4305
J. Phys. Chem. 1987, 91, 4305-4310 of supported Pt metal particles.21 Fast reduction rates in general lead to the formation of the more open Pt(l10)- and Pt( 100)-like crystallographic faces. These open faces tend to be rougher and have a larger number of surface defects (steps and other imperfections). Slow reduction rates, on the other hand, favor the formation of smoother Pt( 111)-like faces having fewer defect sites. Conclusions
The following conclusions emerge from this study: (1) Variations in the nature of the adsorbed phase of NO
depend on surface structure rather than on Pt dispersion. (2) NO is adsorbed on Pt/SiO, to give three distinct surface species. These include linearly adsorbed NO, bridge-bonded NO, and vibrationally coupled NO. (3) The collapse of band A occurs primarily on a smooth Pt surface due to the formation of this vibrationally coupled NO. Pretreatment in H2 promotes the formation of vibrationally coupled N O while pretreatment in O2 hinders it. Registry No. Pt, 7440-06-4; NO, 10102-43-9; H,, 1333-74-0; O,, 7782-44-1.
Photocatalytic Hydrogenation of CH,CCH with H20 on Small-Particle TiO,: Quantization Effects and Reaction Intermediates
Size
Masakazu Anpo,* Takahito Shima, Sukeya Kodama, and Yutaka Kubokawa Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received: November 13, 1986: In Final Form: March 3, 1987)
Photocatalytic hydrogenation reactions of CH3CCH with H 2 0 were investigated with Ti02catalysts of extremely small particle size. Photocatalytic activity increased as the diameter of the TiO, particles becomes smaller, especially below 100 A. Absorption and photoluminescence spectra of catalysts exhibit blue shifts as the diameter of TiO, becomes smaller, also especially below 100-A particle size, suggesting that a size quantization effect is operating. Photoformed, carbon-centered radical species resulting from the fission of the carbon-carbon bond of alkyne molecules and Ti3+ ions as trapped electrons were directly observable. The different dependence of the yields on the particle diameter of TiO, catalyst suggests that the differences in photocatalytic activity arise from the differences in the reactivity but not in the physical properties such as the surface area of these catalysts. This demonstrates a size quantization effect at extremely small Ti02 particles. Reduction of size might result in some electronic modification of Ti02to produce an enhancement of the activities of electrons and holes, and/or a suppression of the radiationless transfer of absorbed photon energies.
Introduction
Photocatalysis on various metal oxides and sulfides has been investigated by a number of workers.' Recent studies have focused on problems with extremely small size colloidal semiconductors,24 as well as on semiconductors supported on inert supports5s6and on various binary metal oxides' and sulfides.8 On colloidal semiconductors with an extremely small size particle, size quantization effects are expected to be observable. Recently, Brus et al.,3 Henglein et al.,4 and Nozik et aL9 have shown that small (1) (a) Photoelectrochemistry, Photocatalysis and Photoreactions, Schiavello, M., Ed.; Reidel: Dordrecht, 1984; and references therein. (b) Energy Resources through Photochemistry and Catalysis, Gratzel, M., Ed.; Academic: New York, 1983; and references therein. (c) Anpo, M.; Kubokawa, Y. Rev. Chem. Intermed. 1987, 8, 105. (2) (a) Duonghong, D.; Ramsden, J.; Gratzel, M. J . Am. Chem. Soc. 1981, 103, 46, 85; 1982, 104, 2977. Ramsden, J. J.; Webber, S. E.; Gratzel, M. J . Phys. Chem. 1985,89,2740. (b) Becker, W. G.; Bard, A. J. J . Phys. Chem. 1983, 87, 4888. (c) Rafaeloff, R.; Tricot, Y. M.; Nome, F.; Tundo, P.; Fendler, J. H. J . Phys. Chem. 1985,89, 1236. (d) Albery, W. J.; Brown, G. T.; Darwent, J. R.; Saiever-Iranizad, E. J . Chem. SOC.,Faraday Trans. I 1985, 81, 1999. (3) Brus, L. E. J . Phys. Chem. 1986, 90, 2555. Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1985,82,552. Brus, L. E. J . Chem. Phys. 1983, 79, 5566. 1984, 80, 4403. Rossetti, R.; Ellision, J. L.; Gibson, J. M.; Brus, L. E. Ibid., 1984,80,4464. Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. Ibid. 1985, 82, 552. (4) Weller, H.; Koch, U.; Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984. Fojtik, F.; Koch, U.; Henglein, A. Ibid. 1984, 88, 969. Henglein, A.; Gutierrez, M.; Fischer, Ch.-H. Ibid. 1984, 88, 170. ( 5 ) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1985, 89, 2720. (6) Anpo, M.; Aikawa, N.; Kubokawa, Y . ;Che, M.; Louis, C.; Giamello, E. J . Phys. Chem. 1985, 89, 5017, 5689, and earlier papers. (7) Anpo, M.; Nakaya, H.; Kodama, S.; Kubokawa, Y.; Domen, K.; Onishi, T. J . Phys. Chem. 1986, 90, 1633. Kodama, S.; Nakaya, H.; Anpo, M.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1986, 59, 257. (8) 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.; Kukuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.; Fox, M. A,; Webber, S.E.; White, J. M. Ibid. 1985, 89, 3828. Kakuta, N.; Park, K. H.; Finlayson, M. F.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J . M. Ibid. 1985, 89, 5028.
0022-3654/87/2091-4305$01.50/0
colloidal particle such as CdS and ZnS (-50 A) exhibit large blue shifts in the absorption and photoluminescence spectra. These studies have indicated that the physical and chemical properties of small semiconductor particles are different from those of bulk materials, in agreement with the prediction of Brus3 that the electronic properties of small semiconductors should be dependent upon the crystallite size and shape due to quantized motion of the electron and hole in a confined space. Although the usefulness of powdered semiconductors as photocatalysts has attracted a great deal of attention, the actual confirmation of the size effect in photocatalytic reactions is still unknown. On the other hand, most of the studies about photocatalysis on powdered semiconductors were concerned with the analyses of reaction products and yields alone.'' Few studies have been made on reaction intermediates and the primary processes of photocatalysis. Exceptions are the studies of Bard et al.ll and of Habour et a1.,12who were able to detect intermediate radical species by ESR techniques by using spin-trapping agents in the solid-liquid reaction systems. In order to design a photocatalytic system with a high conversion ofiight energy into chemical energy, it seems to be important to understand the true nature of the primary processes of photocatalysis through the direct detection of reaction intermediates, in addition to searching for efficient photocatalysts and useful reaction systems. In connection with these problems, we synthesized extremely small size and photocatalytically high efficient TiO, particles and investigated the photocatalytic hydrogenation reaction of unsaturated hydrocarbons with H,O and their reaction intermediates in the gas-solid reaction system; the detailed nature of this system (9) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.; Micic, 0. I. J . Phys. Chem. 1985,89, 397. Nedelijkovic, J. M.; Nenadovic, M. T.; Micic, 0. I.; Nozik, A. J. Ibid. 1986, 90, 12. (10) Anpo, M.; Kubokawa, Y. Hyomen Kuguku 1983,4,200. Kubokawa, Y.; Anpo, M., Shokubai 1981, 23, 89. (11) Jaeger, C. D.; Bard, A. J. J . Phys. Chem. 1979, 8 3 , 3146. (12) Harbour, J. R.; Harr, M. L. J . Phys. Chem. 1979, 83, 652.
0 1987 American Chemical Society
4306 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
r
50c
r
Anpo et al.
'
400k $' 29,
i kI
273
473
673
I
873
1073
Calcination temperature , K Figure 1. Effects of the calcination temperature upon the particle size, and the wavelength of photoluminescence and absorption band of TiO, (rutile) catalysts. (Photoluminescencespectra were recorded at 77 K. The excitation wavelength was 300 f 15 nm. The emission slit width was 7.0 nm.)
is well understood from studies by many
Experimental Section Preparation of pure rutile-type T i 0 2 catalysts was as follows. TiCI, (40 mL) was slowly titrated with continuous stirring at 273 K into a 1000 mL of distilled water in which 40 mL of concentrated HCI had been dissolved (pH 0.62). Titration was carried out in ice-water bath at a speed of about 2 mL/min. During titration, the temperature of the solution was kept below 290 K. The solution was maintained at 333 K for 3 h, cooled to 290 K, and then again kept at the same temperature for 15 h. After these procedures, the pH of the top solution was -0.2. The precipitate was filtered off and washed thoroughly with distilled water until the washings were chloride free. The material was dried a t 303 K in vacuo for 15 h. For the preparation of pure anatase-type TiOz catalysts, 480 mL of distilled water in which 6 mL of concentrated HC1 and 312 g of (NH4)2S04had been dissolved, was added at 280 K to the solution resulting after 125.2 mL of TiCI, had been dissolved in 240 mL of distilled water in an ice-water bath. After the mixture was boiled for 1 h, 220 mL of a 28% aqueous solution of NH,OH was added (6.67). All subsequent procedures were almost same to that employed in the preparation of rutile.18 T i 0 2 samples of different diameter were obtained by carefully controlling the calcination temperature of the dried powders. As shown in Figure 1, increasing the calcination temperature of the powders leads to an increase of the particle size and to a marked change in the wavelengths of the absorption and photoluminescence spectra of the catalysts. Analysis of the X-ray diffraction lines of the catalysts showed that the catalyst consists of only pure rutile- or anatase-type Ti02 crystallites, respectively, except for the anatase sample calcined at 773 K. The particle diameter was determined by X-ray diffraction and/or transmission electron microscopy. Figure 2 shows the strongest X-ray diffraction lines of anatase-type TiOz samples with various particle sizes. ~~
~
~~
(13) Boonstra, A. H.; Mutsaers, C. A. H. A J . Phys. Chem. 1975, 79, 2025.
(14) Schrauzer, G. N.; Guth, T. D. J . Am. Chem. SOC.1977, 99, 7189. (15) Anpo, M.; Aikawa, N.; Kodama. S.; Kubokawa, Y . J. Phys. Chem. 1984, 88, 2569. (16) Anpo, M.; Aikawa, N.; Kubokawa, Y .J.Phys. Chem. 1984,88,3998. (17) Frank, A. J.; Goren, Z.; Willner, I. J . Chem. SOC.,Chem. Commun. 1985, 1029.
(18) Tanabe, K.; Ishiya, C.; Matsuzaki, 1.; Ichikawa, I.; Hattori, H. Bull. Chem. SOC.Jpn. 1912, 45, 47.
degree
Figure 2. X-ray diffraction patterns (at the strongest line) of anatasetype TiO, with various particle sizes (A): 1, 530; 2, 220; 3, 110; 4, 85; 5, 65; 6, 50; 7, 38.
O
s
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n I i\
~
I -
q/( -
,\I
i
Differentia I
273
373
~
473 Temperature
573
,
K
673 ( I O deg./min)
Figure 3. Thermal gravimetric analysis pattern of TiO, (rutile) catalyst (The rate of increasing temperature was 10 OC/min in air.)
Thermal gravimetric analysis of the catalysts using a Rigaku TG-DTA showed that a peak corresponding to the desorption of water contained in the sample appeared around 393 K and that of the surface OH- groups around 623-673 K (Figure 3,, which is in agreement with those obtained with bulk TiO2,I5except that a much larger amount of water was desorbed from the synthesized samples. Photoluminescence and reflectance spectra were measured by using a Shimadzu RF-501 spectrofluorophotometer and a Shimadzu digital double-beam spectrophotometer equipped with an intergrating sphere accessory, respectively. The samples were degassed at 300 K for 5-6 h before measurement. The BET surface area of the catalysts which had been degassed at 300 K for 3-4 h was measured by the adsorption of Ar molecules at 77 K. It should be noted that TiOz samples with extremely high surface area were prepared in the present work. Although it is difficult to neglect completely the effect of the surface area of the catalyst, the change of surface area of the catalysts with particle size is unlikely to be directly related to that of the observed quantum yields of the reactions, since at high levels of catalyst content in the cell, the photoreaction rate is not proportional to the catalyst's mass (Le. surface area). Under these circumstances, the UV irradiated area, rather than the total surface area, is a better variable, being always kept constant in the present experiments. For other photocatalytic reactions such as the photo-Kolbe reaction of CH,COOH, in which a photoelectrochemical mechanism is predominant, the trend of the change in quantum yields on the surface area of TiO, samples was found to be quite different from that of the present results. As will be reported in the near future, the quantum yield of the photo-Kolbe reaction on anatase, for example, increased with increasing particle size, passes through a maximum with TiOz having a particle size of 220 A, and then decreased.lg
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4307
Photocatalysis on Small TiOz
TABLE I: Small-Particle Size Effects upon the BET Surface Area, Wavelength at the Band Gap Position, Magnitude of the Blue Shift of the Band Gap Compared with Bulk Ti02, ,A, of Photoluminescence, and Quantum Yields of the Photocatalytic Hydrogenation Reactions of CHBCCH with H,O over Rutile-TvDe TiO, at 300 K ~~
~
particle size,
BET surface area,
A
m2/e
55 120 400 1800 2000"
533 121 26 4.7 4.0
wavelength at band gap position, nm 398.0 401.5 409.2 410.1 410.4
magnitude of the shift at band gap, eV 0.0934 0.067 0.01 0.002 0.000
Amax of photoluminescence, nm 408.0 412.3 417.1 420.0 420.0
quantum vieldsqb% 3.6 X 0.52 X 0.36 X 0.12 x 10-2 0.10 x 10-2
"The highly pure T i 0 2 (rutile) catalyst was obtained from Ishihara Ind Co. bQuantum yields were measured at 300 nm. Quantum yield = (number of photoformed products)/(number of incident photons). Major products were C2H6 and CH4.6 The quantum yield was determined by using the quantum yields of a potassium ferrioxalate actinometer at excitation wavelength.
TABLE 11: Small-Particle Size Effects upon the BET Surface Area, Wavelength at the Band Gap Position, Magnitude of Blue Shift of the Band Gap Compared with Bulk Ti02, and Quantum Yields of the Photocatalytic Reactions of CHICCH with H20 over Anatase-Type TiO, at 300 K BET magnitude wavelength of blue shift particle surface size, quantum at band gap of band gap, area, A eV yields,a % position, nm m2/g 1068 371.5 0.156 7.18 X 38 941 50 7.02 X 375 0.126 8.80 X 609 380.5 0.079 65 0.041 2.42 X 385 85 430 110 0.024 2.26 X 312 387 1.13 X 220 0.016 137 388 26 0.000 0.264 X 530 389.9 "Quantum yields were measured at 300 nm. Quantum yield = (number of photoformed products)/(number of incident photons). The details are same as those in Table I.
UV irradiation of the catalyst in the presence of CH3CCH and H 2 0 was carried out with a 75-W high-pressure mercury lamp (Toshiba SHL lOOUV) through a color filter and a water filter (A > 290 nm) at 300 K. The quantum yield for the reaction was determined by using a monochromator equipped with a 500-W Xe lamp. ESR measurements were carried out at 77 K with a JES-ME-IX (X-band). Mn2+ions in MgO powder were used for g values and sweep calibrations. UV irradiation for ESR measurements was carried out by using a 500-W high-pressure mercury lamp with a color filter (X > 290 nm) in a ESR cavity. Details of the experiments have been described in previous paper~.'~*'~
Results and Discussion Tables I and I1 show the results of the photocatalytic hydrogenation of CH,CCH with H,O on rutile- and anatase-type T i 0 2 catalysts having various particle sizes, respectively, together with their BET surface area, wavelength at the band gap position, magnitude of the blue shifts of the band gap compared with that of photoluminescence spectrum of bulk T i 0 2 crystallites, and A, at 77 K. The products accompanied by the carbon-arbon fission such as C2H6and CHI were found to constitute the major photohydrogenation products (photohydrogenolysis), while CO, was the major photooxidation product. As described p r e v i o u ~ l y , ' ~ J ~ such features are in good agreement with those observed with metal-free TiO, catalysts, being attributable to the action due to the close existence of photoformed electron and hole, i.e. Ti3+-0; (or Ti3+-OH) pair species. These electron and hole pairs will interact with CH3CCH adsorbed on the surface to form the carbenes and oxygen-containing compounds in a manner similar to that described previously.20 On the other hand, as will be shown later, with Pt-loaded Ti02, where the photoelectrochemical mechanism is operating, hydrogenation products without carbon-carbon bond fission are p r e d ~ m i n a n t . ' ~ J ~ (19) Anpo, M.; Shima, T.; Kubokawa, Y.,unpublished data. (20) A n p , M.; Tanahashi, I.; Kubokawa, Y .J . Chem. SOC.,Faraday1 Trans. 1 1982, 78. 2121.
Particle size of Ti% (rutile), A Figure 4. Effects of the particle size of T i 0 2 (rutile) catalysts upon the quantum yield of the photocatalytic hydrogenolysis of CH,CCH with H20, threshold of absorption band, and BET surface area of the catalyst. (The initial pressure of CH,CCH was 3 Torr. The initial pressure of H20 was 8 Torr. Threshold indicates the highest energy position observed in the absorption spectrum of the catalyst.)
r
I I
300
11200
t! 1t
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0
100
I
-.300
5
4400 v,,-
l
i
500
Particle size of Ti02 (anatase), A Figure 5. Effects of the particle size of T i 0 2 (anatase) catalysts upon the quantum yield of the photocatalytic hydrogenolysis of CH,CCH with HzO, threshold of absorption band, and BET surface area of the catalyst. (The initial pressure of CH,CCH was 3 Torr. The initial pressure of H 2 0 was 8 Torr. Threshold indicates the highest energy position observed in the absorption spectrum of the catalyst.)
As shown in Tables I and 11, for both rutile- and anatase-type TiO, quantum yields for the photocatalytic hydrogenolysis reaction
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
4308
Anpo et al.
TABLE 111: Small-Particle Size Effect upon the Photocatalytic Hydrogenation of CH3CCH with H 2 0 over Pt-Loaded and -Unloaded TiOz (Rutile) at 300 K
TiO, particle size, A
Pt content, wt %
0.0 4.0 0.0 4.0 0.0 4.0 0.0 4.0
55
120 400 2000
14.0 G
il
]
I
photohydrogenation products, IO-* mol/(g*h) selectivity CH4 C& C3H8 (C3H8/C2H6) 60.2 28.9 9.10 0.62 6.82
trace 1.91
trace
290 160 42.2 5.40 28.9 1.00 10.2 0.24
1.10 1540 0.16 135 0.09 56.0 0.04 23.8
0.0039 9.6 0.0038 25 0.0032 56 0.0039 99
TABLE IV: Small-Particle Size Effect upon the Quantum Yields of the Photocatalytic Hydrogenation Reaction of CH3CCH with H 2 0 over Pt-Loaded TiO, (Rutile) at 300 K particle size, A Pt content, wt 3' 6 quantum yields,' % 55 4.0 15.61 X lo-' I20 4.0 1.44 X IO-* 400 4.0 0.58 X lo-' 1800 4.0 0.24 X lo-* 2000 4.0 0.40 X lo-*
'Quantum yield = (number of photoformed products)/(number of incident photons). The major photoformed products were C3H6.9CH4 and C2H, were a minor products. The quantum yield was determined by using the quantum yield of potassium ferrioxalate actinometer at each excitation wavelength. increases with decreasing particle size of Ti02catalysts. Especially in the range of less than 100-A particle size, a significant increase of the yields is observed for both rutile- and anatase-type TiO, catalysts as seen in Figures 4 and 5. These features seem to parallel those in the absorption and photoluminescence spectra rather than those in the BET surface area measurements of the catalysts, the latter increasing monotonously with decreasing particle size of the catalysts (Figures 4 and 5 ) . Tables 111 and IV show the small-particle size effect upon the photocatalytic hydrogenation reaction of CH3CCH with H 2 0 on Pt-loaded (4.0 wt%) Ti02 (rutile) catalysts. As shown in Table 111, in contrast to that with metal-unloaded T i 0 2 catalyst, with Pt-loaded TiO, catalyst, where the photoelectrochemical mechanism is mainly operating," hydrogenation products without the carbon-carbon bond fission of CH,CCH are predominant, Le., C3H8 is a major hydrogenation product. These results are in agreement with those obtained with Pt-loaded TiO, (P-25 Degussa or pure rutile of Ishihara Ind.).I6 CO, was found as a major oxidation product. However, selectivity, i.e., the ratio of photohydrogenation to photohydrogenolysis, (yield of C3H,)/(yields of C2H6 CHJ, is found to be much higher with Pt-loaded Ti02 and much lower with unloaded TiO,, respectively, as compared to those in the previous paperi6*I7in which P-25 Degussa were used. As shown in Table IV, it is clear that the quantum yield of photohydrogenation of CH,CCH with H 2 0 on Pt-loaded TiO, increases with decreasing particle size of the catalysts. In Tables I and 11, and Figures 4 and 5, a steady blue shift in the absorption spectra of the Ti02 catalyst is seen. At particle diameters < 100 A, the blue shift becomes significant. It appears that the large blue shift in the absorption spectra is a reflection of size-quantization e f f e ~ t , ~though . ~ . ~ the effective mass of TiO, is not small as compared to that of sulfides.3a Photoluminescence, which is observable in the region of the absorption band edge, is also found to shift toward the higher energy region with decreasing of particle size, in agreement with the features in the absorption spectra. According to the results obtained with highly dispersed TiO, anchored onto porous Vycor glass6 and Si-Ti binary oxide catalysts,' the photoluminescence observed around 450 nm might be assigned to the radiative decay processes via direct recombination of photoformed electrons and holes. With quantized small particles, a well-defined structure in the absorption and photoluminescence spectra would be expected due
+
1
1 51.1 G
up
to 273 K
Figure 6. ESR signals of carbon-centered radicals, bulk 0; species, and Ti3+ions formed in the photocatalytic reaction of CH3CCH with H,O on TiOz (anatase) at 77 K. (The TiOz used has a BET surface area of 941 m2/g. UV irradiation was carried out at 77 K for I O min. The amount of adsorbed CH3CCH was ca. 1.8 X mol/g. The amount of adsorbed H 2 0 was ca. 4.7 X mol/g. The dotted line (carboncentered radicals) was obtained by subtracting the contribution of 0; species and Ti3+ions from the original signal.)
to the discrete levels. The absence of such peaks suggests that perturbation of the band structure as well as some distribution of particles having various sizes would affect the significant change in the deactivation processes of the photon energy absorbed by the catalyst and the carrier confinement effect. If one estimates the mass to be about 0.22 m,3,30then for rutile Ti0, with a band gap of 3.0 eV for the bulk material, a particle of 100 A would show a change in the band gap of only 0.02 eV. Because much smaller particles may be hidden in the samples which could not be discerned by the methods described above, we think this value is in rather good agreement with those shown in Table I, suggesting a good accord with the predictions of Brus et aL3x30 It is known that if photogenerated carriers are rapidly removed from a particle through a fast charge transfer, then the major effect of small particle size is quantization of band state^.^ It is also well established that with Pt-loaded Ti02 catalysts photogenerated electrons easily transfer from TiO, to Pt particles,'6s2' even from small T i 0 2 particles.22 The results obtained with Pt-loaded TiOz (Tables I11 and IV) as well as with unloaded T i 0 2 catalysts, therefore, suggest that an increase in quantum yields of the photocatalytic hydrogenation reactions with decreasing particle diameter is closely associated with size quantization effect, which results in a large modification in the energy of a localized excited state of T i 0 2 particles generated during photoexcitation. It is worth mentioning that highly dispersed TiO, species anchored onto porous Vycor glass6 and TiO, species in Ti-Si binary oxide c a t a l y ~ t which ,~ might be regarded as a model of an extremely small particle of TiOz, exhibits much larger blue shifts and much higher activities in the same photocatalytic hydrogenation reaction of CH,CCH with HzO than the smallest TiOz crystallites studied in this work. Such high activity with highly dispersed TiO, anchored on inert support surfaces and TiO, species in SiO, matrix with the Ti-Si binary oxides has been attributed to a diminished radiationless transfer of the photon energy absorbed by TiO, species, due to high dispersion of Ti ions and/or coordinative unsaturation of surface Ti ions. (21) (a) Hope, G. A,; Bard, A. J. J . Phys. Chem. 1983,87, 1979. Reich, H.; Dunn, W. W.; Bard, A . J. Ibid. 1981, 85, 2248. Ward, M . D,; Bard, A. J. Ibid. 1982, 86, 3599. (b) Aspenes, D. E.; Heller, A. J . Phys. Chem. 1983, 87. 4919. (22) Gerisher, H J Phys. Chem. 1984, 88. 6096
The Journal of Physical Chemistry, Vol. 91, No. 16. 1987 4309
Photocatalysis on Small TiO,
42
-+ 11.10
I
e
\
+--F--Ju p-20014
T
p l = 13% 7 )
I
I2t
L
Figure 7. ESR signals of carbon-centered radicals, bulk Or, and Ti3+ ions formed in the photocatalytic reaction of CH,CCH with H 2 0 on T i 0 2 (rutile) a t 77 K. (The TiOz used has a BET surface area of 121 m2/g. UV irradiation was carried out for at 77 K for 60 min. Other conditions were the same as in Figure 6.)
UV irradiation of small TiO, catalyst in the presence of alkynes and H 2 0at 77 K was found to lead the appearance of ESR signals. Figure 6 shows the ESR signals obtained at 77 K with the CH3CCH and H 2 0 system on the small anatase-type of Ti02. The intensity of the ESR signal increased with UV irradiation time. On raising the temperature to 273 K, the ESR signals completely disappeared, while small signals due to the photogenerated bulk 0; species (g = 2.0014)23and Ti3+ions (g = 1.988)24remained. These intensities of signals were also dependent upon the temperature, exhibiting a quite different temperature dependence from that of two strong doublet ESR signals. Subtracting the contribution of these signals due to the 01-and Ti3+ ions from the original ESR signal, one can get the real ESR signals due to the unstable radical species formed from CH3CCH. Thus, it was found that two different carbon-centered radicals are produced in this system under UV irradiation of TiO,. They have a center around g = 2.0032 with a hyperfine splitting of 14.0 and 5 1.1 G, respectively. When C H C H was used in place of CH3CCH, only one radical species having a aHhyperfine splitting of 14.0 G was observed. It is well-known that a hyperfine splitting due to the /3 hydrogen atom in vinyl radicals such as CHC is about 40-50 G,26and that due to the a atom is about 10-20 G.24 From these experimental results, it is possible to assign the ESR signals shown in Figure 6 (dotted line) to the photoproduced carboncentered radicals. Radical species with aH hyperfine splitting of 14 G are consistent with the formation of CH,& type radicals while radicals with PH hyperfine splitting of 51 G are due to CHCad, radicals. Figure 7 shows the ESR signals produced at 77 K by UV irradiation of small rutile-type TiO, in the presence of CH3CCH and H 2 0 . It is found that the same radical species as described above for anatase are formed. However, it is seen that the concentration of the radical species is rather low, while that of the photoformed Ti3+ions is remarkedly high, which is in contrast to the behavior of the anatase-type Ti02 catalysts. Formation of these carbon-centered radicals on both rutile and anatase is in good agreement with the product's distribution of the photocatalytic hydrogenolysis of CH3CCH with H 2 0 . Thus, the proposed mechanism involving the fission of C=C or C-C bond of alkyne (or alkene) molecule^'^^^^ is well confirmed through ~~
(23) (a) van Hooff, J. H. C. J . Catal. 1968, 11, 277. (b) Che, M.; Tench, A. J. Adu. Catal. 1982, 31, 77. (24) (a) Cochran, E. L.: Adrian, F. J.; Bowers, V. J . Chem. Phys. 1964, 40, 213. (b) Fessenden, R.; Schuler, R. H. J . Chem. Phys. 1963, 39, 2147. ( c ) Kasai, P. H. J. Am. Chem. SOC.1972, 94, 5950. Kasai, P. H.; McLeode, Jr., D.; Watanabe, T. Ibid. 1980, 102, 179. Kasai, P. H. Ibid. 1983, 105, 6704. (25) Coluccia, S. In Adsorption and Catalysis on Oxide Surfaces, Che, M.; Bond, G . C., Eds.; Elsevier; Amsterdam, 1985; p 59. (26) Anpo, M.; Yamada, Y.; Kubokawa, Y. J . Chem. SOC.,Chem. Commun. 1986, 714. Anpo, M.; Yamada, Y. Advances in Basic Solid Materials, Tanabe, K., Ed.; Elsevier Sequoia S. A., in press.
0
2
4
6
8
1
0
Relative concentrotion of photo-formed Ti3' Figure 8. Relationship between the relative concentration of carboncentered radicals and Ti3+ ions on rutile- and anatase-type TiO, catalysts at 77 K. (The anatase TiO, has a BET surface area of 609 m2/g. The UV irradiation time was 10 min. The rutile Ti0, has a BET surface area of 121 m2/g. The UV irradiation time was 60 min. Other conditions were the same as in Figures 6 and 7.)
the direct detection of unstable intermediate radical species at 77 K. Figure 8 shows the relationship between the concentration of photoformed radical species and Ti3+ ions as a trapped electrons. It is seen that with anatase of small particle size, which exhibited a high photocatalytic activity for hydrogenation of CH3CCH with HzO, the formation of radicals and of Ti3+ ions is efficient and well proportional to their concentrations. On the other hand, with the less photocatalytically active rutile-type Ti02 of small particle size, the detection of radical species is difficult, while Ti3+ ions are easily detectable by ESR. As described in our previous pap e r ~ , ]the ~ ~same ' ~ trend has been also observed with other commercially available TiO, catalysts. Thus, it was found that the concentration of the photoformed carbon-centered radical species parallels the yield of the photohydrogenolysisreaction of CH3CCH with water, while that of Ti3'' ions seems to exhibit an inverse relationship. It was also found that with Pt-loaded TiO,, both carbon-centered radicals and Ti3+ions were of very low intensity. The results shown in Figure 8 also suggest that the particle size effect upon the yields of the photocatalytic hydrogenolysis and hydrogenation reaction could arise from the different nature of the catalysts in their chemical rather than physical properties such as surface area, in agreement with the results shown in Figures 4 and 5. Although further study is necessary to clarify the true nature of the particle size effect, it is likely that decreasing the particle size of Ti02catalysts, especially in the range of less than 100 A, increases the photocatalytic activity and produces remarkable blue shifts of the absorption band in the catalyst. The above considerations imply that an increase in the photocatalytic activity of TiO, crystallites with a decrease in their particle size, especially in the range of less than 100-A diameter, is closely associated with the size quantization effect. Size quantization effects might result in some electronic modification in the T i 0 2 particle and lead an enhancement of the activity of both photoformed electron and hole species and/or a suppression of the radiationless transfer of the absorbed photon energy: In small crystallites in which the photoformed electron (Ti3+) and hole (OH and/or 0;) are close to each other, their interaction should be signifi~ant.'~ This situation results in a high efficiency in photocatalytic hydrogenolysis, as well as a balanced contribution of photoproduced electrons and holes to the surface reaction. The Coulomb energy due to the close interaction of the electron and the hole might be used as the potential energy, which would result in an enhancement in their reactivities. On the other hand, it is also likely that, in coordinatively unsaturated surface sites, ra-
4310
J . Phys. Chem. 1987,91, 4310-4317
diationless transfer of the excitation photon energies takes place having a much larger diameter than those mentioned above, inless efficiently than for saturated coordination, because the ions creased with decreasing diameter of Ti02. However, such an in high coordination sites have a larger number of bonds to the increase may be ascribed to a morphological and/or a compooxide and couple more strongly with the phonon transitions of the sitional effect on the contact between Pt and TiOz particles, not lattice, providing a high probability for nonradiative d e ~ a y . ~ ~ .from ~ ~ the size quantization effect. Therefore, the smaller the particle size of the catalyst, the higher Although it is well-known that the semiconductor particles lose the efficiency of photoreaction, since, on decreasing the particle their semiconductor properties in their photophysics as they besize, the concentration of unsaturated surface sites such as corners come smaller and ~ m a l l e r this , ~ ~is ~the ~ ~first report to indicate and/or edges increases, and photon energies absorbed by the that the photocatalytic activity of the semiconductor particle is catalyst contribute effectively to the surface reactions. dependent on particle size. Sakata et Tsai et and Harada et aLZ9have reported Acknowledgment. M. Anpo is indebted to Professor A. Henthat yields of the photocatalytic reactions on Pt-loaded TiOz, glein of Hahn-Meitner-Institute fur Kernforschung Berlin for drawing his attention to this problem. The authors thank Professor (27) Sakata, T.; Kawai, T.; Hashimoto, K. Chem. Phys. Lett. 1982, 88, 50. T. Minami of the University of Osaka Prefecture for Thermal (28) Tsai, C. C., Chung, Y. W J . Catal. 1984.86, 231 Gravimetric Analysis. Thanks are due to the Ministry of Edu(29) Harada, H.; Ueda, T. Chem. Phys. Lett. 1984, 106, 229. cation of Japan (Grant No. 59470007 and 59550558). The au(30) The authors thank a referee for suggesting a value of a effective mass thors thank referees for their useful comments and suggestion. of TiO:.
~~
Photoionization Mass Spectroscopy with Synchrotron Radiation of Hydrogen-Bonded Alkylamine Clusters Produced in Supersonic Beams Peter G. F. Biding, Eckart Ruhl, Bernhard Brutschy, and Helmut Baumgartel* Institut fur Physikalische und Theoretische Chemie, Freie Universitat Berlin, D- IO00 Berlin 33, FRG (Received: December 1, 1986; In Final Form: April 3, 1987)
Hydrogen-bonded clusters of methyl-, ethyl-, dimethyl-, and diethylamines are synthesized in a seeded supersonic expansion for a mass spectroscopic study. The mass spectra are compared by ionizing either with 21-eV electrons or with photons from dispersed synchrotron radiation. Photoionization efficiency curves are measured to determine threshold energies for the ionization and fragmentation of the clusters. The threshold values yield gas-phase proton affinities, association, and dissociation energies by thermochemical calculations. The derived thermochemical quantities of cluster ions depend on the amount of the association energies of the neutral clusters. The association energy data found in this study are compared with previously calculated values. Lower bounds to the bond dissociation energies of alkylamine cluster ions are presented. The absolute proton affinity values of CH3NH2(930 i 15 kJ/mol), C2H5NH2(940 f 15 kJ/mol), (CH3)2NH(955 f 15 kJ/mol), and (C2H5),NH (965 f 15 kJ/mol) determined in this study are about 30 kJ/mol higher than the currently recommended reference data. The proton affinities of molecular aggregates are determined in order to quantify the first steps of gas-phase proton solvation energetics.
Among the intermolecular interactions between molecules forming weakly bound molecular complexes in gaseous or condensed phases the hydrogen bonding is of fundamental importance to physical, chemical, and biological processes. Hydrogen bonding between molecules is observed when the region of negative charge due to lone-pair electrons of one molecule attracts the proton of another hydrogen-containing molecule. In condensed matter it is often difficult to isolate the intrinsic effects of the hydrogen bond from other solvent effects.] In the last decade investigations of gas-phase van der Waals and hydrogen-bonded clusters were initiated to overcome this difficulty using the supersonic molecular beam method for cluster production.2 Efficacious concentrations of isolated clusters were obtained in a sufficiently low-density region for experimental investigation by spectral dispersed vacuum ultraviolet light.3 The predominant processes such as ionization and dissociation allow to elucidate the energetics, the structures, and frequently the dynamics of clusters, thus promoting the basic understanding of intermolecular interactions.
A large number of gas-phase equilibrium constant measurements is available giving comprehensive information on the central role of proton-transfer reactions in intermolecular proce~ses.~J The free energy change, AG, associated with proton-exchange reactions defines the relative proton affinity, PA, of a molecule M by the negative of the corresponding enthalpy change, -AH. Rarely, absolute PA's have been measured by appearance energy measurements giving heats of formations of MH' ions derived from photon or electron impact induced molecular fragmentation or ionization. For example, new experimental results reconciled previous attempts to determine the HCO' heat of formation for the absolute PA of CO on the basis of a clear analysis about energy conservation associated with the photofragmentation process.6 The ionization energy of the C(CH3)3 radical was carefully analysed by photoelectron spectroscopy.' Following this study, a value of 871.9 kJ/mol was recommended for the PA of NH3, to which the absolute PA scale is most often related as a standard. The molecular beam photoionization mass spectrometry was used to determine the heat of formation of NH4' based on its AE from
(1) Schuster, P.; Zundel, G.; Sandorfy, C., Eds. The Hydrogen Bond North-Holland: Amsterdam, 1976; Vol. 1-3. (2) Mark, T. D.; Castleman, Jr., A. W. Adu. At. Mol. Phys. 1985, 20,65. (3) Ng, C Y . Adu. Chem. Phys. 1983, 52, 263.
(4) Kebarle, P. Annu. Reu. Phys. Chem. 1911, 28, 445. ( 5 ) Taft, R. W. Prog. Phys. Org. Chem. 1983, 14, 247. (6) Traeger, J. C. In!. J . Mass Spectrom. Ion Processes 1985, 66, 271. (7) Houle, F. A.: Beauchamp, J. L. J . Am. Chem. SOC.1979, 101, 4067.
Introduction
0022-3654/87/2091-43 lO$Ol.50/0
0 1987 American Chemical Society
