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manganese oxide Fischer—Tropsch catalysts during start-up and synthesis process
Applied Catalysis, 23 (1986) 339-354 Elsevier Science Publishers B.V., Amsterdam
IRON/MANGANESE PART
OXIDE
III: PHASE
DURING
START-UP
U. LUCHNER,
H. PAPP
(Received
SYNTHESIS
OXIDE FISCHER-TROPSCH
CATALYSTS
PROCESS
and M. BAERNS
Bochum,
D-4630
in The Netherlands
FOR FISCHER-TROPSCH
IN IRON/MANGANESE
AND SYNTHESIS
Ruhr-Universitat POB 102148,
CATALYSTS
CHANGES
339 - Printed
Lehrstuhl
Bochum,
15 October
fur Technische
Chemie
FRG.
1985, accepted
6 March
1986)
ABSTRACT
Structural changes in bulk phase composition of six different iron manganese oxide catalysts were studied under FT-synthesis conditions after 18 hrs, 41 hrs. 72 hrs and 180 hrs of FT operation. Relative amounts of the phases involved andtheir lattice constants were determined by X-ray diffraction. Hausmannite, iron, iron-carbides, manganospinels and manganowustites of varying compositions were identified as the major structural components. The catalysts were found to be structurally highly active solids throughout the synthesis. Catalysts of law and high manganese content exhibited a distinctly different variation of the spine1 lattice constant with synthesis time. Manganese oxide plays an important role as structural modifier by distinctly influencing the structure of a FT catalyst in bulk as well as on the crystal-lattice level. Phase relationships are discussed in terms of the lattice constant variation and a model is proposed to explain the changes observed.
INTRODUCTION
The assessment moter
or structural
served
/4-6/,
has shown,
composition time.
bimetallic
In the
Other
Similar
experiments
paper
addition
for
the problem, oxide
/17,18/
of pure
pro-
light olefins
effects
changes
synthesis
out earlier
iron /11,12/,
of
of manganese
/7,8/.
investigated
under
however
the selectivity
marked
ob-
at 240 oC
Our own research,
does affect
we have
have been carried
the phase
/2,3/.
catalysts
as catalytic
of their catalyst
have also claimed
on the behaviour
/16/ or supported
claims
catalysts
Van Dijk and co-workers
selectivity
selectivity
elucidate
of iron/manganese
either
present
oxide
authors
on the catalyst
to further
were centered
to patent
that manganese
/5,21/.
additions
In order
with
is still controversial.
of MnO on the olefin
is in contradiction
iron catalysts
phase
modifier
no influence
/l/, which
oxide
of the role of MnO in FT iron-based
/1,9-19/, fused
in bulk
conditions but these
iron /13-15/,
catalysts.
changes
in the start-up
stages
of the synthesis
340 reaction
for various
compositions
lysts as determined
by X-ray
diffraction
suggestions
are made
for possible
composition
to ensure
successful
X-ray
investigations
formed
late these
changes
ides /20/.
have done
investigated
with
and selectivity
operation
oxide
of a stable Of course,
species
data,
iron deposited
to eventually
and to correlate
or minor
at this point,
and selectivity
with
furthermore,
of the catalyst.
surface
activity
cata-
in the buildup
has been made,
for catalysts
intended
the catalysts
involved
to determine
thus no attempt
It is, however,
activity
long-time
with quantitative
and Renard
iron/manganese
in the bulk are outlined;
mechanisms
do not allow
near the surface;
Barrault
of co-precipitated
present
these with
phases to corre-
as recently
on manganese
surface
ox-
compositions
bulk composition
of
as well
as
data.
EXPERIMENTAL
Catalysts
and Reactants
Six compositions iron
(atom%);
mixture ments.
of catalysts
designated
of iron nitrate A detailed
account
of measurements
The pelletized of V4A-stainless were
placed
grams.=
pellets
all exposed
riodically
monitored
CO2,C1-C4)
to check
2000 h-1, start-up
was H2:CO:Ar velocity start
= 2:l:l
of approx.
to reach
hydrocarbons changes different
each).
Thus,
syngas
about
produced
reaction
experiments
from a
(500 oC,
while
being
system
raised
and forty-one
the same
pellets
imnediately
Syngas
at 11 bar with
increased
(SV =
by a
hours
was
a space
thereafter.
not to affect
phase
composition
from 15% at the
and was held constant assumed
was pe-
by steps of 5 oC every 8-10
was maintained
were
of the
(partial
(essentially
of the pre-calcined
The start-up
5
(Carle lllH, Ar, H2, CO,
of 270 oC was reached.
hours
made
but the pel-
the reaction
were as follows
of CO conversion
synthesis
eighteen
evaluation
compositions,
to 225 oC), was followed
pressure
catalysts.
(30 g total,
of the reaction
reduction
reactor
The six catalysts
chamber
no quantitative
gas chromatograph
temperature
50% after forty
after
/22,23/.
concentration
conditions
500 h-1. Degree
during
recycling
performance
hydrogen
and syngas
in a Berty-type
for the individual
with an on-line
of the individual
and calcination
the reaction
of course,
to the same syngas
/4,5/):
to synthesis
of 72 hrs, the temperature
the final
co-precipitated
were used for the experi-
of precipitation
tray within
24 h, 300 oC, then cooling phase
hrs until
internal
on the overall
papers
solutions
/4,21/.
subjected
of CO and H2). Reaction
as in previous
97%, 85%, 53%, 40%, and 20%
and equipment were
could be obtained
lets were
pressures
elsewhere
steel with
products
nitrate
of the method
into a partitioned
35-45
reaction
samples
iron, approx.
Fe97 etc. in the following)
and manganese
24 hrs, Ar) has been given
Procedure
(100%
FeiOO,
the structural
interrupted
to sample
The
in two
the catalysts
341
for X-ray investigation. Another experiment completed the start-up phase and in a fourth experiment synthesis was run for a total of 180 hrs. Thus four points (designated -20h-S. -4lh-S. -72h-S. -180h-S. "S" standing for "synthesis") on a time scale were obtained. X-ray diffraction For sampling. the pellets were removed from the reaction chamber at room temperature under an argon flow and immediately dropped into immersion oil (Merck 4699); this procedure effectively protects the samples from oxidation. even over long periods of time. Patterns of samples recorded immediately after synthesis. after two months and after six months of storage in oil were identical. Guinier powder samples (smear-mounts of constant sample volume) were prepared from a bulk average of two of these pellets with silicon powder used as an internal standard. The X-ray diffraction patterns were secured with a Guinier chamber HUBER 621 with evacuab1e protective gas cap using monochromatized Fe-~-radiation and sing1escreen X-ray-fi1m Kodak SB-392. Photometer traces of the patterns (Zeiss-Jena Schne11photometer GIll. slit 1.Ox20mm) served for the determination of lattice constants and approximate phase compositions as detailed elsewhere /24/. The given values are accurate within 5 to 8% for the major constituents. Lattice constants are given in nanometers with the error in the last significant digits in parentheses (doubled numerical error as given by the least-squares refinement program /25/). The compositions of the manganese rich spinel phases MnxFe3_x04 were estimated from the linear regression curve of the lattice parameter for the system Fe304 - MnxFe3_x04 as expressed by a{XMn) = 8.3978 + 0.11833 x. (a~) /26/. RESULTS General The X-ray patterns reveal the catalysts as highly dynamic solids. especially in the early stages of synthesis. Structural disorder is more pronounced in the manganese rich catalysts Fe20. Fe40 and Fe53 and less pronounced for the iron rich compositions Fe85. Fe97 and FelOO. which are of better crystallinity. Crystallinity improves during FT synthesis for all catalysts. Because the patterns defy straightforward crystallographic description as simple mixtures of well defined phases. we have chosen to reproduce here characteristic photometer traces of a relevant portion of three patterns (FelOO. Fe8S. Fe20. Fig. 1) arranged in such a way as to illustrate phase developments with synthesis time (••• h-S) for each composition. Additionally. at the bottom of the set of traces for Fe85 and Fe20 a trace for the freshly reduced catalyst (designated "R") from earlier experiments is added. Although this sample was reduced outside the reactor in a separate furnace and cooled to room temperature. there is no reason to believe that the reduced catalyst in the reactor was any different.
.
342
FIGURE 1 Diffraction patterns of catalyst samples FelOO, Fe85 and Fe20 after different times of FT-synthesis (225 "C - 270 "C, 11 bar, H:CO:Ar = 2:l:l). Bottom trace (R) of Fe85 and Fe20 is the pattern of the reduced catalyst.
343
Fig. 2 and Fig. 3 depict the phase changes in the six catalyst samples in the form of bar diagrams. We have attempted to include information on the degree of intermixing between the phases by graphical representation within the bars (see caption of Fig. 3 and description of the individual compositions). Thus, the inclined boundary lines indicate solid solution between respective phases, with strong intermixing between the wustites leading to the hatched areas representing an unsegregated wustite phase. Fig. 4 shows the variation of the spinel lattice parameter with composition and synthesis time. Pure iron Sample Fe100 stands out from all other samples in that it develops carbide phase very early in the start-up process. After eighteen hours of synthesis it consists of ca 50 vol% o-carbide /27/ and of 50 %of a defect spinel phase as evidenced by the presence of the spinel-forbidden reflexions (200), (210) (both not shown) and (310), the unusually strong reflexion (400) /28/, and the low lattice constant of 0,8387(3) nm. Presence of elemental iron at this stage is highly improbable as judged from the absence of the iron (200) reflexion and from a MoBbauer spectrum /29/. After 41 hrs the carbide phase has changed its character from octahedral carbide to x-carbide /30/ (now ca. 40 %). Minor amounts ( 5 %) of elemental iron can be made out in the MoBbauer spectrum and from a very weak iron (200) reflexion (not shown). The spinel phase (ca. 55%) has markedly improved its crystallinity after 41 hrs. After 72 hrs, at the end of the start-up process, the carbide phase has again changed its character towards o-carbide and decreased to approx. 15% of the total sample volume, the remainder being spinel phase of further improved crystallinity. The consumption of carbide phase continues as the synthesis goes along and after 180 hrs the sample consists of a welJ crystallized spinel phase with a lattice constant of a = 0.8395(5) nm corresponding to almost perfect magnetite with little defect character /31/. Carbide content is not nil but barely detectable on long-time exposures. No indication of an Fe1_xO-wustite phase has been found throughout the synthesis process. 3%
manganese sample The addition of a small amount of manganese ions drastically changes the picture as compared to pure iron. Carbide phase formation in Fe97 (no pattern given) is suppressed early in the start-up process in favour of elemental iron with marked iron (110) and iron (200) reflex ions present. The freshly reduced sample is composed of 30% spinel phase and 70% iron which decreases to approx. 40% after 18 hrs. About 10% of o-carbide have then formed which, like in the Fe100-sample, changes towards x-carbide after 41 hrs but does not increase in mass while elemental iron predominates. At the end of the start-up phase, however, free iron has entirely disappeared and the sample is composed of slightly expanded spinel (a = 0.8406(2) ~
344
100
Fe100
80 60 40 20
Fe97 m ;,” ,“; ;zh Wlistite
fit3mongonese $I!t’n”ss$AFegoted 0
Spine1
q Housmonnite EJ Iron
q Iron
WI00
carbides
Fe85
u-l
6
2 80 Y zi m 60 40 1
20 FIGURE
2
Phase composition
60
100
of iron-rich
IL0
samples
180
thl
rich
345
nm, Fig.2, 85 %) and o-carbide (15 %). This distribution of phases now remains unaltered until the end of the synthesis, where the spinel lattice has shrunk to a = 0.8400(2) nm, showing marked contributions from a defect (or interstitial) lattice superstructure /28/. 15% - manganese sample Stabilization of elemental iron as the major reduced phase during the early stages of synthesis is further enhanced as the manganese content increases to 15%. The reduced sample is a well crystallized spinel (.0.8433(3) nm) with only traces of wustite phase and no apparent iron content. Yet we know from other work /24/ that elemental iron may well be present in a non-detectable form (see discussion). After eighteen hours of synthesis this elemental iron has manifested itself by comprising 20 %of the sample mass while the spinel lattice has shrunk to a = 0.8407(3) nm. Carbide diffraction lines are barely visible. After 41 hrs., iron has further increased to 35% along with some carbide formation (ca. 5%). The spinel phase now amounts to half the sample mass with a recovered lattice constant of 0.8438(8) nm, (Fig.4), and an iron rich wustite phase (a = 0.438(1) nm) /32/ accounts for the remainder 10-15% of the sample. At the end of the start-up phase elemental iron has been consumed almost entirely and the operating catalyst is made up of 60-70% spinel (a = 0.8429(7)nm), 25-30% wustite (a = 0.438(1) nm) and some carbide. As synthesis continues spinel phase increases so~ewhat at the expense of wustite phase which finally amounts to 15-18% and has become enriched with manganese (a = 0.441(1) nm). No more elemental iron and only traces of carbide can be detected at this point. 47% - manganese sample X-ray patterns of sample Fe53 (not shown) ar~ hardest to interpret. Already the freshly reduced oxide mass is an ill-defined mixture of spinel (possibly with contributions from tetragona11y distorted hausmannite /33,34/) and wustite, with poor overall crystallinity. After eighteen hours Of synthesis this picture has hardly changed. One should note that unusually long exposure times (70 hrs) with standard species or very thick mounts (0.3 mm) are necessary to obtain the patterns. The broad peaks can best be explained as a superposition of spinel, wustite and hausmannite lattices with a high degree of structural disorder. Only traces of elemental iron « 5%) are X-ray-detectab1e. High-angle ref1exion bands may be interpreted as arising from two separate wustite phases of a = 0.432(1)nm and a = 0.444(1)nm, e.g. almost pure FeO and MnO. As the start-up process proceeds, after 41 hrs, carbide formation becomes apparent as well as elemental iron (ca. 10% each). Hausmannite is no longer present and the wustites have merged into a solid solution (ca. 55%) of (Fe,Mn)O with a = 0.4388(5) nm. Spinel phase (ca. 25%) has consolidated into a lattice of a = 0.8483(12) nm, e.g. nearly MnFe204. As the start-up process is completed, the wustites have segregated again (a = 0.437(1)
346
~Wustite manganese
rich
~~r%&goted 0
Spine1
q Hausmonnite Q
Iron
m
Iron
corbldes
FIGURE 3 Phase composition of manganese-rich samples. Inclined boundary lines within the histograms indicate degree of intermixing between respective phases. Interpenetrating areas of wustites form symbol for unsegregated wustites (cross-hatched fields).
347 nm and 0.442(I) at the start solution
nm). We now do not find the nearly
(indicated
wustites
by the inclined
of which
over the manganese-rich These
wustite
manganese and/or
phases
phase
thus exhibit more
slightly
(35%) in mass
of approximately
especially
towards
as before.
and the spine1
wustite
(a = 0.8472(7)
has lost
Traces
in manganese
to reach
unsymmetrical
the iron-rich
overexposed
of iron
the spine1
nm). The wustite
are still rather
segregation
of two
reflexions.
phase
nm accordingly.
and also enriched
region,
Even on grossly
by a factor
the end of the synthesis
MnO.63Fe2.3704
in the high-angle
a tendency
abundant
is abundant
of a = 0.8455(g) Towards
that were present
from the high-angle
to 75% of the sample
can only be suspected.
has increased
a composition peaks,
amount
, as determined
pure phases
line in Fig. 3), but two solid-
the iron rich wustite
wustite
ions to form a lattice
carbide
boundary
patterns
and
wustite
being
no iron or carbide
can be found.
60 % - manganese
sample
The Fe40-sample
(no pattern
the X-ray pattern of two wustites duction), nounced
(also the principal
spine1
and hausmannite.
as with the Fe53-sample
the spine1
composition
of 0.8419(5) ganese
content
(IO-15%)
constant.
has almost
solution
After
increased
to ca. 30%
elemental
iron is detectable.
in solid
solution
position
unchanged.
80 % - manganese
phase
(a = 0.4419(5)
of carbide
nm) with
the remainder
comprising
(a = 0.442(l)
is slightly wustite.
phases
in excess
Spine1
has
are left and no
the wustites
are found
of the crystal
phase
com-
(a
nm, 60-70%)
sample only one manganese-rich
in the early
(~18%).
and has been consumed dominates
traces
remain-
nm). Haus-
spine1
are still separate
At the end of the synthesis
constant
as separate
nm and 0.4395(5)
wustite
and
have been formed
appear
than the iron-rich
nm). Only
lattice
thereafter
to less than 5 % with
72 hrs the wustites
(a = 0.8477(g)
With the Fe20-sample can be observed
iron and carbide
is as pro-
amounts),
after 41 hrs the man-
(a = 0.8475(8)nm),
(viz. 0.4415(10)
crystallinity
nm, equal
The spine1
(60-65 X) still
and amounts
after re-
of the two wustites
near MnO.28Fe2.7204;
in that
as a superposition
iron and spine1
nm vs.O.443(1)
nm) but now the manganese-rich
and seems to be of better
to the Fe53 sample
little
far from MnFe204.
Also elemental
solid
nm vs. a = 0.438(l)
plus
to MnO.65Fe2.3504
disappeared
similar
must be described
The segregation
after 41 hrs and the wustites
the rest of the sample.
phase
phases
a composition
is enriched
but have approached mannite
is quite
(0.434(l)
is initially
nm indicates
ing essentially
spine1
shown)
after 18 hrs of synthesis
stages
The hausmannite
along with phase
wustite
hausmannite
diminishes
at the end of the start-up
the picture
it is the only X-ray-visible
at this composition. component
q
0.4429(3)
(15-25%)
and only
to ca. 5% after
process.
Naturally
In the freshly
and shows just a slight
forty
little hours
the wustite
reduced
tendency
catalyst
of
segre-
348
gation
into two separate
lower d-values form, since
(Fig.1).
phases
as evidenced
Still elemental
after several
a mixture
ed field
in Fig. 3). The tailing
of wustite
the sample, conditions wustite rate
this
tes, but the separation
lattice
one finds
parameter
out the early apparent
Only
although
phase
amounting
experiments
MoBbauer
showed
/6/. The observed
catalysts
suggests
remains
these carbides
or to the method
satisfactorily
at present.
the
to sepa-
poorly
period
wusti-
At the end of the
phase
(85%) with defined
does
through-
pure magnetite
parameter
with
little
any carbides,
to an appreciable
degree
in
may be due to non-crystallinity
differences
of investigation
a
it become
lattice
films do not reveal
their presence
Instead,
a tendency
to ca. 15%. The almost
ions. Long-exposure
of manganese
by the reaction
speak of two separate
as the major
phase
cooling
even more pronounced
the Fe40-sample.
wustite
nm. The spine1
nm from the (220)-reflexion
incorporation
and exhibits
after the end of the start-up
as a well crystallized
of 0.8399
similar
of 0.4420(5)
stages.
as with
and dash-
phase without
72 hrs this becomes
solid-solution
in Fig.1
iron be observed.
nm) and one might
is not as clear
again
active
catalyst
has then disappeared.
iron is inhibited
can elemental
to be structurally
reduced
peak
after the reduction
after 41 hrs. After
nm vs. a = 0.4391(I)
(a = 0.443(l)
(dashed
reflexions
of elemental
in the process
continues
into two phases
synthesis
continues
recrystallization
and nowhere
phase
of the wustite
off to
in a non-detectable
the freshly
(80%) and iron (20%)
process
tailing
iron must be present
days at room temperature
exhibits
Yet, as the synthesis
by the reflexions
or both and cannot
of
be explained
DISCUSSION
The following formation phase
behaviour
wustite-spine1 structure
General
first outline
of the individual interaction
the general
in FT iron/manganese samples
as a major
will be examined
factor
with hydrogen
factors
involved
oxide catalysts.
and subsequently
in the buildup
at 300 oC will
in the
Then the the
of the bulk-phase
be dealt with.
considerations interactions
in determining
First
there
catalyst.
which
structural
the behaviour
active
temperature.
teraction
between
is the wustite
the catalytically given
will
phases
after reduction
Two main tant
discussion
of crystalline
influences
of the individual
spine1
"iron"
Secondly,
components
interaction
during
there activity
reduction
as impor-
catalyst.
which
influences
and subsequent
is the iron/iron and possibly
can be identified
carbide
deactivation
the nature
synthesis
- surface
of
at a
carbon
of the operating
in-
349 Paramount mixture
to the overall
at this temperature
manganese
reduction
hausmannite
ite thus produced temperatures
Its ability
to form solid
to either
tween
matrices
that of ferrous
is a source
ions, which
Thus,
solution
manganowustite
an existing
the wustite
external commodate /36/.
"elemental"
cal cubic
spine1
"protected"
suggested
in conjunction
small microdomains
with
/40,41/,
which
in
than
grain boundary
an existing
solid-
manganese
ions
manganese
phase
ions
of changing
are able to ac-
lines of iron conform compatible
having
should
iron ammonia
can occur
ions is higher
can accomodate
with
to a hypotheti-
the spine1
exsolutions
by the oxide matrix
without
be-
level as well as in bulk intergrowths
nm, easily
if such exsolutions
into haus-
interaction
under the influence
of mere 1.3 %. Conceivably "shielded"
variab-
on one hand and
precipitations
and wustite
low
of a(-Fe203.
distortion
solid-state
phase
both spine1
that the diffraction
to carbidization
is not yet clear,
superstructures
field by extracting
and segregate
iron on a sub-lattice
or structurally
product
only to the nearest
the spine1
unstable
difference
less susceptible
is structural
there
reduction
of the manganese
migrate
iron lattice of a = 0.8356
a volume
Third,
phase will be able to drive
Additionally,
It is noteworthy
within
usually
spoken,
become
conditions.
at these
that (Fe,Mn)O
out of its stability
it or, alternatively
should
stable
of continuous
It is well known
of
iron wustite
and/or
and that the mobility
formation
of manganowust-
possible
variants
intermediate
is the possiblility
with
conditions.
as another
oxide
is the lowest possible
ions up to the point of tetragonal
hand
/35,36/.
from
solutions
form defect
the structures.
Fe304-Mn304
phase
manganese
on the other
of an iron/manganese
by thermodynamics, Next there
under non-equilibrium
to incorporate mannite
allowed
notwithstanding.
ility of the iron spine1
characteristics
is the fact that manganowustite
product
spine1
reduction
and will therefore
to loose catalytic
be termed
catalysts
/37-39/,
be
activity.
"paracrystalline",
can also be invisible
lattice
of iron will be
It
as was
or if they occur
for-.-X-rays. depending
as on
size. Finally,
as a determining
ture of the FT-reaction degree
of carbon
Iron-rich
monoxide
samples
Fe100
In the case of pure lattice
instabilities,
with elemental all oS.the
factor
itself,
while
stability,
or oxidative,
iron oxide with no manganese a highly
mentioned
carbides
the synthesis.
As carbon
dering
a distinct
to form
phase
there
is the na-
depending
on the
and Fe97
defective
iron not all of which
reasons
reductive
conversion.
above.
the -wary start of the FT synthesis decreases
for crystal
being
phase
to compensate
is formed
may be visible This elemental
freely
high activity during
carbidized
the reductive
to minor
from
of the catalyst
in the iron lattice,
is very much subject
and
together
beam for one or
iron is readily
iron compete
rather
for valence
by reduction
to the X-ray
and the initially
and elemental atoms move
spine1
changes
phase of
their
or-
in external
350 8,!3l ._...._..... _.._..._......_.. .._,.,,..._,.___._..
MnFe20L 8.L9 t
P. Fe4_..-..4 i' _~_~_.._-o-._-..-.-~'-"P !; 1.
au8b7-
5 &6c ”
20
FIGURE
4
Lattice
conditions. carbon
Also,
atoms
carbide
constant
a carbide
Under the conditions
iron phase
gradually
It is, nevertheless,
spine1
phase
defect
spine1
drops
off again
due either
during
chosen
spine1
the reductive
the value
interstices
the
may again
/24/.
some time before
oxidative
and the carbidic
as synthesis phase
lattice parameter
the low lattice
phase when carbide
for pure Fe304. the spine1
or defects
continues.
Addition lattice
of the spine1
We
itself at this of the
constant
formation
at the end of the start-up
affect
loose
of iron/iron
publication
of the spine1
to follow
synthesis
discussion
in this study,
(Fig. 4). For Fe100
in Fe97 does not strongly to filling
near the surface
the gas phase becomes
that of pure magnetite to approach
phase during
A more detailed
activity
A 180
160
in a forthcoming
instructive
synthesis
increases
spine1
into a stable
catalytic
point.
gest and exceeds
phase
transforms
from discussing
100 120 1LO TIME Ihl
once formed
reaction.
of FT synthesis
of the start-up
manganese
phase
will be presented
completion
during
60 80 SYNTHESIS
of iron-manganese
to the synthesis
interaction
refrain
LO
of the is stron-
phase.
It then
of small amounts constant.
lattice with
This
of
is
the large
351 Mn2+-ions
or to incorporation
not differ currently zation pure
much being
performed
and competition
iron sample,
Thus,
for manganese
phases, oxide
manganese
structure
lence states
samples
samples
high
additional
clearly
spine1
lattice
of synthesis wustite Mn2+
than
ill-defined tected would
lattice,
lattice
iron/iron va-
structural
steep
is well
modifier.
due to its possible
would
especially
between
The "burning"
of the spine1
expansion
will
lattice,
be entirely
fore and because
of loss of Mn2+
of this
in.Fe40
than
in Fe53
Contributions
in Fe53 despite
from carbide/
the ongoing
is sufficiently cannot
due to manganese
is
to stabi-
so they cannot
during
the of
if they occur,
drop OS the lattice
ions to the hausmannite
is smaller
For the stages
incorporation
high enough
but these,
this drop-off
on the
in the early
increase
becomes
of these
the
is not as
18 and 41 hrs while
and are also of poor crystallinity,
for the subsequent
character,
at present.
successive
component.
be excluded,
leads
as an
of phase behaviour
as the absolute
third
defect
reduction
content
This
for manganowustite
in accord with
the manganese
ions becomes
of wustite
be too speculative
increase
is encountered
of mangenese
and to segregation
In Fe40, where carbide formation
increase
of separate
of the basic
lattice parameter
so that considerations
in Fe53 cannot
an explanation
in an expansion
spine1
phase
constant
as an intermediate
in the X-ray patterns.
41 and 72 hrs. show
This
as "phases"
offer
of the spine1
and iron content
by a rather
incorporations
and
is stopped.
lattice.
we first find a drop after
in Fe40, where
lize hausmannite carbon
stages.
phase
of ions in different
Here the concentration
parameter
lattice
constant
into the spine1
Carbidi-
the early
the major
segregation
modifier
its role as a direct
as for spinel,
followed
/42/.
way as with the
by the time the experiment
the distribution
Unfortunately.
lattice
phase develops.
stronger
variation
on the spine1
constituent.
defined
during
becomes
too small to achieve
influencing
to excert
basis of the wustite
in a similar
in excess
acts as an electronic
Fe53 and Fe40.
influences
between
on this point
by oxidation
ideal magnetite
the ionic radii do XPS experiments
- role of manqanowustite
different
Fe85,
sufficiently
relation
the spine1
over the sites of the spine1
A strikingly
to distinct
information
and iron proceeds
concentrations clearly
more
where
configurations;
iron being clearly
almost
by mainly
Manganese-rich
with
may yield
continues
to approach
for Fe 3+-ions,
and low-spin
of carbides
elemental
As the FT-reaction re-orders
of Mn3+-ions
for the high-spin
synthesis
constant
suppressed occur,
between not to
and the
incorporation. phase,
are be de-
There-
the absolute
the higher
value
manganese
content. For the Fe20 ly defined
takes notice rameter,
sample
of its final value,
one might
Pure Fe304
the spine1
as to allow meaningful
phase
during
determinations only sligbtly
the first
in excess
argue that for the iron poor Fe20
as a catalytically
active
component
eighty
hours
is too poor-
of its lattice constant.
occurs
of the Fe304
sample
segregation
If one
lattice
pa-
of almost
out of the basically
inert
352 wustite
matrix
/24/. The general
earlier
/5,21/
would
stant
in Fe85,
the wustite persistent
support
phases
as to their
the start-up
final degree
of the spine1
ions and manganowustite)
hausmannite
component,
itself
ence to the formation Fe53 and Fe40
in activity
of this sample
this view. The variations
Fe53 and Fe40 after
interaction
manganese
increase
with
during
a spine1
of the mixed
and contributes
of the spine1
phase,
its manganese
/43/,
then reflect
modifiers
the now stable
variant
lends
spine1
(e.g.
synthesis
of
the
inherent
process.
its structural
early
The. influ-
in the synthesis
to the drop of the lattice constant
tion and the 18 h-observation
lattice con-
and the "undecidedness"
of salid solution,
iron-manganese
as observed
between
in
reduc-
point.
CONCLUSION
We have presented nese oxide sis that
a tentative
FT-catalysts,
is consistent
iron-manganese Two groups
spine1 phase
the crystal
ability
to form structurally
phase
composition active
position.
More data, especially
to extend
the model
and selectivity. .methods
alone,
and check
at different
The iron/iron deserves
of the manganese
special
carbide
during
be identi-
to the assumption phase
rich catalysts solutions
due to its com-
are needed
data on activity
is not accessible
further
strongly
of varying
of reduction.
with quantitative which
could
The wustite
temperatures
problem,
attention
solid
and synthe-
of the mixed
of these catalysts.
lend some support
(Fel_x,Mnx)O
its consistency
parameter
component
the synthesis.
of iron/manga-
phase
one manganese-poor,
Our results during
changes
the start-up lattice
as the major structural
component
affects
the phase
of the
one manganese-rich,
to this variation.
as an active
to explain
at 300 oC, during
with the variations
of catalysts,
fied according of spine1
model
reduced
investigations
by X-ray in this
respect.
ACKNOWLEDGEMENTS
We thank for running Lehrstuhl
Or. R. Malessa
#or preparing
the FT synthesis
fiir Mineralogie
experiments.
the catalyst Thanks
of the Ruhr-University
This work was.suppori.ed
by the Bundesminister
of the Federal
of Germany.
Republic
samples
and Mr. G. Beerwerth
also go to Prof. for providing
fiir Forschung
Florke
of the
XRD-facilities.
und Technologie
(BMFT)
353
REFERENCES
1 2 3 4 5 6 7
a 9
W.L. van Dijk, J.W. Niemantsverdriet, A.M. van der Kraan and H.S. van der Baan, Appl. Catal., 2 (1982) 273. H. Ktilbel and K.O. Tillmetz, Belg. Patent 837,628 (1976) and DOS 2 507 647 (1976). B. BUssemeier, C.D. Frohning, 6. Horn and W. Klug, DOS 2 518 964 (1975). G.C. Maiti, .R. Malessa and M. Baerns, Appl. Catal., 5 (19831 151. G.C. Maiti, R. Malessa, U. Lbchner, H. Papp and M. Baerns, Appl. Catal., 16 (1985) 215. N.K. Jaggi, L.H. Schwartz, J.B. Butt, H. Papp and M. Baerns, Appl. Catal., 13 (1985) 347. J. Barrault, C. Forquy and V. Perrichon, Appl. Catal., 5 (1983) 119. K.B. Jensen and F-E. Massoth, J. Catal., 92 (1985) 98, and ibid. 109. L.J.E. Hofer, E.M. Cohn and W.C. Peebles,
3. Am. Chem. Sot., 10 :: 13 14
71 (1949) 189.
W. Rahse, J. Heil, M. Ralek, Erdbl und Kohle, 31 (1978) 356. H. Kolbel, Chem.-Ing.-Tech., 23 (1951) 153. J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk and H.S. van der Eaan, J. Phys. Chem., 84 (1980) 3363. R-B. Anderson, L.J.E. Hofer, E.M. Cohn, B. Seligman,
3. Am. Chem. sot., 73 (1951) 944. O.G. Malan, J.D. Louw and L.G. Ferreira, Brennst. Chem., 42 (1961) 209. J.F. Shu1tz;W.K. Hall, 8. Seligman, R.B
Anderson, J. Am. Chem. Sot., 77 (1955) 213. 16 !.Liap;;, T. Fleisch and ti.N. Delgass, . .* 65 (1980) 253. G.B. Raupp, W.N. Delgass, J. Catal., 58 (1979) 348. :: ibid., 58 (1979) 361. I9 H. Schulz, E. Bamberger and H. Gorre,__ Proc. 8th International Congress on Catalysis, Berlin (West), July 1984, Vol. 11-123. 20 J. Barrault and C. Renard, ibid., Vol. 11-101. 21 R. Malessa, Dissertation, Ruhr-Universitat Bochum, 1985. J.M. Berty, Chem. Eng. Progr., 70(5) (I974) 78. ;23 J: Klose, Dissertation, Ruhr-Universitat Bochum, 1982. 24 U;.LBchner, R. Malessa and M, Baerns, Appl. Catal., (to be published). 25 LCLSQ, An IBM-7090 Computer Program for Least-Squares Refinement of crystallographic lattice constants, Charles W. Burnham, Carnegie Institution of,Washington, Washington D.C., USA, 1963, modernized and addpted for a CQC Cyber-175 Computer. 26 -6. Punge,.Diplomarbeit, Lehrstuhl fir Mineralogie, Ruhr-UniversitZt Bochum. 1982. 27 G. Le Caer, J.M. Dubois; M. Pijolat, V. Perrichon and P. Bussiere, J. Phys. Chem,, 86 (1982) 4799. 28 R. Schrader and 6. Biittner, 2. Anorg. Allg. Chem., 320 (1963) 205. P. Deppe, H. Papp and M. Rosenberg (to be published). iit J.P. Senateur, Ann. Chim. (Paris), 2 (1967) 103. 15
l
354 31 32 33 34 35
36 37 38 39 40 41 42 43
M.E. Fleet, Acta Cryst., 637 (1981) 917. P.K. Foster and A.J.E. Welch, Transact. Faraday Sot., 52 (1956) 1626. H.J. van Hook, M.L. Keith, Am. Min., 43 (1958) 69. T. Tanaka, Japan J. Appl. Phys., 13(8) (1974) 1235. H.J. Engell, K. Bohnenkamp, H.K. Kohl, E. Riecke and K.H. Ulrich, Forsch. Ber. NRW Nr. 1909, Westdeutscher Verlag, Koln und Opladen 1968. H. Schmalzried, Ber. Bunsenges. Phys. Chem., 88 (lY84) 1186. R. Hosemann, A. Preisinger and W. Vogel, Ber. Bunsenges. Phys. Chem., 70 (1966) 796. H. Ludwiczek, A. Preisinger, A. Fischer, R. Hosemann, A. Schbnfeld and W. Vogel, J. Catal., 51 (1978) 326. G. Ertl, D. Prigge, R. Schloegl and M. Weiss, J. Catal., 79 (1983) 359. G. van Tendeloo and S. Amelinckx, Acta Cryst., A30 (1974) 431. ii. Wondratschek and W. Jeitschko, Acta Cryst., A32 (1976) 664. T. Grzybek, H. Papp and M. Baerns, (to be published). M. Keller and R. Dieckmann, Bet-. Bunsenges. Phys. Chem., 89 (1985) 1095.
IRON/MANGANESE PART
OXIDE
III: PHASE
DURING
START-UP
U. LUCHNER,
H. PAPP
(Received
SYNTHESIS
OXIDE FISCHER-TROPSCH
CATALYSTS
PROCESS
and M. BAERNS
Bochum,
D-4630
in The Netherlands
FOR FISCHER-TROPSCH
IN IRON/MANGANESE
AND SYNTHESIS
Ruhr-Universitat POB 102148,
CATALYSTS
CHANGES
339 - Printed
Lehrstuhl
Bochum,
15 October
fur Technische
Chemie
FRG.
1985, accepted
6 March
1986)
ABSTRACT
Structural changes in bulk phase composition of six different iron manganese oxide catalysts were studied under FT-synthesis conditions after 18 hrs, 41 hrs. 72 hrs and 180 hrs of FT operation. Relative amounts of the phases involved andtheir lattice constants were determined by X-ray diffraction. Hausmannite, iron, iron-carbides, manganospinels and manganowustites of varying compositions were identified as the major structural components. The catalysts were found to be structurally highly active solids throughout the synthesis. Catalysts of law and high manganese content exhibited a distinctly different variation of the spine1 lattice constant with synthesis time. Manganese oxide plays an important role as structural modifier by distinctly influencing the structure of a FT catalyst in bulk as well as on the crystal-lattice level. Phase relationships are discussed in terms of the lattice constant variation and a model is proposed to explain the changes observed.
INTRODUCTION
The assessment moter
or structural
served
/4-6/,
has shown,
composition time.
bimetallic
In the
Other
Similar
experiments
paper
addition
for
the problem, oxide
/17,18/
of pure
pro-
light olefins
effects
changes
synthesis
out earlier
iron /11,12/,
of
of manganese
/7,8/.
investigated
under
however
the selectivity
marked
ob-
at 240 oC
Our own research,
does affect
we have
have been carried
the phase
/2,3/.
catalysts
as catalytic
of their catalyst
have also claimed
on the behaviour
/16/ or supported
claims
catalysts
Van Dijk and co-workers
selectivity
selectivity
elucidate
of iron/manganese
either
present
oxide
authors
on the catalyst
to further
were centered
to patent
that manganese
/5,21/.
additions
In order
with
is still controversial.
of MnO on the olefin
is in contradiction
iron catalysts
phase
modifier
no influence
/l/, which
oxide
of the role of MnO in FT iron-based
/1,9-19/, fused
in bulk
conditions but these
iron /13-15/,
catalysts.
changes
in the start-up
stages
of the synthesis
340 reaction
for various
compositions
lysts as determined
by X-ray
diffraction
suggestions
are made
for possible
composition
to ensure
successful
X-ray
investigations
formed
late these
changes
ides /20/.
have done
investigated
with
and selectivity
operation
oxide
of a stable Of course,
species
data,
iron deposited
to eventually
and to correlate
or minor
at this point,
and selectivity
with
furthermore,
of the catalyst.
surface
activity
cata-
in the buildup
has been made,
for catalysts
intended
the catalysts
involved
to determine
thus no attempt
It is, however,
activity
long-time
with quantitative
and Renard
iron/manganese
in the bulk are outlined;
mechanisms
do not allow
near the surface;
Barrault
of co-precipitated
present
these with
phases to corre-
as recently
on manganese
surface
ox-
compositions
bulk composition
of
as well
as
data.
EXPERIMENTAL
Catalysts
and Reactants
Six compositions iron
(atom%);
mixture ments.
of catalysts
designated
of iron nitrate A detailed
account
of measurements
The pelletized of V4A-stainless were
placed
grams.=
pellets
all exposed
riodically
monitored
CO2,C1-C4)
to check
2000 h-1, start-up
was H2:CO:Ar velocity start
= 2:l:l
of approx.
to reach
hydrocarbons changes different
each).
Thus,
syngas
about
produced
reaction
experiments
from a
(500 oC,
while
being
system
raised
and forty-one
the same
pellets
imnediately
Syngas
at 11 bar with
increased
(SV =
by a
hours
was
a space
thereafter.
not to affect
phase
composition
from 15% at the
and was held constant assumed
was pe-
by steps of 5 oC every 8-10
was maintained
were
of the
(partial
(essentially
of the pre-calcined
The start-up
5
(Carle lllH, Ar, H2, CO,
of 270 oC was reached.
hours
made
but the pel-
the reaction
were as follows
of CO conversion
synthesis
eighteen
evaluation
compositions,
to 225 oC), was followed
pressure
catalysts.
(30 g total,
of the reaction
reduction
reactor
The six catalysts
chamber
no quantitative
gas chromatograph
temperature
50% after forty
after
/22,23/.
concentration
conditions
500 h-1. Degree
during
recycling
performance
hydrogen
and syngas
in a Berty-type
for the individual
with an on-line
of the individual
and calcination
the reaction
of course,
to the same syngas
/4,5/):
to synthesis
of 72 hrs, the temperature
the final
co-precipitated
were used for the experi-
of precipitation
tray within
24 h, 300 oC, then cooling phase
hrs until
internal
on the overall
papers
solutions
/4,21/.
subjected
of CO and H2). Reaction
as in previous
97%, 85%, 53%, 40%, and 20%
and equipment were
could be obtained
lets were
pressures
elsewhere
steel with
products
nitrate
of the method
into a partitioned
35-45
reaction
samples
iron, approx.
Fe97 etc. in the following)
and manganese
24 hrs, Ar) has been given
Procedure
(100%
FeiOO,
the structural
interrupted
to sample
The
in two
the catalysts
341
for X-ray investigation. Another experiment completed the start-up phase and in a fourth experiment synthesis was run for a total of 180 hrs. Thus four points (designated -20h-S. -4lh-S. -72h-S. -180h-S. "S" standing for "synthesis") on a time scale were obtained. X-ray diffraction For sampling. the pellets were removed from the reaction chamber at room temperature under an argon flow and immediately dropped into immersion oil (Merck 4699); this procedure effectively protects the samples from oxidation. even over long periods of time. Patterns of samples recorded immediately after synthesis. after two months and after six months of storage in oil were identical. Guinier powder samples (smear-mounts of constant sample volume) were prepared from a bulk average of two of these pellets with silicon powder used as an internal standard. The X-ray diffraction patterns were secured with a Guinier chamber HUBER 621 with evacuab1e protective gas cap using monochromatized Fe-~-radiation and sing1escreen X-ray-fi1m Kodak SB-392. Photometer traces of the patterns (Zeiss-Jena Schne11photometer GIll. slit 1.Ox20mm) served for the determination of lattice constants and approximate phase compositions as detailed elsewhere /24/. The given values are accurate within 5 to 8% for the major constituents. Lattice constants are given in nanometers with the error in the last significant digits in parentheses (doubled numerical error as given by the least-squares refinement program /25/). The compositions of the manganese rich spinel phases MnxFe3_x04 were estimated from the linear regression curve of the lattice parameter for the system Fe304 - MnxFe3_x04 as expressed by a{XMn) = 8.3978 + 0.11833 x. (a~) /26/. RESULTS General The X-ray patterns reveal the catalysts as highly dynamic solids. especially in the early stages of synthesis. Structural disorder is more pronounced in the manganese rich catalysts Fe20. Fe40 and Fe53 and less pronounced for the iron rich compositions Fe85. Fe97 and FelOO. which are of better crystallinity. Crystallinity improves during FT synthesis for all catalysts. Because the patterns defy straightforward crystallographic description as simple mixtures of well defined phases. we have chosen to reproduce here characteristic photometer traces of a relevant portion of three patterns (FelOO. Fe8S. Fe20. Fig. 1) arranged in such a way as to illustrate phase developments with synthesis time (••• h-S) for each composition. Additionally. at the bottom of the set of traces for Fe85 and Fe20 a trace for the freshly reduced catalyst (designated "R") from earlier experiments is added. Although this sample was reduced outside the reactor in a separate furnace and cooled to room temperature. there is no reason to believe that the reduced catalyst in the reactor was any different.
.
342
FIGURE 1 Diffraction patterns of catalyst samples FelOO, Fe85 and Fe20 after different times of FT-synthesis (225 "C - 270 "C, 11 bar, H:CO:Ar = 2:l:l). Bottom trace (R) of Fe85 and Fe20 is the pattern of the reduced catalyst.
343
Fig. 2 and Fig. 3 depict the phase changes in the six catalyst samples in the form of bar diagrams. We have attempted to include information on the degree of intermixing between the phases by graphical representation within the bars (see caption of Fig. 3 and description of the individual compositions). Thus, the inclined boundary lines indicate solid solution between respective phases, with strong intermixing between the wustites leading to the hatched areas representing an unsegregated wustite phase. Fig. 4 shows the variation of the spinel lattice parameter with composition and synthesis time. Pure iron Sample Fe100 stands out from all other samples in that it develops carbide phase very early in the start-up process. After eighteen hours of synthesis it consists of ca 50 vol% o-carbide /27/ and of 50 %of a defect spinel phase as evidenced by the presence of the spinel-forbidden reflexions (200), (210) (both not shown) and (310), the unusually strong reflexion (400) /28/, and the low lattice constant of 0,8387(3) nm. Presence of elemental iron at this stage is highly improbable as judged from the absence of the iron (200) reflexion and from a MoBbauer spectrum /29/. After 41 hrs the carbide phase has changed its character from octahedral carbide to x-carbide /30/ (now ca. 40 %). Minor amounts ( 5 %) of elemental iron can be made out in the MoBbauer spectrum and from a very weak iron (200) reflexion (not shown). The spinel phase (ca. 55%) has markedly improved its crystallinity after 41 hrs. After 72 hrs, at the end of the start-up process, the carbide phase has again changed its character towards o-carbide and decreased to approx. 15% of the total sample volume, the remainder being spinel phase of further improved crystallinity. The consumption of carbide phase continues as the synthesis goes along and after 180 hrs the sample consists of a welJ crystallized spinel phase with a lattice constant of a = 0.8395(5) nm corresponding to almost perfect magnetite with little defect character /31/. Carbide content is not nil but barely detectable on long-time exposures. No indication of an Fe1_xO-wustite phase has been found throughout the synthesis process. 3%
manganese sample The addition of a small amount of manganese ions drastically changes the picture as compared to pure iron. Carbide phase formation in Fe97 (no pattern given) is suppressed early in the start-up process in favour of elemental iron with marked iron (110) and iron (200) reflex ions present. The freshly reduced sample is composed of 30% spinel phase and 70% iron which decreases to approx. 40% after 18 hrs. About 10% of o-carbide have then formed which, like in the Fe100-sample, changes towards x-carbide after 41 hrs but does not increase in mass while elemental iron predominates. At the end of the start-up phase, however, free iron has entirely disappeared and the sample is composed of slightly expanded spinel (a = 0.8406(2) ~
344
100
Fe100
80 60 40 20
Fe97 m ;,” ,“; ;zh Wlistite
fit3mongonese $I!t’n”ss$AFegoted 0
Spine1
q Housmonnite EJ Iron
q Iron
WI00
carbides
Fe85
u-l
6
2 80 Y zi m 60 40 1
20 FIGURE
2
Phase composition
60
100
of iron-rich
IL0
samples
180
thl
rich
345
nm, Fig.2, 85 %) and o-carbide (15 %). This distribution of phases now remains unaltered until the end of the synthesis, where the spinel lattice has shrunk to a = 0.8400(2) nm, showing marked contributions from a defect (or interstitial) lattice superstructure /28/. 15% - manganese sample Stabilization of elemental iron as the major reduced phase during the early stages of synthesis is further enhanced as the manganese content increases to 15%. The reduced sample is a well crystallized spinel (.0.8433(3) nm) with only traces of wustite phase and no apparent iron content. Yet we know from other work /24/ that elemental iron may well be present in a non-detectable form (see discussion). After eighteen hours of synthesis this elemental iron has manifested itself by comprising 20 %of the sample mass while the spinel lattice has shrunk to a = 0.8407(3) nm. Carbide diffraction lines are barely visible. After 41 hrs., iron has further increased to 35% along with some carbide formation (ca. 5%). The spinel phase now amounts to half the sample mass with a recovered lattice constant of 0.8438(8) nm, (Fig.4), and an iron rich wustite phase (a = 0.438(1) nm) /32/ accounts for the remainder 10-15% of the sample. At the end of the start-up phase elemental iron has been consumed almost entirely and the operating catalyst is made up of 60-70% spinel (a = 0.8429(7)nm), 25-30% wustite (a = 0.438(1) nm) and some carbide. As synthesis continues spinel phase increases so~ewhat at the expense of wustite phase which finally amounts to 15-18% and has become enriched with manganese (a = 0.441(1) nm). No more elemental iron and only traces of carbide can be detected at this point. 47% - manganese sample X-ray patterns of sample Fe53 (not shown) ar~ hardest to interpret. Already the freshly reduced oxide mass is an ill-defined mixture of spinel (possibly with contributions from tetragona11y distorted hausmannite /33,34/) and wustite, with poor overall crystallinity. After eighteen hours Of synthesis this picture has hardly changed. One should note that unusually long exposure times (70 hrs) with standard species or very thick mounts (0.3 mm) are necessary to obtain the patterns. The broad peaks can best be explained as a superposition of spinel, wustite and hausmannite lattices with a high degree of structural disorder. Only traces of elemental iron « 5%) are X-ray-detectab1e. High-angle ref1exion bands may be interpreted as arising from two separate wustite phases of a = 0.432(1)nm and a = 0.444(1)nm, e.g. almost pure FeO and MnO. As the start-up process proceeds, after 41 hrs, carbide formation becomes apparent as well as elemental iron (ca. 10% each). Hausmannite is no longer present and the wustites have merged into a solid solution (ca. 55%) of (Fe,Mn)O with a = 0.4388(5) nm. Spinel phase (ca. 25%) has consolidated into a lattice of a = 0.8483(12) nm, e.g. nearly MnFe204. As the start-up process is completed, the wustites have segregated again (a = 0.437(1)
346
~Wustite manganese
rich
~~r%&goted 0
Spine1
q Hausmonnite Q
Iron
m
Iron
corbldes
FIGURE 3 Phase composition of manganese-rich samples. Inclined boundary lines within the histograms indicate degree of intermixing between respective phases. Interpenetrating areas of wustites form symbol for unsegregated wustites (cross-hatched fields).
347 nm and 0.442(I) at the start solution
nm). We now do not find the nearly
(indicated
wustites
by the inclined
of which
over the manganese-rich These
wustite
manganese and/or
phases
phase
thus exhibit more
slightly
(35%) in mass
of approximately
especially
towards
as before.
and the spine1
wustite
(a = 0.8472(7)
has lost
Traces
in manganese
to reach
unsymmetrical
the iron-rich
overexposed
of iron
the spine1
nm). The wustite
are still rather
segregation
of two
reflexions.
phase
nm accordingly.
and also enriched
region,
Even on grossly
by a factor
the end of the synthesis
MnO.63Fe2.3704
in the high-angle
a tendency
abundant
is abundant
of a = 0.8455(g) Towards
that were present
from the high-angle
to 75% of the sample
can only be suspected.
has increased
a composition peaks,
amount
, as determined
pure phases
line in Fig. 3), but two solid-
the iron rich wustite
wustite
ions to form a lattice
carbide
boundary
patterns
and
wustite
being
no iron or carbide
can be found.
60 % - manganese
sample
The Fe40-sample
(no pattern
the X-ray pattern of two wustites duction), nounced
(also the principal
spine1
and hausmannite.
as with the Fe53-sample
the spine1
composition
of 0.8419(5) ganese
content
(IO-15%)
constant.
has almost
solution
After
increased
to ca. 30%
elemental
iron is detectable.
in solid
solution
position
unchanged.
80 % - manganese
phase
(a = 0.4419(5)
of carbide
nm) with
the remainder
comprising
(a = 0.442(l)
is slightly wustite.
phases
in excess
Spine1
has
are left and no
the wustites
are found
of the crystal
phase
com-
(a
nm, 60-70%)
sample only one manganese-rich
in the early
(~18%).
and has been consumed dominates
traces
remain-
nm). Haus-
spine1
are still separate
At the end of the synthesis
constant
as separate
nm and 0.4395(5)
wustite
and
have been formed
appear
than the iron-rich
nm). Only
lattice
thereafter
to less than 5 % with
72 hrs the wustites
(a = 0.8477(g)
With the Fe20-sample can be observed
iron and carbide
is as pro-
amounts),
after 41 hrs the man-
(a = 0.8475(8)nm),
(viz. 0.4415(10)
crystallinity
nm, equal
The spine1
(60-65 X) still
and amounts
after re-
of the two wustites
near MnO.28Fe2.7204;
in that
as a superposition
iron and spine1
nm vs.O.443(1)
nm) but now the manganese-rich
and seems to be of better
to the Fe53 sample
little
far from MnFe204.
Also elemental
solid
nm vs. a = 0.438(l)
plus
to MnO.65Fe2.3504
disappeared
similar
must be described
The segregation
after 41 hrs and the wustites
the rest of the sample.
phase
phases
a composition
is enriched
but have approached mannite
is quite
(0.434(l)
is initially
nm indicates
ing essentially
spine1
shown)
after 18 hrs of synthesis
stages
The hausmannite
along with phase
wustite
hausmannite
diminishes
at the end of the start-up
the picture
it is the only X-ray-visible
at this composition. component
q
0.4429(3)
(15-25%)
and only
to ca. 5% after
process.
Naturally
In the freshly
and shows just a slight
forty
little hours
the wustite
reduced
tendency
catalyst
of
segre-
348
gation
into two separate
lower d-values form, since
(Fig.1).
phases
as evidenced
Still elemental
after several
a mixture
ed field
in Fig. 3). The tailing
of wustite
the sample, conditions wustite rate
this
tes, but the separation
lattice
one finds
parameter
out the early apparent
Only
although
phase
amounting
experiments
MoBbauer
showed
/6/. The observed
catalysts
suggests
remains
these carbides
or to the method
satisfactorily
at present.
the
to sepa-
poorly
period
wusti-
At the end of the
phase
(85%) with defined
does
through-
pure magnetite
parameter
with
little
any carbides,
to an appreciable
degree
in
may be due to non-crystallinity
differences
of investigation
a
it become
lattice
films do not reveal
their presence
Instead,
a tendency
to ca. 15%. The almost
ions. Long-exposure
of manganese
by the reaction
speak of two separate
as the major
phase
cooling
even more pronounced
the Fe40-sample.
wustite
nm. The spine1
nm from the (220)-reflexion
incorporation
and exhibits
after the end of the start-up
as a well crystallized
of 0.8399
similar
of 0.4420(5)
stages.
as with
and dash-
phase without
72 hrs this becomes
solid-solution
in Fig.1
iron be observed.
nm) and one might
is not as clear
again
active
catalyst
has then disappeared.
iron is inhibited
can elemental
to be structurally
reduced
peak
after the reduction
after 41 hrs. After
nm vs. a = 0.4391(I)
(a = 0.443(l)
(dashed
reflexions
of elemental
in the process
continues
into two phases
synthesis
continues
recrystallization
and nowhere
phase
of the wustite
off to
in a non-detectable
the freshly
(80%) and iron (20%)
process
tailing
iron must be present
days at room temperature
exhibits
Yet, as the synthesis
by the reflexions
or both and cannot
of
be explained
DISCUSSION
The following formation phase
behaviour
wustite-spine1 structure
General
first outline
of the individual interaction
the general
in FT iron/manganese samples
as a major
will be examined
factor
with hydrogen
factors
involved
oxide catalysts.
and subsequently
in the buildup
at 300 oC will
in the
Then the the
of the bulk-phase
be dealt with.
considerations interactions
in determining
First
there
catalyst.
which
structural
the behaviour
active
temperature.
teraction
between
is the wustite
the catalytically given
will
phases
after reduction
Two main tant
discussion
of crystalline
influences
of the individual
spine1
"iron"
Secondly,
components
interaction
during
there activity
reduction
as impor-
catalyst.
which
influences
and subsequent
is the iron/iron and possibly
can be identified
carbide
deactivation
the nature
synthesis
- surface
of
at a
carbon
of the operating
in-
349 Paramount mixture
to the overall
at this temperature
manganese
reduction
hausmannite
ite thus produced temperatures
Its ability
to form solid
to either
tween
matrices
that of ferrous
is a source
ions, which
Thus,
solution
manganowustite
an existing
the wustite
external commodate /36/.
"elemental"
cal cubic
spine1
"protected"
suggested
in conjunction
small microdomains
with
/40,41/,
which
in
than
grain boundary
an existing
solid-
manganese
ions
manganese
phase
ions
of changing
are able to ac-
lines of iron conform compatible
having
should
iron ammonia
can occur
ions is higher
can accomodate
with
to a hypotheti-
the spine1
exsolutions
by the oxide matrix
without
be-
level as well as in bulk intergrowths
nm, easily
if such exsolutions
into haus-
interaction
under the influence
of mere 1.3 %. Conceivably "shielded"
variab-
on one hand and
precipitations
and wustite
low
of a(-Fe203.
distortion
solid-state
phase
both spine1
that the diffraction
to carbidization
is not yet clear,
superstructures
field by extracting
and segregate
iron on a sub-lattice
or structurally
product
only to the nearest
the spine1
unstable
difference
less susceptible
is structural
there
reduction
of the manganese
migrate
iron lattice of a = 0.8356
a volume
Third,
phase will be able to drive
Additionally,
It is noteworthy
within
usually
spoken,
become
conditions.
at these
that (Fe,Mn)O
out of its stability
it or, alternatively
should
stable
of continuous
It is well known
of
iron wustite
and/or
and that the mobility
formation
of manganowust-
possible
variants
intermediate
is the possiblility
with
conditions.
as another
oxide
is the lowest possible
ions up to the point of tetragonal
hand
/35,36/.
from
solutions
form defect
the structures.
Fe304-Mn304
phase
manganese
on the other
of an iron/manganese
by thermodynamics, Next there
under non-equilibrium
to incorporate mannite
allowed
notwithstanding.
ility of the iron spine1
characteristics
is the fact that manganowustite
product
spine1
reduction
and will therefore
to loose catalytic
be termed
catalysts
/37-39/,
be
activity.
"paracrystalline",
can also be invisible
lattice
of iron will be
It
as was
or if they occur
for-.-X-rays. depending
as on
size. Finally,
as a determining
ture of the FT-reaction degree
of carbon
Iron-rich
monoxide
samples
Fe100
In the case of pure lattice
instabilities,
with elemental all oS.the
factor
itself,
while
stability,
or oxidative,
iron oxide with no manganese a highly
mentioned
carbides
the synthesis.
As carbon
dering
a distinct
to form
phase
there
is the na-
depending
on the
and Fe97
defective
iron not all of which
reasons
reductive
conversion.
above.
the -wary start of the FT synthesis decreases
for crystal
being
phase
to compensate
is formed
may be visible This elemental
freely
high activity during
carbidized
the reductive
to minor
from
of the catalyst
in the iron lattice,
is very much subject
and
together
beam for one or
iron is readily
iron compete
rather
for valence
by reduction
to the X-ray
and the initially
and elemental atoms move
spine1
changes
phase of
their
or-
in external
350 8,!3l ._...._..... _.._..._......_.. .._,.,,..._,.___._..
MnFe20L 8.L9 t
P. Fe4_..-..4 i' _~_~_.._-o-._-..-.-~'-"P !; 1.
au8b7-
5 &6c ”
20
FIGURE
4
Lattice
conditions. carbon
Also,
atoms
carbide
constant
a carbide
Under the conditions
iron phase
gradually
It is, nevertheless,
spine1
phase
defect
spine1
drops
off again
due either
during
chosen
spine1
the reductive
the value
interstices
the
may again
/24/.
some time before
oxidative
and the carbidic
as synthesis phase
lattice parameter
the low lattice
phase when carbide
for pure Fe304. the spine1
or defects
continues.
Addition lattice
of the spine1
We
itself at this of the
constant
formation
at the end of the start-up
affect
loose
of iron/iron
publication
of the spine1
to follow
synthesis
discussion
in this study,
(Fig. 4). For Fe100
in Fe97 does not strongly to filling
near the surface
the gas phase becomes
that of pure magnetite to approach
phase during
A more detailed
activity
A 180
160
in a forthcoming
instructive
synthesis
increases
spine1
into a stable
catalytic
point.
gest and exceeds
phase
transforms
from discussing
100 120 1LO TIME Ihl
once formed
reaction.
of FT synthesis
of the start-up
manganese
phase
will be presented
completion
during
60 80 SYNTHESIS
of iron-manganese
to the synthesis
interaction
refrain
LO
of the is stron-
phase.
It then
of small amounts constant.
lattice with
This
of
is
the large
351 Mn2+-ions
or to incorporation
not differ currently zation pure
much being
performed
and competition
iron sample,
Thus,
for manganese
phases, oxide
manganese
structure
lence states
samples
samples
high
additional
clearly
spine1
lattice
of synthesis wustite Mn2+
than
ill-defined tected would
lattice,
lattice
iron/iron va-
structural
steep
is well
modifier.
due to its possible
would
especially
between
The "burning"
of the spine1
expansion
will
lattice,
be entirely
fore and because
of loss of Mn2+
of this
in.Fe40
than
in Fe53
Contributions
in Fe53 despite
from carbide/
the ongoing
is sufficiently cannot
due to manganese
is
to stabi-
so they cannot
during
the of
if they occur,
drop OS the lattice
ions to the hausmannite
is smaller
For the stages
incorporation
high enough
but these,
this drop-off
on the
in the early
increase
becomes
of these
the
is not as
18 and 41 hrs while
and are also of poor crystallinity,
for the subsequent
character,
at present.
successive
component.
be excluded,
leads
as an
of phase behaviour
as the absolute
third
defect
reduction
content
This
for manganowustite
in accord with
the manganese
ions becomes
of wustite
be too speculative
increase
is encountered
of mangenese
and to segregation
In Fe40, where carbide formation
increase
of separate
of the basic
lattice parameter
so that considerations
in Fe53 cannot
an explanation
in an expansion
spine1
phase
constant
as an intermediate
in the X-ray patterns.
41 and 72 hrs. show
This
as "phases"
offer
of the spine1
and iron content
by a rather
incorporations
and
is stopped.
lattice.
we first find a drop after
in Fe40, where
lize hausmannite carbon
stages.
phase
of ions in different
Here the concentration
parameter
lattice
constant
into the spine1
Carbidi-
the early
the major
segregation
modifier
its role as a direct
as for spinel,
followed
/42/.
way as with the
by the time the experiment
the distribution
Unfortunately.
lattice
phase develops.
stronger
variation
on the spine1
constituent.
defined
during
becomes
too small to achieve
influencing
to excert
basis of the wustite
in a similar
in excess
acts as an electronic
Fe53 and Fe40.
influences
between
on this point
by oxidation
ideal magnetite
the ionic radii do XPS experiments
- role of manqanowustite
different
Fe85,
sufficiently
relation
the spine1
over the sites of the spine1
A strikingly
to distinct
information
and iron proceeds
concentrations clearly
more
where
configurations;
iron being clearly
almost
by mainly
Manganese-rich
with
may yield
continues
to approach
for Fe 3+-ions,
and low-spin
of carbides
elemental
As the FT-reaction re-orders
of Mn3+-ions
for the high-spin
synthesis
constant
suppressed occur,
between not to
and the
incorporation. phase,
are be de-
There-
the absolute
the higher
value
manganese
content. For the Fe20 ly defined
takes notice rameter,
sample
of its final value,
one might
Pure Fe304
the spine1
as to allow meaningful
phase
during
determinations only sligbtly
the first
in excess
argue that for the iron poor Fe20
as a catalytically
active
component
eighty
hours
is too poor-
of its lattice constant.
occurs
of the Fe304
sample
segregation
If one
lattice
pa-
of almost
out of the basically
inert
352 wustite
matrix
/24/. The general
earlier
/5,21/
would
stant
in Fe85,
the wustite persistent
support
phases
as to their
the start-up
final degree
of the spine1
ions and manganowustite)
hausmannite
component,
itself
ence to the formation Fe53 and Fe40
in activity
of this sample
this view. The variations
Fe53 and Fe40 after
interaction
manganese
increase
with
during
a spine1
of the mixed
and contributes
of the spine1
phase,
its manganese
/43/,
then reflect
modifiers
the now stable
variant
lends
spine1
(e.g.
synthesis
of
the
inherent
process.
its structural
early
The. influ-
in the synthesis
to the drop of the lattice constant
tion and the 18 h-observation
lattice con-
and the "undecidedness"
of salid solution,
iron-manganese
as observed
between
in
reduc-
point.
CONCLUSION
We have presented nese oxide sis that
a tentative
FT-catalysts,
is consistent
iron-manganese Two groups
spine1 phase
the crystal
ability
to form structurally
phase
composition active
position.
More data, especially
to extend
the model
and selectivity. .methods
alone,
and check
at different
The iron/iron deserves
of the manganese
special
carbide
during
be identi-
to the assumption phase
rich catalysts solutions
due to its com-
are needed
data on activity
is not accessible
further
strongly
of varying
of reduction.
with quantitative which
could
The wustite
temperatures
problem,
attention
solid
and synthe-
of the mixed
of these catalysts.
lend some support
(Fel_x,Mnx)O
its consistency
parameter
component
the synthesis.
of iron/manga-
phase
one manganese-poor,
Our results during
changes
the start-up lattice
as the major structural
component
affects
the phase
of the
one manganese-rich,
to this variation.
as an active
to explain
at 300 oC, during
with the variations
of catalysts,
fied according of spine1
model
reduced
investigations
by X-ray in this
respect.
ACKNOWLEDGEMENTS
We thank for running Lehrstuhl
Or. R. Malessa
#or preparing
the FT synthesis
fiir Mineralogie
experiments.
the catalyst Thanks
of the Ruhr-University
This work was.suppori.ed
by the Bundesminister
of the Federal
of Germany.
Republic
samples
and Mr. G. Beerwerth
also go to Prof. for providing
fiir Forschung
Florke
of the
XRD-facilities.
und Technologie
(BMFT)
353
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