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Photocatalytic oxidation of alcohol by titanium tetraalkoxide — an analog of heterogeneous photocatalysis on TiO2
447
J. Electroanal. Chem., 260 (1989) 447-450 Elsevier Sequoia S.A., Lausarme - Printed in The Netherlands
Preliminary note
Photocatalytic oxidation of alcohol by titanium tetraalkoxide - an analog of heterogeneous photocatalysis on TiO, Sadamu Yamagata, Boon H. Loo * and Akira Fujishima Department
of Synthetic Chemistty,
The University of Tokyo, Hongo, Bunkyo-ku,
Tokyo I13 (Japan)
(Received 3 January 1988)
INTRODUCTION
The photocatalytic activity of TiO, powders has been applied to water splitting [l] and other synthetically useful reactions [2]. Although there have been several studies [3], detailed mechanisms of photocatalytic activities of semiconductor powders have yet to be fully understood. This is principally due to the fact that light scattering by the heterogeneous photocatalyst limits detailed analysis of it to a few spectroscopic techniques only [4,5]. The light-scattering problem may be circumvented if homogeneous catalyst can be used in lieu of a heterogeneous photocatalyst. Recent works on the use of homogeneous Ti compounds, Ti(OR), (R = (CH,),CH-) and TiCl,, in reactions of the photocatalytic hydrogen evolution from alcohol [6,7], the asymmetric epoxidation of ally1 alcohol [8], and the selective photocleavage of the C-C bond in sugar molecules [9], have been important in the research of catalysis with homogeneous Ti compounds, and have provided an impetus to our present work. In this work, we give the first experimental correlation of photocatalytic activities between TiO, and Ti(OR), (R= CH,CH,or (CH,),CH-) in the photooxidation of ethanol and 2-methyl-2-propanol. EXPERIMENTAL
A given amount of Ti(OR), (R = CH,CH,or (CH,)$H-) was mixed with 5 ml of ethanol without purging in a Pyrex test tube. An anatase TiO, (Nippon Aerosil, P-25) slurry was also prepared in a similar fashion for a comparison of its photocatalytic activity with that of Ti(OR), under the same conditions. The test
* Permanent address: Department of Chemistry, The University of Alabama in Huntsville, Huntsville, AL 35899, U.S.A.
0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
448
tubes were put into slots of a round turntable and were irradiated at the same time with a Hg lamp situated at the center of the turntable. A water jacket around the lamp removed the infrared irradiation. The reaction products were analyzed with a gas chromatograph (Hitachi 163; column: PEG4000). Radical species formed during photocatalysis were detected by ESR using a spin trapping technique. Alcoholic solutions (1 ml) containing 10 mg of TiO, powder or 25 ~1 of Ti(OCH(CH,),), and 5 I_LIof 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were used as the test solutions for the ESR measurements. ESR spectra of the test solutions were recorded immediately after the UV irradiation. RESULTS
AND
DISCUSSION
The Ti(OR), photocatalysts behave much like the TiO, photocatalyst towards ethanol under UV irradiation. Acetaldehyde was the major reaction product detected in both cases, although the yields were different. Acetaldehyde was not detected in solutions without Ti(OR), or TiO, photocatalysts. Figure 1 shows the amounts of acetaldehyde produced after 70 min of irradiation of TiOz + ethanol slurries and Ti(OR), + ethanol solutions. The difference in the photocatalytic activities between TiO, and Ti(OR), was mainly attributed to a difference in the amounts of the photons absorbed by TiOI and Ti(OR), photocatalysts. Ti(OR),
(d)
Amount
-__-.
of catalyst/pg
in EtOE
Fig. 1. The amount of acetaldehyde Ti(OCH(CHM,
produced
-
-
-
(5ml) photocatalytically
10 G by TiOz (a), Ti(OCH$H,),
(b) and
(c).
Fig. 2. ESR spectra of spin adducts observed after UV irradiation of photocatalyst in 2-methyl-2-propanel. (a) TiO, in N,; (b) Ti(OCH(CH,),), in N,; (c) TiOz in air; (d) Ti(OCH(CH,),), in air.
449 TABLE
1
Kinds of trapped radicals and relative concentration photocatalysts in alcohol Alcohol
CH,CH,OH Atmosphere:
Ti(OCH(CHM,
of spin adducts
’
TiO, b
obtained
by UV irradiation
of
(CH,),COH
N,
Air
N*
Air
species intensity
C 2
0 3
C 2
0 8
species intensity
C xl
0 5
C 13
0 13
’ 30 s of irradiation. b 60 s of irradiation.
absorbs only light of a wavelength shorter than 330 nm whereas TiO, (anatase), with a bandgap of 3.2 eV, absorbs light of a wavelength shorter than 390 run. It is estimated from the calibrated intensity of the Hg lamp used that Ti(OR), absorbs only about 15% of the photons absorbed by TiO,. Figure 2 shows the ESR spectra of spin adducts observed after the UV irradiation of 2-methyl-2-propanol + DMPO solutions containing TiO, or Ti(OR),. The same radical intermediates were produced for both TiO, and Ti(OR), photocatalysts in nitrogen (Figs. 2a and b) and in air (Figs. 2c and d). In the absence of photocatalysts, no radical intermediates were detected. The hyperfine structures of spin adducts indicate that C- and O-centered radical intermediates were formed [lo] in the absence and the presence of molecular oxygen, respectively. Table 1 shows the kinds of trapped radicals and the relative concentrations of the spin adducts produced under different conditions. The presence of molecular oxygen affected the kind of radical species formed. Instead of a C-centered species, an O-centered species was formed for both photocatalysts and also the amounts of the spin adducts were increased. In contrast to the oxygen effect, the different types of alcohol affected differently the amount of spin adducts produced for these photocatalysts. The concentrations of spin adducts produced by Ti(OR), in both alcohols were almost the same while those produced by TiO, differed quite significantly. On the TiO, photocatalyst, ethanol gave a lower concentration of spin adduct than 2-methyl-2-propanol [ll]. This was due to subsequent electron injection from the intermediate radical species to the TiO, substrate whereby a current doubling was observed. The present results show that both alcohols gave almost the same amount of spin adducts with the Ti(OR), photocatalyst, which indicates that subsequent electron injection from the radical intermediates to the photocatalyst did not occur. However, it is interesting to note that heteropolytungstate, a homogeneous photocatalyst, was reported to accept an electron from the hydroxyalkyl radical produced [12]. Thus, we conclude that electron injection from an intermediate radical to the photocatalyst may occur if the photocatalyst has a dimension large enough that excess electrons can exist stably by delocalization.
450
The phot~atal~ic oxidation of alcohols by Ti(OR), was found to involve the same radical intermediates as in the case of Ti02. The use of a homogeneous catalyst will permit future investigations with many powerful spectroscopic techniques. On the basis of these obervations, we believe reactions on Ti(OR), will offer a better understanding of the photocatalysis on TiO,.
B.H.L. thanks the Japan Society for the Promotion Fellowship (1988-1989) at the University of Tokyo.
of Science for a Visiting
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
G.N. Schrauzer and T.D. Guth, J. Am. Chem. Sot., 99 (1977) 7189. M.A. Fox, Act. Chem. Res., 16 (1983) 314. R. Baba, S. Nakabayashi, A. Fujishima and K. Honda, J. Phys. Chem., 89 (1985) 1902. D. Duonghong, E. Borgarello and M. GrUzel, J. Am. Chem. Sot., 103 (1981) 4685. B. Kraeutler, CD. Jaeger and A.J. Bard, J. Am. Chem. Sot., 100 (1978) 4903. M. Ohno, H. Uzawa, T. Miyazaki and K. Tarama, Preprint, Photochem. Conf., 1987, p. 331. Y.V. Khokxienko, S.Ya. Kuchmii, A.V. Konhak and A.I. Kryukov, Teor. Eksp. Khim., 23 (1987) 295. T. Katsuki and K.B. Sharpless, J. Am. Chem. Sot., 102 (1980) 5974, T. Sato, K. Takahashi and S. Ichikawa, Chem. Lett., (1983) 1589. E.G. Janzen and J&P. Liu, J. Magn. Reson., 9 (1973) 510. S. Yamagata, S. Nakabayashi, K.M. Saucier and A. Fuji&ma, Bull. Chem. Sot. Jpn., 61(1988) 3429. A. Hiskia and E. Papaconst~tinou, Polyh~n, 6 (1988) 477.
J. Electroanal. Chem., 260 (1989) 447-450 Elsevier Sequoia S.A., Lausarme - Printed in The Netherlands
Preliminary note
Photocatalytic oxidation of alcohol by titanium tetraalkoxide - an analog of heterogeneous photocatalysis on TiO, Sadamu Yamagata, Boon H. Loo * and Akira Fujishima Department
of Synthetic Chemistty,
The University of Tokyo, Hongo, Bunkyo-ku,
Tokyo I13 (Japan)
(Received 3 January 1988)
INTRODUCTION
The photocatalytic activity of TiO, powders has been applied to water splitting [l] and other synthetically useful reactions [2]. Although there have been several studies [3], detailed mechanisms of photocatalytic activities of semiconductor powders have yet to be fully understood. This is principally due to the fact that light scattering by the heterogeneous photocatalyst limits detailed analysis of it to a few spectroscopic techniques only [4,5]. The light-scattering problem may be circumvented if homogeneous catalyst can be used in lieu of a heterogeneous photocatalyst. Recent works on the use of homogeneous Ti compounds, Ti(OR), (R = (CH,),CH-) and TiCl,, in reactions of the photocatalytic hydrogen evolution from alcohol [6,7], the asymmetric epoxidation of ally1 alcohol [8], and the selective photocleavage of the C-C bond in sugar molecules [9], have been important in the research of catalysis with homogeneous Ti compounds, and have provided an impetus to our present work. In this work, we give the first experimental correlation of photocatalytic activities between TiO, and Ti(OR), (R= CH,CH,or (CH,),CH-) in the photooxidation of ethanol and 2-methyl-2-propanol. EXPERIMENTAL
A given amount of Ti(OR), (R = CH,CH,or (CH,)$H-) was mixed with 5 ml of ethanol without purging in a Pyrex test tube. An anatase TiO, (Nippon Aerosil, P-25) slurry was also prepared in a similar fashion for a comparison of its photocatalytic activity with that of Ti(OR), under the same conditions. The test
* Permanent address: Department of Chemistry, The University of Alabama in Huntsville, Huntsville, AL 35899, U.S.A.
0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
448
tubes were put into slots of a round turntable and were irradiated at the same time with a Hg lamp situated at the center of the turntable. A water jacket around the lamp removed the infrared irradiation. The reaction products were analyzed with a gas chromatograph (Hitachi 163; column: PEG4000). Radical species formed during photocatalysis were detected by ESR using a spin trapping technique. Alcoholic solutions (1 ml) containing 10 mg of TiO, powder or 25 ~1 of Ti(OCH(CH,),), and 5 I_LIof 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were used as the test solutions for the ESR measurements. ESR spectra of the test solutions were recorded immediately after the UV irradiation. RESULTS
AND
DISCUSSION
The Ti(OR), photocatalysts behave much like the TiO, photocatalyst towards ethanol under UV irradiation. Acetaldehyde was the major reaction product detected in both cases, although the yields were different. Acetaldehyde was not detected in solutions without Ti(OR), or TiO, photocatalysts. Figure 1 shows the amounts of acetaldehyde produced after 70 min of irradiation of TiOz + ethanol slurries and Ti(OR), + ethanol solutions. The difference in the photocatalytic activities between TiO, and Ti(OR), was mainly attributed to a difference in the amounts of the photons absorbed by TiOI and Ti(OR), photocatalysts. Ti(OR),
(d)
Amount
-__-.
of catalyst/pg
in EtOE
Fig. 1. The amount of acetaldehyde Ti(OCH(CHM,
produced
-
-
-
(5ml) photocatalytically
10 G by TiOz (a), Ti(OCH$H,),
(b) and
(c).
Fig. 2. ESR spectra of spin adducts observed after UV irradiation of photocatalyst in 2-methyl-2-propanel. (a) TiO, in N,; (b) Ti(OCH(CH,),), in N,; (c) TiOz in air; (d) Ti(OCH(CH,),), in air.
449 TABLE
1
Kinds of trapped radicals and relative concentration photocatalysts in alcohol Alcohol
CH,CH,OH Atmosphere:
Ti(OCH(CHM,
of spin adducts
’
TiO, b
obtained
by UV irradiation
of
(CH,),COH
N,
Air
N*
Air
species intensity
C 2
0 3
C 2
0 8
species intensity
C xl
0 5
C 13
0 13
’ 30 s of irradiation. b 60 s of irradiation.
absorbs only light of a wavelength shorter than 330 nm whereas TiO, (anatase), with a bandgap of 3.2 eV, absorbs light of a wavelength shorter than 390 run. It is estimated from the calibrated intensity of the Hg lamp used that Ti(OR), absorbs only about 15% of the photons absorbed by TiO,. Figure 2 shows the ESR spectra of spin adducts observed after the UV irradiation of 2-methyl-2-propanol + DMPO solutions containing TiO, or Ti(OR),. The same radical intermediates were produced for both TiO, and Ti(OR), photocatalysts in nitrogen (Figs. 2a and b) and in air (Figs. 2c and d). In the absence of photocatalysts, no radical intermediates were detected. The hyperfine structures of spin adducts indicate that C- and O-centered radical intermediates were formed [lo] in the absence and the presence of molecular oxygen, respectively. Table 1 shows the kinds of trapped radicals and the relative concentrations of the spin adducts produced under different conditions. The presence of molecular oxygen affected the kind of radical species formed. Instead of a C-centered species, an O-centered species was formed for both photocatalysts and also the amounts of the spin adducts were increased. In contrast to the oxygen effect, the different types of alcohol affected differently the amount of spin adducts produced for these photocatalysts. The concentrations of spin adducts produced by Ti(OR), in both alcohols were almost the same while those produced by TiO, differed quite significantly. On the TiO, photocatalyst, ethanol gave a lower concentration of spin adduct than 2-methyl-2-propanol [ll]. This was due to subsequent electron injection from the intermediate radical species to the TiO, substrate whereby a current doubling was observed. The present results show that both alcohols gave almost the same amount of spin adducts with the Ti(OR), photocatalyst, which indicates that subsequent electron injection from the radical intermediates to the photocatalyst did not occur. However, it is interesting to note that heteropolytungstate, a homogeneous photocatalyst, was reported to accept an electron from the hydroxyalkyl radical produced [12]. Thus, we conclude that electron injection from an intermediate radical to the photocatalyst may occur if the photocatalyst has a dimension large enough that excess electrons can exist stably by delocalization.
450
The phot~atal~ic oxidation of alcohols by Ti(OR), was found to involve the same radical intermediates as in the case of Ti02. The use of a homogeneous catalyst will permit future investigations with many powerful spectroscopic techniques. On the basis of these obervations, we believe reactions on Ti(OR), will offer a better understanding of the photocatalysis on TiO,.
B.H.L. thanks the Japan Society for the Promotion Fellowship (1988-1989) at the University of Tokyo.
of Science for a Visiting
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
G.N. Schrauzer and T.D. Guth, J. Am. Chem. Sot., 99 (1977) 7189. M.A. Fox, Act. Chem. Res., 16 (1983) 314. R. Baba, S. Nakabayashi, A. Fujishima and K. Honda, J. Phys. Chem., 89 (1985) 1902. D. Duonghong, E. Borgarello and M. GrUzel, J. Am. Chem. Sot., 103 (1981) 4685. B. Kraeutler, CD. Jaeger and A.J. Bard, J. Am. Chem. Sot., 100 (1978) 4903. M. Ohno, H. Uzawa, T. Miyazaki and K. Tarama, Preprint, Photochem. Conf., 1987, p. 331. Y.V. Khokxienko, S.Ya. Kuchmii, A.V. Konhak and A.I. Kryukov, Teor. Eksp. Khim., 23 (1987) 295. T. Katsuki and K.B. Sharpless, J. Am. Chem. Sot., 102 (1980) 5974, T. Sato, K. Takahashi and S. Ichikawa, Chem. Lett., (1983) 1589. E.G. Janzen and J&P. Liu, J. Magn. Reson., 9 (1973) 510. S. Yamagata, S. Nakabayashi, K.M. Saucier and A. Fuji&ma, Bull. Chem. Sot. Jpn., 61(1988) 3429. A. Hiskia and E. Papaconst~tinou, Polyh~n, 6 (1988) 477.