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).

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