www.elsevier.com/locate/apcata

Applied Catalysis A: General 310 (2006) 24–30

Studies on precipitated iron catalysts for Fischer–Tropsch synthesis

Hiroshi Hayakawa a,b,*, Hisanori Tanaka b, Kaoru Fujimoto a

a Department of Chemical Processes and Environment, The University of Kitakyushu, 1-1 Hibikino Wakamatsu-ku

Kitakyushu-shi, Fukuoka 808-0135, Japanb Wakamatsu Research Institute, Electric Power Development Co., Ltd., 1 Yanagisakimachi Wakamatsu-ku

Kitakyushu-shi, Fukuoka 808-0111, Japan

Received 8 February 2006; received in revised form 26 April 2006; accepted 27 April 2006

Available online 5 July 2006

Abstract

The silica-containing precipitated iron catalysts had higher STY (C5+ hydrocarbon productivity), but were more difficult to be reduced with

syngas (mixtures of hydrogen and carbon monoxide) than the silica-free precipitated iron catalyst. The results suggested that the added silicate

suppresses the crystal growth of hematite from the structure of catalyst precursor. X-ray diffraction (XRD) studies of catalysts after activation under

various conditions revealed that the presence of iron carbide (CFe2.5) was related to the active sites in the FT reaction, and that the relative quantity

of iron carbide (CFe2.5) to magnetite (Fe3O4) was closely related to the number of active sites available for FT synthesis. The effects of copper on

the silica-containing precipitated iron catalyst were to enhance the activity and the rate of reduction, as judged from the XRD and TPR studies.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Fischer–Tropsch synthesis; Iron catalyst; Reduction behavior; Iron carbide

1. Introduction

Iron catalysts have been used for the commercial Fischer–

Tropsch Synthesis (FTS) process, especially for synthesis of the

coal-derived syngas. Many studies on iron catalysts have been

reported in which efforts were focused on the activation of

precipitated iron oxide by different reductant gases, such as

syngas (H2/CO), hydrogen or CO.

Ma et al. evaluated the influence of syngas, H2, and CO on

silica-supported iron catalysts and found that H2-pretreated

catalysts gave products with lower concentrations of olefins,

whereas CO-pretreated catalysts gave products with higher

concentrations of olefins [1]. Luo and Davis evaluated the

influence of different reductants, reduction temperature and

pressure on the Fe/K/SiO2 catalysts, and found that the CO-

activated catalysts had the highest CO conversions, and H2-

activated catalysts had the lowest ones [2]. Bukur and Sivaraj

studied the reduction behavior on silica- and alumina-supported

iron catalysts. They found that alumina inhibits the reduction of

iron, and that the reduction behavior of the silica-supported

* Corresponding author. Tel.: +81 93 741 0942; fax: +81 93 741 0959.

E-mail address: hiroshi_hayakawa@jpower.co.jp (H. Hayakawa).

0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2006.04.045

catalyst was similar to that of the precipitated iron catalyst [3].

Mansker et al. investigated the active form of catalysts by XRD

analysis using quantitative Rietveld structural refinement, and

claimed that the active Fe catalyst contained hexagonal carbide

(Fe7C3) with a small amount of a-iron [4].

Dry reported that the addition of copper enhanced the rate of

reduction [5]. Luo et al. conducted studies on the effect of

palladium on iron catalysts and found that the FTS rate constant

of the Pd-promoted catalyst was lower than that of the Cu-

promoted catalyst [6].

The purpose of this study was to investigate the activation

behavior of precipitated iron-based catalysts, especially the

effects of added sodium silicate and copper oxide on the

reduction behavior and catalytic performance (activity,

selectivity and productivity).

2. Experimental

2.1. Catalyst preparation

All precipitated iron-based catalysts were obtained by

precipitation from an aqueous nitrate solution by adding an

aqueous solution of NH4(CO3)2; this method was similar to that

of Kunugi et al. [7]. The precipitates were filtered and washed

H. Hayakawa et al. / Applied Catalysis A: General 310 (2006) 24–30 25

Table 1

Physical properties of precipitated iron catalysts

Catalyst Component (on mass basis) BET results

Analyzed Surface area

(m2/g)

Pore volume

(cm3/g)Fe Cu K SiO2

Cat.A 100 0.7 2.2 – 55 0.330

Cat.B0 100 0 N/A N/A N/A N/A

Cat.B1 100 0.3 2.0 8.8 175 0.273

Cat.C0 100 0 N/A N/A N/A N/A

Cat.C1 100 1.2 N/A N/A 250 0.271

Cat.C2 100 2.8 N/A N/A 198 0.264

N/A: not analyzed.

Fig. 1. XRD patterns of the fresh catalysts (a) cat.A and (b) cat.B1.

with distilled water. The washed filter cakes were dried in air

and subsequently calcined in air at high temperatures.

Copper-promoted catalysts were prepared by mixing

aqueous solutions of Cu(NO3)2�3H2O and Fe(NO3)3�9H2O,

prior to precipitation. Potassium-promoted catalysts were

prepared by impregnating heated samples with an aqueous

solution of K2CO3. Silica-containing catalysts were prepared

by adding sodium silicate in a precipitation process.

2.2. Activation and reaction procedure

Activation in the slurry phase was performed in a 100 cm3

continuously stirred tank reactor (CSTR). Three grams of the

catalyst precursor were suspended in 50 ml of n-C16H34. The

system was purged with 150 cm3/min of N2, then

the pressure and temperature were raised to 0.5 MPa and

523 K, respectively, and these conditions were retained for

0.5 h, after which time the temperature was raised to 573 K

at a rate of 2 K/min. Subsequently, catalyst activation was

conducted by switching the feed gas from N2 to syngas.

Activation was carried out at 0.5 MPa, 573 K, and a H2/CO

ratio of 1.0; the space velocity (W/F) was 7.5 g h/mol for a

certain period. The stirring speed in the reactor was

1800 rpm.

Activation in the gas phase was performed in a fixed bed

continuous flow reactor. The quantity of catalyst used was 3 g.

Activation was carried out at 0.1 MPa, 573 K, and a H2/CO

ratio of 1.0; the space velocity (W/F) was 7.5 g h/mol.

Following the activation, the reactor was brought to the

desired reaction conditions, which were 2.0 MPa, 533 K, and a

H2/CO ratio of 1.0; here too the space velocity (W/F) was 7.5 g

h/mol.

Light wax and water were collected by a separator

(cold trap), and heavy wax was collected in the reactor

with the initial solvent. These were analyzed with an off-line

FID gas chromatograph (GC) (column packing: Silicone SE-

30). Effluent gas from the separator was analyzed with an on-

line TCD GC (column packing: Molecular Sieve 5A

and Porapak N) and FID (column packing: PLOT Fused

Silica).

2.3. Characterization of the catalysts

The surface area of the catalysts were measured by the BET

method with QUANTA CHROME AUTOSORB-1MP.

XRD patterns of catalyst samples were measured by a

RIGAKU X-ray diffractometer. The samples after reduction

and reaction for XRD measurements were obtained from the

reactor. The reduced and reacted samples were separated from

the wax product and solvent by filtration in a glove box before

being transferred to the XRD instrument.

The temperature programmed reduction (TPR) study was

performed by a TPD/R/O 1110 Catalytic Surfaces Analyzer.

The catalyst sample was heated and purged with He, and was

then reduced by heating in a flow of 5% H2/95% N2 from 323 to

1023 K at a rate of 10 K/min. Hydrogen consumption was

determined by TCD GC.

3. Results and discussion

3.1. Catalytic performances for precipitated iron catalysts

Precipitated iron catalyst (Fe/Cu/K) and silica-containing

precipitated iron catalyst (Fe/Cu/K/SiO2) were subjected to the

activity tests and the characterization studies. Their physical

properties and the structures of the catalysts before use are

shown in Table 1 and Fig. 1.

From the XRD results, the presence of hematite (Fe2O3) was

confirmed in the precipitated (silica-free) iron catalyst, while

there was no clear peak in the silica-containing catalyst. BET

results shows that the silica-containing catalyst had a much

larger surface area than the silica-free catalyst. These results

suggest that the silica-free iron catalyst contains large

crystallites of hematite. The added sodium silicate or silicate

H. Hayakawa et al. / Applied Catalysis A: General 310 (2006) 24–3026

Fig. 2. Comparison of CO conversion of cat.A for (~) reaction after activation

for 3h, (~) reaction after non-activation: catalyst activity at 260 8C, 2 MPa, H2/

CO = 1 and W/F = 7.5 g h/mol; catalyst activation at 300 8C, 0.5 MPa, H2/

CO = 1 and W/F = 7.5 g h/mol.

Table 2

Catalytic performances of precipitated iron catalysts

Catalyst W/F

(g h/mol)

CO conversion

(%)

Hydrocarbon C5+

selectivity (%)

STY

(g/kg/h)

Cat.A 7.5 85.9 70.3 297

Cat.B1 7.5 66.0 73.5 268

Cat.C1 1.25 30.1 74.5 773

2.5 48.2 74.5 564

7.5 80.1 81.6 336

suppresses the crystal growth of hematite, probably by existing

as the precipitated iron catalyst precursor in the FeOOH (which

is the main species of precipitates of iron). The above

phenomenon may be the reason why hematite was not observed

in the XRD pattern of the silica-containing catalyst. The results

of reaction tests are shown in Figs. 2 and 3.

The results of the reaction tests showed that the activity of

the catalyst after reduction (activation) was higher than that

of non-activated catalysts in all catalysts. Such an activation

procedure at high temperature was effective to enhance

the activity of precipitated iron catalyst. The activity of

the silica-free catalyst (cat.A) was stable from the initial

stage, while the activity of the silica-containing catalysts

(cat.B1 and cat.C1) gradually increased with TOS. The

catalytic performances of all catalysts are summarized in

Table 2.

The silica-free catalyst (cat.A) has higher activity than the

silica-containing catalysts (cat.B1 and cat.C1), whereas the

selectivity of C5+ hydrocarbon on cat.A was lower than that on

cat.B1 and cat.C1. Table 2 shows that cat.C1 had higher STY

Fig. 3. Comparison of CO conversion of cat.B1 for (~) reaction after activa-

tion for 3h, (~) reaction after non-activation, (^) cat.C1 for reaction after

activation for 3 h: catalyst activity at 260 8C, 2 MPa, H2/CO = 1 and W/

F = 7.5 g h/mol; catalyst activation at 300 8C, 0.5 MPa, H2/CO = 1 and W/

F = 7.5 g h/mol.

(C5+ hydrocarbon productivity) than cat.A, due to the high C5+

selectivity.

3.2. Reduction behavior of precipitated iron catalysts

The structures of silica-free and silica-containing catalysts

after reduction are shown in Figs. 4 and 5.

The XRD results of the reduced catalysts (reduction

conditions for 3 h in the slurry phase) showed that iron carbide

(CFe2.5) was present in all reduced catalysts. As all catalysts

have high activity after reduction, the activated phase can be

observed as iron carbide (CFe2.5). Shroff et al. have also shown

that, as the catalyst becomes more active for FTS, iron carbide

is observed [8]. All of the hematite (Fe2O3) in cat.A was

reduced and rapidly transformed into active sites. As the

reduction of the catalyst proceeded, iron oxide was transformed

from Fe2O3! Fe3O4! a-Fe or iron carbide. a-Fe could not

be observed in our reduced catalyst, because the metallic iron

Fig. 4. XRD patterns of (a) fresh cat.A and (b) cat.A reduced in the slurry phase

for 3 h: reduction at 300 8C, 0.5MPa, H2/CO = 1 and W/F = 7.5 g h/mol.

H. Hayakawa et al. / Applied Catalysis A: General 310 (2006) 24–30 27

Fig. 6. XRD patterns of cat.B1 (a) fresh, (b) after reduction for 0.5 h, (c) after

reduction for 1 h and (d) after reduction for 3 h: reduction at 300 8C, 0.5 MPa,

H2/CO = 1 and W/F = 7.5 g h/mol.

Fig. 5. XRD patterns of cat.B1 reduced in the slurry phase (a) fresh, (b) reduced

for 3 h, (c) reduced for 10 h and (d) reduced for 20 h: reduction at 300 8C,

0.5 MPa, H2/CO = 1 and W/F = 7.5 g h/mol.

was fairly reactive to carbon dissociated from carbon

monoxide.

Based on the chain growth mechanism of FTS, production of

carbides! hydrogenation of carbides! production of carbe-

ne! polymerization of carbene, [5,9], i.e. the FTS reaction

proceeds by the hydrogenation of carbides to form carbene. It is

suggested that a catalyst that has much iron carbide on its

surface will have the high activity and C5+ selectivity. The

results of the XRD studies indicate that the carbide on the

catalyst surface exists as CFe2.5.

Some models for FT reaction with the role of iron carbide

have been proposed. The literature suggests that the iron

carbide exists in surface and bulk of catalyst, and that the bulk

carbide is not necessarily the active phase, but carbide

formation need to occur before the catalyst surface can retain

enough carbon to become active [8].

In the case of reduced cat.B1, both magnetite (Fe3O4) and

iron carbide (CFe2.5) were observed. This indicates that

magnetite (Fe3O4) was unable to be reduced completely even

after 3 h reduction in the slurry phase, due to the presence of

Table 3

XRD results of the composition of reduced cat.B1 in the slurry phase with time

XRD result Reduction time (h)

0 3.0 10.0 20.0

Peak ratio of CFe2.5/Fe3O4 – 0.264 0.324 0.338

silica in this catalyst. The reason why the silica-containing

catalyst was difficult to be reduced could be because of the

strong interaction between the silica and iron oxide whose

crystallites are suppressed and exist in the silica network.

The results given so far indicate that the silica-containing

catalyst had the characteristic of being hard to reduce. As the

magnetite phase becomes more active transforms gradually to

a-Fe, the iron carbide phase. We assumed that the increase in

iron carbide corresponded to the appearance of active sites for

FTS. We observed the reduction behavior of the silica-

containing catalyst after activation under sufficiently reducing

conditions. The degrees of reduction in these samples are

shown in Table 3, calculated by the relative amounts of CFe2.5

and Fe3O4 on each peak base. From these results, we see that

the reduction proceeds with time, the amount of Fe3O4

decreased relative to CFe2.5.

Moreover, the reduction behavior of the silica-containing

catalyst was investigated under different conditions, such as the

shorter reduction times and other environments (slurry/gas

phase). The results of XRD analysis of the reduced samples

within 3 h in the slurry phase are shown in Fig. 6, and the

degrees of reduction are shown in Table 4.

As the reduction time became longer, the degree of reduction

and the activity increased, similar to the above results for over

3 h reduction tests. We found that the reduction proceeds

gradually within 3 h in the slurry phase. This indicates that the

presence of iron carbide (CFe2.5) is related to the active sites in

the FT reaction.

The XRD patterns of catalysts reduced for 3/10 h in the gas

phase are shown in Fig. 7. And the degree of reduction in these

samples is shown in Table 5, calculated by a similar method to

that given above. These results show that, as the reduction time

Table 4

XRD results of the degree of reduction for cat.B1 in the slurry phase

XRD result Reduction time (h)

0.5 1.0 3.0

Peak ratio of CFe2.5/Fe3O4 0.043 0.079 0.104

H. Hayakawa et al. / Applied Catalysis A: General 310 (2006) 24–3028

Fig. 7. XRD patterns of cat.B1 reduced in the gas phase (a) fresh, (b) reduced

for 3 h and (c) reduced for 10 h: reduction at 300 8C, 0.1 MPa, H2/CO = 1 and

W/F = 7.5 g h/mol.

Table 5

XRD results of the composition of reduced cat.B1 in the gas phase with time

XRD result Reduction time (h)

0 3.0 10.0

Peak ratio of CFe2.5/Fe3O4 – 1.870 3.286

Fig. 8. Comparison of CO conversion for cat.B1 after (^) reduction in the

slurry phase for 3 h, (~) reduction in the slurry phase for 10 h and (&)

reduction in the gas phase for 3 h: catalyst activity at 260 8C, 2 MPa, H2/CO = 1

and W/F = 7.5 g h/mol; catalyst activation in slurry phase at 300 8C, 0.5 MPa,

H2/CO = 1 and W/F = 7.5 g h/mol; catalyst activation in gas phase at 300 8C,

0.1 MPa, H2/CO = 1 and W/F = 7.5 g h/mol.

Table 6

Effect of different reduction conditions for cat. B1

Catalyst B Results (2 MPa, 260 8C, W/F = 7.5)

CO conversion

(%)

Hydrocarbon

selectivity (%)

STY

(g/kg/h)

CH4 C5+

RD-s3 66.0 4.1 73.5 268

RD-s10 69.9 6.5 60.9 215

RD-g3 81.3 6.3 59.8 247

RD-s(g)x: reduction for x h in the slurry(gas) phase.

was increased, the amount of Fe3O4 decreased relative to

CFe2.5. Catalytic performances under different reduction

conditions are shown in Fig. 8 and Table 6.

Comparing the activity of the gas phase with the slurry phase,

we see that the activity of the sample that was reduced in the gas

phase was higher and more stable initially. The sample reduced

for 3/10 h in the slurry phase slowly increased to stable activity.

The activity transition corresponds to the degree of reduction (the

ratio of iron carbide to magnetite) as given by XRD analysis.

Such data imply that the relative quantity of iron carbide is

closely related to the number of active sites available for FTS.

The catalytic activity after the gas phase reduction was

higher than the activity after the slurry phase reduction, due to

high degree of reduction, as shown in Tables 3–5, which

corresponds to a large amount of active sites.

On the other hand, the CH4 selectivity of the sample reduced

in the gas phase was quite different than that reduced in the slurry

phase for 3 h. These results suggest that the manner in which the

reduction is carried out has an impact on the selectivity in FTS.

3.3. Effect of copper on the reduction behavior of

precipitated iron catalysts

It has been reported that the addition of copper enhances the

rate of reduction of iron catalysts for FTS [5]. We inspected the

effect of copper on the reduction behavior of silica-containing

catalysts. Four samples of silica-containing catalysts were used

in this study. Cat.B0 and cat.C0 were copper-free catalysts,

Fig. 9. XRD patterns of (a) cat.B0, (b) cat.C0, (c) cat.B1 and (d) cat.C1, after

reduction: reduction at 300 8C, 0.1 MPa, H2/CO = 1 and W/F = 7.5 g h/mol.

H. Hayakawa et al. / Applied Catalysis A: General 310 (2006) 24–30 29

Table 7

Effect of copper-promoted precipitated iron catalysts

Catalyst Results

CO conversion

(%)

Hydrocarbon

selectivity (%)

STY

(g/kg/h)

a

CH4 C5+

Cat.A 85.9 4.9 70.3 297 0.805

Cat.B0 34.1 6.8 53.2 114 N/A

Cat.B1 66.0 4.1 73.5 268 N/A

Cat.C0 45.4 7.4 50.9 125 N/A

Cat.C1 80.1 2.1 81.6 336 0.865

N/A: not analyzed.

Table 8

Quantitative TPR results

Catalyst Content

ratio of Cu/Fe

Peak area ratio Rate of reduction

RT to 800 8C (%)Theoretical Experimental

Cat.C0 0 8.00 2.57 39.6

Cat.C1 0.012 7.52 2.87 45.0

Cat.C2 0.028 6.97 3.43 54.8

whereas Cat.B1 and cat.C1 were copper-promoted catalysts.

The XRD results of samples reduced for 3 h in the slurry

phase are shown in Fig. 9, and Table 7 gives their catalytic

performance.

The activity on the copper-promoted catalyst, cat.B1, C1,

was higher than that on the copper-free catalyst, cat.B0, C0,

because copper-promoted catalyst includes a larger amount of

active sites than copper-free catalyst, due to the effect of

promoting by adding copper.

The results of the XRD analysis showed that Fe3O4 was

present in both copper-promoted and copper-free cat.B, while

Fe3O4 was not present in either copper-promoted or copper-free

cat.C. Cat.B and cat.C with variations in the other compositions

(K, SiO2) of catalyst were also examined. The peak of iron

carbide in the promoted catalyst was different from that of the

copper-free catalyst. Hexagonal iron carbide (Fe7C3) appeared

in the reduced cat.B0 and cat.C0 (copper-free), but was not

present in the reduced cat.B1 and cat.C1 (copper-promoted).

This result suggests that Fe7C3 is not required for catalytic

activity (being different than CFe2.5), and so copper may have

an effect on the formation of active sites of a catalyst

(producing different carbides), as well as promoting the

reduction.

ATPR study was carried out with copper-promoted catalysts

containing different amounts of copper. Fig. 10 shows the TPR

profiles. The profiles show two peaks associated with reduction

of oxides.

Fig. 10. TPR profiles of (a) cat.C0, (b) cat.C1 and (c) cat.C2 (different contents

of Cu/Fe).

It has been postulated that the first peak corresponds to

reduction of Fe2O3 to Fe3O4, and the second peak

corresponds to subsequent reduction of Fe3O4 to metallic

iron [10–12]. Bukur and Sivaraj suggested that only a small

portion of iron oxide is reduced to metallic iron during the

first peak, with most of the Fe3O4 being reduced during the

second peak [3].

Our results show that as the amount of copper was increased,

the temperature of the first and second peaks shifted to the

lower values. Especially, copper has a marked effect on the

reduction of Fe2O3 to Fe3O4 (first peak). Quantitative results of

hydrogen consumption are shown in Table 8. Assuming that the

first peak area corresponds to the reduction of Fe2O3 to Fe3O4

and CuO to Cu, and the second peak area corresponds to the

reduction of Fe3O4 to Fe, we concluded the experimental value

of the peak area ratio (Table 8). The rate of reduction was

determined from this experimental value. Compared to the

theoretical value, the experimental value was quite small. These

results are similar to those of a previous study of supported iron

catalysts [3], and suggest that a small amount of Fe2O3 was

reduced to metallic iron. The results clearly show that the effect

of adding copper to silica-containing precipitated iron catalysts

enhances the rate of reduction.

4. Conclusions

Our studies on the precipitated iron catalyst allow us to

conclude the following.

The silica-containing catalyst (Fe/Cu/K/SiO2) has quite a

different structure, in which iron oxide are not observed by

XRD, from the structure of the silica-free catalyst (Fe/Cu/K) in

which hematite is observed definitely.

The silica-containing catalysts have higher STY values (C5+

hydrocarbon productivity) than the silica-free catalysts after

activation procedure at high temperature. Without any

activation procedure, the activities of both catalysts are lower

at initial stage, and then increase gradually.

The XRD results of catalysts after activation show that the

silica-free catalyst transforms from hematite to iron carbide,

whereas the silica-containing catalysts transform to magnetite

and iron carbide. This indicates that silica-containing catalysts

are more difficult to be reduced than the silica-free catalyst.

These results suggest that the iron oxide particles which exist in

the silica-containing catalysts have strong interactions with the

added silica.

The activation (reduction) condition (slurry/gas phase)

impacted the activity and methane selectivity.

H. Hayakawa et al. / Applied Catalysis A: General 310 (2006) 24–3030

According to the results of XRD patterns of copper-

promoted and copper-free precipitated iron catalysts, the

hexagonal iron carbide (Fe7C3) that only appeared in the

reduced sample of the copper-free catalyst might be less active

than CFe2.5.

The TPR results show that the reduction of silica-containing

material is facilitated by the addition of copper.

References

[1] W. Ma, Y. Ding, V.H.C. Vazquez, D.B. Bukur, Appl. Catal. A 268 (2004) 99.

[2] M. Luo, B.H. Davis, Fuel Process. Technol. 83 (2003) 49.

[3] D.B. Bukur, C. Sivaraj, Appl. Catal. A 231 (2002) 201.

[4] L.D. Mansker, Y. Jin, D.B. Bukur, A.K. Datye, Appl. Catal. A 186 (1999)

277.

[5] M.E. Dry, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and

Technology, vol. 1, Springer-Verlag, 1981, p. 159.

[6] M. Luo, R. O’Brien, B.H. Davis, Catal. Lett. 98 (2004) 17.

[7] T. Kunugi, T. Sakai, T. Ose, Y. Hamada, Kogyo Kagaku Zasshi 69 (1966)

2245.

[8] M.D. Shroff, D.S. Kalakkad, K.E. Coulter, S.T. Kohler, M.S. Harrington,

N.B. Jackson, A.G. Sault, A.K. Dayte, J. Catal. 156 (1995) 185.

[9] L. Nowicki, S. Ledakowicz, D.B. Bukur, Chem. Eng. Sci. 56 (2001)

1176.

[10] R. Brown, M.E. Cooper, D.A. Whan, Appl. Catal. 3 (1982) 177.

[11] A.J. Knock, H.M. Fortuin, J.W. Geus, J. Catal. 96 (1985) 261.

[12] I.S.C. Hughes, J.O.H. Newman, G.C. Bond, Appl. Catal. 30 (1987)

303.