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IR spectroscopic studies of the nature of surface sites in hydrotreating catalysts
Journal of Electron Spectroscopy and Related Phenomena, 39 (1986) 11-13 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
11
IR SPECTROSCOPIC STUDIES OF THE NATURE OF SURFACE SITES IN HYDROTREATIN~ CATALYSTS Nan-Yu TopsQe and Henrik Topspe Haldor Tops@e Research Laboratories, DK-2800 Lyngby, Denmark
One of the important relevance of surface science studies of well-defined single crystal surfaces is to provide basis for understanding of the more complicated real catalyst systems. A useful physical technique for studies of real catalysts is the conventional infrzed
transmission spectroscopy (see,
e.g., (1)). We have used this technique to study the very important hydrodesulfurization catalysts used for cleaning of crude oil for sulfur (2-3). A typical industrial hydrodesulfurization catalyst generally consists of MO supported on high surface area Al203 promoted by either Co or Ni. The origin of the promotion has been difficult to establish in the past due to lack of basic structural information. The recent application of novel in-situ techniques, such as Massbauer emission spectroscopy (MES) and EXAFS, has casted some light on the structures present in such catalysts (see, e.g., (4)). The MO has been observed by EXAFS to be present as small (- 10 I\) MoS2-like particles. The pro~tion was shown by MES to be linked to a certain fraction of Co atoms occupying edge positions in the MoS2-like phase. This structure has been termed "CO-MO-S". We will presently discuss the
nature of the active
sites in the CO-MO-S structures. Adsorption of selective probe molecules has frequently been used to elucidate the nature of surface sites. In the case of unpromoted MO based hydrodesulfurization catalysts, oxygen has been successfully used as a probe mofecule (5). The total 02 uptake was found to correlate well with the thiophene hydrodesulfurizationactivities. Such simple correlations are, however, not observed for promoted catalysts (6). This lack of correlation is due to the fact that for the promoted catalysts 02 adsorbs indiscriminatelyon different types of sites which have different catalytic activities. Such difficulties may be avoided by using NO instead of 02 as a probe. The usefulness of NO lies mainly in the possibility of using ir to distinguish between adsorption on different sites (2). The ir bands due to NO adsorbed on Co surface sites (1840 cm-l, 1780 cm-l)
in
an
active sulfided catalyst can
be
easily
distinguished from those on MO (1780 cm-l, 1685 cm-l) (2). In order to understand the nature of the adsorption sites in the catalyst,
036%2048/86/$03.50
0 1986 Elsevier Science Publishers B.V.
12 we
have
made
cluster that
both
species.
for The
electronic
a sulfur
of
ir results
obtained
(7). From the results
the
Co
and
frequencies
environment
frequencies
This
use
complexes
MO
of of
surface
the
the
NO
for
given
rich environment
bands
adsorbent
rather
than
in view of the
NO
sites, are
A
well
defined
adsorbed to
shift
as
dinitrosyl
be
sensitive
to
lower
to
environment
lower electronegativity
the
vibrational
when the Co or MO atoms
in an oxygen
metal
1, it can be concluded
is
seen
atom.
in the NO bands has been observed
is as expected
different
in Table
are in
(see Table 1).
of S as compared
to 0.
TABLE NO
1
stretching
vibrational
cluster
complexes.
in
frequencies
adsorption
systems (sulf.)
1840, 1780
Co/Al203
(oxid.)
1863, 1780
Mo/A1203
(sulf.)
1780, 1685
Mo/A1203
(oxid.)
1804, 1692
Co-Mo/A1203
(sulf.)
1850, 1785, 1690
Co-MolAl203
(oxid.)
1880, 1796, 1695
((C6H5)4P)Co(WS4)2(NO)
1674 1842, 1782
((NO)2Co(MoS4))-
1852, 1770
((NO)2Co(MoO2S2))-
1859, 1781
((NO)~MO(NCS))~)~-
1782, 1661
Figure
la
similar sites.
shows
the
catalysts
MoS2-like
domain
increases
infrared
with
It is seen that
Co NO band
size
upon
with
results
show that
the MO edge sites vities atoms with
Co
atoms
earlier
loadings.
therefore
increasing
mainly
(Fig.
"decorating"
These
constant
decrease
at the MO edge
be present
of
a
series
catalysts
number the
of
of
all
have
total
edge
absorbance
of the
of the MO NO band. Since sites
at the edges
in MoS2,
of MoS2
the
thus
it
infra-
blocking
with the hydrodesulfurization
acti-
lb) shows that the main role of the promoter
more MO edge sites
by MES).
results
the Co loading
(Fig. lc). A comparison
is not to create the
and
adsorption Co
a simultaneous
Co must
of these catalysts
identified
NO
different
is known that NO (8) adsorbs red
metal
\)N(), Cm-’
complexes
((~~~~)~~)~~0(~~4)(~~~~
Co-Mo/Al203
and
cm-l
%O,
Catalyst Co/Al203
Metal cluster
complexes
the
Since
edges
the
but to create (these
Co edge
new sites
are the
sites
have
associated
"CO-MO-S" a much
Co atoms
higher
in-
13 trinsic
activity
than
the
MO
edge
sites,
play a role only at very low coverages
the
unpromoted
MO
edge
sites
of the edges of Co.
.a
-
2 Co
4 Loading
6 (wt%)
8
co -_,
Fig. 1. The effect of Co loading on (a) adsorbances of ir absorption bands of NO adsorbed on surface Co and MO at edges of MoS2-like structures, and (b) the thiophene HDS activities. A schematic model indicating the edge position of Co is shown in (c). REFERENCES L.H. Little, (1966). N.-Y. Tops0e,
(1983). N.-Y. Topsee,
in and
"Infrared H.
Spectra
Topsae,
J.
of Adsorbed
Catal.
H. Topsee, 0. Ssrensen, ;o~.~;;,"e.,Be$,~q, 8i;;,;zn7 (1984). , Catal. . . .
75,
354
Species",
(1982),
B.S.
Clausen,
Rev.
Sci.-Sci.
Academic
Ibid.,
84,
and R. Candia, Eng.
26
Press
386 Bull.
(3,4)
395
$,!?4iauster T A Pecoraro and R R Chianelli J Catal 63 515 (1980). and H. R. Candia, i.S: filausen, J: Barthbidy, N.-Y. !op;@e, B.' Lsgeler, Weinheim, 1984, Topsae, in Proc. 8th Int. Congr. Catal., Verlag Chemie: Vol. II, p. 375. A. Miiller, P. Stolz, N.-Y. TopsBe, B.S. Clausen, and H. Topsoe, to be published. K. Suzuki, M. Soma, T. Onishi, and K. Tamaru, J. Electron Spectros. Relat. Phenom., 24, 283 (1981).
11
IR SPECTROSCOPIC STUDIES OF THE NATURE OF SURFACE SITES IN HYDROTREATIN~ CATALYSTS Nan-Yu TopsQe and Henrik Topspe Haldor Tops@e Research Laboratories, DK-2800 Lyngby, Denmark
One of the important relevance of surface science studies of well-defined single crystal surfaces is to provide basis for understanding of the more complicated real catalyst systems. A useful physical technique for studies of real catalysts is the conventional infrzed
transmission spectroscopy (see,
e.g., (1)). We have used this technique to study the very important hydrodesulfurization catalysts used for cleaning of crude oil for sulfur (2-3). A typical industrial hydrodesulfurization catalyst generally consists of MO supported on high surface area Al203 promoted by either Co or Ni. The origin of the promotion has been difficult to establish in the past due to lack of basic structural information. The recent application of novel in-situ techniques, such as Massbauer emission spectroscopy (MES) and EXAFS, has casted some light on the structures present in such catalysts (see, e.g., (4)). The MO has been observed by EXAFS to be present as small (- 10 I\) MoS2-like particles. The pro~tion was shown by MES to be linked to a certain fraction of Co atoms occupying edge positions in the MoS2-like phase. This structure has been termed "CO-MO-S". We will presently discuss the
nature of the active
sites in the CO-MO-S structures. Adsorption of selective probe molecules has frequently been used to elucidate the nature of surface sites. In the case of unpromoted MO based hydrodesulfurization catalysts, oxygen has been successfully used as a probe mofecule (5). The total 02 uptake was found to correlate well with the thiophene hydrodesulfurizationactivities. Such simple correlations are, however, not observed for promoted catalysts (6). This lack of correlation is due to the fact that for the promoted catalysts 02 adsorbs indiscriminatelyon different types of sites which have different catalytic activities. Such difficulties may be avoided by using NO instead of 02 as a probe. The usefulness of NO lies mainly in the possibility of using ir to distinguish between adsorption on different sites (2). The ir bands due to NO adsorbed on Co surface sites (1840 cm-l, 1780 cm-l)
in
an
active sulfided catalyst can
be
easily
distinguished from those on MO (1780 cm-l, 1685 cm-l) (2). In order to understand the nature of the adsorption sites in the catalyst,
036%2048/86/$03.50
0 1986 Elsevier Science Publishers B.V.
12 we
have
made
cluster that
both
species.
for The
electronic
a sulfur
of
ir results
obtained
(7). From the results
the
Co
and
frequencies
environment
frequencies
This
use
complexes
MO
of of
surface
the
the
NO
for
given
rich environment
bands
adsorbent
rather
than
in view of the
NO
sites, are
A
well
defined
adsorbed to
shift
as
dinitrosyl
be
sensitive
to
lower
to
environment
lower electronegativity
the
vibrational
when the Co or MO atoms
in an oxygen
metal
1, it can be concluded
is
seen
atom.
in the NO bands has been observed
is as expected
different
in Table
are in
(see Table 1).
of S as compared
to 0.
TABLE NO
1
stretching
vibrational
cluster
complexes.
in
frequencies
adsorption
systems (sulf.)
1840, 1780
Co/Al203
(oxid.)
1863, 1780
Mo/A1203
(sulf.)
1780, 1685
Mo/A1203
(oxid.)
1804, 1692
Co-Mo/A1203
(sulf.)
1850, 1785, 1690
Co-MolAl203
(oxid.)
1880, 1796, 1695
((C6H5)4P)Co(WS4)2(NO)
1674 1842, 1782
((NO)2Co(MoS4))-
1852, 1770
((NO)2Co(MoO2S2))-
1859, 1781
((NO)~MO(NCS))~)~-
1782, 1661
Figure
la
similar sites.
shows
the
catalysts
MoS2-like
domain
increases
infrared
with
It is seen that
Co NO band
size
upon
with
results
show that
the MO edge sites vities atoms with
Co
atoms
earlier
loadings.
therefore
increasing
mainly
(Fig.
"decorating"
These
constant
decrease
at the MO edge
be present
of
a
series
catalysts
number the
of
of
all
have
total
edge
absorbance
of the
of the MO NO band. Since sites
at the edges
in MoS2,
of MoS2
the
thus
it
infra-
blocking
with the hydrodesulfurization
acti-
lb) shows that the main role of the promoter
more MO edge sites
by MES).
results
the Co loading
(Fig. lc). A comparison
is not to create the
and
adsorption Co
a simultaneous
Co must
of these catalysts
identified
NO
different
is known that NO (8) adsorbs red
metal
\)N(), Cm-’
complexes
((~~~~)~~)~~0(~~4)(~~~~
Co-Mo/Al203
and
cm-l
%O,
Catalyst Co/Al203
Metal cluster
complexes
the
Since
edges
the
but to create (these
Co edge
new sites
are the
sites
have
associated
"CO-MO-S" a much
Co atoms
higher
in-
13 trinsic
activity
than
the
MO
edge
sites,
play a role only at very low coverages
the
unpromoted
MO
edge
sites
of the edges of Co.
.a
-
2 Co
4 Loading
6 (wt%)
8
co -_,
Fig. 1. The effect of Co loading on (a) adsorbances of ir absorption bands of NO adsorbed on surface Co and MO at edges of MoS2-like structures, and (b) the thiophene HDS activities. A schematic model indicating the edge position of Co is shown in (c). REFERENCES L.H. Little, (1966). N.-Y. Tops0e,
(1983). N.-Y. Topsee,
in and
"Infrared H.
Spectra
Topsae,
J.
of Adsorbed
Catal.
H. Topsee, 0. Ssrensen, ;o~.~;;,"e.,Be$,~q, 8i;;,;zn7 (1984). , Catal. . . .
75,
354
Species",
(1982),
B.S.
Clausen,
Rev.
Sci.-Sci.
Academic
Ibid.,
84,
and R. Candia, Eng.
26
Press
386 Bull.
(3,4)
395
$,!?4iauster T A Pecoraro and R R Chianelli J Catal 63 515 (1980). and H. R. Candia, i.S: filausen, J: Barthbidy, N.-Y. !op;@e, B.' Lsgeler, Weinheim, 1984, Topsae, in Proc. 8th Int. Congr. Catal., Verlag Chemie: Vol. II, p. 375. A. Miiller, P. Stolz, N.-Y. TopsBe, B.S. Clausen, and H. Topsoe, to be published. K. Suzuki, M. Soma, T. Onishi, and K. Tamaru, J. Electron Spectros. Relat. Phenom., 24, 283 (1981).