Kinetic Study of Photoelectrochemical Oxidation of Lignin Model Kinetic Study of Photoelectrochemical Oxidation of Lignin Model Compounds on TiO Compounds on TiO 22 Nanotubes Nanotubes

: In this study, TiO 2 nanotubes were prepared via the electrochemical oxidation of titanium substrates in a non-aqueous electrolyte and their morphology and microstructures were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The photoelectrochemical oxidation of two lignin model compounds, 1-(3,4-dimethoxyphenoxy) -2-(2-methoxyphenoxy)-1,3-propanediol (DMP) and 3-hydroxy-1-(3,4-dimethoxyphenoxy)-2-(2-methoxyphenoxy)-1, 3-propanone (HDM), was investigated. A new band appeared at ~ 304 nm during the photoelectrochemical oxidation of DMP. The rate of DMP intermediate formation was amplified with the increase of initial concentrations, while it was diminished with increased temperature. Despite the similarity in structure between HDM and DMP, there are only small increases in absorbance during the oxidation of HDM, suggesting that HDM is less reactive. Quantum chemical calculations based on the density functional theory (DFT) were performed in order to link photoelectrochemical reactivity with specific molecular properties. Relatively higher E LUMO - E HOMO of HDM makes it more stable and thus more refractory to oxidation, which is consistent with our photoelectrochemical results.


Introduction
Lignin, a tridimensional amorphous phenolic polymer comprised of phenylpropane units linked together by various bonds, is the major byproduct of the pulp and paper industry [1] .After being separated from the cellulosic mass, in most cases lignin is either combusted in energy and caustic recovery boilers, as is the practice in many Kraft mills, or it makes its way into waste streams, which adversely impacts river and lake systems.Various treatment methods such as electrochemical oxidation [2][3][4] , activated electrosorption [5] , chemical oxidation, and biological digestion [6] have been investigated for the modification [7][8] or degradation [ 9] of organic compounds, including lignin [10] , in aqueous systems.
Heterogeneous photocatalys is [11][12] is a promising alternative technique for the elimination of organic pollutants from wastewater [13][14][15] .Among the various oxide semiconductor photocatalysts, titania (TiO 2 ) is one of the most promising because of its biological and chemical inertness, cost effectiveness, and robust oxidizing power via the prevalence of photogenerated holes [16][17] .TiO 2 has three natural phases, namely, brookite, anatase, and rutile.Of these phases, anatase is typically considered to be the most active [18] .When TiO 2 is present in the form of a slurry/suspension, the photochemical reaction is enhanced due to the large available surface area/volume ratio.However, there are two significant problems associated with this approach: (i) TiO 2 must be recovered from the treated water, which is a challenge for large scale applications; and (ii) The efficiency is low due to the recombination of the electron-hole pairs.Supported semiconductors can provide an alternative solution to the separation problem.However, photocatalysis through the use of thin films also has a number of disadvantages when compared with powders.Firstly, the surface area that is available for exposure to the liquid phase is reduced by 2 ~3 orders of magnitude.Secondly, the diffusion boundary conditions are altered from binary collisions to that of particles that are adhered to a substrate, wherein the mass transfer may become the limiting step.Thus, a high surface area to volume ratio [19][20] (e.g., nanoscale TiO 2 [21-22]   ) is desirable for improving the efficacy of photocatalysis using thin films.
Lignin is a complex polymer that is constructed of phenylpropane monomers, which are linked primarily through C要O ether bonds, and to a lesser extent, through various C要C alkyl-aryl and di-aryl bonds.The dominant C要O ether linkages are 茁-Aryl (茁-O-4), 琢-Aryl (琢-O-4) and diphenyl (4-O-5) types.The inherent complexity of the lignin polymer makes it difficult to evaluate in terms of reaction kinetics.Consequently, relatively simple molecules that contain isolated functional groups or typical lignin bonding patterns can be utilized to reveal detailed information of mechanistic processes.
According to the primary chemical structure of lignin, as briefly stated above, and the previous studies [23][24][25] , in this study, we chose the following 2 Experimental

Materials
DMP and HDM were synthesized according to an established procedure [24] , All other chemicals were of reagent grade and were used as supplied.
The water (18.2M赘窑cm) for the preparation of all solutions was purified by a Nanopure Diamond ® water system.Stock solutions were made by dissolving the DMP and HDM in a 0.1 mol 窑L -1 NaOH solution.Subsequent concentrations were obtained by diluting the stock solution with a 0.1 mol窑L -1 NaOH solution.

Preparation and Characterization of TiO 2 Nanotube
Pure Ti substrates were degreased by sonication in acetone for 10 min and then in the pure water for an additional 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, followed by anodization in dimethyl sulfoxide (DMSO) with 2% HF at 40 V for 窑 窑 Tian 等: 木质素模型化合物在二氧化钛纳米管上光电氧化的动力学研究 第 6 期 8 h [13] .After the electrochemical treatment, the samples were rinsed thoroughly with pure water and dried in an argon stream.In order to obtain a defined anatase structure, the samples were annealed for one hour at 450 °C.The crystalline structure and phase of the TiO 2 nanotubes were examined using an X-ray diffraction (XRD, Philips PW 1050-3710 diffractometer with Cu K 琢 radiation), and the morphology was characterized by scanning electron microscopy (SEM, JEOL JSM 5900LV).Gas chromatography/mass spectrometry (GC/MS, Varian 450/300) was used to identify the intermediates that were generated in the course of the photoelectrochemical process.

Photoelectrochemical Experiment
An EG&G 2273 potentiostat/galvanostat was used to apply an anodic potential bias during the photo-degradation of the DMP and HDM.TiO 2 nanotubes were synthesized and used as the working electrode in this study.The counter electrode consisted of a Pt coil.Prior to each experiment the counter electrode was cleaned by flame annealing and then quenched with pure water.The reference electrode was an Ag/AgCl electrode.The solution in the cell was under continuous mixing using a small magnetic stirrer bar.An ADAC Systems TM Cure Spot TM 50 UV spot lamp (main line of emission -365 nm) was utilized for the irradiation of the TiO 2 nanotubes.The intensity of the UV light was ca. 2 mW窑cm -2 .The UV light was introduced into the cell via a fiber optic cable, which was placed above the electrode.The distance between the UV-visible light and the electrode surface was 1 cm.The absorbance measurements were carried out by means of a computer controlled Cary 50 UV-vis spectrometer.

Quantum Chemical Calculation
DFT (Density Functional Theory) methods were carried out using Gaussian 03, a quantum-mechanical program, for molecular modeling.The calculation was based on the B3LYP met hod [26] and 6-31*G basis set [27] at ambient temperature.This basis set provided accurate geometry and electronic properties for a wide range of organic compounds.
The DFT/B3LYP method defines the exchange functional as a linear combination of Hartree-Fock, local, and the gradient-corrected exchange terms.This exchange functional is then combined with a local and/or gradient-corrected correlation functional of Lee, Yang and Parr (LYP).All quantum theoretical calculations were carried out with full geometry optimizations.The following quantum chemical descriptors were calculated: the energy of the highest occupied molecular orbital (E HOMO ), the energy of the lowest unoccupied molecular orbital (E LUMO ), and the dipole moment (滋).

Characterization of TiO 2 Nanotubes
Fig. 2 depicts SEM images of TiO 2 nanotube arrays that were synthesized by the electrochemical oxidation of titanium substrates at 40 V for 8 h in a non-aqueous electrolyte (DMSO/HF).From the SEM images, it is evident that the self-organized nanotubes with lengths of ca. 2 滋m , form regular arrays with pores that have uniform diameters of ca.60 nm.It is also clear that the pore mouths are open at the top of the layer while the nanotubes are closed at the bottom of the layer.The length and diameter of TiO 2 nanotubes can be affected by both the type of solvent and the amount of HF.The enhanced length of the TiO 2 nanotubes that were grown in organic solvent might be attributed to the presence of less water, which decreases the solubility of TiO 2 .
Anatase is known to be the most efficient TiO 2 structure for photocatalysis.In previous work we confirmed that anatase TiO 2 provides a high decomposition rate for salicyclic acid [15] .Thus, the TiO 2 nanotube electrodes were annealed at 450 °C in order to convert them to an anatase structure.The resulting structure of the prepared TiO 2 nanotubes was confirmed by diffraction peaks in the XRD pattern presented in Fig. 2B.It is evident that the peak at 2兹 = 25.4°is the crystal of the tetragonal anatase TiO 2 phase.The peaks marked with an asterisk are derived from the Ti substrate.

Photochemical and Photoelectrochem鄄 ical Oxidation of DMP in Alkaline
The photochemical and photoelectrochemical oxidation of DMP were carried out at the TiO 2 nan otubes in a 0.1 mol 窑L -1 NaOH solution, and monitored by UV-Vis spectroscopy.Fig. 3A shows the spectral absorbance of 100 滋g窑mL -1 DMP in 0.1 mol 窑L -1 NaOH taken at two-minute intervals during 40 min of photochemical oxidation and at ten-minute intervals for the remaining time.It was observed that the absorbance at the range spanning 240 ~400 nm increased with time, indicating that intermediates were produced in the course of the photochemical oxidation of DMP.The absorbance reached its maximum after 40 min, after which it began to decrease, as shown in the inset of Fig. 3A.This demonstrates that the intermediates may also be photochemicalally oxidized.The color of the solution changed to yellow after 40 min upon UV irradiation and became colorless subsequent to 5 h irradiation.The band at ca. 276 nm was blue-shifted to 266 nm, and its height decreased to 1.87 after ca.
3 h (curve a in Fig. 3C), corresponding to a 40% oxidation of DMP.
In order to improve the efficiency of the oxidation process, the recombination of the photogenerated charge carriers must be avoided.This may be accomplished by applying an external potential bias [28][29] .It was assumed that the small size of the TiO 2 particles resulted in only a very weak electric field [30] , which was insufficient to facilitate charge separation.The high degradation rates obtained through the application of a potential to the TiO 2 electrode was attributed to increased charge separation efficiencies driven by the differing rates of electron and hole transfer at the solution interface.
To identify the byproducts that were generated by the lignin model compounds, the solution was acidified to pH 1 ~2 with HCl following 20 min of photochemical and photoelectrocatalytic oxidation.
The intermediates were extracted with CH 2 Cl 2 , which was subsequently dehydrated with anhydrous Na 2 SO 4 and analyzed using GC/MS.3,4-dimethoxybenzaldehyde was identified as the primary intermediate with m/z 166.

Photoelectrochemical Oxidation of HDM in Alkaline
Fig. 4 shows the spectral absorbance of 100 滋g 窑mL -1 HDM in 0.1 mol 窑L -1 NaOH taken at two-minute intervals during 40 min of photochemical oxidation, and at ten-minute intervals for the remaining time.Absorbance at 280 nm increases with the irradiation time in 20 min (Fig. 4A), and then starts to decrease (Fig. 4B) for the remaining time.It is readily seen that a new peak appeared at ca. 260 nm during the HDM oxidation process.Interestingly, there is no increase at ca. 304 nm and the increase at 280 nm is much slower than that of DMP despite the similarity in structure between HDM and DMP (Fig. 1). the arylglycerol ether structure [31] , rather than the oxidation of the 琢-hydroxyl group to C==O.This is an important factor insofar as the light-induced yellowing of high-yield pulps.However, further work is required to evaluate the mechanism of the photochemical processes.

Kinetics of Photoelectrochemical Oxidation of DMP
It is well known that the photoelectroehemical oxidation of most organic compounds can be analyzed using the Langmuir-Hinshelwood model [32][33] . - where -(dC dt) is the photochemical oxidation rate, k is the photochemical oxidation rate constant, k a is the adsorption equilibrium constant, and is the reaction time.If the product of k a and C is significantly smaller than 1, the above equation can be simplified to a first-order reaction: Consequently an integrated form of eq. ( 2) can be represented as follows: 1n 蓸 蔀 = k app (5)   where k app = k r k a is the apparent rate constant in min -1 .The k app value can be obtained by plotting ln ( o ) versus .was also confirmed by the linear relationship of ln ( o ) versus irradiation time (eq.5), giving a first order rate constant value of k app = 0.010, 0.011 and 0.012 min -1 100, 75 and 50 滋g窑mL -1 , respectively, showing that the rate constants are almost independent of the initial concentrations of DMP.governed by the hydroxyl radical reaction [34][35] .

Quantum Chemical Study of HDM and DMP
Quantum chemical calculations were performed in order to correlate the order of the photoelectrochemical reactivity of HDM and DMP with electron structure.Quantum chemistry calculations have been successfully applied to study of reaction mechanisms [36] , to solve chemical reaction ambigui-  [37][38] .Tab. 1 represents the results of E HOMO (eV), E LUMO (eV), E L -E H (eV), 滋(Debye) and total relative energy calculated by the B3LYP method in the gas phase for DMP and HDM.It is clearly seen from Tab. 1 that the substitution of C＝O in HDM by C要OH results in a decrease in dipole moments, and energy difference 驻E (3.0 eV), respectively.The higher energy gap (驻E) of HDM makes it more stable [38][39] and thus more resistant to oxidation.This is consistent with our results, which were derived from the photoelectrochemical oxidation of DMP and HDM.

Fig
Fig. 3B shows a plot of the photoelectrochemical oxidation of DMP at TiO 2 nanotubes in a 0.1 mol窑L -1 NaOH solution.The applied potential was 0.6 V vs. Ag/AgCl.A 0.6 V applied potential did not, without UV, result in any oxidation of DMP.A new band appears again for DMP at ca. 304 nm and reaches the maximum at ca. 20 min.This indicates the formation of intermediates during the process of photoelectrochemical degradation of DMP.Subsequent to increasing to their maximum at 275 nm and 304 nm, absorbance began to decrease and approached to 0.25 at 275 nm (curve b in Fig. 3C), corresponding to 85% removal of the DMP.There are two significant changes that are apparent in the absorption spectra depicted in Fig. 3B.One is the amount of time that is required to attain the maximum (20 min rather than 40 min) for the formation of intermediates.The second is the enhanced degradation of DMP; the efficiencies of DMP degradation in 3 h are 40% and 85% for photochemical and photoelectrochemical oxidation, respectively.The higher efficiency of the photoelectrocatalytic process may be attributed to the suppression of the recombination of photogenerated holes and electrons.It is known that photocatalysts are first excited by UV light, which subsequently initiates the photochemical process.TiO 2 nanotubes absorb light and generate electron/hole pairs (Eq.1).TiO 2 + h淄 → TiO 2 (e cb -+ h vb + )(1)Since the position of the TiO 2 valence band is high, the photogenerated holes can oxidize a wide variety of organic substrates.TiO 2 (h + ) + S → TiO 2 + S .+(2)

Fig. 2
Fig. 2 SEM image (A) and XRD pattern (B) of the titanium oxide nanotubes prepared at 40 V for 8 h in DMSO+ 2% HF.

3. 5
Effect of Temperature We further investigated the kinetics of the photoelectrochemical oxidation of HDM and DMP at various temperatures, which were controlled by a Fisher thermostat.Fig. 6A and 6B presents the spectral absorbance during the photoelectrochemcal oxidation of 100 滋g窑mL -1 DMP at 10 and 5 0 °C, respectively.The absorption band at 304 nm increased with time at both temperatures, as observed in Fig. 3B.It is worth noting that the band at 275 nm in-creased at a much slower rate at 50 °C (Fig. 6B), indicating that a low temperature favors the formation of the intermediate.Fig. 7 displays the plots of the absorbance at 280 nm vs. time for the photoelectrochemic al oxidation of HDM at different temperatures, showing that the increase of the temperature decreased the time for the formation and further degradation of the intermediate.It is found experimentally for many reactions that a plot of lnk app against 1/T gives a straight line.This behavior is normally expressed mathematically by Arrhenius equation: lnk app = lnA 蛳 a 渊6冤 where parameter A , which corresponds to the intercept of the line at 1/T = 0, is called the pre-exponen tial factor.The parameter E a , which is obtained from the slope of line, represents activation energy.Generally speaking, the higher the activation energy, the stronger the temperature dependence of the rate constant.The Arrheninus plots based on the kinetics data 543 Fig. 6B and Fig. 7) determined at 20, 40 and 60 o C yield two straight lines, from which the apparent activation energy was estimated to be 21.8 and 24.6 kJ 窑mol -1 for the photoelectrochemical oxidation of the intermediates resulted from HDM and DMP, respectively.These values are quite close to those of hydroxyl radical reactions, suggesting that the photoelectrochemical oxidation process is

Tab. 1
Molecular properties of DMP and HDM calculated using B3LYB/6were grown directly on a Ti substrate via electrochemical oxidation in a DM-SO/HF solution.The photoelectrochemical oxidation of DMP and HDM was investigated at TiO 2 nanotubes in 0.1 mol窑L -1 NaOH, and the process was monitored using a UV-Vis spectrophotometric method.The results obtained demonstrate that the efficiency of the photochemical oxidation of lignin model compounds can be significantly improved through the application of an external potential bias.In a 100 滋g窑mL -1 solution, 85% of DMP was removed upon three-hour UV irradiation.It is important to note that a new band appeared for DMP at ca. 304 nm, indicating that the intermediates were formed at the onset of the UV irradiation.Our GC/MS results revealed that 3,4-dimethoxyben-Tian 等: 木质素模型化合物在二氧化钛纳米管上光电氧化的动力学研究 第 6 期zaldehyde was the primary intermediate, which can be further degraded completely with the increase of the oxidation time.The photoelectrochemical oxidation of both DMP and HDM follows the first-order kinetics.The rate constants are nearly independent of initial concentrations, while strongly dependant on temperature.The higher energy gap (驻E) of HDM based on the DFT calculation makes it more stable and thus more difficult to oxidize, which is consistent with our results from the photoelectrochemical oxidation of DMP and HDM.