Photophysical Properties of Ag(I)-exchanged Zeolite A and the

Jul 12, 2001 - The photodecomposition of malathion in dichloromethane at room temperature shows different decomposition products in the presence and ...
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J. Phys. Chem. B 2001, 105, 7508-7516

Photophysical Properties of Ag(I)-exchanged Zeolite A and the Photoassisted Degradation of Malathion Sofian M. Kanan, Marsha C. Kanan, and Howard H. Patterson* Department of Chemistry, UniVersity of Maine, Orono, Maine 04469 ReceiVed: January 17, 2001; In Final Form: May 21, 2001

Three colored derivatives of Ag(I) doped A-type zeolite were prepared and analyzed spectroscopically at 77 K. These compounds have luminescence spectra containing several emission bands that become dominant at characteristic excitation wavelengths. The white AgA zeolite shows luminescence bands at 290, 350, 423, 500, and 550-650 nm, whereas the yellow- and red-colored zeolites show bands at 500, 545, and 625 nm. These bands are attributed to the presence of different silver clusters in zeolite A. Both the experimental and theoretical results suggest the formation of Ag-Ag bonded excimers and exciplexes in the zeolite host. The photodecomposition of malathion in dichloromethane at room temperature shows different decomposition products in the presence and absence of the AgA zeolite catalyst. In addition, the decomposition rates depend on the color of the AgA zeolite. The white, yellow, and red AgA zeolites produce photodecomposition rates with malathion that are 35, 18, and 20 times faster, respectively, than malathion alone in solution. The increase in decomposition rates is due to the different reactivities of the silver clusters of various sizes and charges on the white, yellow, and red AgA zeolite derivatives. The large size of malathion prevents its entrapment in the zeolite channels; therefore, the photodecomposition process and the catalytic silver clusters must occur on the AgA surface

Introduction The design of zeolite-derived photoactive catalysts for the degradation of organic pollutants is a promising direction for environmental remediation technologies. Zeolites are important technological materials because of their many applications. They show high catalytic activity and specific selectivity toward the reactants, intermediates, and products of chemical reactions.1-7 Some important applications include catalysts for petroleum refining, petrochemical production,8-12 gas storage materials,13 and the use of photochromic and cathodochromic sodalites for information storage, display, and fiber optics.13 The formation of silver clusters in zeolites has been discussed extensively,14-24 with one particular case involving zeolite A doped with Ag. Zeolite A has a three-dimensional network with a formula of Na+12[(SiO2)12(AlO2)12].25 The sodium ions can be fully or partially exchanged with silver ions. The Agx+Na+12-xA zeolite is colorless in the hydrated state, but upon activation, it changes to a yellow color and then to a brick red color.18a-d,26,27 Ra´lek and co-workers observed this property for the first time after thermal activation under vacuum.27 Upon dehydration of the AgA zeolite, Ag+ ions become reduced and produce colored centers that are assigned to linear Ag32+ clusters.28 Jacobs et al., in a study of activated and reduced AgNaA zeolites, proposed that linear Ag3+ clusters are formed upon activation, the ends of which constitute chemisorption sites for hydrogen and oxygen.29 Further investigations into the structures associated with the color changes in AgA led to assignments of some of the clusters as Agn0. It was shown that the color change was due to the formation of Agn0 clusters in the cavities of silver zeolite A. * To whom correspondence should be addressed. Phone: 207-581-1178. Fax: 207-581-1191. E-mail: [email protected].

These neutral silver species were assumed to form at elevated temperatures via an autoreduction process in which O2 from the zeolite framework was released.30 Wasowica et al. have indicated that the presence of water significantly affected the silver agglomeration process in the A-type zeolite.31 In the Ag6NaA dehydrated zeolite at 373 K, the Ag6n+ cluster is formed after γ irradiation at 77 K.31-33 The same hexameric silver species can also be prepared by hydrogen reduction of AgNaA zeolite at a temperature below 298 K.34 Moreover, Seifert et al. have recently showed that activation at room temperature under low pressure is sufficient to enhance the electronic chargetransfer transition from the oxygen lone pairs of the zeolite framework to the empty 5s orbital of the Ag+ ions leading to the yellow form of Agx+Na+12-xA.35,36 Previous studies have demonstrated the presence of ligandunsupported Ag-Ag interactions in mononuclear silver(I) compounds.37-39 Recent examples include Tl[Ag(CN)2] and [Ag(CN)2-] doped in KCl crystals which have been investigated in our laboratory.40 The photoluminescence bands of these species have been explained in terms of excited-state Ag-Ag interactions that lead to exciplex formation. We have recently reported the role of exciplex formation in Ag(I) doped ZSM-5 zeolite toward the photodecomposition of nitric oxide.41 AgA zeolites are important because they serve as catalysts for the photochemical degradation of organic pollutants.42-46 Pesticides are organic pollutants that continue to harm the environment. To limit their harmful effects, new methods of breaking down pesticides are needed. The new method that we are pursuing involves the photodegradation of pesticides in the presence of metal-doped zeolites as catalysts. Malathion is an organophosphorus insecticide that is widely used all over the world. Entry of this pesticide into the aquatic ecosystem can cause severe damage to many nontarget species such as fish. Previous studies showed that malathion causes

10.1021/jp010184j CCC: $20.00 © 2001 American Chemical Society Published on Web 07/12/2001

Properties of Ag(I)-exchanged Zeolite A damage to liver cells.47 Additionally, malathion (1) reportedly breaks down into its toxic metabolite malaoxon, which builds up more in insects than in mammals. This accounts for the selective toxicity of malathion toward insects.48-50

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7509 1,10-phenanthroline in sodium acetate buffer (pH 4) and measuring the absorbance at 510 nm. The calculated value of the extinction coefficient was 1.15 × 104 L mol-1 cm-1. This is in agreement with the reported value (1.10 × 104 L mol-1 cm-1). The lamp flux was determined by irradiating 5 mL of potassium ferroxylate solution for 12 s and determining the number of the Fe2+ ions produced. Knowing that the quantum yield of Fe2+ is unity, the lamp output (f) can be determined from eq 1

f ) nFe+2/xabsφFe+2 t

In this paper, we report the first luminescence study of silverexchanged zeolite A. The peaks in the emission and synchronousscan luminescence spectra are related to the presence of different silver Agnm+ oligomer sites in the A-type zeolite. The luminescence bands produced by the AgA zeolite result from silversilver bonded excimers and exciplexes with a formula of *Agnm+.51 We also report the enhanced photodecomposition rates of malathion (1) in the presence of Ag zeolite A and draw tentative conclusions about relationships between silver cluster structure, exciplex formation, and photocatalysis. Materials and Methods Preparation of the Silver(I) Doped A-Type Zeolites. NaA zeolite, silver nitrate 99%, and aqueous ammonia were purchased from Aldrich Chemical Co. An aqueous solution of Ag(NH3)2+ was prepared by the reaction of 10.0 M AgNO3 with aqueous ammonia at room temperature. After the addition of a few drops of ammonia, a brown precipitate appears, and then an additional amount of ammonia was slowly added to dissolve the precipitate until a clear solution of Ag(NH3)2+ was produced. Silver(I) anchored in A zeolite was prepared by ion exchange of 3 g of NaA zeolite with Ag(NH3)2+ solution for 24 h at 343 K with continuous stirring. The AgA zeolite samples were filtered, washed three times with distilled and deionized water (ddH2O), and then dried in air at 375 K. ICP analysis of silver loading was determined to be 15.09 wt % after drying at 375 K. The white-colored zeolite was treated as follows: degassed at 295 K for 1 h and heated to 350 K in the presence of 5 Torr of N2 for 30 min. The color changed to yellow upon further heating to 400 K for 1 h under the above-mentioned conditions. A red color was observed after further pretreatment to 700 K for 1 h. The same red color appears upon treatment at 700 K under O2 for 10 min. The samples were immediately sealed in quartz tubing with an oxygen-acetylene torch. The color changes also occur photochemically. The white AgA zeolite changed to a yellow color after exposing it to 266 nm laser light with an average power of 4.6 milliwatts for 3 h. To determine the amount of silver doped in zeolites, ICP analysis was performed using an ICP Perkin-Elmer Optima with rf power of 1300 W. Photochemical Reaction Cell. Malathion (diethyl(dimethoxyphosphinothioylthio) succinate) was purchased from Crescent Chemical Co. and used as received (99.5% purity). A 30 ppm solution of malathion was prepared in 99% CH2Cl2 and irradiated with 254 nm light in quartz tubes with 1 mm thickness. Each tube was 100 mm long with an inside diameter of 12.5 mm. The distance from the light source to the middle of the sample was 3.8 cm. The photochemical reaction cell was characterized by using a potassium ferroxylate actinometer. Ferric sulfate was standardized spectrophotometrically by complexing reduced iron with

(1)

where nFe+2 is the number of Fe2+ molecules, xabs is the fraction of light absorbed, φFe+2 is the quantum yield of Fe2+, and t is the irradiation time. Using eq 1, a flux value of 1.73 × 1017 photon s-1 was calculated. The molar flux, fmolar (photons L-1 s-1), can be obtained as follows:

fmolar ) f(1 - 10A(254))/Vrxn

(2)

where A(254) is the absorbance at the lamp emission wavelength of 254 nm, and V is the volume of the reaction solution (L). Having the flux value, the quantum yield φ for the photodecomposition of malathion can be determined using eq 3

φ ) (∆[C]NAI/ t)/fmolar

(3)

where ∆[C] is the change in concentration (moles L-1) and NA is Avogadro’s number (molecules mole-1). All solutions were prepared immediately before the irradiation experiments. Four reaction tubes were irradiated simultaneously. Each reaction tube contained 4 mL of malathion solution or 4 mL of malathion solution mixed with 10 mg of silver A-type zeolite catalyst. At five minute intervals, two reaction tubes were collected for each data point. Luminescence measurements and GC-MS analysis were performed directly after irradiation. Concentrations of the irradiated malathion were calculated using calibration curves of absorbance versus concentration. Malathion has a maximum absorbance at 230 nm with an extinction coefficient of 4.05 × 103 L mol-1 cm-1. Computational Details. Semiempirical energy calculations were carried out using the Austin Model 1 (AM1) method.52-55 In these calculations, the total energy of the molecule is represented as the sum of the electronic energy and core repulsions. The enthalpy of formation of the molecule is then obtained from its total energy by subtracting the electronic energies and adding the experimental heats of formation of the individual atoms. A molecular mechanics (MM2) calculation was carried out to determine the geometrically optimized structure. Extended Hu¨ckel molecular orbital calculations were carried out using the FORTICON8 program (QCMP011). This program allowed for excited-state calculations, which were selected after examining the pertinent frontier molecular orbitals of the models studied. Instrumentation. Photoluminescence spectra were recorded with a PTI spectrofluorometer equipped with two excitation monochromators and a 75 W xenon lamp. Excitation spectra for each of the emission bands have been recorded and corrected for variations in the intensity of the xenon lamp at different wavelengths by the quantum counter method using rhodamine B. The spectra were recorded at low temperature using liquid nitrogen as a coolant in a model LT-3-110 Helitran cryogenic liquid transfer system equipped with a temperature controller. Lifetime measurements were obtained using a Quasi-CW NanoLaser with λex ) 266 nm, gated microsecond detector, and

7510 J. Phys. Chem. B, Vol. 105, No. 31, 2001

Figure 1. Emission spectra of the white Ag(I) doped A-type zeolite at 78 K with the different excitation wavelengths as indicated. Relative intensities between the different spectra are not comparable.

a LeCroy 9310C Dual 400 MHZ digital oscilloscope. All irradiations were performed with an EF-260C UV lamp from Spectronics Corporation. The lamp emits a narrow band of radiation at 254 nm. The light source contains two 6 W tubes that are 8 cm long. Its typical output is 810 µW/cm2. EPR spectra were recorded using a JEOL -TE 100 EPR spectrometer equipped with an Oxford ESR900A cryostat at a liquid-helium temperature of 10 K. GC-MS measurements were made with a Hewlett-Packard 5890 gas chromatograph with a Hewlett-Packard MS 5970 detector. A 30 m × 0.25 mm i.d. DB-5 GC column from J&W Scientific was used. Components of various samples were separated using the following parameters: injector temperature set at 100 °C and detector temperature set at 320 °C. The initial oven temperature of 100 °C was held for 3 min. The temperature was ramped to 150 °C at a rate of 20 °C/min and held constant for 3 min. Finally, the temperature was then ramped to 280 °C at a rate of 20 °C/min and held constant for 3 min. Helium was used as the carrier gas with a flow rate of 1 mL/min. Samples were filtered through a Gelman 2 mm Acrodisc syringe filter to remove any particles that might obstruct the column. Results 1. Photoluminescence of AgA White, Yellow, and Red Forms. The silver(I) A zeolite has photoluminescence spectra that depend on the excitation wavelength. Figure 1 shows the emission spectra of the AgA white powder at 77 K. Five major luminescence bands at 290, 345, 423, 500, and 550-565 nm are observed upon excitation with the indicated wavelengths and are labeled as A, B, C, D, and E, respectively. Each band becomes dominant at a characteristic excitation wavelength. The excitation spectra show peaks between 220 and 240, 245 and 265, 270 and 300, 310 and 340, and ∼350 nm upon monitoring

Kanan et al.

Figure 2. Emission spectra of the yellow Ag(I) doped A-type zeolite at 77 K with the different excitation wavelengths as indicated. Relative intensities between the different spectra are not comparable.

the emission at the maxima labeled A, B, C, D, and E, respectively. Thermal pretreatment of the white AgA zeolite leads to the yellow form, and then further heating gives the red-colored form. We also discovered that the white AgA zeolite changed to a yellow color after exposure with a 266 nm laser for 3 h. In addition, the red-colored zeolite is converted to yellow upon exposure to air as well as to UV light. Therefore, we were not able to record the emission spectra of the red-colored zeolite because of its rapid conversion to the yellow form. Figure 2 shows the emission spectra of the thermally prepared AgA yellow powder at 77 K. Three luminescence bands at 500, 530550, and 613 nm are observed upon excitation with the indicated wavelengths and labeled as D, E, and F, respectively. The photochemically prepared yellow AgA zeolite showed the same three peaks but with different relative emission intensities. For example, in the thermally prepared yellow AgA zeolite, band E is strong relative to both bands C and D, whereas in the photochemically prepared yellow AgA zeolite, bands C and D are strong, whereas band E is weak. Synchronous scan luminescence spectroscopy (SSLS) involves scanning both the excitation and emission monochromators over a specified range of wavelengths with a constant wavelength difference (∆λ) between the monochromators.41 Figure 3a shows the SSLS of the white AgA-type zeolite as a function of ∆λ after correction for variations in lamp intensity. The spectra show resolved peaks labeled A-F (the same labels were given to the ordinary emission spectra shown in Figures 1 and 2). Each peak in the synchronous scan spectra becomes dominant over the other peaks at a particular ∆λ. Figure 3b shows the ∆λ ) 140 nm SSLS of the white and the yellow AgA zeolites prepared by treatment of the white AgA via both thermal and photochemical methods. The white AgA zeolite shows one luminescence band at 450 nm (band C), whereas the yellow AgA zeolites show the presence of bands C, D, and

Properties of Ag(I)-exchanged Zeolite A

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7511

Figure 4. Example of malathion luminescence photodecomposition as a function of irradiation time: (a) 0, (b) 9, (c) 18, (d) 23, (e) 26, and (f) 30 min.

Figure 3. Low-temperature synchronous scan luminescence spectra at 78 K of (a) Ag(I) doped white A-type as a function of ∆λ and (b) AgA white and yellow samples at ∆λ ) 140 nm (relative intensities between the different spectra are not comparable).

TABLE 1: Tentative Assignment of the Luminescence Bands of AgA Zeolite luminescence band A B C D E F

λem, nm

assignmenta

λex nm

280-290 220-240 330-350 245-265 400-450 270-300 490-550 >330 530-550 >330 >550 >330

+

*Ag2 excimer angular *Ag3+ exciplex linear *Ag3+ exciplex bent *Ag3+ exciplex linear *Ag32+ exciplex *Agn+, n > 3 delocalized exciplexesb

a The peaks that appear within each luminescence band (see Figures 1-3) are due to the cluster ions present in different environments in the zeolite lattice, for example, in major channels, minor channels, and surface sites. b Band F is collectively labeled as delocalized exciplexes due to *Agn+ species with n > 3. The number of isomers of a given oligomer with n > 3 is too large to give a definite assignment for the cluster size.

E. In the SSLS, bands D and E are strong in the thermally prepared yellow AgA zeolite, whereas in the photochemically prepared yellow AgA zeolite, bands C and D are strong, whereas band E is weak. A summary of the luminescence bands of the AgA zeolites is given in Table 1. 2. EPR Study of the AgA Zeolites. EPR spectra of the white and the thermally prepared yellow samples were obtained at 11 K. The EPR spectra were recorded with different samples in sealed tubes. The white and yellow samples show doublets with g values of 2.00-2.14 and a splitting constant of 25-50 G. 3. Luminescence Photodecomposition Rates and Quantum Yields of Malathion. Malathion shows two strong fluorescence

Figure 5. Plot of log[malathion] as a function of irradiation time (a) malathion only, (b) malathion/AgA red, (c) malathion/AgA yellow, and (d) malathion/AgA white.

emission bands at 338 and 354 nm (τ ) 20 and 50 ns, respectively) upon excitation at 290 nm (Figure 4a). Molecular orbital calculations indicate major contributions from sulfur atom orbitals in the HOMO orbital and from phosphorus and sulfur atomic orbitals in the LUMO orbitals. Therefore, the emission bands are assigned to transitions between the phosphorus and sulfur π bonds. Figure 4 shows the emission spectra of malathion before UV irradiation (Figure 4a) and after UV irradiation in the presence of AgA zeolite at different exposure times (Figure 4b-f). The malathion concentration was determined by monitoring the emission intensity at 354 nm with λex ) 290 nm. The formation of nonluminescent products upon irradiation allows us to determine the change in the malathion concentration using the luminescence intensity. Figure 5 is a plot of the logarithm of the malathion concentration versus irradiation time. A decrease in the malathion concentration is seen as a function of the irradiated time in the presence and absence of the AgA zeolites and indicates first-order photodecomposition rates. As shown

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TABLE 2: Rate Constants and Quantum Efficiency at 298 K for the Photodecomposition of Malathion in the Presence and the Absence of AgA Zeolites type

malathion-only

Ag/A white

Ag/A yellow

Ag/A red

rate constant (sec-1) φ molecule/photon

4.0 ( 0.16 × 10-5

1.4 ( 0.2 × 10-3

7.0 ( 9.17 × 10-4

8.04 ( 0.15 × 10-4

2.9 ( 0.11 × 10-5

1.1 ( 0.13 × 10-3

6.3 ( 0.22 × 10-4

7.1 ( 0.21 × 10-4

in Figure 5, the photodecomposition rates of malathion in the presence of the AgA white, yellow, and red zeolites are faster than that in the zeolite-free sample. The quantum yield of malathion was calculated in the absence and presence of the AgA white, yellow, and red zeolite catalysts. The data show that the quantum efficiency is much larger in the presence of the AgA zeolites, indicating that malathion is more efficiently photolyzed in the presence of the silver zeolite catalysts. Moreover, the quantum yield results are in agreement with the decomposition rate constants. The photodecomposition rate constants along with the quantum yields of malathion at 298 K in the absence and in the presence of the different colored zeolites are given in Table 2. The same photolysis experiments were performed for malathion in the presence of the A-type zeolite without silver ions, and the rate constants were similar to those of samples of malathion alone in solution. Decomposition experiments of malathion in the presence of Ag(I) doped zeolites without UV light were also performed with no significant decomposition observed. Furthermore, to study the effect of the silver ion on the decomposition rates, we have performed the photolysis experiments in the presence of three AgNO3 solutions. The photodecomposition rates of malathion in AgNO3 solutions were similar to the decomposition of malathion alone with no dependence on the AgNO3 concentrations. As a result, we conclude that the presence of silver clusters on the surface of the zeolite plays a significant role in the observed strong catalytic activity. 4. GC-MS Analysis. Two samples of the malathion solution were irradiated with 254 nm light for 30 min and subsequently analyzed by GC-MS. Sample A contains 30 ppm of malathion alone, and sample B contains 30 ppm of malathion in the presence of 10 mg of AgA zeolite. GC analysis of samples A and B after UV irradiation for 30 min shows peaks at retention times of 8.203, 8.605, and 9.25 min. The mass spectrum of each GC peak was analyzed. The GC peak at 8.203 min produced mass spectral bands at m/z ) 486, 73 (base peak), 147, 244, and 414. The mass spectra for the 8.605 and 9.25 minute GC bands were the same, having peaks at m/z ) 514, 73 (base peak), 147, 207, 281, and 415. In the previous three GC bands, the molecular ion peaks appeared at m/z ) 486 and 514, indicating the possibility of dimer formation. The structures of the four possible dimers are shown in Chart 1. The heats of formation of the four possible dimers 2-5 were calculated using the AM1 method. The molecular ion peak at m/z ) 486 that is apparent in the mass spectrum of the 8.203 min GC peak is assigned to dimer 2 or 3. However, the calculation indicates that dimer 2 is more stable than dimer 3 by 79.5 kJ mol-1. As a result, dimer 2 has been assigned to the GC peak at a retention time of 8.203 min and the mass molecular ion peaks at m/z ) 486. The GC peaks at 8.605 and 9.25 min show similar molecular ion peaks at m/z ) 514. These peaks are assigned to dimers (4) and (5). The difference in the calculated heats of formation for dimers (4) and (5) is 23.9 kJ mol-1.

CHART 1

CHART 2

Malathion only and malathion with AgA zeolite were each irradiated for 15 h. The irradiated sample in the presence of the AgA zeolite shows the same GC-MS data that was observed for sample B. However, further irradiation of malathion alone shows five different GC bands with retention times of 4.09, 4.01, 4.15, 4.58, and 4.81 min. The mass spectrum of each GC band shows molecular ion peaks at m/z ) 144, 170, 128, 116, and 204, respectively. These bands were assigned to compounds 6-10, respectively. The structures of these products are shown in Chart 2. Scheme 1 shows the step-by-step proposed mechanism for the formation of compounds 6-8 and 10 from malathion (1) in the absence of the Ag-zeolite. As illustrated in Scheme 1, compounds 7 and 8 are produced through the formation of the diethylsuccinate radical. Compound 8 then reacts with the generated sulfur radical to produce compound 6. Finally, we predict that compound 10 might form from compound 6 as illustrated in Scheme 1. The loss of two ethoxy groups followed by an inhibition step leads to the formation of compound 9. Discussion We find strong evidence for the formation of silver clusters doped in A-type zeolite. Ordinary luminescence, SSLS, and EPR experimental results along with the theoretical calculations all indicate the formation of silver clusters in the A-zeolite having different sizes and charges. The strong catalytic activity of the prepared AgA zeolite toward the photodecomposition of malathion is attributed to excimer and exciplex formation between the silver ions and the malathion sulfur atoms. The following sections discuss the analysis of the spectroscopic results and the photocatalytic models of the Ag doped in A-type zeolite and the malathion molecule. 1. Structural Analysis of the Silver Clusters in Zeolite A. 1.1. Assignment of the Luminescence Bands. We have recently

Properties of Ag(I)-exchanged Zeolite A

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7513

SCHEME 1

reported the luminescence properties of Ag(I) clusters doped in ZSM-5 zeolite.41 Conventional luminescence spectra of Ag/ ZSM-5 show three emission bands that are very similar in shape and energy to the high-energy bands that are observed in this study. These bands appear at 290, 345, and 423 nm and are labeled A, B, and C, respectively, in the two systems.41 Because the absorption and luminescence energies of d10 systems are strongly sensitive to metal-metal interactions,40a-e different emissions are expected to occur from different Agnm+ oligomers. In recent studies, Omary et al.40a,b have suggested that energy transfer and direct excitation mechanisms are responsible for the emission bands of silver complexes that exhibit multiple luminescence centers. We apply a similar argument to the Ag(I) doped A zeolite because of the similarity of the observed luminescence bands to those exhibited by Ag(CN)2- doped in KCl40a,b and Ag(I) doped in ZSM-5.41 Therefore, emission bands A-C are assigned to different geometrical isomers of Agn+ (n ) 2 and 3) excimer and exciplexes, respectively. The highest energy band at the 280300 nm region is attributed to a *Ag2+ excimer. Also, the photoluminescence bands at 340-370 nm (band B) and at 380420 nm (band C) are attributed to two geometrical isomers (bent and linear forms, respectively) of a *Ag3+ trimer exciplex. Table 1 summarizes the observed luminescence bands of the AgA zeolite and their assignments. Several studies have indicated the presence of Ag2+ and Ag3+ in the zeolite channels. For example, Morton and Preston have carefully studied γ-irradiated AgA zeolite by ESR and detected three paramagnetic silver clusters: Ag2+, Ag3+2, and Ag6+. Unlike the assignment of the linear trinuclear silver clusters, they found that their trinuclear silver cluster had four isotropic lines, indicating the equivalency of the three silver nuclei. As a result they concluded that this trinuclear silver cluster was cyclic with a 2A1′ electronic

Figure 6. Potential energy diagram of the ground and the lowest electronic excited states of Ag32+ in its linear form.

configuration in D3h symmetry.56 The formation of Agn+ clusters were found to be due to the presence of silver atoms in site I which interacts with a silver ion (Agn-l+) in site I′.19,57,58 Photoluminescence studies of Agn+ clusters have shown that Ag-Ag interactions are much stronger in the first electronic excited state than in the ground state.40 We have performed electronic structure calculations for various silver oligomers with different geometries and charges in the ground and the first electronic excited state. Figure 6 shows the results of extended Hu¨ckel calculations for Ag32+ (linear form) in the ground and the first electronic excited state. In the ground state of the Ag3+2 cluster, the Ag-Ag bond distance is 2.8 Å with a binding energy of 1.23 eV, whereas the Ag-Ag bond distance becomes shorter (2.4 Å) with a Ag-Ag binding energy of 2.02 eV in the first electronic excited state. The results also show that Ag-Ag antibonding character exists in the ground state and bonding

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TABLE 3: Summary of the Results of Extended Hu1 ckel Calculations for the Minimized Structures of the Indicated Compounds Ag-Ag dist, Å

total energy, eV

g. state ex. state Ag30 bent g. state ex. state Ag3+ bent g. state ex. state Ag32+ bent g. state ex. state Ag32+ linear g. state ex. state Ag40 linear g. state

3.0 2.3 3.0 2.6 2.5 2.3 2.9 2.3 2.8 2.4 2.4

-298.20 -293.60 -440.18 -440.43 -445.16 -443.56 -447.03 -439.84 -446.63. -442.52 -589.16.

0.02 2.57 3.03 3.52 2.43 3.72 1.93 -2.53 1.23 2.02 3.21

ex. state

2.3

-587.15

4.35

g. state

2.4

-592.42

5.01

ex. state

2.3

-592.35

5.23

compound

electronic state

Ag2+

Ag4+ linear

binding energy, o.p.a H-Lb eV Ag-Ag eV 0.0097 0.4698 0.3167 0.4134 0.2897 0.3530 0.1590 0.2863 0.2112 0.2166 0.6351c 0.3652d 0.6377c 0.3640d 0.4424c 0.3458d 0.4546c 0.3446d

4.75 2.44 2.80 3.06 1.61 1.79

1.92

a Overlap population. b HOMO-LUMO energy difference. c Ag1Ag2. d Ag2-Ag3.

character is present in the first electronic excited state of all clusters. Also, in the first electronic excited state of each cluster, the Ag-Ag overlap populations are greater and the potential wells along the Ag-Ag bonds are deeper than in the ground state. Table 3 gives a summary of the theoretical electronic structure calculations. The results in Table 3 also show that the Ag-Ag bonding in the Agnm oligomers is highly sensitive to the cluster size (n), the geometry, and the charge (m) of a given oligomer. These factors affect the HOMO-LUMO gap causing different emission bands to exist for various Agnm oligomers. As shown in Table 3, the HOMO-LUMO gap decreases as the oligomer size increases. In addition, the dimer Agn+ shows a HOMOLUMO gap of 4.75 eV, and this gap decreases to 1.92 eV for the exciplex with n ) 4. The calculation also shows that the HOMO-LUMO gap is larger for the bent form than the linear form for a given cluster. The white AgA zeolite has low-energy luminescence bands at 500 and 550 nm (bands D and E, respectively). Extended Hu¨ckel calculations show that the HOMO-LUMO gap decreases as the cluster size increases or as the charge of the silver cluster decreases. As shown in Table 3, the HOMO-LUMO gap of the bent form of Ag30 and the linear Ag32+ oligomers are 2.44 and 1.61 eV, respectively. These energy gaps are relatively similar to the emission energies at 500 and 550 nm, respectively. Therefore, bands D and E are assigned to the bent *Ag30 and linear *Ag32+ exciplexes. Several studies have indicated the presence of Ag30 and Ag32+ clusters in the A-type zeolite. For example, a far-infrared study of the hydrated AgA zeolite shows absorption bands at 149 and 110 cm-1 that are ascribed to the asymmetric stretching and deformational skeletal modes of linear Ag3+2 entrapped in the β-cage sodalite.59 Moreover, Michalik et al. have observed two different trimeric silver sites that show strong interaction with the zeolite lattice as evidenced from the EPR spectra.17 The luminescence properties of the yellow AgA zeolite suggest the formation of larger silver clusters. The 500 and 530550 nm emission bands are similar to bands D and E observed in the white sample and are assigned to bent *Ag30 and linear *Ag32+ delocalized exciplexes, respectively. The low-energy band at 613 nm (labeled as band F) appears in the yellow silver

zeolite luminescence spectra and is attributed to delocalized *Agnm+ exciplexes with n g 4. The high-energy emission bands (bands A-C) that are dominant in the white AgA zeolite have completely disappeared in the colored AgA zeolites. The color changes have been attributed to charge transfer from the oxygen atoms in the zeolite framework to the silver cations.35,36,60 In this study, we discovered that the color changes occur only in AgA samples with a high silver loading of 15.09 wt % with no change in color being observed in AgA having 2.5 wt % and 6.02 wt % silver loadings. Oxygen treatment at elevated temperatures of Ag-exchanged zeolite A with high silver loading induces silver ion migration and subsequent formation of Ag32+ and larger clusters. The disappearance of the high-energy emission bands for the colored AgA zeolites (bands A-C) and the enhancement of the emission intensities of the low-energy bands support the charge transfer from the zeolite framework to the entrapped silver ions thermally or photochemically. 1.2. Synchronous Scan Luminescence Spectra (SSLS). SSLS is a technique used to study chemical systems in which more than one luminophore is present. The advantage of SSLS is that it has more well-defined peaks when compared to normal emission and excitation spectra. We have recently reported the principles of this technique.41 Figure 3a shows the SSLS of the white AgA zeolite as a function of ∆λ. The spectrum shows the resolved peaks labeled A-F. At low ∆λ’s, bands are observed at 290, 330, and 390 nm, whereas at higher ∆λ’s, bands at 450, 480, and 550-620 nm are dominant. The dependence on the ∆λ can be explained by considering that different silver species in the A-type zeolite have different Stokes shifts. The appearance of several peaks as shown in Figure 3a indicates the presence of several environments of Ag(I) ions in the zeolite lattice. The Agnm+ oligomers can be located in major channels, in minor channels, and on the surface of zeolite A. In conventional luminescence spectroscopy, the spectrum shows resolved bands when the luminescence is monitored as a function of the excitation energy. However, in order to observe a narrow peak with the synchronous scan method, either the excitation or the emission spectra have resolved structure in a given spectral range. This increases the chance of obtaining resolved spectral peaks. The highest peak intensities are obtained when ∆λ is equal to the difference between the emission and the excitation maxima. For example, band C at 420-440 nm was dominant in the SSLS of the white AgA zeolite at ∆λ ) 140 nm as shown in Figure 3b. In the conventional luminescence studies, an emission band was observed at 423 nm (band C) upon excitation at 270-300 nm. Figure 3b shows the SSLS of the white and the two yellow AgA samples at ∆λ ) 140 nm and 77 K. The white AgA zeolite shows one luminescence band at 450 nm (band C), whereas the two yellow AgA zeolites show the presence of bands C, D, and E. In the thermally prepared AgA zeolite, band C appears as a weak shoulder, whereas bands D and E are strong. In the photochemically synthesized AgA yellow zeolite, bands C and E are weakly intense, whereas band D is strong. Therefore, large clusters were formed in the colored AgA zeolites. These results also support the idea that thermal and photochemical energy transfer processes activate the formation of *Agn+ excimers and exciplexes. 1.3. Analysis of the EPR Data. The EPR spectra of the white and the thermally prepared yellow AgA zeolites show that an isotropic doublet exists in both samples. The white sample shows an isotropic doublet with g values of 2.13 and 2.14 and a

Properties of Ag(I)-exchanged Zeolite A

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7515

Figure 7. Potential energy curves of the ground and the lowest electronic excited states along the phosphorus-sulfur bond of intermediate I in the absence of the Ag(I) ion.

Figure 8. Potential energy curves of the ground and lowest electronic excited states along the silver-sulfur bond of intermediate I in the presence of the Ag(I) ion.

splitting constant of 25-50 G. The yellow sample shows a doublet with g values of 2.07 and 2.05 with a similar splitting constant to that observed in the white AgA zeolite. One EPR band is expected for a dimer form (only the Ag2+ excimer is paramagnetic), but because two EPR signals were observed, they must be attributed to the trimeric silver species in the AgA zeolite. The trimeric silver clusters may have different charges and/or different geometries. Molecular orbital calculations indicate that both Ag32+ and Ag30 trimers are paramagnetic because of the presence of unpaired electrons. Two EPR signals may be assigned to the presence of both Ag32+ and Ag30 forms or to the presence of one of these forms in different conformations (i.e., linear and bent forms). Brown and co-workers have reported the EPR spectra of the Ag32+ form, which was observed in silver clusters of doped Y zeolite.61 Two EPR signals with giso of 1.974 and a large coupling of 515 G have been observed and assigned to Ag32+.61 Therefore, we assign the doublet EPR signals observed in our study to a reduced form of the trimeric silver cluster (Ag30) in its linear and bent forms. 2. Relationship between Cluster Structure and Catalytic Activity. The large difference in the photodecomposition of malathion in the presence and the absence of AgA zeolite may be attributed to the entrapment of the pesticide molecule inside the zeolite channels or on the high surface area of the AgA catalyst. However, the large size of the malathion molecule (10.8-11.9 Å) makes it is impossible to fit inside the AgA zeolite channels of 6.0-6.5 Å. Therefore, the photodecomposition of malathion occurs only at the surface of the silver zeolite catalyst that facilitates the formation of the dimer products. It has been reported that the Ag3+ exciplex forms on the surface of the zeolite.28,41,62 Moreover, we have tested the photodecomposition rates of the carbofuran pesticide and smaller organic compounds in the presence of AgA catalyst and Ag doped in X and mordenite zeolites. The study indicated that the photodecomposition of carbofuran in the presence of AgA catalyst is much faster than the photodecomposition of malathion/ AgA catalyst. Also, the photodecomposition rate of carbofuran increased as the channel size of the zeolite increased, indicating that the entrapment of the studied molecule in the zeolite channels enhanced the photodecomposition rates.63 Extended Hu¨ckel calculations for the phosphorus-sulfur portion of malathion were performed in the absence and presence of the Ag(I) ion. Compound I was used without silver bonding to understand the stability of the phosphorus-thionyl site upon irradiation. Figure 7 shows the potential energy curves that are obtained upon variation of the P-S bond distance. As indicated in Figure 7, in the first electronic excited state, the

P-S bond separations are shorter, the P-S overlap populations are greater, and the potential wells along the P-S bonds are deeper than in the ground state.

Extended Hu¨ckel calculations of complex II were also carried out for the minimized structure of compound I and the Ag-S bond distance was then varied. Figure 8 shows the potential energy diagram of compound II as a function of the Ag-S bond distance. The Ag-S bond distance was 2.7 Å in the ground state and 2.2 Å in the first electronic excited state. The calculations were also performed for complexes III and IV, respectively. In the ground state, the Ag-S bond distance was 2.8 Å for both the Ag2+ excimer (compound III) and the Ag3+ exciplex (compound IV). On the other hand, in the first electronic excited state, the Ag-S bond distances were found to be 2.3 and 2.5 Å for the Ag2+ excimer and Ag3+ exciplex, respectively. The strong Ag-S interaction in the excited state increases the stability of the dimers and prevents any further photodecomposition. The calculations show excimer formation between the silver and sulfur atoms; that is, the Ag-S bond distance is shorter and the Ag-S binding interaction is stronger in the first electronic excited state than in the ground state. Therefore, Ag-S excimer formation is responsible for the stability of the dimer products that form in the presence of the AgA zeolites. We conclude that the presence of silver clusters on the A-zeolite surface form stable bonds between the sulfur atoms and the silver clusters. The strength of the Ag-S excimers were found to be in the following order: Ag+ > Ag2+ > Ag3+ > larger clusters. Thus, the presence of small size clusters enhance the stability of the dimeric products (shown in Chart 1) by forming strong excimers between the malathion sulfur atoms and the silver atoms on the zeolite surface. This explains the high catalytic activity of the white AgA zeolite (contains Agnm+ clusters with n ) 1, 2, and 3) compared to the yellow and red AgA zeolites (contains only large clusters with n > 3). Conclusions This study highlights the first spectroscopic study of the different colored AgA zeolites showing the presence of Agnm+

7516 J. Phys. Chem. B, Vol. 105, No. 31, 2001 oligomers in the A-type zeolite. The study also shows that silver clusters doped in zeolite can act as photocatalysts to decompose organic molecules such as malathion. Both conventional and synchronous scan photoluminescence results underscore the presence of multiple environments of Agnm+ clusters in the zeolite channels. This luminescence study of the white AgA zeolite indicates the presence of Agn+ oligomers with n g 2. The formation of Ag-Ag bonded excimers and exciplexes in A-type zeolite gives rise to the different luminescence bands. Mild to severe thermal pretreatment of the white AgA zeolite with high silver loading leads to the formation of the yellow and red AgA-type zeolites. Luminescence of the colored samples indicates the presence of Agnm+ oligomers with n g 3. The color changes are attributed to charge transfer from the framework oxygen atoms and silver ions to large silver clusters that activate the formation of *Agn+ excimers and exciplexes. The photodecomposition rate constants of malathion are in the following order: malathion only < malathion/AgA red < malathion/AgA yellow < malathion/AgA white. GC-MS analysis of each photodecomposition reaction shows that with malathion the products change in the presence of the AgA catalyst. Acknowledgment is made to the donors of the Petroleum Research Fund, administrated by the American Chemical Society, for the support of this research. The authors wish to thank Professor Rachel Austin, Bates College in Lewiston, Maine, for assistance with the ICP and EPR experiments. References and Notes (1) Cano, M. L.; Cozens, F. L.; Fornes, V.; Garcia, H.; Scaiano, J. C. J. Phys. Chem. 1996, 101, 18145. (2) Anpo, M.; Matsuoka, M.; Mishima, H.; Yamashita, H. Res. Chem. Int. 1997, 23, 197. (3) Vasenkov, S.; Frei, H. J. Phys. Chem. A 2000, 104, 4327. (4) Liu, X.; Zhang, G.; Thomas, J. K. J. Phys. Chem. 1995, 99, 10024. (5) Burch, R.; Scire, S. Appl. Catal., B 1994, 3, 295. (6) Goryashchenko, S. S.; Alimov, M. A.; Fedorovskaya, E. A.; Slovetskaya, K. I.; Slinkin, A. A. Kinet. Catal. 1994, 35, 588. (7) Panov, G. I.; Sheveleva, G. A.; Kharitonov, A. S.; Romannikov, V. N.; Vostrikova, L. A. Appl. Catal., A 1992, 82, 31. (8) Van Bekkum, H.; Flanigen, J. C. Introduction to Zeolite Science and Practice; Elsevier Science Publishers: Amsterdam, 1991; Vol. 58. (9) Sauer, J. Chem. ReV. 1989, 89, 199. (10) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. ReV. 1994, 94, 2095. (11) Van Santen, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637. (12) Maxwell, I. E.; Naber, J. E.; de Jong, K. P. Appl. Catal., A: General 1994, 113, 153. (13) Stein, A.; Ozin, G. A. Sodalite: An Old Material for Advanced Uses. In AdVances in the Synthesis and ReactiVity of Solids; Mallouk, T. E., Ed.; JAI Press: London, 1991, Vol 3. (14) Gachard, E.; Belloni, J.; Subramanian, M. A. J. Mater. Chem. 1996, 6, 867. (15) Gellens, L. R.; Smith, J. V.; Pluth, J. J. J. Am. Chem. Soc. 1981, 103, 51. (16) Schoonheydt, R. A.; Hall, M. B.; Lunsford, J. H. Inorg. Chem. 1983, 22, 3834. (17) Michalik, J.; Wasowicz, T.; der Pol, A. V.; Reijerse, E. J.; de Boer, E. J. Chem. Soc., Chem. Comm. 1992, 29. (18) (a) Kim, Y.; Seff, K J. Am. Chem. Soc. 1978, 100, 3801. (b) Kim, Y.; Seff, K. J. Am. Chem. Soc. 1978, 100, 6989. (c) Kim, Y.; Seff, K. J. Phys. Chem. 1978, 82, 1307. (d) Kim, Y.; Seff, K. J. Phys. Chem. 1978, 82, 1071. (e) Kim, Y.; Seff, K. J. Phys. Chem. 1978, 82, 921. (f) Seff, K. Acc. Chem. Res. 1976, 9, 121. (19) Gellens, L. R.; Mortier, W. J.; Schoonheydt, R. A.; Uytterhoeven, J. B. J. Phys. Chem. 1981, 85, 2783. (20) Jacobs, P. A.; Tielen, M.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans 1 1976, 72, 2793. (21) Anpo, M.; Tomonari, M.; Fox, M. J. Phys. Chem. 1989, 93, 7300. (22) Anpo, M.; Shima, T.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305. (23) Anpo, M.; Sunamoto, M.; Che, M. J. Phys. Chem. 1989, 93, 1187.

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