J. Phys. Chem. 1985, 89, 4415-4418
The molecular orbital t h e o r i e ~ ~ incorporating v~.~~ the atomic orbitals of halogens give the eg HOMO. In this case, the lowest triplet state becomes 3T2u.When the spin-orbit coupling is introduced, the TI, sublevel is located slightly above the E, sublevel, the splitting between these two levels being due to the second-order spin-orbit coupling only. In this case, the lifetime should have to exhibit a very sharp temperature dependence at a very low (11) Guggenberger, L. J.; Sleight, A. W. Inorg. Chem. 1969, 8, 2041.
4415
temperature range, which again conflicts with the experimental finding. In this way, among the three candidates of HOMO, t2g, tzu, and eg, only the tza H O M O is consistent with the observed temperature dependence. Acknowfedgment. We thank Professor T. Nakajima of this Department and Professor K. Saito of the Institute for Molecular Science for discussions. We also thank Professors T. Mukai and T. Miyashi for the opportunities to use their displex cryogenic system.
Gas-Phase Radical Formation during the Reactions of Methane, Ethane, Ethylene, and Propylene over Selected Oxide Catalysts Daniel J. Driscoll and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: June 6, 1984)
The formation of surface-generated gas-phase radicals during the reactions of methane, ethane, ethylene, and propylene over Bi203,yBi203-Mo03,PbO, MgO, and Li/MgO was examined by EPR matrix isolation spectroscopy. Allyl radicals were detected during the reaction of propylene over all of the oxides examined. Gas-phase methyl and ethyl radicals were detected during the reactions of methane and ethane, respectively, over MgO and Li/MgO while BizO3, y-Bi20,.Mo03, and PbO were essentially inactive. Gas-phase vinyl radicals were not detected during the reaction of ethylene over any of the oxides. These differences in reactivity are attributed to differences in reactivity of the oxide ions on these surfaces toward hydrogen atom abstraction.
Introduction Recently there has been a renewed interest in the formation and detection of surface-generated gas-phase radicals during heterogeneous catalytic reactions. Early work by Hart and Friedli’ and Dolejsek and Novakova2 employed conventional mass spectrometry for detection of these species. With current advances in instrumentation, a number of new spectroscopic techniques have been developed which allow detection of surface-generated gasphase radicals. These include matrix isolation infrared spect r o ~ c o p y laser-induced ,~ fluore~cence,’’~~ modulated beam mass spectrometry: photoelectron spectroscopy,’ resonance-enhanced multiphoton ionization (REMPI),* and matrix isolation EPR.+12 In most cases, investigations with these techniques have been limited to the detection of surface-generated radicals during several specific reactions; thus, at present, no general comparisons of catalysts and reactants have been conducted. Previous reports, from this laboratory, have shown that EPR matrix isolation is a very sensitive and versatile technique for detecting the formation of surface-generated gas-phase radicals.9JoJ2 With this system it was possible not only to detect the formation of gas-phase radicals, but also to determine the amounts (1) Hart, P. J.; Friedli, H. R. J. Chem. SOC.,Chem. Commun. 1970, 1 1 , 621. (2) Dolejsek, Z.; Novakova, J. J. Catal. 1975, 37, 540.
(3) Tevault, D. E.; Lin, M. C.; Umstead, M. E.; Smardewski, R. R. Int. J . Chem. Kinet. 1979, 1 1 , 445. (4) Talky, L. D.; Sanders, W. A.; Bogan, D. J.; Lin, M. C. Chem. Phys. Lett. 1981, 78, 500. ( 5 ) Dulcey, C . S.;Lin, M. C.; Hsu, C. C. Chem. Phys. Lett. 1985, 115, 481. (6) Amorebieta, V. T.; Colussi, A. J. J . Phys. Chem. 1982, 86, 2760. (7) Schultz, J. C.; Beauchamp, J. L. J. Phys. Chem. 1983, 87, 3587. (8) Squire, D. W.; Dulcey, C. S.;Lin, M. C., to be submitted for publi-
cation. (9) Martir, W.; Lunsford, J. H. J. Am. Chem. SOC. 1981, 103, 3728. (10) Driscoll, D. J.; Lunsford, J. H. J . Phys. Chem. 1983, 87, 301. (11) Berlowitz, P.; Driscoll, D. J.; Lunsford, J. H.; Butt, J. B.; Kung, H. H.Combust. Sei. Technol. 1984, 40, 317. (12) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. Chem. SOC.1985, 107, 58.
of the radicals produced. Furthermore, the versatility of the technique allows a variety of different catalysts and gas reactants to be examined without modification of the system. Therefore, this technique is suitable for conducting a general survey on the formation of gas-phase radicals over solid surfaces. In the present study, five oxides (BizO3, y-Bi2O3-MoO3,PbO, MgO, Li/MgO), known to be active for the formation of gas-phase radicals, were compared in their ability to promote the formation of different gas-phase radical species. The hydrocarbons examined were methane, ethane, ethylene, and propylene. In addition to detecting and quantifying the radicals produced, the results obtained also provide insight into the relative reactivity of surface oxygen ions for hydrogen atom abstraction.
Experimental Section The system used to carry out the matrix isolation experiments has been described in considerable detail e l s e ~ h e r e ; ~therefore, J~J~ only the more important aspects of the system will be described. The same conditions and experimental system were used for all the reactions presented here. The reactor was constructed of fused quartz (2.5-cm i.d., 35.8-cm length) and had a thermocouple well centered along its axis which allowed measurement of the temperature along the entire reactor length. A perforated quartz plate was positioned 9 cm from the exit end of the reactor to support the catalyst bed. The reactor was resistively heated over a 23.5 cm length and consisted of a 9.0-cm-long reaction zone and a 14.5-cm-long preheater zone. A temperature profile of the reaction zone showed a maximum temperature variation of f 2 OC. A gas leak was positioned between the exit of the reactor and the collection system creating a pressure gradient. Pressure in the reaction zone was approximately 0.9 torr while in the collection zone the pressure torr. The reaction temperature in all was typically 2 X experiments was 475 OC. The MgO and Li,CO, (99.0%) were obtained from Fisher Scientific (Certified ACS grade). Bismuth oxide (Bi2O3, 99.8%) and lead monoxide (PbO, 99.9%) were obtained from Ventron, Alpha Division and Aldrich Chemical, respectively. The y-bis-
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The Journal of Physical Chemistry, Vol. 89, No. 21. 1985
Letters
TABLE I: Radical Production over Various Oxide Catalysts' radical amount, nmol g-'
surface area, catalyst Biz?, ~-BI,O~*MOO, PbO(I)d PbO(I1)' MgO 14.6 wt % Li/MgO
m2 g-' 0.35 1.2 0.13 0.43 159.2 29.8
C3Hy [C3H, (89)l" 17.90 (51.1)c 6.50 (5.4) 2.80 (21.5) 13.24 (30.8) 14.82 (0.1) 65.52 (2.2)
C2Hs. [C*Hs (98)l" trace CH3., CH,O2. 0 0 0 2.16 (0.01) 10.78 (0.36)
CH3* [CHI (104)l" 0.056 (0.16) 0 0.010 (0.08) 0.036 (0.08) 3.54 (0.02) 38.18 (1.28)
C2H3. [C& ( l o 8 ) l b trace CH,., C H 3 0 2 . 0 0 weak unidentified signal CH3. (0.066) C H y (0.184)
aReactions carried out at 475 OC, collection period = 30 min. Flows: Ar = 3.8 cm3 min-', O2 = 0.024 cm3 min-I, hydrocarbon = 1.08 cm3 min-'. "Reactant and C-H bond strength in kcal mol-'. CNumbersin parentheses are radical amounts based on surface area ( m o l m-2). dPreconditioned at 450 OC, 2.5 h, 300 cm3 of O2 min-' (chips). ePreconditioned under vacuum (powder).
muth molybdate (Bi2Mo0,) was supplied by Professor G. W. Keulks of the University of Wisconsin. The MgO was used after conversion to the high-surface-area form. The MgO, as received, was added to deionized water and the resulting slurry was stirred and heated until only a thick paste remained. The paste was then air-dried overnight at 140 O C . This Mg(OH), was then reconverted to a high-surface-area oxide during the high-temperature pretreatment. The lithium-doped sample was prepared by adding the carbonate to the MgO slurry. The remaining samples were used without further pretreatment or purification. With the exception of the sample labeled PbO(II), all of the oxide powders were pressed into thin wafers and broken into approximately 3 X 3 mm chips which were loaded into the reactor between four layers of quartz wool. The PbO(I1) sample was loaded as a powder between three layers of quartz wool. The argon (99.99%), oxygen (99.8%), methane (99.97%), ethane (99.0%), ethylene (99.5%), and propylene (99.0%) were obtained from Matheson. The oxygen was further purified by passage through a molecular sieve trap. The remaining gases were used as received. Flow rates were regulated with Vacoa precision metering values. The flow rates, for all experiments, were as follows: argon, 3.8 cm3 min-I; oxygen, 0.024 cm3 min-I; hydrocarbon, 1.08 cm3 m i d . The samples, with the exception of the PbO(I1) sample, were loaded into the reactor and conditioned at 450 "C under a flow of oxygen (300 cm3 m i d ) for 2.5 h. The samples were then cooled to 175 "C under flowing 02.After the O2 flow was terminated the reactor was placed in line and evacuated. Evacuation continued for 30 min with the catalyst at 175 "C. The reactor was then heated to 475 O C over a period of ca. 10 min, evacuation was discontinued, and the flow of the reactant mixture was initiated. When the run was completed, the reactor was allowed to cool, removed from the system, and the samples were then reconditioned for the next run. The PbO(I1) sample was not thermally treated in the presence of oxygen, and a fresh sample was used for each run. This sample was evacuated at 175 OC for 30 min, heated to 475 OC under vacuum over a period of ca. 10 min, and then exposed to the reactant gases. The radicals were collected and analyzed in a solid argon matrix formed on a sapphire rod at a temperature of 14 K. The collection period for all samples was 30 min. Two collections were obtained for each run. The first collection was begun after 2 min on stream. A second collection was begun after 4.0 h on stream. At this time the radical production had reached its steady-state value. Analysis of the radicals was accomplished by lowering the matrix into a TEIo2microwave cavity and recording the EPR spectrum with a Varian Model V4500 spectrometer. The g values and spin concentrations were determined relative to a phosphorus-doped silicon standard with g = 1.9987. The spin concentration of a pure spectrum of each radical was calculated by the double integration method. All other spectra of the radical were then compared to this standard spectrum by comparison of peak-to-peak heights. This type of calculation was possible because the shapes of the spectra in each case were identical. We estimate that a determination of spin concentration of ratios for the same type of radical (e.g., comparison of methyl radicals) results in an error of &5%, whereas a determination of ratios for different radicals (e.g., comparison of methyl with allyl radicals) results in an error of *IO%. In both cases, using ratios eliminates the
-2
1 1 -11-1
1-
Figure 1. EPR spectrum of the ethyl radical.
error due to the standard, and in the former case the error due to the double integration step is eliminated. Surface areas were determined by a volumetric BET method with krypton gas as the adsorbate. Lithium concentration was determined by atomic absorption on a Varian A-6 spectrometer.
Results and Discussion Hydrogen atom abstraction was expected to be the predominant process occurring over these oxide surfaces; therefore, the primary radical products expected during the reactions of methane, ethane, ethylene, and propylene over these oxides were the methyl, ethyl, vinyl, and allyl radicals, respectively. The low molecular oxygen partial pressure employed effectively eliminated the formation of alkyl peroxy radicals from secondary gas-phase reactions. The EPR spectra of the methyl radical'* and the allyl radicalg obtained with this system have been previously reported. The EPR spectrum obtained for the ethyl radical ((P)CH3-(a)CHZ.) is depicted in Figure 1. The twelvelined spectrum arises from the interaction of the unpaired electron with the two sets of nonequivalent (Y and /3 hydrogen atoms, and the values for the hyperfine splittings calculated from this spectrum are in excellent agreement with the earlier reported values of a, = 22.2 G and ab = 26.9 G.13 Vinyl radicals were not detected in any of the reactions carried out in this study; thus the EPR spectrum of this radical is not illustrated here. The results of this study, along with the specific surface areas of the oxides examined and the bond strengths of the most labile hydrogen atom of the hydrocarbons,14 are presented in Table I. The radical amounts listed in the table were obtained with a constant catalyst mass of 0.50 g, and the radical amounts have not been normalized to account for the differences in surface area. At the reaction temperatures employed in this study, small (13) Fressenden, R. W.;Schuler, R. H. J . Chem. Phys. 1960,33, 935. (14) Weast, R. C., Ed.'CRC Handbook of Chemistry and Physics", 62nd ed; CRC Press: Boca Raton, FL, 1982; p F-191.
Letters amounts of gas-phase radicals were normally generated through homogeneous thermal decomposition, and the radical amounts presented have already been corrected to account for this fact. All of the oxides examined were capable of promoting gas-phase allyl radical formation from propylene which, based on C-H bond strengths, contained the y o s t easily abstracted hydrogen atom of the four hydrocarbons studied. Allyl radical formation (per g of catalyst) over BizO3, PbO(II), and MgO was nearly equal, even though the surface areas of these samples varied by a factor of ca. 450. Radical production over y-Bi203-Mo03was slightly lower than that observed aver the three preceding samples; however, this result is in complete agreement with the proposal that a surface radical reaction, in contrast to radical desorption, is favored during the partial oxidation of propylene over the bismuth m ~ l y b d a t e s . ~ JThe ~ J ~PbO(1) sample was considerably less active for gas-phase allyl radical formation than any of the other oxides examined. Lithium-doped MgO, on the other hand, was by far the most active catalyst for the production of gas-phase allyl radicals from propylene. Radical formation over this catalyst was a factor of 3.7 moie active than the Biz03 sample and a factor of approximately 23 more active than the PbO(1) sample, which was the least active of the oxides. In contrast to the previous results, Bi2O3, y-Bi203.Mo03, PbO(I), and PbO(I1) were relatively ineffective in generating gas-phase alkyl radicals from the simple alkanes (CH4 and CzH6) or vinyl radicals from ethylepe. The MgO and Li/MgO catalysts, however, still exhibited considerable activity for generating gasphase ethyl and methyl radicals from ethane and methane, respectively, with Li/MgO remaining the most active catalyst. Gas-phase vinyl radicals were, once again, not detected during the reaction of ethylene over these samples. These observations can be interpreted by considering the nature and number of the active sites which promote the hydrogen atom abstractions. The results of the study on the formation of gasphase allyl radicals from propylene indicate that MgO has very few active sites per unit of surface area. Previously, it was proposed that the sites responsible for hydrogen atom abstraction on MgO were 0-ions associated with intrinsic surface cation vacancies in the MgO lattice,I2 and the concentration of these sites would be expected to be low. The active sites for hydrogen atom qbstraction on Li/MgO were also proposed to be surface 0-ions; however, in addition to vacancy-0- sites, 0; ions were also produced at substitutional lithium ion impurities. The latter centers have been designated as [Li+O-].'2*'7 Thus, the concentration of active sites on this catalyst was substantially greater. Therefore, gas-phase radical formation over Li/MgO was also considerably higher. Since the MgO and Li/MgO catalysts were the only oxides capable of producing large amounts ef gas-phase alkyl radicals from alkanes, one may conclude that 0; ions are capable of abstracting hydrogen from hydrocarbons with C-H bond strengths as high as 104 kcal mol:'. The reactivity of 0;with simple alkanes a t low temperatures has been previously demonstrated by stoichiometric reactions.'* It is likely that the number of such 0; species on the other oxides was small, although it is possible that a few oxygen ions of this type existed via the equilibrium M*02~it10-.19
The Biz03 and PbO samples apparently contained a significantly larger number of active sites per unit of surface area which were capable of promoting gas-phase organic radical formation from hydrocarbons with C-H bond strengths 1 8 9 kcal mol-'. The active species on these oxides are generally believed to be lattice 02-ions in sites of low coordination.20 In this respect, it is of interest to note that the average coordination for most of the surface oxide ions on both Bi203 and PbO is lower than the (15) Grasselli, R. K.; Burrington, J. D. Ado. Coral. 1981, 40, 133. (16) Keulks, G. W.; Krenzke, L. D.; Notennann, T. N . Adu. Carol. 1978, 27, 183. (17) Boldu. J. L.; Abraham, M. M.; Chen, Y. Phys. RN. B 1979,19,4421. (18) Aika, K.; Lunsford, J. H. J. Phys. Chem. 1977, 81, 1393. (19) Yang, T.-J.; Lunsford. J. H. J. Card 1980, 63, 505. (20) Margolis, L. Ya. Card Reu. 1973, 8, 241.
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4417
coordination of oxide ions on the flat [ 1001 surface of MgO.*' This factor, coupled with the poor reducibility of Mg2+, could account for the inert behavior of surface 02-ions on MgO toward hydrogen atom abstraction from even propylene. Reactions of a radical on the surface may also play an important role in determining the number of gas-phase radicals detected. As previously mentioned, a surface-radical reaction is favored during the partial oxidation of propylene over the bismuth molybdates, and this accounts for the lower production of gas-phase allyl radicals (vs. Bi203 and PbO(I1)) observed over the yBi203-Mo03sample examined here. Ethyl radicals also appear to undergo further surface reactions on all of the oxide surfaces. Ethane has lower C-H bond strengths than methane and the rate constant for gas-phase ethyl radical recombination is smaller than that of the methyl radicaLZ2 Therefore, since small amounts of gas-phase methyl radicals were detected during the reaction of methane over Biz03 and PbO, it would be expected that at least trace amounts of gas-phase ethyl radicals would be detected during the reaction of ethane over these same oxides. Such was not the case. In addition, gas-phase ethyl radical formation, relative to gas-phase methyl radical production, was also lower over the more active MgO and Li/MgO catalysts. These results likewise indicate that ethyl radicals were undergoing further surface reactions rather than desorbing into the gas phase. Methyl radicals may also undergo surface reactions, but the probability of this occurring was lower than for the ethyl radical. The formation of trace amounts of methyl and methylperoxy radicals during the reactions of ethane and ethylene over Bi2O3 provides additional support for the importance of surface reactions. These samples, after reaction, appeared to be slightly coked as determined by visual inspection. Coke is formed from decomposition of the reactant hydrocarbon on the catalyst surface during reaction. Therefore, it is likely in these cases that small amounts of gas-phase methyl radicals were produced during surface decomposition of the C2 compounds, and not via the formation of gas-phase C2radicals from hydrogen atom abstraction, followed by gas-phase decomposition. The trace amounts of methylperoxy radicals were probably formed through secondary gas-phase reactions of these methyl radicals with molecular oxygen. It should be mentioned that coking was not observed on any of the other catalysts examined in this study. The absence of gas-phase vinyl radicals during the reaction of ethylene over MgO and Li/MgO can be explained by considering the unique reactivity of ethylene with 0; ions on MgO. Previous investigations have shown that ethylene readily reacts with 0; ions on MgO at room t e m p e r a t ~ r e . ~The ~ reaction is believed to proceed via a surface intermediate formed through the process:24v25
-
H2C=CH2
+ 0;
H2C=CH
+ 0:-
-
H2C=CH H2C=C'-
+ OH; - -OH;
(2)
Although vinyl radicals have been proposed as intermediates in this process, their presence has never been detected either on the surface or in the gas phase. Since ethylene has a relatively large C-H bond strength (-108 kcal mol-'), simple hydrogen atom abstraction, resulting in the formation of (OH;) and free vinyl radicals, may be energetically unfavorable. (The 0-H bond strength is 105 kcal mol-'.) Therefore, the driving force for the reaction includes complex formation between the vinyl radical and surface 02-ions. Apparently the lifetime of the surface vinyl radical is too short to permit detection. In previous work, stable products resulted from further reaction of this surface complex,23 while in the present study the trace amounts of gas-phase methyl
-
(21) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. "Chemistry of Catalytic Processes"; McGraw-Hill: New York, 1979; p 369. (22) Kerr, J. A.; Moss, S. J. "Handbook of Bimolecular and Termolecular Gas Reactions"; CRC Press: Boca Raton, FL, 1981; Vol. 2, p 37-41. (23) Aika, K.; Lunsford, J. H. J . Phys. Chem. 1978,82, 1974. (24) Ben Taarit, Y.; Symons, M. C. R.; Tench, A. J. J . Chem. Soc., Faraday Tram. I 1977, 73, 1149. (25) Naccache, C. Chem. Phys. Lett. 1977, 11, 323.
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J . Phys. Chem. 1985, 89, 4418-4420 are most likely lattice Os2-ions associated with Pb(I1) ions, and not Pb(1V).
radicals detected may have resulted from decomposition of a resulting surface species. The remaining oxides (Bi2O3, yBi2O3’MoO3,and PbO) probably were incapable of promoting the initial hydrogen atom abstraction; therefore, neither gas-phase vinyl radicals nor a surface radical species were produced. Lead monoxide (PbO), when heated in the presence of air or oxygen, is easily converted to the mixed oxide Pb2(II)Pb(IV)O, (red lead), which behaves chemically as a mixture of PbO and Pb02.26 The standard high-temperature oxygen pretreatment used in this work would easily convert PbO to this mixed oxide form, and inspection of these samples (labeled PbO(1)) after pretreatment, and after reaction, confirmed this fact. To eliminate this transformation, PbO samples were examined after only a thermal, vacuum pretreatment and inspection of these samples (labeled PbO(II)), after reaction, indicated that the original form of the oxide had been preserved. Considering this chemistry and the radical amounts observed for the PbO samples, it appears that the sites responsible for hydrogen atom abstraction on these oxides
Conclusions The results described here clearly demonstrate that the ability of a surface to generate gas-phase hydrocarbon radicals depends upon the bond strength of the weakest C-H bond, the presence of particular radical-forming sites on the surface, and the propensity of the radicals to either desorb or to undergo further surface reactions. The importance of gas-phase radical coupling reactions is evident in the catalytic conversion of propylene to 1,Shexadiene over Bi20:’ and the conversion of methane to ethane and ethylene over Li/Mg0.28 Subsequent surface reactions of allyl radicals are responsible for the conversion of propylene to acrolein over the bismuth molybdates.’5-16
(26) Cotton, F. A.; Wilkinson, G. “Advanced Inorganic Chemistry”, 4th ed; Wiley-Interscience: New York, 1980; p 399-400.
(27) Swift, H. E.; Bozik, J. E.; Ondrey, J. A. J. Catal. 1971, 21, 212. (28) Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721.
Acknowledgment. We acknowledge financial support of this work by the Division of Basic Energy Science, Department of Energy.
Integrated Chemical Systems: Photocatalysis at TIO, Incorporated Into Nafion and Clay Fu-Ren F. Fan, Hsue-Yang Liu, and Allen J. Bard* Department of Chemistry, The University of Texas, Austin, Texas 78712 (Received: June 7 , 1985)
Ti02 incorporated into Nafion or clay films has been prepared by treatment with Ti(II1) followed by oxidation. Both systems show photocatalytic activity in the reduction of methylviologen (N,N’-dimethyL4,4’-bipyridiniumor MV2+)with oxidation of triethanolamine. The state and the chemical reactivity of titanium species in Nafion were characterized by electron spin resonance (ESR) spectroscopy. The similarity with the solution spectra suggests that Ti(II1) ions tumble in solvent-saturated Nafion but do not in the dry film. Oxygen adsorption on a dry Ti(II1)-incorporated Nafion membrane gives a strong ESR signal with g = 2.0095, which has been assigned as the superoxo complex of Ti(1V).
Introduction The utilization of integrated chemical systems’ to carry out a particular process is of current interest. For example, the photogeneration of hydrogen on a ptype semiconductor electrode (e.g., GaAs and Si) is enhanced by the addition to its surface of a viologen-bearing polymer layer containing finely divided Pt and the photogeneration of oxygen or chlorine on an n-type semiconductor electrode (e.g., Si) is facilitated by the addition of Ru02 to the surface silicide layer.2 The incorporation of a dispersed semiconductor (e.g., CdS) within a polymer membrane to carry out photocatalytic and photosynthetic processes has also been d e ~ c r i b e d . ~In these systems, suitable relays and catalysts can be added to promote photocatalytic reactions on these membranes. Separation of products may also be feasible in these systems. We report here another system based on titanium-exchanged Nafion (1) Bard, A. J.; Fan, F.-R. F.; Hope, G. A,; Keil, R. C. ACSSymp. Ser. 1983, No. 211, 93. (2) (a) Abruna, H. D.; Bard, A. J. J . Am. Chem. SOC.1981, 103, 6898. (b) Dominey, R. N.; Lewis, N. S.;Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S.J. Am. Chem. SOC.1982,104,467. (c) Fan, F.-R. F.; Keil, R. G.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 220. (3) (a) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (b) Krishnan, M.; White, J. R.; Fox,M. A,; Bard, A. J. J . Am. Chem. Sm. 1983, 105,7002. (c) Mau, A. W.-H.; Huang, C. B.; Kakuta, N.; Bard, A. J. J. Am. Chem. Soc. 1984, 106, 6531.
0022-3654/85/2089-4418$01.50/0
or clay (e.g., hectorite) membranes. Ti02-based membrane integrated systems are of special interest because of the widespread use of Ti02 particle dispersions in photocatalytic and photosynthetic s t u d i e ~ . ~
Experimental Section Consider first the incorporation of Ti(II1) into Nafion. A Nafion membrane (type 125, 110 equiv wt, thickness ca. 0.13 mm) was pretreated by boiling it in concentrated H N 0 3 until it became clear and transparent. It was then soaked and stored in deionized distilled water. Ti(II1)-exchanged Nafion was prepared by immersing a clean and thoroughly washed (with deoxygenated MeOH) Nafion membrane in a 0.1 M TiCI3 (Alfa Products, Danvers, MA) in deoxygenated MeOH solution overnight. The light purple Ti(II1)-incorporated Nafion membrane was then washed thoroughly with deoxygenated MeOH. The incorporation of Ti(II1) into hectorite membranes was performed in a similar fashion. The light purple Ti(II1)-Nafion membrane (A), when exposed to moisturized air or immersed in air-saturated H 2 0 , became (4) (a) Frank, S. N.; Bard, A. J. J . Phys. Chem. 1977, 81, 1484. (b) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 5985. (c) Kiwi, J.; Borgarello, E.; Pelizzetti, E.; Visa, M.; Griitzel, M. Angew. Chem., In?.Ed. Engl. 1980, 19, 646. (d) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visa, M.; Gratzel, M. Nature (London) 1981, 289, 158 and references therein.
0 1985 American Chemical Society
