Langmuir 1992,8, 2515-2515
2515
Titania Coating of a Microporous Carbon Surface by Molecular Adsorption-Deposition Akihiko Matsumoto,*yt Kazuo Tsutsumi,: and Katsumi Kaneko Department of Chemistry, Faculty of Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263, Japan, and Department of Materials Science, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan Received April 13, 1992. I n Final Form: July 30, 1992 The preparation of thin titania on activated carbon fiber (ACF)was investigated by molecular adsorptiondeposition (MADD) using controlled reaction of adsorbed Tic4 and water on the ACF surface to obtain the oxide surface of high surface area and specific activity. The TiOz-coated ACF was characterizedwith thin-film X-ray diffraction and nitrogen adsorption, and the adsorptivity of water vapor, NO, and NH3 was measured. The thin-film X-ray diffraction examination showed formation of anatase film on the carbon surface. The micropore width and its volume of TiOz-coated ACF decreased with the Ti02 coating. However, the adsorptivities of HzO and NHs were enhanced drastically. The evidence suggests the oxide formation on the ACF surface. cropore fil1ing”.l2-l4 It has also been observed for adsorption of NO on the ACF when the external surface is Active carbon fibers (ACFs) are highly microporous with modified with other metal oxides.15 small external surface areas; the pore widths of the slitWe have previously modified ACFs with transitionshaped micropores are in the range of 0.7-2 nm.lB2 The metal oxides by immersion in aqueous solutions of the ACFs have a greater pore volume and a more uniform relevant metal salts and successive deposition of their micropore sizedistribution than granular activated carbons oxides.l+16 However, this precipitation method is not a (GACS).~~~ The physical adsorption of vapor in micropores, good way to control the pore size, the pore shape, and the which is known as micropore filling,is enhanced by overlap dispersion state of the metal oxides. of the force field from the opposite walls of the mi~ropore.~ Some types of oxides are prepared directly by decomACFs have a uniform pore size distribution and can adsorb position in vapor phase of their chlorides,17J8 alkoxides, strongly a large amount of vapors in their micropores by and organometallic c o m p ~ u n d s . ~Chemical ~J~ vapor depmicropore filling. ACF can be a suitable model system in osition is one of the useful methods for the formation of the study of micropore filling.213Although micropore filling thin films and single crystals. Titanium tetrachloride is the strongest physical adsorption process, it does not (Tic&)is a liquid with a high vapor pressure of 1.87 kPa take place for adsorption of a supercritical gas whose at 298 K.20 The T i c 4 molecule has a tetrahedral struccritical temperature, T,, is less than the adsorption ture,21and its molecular area calculated by use of the van temperature. der Waals radii of constituent atoms and the covalent bond Metal oxides show characteristic chemical activity not length is 0.132 nm,2,22which is small enough to be adsorbed only for vapors but also for supercritical g a ~ e s . ~The -~ in the micropores of ACF, while alkoxides and organodispersion of transition-metal oxides on ACF leads to metallic compounds cannot enter the micropores. The enhancement of the adsorbate-adsorbate interaction of hydrolysis of TiC4, followed by pyrolysis, proceeds easily supercritical gases as well as the adsorbent-adsorbate at relatively low temperature and gives anatase, one of interaction.’*12 Oxide dispersion on ACF brings about a three cystalline forms of titania.23 Thus, if Tic& is weak chemisorption on the metal oxide followed by adsorbed as a film on both internal and external surfaces micropore fiiing by NO, a supercritical gas near room of ACF, and hydrolyzed with adsorbed water on the ACF temperature for which T, is 180 K, at 303 K. We have surface, the surface would be coated with a thin titania called this phenomenon “chemisorption-assisted milayer. By changing the vapor pressure of the reactants, Introduction
* To whom correspondence should b e addressed. + Prwent address: Department of Materials Science, Faculty of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441,Japan. t Toyohashi Universitv of Technoloev. (1)freeman, J. J.; Gimblett, F. G. R.;Eoberta, R. A.; Sing, K. S. W. Carbon 1987,25,559. (2)Kakei, K.; Ozeki, S.;Suzuki, T.; Kaneko, K. J . Chem.Soc., Faraday Trans. 1990,86,371. (3)Kaneko. K.: Nakahinashi. Y.: Nanata. K. Carbon 1988.26. 327. (4)Jayson, G.G.;Lawless, T.A.; Fairgust; D.J . Colloid Interface Sci.
1982,86,397. (5)Gregg, S.J.;Sing, K. S. W. Adsorption Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982;Chapter 4. (6)Dubinin, M. M.; Stoeckli, H. F. J . Colloid Interface Sci. 1980,75, 34.
(7)Kiselev, V. F.; Krylov, 0.V. AdsorptionProcesses on Semiconductor and Dielectric Surfaces I; Springer Verlag: Berlin, 1985;Chapter 4. (8)Mataumoto, A.; Kaneko, K. Colloids Surf. 1989,37,81. (9)Kaneko, K.; Mataumob, A. J.Phys. Chem. 1989,93,8090. (10)Kaneko, K.; Inouye, K. Adsorpt. Sci. Technol. 1988,5,239. (11) Kaneko, K.; Inouye, K. Carbon 1986,24,772. (12)Kaneko, K. Langmuir 1987,3,357.
0743-7463/92/2408-2515$03.00/0
(13)Kaneko, K. In Characterization of Porous Solids; Unger, K. K., Rouquerol, J., Sing, K. S. W., Kral, H., Eds.; Elsevier: Amsterdam, 1988; p 183.
(14)Kaneko, K.; Ohta, T.; Ozeki, S.; Kosugi, N.; Kuroda, H. Appl. Surf. Sci. 1988,33/34,355. (15)Matsumob,A.; Kaneko, K. J . Chem. Soc., Faraday Trans. I 1989, 85, 3437. (16)Kaneko, K.; Ozeki, S.; Inouye, K. Colloid Polym. Sci. 1987,265, 1018. (17)Ehrlich, P. In Handbook of Preparative Inorganic Chemistry, 2nd ed.; Brauer, G., Ed.; Academic Press: London, 1965;Vol. 2,p 1216. (18)Sakka, S. Sol-Gel hou no Kagaku (Science of Sol-Gel method); Agune-Syoufusya: Tokyo, 1988,Chapter 3. (19)Segal, D.L.Chemical Synthesis of Advanced Ceramic Materials; Cambridge University Press: Cambridge, 1989; Chaper 5. (20)Weast, R. C., Ed. CRC Handbook of Chemistry and Physics; CRC: Florida, 1980; p D-201. (21)Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University Press: Oxford, 1984;p 424. (22)Chemicalsociety ofJapan, Ed. KagakubinranKisohen (Handbook of Chemistry), 3rd ed.; Maruzen: Tokyo, 1987;Vol. 2,Chaper 15. (23)Mataumoto, A.; Tsutsumi, K.; Kaneko, K. J . Mater. Chem., submitted for publication.
0 1992 American Chemical Society
Matsumoto et al.
2516 Langmuir, Vol. 8, No. 10, 1992 the molecular adsorption-deposition method (MADD)can be used to control the thickness of the oxide layer. The titania-coated ACF is expected to be an adsorbent with high adsorptivity and capacity. In this paper, the preparation of thin titania coated on ACF by MADD and its characterization are described. The adsorptive properties of the TiO2-coated ACF for water, NO, and NH3 are also examined in this study.
1500
‘i
g1000 E
%
’&
$
4a
Experimental Section Titania Coating by Molecular Adsorption-Desorption. The pitch-based ACF (Osaka Gas Co.), which has a relatively large pore size?‘ was used in this study. A glass vacuum line with greaselessvalves and flange-typejoints was used to avoid reaction of the chloride vapor with vacuum grease. A part of the ACF sample was placed in a quartz basket which was suspended from a McBain spring balance in order to monitor the amount of adsorption; the reminder of the ACF sample was tied up with a platinum wire and placed at the bottom of the reaction cell. The T i c 4 vapor was adsorbed onto the ACF surface at 298 K. The ACF was evacuated at 383 K and 1 mPa for 3 h prior to the adsorption. The adsorbed T i c 4 film on the ACF was reacted with the saturated water vapor at 298 K after removal of gasphase T i c 4 by a liquid Nz trap. The hydroxylated titania-coated ACF was evacuated at 353 K and 0.1 Pa for 1 h after hydrolysis of TIC4 and then heated at 573 K for 1 h in order to remove HC1 vapor to produce the titania film. The completion of hydrolysis was determined from the weight change of the samples. The TiOz-coatedACF is designated Ti-ACF-n, where n is the amount of coated titania on the ACF in milligrams per unit weight of ACF. Characterization of t h e TiOl-Coated ACF. The microporosity of the titania-coated ACF was determined gravimetrically by nitrogen adsorption at 77 K. The samples were evacuated at 383 K and 1 mPa for 3 h prior to the adsorption measurement. The X-ray diffraction of ACF and Ti-ACF was measured by a highly sensitive diffractometer (MAC Science, MXP) using a low-angle incident X-ray with Ni-filtered CuKa radiation from an 18-kW generator. Therogravimetry (TG) and differential thermal analysis (DTA) data were determined in the temperature range of 303-1073 K at a rate of 10 K/min in a stream of nitrogen (Rigaku, Thermoflex TG/DTA). Diffuse reflectance infrared FT spectra (DRIFT) were measured in vacuo at 293 K using an FT-IR spectrometer (JASCO, FT/IR3) with anMCTdetector andadiffusereflectanceattachment (FDR61H). Samples were ground to a powder which had a mean particle diameter of less than 20 pm and evacuated at 383 K and 1 mPa for 1 h prior to the DRIFT measurement. Adsorption of HzO, NO,and NHs. The adsorption of water vapor, NO, and NH3 was measured gravimetrically at 303 K. NO and NH3 gases used in this study were of 99.9%purity, and each gas was purified by several freeze-thaw cycles in vacuo. Distilled water was used for the water vapor supply after removal of dissolved gases. The samples were evacuated at 383 K and 1 mPa for 3 h before adsorption measurements.
Results and Discussion Microporosity. Figure 1 shows the adsorption isotherms of N2 on Ti-ACFs and ACF. Not all the adsorption isotherms were typical of the type I characteristic of microporous solids. The amount of adsorbed N2 increases graduallywith the relative pressure (PIP& and more than 90% of the N2 adsorption is completed at PIP0 = 0.5 for each sample. The t plob appliedto the adsorption isotherms in Figure 1are shown in Figure 2. The standard thickness of an N2 layer, t, on the graphitized nonporous carbon was used25v26 ~
500
0
Y l u 1.0
0.5
PIP,
Figure 1. Adsorption isotherms of Nz. Amount of coated Ti02 (mg/g): H, 0; 0 , 120; 0,200; 0,1540.
1
5
0
0
r
t Inm
Figure 2. t plots for the N2 adsorption shown in Figure 1. Amount of coated Ti02 (mg/g): H, 0; 0 , 120; 0,200; 0,1540. Table I. Microporosity of Ti-ACFs and ACF Determined from Nitrogen Adsorption coated Ti02 oxide coverage at pore volume (mg/g) (0) (m2/g) (cm3/g) Ti-ACF-120 120 0.04 2340 1.41 0.06 2060 1.21 Ti-ACF-200 200 Ti-ACF-1540 1540 0.46 800 0.46 ACF 2280 1.49
because the micropore of ACF consists of micrographite layers and the Ti02 coverage of Ti-ACFs is relatively low, at most 0.04-0.46, as shown in Table I. In the case of Ti-ACF, it is difficult to elucidate the surface state of Ti-ACF at the present; therefore, we used approximately the standard t of graphitized nonporous carbon. t plots of Ti-ACFs and ACF are linear and pass through the origin in the t region between 0.35 and 0.6 nm, though greater deviation is observed in the lower t region for each sample. The t plots bend near t = 0.6 nm and are nearly parallel to the abscissa. This evidence indicates the existence of micro pore^.^^ The specific surface area (at) and micropore volume of Ti-ACFs, calculated from the t plots, are shown in Table I. at of Ti-ACF-120 is almost equal to that of ACF. at decreases from 2280 to 800 m2/g with the increase in the amount of Ti02 coating up to 1540 mg/g. The increase of
~~
(24) Kqeko, K.; Suzuki,T.;Kuwabara,H.;Kakei, K. InFundamentak
of Adsorptron; Mersmann, A. B., Scholl, S. E., Eds.; American Institute of Chemical Engineering: New York, 1989; p 345. (25) Rodriguez-Reinoeo,F.; Martin-Martinez,J. M.; Prado Burguete, C.; McEnaney, B. J . Phys. Chem. 1987, 91, 515.
(26) Carrot, P. J. M.; Roberts,R. A; Sing, K. S. W. Carbon 1987,25, 769. (27) Broekhoff,J. C. P.;Linsen,B. G. InPhysicaland ChemicalAspects of Adsorbents and Catalysis; Linsen, B.
1970; Chapter 1.
G.; Academic Press; London,
Langmuir, Vol. 8, No. 10,1992 2517
Preparation and Characterization of TiOz-Coated ACF
(b) anatase layer
L
Yka/ 2
Figure 3. Coating model of Ti02 on ACF surface: the patchwise model (a) and thin-film model (b). the Ti02 weight in the sample mainly lowers the at value because the at was determined by the unit weight of ACF and coated TiO2. at of each Ti-ACF sample agrees within 15% of the calculated surface area of ACF contained in 1 g of the Ti-ACF sample. The Ti02 coating scarcely affects the decrease of surface area due to micropores, although it does reduce the micropore volume. Most of at for Ti-ACF is due to the original micropores of ACF. The micropore volume of Ti-ACF also decreases from 1.49 to 0.46 cm3/g. The decrease in pore volume almost coincides with the volume of the anatase coating on the ACF surface; the anatase formationwill be described later. This result suggests that Ti02 is present not only on the outer surface of ACF but also on the micropore walls. The point of inflection in the t plots shown in Figure 2 changes from 0.65 to 0.55 nm with increasing amounts of coated TiO2. This suggests that the micropore width is narrowed by 0.2 nm with the development of oxides. Therefore, the average thickness of the oxide layer of 1540 mg/g on the ACF surface is assumed to be 0.1 nm. As the occupied area of the unit cell of anatase on the ACF surface is presumed to be 0.351 nm2,the surface coverage by anatase on the ACF surface, 8, can be expressed as
8 = {amountof Ti02 (mg/g of ACF) X 10-3/(131.34x X 3.90)) X 0.351 X 10-18/2280 where the value of 131.34 X is the volume of a unit cell expressed in cubic centimeters, 3.90 (10.07) is the specific gravity of anatase, and 2280 is the at of ACF in square meters per gram. The 8 value of each sample is listed in Table I. The 0 changes from 0.04 to 0.46 with the Ti02 coating, which correspondsto a thickness increment of Ti02 from 0.015 to 0.17 nm if the a axis value of the lattice constants are taken as the unit thickness of the anatase. The thickness of 0.17 nm is about half of the a axis value and agrees with the decrease of the micropore width from the t plots. As shown in Figure 3, the oxides for which the thickness is the a constant are probably formed patchwise on both pore walls (a), or the oxide layer with a thickness of half of the a constant is formed on both micropore walls (b). Applicability of the t plot analysis to the micropore system has been criticized because of the absence of a clear determination of the monolayer capacity. Sing et al. proposed replacement of the monolayer capacity as a normalizing factor by the amount adsorbed at a fixed .~ relative pressure of 0.4, which is called the as m e t h ~ das plots for N2 adsorption on ACF and Ti-ACFs are shown
I S
Figure 4. as plots for the Nz adsorption. Amount of coated
Ti02 (mg/g): B, 0; 0,120;0,200;0,1540.
28 / deg, CuKa
Figure 5. X-ray diffraction patterns of ACF (a) and Ti-ACF1540 (b).
in Figure 4. The micropore volume of each sample obtained from the t plot listed in Table I is within 10% of that obtained from the as plot. Therefore, the change in the micropore width determined by the t plot due to developing oxides is reliable in the present case. Ti02 Coating on the ACF Surface. Figure 5 shows the thin-film X-ray diffraction patterns of ACF (a) and Ti-ACF-1540 (b). The broad diffraction peaks at 22.6O and 43.8' of ACF are attributed to the reflections from the (002) plane and both (100)and (101)planes of graphite, respectively. The thin-film X-ray diffractometer used in this study has a sensitivity of 1nm. The crystallite size of each sample is obtainable from diffraction peaks by use of the Scherrer equation. The crystallite sizes of graphiteconsisting ACF are 1.3 and 5 nm along [0021, and both [lo01 and [1011 directions, respectively. The micrographitic structure of ACF does not change with the Ti02 development. There are several peaks due to anatase in the diffraction patterns of Ti-ACF-1540, even though ordinary powder X-ray diffraction measurementsdid not show the presence of anatase. These peaks appeared at 25O, 37.6O, 47.8', and 54.1', and are attributed to reflections from (1011, (0041, (200),and (105) planes of anatase, respectively. The crystallite sizes in the [loll, [0041, and 12003 directions are 2.5, 3.4, and 3.5 nm, respectively. In this case, these reflections are mainly caused by anatase blocking micropores; the crystallite size of anatase produced on the micropore wall must be less than the pore width. However, it is possible that thin-film anatase of 0.2-0.4 nm is produced on the micropore wall from t plot analysis of N2
Matsumoto et al.
2518 Langmuir, Vol. 8, No. 10, 1992
I
I
I
100
200
300
I
I
400 500 temperature I K
I
I
600
700
800
Figure 6. DTA and TG curves for Ti-ACF-1540 (a) and ACF (b) (solid lines and broken lines denote the DTA curves and the TG curves, respectively).
3 PIPo
Figure 8. Adsorption isotherms of HzO at 303 K. Amount of , 0; 0,120; 0,200; 0,1540. coated Ti02 (mg/g): .
1400 1000 wavenumberl crn-'
600
Figure 7. Diffuse reflectance IR spectra of ACF (a) and TiACf-1540 (b).
adsorption, even though thin-film X-ray diffraction did not confirm the formation. DTA and TG curvesof Ti-ACF-1540 and ACF are shown in Figure 6. Ti-ACF-1540 has a sharp endothermic peak a t 331K with arapid weight loss, which were not observed in the case of ACF. The endotherm and weight loss are due to the removal of adsorbed water.28 The water adsorption data, which are described later, demonstrate that more water is adsorbed by the Ti-ACFs than ACF. DRIFT spectra of ACF and Ti-ACF-1540 are shown in Figure 7. Each spectrum has absorption bands a t 1200 and 1320 cm-l which are attributed to C-0 stretching, and 940 and 900 cm-l due to the carbon skelet0n.~*3~ Absorption bands due to the surface OH groups of titania, which appear around 3650 and 1600 ~ m - * , 3were ~ not observed because of the weak intensity of these absoprtion bands. Introduction of Hydrophilicity. Figure 8 shows the adsorption isotherms of water on Ti-ACFs and ACF at 303 K. The adsorption isotherms of ACF are typical type 111,which is characteristic of a weak adsorbent-adsorbate interaction. The original ACF has a small water uptake, 5 mglg, up to the relative pressure PlPo = 0.6, which indicates that it is hydrophobic because its surfaceconsists of microcrystallitesof graphite. However, the development (28) Duval, C. Inorganic Thermalgrauimetric Analysis, 2nd ed.; Elsevier: Amsterdam, 1963; p 289. (29) Papirer, E.; Guyon, E.; Perol, N. Carbon 1978, 16, 133. (30) Friedel, R. A.; Hofer, L. J. E. J. Phys. Chem. 1970, 74, 2921. (31) Bouwman, R.; Frerike, I. L. C.; Wife, R. L. J.Catal. 1981,67,282. (32) Meldrum, B. J.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1990,86, 2997. (33) Zarifyants, Y. A.; Kiselev, V. F.; Petrov, A. S.;Prudnikov, R. V.; Khrustaleva, S. V.; Chukin, G. D. Kinet. Catal. (Engl. Transl.) 1974,15, 325.
equilibrium pressure I kPa
Figure 9. Adsorption isotherms of NH3 at 303 K. Amount of coated Ti02 (mg/g): . , 0; 0, 120; 0,200; 0,1540.
of anatase on the surface leads to a drastic enhancement in adsorption at lower P!Po and to a type I1 isotherm. The amount of water adsorbed per unit micropore volume is 20-30 times greater than that of uncoated ACF. Therefore, the ACF surface with the Ti02 coating is hydrophilic. Adsorptivity of the TiO2-Coated ACF for NHJ. Figure 9 shows the adsorption isotherms of NH3 on TiACFs and ACF at 303 K. The adsorptivity for NH3 increases markedly with the Ti02 coating. Each isotherm is of type I character (Langmuir type), and Langmuir b values obtained from the Langmuir plots are listed in Table 11. Micropore filling for vapors can be described using the Dubinin-Radushkevich (DR) equation,524~35written as
W = Woexp(-t//3E0)* where W is the amount adsorbed at relative pressure PIPO, W Ois the amount of micropore volume filled with vapor, j3 is the affinity coefficient, and Eo is a characteristic adsorpiton energy. E = R T In (PoIP) is the adsorption potential. Figure 10 shows the DR plots for Ti-ACFs and ACF. These plots exhibit good linear relationships. NH3 is adsorbed through the micropore filling. The amount of (34) Dubinin, M. M. In Chemistry and Physics of Carbon; Wlaker, P. L., Ed.; Marcel Dekker: New York, 1966; Vol. 2, p 51. (35) McEnaney, B.; Mays, T. J. In Introduction to Carbon Science; Marsh, H., Ed.; Butterworths: London, 1989; Chapter 5.
Langmuir, Vol. 8, No. 10,1992 2519
Preparation and Characterization of TiOz-Coated ACF
Ti-ACF- 120 Ti-ACF-200 Ti-ACF- 1540 ACF anatase
pore volume (cm3/g) 1.42 1.21 0.46 1.36
Table 11. Adeorptivities of NH3 and NO on Ti-ACFe and Ti02 NH3 adsorbed pore filling ratio b(NH3) on coated Ti02 Wo(NH3) (Wo(NHs)/ mg/g mg/cm3 (mg/g) mg/g mg/cm3 porevolume) 46.3 72.9 153.3 29.9 22.0
32.8 60.2 333.0 20.1
3.2 5.1 18.1
93.8 78.5 147.3 31.4
Langmuir b(N0) mg/g mg/cm3
8.0 x 1.9 x 10-2 4.0 X lo-' 3.4 x 10-2
66.1 64.9 333.3 27.5
32.8 36.5 40.6 84.8
23.1 30.2 88.9 62.3
4=2
0
10
20
30
40
50
tl"(Pl/P)32
Figure 10. DR plots for NH3 adsorption at 303 K. Amount of coated Ti02 (mg/g): B, 0; 0,120;0,200;0,1540.
micropore volume filled with NH3, Wo(NH3),was obtained from the DR plot listed in Table 11, with the assumption that NH3 exists in the micropore as a liquid of the same density as liquid a t 197 K (0.817 mg/cm3). Each Wo(NH3) value (mg/g of adsorbent) is comparable to the Langmuir b except in the case of Ti-ACF-120. The large difference between Wo(NH3) and Langmuir b of Ti-ACF-120 is caused by the difficulty of estimating WOin the highpressure region of the DR plot. Wo(NH3) increases from 37.4 to 147.3 mg/g with Ti02 coating. The pore filling ratio of NH3 was estimated from the ratio of Wo(NH3) to micropore volume determined by Nz adsorption and is shown in the eighth column of Table 11. The ratio of ACF is only 3.4 X 10-2 whereas that of 'Ti-ACF-1540 is 4.0 X10-l; this evidence demonstrates the enhancement of micropore filling by Ti02 coating. As shown in Table 11,Langmuir b of Ti-ACF increased from46.3 mg/g of adsorbent (32.8 mg/cm3 of pore volume) to 153.3 mg/g (333.0 mg/cm3)as the Ti02 coating increased from 120 to 1540 mg/g. The amount of NH3 adsorption on anatase is also listed in comparison with those of TiACFs. The contribution of the Ti02 coating to the NH3 adsorption on Ti-ACF is estimated by use of the amount of Ti02 coating and Langmuir b values of anatase (22.0 mg/g), assuming that the adsorption activity of the coated Ti02 is the same as that of anatase powder. This is listed in the fifth column of Table 11. The contribution of Ti02 itself to adsorption on Ti-ACF is only 7-12 76. In the case of Ti-ACF-1540, for example, the contribution of coated Ti02 to the NH3 adsorption is 18.1 mg/g for the observed amount of adsorption (153.3 mg/g). Thus, enhancement in NH3 adsorption occurs in the Ti-ACF systems. This evidence is also supported by the fact that the amount of NH3 adsorption on the Ti-ACF sample per unit area is much greater than that on ACF. Chemisorption-AssistedMicropore Filling of NO. The adsorption isotherms of NO on ACF and Ti-ACFs at 303 K are shown in Figure 11. Here the ordinate is expressed in terms of milligrams per unit micropore volume because the micropore volume changed with the Ti02 coating. Each adsorption isotherm is of type I (Langmuir type). The saturated amount of NO adsorption, Langmuir b, is determined from the Langmuir plots. The Langmuir b values for NO adsorption, expressed in terms of milligrams per unit weight and unit pore volume, are listed in Table 11. NO adsorption of Ti-ACF-1540 is much greater than that of ACF, though no enhancement was observed in the cases of Ti-ACF-120 and Ti-ACF-200.
equilibrium pressure/ kPa
Figure 11. Adsorption isotherms of NO at 303 K. Amount of coated Ti02 (mg/g): B, 0; 0 , 120; 0,200; 0,1540.
I
0
1
2
3
4
In P
Figure 12. Modified DR plots for NO adsorption at 303 K. Amount of coated TiOz(mg/g): B, 0; 0 , 120; 0,200; 0, 1540. The DR equation mentioned in the previous section is not applicable for micropore filling with supercritical gases because the equilibrium relative pressure at the experiment temperature cannot be calculated. However, if the pore volume againd NO is substituted for the Langmuir b value, WL(NO),the modified DR equation is obtained:l3 [In WL(NO)/WI'/2= (RT/i3Eo)(lnPo(NO)- In P) where W is the amount adsorbed a t pressure P, B is the affinity coefficient estimated ca. 0.26 from c a l ~ u l a t i o n , ~ ~ Eo is a characteristic adsorption energy, and Po(N0) is the saturated vapor pressure in the micropore. Modified DR plots are shown in Figure 12. Each plot is linear; the intercept and slope yield the values of POand EO. The isosteric heat of adsorption a t the coverage of l / e (ca. 0.4), qBtg=l/e,is associated with the following equation:*J6 qBtg=l/e= i3Eo + M v where AHv is the heat of vaporization of NO, 13.8 kJ/mol. The qstg=l/e values of NO in the micropore are ACF, 19.5; Ti-ACF-120,23.3; Ti-ACF-200, 26.2; and Ti-ACF-1540, 28.4 kJ/mol. These results indicate that coated Ti02 stabilizes NO adsorbed in micropores. POof each sample differs widely, from 76 to 823 kPa, because the value is dependent on the slope of the modified DR plot. More precise measurements are needed to evaluate the PO.In the case of NO adsorption, though, the interaction between NO and the sample surface increase with anatase coating because the coating did not sufficiently lead to enhancement of the amount of NO adsorption which was observed in the iron oxide-dispersed ACF. The enhancement of the micropore filling of supercritical NO by dispersion of
2520 Langmuir, Vol. 8, No.10, 1992
iron oxides should be msociated with magnetic interaction between a paramagnetic NO molecule and an Fe(II1) ion of great magnetic moment, giving rise to dimerization of N0.36137 Diamagnetic Ti02 on ACF may not accelerate the dimerization of NO.
Conclusion Development of Ti02 on the ACF surface enhances markedly the adsorptivity of ACFs for NH3 and H20, but not so significantly for supercritical NO. The enhancement of H20 and NH3 adsorption by the oxide coating cannot ~~
(36) Kaneko, K. Colloida Surf. 1989, 37,115. (37) Uchiyama, H.; Ozeki, S.;Kaneko, K. Chem. Phys. Lett. 1990,166, 531.
Matsumoto et al. be explained by the mixture of Ti02 and ACF. So far, we have not found direct evidence for the thin coating of Ti02 on the micropore walls of ACF. However, t plot analysis, the adsorption behavior of NH3 and H20, and thin-fib X-ray diffraction suggest that the micropore walls of ACF are coated with ultrathin oxide layers.
Acknowledgment. We are indebted to Drs. F. Mizukami and K. Maeda for thin-film X-ray diffraction measurements and Dr. D. L.Segal for help in preparation of the paper. The financial support by the Science Research Grant from the Ministry of Education, Japanese Government, and Osaka Gas Research Center is greatly appreciated.
