Evaluation of Microporous Texture of Undecomposed and Decomposed β-FeOOh Fine Particles by Means of Adsorption Isotherms of Nitrogen Gas and Water Vapor

Fundamentals of Adsorption Proc. IVth Int. Conf. on Fundamentals of Adsorption, Kyoto, May 17-22, 1992 Copyright 0 1993 International Adsorption Society

Evaluation of Microporous Texture of Undecomposed and Decomposed P-FeOOH Fine Particles by Means of Adsorption Isotherms of Nitrogen Gas and Water Vapor

Hiromitu Naono, Joji Sonoda, Kiyohide Oka and Masako Hakuman Department of Chemistry, Faculty of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662, JAPAN ABSTRACT The HzO adsorption isotherms at 2OoC were measured on undecomposed 18-Fe00H. It w a s found from the HzO t plot that micropore filling of HzO molecules can occur into the structural tunnel of P-FeOOH. To explain the results of t h e HzO t plot, t h e modified tunnel model (Fig. 8) has been proposed, where F- or Cl--ion w a s coordinated to Fe3+-ion. The tunnel forms the open space int~which HzO molecules can be accommodated. The adsorption isotherms of N2 gas on undecomposed and decomposed /8-FeOOH were measured at 77 K. Nz molecules cannot enter into the tunnel of undecomposed -FeOOH. With the progress of decomposition, the slit-shaped micropores of 0.8 nm in width are successively formed along the c-axis of /-FeOOH.

9

INTRODUCTION I t is well known t h a t j - F e 0 0 H fine p a r t i c l e s can be p r e p a r e d by hydrolysis of Fe&-salt in the presence of Cl--ions. Mackay’) and Szytula et al.2) have proposed the crystal structure of #-FeOOH on the basis of the X-ray and n e u t r o n diffraction p a t t e r n s of powder sample. The proposed unit cell (Fig. 1) is similar to that of hollandite. The symmetry is tetragonal with unit cell dimensions a=b=1.048 nm, ~ 4 . 3 0 2 nm. Cl--ions and HzO-molecules occupy the structural tunnel running parallel to the c axis. Mackayl) has pointed out t h a t such large ions as Cl--ions seem necessary for the formation of the 9 -FeOOH structure. W e found that the proposed tunnel model of#-FeOOH is inconsistent with t h e adsorption data of HzO vapor. Adsorbed amount of HzO vapor per unit area is abnormally large for)B-FeOOH compared with that for d -, / -FeOOH. From t h e t plot of HzO isotherm for /8-FeOOH, it has been clarified that an appreciable amount of HzO molecules can enter into the tunnel. According to the proposed tunnel model, large Cl-ions occupy the I t seems, tunnel and inhibit the filling of HzO molecules into the tunnel. therefore, necessary to modify the position of Cl--ions in the unit cell in order to explain the micropore filling of HzO molecules into the tunnel. On the other hand, since the work of Watson, Cardell, and Heller, the presence of the characteristic porous structure w i t h i n a particle of 1-FeOOH has been believed.$” The pores of ca. 1 nm could be clearly observed in the electron micrograph of 4-FeOOH (cf. Fig. 2). However, the presence of such micropores w a ~denied by Galbraith et a2.6’) and Nmno et Galbraith et al. concluded that the porous structure w a s induced by radiation damage of an electron beam. Naono et al. showed that t plot for the Nz isotherm on undecomposed # -FeOOH gives the straight line passing 467

468

H.Naono, J. Sonoda, K. Oka and M. Hakuman

through the origin, which is an unambiguous evidence that a #-FeOOH particle has no pores larger than the size of nitrogen molecule (0.43 nm). I t is, therefore, necessary to investigate t h e micropore formation as a function of decomposition temperature. In the present work, we have prepared the!-FeOOH samples in which F--ions and Cl--ions are contained and we will examine the two problems mentioned above by measuring the adsorption isotherms of HzO vapor and Nz gas. EXPERIMENTAL

Materials. j-FeOOH i n which F--ion w a s contained (samples A-1 a n d A-2) w a s prepared according to the method reported by Childs et d o ) . N a F was added to the 0.1 M solution of Fe(NOs)s, where the amount of NaF w a ~adjusted to give a F-/Fe* = 1.0. In the solution, free F--ions could not be detected. AU F--ions added to the solution were coordinated to Fes+-ions. P-FeOOH particles were formed through hydrolysis of the hydrated Fe*ions to which F--ions were coordinated. Hydrolysis w a s carried out at 7BC for 7 days. A - l w a s obtained under mild stirring and A-2 under vigorous stirring. ,d-FeOOH in which C1-ion w a s incorporated (sample B) w a s prepared according to the patent reported by Hirai et &.lo) The solution in which 1.0 M FeCls, 2.5 M NH4C1, and 5.0 M urea were contained w a s prepared. Hydrolysis w a s carried out a t 100°C for 2 h. The precipitates of samples A-I, A-2, and B were repeatedly washed with distilled w a t e r and dried at 5OoC in atmosphere. Analysis of F#, F, and Cl- i n #-FeOOH. Powder sample of ca. 0.5 g w a s dissolved in 15 M HNOa. After neutralization by 6 M NHs, the precipitate of Fe(0H)s w a s filtered out and washed. The content of Fe3+ w a s gravimetrically determined after calcining the washed precipitate in a crucible. The concentration of F--ion in the filtrate of samples A-1 and A-2 w a s measured by means of an ion-meter having F--ion electrode (Towa Denpa-kogyo Co.). The content of C1-ion in t h e f i l t r a t e of sample B w a s determined by Volhald method as was described in the previous paper.”

a

k

A- 1

Fig. 1. Unit cell ofp-Fe00H proposed by Mackay and Szytula et al.

,

d

0.2um

Fig. 2. Electron micrographs of P-FeOOH (A-1 and B).

MicroporousTexture of PFeOOH !?om Isotherms 469

Decomposition and water content of )B-FeOOH. Powder sample of ca. 0.5 g w a s decomposed a t temperatures from 25 to 5000C for 2 h under a reduced pressure of 3 x Pa. Water vapor released from the sample during decomposition w a s trapped a t 77 K and then w a s measured gravimetrically. X-ray diffraction measurement o f powder samples. The X-ray powder diffraction patterns of/-FeOOH sample were taken with CuK, radiation under the condition of 50 kV and 40 mA by using the Xray diffractometer of Rigakudenki RAD-RC. Electron microscopic observation o f fine particles. The morphology of fine particles and their porous texture were observed by electron microscope of Xtachi H-300 under the condition of 75 kV and 80 p. Measurement of adsorption isotherms of Nz gas and HZO vapor. N2 isotherms at 77 K and H2O isotherms a t 293 K were determined by means of t h e computer-controlled automatic adsorption apparatus constructed i n our laboratory.11.12) N2 gas of 99.9999% purity w a s used without f u r t h e r purification. Water used in the H2O adsorption w a s purified by distillation and repeated degassing.

RESULTS AND DISCUSSION Character of PFeOOH samples.

The compositions of samples A-1, A-2 and B were determined on the basis of the analytical data of Fe*, X-, and Ha0 content. Samples A-1 and A-2 are described by formula )f-FeO(OH)o..rFo.s.O.3Hz0, whereas sample B corA s will be discussed in later, halide responds to ~-FeO(OH)o.eClo.z- 0.3Hz0. ion is directly bound to Fe*-ion (cf. Fig. 8 ) and HaO molecules (zeolitic H a ) are located in the structural tunnel. When samples are pretreated in vacuo, the zeolitic Hd3 molecules are removed at first (25 - 150°C) from the tunnel. The thermal decomposition occurs a t 100 - 30OOC. Finally, halide ions are separated as FeXwublimate at temperatures higher than 500OC.

I

a

ZB/degree

Fig. 3. Powder X-ray diffraction patterns of /-FeOOH (A-1

and B).

ads.

t/nm Fig. 4. Nz t plots for A-1, A-2, and B.

470

K.Oka and M.Hakuman

H. Naono, J. Son&,

Electron micrograhs of samples A-1 and B are shown in Fig. 2. Paxticle shape of sample A-1 is capsule-like and that of sample B is acicular. Slit-shaped micropores of ca. 1 nm can be observed in the particles of sample B. A s has been pointed out by Galbraith et aL8.'), these micropores are formed by the damage of irradiation of an electron beam. Figure 3 indicates the X-ray diffraction pattern of samples A-1 and B. All peaks can be assigned to thefFeOOH peaks.lg) From Fig. 3, the unit cell dimension w a s calculated to be a=1.06 nm cd.30 nm for sample A-2 and to be ~ 1 . 0 6nm and ~ 0 . 3 3nm for sample B. The cell dimension of sample B in which Cl--ions are coordinated to Fe%-ion is slightly larger than that of sample A-2 in which F--ions are coordinated to Fes+-ion. The difference in unit cell dimension may arise from the difference in ionic size of halide ion (the ionic radii of F- and C1- are 0.133 nm and 0.182 nm). The difference in thermal stability between samples A-1 and B will be considered below. Porosity of undecomposed samples. In Fig. 4, t plots of the Nz isotherm for three samples A-1, A-2, and B are shown, where t h e t c u r v e f o r nonporous PC-FeOOH w a s used as a reference.s*ll) Samples w e r e pretreated in vacuo at 25OC for 12 h before N2 adsorption measurement. As has been clarified from Fig. 4, ell samples give the straight lines passing through the origin. The result is an unambiguous evidence that three samples A-1, A-2, and B prepared in this experiment are nonporous for adsorption of Nz malecules. In other words, in the 25OC-degassed samples, there are no pores larger than the size of Nz molecule (0.43 nm in diameter). The total surface areas (St) calculated from the slope of the straight lines of Fig. 4 are 17, 7, and 24 mZg-l for samples A-1, A-2, and B, respectively. Micropore filzing of Ha0 molecules into the tunnel. In Fig. 5, the adsorption-desorption isotherms of HzO vapor are shown for sample B pretreated in vacuo a t 25°C for 12 h. In these isotherms, slight low pressure hysteresis could be detected. The apparent surface density HzO molecules in the monolayer (VBBT(H~O))calculated from the BET plot Of t h e HzO isotherm w a s found to be 69 HzO/nm2. Such abnormally large 90

60

30

V,,

0

(W,O)

: 69 H20/nmc

0. 5

1.10

P/P' F i g . 5. HzO isotherm for B at 200~.

0

0. 5

1.0

P/P' Fig. 6. HzO t curve on nonporous alumina at 10, 20, 3OoC.

Microporous Texture of PFeOOH from Isotherms

47 I

value cannot be explained by the adsorption of HzO molecules on an external surface of particles. It is, therefore, reasonable to conclude that the adsorption phenomenon for H2O vapor is closely related to the volume filling of HzO molecules into the structural tunnel of a-FeOOH. In order to estimate the volume of Hz0 molecules filled into the tunnel (Vmp), the t plot w a s carried out for the HzO isotherms shown in Fig. 5, where the HzO t curve for t h e nonporous aluminall) shown in Fig. 6 w a s utilized a s a reference curve. The t plot is shown in Fig. 7. Vmp was estimated from extrapohtion of the desorption branch to an ordinate. The measured value of Vmp w a s 16.3 mm3/g. A s w a s mentioned in Introduction, according to the tunnel model so far proposed, large Cl--ions occupy the structural tunnel (6. Fig. 1). If Cl--bns are present in the tunnel, it is impossible to enter HzO molecules into the tunnel. In order to explain the t plot data mentioned above, we Cl--ions are not present in propose the modified tunnel model (Fig. 8). the center of the tunnel, but they are directly coordinated to Fes+-ion in In t h e modified model, the tunnel forms t h e open place of OH- ions. space. It is possible to enter HzO molecules into the open space, because t h e size of t h e open space (0.21 - 0.24 nm) is near to t h a t of a H2O molecule (0.23 nm). The size of a HzO molecules w a s calculated on the basis of ice structure. On the other hand, Nz molecules cannot be accommodated into the open space, because the size of a N2 molecule (0.43 nm) is larger than that of the open space. Accordingly, as has been pointed out above, sample B is concluded to be nonporous from the Nz t plot (cf. Fig. 4). Next, the volume of the open space (V,) in the tunnel w a s estimated Vop w a s ctilculated to be 17 from the structure model shown in Fig. 8. mrn3g-l1 where we assume that the cylindrical pore of 0.2 nm in diameter is present in the tunnel. The value of Vmp determined experimentally from the HzO t plot is in good agreement with that of Vop. This fact suggests that in sample B, most of HzO molecules initially filled in the structural tunnel can be removed by 25°C-evacuation. A s the result, the open space is formed in the tunnel. By the exposure to HzO vapor, the open space w a s again filled with HzO. A s will be mentioned below, in the case of sample A-2, it is difficult t o remove H2O molecules from t h e tunnel by 25°C-evacuation

o

0

ads.

0. 5

1

t/nm Fig. 7. HzO t plot for B.

Fig. 8. Modified model of unit cell

of )8-FeOOH.

472

H. Naono, J. Sonoda. K.Oka and M. Hakuman

In Fig. 9, t h e adsorption-desorption isotherms of HzO vapor are shown. The isotherm I is given for sample A-1 pretreated in vacuo at 25OC for 1 2 h. The isotherm I1 is shown for sample A-I pretreated in vacuo at 15OOC for 2 h. The isotherm I gives a slight low pressure hysteresis, whereas t h e isotherm I1 shows a remarkable low pressure hysteresis. HzO t plots for the I and I1 isotherms are shown in Fig. 10. Vmp, which w a a calculated from the desorption branch of Fig. 10, is found to be 1.7 mm3g-l for the 25OC-treated sample, and 12.0 mm3g-l for 1500C-treated sample. For 25OC-treated sample, the micropore filling of Ha0 molecules into the tunnel is slight. The HzO adsorption occurs mainly on the external surface of particles. I t is evident t h a t it is difficult to remove HzO molecules from the tunnel by 25°C-evacuation. When sample A- I was pretreated in vacuo at 15OoC, a significant amount of HzO molecules could enter into the tunnel. But the Vmp for 150OC-treated sample A-1 (12.0 mm3g-l ) is small in comparison to the V m p for 25OC-treated sample B (16.3 mm3g-l). When sample A-l is pretreated at temperatures higher than 15OoC, the thermal decomposition occurs (cf. Fig. 11). The open space of t h e tunnel of sample A-2 is large in comparison to t h a t in sample B, because t h e ionic radius of Cl--ion (0.181 nm) is larger than that of F--ion (0.133 nm). In spite of the large open space, the higher temperature (15OOC) is necessary to remove the HzO molecules from the tunnel. This suggests that the HzO molecules in the tunnel of sample A-1 interact strongly with the tunnel surface. The strong interaction may arise from the hydrogen bonding between HzO molecule and F-ion. Under such circumstance, the mobility of HzO molecules in the tunnel becomes slow, which may cause the remarkable low pressure hysteresis (the isotherm I1 of Fig: 9). Thermal decomposition of -FeOOH. The thermal stability of samples A-1 and B wiU be examined a t first. In Fig. 11, the total surface area (St) of samples A-1 and B are plotted as a function of the pretreatment temperature. The starting point of the steep rise in the surface area is located at 15OOC for sample A-1 and at 100°C for sample B. From this finding, sample B is considered to be thermally

fl

20r

P/P' Fig. 9. HzO isotherms for A-I at 20°C. A-2 was pretreated at 25OC and 150OC.

t/nm Fig. 10. HzO tplotsfor A - )

Microporous Texture of PFeOOH from Isotherms

473

less stable than sample A-1. Such difference in thermal stability between samples A-1 and B may arise from the difference in ionic size between F-ion and C1-ion. The ionic radius of F--ion (0.133 nm) is nearly equal to that of OH--ion (ca. 0.14 nm), whereas the ionic radius of Cl--ion (0.181 nm) is significantly larger than that of OH-ion. When OH--ion is replaced by Cl--ion, the lattice of p-FeOOH may be strained. The strain of lattice in sample B may lead to the lower thermal stability. The adsorption isotherms and t plots of N2 gas for sample A-1 and its decomposed products are shown in Figs. 12 and 13. Number given in Figs. 1 2 and 13 is the pretreatment temperature (T) in vacua With an increase of pretreatment temperatures, t h e N2 isotherms shift to upper region, being parallel each other. No hysteresis is detected i n these isotherms. The N2 isotherms shown i n Fig. 1 2 indicate t h a t t h e microporous texture is progressively formed in the decomposed products of /-FeOOH. To analyze the microporous texture, the c method reported by de Boer and his coworker14) w a s utilized. The standard t curve used in the present work has been reported previously (d. Fig. 6 of ref. 8).611) t plots for sample A-1 and decomposed products are shown in Fig. 13. The t plots for decomposed products consist of two lines with the bending point at the vicinity of t = 0.4 nm. A s w a s seen from Fig. 2, the micropores formed i n decomposed /-FeOOH a r e slit-shaped. Their width corresponds to twice of an adsorbed thickness (at) a t the bending point. Accordingly, the micropores of 0.8 nm in width are formed by the thermal decomposition. On the basis of the data of Fig. 13, w e determined the total surface area, St, the external surface area, SEX,the micropore area, S p , the micropore volume, Vp, and the pore width, 2t. The data obtained are listed in Table I. St remains constant at the temperature range from 25OC to 15BC. In this temperature range, the thermal decomposition of sample A-1 does not

Table 1. Surface area, micropore volume, and adsorbed thickness for undecomposed and decomposed # -FeOOH (A-1).

-

200

St

Sex

SD

0)

n

a E

m

100

Fig. 11. Relation between total surface

area (St) and pretreatment temperature.

vp

- mm3g--1

rn2g-l

2t nm

-

25

17

17

0

0

150

17

17

0

0

170

66

21

45

16

0.7,

190

91

21

70

26

‘0.8

210

-

127

20

107

40

0.8

230

163

22‘

141

54

0.8

250

195

23

172

69

0.8s

2 70

212

24

188

77

0.9

300

205

27

178

82

0.B.

474

H. Naono, J. Sonoda, K.Oka and M. Hakuman

0

6

0.5

I I.0

t/nm

P/PO Fig. 12.

N2 isotherms for undecomposed and decomposed #-FeOOH (A-1) at 77 K.

Fig. 13.

N2 t plots for undecomposed and decomposed /-FeOOH (A-1).

occur, b u t t h e zeolitic H2O molecules are eliminated from t h e s t r u c t u r a l tunnel. Above 15OoC, t h e decomposition begins and the remarkable increase in St is observed (17 m2g-l at 15OOC - 212 m2g-l at 27OOC). The increase in Sis slight (17 - 24 m2g-’). The remarkable increase in St is, therefore, a t t r i b u t e d t o t h e increase in Sp. A t 27OoC, t h e dehydration almost finishes. A t temperatures higher than 27OoC, t h e microporous texture is destroyed and t h e mesoporous t e x t u r e gradually develops, resulting t h e decrease in St.

REFERENCES

1)A. L. Mackay, Minerd. Magn., 32 (1960) 545 2)A. Szytula, M. Balanda, and 2. Dimitrijevic, P h y s . S t a t . Sol., (a) 3 (1970) 1033 3)J. H. L. Watson, R. R. Cardell, Jr., and W. Heller, J. Amer. Chem. Soc., 66 (1962) 1757 4)K. J. Gallagher, Na t u r e, 266 (1970) 1225. 5)A. T. Howe and K. J. Gallagher, J. Chem. SOC. Faraday Trans. I, 71, (1975) 22 6)s. T. Galbraith, T. Baird and J. R. Fryer, A c t a C r y s t . A35 (1979) 197 7)s. T. Galbraith, T. Baird and J. R. Fryer, I n s t . P h y s . Conf. Ser. No. 52 (1980) 291 8)H. Nmno, R. Fujiwara, H. Sugioka, K. Sumiya, and H. Yanazawa, J. Colloid I n t e r f a c e Sci., 87 (1982) 317 9)C. W. Childs, B. A. Goodman, E. Paterson, and F. W. D. Woodhams, A u s t . J. Chem., 33 (1980) 15 10)s. Hirai, T. Sueyoshi, K. Wakai, Japan Pat., (1980) No. 1009136 11)H. Nmno and M. Hakuman, Hyomen, 29 (1991) 362 12)H. Nmno and M. Hakuman, J. CaLlaid I n ikrfac e Sd., 145 (1991) 405 13) U. Schwertmann and R. M. Cornell, I r o n O x i d e s i n the Laboratory (Preparation and Characterization), VCH, Weinheim, 1991, p.98 14)J. H. de Boer, B.C. Lippens, B. G. Linsen, J. C. P. Broekhoff, A. van den Heuvel, and Th. J. Osinga, J. Colloid I n t er fac e Sci., 21 (1966) 405