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Dependence of the oxide-support interaction on the size and nature of iron oxide particles on SiO2
PII: SOO22-3697(97)00161-3
Pergamon
J. Php. Chm Solids Vol58. No. 12. pp. 2119-2125. 1997 1997 Elsevia Science Ltd. All rights reserwd Printed in Great Britain 0022-?r697/97 $17.00 + 0.00
0
DEPENDENCE OF THE OXIDE-SUPPORT INTERACTION ON THE SIZE AND NATURE OF IRON OXIDE PARTICLES ON Si02 S. G. MARCHETTIa,
M. V. CAGNOLI”,
A. M. ALVAREZ’,
N. G. GALLEGOSa,
J. F. BENGOAa,
A. A. YERAMIANa and R. C. MERCADERb** bCINDECA CONICET, CIC, Facultad de Ciencias Exactas, Fact&ad de Ingenieria, Universidad National de La Plats, 47 No. 257, 1900 La Plant, Argentina ?ENAES, Departamento de Fisica, Universidad National de La Plats, C.C. 67, 1900 La Plats, Argentina (Received I I June 1996; accepted 13 June 1997)
Abstract-The thermal dependence of the hypertine fields and of the relative Mijssbauer factors have been used to investigate the oxide-support interaction in the Fe-.%02 system. The calcination temperature (698, 898 and 1098 K in Nr) has an influence on the nature and particle size of the iron species, while the atmosphere (air and Nz at 698 K) affects the oxide particle size only. a-FerO, particle diameters of = 45 and = !SOA were found for samples calcinated at 698 K in air and nitrogen, respectively. y-FerO, particles of = 90 A were obtained after calcination in air at 898 K and migration of iron ions into the SiOs matrix was verified after the 1098 K heat treatment. It is found that for o-FesOs, the smallerparticleshave a stronger oxide-support interaction and y-Fe r0 1 particles have an even higher strength than o-FezOr. 0 1997 Elsevier Science Ltd. AI1rights reserved Keywords: A. oxides, A. microporous materials, C. Miissbauer Spectroscopy, D. surface properties
1. INTRODUCTION The nature of the oxide-support interaction and the relevant parameters on which it depends is one of the many physico-chemical issues that are still open in systems of small supported particles. This knowledge is of great importance in fields of technological application such as catalysis. For example, the iron oxide-support interaction in the Fe-Si02 system was found to influence the activity of the hydrocarbons synthesis starting from CO and H2 (Fischer-Tropsch reaction) [ 11. Mijssbauer spectroscopy is widely used to study particle-support systems. It has been employed by many authors to investigate the oxide-support interaction in several systems. For example, the play between particle size and thermal dependence of the hyperfine field collapse and superparamagnetism have been used to assess distributions of particles sizes in iron supported on carbon [2]. In gold particles embedded in gelatin, the Mijssbauer recoilless fraction has been found to yield relevant details of the binding of small particles to the medium containing them through its dependence on the size and nature of the particles [3]. In this paper, we have used Mossbauer spectroscopy to investigate the modification of the degree of the iron oxide-SiO2 interaction caused by different preparation methods. The changes of hyperfine parameters and f-factors of precursors of Fe-SiOZ catalysts prepared at three different temperatures and in air or nitrogen *Author to whom correspondence should be addressed.
enabled us to advance the knowledge of the oxidesupport interaction.
2. EXPERIMENTAL
Samples were prepared by the impregnation method with a solution of Fe(N03)s.9H20 on silica gel so as to obtain oxides with an Fe content of 5% wtlwt. The product was divided in four batches subjected to the following thermal treatments: 1. Fe-SiOs calcinated 2. Fe-SiO2 calcinated (698n); 3. Fe-%02 calcinated (898n); 4. Fe-SiOs calcinated (1098n).
in air 8 h at 698 K (698a); in flowing nitrogen 8 h at 698 K in flowing nitrogen 8 h at 898 K in flowing nitrogen 8 h at 1098 K
When samples were exposed to air after calcination, only 898n experienced a colour change from black to dark brown. Mossbauer spectra were recorded with a standard 5 12 channels spectrometer with transmission geometry. Samples were placed in a helium closed-cycle refrigerator at temperatures ranging from 17 to 298 K. A ‘7Co in Rh matrix source of nominally 50 mCi was used. Velocity calibration was performed against a 6 pm thick Or-Fefoil. All isomer shifts are referred to this standard at 298 K. The absorbers of samples 698a. 698n, 898n and 1098n were prepared for Mossbauer spectroscopy, weighing
2119
S. G. MARCHITTI et al.
2120
T=298K
90 mg of Fe-Si02 with 5% wt/wt of Fe, which gives an effective thickness dimensionless parameter t, = 0.6 for our 19 mm diameter absorber holder if all the iron existing in the sample belonged to only one species. All spectra were fitted with a programme [4] capable to analyse hyperfine parameters distributions originating in particles of different sizes and/or the different crystallographic sites of the iron phases present.
3. RESULTS AND DISCUSSION 3.1. Hyperfine parameters Figs 1 and 2 display the spectra taken at 17 and 298 K. At intermediate temperatures, the spectra do not exhibit features different from these two. Tables 1 and 2 show the hyperfine parameters of the fitted spectra. The hyperfine parameters of sample 698a are T=17K
NBa
0.
v
’
I
1.4%
-12
-8
-4
0
4
8
12
velocity(mm/s)
Fig. 2. Room temperature MGssbauer spectra of Fe-SiOz samples cakinated in air and nitrogen at different temperatures.
098n
Fig. I. Mijssbauer spectra taken at 17 K of Fe402 specimens calcinated in air and nitrogen at different temperatures.
characteristic of Fe3+. One quadrupole signal and a magnetic site can be seen at 17 K, while only one doublet is observed at 298 K. These broadened signals have been assigned to diferent Fe3+ species in previous works: very small superparamagnetic microcrystals of cr-Fez03 [5-71; and Fe3+ either exchanged onto the surface of the support, or in a solid solution with the support [8,9]. The magnetic signal of the spectrum of sample 698a at 17 K manifests itself only as a broad relaxation that contributes to the curvature observed in the background. The hyperfine fields distributions programme used does not converge to reasonable values unless this magnetic interaction is also introduced. Since the spectra show an impending magnetic splitting only at 17 K, it is inferred that the Fe3+ species must be in the form of very small particles of cr-Fez03. The most likely value of H is smaller than that of bulk ar-Fez03, but the difference is
Interaction of iron oxide on SiOr
2121
Table 1. Most likely values of hyperhne parameters of Fe-SiOr specimens calcinated at different temperatures (698,898 and 1098 K) and atmospheres (a = air, n = nitrogen) at 17 K Species Fe3+ (sp) Fe*’ Fe’+ (m)
Parameter ’
698a
698n
898n
d (mm/s) A (mm/s) 6 (mm/s) A (mm/s) 6 (mm/s) 2~ (mm/s) H(r) 6 (mm/s) 2~ (mm/s) H(r)
0.43 It 0.01 0.70 2 0.01 0.39 2 0.04b - 0.21 ? 0.03s 51.2 ? 0.3b -
0.34 f 0.04 0.61 + 0.04 0.48 ‘- 0.01 b - 0.21 IL O.Olb 52.6 2 0.1 b -
0.29 t 0.04 0.45 + 0.04 0.41 2 O.OIC - 0.02 ? 0.01 c 52.5 2 0.1’ 0.39 t 0.01 c - 0.22 2 0.01’ 47.5 t 0.2’
-
-
1098n 0.29 2 0.77 + 1.07 t 3.03 2
0.01 0.01 0.01 0.01
-
:A11isomer shifts are referred to cr-Fe at 298 K. cr-Fe203. ‘y-Fe203. Fe’+ (sp): superparamagnetic Fe’+. Fe3+ (m): magnetic Fe.+. accounted for by the collective model (CMEM) [lo].
magnetic
excitation
central signal even in the spectrum
at 17 K and the
hypertine magnetic field is reduced with respect to bulk
To estimate the particle size of cu-FeZOs we followed
a-Fe203. Again, this decrease can be well accounted for
the idea by Bodker er al. [2] who used tbe sizes of the
by CMEM. A magnetic anisotropy constant value (K) of
particles after reduction
to estimate those of the oxide
0.5 X lo5 Jme3 was estimated for o-Fe203 particles of
particles diameter
on the same samples. Using the average of Fe0 crystals obtained after reduction in
average diameters of = 120 A [lo]. Assuming that this
hydrogen
at 698 K [ 111, and judging
particles sintered during reduction because of the slow
value of H (50.5 2 0.1 r) at 298 K, CMEM yields an average particle size of approximately 150 A. Due to the
way in which the reduction
dependence of K with the particle volume, this value must
unlikely that the
was performed,
we expect
that particles in both samples contain the same number of Fe atoms.
Assuming
that the oxidized
and reduced
value applies for 698n too, and using its most likely fitted
be considered
only as a rough estimate.
It is well known that in cr-Fez03 particles of diameters
particles are both spherical and have the same densities
less than 200 A, the Morin transition
as crystalline or-FetOj and bee o-Fe’, respectively, an average diameter of = 45 A results for the cr-Fe203
suppressed [ 121. Hence, in 698n spectra, the quadrupole splitting value of 2e = - 0.21 + 0.01 mm/s at 17 K does
particles in 698a. The spectra belonging to 698n display a magnetic split
not change when the same sample is measured at 298 K, 2~ = - 0.23 + 0.01 mm/s, for our = 150 A diameter
fraction already at room temperature that increases at 17 K. The fitted hyperfine parameters correspond to
cr-Fe203 particles,
or-Fe203. The presence of a magnetic signal at 298 K is
small particles of cr-Fez03 whose Morin transition has
indicative that the particles are, on average, larger than
been suppressed
those of 698a. However, their size is not large enough to yield bulk parameters since there is a superparamagnetic
our result is coincident with the bulk value of 2~ = -0.20 mm/s [ 121. Additional X-ray diffraction data
This behaviour,
bulk one, is, however, coincident
is completely
different
from the
with that obtained for
[ 131. Moreover, at room temperature,
Table 2. Most likely values of hypertine parameters of Fe-SiOr specimens calcinated at different temperatures (698,898 and 1098 K) and atmospheres (a = air, n = nitrogen) at 298 K Species
Parameter”
698a
698n
898n
Fe’+(sp)
6 (mm/s) A (mm/s) 6 (mm/s) A (mm/s) 6 (mm/s) 2~ (mm/s) H(73 6 (mm/s) 2~ (mm/s) H(T)
0.34 IT 0.01 0.60 2 0.01 -
0.30 + 0.02 0.65 2 0.02
0.25 t 0.02 0.73 t 0.03 0.29 -c 0.01’ - 0.01 I? 0.01’ 49.4 t 0.1’ 0.29 2 0.02’ - 0.05 -c 0.02’ 46.5 2 0.3’
Fe*+ Fe’+(m)
aAll isomer shifts are referred to o-Fe at 298 K. bcz-Fe203. ‘y-Fe203. Fe”+ (sp): superparamagnetic Fe”. Fe”+ (m): magnetic Fe.+.
0.38 + 0.01 b - 0.23 + 0.01 b 50.5 2 0.1 b -
1098n 0.22 * 0.80 ? 0.96 + 2.80 + -
0.01 0.01 0.01 0.01
2122
S. G. MARCHETII
obtained for this sample with broad lines at 28 = 33.3, 35.8,49.5 and 54.3” confirm unambiguously the presence of (u-FeZ09 in 698n. Sample 898n spectra (Figs 1 and 2) display the presence of two signals belonging to a superparamagnetic species and to the two magnetic sextuplets. These spectra were fitted in the same way as in Ref. [14], with two sextets with slightly different parameters characteristic of y-FerOj [ 121.Notwithstanding that two magnetic hyperfine fields are used to fit the spectra, the signals are not resolved due to their very similar values, and the relative ratio of the populations between A and B sites is difficult to assess. The assignment of maghemite to the Fe3+ magnetically split signal of the sample 898n spectra is based also on the following reasons. The existence of y-FeZ03 is coherent with the observed change in colour from black to dark brown when the sample was exposed to air just after calcination. This suggests that after the calcination, the sample contained magnetite, which oxidized to y-Fez03 in air, as already observed by other authors [15]. In addition, it is the only sample that is attracted by a permanent magnet. The isomer shift values of both sextuplets are 0.29 C 0.02 mm/s at 298 K, which are very close to the 0.32 mm/s reported in Ref. [16]. Although the magnetic hypertine fields fitted, HA = 46.5 ? 0.3 T and HB = 49.4 2 0.1 T for sites A and B, respectively, show a reduction of 4.7 and 1.O%, respectively, with respect to HA = 48.8 T and HB = 49.9 T measured by Armstrong et al. [ 171,it can be explained on the basis of the collective magnetic excitations model [ 10, 13,181. Moreover, Picone et al. [ 141have found that for y-Fez03 particles of = 65 A of diameter, the hyperfine field is reduced as much as 18% from the bulk value. In spite of this large discrepancy, these authors attribute the observed decrease also to the effect of the small size of the particles in their samples. Using the same arguments as for 698n, the average diameter estimated applying CMEM is approximately 90Aif,asinRef.[12],Kistakenas105Jm-3andthe average of 47.4 T between the two most likely hyperfine magnetic fields fitted at room temperature is used. Additional X-ray diffraction patterns taken for samples 698a gave signals hardly discernible from the background noise. For 898n, the pattern displayed clearly only one broad signal at 20 z 37”, which is a position coincident for three types of oxides, (Y-and y-FeZ03, and Fe304. Comparing these results with those already mentioned for 698n and the subsequent determination of the particle sizes through Mijssbauer spectroscopy, the X-ray results are in complete accord with the particle diameters sequence mentioned above. A drastic change with respect to the above mentioned samples was observed in the Mossbauer spectra of 1098n over all the temperature range. Only two doublets
er al.
assigned to Fe’+ and Fe3+ are observed. Presumably, these ions have diffused into the support as a consequence of the severe thermal treatment. The reduction of some Fe3+ to Fe’+ could be due to the generation of oxygen vacancies caused by the calcination at a high temperature in flowing nitrogen. 3.2. Recoilless fraction The Mossbauer effect gives information on the phonon frequency distribution through its recoilless fraction. For an absorber, the intensity of absorption of resonant gamma radiation is proportional to: f(T)=exp(
- *),
(1)
where A7) is the probability of recoil-free absorption at temperature T and (z*)r is the mean-square vibrational amplitude at temperature T of the absorbing nucleus along the direction of propagation of the gamma radiation of wavelength X.In a Mossbauer experiment, the spectral area belonging to each species is proportional to its f-factor if the thin absorber approximation is verified. The thermal dependence of the f-factor is governed by the (.x2)term, which is a function better analysed as a sum of frequency moments of the phonon spectrum [19]. Although generally solids do not behave like harmonic oscillators with a cut-off frequency, as a first approximation it is customary to compare the experimental data with the behaviour predicted by Debye’s model. If the Debye model for the solid vibrations is assumed, the recoilless fraction has for high temperatures (T 1 t&/2) the asymptotic behaviour:
fLdT)=ev[(f-$&&T],(2) where t9n is the Debye temperature, Ea is the recoil energy of a free atom and ks is Boltzmann’s constant. To check that the Miissbauer results are indeed within the thin absorber limit, we considered the spectra obtained for sample 698n. This is the only one that could display a line broadening caused only by thickness effects and not by those owed to the small particle size [the particle diameters values of the three samples used for the relative 87) analyses, as mentioned above, were: z 45, 150 and 90 A for 698a, 698n and 898n, respectively]. For this spectrum, the magnetically split signal belonging to o-Fe203 with a relative population of 70%, that brings t, down to 3 0.4, yields a linewidth of P = 0.33 ? 0.01 mm/s. The criterion described by Rancourt er al. [20], leads to an equal spectral area attenuation of A/At,,in s 0.97, that falls within the thin absorber approximation. The logarithm of eqn (2) is linear in T, extrapolating to InAr) = 0 for T - 0 K. Fig. 3 shows the thermal dependence of the natural logarithm of the spectral
Interaction of iron oxide on SiOr determine
2123
eo, its graphic solution [21] gives &, values
of z 360 and s 340 K for 698a and 898n, respectively. 0.0
No graphic solution represents thefLArr vs Tdependence for 698n. The values for 698a and 898n are in good agreement with the Debye temperatures obtained from
l.-.).i.\
the high-temperature limit. However, the Debye model does not account for thefLArr(T) behaviour of 698n. These Debye temperatures are much smaller than those of macroscopic
mBa -0.4
crystals,
e.g. eo = 500 K for bulk
c+FezOs [2] and @o = 460 K for bulk y-Fe203 [12]. If
the decrease in the eo values were due to a true variation of the phonon frequencies
0.0
dependence
of the samples, because of its
on Bo, the second-order
Doppler shift should
with respect to the bulk values [ 141.
also be diminished
However, no noticeable changes in the isomer shifts of the iron species were found compared with the values for bulk or-Fez03 and y-FezOs.
-0.4
From this evidence,
assume that the Debye temperatures
we
of the crystallites
must be similar to those of bulk specimens as reported in [14, 221. The differences in the Debye temperatures
-0.6
obtained from the InAT) data must be ascribed to the -0.8
oversimplification particles
implied in eqn (2). i.e. j(T) for small
may not be only the result of the phonon
spectrum of the particle itself, but also it may be sensitive
0.0
to the coupling of the particle with the support (or to
t
the motion of the particle as a whole when the particle-support interaction is small). Indeed, the influence of the vibrations of the particle as
-0.2
a whole on the thermal dependence BB6n \
3.
-0.4 0
50
100
of the recoilless
fraction has been studied in previous works [2, 3, 141 where it has been assumed that the measuredf-factor can 150
200
250
be written as:
1 300
350
(4)
fTOT=fLATTfPART,
Temperature (K) where LATT and PART refer to the inner vibrations and Fig. 3. Thermal dependence of spectral areas of Fe-SiOl samples calcinated at 698 K in air and nitrogen and 898 K in nitrogen. The vertical scale has been normalized so that InA(T)=OforT=OK. areas for samples 698a, 698n and 898n normalized to zero at T = 0 K as was performed in Ref. [ 21. The data suggest
to the particle as a whole, respectively. is a consequence
the lattice and particle vibrations hence
the
This factorization
of the assumption that for those systems
mean
square
are uncoupled,
vibration
of the
and
resonant
absorbing nucleus can be written as:
that the Debye model might describe the lattice behaviour
G?&or
=~%ART+GI*)LA~~
(5)
for the three samples studied. Therefore, after the change of slope of ln(A/Ar,) around 100 K, the data were fitted with straight lines up to room temperature. The following Debye temperatures Sample ~DW
Bu were obtained:
698a 332 2 13
698n 247 + 12
89Sn 331 + 16
eqn (4). For these systems, if only
fLAnwere used instead of fToT, the flo obtained would be lower than the bulk values. Instead, if only fpART wereused, a hea temperam'e dependence
If, instead, the complete expression: +
where PART and LATT have the same meaning as in
-11 XdX
e*-1
(3)
forfLATT(7) deduced from the Debye model is used to
over the whole temperature range should be
observed [2]. The above
assumptions
are appropriate
when the
average distance between particles is large compared with their diameter, and hence the vibrational coupling between the particles can be neglected. This is the case of catalysts with low iron content and high surface area. In such circumstances,
the vibration of the particles as a
S. G. MARCHE’ITI ef nl.
2124
Table 3. Force constant between oxide particles and the support (q), angular frequency for particles vibrations as a whole (w) and Einstein temoerature (0~) for 698a. 698n and 898n. Sample
q (J/m*) w (s-l) ‘jE 6)
898n
69811
698a
44 6.9 x 10” 0.5
107 6.5 x 10” 5.0
I23 2.4 X 10” 1.9
must be low enough to allow the further reduction of the supported oxide and high enough to impair the sinterization of the oxide particles. The previous discussion shows that by an appropriate choice of the heat treatment it is possible to regulate the oxide-support interaction.
4. CONCLUSIONS whole are better described by a localized mode and the Einstein model is applicable [3]. Assuming that the particle vibrations as a whole can be described as an harmonic oscillator with an angular frequency: q
w=-
112
(> M
(6)
)
where q is the force constant between the particle and the support and M is the particle mass. At temperatures above the Einstein temperature (13~= hw/2aka), one finds:
The calcination atmosphere (air or nitrogen) does not modify the iron species present on the support, but influences the degree of interaction through the modification of the size of the oxide particles obtained, namely, = 45 and 150 a for cu-Fe20j. The temperature dependence of thef-factors indicates that the smaller the crystals the stronger the oxide-support interaction is. The present experiments are a ‘quantitative’ evidence of the trends mentioned in previous works [8], that the higher area/volume
ratio of the crystals increases the particle-
support interaction strength.
The condition T > flEis fulfilled at T 2 17 K. The values of q, w and f& shown in Table 3 have been estimated with eqns (4) and (6) taking Bn for bulk species to calculate fLAn since, as described above, the Debye temperatures of small particles do not differ much from bulk values [ 14,221. The high q values, especially for 698a and 898n, are indicative that the applied model does not fulfil the assumptions about the inexistence
of a coupling between
the particle phonon spectrum and the vibrations particle
as a whole.
For these intermediate
support
interactions,
a special
account the coupling between
model
of the
particle-
that take into
the support and particle
phonon spectra would be necessary.
To our knowledge
The calcination temperature in flowing nitrogen affects the iron oxide species obtained: at 898 K, y-FeZ03 is produced instead of the cu-FeZOj produced at 698 K. The strength of the interaction is higher for y- than for cr-FeZ03. Calcination at higher temperatures (1098 K) causes a diffusion of the iron ions into the support and the system no longer has the features of small supported oxide particles. Acknowledgemenrs-The
authors acknowledge support of this work by Consejo National de Investigaciones Cientificas y TBcnicas, Comisi6n de Investigaciones Cientificas Pcia. Bs. As. and Universidad National de La Plata, Argentina. S.G.M., M.V.C., N.G.G., A,A.Y. and R.C.M. are members of Carrera de1 Investigador Cientifico y TecnolBgico, CONICET. A.M.A. and J.F.B. are members of Cara de1 Personal de Apoyo, CONICET and CIC, respectively.
there is no such model available in the literature.
Notwithstanding being’ obtained from an oversimplified model, the q values of Table 3 allow one to perform a qualitative analysis of the particle-support interaction. It can be seen that 698a, with smaller crystal sizes, has a higher q than 698n. This result is a confirmation that for the same species, a smaller particle size produces a stronger interaction with the support. On the other hand, bearing in mind that the crystal size of a-Fe203 in 698a is about one half than in 898n, one can deduce that y-FeZOs interacts more strongly with the support than ol-Fe203. The comparison between 698n and 898n confirms that the y-FezOj crystals with the smaller size have a stronger interaction. Sample 1098n could not be analysed along these lines since, as a consequence of the higher calcination temperature, Fe migrated into the support actually forming a compound that no longer had the features of small supported particles. In the search for better catalysts, it is important to obtain an optimum degree of interaction; it
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Pergamon
J. Php. Chm Solids Vol58. No. 12. pp. 2119-2125. 1997 1997 Elsevia Science Ltd. All rights reserwd Printed in Great Britain 0022-?r697/97 $17.00 + 0.00
0
DEPENDENCE OF THE OXIDE-SUPPORT INTERACTION ON THE SIZE AND NATURE OF IRON OXIDE PARTICLES ON Si02 S. G. MARCHETTIa,
M. V. CAGNOLI”,
A. M. ALVAREZ’,
N. G. GALLEGOSa,
J. F. BENGOAa,
A. A. YERAMIANa and R. C. MERCADERb** bCINDECA CONICET, CIC, Facultad de Ciencias Exactas, Fact&ad de Ingenieria, Universidad National de La Plats, 47 No. 257, 1900 La Plant, Argentina ?ENAES, Departamento de Fisica, Universidad National de La Plats, C.C. 67, 1900 La Plats, Argentina (Received I I June 1996; accepted 13 June 1997)
Abstract-The thermal dependence of the hypertine fields and of the relative Mijssbauer factors have been used to investigate the oxide-support interaction in the Fe-.%02 system. The calcination temperature (698, 898 and 1098 K in Nr) has an influence on the nature and particle size of the iron species, while the atmosphere (air and Nz at 698 K) affects the oxide particle size only. a-FerO, particle diameters of = 45 and = !SOA were found for samples calcinated at 698 K in air and nitrogen, respectively. y-FerO, particles of = 90 A were obtained after calcination in air at 898 K and migration of iron ions into the SiOs matrix was verified after the 1098 K heat treatment. It is found that for o-FesOs, the smallerparticleshave a stronger oxide-support interaction and y-Fe r0 1 particles have an even higher strength than o-FezOr. 0 1997 Elsevier Science Ltd. AI1rights reserved Keywords: A. oxides, A. microporous materials, C. Miissbauer Spectroscopy, D. surface properties
1. INTRODUCTION The nature of the oxide-support interaction and the relevant parameters on which it depends is one of the many physico-chemical issues that are still open in systems of small supported particles. This knowledge is of great importance in fields of technological application such as catalysis. For example, the iron oxide-support interaction in the Fe-Si02 system was found to influence the activity of the hydrocarbons synthesis starting from CO and H2 (Fischer-Tropsch reaction) [ 11. Mijssbauer spectroscopy is widely used to study particle-support systems. It has been employed by many authors to investigate the oxide-support interaction in several systems. For example, the play between particle size and thermal dependence of the hyperfine field collapse and superparamagnetism have been used to assess distributions of particles sizes in iron supported on carbon [2]. In gold particles embedded in gelatin, the Mijssbauer recoilless fraction has been found to yield relevant details of the binding of small particles to the medium containing them through its dependence on the size and nature of the particles [3]. In this paper, we have used Mossbauer spectroscopy to investigate the modification of the degree of the iron oxide-SiO2 interaction caused by different preparation methods. The changes of hyperfine parameters and f-factors of precursors of Fe-SiOZ catalysts prepared at three different temperatures and in air or nitrogen *Author to whom correspondence should be addressed.
enabled us to advance the knowledge of the oxidesupport interaction.
2. EXPERIMENTAL
Samples were prepared by the impregnation method with a solution of Fe(N03)s.9H20 on silica gel so as to obtain oxides with an Fe content of 5% wtlwt. The product was divided in four batches subjected to the following thermal treatments: 1. Fe-SiOs calcinated 2. Fe-SiO2 calcinated (698n); 3. Fe-%02 calcinated (898n); 4. Fe-SiOs calcinated (1098n).
in air 8 h at 698 K (698a); in flowing nitrogen 8 h at 698 K in flowing nitrogen 8 h at 898 K in flowing nitrogen 8 h at 1098 K
When samples were exposed to air after calcination, only 898n experienced a colour change from black to dark brown. Mossbauer spectra were recorded with a standard 5 12 channels spectrometer with transmission geometry. Samples were placed in a helium closed-cycle refrigerator at temperatures ranging from 17 to 298 K. A ‘7Co in Rh matrix source of nominally 50 mCi was used. Velocity calibration was performed against a 6 pm thick Or-Fefoil. All isomer shifts are referred to this standard at 298 K. The absorbers of samples 698a. 698n, 898n and 1098n were prepared for Mossbauer spectroscopy, weighing
2119
S. G. MARCHITTI et al.
2120
T=298K
90 mg of Fe-Si02 with 5% wt/wt of Fe, which gives an effective thickness dimensionless parameter t, = 0.6 for our 19 mm diameter absorber holder if all the iron existing in the sample belonged to only one species. All spectra were fitted with a programme [4] capable to analyse hyperfine parameters distributions originating in particles of different sizes and/or the different crystallographic sites of the iron phases present.
3. RESULTS AND DISCUSSION 3.1. Hyperfine parameters Figs 1 and 2 display the spectra taken at 17 and 298 K. At intermediate temperatures, the spectra do not exhibit features different from these two. Tables 1 and 2 show the hyperfine parameters of the fitted spectra. The hyperfine parameters of sample 698a are T=17K
NBa
0.
v
’
I
1.4%
-12
-8
-4
0
4
8
12
velocity(mm/s)
Fig. 2. Room temperature MGssbauer spectra of Fe-SiOz samples cakinated in air and nitrogen at different temperatures.
098n
Fig. I. Mijssbauer spectra taken at 17 K of Fe402 specimens calcinated in air and nitrogen at different temperatures.
characteristic of Fe3+. One quadrupole signal and a magnetic site can be seen at 17 K, while only one doublet is observed at 298 K. These broadened signals have been assigned to diferent Fe3+ species in previous works: very small superparamagnetic microcrystals of cr-Fez03 [5-71; and Fe3+ either exchanged onto the surface of the support, or in a solid solution with the support [8,9]. The magnetic signal of the spectrum of sample 698a at 17 K manifests itself only as a broad relaxation that contributes to the curvature observed in the background. The hyperfine fields distributions programme used does not converge to reasonable values unless this magnetic interaction is also introduced. Since the spectra show an impending magnetic splitting only at 17 K, it is inferred that the Fe3+ species must be in the form of very small particles of cr-Fez03. The most likely value of H is smaller than that of bulk ar-Fez03, but the difference is
Interaction of iron oxide on SiOr
2121
Table 1. Most likely values of hyperhne parameters of Fe-SiOr specimens calcinated at different temperatures (698,898 and 1098 K) and atmospheres (a = air, n = nitrogen) at 17 K Species Fe3+ (sp) Fe*’ Fe’+ (m)
Parameter ’
698a
698n
898n
d (mm/s) A (mm/s) 6 (mm/s) A (mm/s) 6 (mm/s) 2~ (mm/s) H(r) 6 (mm/s) 2~ (mm/s) H(r)
0.43 It 0.01 0.70 2 0.01 0.39 2 0.04b - 0.21 ? 0.03s 51.2 ? 0.3b -
0.34 f 0.04 0.61 + 0.04 0.48 ‘- 0.01 b - 0.21 IL O.Olb 52.6 2 0.1 b -
0.29 t 0.04 0.45 + 0.04 0.41 2 O.OIC - 0.02 ? 0.01 c 52.5 2 0.1’ 0.39 t 0.01 c - 0.22 2 0.01’ 47.5 t 0.2’
-
-
1098n 0.29 2 0.77 + 1.07 t 3.03 2
0.01 0.01 0.01 0.01
-
:A11isomer shifts are referred to cr-Fe at 298 K. cr-Fe203. ‘y-Fe203. Fe’+ (sp): superparamagnetic Fe’+. Fe3+ (m): magnetic Fe.+. accounted for by the collective model (CMEM) [lo].
magnetic
excitation
central signal even in the spectrum
at 17 K and the
hypertine magnetic field is reduced with respect to bulk
To estimate the particle size of cu-FeZOs we followed
a-Fe203. Again, this decrease can be well accounted for
the idea by Bodker er al. [2] who used tbe sizes of the
by CMEM. A magnetic anisotropy constant value (K) of
particles after reduction
to estimate those of the oxide
0.5 X lo5 Jme3 was estimated for o-Fe203 particles of
particles diameter
on the same samples. Using the average of Fe0 crystals obtained after reduction in
average diameters of = 120 A [lo]. Assuming that this
hydrogen
at 698 K [ 111, and judging
particles sintered during reduction because of the slow
value of H (50.5 2 0.1 r) at 298 K, CMEM yields an average particle size of approximately 150 A. Due to the
way in which the reduction
dependence of K with the particle volume, this value must
unlikely that the
was performed,
we expect
that particles in both samples contain the same number of Fe atoms.
Assuming
that the oxidized
and reduced
value applies for 698n too, and using its most likely fitted
be considered
only as a rough estimate.
It is well known that in cr-Fez03 particles of diameters
particles are both spherical and have the same densities
less than 200 A, the Morin transition
as crystalline or-FetOj and bee o-Fe’, respectively, an average diameter of = 45 A results for the cr-Fe203
suppressed [ 121. Hence, in 698n spectra, the quadrupole splitting value of 2e = - 0.21 + 0.01 mm/s at 17 K does
particles in 698a. The spectra belonging to 698n display a magnetic split
not change when the same sample is measured at 298 K, 2~ = - 0.23 + 0.01 mm/s, for our = 150 A diameter
fraction already at room temperature that increases at 17 K. The fitted hyperfine parameters correspond to
cr-Fe203 particles,
or-Fe203. The presence of a magnetic signal at 298 K is
small particles of cr-Fez03 whose Morin transition has
indicative that the particles are, on average, larger than
been suppressed
those of 698a. However, their size is not large enough to yield bulk parameters since there is a superparamagnetic
our result is coincident with the bulk value of 2~ = -0.20 mm/s [ 121. Additional X-ray diffraction data
This behaviour,
bulk one, is, however, coincident
is completely
different
from the
with that obtained for
[ 131. Moreover, at room temperature,
Table 2. Most likely values of hypertine parameters of Fe-SiOr specimens calcinated at different temperatures (698,898 and 1098 K) and atmospheres (a = air, n = nitrogen) at 298 K Species
Parameter”
698a
698n
898n
Fe’+(sp)
6 (mm/s) A (mm/s) 6 (mm/s) A (mm/s) 6 (mm/s) 2~ (mm/s) H(73 6 (mm/s) 2~ (mm/s) H(T)
0.34 IT 0.01 0.60 2 0.01 -
0.30 + 0.02 0.65 2 0.02
0.25 t 0.02 0.73 t 0.03 0.29 -c 0.01’ - 0.01 I? 0.01’ 49.4 t 0.1’ 0.29 2 0.02’ - 0.05 -c 0.02’ 46.5 2 0.3’
Fe*+ Fe’+(m)
aAll isomer shifts are referred to o-Fe at 298 K. bcz-Fe203. ‘y-Fe203. Fe”+ (sp): superparamagnetic Fe”. Fe”+ (m): magnetic Fe.+.
0.38 + 0.01 b - 0.23 + 0.01 b 50.5 2 0.1 b -
1098n 0.22 * 0.80 ? 0.96 + 2.80 + -
0.01 0.01 0.01 0.01
2122
S. G. MARCHETII
obtained for this sample with broad lines at 28 = 33.3, 35.8,49.5 and 54.3” confirm unambiguously the presence of (u-FeZ09 in 698n. Sample 898n spectra (Figs 1 and 2) display the presence of two signals belonging to a superparamagnetic species and to the two magnetic sextuplets. These spectra were fitted in the same way as in Ref. [14], with two sextets with slightly different parameters characteristic of y-FerOj [ 121.Notwithstanding that two magnetic hyperfine fields are used to fit the spectra, the signals are not resolved due to their very similar values, and the relative ratio of the populations between A and B sites is difficult to assess. The assignment of maghemite to the Fe3+ magnetically split signal of the sample 898n spectra is based also on the following reasons. The existence of y-FeZ03 is coherent with the observed change in colour from black to dark brown when the sample was exposed to air just after calcination. This suggests that after the calcination, the sample contained magnetite, which oxidized to y-Fez03 in air, as already observed by other authors [15]. In addition, it is the only sample that is attracted by a permanent magnet. The isomer shift values of both sextuplets are 0.29 C 0.02 mm/s at 298 K, which are very close to the 0.32 mm/s reported in Ref. [16]. Although the magnetic hypertine fields fitted, HA = 46.5 ? 0.3 T and HB = 49.4 2 0.1 T for sites A and B, respectively, show a reduction of 4.7 and 1.O%, respectively, with respect to HA = 48.8 T and HB = 49.9 T measured by Armstrong et al. [ 171,it can be explained on the basis of the collective magnetic excitations model [ 10, 13,181. Moreover, Picone et al. [ 141have found that for y-Fez03 particles of = 65 A of diameter, the hyperfine field is reduced as much as 18% from the bulk value. In spite of this large discrepancy, these authors attribute the observed decrease also to the effect of the small size of the particles in their samples. Using the same arguments as for 698n, the average diameter estimated applying CMEM is approximately 90Aif,asinRef.[12],Kistakenas105Jm-3andthe average of 47.4 T between the two most likely hyperfine magnetic fields fitted at room temperature is used. Additional X-ray diffraction patterns taken for samples 698a gave signals hardly discernible from the background noise. For 898n, the pattern displayed clearly only one broad signal at 20 z 37”, which is a position coincident for three types of oxides, (Y-and y-FeZ03, and Fe304. Comparing these results with those already mentioned for 698n and the subsequent determination of the particle sizes through Mijssbauer spectroscopy, the X-ray results are in complete accord with the particle diameters sequence mentioned above. A drastic change with respect to the above mentioned samples was observed in the Mossbauer spectra of 1098n over all the temperature range. Only two doublets
er al.
assigned to Fe’+ and Fe3+ are observed. Presumably, these ions have diffused into the support as a consequence of the severe thermal treatment. The reduction of some Fe3+ to Fe’+ could be due to the generation of oxygen vacancies caused by the calcination at a high temperature in flowing nitrogen. 3.2. Recoilless fraction The Mossbauer effect gives information on the phonon frequency distribution through its recoilless fraction. For an absorber, the intensity of absorption of resonant gamma radiation is proportional to: f(T)=exp(
- *),
(1)
where A7) is the probability of recoil-free absorption at temperature T and (z*)r is the mean-square vibrational amplitude at temperature T of the absorbing nucleus along the direction of propagation of the gamma radiation of wavelength X.In a Mossbauer experiment, the spectral area belonging to each species is proportional to its f-factor if the thin absorber approximation is verified. The thermal dependence of the f-factor is governed by the (.x2)term, which is a function better analysed as a sum of frequency moments of the phonon spectrum [19]. Although generally solids do not behave like harmonic oscillators with a cut-off frequency, as a first approximation it is customary to compare the experimental data with the behaviour predicted by Debye’s model. If the Debye model for the solid vibrations is assumed, the recoilless fraction has for high temperatures (T 1 t&/2) the asymptotic behaviour:
fLdT)=ev[(f-$&&T],(2) where t9n is the Debye temperature, Ea is the recoil energy of a free atom and ks is Boltzmann’s constant. To check that the Miissbauer results are indeed within the thin absorber limit, we considered the spectra obtained for sample 698n. This is the only one that could display a line broadening caused only by thickness effects and not by those owed to the small particle size [the particle diameters values of the three samples used for the relative 87) analyses, as mentioned above, were: z 45, 150 and 90 A for 698a, 698n and 898n, respectively]. For this spectrum, the magnetically split signal belonging to o-Fe203 with a relative population of 70%, that brings t, down to 3 0.4, yields a linewidth of P = 0.33 ? 0.01 mm/s. The criterion described by Rancourt er al. [20], leads to an equal spectral area attenuation of A/At,,in s 0.97, that falls within the thin absorber approximation. The logarithm of eqn (2) is linear in T, extrapolating to InAr) = 0 for T - 0 K. Fig. 3 shows the thermal dependence of the natural logarithm of the spectral
Interaction of iron oxide on SiOr determine
2123
eo, its graphic solution [21] gives &, values
of z 360 and s 340 K for 698a and 898n, respectively. 0.0
No graphic solution represents thefLArr vs Tdependence for 698n. The values for 698a and 898n are in good agreement with the Debye temperatures obtained from
l.-.).i.\
the high-temperature limit. However, the Debye model does not account for thefLArr(T) behaviour of 698n. These Debye temperatures are much smaller than those of macroscopic
mBa -0.4
crystals,
e.g. eo = 500 K for bulk
c+FezOs [2] and @o = 460 K for bulk y-Fe203 [12]. If
the decrease in the eo values were due to a true variation of the phonon frequencies
0.0
dependence
of the samples, because of its
on Bo, the second-order
Doppler shift should
with respect to the bulk values [ 141.
also be diminished
However, no noticeable changes in the isomer shifts of the iron species were found compared with the values for bulk or-Fez03 and y-FezOs.
-0.4
From this evidence,
assume that the Debye temperatures
we
of the crystallites
must be similar to those of bulk specimens as reported in [14, 221. The differences in the Debye temperatures
-0.6
obtained from the InAT) data must be ascribed to the -0.8
oversimplification particles
implied in eqn (2). i.e. j(T) for small
may not be only the result of the phonon
spectrum of the particle itself, but also it may be sensitive
0.0
to the coupling of the particle with the support (or to
t
the motion of the particle as a whole when the particle-support interaction is small). Indeed, the influence of the vibrations of the particle as
-0.2
a whole on the thermal dependence BB6n \
3.
-0.4 0
50
100
of the recoilless
fraction has been studied in previous works [2, 3, 141 where it has been assumed that the measuredf-factor can 150
200
250
be written as:
1 300
350
(4)
fTOT=fLATTfPART,
Temperature (K) where LATT and PART refer to the inner vibrations and Fig. 3. Thermal dependence of spectral areas of Fe-SiOl samples calcinated at 698 K in air and nitrogen and 898 K in nitrogen. The vertical scale has been normalized so that InA(T)=OforT=OK. areas for samples 698a, 698n and 898n normalized to zero at T = 0 K as was performed in Ref. [ 21. The data suggest
to the particle as a whole, respectively. is a consequence
the lattice and particle vibrations hence
the
This factorization
of the assumption that for those systems
mean
square
are uncoupled,
vibration
of the
and
resonant
absorbing nucleus can be written as:
that the Debye model might describe the lattice behaviour
G?&or
=~%ART+GI*)LA~~
(5)
for the three samples studied. Therefore, after the change of slope of ln(A/Ar,) around 100 K, the data were fitted with straight lines up to room temperature. The following Debye temperatures Sample ~DW
Bu were obtained:
698a 332 2 13
698n 247 + 12
89Sn 331 + 16
eqn (4). For these systems, if only
fLAnwere used instead of fToT, the flo obtained would be lower than the bulk values. Instead, if only fpART wereused, a hea temperam'e dependence
If, instead, the complete expression: +
where PART and LATT have the same meaning as in
-11 XdX
e*-1
(3)
forfLATT(7) deduced from the Debye model is used to
over the whole temperature range should be
observed [2]. The above
assumptions
are appropriate
when the
average distance between particles is large compared with their diameter, and hence the vibrational coupling between the particles can be neglected. This is the case of catalysts with low iron content and high surface area. In such circumstances,
the vibration of the particles as a
S. G. MARCHE’ITI ef nl.
2124
Table 3. Force constant between oxide particles and the support (q), angular frequency for particles vibrations as a whole (w) and Einstein temoerature (0~) for 698a. 698n and 898n. Sample
q (J/m*) w (s-l) ‘jE 6)
898n
69811
698a
44 6.9 x 10” 0.5
107 6.5 x 10” 5.0
I23 2.4 X 10” 1.9
must be low enough to allow the further reduction of the supported oxide and high enough to impair the sinterization of the oxide particles. The previous discussion shows that by an appropriate choice of the heat treatment it is possible to regulate the oxide-support interaction.
4. CONCLUSIONS whole are better described by a localized mode and the Einstein model is applicable [3]. Assuming that the particle vibrations as a whole can be described as an harmonic oscillator with an angular frequency: q
w=-
112
(> M
(6)
)
where q is the force constant between the particle and the support and M is the particle mass. At temperatures above the Einstein temperature (13~= hw/2aka), one finds:
The calcination atmosphere (air or nitrogen) does not modify the iron species present on the support, but influences the degree of interaction through the modification of the size of the oxide particles obtained, namely, = 45 and 150 a for cu-Fe20j. The temperature dependence of thef-factors indicates that the smaller the crystals the stronger the oxide-support interaction is. The present experiments are a ‘quantitative’ evidence of the trends mentioned in previous works [8], that the higher area/volume
ratio of the crystals increases the particle-
support interaction strength.
The condition T > flEis fulfilled at T 2 17 K. The values of q, w and f& shown in Table 3 have been estimated with eqns (4) and (6) taking Bn for bulk species to calculate fLAn since, as described above, the Debye temperatures of small particles do not differ much from bulk values [ 14,221. The high q values, especially for 698a and 898n, are indicative that the applied model does not fulfil the assumptions about the inexistence
of a coupling between
the particle phonon spectrum and the vibrations particle
as a whole.
For these intermediate
support
interactions,
a special
account the coupling between
model
of the
particle-
that take into
the support and particle
phonon spectra would be necessary.
To our knowledge
The calcination temperature in flowing nitrogen affects the iron oxide species obtained: at 898 K, y-FeZ03 is produced instead of the cu-FeZOj produced at 698 K. The strength of the interaction is higher for y- than for cr-FeZ03. Calcination at higher temperatures (1098 K) causes a diffusion of the iron ions into the support and the system no longer has the features of small supported oxide particles. Acknowledgemenrs-The
authors acknowledge support of this work by Consejo National de Investigaciones Cientificas y TBcnicas, Comisi6n de Investigaciones Cientificas Pcia. Bs. As. and Universidad National de La Plata, Argentina. S.G.M., M.V.C., N.G.G., A,A.Y. and R.C.M. are members of Carrera de1 Investigador Cientifico y TecnolBgico, CONICET. A.M.A. and J.F.B. are members of Cara de1 Personal de Apoyo, CONICET and CIC, respectively.
there is no such model available in the literature.
Notwithstanding being’ obtained from an oversimplified model, the q values of Table 3 allow one to perform a qualitative analysis of the particle-support interaction. It can be seen that 698a, with smaller crystal sizes, has a higher q than 698n. This result is a confirmation that for the same species, a smaller particle size produces a stronger interaction with the support. On the other hand, bearing in mind that the crystal size of a-Fe203 in 698a is about one half than in 898n, one can deduce that y-FeZOs interacts more strongly with the support than ol-Fe203. The comparison between 698n and 898n confirms that the y-FezOj crystals with the smaller size have a stronger interaction. Sample 1098n could not be analysed along these lines since, as a consequence of the higher calcination temperature, Fe migrated into the support actually forming a compound that no longer had the features of small supported particles. In the search for better catalysts, it is important to obtain an optimum degree of interaction; it
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