ELSSVIER Solid State Ionics 101-103 (1997) 697-705

Reduction of MgO-supported iron oxide: formation and characterization of Fe /MgO catalysts

Gary Bond’, Kieran C. Molloy, Frank S. Stone*

School of Chemistry, University of Bath, Bath BAZ 7AY, UK

Abstract

Fe-oxide/MgO (5-30 mol% Fe) was prepared by calcination of coprecipitated precursors. Reducibility in H, at 693 K

was measured gravimetrically and volumetrically. All samples yielded metallic iron (Fe”) suitable for catalytic use but much unreduced Fe*+ remained, stabilized by the MgO. The mean particle size of the supported metallic iron was determined by X-ray line-broadening and its surface area by CO chemisorption. Mossbauer spectroscopy confirmed the presence of Fe’, Fe’+ and Fe3+ in the reduced material.

Keywords: Iron; Iron oxide; Reduction; Magnesium oxide support

Materials: FelMgO

1. Introduction

The reduction of oxide-supported iron oxide at-

tracts attention because of the widespread use of iron as a catalyst. A high surface area support can influence the catalytic activity and selectivity of iron

by both physical effects, such as control of particle

size or crystal morphology, and specific chemical

effects, such as control of the oxidation state. Although the presence of a support invariably im- pairs the reducibility of the iron, this is often more

than offset by the increase in active surface area and

benefit for the catalysis of the chemical environment imposed.

*Corresponding author. Fax: + 44-1225-826-231.

‘Present address: Department of Chemistry, University of Central

Lancashire, Preston PRl 2HE, UK.

The subject has both fundamental and applied

interest. The first detailed study of the reduction of MgO-supported iron oxide (Fe-oxide/MgO) was that

of Boudart et al. [I] in the context of iron as a catalyst for ammonia synthesis. Although less than half of the iron in Fe-oxide/MgO with loadings up to

16% Fe could be reduced to the metallic state (Fe”)

at 700 K, the size of the Fe” particles was limited to

15 nm or less and the relation between surface structure and activity could be examined. Later

studies of the reduction of Fe-oxide/MgO [2-61

have been orientated not only towards supported metallic iron, as a catalyst for hydrogenations [2,5,6],

but also towards the partially reduced system, a

catalyst for dehydrogenation [3,4]. In both cases there is a compromise to be found in reducing sufficiently to generate the optimum number of active and selective sites without unduly sintering. This brings the challenge to understand the reduction

0167.2738/97/$17X@ 0 1997 Elsevier Science B.V. All rights reserved.

PII SO167-2738(97)00179-3

698 G. Bond et al. / Solid State Ionics 101-103 (1997) 697-705

mechanism and the physical and chemical reasons for the optimization.

The later studies listed above [2-61 have con-

firmed the initial findings [l] that in contrast to bulk

Fe,O,, which can be reduced completely to metallic iron at 700 K, only a fraction of the Fe3+ in MgO-

supported Fe-oxide can be reduced to Fe” at this temperature. However, opinions differ as to which

phases are present together with metallic iron in the

reduced material and the extent to which their

amounts depend on the iron loading. This is not

surprising, since a variety of precursors have been used and different calcination conditions adopted.

Calcination of Fe-oxide/MgO is potentially able to

produce MgFe,O, as well as Fe,O,, and reduction

of MgFe,O, is then capable of forming the solid

solution Fe,Mg , _,O, thereby stabilizing some of the

iron as Fe’+ and leading to the formation of very

finely-divided iron. This has been especially well

highlighted by the work of Stobbe et al. [4].

In the present study we contribute to the subject

by preparing Fe-oxide/MgO by different routes and at several different iron loadings. We compare the

extents of reduction achieved under standard con-

ditions and use a combination of techniques to

investigate the metallic iron in the reduced products.

We have employed X-ray diffraction and Mossbauer

spectroscopy for phase analysis, gravimetric and volumetric analysis for extents of reduction, and

selective chemisorption for estimating the accessible surface area of the resulting metallic iron. We report

elsewhere [7] on the use of the solids for the iron-

catalyzed hydrogenation of nitriles to primary

amines, a reaction in which we have exploited the

particular selectivity advantages of having basic

MgO as the support.

2. Experimental

2.1. Preparation of oxides

MgO-supported iron oxide, with iron loadings in the range 5-30 mol% Fe has been prepared by two

methods, both based on coprecipitation procedures. Preparation was performed as follows:

(a) Carbonate coprecipitation (CCP). 1 M solu-

tions of Fe(NO,), and Mg(NO,), (BDH) were

mixed in appropriate ratios and heated to 360 K. 1 M (NH,),CO, solution was added drop-wise until precipitation was complete. After washing and fil-

tering, the precipitate was dried at 373 K and calcined to the oxide at 773 K.

(b) Hydroxide coprecipitation (OH). A mixture of

Fe(NO,), and Mg(NO,), solutions as in (a) was added drop-wise into a stirred vessel containing

ammonia solution at 303 K. The pH was maintained

constant at pH 11 by further addition of ammonia.

The resulting precipitate was washed, dried and

calcined as in (a) above.

The carbonate coprecipitated materials are denoted CCPx, while the hydroxide precipitated materials are

referred to as OHX, where, in both cases x = 100

[mol fraction Fe/(Fe + Mg)].

2.2. Reduction procedure and characterization

methods

The extent of reduction of the oxide has been

monitored by two methods. One was performed with a vacuum microbalance and used weight loss to

determine the conversion of iron ions to metallic

iron, and the other was based on the volumetric

uptake of hydrogen. Both methods consisted of

heating the calcined material in a static system in

hydrogen at ca.30 Torr for 24 h at 553 K, 24 h at 623 K and 24 h at 693 K, the water vapour produced

being frozen out in a 77 K trap. Spectroscopic measurements were made as fol-

lows. (A) For i.r. study of adsorbed CO, the Fe-

oxidelMg0 sample was pressed into a wafer and mounted in a heatable cell. Reduction was effected

in situ and the sample was cooled to ambient

temperature before admitting CO (120 Tort). Spectra were recorded using a dispersion spectrometer (Per-

kin Elmer 983) and associated data station (Perkin Elmer 3600). (B) For Mossbauer spectroscopy,

samples (ca. 200 mg) were placed in a thin-walled Pyrex cell and attached to a gas-handling/vacuum manifold for treatments. The cell was isolated,

disconnected and installed in the spectrometer witb- out exposure to air. Spectra were collected at 295 K

a constant acceleration spectrometer

T&yophysics) operating in a saw-tooth wave mode

and using a 57Co/Rh source (Amersham Internation- al). Isomer shift values were referenced to o-Fe.

G. Bond et al. I Solid State Ionics 101-103 (1997) 697-705 699

Spectra were curve-fitted using a conventional least-

squares technique to standard Lorentzian line shapes, with prior correction for parabolic background curva-

ture. X-ray diffraction was carried out using a Philips

diffractometer (PW 1050) or similar equipment. Cu

Ko radiation was used. XRD was not measured in

situ: reduced samples were accordingly passivated with N,O before X-ray analysis.

3. Results

3.1. X-ray analysis of MgO-supported Fe oxide

Oxide prepared as described in Section 2.1, here-

after designated as CCP and OH oxide respectively, was examined by X-ray diffraction. Examples of

diffractograms are shown in Fig. 1. CCP oxide is distinctive in showing a definite o-Fe,O, (haematite)

pattern along with MgO. There are also weak

reflections in CCP oxide at 28 values ascribable to

MgFe,O,. OH oxide, on the other hand, shows broader reflections and the iron-containing phase or

phases are less easily identified. However, a reflec-

tion at 28 = 35.6” in OH 20 (Fig. 1) corresponds to

the position of the strongest line in the pattern of

y-Fe,O, cd,, , = 2.52 A; 28 = 35.5”). Although a

reflection for o-Fe,O, is also expected close to this value, there is no evidence in the OH diffractograms

for other c-w-Fe,O, reflections, notably the strong

Fig. 1. X-ray diffractograms of Fe-oxide/MgO after calcination at

773 K of precursors coprecipitated by carbonate (CCP 20) and

hydroxide (OH 20) routes.

d , ,,, = 2.70 A at 28 = 33.2”. We therefore conclude

that OH oxide comprises mainly y-Fe,O, (maghe- mite) supported on MgO.

3.2. Gravimetric and volumetric study of reduction

by hydrogen

The reduction regime (Section 2.2) was that used by Boudart et al. [l], designed to produce finely-

divided Fe/MgO. The final stage was 24 h at 693 K. The principal interest here was to study the effect of

iron loading on the extent of reduction and to

compare the behaviour of oxide prepared by different

routes. Results of the extent of reduction measured

gravimetrically and volumetrically are shown in Fig.

2. The upper and lower dashed curves in each part

show the expected changes for full reduction to metallic iron and reduction of all iron to Fe*+,

respectively, on the assumption that the iron is

present initially as Fe3+, either as Fe,O, or MgFe,O,, as shown by XRD. The first point to note

is that in all cases the results lie between these

boundaries, showing that on average the Fe3+ pres-

ent is reduced beyond the Fe’+ stage, but falls well short of full reduction to metallic iron. Secondly,

note the good agreement between the gravimetric

and the volumetric data. The third important result is

that the experimental curves are proportionately

much closer to the curve for full reduction to Fe” at higher loadings than at lower loadings: the fraction

of iron reduced increases with increased loading.

3.3. Determination of the surface area of metallic

iron

The results in Fig. 2 imply that some metallic iron is present in all the reduced oxides studied. This was

readily confirmed by X-ray diffractometry. For the reduced materials of 20 and 30 mol% Fe content, the

mean particle size d could be determined by a

line-broadening analysis of the o-Fe (110) reflection.

This was done by the conventional method using the Scherrer equation [8], samples having first been

passivated by exposure to N,O at ambient tempera- ture. The (5 values obtained were 45 nm (CCP 20), 46 nm (CCP 30), 38 nm (OH 20) and 36 nm (OH 30). Measurements could not be made with sufficient accuracy on the 5 and 10 mol% samples to justify an

700 G. Bond et al. I Solid State Ionics 101-103 (1997) 697-705

0 5 10 15 20 25 30 35

Fe content 1 mol %

Fig. 2. Reduction by hydrogen of calcined Fe-oxide/MgO obtained via carbonate (CCP) and hydroxide (OH) routes. Extent of reduction at

693 K as a function of iron loading, determined by (a) gravimetric measurements and (b) volumetric measurements.

evaluation of d: the Fe( 110) reflection was too weak

and too broad. From the standpoint of catalysis, however, the

quantity of greater interest is the surface area of

metallic iron in the reduced materials. It was possible to determine this for all samples, irrespective of the amount of iron, by means of carbon monoxide

chemisorption. The generally-assumed surface stoi- chiometry in the experiment is Fe:CO = 2:l [I]. Infra-red spectroscopy provided evidence for this stoichiometry, as well as confirming the presence of iron in the dilute samples. Fig. 3 shows the infra-red spectrum of adsorbed CO in the 1700-2200 cm-l range for one of the most iron-dilute samples, CCP 5, measured after reduction at 693 K in situ in the infra-red cell. The spectrum of the disc prior to CO

adsorption has been subtracted, as has the spectrum

of the residual gaseous CO, using the software facilities of the spectrometer assembly. In spite of

the low Fe content, there is seen to be a strong absorption envelope peaking at 1889 cm-‘. A CO

stretching frequency at this value is typical for CO strongly chemisorbed on a metal. A prior experiment

with pure MgO established that there is no signifi- cant i.r. absorption in this region from CO adsorbed on the support, so the presence of metallic iron in CCP 5 and exposed at the surface can be inferred. Absorption at 1850-1950 cm-’ is characteristic of CO chemisorbed in bridged form, and hence the assumption of 2:l Fe:CO surface stoichiometry is justified.

CO chemisorption was measured volumetrically

G. Bond et al. I Solid State Ionics IOI-103 (1997) 697- 705 701

;-:-.i^ Ii89

2200 2om 1900 1800 :

Wawnnmbor I cm-’

Ll

Fig. 3. Infra-red spectrum of CO chemisorbed on Fe/MgO after

reduction of CCP 5 in situ (change of wavenumber scale at 2000

cm-‘).

for all the reduced oxides by observing the difference

between successive 195 K adsorption isotherms in the pressure range between 75 and 150 Torr, the

region where the difference was constant. By con- verting the chemisorbed volume to number of CO

molecules, doubling to obtain the number of surface Fe” atoms (2: 1 Fe:CO stoichiometry) and taking

1.2 X lOI atoms Fe per m2 as an average for the principal crystal faces of a-Fe, the Fe” surface areas

of the reduced materials were derived. The values are shown in Fig. 4. The results reveal a striking

difference between the OH series and the CCP series. The latter, whilst having a similar (in some

cases, at high loading, a somewhat greater) amount

of reduced iron, have markedly lower surface areas.

3.4. Miissbauer spectroscopy

The phases present in the calcined and reduced Fe-oxide/MgO were also investigated briefly by

Mossbauer spectroscopy. The study was limited to CCP 30, an example of a high-loaded material containing haematite, and a specially-prepared low- loaded OH material containing 1 mol% Fe enriched in the Mossbauer isotope 57Fe. The techniques so far

Fo content / mol %

Fig. 4. Surface area of metallic iron component of reduced oxide

as a function of Fe content (surface area is referenced to unit

weight of calcined material before reduction).

described were not suitable for studying loadings below 5 mol %, but by using enriched 57Fe in the Fe nitrate in the OH coprecipitation route it was pos-

sible to use Mossbauer spectroscopy to investigate

oxide with this much lower loading. A matter of interest in view of the low extent of reduction at 5

mol % loading (Fig. 2) was whether there would be

any reduction at all to Fe” at 1 mol % loading. Fig. 5 shows the Mossbauer spectra for CCP 30

in the calcined state (Fe-oxide/MgO) and after

reduction at 693 K. The Mossbauer parameters for the deconvoluted spectra are shown in Table 1. The

calcined oxide spectrum consists of two overlapping sextets. The reduced oxide spectrum (693 K reduc-

tion) resolves with two new outer sextets and a central doublet, in addition to the inner sextet with

H = 33.3 T characteristic of o-Fe [9]. A further reduction (723 K for 24 h) gave a spectrum (not

shown) comprised of the same components but in

different proportions (Table I ). Results with 57Fe-enriched OH 1 (Fig. 6) are quite

different. The calcined state resolves as two overlap- ping doublets with similar isomer shifts (IS). The

693 K reduced spectrum computer-fits with a lower proportion of these two doublets, plus two new

doublets with larger IS, and also a-Fe. The spectrum of the further-reduced oxide possesses the same components in different proportions. The immedi- ately obvious feature of the reduced spectrum (Fig. 6) is the unmistakable presence of the o-Fe sextet,

702 G. Bond et al. I Solid State Ionics 101-103 (1997) 697-705

100

98

96

% 94 .

%

1

100

911

96

94

I I I I I I I

-8.00 0.00 8.00

vslodty / mm s-1

Fig. 5. Mijssbauer spectra at 295 K of CCP 30 in (top) calcined state and (bottom) after reduction in situ at 693 K.

testifying to the full reducibility of an appreciable

fraction of the total iron content at 693 K, even at low iron loading.

4. Discussion

The 773 K calcined Fe-oxide/MgO, as expected, contains all its iron in the form of Fe3+ ions. The XRD results, however, show that the CCP and OH preparation routes do not give rise to the same Fe3+ phases (Fig. 1). The distinction between the patterns

indicates that the methods of preparation adopted lead to different distributions of Fe3+ ions. The CCP oxide is biphasic, the major phase being haematite

(o-Fe,O,). This is confirmed by the Miissbauer

results for CCP 30: literature values for the Mdssbauer parameters of a-Fe,O, are IS = 0.37 IWIls-' and H= 51.8 T [lo], in good agreement with our data for the first sextet (Table 1). The second sextet, that with H = 45.9 T, agrees well with values of 45.0 T [l l] and 44.5 T [12] reported for

MgFe,O,. This phase proves the intimate nature of the interaction of Fe3+ with the MgO support. The OH oxide has broadened XRD reflections indicative of poor crystallinity, consistent with it containing y-Fe,O,, the highly non-stoichiometric spinel-struc-

tured Fe3+ oxide. Low crystallinity and defects also foreshadow paramagnetic rather than spin-ordered

Fe3+, which accounts for the doublet (rather than sextet) character of the Mossbauer spectrum of OH 1

(Fig. 6).

G. Bond et al. I Solid State lonics 101-103 (1997) 697-705 703

Table 1

Mossbatter parameters

Sample

and

treatment

CCP 30

calcined

Isomer

shift

IS

(mm SC’)

0.41

0.35

Quadrupole

shift

QS (mm SC’)

- 0.31

- 0.10

Linewidth

(mm SC’)

0.27

0.71

Hyperfine

field

H

(T)

52.0

45.9

Spectral

contribution

(%I

76

24

Species (phase)

Fe” (o+Fe,O,)

Fe’+ (MgFe,O,)

CCP 30

reduced at

693 K

0.35 - 0.43 49.7 30 Fe’+ (Fe304)

0.70 _ 0.43 46.4 30 Fe*+ (Fe,O,)

1.14 0.72 0.49 _ 48 Fe”

0.05 - 0.34 33.3 22 Fe” (o-Fe)

CCP 30

reduced at

723 K

0.36 - 0.41

0.70 - 0.41

1.13 0.75 0.46

0.04 - 0.31

49.9

46.4

33.4

14 Fe’+ (Fe,O,)

14 Fe2+ (Fe,O,)

45 Fe”

41 Fe” (a-Fe)

OH 1 0.34 0.57 0.41

calcined 0.30 1.03 0.65

_ 34 Fe”

66 Fe3+

OH 1 0.37 0.61 0.47 20 Fe3+

reduced at 0.28 1.10 0.73 20 Fe’+

693 K 0.89 1.69 0.77 10 Fe’+

0.81 0.89 0.86 _ 28 Fe2+

0.02 - 0.40 32.6 21 Fe” (o-Fe)

OH 1 0.42 0.72 0.28 10 Fe’+

reduced at 0.32 0.97 0.35 7 Fe3+

723 K 0.91 1.55 0.77 _ 19 Fe’+

0.87 0.90 0.82 _ 43 Fe2+

0.02 _ 0.40 32.7 21 Fe” (a-Fe)

Strong interaction between a substantial proportion of the Fe3+ ions and the support is also manifested in the reduction behaviour (Fig. 2). Whereas bulk Fe,O, is readily reduced to metallic iron at ca. 700 K, the present Fe-oxide/MgO still contains much unreduced iron after reduction at 693 K. This is especially true at 5 mol% Fe, where the overall reduction is less than 50%. This difficulty of reduc- ing supported iron oxide is already well documented [l-6], but in contrast to some earlier work [ 1,2] we do not find the extent of reduction to metallic iron to be independent of the iron loading. There is a higher proportion of reduction when the loading is high. This is readily understandable if there are two phases in the calcined material, one of which is pure Fe,O, and the other involves ions of the support. The dispersing effect of the support is limited and as Fe content increases, the proportion of iron as Fe,O,

(the more easily reduced component) can be ex- pected to increase. This may account for the CCP curve steepening above 10% Fe to rise above the OH curve. In general, however, there is only a small effect of preparation route on the reducibility. This suggests a similar strong stabilization of iron ions by the support in the two cases.

The Mossbauer data show an unexpected result in regard to the amount of Fe3+ retained in the reduced material. Our assumption made previously [7] that one could express the percentage reduction to Fe” by assuming that in Fe-oxide/MgO all Fe3+ reduced to Fe*+ before any reduction to Fe” occurred is not supported. The Miissbauer parameters for the re- duced CCP 30 (Table 1) show clear evidence for Fe,O,. The overlapping sextets, one due to the Fe3+ ions (H=49.7 T) and the other to the Fe*+ ions (H = 46.4 T), agree well with published values for

704 G. Bond et al. I Solid State tonics 101-103 (1997) 697-70.5

98

aa

ae .

1 78

2 99

95

91

-8.00 0.00 8.00

vebdty I mm s-’

Fig. 6. Miissbauer spectra at 295 K of 57Fe-enriched OH 1 in (top)

calcined state and (bottom) after reduction in situ at 693 K.

Fe,O, [lo], also as regards the IS and QS. It is the phase with the doublet at IS = 1.14 mm s-‘, how- ever, which is predominant. The large isomer shift

identifies it as a Fe’+ phase, but not necessarily

wiistite (FeO). In fact, we note that Fe,Mg, _,O solid solution has virtually the same parameters as Fe0 [13], and the fact that enhanced reduction (Table 1) decreases the Fe,O, sextets without appreciably

decreasing the Fe’+ doublet intensity implies that Fe’+ is mainly present in stabilized form as

Fe,Mg , _,O. The proportion of o-Fe determined by Mossbauer spectroscopy (22%) is lower than that corresponding to Fig. 2 for CCP 30, but this is no

doubt due to a less efficient in situ reduction in the confined space of the Mossbauer cell: note that there

is a marked increase in Fe” concentration after treatment at 723 K. The Mossbauer results for

reduced OH 1 can be interpreted similarly, the main

interest in that case being the proof of appreciable Fe” formation already at 693 K. The remaining Fe’+,

however, is evidently stabilized very strongly by the

support, since Fe” is not increased on extended reduction.

The results for the surface area of metallic iron (Section 3.3) reveal a marked difference between the

CCP and OH materials. The average crystallite size of Fe” particles by XRD is smaller for the latter

(36-38 nm vs. 45-46 nm for CCP), implying a larger specific surface area. This is reflected with

greater force by the results from CO chemisorption

(Fig. 4) which show that the surface areas of metallic

iron in the reduced OH oxides are consistently higher than those for the CCP case. Comparison of the

XRD and CO data by the method of Boudart et al. [l] gives a lower d value by CO chemisorption than by XRD. This suggests the presence of a consider-

able amount of very small Fe” particles, whose contribution is registered by chemisotption but which are lost in the line-broadening tail. The drop in Fe”

surface area for OH 30 (Fig. 4) suggests a limit to

the enhancing effect of the OH route, with excess finely-divided iron being preferentially sintered.

The results overall strongly support a mechanism

of reduction similar to that advocated by Stobbe et

al. [4]. Fe,O, particles as a separate phase on the

surface of the MgO support reduce via Fe,O, to give a stabilized Fe0 phase, which then reduces at ca. 700 K to Fe”. A proportion of the iron, however, is more

intimately locked into MgO, probably as MgFe,O,, and this reduces to Fe,Mg, _,O solid solution from which Fe” is extracted slowly to decorate the MgO as

very small particles. The generation of small iron

particles is favoured by the preparation route from coprecipitated hydroxide.

Acknowledgements

The authors thank the Science and Engineering Research Council and also ICI Chemicals and Poly- mers Ltd for their support of this work.

G. Bond et al. I Solid State Ionics 101-103 (1997) 697-705 IQ5

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