J. Phys. Chem. 1988, 92, 438-440
438
Photocatalysts on Ti-AI Binary Metal Oxides: Enhancement of the Photocatalytic Activity of TiOp Species Masakazu Anpo,* Takafumi Kawamura, Sukeya Kodama, Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan
Kenichi Maruya, and Takaharu Onishi Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259-Nagatsuta, Midori- ku, Yokohama 227, Japan (Received: January 29, 1987; In Final Form: July 15, 1987)
Titanium-aluminum binary metal oxides prepared by coprecipitation have been used as photocatalysts for the hydrogenation reaction of propene with water and isomerization of cis-2-butene in the gas-solid system. Photocatalytic activity of TiO, species in the oxide was found to be significantly enhanced by forming surface layers where TiO, species are surrounded by A120, carrier acting as cocatalyst.
Introduction Recent studies on photocatalysis focus on colloidal semiconductors,' on extremely small-sized particles in which quantization effects are expected,2 as well as on semiconductors supported on inert support^.^-^ Various binary oxide catalysts also seem to be potentially promi~ing,~ since it is well-known that binary catalysts often exhibit higher catalytic activity and selectivity than what one can predict from the properties of their components. White et a1.6 showed that in the photodecomposition of water the photocatalytic activity of ZnS.CdS/SiO, is higher than those of its components. As an additional advantage, with the binary catalysts, it is convenient to get detailed information about the relationship between the structure and the photocatalytic activity by changing the composition continuously from 0 to 100%. However, few studies have been made on binary catalysts. In the present work, we report on the photocatalytic activity of Ti-A1 binary oxide catalysts prepared by coprecipitation for the reaction of H 2 0with C3H6as well as isomerization reaction of cis-2-C4Hs, since the characteristics of these reactions were clarified by many workers.'+
Experimental Section Catalysts. Ti-A1 oxide catalysts having different mole percent of Ti ions were prepared by coprecipitation of desired amounts of mixed solution of TiC14 and AlC13, by addition of an aqueous solution of ammonia. The washed and dried material was con(1) For example: Moser, J.; Gritzel, M. J . Am. Chem. SOC.1983, 105, 6547 Enea, 0.; Bard, A. J. J. Phys. Chem. 1986, 90, 301. (2) (a) For example: Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984,80,4464 Brus, L. E. J. Phys. Chem. 1986,90,2555. (b) For example: Fojtik, A.; Weller, H.; Koch, H.; Henglein, A. Ber. Eunsen-Ges. Phys. Chem. 1984,88,649. (c) A n p , M.; Shima, T.; Kodama, S . ; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305. (3) Kuczynski, J.; Thomas, J. K. J. Phys. Chem. 1985, 89, 2720. (4) A n p , M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.; Giamello, E. J. Phys. Chem. 1985.89, 5017; 5689. (5) Kodama, S.; Nakaya, H.; Anpo, M.; Kubokawa, Y . Bull. Chem. SOC. Jpn. 1985, 58, 3645. Anpo, M.; Nakaya, H.; Kodama, S.; Kubokawa, Y.; Domen, K.; Onishi, T. J. Phys. Chem. 1986, 90, 1633. (6) (a) Mau, A. W. H.; Huang, C. B.; Kakuta, N.; Bard, A. J.; Campion, A.; Fox, M. A.; White, J. M., Webber, S. E. J. Am. Chem. SOC.1984, 106, 6537. Kakuta, N.; Park, K. H.; Finlayson, M. F.; Ueno, A,; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 732. Ueno, A.; Kakuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J. Phys. Chem. 1985, 89, 3828. (b) Domen, K.; Naito, S.; Soma, M.; Onishi, T.; Tamaru, K. J . Phys. Chem. 1982,86, 3657; Chem. Phys. Lett. 1982,92,433. Domen, K.; Kudo, A,; Onishi, T. J. Cafal. 1986, 102, 92. (7) Schrauzer, G. N.; Guth, T. D. J. Am. Chem. SOC.1977, 99, 7189 (8) Anpo, M.; Aikawa, N.; Kodama, S.; Kubokawa, Y. J . Phys. Chem. 1984, 88, 2569, 3998. (9) Anpo, M.; Yabuta, M.; Kodama, S.; Kubokawa, Y. Bull. Chem. SOC. Jpn. 1986, 59, 259.
0022-3654/88/2092-0438$01.50/0
verted to oxide by thermal decoposition at 773 K for 3 h in air. The binary oxide catalysts were degassed under vacuum (ca. 1 X Torr) at 773 K, heated in O2 at the same temperature, and then finally degassed just before use as photocatalysts. Apparatus and Procedure. X-ray diffraction patterns of catalysts were obtained on a Rigaku RAD-rA X-ray diffractometer using Cu Ka radiation with a Ni filter. UV-diffusion reflectance and photoluminescence spectra of catalysts were measured by a Shimadzu UV-2 1OA double-beam digital spectrophotometer equipped with conventional components of a reflectance spectrometer and a Shimadzu RF-501 spectrofluorophotometer equipped with color filters, respectively. XPS was measured a t 298 K with a Shimadzu ESCA-750 photoelectron spectrometer. The degassed samples were mounted on a double-sided adhesive tape in an Ar-filled glovebox. Binding energies were corrected for samples by reference to Au evaporated onto the sample by vacuum depo~ition.~ UV irradiation was carried out with a 75-W high-pressure mercury lamp (Toshiba SHLIOOUV) at 298 K for the photocatalytic hydrogenation and at 273 K for the photocatalyzed isomerization. Hydrogenation in the dark was negligible. The yield of photocatalyzed isomerization was obtained by subtracting the yield of dark isomerization from the total yield of the isomerization under UV irradiation, since isomerization in the dark was not negligible. Catalysts were spread on a quartz window having an area of about 15 cm2. For the photocatalyzed isomerization, cis-2-C4Hs was introduced at a pressure of 10 Torr. For the photocatalytic hydrogenation, H 2 0 was adsorbed onto the catalyst at 298 K until a pressure of about 10 Torr was reached, after which the evacuation was carried out at the same temperature. At this point, C3H6was introduced at a pressure of 10 Torr at 298 K. There was no correlation between activity of binary catalysts and their surface area, being similar to the case of Ti-Si binary oxide catalyst^.^ Results and Discussion
Figure 1 shows the yields of the photocatalytic isomerization of cis-2-C4H8to trans-2-C4Hs (geometrical isomerization) and to l-C4H8(double bond shift isomerization) on Ti-AI binary oxide catalysts with various different compositions. It may be seen that the addition of only 2 mol % of alumina to Ti02 leads to a significant enhancement in the yield of both photocatalyzed isomerization reactions. However, the selectivity of the isomerization reaction was found to be almost same as that obtained on pure anatase TiO, catalysts. Increasing the mole fraction of alumina causes an increase of the yields of both isomerization reactions again with maxima at the same composition (Ti (50%):A1(50%)). The ratio of the geometrical isomerization vs the double bond shift isomerization was roughly constant. 0 1988 American Chemical Society
Photocatalysis on Ti-AI Binary Metal Oxides
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 439 3
9
0
7
A
11
Mole % of Ti02
Figure 1. Effects of changing mole percent of Ti02 in the Ti-A1 binary oxides upon the yields of photocatalyzed isomerization reactions of cis-
2-C4Hs to trans-2-CpH8(-+) and to 1-C4Hsformation (-0-), and photocatalytic hydrogenation reaction of C3H6 with H20 (--@--). (Amount of catalyst used, 0.36 g; UV irradiation time, 3 h; (--@--) catalyst was calcined at 973 K.) TABLE I: Particle Diameters and Absorbance at the 380-nm Band of the Ti-AI Binarv Oxide Catalvsts ~~
catalyst compsn, mol 5% Ti02(100) Ti02(98.5):A1203(1.5) TiO2(97):AI2O3(3.0) Ti02(94):A1203(6.0) TiO2(90):AI2O3( 10) TiO2(75):AI2O3(25) TiO2(50):AI2O3(50) TiO2(25):AI20,(75) Ti02(12):A1203(88) Ti02(3):A1203(97)
particle dia, A 330 f 5 200 f 5 180 f 5 160 f 5 145 f 5
absorbance,
113 f 5 100 f 5
37 f 2 29 f 2
90f 10 80f 10 a
Mole % of TIOp
Figure 2. Effects of changing mole percent of Ti02 in the Ti-A1 binary
oxides upon the A, of the absorption band of the catalyst (-0-),the intensity of X-ray diffraction lines of the catalyst (-+-), and the values of the enrichment of Ti ions at the surface regions of the catalyst (--@--). (1.0 in value of Ti enrichment means that the surface composition is equal to the bulk composition, and a value smaller than 1.0 means that A1203composition is larger at the surface region that what is predicted from the bulk composition; intensity in the X-ray diffraction lines was measured at 20 = 25.2' (anatase).)
414:.
%
50 f 2 56 f 2 55 f 2 57 f 2
460
57 f 2 18 & 2
9f3 2 f l
"Spectrum was very weak and the particle size was not determined. 0
The major products of the photocatalytic hydrogenation reaction of C3H6with H20on the Ti-AI binary oxide catalysts were C2H6, in addition to CH,, and only little C3Hswas formed. Thus, the hydrogenation reactions accompanied by the fission of C = C bond of C3H6operate mainly on Ti-AI oxide catalysts. This was also observed on pure anatase TiO,, but with different efficiency.8 The yield of hydrogenation products drastically increased first on addition of a small amount of alumina to the TiO, component having a maximum at the composition of Ti (98.5%):A1 (1.5%). On increasing the mole fraction of alumina, the yield again increased, passing through a maximum a t the composition of Ti (50%):A1 (50%), and then decreased with the same product distribution. The results for the photoisomerization reactions were similar, as shown in Figure 1. In order to understand the nature of such an enhancement in the photocatalytic activity of TiO, species in the titanium-alumina oxide catalysts, the UV reflectance spectra, XPS, and X-ray diffraction of the catalysts have been measured. The X-ray diffraction patterns of all Ti-AI oxide catalysts showed only the diffraction lines due to anatase. As shown in Figure 2, with increasing mole fraction of alumina, the diffraction lines broadened and decreased in intensity. As shown in Table I, the particle diameter of crystalline TiO,, which was determined from the width of X-ray diffraction lines, gradually decreases from 330 8,for pure TiOz to about 80 8, for Ti (12%):A1 (82%) binary oxide. These results clearly indicate that the crystallinity of TiO, species decreases and the dispersion of TiOz species increases by the addition of alumina to Ti02. Figure 2 also shows the A, of the absorption band of the Ti-A1 of the absorption binary oxide catalysts. It is seen that the A, band of catalysts is almost constant in the lower content regions of alumina, being equal to the value of anatase. However, it may be seen that with increasing alumina content the absorption band of the catalysts remarkably shifts toward a shorter wavelength. As has been reported previously, a considerable blue shift is
25
50
75
100
Mole % of T i 0 2
Figure 3. Effects of changing mole percent of Ti02in the Ti-AI binary oxides upon the binding energies of Ti(2pI12)and Ti(2pll2).
observed with the absorption band of the highly dispersed TiO, anchored onto porous Vycor glass4 and of the Ti-Si binary oxide
catalyst^.^ Figure 3 shows the binding energy of the Ti(2p3/,) and Ti(2p,@ bands of Ti-A1 oxide catalysts used. These binding energies initially shift to higher values by 1 eV at the composition of Ti(98.5%):AI (1.5%), and then decrease. At the higher mole fractions of alumina, these binding energies again shift to higher values with increasing content of alumina. Figure 2 also shows the extent of the enrichment of Ti ions at the surface regions, which was determined by dividing the ratio of the Ti(2p312)to Al(2pljz) band intensity by the ratio of Ti to A1 of the bulk composition. It is clearly seen that there is a significant enrichment of AI ions at the surface regions in the lower fractions of alumina. Conversely, a steady enrichment of Ti ions with increasing the fraction of alumina is found in the higher content regions of alumina. Positive chemical shifts, therefore, might be attributable to the change of relaxation energy of TiO, species due to the high dispersion of TiO, species in A1203carrier m a t r i ~ . ~ The specific photocatalytic activity of TiO, species in the binary oxide catalysts can be cal~ulated,~ since the catalysts involve only anatase type TiO, species and A1203species which by themselves scarcely exhibit any activity for these reactions, except for some activity for only the double bond shift isomerization. Figure 4 shows the specific photocatalytic activity of TiO, species for the photocatalyzed isomerization reactions and photocatalyzed hydrogenation reaction, Le., (yields of photoformed products)/(TiO, content) of the titanium-alumina oxide catalyst^.^ As shown in Figure 4, activity of TiO, species in Ti-A1 oxide catalysts drastically increases with increasing the mole fraction of alumina. It should be also emphasized that the addition of a small amount
J . Phys. Chem. 1988, 92, 440-445
440
Mole X of Ti02
Figure 4. Effects of changing mole percent of Ti02 in the Ti-A1 binary oxides upon the specific photocatalytic activity of Ti02 species for the photocatalyzed isomerization of cis-2-butene (--0--) and photocatalyzed hydrogenation of propene with water. (The specific photocatalytic activity of Ti02 species was given by dividing the yields of photoformed products by the content of Ti02in the used catalysts. --0--, catalyst was calcined at 773 K; --0--, catalyst was calcined at 973 K -0-, catalyst was calcined at 773 K.)
of alumina to TiOz component leads to an enhancement of activity. As seen in Figure 4, photocatalytic activity of the Ti-A1 binary oxide calcined at 973 K is higher than that of calcined at 773 K. Although the details are unclear at present, it was found that the optimum calcination temperature of the oxides was around 973 K, and even after the calcination at this temperature all TiOz species in the Ti-A1 binary oxides were found to keep the anatase form, in contrast with the pure anatase type TiOz which easily change into rutile by the calcination at around 923 K. From these results the following conclusions are possible. The Ti-AI oxide catalyst consists of surface layers in which TiOz species are surrounded by A1203carrier acting as cocatalyst, these structure resulting in the enhancement of the activity of TiOz species. In this situation, the photon energy absorbed by the oxide is utilized for the reaction with high efficiency due to the less efficient radiationless energy transfer on the surface with coordinatively unsaturated surface ions.1° On the other hand, the
ions in saturated or high coordination have a larger number of bonds to the oxide and couple more strongly with the phonon transitions of the lattice, providing a high probability of radiationless decay.2c~4*11~12 It is of interest to point out that, even with catalysts having a much lower content of alumina, such surface layers seem to be formed on some special parts of the catalyst, by considering the facts that in this region a peculiar enhancement is observed and the results obtained by XPS indicate the enhancement of A1203on the surfaces. Of course, such a peculiar enhancement in the photocatalytic activity would also be explicable by assuming A1203to act as an impurity of the trapping sites in TiOz. Thus, the present work not only provides useful information on the nature and properties of the Ti-A1 binary oxide catalysts but also shows the significant enhancement of photocatalytic activity of TiOz species located in the Alz03 carrier matrix as cocatalyst in the Ti-A1 binary oxides. Details of this system will be clarified in the near future by means of photoluminescence spectroscopy, because the preliminary experiments indicate that the Ti-A1 binary oxides exhibit a photoluminescence at around 450 nm,which depends upon the composition of the catalysts.
Acknowledgment. M.A. thanks Professor Y. Kubokawa for his continued encouragement. Thanks are due to the Ministry of Education, Science and Culture of Japan for Grant-in-Aid for Scientific Research No. 5947007 and Grant-in-Aid for Special Project Research, No. 61223022. Registry No. A1203,1344-28-1;Ti02, 13463-67-7;H20, 7732-18-5; propene, 115-07-1;cis-2-butene, 590-18-1; aluminum titanium oxide, 37220-25-0; ethane, 74-84-0; methane, 74-82-8. (IO) Although it is difficult to determine the exact efficiency (quantum yield) of photons absorbed by the catalysts because of difficulty to determine the number of photons absorbed, their relative efficiencies were estimated by considering the reflectance spectra of catalysts and the yields of products. These values are given in Table I and Figure 1. (1 1) Coluccia, S . In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; p 59. (12) Anpo, M.; Yamada, Y.; Kubokawa, Y. J . Chem. SOC., Chem. Commun. 1986, 714. Anpo, M.; Kubokawa, Y. Rev. Chem. Intermed. 1987,8, 105. Anpo, M.; Yamada, Y. In Advances in Basic Solid Materials; Tanabe, K., Ed.; Elsevier: Amsterdam, in press.
Size and Shape of Micelles in the Ternary System n-Dodecylbetaine/Water/l-Pentanol J. Marignan,* F. Gauthier-Fournier, J. Appell, F. Akoum, Groupe de Dynamique des Phases Condensees (UA233) and Greco MicroZmulsions, USTL Place E . Bataillon, 34060 Montpellier, Cedex, France
and J. Lang CNRS Institut Charles Sadron (CRM-EAHP) and Greco MicroZmulsions, 67083 Strasbourg, Cedex, France (Received: February 20, 1987; In Final Form: July 29, 1987)
The phase diagram of the system n-dodecylbetaine/water/ 1-pentanol is first presented. The use of several experimental techniques, namely (principally) small-angle X-ray scattering and fluorescence probe studies, allows us to obtain the size and the shape of the micelles in the LI phase. It is shown that the most probable shape corresponds to elongated aggregates, the self-consistency of the results leading to exclude the other shapes.
Introduction The phase diagrams of systems involving ionic surfactants alcohol and brine have the same characteristic structure,'v2 especially in the water-rich region. It can be thought that this behavior is principally due to the presence of salt and, therefore, to the screening effect of the salt on electrostatic interactions. It is thus not surprising to obtain a very similar phase diagram when *To whom correspondence should be addressed.
dealing with a zwitterionic surfactant. Such a phase diagram is first presented in the case of n-dodecylbetaine. Evolution of the shape of the aggregates when alcohol is added to binary solution is beginning to be known in the case of ionic surfactants diluted in water or brine. The aim of this paper is (1) Porte, G.; Gomati, R.; El Haitamy, 0.; Appell, J.; Marignan, J. J . Phys. Chem. 1986, 90, 5746. (2) Appell, J.; Gomati, R.; Bassereau, P.; Marignan, J.; Porte, G.J . Phys. Chem., in press.
0022-365418812092-0440!§01.50/00 1988 American Chemical Society
