Adsorption of water vapor by iron oxides. 1. Preparation and

Langmuir , 1991, 7 (4), pp 672–677. DOI: 10.1021/la00052a014. Publication Date: April 1991. ACS Legacy Archive. Cite this:Langmuir 7, 4, 672-677. No...
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Langmuir 1991, 7, 672-677

672

Adsorption of Water Vapor by Iron Oxides. 1. Preparation and Characterization of the Adsorbents N. S. Clarke? and P. G. Hall*J Department of Chemistry, University of Exeter, Exeter E X 4 4QD, United Kingdom, and Atomic Weapons Research Establishment, Aldermaston, United Kingdom Received April 2, 1990. I n Final Form: September 11, 1990 The preparation and interconversion of iron(II1) oxides and hydroxides and their characterization by X-ray diffraction and small angle neutron scattering (SANS) are described. The solids, Le. a - F e 2 0 3(haematite), ?-Fez03 (maghemite), FesO4 (magnetite), a-FeOOH (goethite), @-FeOOH (akaganeite), and y-FeOOH (lepidocrocite) were all characterized by X-ray diffraction. The various chemical transformations are compared with nominally similar processes published in the literature and some significant discrepancies are noted. With SANS, (u-FezO3, y-FenO3, y-Fe203 (calcined), and Fe304 show nonlinear Guinier behavior but the Debye analysis gives linear plots indicating random structure; the corresponding specific surface areas generally show good agreement with gas adsorption values. u-Fe203 and y-FezO3 (calcined) follow the Porod law, but y-FezO3 and Fe304 show deviation.

Introduction

iormal

T h e iron(II1) oxides a n d hydroxides exist in several polymorphic forms. In t h e present work, they have been prepared to use as adsorbents for water vapor adsorption studies. In p a r t 1 we report t h e preparation a n d characterization by X-ray diffraction of t h e adsorbents in some detail because t h e r e are significant differences between some of our results a n d those in t h e literature. Also in P a r t 1we report t h e characterization of some of t h e oxides by small angle neutron scattering (SANS). Subsequent parts (2 a n d 3) deal with t h e water vapour adsorption studies using isotherm measurements, X-ray photoelect r o n spectroscopy, laser ionization mass analysis, a n d inelastic neutron scattering. T h e adsorbents were prepared under controlled conditions so t h a t t h e chemical a n d thermal histories of each material were accurately known. T h i s is because i t has been demonstrated t h a t t h e surface chemistry of iron oxides a n d oxyhydroxides is strongly dependent on t h e mode of preparation a n d on t h e thermal history of t h e sample. T h e syntheses a n d interconversions effected in this work are discussed a n d compared with corresponding reactions reported in t h e literature. Figure 1 shows a reaction scheme t h a t summarizes t h e synthesis a n d interconversions of some iron oxides a n d oxyhydroxides. T h i s scheme is given by Wells.' Well's reaction scheme, however, is a simplistic one a n d in general refers t o simple synthetic routes to t h e oxides of interest. T h e reactions a n d interconversions carried out in t h e present work are summarized below T h e preparation of goethite:

i"

pure soh,

+

-

500 o

0-FeOOH(s)

(1)

s l o w l y lheat

Fel0Hl3

gel or sol

Figure 1. Reaction scheme (Wells) for iron oxides and oxyhydroxides. T h e preparation of akaganeite:

+ NH,CONH,(aq) FeCl,(aq) + NaOH(aq) + O,(g) FeCl,(aq)

-

-

P-FeOOH(s)

a-Fe,O,

T h e preparation of lepidocrocite: FeC1,-4H20(aq) + (CH,),N,(aq) + NaOH(aq)

(4i)

P-FeOOH(s) (4ii)

T h e preparation of maghemite:

+ XSNaOH(aq) + (CH,),N,(aq) +

FeSO,.'iH,O(aq) FeC1,.4H,O(aq)

-

02k)

y-Fe,03(s)

-

(5i)

y-Fe,O, (5ii)

T h e preparation of magnetite:

-

heat

c

in air

/heat

/I 1

?eat

water

NaNO,(aq) a-FeOH(s)

FeOOH

hydrol y sis

OPk)

FeS04.7H,0(aq) NaOH(aq) T h e preparation of haematite:

+ Atomic

-

-

usting

-

(2)

Fe,C,O,(s)

in air

Fe304

2Fe3+(aq)+ Fe2+(aq)+ NaOH(aq) y-FeOOH(s) (3)

Weapons Research Establishment.

* [Jniversity of Exeter.

(1) Wells, A. F. Structural Inorganic Chemistry; Oxford University Press: Oxford, 1962.

0743-7463/91/2407-0672$02.50/0

-

(6i) Fe304(s)

(6ii)

Some of t h e preparations summarized above confirm results published in t h e literature b u t others a r e in marked disagreement. Reaction 1,which refers t o t h e preparation of goethite, is in accordance with t h e literature a n d is quite reproducible. I t is a general equation where N a O H could be any base a n d t h e iron(I1) sulfate heptahydrate could

0 1991 American Chemical Society

Adsorption of Water Vapor by Iron Oxides

Langmuir, Vol. 7, No. 4, 1991 673

be a n y iron(I1) s a l P 4 except t h e chloride. T h i s reaction was routinely used t o make goethite but had originally been reported t o produce maghemite.5 Reaction 2 is ~ e l l - k n o w n ~ ~a n~d~ gives ~ - ' ~ well-crystallized haematite samples. Reaction 3 agrees with published r e ~ u l t s ~ J ' - as ' ~ do reactions 4i a n d 5ii giving reasonably pure products.2 Reaction 5i was not always reproducible a n d frequently resulted in nonspecific mixtures of products. Reaction 5ii was found accidentally t o produce y-FezO3; t h e original preparation was for y-FeOOH, but t h e substitution of nitrite for nitrate produced y-FeeO3. Reaction 6i was used by R a o e t al.14t o produce maghemite. I n t h e present work three variations of their method produced yFe2O3 a s a minor impurity only, with F e s 0 4 as t h e major product. Reaction 6ii was reported t o produce poorly crystallized nonstoichiometric magnetite.15 However, in this work t h e magnetite e n d product was found t o be well crystallized a n d pure. T h e iron oxides a n d oxyhydroxides can be interconverted by solid-state transformations, i.e. without any solution phase intermediates. T h e following transformations have been purposely carried o u t or exploited in these studies

-

500 "C

n-FeOOH(s)

cu-Fe,O,(s)

(7)

+

(84

air

rFeOOH (s)

I

500%

Fe3O4 (s)

a-Fep03 (s)

(8ii)

T h e following transformations were observed during t h e course of equilibration of maghemite with water vapor

a-FeOOH (s)

+

y-Fez03 (s)

1

HP (g)

fewweeks

+

a-FezO3

+

Fe304

I / I0 10 100 10

30 8

4 16 25 10 2

(hkl) 020 110 120 130 021 101 040 111

121 140

dlA 4.97 4.226 3.38 2.64 2.587 2.46 2.309 1.32

1/10 W

str W

m W

m W W

From refs 16 and 17. Table 11. Crystal Structure Confirmation: Akaganeite. standard tables observed

dlA

1110

7.40 5.25 3.70 3.311 2.616 2.543 2.343 2.285 2.097 2.064 1.944 1.854 1.746 1.719 1.635 1.515 1.447 1.480 1.459 1.438 1.374

100

0

40 10 100 40 80 20 40 20 20

60 10 40 10

100 40 20 20 10 80 40

(hkl) 110 200 220 310 400

211 420 301 321 510 411 440 600 501, 431 521 002 611 112, 710+ 640 5, 416 730, 312

dlA 7.40 5.26 3.85 3.324 2.63 2.547 2.36 2.29 2.101 2.069 1.954 1.879 1.754 1.732 1.591

1/10 str m W

str m m W

m W W

m W

m W

str

From refs 16 and 18.

Experimental Section (9ii)

Transformation 7 above has been discussed extensively elsewhere. Transformation 8i does n o t conform with Wells's reaction scheme with respect t o t h e dehydration of lepidocrocite. E q u a t i o n 8ii agrees with Lux13 a n d eqs 9i a n d 9ii are discussed elsewhere in this work. (2) Ishikawa, T.; Inouye, K. Bull. Chem. SOC.Jpn. 1972, 45, 2350-

2354.

(3) Ishikawa, T.; Inouye, K. Bull. Chem. SOC.Jpn. 1973,46(9), 2665-

2668.

dlA 4.98 4.183 3.38 2.693 2.583 2.52 2.49 2.452 2.303 1.719

yFe203 (s) (9)

heal air

y-Fez03

Table I. Crystal Structure Confirmation: Goethitea standard tables observed

(4) Rochester, C. H.; Topham, S. A. J . Chem. Soc., Faraday Trans. I 1979, 75, 591. (5) Abeck, W.; Kober, H.; Seidel, B. Gevaert Agfa NV Belg., 725,759, June 1969. (6) Sapiesko, R. S.; Matijevic, G. J . Colloid Interface Sci. 1980,74 (2), 405. (7) Gout, R.; Ferrer, M. Acad. Sci. C. R., Ser. 0 1972, 275, 6, 273. (8) Saraswat, I. P.; Vajpei, A. C.; Garg, V. K.; Sharma, V. K.; Prakash, N. J. Colloid Interface Sci. 1958, 73 (21, 373. (9) Maeda, Y.; Igarishi, M.; Terada, A.; Yoshimura, F. Jpn. J . Appl. Phys. 1974, 13 (2). (10) Langmuir, D. Am. J . Sci. 1972,272, (lo), 972. (11)Kaneko, K.; Serizawa, M.; Ishikawa, T.; Inouye, K. Bull. Chem. SOC.Jpn. 1975, 48 (6), 1764-1769. (12) Baudisch, 0.;Albrecht, W. H. J . Am. Chem. SOC.1932,54,943. (13) Lux, H. In Handbook ofPreparatiueInorganic Chemistry;Brauer, Georg, Ed.; Academic Press: New York, 1965; p 1500. (14) Rao, V.; Shashimohan, A. L.; Biswas, A. B. J . Mater. Sci. 1974, 9, 430. (15) Segal, D. L. Chemistry Division, AERE Harwell, private communication.

The detailed accounts and observations of the preparations are given in the followingdiscussion. X-ray diffraction was carried out in the Materials Development Division's Laboratories at AERE Harwell by M. Fones and F. Cullen using a Philips PW 1050 vertical diffractometer with pulse height discrimination, and a curved graphite secondary monochromator. The diffractometer scan details were as follows. Cu Ka X = 1.54 A, P.H.D. monochromator, fsd (counts/s-'): goethite, 200; lepidocrocite, 25,50; haematite, 20; magnetite, 50, 200; akaganeite, 100. T.C. values were 5, 25, and 125 and scan speeds selected were 2,6.5, 12, and 3620 h-l. The corresponding crystal structure confirmations are shown in Tables I-VIII. The detection limit for noncrystalline, amorphous materials is better than 5 % and for crystalline impurities better than 1%. Since we only expect crystalline impurities, we conclude that the materials were certainly better than 99% pure. The SANS measurements were made by H. M. Langran-Goldsmithusing the SANS spectrometer at A.E.R.E., Harnell. Preparation of Goethite (a-FeOOH). Goethite was prepared by the reaction of a sodium hydroxide solution with a solution of iron(I1)sulfate heptahydrate in the presence of oxygen. This method5 was reported to produce fine particles of ?-Fez03 for use in magnetic recording tapes. Nevertheless, the end product was confirmed as goethite by X-ray diffraction. Iron(I1) sulfate heptahydrate (AnalaR, 923 g) was dissolved in 6.5 dm3 demineralized water (DMW). Sodium hydroxide pellets (16) J.C.P.D.S. Powder Diffraction file. (17) Sampson, C. F. Acta Crystallogr. 1969, B25, 1683. (18) Mackay, A. L. Miner. Mag. 1962,33, 270. (19) Rooksby, L. X-ray Identification and Crystal Structures of Clay Minerals; Chapman and Hall: London, 1951.

Clarke and Hall

674 Langmuir, Val. 7, No. 4, 1991

Table V. Crystal Structure Confirmation: Haematiten standard tables observed

Table 111. Crystal Structure Confirmation: Lepidocrocites observed standard tables dlA IJI (hkl) 6.26 100 020 3.29 90 120 3.79 10R 011 2.47 80 031 2.36 20 111 2.09 20 131, 060 1.937 70 051, 200 1.848 20 220 1.732 40 151 1.566 20 080 1.535 20 002 1.524 40 231 1.496 10R 022 1.499 10 180 1.433 20 171 1.418 10 260 1.389 10 122 1.367 30 251 1.261 10 091, 320 1.213 10 280

crustb

PPtb

dlA

dlA 6.24 3.286

str str

2.46 2.36

str m

1.935 1.843 1.726 1.562 1.536

m

6.24 3.286

str str str m

2.47

2.357 1.936 1.850 1.732 1.561 1.534

w

w w m

m w 2 w

m

6.26 3.29 -3.79

100 90 10R 80 20 20 70 20 40 20 20 40 10R 10

2.47

2.36 2.09 1.937 1.848 1.732 1.S66 1.535

25 100 50 2

30 2

40 60 4

211

16 35 35 4 20

018 214 300 208 l00/119

dlA

1110

3.765 2.688 2.512

W

2.199

m

1.836 1.691 1.624

m m

str str

W

1.48 1.451

W

1.308

W

m

Table VI. Crystal Structure Confirmation: Magnetites standard tables observedb

dlA 4.852 2.967 2.532 2.4243 2.0993 1.7146 1.6158 1.4845 1.4192 1.3277 1.2807 1.2659 1.2119

~~

1/10

3.66 2.69 2.51 2.285 2.201 2.070 1.838 1.690 1.634 1.596 1.484 1.452 1.349

(hk0 012 104 110 006 113 202 024 116

From ref 16.

Table IV. Crystal Structure Confirmation: LeDidocrociteO standard tables observedb

dlA

1/10

1.310

OFrom refs 16 and 19. Ishikawa and Inouye's method of preparation.* Notes: admixed with magnetite and goethite. ~

dlA

(hk0

020 120 011 031 111

131, 060 051, 200 220 151 080 002 231 022 180 171 260

dlA 6.32 3.257

str str

2.469 2.357 2.081 1.932 1.846 1.732

str m

1.1221

W

1.0930 1.0496 0.9896 0.9695 0.9632 0.9388 0.8952 0.8802 0.8569 0.8233

str W

m

1.530 m 1.524 1.496 1.449 1.433 20 1.418 10 1.389 10 122 1.367 30 251 1.261 10 091, 320 1.213 10 280 From refs 16 and 19. Lux's method of ~reparati0n.l~Notes: very pure material, broad peaks. Small peak at 20 = 30.1' due to magnetite (220)? (AnalaR, 122 g) were dissolved in DMW (500 cm3). The sulfate solution (blue/green) was poured into a IO-dm3 flat-bottomed flask and was stirred magnetically while being heated until the temperature ofthe liquid rose to40 k 5 "C. The sodium hydroxide solution was then added and the liquid turned from blue/green to blue/black. Oxygen from the laboratory supply was bubbled, without further purification, via a sintered glass nozzle, into the reaction vessel in such a manner as to agitate the mixture. The presence of oxygen, however was primarily to provide an oxidizing environment. The volume flow rate of the oxygen was 1000 dm3 h-l. Oxygen was passed through the heated reaction mixture for 3-4 h by which time an orangelbrown precipitate had been produced. This was filtered off with suction and washed with water until there was no trace of sulfate in the washings (tested with BaClz(aq)) and the pH of the washings was neutral (tested with universal indicator paper). The solid material produced by this method was green when compacted and wet, yellow-brown when dry. It was dried in an oven a t 100 "C and then ground in a rotary ball mill until a fine powder was produced, with no large aggregates remaining. P r e p a r a t i o n of Akaganeite (P-FeOOH). This material was

1/10 8

30 100 8 20 10 30 40

(hkl) 111

d/A

1/10

220 311

2.957 2.520 2.410 2.086 1.703 1.605 1.475

m str

222

400 422

4 12

511 440 531 620 533 622 444 642 731

6

800

2

822 751 662 840 664 931 844 1020

2

4 10

4 2

6 4 4 2

6 8 4

W

m W

m m

a From ref 16. Rao et al.'s method of preparing maghemite.14 Notes: Maghemite + haematite impurities. prepared by the method of Ishikawa and InouyeS2 Iron(II1) chloride solution (0.1 M, 1 dm3) was hydrolyzed with 100 g of urea a t 100 "C for 15 h. The precipitate, a light brown solid, was washed briefly with water. The material was confirmed as pure crystalline akaganeite by X-ray diffraction. The reaction proceeds according to the equation

b

-

FeCl,(aq) + (base)(aq) + H,0(1) P-FeOOH(s) + (salt)(aq) Another method for the synthesis of 0-FeOOH was as follows. Iron(I1) chloride tetrahydrate (BDH Laboratory Reagent Grade) was dissolved in DMW (1 dm3). A 500-cm3 portion of a 1 M sodium hydroxide solution was heated to 45 'C and the chloride solution added. A brown precipitate formed immediately. Air was bubbled through the mixture for 4 h while the mixture was heated and stirred magnetically. The precipitate was washed with water by decantation and dried in air a t 80 "C. A light brown powder was produced. X-ray powder diffraction showed that this material was pure crystalline 0-FeOOH. The reaction proceeds according to the equations FeC1,.4H20(aq) + BNaOH(aq) -r Fe(OH),(aq) 3Fe(OH),(aq)

+ O,(g)

CI-

+ 2NaCl(aq) + 4H20(1)

3@-FeOOH(s)+ 2H20(1)

P r e p a r a t i o n of Lepidocrocite (y-FeOOH). Lepidocrocite was prepared by a well-established method.*J*-13Iron(I1) chloride

Adsorption of Water Vapor by Iron Oxides

Langmuir, Vol. 7, No. 4, 1991 675

Table VII. Crystal Structure Confirmation: Magnetites standard tables observedb dlA 4.852 2.967 2,.S32

2.4243 2.0993 1.7146 i.615a 1.4845 1.4192 1.3277 1.2807 1.2659 1.2119 1.1221 1.0930 1.0496 0.9896 0.9695 0.9632

(hkl)

dlA

a

111

30 100

220 311 222 400 422 511 440 531 620 533 622 444 642 731 800 822 781 662

4.844 2.966 2.526 2.421 2.097 1.711 1.614 1.483

1/10

8 20

10 30

40 2 4 10 4 2 4 12 6 2 6

4

W

m str W

m W

m m

standard tables observedb dlA It10 (hkl) dlA 4.852 a 111 4.896 W 2.967 30 220 2.957 m 2.532 100 311 2.513 str 2.4243 8 222 2.421 W 2.0993 20 400 2.083 m 1.7146 10 422 1.708 W 1.6158 30 511 1.604 W 1.4845 40 440 1.4192 2 531 1.3277 4 620 1.2807 10 533 1.2659 4 622 From ref 16. Segal's ~reparati0n.l~Notes: Smallimpuritypeaks at 20 = 27.4", 47.6", 48.75". tetrahydrate (40 g, Hopkin and Williams laboratory reagent grade) was dissolved in DMW (1dm3). Hexamine, ( C H Z ) ~(28 N~ g BDH AnalaR), was dissolved in 200 cm3 DMW to give a 2 M solution. A solution of sodium nitrate (1 M) was prepared. The iron(I1) chloride solution was heated to 60 "C and stirred magnetically. The hexamine (hexamethylene tetramine, or urotropine) solution was added and then the mixture was stirred for 45 min a t 60 "C. An orange-brown precipitate was produced which was filtered and washed until the pH of the washings was neutral (universal indicator) and the washings were free of C1(negative reaction with silver nitrate solution). The reaction proceeds according to the following scheme:

Fe(OH),(aq)

+ NaN03(aq)

-

2Fe(OH),(aq)

+ 4NH,Cl(aq)

y-FeOOH(s) + NO,(g)

+ NaOH(aq)

The material was confirmed as crystalline y-FeOOH with minor impurities of magnetite and goethite. Substituting nitrite for a nitrate as the oxidizing agent13 also produced lepidocrocite with magnetite impurity. According to some a~thors1~J~,20 y-FeOOH can be converted to y-Fe20Yin air. To check this observation a sample of lepidocrocite was prepared as above2 and heated a t 265 "C in air for 5 h. At the end of this time a red-brown powder has been produced. X-ray diffraction showed this sample to consist of a mixture of magnetite and haematite. However, dehydration in (20)Sakash, G. S.; Solntseva, L. S. Zh. Prikl. Spectrosk. 1972,16,4,

551.

oc

air

Table VIII. Crystal Structure Confirmation: Magnetitee

-

-

500

2a-FeOOH(s)

"From ref 16. Third variation in Rao's method of preparing y-FeZO3. * Notes: scale change at 28 = 49.4 pure material.

2FeC12(aq)+ (CH),N,(aq)

vacuo at 265 "C for 5 h produced maghemite. This was confirmed by X-ray diffraction. P r e p a r a t i o n of Haematite (a-FezO3). Haematite was prepared by heating goethite in air a t 500 "C for approximately 6 h. It is well-known3~9~21 that this treatment produces haematite from goethite. Starting from pure goethite there was no need for further purification. Haematite is also reportedly produced by dehydration of other oxyhydroxides and calcination of other oxides1Sz2in air. X-ray diffraction confirmed the dehydration product of pure goethite as pure crystalline haematite. The reaction proceeds according to the equation a-Fe203(s)+ H,O(g)

Preparation of Maghemite (y-FezO3). Maghemite was prepared according to the following method. Iron(I1) sulfate heptahydrate (AnalaR, 400 g) was dissolved in DMW (8 dm3). A green-brown solution was produced. Sodium hydroxide pellets (100 g) were dissolved in DMW (500 cm3). The sulfate solution was heated to 45 f 5 "C and stirred magnetically. The sodium hydroxide solution was added quickly and oxygen from the laboratory supply was bubbled through the blue-black mixture. This was done in similar fashion to the preparation of goethite (qv). After 3-4 h a black solid had been produced. This was filtered off with suction until the filtrate was neutral (universal indicator) and free of sulfate (negative reaction with barium chloride solution). The solid was dried a t 70 "C in air and was ground with a rotary ball mill. This material was observed to exhibit magnetic behavior in the presence of a small horseshoe magnet. The crystal structure of this material was confirmed as being that of y-FezO3 by a Debye-Scherrer photograph. This method however was not always reliable and sometimes produced goethite only. In view of the unreliability of this method, another approach for the preparation of maghemite was adopted. This was similar to a method2 used for the production of 7-FeOOH using sodium nitrate to oxidize Fez+to Fe3+ in a basic solution. However sodium nitrite was substituted for nitrate and the end product was maghemite. An iron(I1) hydroxide suspension obtained from 1 dm3 of FeC12.4Hz0 (40 g) aqueous solution and 200 cm3 of a 2 M hexamine solution was heated with 27 cm3 of a 1 M sodium nitrite solution and stored a t 45 "C for 1h. A black solid was produced, which was washed with water until the pH of the washings was neutral (universal indicator paper) and free of chloride (negative reaction with silver nitrate). A Debye-Scherrer photograph showed this material to be maghemite. Preparation of Magnetite (Fe304). Magnetite was prepared by two main methods. The first,14whichwas reported to produce maghemite (y-FeZOa),was shown repeatedly to produce magnetite (Fe304),which was confirmed by X-ray diffraction. This solidstate transformation was employed because solution phase syntheses had been thought to introduce a significant degree of nonstoichiometry in the final product.15 The method was as follows: Iron(I1) oxalate (BDH Laboratory Reagent Grade, 50 g) and a drop of water (to render the powder slightly moist) were placed in a large porcelain dish and kept in a furnace a t 300 "C for 1-2 h. Periodically the dish was removed from the furnace and the contents were mixed. A thin layer of black powder was produced on top of the bulk of the material, but this color disappeared on mixing with the body of the powder. After 1-2 hall of the material had changed to a brown color and was found to be magnetic when tested with a bar magnet. Variations of this method were tried in an unsuccessful attempt to produce maghemite. These variations consisted of heating the iron(I1) oxalate in a narrow tube, which was mounted in a tube furnace. Pure magnetite was produced when the oxalate was heated in a large test tube loosely plugged with glass wool, over a Bunsen burner. The thermal decomposition of iron(I1) oxalate liberates gases such as CO, COz, and steam, causing the powder to be violently agitated in the manner of a fluidized bed reactor. When all such agitation had ceased the powder had all turned black (21) Naona, M.; Fujiwara, R. J. Colloid Interface Sci. 1983, 73 (2). (22) Lichtner, E.; Szalek, G. 2.Anorg. Allg. Chem. 1968, 360.

Clarke and Hall

676 Langmuir, Vol. 7, No. 4 , 1991

I

/. 5

/O‘ Q‘

3

-6

0

A‘ ‘

-3 Ah

Figure 2. Guinier plot of 7-Fe203: Y axis 0 to 10.5 labeled as In I; X axis 0 to 3 labeled as 10ZQ2, A 2 .

Q

F i g u r e 4. Porod plot of Fe304: Y axis 0 to 10.2 labeled as In I; x axis -6 to 0 labeled as ln Q.

I

0 20

-

4

AI

*+&

5 3

-Q

-3

A

C

0

F i g u r e 3. Porod plot of y-Fe203: Y axis 0 to 10.5 labeled as In I ; X axis -6 to 0 labeled as In Q. and the decomposition was considered to be complete. The resultant material was confirmed as being pure crystalline magnetite by X-ray diffraction. A second major method23 for the preparation of magnetite was a solution phase synthesis. A solution containing aqueous Fe3+ and Fez+ in the mole ratio 2:l was reacted with sodium hydroxide to form the corresponding hydroxides as intermediates, and subsequently the “ferroso-ferric” oxide, magnetite. Iron(II1) sulfate (200 g) and iron(I1) sulfate heptahydrate (136 g), both BDH “AnalaR”, were dissolved in DMW (2.5 dm3). Sodium hydroxide solution (3 dm3, 2.1 M) was heated to 90 “C and stirred magnetically in a 10-dm3 beaker. The Fe3+/Fez+ solution was added to the hydroxide solution and a black precipitate formed immediately. This was kept at near boiling for 1 h, stirring continuously. The precipitate was allowed to settle and was washed by repeated decantation until the pH of the washings was neutral (universal indicator paper) and free of sulfate (negative reaction with barium chloride solution). The powder was dried and X-ray diffraction confirmed the identity of the material as pure crystalline magnetite. (23) Segal, D. L.

AERE Harwell Report No. 9976, October 1980.

O0