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TiO2 catalysts after reduction and fischer-tropsch synthesis by Mo˝ssbauer spectroscopy
285
Applied Catalysis, 27 (1986) 285-298 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CHARACTERIZATION TROPSCH
OF FeRu/Ti02
SYNTHESIS
BY MaSSBAUER
A.M. van der KRAANa, aInteruniversitair bLaboratory
Reactor
AFTER
REDUCTION
AND FISCHER-
SPECTROSCOPY
F. STOOPb
NONNEKENSa, Instituut,
and J.W. NIEMANTSVERDRIETC
2629 JB Delft, The Netherlands.
Technology,
for Inorganic
Chemistry
5600 MB Eindhoven,
(Received
CATALYSTS
Eindhoven
University
of Technology,
5600 MB
The Netherlands.
'Laboratory nology,
R.C.H.
for Chemical
Eindhoven,
AND Fe/Ti02
4 April
Eindhoven
and Catalysis,
University
of Tech-
The Netherlands.
1986, accepted
28 May 1986)
ABSTRACT Ti02-supported bimetallic catalysts, consisting of iron and ruthenium with metal loading 5 wt.? and molar ratios Fe :Ru = m, 10:1, 3:1, I:1 and I:3 have been investigated with ix ;itw Mtlssbauer spectroscopy. Ruthenium has a strong influence on the behaviour of the iron in the catalysts. Partial reduction of Fe3+ to Fe2+ takes place at a lower temperature for FeRu/Ti02 than for Fe/Ti02. For reduced FelTi and IO:1 FeRu/Ti02 catalysts a magnetic sextuplet of a-Fe and bee-FeRu alloy has been observed and carburization of the iron in these catalysts to x-Fe5C2 during Fischer-Tropsch synthesis. The 3:1, I:1 and 1:3 FeRu/Ti02 catalysts are only partially reduced to Feo, the latter being present in a hcp-FeRu alloy. With increasing ruthenium concentration and increasing reduction temperat re, the initially formed Fe2+-phase is transformed back to an Fe3+ phase. This Fe Y+ -phase changes partially to an Fez+-phase upon chemisoption of CO at 295 K, indicating that the Fe3+-phase is highly dispersed. Passivation of the catalysts in air at 295 K results in oxidation of Fe0 nd Fe2+ to Fe3+. All passivated FeRu/Ti02 catalysts show reduction of Fe3+ to Fe ?+ by Hp and by CO at 295 K, which is promoted by the noble metal ruthenium.
INTRODUCTION Alloying catalysts
of metals
provides
[il. Supported
interesting
bimetallic
selectivities
in the Fischer-Tropsch
a powerful
that FeRu on Ti02 and Si02 with
[2]. Recent an atomic
higher
metals
on Ti02 or SiO2 in Fischer-Tropsch
Mossbauer
and lower methane
spectroscopy
they are active.
The purpose
of the iron in a series function synthesis.
of reduction,
Mllssbauer investigations
0166-9834/86/$03.50
ethylene
by Stoop
have shown
et al. [3] revealed
= 3:l exhibited
than either
technique
to investigate
study
catalysts
2 chemisorption
is to compare
0 1986 Elsevier Science Publishers B.V.
in which
the chemical
Fe/Ru
of CO and after
of FeRu supported
iron-containing
environments
with different
signifi-
of the single
at 575 K and 1 atm.
in the gaseous
of
and propylene
synthesis
of the present
oxidation,
work
ratio of Fe:Ru
can be studied
of FeRu/TiO
the performance
of iron and ruthenium
selectivities
is an excellent
as the catalysts
to optimize
of light olefins,
cantly
catalysts,
olefin
catalysts
for formation
synthesis
means
ratios
state
as a
Fischer-Tropsch
on Si02 [2,4-71
and of Fe
286 on Ti02 [8,9] have been reported discussion
in the literature
and provide
a brackground
for
of the results.
Our results
indicate
that ruthenium
of the iron in the catalysts. in the bimetallic
catalysts
has a strong
Below a certain is not observed
influence
Fe/Ru
during
on the behaviour
ratio carburization the Fischer-Tropsch
of the iron synthesis.
In addition
it is observed that at increasing temperature for reduction in H2-gas 3+ 2+ an initially formed Fe -phase is reoxidized to an Fe -phase [IO]. This unexpected behaviour
is more pronounced
found for I:1 FeRu/Si02. catalysts
at different
with
increasing
A systematic
ruthenium
concentration
study of this behaviour
kinds of support
is in progress
and is also
for bimetallic
[II].
EXPERIMENTAL The FeRu/TiO2 An aqueous
catalysts
solution
was added dropwise stirring
were prepared
to the TiO2 support
until the incipient
used Fe(N03)3.9H20
(Merck,
had been reduced
P.A.),
in flowing
loading of 5 wt%, whereas isotope
wetness
samples
samples
vacuum
(Aerosil
(10.9% 57Fe),
were
As starting
Matthey,
materials
41.8%
in 2.2 N HN03,
catalysts
was enriched Fe/Ti02
Ru and Fe203
a total metal
in the MBssbauer
(4.9% 57Fe),
I:1 FeRu/Ti02
were
the Fe203
contained
(10.7%
lysts took place
in air at room temperature
carried
300 V), pressed
22 mm using a pressure
IO:1 FeRu/
57Fe) and I:3
of IO MPa.
purity > 99.5%) were each sieve
in situ
H2 (Hoekloos,
purified
(Union
for 24 h and finally
wafers
For the investigations
were cut to a diameter
The gases
reactors
in air at
mixed
of the catalysts
which
100 of
at 77
of the cata-
have been described
purity > 99.9%) copper
with
with a diameter
of 17 mm. The treatments
over a reduced
Carbide,
for three days,
out on 200 mg of catalyst
into self-supporting
in the MHssbauer
[12-141.
and a molecular
(Johnson
(100 Pa) at room temperature
and 4.2 K the wafers
elsewhere
, 50 m2 g-l) under frequent
dissolving
were prepared:
were dried
400 K for 24 h. Experiments mg Si02
Before
impregnation.
of iron and ruthenium
57Fe).
The impregnated next under
P25
the iron in the catalysts
Ti02 (7.:: 57 Fe), 3:l FeRu/Ti02 (14.9%
(Degussa,
amounts
point was reached.
H2. The impregnated
Fe. The following
FeRu/Ti02
of pore volume
the desired
RuC13.xH20
in 57 Fe, Oak Ridge).
(90% enriched
by means
of pH = 1 containing
and CO (Hoekloos,
catalyst
(BASF, R3-11)
SA) at room temperature.
The Mtlssbauer experiments were done in situ with a constant acceleration 57 Co in Rh source. The spectra were not corrected for the spectrometer with a varying
distance
the measured
between
spectra
source
relative
to the NBS standard
Magnetic
fields
Mdssbauer
spectra
were fitted
of Lorentzian-shaped iterative
minimization
lines,
description
with
nitroprusside
with
the curved shifts
of a-Fe203
calculated
the Mossbauer
of the two lines were is given
in
in Ref.
consisting
in a nonlinear,
doublets
constrained [13].
at 295 K.
subspectra
parameters
In the case of quadrupole
of this method
background
(IS) are reported
(SNP) at room temperature.
the 51.5 T field
by varying
areas
Isomer
origin.
by computer
routine.
as well as the absorption detailed
sodium
were calibrated
and hence
and detector,
is of instrumental
the line widths to be equal.
A
287
fresh
I I
I 770 I 16
406 600
/ /
1
Id
-10 -8 -6 +
FIGURE
1
as indicated
-2
DOPPLER
Mtlssbauer spectra
treatments
-4
!
2
4
I
I
I
6
8
IO
VELOCITY
b-rims-‘)
catalysts
of Fe/Ti02
measured
I 0
after
a series
of subsequent
i*: o<:;-cat room temperature.
RESULTS Mllssbauer spectra after
drying,
shown
in the Figures
contributing the spectra
at room temperature Fischer-Tropsch
reduction,
of the Fe/T;02 synthesis
1 and 2. The Mdssbauer
to the spectra into these
are listed
iron phases
FeRu/TiO2
and I:1 FeRu/Ti02
in Figure
2.
is given
appeared
and passivation
parameters
in Table
and FeRu/TiO2
in Table
in air are
of the different
1, whereas
very similar,
catalysts
iron phases
the decomposition
2. As the spectra
of
of 3:l
we only show those of the former
Fresh catalysts The Mossbauer in air, consist contribution
spectra
of all fresh
of a doublet.
of a six line component
45.0 T. The values are characteristic
catalysts,
The spectrum
i.e., the samples
of Fe/Ti02
with an average
contains magnetic
after drying
also a small splitting
of about
for the Mtlssbauer parameters of both the doublet and the sextet 3+ Fe Tons in highly dispersed iron (III) oxide ot
of high-spin
288
180 fresh
I65 400
n2 675 K 18 h
air 300h
-0-6-6-4-Z
0
2 4
6
8 IO
-D-6-6-4-2
0
2
4
6
6
-DOPPLER
FIGURE
2
MHssbauer
of subsequent
TABLE
spectra
treatments
10
-IO-8-6-4-Z
VELOCITY
of lO:l, 3:l and I:3 FeRu/Ti02
as indicated
measured
0
2
4
6
6
IO
(mm.,-‘)
i7 situ
catalysts
after
a series
at room temperature.
1
Average
values
of Figures
of the iron phases
observed
QS
E’
H
r
/mm s-l
/mm s-'
/kOe
/mm s
for Mllssbauer parameters
in the spectra
1 and 2. IS
Iron phase
/mm s 3+
-1
-1
0.63+0.02
0.82i-0.05
-
0.4iO.l
Fe3+ a
0.62kO.02
0.68AO.05
-
0.5+0.1
Fe2'
1.36kO.03
2.10k0.15
-
0.7+0.1
in FeRu alloy
0.26kO.03
0.20+0.04
in B-Fe
0.26+0.02
0.0
331*2
0.33+0.05
0.45+0.04
0.0
189*2
0.37~0.10
II
0.53kO.04
-0.05
216+2
0.3orio.05
III
0.4350.06
11355
0.5
Fe
Fe0
in x-Fe5C2-I
ain bimetallic
catalysts
after
reduction
0.27+0.05
0
in H2 at 400 K.
to.1
289 TABLE
2
Composition
("4) of the MHssbauer
the treatments
spectra
and FeRulTi02
catalysts
after
indicated.
Catalyst
Fe3+
Treatment
Fe/Ti02
fresh
100
H2, 400 K, 1 h
100
a-Fe
bee-FeRu
+
x-Fe5C2
66
13
3
33
64
28
12
60
100 3
97
H2, 675 K, 18 h
4
31
7
58
FTS, 575 K, 6 h
4
41
6
6
43
44
7
7
42
air, 295 K fresh
100
Hp. 400 K, 1 h
5
95
h
27
66
7
FTS, 575 K, 6 h
11
84
5
air, 295 K
90
10
H2, 675 K;18
fresh
100
H2, 400 K,
I:3 FeRu/Ti02
hcpFeRu
21
H2, 400 K, 1 h
I:1 FeRu/Ti02
2+
FTS, 575 K, 6 h
fresh
3:l FeRu/Ti02
Fe
H2, 675 K, 18 h
air, 295 K IO:1 FeRu/Ti02
1 h
4
96
H2, 675 K, 18 h
26
68
6
FTS, 575 K, 5 h
13
81
6
air, 295 K
90
10
100
fresh H2, 400 K,
18
1 h
76
6
H2, 675 K, 18 h
74
19
7
FTS, 575 K, 6 h
43
47
IO
air, 295 K
86
14
oxyhydroxide
[15]. MBssbauer
temperatures
(not shown)
spectra
showed
at 4 K, with a distribution confirms
of Fe/Ti02
that the iron (III)
of the 1:l FeRu/Ti02
a doublet
of hyperfine
catalyst
at 77 K and partial
fields
between
oxide or oxyhydroxide
at cryogenic
magnetic
splitting
40.0 and 50.0 T. This
is present
in a highly
dispersed
state.
Reduced
catalysts
As the corresponding 1 and 2 show, exposure
spectra
in Figures
of the catalysts
state of iron in monometallic
Fe/Ti02,
1 and 2 and their analysis
to H2 at 400 K does not affect but leads to partial
reduction
in Tables the chemical 3+
of the Fe
290
in the bimetallic FeRu/Ti02 catalysts. In lO:l, 3:l and I:1 FeRulTiO2 almost 3+ present in the fresh catalyst is converted into Fe '+ by H2 at 400 K, all Fe 2+ and Fe0 occurs in I:3 FeRu/Ti02. The Mbssbauer parameters whereas reduction to Fe -1 -1 and QS = 2.10 f 0.15 mm s are of the ferrous iron, IS = 1.36 * 0.03 mm s 2+ characteristic of high-spin Fe in an octahedral coordination 1161. The parameters of the zero-valent iron present in the I:3 FeRu/Ti02 catalysts, IS = 0.26 f 0.03 -1 -1 and QS = 0.20 + 0.04 mm s , correspond to those of Fe0 atoms in hcp-FeRu mm s alloys,
as reported
The final
Mllssbauer spectrum pattern
of a-Fe,
In the reduced present
by Rush et al. 1171.
reduction
of the catalysts
of reduced
and contains
IO:1 FeRu/Ti02
Fe/Ti02
state.
in IO:1 FeRu/Ti02 iron compounds
catalyst
[18], degrees of reduction
than in Fe/Ti02.
of reduction
in our 5 wt% Fe/Ti02
than observed
with Fe/Si02
The Mtissbauer spectra
1) is dominated
almost
all unreduced
of reduction
may differ
be obtained
fractions
accurately.
of comparable
iron is present
of the different
Nevertheless,
catalysts loading
higher
at room temperature the degree
issignificantlyhigher
[19,20].
3:1, 1:l and 1:3 FeRu/Ti02
of the reduced
iron is
of iron is somewhat
considerably
and IO:1 FeRu/Ti02
catalysts
by the six line 2+ 31 and Fe .
of Fe
2) the zero valent
As the recoilless
catalysts cannot
while
degree
out at 675 K for 18 h. The
small contributions
(see Figure
alloy,
The overall
in supported
(see Figure
furthermore
as a-Fe and as bee-FeRu
in the ferrous
was carried
catalysts
indicate
that iron is present in three different states: Fe0 in hcp-FeRu alloy, 3+ ..The contribution of the zero-valent iron to the spectra is only Fe2+ and Fe about
6-7X. This
is surprisingly
reduction
in Fe/Ti02
supported
FeRu catalysts
and by our group A remarkable that almost whereas
in view of the relatively
[4-7, result
However,
has also been reported
high degree
poor reducibility
for FeRu/Si02
of
of iron in
by Guczi
et al.
IO]. concerning
all Fe3+ in . the fresh
after
small
and IO:1 FeRu/Ti02.
the 3:1, catalysts
I:1 and I:3 FeRu/TiO2 can be reduced
catalysts is 2t to Fe by H2 at 400 K,
at 675 K a considerable fraction of the iron is in the 3+ ferric state. The presence of Fe IS inferred from the peak in the MBssbauer -1 spectra at 1 mm s (see Figure 3a) which is considered as the right-hand part -1 of a doublet with the corresponding counterpart around 0 mm s . The parameters 2+ 3+ of this doublet are characteristic for either high-spin Fe or low-spin Fe . -1 by itself represents an Another possibility would be that the peak at 1 mm s 2+ unresolved doublet of high-spin Fe , as has been observed in carbon-supported iron catalysts
reduction
[21,22].
In the latter
case a resolved
doublet
would
appear
at
77 K [223. Figure
3a shows the Mdssbauer
spectra
of the reduced
I:1 FeRu/Ti02
catalysts
at 295, 77 and 4 K. The 77 K spectrum in particular ment of the spectrum
1
is similar to the 295 K spectrum and shows -1 has not split. This excludes the assignthat the peak at 1 mms -1 mm s peak to an unresolved doublet of high-spin Fe 2t. The 4 K
shows that the doublets
of FeRu alloy and Fe 2+ have not changed,
but
1
I1
1
/
,
,
,
,
,
,
II,,,,,,,,,
III FeRu/TiOz FTS, 575K,6h
I:I FeRu/TiOg reduced 400 K, I h -675
K, 18 h
r-7?+ r-b* no 1.45-
I 21 0.58
1.08 1.20-
b I
1
I
I
1
-10-8 -6 -4 -2 0
1
I
I
I
2
4
6
8
-DOPPLER FIGURE
3
Mossbauer
77 and 4.2 K after subsequently
spectra
II
/
VELOCITY
of I:1 FeRu/Ti02
(a) reduction
jb) additional
I
1
I
-10 -8 -6 -4-2
IO
I
I
II
0
2
4
/
6
I
8
IO
(mm.s-'1
catalysts
measured
in situ at 300,
in H2 at 400 K for 1 h and at 675 K for 18 h
Fischer-Tropsch
synthesis
(H2:C0
= 2:l) at 575 K
for 6 h.
that the doublet magnetically
associated
split.
This
with
implies
the 1 mm s
-1
peak at 295 and 77 K has become
that the subspectrum
cannot
be due to low-spin
Fe2+ as this state does not possess noted that the intermediate however, confirm
a magnetic moment. For completeness 2+ spin Fe would exhibit magnetic splitting.
has no known occurrences that the reduced
Fe 3+. The magnetic
hyperfine
fields.
iron (III)
oxides
the MBssbauer
catalyst 2 of this ferric phase
I:1 FeRu/TiO
pattern
area of the 4 K spectrum
[16]. Thus,
and is characteristic
The pattern
Fe304 or FeOOH.
This state,
at 77 and 4 K
contains
significant
accounts
for about 40-501
of a broad distribution
has no resemblance
such as Fe203,
spectra
it is
amounts
of
of the
in magnetic
to the Ml)ssbauer spectra
of known
292 Catalysts
under synthesis
gas
The state of iron in Fe/Ti02 been investigated
by treating
for 6 h, after which The I:1 FeRu/TiD2 cryogenic
123-251,
at room temperature.
Fe/Ti02
after
Fischer-Tropsch
shows carburization
is also observed
conditions,
the major
in the reduced alloy
been reported
Figure measured
of all a-Fe
was also studied
at
as for the reduced
synthesis
catalyst.
Similar
catalysts
of the magnetically less than
1:l FeRu/Ti02
split pattern
As follows
catalyst
synthesis,
spectra,
the same argu-
Comparison
of the spectra
indicate
contains reveals
that after
less Fe3+ than the
that the intensity
of I:1 FeRu/Ti02
This confirms
to the magnetically
and chemisorption
from Figures
catalyst.
in the spectra catalyst.
of iron has
after Fischer-Tropsch
3a and 3b, respectively,
at 295 K and 77 K corresponds
rereduction
behaviour
of these
of the two 4 K spectra
in that of the reduced
Passivation,
into
[18,26].
of I:1 FeRu/Ti02
the I:1 FeRu/Ti02
Comparison
of a-Fe
catalysts
by the syngas.
at 295 K or at 77 K in the Figures Fischer-Tropsch
2). Carburization
are exposed to Fischer-Tropsch 3+ is that of Fe , which is abundantly 2+ into Fe , whereas the zero-valent iron in
for FeRh/Si02
3b shows the spectra
x-Fe5C2
in the reduced
change
catalysts,
is not affected
into the carbide
iron present
catalyst.
at 300, 77 and 4.2 K. For the analysis
apply
doublet
synthesis
for the IO:1 FeRu/Ti02
the 3:1, I:1 and I:3 FeRu/Ti02
hcp-FeRu
reduced
has
was recorded
recently
ments
synthesis
a Mllssbauer spectrum
catalyst
simultaneously the residual ferric 2c is converted to Fe (see Figure 1, Table
synthesis present
Fischer-Tropsch
gas (H2:CO = 2:l) at 575 K
while
catalyst
When
after
in synthesis
temperatures.
Monometallic
x-Fe5C2
and FeRu/Ti02
the samples
once more
catalyst
is
that the Fe3+
split pattern
at 4 K.
at room temperature
1 and 2 and Table
2, upon exposure
of the catalysts
to air at 295 K after Fischer-Tropsch synthesis, all samples show oxidation 2+ 3+ substantial fraction of Fe to Fe . In all bimetallic FeRu/Ti02 catalysts investigated
here the zero-valent
iron in hcp-FeRu
at room temperature.
The carbide
appears
in air at 295 K.
to be stable
Exposure
x-Fe5C2,
alloy
as present
is also oxidized
in Fe/Ti02
of a
by air
and IO:1 FeRu/Ti02
of passivated
leads to a partial all passivated
lO:l, 3:l and I:1 FeRu/Ti02 to H2 at room temperature 2+ . This process has been observed in reduction of Fe3+ to Fe
bimetallic
combinations
of iron and a more noble
group VIII metal
[2,7,14,19,27,28-J. When 3:l and I:1 FeRu/Ti02 at 295
K, reduction
induced
conversion
of Fe 3+ to Fe2+ has been observed
FeRh, FeIr and FePt catalysts takes
place
involved
catalysts reduced in H2 at 675 K are exposed to CO 2c occurs, as is shown in Table 3. The same CO-
of Fe 3+ to Fe
at room temperature
are exposed
[6,7,27].
in silica-supported
The fact that this transition
is considered
as evidence
FeRu.
already
that the Fe 3+ ions
to the gas phase and, thus, are located
at the surface.
It
293 TABLE
3 (% ) of the Mbssbauer
Composition
spectra
of
I:1 and 3 :I FeRu/Ti02
catalysts
after
indicated
the treatments
3:l FeRu/Ti02
I:1 FeRu/Ti02 Treatment Fe'
Fe*+
Fe3+
Fe'
Fe*+
Fe3+
HE, 675 K, 1 h
7
61
32
5
73
22
CO, 295 K
8
77
15
5
84
11
TABLE
4
Composition
(%) of the MBssbauer
treatments
spectra
Fe*+
Fe3+
passivated
11
89
CO, 295 K
83
17
passivated
8
92
H2, 295 K
75
25
is remarkable
that the Fe
the reduction
treatment
2 and Table
Also on exposing Fe3+ to Fe2+
species
in Table
passivated
concerning
that the degree
after
the
by additional
the results.
of the respective
2+
phase during catalysts
(see
by the CO-gas
3. FeRu/Ti02
catalysts
catalyst,
such experiments
with
the reduction
limitations
exposure
catalysts
reported
4.
It is our for CO as
time of CO and H2. These
in the pressed on whether
of
by CO at
in Table
may be appropriate.
of the passivated
increasing
measurements
All spectra
to CO at 295 K a reduction
by H2 at the same temperature
of reduction
may be due to diffusion
influences
from the Fe
to an Fe 2+ phase again
For a 1:l FeRu/Ti02
well as H2 at 295 K, increases
one has to check
is formed
converted
to the reduction
A note of caution
which
at 675 K in H2 of the 3:l and I:1 FeRu/Ti02
IS observed.
295 K is compared
experience
3+
2), is partly
at 295 K, as is shown
effects
catalysts
indicated
Treatment
Figure
of I:1 FeRu/Ti02
catalysts
wafers.
or not the exposure
here correspond
So
time
to the final states
reactions.
DISCUSSION Reduction
of FeRu/Ti02
Reduction
of bimetallic
reduction
of almost
reduction
does not occur
in the bimetallic
FeRu/Ti02
catalysts
all Fe3+ in the fresh in monometallic
catalysts
is responsible
in H2 at 400 K results
catalysts Fe/Ti02.
in partial
to Fe2+ (see Figure
Apparently,
for the reduction
2). This
the presence
of Ru
of iron at relatively
294
low temperatures. bimetallic detail
Promotion
FeRh,
of the reduction
FePd, FeIr, and FePt catalysts
for the case of FeRh/SiO2
gen spillover" centres
where
[29]. In this explanation H2 is dissociated
Hence the temperature
at which
determines
freshly
prepared
also whether
of MLlssbauer spectroscopy
the initial
step
H2 and iron oxide,
is more
particles
The Mossbauer
difficult
also contain,
spectra
[30]. Higher
for the reduction
in the reduction
indicates,
of the Fe/Ti02
and FeRu/Ti02
differences.
and IO:1 FeRu/Ti02
of 3:1, 1:l and I:3 FePu/Ti02.
iron in the spectra
of Fe/Ti02
is less than 10% in the more The magnetic
sextet
of a-Fe. The magnetic to the outside.
in the spectrum
which
of magnetic model
fields
of Vincze
a composition
higher
that for FeRu alloys magnetic expected
ordering
enhances
1:3 FeRu/Ti02
higher
Fe/Ti02
whereas
which
in the
of zero-valent 65%, whereas
it
is characteristic
associated
to iron-rich
the hcp structure
reduction
is broadened
with
this pattern
than the 33.0 T field
is in qualitative
alloy.
Sextuplets
agreement
with the
alloys
with
[32] indicates
prevails,
in which
are, therefore,
not
catalysts.
a remarkable
is only below
of a-Fe.
The occurrence
bee-FeRu
at 400 K demonstrate
of iron. Therefore,
of iron reduction
catalysts,
sextuplets
however,
to iron in bee-FeRu
at room temperature.
after
reduction
absent
is high, about
of 3:1, 1:l and I:3 FeRu/Ti02
of FeRu/Ti02
the reducibility
is that the degree
the magnetic
fields
[31] when applied
with 23% Ru or more
is absent
ruthenium.
after
are virtually
96% Fe and 4% Ru. The FeRu phase diagram
in the spectra
The spectra
with,
catalysts
IO:1 FeRu/Ti02,
than that of a-Fe
and Campbell
of about
is assigned
molecular
[28]. The easy
3~1, 1:I and 1:3 Fet?u/Ti02 catalysts.
of hyperfine
are significantly
This part of the distribution
iron catalysts,
that all iron-
the contribution
of reduced
of reduced
The distribution
to values
Second,
and 1O:l FeRu/Ti02
ruthenium-rich
pattern
First,
in the
between
catalysts
therefore,
in H2 at 675 K show significant of Fe/Ti02
the reaction
or are at least in contact
dominate
the spectra
temperatures,
of monometallic
process,
than on noble metal
of Fe3+ to Fe2+ in FeRu/Ti02
containing
extends
hydro-
act as nucleation
the noble metal ions in the fresh catalyst become 3+ or not Fe will be reduced. For&le, in
range 550 to 800 K, are necessary
spectra
atoms
FePd/SiO
by means
reduction
noble metal
in
"intra-particle
Pd reduction starts already below room temperature, 2' from TPR work, and partial reduction of iron at room temperature is
as follows
because
called
into H atoms which diffuse to iron oxide in contact 3+ 2+ reduction of Fe to either Fe or Fe0 occurs.
after which
reduced
in
[7] and has been explained
[28] by a mechanism
with the noble metal,
observed
of iron has also been observed
clearly
result
10% in the Ru-rich
it is high in the Fe-rich
that Ru
of this work
Fe/Ti02
and
3:1,
1:l and
10:1 FeRu/
Ti02 catalysts. Although
a fully
yet been given,
reasoned
evidence
explanation
exists
of the phenomena
that particle
described
size is an important
above
has not
factor.
Guczi
et al. [33] have found that in FeRu/Si02 catalysts with metal loadings below 1% 2+ 3+ or Fe0 is suppressed, whereas in monometallic reduction of Fe to either Fe
295 Fe/SiO2
reduction
of Fe
[6] has proposed particle
by impeding
results,
not, or to a lesser As has first exposing
of particle
size.
bimetallic
By applying
the particles is found,
FeRu/Ti02
catalysts,
mutually
particle
and IO:1 FeRu/Ti02,
but
of 3:1, I:1 and I:3 FeRu/Ti02. [34], the extent
to the FeRu/Ti02
does not oxidize
all iron is converted
and IO:1 FeRulTiO2,
the degree
to
growth
to 02 at 295 K is a qualitative
larger
prevent
When applied
by Garten
are indeed much in which
that significant
of Fe/Ti02
this method
almost
surface.
catalysts
and IO:1 FeRu/TiO2
in Fe/ii02
reduction
suggests
as low as 0.1%. Guczi
iron and ruthenium
over the support
in the reduction
been demonstrated reduced
I:1 and I:3 FeRu/Ti02
Thus,
picture
even at loadings
unreduced
in the reduction
extent,
that iron in Fe/Ti02 3:1,
occurs
migration
Guczi's
have been involved
after
to Fe"
that at low loadings
growth
the present must
3+
of iron oxidation
catalysts
very much,
measure
it is observed whereas
in
into Fe 3t by air at 295 K.
in which
a high degree
of iron
than those of the 3:1, I:1 and I:3
of iron reduction
is lower than
10%.
Although
Ru enhances the reducibility of iron, it appears that a considerable 2t 3+ of the Fe present after 400 K reduction has oxidized back to the Fe
fraction
state after intuition, increases similar
the reduction Table
with
increasing
trends
675 K increased
unequivocally
established
and I:1 FeRh/Si02
support. will
Solid
certainly
state diffusion become
In their work the fraction after
reduction
of at
= 1, to 86% for a catalyst with 3+ is of Fe
of the reduced
I:1 FeRu/Ti02
(see Figure
3a)
temperatures.
will
hardly
of Fe
take place
In analogy
segregation
the component
et al. [26] observed
catalysts
3+
during
of iron to the interface
significant. surface
catalysts
Minai
of large amounts
in the formation
is segregation
chemisorption-induced bimetallic
by spectra
may be involved
temperatures
with Fe/Rh
be noted that the presence
[18] at cryogenic
which
catalysts.
at 295 K of FeRh/Si02
from 51% for a catalyst
It should
to what would be expected by 3+ in the reduced catalysts
of Fe
of the catalysts.
of FeRh/SiO2
spectra
Fe/Rh = 0.03.
higher
Ru content
for a series
Fe3+ in the MBssbauer
A factor
at 675 K. As opposed
2 shows that the fraction
which
at
particle
and
at 400 K, but at 675 K it
to the well-known
in alloys, forms
reduction
between
phenomenon
it is likely
the strongest
of
that in supported
bonds with
the support,
iron in the present where become
case, will become enriched at the particle-support interface, 3+ it is stabilized in the Fe state. Enrichment of iron at the support will more
important
and I:3 FeRu/Ti02 A systematic during
reduction
different
when
the bimetallic
particles
are small,
as in the 3:1, I:1
catalysts.
study of the remarkable at temperatures
supports
is in progress
between [II].
formation
of ferric
from ferrous
400 and 675 K for FeRu catalysts
iron on
296 Catalysts
under
synthesis
From the Mossbauer it follows
gas
spectra
of FeRu/Ti02
that in the cases of Fe/Ti02
and bee-FeRu
alloy
in the reduced
catalysts
The same carbide
has been observed
of carburization
of the IO:1 FeRu/Ti02
metallic
Fe/Ti02
in the bee-FeRu
catalysts, alloy
Carburization
which
during
in unsupported
Fischer-Tropsch
is probably
nearly
synthesis
all the a-Fe
into the carbide
Fe catalysts
catalysts
catalysts.
conditions
is smaller
x-Fe5C2.
[23]. The extent
than that of the mono-
due to a slower
converted
valent
unaffected.
iron remains
catalysts
[18, 261, also occurs
synthesis
gas and has been attributed
synthesis
carbon
diffusion
has not been observed
The effect
appears
at 295 K: Fe3+ is partially
et al. [26] reported
is converted
the Fischer-Tropsch
to synthesis
alloyed
after
than in a-Fe.
the 3:1, I:1 and I:3 FeRu/TiO2 Ti02 catalyst
catalysts
and iO:l FeRu/Ti02
of exposing
to be similar
into a ferrous This
species
behaviour,
in the monometallic
formed
while
found
FeRu/ of CO
the zero-
before
in FeRh/Si02
Fe/Ti02
to the formation
that the Fe2+ species
a reduced
to the effect
in
catalysts under 2t of Fe -CO complexes.
in FeRh/Si02
under
Minai
synthesis
gas are not stable towards reduction in H2 at 675 K, but were converted back to 3+ state. This result in combination with the interpretation given above the Fe would
imply that the Fe 2+-C0 complex on the support is destroyed by H2, as a 2+ 3+ is no longer stabilized and oxidizes back to the Fe result of which the Fe state. Dependent
formation
on the Fe/Ru
of low olefins
[3]. It is interesting positional bee-FeRu
region alloy
tribution
species.
Passivated
is observed
the main
noble metal
contributions
bimetallic
M remains
combination
[7]. Oxidation
catalysts
for olefins
catalysts
where
for
by Stoop et al,
is found neither
at 675 K. In this region
in a coma-Fe nor
the con-
alloy to the spectra
to the spectra of these
is only about 2+ 3+ are due to Fe and Fe
catalysts
FeM/Si02
catalysts
has been observed.
are passivated
most of the iron is oxidized
in the zero-valent FeRu/Si02
in which
of iron is also observed
synthesis
are exposed
state, about
except
by exposing
to iron (III)
when FeRu/Ti02
The
in the least noble bi-
half of the ruthenium
to air. Only the carbide
them to
oxide.
catalysts x-Fe5C2
is oxidized
used in Fischer-
appears
to be
in air at 295 K.
The observed
partial
3:l and 1:l FeRu/Ti02 A remarkable between
reduction
iron in a hcp-FeRu
no carburization
air at room temperature,
stable
after
FeRu/Ti02
and selectivity
catalysts
When reduced
Tropsch
has been found for FeRu/Ti02
of the bimetallic
Furthermore
high activity
that the high selectivity
of zero-valent
6-7%, whereas
metallic
ratio an interesting
rereduction
difference
1:l FeRu/Ti02
of Fe 3+ by H2 at room temperature
is in agreement
with the literature
in the degree
reported
of rereduction
here, and I:1 FeRu/Si02
in passivated
[5,7,19,20,35,36]. by H2 at 295 K exists
catalysts
[7]. Although
297 the specific
surface
areas for the two kinds of supports
310 m2 g-l and for TiO 2' 50 m2 g-') the much for I:1 FeRu/Ti02 SiO2 support. a specific justified
may
surface
a stronger
an investigation
comparable
by H
FeRu/Si02
with Ti02
catalyst
is necessary
with
for a
comparison. FeRu/Ti02
catalysts
IS formed than by exposure
are exposed
at room temperature.
It has been argued
version
under
by CO occurs
indicates
that the Fe3+
Structure
of the catalysts
The present
present,
is converted
but yield
extent
phase
cover
Similar
and
relating
to the structure
of the cata-
3:1, I:1 and 1:3 FeRu/TiO2
that these
as evidence
on the type of iron
ions are accessible
that they form a surface
area of the Fe 3c phase between
phase.
295 and 4 K
(see Figure 3a) is also indicative of surface 2 31 2+ the fact that this Fe to Fe transition occurs
in air-passivated
conclusions
explanation
reduced
significant
in the Si02-supported
discussion
for the structure
but forms
of this argument
between
is that thelattercontaina
has not been observed
indicates
of the present
a separate
of FeM/Si02
catalysts
surface
phase on the
FeRu, FeRh,
in 161.
FeIr and
FeM/Si02
and the present FeRu/TiO2 2+ of Fe , a phase which
amount
catalysts. results
that the ferric
has been presented
have been drawn on Si02-supported
[7]. A difference
not permit
catalysts,
the FeRu alloy particles,
A detailed
FePt catalysts catalysts
information
of the Fe3+ ions in reduced
in spectral
Moreover,
to a greater cannot
noble metal
of 1:l FeRu/TiO
[12,18].
Ti02 support.
not only yield
is regarded
increase
in the spectra behaviour
are exposed to CO 2+ [6,7] that the Fe3+ to Fe con-
by CO at 295 K. The fact
for CO at room temperature The substantial
reductor
catalysts
of the zero-valent
also information
a2rraction
into Fe
before
the influence
that CO is a stronger
reduced
more
ions involved are located at the surface.
Mtlssbauer results
lyst. For example,
to CO at room temperature,
to H2, illustrating
is even more clear when
does
a lot (for SiO2,
of rereduction
of Fe 3+ in the case of's
stabilization
than Hp. This effect
species
differ
degree
of a bimetallic
area of the support
When passivated Fe'+
indicate
However,
higher
The presence
of this Fe2+
in terms of the models
proposed
[6,7,14,37].
ACKNOWLEDGEMENTS We thank Mr. E. Gerkema ments.
J.W.N.
Organization
acknowledges
for the skilful support
for the Advancement
assistance
with the MBssbauer
from a Huygensfellowship
of Pure Research
experi-
of the Netherlands
(ZWO).
REFERENCES 1 2 3 4
V. Ponec, Catal. Rev. Sci. Eng., 11 (1975) 41; Surf. Sci., 80 (1979) 352. M.A. Vannice, Y.L. Lam and R.L. Garten, Adv. Chem., 178 (1979) 15. F. Stoop and K. van der Wiele, Appl. Catal., to be published; F. Stoop, in preparation. I. Oeszi, D.L. Nagy, M. Eszterle and L. Guczi, J. de Phys., 40 (1979) C2-76.
298 5 6
7 8 9 IO 11 12 13 14
15 16
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
L. Guczi, Cat. Rev-Sci. Eng., 23 (1981) 329. Niemantsverdriet, D.P. Aschenbeck, F.A. Fortunato and W.N. tlelgass, J. Mol. Catal., 25 (1984) 285. J.W. Niemantsverdriet, J.A.C. van Kaam, C.F.J. Flipse and A.M. van der Kraan, J. Catal., 96 (1985) 58. B.J. Tatarchuk and J.A. Dumesic, J. Catal., 70 (1981) 308; J. Catal., 70 (1981) 323; J. Catal., 70 (1981) 335. L.M. Tau and C.O. Bennett, J. Catal., 89 (1984) 285. A.M. van der Kraan, R.C.H. Nonnekens and J.W. Niemantsverdriet, Hyperf. Int. 28 (1986) 899. A.M. van der Kraan and J.W. Niemantsverdriet, to be published. J.W. Niemantsverdriet, C.J.F. Flipse, A.M. van der Kraan and 3.5. van Loef, Appl. Surface Sci., IO (1982) 302. J.W. Niemantsverdriet, Thesis University of Technology, Delft, 1983. A.M. van der Kraan and J.W. N;emantsverdriet in "Industrial Applications of the Mtissbauer Effect", G.J. Long and J.G. Stevens, editors, to be published by Plenum Publishing Company. H.M. Gager and M.C. Hobson, Catal. Rev. Sci. Eng., 11 (1975) 117. J.C. Travis in "An Introduction to Mtlssbauer Spectroscopy", L. May, editor, Plenum, New York, 1971. J.D. Rush, C.E. Johnson and M.F. Thomas, J. Phys. F. 6 (1976) 2017. J.W. Niemantsverdriet, A.M. van der Kraan and W.N. Delgass, J. Catal., 89 (1984) 138. H. Topsde, J.A. Dumesic and S. Mdrup, in "Applications of Mossbauer Spectroscopy", R.L. Cohen, Editor, Academic Press, New York, 1980, Vol.11, p.55. J.W.Niemantsverdriet, A.M. van der Kraan, J.J. van Loef and W.N. Delgass, J. Phys. Chem., 87 (1983) 1292. H.J. Jung, M.A. Vannice, L.N. Mulay, R.M. Stanfield and W.N. Delgass, J. Catal., 76 (1982) 208. J.W. Niemantsverdriet, A.M. van der Kraan, W.N. Delgass and M.A.Vannice, J. Phys. Chem., 89 (1985) 67. J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk and H.S. van der Baan, J. Phys. Chem., 84 (1980) 3363. G. Le Caer, J.M. Dubois and J.P. Senateur, J. Sol. State Chem., 19 (1976) 19. L.M. Tau, S. Borcar, D. Bianchi and C.O. Bennett. J. Catal.. 87 (1984) 36. Y. Minai, T. Fukushima, M. Ichikawa and T. Tominaga, 3. Radional. Nuci. Chem. Lett.._ 87 (1984) 189. J.W. Niemantsverdriet and A.M. van der Kraan, Hyperfine Int., 28 (1986) 869. H.F.J. van 't Blik and J.W. Niemantsverdriet, Appl. Catal., 10 (1984) 155. N.W. Hurst, S.J. Gentry, A. Jones and B.D. McNicol, Catal. Rev.-Sci. Eng., 24 (1982) 233. J.W. Niemantsverdriet and A.M. van der Kraan, unpublished results. I. Vincze and I.A. Campbell, J. Phys., F.3 (1973) 647. "Iron Binary Phase Diagrams", Springer-Verlag, Berlin (1982). 0. Kubaschewski, L. Guczi, K. Matusek, I. Manninger, J. Kiraly and M. Eszterle, in "Preparation of Catalysts", Vol.11, Elsevier, Amsterdam, p. 391. R.L. Garten, J. Catal., 43 (1976) 78. C.H. Bartholomew and M. Boudart, J. Catal., 29 (1973) 278. R.L. Garten in "Mdssbauer Effect Methodology", I.J. Gruverman, editor, Vol. 10, Plenum Press, New York, 1976, p.69. Y. Minai, T. Fukushima, M. Ichikawa and T. Tominaga, in "Industrial Applications of the Mtlssbauer Effect", G.J. Long and J.G. Stevens, editors, to be published by Plenum Publishing Company.
J.W.
Applied Catalysis, 27 (1986) 285-298 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
CHARACTERIZATION TROPSCH
OF FeRu/Ti02
SYNTHESIS
BY MaSSBAUER
A.M. van der KRAANa, aInteruniversitair bLaboratory
Reactor
AFTER
REDUCTION
AND FISCHER-
SPECTROSCOPY
F. STOOPb
NONNEKENSa, Instituut,
and J.W. NIEMANTSVERDRIETC
2629 JB Delft, The Netherlands.
Technology,
for Inorganic
Chemistry
5600 MB Eindhoven,
(Received
CATALYSTS
Eindhoven
University
of Technology,
5600 MB
The Netherlands.
'Laboratory nology,
R.C.H.
for Chemical
Eindhoven,
AND Fe/Ti02
4 April
Eindhoven
and Catalysis,
University
of Tech-
The Netherlands.
1986, accepted
28 May 1986)
ABSTRACT Ti02-supported bimetallic catalysts, consisting of iron and ruthenium with metal loading 5 wt.? and molar ratios Fe :Ru = m, 10:1, 3:1, I:1 and I:3 have been investigated with ix ;itw Mtlssbauer spectroscopy. Ruthenium has a strong influence on the behaviour of the iron in the catalysts. Partial reduction of Fe3+ to Fe2+ takes place at a lower temperature for FeRu/Ti02 than for Fe/Ti02. For reduced FelTi and IO:1 FeRu/Ti02 catalysts a magnetic sextuplet of a-Fe and bee-FeRu alloy has been observed and carburization of the iron in these catalysts to x-Fe5C2 during Fischer-Tropsch synthesis. The 3:1, I:1 and 1:3 FeRu/Ti02 catalysts are only partially reduced to Feo, the latter being present in a hcp-FeRu alloy. With increasing ruthenium concentration and increasing reduction temperat re, the initially formed Fe2+-phase is transformed back to an Fe3+ phase. This Fe Y+ -phase changes partially to an Fez+-phase upon chemisoption of CO at 295 K, indicating that the Fe3+-phase is highly dispersed. Passivation of the catalysts in air at 295 K results in oxidation of Fe0 nd Fe2+ to Fe3+. All passivated FeRu/Ti02 catalysts show reduction of Fe3+ to Fe ?+ by Hp and by CO at 295 K, which is promoted by the noble metal ruthenium.
INTRODUCTION Alloying catalysts
of metals
provides
[il. Supported
interesting
bimetallic
selectivities
in the Fischer-Tropsch
a powerful
that FeRu on Ti02 and Si02 with
[2]. Recent an atomic
higher
metals
on Ti02 or SiO2 in Fischer-Tropsch
Mossbauer
and lower methane
spectroscopy
they are active.
The purpose
of the iron in a series function synthesis.
of reduction,
Mllssbauer investigations
0166-9834/86/$03.50
ethylene
by Stoop
have shown
et al. [3] revealed
= 3:l exhibited
than either
technique
to investigate
study
catalysts
2 chemisorption
is to compare
0 1986 Elsevier Science Publishers B.V.
in which
the chemical
Fe/Ru
of CO and after
of FeRu supported
iron-containing
environments
with different
signifi-
of the single
at 575 K and 1 atm.
in the gaseous
of
and propylene
synthesis
of the present
oxidation,
work
ratio of Fe:Ru
can be studied
of FeRu/TiO
the performance
of iron and ruthenium
selectivities
is an excellent
as the catalysts
to optimize
of light olefins,
cantly
catalysts,
olefin
catalysts
for formation
synthesis
means
ratios
state
as a
Fischer-Tropsch
on Si02 [2,4-71
and of Fe
286 on Ti02 [8,9] have been reported discussion
in the literature
and provide
a brackground
for
of the results.
Our results
indicate
that ruthenium
of the iron in the catalysts. in the bimetallic
catalysts
has a strong
Below a certain is not observed
influence
Fe/Ru
during
on the behaviour
ratio carburization the Fischer-Tropsch
of the iron synthesis.
In addition
it is observed that at increasing temperature for reduction in H2-gas 3+ 2+ an initially formed Fe -phase is reoxidized to an Fe -phase [IO]. This unexpected behaviour
is more pronounced
found for I:1 FeRu/Si02. catalysts
at different
with
increasing
A systematic
ruthenium
concentration
study of this behaviour
kinds of support
is in progress
and is also
for bimetallic
[II].
EXPERIMENTAL The FeRu/TiO2 An aqueous
catalysts
solution
was added dropwise stirring
were prepared
to the TiO2 support
until the incipient
used Fe(N03)3.9H20
(Merck,
had been reduced
P.A.),
in flowing
loading of 5 wt%, whereas isotope
wetness
samples
samples
vacuum
(Aerosil
(10.9% 57Fe),
were
As starting
Matthey,
materials
41.8%
in 2.2 N HN03,
catalysts
was enriched Fe/Ti02
Ru and Fe203
a total metal
in the MBssbauer
(4.9% 57Fe),
I:1 FeRu/Ti02
were
the Fe203
contained
(10.7%
lysts took place
in air at room temperature
carried
300 V), pressed
22 mm using a pressure
IO:1 FeRu/
57Fe) and I:3
of IO MPa.
purity > 99.5%) were each sieve
in situ
H2 (Hoekloos,
purified
(Union
for 24 h and finally
wafers
For the investigations
were cut to a diameter
The gases
reactors
in air at
mixed
of the catalysts
which
100 of
at 77
of the cata-
have been described
purity > 99.9%) copper
with
with a diameter
of 17 mm. The treatments
over a reduced
Carbide,
for three days,
out on 200 mg of catalyst
into self-supporting
in the MHssbauer
[12-141.
and a molecular
(Johnson
(100 Pa) at room temperature
and 4.2 K the wafers
elsewhere
, 50 m2 g-l) under frequent
dissolving
were prepared:
were dried
400 K for 24 h. Experiments mg Si02
Before
impregnation.
of iron and ruthenium
57Fe).
The impregnated next under
P25
the iron in the catalysts
Ti02 (7.:: 57 Fe), 3:l FeRu/Ti02 (14.9%
(Degussa,
amounts
point was reached.
H2. The impregnated
Fe. The following
FeRu/Ti02
of pore volume
the desired
RuC13.xH20
in 57 Fe, Oak Ridge).
(90% enriched
by means
of pH = 1 containing
and CO (Hoekloos,
catalyst
(BASF, R3-11)
SA) at room temperature.
The Mtlssbauer experiments were done in situ with a constant acceleration 57 Co in Rh source. The spectra were not corrected for the spectrometer with a varying
distance
the measured
between
spectra
source
relative
to the NBS standard
Magnetic
fields
Mdssbauer
spectra
were fitted
of Lorentzian-shaped iterative
minimization
lines,
description
with
nitroprusside
with
the curved shifts
of a-Fe203
calculated
the Mossbauer
of the two lines were is given
in
in Ref.
consisting
in a nonlinear,
doublets
constrained [13].
at 295 K.
subspectra
parameters
In the case of quadrupole
of this method
background
(IS) are reported
(SNP) at room temperature.
the 51.5 T field
by varying
areas
Isomer
origin.
by computer
routine.
as well as the absorption detailed
sodium
were calibrated
and hence
and detector,
is of instrumental
the line widths to be equal.
A
287
fresh
I I
I 770 I 16
406 600
/ /
1
Id
-10 -8 -6 +
FIGURE
1
as indicated
-2
DOPPLER
Mtlssbauer spectra
treatments
-4
!
2
4
I
I
I
6
8
IO
VELOCITY
b-rims-‘)
catalysts
of Fe/Ti02
measured
I 0
after
a series
of subsequent
i*: o<:;-cat room temperature.
RESULTS Mllssbauer spectra after
drying,
shown
in the Figures
contributing the spectra
at room temperature Fischer-Tropsch
reduction,
of the Fe/T;02 synthesis
1 and 2. The Mdssbauer
to the spectra into these
are listed
iron phases
FeRu/TiO2
and I:1 FeRu/Ti02
in Figure
2.
is given
appeared
and passivation
parameters
in Table
and FeRu/TiO2
in Table
in air are
of the different
1, whereas
very similar,
catalysts
iron phases
the decomposition
2. As the spectra
of
of 3:l
we only show those of the former
Fresh catalysts The Mossbauer in air, consist contribution
spectra
of all fresh
of a doublet.
of a six line component
45.0 T. The values are characteristic
catalysts,
The spectrum
i.e., the samples
of Fe/Ti02
with an average
contains magnetic
after drying
also a small splitting
of about
for the Mtlssbauer parameters of both the doublet and the sextet 3+ Fe Tons in highly dispersed iron (III) oxide ot
of high-spin
288
180 fresh
I65 400
n2 675 K 18 h
air 300h
-0-6-6-4-Z
0
2 4
6
8 IO
-D-6-6-4-2
0
2
4
6
6
-DOPPLER
FIGURE
2
MHssbauer
of subsequent
TABLE
spectra
treatments
10
-IO-8-6-4-Z
VELOCITY
of lO:l, 3:l and I:3 FeRu/Ti02
as indicated
measured
0
2
4
6
6
IO
(mm.,-‘)
i7 situ
catalysts
after
a series
at room temperature.
1
Average
values
of Figures
of the iron phases
observed
QS
E’
H
r
/mm s-l
/mm s-'
/kOe
/mm s
for Mllssbauer parameters
in the spectra
1 and 2. IS
Iron phase
/mm s 3+
-1
-1
0.63+0.02
0.82i-0.05
-
0.4iO.l
Fe3+ a
0.62kO.02
0.68AO.05
-
0.5+0.1
Fe2'
1.36kO.03
2.10k0.15
-
0.7+0.1
in FeRu alloy
0.26kO.03
0.20+0.04
in B-Fe
0.26+0.02
0.0
331*2
0.33+0.05
0.45+0.04
0.0
189*2
0.37~0.10
II
0.53kO.04
-0.05
216+2
0.3orio.05
III
0.4350.06
11355
0.5
Fe
Fe0
in x-Fe5C2-I
ain bimetallic
catalysts
after
reduction
0.27+0.05
0
in H2 at 400 K.
to.1
289 TABLE
2
Composition
("4) of the MHssbauer
the treatments
spectra
and FeRulTi02
catalysts
after
indicated.
Catalyst
Fe3+
Treatment
Fe/Ti02
fresh
100
H2, 400 K, 1 h
100
a-Fe
bee-FeRu
+
x-Fe5C2
66
13
3
33
64
28
12
60
100 3
97
H2, 675 K, 18 h
4
31
7
58
FTS, 575 K, 6 h
4
41
6
6
43
44
7
7
42
air, 295 K fresh
100
Hp. 400 K, 1 h
5
95
h
27
66
7
FTS, 575 K, 6 h
11
84
5
air, 295 K
90
10
H2, 675 K;18
fresh
100
H2, 400 K,
I:3 FeRu/Ti02
hcpFeRu
21
H2, 400 K, 1 h
I:1 FeRu/Ti02
2+
FTS, 575 K, 6 h
fresh
3:l FeRu/Ti02
Fe
H2, 675 K, 18 h
air, 295 K IO:1 FeRu/Ti02
1 h
4
96
H2, 675 K, 18 h
26
68
6
FTS, 575 K, 5 h
13
81
6
air, 295 K
90
10
100
fresh H2, 400 K,
18
1 h
76
6
H2, 675 K, 18 h
74
19
7
FTS, 575 K, 6 h
43
47
IO
air, 295 K
86
14
oxyhydroxide
[15]. MBssbauer
temperatures
(not shown)
spectra
showed
at 4 K, with a distribution confirms
of Fe/Ti02
that the iron (III)
of the 1:l FeRu/Ti02
a doublet
of hyperfine
catalyst
at 77 K and partial
fields
between
oxide or oxyhydroxide
at cryogenic
magnetic
splitting
40.0 and 50.0 T. This
is present
in a highly
dispersed
state.
Reduced
catalysts
As the corresponding 1 and 2 show, exposure
spectra
in Figures
of the catalysts
state of iron in monometallic
Fe/Ti02,
1 and 2 and their analysis
to H2 at 400 K does not affect but leads to partial
reduction
in Tables the chemical 3+
of the Fe
290
in the bimetallic FeRu/Ti02 catalysts. In lO:l, 3:l and I:1 FeRulTiO2 almost 3+ present in the fresh catalyst is converted into Fe '+ by H2 at 400 K, all Fe 2+ and Fe0 occurs in I:3 FeRu/Ti02. The Mbssbauer parameters whereas reduction to Fe -1 -1 and QS = 2.10 f 0.15 mm s are of the ferrous iron, IS = 1.36 * 0.03 mm s 2+ characteristic of high-spin Fe in an octahedral coordination 1161. The parameters of the zero-valent iron present in the I:3 FeRu/Ti02 catalysts, IS = 0.26 f 0.03 -1 -1 and QS = 0.20 + 0.04 mm s , correspond to those of Fe0 atoms in hcp-FeRu mm s alloys,
as reported
The final
Mllssbauer spectrum pattern
of a-Fe,
In the reduced present
by Rush et al. 1171.
reduction
of the catalysts
of reduced
and contains
IO:1 FeRu/Ti02
Fe/Ti02
state.
in IO:1 FeRu/Ti02 iron compounds
catalyst
[18], degrees of reduction
than in Fe/Ti02.
of reduction
in our 5 wt% Fe/Ti02
than observed
with Fe/Si02
The Mtissbauer spectra
1) is dominated
almost
all unreduced
of reduction
may differ
be obtained
fractions
accurately.
of comparable
iron is present
of the different
Nevertheless,
catalysts loading
higher
at room temperature the degree
issignificantlyhigher
[19,20].
3:1, 1:l and 1:3 FeRu/Ti02
of the reduced
iron is
of iron is somewhat
considerably
and IO:1 FeRu/Ti02
catalysts
by the six line 2+ 31 and Fe .
of Fe
2) the zero valent
As the recoilless
catalysts cannot
while
degree
out at 675 K for 18 h. The
small contributions
(see Figure
alloy,
The overall
in supported
(see Figure
furthermore
as a-Fe and as bee-FeRu
in the ferrous
was carried
catalysts
indicate
that iron is present in three different states: Fe0 in hcp-FeRu alloy, 3+ ..The contribution of the zero-valent iron to the spectra is only Fe2+ and Fe about
6-7X. This
is surprisingly
reduction
in Fe/Ti02
supported
FeRu catalysts
and by our group A remarkable that almost whereas
in view of the relatively
[4-7, result
However,
has also been reported
high degree
poor reducibility
for FeRu/Si02
of
of iron in
by Guczi
et al.
IO]. concerning
all Fe3+ in . the fresh
after
small
and IO:1 FeRu/Ti02.
the 3:1, catalysts
I:1 and I:3 FeRu/TiO2 can be reduced
catalysts is 2t to Fe by H2 at 400 K,
at 675 K a considerable fraction of the iron is in the 3+ ferric state. The presence of Fe IS inferred from the peak in the MBssbauer -1 spectra at 1 mm s (see Figure 3a) which is considered as the right-hand part -1 of a doublet with the corresponding counterpart around 0 mm s . The parameters 2+ 3+ of this doublet are characteristic for either high-spin Fe or low-spin Fe . -1 by itself represents an Another possibility would be that the peak at 1 mm s 2+ unresolved doublet of high-spin Fe , as has been observed in carbon-supported iron catalysts
reduction
[21,22].
In the latter
case a resolved
doublet
would
appear
at
77 K [223. Figure
3a shows the Mdssbauer
spectra
of the reduced
I:1 FeRu/Ti02
catalysts
at 295, 77 and 4 K. The 77 K spectrum in particular ment of the spectrum
1
is similar to the 295 K spectrum and shows -1 has not split. This excludes the assignthat the peak at 1 mms -1 mm s peak to an unresolved doublet of high-spin Fe 2t. The 4 K
shows that the doublets
of FeRu alloy and Fe 2+ have not changed,
but
1
I1
1
/
,
,
,
,
,
,
II,,,,,,,,,
III FeRu/TiOz FTS, 575K,6h
I:I FeRu/TiOg reduced 400 K, I h -675
K, 18 h
r-7?+ r-b* no 1.45-
I 21 0.58
1.08 1.20-
b I
1
I
I
1
-10-8 -6 -4 -2 0
1
I
I
I
2
4
6
8
-DOPPLER FIGURE
3
Mossbauer
77 and 4.2 K after subsequently
spectra
II
/
VELOCITY
of I:1 FeRu/Ti02
(a) reduction
jb) additional
I
1
I
-10 -8 -6 -4-2
IO
I
I
II
0
2
4
/
6
I
8
IO
(mm.s-'1
catalysts
measured
in situ at 300,
in H2 at 400 K for 1 h and at 675 K for 18 h
Fischer-Tropsch
synthesis
(H2:C0
= 2:l) at 575 K
for 6 h.
that the doublet magnetically
associated
split.
This
with
implies
the 1 mm s
-1
peak at 295 and 77 K has become
that the subspectrum
cannot
be due to low-spin
Fe2+ as this state does not possess noted that the intermediate however, confirm
a magnetic moment. For completeness 2+ spin Fe would exhibit magnetic splitting.
has no known occurrences that the reduced
Fe 3+. The magnetic
hyperfine
fields.
iron (III)
oxides
the MBssbauer
catalyst 2 of this ferric phase
I:1 FeRu/TiO
pattern
area of the 4 K spectrum
[16]. Thus,
and is characteristic
The pattern
Fe304 or FeOOH.
This state,
at 77 and 4 K
contains
significant
accounts
for about 40-501
of a broad distribution
has no resemblance
such as Fe203,
spectra
it is
amounts
of
of the
in magnetic
to the Ml)ssbauer spectra
of known
292 Catalysts
under synthesis
gas
The state of iron in Fe/Ti02 been investigated
by treating
for 6 h, after which The I:1 FeRu/TiD2 cryogenic
123-251,
at room temperature.
Fe/Ti02
after
Fischer-Tropsch
shows carburization
is also observed
conditions,
the major
in the reduced alloy
been reported
Figure measured
of all a-Fe
was also studied
at
as for the reduced
synthesis
catalyst.
Similar
catalysts
of the magnetically less than
1:l FeRu/Ti02
split pattern
As follows
catalyst
synthesis,
spectra,
the same argu-
Comparison
of the spectra
indicate
contains reveals
that after
less Fe3+ than the
that the intensity
of I:1 FeRu/Ti02
This confirms
to the magnetically
and chemisorption
from Figures
catalyst.
in the spectra catalyst.
of iron has
after Fischer-Tropsch
3a and 3b, respectively,
at 295 K and 77 K corresponds
rereduction
behaviour
of these
of the two 4 K spectra
in that of the reduced
Passivation,
into
[18,26].
of I:1 FeRu/Ti02
the I:1 FeRu/Ti02
Comparison
of a-Fe
catalysts
by the syngas.
at 295 K or at 77 K in the Figures Fischer-Tropsch
2). Carburization
are exposed to Fischer-Tropsch 3+ is that of Fe , which is abundantly 2+ into Fe , whereas the zero-valent iron in
for FeRh/Si02
3b shows the spectra
x-Fe5C2
in the reduced
change
catalysts,
is not affected
into the carbide
iron present
catalyst.
at 300, 77 and 4.2 K. For the analysis
apply
doublet
synthesis
for the IO:1 FeRu/Ti02
the 3:1, I:1 and I:3 FeRu/Ti02
hcp-FeRu
reduced
has
was recorded
recently
ments
synthesis
a Mllssbauer spectrum
catalyst
simultaneously the residual ferric 2c is converted to Fe (see Figure 1, Table
synthesis present
Fischer-Tropsch
gas (H2:CO = 2:l) at 575 K
while
catalyst
When
after
in synthesis
temperatures.
Monometallic
x-Fe5C2
and FeRu/Ti02
the samples
once more
catalyst
is
that the Fe3+
split pattern
at 4 K.
at room temperature
1 and 2 and Table
2, upon exposure
of the catalysts
to air at 295 K after Fischer-Tropsch synthesis, all samples show oxidation 2+ 3+ substantial fraction of Fe to Fe . In all bimetallic FeRu/Ti02 catalysts investigated
here the zero-valent
iron in hcp-FeRu
at room temperature.
The carbide
appears
in air at 295 K.
to be stable
Exposure
x-Fe5C2,
alloy
as present
is also oxidized
in Fe/Ti02
of a
by air
and IO:1 FeRu/Ti02
of passivated
leads to a partial all passivated
lO:l, 3:l and I:1 FeRu/Ti02 to H2 at room temperature 2+ . This process has been observed in reduction of Fe3+ to Fe
bimetallic
combinations
of iron and a more noble
group VIII metal
[2,7,14,19,27,28-J. When 3:l and I:1 FeRu/Ti02 at 295
K, reduction
induced
conversion
of Fe 3+ to Fe2+ has been observed
FeRh, FeIr and FePt catalysts takes
place
involved
catalysts reduced in H2 at 675 K are exposed to CO 2c occurs, as is shown in Table 3. The same CO-
of Fe 3+ to Fe
at room temperature
are exposed
[6,7,27].
in silica-supported
The fact that this transition
is considered
as evidence
FeRu.
already
that the Fe 3+ ions
to the gas phase and, thus, are located
at the surface.
It
293 TABLE
3 (% ) of the Mbssbauer
Composition
spectra
of
I:1 and 3 :I FeRu/Ti02
catalysts
after
indicated
the treatments
3:l FeRu/Ti02
I:1 FeRu/Ti02 Treatment Fe'
Fe*+
Fe3+
Fe'
Fe*+
Fe3+
HE, 675 K, 1 h
7
61
32
5
73
22
CO, 295 K
8
77
15
5
84
11
TABLE
4
Composition
(%) of the MBssbauer
treatments
spectra
Fe*+
Fe3+
passivated
11
89
CO, 295 K
83
17
passivated
8
92
H2, 295 K
75
25
is remarkable
that the Fe
the reduction
treatment
2 and Table
Also on exposing Fe3+ to Fe2+
species
in Table
passivated
concerning
that the degree
after
the
by additional
the results.
of the respective
2+
phase during catalysts
(see
by the CO-gas
3. FeRu/Ti02
catalysts
catalyst,
such experiments
with
the reduction
limitations
exposure
catalysts
reported
4.
It is our for CO as
time of CO and H2. These
in the pressed on whether
of
by CO at
in Table
may be appropriate.
of the passivated
increasing
measurements
All spectra
to CO at 295 K a reduction
by H2 at the same temperature
of reduction
may be due to diffusion
influences
from the Fe
to an Fe 2+ phase again
For a 1:l FeRu/Ti02
well as H2 at 295 K, increases
one has to check
is formed
converted
to the reduction
A note of caution
which
at 675 K in H2 of the 3:l and I:1 FeRu/Ti02
IS observed.
295 K is compared
experience
3+
2), is partly
at 295 K, as is shown
effects
catalysts
indicated
Treatment
Figure
of I:1 FeRu/Ti02
catalysts
wafers.
or not the exposure
here correspond
So
time
to the final states
reactions.
DISCUSSION Reduction
of FeRu/Ti02
Reduction
of bimetallic
reduction
of almost
reduction
does not occur
in the bimetallic
FeRu/Ti02
catalysts
all Fe3+ in the fresh in monometallic
catalysts
is responsible
in H2 at 400 K results
catalysts Fe/Ti02.
in partial
to Fe2+ (see Figure
Apparently,
for the reduction
2). This
the presence
of Ru
of iron at relatively
294
low temperatures. bimetallic detail
Promotion
FeRh,
of the reduction
FePd, FeIr, and FePt catalysts
for the case of FeRh/SiO2
gen spillover" centres
where
[29]. In this explanation H2 is dissociated
Hence the temperature
at which
determines
freshly
prepared
also whether
of MLlssbauer spectroscopy
the initial
step
H2 and iron oxide,
is more
particles
The Mossbauer
difficult
also contain,
spectra
[30]. Higher
for the reduction
in the reduction
indicates,
of the Fe/Ti02
and FeRu/Ti02
differences.
and IO:1 FeRu/Ti02
of 3:1, 1:l and I:3 FePu/Ti02.
iron in the spectra
of Fe/Ti02
is less than 10% in the more The magnetic
sextet
of a-Fe. The magnetic to the outside.
in the spectrum
which
of magnetic model
fields
of Vincze
a composition
higher
that for FeRu alloys magnetic expected
ordering
enhances
1:3 FeRu/Ti02
higher
Fe/Ti02
whereas
which
in the
of zero-valent 65%, whereas
it
is characteristic
associated
to iron-rich
the hcp structure
reduction
is broadened
with
this pattern
than the 33.0 T field
is in qualitative
alloy.
Sextuplets
agreement
with the
alloys
with
[32] indicates
prevails,
in which
are, therefore,
not
catalysts.
a remarkable
is only below
of a-Fe.
The occurrence
bee-FeRu
at 400 K demonstrate
of iron. Therefore,
of iron reduction
catalysts,
sextuplets
however,
to iron in bee-FeRu
at room temperature.
after
reduction
absent
is high, about
of 3:1, 1:l and I:3 FeRu/Ti02
of FeRu/Ti02
the reducibility
is that the degree
the magnetic
fields
[31] when applied
with 23% Ru or more
is absent
ruthenium.
after
are virtually
96% Fe and 4% Ru. The FeRu phase diagram
in the spectra
The spectra
with,
catalysts
IO:1 FeRu/Ti02,
than that of a-Fe
and Campbell
of about
is assigned
molecular
[28]. The easy
3~1, 1:I and 1:3 Fet?u/Ti02 catalysts.
of hyperfine
are significantly
This part of the distribution
iron catalysts,
that all iron-
the contribution
of reduced
of reduced
The distribution
to values
Second,
and 1O:l FeRu/Ti02
ruthenium-rich
pattern
First,
in the
between
catalysts
therefore,
in H2 at 675 K show significant of Fe/Ti02
the reaction
or are at least in contact
dominate
the spectra
temperatures,
of monometallic
process,
than on noble metal
of Fe3+ to Fe2+ in FeRu/Ti02
containing
extends
hydro-
act as nucleation
the noble metal ions in the fresh catalyst become 3+ or not Fe will be reduced. For&le, in
range 550 to 800 K, are necessary
spectra
atoms
FePd/SiO
by means
reduction
noble metal
in
"intra-particle
Pd reduction starts already below room temperature, 2' from TPR work, and partial reduction of iron at room temperature is
as follows
because
called
into H atoms which diffuse to iron oxide in contact 3+ 2+ reduction of Fe to either Fe or Fe0 occurs.
after which
reduced
in
[7] and has been explained
[28] by a mechanism
with the noble metal,
observed
of iron has also been observed
clearly
result
10% in the Ru-rich
it is high in the Fe-rich
that Ru
of this work
Fe/Ti02
and
3:1,
1:l and
10:1 FeRu/
Ti02 catalysts. Although
a fully
yet been given,
reasoned
evidence
explanation
exists
of the phenomena
that particle
described
size is an important
above
has not
factor.
Guczi
et al. [33] have found that in FeRu/Si02 catalysts with metal loadings below 1% 2+ 3+ or Fe0 is suppressed, whereas in monometallic reduction of Fe to either Fe
295 Fe/SiO2
reduction
of Fe
[6] has proposed particle
by impeding
results,
not, or to a lesser As has first exposing
of particle
size.
bimetallic
By applying
the particles is found,
FeRu/Ti02
catalysts,
mutually
particle
and IO:1 FeRu/Ti02,
but
of 3:1, I:1 and I:3 FeRu/Ti02. [34], the extent
to the FeRu/Ti02
does not oxidize
all iron is converted
and IO:1 FeRulTiO2,
the degree
to
growth
to 02 at 295 K is a qualitative
larger
prevent
When applied
by Garten
are indeed much in which
that significant
of Fe/Ti02
this method
almost
surface.
catalysts
and IO:1 FeRu/TiO2
in Fe/ii02
reduction
suggests
as low as 0.1%. Guczi
iron and ruthenium
over the support
in the reduction
been demonstrated reduced
I:1 and I:3 FeRu/Ti02
Thus,
picture
even at loadings
unreduced
in the reduction
extent,
that iron in Fe/Ti02 3:1,
occurs
migration
Guczi's
have been involved
after
to Fe"
that at low loadings
growth
the present must
3+
of iron oxidation
catalysts
very much,
measure
it is observed whereas
in
into Fe 3t by air at 295 K.
in which
a high degree
of iron
than those of the 3:1, I:1 and I:3
of iron reduction
is lower than
10%.
Although
Ru enhances the reducibility of iron, it appears that a considerable 2t 3+ of the Fe present after 400 K reduction has oxidized back to the Fe
fraction
state after intuition, increases similar
the reduction Table
with
increasing
trends
675 K increased
unequivocally
established
and I:1 FeRh/Si02
support. will
Solid
certainly
state diffusion become
In their work the fraction after
reduction
of at
= 1, to 86% for a catalyst with 3+ is of Fe
of the reduced
I:1 FeRu/Ti02
(see Figure
3a)
temperatures.
will
hardly
of Fe
take place
In analogy
segregation
the component
et al. [26] observed
catalysts
3+
during
of iron to the interface
significant. surface
catalysts
Minai
of large amounts
in the formation
is segregation
chemisorption-induced bimetallic
by spectra
may be involved
temperatures
with Fe/Rh
be noted that the presence
[18] at cryogenic
which
catalysts.
at 295 K of FeRh/Si02
from 51% for a catalyst
It should
to what would be expected by 3+ in the reduced catalysts
of Fe
of the catalysts.
of FeRh/SiO2
spectra
Fe/Rh = 0.03.
higher
Ru content
for a series
Fe3+ in the MBssbauer
A factor
at 675 K. As opposed
2 shows that the fraction
which
at
particle
and
at 400 K, but at 675 K it
to the well-known
in alloys, forms
reduction
between
phenomenon
it is likely
the strongest
of
that in supported
bonds with
the support,
iron in the present where become
case, will become enriched at the particle-support interface, 3+ it is stabilized in the Fe state. Enrichment of iron at the support will more
important
and I:3 FeRu/Ti02 A systematic during
reduction
different
when
the bimetallic
particles
are small,
as in the 3:1, I:1
catalysts.
study of the remarkable at temperatures
supports
is in progress
between [II].
formation
of ferric
from ferrous
400 and 675 K for FeRu catalysts
iron on
296 Catalysts
under
synthesis
From the Mossbauer it follows
gas
spectra
of FeRu/Ti02
that in the cases of Fe/Ti02
and bee-FeRu
alloy
in the reduced
catalysts
The same carbide
has been observed
of carburization
of the IO:1 FeRu/Ti02
metallic
Fe/Ti02
in the bee-FeRu
catalysts, alloy
Carburization
which
during
in unsupported
Fischer-Tropsch
is probably
nearly
synthesis
all the a-Fe
into the carbide
Fe catalysts
catalysts
catalysts.
conditions
is smaller
x-Fe5C2.
[23]. The extent
than that of the mono-
due to a slower
converted
valent
unaffected.
iron remains
catalysts
[18, 261, also occurs
synthesis
gas and has been attributed
synthesis
carbon
diffusion
has not been observed
The effect
appears
at 295 K: Fe3+ is partially
et al. [26] reported
is converted
the Fischer-Tropsch
to synthesis
alloyed
after
than in a-Fe.
the 3:1, I:1 and I:3 FeRu/TiO2 Ti02 catalyst
catalysts
and iO:l FeRu/Ti02
of exposing
to be similar
into a ferrous This
species
behaviour,
in the monometallic
formed
while
found
FeRu/ of CO
the zero-
before
in FeRh/Si02
Fe/Ti02
to the formation
that the Fe2+ species
a reduced
to the effect
in
catalysts under 2t of Fe -CO complexes.
in FeRh/Si02
under
Minai
synthesis
gas are not stable towards reduction in H2 at 675 K, but were converted back to 3+ state. This result in combination with the interpretation given above the Fe would
imply that the Fe 2+-C0 complex on the support is destroyed by H2, as a 2+ 3+ is no longer stabilized and oxidizes back to the Fe result of which the Fe state. Dependent
formation
on the Fe/Ru
of low olefins
[3]. It is interesting positional bee-FeRu
region alloy
tribution
species.
Passivated
is observed
the main
noble metal
contributions
bimetallic
M remains
combination
[7]. Oxidation
catalysts
for olefins
catalysts
where
for
by Stoop et al,
is found neither
at 675 K. In this region
in a coma-Fe nor
the con-
alloy to the spectra
to the spectra of these
is only about 2+ 3+ are due to Fe and Fe
catalysts
FeM/Si02
catalysts
has been observed.
are passivated
most of the iron is oxidized
in the zero-valent FeRu/Si02
in which
of iron is also observed
synthesis
are exposed
state, about
except
by exposing
to iron (III)
when FeRu/Ti02
The
in the least noble bi-
half of the ruthenium
to air. Only the carbide
them to
oxide.
catalysts x-Fe5C2
is oxidized
used in Fischer-
appears
to be
in air at 295 K.
The observed
partial
3:l and 1:l FeRu/Ti02 A remarkable between
reduction
iron in a hcp-FeRu
no carburization
air at room temperature,
stable
after
FeRu/Ti02
and selectivity
catalysts
When reduced
Tropsch
has been found for FeRu/Ti02
of the bimetallic
Furthermore
high activity
that the high selectivity
of zero-valent
6-7%, whereas
metallic
ratio an interesting
rereduction
difference
1:l FeRu/Ti02
of Fe 3+ by H2 at room temperature
is in agreement
with the literature
in the degree
reported
of rereduction
here, and I:1 FeRu/Si02
in passivated
[5,7,19,20,35,36]. by H2 at 295 K exists
catalysts
[7]. Although
297 the specific
surface
areas for the two kinds of supports
310 m2 g-l and for TiO 2' 50 m2 g-') the much for I:1 FeRu/Ti02 SiO2 support. a specific justified
may
surface
a stronger
an investigation
comparable
by H
FeRu/Si02
with Ti02
catalyst
is necessary
with
for a
comparison. FeRu/Ti02
catalysts
IS formed than by exposure
are exposed
at room temperature.
It has been argued
version
under
by CO occurs
indicates
that the Fe3+
Structure
of the catalysts
The present
present,
is converted
but yield
extent
phase
cover
Similar
and
relating
to the structure
of the cata-
3:1, I:1 and 1:3 FeRu/TiO2
that these
as evidence
on the type of iron
ions are accessible
that they form a surface
area of the Fe 3c phase between
phase.
295 and 4 K
(see Figure 3a) is also indicative of surface 2 31 2+ the fact that this Fe to Fe transition occurs
in air-passivated
conclusions
explanation
reduced
significant
in the Si02-supported
discussion
for the structure
but forms
of this argument
between
is that thelattercontaina
has not been observed
indicates
of the present
a separate
of FeM/Si02
catalysts
surface
phase on the
FeRu, FeRh,
in 161.
FeIr and
FeM/Si02
and the present FeRu/TiO2 2+ of Fe , a phase which
amount
catalysts. results
that the ferric
has been presented
have been drawn on Si02-supported
[7]. A difference
not permit
catalysts,
the FeRu alloy particles,
A detailed
FePt catalysts catalysts
information
of the Fe3+ ions in reduced
in spectral
Moreover,
to a greater cannot
noble metal
of 1:l FeRu/TiO
[12,18].
Ti02 support.
not only yield
is regarded
increase
in the spectra behaviour
are exposed to CO 2+ [6,7] that the Fe3+ to Fe con-
by CO at 295 K. The fact
for CO at room temperature The substantial
reductor
catalysts
of the zero-valent
also information
a2rraction
into Fe
before
the influence
that CO is a stronger
reduced
more
ions involved are located at the surface.
Mtlssbauer results
lyst. For example,
to CO at room temperature,
to H2, illustrating
is even more clear when
does
a lot (for SiO2,
of rereduction
of Fe 3+ in the case of's
stabilization
than Hp. This effect
species
differ
degree
of a bimetallic
area of the support
When passivated Fe'+
indicate
However,
higher
The presence
of this Fe2+
in terms of the models
proposed
[6,7,14,37].
ACKNOWLEDGEMENTS We thank Mr. E. Gerkema ments.
J.W.N.
Organization
acknowledges
for the skilful support
for the Advancement
assistance
with the MBssbauer
from a Huygensfellowship
of Pure Research
experi-
of the Netherlands
(ZWO).
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