J. Phys. Chem. 1982,86,3657-3661
of the rest, just as the w1I2 relation can be used in rotating-disk experiments. As shown in Figure 2, the apparent value of k actually increases as internal resistance limited transfer proceeds; then, over a rather short period, transfer stops completely. By starting with a wide range of initial (A-) values, it was shown that this behavior is governed not by (A-) or the organic ion pair concentration but by length of time that the membrane has been in operation and the pumping rate. Aqueous hydrodynamically limited transfer is also terminated rather abruptly, but, in that case, termination is not preceded by acceleration. These observations can be understood if the breakdown is caused by the removal of the active liquid from its support. Since the volume of liquid is small, it may be absorbed by the plastic tubing connecting the transfer cell to the reservoirs. In the case of aqueous hydrodynamically limited rates, the thinning of the membrane makes no difference until the membrane is actually worn through and becomes inactive. Internal resistance, however, is reduced by thinning the membrane (eq 5) so the transfer accelerates. This has been quantified in eq 10, which is based on the assumption that the d(A-) dt
--
---kal(A-)
~ ( -lc t )
(10)
membrane thickness at time t is the initial, measured thickness, less c t , where c is a parameter characteristic of the pumping rate. Equation 10 integrates to eq 11,and
In (A-) = In (A-)o
3657
+ (kaT/uc)(ln (t - c t ) - In 7)
(11)
Figure 2 shows the satisfactory manner in which eq 11 describes the results. The fit was optimized by adjusting c. Other experiments at the same pumping rate gave about equally good results with similar, though not identical, values of c. Although it is undoubtedly somewhat oversimplified, the goodness of the fit suggests that the model is essentially correct. Addition of small concentrations of linear polystyrene to the membrane liquid had no effect either on k or c, although the macroscopic viscosity of the liquid increased very markedly. This excludes failure of cohesion as the cause of the breakdown.
Conclusions a thin layer of hydrophobic amine, dissolved in phenyl ether, and supported on a porous, inert plastic film, will transfer anions from a solution of low pH to a solution of high pH (on the other side) even though the anion concentration is much higher on the high pH side (the stripping side). The major resistances to transport across the membrane are the hydrodynamic resistance on the aqueous side of the loading face and the internal resistance in the membrane. The former changes in a systematic way with the pumping rate in the cell. The latter is proportional to the equilibrium constant for ion pair formation in the membrane, the (H+)in the loading aqueous solution, and the amine concentration in the membrane. Membrane lifetime is limited by liquid loss.
Study of the Photocatalytic Decomposition of Water Vapor over a NiO-SrTiO, Catalyst Karunarl Domen, Shulchl Nalto, Takaharu Onlshl, Kenrl Tamaru, Department of Chemistry, Facuw of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
and Mltsuyukl Soma National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan (Received: December 1 5 198 1; In Final Form: April 10, 1982)
Photocatalyticdecomposition of H20 vapor (and liquid) on SrTiOs powder impregnated with NiO was studied by using a closed gas circulation system and infrared spectroscopy. The activity for the photodecomposition of H20 was increased by the pretreatments of reduction of the catalyst by hydrogen, and reoxidation by oxygen before the reaction. The dependence of H20 decomposition upon H20vapor pressure indicated that an adsorbed water molecule is necessary for the evolutions of H2 and 02.A plausible mechanism of photocatalytic decomposition of H 2 0 vapor is proposed.
Introduction The conversion of a photon's energy, especially of solar energy, to chemical energy has recently attracted much attention. The photodecomposition of water to hydrogen and oxygen is one of the attractive candidates for this purpose and has been investigated extensively in homogeneous as well as heterogeneous systems. Since the report of Fujishima and Hondal in 1971, the possibility of the photodecomposition of water using semiconductors as the active component has been examined by many workers. So far, the reaction has been mainly investigated in the photoelectrochemical cell using a semiconductor such as (1)K. Honda and A. Fujishima, J. Chem. SOC.Jpn., 74, 355 (1971); Nature (London),238, 37 (1972).
Ti02,SrTi03,or KTa032-8as the photoelectrode together with a Pt counterelectrode. On the other hand, experiments of the catalytic decomposition of water over single crystalsgJOand p ~ w d e r s ' l - ~ ~ (2)M. S. Wrighton, A. B. Ellis, P. T. Wolcznski, D. L. Mores, H. B. Abrahamson, and D. S. Ginley, J. Am. Chem. SOC.,98, 2774 (1976). (3) J. G. Maviroides, J. A. Kafalas,and D. F. Kolesar, Appl. Phys. Lett., 28, 241 (1976). (4) A Fujishima, K. Kohayakawa, and K. Honda, Bull. Chem. SOC. Jpn., 48, 1041 (1975). (5)T.Watanabe, A. Fujishima, and K. Honda, Bull. Chem. SOC.Jpn., 49, 355 (1976). (6)M. S.Wrighton, D. S. Ginley, P. T. Wolczanski, A. B. Ellis, D. L. Morse, and A. Linz, R o c . Natl. Acad. Sci. U.S.A., 72, 1518 (1975). (7)A. B. Ellis, S. W. Kaiser, and M. S. Wrizhton, J.Phvs. Chem., 80, 1325 (1976). (8) A. J. Nozik, Annu. Rev. Phys. Chem., 29, 189 (1978).
0022-3654/82/2086-3657$01.25/00 1982 American Chemical Society
3858
The Journal of Physical Chemistry, Vol. 86, No. 18, 1982 to vacuum line m
to G.C
Circulation
m 1
mmeter
catalyst
highpressure Hg lamp
J
Figure 1. Schematic view of the apparatus.
of semiconductors have been recently studied. The use of a powdered semiconductor catalyst has such merits as the simplicity of treatments or of modification of the catalyst compared with that of a semiconductor electrode. Wrighton et al.9 and Somorjai et al.1° reported the photodecomposition of water in concentrated alkaline solution by a single crystal of SrTi03which was mounted with Pt on the surface. In these systems, semiconductors were usually used in the presence of platinum, and, accordingly, the reverse reaction, Le., the formation of water from H2 and 02,may occur simultaneously on the catalyst in addition to the decomposition of water. We have already reported the photocatalytic decomposition of water vapor or liquid water on a SrTi03 powder impregnated with NiO or cobalt oxide.16 The reaction proceeded steadily for more than 100 h, and the reverse reaction of water formation from H2 and O2 did not occur. This is a great advantage in such systems, which may lead to further development of the catalytic system to obtain a higher and a more stable activity. In this paper, the photocatalytic decomposition of water vapor on a NiO-SrTi03 powder was studied in detail by kinetic measurements and infrared spectroscopy. The reaction system was also examined under working conditions, which has not been done so far although there have been a few reports on the photodecomposition of water vapor."-14 The difference between the gas-phase reaction and the liquid-phase reaction was also investigated. Experimental Section A closed gas circulation system (ca. 350 cm3) was used to investigate the photodecomposition of water in the gas or liquid phase, and it was equipped with a quartz or Pyrex reaction vessel with a flat bottom (ca. 15 cm2) (Figure 1). The catalyst (ca. 2 g), spread over the bottom of the vessel, was irradiated through the bottom by a 450-W highpressure mercury lamp (Ushio, UM-452). The temperature of the vessel was 308 K in the stationary state under irradiation. ks the lamp was covered by a cooling tube made (9)M.S.Wrighton, P. T. Wolczanski, and A. B. Ellis, J. Solid State Chem., 22, 17 (1977). (10)F. T.Wagner and G. A. Somorjai, Nature (London),285, 559 (1980);J. Am. Chem. SOC.,102,5494 (1980). (11)G . N. Schrauzer and T. D. Guth, J. Am. Chem. SOC.,99,7189 (1977). (12)H.V. Damme and W. K. Hall.J. Am. Chem. Soc., 101, 4373 (1979). (13)S.Sat0 and J. M. White, Chem. Phys. Lett., 72, 83 (1980). (14)T. Kawai and T. Sakata, Chem. Phys. Lett., 72,87 (1980). (15)E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, and M. Griitzel, Nature (London),289,158 (1981). (16)K. Domen, S. Naito, M. Soma, T. Onishi, and K. Tamaru, J. Chem. SOC.,Chem. Commun., 543 (1980).
Domen et ai.
of Pyrex glass, the catalyst was irradiated only by the light which transmitted through the Pyrex glass. The amounts of H2 and O2 produced were measured by gas chromatography with a molecular sieve 5A column at room temperature using Ar carrier gas. Infrared spectroscopic measurements (a JASCO IRA-2 spectrometer for measurement of the H-0-H bending region, and a Jeol JIR 10 Fourier transform infrared spectrometer for the 0-D stretching region) were carried out by using a disk of the catalyst powder, which was placed in the infrared cell attached to a closed circulation system, and was irradiated through a NaCl window after pretreatments, X-ray photoelectron spectra of the catalysts were measured with a Mcpherson ESCA 36 spectrometer. The samples were prepared in a closed gas circulation apparatus with some side arms of sampling tubes. After each treatment, a part of the catalyst was transferred into the sampling tube and sealed off by a gas burner. Then the tubes were put into a nitrogen-purged drybox, which was attached to the sampling chamber of the spectrometer. After the seal of the tube was broken, the sample was mounted on the sample holder of the apparatus. The SrTi03powder used was 98.5% purity (Ventron), whcee average particle size was 2 X lo4 m, and the surface area determined by the BET method using N2 adsorption was 3.9 m2 g-l. After evacuation at 1000 K for 3 h, the powder was impregnated with Ni(N03)2aqueous solution followed by calcination in air at ca. 700 K for 1 h. Then the catalyst was placed in the reaction vessel and pretreated by H2 and Op H 2 0 used for the reaction was degassed by repeated freezing, evacuation, and thaw cycles.
Results Effects of Pretreatments. At first the activity of SrTi03 alone for the photocatalytic decomposition of water vapor was investigated. After evacuation at 700 K for 20 h, only a small amount of H2 (4 X cm3 h-l) was detected by the irradiation in water vapor of 2.7 X lo3 Pa. When the catalyst was reduced by H2 (2.7 X lo4 Pa) for 20 h and reoxidized by O2 (1.3 X lo4 Pa) for 3 h at 520 K, the rate of H2 evolution increased to 1 X cm3 h-l, but the activity was still about 1% of that of SrTiO, impregnated with NiO. After reduction by H2 (2.7 X lo4 Pa) at 1000 K for 3 h, H2 was evolverd in the dark when H 2 0 vapor was introduced, but the activity decayed within 10 h and the irradiation effect was not clear in this case. Next, the photodecomposition of water vapor on SrTiO, powder mounted with NiO was studied. The effect of pretreatments on the activity of NiO (2 w t %)-&"io3 was investigated. When the catalyst was evacuated at 700 K for 20 h, the activity of photodecomposition of water vapor was rather low (1 X cm3 of H2 h-l) and the amount of evolved O2was less than the stoichiometric amount (2 X cm3of O2h-l). Infrared spectroscopic observation showed that the nitrate or carbonate could be removed from the catalyst surface by this treatment. When the catalyst was reduced by H2 (2.7 X lo4 Pa) for 20 h and reoxidized by O2 (1.3 X lo4 Pa) for 3 h at 520 K, after the evacuation at 700 K, the activity was increased (6 X cm3 of H2h-l, 2.8 X cm3 of O2h-l), and the amounts of evolved H2 and O2were stoichiometric. A typical time course of the reaction of gas (and liquid) phase water over this catalyst is shown in Figure 2. Since SrTiOs is not reduced by H2 at 520 K, only NiO would be reduced and reoxidized by this pretreatment. In fact, the amounts of consumed H2 and O2 almost correspond to the amounts necessary for the reduction and oxidation of NiO supported on SrTiO,. After the pretreatments by H2and 02, the system was evacuated for 30 min at the reoxidation
Photocatalytic Decomposition of Water Vapor
1
I
20
The Journal of Physical Chemistry, Vol. 86, No. 18, 1982 3650
KJt
31 0
5 1 0 1 5 2 0 amwnt of supported NiO I w t %
Flgure 4. Dependence of the activity upon the amount of NiO supported on SrTiO,; PH8 = 2.7 X lo3 Pa, T = 308 K.
Time I h
Flgwe 2. Photocatalyticdecomposition of H 0 s and liquid) on NiO (1.7wt %)-SrTi03 at 308 K. H,O(g) = 2.3 X2P 10 Pa, after 70 h, H,O(I) was introduced during the reaction; (0)H, (A)02.
7
75
i
Q
-\
50-
reoxidation temperature I K
Figure 3. Effect of temperature of reoxidation by 0,on the activity. Po, = 1.3 X lo4 Pa, reoxidation time = 3 h, PH,O = 2.7 X lo3 Pa, T = 308 K; (0) rate of H, evolution, (A)rate of 0,evolution.
temperature. When liquid or vapor water was introduced on this pretreated catalyst without irradiation, no H2 or O2 could be detected by gas chromatography for a prolonged reaction period. The effect of reoxidation temperature was also studied by using NiO (1.5 wt. %)-SrTi03. After reduction by H2 a t 520 K for 20 h, the catalyst was reoxidized for 3 h at various temperatures. The activities of H2 and O2 evolution under irradiation in the water vapor of 2.7 X lo3Pa after oxidations at various temperatures are shown in Figure 3. The activity of H2 evolution increased with the decrease of reoxidation temperature. On the other hand, O2 was not evolved when the catalyst was reoxidized at room temperature. At a reoxidation over 470 K, the ratio of O2to H2 became stoichiometric. Probably by the oxidation below 370 K for 3 h, Ni(0) still remained on the surface and was oxidized by water during the photodecomposition. The catalytic decomposition of water vapor proceeded after the oxidation above 470 K, but the activity decreased gradually with the increase of oxidation temperature. X-ray photoelectron spectroscopic measurements of some NiO (2 wt %)-SrTiO, catalysts under different pretreatment conditions were also carried out. When the catalyst was reduced by H2 a t 520 K for 20 h, the peak of Ni 2P3/2 was detected at 852.7 eV of binding energy; that means that Ni on the surface of the catalyst was reduced to Ni(0). After reoxidation of the catalyst by O2 at 520 K for 3 h, the peak of Ni 2P3/2shifted to 855.2 eV; that is, Ni(0) was reoxidized to Ni(I1). Under all of
Figure 5. Dependence of the activity of photodecompositionof H20 on P ,+,; NIO (1.5wi %)-SrTO, at 308 K (0)rate of H, evolution, (A) rate of O2 evolution.
the pretreatment conditions in this study, the peaks of Ti 2P,/,were detected only at about 458.7 eV (Ti(IV)), and the existence of Ti(1II) or Ti(I1) could not be confirmed clearly even after the reduction by H2 a t 1000 K for 3 h. The Catalysts used hereafter in this study were reoxidized at 470-520 K. Dependence of the Activity upon the Amount of NiO on SrTiOs. The dependence of the activity of H2 evolution upon the amount of NiO impregnated on SrTiO, is shown in Figure 4. The activity increased proportionally with the amount of supported NiO below 2 wt % and, through a maximum value, decreased gradually with further increase of the amount of supported NiO. At higher percentages of NiO the color of catalyst was changed from gray to black, and, consequently, the number of photons absorbed in SrTi03 became attenuated. Dependence of the Actiuity upon H 2 0 Pressure and Infrared Spectra of the Catalyst. A typical time course of the reaction has been reported in a previous paper,16 which demonstrates that the evolution of H2 and O2 stopped immediately when water vapor was trapped by a solid C02-methanol trap during the reaction. The result suggests that adsorbed water which is equilibrated with water vapor under irradiation is necessary for the continuous decomposition of water. The dependence of the photocatalytic activity upon H20 pressure was studied in more detail, as shown in Figure 5. The catalyst, NiO (1.5 wt %)-SrTi03, was reduced at 520 K for 20 h and reoxidized at 470 K for 3 h before the reaction. Both H2 and O2evolution depended positively upon the H20 pressure, though the dependence was different for H2 and 02.H2 evolution occurred even at low H 2 0 pressures below 4 X 102 Pa, but O2evolved only above about 9 X lo2 P a of H 2 0 pressure. Stoichiometric H2 and O2evolution was realized in the H 2 0 pressure range higher than 2 X lo3Pa. A t lower H20 pressures (below 9 X lo2Pa), little O2 was detected by gas chromatography, which did not increase in 100 h. these results suggest that, after the dissociation of H20,reduced species can easily desorb as H2, whereas oxidized species cannot readily desorb as 02.
3660
Domen et ai.
The Journal of Physlcal Chemistry, Vol. 86,No. 18, 1982
intensity d light(365m-n) I
-noo
2600 m f i 1 e O o
Flgure 7. Dependence of the rate of H2 evolution upon the intensity of U&it (365 nm) on NO (2 wt %)-SrT103. H20(I),450-W highpressure mercury lamp.
TABLE I: Photocatalytic Decomposition of Water Vapor on Various Oxide-Supported SrTiO, Powdersa 1500
pretreatments
wavenumber Icm-1
Flgure 6. Infrared spectra of adsorbed specbs on NIO (1.5 wt %)-SrTI03. (1) D,O ( 0 4 stretd-~ingregion). (2) H@ (H-O-H bendhg reglon): (a) Background after the following pretreatments: reduction by H2 (2.7 X lo4 Pa) at 520 K for 20 h and reoxidetion by 0, (1.3 X lo4 Pa) at 520 K for 3 h; (b) b0 (or D,O) = 2.3 X lo3 Pa was added at room temperature; (c) HpO (or D,O) was trapped by solid COPmethanol for 10 h In the dark; (d) H,O (or D,O) was trapped by solid Cop-methanol for 1 h under illumination.
To elucidate the mechanism of the process, we examined the catalyst surface by infrared spectroscopy under the reaction conditions. The catalyst was the same as that used for volumetric measurements. D20was used to study the stretching vibrational regions of the surface hydroxyl groups, since the transmission in the 3000-cm-' region of this sample was very low. The peak at about 1640 cm-' in Figure 6 is due to the H-0-H bending mode of adsorbed H20. The bands at 2530 and 2620 cm-l are attributed to the 0-D stretching modes of adsorbed D20,and that at 2680 cm-' may be attributed to surface hydroxyl groups, 0-D, although it was rather broad. After the pretreatment described above using H2or Dz, surface hydroxyl groups still seem to remain on the surface (Figure 6a), though no adsorbed water existed. When water vapor of 2.3 X lo3 Pa (H20or D20) was introduced, strong bands caused by adsorbed water were observed in spectrum b. The bands a t about 2700 cm-'in spectrum b are due to water in the gas phase. Even after the water vapor was trapped in the dark by a solid C0,-methanol trap for 10 h (c), a considerable amount of adsorbed water remained on the surface. However, when trapped under irradiation (d), the adsorbed water desorbed completely within 1h, and only surface hydroxyl groups remained, the amount of which was almost the same as that before the introduction of water vapor. As mentioned already, when the water vapor was trapped during the reaction, the evolution of H2 and 0,stopped a t once. Consequently the hydroxyl groups on the surface could not produce H2 and/or 0,under irradiation, and the adsorbed water was necessary for their evolution. Furthermore, the activity of the catalyst which had more surface hydroxyl groups was investigated, by treating the catalyst with saturated water vapor at 520 K for 15 h after the pretreatment stated above. Its activity was almost the same as, or a little less than, that of the ordinary pretreated catalyst, and no increase of the activity was observed by this treatment. It should be regarded consequently that surface hydroxyl groups are not directly involved in the photocatalytic decomposition of water vapor. Decomposition of Liquid Water. Photodecomposition of water in the liquid phase was also studied and was
supported wt oxide % Fe,O, Co,O, Co,O,
NiO CuO Cu,O
2.0 1.5 5.0 2.0 2.0 2.0
103(rate of H, evol)/ (cm3 h - l )
0.15 1.0 1.2 3.9-6.0 0.08 0.17
H, red, 0, oxid T/K time/h T/K time/h 693 753 643 523 533 523
18 4 22 17 24 16
523 523 543 513 523
413b
3 1.5 2 3 3 3
a 450-W high-pressure mercury lamp, PH,-,= 2.7 X lo3 Cu,O layer was produced by oxidation under H, + Pa. 0 , reaction on Cu-SrTiO,.
compared with that in the vapor phase on the same catalyst. The result is shown in Figure 2. At first, water vapor (2.7 X lo3 Pa) was introduced onto NiO-SrTi03 and the gas-phase reaction was followed for 70 h. Then liquid water was introduced during the reaction. Liquid water decomposition also proceeded steadily, and the rate of H20 and O2evolution from liquid water was about 3 times faster than that from water vapor of 2.7 X 103Pa. The difference in the rates in the vapor and gas phases seems to be understandable as an extension of the rate dependence upon the surface concentration of the adsorbed water, as shown in Figure 5. The dependence of the activity on the light intensity is studied in Figure 7. The wavelength of the light used was an emission line at 365 nm of a high-pressure mercury lamp, which has a higher energy than the band gap of SrTi03 (3.2 eV). The value at 100% intensity of light indicates the reaction rate under irradiation without a filter (through a Pyrex glass only); the light intensity was reduced by a proper neutral density filter (HOYA Glass Works). The estimated value of the light intensity contains 10-20% ambiguity because of the line width at the 365-nm band. The rate of Hz evolution was proportional to the intensity of the 365-nm light under the investigated conditions. The same dependence was also obtained in the case of water vapor decomposition. The possibilities of the substitution of NiO for some other transition-metal oxide such as iron, cobalt, and copper oxides were investigated in the vapor-phase reaction. The activity of H2 evolution and pretreatment conditions are summarized in Table I. Though the pretreatment condition of each catalyst might not be optimum, cobalt oxide seems to be effective in a way similar to NiO. Iron and copper oxides were not so effective, but the activities were still higher than that of SrTiOa alone.
Discussion NiO-doped SrTiO, powder exhibited activities for the
J. Phys. Chem. 1982, 86, 3661-3667
photocatalytic decomposition of water in the vapor as well as liquid phase about 100 times higher than that of SITio3 alone. In this reaction the initial step is the absorption of a photon in the semiconductor, SrTi03, and production of an electron (e-) in its conduction band and a hole (h+) in its valence band. According to the electrochemical results, on a n-type semiconductor such as SrTi03 (E b = 3.2 eV), a hole can migrate easily to the surface and an electron to the bulk, because of the band bending near the surface. Accordingly, it is necessary to have some short circuit for the electron to migrate with ease to the surface of SITio3.Consequently, it is possible that the conduction band electron reacts with H+ through NiO and H2 is produced. The pretreatments of reduction and reoxidation may make the contact between NiO and SrTi03better for the electron transfer. The effect of supported NiO was detected clearly even when the amount of NiO was small (4 w t %), as shown in Figure 4, which indicates that highly dispersed NiO is an effective component, and bulky NiO is not necessary to get activity (0.6 wt % of NiO is required to form a monolayer of NiO on the surface of SrTiO,,calculatedfrom the results of BET measurements). Adsorbed water was necessary to decompose water molecules to H2and O2and surface hydroxyl groups alone; i.e., in the absence of adsorbed water, we could not produce any H2 or 02,as shown in Figure 6. On the other hand, direct oxidation of H20 molecules to H20+by a hole in the valence band of SrTi03 is energetically unfavorab1e.l' It seems reasonable to suppose a feasible reaction between a hole (h+) and OH-(a), which is produced by a rapid equilibrium with H20(a) in eq 1. The OH-(a) species in (17) The ionization potential of H20 = 12.6 eV; the hydration energy of H20t = ca.3 eV; the energy of ht in the valence band of SrTiOs = 7.2 eV (vs. vacuum level); 12.6 eV > 3 + 7.2 eV. Another possibility for the reaction between H20(a)and h+ may be H20(a) ht Ht(a) + OH.(a).
+
-
388 1
H20(a) s H+(a) + OH(a)
(1)
eq 1 should be distinguished from that in stable Ti-OH which remains even after the pretreatments and exhibits no activity for the photodecomposition. Damme et al.12 also suggested that the formation of the Ti-OH bond on the surface is a very slow step. In the lower H20 pressure region (9 X lo2Pa), only H2, not 02,evolved; that is, the reaction was not catalytic, as shown in Figure 5. This suggests that much H20(a) is necessary for the evolution of O2 When the H20 pressure was increased from 4 X lo2 to 2.7 X lo3 Pa, H2 and O2 began to evolve in stoichiometric amounts. The oxidized species which could not evolve to the gas phase as O2under 4 X lo2P a of H 2 0 seems to be trapped irreversibly in the surface. On the basis of the results obtained so far, the following mechanism is tentatively proposed for the photodecomposition of water vapor on NiO-SrTiOS H20(g) e H20(a) s H+(a) SrTi03 + hu On NiO
h+
40H.
h+
+ e-
2H4a)
H2(a)
H2(g)
OH- -* OH-
02(a)
2H20(a)
(1')
(2)
- - + - ... - + -
2H+(a) + 2eOn SrTiO,
-
+ OH-(a)
(3) (4)
02(g)
(5) The rate of photodecomposition of pure liquid water was about 3 times faster than that of water vapor. Although this seems attributable to the difference in the amount of H20(a),extrapolating the results in Figure 5, a detailed study will be reported in a later publication.
One-Electron Redox Potentials of Phenols. Hydroxy- and Aminophenols and Related Compounds of Biologlcal Interest
'
S. Steenken MeX-pbnck-InstlM fur Strahienchemie, D-4330 Miihlm, West Germany
and P. Neta" Radlatlon Laboratory and Depertment of Chemistry, Univwtty of Notre Dame, Notre Dame, Indlena 46556 (Received: February 23, 1982; In Final Fcfm: March 2, 1982)
The rate constants for reversible electron transfer between a series of substituted phenolate ions and anilines and various substituted phenoxy1 or anilino radicals in aqueous solution were measured by observing the formation or depletion of the radicals involved. Nonequilibrium concentrationsof the radicals were produced in the presence of the corresponding phenols or anilines by using the pulse radiolysis technique. The relaxation of the system to equilibrium was monitored by optical detection methods. From the equilibrium constants for one-electron transfer, the one-electron redox potentials ( E )for 38 phenolic or anilino type compounds were determined, many of which are natural products. The redox potentials are strongly influenced by electron-donating or -withdrawing substituents at the aromatic system. . Introduction The oxidation of phenols is of great interest because of their involvement in important biological and industrial
processes. Phenolic compounds are members of the groups of hormones, vitamins, and food antioxidants. The latter group consists almost exclusively of phenols, many of which are natural products.24 The mechanism of their
(1) The reeearch described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2312 from the Notre Dame Radiation Laboratory.
(2) Howard, J. A. Ado. Free-Radical Chem. 1972,4,49. Ingold, K. U. Adv. Chem. Ser. 1968, No. 75,296. Mahoney, L. R. Angew. Chem., Int. Ed. Engl. 1969,8, 547.
QQ22-3654/82/2086-366~$Q1.25/O0 1982 American Chemical Society
