Probing Methylene Blue Photocatalytic Degradation by Adsorbed

Aug 29, 2007 - S. W. Daniel Ong , Jianyi Lin , and Edmund G. Seebauer. The Journal of Physical Chemistry C 2015 119 (48), 27060-27071. Abstract | Full...
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J. Phys. Chem. C 2007, 111, 13813-13820

13813

Probing Methylene Blue Photocatalytic Degradation by Adsorbed Ethanol with In Situ IR Zhiqiang Yu and Steven S. C. Chuang* Department of Chemical and Biomolecular Engineering, UniVersity of Akron, Akron, Ohio 44325-3906 ReceiVed: February 24, 2007; In Final Form: July 5, 2007

The dynamic behavior of infrared (IR)-observable species during the methylene blue [i.e., MB, (CH3)2N(C6H3)NS+(C6H3)N(CH3)2Cl-] photocatalytic degradation (PCD) on TiO2 has been investigated at 30 °C. Exposure of MB/TiO2 to UV illumination led to the scission of the N-CH3 bond followed by breaking of the C-H and CdN bonds in the MB central aromatic ring and the side aromatic rings, indicating demethylation as the first step of the MB PCD. The bond breaking in the MB molecule and subsequent reactions produced charge-containing intermediates (i.e., carboxylate (RCOO-) and R-NH3+), slowing down the conversion of MB to CO2, H2O, NH4+, and SO42-. Probing the MB PCD by adsorbed ethanol revealed that the demethylation step was initiated by the OH/OD radical (‚OH/‚OD) and the breaking of CdN and CdS-C in the MB central aromatic ring by H+/electron transfer. In situ IR coupled with the use of ethanol as a probe molecule provides an excellent method for investigating the PCD mechanism.

TABLE 1: Ethanol Probing Reactions and Their Functionsa

Introduction Heterogeneous photocatalytic degradation (PCD) provides a promising approach for the conversion of organic wastes to CO2 and H2O.1-8 A simple method for determining the PCD activity of a semiconductor catalyst is to measure the decrease in the UV-visible absorption intensity of methylene blue [MB, (CH3)2N(C6H3)NS+(C6H3)N(CH3)2Cl-)] at 664 nm as a function of reaction time.9-12 In addition to serving as a model compound for PCD, MB has been studied widely for its redox properties and hydride-related reactions.13,14 The PCD of organics involves a series of steps: (i) the generation of charge carriers (i.e., electron and hole) on the surface of semiconductor catalysts under UV illumination, and (ii) the transfer of charge carriers to adsorbed reactants to initiate a series of bond-breaking and formation steps, ultimately leading to the formation of CO2 and H2O.15 Other elements such as nitrogen and sulfur in organics are typically oxidized to NH4+/NO3- and SO42-, respectively.16 The MB molecule consists of several types of bonds, such as C-H, N-CH3, CdN, CdS, CAr-N, and aromatic rings. A fundamental understanding of the MB PCD mechanism could assist in the design of a highly active photocatalyst for PCD of those recalcitrant organics containing N and S elements. One effective approach to study complex reaction mechanisms is the use of probe molecules to perturb the reaction pathways and then elucidate reaction mechanisms from the resulting products. The probe-molecule approach has been used widely to determine reactive intermediates, active sites, secondary reactions, reaction networks, and chemical properties of the catalyst surface.17-19 In the present study, we have employed ethanol as a probe molecule to determine the mechanism of the MB PCD on TiO2. The probe-molecule approach was, for the first time, coupled with the in situ IR technique to investigate the PCD mechanisms. Table 1 shows the possible reaction steps of ethanol during the MB PCD and lists their specific functions for probing the MB PCD mechanism. The ethanol probe molecule could (i) displace adsorbed water (H2Oad) and the * Corresponding author. Tel: +1-330-972-6993; fax: +1-330-972-5856; E-mail: [email protected].

reactions CH3CH2OH + H2Oad f CH3CHOHad + H2O CH3CH2OH + -OH f CH3CHOad + H2O CH3CH2OHad + h+ f CH3C˙ HOHad + H+ CH3CH2Oad + h+ f CH3C˙ HOad + H+ CH3CH2OHad + ‚OH f CH3C˙ HOHad + H2O CH3CH2Oad + ‚OH f CH3C˙ HOad + H2O CH3C˙ HOHad f CH3CHO + H+ + eCH3CHO + Oad or O(lattice) f CH3COO+ H+ a

functions displacing H2Oad displacing -OH consuming h+, providing H+ consuming h+, providing H+ scavenging ·OH scavenging ·OH providing H+ and eformation of carboxylate

CH3C˙ HOHad: adsorbed hydroxyalkyl radical.

hydroxyl group (-OH) from the TiO2 surface,20 (ii) consume photogenerated holes,21 making them unavailable for other PCD steps, (iii) suppress those steps involving the OH radical (‚OH) by scavenging ‚OH,3 (iv) donate electrons to either the TiO2 conduction band or other adsorbed reactants on the TiO2 catalyst surface, and (v) donate the protons to its acceptors.22 The MB PCD with the ethanol probe molecule on TiO2 was studied by in situ IR at 30 °C with 1 atm of O2. The pathway for the MB PCD was elucidated from the evolution of IR intensities of MB, ethanol, reaction intermediates, CO2, H2O, the ammonium ion (NH4+), and the sulfate ion (SO42-) in the absence and presence of the ethanol probe molecule. Experimental Methods Titanium dioxide (TiO2) was supplied by Degussa (P25, ∼50 m2/g, approximately 70% anatase and 30% rutile). MB and CaF2 powder (325 mesh, Alfa Aesa) were purchased from Alfa Aesar. Ethanol (reagent, anhydrous) was purchased from Aldrich. All of the compounds were used without further treatment The experimental apparatus for the in situ IR study of the MB PCD has been reported in detail elsewhere.20 Briefly, the apparatus included the following: (i) a diffuse reflectance chamber (i.e., DRIFTS cell from HVC-DRP, Harrick Scientific), which can be operated from room temperature to 600 °C; (ii) a Xe 350 W mercury lamp (Oriel 6286) with a light condenser

10.1021/jp0715474 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2007

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Figure 1. IR spectra during 120 min of the photocatalytic degradaion of MB on TiO2.

(Oriel 77800); and (iii) two flow meters (Brooks 5850E) to control the inlet flows into the DRIFTS cell. The adsorption of MB on TiO2 (i.e., MB/TiO2) was achieved by mixing the 20 mg of TiO2 catalyst and 1 mL of MB aqueous solution with a concentration of 1 mg/mL and then vacuumdrying at 80 °C for 24 h. To determine the role of water, D2O was used to prepare MB aqueous solution to obtain the D2Ocontaining MB/TiO2 mixture. An amount of 15 mg of MB/TiO2 mixture was placed on top of 80 mg of inert CaF2 powder in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell that possesses two IR transparent ZnSe windows and a UV-transparent CaF2 window. The IR spectra of adsorbed species during the PCD were collected by co-added 32 scans at a resolution of 4 cm-1 using a FTIR (DigiLab FTS 4000) bench. The MB PCD was carried out on TiO2 with ultrahigh purity O2 (Praxair, 99.999%) in the DRIFTS cell at 30 °C and 1 atm of O2 for 120 min. Prior to the reaction study, the MB/TiO2 mixture, which contained either H2Oad or D2Oad, was purged with O2 for 120 min. The reaction was initiated by UV illumination with an intensity of 25 mW/cm2. The amount of the ethanol probe molecule that was admitted to the DRIFTS cell for adsorption on the MB/TiO2 surface was controlled by duration of flowing O2 stream containing 10.3 mol % of ethanol vapor. Results Photocatalytic Degradation of MB on TiO2. Figure 1 shows the infrared (IR) absorption spectra of MB/TiO2 before and during the MB PCD. The spectra were obtained by subtracting the TiO2 spectrum from those of MB/TiO2 at different reaction times. Prior to the PCD, MB/TiO2 exhibited prominent bands for the CdN central ring stretching at 1600 cm-1, the CdC side ring stretching at 1488 cm-1, the multiple ring stretching at 1388 cm-1, the CAr-N (i.e., the bond between the side aromatic ring and the nitrogen atom) stretching at 1333 cm-1, the N-CH3 (i.e., the bond between CH3 and the nitrogen atom) stretching at 1247, 1178, and 1143 cm-1, the C-H asymmetric

Yu and Chuang stretching of CH3 at 2926 cm-1, and the C-H symmetric stretching of CH3 at 2858 and 2815 cm-1. The isolated linear -OH at 3694 cm-1, bridged -OH at 3663 cm-1, and H2Oad at 3550 cm-1 exhibited negative intensities, indicating that a fraction of these species were displaced from the TiO2 surface by the MB adsorption. The inset in Figure 1 shows the IR responses of pure TiO2 and MB/TiO2, further confirming that MB adsorption only displaces part of -OH and H2Oad on the TiO2 surface. The band assignments of these species are listed in Table 2.23-31 Comparing the IR spectrum of MB/TiO2 with that of pure MB (not shown) revealed that adsorption of MB on TiO2 did not result in any variation in the wavenumbers of the MB characteristic IR bands, suggesting that MB was physisorbed on the TiO2 surface. The amount of adsorbed MB that gave rise to those IR bands in Figure 1 was determined to be 1.17 µmol on the basis of carbon balance between adsorbed MB and CO2 formation from the MB PCD. Exposure of MB/TiO2 to UV illumination caused the intensity of the MB bands to decay and their broad shoulder bands between 1700 and 1100 cm-1 to rise gradually, as shown in Figure 1. Plotting the normalized intensities of MB bands (i.e., band maximum height) and the amount of CO2 produced as a function of time in Figure 2 shows the intensities of MB bands decayed at different rates. The decay rates of the MB IR intensity decreased in the order: N-CH3 at 1247 cm-1 > CAr-N at 1333 cm-1 > CdN at 1600 cm-1 > the multiple ring stretching at 1388 cm-1 > CdC at 1488 cm-1 during the first 10 min, as shown in Figure 2a. The initial rapid scission of the N-CH3 bond was accompanied by the decreases in the intensity of the CH3 band at 2926, 2858, and 2815 cm-1 in Figure 1, revealing demethylation as the first step of the MB PCD. The detailed variations of IR bands during the MB PCD can be further discerned from the difference spectra in Figure 3. Parts a1 and b1 of Figure 3 were obtained by subtracting the spectrum at 0 min (i.e., prior to the MB PCD) from the subsequent spectra during the MB PCD. Parts a2 and b2 of Figure 3 were obtained by subtracting each spectrum from its successive spectrum, highlighting the subtle change in the concentration of intermediates during the MB PCD. The negative intensity manifests the scission of the specific bonds in the MB, whereas the positive intensity reflects emergence of intermediate species and products. The discernible bands emerged during the MB PCD were isolated -OH at 3687 and 3634 cm-1, NH2 of aniline at 3338 cm-1, NH4+ at 3163 cm-1, CO2 at 2362 cm-1, CdO (i.e., the carbonyl bond of an aldehyde) at 1721 cm-1, R-NH3+ at 1648 cm-1, carboxylate (RCOO-) at 1566 cm-1, CAr-O of phenol at 1265 cm-1, and S-O of sulfate ion (SO42-) at 1228 cm-1. The detailed IR band assignments are summarized in Table 2. A relatively small variation in the intensity of these species was observed from the 80 to 120 min of the reaction, as shown in Figure 2b, suggesting that the MB PCD has slowed down after 80 min of the PCD. During the MB PCD, all of the constituent elements of MB can be converted to the final oxidation products including CO2, H2O, NH4+ and SO42-.11,32,33 Thus, it can be concluded that aniline, aldehydic species, R-NH3+, carboxylate, and phenol observed during the MB PCD were the intermediate species that should be eventually converted to the final oxidation products. The behavior of these intermediates can be further unraveled from the sequential difference spectra in parts a2 and b2 of Figure 3, which show that the intensities of R-NH3+ and phenol continued to grow from 10 to 120 min, whereas the intensity of carboxylate increased from 0 to 40 min and began

Probing Methylene Blue Photocatalytic Degradation

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13815

TABLE 2: Band Assignments and Their Vibration Modes species MB C2H5OHad C2H5Oad

C6H5NH2ad NH4+ R-NH3+ CH3CHOad RCOO-ad HCOO-ad CH3COO-ad C6H5OHad SO42isolated -OH isolated -OD H2Oad CO2

bands (cm-1) and modes νas(CH3)/2926; νs(CH3)/2858, 2815; ν(CdN)/1600; ν(CdC)/1488; ν(multiple ring)/1388; ν(CAr-N)/1333; ν(N-CH3)/1243, 1247, 1178, 1143; ν(HO‚‚‚×b0×b0×b0H)/3440; νas(CH3)/2969; νas(CH2)/2930; δs(CH3)/1400; δ (OH)/1251; ν(C-O)/1050; νas(CH3)/2969; νas(CH2)/2930; νs(CH3)/2868; δas(CH2)/1445; CH2 wagging/1355; ν(C-O) monodentate/1118; ν(C-O) bidentate/1050; ν(N-H)/3338; ν(N-H)/3163; δas(N-H)/1648; ν(CdO)/1721; ν(COO)/1566, 1514, 1473; ν(C-H)/2840; ν(C-C)/1026; ν(CAr-O)/1265; ν(S-O)/1228; ν(OH) linear/3694, 3687; ν(OH) bridged/3663; ν(OH) triple coordinated/3634, 3600; 2705 ν(HO‚‚‚H)/3550; 1630; νas(CdO)/2362;

decreasing after 40 min of the reaction. Carboxylate has long been recognized as a precursor for the formation of CO2 from the PCD of organics.29,34 Photocatalytic Degradation of MB on D2O-Containing TiO2. The results shown in Figures 1-3 were obtained from studies on the H2O-containing TiO2, which exhibits linear/ bridged -OH and H2Oad bands. To verify the role of -OH, the MB PCD was also studied on the D2O-containing TiO2. A fraction of linear/bridged -OD and D2Oad were displaced by the MB adsorption on the D2O-containing TiO2, resembling the behavior of -OH and H2Oad on TiO2. Figure 4 shows the difference spectra of MB/TiO2 during the MB PCD on the D2Ocontaining TiO2. The decrease in the intensity of the MB bands and the increase in the intensity of their product bands, in Figure

Figure 2. Variation of the normalized IR intensity of MB characteristic bands and mol amount of CO2 production during (a) 10 min and (b) 120 min of the photocatalytic degradation of MB. Normalized intensity ) I(t)/Iinitial, where I(t) is the intensity of a band at a specific time of t and Iinitial is the intensity of a band at 0 min.

ref 25, 31 23, 24, 30 23, 24, 30

25, 28 25, 27, 28 27, 28 30 23, 29, 30 24, 29, 30 30 26 25 25, 28, 30 25 30 30

4a1 and b1, were accompanied by a conspicuous decline in the intensity of the -OD band at 2705 cm-1 and a marked increase in the intensity of the linear -OH band at 3687 cm-1. Zooming in changes in the intensity of -OH/-OD and C-H bands during the first 10 min of the MB PCD reaction, as shown in Figure 5, revealed the decrease in C-H, -OD, and triple coordinated -OH occurred prior to the growth of linear -OH. Comparison of the rates of changes in -OD and linear -OH in the presence of MB in Figure 5 with those in absence of MB shows that the rate of decrease in the -OD intensity and the rate of increase in the linear -OH intensity during the MB PCD are about 5 times greater than those of the -OD decrease and linear -OH increase in the absence of MB under UV illumination. This observation indicates that -OD participates in the MB PCD reaction and the linear -OH is a product of the reaction. Further comparing variations of the MB band and its product intensities in Figures 3b1 and 4b1 revealed that the decreasing rates of the MB band intensities are nearly identical on both H2O- and D2O-containing TiO2. The major effects of D2O, as shown in Figures 3a1 and 4a1, are to increase the rate of NH4+ and C6H5NH2 production and to decrease the rate of CO2 formation. Proposed Photocatalytic Degradation Pathways. The sequence of MB bonds breaking as well as the formation of intermediates and final products can be summarized in the proposed reaction scheme in Figure 6. The MB PCD proceeds via (i) demethylation, (ii) breaking of the MB central aromatic ring and then the side aromatic rings, (iii) conversion of the fragments produced from the first two steps to intermediates species such as R-NH3+, aldehydic/carboxylate species, aniline, and phenol, and (iv) further conversion of these intermediates to CO2, H2O, NH4+ and SO42- as the final oxidation products.11,32,33 In general, the PCD of organics is initiated through abstraction of hydrogen from the C-H bond via photogenerated holes or radical intermediates.35 It is not likely that MB could interact with the photogenerated hole directly because of the cationic nature of MB molecule and the presence of H2Oad on the TiO2 surface. The radical-initiating PCD can be considered as an indirect pathway because radicals are produced by the interaction of h+ with H2Oad.35 The interaction of h+ with H2Oad can also lead to the production of O2 through Ti-O‚ and Ti-O-O-Ti

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Figure 3. (a1) Difference spectra and (a2) sequential difference spectra during the 120 min of photocatalytic degradation of MB. The difference spectra were obtained by subtracting the spectrum at 0 min from the subsequent spectra; the sequential difference spectra were obtained by subtracting each spectrum from its successive spectrum. Range of IR spectra: 3850-2250 cm-1. (b1) Difference spectra and (b2) sequential difference spectra during the 120 min of photocatalytic degradation of MB. Range of IR spectra: 1800-1000 cm-1.

intermediates.36,37 ‚OH, which has been proposed as a key radical to initiate the PCD through hydrogen abstraction as illustrated in the inset of Figure 6, can be produced via the following reactions:38

H2Oad f OH- + H+

(1)

OH- + h+ f ‚OH

(2)

Figure 4. (a1) Difference spectra and (a2) sequential difference spectra during the 120 min of photocatalytic degradation of MB with the absorbance of D2O on TiO2. Range of IR spectra: 3850-2250 cm-1. (b1) Difference spectra and (b2) sequential difference spectra during the 120 min of photocatalytic degradation of MB with the absorbance of D2O on TiO2. Range of IR spectra: 1800-1000 cm-1.

A number of recent studies have further confirmed the presence of ‚OH on the TiO2 surface39 and its reactivity to abstract hydrogen from hydrocarbons.12,40-43 The results in Figure 5 show the parallel decrease in -OD with C-H, supporting the proposed role of ‚OH (or ‚OD) in the abstracting hydrogen from C-H. An alternative mechanism for hydrogen abstraction via TiO‚ is also probable, as shown in the following reaction step:35

(CH3)2-N - + Ti-O‚ f ‚CH2CH3-N - + Ti-OH

(3)

Probing Methylene Blue Photocatalytic Degradation

Figure 5. Difference spectra during the 10 min of photocatalytic degradation of MB with the absorbance of D2O on TiO2. Range of IR spectra: 3800-2500 cm-1.

Figure 6. Proposed reaction scheme for photocatalytic degradation of MB.

The lack of the correlation between the increase in the Ti-OH intensity and decrease in the C-H intensity in Figure 5 suggests that Ti-O‚ may not play a key role in hydrogen abstraction. Regardless of the type of initiating species, hydrogen abstraction produces the ‚CH2- radical, which can be further attacked by either O2 or O2-, resulting in demethylation. Although the detailed mechanistic steps of the demethylation reaction cannot be discerned by the in situ infrared techniques used in this study, the occurrence of demethylation is evidenced by the decrease in the N-CH3 intensity. The radical ring structure resulting from demethylation would be unstable, leading to breakup of the ring structure. The radical ring structure can also be further attached by O2 or O2-, resulting in the formation of carboxylate, giving the IR bands at 1514 and 1566 cm-1, as shown in Figure 4b. The important role of oxygen species in the production of

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13817

Figure 7. (a) IR spectra of MB/TiO2 and MB/TiO2 with low (5.1 µmol) and high coverage (11.4 µmol) of ethanol and (b) the difference spectra between the MB/TiO2 and MB/TiO2 with low and high coverage of ethanol.

carboxylate has been demonstrated by the formation of 18O carboxylate from the use of 18O2 as an oxidant.43,44 In addition to destabilization of the ring structure via demethylation, the proton produced from step 1 and photogenerated electron could reduce CdN and CdS+-C in the central aromatic ring to C-N and C-S bonds,13,14 weakening the ring structure. The IR band of C-N is in the range of 1000-1200 cm-1, which is overlapped with those of the N-CH3.25 The C-S band is in the range of 700-570 cm-1,25 being cut off by the CaF2 window. Although the formation of the C-N and C-S bonds cannot be unambiguously identified, the easy of conversion of methylene blue (MB) to leuco-methylene blue13 and the abundance of H+ suggest that reduction of CdN and CdS+-S to C-N and C-S could facilitate the breakup of the central aromatic ring and further opening of the two side aromatic rings. Probing MB Photocatalytic Degradation with Adsorbed Ethanol. Figure 7a displays the initial IR spectra of MB/TiO2 and MB/TiO2 with 5.1 µmol and 11.4 µmol of ethanol probe molecules, respectively. For convenience, the former is designated as a low coverage of ethanol, and the latter a high coverage of ethanol. The amount of adsorbed ethanol was determined by the carbon balance between adsorbed ethanol and CO2 product with their IR intensities and extinction coefficients.20 Adsorption of ethanol on MB/TiO2 further removed -OH and H2Oad. The negative 3634 cm-1 band, which can be assigned to a triple coordinated -OH on TiO2, indicating that ethanol was able to reach those sites that were not occupied by physisorbed MB. The MB characteristic IR bands remained the same despite the coadsorption of ethanol on MB/TiO2, allowing a reasonable comparison of the MB and ethanol spectra during the PCD. The difference spectra between MB/TiO2 and MB/ TiO2 with the low and high coverage of ethanol in Figure 7b show ethanol was adsorbed on the MB/TiO2 surface as molecularly adsorbed ethanol (C2H5OHad) and ethoxy (C2H5Oad) species.20,30 The band assignments for the molecularly adsorbed ethanol and ethoxy (i.e., dissociatively adsorbed ethanol) are listed in Table 2. Figure 8 shows the difference spectra taken during the MB PCD and the MB PCD with the low and high coverage of

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Figure 9. Variation of the normalized IR intensity of MB bands at (a) 1600, (b) 1388, (c) 1488, (d) 1247, and (e) 1333 cm-1, (f) coadsorbed ethanol at 2969 cm-1, (g) H2Oad at 3550 cm-1, and (h) mole amount of CO2 formation during the 10 min of photocatalytic degradation of MB/ TiO2 and MB/TiO2 with the low (5.1 µmol) and high coverage (11.4 µmol) of ethanol.

Figure 8. (a) Comparison of difference spectra during the 120 min of photocatalytic degradation of MB/TiO2 and MB/TiO2 with low (5.1 µmol) and high coverage (11.4 µmol) of ethanol. Range of IR spectra: 3850-2250 cm-1. (b) Comparison of difference spectra during the 120 min of photocatalytic degradation of MB/TiO2 and MB/TiO2 with low (5.1 µmol) and high coverage (11.4 µmol) of ethanol. Range of IR spectra: 1800-1000 cm-1.

ethanol on the TiO2 surface from 0 to 120 min. Upon UV illumination, the ethanol probe molecules accelerated the growth of the negative bands significantly for CdN at 1600 cm-1, the multiple ring vibration at 1388 cm-1, CAr-N at 1333 cm-1, and N-CH3 at 1247 cm-1. The growing negative intensities of the CdN and multiple ring stretching bands indicate the occurrence of the breaking of the central and side aromatic rings,

respectively. The increasing negative intensity for CAr-N and N-CH3 is indicative of the demethylation step. Plotting the decay of these MB band intensities as a function of time in Figure 9 revealed that the ethanol probe molecules accelerated the rates of all MB bonds breaking during the first minute of the reaction, decelerated the rate of demethylation (i.e., N-CH3 at 1247 cm-1 in Figure 9d) after the first minute of the reaction and then the rate of deamination (i.e., CAr-N at 1333 cm-1 in Figure 9e) after 4 min of the reaction. The growing negative intensity of the MB IR bands in Figure 8 was accompanied by the increasing intensity of the carboxylate-related bands at 1566, 1514, and 1473 cm-1 as well as those of R-NH3+ and the final products including CO2, H2O, NH4+, and SO42-. The conspicuous 1566 cm-1 band assigned to the asymmetric stretching vibration of O-C-O has been observed in a good number of organic PCD reactions. Because of the overlapping of these carboxylate bands with the MB bands, it is difficult to quantify the trend of their variations. These carboxylate bands can be related to either formate or acetate. In addition to the IR bands in the 1400-1600 cm-1 region, formate exhibited a C-H stretching band at 2840 cm-1 in Figure 8a2; acetate showed a C-C stretching band at 1026 cm-1 at Figure 8b3. The growing patterns of the 2840 and 1566 cm-1 bands in Figure 8a2 and b2 as well as those of 1566 and 1026 cm-1 bands in Figure 8b3 suggest that a large fraction of carboxylate was in the form of formate produced from the MB PCD with the low coverage of ethanol and in the form of acetate produced from the MB PCD with the high coverage of ethanol.20

Probing Methylene Blue Photocatalytic Degradation Discussion The MB PCD studies reported in the literature were carried out in a slurry mode where MB and O2 were dissolved in a TiO2-suspended aqueous solution at room temperature. The disappearance of MB was found to follow first-order kinetics; the rate constant was determined to be in the range of 0.0140.053 min-1.10,11,45,46 In the present study, solid MB was in direct contact with the TiO2 catalyst particle through a wetting and drying process that allowed the physisorption of MB on the TiO2 surface, resulting in displacement of isolated -OH and H2Oad. The first-order rate constant for breaking down the MB central aromatic ring (i.e., the CdN at 1600 cm-1) on the TiO2 surface was determined to be 0.022 min-1, which is surprisingly in agreement with those reported for the aqueous-phase MB PCD. The consistency in the reaction kinetics suggests that both the slurry and solid-solid modes of the MB PCD could be operated with the same mechanism. The proposed reaction scheme in Figure 6 postulates that the MB PCD is initiated by the abstraction of hydrogen from the methyl group (-CH3) by ‚OH, which is produced from the reaction of hole with H2Oad on the TiO2 surface (i.e., reactions 1 and 2). The hydrogen abstraction resulted in demethylation. The proposed ‚OH-initiating demethylation is further supported by the depletion of -OD in the MB PCD on the D2O-containing TiO2 in Figure 5 and the suppression of N-CH3 bond breaking brought about by the ‚OH scavenging effect of the ethanol probe molecule shown in Figure 9d. Because the MB PCD with the high coverage of ethanol proceeded beyond 4 min, ‚OH was effectively scavenged by coadsorbed ethanol, significantly slowing down the ‚OH-initiating demethylation step. An alternative initiating step for the MB PCD is the direct injection of electrons from an excited MB (MB*) to the TiO2 conduction band. Although the thermodynamic driving force for the electron injection is low [E0(MB+/MB*) ) -0.71 V],12 the occurrence of the electron transfer from MB* to the N-doped TiO2, of which the conduction band potential is at the same level of the TiO2 catalyst, has been reported.47,48 In this study, the mercury lamp provided the photon in the 260-800 nm range. The visible range of the photon could photosensitize MB, causing the transfer of electron from the excited MB* to the TiO2 conduction band and producing MB+.12 The subsequent reaction of MB+, as shown in the inset of Figure 6, would result in the formation of -CH2‚ by hydrogen abstraction,40,49 ultimately leading to demethylation. Because ithe n situ IR technique used in this study can only observe the variation of the intensity for the bonds such as CAr-N and N-CH3, we were not able to determine the extent of the contribution of photosensitizing to the hydrogen abstraction process. Following the ‚OH scavenging step (i.e., abstraction of hydrogen from ethanol in Table 1), the ethanol probe molecule was converted to a hydroxyalkyl radical3,50 possessing a sufficiently high negative potential that could (i) drive the electrons from the radical to the TiO2 conduction band and (ii) provide electrons and protons to the MB cationic central ring, facilitating the breaking of the MB ring structure. The acceleration of the central ring breaking in the presence of ethanol in Figure 9a and b confirmed the occurrence of the electron/protoninitiating MB ring breakup step. The electron/proton donating effect of the hydroxyalkyl radical vanished after 4 min of reaction, leading similar decay rates for the MB ring breaking in the MB PCD and the MB PCD with coadsorbed ethanol, as evidenced by the paralleled decay curves for the CdN, multiple ring stretching, and CdC bands in Figure 9a-c. The diminished electron/proton transfer can be attributed to the accumulation

J. Phys. Chem. C, Vol. 111, No. 37, 2007 13819 of charged intermediate species such as carboxylate at 1566 cm-1, and final products such as NH4+ at 3338 cm-1, and SO42at 1228 cm-1, which could strongly interact with H+. The typical charged intermediates produced from organic PCDs include carboxylate species and O2-.51 Parts b1 and a2 of Figure 8 show that carboxylate at 1566 cm-1 and formate at 2840 cm-1 increased with time to a maximum and then decreased gradually during the MB PCD and the MB PCD with the low coverage of ethanol. Decreases in carboxylate intensity indicate that the rate of its conversion to CO2 is greater than the rate of its formation. Plotting the amount of CO2 formation as a function of time in Figures 9h and 10a shows CO2 formation from the MB PCD with the low coverage of ethanol increased dramatically at 40 min, corresponding to the observation of a conspicuous linear -OH band at 3687 cm-1 and an H2Oad band at 3550 cm-1 in Figure 8a2. The -OH results from the dissociation of the H2O molecules: -OH is bonded to the Ti cation sites, whereas the H atom is bonded to the O anion site, as shown in the following reaction:4

The recovery of the -OH band that was previous replaced by adsorbed ethanol and MB in Figure 7 suggests that the adsorbed ethanol, MB, and -OH occupied on the same Ti sites. As the adsorbed ethanol and MB on these Ti sites were oxidized and removed, H2Oad produced from the reaction returned to form isolated -OH. It appears that a proper coverage of H2Oad and organics including carboxylate is essential for the high rate of CO2 formation.52 Although the high coverage of ethanol greatly increased the initial rate of the MB central ring breakup step, it caused displacement of H2Oad on TiO2, producing an H2O-deficient environment and allowing the photogenerated hole to directly react with ethanol (i.e., step 1 in Table 1). Our recent study has shown that the hole-initiating ethanol PCD produced acetate as a major intermediate in an H2O-deficinet environment.20 Figure 8b3 shows that the MB PCD with the high coverage of ethanol also produced a prominent acetate band at 1026 cm-1, of which IR intensity increased as the reaction proceeded, suggesting the reaction was operated with a hole-initiating mechanism. Consumption of holes by the hole-initiating process would result in the buildup of photogenerated electrons if adsorbed O2 and H2Oad were not available to remove the photogenerated electrons at a sufficient rate, as shown in the following reactions:3

O2ad + e- f O2-

(5)

2O2- + 2H2Oad f 2OH‚ + 2OH- + O2

(6)

The buildup of photogenerated electrons was indeed observed for the MB PCD with the high ethanol coverage in Figure 10b, which exhibited the increasing IR intensity at 2000 cm-1 (i.e., a measure of accumulated electrons in or close to the conduction band53-55) with the reaction time. The lack of adsorbed O2 and H2Oad for the removal of photogenerated electrons by reactions 5 and 6 could be attributed to the blockage of their adsorption sites by the high coverage of ethanol and acetate produced from the PCD of ethanol. The observation of site blockage in photocatalysis, in fact, resembles those in the conventional heterogeneous catalysis. High coverage of organics and H2Oad on the TiO2 surface blocked the adsorption of those reactants such as O2, which tends to be weakly adsorbed.15,56 Because

13820 J. Phys. Chem. C, Vol. 111, No. 37, 2007

Figure 10. (a) Variation of mole amount of CO2 formation and (b) IR background at 2000 cm-1 during the 120 min of MB/TiO2 and MB/ TiO2 with the low (5.1 µmol) and high coverage (11.4 µmol) of ethanol.

adsorbed O2 is responsible not only for removing photogenerated electrons but also for oxidizing the MB, ethanol, and their intermediates such as carboxylate, the lack of adsorbed O2 would hinder the steps for oxidation of carboxylate, resulting in their accumulation from the hole-initiating reaction. Conclusions The ethanol probe molecule plays two key roles in perturbing the MB PCD: (i) displacing H2Oad and isolated -OH as well as scavenging ‚OH, resulting in the suppression of the ‚OHinitiated demethylation, and (ii) donating of electrons and protons from the hydroxyalkyl radicals to initiate breaking of the MB ring structure. The results of in situ IR coupled with the ethanol probe molecule study revealed the MB PCD proceed via the scission of the N-CH3 bond followed by breaking of the CAr-N, CdN of the MB central aromatic ring, and the side aromatic rings, indicating demethylation as the first step in MB PCD. The breaking of the central aromatic ring was initiated by the transfer of photogenerated electrons and H+ produced from the reaction of H2Oad with hole. The fragments resulted from demethylation and ring breaking could further react with O2 to produce charge-containing intermediates, slowing down the conversion of MB to CO2, H2O, NH4+, and SO42-. Improvement of the PCD of recalcitrant S-, N-, and C-containing organics could be achieved by accelerating the removal of charge-containing intermediates from the catalyst surface. Acknowledgment. This work was partially supported by the Ohio Board of Regents (Grant R4552-OBR) and the U.S. Department of Energy (Grant DE-FG26-01NT41294). References and Notes (1) Kamat, P. V. Chem. ReV. 1993, 93, 267. (2) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (5) Mills, A.; Le Hunte, S. J. Photochem. Photobiol., A 1997, 108, 1. (6) Ollis, D. F. Cattech 1998, 2, 149. (7) Heller, A. Acc. Chem. Res. 1995, 28, 503. (8) Sun, B.; Reddy, E. P.; Smirniotis, P. G. J. Catal. 2006, 237, 314.

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