Characterization of supported bimetallic FeIr/SiO2 catalysts by
Mössbauer spectroscopy, temperature-programmed reduction
and X-ray photoelectron spectroscopy
Citation for published version (APA):
Niemantsverdriet, J. W., & Kraan, van der, A. M. (1986). Characterization of supported bimetallic FeIr/SiO2 catalysts by Mössbauer spectroscopy, temperature-programmed reduction and X-ray photoelectron spectroscopy. Surface and Interface Analysis, 9(4), 221-225. https://doi.org/10.1002/sia.740090405
DOI:
10.1002/sia.740090405 Document status and date: Published: 01/01/1986 Document Version:
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S U R F A C E A N D I N T E R F A C E ANALYSIS, VOL. 9, 221-225 (1986)
Characterization of Supported Bimetallic FeIr/
Si02
Catalysts by Mossbauer Spectroscopy,
Temperature-programmed Reduction and X-ray
Photoelectron-S pectroscop y
J.W. Niemantsverdriet" and A.M. van der Kraant
'Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, the Netherlands
'Interuniversitair Reactor Instituut, 2629 J B Delft, the Netherlands
Supported bimetallic FeIr/SiOz catalyst have been chacterized by in situ Mossbauer spectroscopy, temperature-programmed reduction (TPR) and XPS. The reduction of iron in freshly prepared FeIr/SiOz catalysts is greatly facilitated by the noble metal iridium. Reduced catalysts consist of Ir-rich fcc FeIr alloys, possibly some isolated Ir metal, and significant amounts of iron (111) oxide. The Mossbauer parameters of the latter change when the catalyst is exposed to CO gas at room temperature, indicating that the iron (111) oxide is present in a highly dispersed phase. Evacuation of reduced samples at 475 or at 775 K leads to significant oxidation of the iron in the catalyst, most likely by OH groups on the S i 0 2 support.
INTRODUCTION
In catalysis it is well known that alloying metals provides a powerful means to optimize the performance of catalysts.
'
As catalysts should prefereably have a large fraction of their atoms at the surface, alloys in the form of small particles stabilized by an inert support, also called supported bimetallic catalysts, are of great practical interest. Supported bimetallic catalysts consisting of iron and one of the more noble group VIII metals have shown attractive improvement of selectivity for reactions of CO and H2to light olefins and
Investigations of the structure of supported bimetallic catalysts are seriously hindered by their small dimensions and by the presence of a nonconducting, porous support. These catalysts are, therefore, not amenable to study by techniques such as x-ray diffraction or Auger electron spectroscopy. In this respect, iron-containing catalysts have the great advantage that Mijssbauer spectroscopy can be applied. This technique yields information on oxidation state, lattice symmetry, magnetism and lattice vibrations of the isotope"7Fe (natural abundance 2%) in the catal- yst, and can be applied in
sit^.^
Although in principle a bulk technique, Mossabauer spectroscopy can be made surface sensitive by analyzing the temperature dcpen- dence of the lattice vibrations5 and by studying the changes in the spectra which are the result of exposing the catalyst to a strongly adsorbing gas at room temperature."Little is known about FeIr catalysts. Preliminary catalytic tests show that methane is by far the dominant product in the CO
+
H2 reaction over Felr/Si02 at atmospheric pressure. At higher pressures, however, methanol and ethanol are formed as well.' Mijssbauer 0142-242 l/86/120221-05 $05.000
1986 by John Wiley & Sons Ltdinvestigations of supported FeIr systems have been r e p ~ r t e d . ~ . ~ These studies, however, concern the use of iron as a probe for the noble metal rather than the composition and structure of a supported bimetallic FeIr catalyst.
In this paper we present a characterization of FeIr/Si02 catalysts by XPS, temperature-programmed reduction (TPR), and in situ Mossbauer spectroscopy. The results illustrate how the latter technique can reveal information on the surface of iron-containing catalysts.
EXPERIMENTAL
The Fe1r/SiO2 catalyst was prepared by adding an aqueous solution of H21rClh.6H20 (Merck. PA), Fe(N03),.9H20 (Baker JT) and "Fe (Oak Ridge) in 2N H N 0 3 , dropwise and under frequent stirring to the S i 0 2 support (Cab-0-31, EH-5, 310 m2 g-I), until the incipient wetness point was reached. The catalyst contained 1.1 wt% iron, 10% of which was 57Fe, and 3.9 wt% iridium (atomic ratio Fe:Ir = l : l ) . The impregnated sample was dried in air at 295 K for several days, at 325 K for 24 h, and at 400 K for 72 h . About 300 mg of dried catalyst was pressed into a wafer with a diameter of 20 mm. All further treatments were carried out in the Mijssbauer irr .situ reactors dcscribed elsewhere. ' O . '
'
Mossbauer spectra were measured with a constant acceleration spectrometer. Isomer shifts are reported with respect to the NBS standard sodium nitroprusside (SNP).
XPS spectra were recorded with a Perkin Elmer PHI 550 spectrometer, equipped with a Mg Ka x-ray source and a double pass cylindrical mirror analyzer operating
222 J . W. NIEMANTSVERDRIET AND A. M. VAN DER KRAAN at a pass energy of 50 eV. Catalysts were reduced in a
pretreatment chamber in flowing H2. After evacuating the latter to a pressure of Torr, the sample was transferred to the main chamber. Typical pressures during measurement were about lo-' Torr.
TPR experiments were carried out by measuring the H2 consumption of a freshly prepared FeIr/SiO catalyst as a function of temperature, which was increased at a rate of 5 K min-I. Further details can be found in Ref. 12.
~ ~~
RESULTS AND DISCUSSION
The freshly prepared FeIrlSiO, catalyst, that is, after impregnation and drying but before reduction, has a Mossbauer spectrum which consists of a quadrupole doublet with parameters IS = 0.63
*
0.03 mm s-' and QS = 0.74 k 0.05 mm s-'. These values are characteristic of high-spin Fe'+ ions in an asymmetric environment as, for example, in highly dispersed iron (111) oxides." The XPS spectra of the fresh catalyst (binding energies in Table 1) indicate that the noble component, iridium, is also present in an unreduced state, as the Ir 4f7,2 binding energies are characteristic of lr'+ and Ir4+ ions.I4We will now discuss the reduction of the FeIriSiO;? catalyst. Fig. 1 shows the H2 consumption of fresh FeIr/Si02 and Fe/Si02 catalysts as a function of reduction temperature in a TPR experiment. Reduction of FeIriSiO, starts at about 3.50 K and is largely completed at 500 K. As opposed to this, reduction of Fe/Si02 starts around 500 K and has not yet been completed at 800 K , as can be inferred from the deviation of the TPR curve from the baseline at 800 K.
Both TPR curves consist of two peaks. TPR profiles of fully oxidized samples consisted of a single peak, for FeIr/Si02 at 485 K and for FeiSiO at 62.5 K. Hence the peaks at the lower temperatures in the TPR patterns of Fig. 1 can be attributed to reduction of the metal falts and the higher peaks represent the reduction of the metal oxides. Apparently, the drying procedure employed was not sufficiently severe to decompose the metal precursor salts entirely. This in agreement with the presence of an intense C1 2p signal in the XPS spectrum of the fresh FeIr catalyst.
The Mbssbauer spectra in Fig. 2 show how the state of iron changes as function of reduction temperature. A small fraction of the initially present Fe'+ has been reduced to Fez+ by H2 at 375 K , and a larger fraction at 475 K. Formation of zero-valent iron, visible by the single peak in the spectra around 0.35 mm s-' has already occurrcd at 475 K, but becomes increasingly important at higher temperatures. The maximum extent to which iron in FeIr/Si02 is reduced is reached
-
Table 1. XPS binding energies of 1:l FeIrlSi02 catalysts
Si 2p 103.4 103.4 103.4
0 I s 532.6 532.7 532.6
Ir 4f7,, 62.2 60.7 61.1
Ir 4fSl2 65.1 63.4 63.9
Fresh Reduced Passivated
199.1
-
- CI 2p I I I I I I' I Fe I r/Si02 0z
1
1 1 I I I 300 400 500 600 700 800 T ( K 1Figure 1. Temperature-programmed reduction of freshly prepared Felr/Si02 and Fe/SiO, catalysts.
I: I Fe Ir/SiO, Fe3' n Fez' n I 4 5 1I41 4 2 F q I 3 8 I 4 5 140 I 46 l 4 0 L
[
:,,I
:1, III(IIIIII/lli -10 -5 0 5 10 DOPPLER VELOCITY hrn/s)Figure 2. Mossbauer spectra measured in situ at 295 K, of the FelriSiO, catalyst after reduction i n H2 at the indicated temperatures. Stick diagrams indicate the positions of the peaks for the symmetrical doublets of Fez' and Fe3' and the singlet of Fe" in fcc Felr.
CHARACTERIZATION OF SUPPORTED BIMETALLIC CATA1.Y STS 223
at 675 - 775 K , and corresponds t o a Mossbauer
spectrum consisting of a singlet due to fcc FeIr alloy15 and a doublet of Fe3+. Each component accounts for about 50% of the spectral area.
XPS spectra of the catalyst after reduction at 875 K (Table 1 j indicate that Ir is present in the reduced form.I4 The Fe 2p spectra are broad and poorly resolved. Hence, no binding energies for Fe 2p have been included in Table 1. We note, however, that the Fe 2p,,, peak extends from about 706 to 712 eV, indicating that iron is present in more than one oxidation state.
The combination T PR , XPS and Mossbauer spectroscopy shows that reduction of Fe and Ir occur in the same temperature region and that iron in FeIr catalysts reduces at significantly lower temperatures than in Fe/Si02. Apparently, the presence of the noble metal Ir enhances the reducibility of iron greatly.
In general, reduction of metal oxides takes place in two steps. First, molecular H2 must react with metal ions to form zerovalent metal, in what is called the nucleation step. Once metal atoms are available, H2 can easily be dissociated to yield reactive H atoms which take care of further reduction.16 In the reduction of iron catalyst, nucleation occurs at rather elevated temperatures, around 500 K. In noble metals, on the other hand, nucleation takes place at lower temperatures, generally in the range of 200 - 400 K.'' Thus, in Felr/SiO2 catalysts Ir provides the sites where H2 is dissociated, after which H atoms diffuse to and reduce ionic iron in contact with Ir.
In the discussion of Fig. 2 we have assumed that the Mossbauer spectra of FeIr/Si02 after reduction at 675 -
875 K consist of a singlet and a doublet, representing Fe" in fcc FeIr alloy and Fe3+ in an oxidic environment, respectively. This interpretation, however, needs to be verified as other assignments cannot be excluded based on the room temperature spectra alone. The Mossbauer doublet with IS = 0.75 f 0.05 mm s-' and
QS = 0.75 k 0.10 mm s-' can in principle be due to high-spin Fe3+ ions, low-spin Fe2+ ions, and, according to Garten and Sinfelt' and Murrell and Garten', also to zero-valent iron in the surface of FeIr alloy. Yet another possibility which cannot be discarded without additional information is that the peak in the 295 K spectra at 1 mm s-l represents by itself an unresolved doublet of Fe2+, as in carbon-supported Fe/C catalysts.
''
Mossbauer spectra at cryogenic temperatures, as shown in Fig. 3, can distinguish between the possibilities mentioned above. The spectrum of reduced FeIr/Si02 at 77 K is very similar to the one at 295 K and shows in particular that the peak at 1 mms has not split at 77 K, as would have been expected for an unresolved Fez' doublet. Hence, the peak at 1 mms-' is indeed the right half of a doublet. The spectrum of FeIr/Si02 at 4 K shows that both the singlet and the doublet present at 77 and 295 K have become magnetically split, with a broad distribution in magnetic splittings. As a low-spin Fez+ ion has no magnetic moment, it cannot exhibit magnetic splitting. So the presence of low-spin Fez+ can be discarded.
The maximum values of the magnetic splittings in the
4
K spectrum of FeIr/SiO, are about 500 kOe. This is much higher than the magnetic fields of fcc FeTr, bcc1 - I 1 . 1 Fe Ir/SiO,
-
t
i l1
-10 -5 0 5 10
DOPPLER VELOCITY ( m r n / s )
Figure 3. Mossbauer spectra of the reduced Felr/SiO, catalyst, measured in situ at the temperatures indicated. The spectra a t 77 and 4 K allow for an unequivocal interpretation of the spectrum at 295 K. Analysis of the spectral intensities as a function of temperature yield qualitative information on the dispersion of the sample, see text.
FeIr, or a-Fe. However, the presence of a broad distribution of magnetic hyperfine fields with values upto 500 kOe is consistent with the assignment of thc corresponding phase to ferric iron in highly dispersed iron (111) We therefore conclude that the Mossbauer spectrum at 295 K of a reduced FeIr/SiOz catalyst consists, as anticipated in the discussion of Fig. 2, of a singlet due to fcc FeIr alloy and a doublet of Fe3+, most likely in the oxidic environment of the SiOz support.
Comparison of Mossbauer spectra at different temperatures provides a way to recognize surface behaviour. The Mossbauer intensity of a certain compound is determined by its concentration times the so-called recoilless fraction,
f.
I' This quantity dependson the energy of the lattice vibrations. The temperature dependence of f is satisfactorily described by t h e Debye model, in which the Debye temperature OD characterizes the lattice vibrations such that a high Or:, corresponds to a rigid lattice and a low OD to a lattice with soft vibrations. As Fig. 4 shows, the recoilless fraction of typical iron bulk compounds (0, 450 - 500
K j does not depend on temperature very much, and hence the intensity of the Mossbauer spectrum decreases only slighly between 4 and 300 K.
According to Somorjai,'" the OD of surfaces is roughly equal to half the value of the corresponding bulk material. The implications of this simple concept for iron atoms at the surface are shown in Fig. 4: the Mossbauer intensity of surface compounds decreases strongly with increasing temperature. Hence, by analyzing Mossbauer intensities as a function of
224
2.22
2.00
L
Y
;
J. W. NIEMANTSVERDRIET AND A . M. VAN DER KRAAN
2.22
2.00
L
Y
;
0.0
-
0 100 200 300 400 500 T ( K )
Figure 4. The recoilless fraction f (Mossbauer efficiency) as a function of temperature in the Debye model. The Debye temperature, OD, characterizes the lattice vibrations of the iron compounds. OD is high for a rigid lattice and low for a lattice with
soft vibrational modes.
temperature, surface behaviour can be inferred. This approach has been successfully ap lied in a study of passivation layers on iron catalysts' and of bimetallic FeRh/Si02
catalyst^.^
A full account on the theory andits limitations has been presented in.'
The areas of the FeIr/Si02 spectra, normalized to unity at 4 K, at 295 K, 77 and 4 K are 0.64, 0.91, and 1.00 respectively, and correspond to an effective OD of 275 4 25 K. This value, which is significantly lower than the O D of iron bulk compounds, indicates that the dispersion of the catalysts is high.
A disadvantage of XPS and other surface techniques is that the sample has to be evacuated prior to measurement. A useful application of in situ techniques such as Mossbauer spectroscopy, which can be applied both in situ and in vacuum, is to verify if the chemical state of the sample is preserved under evacuation. Fig.
5 shows Mossbauer spectra of a reduced FeIr/Si02
1 I I I I Fe3+n I : I Felr/SiO, 2.28
H-A
1.07 - -I0 -5 0 5 10 DOPPLER VELOCITY ( m m / s )Figure 5. Influence of evacuation on the state of iron in Felr/SiO, catalysts, as reflected in the Mossbauer spectra measured in siru
at 295 K after the treatments indicated.
catalyst under H2 and after evacuation of the in situ reactor to lo-' Torr at different temperatures. As Fig.
5 shows, the spectra of evacuated samples may differ significantly from the spectrum measured under H2, particularly when evacuation is carried out at higher temperatures. Evacuation leads to an increase of Fe3' at the expense of Fe', indicative of oxidation of the iron when H2 is removed from the gas phase. Comparison with the spectrum of the air-passivated catalyst in Fig. 6 learns that evacuation of reduced FeIr/Si02 at 775 K gives rise to the same extent of oxidation as does exposure to air at 295 K. Most likely, the oxidation of iron upon removal of adsorbed hydrogen is due to OH groups on the S i 0 2 support, according to a mechanism described by Dutartre et and Dalmon et al." The results imply that care should be taken when XPS is
applied to study the state of nonnoble metals such as iron in supported catalysts. We believe that the XPS
spectra of Ir reflect correctly the state of the noble metal in the reduced FeIr/SiOz catalyst, as the binding energies of the Ir 4f7/* are entirely characteristic of Ir metal.
Another way to obtain information on the surface from Mossbauer spectroscopy is to study changes caused by chemisorption of a gas such as CO or NH3.6 Figure 6 shows that when a reduced FeIr/Si02 catalyst is exposed to CO at 295 K, a considerable fraction of the Fe3+, visible by the peak at 1 mms-', is converted to Fez+, corresponding to the new peak at 2 mms-I. The fact that this conversion takes place at room temperature is taken as evidence that the iron ions involved are exposed to the gas phase, and thus are located at the surface. Similar results have been obtained with Si02-supported FeRu, FeRh, and FePt catalysts.22 The results suggest that a supported
I VJ I- 2 3 0 V In P t Y t VJ z W I-
z
I I I I Fe3:n I : ( FeIr/SiO, Fez-
Feo 3.25I
\/
CO,295K4
-10 -5 0 +5 +I0 DOPPLER VELOCITY fmm/s)Figure 6. Mossbauer spectra in situ at 295 K of a reduced FelrlSiO, catalysts (top) after exposure to CO at 295 K, oxidation in air at 295 K, and again exposure to CO at 295 K.
CHARACTERIZATION OF SUPPORTED BIMETALLIC CATALYSTS 22s
bimetallic catalyst consists of Ir rich FeIr alloy particles which rest on a thin layer of iron (111) oxide in contact with and stabilized by the S i 0 2 support. The alloy
particles do not cover the iron oxide completely but leave a fraction of the latter exposed to the gas phase. These are the sites where Fe3+ can be converted to Fe2+ by CO chemisorption. Minai et ~ 1have recently . ~ ~ obtained additional support for this model from a combination of Mossbauer spectroscopy and EXAFS (extended x-ray absorption fine structure). We refer to Refs; 22-24 for further details on the structure of a supported bimetallic catalyst.
As Fig. 6 also shows, passivation of the reduced
FeIr/Si02 in air at 295 K gives rise to considerable oxidation of the iron to Fe3+. This is another indication that the alloy particles are small. The XPS results in Table 1 indicate that Ir does not oxidize to a large extent during passivation. Combination of the XPS and Mossbauer results suggest that Fe is partially oxidized upon passivation, whereas Ir remains reduced, which suggests that the passivated alloy particle consists of an Ir rich core and an iron oxide shell around it.
CONCLUDING REMARKS
The present work shows that the combination of TPR, XPS and Mossbauer spectroscopy yields information on the composition, structure and reduction of bimetal- lic FeIr/Si02 catalysts. The fact that different gaseous environments of the catalysts are reflected in the 57Fe Mossbauer spectra demonstrates that Mossbauer spectroscopy contains information on the surface of well dispersed samples. A particularly useful application of this technique is that in situ spectra can be compared with spectra obtained under vacuum. In this way Mossbauer spectroscopy may contribute to bridge the gap between UHV investigations of catalysts by surface techniques and the high pressures under which catalysts are active.
Acknowledgement
We thank Mr. J . van Grondelle for measuring TPR profiles, and Mr.
E. Gerkema for assistance with the Mossbauer expcriments. J.W.N. acknowledges support from a Huygensfellowship of the Netherlands Organization for the Advancement of Pure Research (ZWO).
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