J. Phys. Chem. 1989, 93, 4833-4837 Surfynol465 solutions with both types of solvents and obtained the values of R in DzOand H 2 0at 2.4 and 2.5 nm, respectively,26 which shows no significant difference as described above. To elucidate further the variation of R, which seems to depend on the kind of measurement, we are now preparing the N M R relaxation and thermodynamic measurements of Surfynol 465 and (26) Sato, S . ; Kishimoto, H., manuscript in preparation.
4833
other nonionic surfactants in D20,H 2 0 , and their composite solvents. Acknowledgment. I express my grateful thanks to Professor Hiroshi Kishimoto, Nagoya City University, for valuable discussions. I also thank Miss Setsuko Kat0 for her assistance in the N M R measurements. Registry No. Surfynol 465, 9014-85-1.
Photocatalytic Activities of Microcrystalline TiO, Incorporated in Sheet Silicates of Clay Hiroshi Yoneyama,* Shigeo Haga, Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka 2- 1 , Suita, Osaka 565, Japan
and Shoji Yamanaka Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan (Received: May 26, 1988; In Final Form: October 31, 1988)
Photochemical and photocatalytic properties of microcrystallineTiOz incorporated in the interlayer space of montmorillonite were investigated in reference to those of TiOz powder particles which were prepared by agglomeration of a titania sol that was used in the preparation of the titania-pillared clay. The pillared Ti02 microcrystallites having ca. 15-A pillar height showed a ca. 0.58-eV blue shift in its absorption and fluorescence spectra compared with those of the TiO, particles, exhibiting the quantum size effect. The excited electronic states of the pillared Ti0, were determined to be 0.36 V more negative than that of the TiOz powder particles. Photocatalytic activities of the pillared TiO, were greater than those of the Ti02 powder particles for decomposition of 2-propanol to give acetone and hydrogen and of n-carboxylic acids with up to eight carbons (from acetic acid to caprylic acid) to give the corresponding alkanes and carbon dioxide, though the pillared Ti02 exhibited lower activities for decomposition of capric acid, having 10 carbons in a molecule.
Introduction Photochemical properties of semiconductor microcrystallites in solution have become popular in view of their quantum size effects,’-’ which are recognizable by blue shifts in absorption and emission Recently, it has been shown that size(1) (a) Rossetti, s.; Nakahara, s.;Brus, L. E. J . Chem. Phys. 1983, 79, 1086. (b) Brus, L. E. J . Chem. Phys. 1983, 79,5566. (c) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1985, 82, 552. (d) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 83, 1406. (2) Kanata, T.; Murai, H.; Kubota, K. J . Appl. Phys. 1987, 81, 969 (3) Abraham, M.; Puasch, G.; Tadjeddine, A.; Hakiki, N. Solid State Commun. 1986, 60, 397. (4) (a) Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J . Chem. Phvs. 1984.80. 4464. (b) Rossetti. R.: Hull. R.: Gibson. J. M.: Brus. L. E. J . Chem. i h y s : 1985, 8 i 552. (c) Chestnoy, N.;Hul1,~R.;Brus, L: E. J. Chem. Phys. 1986,85, 2237.
( 5 ) (a) Waller, H.; Koch, U.; Gutieretz, M.; Henglein, A. Ber Bunsen-Ges. Phys. Chem. 1984.88, 649. (b) Fojtik, A.; Weller, H.; Koch, U.; Henglein, A. Ber. Bunsen-Ges. Phvs. Chem. 1984. 88, 967. (c) Koch. U.: Foitik. A.: Weller, H.; Henglein, A: Chem. Phys. Lett. 1985, 122, 507. (d) Weiler, H.; Fojtik, A,; Henglein, A. Chem. Phys. Lett. 1985, 117, 485. (e) Baral, S.; Fojtik, A,; Weller, H.; Henglein, A. J. Am. Chem. Soe. 1986, 108, 375. (0 Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A,; Baral, S.; Henglein, A.; Kunath, W.; Weiss, K. Chem. Phys. Lett. 1986, 224, 557. (9) Fischer, Ch. E.; Weller, H.; Fojtik, A.; Lume-Pereira, C.; Jananta, E.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 46. (h) Spanhel, L.; Weller, H.; Fojtik, A.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 88. (6) (a) Nedeljkovic, J. M.; Nenedovic, M. T.; Micic, 0. I.; Nozik, A. J. J . Phys. Chem. 1986, 90, 12. (b) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I. J . Phys. Chem. 1985, 89, 397. (c) Micic, 0. I.; Nenadovic, M. T.; Petersm, N. W.; Nozik, A. J. J . Phys. Chem. 1987, 91, 1295. (d) Williams, F.; Nozik, A. J. Nature 1984, 311, 121. (7) Stramel, R. D.; Nakamura, T.; Thomas, J. K. Chem. Phys. Lett. 1986, 130, 423. (8) Wang, Y.; Herron, N. J. J. Phys. Chem. 1987, 91, 257.
0022-3654/89/2093-4833$01 S O / O
quantized HgSe and PbSe can catalyze reduction reactions that cannot occur in their bulk materials.6a Furthermore, it has been reported that the quantum size effect of TiOz microcrystallites operated in photocatalytic hydrogenation of propyne with water vapor.I2 These results encourage us to investigate further photocatalytic activities of semiconductor microcrystallites that exhibit the quantum size effect. The present paper deals with photochemical and photocatalytic properties of Ti02microcrystallites having ca. 15-8, diameter that were intercalated in sheet silicates of sodium montmorillonite, the preparation method and the characterization of which were already published.I3 By the intercalation in the clay, the TiOz microcrystallites must be stable against aggregation in solution, and then it will not be required to use any stabilizing agents. The first photochemical studies on semiconductor particles retained in clay were made by Ena and Bard for CdS.I4 They obtained several interesting results such as operation of electrostatic interaction between the clay layers and reactants, but their studies did not focus on the evaluation of photocatalytic activities of the retained semiconductor particles. Stramel et al.’ prepared CdS microcrystallites in sheet silicates of a synthetic clay to investigate its spectroscopic properties, but no photocatalytic work was reported. (9) Hodes, G.; Albu-Aharon, A.; Decker, F.; Motisko, F. Phys. Rev. B 1987, 36, 4215.
(10) Warnok, J.; Awschalom, D. D. Phys. Rev. B 1985, 32, 5529. (1 1) Dannhauser, T.: O’Neil. M.; Johanson, K.;Whitten, D.; McLendon, G. J . Phys. Chem. 1986, 90, 6047. (12) Anpo, M.; Shima, T.; Kodama, S. J. Phys. Chem. 1987, 91,4305. (13) Yamanaka, S.; Nishihara, T.; Hattori, M. Mater. Chem. Phys. 1987, 17, 87. (14) Ena, 0.;Bard, A. J. J. Phys. Chem. 1986, 90, 301
0 1989 American Chemical Society
4834 The Journal of Physical Chemistry, Vol. 93, No. 12, 1989
Yoneyama et al.
r -
Experimental Section
Preparation of Ti02-Pillared Clay. Two papers have been published for the preparation of Ti02-pillared clay.13Js We adopted the procedures reported by Yamanaka et al.I3 Titanium tetraisopropoxide was added dropwise to vigorously stirred 1 M HCI so as to give a final molar ratio of 0.25 of the alkoxide to HCI. The resulting slurry was peptized by further stirring for 3 h to give a clear titania sol and then mixed with an aqueous suspension of 1 wt % clay. The clay used was sodium montmorillonite (Kunimine Industrial Co.), having an exchange capacity of 100 mequiv/100 g. The amount of clay mixed with the titania sol was such that its cationic exchange capacity was one-fortieth the number of moles of Ti02in the sol. The concentration of HCI of the resulting clay suspension was 0.61 M. The suspension was continuously stirred for 3 h at 50 "C: then the clay was centrifuged, followed by washing with deionized water, and finally dried in air at room temperature.16 X-ray diffraction of the resulting clay gave a diffraction peak at 28 = 3.6', from which the basal spacing of 24.6 A was determined. By subtracting the silicate layer thickness of 9.6 A from the determined basal spacing, we obtain 15 A as the interlayer spacing of the clay which must be equal to the pillar height of the incorporated Ti02. In the obtained diffraction patterns there was no diffraction peaks assignable to TiOz. Hereinafter the TiO,-pillared clay will be denoted as Ti02/clay. The amount of TiO, of the Ti02/clay, which was determined by elemental analysis of the Ti02/clay calcined at 800 OC, was 50 wt %.13 Presently, we have no information on the number of Ti02molecules of each pillar, because the size and shape of the TiO, pillars have not yet been determined. The specific surface area of the Ti02/clay was 347 m2 g-], as determined by a Shimadzu-Micromeritics Model 2205 surface area analyzer, and that of the clay support alone was 72 m2 g-I. Since the BET method gives the specific surface area of the clay outer surfaces only," the specific surface area of the clay support including the interlayer area would be twice the above value. If this value, 174 m2g-l, is simply subtracted from the specific surface area of the Ti02/clay, one obtains 203 m2 g-l, which gives the lowest limit of the specific surface area of the Ti0, microcrystallites incorporated in the clay interlayers. On the other hand, the value of the highest limit would be 275 m2 g-l, which is obtained by subtracting the contribution of the outer surface of the clay from the specific surface area of the TiO,/clay. Besides the Ti02/clay, a TiOz powder was prepared as its reference. For this purpose the titania sol used for the preparation of the Ti02/clay was allowed to stand for long time to give a cloudy white sol, which was centrifuged, and the resulting solid was washed with deionized water, followed by drying in air. The resulting Ti02 powder had the rutile modification, and its specific surface area was 160 m2 g-I. Spectral Measurements. Absorption spectra of the titania sol were measured intermittently during its standing with a Shimadzu MPS 5000 spectrophotometer after dilution of the sol with 1 M HCI. Absorption spectra of the TiO,/clay were also measured by using a suspension. Fluorescence spectra were measured with a Hitachi MPF-3 fluorescence spectrophotometer. Electrochemical Measurements. Energy diagrams of the TiOz/clay were determined by employing the technique reported by Ward et a1.,'* which is based on photocurrent measurements of irradiated semiconductor suspensions at an inert collector (15) Sterte, J. Clays Clay Miner. 1986, 34, 658. (16) One of the reviewers pointed out possibilities of destruction of clay by contact with the acidic titania sol. However, the preparation of Ti02-incorporated clay was successfully achieved in the presence of 0.61 M HCI, as reported previously. SterteIs also investigated the preparation of Ti0,-pillared clay with the use of HCI solutions ranging from 1 to 0.1 1 M. In his preparation, the clay was left in the acid solutions for 16 h, 13 h longer than in the present study; but according to his results on the effect of acid concentration on the TiOl incorporation, the acid concentration did not bring about any serious problem in the preparation of the Ti02-pillared clay. It seems that acid concentrations much greater than 1 M are required to destroy clay. (17) Sears, W. M.; Morrison, S . R. J . Phys. Chem. 1985, 89, 3295. (18) Ward, M. D.; White, J. M.; Bard, A. J. J . Am. Chem. SOC.1983, 105, 27.
zt
I
I 200
300
400
Waveiengfh I n n
Figure 1. Absorption spectra of a titania sol (a), TiO,/clay (b), and TiO, powder (c) suspensions. Concentrations: (a) 2.5 X IO4 M, (b) 7.5 X M in T i 0 2 base, and (c) 4 X lo-' M. For preparation, see text.
electrode as a function of pH. A two-compartment cell divided by a fine porosity glass frit was used. One compartment contained a 5-cm3 Au plate as a working electrode and the Ti02/clay suspension. A 4-cm2 Pt plate serving as a counter electrode was included in the other compartment separated from the suspension. The electrolyte solution used consisted of 0.1 M KNO,, 1 M sodium acetate, and 1 mM methylviologen, and its pH was adjusted by adding appropriate amounts of 6 M NaOH or 6 M HNO,. The amount of the photocatalyst suspended was 30 mg in TiO, base for 100 mL of electrolyte solution. Before measurements, dissolved oxygen was purged off by bubbling argon for 30 min and then illuminated with a 500-W high-pressure mercury arc lamp (Ushio Electric, Model USH 500). Transient photocurrents induced by the illumination were recorded on a Toa EPR- 15 1A electronic recorder. Photocatalytic Experiments. The experiments were carried capacity having 1.5-cm diameter and 18-cm out in a cell of 30" height. Photocatalytic decompositions of 2-propanol and ncarboxylic acids were investigated. In the former case, 8 mL of 2-propanol was mixed with 0.2 mL of water, while in the latter 2 mL of reactants and 8 mL of water were mixed, except for capric acid decomposition where 2 mg of it was mixed with 8 mL of water. The amount of photocatalyst used was 3 mg in TiO, weight base, and it was suspended in the above-mentioned reactant solutions. The suspension was deaerated by bubbling argon for 2 h, and then the cell was closed and illuminated with the mercury arc lamp while magnetically stirring the suspension. The illumination time was 24 h except where specially noted. The products were determined by gas chromatography; hydrogen and hydrocarbons in gas phase were determined with the use of Molecular Sieve SA column and Porapak Q column, respectively, at 130 "C with argon carrier gas, while organic species in the liquid phase were analyzed with the use of a Gaskuro Kogyo Inc. PEG-HT column at 140 OC with helium carrier gas, except for the determination of acetone where a Gasukuro Kogyo EX-10 column was used at 80 OC with helium carrier gas. The quantum efficiency for photocatalytic decomposition of 2-propanol was determined at 280 nm on the basis of hydrogen produced. Lights from the mercury arc lamp was passed through a monochromator (JASCO, Model CT-10) which contained a diffraction grating made by ruled 1200 lines mm-l. The slit width chosen was 3 mm, and the number of photons in the monochromatic light was determined by ferrioxalate actinometry. Platinization of the photocatalysts was performed photochemically when required. The photocatalyst (10 mg in TiO, weight base) was suspended in 20 mL of a HCI solution of pH 1.5, and the suspension was mixed with a chloroplatinic acid solution which contained 0.53 mg of H2PtC16and 1 mL of 40% formic acid.19 After bubbling argon for 2 h, the suspension was illuminated with the mercury arc lamp for 4 h to give complete deposition of (19) Duounghong, D.; Borgarello, E.; Gratzei, M. J . Am. Chem. Soc. 1981, 103, 4685.
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989 4835
Ti02 Incorporated in Sheet Silicates of Clay
pH 2.1
50
pH 1.0
0 300
400
500
off
5 oc
Wavelength / n m
Figure 2. Fluorescence spectra of the Ti02 powder and the Ti02/clay suspended in deaerated water: (a) Ti02 powder, 7.5 X lo” M; (b) Ti02/clay, 7.5 X 10“ M Ti02. Excitation: 280 nm.
0
PH 0.4
On
off
platinum. The resulting platinized TiO, contained 2 wt ,5% Pt.
50
0
100
Results and Discussion Absorption Spectra and Fluorescence Spectra. The titania sol used for the preparation of the Ti02/clay was transparent, as suggested from an absorption spectrum given by curve a of Figure 1, where absorption in the UV region only can be seen. The spectrum was taken after dilution of the titania sol with 1 M HCl to 1000 times. When the sol was allowed to stand at room temperature, its transparency was gradually lost and simultaneously the absorption spectrum of the sol showed red shifts to finally give spectrum c, which is in good agreement with the spectrum of bulk Ti02. The Ti02/clay gave an absorption spectrum that has a long tail in longer wavelengths as given by spectrum b. The long tail may result from the presence of large particles on the clay surfaces and/or at edges of clay platelets. The absorption threshold of the TiOz microcrystallites held in the interlayer spacings of clay is estimated to be 348 nm from the extrapolation of the rising portion of the spectrum to abscissa, as shown in Figure 1. It is suggested from these results that the quantum size effect operates in the TiO,/clay. Anpo et a1.12 measured the absorption threshold of Ti02 particles covering from 38 to 530 A and recognized that blue shifts in the spectrum occurred with decreasing the particle size at least from 220 A. Fluorescence spectra obtained by excitation a t 280 nm are shown in Figure 2, which were not corrected for the spectral sensitivity of the fluorimeter. These spectra were obtained in deaerated water, but if there was dissolved oxygen the emission intensity was dropped to more than 1000 times that obtained in the absence of oxygen due to the electron scavenging action of the oxygen. According to Figure 2, the fluorescence intensity maximum of the TiOz/clay appears at 332 nm, while that of the Ti02powder appears at 397 nm. A blue shift of 0.58 eV resulted from the size quantization, suggesting that the bandgap of the TiO, microcrystallites is 3.58 eV, being 0.58 eV greater than that of the Ti02 bulk material. The bandgap value estimated here is in good agreement with that expected from the absorption threshold (348 nm). The bandgap (E,) and thus the absorption threshold of sizequantized semiconductor particles can be predicted by using, for example, eq 7 of ref le, if the particles are circular and data on the effective mass of electrons, me*, and positive holes, mh*,are available. If it is assumed that me* = 20ma’ and mh* = 3mo?0 where mo is the mass of a free electron, TiOz microcrystallites having 15-Adiameter would have E, = 3.39 eV, which is smaller than the energy gap estimated from the observed blue shift in the absorption and fluorescence spectra of the TiOJclay. One (20) Itakura, M.; Niizuka, N.; Toyoda, H.; Iwasaki, H. Jpn. J. Appl. Phys. 1967, 6, 31 1. (21) Kasinski, J. J.; Gorenez-Jahn, L. A.; Miller, R.J. D., to be. published.
200
Time/s
Figure 3. Transient anodic photocurrents due to oxidation of methylviologen cation radicals at the Pt working electrode polarized at -0.2 V vs SCE. The Ti02/clay was suspended in 100 mL of aqueous solution containing 1 M sodium acetate, 0.1 M sodium nitrate, and 1 mM methylviologen so as to give 30 mg of Ti02. Solution pH was adjusted by adding either 6 M HN03 or 6 M NaOH. A 500-W high-pressure mercury arc lamp was used as a light source.
.-c
E
4,
-
v
2< .4
6
7
8
PH
9
1
0
PH
Figure 4. Plots of the rate of increase of transient photocurrents Ai/Af as a function of pH: (a) Ti02 powder, (b) Ti02/clay. All the Ai/Af values were time-averaged ones which were determined from transient photocurrent behaviors measured just after illumination of semiconductor
suspensions. plausible reason for the discrepancy observed is that the Ti02 microcrystallites in the interlayer spacings of the clay are not circular and each pillar contains fewer TiOz molecules than each circular particle having 15-8, diameter. Energy Diagrams of the Ti02/Clay. Figure 3 shows transient anodic photocurrent behaviors at the working electrode due to oxidation of methylviologen cation radicals that were produced at illuminated TiO, microcrystallites incorporated in the clay. It was reported for CdS incorporated in m~ntmorillonite’~ that methylviologen was not useful as a mediator to carry photogenerated electrons from the semiconductor particles for the reason that a large fraction of methylviologen cation radicals produced were trapped at anion sites of the clay. In the case of the TiOJclay prepared in the present study, however, methylviologen worked effectively as a mediator, as several examples of transient photocurrent behaviors are shown in Figure 3. It seems important to stress here that the present experiments were carried out in relatively high acidic solutions, and the concentration of protons was greater than that of methylviologen. Accordingly, not methylviologen molecules but protons must have been trapped at anion sites of the clay. Figure 4 shows plots of the rate of increase of transient anodic photocurrents as a function of pH for both Ti02/clay and TiO,
4836
Yoneyama et al.
The Journal of Physical Chemistry, Vol. 93, No. 12, 1989
TABLE 11: Photocatalytic Decomposition of n -Carboxylic Acids'
production rate of major productsb/wmol h-'
Illumination t i m e / h
Figure 5. Time course of hydrogen evolution from 2-propanol decomposition. The TiO,/clay photocatalyst was suspended in 8 mL of 2propanol so as to give 3 mg of Ti02. Illumination was conducted with a 500-W high-pressure mercury arc lamp. TABLE I: Photocatalvtic Decomposition of 2-Propanol" production rate/wmol h-' catalvst Hz CH, CO COz (CHp)zCO trace 0.28 Ti02 0.26 0.07 0.07 Ti02/clay 0.12 0.15 1.34 0.35 1.40 Pt/TiO," 172 183 trace trace trace trace trace Pt/TiOz/clay 697 4.0 661 a Illumination with a 500-W high-pressure mercury arc lamp for 24 h of 3 mg of Ti02 suspended in 8 mL of 2-propanol containing 0.2 mL of water. The data were given as time-averaged values for 24 h. bThe amount of loaded Pt was 2 wt %.
powder. It is seen that the threshold pH values at which the semiconductor loses its photocatalytic activities are different between the two kinds of photocatalysts: pH 0.75 for the Ti02/clay and pH 6.85 for the TiOz powder. At these critical pHs the Fermi level of the photocatalysts must be equal to the redox potential of methylviologen/methylviologen cation radical (-0.69 V vs SCE). With the well-established assumption that electronic energy levels of semiconductors are varied with pH at the rate of -59.1 mV/pH at 25 OC, the Fermi level of the TiOz/clay and the Ti02 powder at pH 0 is estimated to be -0.64 and -0.28 V vs SCE, respectively. In fact, the latter value is in fairly good agreement with the flat-band potential of Ti02.22 If it is assumed that the Fermi level determined above roughly gives the lowest excited energy level (the bottom of the conduction band) of the TiOz particles, the difference in such excited energy levels between the two kinds of photocatalysts amounts to 0.36 eV, the Ti02/clay being higher than the T i 0 2 powder. According to the theory of size quantization,Id-' a ratio of the shift of the conduction bandedge energy AEc caused by the size quantization to the shift of the valence bandedge energy AEv is given by AE, (mh*/m,*), where AEKis an increase in the bandgap of semiconductors by the size quantization. In the case of the TiO,/clay, therefore, a shift of 0.08 eV is expected for the conduction band edge of the Ti02 microcrystallites if AE, is assumed to be 0.58 eV, being in disagreement with the experimentally determined shift of 0.34 eV. Presently, the cause responsible for bringing about such a discrepancy is not known. Photocatalytic Decomposition of 2-Propanol. As shown in Figure 5, hydrogen evolution at the Ti02/clay took place almost linearly with the illumination time, indicating that the activity of the TiOz in the clay was not greatly changed during the course of the photocatalytic experiments. Furthermore, the rate of the decomposition reaction was higher at the Ti02/clay than at the TiOz powder as Table I shows, though the amount of photons absorbed in the photocatalysts should be greater in the former (22) Cooper, G.; Turner, J. A,; Nozik, A. J. J . Electrochem. Soc. 1982, 129, 1973, and references cited therein.
carboxylic acid catalyst acetic acid Ti02 Ti02/clay Pt/Ti02 Pt/TiOz/clay propionic acid TiOz Ti02/clay Pt/Ti02 Pt/Ti02/clay lactic acid Ti02 Ti02/clay Pt/TiOz Pt/TiOz/clay valeric acid TiO, Ti02/clay Pt/T102 Pt/Ti02/clay caprylic acid Ti02 Ti02/clay Pt/T102 Pt/TiOl/clay capric acid Ti02 Ti02/clay Pt/Ti02 Pt/Ti02/clay
co2 6.1 11.6 38.0 84.6 18.5 44.9 70.0 162 3.7 20.1 29.0 92.3 1.4 8.4 21.9 70.0 2.98 14.5 19.7 56.6 2.57 1.33 9.67 4.8 1
H2 4.3 4.9 5.7 13.1 2.8 5.0 2.2 4.2 1.9 2.8 0.8 0.4
'Illumination with a 500-W high-pressure mercury arc lamp of 3 mg of Ti02 suspended in 2 mL of organic reactant and 8 mL of water except for capric acid where 2 mg of it was suspended in 8 mL of water. bUsually illumination for 24 h was carried out, and the rate was evaluated by dividing the produced amount by the illumination time. than in the latter as judged from their absorption spectra. The quantum efficiency determined at 280 nm was 0.045% for the TiO,/clay and 0.0028% for the TiO, powder. It has been well-established that platinization of TiOZis effective in enhancing photocatalytic activities for reactions involving hydrogen evolution. This was confirmed in the present case, as Table I shows. The quantum efficiency determined at 280 nm was 23.5% for the TiOz/clay and 1.98% for the TiO, powder. The higher activity of the TiOz/clay observed in this case may not always be due to its higher surface area, because the activity difference between the TiOz/clay and the TiOz powder is much greater than the surface area difference between the two kinds of photocatalysts. Anpo et a1.12 also discovered for photocatalytic hydrogenation of CH3CCH with water vapor that the size quantization enhanced photocatalytic activities, the degree being dependent on the magnitude of the size quantization. It is thought that the higher excited electronic states contribute to the appearence of the higher activity of the TiOz/clay. Judging from the relative positions of the Fermi levels to the hydrogen redox potential, the conduction bandedge of the TiOZbulk powder is rather critical to induce hydrogen evolution. The 0.36-eV higher excited electronic states of the Ti02/clay are favorable for hydrogen evolution to take place. Photocatalytic Decomposition of Carboxylic Acids. As already reported by Kraeutler and Bard,23and later by Sakata et al.,24 major reaction products in acidic solutions were alkanes and carbon dioxide. Results obtained at both platinized and nonplatinized photocatalysts are summarized in Table 11. It is found that as in the case of 2-propanol decomposition, the Ti02/clay was more active than the T i 0 2 powder except for the case of capric acid decomposition where the upset activities were observed. The higher activities of the TiOz/clay are rationalized by the discussion described above for the 2-propanol decomposition. Then a question arises why the activity of the Ti02/clay was lower than the T i 0 2 powder for the capric acid decomposition. Since the results were reproducible, it is thought that the space of the (23) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 5985. (24) Sakata, T.;Kawai, T.;Hashimoto, K. J . Phys. Chem. 1984,88,2344.
J. Phys. Chem. 1989, 93, 4837-4843 clay interlayers is too narrow to accommodate large molecules such as capric acid, resulting in apparent poor activity of the photocatalyst. Studies to confirm this idea are under way. Acknowledgment. Stimulating discussion with Dr. A. J. Nozik at the Solar Energy Research Institute is gratefully acknowledged. This work was supported by Grant-in-Aid for Scientific Research
4837
No. 61470080 and for Scientific Research on Priority Area No. 6203537, from the Ministry of Education, Science, and Culture. Registry No. Ti02, 13463-67-7;H2, 1333-74-0;CH4, 74-82-8; CO, 630-08-0; CO2, 124-38-9; (CH,)2CO, 67-64-1; C2H6r 74-84-0; C,Hg, 74-98-6; c ~ H , 106-97-8; ~, c,H]~,142-82-5;c~H,, 111-84-2;2-propano1, 67-63-0; acetic acid, 64-19-7; propionic acid, 79-09-4; lactic acid, 50-21-5; valeric acid, 109-52-4; caprylic acid, 124-07-2; capric acid, 334-48-5.
Adsorption of Water on ZSMS Zeolites Andreas Jentys, Gerhard Warecka, Miroslav Derewinski, and Johannes A. Lercher * Institut fur Physikalische Chemie, Technische Universitat Wien, Getreidemarkt 9, A - I060 Vienna, Austria (Received: June 22, 1988; In Final Form: January 30, 1989)
The adsorption of water on HZSM5 and alkali-metal-exchanged ZSMS was studied in the pressure range from to 1 mbar. At equilibrium pressures below 10-4mbar the most important adsorption sites are Lewis acid sites (presumably octahedrally coordinated aluminum). At higher equilibrium pressures strong Bronsted acid sites (bridging hydroxyls) are the most important sites for adsorption. At these bridging hydroxyl groups one water molecule is adsorbed at lower pressures and tends to form larger clusters at higher pressures. Up to a number of three water molecules per cluster, the IR spectra show bands at 2885 and 2463 cm-l characteristic for hydroxonium ions. In contrast, on Li-, Na-, and KZSM5 several water molecules may be adsorbed strongly on the metal cations without pronounced intermolecular interactions. At higher coverages the formation of a second shell of water molecules takes place leading to bands of perturbed (water in the cluster) and unperturbed (water at the boundary of the cluster) OH vibrations. These very different types of interactions are also reflected in the Freundlich-type isotherm for HZSMS and the Langmuir-typeisotherm for the Li-, Na-, and K-exchanged ZSMS. With R b and Cs-exchanged ZSMS the interactions between water molecules are of high importance for adsorption at water equilibrium pressures below mbar.
Introduction
The selectivity of zeolites to interact with water in the presence of other molecules decreases with decreasing concentration of aluminum per unit ce11.'V2 Consequently, materials with low concentrations of aluminum per unit cell, e.g., ZSMS, were characterized as being The reason for this (behavior) was attributed to the low concentration of (hydrophilic) protons in these materials and the weak interaction of the oxygen in Si-0-Si groups with water. As expected, we found that the interactions between water and HZSMS are The presence of alkali-metal cations, however, increased the strength of the interaction with water considerably.* The significance of this enhancement decreased with increasing atomic number of the alkali-metal cation. Hence, without accounting for adsorption on lattice defects, two causes for the weak interactions between water and adsorption sites in high silica zeolites should be considered: (i) the decrease in concentration of the adsorption sites and (ii) the decrease of the strength of interaction between the individual sites and the water molecule. Provided that this second effect is caused by different adsorption structures, vibrational spectroscopy of adsorbed water may help to understand the variation of the strength of the adsorbate bonding as a function of the concentration of aluminum in the lattice. Early IR spectroscopic investigations of nondissociative adsorption of water on X-and Y-type zeolitesg-I2suggest that water (1) Chen, N. Y.J . Phys. Chem. 1971,80, 60. (2) Chen, N. Y . US.Parent 3,732,326, 1973. (3) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.;Patton, R. L.; Kirchner, R.; Smith, J. V. Nature 1978, 272, 512. (4) Olson, D. H.; Haag, W. 0.; Lago, R. M. J . Caral. 1980, 62, 390. (5) Nakamoto, H.; Takahashi, H. Zeolites 1982, 2, 67. (6) Lercher, J. A.; Rumplmayr, G. 2.Phys. Chem. N.F. 1985, 246, 113. (7) Lercher, J. A,; Rumplmayr, G. Appl. Caral. 1986, 25, 215. (8) Derewinski, M.; Haber, J.; Ptaszynski, J.; Lercher, J. A,; Rumplmayr, G. Stud. Surf.Sei. Caral. 1986, 28, 975.
0022-3654/89/2093-4837$01.50/0
adsorbs on alkali-metal zeolites in such a way that the oxygen atom of the water is close to the alkali-metal cation with one proton freely vibrating, while the other is hydrogen bonded to lattice oxygen. In essence, this was concluded from the presence of one sharp and two rather broad bands of stretching vibrations of hydroxyl groups and the simultaneous presence of the band of the deformation vibration of molecular water at 1643 cm-'. The wavenumber of the stretching band of the free hydroxyl group was found to increase from 3640 cm-I with CsY to 3714 cm-' with LiY. IR spectra of water adsorbed on HZSMS and NaZSM5l3-l5 differ from the spectra reported for faujasites, but neither were the contrasting spectra recorded under comparable conditions, nor were assignments of the observed spectral features made. In general, it seems that upon contact with water at lower vapor pressures or higher temperatures a band around 3690 cm-I is generated with HZSMS, at higher pressures or lower temperatures a broad and ill-defined band. NaZSM5, in contrast, showed two relatively narrow bands.I4 The purpose of the present communication is, therefore, to describe the surface chemistry of water adsorbed on HZSMS and a series of alkali-metal ZSMS. We will mainly use (time resolved) transmission absorption IR spectroscopy for analysis. Experimental Section
Zeolites. ZSMS was synthesized according to Mobil patentsI6 and subsequently ion exchkged with alkali-metal nitrate Holutions. (9) Bertsch, L.; Habgood, H. W. J . Phys. Chem. 1963, 67, 1621. (10) Angell, C. L.; Schaffer, P. C. J . Phys. Chem. 1965, 69, 3436. (11) Ward, J. W. J . Phys. Chem. 1968, 72, 4211. (12) Ward, J. W. J . Catal. 1968, 2 2 , 238. (13) Ison, A.; Gorte, R. J. J . C a r d 1968, 89, 150. (14) Schnabel, K. H.; Peuker, Ch.; Parlitz, B.; LGffler, E.; Kurschner, U.; Kriegsmann, H. 2.Phys. Chem. Leipzig 1987, 268, 225. (15) Brunner, G . 0. Zeolites 1987, 7, 9. (16) Argauer, R. G.; Landolt, G. R. U S . Patent 3,702,886, 1972.
0 1989 American Chemical Society
