 2030 cm~1 (Fig. 4). The 1990 cm~1 band is
more resistant towards evacuation than is the absorptions at
higher wavenumbers.
Absorptions at 1900 and 1805 cm~1 were observed immediately after introduction of 1 mbar CO gas. (Figs. 1, 5(a) and
8). By increasing the exposure time and/or temperature, both
bands changed and reshaped themselves into a broad, low
wavenumber ““ tail ÏÏ which showed no clear intensity maxima
and was indistinguishable from the continuous bandstructure
dominating the 2100È1700 cm~1 region. The degree of
reduction obtained prior to the introduction of CO, determined whether the absorptions at 1900 and 1805 cm~1,
existed as distinct bands or not.
T emperature dependence. The temperature did not have any
dramatic e†ect on the band shapes (Fig. 3). However, after
prolonged exposure to CO(g) at higher temperatures the CO
adsorption process was no longer completely reversible due to
Fig. 9 FTIR spectra of CO on 12% Co/c-Al O after exposure to
2 3
150 mbar CO at 250 ¡C : (a) immediately after introduction
of CO(g),
(b) after 50 min [identical to 7(a)]. For comparison, a spectrum of
CO (g) over KBr is presented (c).
2
1838
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
other factors such as the dissociation of CO. There were also
small variations in the wavenumbers of the intensity maxima.
In general, the apparent band maximum, l
, Ðrst shifts to
Imax
lower wavenumbers following heating from room temperature
to about 125È150 ¡C ; then blue shifts with temperature up to
200 ¡C ; and Ðnally red shifts upon further heating to 250 ¡C.
The spectral changes as a function of time after CO introduction also varies with the actual temperature. After CO(g)
introduction at room temperature, the temperature was raised
to 150 and 200 ¡C, and at each temperature level there was an
interval where we studied the time dependence of the CO
absorptions. From this, we found that l
increases with
Imax
time in the region from room temperature to 150 ¡C and
mainly decreases as a function of time at higher temperatures.
When CO was introduced directly at 200 ¡C, we observed that
l
increased, decreased, increased once more and then
Imax
Ðnally decreased again as a function of time. Furthermore, we
found that adsorbed CO which dissociated at higher temperatures may have recombined when the temperature was
lowered (Fig. 10).
Introduction of O and H . The introduction of O after CO
2
2
2
adsorption had a marked e†ect on the spectrum. Even small
amounts of O (g) at room temperature made all the Co(CO)
2
x
bands disappear, the more resistant species having the lowest
wavenumbers (Fig. 5). We also observed that evacuation and
oxidation at room temperature, followed by heating to at least
300 ¡C and then subsequent cooling, reestablished a band at
1920 cm~1 (Fig. 11).
Introduction of H at higher temperatures, where the
2
FischerÈTropsch reaction is active, resulted in developments
very similar to what was observed during the oxidation
process. The bands at the highest wavenumbers are the Ðrst to
disappear. Those are followed by the bands of lower wavenumbers until (nearly) all molecularly adsorbed CO is consumed by the reaction (Fig. 9). The CO(g) in the reaction
chamber is not always all consumed in the reaction, since the
reaction products are not removed from the surface. Therefore, at 200 ¡C, the reaction eventually ceases upon the introduction of syn gas at higher pressures, see Fig. 6. The spectral
region from 2100È1700 cm~1 then undergoes no more change,
the intensity maximum being typically around 2020 cm~1.
With evacuation, the band maximum decreases to 1920 cm~1
before it vanishes. In contrast, all molecularly adsorbed CO
Fig. 10 FTIR spectra of CO on 12% Co/c-Al O recorded after 1 h
2 the
3 situation in Fig.
of FischerÈTropsch activity at 250 ¡C [i.e. after
6(c)], showing how CO that is dissociated at 250 ¡C may recombine at
lower temperatures : (a) T \ 250 ¡C, (b) T \ 150 ¡C, (c) T \ room
temperature.
Fig. 11 FTIR spectrum of CO on 12% Co/c-Al O showing that
2 3
dissociated CO may recombine after annealing. After oxidation at
room temperature all bands in the region 2100È1800 cm~1 had disappeared, and the sample was annealed at 300 ¡C during the night.
When the temperature was lowered back to room temperature,
recombined CO gave rise to the band at 1920 cm~1.
seemed to be dissociated after 1 h of FischerÈTropsch activity
at 250 ¡C. If the temperature was subsequently reduced to
200 ¡C or less, a band appeared at 2030 cm~1 ; this was due to
the recombination of CO molecules.
Calculation of carbonyl frequencies
The method of calculating vibrational wavenumbers of
adsorbed CO is described above, and the predicted vibraTable 1 Vibrational wavenumbers for linearly adsorbed monocarbonyl, Co(CO)B, calculated directly from the equation for metal
complexes
No of valence electrons
Atomic charge
Wavenumber
6.0
7.0
8.0
8.5
9.0
9.5
10.0
10.5
]3.0
]2.0
]1.0
]0.5
0
[0.5
[1.0
[1.5
2213
2133
2049
2006
1962
1916
1870
1823
Fig. 12 Cluster model for calculation of the maximum chemical
shifts that can result from deposition of CO, carbide or atomic oxygen
on a Co(0001) surface. The adsorbed species are on the Co atoms
shown as …. The Co atom marked with * is the one for which the
number of electrons has been studied. (The cluster model consists of 4
layers containing 49/36/25/16 atoms respectively.)
tional wavenumbers for monocarbonyl species (calculated
from eqn. 1, Appendix A) are shown in Table 1. From Table 1,
the di†erence in wavenumbers per valence electron (or per
atomic charge) is about 80 cm~1. The number of valence electrons, typical of cobalt atoms which were situated at various
planes, edges and corners, were estimated by ASED-MO calculations. The results are presented in Table 2 together with
the corresponding theoretical wavenumbers of adsorbed
mono- and polycarbonyl species. The cluster models are
shown in Appendix B. A comparison between calculated
wavenumbers and experimental data for CO adsorbed on
single crystals is presented in Table 3.
A single CO molecule adsorbed on Co(0001) is estimated to
give rise to a band around 1978 cm~1. Accordingly, a single
CO molecule adsorbed on any other plane will give rise to
absorption bands at wavenumbers lower than this. On edges
and corners single CO molecules may even have wavenumbers as low as 1835 cm~1 (Table 2). For the dicarbonyl
species, l will be in the region 2017È1879 cm~1, l is predicts
a
ed to be between 1977 and 1831 cm~1 and l [ l for a spes
a
ciÐc species is D50 cm~1. For the tricarbonyl species l is
s
expected to be in the region of 2052È1918 cm~1, l in the
d
region 1997È1853 cm~1 and l Èl is D55 cm~1.
s d
To estimate the wavenumber shifts resulting from the dissociative adsorption of CO and from full surface coverage, the
cluster model was expanded with non-metal atoms/molecules
such as carbon, oxygen or carbon monoxide. These species
Table 2 Number of valence electrons characteristic of di†erent Co atoms situated at various planes, edges or corners estimated by ASED-MO
calculations ; as well as vibrational wavenumbers calculated for low coverages of mono- and polycarbonyl species on various planes, edges and
corners, see Appendix B
Calc. no. of valence electrons
Surface
No. of
atoms
s
p
d
Co(0001)
Co(10-10)a
Co(01-10)
Co(10-12)
Edge
Corner (a)
Corner (b)
126
160
148
169
110
125
125
0.67
0.67
0.64
0.66
0.64
0.69
0.55
0.20
0.19
0.16
0.16
0.10
0.11
0.05
7.94
8.23
8.68
8.69
9.30
8.80
9.77
Total
Co (CO)
l m
co
Co (CO)
m
2
l
l
s
a
Co (CO)
m
3
l
l
s
d
8.81
9.10
9.47
9.51
10.03
9.60
10.37
1978
1953
1919
1915
1867
1907
1835
2017
1992
1960
1956
1910
1948
1879
2052
2028
1996
1993
1948
1985
1918
1977
1951
1917
1913
1864
1905
1831
1997
1971
1938
1934
1886
1926
1853
a There are two possible versions of this plane. This one is that parallel to the fcc (110).
Table 3 Calculated vibrational wavenumbers for linearly adsorbed monocarbonyl species compared to the corresponding experimental values
Cobalt
crystal plane
l /cm~1
calc work
This
l /cm~1
calc
ASED-MO
data, ref. 53
l /cm~1
calc calc., ref. 44
DFT
l /cm~1
exp 22, 24
Refs.
Co(0001)
Co(10-10)
1978
1953
1915
È
2035
È
2012 at 10 L
1972 at 0.05 L
2020 at 10 L
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
1839
Table 4 Calculated chemical shifts due to carbide, oxygen or carbon
monoxide deposited on the 18 nearest atoms
Species adsorbed
in neighbouring
position
*l/cm~1
Calculated
(ASED-MO)
*l/cm~1
Experimental,
CO/Mo(110), ref. 14
ÈC
ÈO
ÈCO
38
22
12
40
19
È
were added in ““ on-top ÏÏ positions to 18 Co atoms, surrounding the central atom, (Fig. 12). A Co cluster model consisting
of four layers with 49/36/25/16 Co atoms was employed for
the calculations. As before, the calculated number of electrons
characteristic of the central atom in the upper layer was then
used to predict the CO stretching frequency. The results from
these calculations are compared to experimental data from the
CO/Mo(110) system in Table 4.
Discussion
It is generally accepted that the vibrational frequency of CO
adsorbed on an oxidized metal lies above 2100(È2130) cm~1,
while metal carbonyls of the reduced metal absorb below this
value.45h49 Linear carbonyls absorb at higher wavenumbers
than the bridged ones, which are expected to absorb below
about 1950 cm~1. There is evidently more controversy concerning this limiting wavenumber, as some authors would say
below 2000 cm~1,46h48,50 and others again below 1880È1930
cm~1.45,49,51 Triply coordinated species are seldom assigned,
but they are assumed to absorb at lower values than 1900
cm~1.46h48,50 Already in 1964, Blyholder pointed out that
CO adsorbed at exposed metal atoms on the edges and
corners of metal crystallites, may give rise to bands as far
down as 1800 cm~1.52 The possibility of polycarbonyl species
contributing in this region has also been suggested.34
The assignment of the vibrational bands of CO, adsorbed
on a speciÐc supported metallic catalyst, is often based on
various infrared studies of ideal systems. The spectra of CO,
adsorbed on the corresponding metal Ðlms, speciÐc crystal
planes or metal oxide surfaces are informative, as are also
studies of metal carbonyl complexes. Nevertheless, unsupported, such data does not allow one to distinguish between
the characteristic wavenumbers of CO that is multicoordinated to the metal, and CO that is adsorbed on various
planes/edges/corners. To attain this information, it is necessary to combine the infrared studies with results from corresponding LEED (Low Energy Electron Di†raction) studies
and/or with theoretical calculations.
Calculated vibrational wavenumbers
The estimated numbers of valence electrons typical of atoms
situated at various planes, edges and corners are presented in
Table 2. An extended HuŽckel calculation of the Co(0001)
surface has also been presented by Sung and Ho†mann.53
Their calculated value for the number of valence electrons is
9.5, or 0.7 electrons higher than our results. Although signiÐcant, this is easily explained on basis of the di†erences in the
cluster models. Sung and Ho†mann used a model based on a
two-dimensional inÐnite surface with four atomic layers. In
our smaller clusters the atoms situated in the outer rim of
such clusters are coordinately unsaturated and will therefore
attract electrons from the neighboring atoms. Real metal particles consist of Ðnite surfaces that are also less than ideal. In
addition, the changes in the important surface properties are
small on going from a 3- to a 4-layer slab.54 A comparison
between the vibrational wavenumbers predicted from the
number of valence electrons calculated by Sung and Hoffmann and by us is presented in Table 3, together with the
1840
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
corresponding experimental values.22,24 Theoretical wavenumbers found by Density Functional Theory (DFT)44 are
also included in this table. The CO adsorption on Co(0001) at
pressures below 10 L has not been studied by IR before this
publication. An increase in the CO pressure from 0.05 to 10 L
has been shown to give a blue shift in the peak position by 50
cm~1 for Co(10-10).24 We expect that CO adsorbed on
Co(0001) may display a signiÐcant shift in the same direction
given a similar increase in the CO pressure. On this basis, our
theoretical values are therefore quite consistent with the
experimental measurements.
We now compare the experimental data from refs. 22 and
24 to the estimated vibrational wavenumbers for monocarbonyl species. From the spectra of CO adsorbed on
Co(0001)22 and Co(10È10),24 we Ðnd that 10 L of CO gives
rise to absorptions at 2012 cm~1 and 2020 cm~1, respectively
(Table 2). Since LEED studies indicate that the density of
adsorbed CO molecules is also approximately the same, this
result is unexpected as CO adsorbed on Co(0001) is believed
to give rise to the higher wavenumber absorbances. (See also
ref. 22.) This apparent contradiction to theory may Ðnd at
least two explanations :
(i) Our expectations concern the Ðrst molecule that adsorbs
on the actual surface. Moreover, we have no indications that
the shifts due to chemical¤ or coupling” e†ects46,55 will be of
exactly the same size for di†erent crystal planes and di†erently
arranged CO molecules. In this connection, the absorptions at
2012 and 2020 cm~1 are relatively close in wavenumbers. The
CO molecules adsorbed on Co(10-10) might therefore give rise
to larger positive shifts than do the CO molecules adsorbed
on Co(0001).
(ii) Dicarbonyls could be adsorbed on Co(10-10) instead of
monocarbonyls (see Table 2), as mono- and dicarbonyls are
indistinguishable in LEED unless extensive and complicated
calculational work is carried out.56
The estimated chemical shifts for full CO coverage, for carbidization and oxidation are presented in Table 4. Carbidization seems to lead to a signiÐcantly larger shift towards
higher wavenumbers than oxygenation, in accordance with
reports from IRAS (Infrared ReÑection Absorption
Spectroscopy) studies of Mo(110).14 However, the total shift
caused by the increasing surface coverage is underestimated.
We can only Ðnd the e†ect of the chemical shift, while the shift
due to coupling e†ects is neglected.
It must also be kept in mind that the extended HuŽckel
parameters that have been employed are developed for the
clean metal. When non-metal species are added to the metal
surface, the resulting cluster may possess characteristics from
both complexes and metals. In consequence, the cluster may
be characterized by modiÐed extended HuŽckel parameters.
Finally, we have neglected surface restructuring in our model.
The calculated chemical shifts are therefore only qualitative
and should not be used rigorously.
Our model is restricted to the prediction of the wavenumbers for mono- and polycarbonyl species only. Although
bridged species are not directly implemented, it is possible to
make some deductions concerning their likely vibrational
wavenumbers. We infer from the observed wavenumbers of
CO adsorbed on the Co(0001) plane that the Co atoms have a
number of valence electrons that are approximately the same
as in neutral Co carbonyl complexes. From this, we deduce
that bridged species, situated on this surface and at low coverage, will be close in wavenumbers to the corresponding
¤ Changes in the electronic environment of the molecules, such as
competition for the electrons involved in synergetic r- and p-bonding
limited to a few lattice spacings, may cause shifts of either sign.
” Collective term for long range dipolar couplings or much shorter
range van der Waals type of interactions ; includes shifts due to
dipoleÈdipole interactions.
doubly or triply bridged carbonyl groups in Co clusters like
Co (CO) or Co (CO) . In addition, we know that the CO
2
8
4
12
adsorbed on the Co(0001) surface gives rise to the highest
wavenumbers. At low coverage one would therefore expect the
wavenumbers of well deÐned, doubly coordinated carbonyl
species to appear below D1900 cm~1 and triply coordinated
species below D1830 cm~1. This is in accordance with our
observations. Nevertheless, on surfaces, in contrast to complexes, there exists a possibility for so-called ““ half-bridged ÏÏ
situations. It must therefore be emphasized that the discussion
above concerns the well-deÐned covalent species.
We shall Ðnally make some additional comments on the
density functional theory (DFT) calculations on a symmetric
Co CO cluster mimicking CO adsorbed on a Co(0001)
13
surface.44 The Co cluster consists of three c.c.p. layers with 3,
7 and 3 Co atoms, respectively. The calculated absorbances
for the CO stretching mode is 2035, 1860 and 1751 cm~1 for
CO adsorbed in an ““ on-top ÏÏ, a bridged and a three-foldhollow position, respectively. Wavenumbers calculated at this
level of approximation are normally 2È3% too high, although
for this speciÐc type of system, the error is less systematic,
probably due to complexities in electronic structure.57
Toomes and King24 have assigned absorption bands at 1900
cm~1 (10~3 mbar) and 1850 cm~1 (10È300 mbar) to CO molecules adsorbed in a bridged and a three-fold hollow position
on Co(0001), respectively. Two to three percent lower values
for the 2035 cm~1 band would result in an absorption in the
range 1994È1973 cm~1 for linearly adsorbed CO, which is
well within our results and with the observed spectra. On the
other hand, values 2È3% lower for the 1860 and the 1751
cm~1 bands calculated for bridged and triply coordinated carbonyl species are clearly inconsistent with the experimental
data.
The Co/c-Al O catalyst
2 3
Results from temperature programmed desorption (TPD)
have shown that the Co/c-Al O catalyst consists of four
2 3
phases : 58h60 (i) the alumina support itself ; (ii) a CoAl O
2 4
spinel phase ; (iii) an amorphous Co O phase interacting with
3 4
the support ; and (iv) a crystalline Co O phase. The divalent
3 4
Co atoms existing in the spinel phase are rather inactive
towards adsorption of CO ; their tetrahedral coordination and
surface shielding61 is a rational used to explain this. The
extent of reduction is found by temperature programmed
reduction (TPR) to be around 40È50% after 14 h at
350 ¡C.62,63
Bands above 2100 cm—1 and below 1700 cm—1. The CÈH
stretching bands that were observed at ca. 3000 cm~1 conÐrmed that FischerÈTropsch reactions were taking place. In
addition to the absorption due to CH (g) at 3017 cm~1, we
4
also distinguished a band at 2908 cm~1. We recognize this
band to be identical to the 2915 cm~1 band that is assigned to
formic acid on alumina,64 which is an intermediate in one of
the two reaction paths for producing hydrocarbons.65
The observed increase in the amount of CO and the simul2
taneous decrease in the amount of adsorbed CO subsequent
to the CO(g) introduction at 200È250 ¡C is caused by disproportionation of CO (the Boudouard reaction) :
2CO(ads) ] C(ads) ] CO (g, ads)
2
where the produced CO may be in form of gas or loosely
2
adsorbed molecules (Figs. 7 and 9).
The observed bands at 2250(sh) and 2230 cm~1 are present
in the spectra of CO adsorbed on Al O ,45 while they are
2 3
absent in the corresponding spectra of CoAl O 39 and
2 3
Co O .34 Accordingly, the two bands are assigned to mono3 4
carbonyls situated on tetrahedrally and octahedrally coordinated Al3`, respectively.45 The observed increase in the band
intensity with increasing time and temperature indicate that a
progressively larger area of the bare support (not spinel phase)
is exposed and thereby is accessible for CO adsorption. This
progress is ascribed to a restructuring of the catalyst surface,
which includes sintering of the Co particles. The disappearance of the 2250(sh) and 2230 cm~1 bands after treatment
with H is in accordance with other reports.66,67 A linear
2
correlation between the loss of hydroxy groups and the
increase in the amount of CO chemisorbed to the Al3` sites of
pure c-Al O has been demonstrated. V ice versa, an increase
2 3
in the amount of hydroxy groups due to treatment with H
2
should then result in a decrease in the amount of CO
adsorbed on Al3` atoms.
When CO was adsorbed on bare alumina, the band at 2180
cm~1 was not observed. Theoretically, the absorption should
have agreed with that due to the CO adsorbed on a Co atom
with a positive charge of 2.6, corresponding to coordinately
unsaturated Co3` atoms at the surface (Table 1). However,
when the metallic charge is high, the character of the MÈCO
bonding may have some (or complete) electrostatic character.
The d-2p back-donation, which our model is based on, is then
lacking.45 A correlation between the stretching frequency of
adsorbed carbon monoxide and the modiÐed electric Ðeld
strength produced by the cation adsorption site is then
expected. The actual atomic charge is not necessarily the same
as the formal charge. Accordingly, the band at 2180 cm~1
might be assigned to CO adsorbed at Co atoms both with
formal charge ]2 and ]3, but with di†erent coordination to
neighboring metallic atoms or di†erent degree of coordinative
unsaturation. From the literature, we Ðnd that the absorption
has both been assigned to CO adsorbed on Co2`,6 and
Co3`.34 The 2180 cm~1 band is also characteristic of
CoAl O ,39 where the cobalt exists as Co(II).
2 3
Bands in the region 1700È1200 cm~1 are assigned to carbonate species predominantly found on the alumina support.
No further description or discussion of these bands will be
attempted here.
Bands in the region 2100–1700 cm—1. The di†erent bands
appearing in the CO stretching region indicate the existence of
various structures. The di†erence in extinction coefficient
between bands originating from linear and bridged species is
not known, but we assume that it is small. More important,
the possibility of intensity transfer from the low to the high
wavenumbers has been demonstrated.46,55 This complication
arises when species are closely adsorbed on a catalyst segment
and individual molecular vibrations have wavenumbers that
are close to each other. We observe that the relative intensity
of the infrared bands below 2000 cm~1 (Fig. 1) are signiÐcantly lower compared to observations from similar studies of
Rh containing catalysts.41 In spite of this, we have no certain
indication of the relative amounts of bridged species (or other
species that absorb at the lower wavenumbers). The reason is
that we do not know which surface planes are exposed or the
degree of surface restructuring we are dealing with. The nature
of the surface is important, as better metal conductors give
rise to larger extent of intensity transfer. In short, the amount
of species in the lower wavenumber region might easily be
underestimated.
According to our calculations, polycarbonyl species may
contribute to the absorptions for most of the region 2000È
1800 cm~1 (Table 2). Nevertheless, for good metal conductors
““ the metal surface selection rule ÏÏ46,68 should also be obeyed.
According to this rule, only those modes that have vibrational
dipole moments perpendicular to the metal surface (in general
the symmetric modes) will have measurable intensities.
Although the surface of cobalt catalyst is not ideal, we assume
that the intensity of l and l in most cases shall be (very) low
a
d
compared to l . From our own spectra, we therefore conclude
s
that subcarbonyls do not make any signiÐcant contributions
to the observed intensities below 1900 cm~1. Accordingly, the
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
1841
absorptions below 1900 cm~1 are expected to arise either
from CO adsorbed in a bridged position (see the previous
section) or from monocarbonyls adsorbed on defects, steps or
corners.
The 2060 cm~1 band dominates most spectra, by virtue of
its high intensity and broadness. Clearly many di†erent
species may contribute to this absorption signal, and all of
them are consumed during the FischerÈTropsch process. That
is, they are either actually part of an active site, or they are
able to migrate to such a site during the reaction. According
to the continuous-Ñow study performed by Fredriksen et
al.,3,4 the band may also be present while there is FischerÈ
Tropsch activity. (See the next section.) On the other hand, in
our study we have not been able to reestablish the band once
it has disappeared during the hydrocarbon formation. Further
introduction of CO(g) only gives rise to band structures that
are both lower in wavenumbers and in intensity, even when
the CO gas pressure is substantially higher than at Ðrst. This
implies that a less dense arrangement of adsorbed CO,
obtained by evacuation or by CO being consumed in the
FischerÈTropsch process, permits some kind of restructuring
of the surface layer.
The absorption at 2060 cm~1 is also characteristic of the
Co(0001) surface,22 which has been shown to be active in the
FischerÈTropsch process.21,23 Exposure to CO(g) at higher
temperatures leads to reconstruction of this surface.22,69
Changes in the morphology of the Co(0001) surface following
CO hydrogenation at pressures above 1 bar have also been
detected by scanning tunneling microscopy (STM).69 From
the latter observations it has been suggested that cobalt mass
transport occurs via an etchÈregrowth process, leading to the
formation of triangular shaped cobalt islands and thereby to a
signiÐcant increase in the number of coordinativly unsaturated sites. (The fraction of all cobalt atoms in the uppermost
atomic layer occupying edged sites is calculated to be around
50 percent after the restructuring.) This mechanism is consistent with the fact that only one type of Co site is found to be
present on the surface, and that CO hydrogenation is a structurally insensitive reaction.70,71
Beitel et al.22 investigated the e†ect of CO on the Co(0001)
surface by RAIRS (infrared reÑection absorption
spectroscopy), LEED and XPS (X-ray photoelectron
spectroscopy) and were able to detect that the presence of CO
led to formation of defects on the surface, which we, in spite of
the di†erence in pressure, attribute to the cobalt mass transport mechanism described above. The CO adsorbed on these
defects is found to give rise to an infrared absorption
maximum appearing in the region 2055È80 cm~1, while CO
adsorbed on the ideal surface is expected to absorb at lower
wavenumbers. The authors ruled out that the band at 2080
cm~1 is due to the direct inÑuence of adsorbed carbon or
oxygen, and reported the indications that the absorption
arises from species adsorbed on the active sites for the carbon
chain growth.21,22 This is an unexpected result, and the
authors also state that : ““ This, for cobalt, unexpected result
shows that our current understanding of CO bonding to
group VIII metals is incomplete ÏÏ. According to the Blyholder
model,52,72 the vibrational band arising from CO adsorbed
on the more electron rich site should appear at the lower
wavenumber.
The absence or presence of di- or tricarbonyls has not yet
been proven. These species are not normally considered as a
typical starting point for CÈO dissociation, but the existence
of polycarbonyls on some defects does not exclude a simultaneous presence of monocarbonyls on others. If the monocarbonyls adsorbed on defects and on terrace sites give rise to
absorbances that are rather close in wavenumbers, these may
be difficult to distinguish from each other. In a study performed on a 5% Co/Al O catalyst,1 the 2060 cm~1 band is
2 3
also assigned to overlapping contributions from Co(CO) ,
n
1842
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
CoHCO and Cod`ÈCO. We have calculated l for CoHCO
CO
to be very close to (or 5 cm~1 higher than) l
for
CO, symm
Co(CO) , assuming the bonding angle between the substit2
uents to be 90¡. We therefore concur in the interpretation by
Kadinov et al.,1 although we shall not rule out the possibility
that the relatively broad 2060 cm~1 band also contains a contribution from CO linearly adsorbed on Co(0001).
We now turn to the bands at the lower wavenumbers. The
absorptions around 1920È1900 cm~1 and 1805È20 cm~1
appear as maxima on the low wavenumber tail of the 2060
cm~1 band for a short time immediately after the introduction
of CO (Fig. 1). Both bands Ðt well with the absorptions arising
from bridged species. Following the discussion above, bridged
species situated at Co(0001) at low coverage should give rise
to absorbances that might be a little higher in wavenumbers
than the corresponding bridged carbonyl groups in Co clusters. A pair of bands at 1868 and 1857 has earlier been assigned to the bridging CO groups in Co (CO) .35,36 In
2
8
accordance with this, bands at 1900 and 1850 cm~1 have been
assigned to CO adsorbed on Co(0001) in a bridged and in a
3-fold hollow position, respectively.22 An absorption band at
1900È1967 cm~1 (0.5 \ h \ 1.0) has also been interpreted as
CO
resulting from CO adsorbed in a bridged position on Co(1010).24 The band around 1900 cm~1 can therefore be assigned
to CO adsorbed in a bridged position (l ) on the catalyst
2
surface, and the band around 1820 cm~1 to CO molecules
adsorbed in 3- or 4-fold hollow positions (l or l ). In addi3
4
tion, the absorptions in the higher wavenumber part of the
1900È20 cm~1 band may contain contributions from monoand polycarbonyl species.
T emperature dependence. The spectra in Figs. 2 and 3 illustrate the competition of various e†ects and how one (or more)
of these may dominate at di†erent temperatures and as a function of time. At the lower temperatures, e†ects resulting from
the dense arrangement of the adsorbed CO molecules dominate. With increasing temperature, carbidization and surface
restructuring lead to a breakdown of these large (ideal) surface
structures, and give rise to other spectral phenomena. It is
natural to discuss the temperature dependence of l
and
Imax
I in three temperature regions :
max
T he temperature region 25È150 ¡C. The red-shift and the
accompanying decrease in the intensity of the absorption
around 2060 cm~1 when the temperature is raised from 25 to
150 ¡C, are consistent with one or more of the following interpretations :
(i) A decrease in the component of the dynamic dipole
moment normal to the surface may lead to a decrease in the
integrated absorbance. This e†ect can be explained as a consequence of the tendency for the CO molecules to tilt away from
each other at surface coverages higher than 0.5 as proposed by
Ryberg.46 Alternatively, it may be explained by an increase in
the amplitude of the frustrated translation, which occur at
higher temperatures.
(ii) A decrease in the amount of the CO linearly adsorbed
on the surface. In this case, there may be an increase in the
total amount of adsorbed CO, due to a structural transition
and a simultaneous change of adsorption sites from on top to
bridge-bonded sites.22
(iii) Formation of polycarbonyls.
(iv) An increasing amount of hydrogen may be adsorbed on
the surface, see later. In this case, remains of hydrogen from
the reduction procedure must migrate from the catalyst pores
or sublayers up to the surface and adsorb there. Small
amounts of hydrogen are located somewhere in the catalyst
system. This has been conÐrmed by the observation of
methane formation (band at 3017 cm~1) after the catalyst had
been exposed for 1 mbar CO at 200 ¡C overnight.
Interpretation (i) and (ii) are regarded as the most probable,
although (iii) and (iv) cannot be excluded. The blue shift in
wavenumbers as a function of time, at constant temperature,
is predicted to arise from a general increase in the coverage of
adsorbed CO. Hence, there must be a competition between
the mechanism that leads to the restructuring of the adsorbed
CO and the mechanism favoring further adsorption of linearly
bonded CO.
T he temperature region 150È200 ¡C. The blue shift in wavenumbers with increasing temperature and the simultaneous
increase in the intensity of the higher wavenumbers (around
2060 cm~1) can be due to :
(i) A further increase in the linearly adsorbed CO and in the
total CO coverage. This includes the possibility of CO linearly
adsorbed at defect sites. Annealing at 180È220 ¡C and 100
mbar of CO pressure has earlier been shown to lead to the
creation of defects at the Co(0001) surface.22
(ii) CO adsorbed on more oxidized sites as a result of the
increased CÈO dissociation activity at higher temperatures.
(iii) Formation of new surface species.
The alternate blue and red shifts in wavenumbers, as a function of time after CO introduction at 200 ¡C, are interpreted
to be a consequence of the following e†ects in competition :
(i) The initial blue shift results from an increase in the total
CO coverage as a function of time.
(ii) As CO starts to dissociate, the following red shift may
arise from a decrease in the coverage of linearly adsorbed CO
(the total coverage of CO may still increase). For Co(0001), a
decrease in maximum intensity and a simultaneous red shift
(in the pressure range 1È300 mbar) has been shown to result
from a dilution of linearly bound CO in bridge-bonded CO.22
(iii) Due to CO dissociation, the adsorption sites are gradually oxidized leading to higher wavenumbers and lower
intensities.
(iv) Upon further surface carbidization, CO adsorption is
inhibited and a decrease in the coverage of adsorbed CO
Ðnally leads to a decrease both in the observed wavenumbers
and in the intensities of the absorption signal.
T he temperature region 200È250 ¡C. The observed red-shift
and the decrease in the intensity maximum in this region most
probably result from an increase in the CÈO dissociation
activity that inhibits CO adsorption.
Oxygenation and hydrogenation. Adsorbed molecules of CO
are not resistant towards surface oxidation. Nevertheless, oxidation and evacuation, followed by annealing and subsequent
cooling, reestablishes a band around 1940 cm~1. This is in
accordance with the theory that carbon and oxygen, coming
from dissociative adsorption of CO, may recombine at this
temperature.9
We have also observed that the absorptions remaining after
prolonged FischerÈTropsch activity could have an intensity
maximum as low as 2020 cm~1, which decreases to 1920
cm~1 during evacuation. Since bridged CO seems only to
form on islands of a certain size ;22 these low wavenumber
bands are interpreted as small islands of linearly adsorbed CO
in accordance with the refs. 22 and 46.
Comparison with infrared studies performed in-situ. It is of
interest to compare our results with observations from in-situ
systems, and a study has been performed on a 4.7 wt.% Co/cAl O catalyst almost identical to our own sample.3,4 In this
2 3
particular experiment the catalyst was exposed to a Ñow of
syn gas (H /CO \ 2, P \ 6 bar, T \ 200 ¡C). Any di†er2
syn
ences between the results might therefore be explained in
terms of the di†erences in pressure, di†erences in metal
content or the Ñow vs. the static system.
In spite of considerable di†erences in the experimental conditions, we Ðnd strong resemblances in the spectral observations. The main di†erences are to be found in the spectra
obtained immediately after the introduction of syn gas. Fredriksen et al.3,4 observed, at this stage, well-deÐned bands at
2060, 1990 and 1950 cm~1. The latter two bands increased in
intensity in the Ðrst ten minutes. Then, during a couple of
hours the bands diminished and almost vanished into a broad,
low wavenumber ““ tail ÏÏ with no clear intensity maxima. The
authors interpreted the 2060 cm~1 band as due to linearly
adsorbed CO, and the pair of bands at 1990 and 1950 cm~1
to bridged species, adsorbed on the more oxidized parts of the
catalyst.
The appearance of the bands at 1990 and 1950 cm~1 in the
in-situ study may be explained on the basis of either the high
CO pressure or the low metal content in the catalysts. In a
study by Choi et al.9,10 a low pressure (0.35 mbar) of CO was
introduced to both 5 wt.% and 12 wt.% Co/Al O samples.
2 3
The 5 wt.% catalyst showed absorption bands at 2176, 2056
and 2030 cm~1 and the 12 wt.% catalyst at 2045 and 2028
cm~1. In a study by Kazansky et al.6 bands were observed at
2160, 2060, 1990 and 1950 cm~1 when a 1.3 wt.% Co/Al O
2 3
catalyst was exposed to a CO pressure of 10 mbar. It seems
therefore, that the appearance of the two bands is favored
both by low metal content and by high CO pressure, which
could indicate that the bands are due to CO adsorbed on
defects. High CO pressures lead to a reconstruction of the Co
surface and thereby to both an increase in the amount of
defects and a decrease in the extensive long-range dipoleÈ
dipole interactions. A more thorough discussion of these
bands is presented in the following section. A short summary
of our assignment of the observed absorptions due to CO
adsorbed on Co is given in Table 5.
Reaction pathway
As discussed earlier, two reaction paths are possible on the
Co/c-Al O catalyst. The one involving only the Co crys2 3
tallites is the most important. Only one type of Co site is
Table 5 Assignment of the vibrational absorbances observed in the spectra of CO adsorbed on the cobalt surface in a Co/Al O catalyst
2 3
Vibrational band
Assignment
2230, 2250(sh)
2180
Assigned to CO linearly adsorbed on Al3` sites on the alumina support
May be due to CO linearly adsorbed on Co atoms with formal charge ] 2 or ]3, with a
varying coordination of neighboring atoms. More probably caused by CO adsorbed on Co 2`
Cod`ÈCO ; Co0ÈCO ; Cod~ÈCO ; Co(CO) ; CoHCO. High coverages of CO
Cod`ÈCO ; Co0ÈCO ; Co(CO) ; CoHCO n
n
Co0CO ; Co(CO)
n unoxidized sites ; Co(CO) situated on more coordinately unsaturated
CoÈCO on various
n
sites. Low CO coverages
Co0ÈCO ; Cod~È(CO)
n
Cod~È(CO) . Might arise
from the same species as the 1990 cm~1 band
n
When observed
after oxidation and subsequent recombination of CO : CoÈCO
Cod~ÈCO and Co (CO) on defects, edges or corners
Co CO, where n \m2È4 ; nCod~ÈCO (on defects, edges or corners)
COn adsorbed in bridged (l2) position e.g. on Co(0001)
CO adsorbed in a 3- or 4-fold hollow position (l3 or l4) e.g. on Co(0001)
2070È1990
2060
2030
1990È1900
1990
1950
1940
1920
1900È1800
1900È1880
1820È05
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
1843
found present on the Co surface, indicating that CO hydrogenation is a structure insensitive reaction.70,71 Furthermore,
the active sites are shown to be surface defects that are
responsible for CO absorptions around 2055È2080 cm~1.22
According to our interpretation this band is due to Co(CO)
n
and CoHCO species adsorbed on defects including steps/
corners.
One possible reaction path in the hydrogenation process
could therefore involve these Co(CO) and CoHCO species,
n
with the unstable CoHCO species as an intermediate in the
FischerÈTropsch process. However, this is not in agreement
with the general understanding of the CO hydrogenation
mechanism.65,69h71 We therefore suggest that monocarbonyl
species are also found on defects, where they give rise to
absorptions at lower wavenumbers and are overlapped by
other bands. They may be of low intensity as a result of rapid
CO dissociation (when the steric conditions permit this) or
due to further CO adsorption giving rise to polycarbonyl
species (when there is a lack of free space around a defect). We
conclude that the main FischerÈTropsch reaction path is
through the carbide species formed from the dissociation of
these monocarbonyls.
Conclusions
The results from DRIFTS studies in combination with semiempirical calculations are promising and helpful in assigning
vibrational bands arising from carbonyl species on metal surfaces. The results emphasize how various adsorbed species
may contribute to absorption bands in the same spectral
region, and open the way for new spectral interpretations that
do not exclude the prior ones. The broadness of the infrared
bands may be explained not only by the dipoleÈdipole e†ect,
but also by various subcarbonyls adsorbed on surface defects
and by the varying oxidation state of the metal atoms. In
addition, it is possible to explain the low wavenumber absorptions by species other than the bridged ones.
When we compare our DRIFTS results to those at an
in-situ investigation,3,4 we Ðnd that there are strong resemblances in the observations, in spite of considerable di†erences
in experimental conditions. Also, hydrocarbon formation is
observed at the lower pressures, and an absorption band with
a maximum in the wavenumber region 2060È2080 cm~1 is
supposed to rise from the active sites in the FischerÈTropsch
process.21 From this, we conclude that our observations give
a better understanding also of the commercial process.
Acknowledgements
The authors gratefully acknowledge the Norwegian Research
Council and Statoil for Ðnancial support and the Norwegian
Institute of Technology for supplying catalyst samples. We
thank Christian Richard for his critical review of the manuscript.
Appendix
Empirical method for predicting the wavenumbers
characteristic of the carbonyl stretching mode of transition
metal carbonyls
The method for predicting carbonyl stretching frequencies in
transition metal compounds is taken from ref. 42. The force
constants for complexes of the type M(CO)
LZ, are given
n~m m
by the equation :
k \ k ] Z*e ] &ea
(1)
co
n
c
L
where k is the CÈO stretching constant, k is the force conco
n
stant for the isolated metal monocarbonyl fragment, M(CO),
1844
Phys. Chem. Chem. Phys., 2000, 2, 1835È1846
e is the charge e†ect constant (\1.97 N cm~1), Z is the ionic
c
charge and ea are ligand e†ect constants, characteristic of the
L
particular ligand L and the angle nLM(CO). When M
belongs to group VIB-VIIIB, k is given by the equation :
n
k \ 10.54 ] 0.556*n (N cm~1) where n is the number of
n
valence electrons within the metal.
The COÈCO interaction constants k
are given by the
co, co
equation :
A
k \A[B
ab
B
k ]k
a
b [ Z*c
C
2
(2)
where k is the interaction between two carbonyls with
ab
stretching constants k and k , A and B are functions of the
a
b
CMC bonding angle only and c is a charge correction conC
stant (\1.20 N/cm~1).
When calculating the wavenumbers the following relationships have been employed :
Co(CO)
F11 \ k
co
Co(CO) F11 \ k ] k
2
co
co, co
F22 \ k [ k
co
co, co
Co(CO) F11 \ k ] 2k
3
co
co, co
F22 \ k [ k
co
co, co
Further, the wavenumber, j~1, may be expressed in terms of
the G and F matrices :
j~1 \ factor*J(G*F)
where factor \ 803.123 and G \ 0.382.
Appendix B
Cluster models used in the ASED-MO calculations
Below is shown some cluster models used in the ASED-MO
calculations. The total number of metal atoms is indicated as
well as the number of atoms in each closest packed layer.
f \ s or p electron exponent and f , C and C are used for
1
2 1
2
the contracted d-orbitals of Co :
R (r) \ N(nd~1)(C É e~f1 Õ r ] C É e~f2 Õ r),
nd
1
2
n being the d-electron principal quantum number (i.e. n \ 3
d
d
in the case of Co)
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Interatomic distances used in the calculations
CoÈCo 2.51 ; CoÈC 1.82 ; CÈO 1.13 ; CoÈO 2.13.
26
27
Extended HuŽ ckel parameters
Orbital
H eV
f
ii
1
Co 4s
Co 4p
Co 3d
C 2s
C 2p
O 2s
O 2p
O 2s
O 2p
[7.80
[3.80
[9.70
[21.40
[11.40
[28.20
[12.40
[32.30
[14.80
2.000
2.000
5.550
1.710
1.625
2.575
2.275
2.575
2.275
f
2
1.90
28
C
1
C
0.5550
0.6680
2
The diagonal Hamiltonian matrix elements are given by
H \ [VSIE(Q) where VSIE(Q) is the valence state ionizaii
tion energy of orbital i when the atom has a total charge of Q,
29
30
31
32
33
34
35
36
37
38
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