J. Phys. Chem. C 2008, 112, 8311–8318
8311
Selective Photocatalytic Decomposition of Nitrobenzene Using Surface Modified TiO2 Nanoparticles Donald Cropek,*,† Patricia A. Kemme,† Olga V. Makarova,‡ Lin X. Chen,‡ and Tijana Rajh*,‡ U.S. Army Corps of Engineers, Construction Engineering Research Laboratory, Champaign, Illinois 61822, and Chemistry DiVision and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: December 28, 2007; ReVised Manuscript ReceiVed: March 3, 2008
Adsorption and photocatalytic degradation of nitrobenzene (NB) in the presence and absence of phenol (Ph) over UV-illuminated arginine-modified TiO2 colloids have been investigated by infrared absorption, electron paramagnetic resonance spectroscopy, and X-ray absorption spectroscopy. High performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry were used for monitoring degradation conversion rates and byproduct identification. It was found that photodegradation of NB and Ph strongly depends on the nature of the TiO2 surface. Through the use of the HPLC peak area ratio before and after illumination, the photocatalytic decomposition rate of NB and Ph individually using bare TiO2 is nearly identical (1.7 and 1.5, respectively) and occurs via oxidative mechanism. Through the use of arginine-modified TiO2 nanoparticles, a three-fold increase in the NB decomposition rate is observed while no Ph decomposition is observed. Furthermore, the degradation pathway using the arginine-modified photocatalyst is completely altered to a reductive mechanism, providing a more efficient means to degrade nitrocompounds that are already in a highly oxidized state and limiting the number of byproduct. These results indicate that a critical parameter in the photocatalytic decomposition of NB and Ph is their specific adsorption and coupling to the TiO2 surface. Modification of the TiO2 particle surface with chelating agents demonstrates enhanced interaction with the desired target contaminant to impart selectivity to photocatalysis. Introduction The United States military is the dominant user of energetic and explosive materials as well as the driver for development of new munitions formulations. Nearly all of the mature explosives as well as the nascent compounds currently in testing have multiply nitrated structures that can rapidly decompose under conditions of high temperature, high pressure, or impact.1 The most widely used class of explosives is the nitroaromatic compounds that include benzene, toluene, and phenol with varying degrees of nitration.2 Issues surrounding pollution prevention from these compounds and end-of-pipe treatment technologies are of continual interest to Army environmental research. Waste streams generated from the production, storage, and demilitarization of munitions can be contaminated with these energetic compounds3 and, by the Resource Conservation and Recovery Act (RCRA) definition, characterize these streams as hazardous.4 The nitroaromatic compound is frequently present in only trace amounts yet it contributes the bulk of the toxic nature. For instance, the presence of as little as 0.1 ppm of any nitrobody in wastewater is sufficient to define this as a hazardous waste according to K044 RCRA identified waste streams.4 Targeted decomposition of nitroaromatic components is attractive to focus the treatment effort at only the toxic component, increase the remediation rate by ignoring the more innocuous constituents, and permit possible recycle of less hazardous constituents.5 Many treatment processes rely on an oxidative pathway due, in large part, to the high reactivity of the hydroxyl radical and * To whom the correspondence should be addressed. E-mail: (T.R.)
[email protected]; (D.C.)
[email protected]. † U.S. Army Corps of Engineers. ‡ Argonne National Laboratory.
the potential for complete mineralization of contaminants to nontoxic end products of carbon dioxide, nitrogen oxides, and water.6–8 Unfortunately, highly nitrated energetic compounds tend to be resistant to oxidative degradation.9–13 As such, advanced oxidation processes that produce reactive hydroxyl radicals such as Fenton’s reagent, UV/ozone, UV/H2O2, wet air oxidation, and ultrasound have had only limited success in treating energetic compounds.7,14–17 The combination of a contaminant with a semiconductor photocatalyst, followed by illumination with photons energetic enough to produce an electron/hole pair, is also a well-known treatment technique for purification of air and water.18–20 Titanium dioxide, TiO2, is the most common semiconductor photocatalyst and has been used for degradation of energetic compounds.21–25 The resistance of highly nitrated energetic compounds to oxidative degradation as well as the complex array of byproduct has swayed research toward the reductive degradation of these compounds. While oxidative methods tend to attack the cyclic structure in energetic compounds, reductive techniques aim at the nitro group. Reductive methods such as zero-valent iron, electrolytic reduction at electrodes, and bioremediation have been shown to chemically transform energetic contaminants.26–34 These methods are more efficient for controlled degradation of the energetic nitrocompound and produce known end products. Furthermore, reductive methods result in byproducts that are more amenable to secondary biological treatment.9,27,33 Our previous work has illustrated that surface modification of nanoparticle TiO2 photocatalyst with arginine completely alters the degradation pathway from oxidation to reduction during treatment of nitrobenzene (NB) as compared with bare TiO2.35 Arginine acts as a hole trap to prevent electron/hole recombination and transfers electrons from the conduction band of TiO2 to NB with small activation energy.
10.1021/jp712137x CCC: $40.75 2008 American Chemical Society Published on Web 05/02/2008
8312 J. Phys. Chem. C, Vol. 112, No. 22, 2008 This paper reports the competition between NB and phenol (Ph) for adsorption and photodecomposition with bare and arginine-modified nanocrystalline TiO2. NB serves as a substitute for the more highly nitrated explosive compounds such as 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT). From previous work, arginine was an excellent amino acid surface modifier for the reduction of NB and is therefore used in this work.35 Ph is chosen as a competing contaminant due to its simple molecular structure and its well-studied oxidative degradation over TiO2. Fourier transform infrared spectroscopy (FTIR) is used to study adsorption of NB and Ph onto the modified TiO2 surface. Electron paramagnetic resonance spectroscopy (EPR) was used to identify radical species formed at the illuminated TiO2 surface and to study the decomposition mechanism. Chemical analysis of the degradation byproduct was performed using high performance liquid chromatography (HPLC) and gas chromatography/mass spectrometry (GCMS). Experimental Section Photocatalyst Preparation. All chemicals used (SigmaAldrich, Milwaukee, WI) were reagent grade and used without further purification. Triply distilled water was used. Colloidal TiO2 was prepared by dropwise addition of titanium(IV) chloride to ice cold water followed by TiCl4 dialysis against distilled water at 4 °C as previously described.36 The pH of the colloid after dialysis was 3.5, and the mean particle size of TiO2 particles was 45 Å as determined by Rajh et al.36 The concentration of TiO2 in the final solution was determined from the concentration of the Ti(IV) peroxide complex obtained after dissolving the colloid in concentrated sulfuric acid.37 Surface modification of nanocrystalline TiO2 colloid with L-arginine (Arg) was performed as described previously.35 An aqueous solution of Arg was added to the TiO2 colloid and allowed to stand at least two hours for complete complexation. NB and Ph aqueous solutions were added to the bare or surfacemodified TiO2 colloids and allowed to stand for 24 h to ensure complete equilibrium. The final concentrations were [TiO2] ) 0.1 M, [Arg] ) 3 × 10-2 M, [NB] ) 4 × 10-4 M (∼50 ppm), [Ph] ) 5 × 10-4 M (∼50 ppm). These colloids were used for EPR study and steady-state illumination. EPR Experiments. EPR spectra were recorded on a Varian E-9 EPR spectrometer (Palo Alto, CA) in a setup described in detail elsewhere.35 Samples at 10 K were illuminated directly in the spectrometer cavity with a LX 300 UV xenon lamp (Atlas Specialty Lighting, Tampa, FL) for 10 min. Charge separation at elevated temperatures was determined by illuminating a sample at 10 K then warming it in the dark to 80, 120, and 200 K. The sample was cooled to 10 K between each warm-up step to obtain a high signal-to-noise ratio. The background EPR signal was measured for each sample before illumination. The g-values were calibrated by comparison to the Mn2+ standard in a SrO matrix (g ) 2.0012 + 0.0002).38 Diffuse Reflectance Fourier Transform (DRIFT) Measurements. Bare and surface-modified TiO2 colloids were filtered through a cellulose membrane (YM 10, Millipore, Bedford, MA), washed with water while on the filter and dried. Samples were mixed with dried KCl (10 wt % sample in KCl), spread on watch glasses, and placed in a desiccator that also housed separate watch glasses with 2.5 µL of NB or 3 × 10-3 g of Ph for 24 h to examine vapor phase adsorption. FTIR measurements were performed on a Nicolet 510 (Madison, WI) Fourier transform infrared spectrometer equipped with a SpectraTech Inc. (Stamford, CT) diffuse reflectance accessory. The resolution for these experiments was 4 cm-1. Typically, 100
Cropek et al. scans were averaged for each spectrum. All results are presented as normalized Kubelka–Munk plots. XAS. X-ray absorption near-edge spectroscopy (XANES) measurements were carried out at a bending magnet beamline, 12BM, of the Advanced Photon Source at Argonne National Laboratory. Si (111) crystals were used in the double-crystal monochromators. The incident X-ray intensity was kept constant via a feedback system with 30% detuning to remove higher order harmonics. All spectra were collected at room temperature in the transmission mode. The monochromator energy was calibrated with the K-edge position of a Ti foil at 4.996 keV and was reproducible within 0.1 eV between the scans by simultaneously collecting the transmission of the sample and the reference foil. The samples were ground into fine powders and pressed into thin pellets. Each sample was sandwiched between two pieces of Kapton tape. The data analysis was carried out with WinXAS.39 Each spectrum was normalized by the edge jump defined by the pre-edge fit with a linear function and the postedge fit with a third order polynomial. The calculated XANES spectra were produced based on the Cartesian coordinates of anatase using the FEFF8.2 program.40 Photocatalytic Degradation. The photodegradation of NB and Ph over illuminated TiO2 colloids was determined from the comparison of the NB and Ph concentrations in the sample before and after illumination. Steady-state illumination of a magnetically stirred sample was performed for 30 min using a LX 300 UV xenon lamp. The total photonic flux over the 250-400 nm wavelength regime was 7.5 × 10-5 einstein min-1. The sample was centrifuged, and the NB and Ph concentrations in the supernatant were determined by chromatographic analysis. HPLC analysis was done on a Waters LC Module 1 (Milford, MA) with a Supelco ABZ+ column (Bellefonte, PA), 150 mm × 4.6 mm. The mobile phase was 70% water/30% acetonitrile, and the detection wavelength was 254 nm. The aqueous solutions of the samples were extracted three times with CH2Cl2. The organic extracts were concentrated in conical vials under a stream of nitrogen and analyzed by gas chromatography/mass spectrometry (GCMS) using a Hewlett-Packard 6890 GC/5973 MSD (Palo Alto, CA) equipped with a HP5MS (5% phenyl/ 95% methyl) 30 m × 0.25 mm × 0.25 µm capillary column. The oven temperature was programmed as follows: isothermal at 40 °C for 3 min, from 40 to 300 °C at 10 °C/min, and isothermal at 300 °C for 1 min. Results and Discussion Photocatalytic Activity. The photocatalytic activity of the bare and arginine-modified TiO2 for NB or Ph degradation was measured and the ratio of the initial contaminant concentration to the final contaminant concentration after illumination was compared. The initial and final concentrations were determined using the HPLC peak area. The measured ratios between initial contaminant concentrations and the final concentrations after 30 min of illumination for different samples are presented in Table 1. Bare TiO2 easily degrades both NB and Ph with similar efficiencies, 1.7 and 1.5, respectively. As noted previously35 and in agreement with others,41–44 the GC analysis of the byproduct of NB photocatalytic degradation includes primarily nitrophenols but also small amounts of benzoquinone, phenol, and aniline; dinitrobenzene has also been observed indicating that both reductive and oxidative pathways are present. Little control is exerted over the degradation mechanism resulting in this wide array of transformation products. Ph is also readily degraded at the bare TiO2 surface. Hydroxylation of the aromatic ring is
Selective Photocatalytic Decomposition of Nitrobenzene TABLE 1: Ratio of HPLC Peak Areas of the Initial Compound Concentration to the Final Concentration after 30 Minutes Of Illumination sample
NBbefore/NBafter
TiO2 + NB TiO2 fresh + NB TiO2 + NB, degassed TiO2 + Ph TiO2 + NB +Ph Arg-TiO2 + NB Arg-TiO2 fresh + NB Arg-TiO2 +NB, degassed Arg-TiO2 + Ph Arg-TiO2 + NB + Ph
1.7 1.2 1 (no reaction) 1.5 4.7 2.6 5.7 6.2
Phbefore/Phafter
1.5 1.4
1 (no reaction) 1.1
the primary reaction to produce hydroquinone. Dimerization products are also observed including phenoxyphenol and dihydroxybiphenyl. These oxidative byproducts are typical of phenol photocatalytic degradation.45–47 Illumination of a bare TiO2 slurry with a combination of both NB and Ph in solution leads to the same byproduct and only slight decreases in the degradation ratios due to competitive adsorption onto active surface sites. This negligible effect is in contrast to the conflicting results of Bhatkhande et al.,48 who found that NB degrades much faster than Ph due to improved coupling of NB to TiO2, and Maillarddupuy et al.44 who noted a decrease in NB degradation in the presence of Ph due to greater Ph adsorption. The use of arginine-modified TiO2 increases the photodegradation efficiency of NB by nearly a factor of 3 from 1.7 to 4.7 and only the single byproduct, aniline, is observed indicating that the oxidative pathway for nitroaromatic degradation is eliminated. Only the reduction pathway of nitro to amino group is active; furthermore, the absence of the nitroso and hydroxyamino intermediates indicates how facile this pathway becomes in the presence of the arginine as a surface modifier. Illumination of arginine-modified TiO2 has no effect on Ph in solution (ratio of Phbefore/Phafter ) 1) thus illustrating a selectivity factor for nitroaromatic compounds. Because Ph is readily oxidized at the bare TiO2 surface, it is deduced that (1) arginine has efficiently blocked the TiO2 surface, (2) there is no electronic coupling between Ph and arginine, and (3) while arginine acts as a hole trap, it does not transfer photogenerated holes to phenol as it does to methanol.35 During illumination of the arginine-modified TiO2 in the presence of both NB and Ph, NB is transformed to aniline at an even faster rate than in the solution containing NB alone; the ratio NBbefore/NBafter becomes 6.2 in the presence of Ph versus 4.7 in the absence of Ph. Fu et al.49 has noted that the presence of Ph during the wet air oxidation of NB significantly enhances the oxidative degradation of NB due to creation of Ph-initiated free radicals that can take part in NB transformation. However, this is an unlikely scenario in our measurements because no transformation or degradation of Ph was observed. Currently, we do not have an explanation for this increased rate, but coadsorption of phenol may induce redistribution of the electron density in NB molecules to facilitate its reductive degradation. It is worth noting that reductive degradation of NB to aniline occurs even in the presence of oxygen that has high affinity for photogenerated electrons. Experiments were performed using solutions purged with nitrogen to rid the system of oxygen. Under these anaerobic conditions, NB oxidation over bare TiO2 is completely suppressed (NBbefore/NBafter ratio ) 1) indicating that the fast reaction of oxygen with conduction band electrons
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8313 is a critical step for efficient oxidation of contaminants, because it suppresses electron–hole recombination and increases the amount of photogenerated holes at the TiO2 surface available for oxidative photodecomposition of NB. On the other hand, removing oxygen from the arginine-modified TiO2 system increases the rate of NB reduction to aniline from 4.7 to 5.7 as expected for the reactions that proceed via reductive chemistry. The Role of Nanoparticle Aging in Photocatalytic Activity of TiO2. It should be pointed out that the yield of photodegradation depends on the treatment of the nanoparticles after preparation; aged and thermally treated nanoparticulate TiO2 is a more active photocatalyst than freshly prepared colloids. Freshly prepared TiO2 photodecomposes NB with an efficiency of 1.2 compared to 1.7 for aged TiO2. Similarly, argininemodified fresh TiO2 degrades NB with an efficiency of 2.6 compared to 4.7 for arginine-modified aged TiO2. We have employed EPR spectroscopy to understand processes associated with aging of TiO2 nanoparticles. EPR spectroscopy has been widely used to examine paramagnetic species formed upon band gap excitation of TiO2 including colloidal anatase particles,50–56 anatase and rutile powders,57–61 as well as mixed phase titania such as Degussa P25.62,63 When nanocrystalline anatase TiO2 particles are illuminated at liquid helium temperatures, stable photogenerated holes and electrons at 10 K are observed due to suppressed diffusion of photogenerated radicals that results in slow radical recombination (Figure 1a). The width and the shape of this EPR signal directly reports on the distribution of the coordination environment around photogenerated radicals and can be used to monitor the crystalline quality of nanoparticles. In principle, two sets of signals are observed for TiO2 nanoparticles at helium temperatures: g > 2 for trapped holes, and g < 2 for trapped electrons (Figure 1a). The signal for photogenerated holes consists of the oxygen centered radical with gx ) 2.024, gy ) 2.014, and gz ) 2.007,52 while the signal for trapped electrons is composed of axially symmetric signals for bulk Ti(III) in anatase at g⊥ ) 1.988 and g| ) 1.957 and the much smaller signals for surface Ti(III) with g⊥ ) 1.925 and g| ) 1.885.50 Figure 1a shows that the signal of radical species formed upon illumination of the aged nanoparticulate TiO2 has a much more intense signal for photogenerated electrons than freshly made TiO2. Careful investigation of the EPR spectra of the colloids at different times after preparation shows that as the particles age, the signal becomes more intense. One of the reasons for signal enhancement is the increase in the domains of crystallinity and ordering within nanoparticles upon aging. It has been shown recently that because of the small diffusion length in nanoparticles, weakly bound defect lattice sites are easily healed upon aging,64,65 and as a consequence bond lengths and bond angles in which photogenerated radicals reside become ordered, removing broadening of the EPR signals.66 Removing the heterogeneous broadening causes the signals of different radicals to overlap, contributing to the signal sharpness and enhancement of the signal intensity. We explored this model by direct determination of the change of the local crystal environment of Ti with aging by using XANES. Figure 1b shows XANES spectra of the fresh and aged surface passivated TiO2 nanoparticles (4.5 nm as determined by TEM) in conjunction with the spectrum of bulk anatase. The main difference in the spectra occurs in the region of D1 and D2 transitions that are sensitive to long-range crystallinity.67,68 These transitions broadened in the fresh sample due to the structural heterogeneity but they emerge with aging when the crystallinity increases. Modeling of XANES spectra suggests that upon aging, the domain of crystallinity increases and when crystallization domains reach
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Cropek et al.
Figure 2. DRIFT spectra of TiO2 samples before (solid line) and after (bold) adsorption of NB vapor on bare TiO2 (black) and argininemodified TiO2 (red). In each case, the lower spectrum is before adsorption and the upper spectrum is after adsorption of NB. The nitro group stretching bands are clearly marked by arrows. DRIFT spectra of NB saturated TiO2 samples after 45 h in air is also shown (dashed lines); bare TiO2 (black) and arginine-modified TiO2 (red).
Figure 1. (a) EPR spectrum of freshly prepared (black) and aged (red) nanoparticulate TiO2 at liquid helium temperatures. (b) XANES spectrum of freshly prepared (black) and aged (red) nanoparticulate TiO2 and bulk anatase (orange) in conjunction with simulated XANES spectra for anatase clusters with sizes 8 and 15 Å.
∼1.5 nm, the transitions associated with higher lying p states sharpen, leading to resolved D1 and D2 features in XANES spectra and, concomitantly, to the fully developed bulk-like EPR spectrum of TiO2 nanoparticles. Adsorption of NB and Ph from Vapor Phase. Adsorption of reactants on nanoparticle surfaces is a necessary (but not sufficient) step for efficient photocatalysis. To understand the mechanism of photocatalytic degradation, information on the availability of the reactants for photogenerated charges must be obtained. Adsorption of NB, Ph, and their mixture from vapor phase onto bare and surface-modified nanocrystalline TiO2 was studied using FTIR spectroscopy. The spectra were taken after 24 h of photocatalyst exposure to the NB/Ph vapor in the closed desiccator. Quantitative determination of species from DRIFT IR spectra is rather complicated because of dependence of the adsorption band intensity on many parameters such as light penetration depth and self-absorption.69 Usually one of the least affected bands is chosen as a reference band for normalization of the spectra. To compare different samples, our assumption was that samples from the same preparation batch contain the same amount of adsorbed water, and all spectra were normalized to
the adsorbed water band in the region of 3000–3500 cm-1. The relative intensity of the asymmetric and symmetric NO2 stretching bands at 1525 and 1347 cm-1 was used as a measure of adsorption capacity. Figure 2 shows the DRIFT spectra of aged bare and arginine modified TiO2 taken both before and immediately after removing the sample from the desiccator containing NB vapor. The presence of NB on the samples is clearly seen in these samples, and based on the difference in absorbance before and after exposure of NB onto bare TiO2 is comparable to the electron donor/electron acceptor interaction of arginine and NB in aged samples. However, freshly prepared samples show much weaker adsorption indicating that a welldeveloped crystalline surface is needed for efficient adsorption of NB. The strength of the surface complex was estimated from the decrease in the nitro group intensity after 45 h exposure to air, and the spectra are shown as dashed lines in Figure 2. NB peaks are substantially reduced but still visible on the bare TiO2, whereas on Arg-TiO2 they are broad and barely visible, suggesting a weaker chemical attraction of NB to Arg compared to bare TiO2. Adsorption of Ph vapor onto dried bare TiO2 turns this white colloid to a bright yellow color suggesting strong charge transfer interaction of phenols with the TiO2 surface.70 The DRIFT spectra of Ph sorbed onto bare TiO2 (Figure 3) shows the presence of Ph-stretching modes undergoing changes in the hydroxyl group vibration due to coordination to the titanium. This monodentate configuration creates an efficient charge transfer complex similar to the one obtained in salicylate35 or enediol-modified TiO2.71 This chemisorption is strong, and there is no loss of Ph over time. As shown in Figure 3, adsorption bands of Ph at 3030 cm-1 and 1480 cm-1 assigned to CH stretching and bending of the benzene ring remain unchanged, while the Ar-O stretching vibration at 1225 cm-1 is shifted to 1260 cm-1 due to binding to a heavy mass (Ti atoms). The OH
Selective Photocatalytic Decomposition of Nitrobenzene
Figure 3. DRIFT spectra of TiO2 samples before and after adsorption of Ph vapor. (a) Ph crystals, (b) bare TiO2, (c) bare TiO2 + Ph, (d) arginine-modified TiO2 + Ph, and (e) arginine-modified TiO2. Characteristic Ph bands are marked by dotted lines.
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8315
Figure 5. EPR spectra of bare nanoparticulate TiO2 in the presence of Ph at different times after illumination from 1 to 60 min. Illumination at 10 K leads to formation of phenoxyl radicals and reduced Ti centers in TiO2 at short times of illumination (up to 10 min). Phenoxyl radical signals are denoted by dashed vertical lines. Subsequent illumination leads to the change of hole centers into resorcinol radicals denoted by solid vertical lines.
SCHEME 1: Schematic Presentation of Phenol Oxidation with Nanoparticulate TiO2 Proposed from Low Temperature EPR Measurements
Figure 4. EPR spectra of bare nanoparticulate TiO2 in the presence of NB at different temperatures from 10 to 200 K. Illumination at 10 K leads to the formation of holes and electrons in TiO2 that recombine upon annealing.
stretching bands in the region of 3500 and 1358 cm-1 disappear upon Ph adsorption onto TiO2. The color of arginine-modified TiO2 changed only slightly from white to very light yellow upon exposure to Ph vapor indicating weaker adsorption of Ph to Arg-TiO2. Adsorption bands characteristic for Ph at 1480 and 1230 cm-1 are also far less intense for arginine-modified TiO2 compared with the bare TiO2. Arginine is a poor attractor for Ph and protects the TiO2 surface from Ph chemisorption. Adsorption of Ph on argininemodified TiO2 was also weak when coadsorbed with NB in the vapor phase (not shown). EPR Experiments on NB and Ph Degradation with Bare or Arginine-Modified TiO2. In the presence of organic molecules that interact with photogenerated electrons or holes, the EPR spectrum changes, reflecting the reactions that occur
at the TiO2 surface. The presence of NB does not affect the EPR signals at liquid helium temperatures for photogenerated electrons and holes that remain confined to the TiO2 lattice (Figure 4). Although there is significant adsorption of NB on the bare TiO2 surface, there is no charge transfer at 4 K or at elevated temperatures (
