Photocatalytic Oxidation of Aqueous 1,2-Dichlorobenzene by

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J. Phys. Chem. B 2002, 106, 4336-4342

Photocatalytic Oxidation of Aqueous 1,2-Dichlorobenzene by Polyoxometalates Supported on the NaY Zeolite Ruya R. Ozer and John L. Ferry* Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: October 15, 2001; In Final Form: February 6, 2002

In this work, we report the effectiveness of the NaY zeolite as a solid support for several photocatalytically active polyoxometalate (POM) salts (H2NaPW12O40, H4SiW12O40, or H3PMo12O40) as indicated by the photodegradation kinetics of the probe compound 1,2-dichlorobenzene (DCB) in water. The photooxidation of DCB was carried out in illuminated (254-370 nm), oxygenated solution at pH 1.0. NaY zeolite-supported POMs exhibited photocatalytic activity over the wavelength range 254-350 nm. Photocatalytic activity did not drop off significantly until λ > 350 nm. At 350 nm, kobs values at an optimum NaY zeolite/POM combination were 1.02 × 10-2 , 6.5 × 10-3 , and 5.1 × 10-3 min-1 for PW12O403-, SiW12O404-, and PMo12O403-, respectively. Unsupported POMs exhibited no detectable photocatalytic activity at λ > 340 nm. At all wavelengths, photooxidation of DCB in the presence of 0.1% NaY zeolite with each of three different 0.5 mM polyoxometalates (POMs) was first order in DCB. The optimum POM/NaY zeolite ratios at 254 nm for DCB oxidation were 0.5 mM PW12O403-/0.2 wt % NaY zeolite (kobs ) 1.69 × 10-2 min-1), 0.75 mM SiW12O404-/0.1 wt % NaY zeolite (kobs ) 8.1 × 10-3 min-1), and 0.5 mM PMo12O403-/0.1 wt % NaY zeolite (kobs) 6.8 × 10-3 min-1). The kobs values are a factor of 4-8 times higher than those with PW12O403-, SiW12O404-, and PMo12O403- alone, respectively. Although the system is clearly complex, with oxidation rates dependent on [POM], [DCB]o, zeolite loading, and λ, the results indicate a promising approach for enhancing polyoxometalate photochemistry and for developing new photocatalytic materials.

Introduction A fundamental limitation in the development of photocatalytic technologies has been the lack of robust, high quantum yield photocatalytic materials. A cursory search of the photocatalysis literature reveals that although several thousand studies have been published on the subject of photocatalysis the majority of them revolve around just a few semiconductor materials: TiO2, WO3, ZnO, CdS, and FeOx. In large part, this is because these materials have the highest quantum yields for the general purpose oxidation of organic chemicals in the liquid or gas phase. However, they are still limited, with liquid phase quantum yields usually well below 0.01. A more complex class of photocatalytically active material that has received less attention is the polyoxometalate salts. Several thousand of these chemicals (generated during the condensation/polymerization of simple metal oxides) have been synthesized and characterized (for an excellent review, see ref 1). Many of them share the same general photochemical characteristics of the semiconductor photocatalysts, that is, they can be photoexcited, abstract an electron from the substrate, be reoxidized by an electron acceptor, and return to a “resting state” that can be photoexcited again. However, they have received little attention because their excited-state lifetimes (those that have been measured) are on the picosecond time scale and so are too brief to engage in many chemical reactions.2 In this article, we report a novel concept: supporting several polyoxometalate ions on zeolite supports to enhance the “local” concentration of the oxidizable substrate to levels high enough that bimolecular reactions may occur even on very short time scales. * Corresponding author. E-mail: [email protected].

Zeolites are porous aluminosilicate materials with high surface area, uniform cavities, and channels of nanometer dimensions that are widely used to catalyze a variety of reactions.3,4 By employing zeolites as “solvents” for photochemical reactions, the selectivity and efficiency of many photoinduced electrontransfer reactions can be controlled.5-14 For instance, Turro has reported that alkyl radicals generated from the photolysis of zeolite-confined phenyl ketones experience different fates as a function of the pore size of the zeolite supercage.15 Kochi and co-workers observed that back electron transfer following excitation of the charge-transfer band of pyridinium-arene complexes is retarded in zeolites.16-19 Stabilization of the charge-transfer state of many reactions10,20-23 and transient species such as •OH and O2•-,4,24-26 in the presence of zeolites has also been observed. These observations suggest that zeolites are intriguing hosts for photocatalytic processes. Excellent reviews of zeolite photochemistry have been published recently.6,15,27,28 Some polyoxometalate salts (POM) have been combined with photoactive and inactive support materials29-43 to assist in POM retrieval and to improve thermal catalytic activity.29-43 By supporting POMs on the solid surface, their specific surface area is largely increased (BET specific surface areas of POMs are lower than 10 m2/g).44 This greater surface area may result in an increase in the catalytic activity of POMs by providing more contact area between catalyst and substrate for the surfacemediated electron-transfer reactions to take place. Although the effects of supports on the thermal catalytic activity of POMs have been investigated, 45-48 we are unaware of any reports of the effects of solid supports on POM photochemistry.

10.1021/jp0138126 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/27/2002

Oxidation of Aqueous 1,2-Dichlorobenzene SCHEME 1: Proposed Photocatalytic Cycle of Polyoxometalates

The proposed photocatalytic cycle of POMs, which is analogous to the well-known photocatalytic cycle of semiconductor photocatalysts, is shown in Scheme 1 along with the two possible mechanisms (A and B) for the photocatalyzed oxidation of organic substrates by illuminated POMs. Illumination of the ligand-to-metal charge-transfer band (λmax ≈ 260 nm) of POMs used in this study generates a strongly oxidizing excited-state POM*, with reduction potentials exceeding 2.5 V versus the NHE (path A) for many POMs.49,50 After oxidizing the electron donor, the corresponding reduced POMs are usually reoxidized to their original oxidation state by an electron acceptor such as dioxygen.1 There can also be precomplexation between the substrate and photocatalyst prior to photoactivation (path B). Spectroscopic and kinetic evidence has been provided for the existence of preassociation complexes in the photooxidation of several alcohols and tetramethyl urea.51-55 Photoexcitation of the complex then produces an excited state in which electron/ hydrogen atom transfer from the substrate to POM occurs.51-55 Both pathways also include the relaxation of either POM* or [POM-substrate]*. This relaxation or back electron-transfer process is proposed to be the limiting factor in the efficiency of photoinduced electron-transfer reactions. In this study, we investigated the effect of the NaY zeolite support (NaY, see Materials in the Experimental Section for detailed information) on the photocatalytic activities of three polyoxometalates, H2NaPW12O40, H4SiW12O40, and H3PMo12O40. Photooxidation kinetics of the index chemical 1,2-dichlorobenzene (DCB) in aqueous solution were used as a probe to study the effect of the NaY support on the photophysical and photocatalytic properties of POMs. To investigate the effect of NaY on the mechanism of the POM-mediated photooxidation of DCB, separate experiments were conducted in the presence of the •OH radical scavenger Br- (6.66 and 20 µM). •OH radical scavenger experiments revealed that the NaY supports do not change the POM-mediated photooxidation mechanisms of organic compounds in aqueous solution. Photocatalytic activity of the NaY/POM systems in this study was observed over the wavelength range 250-350 nm.

J. Phys. Chem. B, Vol. 106, No. 16, 2002 4337 Experimental Section Materials. HNa2PW12O40, H4SiW12O40, and H3PMo12O40 (reagent grade) were obtained from Fluka. NaY zeolite (pore sizes 24.7 Å, Si/Al ratio 2.6, average particle size 3.26 ( 0.87 (n ) 116) microns (supplemental), BET surface area 669m2/ g), 1,2-dichlorobenzene (DCB, 99%), HClO4 (99+%), and 4-bromoanisole (BA, 99%) were obtained from Aldrich. ASTMgrade water (18.1 MΩ) was used for all experiments. Aqueous DCB solutions were prepared from a stock DCB-saturated aqueous solution. Analytical DCB stocks were prepared in GC2grade methanol. The internal standard for all quantifications was BA spiked into the extracting solvent. All reagents were used without further purification. Experimental Procedure. The reactor used in all experiments was an Ace Glass Schlenk reactor. The light source was a 100-W Hg arc lamp (Osram) filtered through a monochromator (Photon Technologies Incorporated). The slits were set at 0.6 mm with a (2.4-nm band-pass. Lamp output (12.6 × 10-6 einstein/min at 254 nm, 5.2 × 10-6 einstein/min at 350-370 nm) was constant over the time frame of the experiments, as determined by ferrioxalate actinometry.56 The effective path length was determined to be 2.7 cm by following the method described by Leifer; the method and a schematic of the reactor are provided in Supporting Information.57 Aqueous NaY suspensions were prepared (0.05, 0.1, 0.2, or 0.3 wt %) and adjusted to pH 1.0 with HClO4 following POM addition. The suspension was stirred for 15 min to ensure complete mixing. Aqueous DCB was added to achieve the desired substrate concentration. The reactor was allowed to equilibrate in the dark with stirring for an additional 30 min prior to the beginning of illumination. Samples (2.0 mL) were removed from the sample port with an adjustable spring-loaded syringe (Manostat). Samples were immediately placed in 20-mL EPA vials that had been precharged with 5 mL of chilled extraction solvent (MTBE with BA as an internal standard). HF (500 µL) was added to dissolve the NaY and allow quantitative DCB recovery. The samples were then buffered at pH 7.0 by the addition of 0.2 mL of 1.0 M Na2CO3, and DCB was immediately extracted from the aqueous phase by rapidly mixing samples on a vortex mixer for 30 s. The organic layer was removed and stored at 4 °C until analysis by GC-ECD (gas chromatography with electron capture detection).58 POM adsorption isotherms were obtained by mixing an appropriate amount of NaY in 20-mL EPA vials with varying concentrations of aqueous POM solution at pH 1.0. The resulting suspensions were shaken for 24 h, centrifuged, and filtered through Micropore filters (2.5 mm). UV-vis absorbance spectra of the filtrate were recorded on a Perkin-Elmer Lambda 4 spectrophotometer and used to quantify the extent of POM adsorption. DCB adsorption isotherms were obtained by mixing an appropriate amount of NaY in 20-mL EPA vials with varying concentrations of methanol-dissolved DCB at pH 1.0. The resulting suspensions were shaken for 24 h, and DCB was extracted with MTBE from the aqueous phase by mixing on a vortex mixer for 30 s. The organic layer was analyzed by GCECD. GC-ECD Operating Conditions. GC-ECD analyses were done on a Hewlett-Packard 5890 GC equipped with a 7672A autoinjector, Chemstation integration package, and electron capture detector. The carrier gas was He, and N2 was used as makeup. The carrier gas flow rate was 1.3 mL/min, and the method detection limit for DCB was 0.018 µM. The injector

4338 J. Phys. Chem. B, Vol. 106, No. 16, 2002

Figure 1. Schematic diagram of DCB photooxidation.

port was set for splitless operation at 250 °C. The autoinjector volume was set at 1 µL. The analytical column was a 30-m DB-5 with 0.25-µm film thickness. The temperature program was as follows: 1 min at 80 °C, increased at a rate of 3 °C/min to 180 °C, isothermal at 180 °C for 1 min, increased at 20 °C/ min to 250 °C, and isothermal for 7 min. Results and Discussion A baseline for DCB degradation was established by illuminating aqueous solutions of DCB (λ ) 254 nm; I ) 12.6 × 10-6 einstein/min). DCB solutions with added NaY were also irradiated. None of these solutions exhibited a detectable loss of DCB over the time scale of our experiments. Illumination of solutions of DCB with POMs showed slight photodegradation of DCB, with apparent first-order kinetics consistent with previous observations by Ozer and Ferry (Figure 1).58 However, illumination of solutions of DCB, NaY, and POM combined resulted in rapid oxidation of DCB (Figure 2). 2,3-Dichlorophenol and 3,4-dichlorophenol were the detectable products, but they occurred only in low yields ( SiW12O404> PMo12O403-, which is consistent with the ranking suggested by the DCB photodegradation rate constants with POMs in the absence of NaY (kobs) 1.00 × 10-3, 8.00 × 10-4, and 3.00 × 10-4 min-1 for 0.5 mM PW12O40-3, 0.5 mM SiW12O40-4, and 0.5 mM PMo12O40-3, respectively). This ranking suggests that the mechanism of POM-mediated photooxidation is independent of the support. Catalyst Loading. Typically, photocatalyic activity for supported photocatalysts increases with catalyst loading up to a certain value beyond which photoactivity decreases.59,60 We studied the dependence of kobs while varying NaY loading (0.05-0.3 wt %) and [POM] (0.25-1.0 mM), with a constant [DCB]o of 39.0 µ M. Figure 3 a-c shows how kobs varied with NaY loading and [POM]. The maximum kobs for DCB degradation occurred at 0.2 wt % NaY + 0.5 mM PW12O403- (1.69 × 10-2 min-1), 0.1 wt % NaY + 0.75 mM SiW12O404- (8.1 × 10-3 min-1), and 0.1 wt % NaY + 0.5 mM PMo12O403- (6.80 × 10-3 min-1). Each system displayed a rate maximum at similar conditions. The fall of the photooxidation rate at high [POM] or NaY loading is attributed to competitive adsorption and is consistent with similar observations made by other researchers.59-61 Quantum Efficiency. Quantum efficiencies for DCB photodegradation at those specific POM/NaY combinations were calculated to be 9.75 × 10-2, 4.67 × 10-2, and 3.92 × 10-2, respectively (assuming no reflectance losses). The quantum efficiencies were calculated according to eq 1 (Table 1):57

quantum efficiency )

kobs[DCB]o I/Io

(1)

Quantum efficiency is difficult to determine for each wavelength measured in this study because (a) the reactor is irregularly shaped and (b) the polyoxometalate solutions are not optically “thick” at all wavelengths studied. Accordingly, we normalized the experimental rate against the optical density of the suspension whenever it fell below 2 absorbance units. The resulting plot of quantum efficiency versus wavelength showed that the

Figure 2. DCB photodegradation is dramatically enhanced by the presence of NaY ), 0.1 wt % NaY zeolite; b, 0.5 mM PMo12O403-; +, 0.5 mM SiW12O404-; -, 0.5 mM PW12O403-; 0, 0.5 mM PMo12O403-/0.1 wt % NaY zeolite (R2 ) 0.98, kobs ) 6.80 × 10-3 min-1); 4, 0.5 mM SiW12O404-/ 0.1 wt % NaY zeolite (R2 ) 0.98, kobs ) 7.90 × 10-3 min-1); ×, 0.5 mM PW12O403-/0.1 wt % NaY zeolite (R2 ) 0.98, kobs ) 1.18 × 10-2 min-1). All experiments were carried out at pH ) 1.00 (HClO4), T ) 25 °C, and λ ) 254 nm.

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Figure 3. (a) Photodegradation rate constants of DCB as a function of [PW12O403-] and zeolite loading. (b) Photodegradation rate constants of DCB as a function of [SiW12O404-] and zeolite loading. (c) Photodegradation rate constants of DCB as a function of [PMo12O403-] and zeolite loading. All experiments (parts a-c) were carried out at pH ) 1.00 (HClO4), T ) 25 °C, and λ ) 254 nm

TABLE 1: Optical Densities of Polyoxometalates in a 2.7-cm Path-Length Cell and Quantum Efficiencies (O) of NaY Zeolite/POM-Catalyzed DCB Photodegradation PW12O403wavelength (nm)

optical density

φ

254 280 300 320 340 350 360 370

65.3 36 13.9 4.62 0.915 0.251 0.0351 0

0.0975 0.0858 0.0806 0.0811 0.0920 0.0968 0 0

SiW12O404wavelength (nm)

optical density

φ

254 350

43.9 0.459

0.0467 0.0338

PMo12O403wavelength (nm)

optical density

φ

254 350

41.6 1.74

0.0392 0.0221

efficiency was essentially constant over the wavelengths where photoactivity was observed (Figure 4). This result suggests that the photocatalytic process arises from the same electronic transition across the wavelength range studied. The observed rate enhancement of DCB photodegradation can be attributed to the fact that the high concentration of analyte (due to sorption on the NaY framework) in the vicinity of the

Figure 4. Quantum efficiencies of DCB photodegradation are essentially constant as a function of wavelength. All experiments were carried out at pH ) 1.00 (HClO4) and T ) 25 °C with 0.2 wt % NaY/ 0.5 mM PW12O403-.

photocatalyst (which is also sorbed on the NaY framework) increases the encounter probability between short-lived POM* and DCB by reducing diffusion from 3-D to 2-D, which is supported by the observation that kobs is proportional to the extent of DCB surface coverage (Figure 5). These results are consistent with reports of the enhanced photoactivity of NaYsupported TiO2 that was also ascribed to a high concentration of substrate around the photoactive material.59-61 The high oxygen affinity of the zeolite surface may also account for the increase in the observed rate of DCB photodegradation. Oxygen is essential for the catalytic cycling of POMs in our system because it reoxidizes reduced POMs to

4340 J. Phys. Chem. B, Vol. 106, No. 16, 2002

Ozer and Ferry

Figure 5. Plot of DCB photodegradation rates vs [DCB]o shows saturation at [DCB]o > 30 mM, coincident with the maximum [DCB]ads measured by using the Langmuir adsorption isotherm. All experiments at were carried out at pH ) 1.00 (HClO4), T ) 25 °C, and NaY/0.5 mM PW12O403-.

Figure 6. DCB adsorption on the 0.2 wt % NaY, with pH ) 1.00 (HClO4) and T ) 25 °C.

their original oxidation state (Scheme 1).62 The sorption of O2 onto NaY has been shown to make it a stronger oxidant, with an E° value that was shifted from -0.12563 to -0.4 V64 upon adsorption. Paths A and B would both be affected by this phenomenon. Last, it is also possible that NaY increases the apparent rate of DCB oxidation by suppressing back electron transfer. The excitation of a charge transfer (CT) complex is17,65

D+Ah

hν

k2

z [D• +, A• -] 98 D• + + A• [D, A] y\ k-1 CT complex ion pair

where D and A represent electron donor and electron acceptor, respectively. In the interior of a zeolite, radical ions have been shown to complex the zeolite surface and become highly stabilized.17 Similarly, our NaY-supported POM-DCB system may be viewed as a CT complex, with DCB being the donor and POM acting as the acceptor. We speculate that the back electron transfer from one-electron-reduced POM to oxidized DCB is retarded on the NaY surface, which accounts for the observed increase in the photodegradation rate of DCB (Scheme 1, Path B). Experiments are ongoing to resolve the relative magnitude of the contributions of these three mechanisms to rate enhancement. Adsorption of POM and DCB onto NaY. Batch adsorption experiments revealed that DCB has a strong affinity for NaY. The slope of the Langmuir isotherm for DCB on NaY indicated that the adsorption coefficient (K) for DCB on NaY in aqueous medium is 3630 M-1 (Figure 6). The adsorption of POMs on NaY was also investigated at the optimum NaY loading for DCB photooxidation (Figure 7). NaY/POM association constants were

Figure 7. Adsorption isotherms of (×) 0.2 wt % NaY-PW12O403(R2 ) 0.98); 4, 0.1 wt % NaY-SiW12O404- (R2 ) 0.98); and 0, 0.1 wt % NaY-PMo12O403- (R2 ) 0.98). All experiments were carried out at pH ) 1.00 (HClO4), and T ) 25 °C.

obtained from Langmuir isotherms (eq 2),66 and were determined to have values of 844 (PW12O403-),

[A] [A] 1 ) + max [A]ad [A]ad Kad[A]max ad

(2)

831 (SiW12O404-), and 819 M-1(PMo12O403-). A is the POM concentration, Aad is the amount of POM adsorbed, Amax ad is the maximum amount of POM adsorbed, and Kad is the adsorption constant. These comparable binding constants suggest that differential sorption of POMs is not a significant factor in the observed ranking on the DCB photodegradation rate among NaY/POM systems. Rather, we postulate that the ranking is reflective of the reduction potentials of POM*. Effect of •OH Scavengers. In a previous report, the POMmediated aqueous photooxidation mechanism of DCB was found to be a one-electron oxidation rather than an •OH-mediated process.58 The effect of the NaY support on this process was tested in this study. The effect of Br- on the photodegradation rate of DCB has been calculated (eq 3) on the basis of its known bimolecular rate constant with •OH (1.1 × 1010 M-1 s-1), assuming that free •OH is responsible for the observed degradation.

kpredic HO• )

(

kobs

kHO+DCB [DCB] kHO+DCB [DCB] + kHO+scavenger [scavenger]

)

(3)

Experimental photooxidation rate constants of DCB by each NaY/POM system in this study are reported (Table 2) as well

Oxidation of Aqueous 1,2-Dichlorobenzene

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TABLE 2: Degradation Rate Constants (kobs) of DCB without Any Scavenger and Predicted Degradation Rate Constants (kpredic) of DCB by Eq 1a experimental conditions 0.5 mM PW12O403- + 0.2% zeolite (no scavenger) 0.5 mM PW12O403- + 0.2% zeolite (6.66 µM KBr) 0.5 mM PW12O403- + 0.2% zeolite (20 µM KBr) 0.75 mM SiW12O404- + 0.1% zeolite (no scavenger) 0.5 mM PW12O403- + 0.2% zeolite (6.66 µM KBr) 0.5 mM PW12O403- + 0.2% zeolite (20 µM KBr) 0.5 mM PMo12O403- + 0.1% zeolite (no scavenger) 0.5 mM PW12O403- + 0.2% zeolite (6.66 µM KBr) 0.5 mM PW12O403- + 0.2% zeolite (20 µM KBr)

kobs (min-1)

kpredic (min-1) (based on OH model)

0.0169 0.0152

0.00966

0.0135

0.00745

0.00810 0.00750

0.00463

0.00680

0.00356

0.00680 0.00550

0.00389

0.00480

0.00299

a All experiments were conducted at pH ) 1.00 (HClO4), T ) 25 °C, and λ ) 254 nm.

as those that are predicted and observed in the presence of •OH scavenger Br-. The scavenger is not as effective as predicted by the free •OH model, which is consistent with earlier findings supporting charge-transfer oxidation rather than indirect oxidation by •OH.58 Wavelength Dependence of Photocatalytic Activity. The NaY-supported POMs exhibited photocatalytic activity toward aqueous DCB over the wavelength range 254-350 nm. Quantum efficiencies of DCB degradation at 350 nm by the optimum NaY/POM combination were 9.68 × 10-2 (0.2 wt % NaY + 0.5 mM PW12O403-), 3.38 × 10-2 (0.1 wt % NaY + 0.75 mM SiW12O404-), and 2.21 × 10-2 (0.1 wt % NaY + 0.5 mM PMo12O403-). However, DCB did not undergo any detectable degradation with any of the unsupported POMs used in this study at λ > 340 nm. This observation indicates that employing NaY as a solid support extends the photocatalytic activity of POMs to the longer wavelength region of the spectrum (Figure 4). This is a significant step toward the justification of developing technologies that apply POMmediated photochemical processes. Conclusion The aim of this study was to investigate the effect of a NaY support on the photocatalytic properties of POMs by employing kinetic probes. A significant rate enhancement in the photodegradation of the probe molecule 1,2-dichlorobenzene in the presence of NaY/POM was observed. This enhancement is attributed to the increased encounter probability for coadsorbed DCB and POM*, high oxygen-binding ability of zeolites, and suppressed back electron-transfer reactions. NaY/POM systems maintained their photocatalytic activity up to 350 nm upon systematic illumination between 254 and 370 nm. The use of NaY as a solid support for POM photocatalysis is shown to be a very promising way to improve the photocatalytic efficiency and shift the photocatalytic activity of POM to longer wavelengths. Acknowledgment. We thank the South Carolina Commission for Higher Education for support of this work, the Carolina Nanocenter, Dr. Dana G. Dunkelberger of the USC Electron

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