J. Phys. Chem. C 2008, 112, 825-831
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Kinetics of Photoelectrocatalytic Degradation of Nitrophenols on Nanostructured TiO2 Electrodes Min Tian, Guosheng Wu, Brian Adams, Jiali Wen, and Aicheng Chen* Department of Chemistry, Lakehead UniVersity, Thunder Bay, Ontario P7B 5E1, Canada ReceiVed: September 7, 2007; In Final Form: October 28, 2007
In the present work, titanium oxide (TiO2) nanotubes were directly grown by the electrochemical oxidation of titanium substrates at 20 V in a nonaqueous electrolyte (DMSO/HF). The morphology and microstructure of the synthesized TiO2 photocatalysts were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The kinetically photoelectrochemical degradation of 2-nitrophenol (2-NPh) and 4-nitrophenol (4-NPh) at the TiO2 nanotubes was examined, individually and when they were mixed together. It was found that the photoelectrochemical degradation of 2-NPh is faster than that of 4-NPh. In order to determine the kinetic behavior of 2-NPh and 4-NPh in the course of the photoelectrocatalytic oxidation of their mixtures, an experimental design with 36 samples and a test set with 6 samples were used to build up a partial leastsquares (PLS) model. The degradation of both 4-NPh and 2-NPh became slower in the binary mixture compared with the individual degradation rates of the 4-NPh and 2-NPh. The present work has demonstrated that UVvis spectroscopy coupled with PLS calibration can be used to in situ monitor the concentration changes, providing a novel approach to determine the competitive effects of different organic pollutants during water purification and wastewater treatment.
1. Introduction This paper describes the kinetics of the photoelectrocatalytic degradation of nitrophenols on titania (TiO2) nanotubes studied by in situ UV-vis spectroscopy and chemometrics. Nitrophenols are involved in the synthesis of many chemicals, particularly pesticides. They are widely present in waste streams originating from the manufacturing activities of the mining, plastics, and pharmaceutical industries and from the degradation of pesticides like parathion and nitrofen. Nitrophenols have been listed by the U.S. Environmental Protection Agency as priority pollutants. Thus, the removal, destruction, or modification to less noxious structures of nitrophenolic compounds is necessary for the purification of both wastewater and groundwater. Traditional wastewater treatment techniques include electrochemical oxidation,1-4 activated electrosorption,5 chemical oxidation, and biological digestion.6 The degradation of nitrophenols is difficult as the presence of the nitro group increases the resistance of the phenolic compounds to traditional treatment techniques. Heterogeneous photocatalysis is a promising alternativetechniqueforeliminatingorganicpollutantsfromwastewater.7-9 Among various oxide semiconductor photocatalysts, titania (TiO2) is one of the most promising photocatalysts because of its biological and chemical inertness, cost effectiveness, and the strong oxidizing power of the photogenerated holes.10-12 The generation of electron (e-)-hole (h+) pairs is initiated upon irradiation of photons with energy greater than or equal to the band gap energy of the photocatalyst. The photogenerated electrons and holes can either recombine in the bulk or migrate to the surface to initiate various redox reactions. A high surface area is thus desirable to increase photocatalytic efficiency. In addition, it is known that the recombination between the photogenerated charge carriers can be effectively suppressed * Corresponding author. Fax: 1-807-346-7775. E-mail: aicheng.chen@ lakeheadu.ca.
by applying an external anodic potential bias, thus greatly increasing the efficiency of the purification process.13,14 In this study, we have directly grown TiO2 nanotubes on Ti substrates for the photoelectrochemical degradation of nitrophenols. In addition, more than one organic compound is frequently present in wastewater; thus, competitive effects are expected in wastewater treatment. However, very few studies have addressed the competitive effects,15,16 and no kinetic information has been reported on the photoelectrocatalytic degradation of a mixture of 2-nitrophenol (2-NPh) and 4-nitrophenol (4-NPh). Competitive adsorption of phenoxide anion and phenol at higharea carbon felt electrodes has been studied using in situ UV spectroscopy.5 This approach is only applicable for reaction systems where the UV peaks of different species involved are well-separated. However, in UV absorption spectra, most bands overlap; multivariate calibration methods are thus often required in order to obtain useful information from large amounts of complex measurements. Among the multivariate methods, principal component regression (PCR) and partial least-squares (PLS) have been proposed and applied in multicomponent determinations. PLS possesses two important features: (i) it uses whole kinetic curves at a number of wavelengths in both the calibration and the predication modes, and (ii) it can be applied to chemical systems with a high degree of nonlinearity. Here, we report on the photoelectrocatalytic degradation of 2-NPh, 4-NPh, and their mixture using TiO2 nanotubes. In situ UVvis spectroscopy coupled with PLS was used, for the first time, to determine the kinetics and the competitive effects of the photoelectrochemical degradation of the mixture of 4-NPh and 2-NPh. 2. Experimental Section 2.1. Materials and Chemicals. 4-NPh (99%), 2-NPh (98%), and maleic acid (99%) were purchased from Aldrich and used
10.1021/jp077191d CCC: $40.75 © 2008 American Chemical Society Published on Web 12/29/2007
826 J. Phys. Chem. C, Vol. 112, No. 3, 2008 as received. All other chemicals were of reagent grade and were used as supplied. The water (18.2 MΩ cm) used to prepare all solutions was purified by a Nanopure Diamond water system. Stock solutions were made by dissolving the NPhs in a 0.5 M NaOH solution. Subsequent concentrations were obtained by diluting the stock solution with a 0.5 M NaOH solution. 2.2. Instrumentation. An EG&G 2273 potentiostat/galvanostat was used to apply an anodic potential bias during the photodegradation of the nitrophenols. TiO2 nanotubes synthesized in this study were used as the working electrode (Ti/TiO2). The geometric surface area of the working electrode was 1 cm2. The counter electrode was a Pt coil. Before each experiment, the counter electrode was cleaned by flame annealing and then was quenched with pure water. The reference electrode was a Ag/AgCl electrode. An anodic potential, 0.6 V (Ag/AgCl), was applied to the nanostructured Ti/TiO2 photoelectrode during the photodegradation of nitrophenols to suppress the recombination of photogenerated electrons and holes. The solution in the cell was continuously stirred with a small magnetic stirrer bar. An ADAC Systems Cure Spot 50 UV spot lamp was used for excitation of the TiO2 nanotubes. The primary wavelength distribution of the lamp was at 365 nm, and the intensity of the UV light was approximately 2 mW cm-2. The UV light was introduced into the cell using a fiber optic cable and placed above the electrode. The distance between UV fiber optic cable and the electrode surface was 1.5 cm. All of the photocatalytical experiments were carried out at room temperature and in the presence of air. In situ UV-visible spectra were recorded by an EPP2000C spectrometer coupled with a dipping probe connected by fiber optic cables. Qualitative determinations of reaction intermediates produced during the photodegradation of 2-NPh and 4-NPh were carried out by high performance liquid chromatography (HPLC) using a Varian Prostar chromatograph equipped with a symmetry C8 HPLC column. A mixture of acetonitrile (20%) and HPLC grade ultrapure water (80%) was used as the eluant at 0.8 mL/min flow rate. The intermediates were identified by comparison with standards. The detection wavelength was 270 nm. The volume of the samples injected was 20 µL. The change of the total organic carbon (TOC) of the 2-NPh and 4-NPh before and after the photodegradation was measured using a Thermo ELS2100 total organic carbon analyzer. The volume of the samples automatically injected was 100 µL. 2.3. Preparation and Characterization of the Synthesized TiO2 Photocatalysts. Pure Ti substrates were degreased by being sonicated in acetone for 10 min and then in the pure water for another 10 min. The Ti substrates were then etched in 18% hydrochloric acid at 85 °C for 15 min. The etched titanium plate was rinsed thoroughly with pure water and then anodized in DMSO with 2% HF at 20 V.17 After electrochemical treatment, the samples were rinsed thoroughly with pure water and dried in an argon stream. Finally, the samples were baked at 450 °C for 1 h. The synthesized TiO2 photocatalysts were characterized by X-ray diffraction (XRD, Philips PW 1050-3710 diffractometer with Cu KR radiation) and scanning electron microscopy (SEM, JEOL JSM 5900LV). 3. Results and Discussion 3.1. Characterization of TiO2 Nanotubes. Figure 1A,B shows the low and high magnification SEM images of TiO2 nanotube arrays synthesized by electrochemical oxidation of titanium substrates at 20 V for 14 h in a nonaqueous electrolyte (DMSO/HF). The average pore diameter of as-prepared TiO2 nanotubes estimated from the SEM images is approximately
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Figure 1. SEM images (A,B) and an XRD pattern (C) of the as-prepared titanium oxide nanotubes fabricated at 20 V for 14 h in DMSO + 2% HF.
60 nm, with a length of around 5 µm. The long TiO2 nanotubes grown in the DMSO organic solvent could be attributed to the presence of less water which decreases the solubility of TiO2.18 Figure 1C presents the corresponding XRD pattern of the synthesized TiO2 nanotubes. The peaks marked with a star are derived from the Ti substrate; all other diffraction peaks are attributed to those of the tetragonal anatase TiO2 phase, showing that the prepared TiO2 nanotubes possess anatase structure, which is in agreement with the literature.19 Figure 2 shows the effect of the anodization time on the photocurrent of the synthesized TiO2 nanotubes at an applied
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J. Phys. Chem. C, Vol. 112, No. 3, 2008 827
Figure 2. Relationship between photocurrent and time for the anodization of Ti samples. Inset: the photocurrent response of TiO2 nanotubes following UV illumination at an applied bias of 0.6 V vs Ag/AgCl.
electrode potential of 600 mV versus Ag/AgCl in a 0.5 M NaOH solution. The inset to Figure 2 presents the photocurrent response of the TiO2 nanotube arrays synthesized by electrochemical oxidation of titanium substrates at 20 V for 14 h in the nonaqueous electrolyte (DMSO/HF). In the dark, as expected, the electrochemical current is very low as TiO2 is a poor electrochemical catalyst for the electrolysis of water. A prompt generation of photocurrent is seen when the electrode is illuminated with UV light. Intensive gas evolution is observed on both the TiO2 photocatalyst and the Pt counter electrode because of the water splitting reaction. The photocurrent is pretty high, 4.1 mA/cm2, although the illuminated UV intensity is relatively low, ∼2 mW/cm2. The photocurrent quickly falls to zero once the UV light is switched off. As seen in Figure 2, the photocurrent slightly increases when the anodization time is increased from 2 to 8 h. By further increasing the anodization time, the photocurrent reaches the maximum at 14 h and then decreases. The TiO2 nanomaterial prepared by 14 h anodization was thus used in our further photocatalytic studies. 3.2. Univariate Calibration. The univariate calibration was conducted in order to convert the UV absorbance into concentration. Figure 3 shows the concentration dependence of the spectral absorbance of 4-NPh (A) and 2-NPh (B) recorded in the range of 200∼600 nm. Generally, aromatic hydrocarbons exhibit three main UV absorption bands with maximums at approximately 183 nm (band I), 207 nm (band II), and 264 nm(band III). All three bands are associated with the π electron system of benzene and are strongly affected by substitutions (i.e., resonance and inductive effects) and pH. Two strong bands centered at 200 nm (peak a) and 400 nm (peak b) are observed in Figure 3A. The inset of Figure 3A shows that the linear dynamic range for 4-NPh is 0.008∼0.12 mM with the linear regression equation Aa ) 3.25 c (mM) + 0.001 with a correlation coefficient of 0.996 and Ab ) 7.54 c (mM) + 0.021 with a correlation coefficient of 0.999. As seen in Figure 3B, three strong bands centered at 225 (peak a), 282 (peak b), and 416 nm (peak c) appear in the 2-NPh spectra over a wavelength range from 200 to 600 nm. The absorption of peaks b and c linearly increases with the increase of the 2-NPh concentration. However, there is no linear relationship between peak a and the concentration of 2-NPh. As shown in the inset to Figure 3B, for peaks b and c, the linear dynamic range is 0.05∼0.45 mM with the linear regression equations Ab ) 1.99 c (mM) +
Figure 3. Calibration curves for 4-NPh (A) and 2-NPh (B). Arrows show the direction of movement of curves. All of the spectra are referenced by their corresponding blank 0.5 M NaOH aqueous solution. Insets: relationship between absorbance and concentration.
0.03 (peak b) and Ac ) 1.80 c (mM) + 0.061 (peak c), and both have correlation coefficients of 0.999. 3.3. Photoelectrocatalytic Degradation of 4-NPh and 2-NPh. The photoelectrocatalytic oxidation of the nitrophenols was monitored by in situ UV-vis spectroscopy. Figure 4A shows the time dependence of the spectral absorbances of 0.1 mM 4-NPh in 0.5 M NaOH taken at 10 min intervals during the first hour of photodegradation and recorded at 30 min intervals during the remaining time. The absorbance of 4-NPh decreases with the time and approaches 0.04 after 2 h of photoelectrocatalytic degradation, corresponding to 95% removal of 4-NPh from the solution. In addition, a new band appears at approximately 320 nm, indicating the formation of some intermediates during the photoelectrocatalytic oxidation of 4-NPh. Our further HPLC analysis reveals that maleic acid was the primary intermediate, consistent with the previous studies4 and that no nitrocatechol and hydroquinone were detected. After rising to its maximum, the new band begins to decrease and eventually disappears, demonstrating that the intermediates can also be photoelectrochemically oxidized. This is further supported by our TOC measurements. While the nitrophenols totally disappear within 2 h, the complete removal of TOC is achieved after 4 h photodegradation. For comparison, P-25 was used as the benchmark to evaluate the photoelectrochemical activity of the synthesized TiO2 nanotubes. P-25 is a mixture of anatase (∼79%) and rutile (∼21%) TiO2, and it is currently considered as one of the best commercial TiO2 photocatalysts. The inset to Figure 4A presents the time dependence of the spectral absorbances of 4-NPh during the photoelectrochemical degradation of 0.1 mM 4-NPh on the P-25 thin film (with a thickness of 5 µm). Obviously, the photodegradation of the 4-NPh on
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Figure 5. Scanning kinetics data for photoelectrochemical oxidation of 2-NPh at different initial concentrations: (a) 0.6 mM, (b) 0.4 mM, and (c) 0.2 mM. Inset: the corresponding ln C/Co vs t plots.
Figure 4. (A) In situ scanning kinetics data for the photoelectrochemical oxidation of 0.1 mM 4-NPh at the TiO2 nanotube electrode in 0.5 M NaOH. Inset: photoelectrochemical oxidation of 0.1 mM 4-NPh at the P-25 electrode. (B) In situ scanning kinetics data for the photoelectrochemical oxidation of 0.1 mM 2-NPh at the TiO2 nanotube electrode in 0.5 M NaOH. Inset: photoelectrochemical oxidation of 0.1 mM 2-NPh at the P-25 electrode. (C) Variation of 2-NPh and 4-NPh concentrations vs time during the photo degradation on the TiO2 nanotubes and P-25 thin film. The applied potential bias was 0.6 V (Ag/AgCl).
the P-25 thin film is much slower than that on the synthesized TiO2 nanotubes. Figure 4C presents the scanning-kinetics plot obtained at 400 nm versus time for the photoelectrocatalytic oxidation of 4-NPh. The simulation based on pseudo-first-order kinetics gives a solid line which fits the experimental data (open circles) very well and provides an apparent first-order rate constant of 0.021 min-1 which is much larger than rate constant obtained with P-25 (0.006 min-1). The pseudo-first-order kinetics was further confirmed by the linear relationship of ln(C/Co) versus time. Figure 4B presents the spectral absorbance during the photoelectrocatalytic oxidation of 0.1 mM 2-NPh. The insert to Figure 4B shows time-dependence of the spectral absorbances of 2-NPh during the photodegradation of 0.1 mM 2-NPh on
the P-25 thin film. Peak a and peak c decrease with time and approach zero after approximately 90 min, whereas peak b centers at approximately 282 nm and red shifts to 305 nm as its relative absorbance decreases. The scanning-kinetics plot obtained at 416 nm as a function of time for the photoelectrocatalytic oxidation is shown in the inset to Figure 4C. The simulation, based on pseudo-first-order kinetics, gives the solid line in Figure 4C and is consistent with the experimental data (open squares), giving an apparent first-order rate constant value of 0.037 min-1 which is again much larger than the apparent rate constant obtained with P-25 (0.01 min-1). The apparent rate constant for the photoelectrocatalytic oxidation of 2-NPh is 1.5 times larger than that for the photoelectrocatalytic oxidation of 4-NPh, in good agreement with the literature.20,21 We further investigated the effect of the initial concentrations of 2-NPh on the photoelectrochemical degradation rate. Figure 5 presents the 2-NPh absorbance at 416 nm versus the irradiation time for three different initial concentrations: 0.6, 0.4, and 0.2 mM 2-NPh. The experimental data (symbols) in the three plots (a-c) was effectively fitted using pseudo-first-order kinetics (solid lines). This is also confirmed by the evidence of the straight line relationship of ln(C/Co) versus irradiation time as shown in the inset to Figure 5. The rate constant of photoelectrochemical oxidation of 2-NPh strongly depends on the initial concentrations, decreasing from 0.037 min-1 at 0.1 mM (Figure 4C) to 0.010 min-1 at 0.6 mM. Increasing the initial concentration of 2-NPh results in a decrease of the rate constant. This might be attributed to (i) the competition adsorption for active sites from intermediate compounds deriving from oxidation with the substrate22 and (ii) the significant UV attenuation by the nitrophenol solution.20 After understanding the kinetics of the photoelectrocatalytic oxidation of 4-NPh and 2-NPh separately, we further studied the photoelectrocatalytic degradation of the mixture of 4-NPh and 2-NPh. 3.4. Photoelectrocatalytic Oxidation of the Mixture of 2-NPh and 4-NPh. Figure 6 shows the time dependence of spectral absorbance taken at 10 min intervals during the photoelectrocatalytic oxidation of 0.1 mM 4-NPh + 0.1 mM 2-NPh + 0.5 M NaOH aqueous solution. The absorbance of the mixture decreases from 0.9 to approximately 0.10 after 2 h of photoelectrochemical oxidation. An isosbestic point at 325 nm is observed which is similar to the results obtained from the electrochemical oxidation of 2-NPh and 4-NPh.2 For systems exhibiting little spectral overlap, univariate calibration can be performed with sufficient accuracy. However, problems arise if the absorption bands seriously overlap. By comparing Figure 3A,B, although it seems that the concentrations of 2-NPh could
Photoelectrocatalytic Degradation of Nitrophenols
J. Phys. Chem. C, Vol. 112, No. 3, 2008 829 TABLE 1: Concentration Data for the Calibration Set (mM)
Figure 6. In situ scanning kinetics data for the photoelectrochemical oxidation of the mixture of 0.1 mM 4-NPh and 0.1 mM 2-NPh at the TiO2 nanotube electrode in 0.5 M NaOH. Arrows show the direction of movement of curves.
be estimated using the absorption band at 282 nm, the slight decrease of the absorption band here makes it impossible. PLS is an alternative method to resolve mixtures of compounds with strong overlap peaks23-26 and has been successfully applied in infrared, UV-vis,27 and fluoresce spectral data. PLS was employed in this study to determine the concentration of 4-NPh and 2-NPh during the photoelectrocatalytic oxidation of their mixture. 3.5. Application of PLS in Predicting the Concentration Profiles of 4-NPh and 2-NPh. 3.5.1. Calibration. The spectra acquired by the diode-array spectrophotometer consist of absorbance data (A) at a number of wavelengths (k) for m mixtures, A(m,k):
The aim of the calibration is to build up a model that relates the spectra of the calibration mixtures to the concentration data of p analytes in each mixture, C(m,p):
mixture No.
4-NPh
2-NPh
mixture No.
4-NPh
2-NPh
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0.015 0.030 0.045 0.060 0.075 0.090 0.015 0.030 0.045 0.060 0.075 0.090 0.015 0.030 0.045 0.060 0.075 0.090
0.02 0.02 0.02 0.02 0.02 0.02 0.04 0.04 0.04 0.04 0.04 0.04 0.06 0.06 0.06 0.06 0.06 0.06
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
0.015 0.030 0.045 0.060 0.075 0.090 0.015 0.030 0.045 0.060 0.075 0.090 0.015 0.030 0.045 0.060 0.075 0.090
0.08 0.08 0.08 0.08 0.08 0.08 0.10 0.10 0.10 0.10 0.10 0.10 0.12 0.12 0.12 0.12 0.12 0.12
matrices E(m,k) and F(m,p) are the errors associated with modeling A(m,k) and C(m,p) with the PLS model. In this study, an experimental design of 36 samples (Table 1) and a test set with 6 samples was used to build a PLS calibration model to predict the concentration profiles of the photoelectrocatalytic oxidation of the mixture of 4-NPh and 2-NPh. The concentration of 4-NPh and 2-NPh were varied from 0.02 to ∼0.12 mM. The objectives of this design are to span the concentrations of 4-NPh and 2-NPh in the calibration set and to obtain the highest level of variability. 3.5.2. Number of Principal Components. The number of principal components (PC) needs to be selected in order to achieve the best predictive capacity; however, it is not easy to determine how many principal components should be used to develop a PLS model. Different criteria including empirical functions and methods based on the theoretical study of experimental errors have been proposed.31 Xu and Schechter developed an algorithm for factor analysis,32 which is based on the useful concept of net analyte signal. To accomplish this goal, cross-validation was commonly employed to determine how many factors were needed for a good predictive ability.33 In this procedure, one sample is excluded, and the model is constructed and tested out by how well it predicts the sample. Each sample is predicted once, and the root-mean-square error defined as
RMSE )
The algorism of PLS is given elsewhere.28-30 The PLS breaks down both matrices into smaller ones:
A(m,k) ) T(m,a)P(a,k) + E(m,k) where T(m,a) and U(m,a) are the absorbance and concentration
C(m,p) ) U(m,a)Q(a,p) + F(m,p) “score” matrices, respectively, and P(a,k) and Q(a,p) are the absorbance and concentration “loading” matrices, respectively; a is the number of principal component or factors (PC). The
x
∑ (c - ci)2 D
is calculated; where ci is the added analyte concentration, c is the predicted analyte concentration, and D corresponds to the number of degrees of freedom. The number of factors is chosen to minimize RMSE. Once the PLS model has been applied to the training set, validated using the test set, and demonstrates high predictive abilities, it can be used to predict the concentration profiles in the photoelectrocatalytic degradation of 2-NPh and 4-NPh. Figure 7 shows a plot of RMSE versus the number of PCs in the case of the binary mixtures of 2-NPh and 4-NPh. The shape of this plot is common. The minimum value of RMSE is usually difficult to find. If a small number is selected, prediction errors arise because of systematic effects that are unaccounted; on the other hand, if too large a number of PCs is taken, the predication errors may be caused by overfitting. The right number of PCs
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Figure 7. Plot of RMSE vs number of PCs for 4-NPh and 2-NPh. Figure 9. In situ scanning kinetics data for the recovery test. Electrolyte: 0.1 mM 4-NPh + 0.1 mM 2-NPh + 0.5 M NaOH, (....) initial; (---) after 90 min photoelectrochemical oxidation. 0.015 mM 4-NPh and 0.015 mM 2-NPh were consecutively spiked into the photoelectrocatalytic degraded 2-NPh and 4-NPh mixture four times (curve a-d).
TABLE 2: Recovery Test Results for Real Samples Obtained Using PLS 4-NPh
2-NPh
added mM
found mM
recovery %
added mM
found mM
recovery %
0.015 0.030 0.045 0.060
0.0145 0.034 0.049 0.062
96.7 113.3 108.9 103.3
0.015 0.030 0.045 0.060
0.014 0.032 0.042 0.058
93.3 106.7 93.3 96.7
test were prepared by the photoelectrochemical oxidation of a mixture of 4-NPh and 2-NPh for 90 min first, and then 0.015 mM 4-NPh + 0.015 mM 2-NPh were consecutively spiked into the cell four times (as illustrated in Figure 9). The results predicted by the PLS model are given in Table 2. The recoveries for the two analytes are between 90 and ∼105%, demonstrating that the developed PLS mode can be satisfactorily applied in the analysis of real samples without interference of intermediates produced during the photoelectrocatalytic oxidation of the 2-NPh and 4-NPh. Figure 8. Modeling performance of the optimized PLS for the external test set. Expected concentrations are plotted against those obtained by the PLS model. (A) 4-NPh, (B) 2-NPh.
is the lowest number whose RMSE value is close to the minimum value of RMSE. The relatively high number of PCs (4 for the mixture of 4-NPh and 2-NPh) for UV-vis data is due to the pure spectral similarity of the two compounds studied. In order to obtain a broader picture of the PLS model, the concentrations of 4-NPh and 2-NPh predicted versus their true concentrations used in the mixture are plotted in Figure 8A,B, respectively. The high correlation coefficient obtained for 4-NPh and 2-NPh (>0.999) indicates the absence of random errors; and the near-unity slopes obtained confirm a close linear relationship between the amounts predicted and those actually used. The near zero intercepts suggest the absence of errors associated with matrix effects. 3.5.3. RecoVery Tests for Real Samples. To further evaluate the applicability of the built PLS model, recovery tests were carried out with real samples. The real samples for the recovery
3.5.4. Simultaneous Determination of 2-NPh and 4-NPh in the Mixture. The developed PLS model was employed to determine the concentrations of 4-NPh and 2-NPh during the photoelectrocatalytic oxidation of the mixture of 0.1 mM 4-NPh + 0.1 mM 2-NPh in 0.5 M NaOH solution as shown in Figure 6. The calculated concentrations are plotted in Figure 10. The concentrations of both 4-NPh and 2-NPh decrease with the time of photocatalytic oxidation. The simulation, based on pseudofirst-order kinetics, gives the solid line in Figure 10 and is consistent with the experimental data, giving an apparent firstorder rate constant value of k ) 0.018, 0.012 min-1 for 2-NPh, 4-NPh. This indicates that 2-NPh degrades nearly 1.5 times faster than 4-NPh when their mixtures are photodegraded. The rate constants of 4-NPh and 2-NPh in the mixture are smaller than those illustrated in Figure 4 when they are alone in the solution, which is consistent with the recent study by Priya and Madras.16 This could be attributed to the interaction of 4-NPh and 2-NPh, between the intermediates and the competition for the catalytic sites on the TiO2 nanotubes.
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J. Phys. Chem. C, Vol. 112, No. 3, 2008 831 Acknowledgment. This work was supported by a Discovery grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). A.C. acknowledges NSERC and the Canada Foundation of Innovation (CFI) for the Canada Research Chair Award in Material and Environmental Chemistry. References and Notes
Figure 10. C-t curves for the photoelectrochemical oxidation of the mixture of 2-NPh (a) and 4-NPh (b) in 0.5 M NaOH solution. Inset: the corresponding ln C/Co vs t plots. Individual concentrations were calculated using the PLS model.
4. Conclusions Titanium oxide nanotubes were directly grown by electrochemical oxidation of Ti substrates. The synthesized TiO2 nanotubes show high photocatalytic activity for the degradation of nitrophenolic compounds. Our HPLC analysis has revealed that maleic acid was the primary intermediate and that no nitrocatechol nor hydroquinone were detected. Our TOC measurements further show that the formed intermediate, maleic acid, can be completely oxidized photoelectrocatalytically. The present work shows how the UV-vis spectrophotometric method can be effectively used to in situ monitor the photoelectrochemical oxidation of nitrophenols. In 0.1 mM 4-NPh and 0.1 mM 2-NPh solutions, 95% and 85% of 4-NPh and 2-NPh was removed over a 2 h period, respectively. The apparent rate constant of the photodegradation of 2-NPh decreases with the increase of the initial concentration. A PLS model with a high predictive ability was built up on the basis of an experimental design with 36 samples and a test set with 6 samples to determine the concentration profiles of 2-NPh and 4-NPh during the photoelectrocatalytic oxidation of the mixture of 4-NPh and 2-NPh. The rate of degradation of 4-NPh and 2-NPh was retarded by the presence of one another. The present work shows that UV-vis spectroscopy coupled with PLS calibration can be used to in situ monitor the concentration changes, providing a novel approach to study the competitive effects of organic pollutants during the photoelectrochemical treatment of wastewater.
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