Journal of
ELSEVIER
MOLECULAR
STRUCTURE
Journal
of Molecular
Structure
349 (1995)
325-328
Diffuse reflectance IR studies of bimetallic Fischer-Tropsch catalysts
L. E. Skaare Rygha, I. Gausemela, 0. H. Ellestadapb, P. Klaeboea, C. J. Nielsena
and E. Rytte6
Department of Chemistry, University of Oslo, P.O.Box 1033 Blindern,
N-0315 Oslo, Norway
SINTEF Oslo, P.O.Box 124 Blindern, N-0314 Oslo, Norway
Statoil Research Centre, Rotvoll, Postuttak, N-7004 Trondheim, Norway
The._promoting
effects of Re in a bimetallic Fischer-Tropsch
catalyst, Co&e
_
on a y-Al203
support, has been studied by diffuse reflectance infrared spectroscopy. It is concluded (1) that Re promotes the reduction of Co, (2) that Re for
the first time is observed in the surface layer, and (3) that Re most probably
forms a tricarbonyl entity.
1. INTRODUCTION
In the Fischer-Tropsch process hydrocarbons are synthesized by catalytic
hydrogenation of carbon monoxide. Very effective catalysts are metal particles
of Fe, Co and Ru dispersed on a metal oxide support.
Several bimetallic Co containing Fischer-Tropsch
catalysts increase the
rate of CO hydrogenation compared with the corresponding monometallic catalysts. Detailed knowledge of the active sites and their reaction mechanism is
still not fully available. The present investigation addresses the role of Re atoms
in a Co,Re/r_Al2O3 catalyst by studying the adsorption of CO and by correlating
the spectral shifts.
The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)
employed, is particularly convenient for studying catalysts with high metal
loading and accordingly high optical density.
2.1. Samples
In this study the following catalysts were investigated: (1) 12%Co,l%Re on
r-Al203 (2) 12%Co on r-Al203 (3) l%Re on rAl2O3 (4) r_Al2O3. All the catalysts
were made by coimpregnation, and they were calcinated at 300°C.
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326
26. Il&ared spectra
The IR spectra were recorded on a
Perkin Elmer model 2000 FTIR instrument
with a spectral resolution of 8 cm-l. The
DRIFT unit including a vacuum chamber
was delivered from Harrick Sci. Corp.,
while the temperature
controller
came
from Bruker Anal. Messtechn. GmbH.
2100
2000
wavenumberhi’
1900
I
2.3. Experimental procedure
Co/y-AlgO and Co,Re/y-A1203 were reduced in situ for 16-20 hours under atmospheric pressure by a stream of Hz at 400°C.
Rely-A1203 was treated similarly but the
reduction time was only 2 hours. The sample was cooled to the desired temperature
before evacuation and subsequent introduction of CO(g). The CO pressure was typically l-150 mbar, and the CO adsorption was
studied as a function of temperature (304OO’C) and time after exposure to H2 or 02.
3. RESULXSAND
DISCUSSION
wavenumberkni’
When CO(g) (2143 cm-l) adsorbs on a
metal surface, the resulting wavenumber
shift depends on the oxidation state of the
metal. In general, the adsorption
on a
reduced metal surface will result in shifts
to lower wavenumbers, normally from 2100
to 1800 cm-l.
KM
In order to detect Re in the surface
layer, one must be able to distinguish between possible Re(CO), and Co(CO), species
formed. Unfortunately, bands characteris0,OO’
m
’
a
tic of these species overlap to a high extent,
2100
2000
2200
1900
as demonstrated
by the spectrum of CO
wavenumbedcm-’
adsorbed at room temperature
(Fig. 1).
Accordingly, the CO bands were investiFigure 1. CO adsorbed at room temperature at the mono- and bi-metallic gated after evacuation and oxidation. It can
catalysts (all on y-Al203 support). be seen from Fig. 1 that Co(CO), (top) and
P
is 1 mba_r(upper, middle) and Re(CO)x (bottom) showed quite different
l!$?mbar (lower). The effects of short stability upon removal of CO(g). However,
evacuationand oxidation are shown.
evacuation alone does not reveal any bands
327
which can be assigned to the CO stretching vibration of Re(COX, on the bimetallic catalyst (middle).
After careful oxidation at room temperature, the Co(CO)x lines of Fig. 1 (top)
completely disappeared. The Re(CO), IR
bands of Fig. 1 (bottom) were much more
resistant. Some of the carbonyl bands disappeared, but two very resistant Re(CO),
absorption bands at 2030 and 1920 cm-l
remained. They could also be recognized in
the bimetallic catalyst (Fig. 1 (middle)),
although their intensities were rather low
in this spectrum.
In another approach CO(g) and subse2000
1900
quently
Ha(g) were introduced at 200°C. By
wavenumber/cd
this procedure CO was consumed in the
Figure 2. Comparison between reFischer-Tropsch
reaction and disappearmaining carbonyl structures after
hycirotreatmentat about 2ooOC.
ed. The spectral results were almost identical to the observations made after oxidation at room temperature; only the bands associated with Re(CO)x had a higher
intensity, since the CO adsorption on Re is an activated process. A comparison
between
the resulting
Re(CO)x band structure from ReIy-A1203
and
Co,Ren-Al203 is given in Fig. 2. Evidently, Re is present at the surface of the bimetallic catalyst.
38. Co(CO)x and Re(CO)x strucm
Linearly adsorbed CO is normally expected to show an absorption above
2000 cm-l. Bands appearing below this wavenumber are commonly assigned to
either bridged species (nonexistent for Re), to CO on coordinatively unsaturated
sites (stepped metal planes/defects) or to polycarbonyl species.
The bands appearing from Co(CO), (Fig. 1 (top)) indicate the existence of
various structures. Bridged species may exist, but apparently in very low
amounts if we compare with the relative vibrational intensities resulting from
linear or bridged CO in a molecule like Co2(CO)s [ll. The possibility of polycarbonyl structures cannot be excluded. Matrix isolation studies of various
Co(C!O), structures [2], revealed the characteristic IR gas phase vibrational
wavenumbers for x=1-4 to be respectively 1959,1925,1989 and 2017 cm-l.
An empirical method for predicting the carbonyl stretching frequencies in
mononuclear transition metal compounds has previously been presented [31.
The force constants are here more related to the number of available delectrons than to the actual metal atom involved. In metallic Co, the bulk atoms
have about 8 available d-electrons,
while the calculations indicate that a
surface atom on the (0001) plane has approximately 9 141, which is the same as
for a single Co atom. It is therefore probable that at very low surface coverages
of CO, there will be small differences between the frequencies of the Co,(CO),
(m=metal) and the Co(CO)x compounds. Furthermore, no surface metal atom
328
can be coordinatively more unsaturated than the Co atom in Co(C0). This indicates that v(C0) for linearly adsorbed CO, should not appear below 1959 cm-l,
regardless of how the Co atom is situated. The effect of higher CO coverages
must also be taken into account, but is expected to give a shift to higher wavenumbers.
The exact type of the Re(CO), species
and their structures are still not known.
However, force field calculations and spectral analogies with cluster data suggest a
tricarbonyl
entity. An IR spectrum [5]
presumably
of a tricarbonyl
species,
Re(C0)3(0-Al)(HO-Al)2
is given in Fig. 3.
The resemblance with the Re(CO), structure obtained on oxidation (Fig. 1 (bottom))
or after hydro treatment (Fig. 2) is striking.
3.3.l?romotingeffectofRe
The most prominent effect of Re is that
wavenumbedcd’
it leads to an increased number of availFigure 3. Spectrum of a Re tricarboable sites. This is seen by comparing the
nyl compound from ref. (51.
intensities from Co(CO)x in the mono- and
bimetallic Co catalysts, respectively (Fig. 1
(top, middle) before oxidation). This is also consistent with the results from
temperature programmed desorption (TPD) [6], demonstrating that adding
small amounts of Re promotes the reduction of cobalt.
Furthermore, Re seems to have no influence on the various IR bands of
Co(CO),. The absorptions emerging when CO is introduced to Co,Re/y-Al203
are stronger but otherwise identical to the bands observed with Co/r-AlaOs.
Apparently, Re(CO)x has very low influence on the Co catalytic centres.
Finally, Re in a pure l%Re/r-Al203
sample seems to have no FischerTropsch catalytic activity, since bands in the C-H stretching region around 3000
cm-l were completely lacking.
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2. L. A. Hanlan, H. Huber, E. P. Kiindig, B. R. McGarvey, G. A. Ozin, J. Am.
Chem. Sot., 26 (1975) 7054.
3. J. A. Timney, Inorg. Chem., 18 (1979) 2502.
4. C. Zheng, Y. Apeloig, R. Hoffmann, J&n. Chem. Sot., 110 (1988) 749.
5. P. S. Kirlin, F. A. De Thomas, J. W. Bailey, H. S. Gold, C. Dybowski and B.
C. Gates, J. Phys. Chem., 90 (1986) 4882.
6. A. Hoff, Dissertation, NTH, University of Trondheim, Norway, 1993.