Characterization by Moessbauer spectroscopy of iron carbides formed

J. Phys. Chem. 1982, 86, 4799-4808

4799

Characterization by Mossbauer Spectroscopy of Iron Carbides Formed by Fischer-Tropsch Synthesis G. Le Caer, J. M. Dubols, Labwetoire de h46tallurgie associ6 au C.N.R.S.,L.A. 159, Ecole des mlnes, Parc de Saurupt, 54042 Nancy C&x,

France

M. PIJolat,V. Perrichon, and P. Bussl6re' Instltut de Recherches sur la Catalyse, C.N.R.S.,89828 Vllleurbenne Cedex, France (Received: Merch 15, 1982; In Final Fwtn: July 8, 1982)

Recent Mossbauer spectroscopy results on iron carbides formed by Fischer-Tropsch synthesis are discussed. For carbides with carbon in octahedral sites (named here 0 carbides), we show that "C'-F~~,~C'' and "c-Fe2C" really correspond to Fe2C and -Fe2.J2, respectively. A departure from hexagonal symmetry of the C atom arrangement is deduced for -Fe2C while the limiting compositions are Fe2.4M.1C and Fe2C. Simple relations are given to calculate the mean carbon concentration in 0 carbides or in any mixture of 0 carbides or of 0 and x,6 carbides from the intensities of the Mossbauer subspectra. The model of Andersson et al. is used to describe the 0 x 6 transformation and compared with literature results. A study of 0 carbides formed by Fischer-Tropsch synthesis in Fe/A1203and Fe(Cr)/A1203catalysts at 523 K is detailed, especially their change in composition and structure as a function of time. A correlation between the hyperfine fields of Fe atoms with three carbon atom neighbors and the mean carbon content is observed and interpreted from Mossbauer spectra, Curie temperature, and specific area measurements on -FezC carbide. A model for carburization of iron catalysts in the Fischer-Tropsch reaction accounting for the observed behavior and the literature results is proposed.

--

I. Introduction Iron carbides which are formed in catalytic reactions (Fischer-Tropsch (FT) synthesis,14 diamond synthesis6), during the tempering of supersaturated iron-carbon martensite6-8or during the crystallization of amorphous Fel,C, ( x 2 0.25) sputtered alloys? adopt various structures. These structures may be classified according to the sites occupied by the carbon atoms: structures with carbon atoms in trigonal prismatic interstices (noted TP structures for brevity), and structures with carbon atoms in octahedral interstices (0 structures). Cementite (Fe3C), Hagg carbide (x-Fe6C2),and Fe& are TP carbides whose structures have been clearly established.1° This is not the case at all for 0 metastable carbides for which a complete characterization is difficult, mainly because of the particle sizes which are involved. Precise X-ray diffraction or electron microscopy results, accurate determinations of carbon contents, or information about carbon ordering in 0 carbides produced in catalytic reactions (principally in Fischer-Tropsch synthesis) are practically nonexistent. Mossbauer spectroscopy is the main technique which has been recently used to charac~

~~~~

~

(1) Raupp, G. B.; Delgass, W. N. J. Catal. 1979, 58, 337, 348. (2) Unmuth, E. E.; Schwartz, L. H.; Butt, J. B. J. Catal. 1980,63,404. (3) SchUer-Stahl, H. Angew. Chem., Int. Ed. Engl. 1980, 19, 729. (4) Niemantsverdriet, J. W.; Van Der Kraan, A. M.; Van Dijk, W. L.; Van Der Baan,H. S. J. Phys. Chem. 1980,84, 3363. (5) Kohn, J. A,; Eckart, D. W. Am. Mineral. 1962,47, 1422. (6) Hirotsu, Y.; Nagakura, S. Acta Metall. 1972,20,645. Trans. Jpn. I m t . Met. 1974, 15, 129. (7) Williamson, D. L.; Nakazawa, K.; Kraus, G. Met. Tram. 1979, IOA, 1351. (8) Ino, H.; Ito, T.; Nasu, S.; Gonser, U. J. Jpn. Inst. Met. 1980, 44, 1171. Acta Metall. 1982, 30, 9. (9) Bauer-Grosse, E.; Le CaBr, G.; Frantz, C.; Heiman, N. J. NonCryst. Solids 1981, 44,277. (10) Nagakura, S.; Oketani, S. Trans. Iron Steel Inst. Jpn. 1968, 8, 265. 0022-365418212086-4799$01.25/0

terize such carbides. The aim of the present paper is to critically discuss the conclusions which have been drawn from Mossbauer spectroscopy. In particular, we shall show that the results are inconsistent with the proposed carbon contents. We shall propose an interpretation of already published data and characterize the change in mean carbon contents of 0 carbides formed in Fe/A1203 FischerTropsch catalysts at 523 K.

11. Previous Investigations of 0 Carbides Formed During FT Synthesis Three different types of 0 carbides formed in catalytic reactions are reported as c-Fe2C, t'-Fe2&!, and Fe,C. c-Fe2Chas a Curie point of 653-673 K'l and an hexagonal or approximately hexagonal close-packed array of iron atoms.12 Arents et al.13 and Maksimov et al.14 have analyzed the Mossbauer spectrum o f t carbide as a combination of three different sites with respective hyperfine fields of 170 f 3,237 f 3, and 130 f 6 kOe and intensities 4,1.6, and 1,respectively. They then concluded that the structure of E carbide can be treated as intermediate between the hexagonal close-packed structure of t'-Fe2.2Cand the monoclinic structure of Hiigg carbide. However, Niemantsverdriet et al.4 have emphasized that it is doubtful whether this detailed analysis is correct because of the small contribution of the carbide phase to the spectrum and of the poor velocity resolution. Gridner et al.15 have also indicated that the results of Arents et al.13 do not (11) Loktev, S. M. Makarenkova, L. I.; Slivinskii, E. V.; Entin, S. D. Kinet. Katal. 1972, 13, 1042. Hofer, L. J. E.; Cohn, E. M.; Peebles, W. C. J. Am. Chem. SOC.1949, 71, 189. (12) Barton, G. H.; Gale, B. Acta Crystallogr. 1964,17, 1460. (13) Arents, R. A.; Makaimov, Yu.V.; Suzdalev, I. P.; Imshennik, V. K.: KruDvanskiv. Yu. F. Fiz. Met. Metalloued. 1973. 36. 277. '(14) Maksimbv Yu. V.; Suzdalev, I. P.; Arents, R.' A.f Loktev, S. M. Kinet.Katal. 1974, 15, 1293. (15)Gridnev, V. N.; Gavrilyuk, V. G.; Nemoshkalenko, V. V.; Polushkin Yu. A.; Razumov, D. N. Phys. Met. Metall. 1978,46, 107.

0 1982 American Chemical Society

4800

Le Caer et al.

The Journal of Physical Chemistry, Vol. 86, No. 24, 1982

satisfy the criterion of invariance of nuclear constants so the positions of the individual lines have not been accurately determined. The lines 1 and 6 (from 1 to 6 from negative to positive velocities) of the H 130 kOe site are close to line 1 of the H = 170 kOe site and line 5 of the H = 237 kOe (and 170 kOe) site, respectively. This may be accounted for by a dissymmetric broadening of the peaks as discussed below. Using the same analysis Raupp and Delgass' have found a site with H = 134 kOe which accounts for only about 5% of the total area and is not essential to the fit.16 We conclude that the existence of the H = 130 kOe site has never been convincingly demonstrated. The most accurate results obtained from t carbides (free from x carbide) formed during tempering of iron-carbon martensite have never shown the presence of this site (next section) while no intermediate E structure has been observed with electron microscopy. We shall later show that the assumption of an intermediate (t'-x) = t structure is not needed at all to account for the transformation behavior of t carbide. t'-Fe2& has a Curie point of about 723 K" and an hexagonal close-packed str~cture.'~~'~J' This carbide has only been observed up to now in catalytic reactions. According to Niemantsverdriet et al.? the X-ray diffraction pattern published by Barton and GaleI2 should not be attributed to almost hexagonal t-FezC but to hexagonal t'-Fez,2C. Fe,C was studied by Niemantsverdriet et al.4 who assumed that a Fe,C carbide explains the distribution of hyperfine fields up to 275 kOe at room temperature in metallic iron catalysts after Fischer-Tropsch synthesis at moderate temperatures (1523 K). The field limit of 275 kOe has actually no meaning as the hyperfine field distribution is not known and as sources of broadening exist as shown in section IV. The mean field value (-250 kOe), which is measured on Figure 2a on ref 4, is the only significant field in the absence of a detailed analysis. The only general conclusion which may really be drawn is that the iron atoms form an hexagonal or approximately hexagonal close-packed array while nothing has been established for the ordering of carbon atoms in the octahedral interstices of this array.

(16) Delgass, W. N., personal communication. (17)Amelse, J. A.; Butt, J. B.; Schwartz, L. H. J. Phys. Chem. 1978,

D

0

*:j--+;

-

111. t Phases and 0 Carbides Formed from Iron-Carbon Martensite In this section, we show that an unique carbon neighborhood of iron atoms cannot exist except at some particular carbon contents and that all the published hyperfine fields in 0 carbides may be grouped around only two values. Ruhl and Cohenls have revealed the existence of an hexagonal close-packed (hcp) t phase, considered to be an interstitial solid solution of carbon in hcp iron, in splatquenched Fe-C alloys. This phase has to be distinguished from t carbidela and exists between Fe6C and Fe3C.l9 Different stages of ordering of carbon atoms characterized by the ordering filling up of the free octahedral sites, in particular for Fe6C, Fe4C, and Fe3C, have been observed by electron diffra~tion.'~Fe3C is isomorphous with Fe3N. Each iron atom has two carbon neighbors. As very strong interactions between interstitial atoms are known to exist in the c direction in hcp metals,20the highest carbon content in hcp iron would correspond to Fe,C.

O

0

0

0

e

e

oI . f .L. O 0

e

0

.

.

0

.

Figure 1. (a) Basal projection of the structure of hcp iron with octahedral interstices. A (0)and B (0) sites are iron atoms at the respective height l / p c oan 3/4c,. Small filled circles are octahedral sites whose heights vary according to the structure type (ref 10). (b) Basal plane projection of V-Fe,C (ref 6).

A strong tendency to repulsion between carbon atoms is also observed in ferrite21 and in austenite22while the formation of an Fe4Csuperlattice during martensite ageing has also been reportedSa It is reasonable to assume that repulsive interactions and a long-range ordering tendency will also exist in 0 carbides formed in catalytic reactions or during the tempering of iron-carbon martensite. As the iron atoms also form an hcp or approximately hcp array, each iron atom will have at most n = 3 carbon neighbors while the possible n values are 0 In 13. Figure l a (see also ref 10) represents the basal projection of an hexagonal iron lattice and of the octahedral sites. The ordering tendency will tend to minimize the number of different n values as observed in t-Fe3C phase (n = 2). Equating the number of C-Fe and Fe-C pairs in an Fe,C carbide leads to 3

6/x = C np,

(1)

n=O

whatever the carbon distribution on octahedral sites, pn is the fraction of iron atoms with n carbon neighbors. Eguation 1clearly shows that a single carbon neighborhood of iron atoms cannot exist for 2 < x < 3 while only n = 3 exists for Fe2C (Figure lb). This does not imply that a unique structure exists for x = 2 because there are many ways of distributing three carbon atoms around each iron atom.1° Equation 1 will also hold in (Fe-M),C (where M is any iron-substituting element) if the Fe-C and M-C interactions do not differ. Hirotsu and Nagakura6 and Williamson et ala7have shown that the carbide precipitated from martensitic high carbon steel (Fel.1-1.2 wt % C) is isomorphous with Co2C and Co2N. They have emphasized the similarity of the carbide electron diffraction pattern to 7 carbide diffraction patterns. The distance between the close-packed layers of iron does not change appreciably while the main difference between t and q carbide lies in the carbon atom arrangement which has hexagonal and orthorhombic symmetry in t and q carbide, respectively! In q-Fe2Ceach iron atom has n = 3 C neighbors, as expected, at 1.904 and 1.945 A (2 neighbors). Ino et aL6 have noted that 7 carbide is not stoichiometric from the differences between the calculated and observed superlattice line intensities and from Mossbauer spectroscopy. A f q P has recently observed the following sequence of carbide transformations t q x B in steels with about 0.6 wt % C and 1.6 wt % Si using both X-ray and electron diffraction. This result shows that carbon atom ordering may depend both on temperature

-

--

82, 558.

(18) Ruhl, R. C.; Cohen, M. Tram. TMS-Aime 1969,245, 241. (19) Dubois, J. M.; Le Caer, G. Acta Metal. 1977, 25,609. (20) Hillert, M.; Jarl, M. Acta Metal. 1977, 25, 1.

0

(21) Murch, G. E.; Thorn, R. J. Acta Metal. 1979, 27, 201. (22) Bhadeshia, H. K. D. H. Met. Sci. 1980, 14, 230.

The Journal of Physical Chemistry, Voi. 86, No. 24, 1982 4801

Mossbauer Spectroscopy of Iron Carbides

TABLE I: Hyperfine Fields H (kOe) in 0 Carbides carbide

prepn”

characterznb T,K

H

re1 int

232 * 2 0.49 i 0.12 174 + 1 0.51 i. 0.12 E Te(Fe-C-Si-Mn), Ex X 4.2 185 176 80 162 i. 5 300 ED 178 ? 4 R Te( Fe-C) 300 250 ? 5 E Te(Fe-C) 0.67 191 t 2 65 0.33 261 k 4 167 0.37 (4) E (or rl) Te( Fe-C) 300 253 0.37 (4) 256 0.26 (3) E Te(Fe-C) 240 i. 5 300 cc€” FT (fused iron) X, TMA 130k 6 0.15 300 0.61 170 i 3 237 ? 5 0.24 “E” FT (10Fe/SiO,) X 136.1 0.13 300 172.4 0.59 241.2 0.28 ‘ ‘ e ” t “e”’ X 134.0 0.05 FT (10Fe/SiO,) 300 172.6 0.87 243.0 0.08 “f” FT (10 Fe/MgO) X 134.1 0.14 300 173.0 0.57 239.0 0.29 FT (10Fe/SiO,) X 172.5 300 1 “ ,I> FT (fused iron) X 170 1 300 “ FT (5 Fe/SiO,) X 189 i. 1 1 77 173 i. 1 300 191i.3 1 4 ‘I FT (10 Fe/Al,O,) 184 1 4 173.5 295 FT (10 Fe/A1,0,) 187 t 2 4 251 i. 1 300 173t 1 0.93 0.07 238 f 1 a Te(A) = tempering of A alloy, Ex = extracted from tempered A, FT = Fischer-Tropsch fraction, X = X-ray diffraction, TMA = Thermomagnetic analysis. Te( Fe-C-Si), Ex

f

X, ED

300

I‘€!’’

r , ,

$ 9 ,

and time. Using the volume per matrix atom, Ruhl and Cohen’* have proposed Fe2.4of0.05 C as the t carbide composition. Table I summarizes the reported hyperfine fields of 0 carbides. The column “calculated composition” will be explained in section IV. Williamson et al.’ have emphasized the lack of agreement, particularly for the parameters o f t carbide. First, if we consider the results obtained from carbides formed during martensite tempering, we note that two groups of fields H 250 f 10 kOe and H 173 f 5 kOe are measured except in ref 25 and 26 where only one field is reported. Mathalone et alaEhave measured a Curie temperature Tc = 520 K which is much lower than the Tc values reported for c carbide (section 11). According to these authors the carbon content, alloying elements (Mn), and particle sizes may explain these results. The effect of Mn on the Mossbauer spectrum of 8-Fe3Cis in fact clear in this work. The complexity of the spectra of tempered martensite makes analysis of the e carbide spectrum difficult. This may explain why only one field was reported in ref 26. According to Williamson et al.? this field may be attributed to Fe atoms with one carbon nearest neighbor in low carbon martensite. Mossbauer spectra of 0 carbides extracted from Fe-C-Si tempered alloys are not s y m m e t r i ~and ~ ~ the , ~ ~lines are

-

-

(23)Le CaCr, G., unpublished results. (24) Le Caiir, G.; Simon, A.; Lorenzo, A.; Genin, J. M. Phys. Status Solidi a 1971,6,K97. (25)Mathalone, Z.;Ron, M.; Pipman, J.; Niedzwiedz, J. Appl. Phys. 1971,42, 687. (26)Choo,W.K.; Kaolow, R. Acta Metal. 1973,21, 726.

calcd comp from section IV Fe,,t,,C

ref

23, 24 25

26 Fe,.,,C (2.18-2.30) 13,14 Fe,,,C (2.21-2.32) 1 Fe,.,C (2.05-2.09) 1 Fe,,,C (2.21-2.33) 1 Fe,C Fe,C Fe,C

1 13 17a

Fe,C

28 21 29

Fe,.,,C

55

17b

synthesis.

ED = electron dif-

TABLE 11: Fwhm r (mm s-l) in 0 Carbide Extracted from Fe-C-Si Tempered Alloys ~.

H , kOe (mean field) 174

a

r,

r2

1.08 i. 0.59 t 0.06 0.06 232 1.03 0.43 i. 0.07 0.03 References 23 and 24.

rs 0.77 i 0.06 0.61 i. 0.06

r6

0.71 i. 0.03 0.92 f 0.06

broad. Table I1 gives the full line widths at half-maximum for both sites at room temperature. The strong broadening of the lines may be accounted for by a hyperfine field distribution with a standard deviation CTH = 17 kOe. This has to be attributed to the influence of silicon on the iron hyperfine fields as clearly shown in t phasedg where r1.6 1.5and 0.5 mm s-l with and without silicon, respectively. The broadening will depend on the Si content of the carbide. This interpretation is also consistent with the stabilization of t carbide (and t phases) in the presence of silicon through inhibition of the formation of eme en tit el^ which does not dissolve this element.30 Si does not seem to be at the origin of the dissymmetries as they are not observed in e phases with sili~on.’~ This effect is characteristic of carbide and is clearly visible in

-

(27)Nahon, N. Thesis, Lyon, 1979. (28)Nahon, N.; Perrichon, V.; Turlier, P.;Bussicre,P. J.Phys., Colloq. (Orsay, Fr.) 1980,41, C1-339. (29)Pijolat, M.,unpublished results. (30)Okamoto, T.; Kagawa, A. Met. Sci. 1977,11, 471.

4802

3oor;


published spectra of Fe-C carbides.'P8 Ino et a1.: who have also performed Mossbauer spectra under an externally applied field, have in fact found two sites with H = 253 and 256 kOe with similar isomer shifts but different apparent quadrupole splittings (6 = 0.05; 0.09 mm s-l, 2t = -0.38; 0.41 mm s-l, respectively). An interpretation of these results will be given in the next section. We finally agree with the conclusions of Ino et al. who have suggested that the two latter sites were caused as a result of carbon deficiency in the observed FezC structure. If one excepts the site with H 130 kOe, all the hyperfine fields reported in Table I may be grouped around Rl = 245 f 8 kOe and R2 = 172 f 3 kOe within one standard deviation.

HNlkD ,

400

E\$

H = Ho - CKx(rj)n2 I

(2)

where the sum runs over the various metalloid coordination shells of the Fe atom considered (X = B, C, N, P). Assuming a rapid decrease of Kx with distance r, and considering only the first coordination shell gives H = Ho - Kxnx

(3)

Figure 2 show the hyperfine field variation as a function of nx in iron nitrides at 77 K, iron carbides at 77 K, and iron borides at 4 K. An approximately linear variation is (31)Moriya, T.;Sumitomo, Y.; Ino, H.; Fujita, F. E. J . Phys. SOC.Jpn 1973,35, 1378. (32) Fatseas, G.Phys. S t a t u Solidi b 1973,59,K23. (33)Foct, J.;Le Caer, G.; Dubois, J. M.; Faivre, R. In 'International Conference on Carbides, Borides and Nitrides in Steels", Kolobrzeg, Poland, 1978,p 225. (34)Lines, M. E.Solid State Commun. 1980,36,457. (35)Haydock, R.;You, M. V. Solid State Conmun. 1980,33, 299. (36)Messmer, R. P.Phys. Reu. B 1981,23,1616. (37)Adachi, H.;Imoto, S. Trans. Jpn Inst. Met. 1977,18,375.

,

;,*

200

:'!\

tooo HC

-

IV. Correlation between Hyperfine Field and Carbon Neighborhood of Iron Atoms The general behavior of iron hyperfine fields in the iron-metalloid compounds allows us to show that the effect of the carbon neighborhood or iron atoms provides an explanation of the results of the previous section. Alloying elements, particles sizes, and carbon content may account for the observed scattering around the mean value. As already observed by many a u t h o 1 ~ , 8 ,the ~ ~ hyperfiie -~ fields in metalloid (B, C, N, P)-Fe compounds decrease with the number of metalloid neighbors of iron atoms. Theoretical calculations explain these features. Haydock and have shown that the decrease of the magnetic moment in Fe3A1(2.18 I.(B for 8 Fe neighbors to 1.5 p B for 4 Fe and 4 A1 neighbors) is not due simply to there being fewer iron nearest neighbors but rather to more A1 neighbors. The strongly interacting Ai sp bands increase the d band width and lower the Fe moment. A strong interaction has been calculated between Fe, Ni, and B in tetrahedral sites36and between Fe and C in octahedral sites.37 The magnetic moment decreases. We then expect that the iron magnetic moment is strongly influenced by the number nx of metalloid X neighbors of Fe. In the various compounds discussed below, the coordination number of Fe varies only slightly. In order to obtain quantitative estimates of the influence of n, in various Fe-X compounds, we only consider the hyperfine fields measured at temperatures such that T / T , 5 0.25 which ensure that the field is at least about 0.98 times the saturation field at 0 K. Hyperfine fields at T / T, N 0 are only known for borides. We assume that the hyperfine fields ( T 5 T,) vary as

Le Caer et al.

400

4

I

2

3

4nh

2

3

4nc

k@

3200 j\*

100 0 I

(5)

where the sum runs up to rj 5 3.5 A, m = 4.7, and Cx is a constant depending on the metalloid X. The exponent m is calculated from experimental results for Fe3C and FeB. Lines then deduces a single function describing the magnetic moment I.( = f(nefi)for B, C, and P and a unique related function H = g(nBfl).However, the neffcalculated by Lines for FeB is less than the actual value by 1 and the agreement between the calculated and observed values is then better than it should be for neffk 6. Moreover, Lines has taken p ( 0 ) and H(0) from a-Fe. 'It seems better to take H(0) and ~ ( 0in) Fe-X compounds which show an environment with neff i= 0: and consequently H(0)

-

-

y-Fe,C, (300 K ) y-Fe,N (300 K) Fe,,N. c-Fgsfi (77 K, Tc

- 300 K )

H ( 0), kOe

ref

37 1 386 375 ( p ( o ) = 3WB) 403 365

8 37 32 32 32

380-400 kOe. Another advantage of this choice is that all the structures considered then derive from hcp and fccub structure^.^^ Choosing CB = 2.18 A34equal to the mean (38)De Cristofaro, N.;Kaplow, R.M e t . Trans. 1977,A8, 35.

The Journal of Physical Chemistry, Vol. 86, No. 24, 1982 4803


TABLE 111: Hyperfine Fields in Iron Carbides

T,K

H,kOe

e -$e3>

300 4.2

x-Fe,C,

4.2

0 carbide

4.2

371-386 249 249 252 220 134-147 251 f 2 251 * 1 187 ?: 1 184 191 i. 3

carbide Fe.C,

nC

0 22 -2-3 22 2-3 4 2

3

H-,

Hcald, kOe (es 4)

376 262 205-262 262 205-262 148 262

neff

ref

3.91 3.93 4.05 4.63 5.79 3.7-3.9

397 246 246 241 218 174 246-254

40

5.75

175

29 28 17b

-0

205

kOe (eq 6 )

8, 38 40

24

Fe-B distance in Fe borides, we et a unique H = g(neff) curve for Cc = 2.22 A, CN = 2.24 Cp = 2.46 A very close to the Cx values of Lines34 and we deduce 1 = f(n,ff) (Figure 3). Figure 3a further shows that H 129 pg, as usually found in such compounds, and that Hkoe = 397 - 38.6neff for nsff5 7 (6)

1,

-

We then attribute the hyperfine fields Rl and R2 (as defined at the end of the preceding section) to iron atoms with 2 and 3 C neighbors, respectively (Figure 2). Table I11 summarizes the results of this section for iron carbides. The site with 2 C neighbors in Fe4C, (51) has not been included because a controversy still exists about its hyperfine field.* The neffvalues for 0 carbides have been calculated from the Fe-C distances of q carbidea6 The preceding discussion, which shows the common general behavior of hyperfine fields in Fe-metalloid compounds, and Table I11 establish that the assignment of the sites in 0 carbides is correct. One may argue that the hypothetical site with H -130 kOe could correspond to nc = 4, as in x carbide. As the existence of an intermediate structure between Ut’n and x assumed by Arents et al.13 has never been proved, the arguments of the preceding section remain valid. Moreover, if one observes the x nc = 4 site, one also has to observe the two other sites of x carbide (next section). We conclude that the hyperfine fields in 0 carbides are mainly sensitive to the C nearest neighbors of Fe atoms. Broadening of the peaks may be due to more remote C atoms, to alloying elements which may also change the fields, or to a carbon content distribution dependent on particle size distributional The hyperfine field may also slightly decrease with decreasing particle size due to collective magnetic excitation^.^' Finally, the ordering of carbon atoms may also contribute to a dissymmetric broadening of the peaks through a slight change of the field, various apparent quadrupole splittings related to different configurations of n C atoms around Fe, and perhaps also through small anisotropic hyperfine fields. Assuming no further distortions from q-Fe,C to q-Fe2+,C, to get an order of magnitude of the field variation, we expect to observe two main n = 2 sites with d = 1.904, 1.945 A and with d = 1.945 A, respectively. The neffare then 3.9 and 3.7 for these two sites. The fields will then differ by AH 9 kOe (Table 111). We attribute to such effects the two different sets of hyperfine parameters (AH = 3 kOe) reported by Ino et ala8(see Table I). The preceding discussion may also explain the broad sextuplet attributed by Niemantsverdriet et aL4to Fe,C. The latter

-

(39) Hyde, B. G.;Bagshaw, R. N.; Anderason, S.; O’Keeffe, M.Prog. Solid State Chem. 1979, 12, 273. (40) Le Cagr, G.; Duboia, J. P.; Senateur, J. P. J.Solid State Chem. 1976, 19, 19. (41) Molrup, S.; Topsole, H.; Lipka, J. J. Phys., Colloq. (Orsay, Fr.) 1976, 37, C6-287.

b OB O C

? o

I

2

3

4

5

6

7

8

9nett

Figure 3. (a) Hyperfine flelds and (b) magnetic moments in iron borldes, carbides, nitrides, and phosphides as a function of n,, at low temperature.

authors have noticed that their Mossbauer spectrum resembles the spectrum measured by Maksimov et al.14 From the above detailed results, the decomposition of complex spectra of iron carbides often needs accepting sextets deviating from the usual perfect shape. The mean carbon content can now be calculated from eq 1by assuming equal Debye-Waller factors for all sites. The compositions given in parentheses in Table I are calculated by attributing the whole relative intensity of the H = 130 kOe site either to the n = 2 or to the n = 3 site while the other composition is obtained by normalizing to 1 the intensities of the n = 2 , 3 sites. Table I shows that the so-called E’-F~,.~C (one site) can only correspond to Fe2C while ( E or q)-Fe2C (two sites) has a composition Fe,+,C (y > 0). Cohn and Hofef’ already noted e as Fe2+C to indicate that the carbide contains more iron than corresponds to Fe2C. Finally, it is hard to imagine a unique site in Fe2.2Cbecause it is impossible to distribute C atoms

4004

TABLE IV: Calculated and Observed Carbon Content (wt W )of a Metallic Iron Catalyst during Fischer-Tropsch Synthesis at 513 Ka wt % c

synthesis time, h 2.5

6.3 24 48 a

Le Caer et al.


measd (Figure 8 of ref 4 ) 7.5 8.4-8.5 8.6 8.7

calcd (eq 1) 8.27-8.42 8.43 8.56 8.61

calcd ‘‘C’-Fe2.2C” 8.20 8.21 8.30

Reference 4.

uniformly around Fe atoms (eq 1). A further confirmation of this interpretation may be deduced from the study of the time dependence of the behavior of a metallic iron catalyst during FT synthesis at 513 K done by Niemantsverdriet et al.4 They have measured the relative contributions of the various sites to the Mossbauer spectra and they have determined the carbon weight as a function of time (Table IV and Figure 8 of ref 4). Using eq 1and attributing the Fe,C component to n = 2 and the “ef-Fe2,2C” component to n = 3, we get the results of Table IV. The relative intensities of the Fe,C component are too imprecise at t = 0.5 and 1.1 h and the results for these synthesis times have not been included in Table IV. The discrepancy for t = 2.5 may also be attributed to the problem of the determination of the relative intensity of a broadened component from a complex Mossbauer spectrum. As seen in Table IV, the agreement is much better for the Fe2Ccompositions than for the Fe2.$ composition used in ref 4. However, the slight difference (0.1%) between the calculated and observed wt % C may be attributed to a small amount of free carbon present on the surface as usually observed in FT synthesis and as proposed by Niemantsverdriet et al.4

V. Carbon Concentration Range of 0 Carbides Table I shows that the composition of 0 carbide (t or 7) precipitated from Fe-C or Fe-C-Si alloys is Fe2.4ao.lC within one standard deviation, in good agreement with the carbon content deduced by Ruhl and Cohen.ls The mean composition of 0 carbides formed in Fischer-Tropsch synthesis seems to be at most about Fe2.25a0.05 C while the lower iron concentration is close or equal to Fe2C. It is reasonable to assume, as done by Jack,43that there exists a range of carbon composition for 0 carbides from FT synthesis and martensite tempering between Fe2.4*o.lC (instead of Fe3C) and Fe2C. The upper limit is close or perhaps equal to the composition of x carbide. Mossbauer spectroscopy mainly gives the mean carbon content. In some cases, it should be difficult to distinguish between a mixture of carbon-rich and carbon-poor 0 carbides and a continuous range of carbon concentration. The stability increases when the carbon concentration decreases:lJ7 Fe2C C Fe2.2..2.4C < x-Fe5Cz< O-Fe,C This scheme may be modified by various effects due to strain, surface effects, very small particles, or to 0 carbide stabilizing elements such as Si, A144which are insoluble in cementite or nitrogen. The carbides formed by FT (42) Cohn, E. M.; Hofer, L. J. E. J. C h e n . Phys. 1953, 21, 354. (43) Jack, K. H. J . Iron Steel Inst. London 1951, 169, 26. (44) Kumar, R. In “Physical Metallurgy of Iron and Steel”; Asia Publishing House: London, 1968.

TABLE V: Hyperfine Fields and x i Values H,kOe 4.2 K 252 i 2 249 i 2 248 i 5 220 i 2 186 i 4 140 i 6

296 K 216 i 2 208 * 2 -245 i 8 185 * 2 172 f 3 118 i 6

Xi

2 2 2 2 3 4

synthesis will result from competing effects between the tendency to have a high carbon content, in connection with the relative rates of surface reactions and diffusion of free carbon into iron particle, and a decreasing stability when the carbon content increases. For 0 carbides, the strain energy is likely to increase with carbon content.

VI. Ordering of Carbon Atoms in 0 Carbides As shown by the various results reported in the literature and as already emphasized6,’ X-ray diffraction is inappropriate for a detailed structure analysis of 0 carbides. For instance, the X-ray diffraction pattern of a metallic iron catalyst after 24 h of FT synthesis at 433 K4 would be a combination of the patterns of “eFezC”and “t’-Fe22Cn while Mossbauer spectra were interpreted as a combination of “FexC”and “ef-Fe2,2C”.In 10 Fe/Si02 catalysts carbided in 3.3 H2/C0 6 h at 523 K, Raupp and Delgassl only detected the three major reflections of “e” carbide while the Mossbauer spectrum was interpreted as a mixture of “e” and “E‘” carbides. The long-range ordering tendency of carbon atoms in 0 carbides is clear from Mossbauer spectroscopy for carbon contents in the range Fe2,2C-Fe2,4C.Table V shows that the calculated probabilities for a random distribution of carbon atoms on three available sites around each iron atom are very different from the calculated and observed (Table I) probabilities for an ordered arrangement which includes only sites with 2 and 3 C neighbors for Fe2.4C. However, Mossbauer spectroscopy does not allow one to give a description of the C atoms arrangement. For r I 2.2, it is no longer possible to distinguish between a random and an ordered distribution of C atoms with Mossbauer spectroscopy as the probabilities PoRand PIRbecome vanishingly small (Table V). In both 7-Co2Nand f-Fe2N, the nitrogen atom arrangement is no longer hexagonal but o r t h o r h o m b i ~ . This ~ , ~ ~symmetry change is accompanied by displacement of both metal (Co, Fe) and N atoms from the ideal hcp positions. In iron nitrides45these structural changes appear in a very narrow concentration range. The departure from hexagonal symmetry has been observed to be gradual and Jack46has suggested that both e and f nitrogen atom arrangements coexist within the same crystal. Distortions have also been observed for q-FezC6 and for an iron catalyst extracted after 20 h at 538 K FT synthesis.12 According to previous sections and to Niemantsverdriet et al.4 the latter carbide should correspond to -Fe2C while Hirotsu and Nagakura6 have suggested that its structure is closely related to the structure of {Fe2N. From the results of ref 6 and 7 it seems that the 7-type arrangement of C atoms persists over a wide concentration range in 0 carbides formed during the tempering of martensites. Very small particle sizes (50 X 100 20 8’)are observed and only little coarsening of the carbides took place even after 100 days of tempering at 393 K. The ~

~~~

(45) Jack, K. H. Acta Crystallogr. 1952, 5, 404. (46) Hofer, L. J. E. In “Catalysis”;Emmett P. H., Ed.: Reinhold: New York, 1956; Vol. IV, p 373.



cementite correspond to adjacent slabs (3,2,3,2)and (3,3,3), respectively. This model gives a very simple and attractive description of the t x 0 transformation. It also predicts that intermediate carbides, which may be called, according to the carbon content, faulted Hagg carbide or faulted cementite, may also exist between Fe5C2and Fe3C. For Fe,,,C TP carbide with only (3,Z)and (3,3) slabs, the fraction of iron atoms with LC = 4 is simply given by (1 - 2y)/(5 + 2y). It is also clear that one cannot observe the site with LC = 4 in an hypothetical intermediate structure between t and x13J4without observing the LC = 2 Fe atoms belonging to the same trigonal prisms. The detection of an LC = 4 site (i.e., an hyperfine field of 130 kOe) then means that the t carbide studied is already partially decomposed into x. The assumption of possible solid solutions of x and 0 VII. 0 TP and TP TP Carbide was rejected by Cohn and H o f e P from thermomagnetic Transformations measurements. However, three different experiments confirm the validity of the preceding model (see also the A model of the transformation of 0 carbide into TP conclusion of ref 39). The existence of highly faulted carbides, which may be checked by using in particular cementite with a lot of (001) faults has been observed by Mossbauer spectroscopy, is described. It is also shown how electron microscopy53 while Mossbauer spectra have shown the latter technique allows calculating the mean carbon that the decomposition product of “t” carbide possesses content from a mixture of 0 and TP carbides. Fe sites whose hyperfine characteristics are close to those The driving force for the formation of x and 0 carbides of sites found in cementite and Hagg carbide. The comfrom 0 carbides has been associated with the existence of position of the TP carbide is calculated as W F ~ ~ . Fi~C. a stronger chemical bond between Fe and C in TP sites4’ nally, Curie temperatures of 515,509, and 495 K have been as also deduced by Goodwin and P a r r a v a n ~from ~ ~ demeasured during successive heatings and coolings. carburization experiments of t, x, and 0 carbides. However, Carbides called x’ and x” with Tc = 493 f 10 and 483 a strong interaction is calculated for metalloid atoms in f 10 K,respectively, have also been observed by Arents various coordination polyhedra (Fe-B,3e Fe-C3’). I t may et alai3during the x 0 transformation. Equation 7, Table then also be assumed, as done by Hyde et al.,39that the VI, and the relative intensities given by the latter authors need for larger interstices, which relieve what they have allows calculation of the compositions and Fe2.40C called “chemical stresses”, is at the origin of the transfor x’ and x” carbides, respectively. This result is obviformation. ously meaningless. Well-resolved spectra are needed to The Fe7C3TP carbideg which is very difficult to synapply eq 7 because the x value is strongly dependent on thesize is, once formed, apparently as stable as a x carbide. the LC = 4 site (Table VI) whose external peaks are comThe decomposition behavior of iron carbides demonstrates pletely hidden in the peaks 2 and 5 of the LC = 2 sites. that the TP polyhedron is the more stable coordination The relative intensity of the LC = 4 site is probably impolyhedron of C atoms. This polyhedron is also commonly precise in the work of Arents et al. (Figure 3 of ref 13). found in many transition-metal-metalloid and rareearth-transition metal compounds.49Parthe and M o r e a ~ ~ ~ Room temperature Mossbauer spectra of 10 Fe/Si02 carbided in 3.3 H2/C0 6 h at 523 K with an average have defined a trigonal prism linkage coefficient (LC) particle size of the reduced metal of 101 A1 have been which is, in the present case, the number of C-centered calculated with three sextets with hyperfine field values prisms which share one Fe atom. LC values are 2,2 and of 116,181, and 216 kOe close to those for bulk x carbide. 2,2,4 for the various sites of O-Fe3C and x-Fe5C2respecThe Curie temperature is about 505 K. These spectra were tively while the mean linkage coefficient LC is simply even assigned to x carbide. Niemantsverdriet et al.4 have, by however, suggested that this Mossbauer spectrum repre=6/x (7) sents a combination of x-Fe5Czand “ E ’ - F ~ ~ . the ~ C ”latter , phase being present in small particles with a superparamfor a Fe,C TP carbide in which all the Fe atoms participate agnetic transition temperature below 505 K. Only the site in the formation of trigonal prisms. Equation 7 then allows with H = 181 kOe may be ascribed to ferromagnetic Fe&. one to calculate x when the fraction of iron atoms pi with As this field is significantly higher than the room tema linkage coefficient LCi can be determined. perature hyperfine field of FezC (Table I), the proposed Andersson et al.50y51 and Hyde et al.3ghave shown that explanation may only be valid for the central doublet but twinning on (1122) planes in hcp generates trigonal prisms cannot account for the relative intensities of the three in the twin plane. A great number of structures may be sextets. Equation 7 gives Fe2,54Cwhich may in part acobtained when the twinning operation is regularly recount for the observed decrease of Tc. The latter catalyst peated. Nonstoichiometry can be achieved by an irreguwas partially hydrogenated at 523 K for 10 h and a siglarly repeated “chemicaltwinning”. The twin lamellae are nificant difference was observed in the relative intensities characterized by the number of atom layers parallel to the of the sites yielding Fe2.63Cwith a lower carbon content twin plane within a twin i n d i ~ i d u a l .Hagg ~ ~ carbide and as expected. Finally, a composition Fez,,,$ is calculated for an uncompletely carbided catalyst (5 h in 3.3 H2/Cq at 523 K) while the assumption of Niemantsverdriet et al. (47)Moriya, T.;Ino, H.; Fujita, F. E.; Maeda, Y. J.Phys. SOC.Jpn.

C/Fe radius ratio (0.61) exceeds the value of 0.59 below which simple interstitial structures are observed.46 As suggested by Moriya et al.47 the driving force for the precipitation of t carbide is merely the release of strain energy and t is formed first because it has a better lattice matching with the matrix than x or 0 carbides.44 The distortions in t or 7 carbide are more easily accommodated in the strain fields of defects. 7 carbides were observed to precipitate along dislocation^.^,' In 0 carbides formed during catalytic reactions, a departure from hexagonal symmetry occurs, at least for Fe/C 2. Further experiments, if possible using electron diffraction, are needed to characterize the C atom arrangements as a function of carbon content and treatment, in particular close to Fe2.2C.

--

-

-

-

-

-+

1968,24,60. (48)Goodwin, J. G.; Parravano, G. J. Phys. Chem. 1978,82,1040. (49)ParthB, E.; Moreau, J. M. J . Less-Common M e t . 1977,53, 1. (50)Andersson, S.; Hyde, B. G . J. Solid State Chem. 1974,9, 92. (51)Andersson, S. In ‘Summer School on Inorganic Crystal Chemistry“; Geneva, Jly 21-24, 1980.

(52)Cohn, E. M.; Hofer, L. J. E. J . Am. Chem. SOC.1950,72,4662. (53)Genin, J. M.; Le Caer, G.; Simon, A. In “Proceedings of the 5th International Conference on Massbauer Spectroscopy”; Czechoslovak Atomic Energy Commission: Prague, 1973;Vol. 2,p 318.

4606

Le Caer


Iron Compounds Formed by Fischer-Tropsch Synthesis and Observed by Mossbauer Spectroscopya catalyst ratio T,K time, h carbides other iron compd 6 Fe,C Fez+ 10 Fe/SiO, ( 6 1 A ) 3.3 HJCO 523 6 10 Fe/SiO, (74 A ) 3.3 H,/CO 523 Fe ~.06-2.09 c FeO 0.67 FeZ.,,C 10 Fe/SiO, (101 A ) 3.3 H,/CO 523 6 x-Fe,,,C + S carbide a very small amount of FeO 6 -Fe,C + S Fe,C 5 Fe/SiO, (160 A for cu-Fe,O,) 3 HJCO 523 0.07 Fe,.,;C FeO, Fez+ 10 Fe/MgO ( < 4 0 A ) 3.3 Hz/CO 523 Fez+ x-Fe,,,C + S carbide 6 x-Fe,C, + FezC + S Fe,C Fe/TiO, /CaO ( 39/32 / 9 ) 3 H,/CO/He 513 48 HJCO 523 2 FeO t Fez++ Fe3+ 1 0 Fe/Al,03 Fez.& Fea++ Fe3+ 3.5 Fea.ioC Fez++ Fe3+ 4.5 Fea.osC Fe.C 19 Fez++ Fe3+ metallic Fe -300 A (for or-Fe,O,) 3 H,/CO/He 513 $e$ + Fe,C + x-Fe,C, FeO 0.5 Feo 1.1 2.5 6.5 Fe,C + x-Fe,C, 24 48 fused Fe -Fe,.zC FeO 6 H,/CO 433 132 Fe,C t S carbide Fe0 fused Fe 6 H,/CO, 200 atm 388 100 FeO Fe,C + FezC + x-Fe,C, metallic Fe 3 H,/CO/He 433 24 463 3 623 Fe,C t x-Fe,C,

et al.

T A B L E VI:

a

ref 1

1 1 17 1 4 27

4

14 14 4

S = superparamagnetic.

leads to Fe2.2s-2,32C and Fe2.33-2.38Cafter 5 and 6 h, respectively, that is a lower carbon content after 6 h. Mossbauer spectroscopy is a powerful method for calculating the mean carbon content Fe,C of any mixture of 0 and TP carbides from 6 / x = Cpixi i

(8)

where p i is the fraction of iron atoms and x i is the number n of C neighbors for 0 carbides and the linkage coefficient LC for TP carbides. In order to use relation 8, one must carefully check that the Debye-Waller factor of the various carbides are nearly equal and that the area ratios do not vary with temperature.16 The xi values and corresponding, experimental hyperfine fields at 4.2 and 296 K for x , 0, and 0 carbides are given in Table VI. The hyperfine fields at 77 K are very close to those a t 4.2 K. If we allow for hyperfine field variations due to various effects, it is more simply stated that at T 5 77 K x i = 2 for H 2 200 kOe

x i = 3 for 160 5 H 5 200 kOe x i = 4 for H 5 160 kOe

(9)

It will sometimes be difficult to attribute x i values for fields of about 170-180 kOe at 296 K. A spectrum at 77 K should normally help to overcome this ambiguity. The mean carbon content of 0 and TP carbides may also be calculated separately from a combination of Mossbauer spectra at 77 and 296 K. Only if superparamagnetic effects persist at 77 K will it be necessary to record Mbssbauer spectra at 4.2 K.

VIII. 0 Carbides Formed During Fischer-Tropsch Synthesis The results of the previous sections are applied to carbides formed during FT synthesis. Table VI summarizes all the literature resulta concerning the carbides obtained in such conditions. The column “carbide” corresponds to the reinterpreted data according to the preceding sections. We have followed the change in carbon content, as a function of time, in Fe/A&O3and Fe(Cr)/A1203during FT synthesis at 523 K using in situ Mossbauer spectroscopy.

1

3

0

0

~ 4

’

’ 8

’

i ’ 12

’ ’ 16

’

’ 20

’

1’

Time ( h )

Flgurs 4. Hyperfine field of the 3 C site of 0 carbides as a function of tlme at 523 K for Fe/AI,O, (0)and Fe(Cr)/Al,O,(+) catalysts.

The precursor of Fe/Al203 was obtained by precipitation from a FeC12/FeC13solution and y-A1203.54 The dried precursor was modified by impregnation with a (NH,),Cr2O7solution to give the promoted Fe(Cr)/Alz03precursor. The final compositions are 10 wt % of iron and 0.6 w t % of chromium. Prior to reaction, the samples were reduced in the cell in H2 (4 L/h) for 18 h at 873 K and then brought to 523 K in HO. A premixed 1:l gas of CO/H2 (1L/h) was passed over the reduced catalysts for about 2 h while in situ Mossbauer spectra were recorded successively for 1 or 2 h. Two six-line patterns and a doublet are ascribed to magnetic and superparamagnetic 0 carbides, respective1y.**66 Figure 4 shows the hyperfine field H3c of the most intense n = 3 site in the observed 0 carbides as a function of time at 523 K. H3c decreases rapidly between 0 and -6 h, and then slowly. The mean composition calculated from eq 1decreases rapidly from -Fez& to Fez,& during the first -6 h and then more slowly down to -Fe2.05C for Fe/A1203and -Fe2.0zC for Fe(Cr)/A1203catalysts. (54) Nahon, N.; Perrichon, V.; Turlier, P.; BussiPre, P. React. Kinet. Catal. Lett. 1979,11, 281. (55) Pijolat, M.; Le CaBr, G.; Perrichon, V.; Bussisre, P., presented at the International Conference on the Applications of the MBssbauer Effect, Jaipur, Dec 1981, to be published.



130:

2.05

2.10

215 x( Fe ,C

J

2.20

1

Flgure 5. Correlation between the H, hyperfine field and the mean carbon content in 0 Fe,C carbide at 523 K for Fe/AI,O, (0)and Fe(Cr)/Al,O, (+) catalysts.

0

I

! A

273

473

673

073 Temperature ( K )

Figare 8. Thermomagnetic curves for -Fe,C carbides catalyst (-20 h, 523 K, 1:l CO/H,): curve A, under 1:l CO/H, (300 torr): curve B, under vacuum (- 10" torr).

Figure 5 shows that H3c and x (Fe,C) are correlated. A linear regression line has been tentatively drawn through the experimental points. The decrease of HSc with x may be interpreted as a decrease either of the Curie temperature or of the particle sizes. A decrease of the Curie temperature has already been observed for hexagonal iron nitrides between Fe3N and Fe2N.= However, according to Loktev et al.,ll the Curie temperature value would be 653 K for -Fe2.2C and about 723 K for Fe2C. To clarify this point, we have performed thermomagnetic measurements on the carbide obtained as described above from CO/H2 synthesis on Fe/A1203catalyst. The experiments were carried out with an 8 K min-' heating rate. The results are reported in Figure 6. Curve A corresponds to the catalyst heated under 300 torr of a 1:l CO/H2 mixture in order to prevent a fast carbide decomposition. The Curie point is observed at 653 K with good accuracy. However, the perturbation observed on the curve at -583 K raises the question of the actual composition of the carbide just before the 653 K Curie temperature. Small changes in carbon content may indeed come from reaction of the carbide phase with CO and (or) H2. Another similar perturbation occurs near 713 K after the magnetic transition. On subsequent cooling, it appears that only the stable Fe3C phase is present. Curve B, obtained for the solid heated under (56) De Cristofaro, N.; Kaplow, R. Met. Tram. 1977, A8, 425.

vacuum (residual pressure lo-' torr), shows that the carbide begins to be transformed from -633 K. This is in fact a transient change since it is followed at 713 K by a complete decomposition into metallic iron as evidenced by the Curie point at 1043 K. The extrapolated value of the Curie temperature of the carbide is very close to 653 K. Therefore it can be concluded that -Fe2C and Fe2,2C carbides have the same Curie temperature. The 713-K perturbation observed in curve A may then be interpreted as the result of a partial transformation of -FezC carbide which can lead either to Fe3C or to Fe depending on the operating conditions. Further experiments are under way to explain this perturbation. The 583-K perturbation may be explained in following manner on heating above 523-573 K the superficial layers of the carbide may be involved in hydrocarbon production leading to a momentarily reduced iron surface, which explains the transient magnetization increase. As the temperature is further raised, the process competes with the reaction of the reduced surface with the gaseous atmosphere, forming again the initial -Fe2C carbide. Thus these measurements seem to prove that the H3c variation cannot be attributed to a change of the Curie temperature. Consequently, we tried to see if any change of particle size could be observed during the synthesis. As the reaction occurs, the successive Mdsbauer spectra show an increase of the relative spectra area due to the superparamagnetic carbide doublet. For reaction times between 30 min and 21 h, this area goes from 4 to 10% for Fe/A1203 and from 4 to 14% for Fe(Cr)/A1203 Even when metallic iron has completely disappeared this increase is still noticeable. This superparamagnetic carbide is often quoted in the literature results as can be seen from Table VI. The quadrupole splitting is of the order of 0.7-0.9 mm s-l (0.74 rmn s-' in the present study). Such a carbide may also exist in other cases from the published spectra. In particular the spectra of ref 27 (cf. Table V) may be reinterpreted by taking superparamagnetic carbide instead of an Fe3+ component. We think it corresponds to very thin layers of carbide or to precipitation of -Fe2C clusters (or both) due to a complete carburization of some regions of the initial particle. The increase of this phase as carburizatior. proceeds can therefore be attributed to a decrease of the mean size of the carbide particles. Moreover the BET specific surface areas of a reduced and carburized (-20 h) Fe/A120, catalyst are respectively 100 and 118 m2/g ( h 3 m2/g). The measured BET area of the alumina support is 97 m2 and is assumed to remain the same before and after the CO/H2 reaction. The area of the iron containing phases would thus increases from 30 to 210 m2[g (30 m2/g value is in good agreement with the metallic area measured by CO and H2 chemisorption methods54). Although the experimental error may be as high as 60 m2/g, the area significantly increases. We can conclude from these studies that the carburization seems to proceed via a fragmentation-like mechanism by migration of carbon atoms along the defects (grain boundaries, dislocations, ...) of the initial particle, creating smaller carbide particles. The increase of specific surface area may be explained by the addition of two effects; first, a carbon deposition is generally assumed5' to be responsible for catalytic deactivation, which is indeed observed with this catalyst between 5 min and 20 h of reaction; (57) D y e r , D.J.; Somorjai, G. A. J. Catal. 1978, 52,231. (58) Afqir, Thesis, Nancy, 1982. (59) Bauer-Grosse, E.;Frantz, C.; Le Caer G. Extended Abstracts, VI11 International Conference on Solid Compounds of Transition Elements, Grenoble, France, June 21-25, 1982, to be published.

4808

Le Caer et al.


second, the formation of the small carbide particles (as evidenced by the Mossbauer spectra) may develop microcracks with dimensions too small to be observed by electron microscopy. It seems very difficult to measure the specific area of the carbide and consequently to deduce the relative importance of the latter effect. Nevertheless we think that the variation of the hyperfine field observed in the present study must be interpreted as a decrease of the mean size of carbide particles as long as the carbon content increases. Due to the surrounding alumina support, the fragmentation is not complete. 0 carbides, particularly C-rich carbides ( -FezC), may be stabilized by surface effects, structural defects, and the influence of Al and Si through interaction with the support. Raupp and Delgass have also shown that smaller iron particles on SiOz are carbided at a faster rate than larger particles and that they favor less stable carbides as seen in the first three row of Table VI. They have attributed the stabilizing effect on Si02to interaction with the support. In the early stages of carburization, a carbon concentration range related to the carbide size distribution may exist and will give rise to Mossbauer lines whose widths will be related to this distribution. The differences of the experimental reaction conditions (temperature, partial pressures of Hzand CO, CO/Hz ratio, duration) must be also taken into account to explain the various carbon contents observed in the carbides reported in Table VI, since the rate of chemical reactions at the surface is expected to compete with the diffusional rates of C which is at the origin of carbide formation. The 0 -FezC carbides may be stabilized as very small particles not embedded in a ferromagnetic matrix, which are superparamagnetic or just ferromagnetic;this assumption is supported by the analogy with 7 carbide particles of only -20 A thick, which are magnetic when they are embedded in a ferromagnetic matrix.’ When carburization proceeds further, and depending on reaction conditions, either carbon richer carbides possibly with smaller particle sizes (this study) or more stable x or faulted x carbides (more generally TP carbides) will be formed according to the nature and strength of the various stabilizing interactions. The initial particle size and the nature of the support appear to be the most determining effects. Only stabilized superparamagnetic or just ferromagnetic N Fe2Cparticles are expected to remain in the same state. Elevated temperature of reaction would favor 0 TP transformations or direct TP carbide formation due to their own respective stabilities and nature and rate of chemical reactions at the surface. Such a description accounts reasonably for the

-

results of Table VI at temperatures of about 400-600 K.

IX. Conclusions Information about the structure of iron carbides formed from iron-carbon martensite and the general behavior of iron hyperfine fields in iron-metalloid compounds were used to characterize the phases formed in iron catalysts during FT synthesis. In particular, the mean carbon content of 0 (octahedral) carbides or mixtures of 0 and TP (trigonal prismatic: x , 8, or faulted x,8) carbides may be calculated, using eq 8, from the intensities of the Mossbauer subspectra assigned to corresponding C surroundings of Fe atoms according to relations 9. Due to the strong ordering tendency of carbon atoms only two sites exist in magnetic 0 carbides. Therefore, the occurrence of a hyperfine field of 130 kOe together with other larger ones evidences the presence of x-Fe6C2. Mean carbon content calculations show that the carbides previously reported as Ut’-Fez.2Cn and “E-Fe2C”formed in FT synthesis correspond to FezC and -Fe,,C, respectively. The limiting compositions of 0 carbides obtained in FT synthesis and martensite tempering are FezC and Fe2.4M.1C in good agreement with previous determinations. No well-established information is available concerning the carbon atom ordering in 0 carbides formed by FT synthesis. This justifies the use of “0” instead of ‘‘E” which implies carbon atom arrangements with hexagonal symmetry. From the results of ref 4 and 11, a departure from hexagonal symmetry is deduced for FezC although it is not known whether the ordering is of the 7-Fe2C type as in carbide formed during the tempering of Fe-C martensite. Only electron microscopy would allow one to determine C atom ordering provided that this technique did not alter the carbides. Literature Mossbauer spectroscopy results on iron carbides formed in FT synthesis have been reinterpreted in the light of the above considerations (Table VI). Results with Fe/A1203and Fe(Cr)/A1203are especially illustrative of these. In addition, they support the idea of a fragmentation of the particles during carburization. 0 carbide of composition Fe2C has the same Curie temperature as Fe2& (653 K). The t or 0 x 8 transformation may be interpreted within the framework of the model of Andersson et al. Finally, a schematic description of the evolution of iron carbides during FT synthesis at -450-550 K is proposed.

--

Acknowledgment. We gratefully acknowledge support for this research from C.N.R.S.-GRECO “Oxydes de carbone”.

Characterization by Moessbauer spectroscopy of iron carbides formed

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