J. Phys. Chem. 1987, 91, 5961-5968
5961
Dynamic Phase Behavior of Graphite-Supported Iron-Rhodium Catalysts during Oxidation-Reduct ion Robert R. Gatte and Jonathan Phillips* Department of Chemical Engineering, The Pennsylvania State University, I33 Fenske Laboratory, University Park, Pennsylvania I6802 (Received: April 8, 1987)
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Graphite-supported Fe-Rh particles were produced by decomposition of an organic bimetallic precursor. The particles were studied with Miissbauer spectroscopy, X-ray diffraction, and transmission electron microscopy after various oxidation and reduction treatments. It was found that high-temperature oxidation ( T > 473 K) led to segregation of the metals into separate iron oxide and rhodium oxide phases, which exist as microcrystals within a single particle. Low-temperature hydrogen reduction (T< 373 K) of the oxide phases produced small, separate, reduced iron-only and rhodium-only crystallites within single particles. No bimetallic alloy particles were formed during the low-temperature reduction. High-temperature hydrogen reduction ( T = 673 K) led to formation of bcc bimetallic alloy particles and small amounts of both segregated rhodium and segregated iron. There was no significant interaction between the graphite support and the metal particles during any stage of the procedure. These results provide possibly important implicationsfor the interpretation of previous studies of iron-containing bimetallics.
Introduction Despite numerous the structure of supported bimetallic catalyst particles containing iron and any noble metal remains uncertain. There are two basic models of the structure of the fully reduced particles, one which includes an oxide phase1-15 and one that does not.'G22 Moreover, there are a variety of models of the structure of the oxide, as discussed recently by van der Kraan and Niemantsverdriet.' However, none of these models are supported unequivocally by the available data. There are several reasons why a definitive rendering of the structure has eluded workers to date. First, X-ray diffraction has not been widely used since the small particle sizes and complicated patterns arising from the support inhibit the collection of reliable X-ray data. In addition, X-ray line broadening, which is usually used to estimate particle sizes, can be further complicated by the composition-dependent lattice parameters of some alloys. Second, Miissbauer spectroscopy, which has been widely used for these systems, has yielded spectra which are difficult to interpret. That is, the Mossbauer spectrum of the hydrogen reduced particles is usually an asymmetric doublet which is open to. a number of interpretations. There is general agreement over the assignment of the strong zero-velocity peak to a zerovalent bulk alloy species which is either nonmagnetic or superparamagnetic. However, the assignment of the peak or shoulder near 1 mm/s has been less straightforward. For example, Garten and co-workers162' have consistently suggested that the peak is the right half of a doublet from a low-symmetry, but zerovalent, surface phase. Bartholomew and BoudartZ2also came to this conclusion. In contrast, other workers have proposed that the doublet is caused by an iron(II1) oxide of uncertain ~tructure,'-'~postulated to exist a t a number of locations relative to the particle surface and/or the particlesupport interface. Also, it is generally assumed that the structures formed during high-temperature hydrogen treatment and lowtemperature hydrogen treatment are the same since the Miissbauer parameters are nearly the same. As shown later, this is an unjustifiable assumption. In addition, several important effects, such as superparamagnetic relaxation, which may be able to explain some features of the Mossbauer spectra2ss26have largely been ignored. Third, the particles have generally been produced on hydroxylated substrates, which introduces the possibility of particle-support interactions strong enough to significantly influence the particle structure. This possibility has also been included in the structure models of other workers.*J4 Fourth, the interpretation of surface probes, from simple chemisorption to activity-reactivity measurements, is made difficult by uncertainty regarding the surface composition of the particles. That is, it is *Author to whom correspondence should be addressed.
0022-3654/87/2091-5961$01.50/0
usually not possible to determine with certainty if only one metal, both metals, or an alloy structure is responsible for the observed phenomenon. This current study was designed to overcome some of the difficulties outlined above. Specifically, large Fe-Rh particles (ca. 3 0 0 A in diameter) were produced on a noninteractive dehydroxylated substrate (graphite) via decomposition of a bimetallic organic cluster. This allowed very clear in situ X-ray spectra to be obtained. It also simplified the interpretation of the Miissbauer spectra, since the uncertain contributions of metal-support interactions and relaxation effects arising from small particle sizes could be neglected. A concomitant consideration of the X-ray and Miissbauer data proved to be a very powerful tool for obtaining (1) Van der Kraan, A. M.; Niemantsverdriet, J. W. In Industrial Applicarions o f f h e Miissbauer Effect; Long, G. J., Stevens, J. G., Eds.; Plenum, in press. (2) Niemantsverdriet, J. W.; van der Kraan, A. M.; Delgass, W. N . J . Catal. 1984, 89, 138. (3) Niemantsverdriet, J. W.; Axhenbeck, D. P.; Fortunato, F. A.; Delgass, W. N . J . Mol. Caral. 1984, 25, 285. (4) Van't Blik, H. F. J.; Niemantsverdriet, J. W. Appl. Catal. 1984, 10, 155. (5) Niemantsverdriet, J. W.; van Kaam, J. A. C.; Flipse, C. F. J.; van der Kraan, A. M. J. Caral. 1985, 96,58. (6) Niemantsverdriet, J. W.; van Grondelle, J.; van der Kraan, A. M. Hyperfine Interact. 1986, 28, 867. (7) Niemantsverdriet, J. W.; Stoop, F.; Nonnekens, R. C. H. Hyperfine Interacr. 1986, 28, 899. (8) Van der Kraan, A. M.; Nonnekens, R. C. H.; Stoop, F.; Niemantsverdriet, J. W. Appl. Catal. 1986, 27, 285. (9) Guczi, L. Catal. Reu.-Sci. Eng. 1981, 23(3), 329. (10) Guczi, L.; Matusek, K.; Eszterle, M. J . Catal. 1979, 60,121. (11) Schay, Z.; LAzBr, K.; Mink, J.; Guczi, L. J. Caral. 1984, 87, 179. (12) LA&, K.; Reiff, W. MBrke, W.; Guczi, L. J. Carol. 1986, 100, 118. (13) Lazar, K.; Reiff, W. M.; Guni, L.Hyperfine Interact. 1986,28, 871. (14) Berry, F. J.; Liwu, L.; Chengyu, W.; Renyuan, T.; Su,Z.; Dongbai, L. J . Chem. SOC.,Faraday Trans. I 1985, 81, 2293. (15) Minai, Y.; Fukushima, T.; Ichikawa, M.; Tominaga, T. J. Radioanal. N u l . Chem. 1984,87(3), 189. (16) Burton, J. J.; Garten, R. L. Advanced Materials in Catalysis; Academic: New York, 1977, Mater. Sci. Ser., p 33. (17) Garten, R. L. Miissbauer Effecf Methodology; Gruverman, I. J., Ed.; Plenum: New York, 1976; Vol. 10, p 69. (18) Garten, R. L.; Ollis, D. F. J. Carol. 1974, 35, 232. (19) Garten, R. L. J. Caral. 1976, 43, 18. (20) Garten, R. L.; Sinfelt, J. H. J . Catal. 1980, 62,127. (21) Vannice, M. A.; Lam, Y. L.; Garten, R. L. Adu. Chem. 1979, 178, 25.
(22) Bartholomew, C. H.; Boudart, M. J . Caral. 1973, 29, 278. (23) Kaminsky, M.; Yoon, Ki. J.; Geoffroy, L.; Vannice, M. A. J . Catal. 1985, 91, 338. (24) Hurst, N. W.; Gentry, S. J.; Jones, A.; McNicol, B. D. Catal. Rev.-Sci. Eng. 1982, 24, 233. (25) Gatte, R. R.; Phillips, J. J. Caral. 1987, 104, 365. (26) Ltn, S.-C.; Phillips, J. J . Appl. Phys. 1985, 58(5), 1943.
0 1987 American Chemical Society
5962
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987
convincing structural information. On the basis of the results presented below, several important features of the structure of FeRh particles formed from the thermal decomposition of bimetallic carbonyl derivatives on a hydroxyl-free graphitic carbon can be determined with a high degree of certainty. In this work we have been able not only to explain the structure of the fully reduced particles but also to determine the dynamic changes in the structure which occur as a function of gas atmosphere and temperature. First, following oxidation a t 473 K or higher, iron and rhodium in the particles are segregated and form separate but adjacent oxide phases: iron oxide (Fe203) and rhodium oxide (Rh203). Very small domains of each coexist within a single particle and form via phase segregation during oxidation of the alloy. The mechanism of the segregation may be similar to spinodal decomposition, which has been seen in many bulk metal oxide system^.^'-^^ Second, the structures which form during hydrogen reduction of the oxides depend on the temperature of the reduction. Low-temperature reduction (
