Mössbauer spectroscopy of aged ferric oxide gels

Mi~ssbauer Spectroscopyof Aged Ferric Oxide Gels K E N N E T H KAUFFMAN I AND FRED HAZEL Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19174 Received August 22, 1974; accepted March 24, 1975 INTRODUCTION Ferric oxide gels are formed by the addition of aqueous ammonia or other base to concentrated aqueous ferric salt solutions. These gels have been shown to have very small particle size (1-3) and great reactivity to chemical reagents (3-6) and are considered by most investigators to be amorphous (1, 3-13). This paper is concerned with two aspects of the aging of these gels. First, we report the observation with M6ssbauer spectroscopy of an intermediate in the thermal transformation of the gel to a-Fe20~. Secondly, M6ssbauer measurement of magnetic hyperfine splittings (HFS) is presented as a sensitive indication of the degree of crystallinity of the products of aging. Thermal Transformation Transformation of the freshly washed and dried gel to a-Fe203 occurs between 300 ° and 400°C (2, 4, 11-13). When rapidly heated, DTA indicates a sharp exotherm at or about 400°C (14) so great in magnitude as to induce visible red glowing. This glow phenomenon has been attributed to a sudden loss of surface area (14, 15) or a sudden phase change (16). Probably both processes would occur simultaneously. Bartenev et al. (19) observed with TGA and DTA that the gel is completely dehydrated prior to a strong exotherm. 1Present Address: National Center for Energy Management and Power, University of Pennsylvania, Philadelphia, Pennsylvania.

Freund has noted sharp exotherms on heating aluminum hydroxide gels and kaolinite which he attributed to the autocatalytic collapse of intermediate low density framework structures left after dehydration (12, 17). An intermediate has never been directly observed in the transformation of iron oxide gels, but may have been observed in the thermal decomposition of /~-FeOOH and ~-FeOOH. Bernal et al. (18) isolated Fe203 in a magnetically disordered state at 300°C in the transformation of f3-FeOOH to a-Fe203. Heating this material gave a sharp exotherm at 420°C, which was subsequently interpreted as due to the collapse of the dehydrated skeleton (20). 2 The only case in which an intermediate structure has been observed with MSssbauer spectroscopy is in the decomposition of ~-FeOOH to a-Fe203 (22). At 195°C Vlasov et al. obtained an intermediate that gave no observable M6ssbauer spectrum. This same type of behavior has been found for the gel in the present work. Crystallinity of Aging Products When kept wet the gels age even at room temperature into mixtures identificable by x-ray diffraction as a-Fe203 and a-FeOOH. The rate of transformation and the distribution of products depends on the temperatures during preparation and aging (6-8) and on the pH and the presence of anions, e.g., chloride, during aging (1, 6, 9). This frameworkmay be related to the report of the preparation of j3-Fe~O3(21).

422 Journal of Colloid and Interface Science, Vol. 51, No. 3, June 1975

Copyright ~ 1975 by Academic Press, Inc. All rights of reproduction in any form r c ~ ' v c d .

MOSSBAUER OF FERRIC OXIDE GELS Reactivity with chenfical reagents has been used to determine the percentage of amorphous fraction of iron oxide gels during or after aging: Schwertmann (7) and Landa and Gast (5) using ammonium oxalate; Krause (23) using nitric acid; and Okamoto (4) using hydrazine. Results of investigating the gel with these reagents showed that even well-aged material reacts much more rapidly with all reagents than any well-defined phases with hexagonally packed oxide or oxyhydroxide lattices (3). MATERIALS AND METHODS

Preparation of the Gel At room temperature 0.5 M FeCI3 was rapidly mixed with a twofold stoichiometric excess of 8.5 M NH3, the product was washed free of C1- with dilute NH~ and distilled water, and it was dried at room temperature.

Electron Microscopy Samples were sprayed with ultrafiltered Freon onto evaporated carbon films supported on copper grids. Samples were mixed with a very small amount of a gold sol to provide contrast for focusing. A Philips EM300 microscope with a 100-kV beam was employed at an instrument enlargement on the plates of 204 000 diameters.

MOssbauer An Elron Electronics Corp. driving unit was employed with the function generator operating in constant acceleration mode. The drive, sample, and proportional counter detector (Reuter Stokes G-61-M2) were aligned in transmission geometry. The Victoreen PIP400A analyzer with a 400-channel capacity was coupled to a Teletype and the data output was used to plot spectra. Reference materials used for obtaining isomer shift (IS), quadrupole splitting (QS), and HFS parameters were a single crystal of SNP calibrated by the National Bureau of Standards and annealed 99.99% iron foil. The source employed was 10 mC 6~Co in annealed copper foil, obtained

423

from New England Nuclear Corp. Samples were supported between sheets of cellulose acetate or on Scotch cellophane tape. All spectra were taken with source and absorber at room temperature. RESULTS AND DISCUSSION

Electron Microscopy Figure 1 shows that the preparation was homogeneous with the bulk of the material composed of irregularly shaped particles of 25-30 A diameter. There is slight indication of the formation of linear agregates which appear similar to/~-FeOOH subcrystals (24, 25).

Miissbauer M~Sssbauer effects associated with particles in the colloidal size range are discussed in detail in various review articles (26-29). The fresh air-dried gel spectrum is given in Fig. 2a. The measured values were IS relative to SNP 0.63 -4- 0.02 mm/sec; QS, 0.60 ± 0.05 mm/sec; and line width at half maximum, 0.65 4-0.05 ram/see for both peaks. The IS and QS were both characteristic of Fe (III) and indicated that all iron atoms were octahedrally coordinated. As discussed by Armstrong et al. (30), tetrahedrally coordinated iron is expected to be more covalent than octahedral iron and therefore it should have a smaller IS. In addition, the peaks were not broad enough to indicate both octahedral and tetrahedral iron; however, the peaks were broader than expected for a well-defined phase, being 2.3 times the line width of the source. This broadness probably is best interpreted as indicating variations in the octahedral complexing parameters, i.e., the number and arrangement of oxy and hydroxy-groups about each iron atom. Samples of the fresh gel air-dried at room temperature were left in an oven or muffle furnace for long periods of time and the M~Sssbauer spectra of the products are given in Figs. 2b-2d and 3a. The spectra in Fig. 2 all have the same number of counts per channel and all samples had the same initial sample Journal of Colloid and Interface Science, Vol. 51. No. 3, June 1975

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FIO. 1. Fresh Gel. (;< 592 000).

weight. Fig. 2d is shown on a 2.5 times expanded vertical scale. Heating to 200°C for 64 hr caused an increase in QS from 0.60 to 0.76-t-0.05 m m / s e c and both lines were broadened from 0.65 to 0.82 4- 0.05 mm/sec. As water was removed, all iron sites were not affected equally as evidenced by the increased line width, and at least some fraction of the iron sites became less symmetric (by the increased QS). Relaxation broadening b y a weak magnetic field is rejected as an explanation because of the s y m m e t r y of the quadrupole doublet. If there were magnetic relaxation broadening, the 4-~ to 4-½ transitions, because of the larger Zeeman interaction, would be expected to broaden more than the 4-½ to 4-½ and zV½ transitions and the peaks would not be symmetric (31). Similar behavior of the QS on dehydration has been reported for 6-FeOOH (22) and in another study of the gel (13). Journal of Colloid and Interface Science, Vol. 51, No. 3, J u n e 1975

After heating the gel at 320°C for 18 hr, the spectrum of the resulting sample is a very broad almost featureless absorption; see Fig. 2d? This is thought to be the result of both a very large distribution of magnetic fields at the iron sites and considerable variety in the nature of the sites and is precisely the behavior found by Vlasov et al. for 6-FeOOH heated 3 It is interesting to contrast the featureless absorption with the behavior of amorphous materials predicted by the magnetic cluster theory of Trousdale et al. (32). That theory predicts a magnetic ordering temperature at which the M6ssbauer absorption is a single broad peak. This type of peak is apparent in the spectrum shown for the fresh gel at 45 K by Mathalone (33) and is indicated along with an HFS at 4.5 K for ferric perchlorate hydrolysis products by Vertes (34). The different behavior is most likely due to the fact that the gel intermediate, being dehydrated, is as ordered magnetically at room temperature as the lattice structure will allow. The fresh gel transition state on the other hand, is from a disordered to an ordered magnetic state.

MI~SSBAUER OF FERRIC OXIDE GELS to 195°C (22). The nature of this intermediate state is not known except that it is almost completely dehydrated. Vlasov explains the featureless spectrum by proposing a state in which the iron atoms have sufficient freedom of motion (at room temperature) to preclude recoilless absorption, but this is not convincing since such a state would not be expected to have the stability to exist at higher temperatures. After heating the fresh air-dried gel at 430°C for 48 hr, the product is identifiable as a-Fe~Oz in agreement with previous D T A and x-ray studies. However, the Mt~ssbauer spectrum, Fig. 3a, indicates a HFS of only 480 4- 5 kOe, 70-/0 less than the generally accepted value of 517 kOe for well crystallized c~-Feo.O~ (35). A sample of the gel was boiled in distilled water for 20 hr and Fig. 3b supports the expectation (1, 5, 6) that the product is a mixture of a-Fe~O~ and cz-FeOOH. The HFS for a-Fe~O~ and cz-FeOOH of 487 4- 5 kOe and 360 4- 5 kOe, respectively, are similarly reduced in comparison to the accepted values at room temperature of 517 kOe for a-FezOz and 388 kOe for a-FeOOH (36-38). We have observed similar reductions in the effective field in studies of t

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the products of boiling hydrolysis of ferric chloride solutions (3), and consider these reductions the result of poor crystallinity. Van der Woude and Dekker (36) interpreted reductions in the HFS for some synthetic samples of a-FeOOH as the result of poor crystallinity. Loseva (13) found the product of ferric oxide gel decomposition at 400°C to be c~-Fe203 with a HFS of 504-4-5 kOe, which increased with subsequent heating to higher temperatures. Vlasov (22) found that c~-Fe203 (x-ray identified) resulting from &FeOOH decomposition at 225°C had a HFS of 409 4- 30 kOe which gradually increased to 515 4- 10 kOe by 650°C. All of the above spectra were taken at room temperature. Some attempts were made without success to quantify the HFS data in a way that would give an index of crystallinity, for example, an effective mean volume within which the a-Fe208 structure is well ordered. SUMMARY

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FIG. 2. Mtissbnuer Spectra of Thermally Aged Gel at 300 K. (a) Fresh Gel. (b) 112°C for 20 hr. (c) 200°C for 64 hr. (d) 320°C for 18 hr.

Mt~ssbauer spectroscopic results indicated that aging the gel in a variety of ways produced poorly crystalline products (in agreement with their chemical reactivity) which had previously been identified as a-Fe203 or mixtures of a-Fe20~ and a-FeOOH. The HFS of aged products as observed by M~Sssbauer spectroscopy is believed to be a sensitive indication of crystallinity; a-Fe203 produced by heating the gel at 430°C for 48 Journal of Colloid and Interface Science,

Vol. 51, No. 3, June

1975

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KAUFFMAN AND HAZEL

hr showed a 7% reduction in H F S compared to well-crystallized hematite. Finally, the thermal conversion of the gel to ~-Fe203 involved an intermediate structural state that was interpreted to mean considerable variety in the number and arrangement of ligands around the iron atoms, and a large distribution of magnetic fields. The state, observed after heating the gel for 18 hr at 320°C, gave an almost featureless MSssbauer absorption at room temperature. REFERENCES 1. FEITKNECHT,W., MICHAELIS,W., ttelv. Chim. Acta

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