J. Phys. Chem. 1983, 87, 1292-1294
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I n Situ Mossbauer Spectroscopy of Bimetallic FeRhISiO, Catalysts at 295 K J. W. Niemantsverdrlet," A. M. van der Kraan, J. J. van Loef, Interuniversitair Reactor Instituut, 2629 JB Delft, The Netherlands
and W. N. Delgass School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907 (Received: December 14, 1982)
The reduction behavior of bimetallic FeRh/Si02 catalysts, prepared by means of the incipient wetness technique, has been studied with in situ Mossbauer spectroscopy at 295 K. The spectra demonstrate that coclustering of iron and rhodium occurs and that rhodium enhances the reducibility of iron. The appreciable changes in the spectra of the oxidized FeRh/Si02catalyst which occur upon chemisorption of H2or CO indicate that the dispersion of the iron is high.
Introduction Mossbauer spectroscopy has successfully been applied to verify clustering between the metals in supported bimetallic FeRu, FePd, FeIr, and FePt catalysts.14 For a review of these investigations we refer to Topsere et al.' Another interesting bimetallic cluster catalyst is FeRh/ Sios, as has been reported by Bhasin et aL,* who investigated this catalyst for Fischer-Tropsch synthesis at 300 "C and 68 atm. Under these conditions methane, methanol, and ethanol were the only products. In particular the formation of ethanol is interesting because a direct route from synthesis gas to an added-value chemical such as ethanol might be attractive on a commercial scalesg To our knowledge no Mossbauer studies of supported bimetallic FeRh catalysts have been published in the literature. In this paper we report the use of in situ Mossbauer spectroscopy a t room temperature to demonstrate coclustering of Fe and Rh in FeRh/Si02 catalysts and to investigate the reduction behavior. A subsequent publication will deal with in situ Mossbauer spectroscopy at cryogenic temperatures in order to study the surface composition of the FeRh/Si02 catalysts in more Experimental Section The catalysts were prepared by means of pore volume impregnation. An aqueous solution of Fe(N03),.9H20 (Merck P.A.) and RhCl3.rH2O(39 wt 5% Rh, Drijfhout) was added dropwise to the silica support (Grace S.P. 2-324.382, 290 m2/g) which was stirred continuously until the incipient wetness point was reached. The FeRh/Si02 catalyst prepared in this way contained 1.76 wt % of iron and 3.24 wt 70of rhodium (atomic ratio Fe:Rh = 1:l). A supported iron catalyst was prepared similarly and contained 3.52 wt % of iron. The impregnated samples were dried at room temperature overnight, subsequently at 335,355, and 375 (1) Garten, R. L. In *Mossbauer Effect Methodology"; Gruverman, I. J., Ed.; Plenum Press: New York, 1976; Vol. 10, p 69. (2) Lam, Y. L., Garten, R. L. Proceedings of the 6th Ibero-American Symposium on Catalysis, Rio de Janeiro, 1978. (3) Vannice, M. A.; Lam, Y. L.; Garten, R. L. Adu. Chem. Ser. 1979, No. 178, 15. (4) Garten, R. L.; Sinfelt, J. H. J. Catal. 1980, 62, 127. (5) Bartholomew, C. H.; Boudart, M. J . Catal. 1973, 29, 278. (6) Guczi, L. Catal. Reu.-Sci. Eng. 1981, 23, 329. (7) Topsae, H.; Dumesic, J. A.; Msrup, S. In Applications of Mossbauer Spectroscopy"; Cohen, R. L., Ed.; Academic Press: New York, 1980; VOl. 11, p 55. (8) Bhasin, M. M.; Bartley, W. J.;Ellgen, P. C.; Wilson, T. P. J. Catal. 1978, 54, 120. (9) King, D. L.; Cusumano, J. A.; Garten, R. L., Catal. Reu.-Sci. Eng. 1981, 23, 233. (10) Niemantsverdriet, J. W., Thesis, University of Technology, Delft, 1983. (11) Niemantsverdriet, J. W., to be published
TABLE I: Mossbauer Parameters of FeRh/SiO, at 295 Ka % of
treatment impregnation and drying H,, 295 K
H,, 400 K , 1 h H,, 725 K , 6 h air, 295 K
H,, 295 K CO. 295 K
IS,b iron state mmis
QS, spectral mmis area
Fe3+
0.64
0.74
100
Fez+ Fe3+ Fez+ Fez+ Fe3+ FeG Fe3+ FeO Fe3+ FeG Fez+ Fe3+ FeO Fez+ Fe3+
1.43 0.64 1.44 1.35 0.64 0.36 0.64 0.34 0.64 0.34 1.44 0.63 0.34 1.44 0.63
2.36 0.69 2.08 0.90 0.65
34 66 28 34 38 66 34 10 90 27 30 43 24 49 27
0.81 0.92 2.04 0.72 2.24 0.65
a Accuracies in IS, 0.04 mm/s, QS, 0.08 mm/s, spectral IS relative t o SNP. area, 5%.
K for 2 h at each temperature, and finally at 395 K for 60 h. A quantity of 320 mg of catalyst was pressed into a wafer with a diameter of 20 mm, using a pressure of 100 atm. The Mossbauer in situ reactor has been described elsewhere.l0 The gases H2 (Hoekloos,purity >99.9%) and CO (Hoekloos, >99.5%) were each purified over a reduced copper catalyst (BASF, R3-11) and a molecular sieve (Union Carbide, 5A). Reduction was carried out in a Hz flow of 6 L/h. Mijssbauer spectra were obtained in situ with a constant acceleration spectrometer, which uses a 57C0in Rh source. The spectra are not corrected for the varying distance between source and detector, hence the origin of the curved background is instrumental. Isomer shifts (IS) are reported relative to the NBS standard sodium nitroprusside (SNP) a t room temperature. Mossbauer spectra were fitted by computer with calculated subspectra consisting of Lorentzian shaped lines by varying the Mossbauer parameters in a nonlinear, iterative minimization routine.'O In the case of quadrupole doublets the line widths and the absorption areas of the constituent peaks were constrained to be equal. Results In order to demonstrate the influence of clustering of Rh with Fe both the FeRh/SiOz and the Fe/Si02 catalyst were reduced under the same conditions. Figure 1 shows
0022-3654/83/2087-1292$01.50/00 1983 American Chemical Society
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The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
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3 23
H2
Z%K
I25
H2
2 I7
I
FeRhISid,
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2 14-
HZ
725 K 6h
3 50 air
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0 *5 *IO Doppler veloclty Immis)
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Figure 1. I n situ Mossbauer spectra at 295 K of FeRh/SiO, and Fe/SiO, catalysts after the subsequent series of the treatments indicated.
the in situ Mossbauer spectra of the two catalysts at room temperature after different stages of reduction, as indicated in the figure. The Mossbauer parameters of the bimetallic catalyst are listed in Table I. The Mossbauer spectra of the impregnated and dried FeRh/Si02 and Fe/Si02 catalyst have identical Mossbauer parameters, which correspond to Fe3+ions. Subjecting the catalysts to H2 a t various temperatures, however, shows considerable differences in reduction behavior between the bimetallic and the monometallic catalyst. The Mossbauer spectrum of the FeRh/Si02 catalyst exposed to H2 at room temperature (Figure 1)indicates that a significant fraction of the original Fe3+ions has been converted into Fe2+ions, as shown by the peak a t about 2.5 mm/s. This high velocity peak is attributed to an Fe2+ doublet, the low velocity counterpart of which is hidden under the peaks in the 0-1 mm/s region of the spectrum. The quadrupole splitting of this doublet is about 2.4 mm/s. The corresponding spectrum in Figure 1 shows that the unreduced monometallic Fe/Si02 catalyst is not at all affected by H2 a t 295 K. Reduction in H2 at 400 K for 1h induces further changes in the spectrum of the bimetallic catalyst, whereas that of the monometallic catalyst remains unaltered. The Mossbauer spectrum of the FeRh/Si02 catalysts is characteristic of a combination of three doublets, two of which are identical with the Fe2+and Fe3+doublets of the previous spectrum. The isomer shift and quadrupole splitting of the third doublet are characteristic of Fe2+in a coordination less than octahedral. The Mossbauer spectrum of the FeRh/Si02 catalyst after reduction a t 725 K in H2 for 6 h (Figure 1) consists of a large single line and a smaller poorly resolved contribution. Additional reduction at 725 K or at 775 K does not induce further changes in the Mossbauer spectrum. The spectrum has been analyzed with a computer under the assumption that the poorly resolved spectral contribution belongs to a doublet, the left peak of which coincides with the singlet. The isomer shift of the singlet (0.36 mm/s) is within the range of those for FeRh bulk alloys.I2 The Mossbauer parameters of the doublet are characteristic of high-spin Fe3+ions. A more detailed interpretation of this spectrum will be discussed elsewhere." (12) Chao, C. C.; Duwez, P.; Chang, C. F. J. J.Appl. Phys. 1971,42, 4282.
-10
-5
0
+5
+IO
Doppler velocity (mm/s) Figure 2. Mossbauer spectra at 295 K of the air-exposed FeRh/SiO, catalyst after chemisorption of the indicated gases at 295 K.
The Mossbauer spectrum of the monometallic Fe/SiO, catalyst after 6 h of reduction in H2 a t 725 K shows that only a little magnetically split a-Fe (10-15% of the spectral area) formed; the four outer lines of its sextuplet are just visible. The magnetic hyperfine field of this sextuplet is about 324 kOe, which is smaller than that of bulk a-Fe, 330 kOe at 295 K. This indicates that the dimensions of the iron crystallites in the reduced Fe/Si02 catalyst are very small.13 Agreement seems to exist in the literature7 that iron in reduced Fe/Si02 catalysts occurs 85 FeO and/or Fe2+. The spectrum, however, indicates that most of the iron is still in the Fe3+ state. We attribute this to the relatively short reduction period of only 6 h. Both the reduced FeRh/Si02 and the Fe/Si02 catalyst are not stable with respect to oxygen. The lower two spectra of Figure 1, recorded ex situ, show that most of the reduced iron is oxidized upon exposing the catalysts to air at room temperature. This is regarded as further evidence that the particles in the reduced catalysts are well dispersed. It has been reported in the literature that reoxidized supported FeRu, FePd, and FePt catalysts can partially be reduced by H2 a t room temperat~re.l-~>"~ In order to investigate whether FeRh/Si02 catalysts show the same behavior, we placed the sample that was reduced a t 725 K and subsequently exposed to air at room temperature (Figure 2, upper spectrum) under an H2 atmosphere at room temperature. Its Mossbauer spectrum (Figure 2) indicates that a considerable fraction of the original Fe3+ ions is converted to Fez+ ions upon H2 chemisorption. Reexposure to air restored the original spectrum of the catalyst under air. Chemisorption of CO resulted in a catalyst that is even more reduced than that after chem(13) Marup, S.; Topsae, H.Appl. Phys. 1976, 11, 63.
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isorption of H2. Again the original spectrum appeared on exposing the catalyst to air. The spectra of Figure 2 were analyzed by computer with the assumption that the spectrum of the catalyst under air consist of a singlet and a doublet. The spectra of the catalyst under H2 or CO were fitted with a combination of a singlet and two doublets. See Table I for results.
Discussion If we compare the Mossbauer spectra of the bimetallic FeRh/Si02 and the monometallic Fe/Si02 catalyst after the different reduction steps, it is clear that clustering of Fe and Rh in the former has indeed occurred. The partial reduction which takes place a t room temperature and the relatively large contribution of zerovalent iron in the Mossbauer spectrum of the catalyst after reduction at 725 K both demonstrate that the more noble metal, rhodium, greatly enhances the reducibility of the less noble metal, iron. The isomer shift of the single peak in the spectrum of FeRh/Si02 after reduction at 725 K is within the range expected for FeRh bulk alloys, as reported by Chao et a1.12 Furthermore, the absence in this spectrum of any contribution due to either magnetically split cy-Fe or to Fe2+ ions, which are commonly encountered in monometallic Fe/Si02 catalysts,' indicates that isolated iron particles were not formed. Interesting changes occur in the bimetallic catalyst upon reduction in H2at relatively low temperatures, 295 and 400 K. A t 295 K H2 exposure yields a ferrous compound with Mossbauer parameters which are in the range of those for iron(I1) bulk compounds. Bartholomew and Boudart5 observed a similar Fez+doublet in the Mossbauer spectrum of an oxidized FePt/C catalyst which was exposed to H2 at room temperature. They suggest that the Fe2+doublet is due to a Fe-OH surface complex. Following this in-
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terpretation it is conceivable that the coordinatively unsaturated Fez+ions, which are observed after reduction of the FeRh/Si02 catalyst in H2 at 400 K, are formed from these Fe-OH surface complexes by means of a reaction between OH groups and the subsequent desorption of water. Chemisorption of H2and CO induced significant changes in the Mossbauer spectra of the air-exposed FeRh/Si02 catalyst. The great sensitivity of the iron ions in the catalyst to their gaseous environment implies that the degree of dispersion in these catalysts must be very high. As the recoilless fraction of surface and bulk atoms may differ considerably, the use of Mossbauer spectra which are taken at 295 K only cannot yield a reliable quantitative measure of the dispersion. The changes in the Mossbauer spectra which are the result of chemisorbing a reducing gas like H2or CO on the one hand and O2 on the other hand are reversible. This reversibility appears to be characteristic for supported bimetallic clusters of iron and a more noble group 8 We further note that CO chemisorption converts significantly more Fe3+into Fe2+ions than does H2 chemisorption. In the future we will show that this property can be used to obtain structural information on bimetallic FeRh/Si02 catalysts.'l
Acknowledgment. The authors thank H. F. J. van't Blik of the Laboratory for Inorganic Chemistry, Eindhoven University of Technology for the skillful preparation of the catalysts. The authors also thank the NSF Materials Research Laboratory a t Purdue University (Grant DMR80-20249), NSF (Grant ENG 7920847), and the Netherlands Organization for the Advancement of Pure Research (Z.W.0) for enabling J.W.N. to work at Purdue University in the spring of 1982.
