J. Phys. Chem. B 1997, 101, 2621-2624
2621
Increasing the Efficiency of the Photocatalytic Oxidation of Organic Films on Aqueous Solutions by Reactively Coating the TiO2 Photocatalyst with a Chlorinated Silicone Jo1 rg Schwitzgebel, John G. Ekerdt, Futoshi Sunada, Sten-Eric Lindquist,† and Adam Heller* Department of Chemical Engineering, The UniVersity of Texas at Austin, Austin, Texas 78712-1062 ReceiVed: August 14, 1996; In Final Form: NoVember 1, 1996X
In order to reduce the fraction of water and salt accessible surface OH sites on TiO2, the photocatalytic crystallites were reacted, in an HCl-releasing reaction, with a chlorine-terminated chlorinated poly(dimethylsiloxane) telomer, Glassclad 6C. When the overcoated photocatalyst was applied in the oxidation of organic films on aqueous 0.5 M NaCl, the number of O2 molecules consumed per photon drastically increased. In the oxidation of n-octanal films, 1.25 molecules of O2 were consumed (five electrons were transferred to oxygen), and in the oxidation of n-octanoic acid, 0.15 molecules of O2 were consumed (0.6 electrons was transferred) per photon. The overcoating reduced the O2-consumed/CO2-produced ratio in the initial reaction period when most of the reactant was intact. The enhancement of O2 uptake per photon and the production of oxidized organic compounds, instead of CO2, are attributed to chloride ion exclusion from the reaction zone and to fast hole oxidation, producing a radical, followed by prompt electron injection by the radical, resulting in two-electron oxidation.
Introduction When the chlorine-terminated chlorinated poly(dimethylsiloxane) telomer, Glassclad 6C, reacts with surface OH groups on TiO2, HCl is released and the polymer, part of which is not oxidized to volatile or soluble products, is covalently bound to the surface. Access of ions to the TiO2 interface is reduced, yet oxygen and organic reactants permeate through the silicone layer. We show here that such surface modification substantially enhances the number of oxygen molecules consumed per photon and changes the distribution of products. The change in distribution is consistent with the formation of compounds known to be produced in current-doubling two-electron oxidation reactions, involving formation of a radical through hole oxidation followed by prompt electron injection by the radical.1-4 The two-electron oxidation results in products that differ from the one-electron oxidation products of sequences initiated by holes or OH radicals.5-14 Experimental Section The experimental setup was similar to the one described earlier.15 A cylindrical, water-jacketed, gastight Pyrex cell (volume 360 cm3, cell surface 50 cm2) was exposed to 4 mW cm-2, 300-400 nm (λmax ) 360 nm) UV light. The irradiance was measured with a JBL Model 110 UV meter, confirmed by measurement with an International Light (Newburyport, MA) Model 1L 1700/760D/782A spectroradiometer system and then reconfirmed by ferrioxalate actinometry. The cell was maintained at 27 ( 2 °C by circulating water through the cell jacket. During the experiment the cell was shaken orbitally at 115 rpm. The O2 consumption was measured in 3 h long experiments using a water manometer. The cell was charged with 60 ( 3 mL of an aqueous NaCl solution (0.5 M NaCl concentration), 5 mL of the organic compound, and 0.5 g of TiO2-coated, ∼100 µm diameter glass microbubbles of 0.4 g cm-3 density that were “bare”, i.e., uncoated in half of the experiments and reactively overcoated with the chlorine-terminated poly(dimethylsiloxane) telomer, Glassclad 6C (United Chemical Technologies, Inc., † Permanent address: Department of Physical Chemistry, Uppsala University, Uppsala, Sweden. X Abstract published in AdVance ACS Abstracts, March 1, 1997.
S1089-5647(96)02492-3 CCC: $14.00
Bristol, PA), in the other half. The bare photocatalytic TiO2coated bubbles were prepared as described from 3M K20 glass bubbles and Degussa P25 TiO2.15 The TiO2-coated bubbles were reactively silicone overcoated by mixing 5 g of the bubbles with a solution of 2.5 mL of Glassclad 6C in 97.5 mL of dichloromethane, collection by skimming, washing in water, and drying at 120 °C. All bubbles contained about 50 wt % TiO2, and the silicone-overcoated ones contained 5 wt % of the silicone. The readings of the water manometer were corrected for changes in the barometric pressure and for the CO2 generated in the photooxidation process that was assayed by gas chromatography. In the orbitally shaken cell the photocatalytic microbubbles covered in all experiments at least 90% of the 50 cm2 area of the air-liquid surface, except in one set of experiments where n-octanoic acid was oxidized on silicone-overcoated photocatalytic microbubbles. In this experiment the bubbles and the octanoic acid aggregated, and only about 40% of the 50 cm2 surface was photocatalyst covered when the cell was shaken. The measurements for this case were corrected for full surface coverage by the photocatalyst. Results Control experiments showed that in the photooxidation of the coating of the Glassclad 6C coated bubbles the rate of O2 consumption was not significant relative to the rate of the O2 consumption in the presence of the added eight-carbon compounds. Hydrogen could not be detected by GC among the products of the reactions. The number of moles of CO2 generated per 100 einsteins in the photocatalytic oxidation of 3-octanone where CO2 is promptly evolved15 was, for the bare photocatalyst, 2.5 in pure water and 1.1 in 0.5 M NaCl; with the reactively siliconeovercoated photocatalyst, 1.9 mol of CO2 was generated per 100 einsteins in pure water and 2.1 mol in 0.5 M NaCl. Figure 1 shows the rate of pressure drop in the reaction cell, filled initially with air to atmospheric pressure and containing n-octane, for three cases: without any photocatalyst, with the bare catalyst, and with the silicone-overcoated catalyst. As is evident, silicone overcoating increased the net pressure drop © 1997 American Chemical Society
2622 J. Phys. Chem. B, Vol. 101, No. 14, 1997
Schwitzgebel et al. TABLE 2: Evolved CO2/Consumed O2 Molar Ratio in the Initiala Photocatalytic Oxidation of Linear Eight-Carbon Aliphatic Compounds on Bare and Silicone-Overcoated Photocatalysts photocatalyst/ compound oxidized
CO2 evolved/ O2 consumed on Bare TiO2
CO2 evolved/O2 consumed on siliconeovercoated TiO2
3-octanal n-octanoic acid 3-octanol 3-octanone n-octane
0.028 0.93 0.48 0.60 0.31
0.022 0.63 0.21 0.35 0.21
a
Figure 1. Rates of pressure drop (O2 consumption-CO2 evolution) upon photocatalytic oxidation of an n-octane film on aqueous 0.5 M NaCl in the absence of photocatalyst (solid triangles), with the bare photocatalyst (solid circles), and with the silicone-overcoated photocatalyst (open circles). Conditions: 300 mL gas volume, 50 cm2 illuminated area, 4 mW cm-2 360 nm irradiance, 115 cycle per minute orbital shaking.
Less than 0.1% of the organic compound was oxidized.
for 3-octanol by a factor of 4.0, for 3-octanone by a factor of 3.2, and for n-octane by a factor of 3.6. The increase for n-octanal resulted in 1.25 O2 molecules being consumed (transfer of five electrons) per photon. Table 2 shows the ratio of the number of moles of CO2 evolved per mole of O2 consumed in the photocatalytic oxidations on the bare and silicone-overcoated photocatalysts. In all of the five compounds that were photocatalytically oxidized, the silicone overcoating decreased the CO2-evolved/ O2-consumed molar ratio. Thus silicone overcoating produced preferentially oxygen-enriched organic compounds, rather than CO2. The shift from CO2 production to production of an oxidized organic compound was greatest for 3-octanol, where the CO2/O2 ratio decreased by a factor of 2.3. Organic compounds that were added to the organic phase through its photocatalytic oxidation are listed in Table 3. In the oxidation of n-octanoic acid there was a remarkable 11fold increase in the amount of heptanoic acid formed upon overcoating the photocatalyst, making this compound the dominant organic product in lieu of a mixture of about equal amounts of n-heptanol and n-heptanal produced with the bare photocatalyst. Discussion
Figure 2. Dependence of the amount of CO2 produced in the photocatalytic oxidation of n-octane on the exposure time. Conditions are as in Figure 1.
TABLE 1: Moles of O2 Consumed per 100 einsteins in the Photocatalytic Oxidation of Eight-Carbon Aliphatic Compounds with Bare and Silicone-Overcoated Photocatalysts photocatalyst/ compound oxidized
bare TiO2
silicone-overcoated TiO2
ratio
n-octanal n-octanoic acid 3-octanol 3-octanone n-octane
53 2.9 1.2 2.0 0.67
125 14.7 4.8 6.5 2.4
2.4:1 5.1:1 4.0:1 3.2:1 3.6:1
rate which is the difference between the amounts of O2 consumed and CO2 evolved. In Figure 2, the amount of photooxidatively produced CO2 is plotted as a function of exposure time for the initial 50 h period for the reaction catalyzed by the silicone-overcoated bubbles. No time dependent decrease in the rate of CO2 evolution was seen. Table 1 shows the number of moles of O2 consumed per 100 einsteins, with less than 0.1% of the organic layer oxidized, for reactions photocatalyzed by (a) bare (“hydrophilic”) and (b) by overcoated (“oleophilic”) bubbles. For 3-octanal the increase was by a factor of 2.4, for n-octanoic acid by a factor of 5.1,
Increase in Yield. Abdullah, Low, and Matthews found that chloride anions suppress the rates of photocatalytic oxidation of salicylic acid, aniline, and ethanol to CO2.17 They proposed that organoperoxy radicals oxidize Cl- to chlorine, a process whereby they are reduced to organohydroperoxides. We have earlier proposed that organoperoxy radicals are precursors of labile organohydrotetraoxides that can decompose through CO2evolving reactions.15,16 Comparison of the yield of CO2 molecules per photon in the photocatalytic oxidation of 3-octanone on pure water and on 0.5 M aqueous NaCl shows that the presence of the ions reduced the yield by a factor of 2.5. Because the silicone overcoating excluded the Cl- anions from the reaction zone, no such reduction was observed when the photocatalyst was reactively overcoated with the silicone. As a result, the CO2 yield in 0.5 M NaCl was, for the overcoated catalyst, twice that of the bare one. This doubling, however, accounted only for part of the up to 5-fold increase in the O2 consumption per einstein that was observed after reactive overcoating with the silicone (Table 1) with the five organic reactants. The excess increase and the change in the reaction products are consistent with initiation by two-electron oxidation associated with current doubling on the overcoated surface. Current-doubling photocatalytic oxidation of formaldehyde, methanol, and formic acid, on monocrystalline C-axis cut (rutile) TiO2 has been reported by Nogami and Kennedy.18 The respective two-electron oxidation products were formic acid, formaldehyde, and CO2. Their study established that the radicals generated in the initial one-electron oxidation reactions of an aldehyde, an alcohol, and a carboxylic acid were reducing
Photocatalytic Oxidation of Organic Films
J. Phys. Chem. B, Vol. 101, No. 14, 1997 2623
TABLE 3: Major Initial Reaction Products in the Photocatalytic Oxidation of Linear Eight-Carbon Aliphatic Compounds on Bare and Silicone-Coated Catalystsa photocatalyst/product n-octanal n-octanoic acid 3-octanol 3-octanone n-octane a
bare TiO2
silicone-overcoated TiO2
n-octanoic acid (0.13); CO2 (0.02) CO2 (0.03); n-heptanol (0.006); n-heptanal (0.005) CO2 (0.006); 3-octanone (0.01) CO2 (0.01) CO2 (0.002); octanones (0.002)
n-octanoic acid (0.19); CO2 (0.03) CO2 (0.08), n-heptanoic acid (0.03); n-heptanol (0.02) 3-octanone (0.05); CO2 (0.01) CO2 (0.02) octanones (0.01); CO2 (0.005)
The values in parentheses represent the number of moles produced per einstein and are taken from ref 15.
enough to inject into the conduction band of TiO2 (rutile) electrons at pH 3. At the concentration of oxygen in airsaturated water at 25 °C (∼2.4 × 10-4 M), the effective pseudofirst-order rate constant for organoperoxy radical formation by the diffusion-controlled bimolecular combination of O2 with a radical, even if not surface bound but dissolved, cannot exceed 107 s-1. Excited Coumarin343 has been reported to inject electrons into the TiO2 conduction band with a rate constant of 5 × 1012 s-1.19 Other organic radicals that are equally or more reducing will also inject electrons at rates that are much faster than the diffusion-controlled rate of combination of a radical with dissolved O2. Recombination-like processes, such as combination of •OH radicals with photogenerated electrons to form OH- or •OH radicals with one-electron-reduced and -protonated O2 (the •OOH radical) to form O2 and H2O, become irrelevant and the yield, measured by the number of O2 molecules consumed per photon, increases when a stable twoelectron oxidation product (e.g., 3-octanone from 3-octanol) or intermediate (e.g., a carbene from an aliphatic carboxylic acid) is produced. Furthermore, if the injection is this fast, it will also effectively compete with electron-hole recombination. Increase in efficiency was seen in the oxidation of all of the five eight-carbon aliphatic compounds tested, including a hydrocarbon, an alcohol, a ketone, an aldehyde, and a carboxylic acid upon reactive coating of the TiO2. The increase, though substantial for all compounds, was particularly pronounced in the oxidations of n-octanoic acid and 3-octanol where the photon efficiencies of oxygen consumption increased, respectively, by factors of 5.1 and 4.0. Two-Electron Oxidation Products. Inhibition of the organohydrotetraoxide path through reduction of the organoperoxy radical by Cl-, i.e., oxidation of Cl- to chlorine by the organoperoxy radical, lowers the CO2 yield.17 If, however, the silicone overcoating were merely preventing the lowering of the yield by excluding Cl- from the reaction zone, overcoating would have increased the CO2/organic product ratio. We find, however, that the overcoating substantially decreased the ratio. The observed changes are consistent with enhancement of the two-electron oxidations which become dominant over the oneelectron processes, initiated by OH radicals, on the bare catalyst. n-Octanoic Acid. Upon silicone overcoating, the number of moles of O2 consumed per einstein jumped to 0.15 from 0.029 (Table 1), and a much larger fraction of the O2 ended up in organic products instead of CO2 (Table 2). The dominant organic phase soluble product was now the two-electron oxidation product, n-heptanoic acid (see equations), the yield of which increased 11-fold to 0.027 mol/einstein. The yield of heptanal produced via the organohydrotetraoxide involving a one-electron oxidation-initiated sequence15 increased only by about 30% to 0.007 mol/einstein. Also on the bare, well-hydrated surface, the major path, involving an organotetraoxide, produced mostly CO2 and n-heptanal,15 not the many possible compounds that would have been formed if the OH radical reacted with hydrogens other than that of the COOH function. For the bare photocatalyst, the products were consistent with those of the following
tetraoxide sequence, initiated by a one-electron oxidation reaction:
CH3(CH2)6COOH + •OH f CH3(CH2)6 (‚) + CO2 + H2O CH3(CH2)6(‚) + O2 f CH3(CH2)6OO• CH3(CH2)6OO(‚) + •OOH f CH3(CH2)6OOOOH CH3(CH2)6OOOOH f CH3(CH2)5CHO + O2 + H2O When the catalyst was silicone overcoated, the products were consistent with those of the following two-electron oxidation initial sequence via a transient carbene.
CH3(CH2)6COOH S [CH3(CH2)6COOH]surface [CH3(CH2)6COOH]surface + h+ f [CH3(CH2)6•]surface + CO2 + H+ [CH3(CH2)6•]surface f [CH3(CH2)5CH]surface + H+ + e-CB [CH3(CH2)5CH]surface f CH3(CH2)5CH CH3(CH2)5CH + O2 f CH3(CH2)5COOH CH3(CH2)5CH + H2O f CH3(CH2)5CH2OH The carbene apparently reacted with O2 to produce nheptanoic acid and with water to produce n-heptanol, the only two compounds produced with substantially increased yields. 3-Octanol. Reactive overcoating with the chlorinated poly(siloxane) telomer increased the O2 consumption efficiency 4-fold to 0.048 mol of O2 per einstein (Table 1) and decreased the ratio of the number of molecules of CO2 evolved per O2 molecules consumed by a factor of 2.3 (Table 2). The dominant organic oxidation product was 3-octanone (Table 3). The products formed on bare TiO2 were consistent with •OH radical oxidation and the earlier proposed organohydrotetraoxide path15,16 and those on the silicone-overcoated photocatalyst with the following two-electron oxidation sequence:
CH3CH2CH(OH)(CH2)4CH3 S [CH3CH2CH(OH)(CH2)4CH3]surface [CH3CH2CH(OH)(CH2)4CH3]surface + h+ f [CH3CH2C(•)OH(CH2)4CH3]surface + H+ [CH3CH2C(•)OH(CH2)4CH3]surface f [CH3CH2CO(CH2)4CH3]surface + H+ + e-CB Conclusion When the TiO2 photocatalyst was reactively overcoated with a chlorinated silicone telomer, the O2 consumption per photon
2624 J. Phys. Chem. B, Vol. 101, No. 14, 1997 increased drastically in photocatalytic oxidations of an alcohol, an aldehyde, a ketone, and a carboxylic acid and the ratio of O2 consumed/CO2 produced decreased. These changes are explained by fast and efficient two-electron oxidation, involving hole oxidation and prompt electron injection. Two-electron oxidation yields either stable products or intermediates that, unlike the simple hole or OH radical reaction-initiated reaction intermediates, do not undergo efficiency-reducing homogeneous or heterogeneous radical combination reactions, equivalent to electron-hole recombination. Unlike the one-electron oxidations, where CO2 is produced via labile organohydrotetraoxide intermediates, two-electron oxidation reactions produce stable organic compounds or intermediates reacting with dissolved O2 or with water to form stable organic compounds. The results establish that by overcoating the photocatalyst it is possible to increase the efficiency of photocatalytic oxidation efficiencies in aqueous solutions well beyond those of the 4% quantum efficiency OH radical production20 and at the same time increase the yield of specific organic compounds. Acknowledgment. We acknowledge with thanks support of this work by the National Science Foundation and the Texas Advanced Technology Project. References and Notes (1) Frank, S. N.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 4667. (2) Maeda, Y.; Fujishima, A.; Honda, K. J. Electrochem. Soc. 1981, 128, 1731.
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