J. Chem. SOC., Faraday Trans. I , 1989, 85(4), 869-881 Characterization of Iron Oxide-dispersed Activated Carbon Fibres with Fe K-Edge XANES and EXAFS and with Water Adsorption Katsumi Kaneko" Department of Chemistry, Faculty of Science, Chiba University, Yayoi, Chiba 260, Japan Nobuhiro Kosugi and Haruo Kuroda Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan The Fe K-edge X-ray absorbtion near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) and the water adsorptivity of iron oxide-dispersed activated carbon fibres (ACF) have been investigated. Also, Fe K-edge XANES and EXAFS of various kinds of powdered iron oxides have been measured in order to characterize the species dispersed on the ACF. The XANES and EXAFS indicated that a- Fe00H-like ultrafine particles transform into a-Fe,O,-like particles on the ACF through heating in UCICUO.The adsorption isotherms of water on the a-Fe00H-dispersed ACF were of type V. The water adsorption isotherms were analysed by the Dubinin-Serpinsky equation; the number of polar sites on the surface was estimated and compared with the data from an analysis of the evolved gas. The relationship between the water adsorptivity and the states of the iron oxides on the ACF is discussed. A comparison of nitrogen and water adsorption shows that the layer of water adsorbed on iron oxides dispersed on the ACF has a more sparse structure than normal water, and that the iron oxide-dispersed and preoxidized ACF samples exhibit an excess of water adsorptivity owing to both molecular-sieve effects and surface fractal structure. The surface-chemical properties and the characterization of activated carbon fibres (ACF) have been actively The ACF are highly microporous, with small external surface areas and very little mesoporosi ty, having characteristic adsorption properties.Consequently, an ACF can be a good model system in the study of micropore filling. We are interested in the micropore filling of supercritical NO on metal oxide- dispersed ACF. As the two-dimensional critical temperature of NO is estimated to be 90 K by de Boer's equation,' and the micropore filling is a limited case of physical adsorption which is effective for vapours,8 normal activated carbons cannot adsorb abundant NO by micropore filling. The surface modification of an ACF with iron oxides enhances the NO micropore filling; these sites adsorb large amounts of NO up to 320 mg g-' at 303 K by chemisorption-assisted micropore filling.'^ lo Inter-NO molecular interactions are enhanced in the micropores.l1 Also, surface modification with iron oxides increases the adsorptivity of the ACF for CO, and NH,.12 Characterization of the iron oxides dispersed on the ACF is necessary, therefore, to unveil the mechanism of c hemisorp tion- assis ted micropore filling. Carbons are almost transparent to hard X-rays, and metal K-edge absorption spectroscopy (XANES and EXAFS) is a very useful method for the characterization of metal oxides on carbons ; X-ray absorption near-edge structure (XANES) and extended 869870 Surface Properties of Active Carbon Fibres X-ray absorption fine structure (EXAFS) give important information on the local environment of a specific atom in finely dispersed metal oxides on the ACF.Thus we have applied X-ray absorption spectroscopy to characterize the ultrafine metal oxides on the ACF of interest. The preliminary EXAFS results on Fe,O,-dispersed ACF have already been reported, together with NO adsorption data,', and an EXAFS study of Cu(OH),-dispersed ACF appeared in the previous paper. l4 A number of studies of water adsorption on carbon^^^-^' have shown that the interaction energy of water with the carbon surface is unusually small and that the adsorption isotherms of water on carbons, which change sensitively with the surface oxidized state, are unusual in comparison with nitrogen isotherms.Dubinin and Serpinsky,' assumed that water molecules adsorbed on polar sites act as sites for further water adsorption ; the number of active sites for water can be roughly estimated from the Dubinin-Serpinsky equation.lg a-FeOOH and a-Fe,O, are active to water adsorption through hydrogen-bond formation with surface h y d r o ~ y l s . ~ l - ~ ~ The crystallite size of a- FeOOH can be controlled by changing the time for hydrolysis of the Fe3+ solution. Thus iron oxides dispersed on the ACF are expected to be associated with the water adsorptivity of the modified ACF. Furthermore, a water molecule is smaller than a nitrogen molecule : hence the water adsorption of iron oxide-dispersed ACF can lead to information on the microporosity different from that obtained from nitrogen adsorption.In this work Fe K-edge XANES and EXAFS of iron oxides dispersed on ACF and various kinds of iron oxides and iron oxide hydroxides were measured, together with the corresponding water adsorptivity, so as to characterize the micropore structure of the iron oxide-dispersed ACF systems. Experiment a1 Materials Cellulose-based ACF (Toyobo KF- 1500) and ACF samples preoxidized with 6 mol dm-, HNO, at 373 K, denoted ox-ACF, were used. a-FeOOH particles of different crystallite sizes were dispersed on the ACF by hydrolysing 0.6 mol dm-, Fe,(SO,), solution at 303 K and pH 13 for various hydrolysis times, t, of 0-10 days. These treated ACF samples are designated a-t-ACF. a-FeOOH was also dispersed on the ox-ACF by a 6 h hydrolysis of the Fe3+ solution (a-ox-ACF).a-FeOOH was dispersed on the ACF and ox-ACF after outgassing at 383 K and a reduced pressure of 1 mPa for 15 h. a-ox- ACF-T were obtained by heating a-ox-ACF samples at T K (573 and 773 K) under 1 mPa for 15 h. The Fe content in the ACF and ox-ACF samples was determined by titrating Fe3+ ions eluted in HCl solution with a 0.005 mol dmP3 K,Cr,O, solution. a-Fe00H powders of different crystallite sizes, which are denoted by a-FeOOH-t where t is the hydrolysis time, were prepared for comparison with the ACF-supported samples;24 the (110) crystallite sizes of these a-FeOOH samples are as follows: a-FeOOH-0, 3 nm; a-FeOOH-3h, 7 nm; a-FeOOH-6h, 8 nm; a-FeOOH-2d, 9 nm and a-Fe00H-lOd, 11 nm.Also, a-FeOOH synthesized according to the method of Atkinson rt uZ.~~(~-F~OOH-AT) was examined. Synthetic P-FeOOH,23 y-FeOOH2:3 and 6-Fe00H26 samples were prepared in the same way as described elsewhere. Here we used two kinds of y-FeOOH (y-FeOOH-2 and y-Fe00H-25) having different (002) crystallite sizes, 4.5 and 12 nm, respectively. Four kinds of a-Fe,O, were obtained by decomposition of (i) the amorphous hydroxides from an Fe(NO,), solution at 873 K (a-Fe,O,-I), (ii) a-FeOOH from an FeSO, solution at 673 K (a-Fe,O,-IT), and a-Fe00H from an Fe,(SO,), solution at (iii) 673 K (a-Fe,O,-III) and (iv) 573 K (cc-Fe,0,-IV).27Commercial Fe,O, (Wako Chemicals) and y-Fe,O, (Toda Industry) powders were used for comparison.J . Chem. Soc., Favaday Trans. I , vol.85, part 4 Plate 1 Plate 1. Crystal structures of iron oxides and iron oxide hydroxides: (A) cz-FeOOH, (B) /3- FeOOH, (C) y-FeOOH, (D) Fe(OH),, (E) a-Fe,O, and (F) y-Fe,O, and Fe,O,. K. Kameko, N. Kosugi and H. Kuroda ( Facing p . 87 1 )K. Kaneko, N . Kosugi and H. Kurodu 87 I Water Adsorption and Nitrogen Adsorptions The adsorption isotherms of water on the ACF samples were measured gravimetrically at 303 +O. I K. The entire adsorption apparatus was held at 303 f 0.5 K. The adsorption isotherms of nitrogen on the ACF samples at 77 K were measured in a similar way to the water adsorption. As almost all data on nitrogen adsorption have been reported previously,1° only a brief description of the nitrogen adsorption is given here. The samples were evacuated at 373 K and 1 mPa for 15 h prior to the adsorption experiments. Characterization with XANES and EXAFS and Evolved Gas Analysis The Fe K-edge XANES and EXAFS spectra of a-FeOOH, p-FeOOH, y-FeOOH, 6- FeOOH, a-Fe,O,, y-Fe,O, and Fe,O, powders and a-6h-ACF, a-ox-ACF, a-ox-ACF- 573 and a-ox-ACF-773, were measured using the EXAFS apparatus at BL-7C of the Photon Factory in the National Laboratory for High Energy Physics (Tsukuba, Japan).,* The phase-uncorrected Fourier transforms of k 3 ~ ( k ) were determined from the EXAFS oscillation ~ ( k ) .~ ' The evolved-gas analysis (EGA) spectra of ACF, a-6h-ACF, a-ox-ACF and a-ox-ACF-773 up to 773 K were obtained at a heating rate of 10 K min-' with the aid of a mass filter (ULVAC, MSQ-15OA). Results and Discussion Species Produced oir ACF with XANES and EXAFS Various kinds of iroii oxides and iron oxide hydroxides are known.a-Fe,O, has a corundum structure and y-Fe,O, a spinel-like structure with a deficiency of metal ions. Fe,O, has an inverse spinel structure. The representative four polymorphs of FeOOH are a-FeOOH, P-FeOOH, y-FeOOH and 6-FeOOH. These FeOOH crystals have characteristic structures. Plate 1 shows the crystal structures of these iron oxides and iron oxide hydroxides. The structural unit of these compounds is an iron-centred octahedron with oxygens or hydroxyls at the corners; the octahedra are linked together by sharing corners, edges, faces and hydrogen bonds, giving rise to each characteristic crystal structure. Fe,O, and y-Fe,O, have tetrahedra embracing a central iron atom rather than the octahedra.It is said that the fundamental structure of 6-FeOOH is the same as that of Fe(OH),, with S-FeOOH more defective, having a few tetrahedral Fe sites. These iron oxides have different Fe-0 and Fe-Fe distances, as collected in table 1 ; each compound has an inherent local structure. The interatomic distances are cited or calculated from the literature from the X-ray diffraction studies. The range of data for each bond arises from the existence of different sorts of bonds identified by accurate X-ray examination. Fig. 1 shows the XANES of the Fe IS edge of a-FeOOH, p-FeOOH, y-FeOOH, a- Fe,O,, a-6h-ACF, a-ox-ACF and a-ox-ACF-773. All the XANES spectra are similar; each has three main peaks (A), (B) and (C). A weak pre-edge peak (A) arises from the 1s-3d transition which is dipole-forbidden in the case of the 0, symmetry around an Fe3+ ion.The (A) peak of a-FeOOH-0 is the strongest in these samples; the octahedral unit around Fe3+ of a-FeOOH-0 is therefore much distorted. The intensities of the (A) peaks of a-6h-ACF, a-ox-ACF and a-ox-ACF-773 are so weak that the 0, symmetry of Fe3+ is retained irrespective of the state of dispersion on the ACF. Peaks (B) and (C) are caused by the ls-4pa transition and multiple scattering, respectively. Each sample shows slightly different features in (B) and (C). Only a-Fe,O, has a (B) peak with a shoulder and a relatively high (C) peak. Although these XANES spectra can give no definite identification of the species dispersed on the ACF, XANES spectra of a-6h-872 Surface Properties of Actitle Carbon Fibres Table 1.The nearest Fe-O and Fe-Fe distances (nm) of iron oxides obtained from X-ray dif- fraction studies in the literature Fe-O/nm Fe-Fe/nm ref. a-Fe,O, 0.199 0.206 y-Fe,O, 0.186" 0.205b Fe,O, 0.187" a-FeOOH 0.197 0.202 0.214 B-FeOOH 0.1 89 0.21 1 y-FeOOH 0.189 0.199 0.206 0.207 6-FeOOH 0.20 0.288 0.295 0.297 0.286 0.302 0.301 0.288 - - - - - - - 0.23 30, 31 30, 32 30, 33, 34 30, 35 - - - - 36, 37 30, 38 - - - - 39, 40 a Tetrahedral. Octahedral 7090 7130 7170 photon energylev Fig. 1. XANES of the Fe K edge of iron oxides and iron oxides dispersed on the ACF: (a) a-ox- ACF-773, (b) a-ox-ACF, (c) a-6h-ACF7 ( d ) a-Fe,O,-111, (e) y-Fe00H-25, (f) /I-FeOOH, ( g ) a- FeOOH-2d, (h) a-FeOOH-6h and (i) a-FeOOH-0.K. Kaneko, N.Kosugi and H . Kuroda 873 0 0.2 0.4 0.6 0 0.2 0.4 0.6 distancelnm Fig. 2. Fourier transforms of the EXAFS oscillation for (a) a-Fe,O,-I, (6) a-Fe,O,-11, (c) a-Fe,O,-111, ( d ) a-Fe,O,-IV (e) y-Fe,O, and (f) Fe,O,. ACF, a-ox-ACF and a-FeOOH are similar to each other, with the (C) peak of the XANES spectrum of a-ox-ACF-773 resembling that of a-Fe,O,. We can obtain more information on the local structure of these iron oxides by EXAFS than by XANES, but we should note that the nearest-neighbour bond distance from the EXAFS data without phase correction does not coincide with that obtained from X-ray diffraction. Fig. 2 shows phase-uncorrected Fourier transforms of k 3 ~ ( k ) from the EXAFS oscillation ~ ( k ) for four kinds of a-Fe,O,, y-Fe,O, and Fe,O,.There are two main structures, [A] (0.1-0.2 nm) and [B] (0.24.35 nm), which are ascribed to Fe-0 and Fe-Fe distances, respectively. The low-intensity structure below 0.1 nm comes from errors in finite Fourier transformation, background subtraction etc. The four spectra of the obvious a-Fe,O, samples have similar features, although their intensities are different from each other. On the other hand, y-Fe,O, and Fe,O, exhibit distinctly different EXAFS spectra. Fig. 3 shows Fourier transforms of k 3 ~ ( k ) for a-FeOOH, D- FeOOH, y-FeOOH and 6-FeOOH. These FeOOH modifications have the [A] and [B] structures ;imilar to those of a-Fe,O,. The peak positions of [A] and [B] are similar, but the intensides and the intensity ratio of [A] and [B] are characteristic of each structural modification, Accordingly, we can obtain useful information on the polymorphism from the EXAFS data.The Fourier transforms of k 3 ~ ( k ) for a-FeOOH of different crystallite sizes are shown in fig. 4. The peak positions of [A] and [B] change little with the874 Surface Properties of Active Carbon Fibres 0 0.2 0.4 0.6 - A B 0 0.2 0.4 0.6 distance/nm Fig. 3. Fourier transforms of the EXAFS oscillation for (a) a-FeOOH- 10d, (b) a-FeOOH-AT, (c) a-FeOOH, ( d ) y-FeOOH-2, (e) y-Fe00H-25 and (f) 6-FeOOH. -2 3 (I .r( a 0 d g 9 6 - 3 - t A A B (4 gt A 0 0.2 0.4 0.6 0 02 0.4 0.6 distancehm Fig. 4. Fourier transforms of the EXAFS oscillation for a-Fe00H aged for various periods : (a) none, (b) 6 h, (c) 2 days and (d) 10 days.K. Kaneko.N . Komgi and H. Kuroda 9 - 6 - 3 - A (4 B 0 0.2 0.L 0.6 0 0.2 0.4 0.6 875 distancefnm Fig. 5. Fourier transforms of the EXAFS oscillation for (a) a-FeOOH-fih, (b) a-6h-ACF7 (c) a-ox- ACF, (6) or-ox-ACF-573, (e) a-ox-ACF-773 and (f) a-Fe,O,-111. crystallite size, but we can see the relative change of [A] and [B] peak intensities, which reflects the particle size; the smaller the intensity ratio of [B]/[A], the smaller the particle size. In particular, the [B] peak of a-FeOOH-0 is rather weak, indicating that the a- FeOOH-0 lattice has a low coordination number of nearest-neighbour Fe ions, i.e. a- FeOOH-0 is an ultrafine a-FeOOH particle. Fig. 5 shows the Fourier transforms of k 3 ~ ( k ) for a-FeOOH, a-Fe,O, and iron oxides dispersed on ACF. The EXAFS spectrum of a-ox-ACF is comparable with that of a-FeOOH; the [B] peak of a-ox-ACF is much weaker than that of a-FeOOH, indicating that a-Fe00H-like species, with a lower coordination number of nearest Fe ions, are dispersed on ox-ACF.The EXAFS of a- 6h-ACF leads to a similar conclusion to that for a-ox-ACF, although the [B] peak of a- 6h-ACF is more intense than that of a-ox-ACF. Heating a-ox-ACF brings about a significant change in the EXAFS spectra, as shown in fig. 5(4 and (e). The features of a-ox-ACF-573 and a-ox-ACF-773 resemble those of both a-FeOOH and a-Fe,O,; it is difficult to identify the species. However, the [B] peak grows on heating. It is probable that ultrafine a-Fe00H-like species on the ACF decompose into a-Fe,O,-like particles through heating; these then aggregate.Water Adsorption The adsorption isotherms of water on ACF and a-ACF are of type V, having hystereses (fig. 6); the amount of adsorption is slight until relative pressures of 0.4 are reached. The starting relative pressure associated with steep uptake moves to higher values with the growth of a-FeOOH. As we obtained readings which were invariant over 1 h, there is a possibility that the hystereses disappear after much longer adsorption times, as indicated by McBain.’’ Fig. 7 shows the adsorption isotherms of water on ox-ACF, a-ox-ACF 30 FAR 1876 Surface Properties of Active Carbon Fibres 0 0.5 PlPo 1.0 Fig. 6. Adsorption isotherms of water on ACF and a-ACF: 0, ACF; 0, a-6h-ACF; A, a-2d-ACF. - , O O t 0 0.5 1.0 PIP0 Fig. 7. Adsorption isotherms of water on the preoxidized ACF samples: 0, ox-ACF; 0, CZ-OX-ACF; A, a-OX-ACF-773.and a-ox-ACF-773. The adsorption isotherm of ox-ACF is like that of a-ox-ACF, whereas the adsorption isotherm of a-ox-ACF-773 is clearly different from the latter. The amount of water adsorption of ox-ACF and a-ox-ACF increases monotonically with the relative pressure even below 0.4; the preoxidized ACF interacts more strongly with water than do ACF and a-ACF. We determined the micropore volume for water, W,(H,O), from extrapolation to p / p o = 1. W,(H,O) data are collected in table 2. The type V isotherm can be approximated by the quadratic form of the Dubinin-Serpinsky equation :19* 2o Here w represents the amount of water adsorption at p/p,, a, is the amount of adsorption on the polar sites and c is a constant.Eqn (1) is expressed by a parabola; aK. Kaneko, N . Kosugi and H. Kuroda 877 Table 2. Comparison of water and nitrogen adsorption data and the amount of dispersed Fe ACF a-0-ACF a-3 h-ACF a-6h-ACF a-2d-ACF a- 1 Od-ACF OX-ACF ~z-ox-ACF a-OX-ACF-573 a-OX-ACF-773 W,(H,O) /cm3 g-I 0.62 0.63 0.62 0.55 0.58 0.52 0.48 0.47 0.47 0.48 a0 /cm3 g-I 0.12 0.18 0.18 0.14 0.04 0.08 0.13 0.08 0.10 0.09 4)" /cm3 g-I 0.61 0.6 1 0.62 0.62 0.62 0.59 0.57 0.36 0.37 0.44 Fe (wt Yo) 1 .o I .o 1 .o 0.90 0.94 0.88 0.84 1.3 1.3 1.1 - - 11 8.6 3.5 4.5 2.6 3.0 3.1 - n 0 200 400 adsorbed HzO/mg g-* 500 500 600 300 200 Fig. 8. Dubinin-Serpinsky plots for the adsorption isotherms of water on ox-ACF (0) and a-OX-ACF-773 (0). plot of w(p,/p) us. w should be an inverted parabola if the adsorption isotherms obey eqn (1).Fig. 8 shows Dubinin-Serpinsky plots for the adsorption isotherms of water on ox-ACF and 3-ox-ACF-773. The relationship for ox-ACF is a good parabola, but the Dubinin-Serpinsky plot of a-ox-ACF-773 deviates. We estimated the number of polar sites from the coordinates of the top of the approximate parabola; the a, data are collected in table 2. These a, values decrease with the hydrolysis time of a-FeOOH in the a-ACF system, whereas a, is almost constant irrespective of the heating temperature in the case of the a-ox-ACF system. The surface functional groups on a-ACF must be 30-2878 Surface Properties of Active Carbon Fibres 300 400 500 600 700 T / K Fig. 9. Temperature profile of evolved CO and CO,.(a) open symbols, a-ox-ACF; solid symbols, a-OX-ACF-773; (b) a-6h-ACF; (c) ACF. covered with a-Fe00H-like species that grow as fine particles. As for the a-ox-ACF system, heating removes the surface functional groups and hydroxyls of a-FeOOH on a-ox-ACF, producing hydrophilic a-Fe,O,-like species on the surface. Although a-ox- ACF has a more hydrophilic nature than ACF because of the uptake of water in the low p / p o region, the a, value does not necessarily reflect the water adsorption behaviour. The value of a, is not absolute, but leads to only a qualitative tendency. The EGA spectra give more definitive information on the surface polar groups present on ACF. Fig. 9 shows temperature profiles of the evolved CO and CO, from ACF, a- ACF, a-ox-ACF and a-ox-ACF-773. All samples start to evolve both gases near 400 K.a-ox-ACF has the largest CO and CO, peaks, with a CO, evolution three times larger than that of CO. In the case of a-6h-ACF and ACF, similar amounts of CO and CO, are evolved in the 40&700 K range. However, ACF evolves more CO than CO, above 700 K, whereas a-ACF evolves more CO, than CO. CO, is derived from COOH and lactone groups on the carbon surface and CO is caused by phenolic OH and quinone- type oxygen^.^^ Consequently, a-ox-ACF has more COOH and lactone groups than phenolic OH and quinone-type oxygen on the surface. The amounts of COOH and lactone groups are comparable to those of phenolic OH and quinone-type oxygens on the surface of ACF and a-ACF. a-ox-ACF-773 has no gas evolution peak below 700 K ; hence only a few surface functional groups remain on the micropore surface of a-ox- ACF-773.However, an accurate determination of the surface polar groups requires EGA up to 1300 K. Micro porosity A water molecule (0.108 nm2) is smaller than a nitrogen molecule (0.162 nm2). The difference in saturated adsorption amounts of water and nitrogen should be related to two factors, i.e. a molecular-sieve effect and the surface fractal structure of the micropore walls. The molecular-sieve effect represents the presence of narrower micropores, which are not filled with nitrogen but are filled with water. The molecular fractal nature of solid surfaces developed by Avnir and Pfeifer42.4'3 is also important forK. Kaneko, N . Kosugi and H. Kuroda 879 0.02 cm3 g-' I 3 4 5 ~n [u/(Io-' nm3>1 Fig.10. Separation of the molecular-sieve effect and fractal structure for the difference of water and N, adsorptions on a-ox-ACF; m, observed point of water adsorption; bold line, W cc t j - D r J 3 (Dp = 2.5)." the micropores, the widths of which are greater than the diameter of a nitrogen molecule. One of the present authors determined the exponent, D,, for micropores of the ACF systems by organic vapour a d s ~ r p t i o n . ~ ~ When the ratio of Wo(H20) to Wo(N,) is > 1, the above two factors must be taken into account. The Wo(H2)/Wo(N2) values for a-ACF and ox-ACF are one or less, and are ascribed to causes other than the above two factors. The value Wo(H20)/Wo(N,) < 1 for a-ACF and ox-ACF could be due to the decrease of the density of water in the micropores, as Gregg and Sing suggested;45 the presence of fine a-Fe00H-like particles should lead to a sparser structure for the adsorbed water than that of ordinary liquid water or ice in the micro pore^.^^ On the other hand, the Wo(H20)/Wo(N2) ratios for a-ox-ACF and a- ox-ACF-T series are 1.1-1.3. Accordingly, a-ox-ACF and a-ox-ACF-T probably have narrower micropores and/or a fractal nature.We can distinguish the two factors using the molecular resolution relationship of a-ox-ACF reported earlier.44 Here we extend this fractal relationship to the size of the water molecule, as shown in fig. 10. The ordinate and abscissa of fig. 10 are the saturated amount of adsorption, W; in mmol g-l for various vapours and the molecular volume u, respectively.Here we obtained the u for water from the molecular area of 0.108 nm,. The observed pore volume for water deviates from the fractal linear plot; the amount of deviation (0.09 cm3 g-') probably corresponds to the increment of water adsorption due to the molecular-sieve effect. The difference (0.02 cm3 g-I) between water and N, adsorptions calculated from the molecular resolution relationship in fig. 10 is the adsorption increment due to the fractal nature of the micropore wall. We assume that a-ox-ACF-537 and a-ox-ACF-773 have similar narrow micropores and fractal structures to a-ox-ACF, although we did not obtain such relationships for a-ox-ACF- T. In situ X-ray diffraction and small-angle scattering studies are necessary to elucidate the structural character of the molecular- sieve effect.880 Surface Properties of Active Carbon Fibres Conclusions The Fe K-edge XANES and EXAFS of various kinds of powdered iron oxides of known crystal structures show characteristic features, assisting the identification of species of iron oxides dispersed on activated carbon fibres.Ultrafine a-FeOOH dispersed on activated carbon fibres is probably transformed into fine a-Fe,O,-like particles upon heating at 573 and 773 K in vacuo. All the adsorption isotherms of water on ACF samples have hystereses. ACF surfaces without preoxidation with HNO, are hydrophobic, whereas ACF surfaces preoxidized with HNO, are considerably hydrophilic. The Dubinin-Serpinsky analyses and EGA examinations are available for the assessment of surface properties.In the case of water adsorption of ox-ACF and a-ACF, the density of water in the micropores is lower than that of ordinary water. A comparison of nitrogen and water adsorption on a-ox-ACF and a-ox-ACF- T strongly indicates the presence of molecular- sieve effects and the defective nature of the micropore walls. This work was partly supported by a Grant in Aid for Fundamental Scientific Research from the Ministry of Education of Japan. Special thanks are due to Prof. Y. Iwasawa for suggestions on the EGA experiments and to Mr H. Kuwabara for assistance in preparing plate 1. References 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 J. N. Bohra and K. S. Sing, Acisorption Sci. Tech. 1985, 2, 89.S. J. Hichchock, B. McEnaney and S. J. Waltling, J. Chem. Tech. Biotech., 1983, 33A, 157. J. Koresh and A. J. Soffer, J. Chem. SOC., Faraday Trans. 1 , 1980, 76, 2472. J. J. Freeman, F. G. R. Gimblett, R. A. Roberts and K. S. W. Sing, Carbon, 1987, 25, 559. G. G. Jayson, J. A. Sangster, G. Thompson and M. C. Wilkinson, Carbon, 1987, 25, 523. K. Kaneko, Y. Nakahigashi and K. Nagata, Carbon, 1988, 26, 327. J. H. de Boer, The Dynamical Character of Adsorption (Clarendon Press, Oxford, 1968), p. 147. M. M. Dubinin, Carbon, 1983, 21, 359. K. Kaneko and K. Inouye, Carbon, 1986, 24, 772. K. Kaneko, Langmuir, 1987, 3, 357. K. Kaneko, N. Fukuzaki and S. Ozeki, J. Chem. Phys., 1987, 87, 776. K. Kaneko, S. Ozeki and K. Inouye, Colloid Polym. Sci., 1987, 265, 1018. K. Kaneko, Characterization of Porous Solids, Proc.IUPAC Symp. (Elsevier, Amsterdam, 1988), p. 183. K. Kaneko, A. Kobayashi, T. Suzuki, S. Ozeki, K. Kakei, N. Kosugi and H. Kuroda, J. Chem. SOC. Faraday Trans. 1, 1988, 84, 1795. J. W. McBain, J. L. Porter and R. F. Sessions, J. Am. Chem. Soc., 1933, 55, 2294. P. H. Emmett and R. B. Anderson, J. Am. Chem. Suc., 1945,67, 1492. P. L. J. Walker and J. Janov, J. Colloid Interface Sci., 1968, 28, 449. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity (Academic Press, London, 2nd edn, 1982), p. 262. M. J. B. Evans, Curbon, 1987, 25, 81. M. M. Dubinin and V. V. Serpinsky, Carbon, 1981, 19, 402. E. McCafferty, V. Pravidic and A. C. Zettlemoyer, Trans. Faraday Soc., 1970, 66, 1720. K. Kaneko, M, Serizawa, T. Ihikawa and K. Inouye, Bull. Chem. SOC. Jpn, 1975, 48, 1764. K. Kaneko and K. Inouye, Bull. Chem. SOC. Jpn, 1979, 52, 315. K. Kaneko and K. Inouye, J. Chem. Tech. Biotech., 1987, 37, 11. R. J. Atkinson, A. M. Posner and J. P. Quirk, J. Inorg. Nucl. Chem., 1968, 30, 2371. K. Kaneko and K. Inouye, Acisorption Sci. Tech., 1986, 3, 1 1 . T. Ishikawa, M. Okamoto, Y. Ito and K. Inouye, Nippon Kagaku Kaishi, 1972, 1751. M. Nomura, KEK Report 87-1, National Laboratory for High Energy Physics (1987). N. Kosugi and H. Kuroda, Program EXAFSl/V04 and EXAFS/V03, Research Center for Spectrochemistry, The University of Tokyo (1987). E. J. Fasiska, Corrosion Sci., 1967, 7, 833. L. Pauling and S. B. Hendricks, J. Am. Chem. Soc., 1925, 47, 781. A. K. Nikumbh, K. S. Rane and A. J. Mukhedhar, J. Mater. Sci., 1982, 17, 2503.K. Kaneko, N . Kosugi and H . Kuroda 88 1 33 E. J. Verway, P. W. Haayman and F. C. Romeijn, J. Chem. Phys., 1947, 15, 181. 34 E. Z. Basta, Miner. Mag., 1957, 31, 431. 35 F. J. Ewing, J. Chem. Phys., 1935, 3, 203. 36 A. L. Mackay, Miner. Mag., 1960, 32, 545. 37 A. Szytula, M. Balanda and Z. Dimitrijevic, Phys. Stat. Sol. (a), 1970, 3, 1033. 38 F. J. Ewing, J. Chem. Phys., 1935, 3, 420. 39 M. H. Francombe and H. P. Rooksby, Clay Miner., 1959, 4, 1. 40 0. Muller, R. Wilson and W. Krakow, J. Mater. Sci., 1979, 14, 2929. 41 D. Rivin, Rubber Chem. Technol., 1971, 44, 307. 42 P. Pfeifer and D. Avnir, J. Chem. Phys., 1983, 79, 3558. 43 D. Avnir, D. Farin and P. Pheifer, Nature (London), 1984, 308, 261. 44 K. Kaneko, Langmuir, unpublished work. 45 S. J. Gregg and K. S. W. Sing, Adorption, Surface Area and Porosity (Academic Press, London, 2nd edn, 1982), p. 267. Paper 8/01 7 15H; Received 3rd May, I988