Langmuir 1998, 14, 3551-3555
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Photocatalytic Reduction of Nitro Organics over Illuminated Titanium Dioxide: Role of the TiO2 Surface John L. Ferry* Department of Chemistry and Biochemistry, University of Texas at Austin, A5300, Austin, Texas 78712
William H. Glaze Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, 7400, Chapel Hill, North Carolina 27599 Received October 1, 1997. In Final Form: March 22, 1998
The present study investigates the mechanism of photocatalytic reduction of nitro organics (aromatic and aliphatic) at the surface of titanium dioxide slurries (Degussa P25) in the absence of oxygen and in the presence of the sacrificial electron donor methanol (MeOH) or isopropanol (i-PrOH). Photocatalytic reduction was verified by identifying reduction products as the reaction progressed. Byproducts were observed to occur in the sequence RNO2 - RNO - RNH2; with the hydroxylamine intermediate present by inference but not detected directly. Mass balances ranged between 90+% and 70+%. Comparison of the reduction kinetics in aqueous i-PrOH vs aqueous MeOH revealed that secondary radicals generated from alcohol oxidation played no role in the observed reduction kinetics. For the nitroaromatic compounds studied, the values of the experimental reduction rate constants using i-PrOH as the electron donor divided by the rates using MeOH as the donor averaged 1.03 ((0.19), indicating that the photocatalyst surface was responsible for the observed chemistry. The nitroaliphatic compounds gave an average value of 1.63, with a wide standard deviation of 1.10.
Introduction The development of new processes for the transformation of organic compounds is an important goal for chemists pursuing environmentally “benign” or “green” chemistry. As an example, it may be desirable to develop processes based on a renewable energy source, viz. solar energy. Within that regime, solar-driven “photocatalysts”, catalysts whose chemistry is initiated by the absorption of a photon, may show promise. Accordingly, more information on the fundamental nature of these materials is needed. When a semiconductor such as TiO2 absorbs a photon and is promoted to an excited state, an electron is promoted from the semiconductor valence band to the conduction band, where it can function as a reducing agent, and leaves an electronic vacancy behind that is strongly oxidizing. The forward and reverse reactions are given in eq 1: hν
TiO2 798 h+ + ecb
(1)
The excited state of TiO2 is obtained by absorption of photons with wavelengths less than 365 nm, making it a material whose chemistry could potentially be driven by solar photons.1-3 Since photoexcited TiO2 may function as a strong oxidant (capable of producing the hydroxyl radical in water), a great deal of research into the potential applications of TiO2 as a photooxidant for the oxidation of organic materials has been reported.1,4 Much less attention has been paid to the reductive photochemistry of titanium dioxide, that is, chemistry (1) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (2) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. (3) Martin, C. A.; Baltanas, M. A.; Cassano, A. E. J. Photochem. Photobiol., A: Chem. 1993, 76, 199. (4) Mehos, M. S.; Turchi, C. S. Environ. Prog. 1993, 12, 194.
derived from conduction band electrons. Although they are potentially useful reagents, most photocatalysis studies have sought to remove conduction band electrons from the system using sacrificial electron acceptors such as dioxygen, and comparatively little research has been conducted on the fundamental nature of photocatalytic reduction. Compounds which have been photocatalytically reduced include dioxygen,5 halogenated alkanes,6,7 chloroethylenes,8,9 viologens,10 and nitro compounds.11 The photocatalytic reduction of organic electron acceptors such as carbon tetrachloride or nitroaromatics can be carried out in the presence of a large excess of an electron donor, such as isopropanol, and in the absence of oxygen.7,11 The purpose of the electron donor is to scavenge valence band holes, thereby reducing the degree of recombination within the particle and freeing more reductive equivalents. Removing oxygen from the system improves reduction efficiency by removing a competitive electron scavenger. It has been unclear in previous studies if the agent responsible for organic reduction was conduction band electrons (either directly or indirectly through trapped Ti(III)) or the R-hydroxyalkyl radicals generated during the one-electron oxidation of alcoholic donors. Such radicals are known to be powerful reducing agents, with (5) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (6) Bahneman, M. J.; Moenig, J.; Chapman, R. J. Phys. Chem. 1987, 91, 3782. (7) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1995, 27, 1646. (8) Glaze, W. H.; Kenneke, J. F.; Ferry, J. L. Environ. Sci. Technol. 1993, 27, 177. (9) Kenneke, J. F.; Ferry, J. L.; Glaze, W. H. The TiO2-Mediated Photocatalytic Degradation of Chloroalkenes in Water. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (10) Duonghong, D.; Ramsden, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1982, 104, 2977. (11) Mahdavi, F.; Bruton, T. C.; Li, Y. J. Org. Chem. 1993, 58, 744.
S0743-7463(97)01079-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/20/1998
3552 Langmuir, Vol. 14, No. 13, 1998
Figure 1. Reactor schematic: A, lamp housing; B, monochromator; C, reactor; D, reactor housing; E, magnetic stirrer; F, reactor assembly mounting; G, light path.
reduction potentials lower than -1.0 V vs the NHE, and are quite capable of reacting with halogenated organics and nitro organics at nearly diffusion-controlled rates.12 Thus, the present study was carried out with the principal purpose of determining the identity of the active reducing agent responsible for the reduction of substituted nitrobenzenes and nitropropanes in illuminated TiO2 slurries containing MeOH or i-PrOH. Materials and Methods Materials. 2-Nitropropane (2NP) (97%), 2-methyl-2-nitropropane (2M2NP) (99%), nitrobenzene (NB) (99+%), 4-nitrotoluene (4-NT) (99%), 3-nitrotoluene (3-NT) (99%), 4-nitrobenzonitrile (4-NBN) (97%), 3-nitrobenzonitrile (3-NBN) (98%), 4-aminotoluene (4-AT) (99%), 3-aminotoluene (3-AT) (99%), 4-aminobenzonitrile (4-ABN) (98%), 3-aminobenzonitrile (3-ABN) (98%), nitrosobenzene (NSB) (98%), aniline (AN) (99%), 2-aminopropane (2-AMP) (99%), 2-methyl-2-aminopropane (2M2AP) (99%), dibasic sodium phosphate (98%), sodium bicarbonate (98%), p-toluenesulfonyl chloride (97%), and tribromomethane (99%) were supplied by Aldrich Chemical Co. and used as supplied. Methyl tert-butyl ether (MTBE, GC2 quality), isopropyl alcohol (i-PrOH, GC2 quality), and methanol (MeOH GC2 quality) were supplied by Burdick and Jackson. Degussa P25 was a gift from Dr. Mike Prairie, of Sandia National Laboratories. ASTM grade water (Dracor) was used for all experiments and solutions. Reactor. The reactor used in these experiments was an Ace Glass Microphotochemical Schlenk reactor (Figure 1), mounted on an optical rail (Newport-Klanger) and illuminated by a 150-W Xe arc lamp (Osram). Light was filtered through a monochromator (Photon Technologies). The center of the slit was set at 350 nm with a (1.50-nm band-pass. Lamp output did not change over the time frame of the experiments, as determined by ferrioxalate actinometry. All glass components of the reactor were made of Pyrex. All wetted reactor components were glass, Teflon, or Viton. Ultrahigh-purity nitrogen (Sunox, 99.999%) was admitted to equalize pressure during sampling. Experimental Procedure. An aqueous suspension of TiO2 (0.1 wt % Degussa P25, 0.001 M LiClO4) was degassed under vacuum (5 µmHg) for 1 h. The stock suspension was stored in a glovebox under nitrogen and used for several experiments. All other stock solutions were treated similarly. Before the reactor was assembled, 50 mL of the suspension was added to the test tube body, followed by sufficient sacrificial (12) Neta, P.; Grodkowski, J.; Ross, A. B. J. Phys. Chem. Ref. Data 1996, 25, 709.
Ferry and Glaze donor (i-PrOH or MeOH) to make a 0.20 M alcohol solution. Substrates were added from concentrated isopropanolic or methanolic stock solutions, depending on which alcohol was being used as the donor for the experiment. The reactor was then sealed, moved out of the glovebox, and allowed to equilibrate under N2 for 30 min before illumination. For nitro organic analysis, samples (2.5 mL) were removed from the top sample port, using a 4-mL syringe (Manostat). Samples were immediately placed in 40-mL EPA vials (I-Chem) that contained 2.5 mL of chilled extraction solvent (MTBE with CHBr3 internal standard) and stored sealed at 4 °C until extraction. Extraction was carried out on a vortex mixer for 40 s. When byproduct analysis was desired, the vial contained 2.5 mL of chilled MTBE and 100 µL of saturated dibasic sodium phosphate to adjust the sample pH > 9. The organic layer of extracted samples was split into two samples, one for analysis by GC-ECD and the other for analysis by GC-ITMS. Analysis of Starting Materials (Hewlett-Packard 5890 GC-ECD). The injector port was set for splitless operation at 240 °C. The autoinjector volume was set at 3 µL. The analytical column was a 30-m DB-5, with a 0.25-µm film thickness. The temperature program was as follows: 4 min at 80 °C, ramp at 12 °C/min to 230 °C, ramp at 50 °C/min to 280 °C, and hold at 280 °C for 1 min. This analysis was useful for quantifying the loss of starting material but not for analysis of byproducts. Analysis of Byproducts (Varian Saturn II Ion Trap Mass Spectrometer). The injector port was set at 220 °C and was ramped to 260 °C 1 min after sample injection. The autoinjector volume was set at 1.5 µL. The analytical column was a 60-m DB-5 with a 0.25-µm film thickness. The oven program was as follows: 60 °C for 2 min and ramp at 15 °C/min to 240 °C. The ITMS was tuned with perfluorotributylamine before each use. The low boiling points and molecular weights of 2AMP and 2A2MP, the fully reduced forms of 2NP and 2M2NP, made it necessary to derivatize them to obtain their mass spectra. Qualitative byproduct determination was performed for 2AMP and 2A2MP by analyzing their p-toluenesulfonamide derivatives by GC-ITMS, using the same GC program as for the nitroaromatics.13 Results were verified by comparing the retention time and spectra of unknowns to those of prepared standards. Derivatization of unknowns and standards was performed by adding 30 mL of reactor contents (or an identical solution spiked with the corresponding amine, in the case of standards) to a 40-mL EPA vial and adding 250 µL of saturated NaHCO3 solution. This was immediately followed by 2 mL of MTBE spiked with bromoform as the internal standard and 1 g of NaCl to aid in extraction. After extracting for 1 min with a vortex mixer, 500 µL of a 0.2 M solution of p-toluenesulfonyl chloride in MTBE was added. The reaction mixture was kept at room temperature and extracted on a vortex mixer once every 30 min over a period of 2 h, after which the reaction was quenched with the addition of 750 µL of glacial acetic acid. The MTBE extracts were removed and analyzed. Analytical Standards. Standards were dissolved in MeOH (GC2 grade) and spiked into an aqueous matrix that was a duplicate of the experimental sample matrix (2.5 mL of 0.2 M alcohol, 0.1 wt % TiO2, 0.001 M LiClO4). Standards were extracted in an identical manner to samples.
Results and Discussion Figure 2 shows the decrease in the concentration of nitrobenzene and the formation of two reduction products as a result of photocatalytic reduction. Figures 3 and 4 show similar data for 4-nitrobenzonitrile and 4-nitrotoluene. Product formation and mass balances were determined similarly for 3-nitrotoluene, 3-nitrobenzonitrile, and 4-nitrobenzonitrile. Qualitative byproduct determination was performed for the compounds 2-nitropropane and 2-methyl-2-nitropropane, without quan(13) Fielding, M.; Hutchison, J.; Hughes, D. M.; Glaze, W. H.; Weinberg, H. S. Analytical Methods for Polymers and their Oxidative Byproducts; AWWARF, Denver, CO, 1996.
Reduction of Nitro Organics over Illuminated TiO2
Figure 2. Photocatalytic reduction of nitrobenzene: pH 4.85, 0.20 M i-PrOH, 0.001 M LiClO4, 0.1 wt % TiO2; [, nitrobenzene; 9, aniline; 2, nitrosobenzene; b, sum of all species detected.
Figure 3. Photocatalyic reduction of 4-nitrobenzonitrile: pH 4.85, 0.20 M i-PrOH, 0.001 M LiClO4, 0.1 wt % TiO2; [, 4-nitrobenzonitrile; 9, 4-aminobenzonitrile; 2, sum of all species detected.
Figure 4. Photocatalytic reduction of 4-nitrotoluene: pH 4.85, 0.20 M i-PrOH, 0.001 M LiClO4, 0.1 wt % TiO2; [, 4-nitrotoluene; 9, 4-aminotoluene; 2, sum of all species detected.
tifying those same products. Illumination of the suspensions apparently resulted in the sequential reduction of the nitro group (there was no evidence for ring reduction), as shown for nitrobenzene in Scheme 1. This is in agreement with the hypothesis of Pelizzetti and coworkers, that the photocatalytic reduction of nitroaromatics involves essentially no net loss of organic nitrogen, whereas the photocatalytic oxidation of nitroaromatics generates inorganic nitrogen products (NH3, NO3-).14 The hydroxylamine intermediates were not detected by our analytical techniques (GC-MS); however, Mahdavi and co-workers detected hydroxylamine formation during the photocatalytic reduction of p-nitroacetophenone in neat 2-propanolic suspensions of TiO2 (using HPLC techniques), so the difference between the predicted and observed mass balance was assigned to the hydroxylamine intermediate.11 Although speculative, this approach is supported by the following observations: (a) p-nitroacetophenone is structurally quite similar to the nitroaromatics included in this study and therefore an acceptable analogue, and (b) nitroso intermediates typically do not accumulate to significant levels during the transition metal-catalyzed (14) Paola, P.; Minero, C.; Vincenti, M.; Pelizzetti, E. Catal. Today 1997, 39, 187.
Langmuir, Vol. 14, No. 13, 1998 3553
reduction of nitroaromatics.15 However, this does not rule out the possibility that some of the discrepancy could arise from the presence of some other undetected intermediate. All of the observed species and the calculated mass balances are tabulated in Table 1. It is notable that the percentage value assigned to the hypothesized hydroxylamine intermediate increases with the acidity of the hydroxylamine proton. Presumably, the electron-withdrawing nature of the substituent increases the acidity of the hydroxylamine hydrogen, increasing the electron density on the nitrogen and making reduction less favorable. This hypothesis is supported by a plot of presumed hydroxylamine yield against the Hammet σ value for the substituent, as is shown in Figure 5 (r2 ) 0.99). This implies a strong dependence of the inductive electron-withdrawing properties of other ring substituents on the rate of the reaction (RNHOH f RNH2), although the rates of amine production could not be determined in such a kinetically complex system. It suggests that the nitro compounds that are most easily reduced initially may be the most difficult to fully reduce to the corresponding amine. This is consistent with the observation that electron-withdrawing groups hinder the full reduction of nitroaromatics by a variety of ruthenium complexes.16,17 Accordingly, the complete photocatalytic reduction of the nitro group on compounds such as TNT or nitrated azo dyes is likely to prove difficult. The nitropropanes 2-nitropropane and 2-methyl-2nitropropane appeared to produce very little amine product (
