J. Phys. Chem. B 2001, 105, 9441-9448
Photoluminescence and Raman Spectroscopy as Probes to Investigate Silver and Gold Dicyanide Clusters Doped in A-Zeolite and Their Photoassisted Degradation of Carbaryl Sofian M. Kanan,†,‡ Carl P. Tripp,†,‡ Rachel N. Austin,§ and Howard H. Patterson*,‡ Laboratory for Surface Science & Technology, UniVersity of Maine, Orono, Maine 04469, Chemistry Department, UniVersity of Maine, Orono, Maine 04469, and Chemistry Department, Bates College, Lewiston, Maine 04240 ReceiVed: May 30, 2001; In Final Form: July 16, 2001
Dicyanoaurate and dicyanoargentate ions doped in A-type zeolite were prepared and analyzed spectroscopically. Data from luminescence and Raman spectroscopies along with extended Hu¨ckel calculations indicate the formation of [M(CN)2-]n oligomers (M ) Ag, Au) in the zeolite host. Variations in luminescent properties as a function of excitation wavelength, dopant concentration inside the zeolite host, and temperature facilitate assignment of cluster structures. The data obtained from a comparison of Ag- and Au-doped silica and γ-alumina show that the [M(CN)2-]n clusters form in the channels of the zeolite rather than on the surface. The [M(CN)2-]n clusters in A zeolite show strong catalytic activities toward the photodecomposition of carbaryl in aqueous solution. The photodecomposition of carbaryl at room temperature shows different decomposition products in the presence and absence of the M(CN)2- doped A zeolite catalyst. In addition, the decomposition rate constants increase as the metal content increases. The M(CN)2- doped in A zeolite samples with M ) 24.2 Ag% and 42.3Au% produce photodecomposition rates with carbaryl that are 40 and 60 times faster, respectively, than for carbaryl alone in aqueous solution.
Introduction Zeolites with encapsulated transition metals have been the subject of many recent studies because they can efficiently and selectively catalyze the partial oxidation of hydrocarbons.1-10 Recent synthetic developments have extended the field of transition metal/zeolite chemistry. The application of solid-state reactions of zeolites with various inorganic salts or metal oxides has facilitated the preparation of new catalysts. These techniques have been especially important for incorporating transition metal elements that are difficult to introduce into zeolite cavities by conventional ion-exchange methods.3-5 The anchoring of organometallic compounds to the accessible extraframework cation sites in zeolite hosts is also a new phenomenon, with important implications in molecular separation, catalysis, and materials science.11 Many coordination complexes can form within zeolite pores by simply reacting the exchanged metal cations with various organic molecules. The first characterized coordination compounds formed in zeolite channels were the penta and hexamethyl isocyanide complexes of cobalt prepared by exposing a dehydrated sample of Co-CaY to excess CH3CN at room temperature.12 Many studies have recently been conducted on the interaction of metal carbonyls with zeolites. The incorporation of carbonyl complexes within the large pores of zeolites and their intrazeolite interactions have been investigated with Fe(CO)5, Fe2(CO)9, Fe3(CO)12,13-15 Co2(CO)8,16,17 and Ni(CO)4.18-20 M(CN)2- (M ) Au, Ag) doped in A zeolite samples are important because they serve as catalysts for the photochemical * To whom correspondence should be addressed. Phone: 207-581-1178; Fax: 207-581-1191; E-mail: [email protected]
† Laboratory for Surface Science & Technology, University of Maine. ‡ Chemistry Department, University of Maine. § Chemistry Department, Bates College.
degradation of organic pollutants such as pesticides. For example, carbaryl (1-naphthyl N-methylcarbamate) is widely used as an insecticide throughout the world. Carbaryl is used for cotton, fruit, vegetables, nuts, and other crops and is inherently toxic to humans by skin contact, inhalation, and ingestion.21
We now report the first luminescence study of [M(CN)2-]n (M ) Au, Ag) clusters anchored in A-type zeolite. The luminescence peaks in the emission spectra are related to the presence of different [M(CN)2-]n oligomer sites in the A-type zeolite. The emission spectra for [M(CN)2-] doped in A zeolite results from M-M bonded excimers and exciplexes with a formula of *[M(CN)2-]n. We have previously reported the formation of Ag-Ag bonded exciplexes for Ag(CN)2- doped in alkali halide crystals.22-24 The presence of several bands in the νCN stretching and bending modes in comparison to the pure KM(CN)2 (M ) Ag, Au) starting material indicate the formation of [M(CN)2]n clusters with n g 3. Also, in this study we report photodecomposition studies of carbaryl in the absence and presence of A-zeolite containing different amounts of Au(CN)2and Ag(CN)2- ions. Experimental Section Preparation of the M(CN)2- Doped in A Zeolite Samples (M ) Ag, Au). KAg(CN)2, KAu(CN)2, and potassium acetate were purchased from Alfa-Aesar Chemical Company. NaA
10.1021/jp012038j CCC: $20.00 © 2001 American Chemical Society Published on Web 09/08/2001
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TABLE 1: Relativistic Parameters Used in the Extended Hu1 ckel Calculations atom
6s 6p 5d 5s 5p 4d 2s 2p 2s 2p
-7.937 -3.466 -12.37 -6.453 -3.289 -13.91 -19.39 -11.07 -26.25 -13.83
2.124 1.496 3.471 1.594 1.170 3.248 1.577 1.434 1.886 1.728
Ag C N
zeolite was provided by Professor Douglas Ruthven of the Department of Chemical Engineering, University of Maine. Fumed silica (Aerosil A380) and alumina (Aluminum oxide C) were purchased from DeGussa AG. NaA zeolite was exchanged with potassium acetate at 75 °C for 12 h to facilitate transition metal loading. Potassium zeolite were filtered and washed with distilled deionized water (ddH2O). Then, 3 g of KA-zeolite was exchanged with 50 mL of either 4.0 M aqueous KAg(CN)2 or KAu(CN)2 for 12 h at room temperature. The reaction mixture was filtered, washed and dried in air at 298 K. The same procedure was performed using different KAg(CN)2 and/or KAu(CN)2 concentrations to prepare different metal loadings. Instrumentation. Photoluminescence spectra were collected on a model QM-1 Luminescence spectrometer from Photon Technologies International (PTI). The instrument is equipped with two excitation monochromators and a 75 W xenon lamp. Excitation slit widths were typically set at 1.5 mm with the emission slits set at 2.5 mm. Quartz fluorimetry cuvettes with a path length of 1 cm were used for all luminescence and absorption measurements. Excitation and synchronous-scan spectra were corrected for the lamp background using the quantum counter rhodamine B. Luminescence spectra were recorded as a function of temperature, using liquid nitrogen as the coolant in a model LT-3-110 Heli-Tran cryogenic liquid transfer system equipped with a temperature controller. Raman spectra were recorded on a Renishaw Raman Imaging microscope System 1000. The system is equipped with a diode laser having λex ) 785 nm. All Raman spectra were recorded at room temperature. All irradiations for the photochemical experiments 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-watt tubes that are 8 cm long. Its typical output is 810 µW/cm2. Finally, GC-MS analysis was made on a Hewlett-Packard 5890 gas chromatograph with a Hewlett-Packard MSD 5970 detector. A 30 m × 0.25 mm ID DB-5 MS column from J&W scientific was used. The sample was injected at an initial temperature of 150 °C, then the oven temperature was ramped to 180 °C and held constant at that temperature for 5 min, and ramped to a final temperature of 250 °C and held constant for 5 min. Computational Details. Ground and excited state extended Hu¨ckel calculations were carried out using the FORTICON8 program. The calculations were carried out for different oligomers and geometrical isomers of [M(CN)2-]n (M ) Au, Ag). Relativistic parameters were used for all atoms as shown in Table 1.25 Results and Discussion Two series of samples containing different metal [M(CN)2-]n (M ) Ag, Au) ions doped in A zeolite were prepared and
Figure 1. Emission spectra of the dicyanoargentate ion doped in A zeolite with high silver loading at the indicated excitation wavelengths and 77 K.
characterized based on their photoluminescence properties and Raman spectroscopy. The first set consist of [Ag(CN)2-]n oligomers doped in A zeolite. ICP analysis showed that the silver loadings in these samples were 24.2, 4.6, 2.2, and 0.22 wt % Ag. The second set consisted of [Au(CN)2-]n clusters doped in A zeolite and the gold content in these samples were found to be 42.3, 5.2, 1.5, and 0.22 wt % Au. 1. Dicyanoargentate Clusters Doped in A Zeolite. Figure 1 shows the emission spectra of Ag(CN)2- doped in A zeolite at 77 K and various excitation wavelengths. The spectra depend strongly on the excitation wavelengths. Emission bands at 354, 378-390, and 410 nm occur upon excitation at the indicated wavelengths and are labeled as A, B, and C, respectively. Each of these bands becomes dominant over the others at a characteristic excitation wavelength. Excitation peaks between 250270, 275-290, and 300-330 nm are observed when the emission wavelength is fixed at the maximum values labeled A, B, and C, respectively. The absorption spectum of a dilute aqueous solution of Ag(CN)2- shows a peak only above 50 000 cm-1.26,27 This absorption band has been attributed to a metal-ligand chargetransfer transition.26 In contrast, the observed luminescence bands in Ag(CN)2- doped in A zeolite are strongly red-shifted from the absorption band of dilute aqueous solutions of Ag(CN)2-. Therefore, the luminescence bands are not metal to ligand charge transfer (MLCT) bands but are instead due to M-M interactions. In support of this assignment, it is noted that a red-shifted excitation spectrum has been observed in the luminescence studies for Ag(CN)2- doped in alkali halides.22-24 Both theoretical and experimental results support the formation of excimers and exciplexes between adjacent Ag(CN)2- ions in the alkali halide lattice. In this work, we refer to an excitedstate dimer as an “excimer”, whereas excited-state trimers and longer oligomers are called “exciplexes”. Therefore, we expect these species to form also in zeolites. Because the absorption and luminescence energies of d10 systems are strongly sensitive to metal-metal interactions,22,23,28-31 different emissions are expected to occur from different [M(CN)2-]n oligomers. In recent studies, Omary et al.22,23 have suggested that energy transfer and direct excitation mechanisms are responsible for the emission bands of silver complexes that exhibit multiple luminescence centers. As shown in Figure 1, similar luminescence properties are obtained for Ag(CN)2- doped in A-zeolite. The assignment of the luminescence bands due to different [M(CN)2-]n oligomers are supported by the electronic structure calculations for the ground and the lowest excited states of different [M(CN)2-]n oligomers.
Silver and Gold Dicyanide Clusters
J. Phys. Chem. B, Vol. 105, No. 39, 2001 9443
Figure 2. Potential energy curves for the ground and the first electronic excited states of [Ag(CN)2-]2 (eclipsed form) plotted from extended Hu¨ckel calculations.
TABLE 2: Summary of the Ground and the Electronic Excited States Extended Huckel Calculations of the Eclipsed Forms of Dicyanoargentate and Dicyanoaurate Ions oligomer
M-M distance, Å
binding energy, eV
[Au(CN)2-]2 *[Au(CN)2-]2 [Ag(CN)2-]2 *[Ag(CN)2-]2 lin-[Au(CN)2-]3 lin-*[Au(CN)2-]3 lin-[Ag(CN)2-]3 lin-*[Ag(CN)2-]3 ang-[Au(CN)2-]3 ang-*[Au(CN)2-]3 ang-[Ag(CN)2-]3 ang-[Ag(CN)2-]3
3.45 3.00 3.57 3.02 3.44 3.07 3.48 3.08 3.47 3.14 3.53 3.14
0.131 0.881 0.130 1.121 0.302 1.212 0.332 1.471 0.266 0.877 0.29 1.14
0.0224 0.0735 0.0031 0.0351 0.0117 0.0571 -0.0081 0.0251 0.0201 0.0680 0.0031 0.0479
3.77 3.40 4.36 3.97 3.43 2.95 4.01 3.54 3.61 3.29 4.16 3.82
a Overlap population. b HOMO-LUMO gap. *electronic excited state. lin ) linear. ang ) angular.
We have performed electronic structure calculations for various [M(CN)2-]n oligomers with different geometries in the ground and the first electronic excited state. Figure 2 shows the results of extended Hu¨ckel calculations for potential curves for [Ag(CN)2-]2 (eclipsed configuration) in the ground and the first electronic excited states. In the ground state of the [Ag(CN)2-]2 cluster, the Ag-Ag bond distance is 3.57 Å whereas, the Ag-Ag bond distance becomes shorter (3.02 Å) in the first electronic excited state. Also, in the first electronic excited state, the Ag-Ag overlap population is greater, and the potential well of the Ag-Ag bond is deeper than in the ground state. Table 2 gives a summary of the theoretical electronic structure calculations. As shown in Table 2, a clear trend in the reduction of the HOMO-LUMO energy gap occurs as the oligomer size increases. The luminescence bands for Ag(CN)2- doped in A zeolite are similar to the observed luminescence bands for silver dicyanide ions doped in KCl crystals. Therefore, the emission bands are assigned to different geometrical isomers of [Ag(CN)2-]n exciplexes in the zeolite host. Using results from luminescence studies with Ag(CN)2- doped in KCl, we attribute the photoluminescence bands at 340-360 nm (band A) and at 375-390 nm (band B) to two geometrical isomers (angular and linear forms, respectively) of a *[Ag(CN)2-]3 trimer exciplex. The lowest energy band at 410 nm (band C) is assigned to the exciplex *[Ag(CN)2-]4. A summary of the luminescence bands and their assignments are given in Table 3. It is found that the luminescence properties also depend strongly on the dopant concentrations. As shown in Figure 3a,
the spectra of the Ag(CN)2- doped in A zeolite with low Ag loading shows two luminescence bands in the high energy region at 350 and 375 nm. The intensity of these two bands decreases as the silver loading increases. At higher silver loadings, bands also appear in the low energy region at 410 and 445 nm (see Figure 3d) assigned to *[Ag(CN)2-]4 exciplexes. Thus, an increase in dopant concentration leads to the formation of larger clusters. The statistical distribution of the larger oligomers increases as the total concentration of dicyanoargentate ion in the zeolite host increases.22,23 This leads to a higher population of the *[Ag(CN)2-]n excitons. Data from Raman spectroscopy support the formation of [Ag(CN)2-]n clusters in the A-type zeolite. Figure 4a-c (left) shows the Raman spectra of the Ag(CN)2- samples doped in A zeolite as well as the pure KAg(CN)2 starting material (Figure 4d) in the νCN stretching frequency region. As shown in Figure 4a-c, the Raman spectra in the νCN region (left) strongly depend on the metal loading. Similarly, the low-frequency region also shows three Raman bands at 248, 250, 258 cm-1 that are assigned to the δMCN bending modes as shown in Figure 4 (right). The Raman spectrum of the KAg(CN)2 starting material shows a strong and sharp Raman band at 2146 cm-1 (see Figure 4d) along with a band at 251 cm-1 assigned as νC-N stretching and δMCN bending modes, respectively. The Raman band at 2146 cm-1 for the linear KAg(CN)2- molecule (D∞h) is assigned to the Σg+ mode.32 This mode is non degenerate, therefore, no increase in the number of Raman bands is expected by lowering the symmetry. In contrast, the band at 251 cm-1 is the doubly degenerate M-CN bending mode and this band can split to produce two Raman active modes upon lowering of the symmetry. Two bands at 2159 and 2153 cm-1 are observed and the relative intensity of these bands depend on the metal loadings. The band at 2146 cm-1 seen in the pure KAg(CN)2 starting material is not seen in the zeolite doped materials. One possible explanation is that the KAg(CN)2 band at 2146 cm-1 shifts to lower frequency due to adsorption or interaction with sites on the zeolite surface. To test this possibility, we prepared samples of high surface area, nonporous silica and alumina powders impregnated with KAg(CN)2. In both these samples, the Raman spectra showed bands due to the pure KAg(CN)2 material. There was no shift in the νC-N or splitting in the δMCN modes. Therefore, we conclude that the observed shift and splitting in the νC-N stretching mode in the zeolite impregnated samples is due to the presence of different oligomers. The shift of the νC-N band is due to a change in the Ag-Ag bond distances induced by the cage size in the zeolite. It is known that from Extended Hu¨ckel calculations, the Ag-Ag bond distance decreases as the oligomer size increases.28 The increase in the Ag-Ag strength reduces the back-donation from the silver ion to the empty π* of the CN molecular orbitals and this produces a shorter C-N bond giving rise to a higher shift in the νC-N stretching mode. Thus, the Raman bands at 2159 and 2154 cm-1 are shifted to higher frequency than the Ag(CN)2starting material indicating the presence of Ag-Ag oligomers. 2. Dicyanoaurate Clusters in A Zeolite. Figure 5 shows the emission spectra of the Au(CN)2- doped in A zeolite at 77 K with a gold content of 42.3% at different excitation wavelengths. The emission bands again depend on the excitation wavelength. Bands at 390-398, 410, and 423 nm were observed upon excitation at the indicated wavelengths. The luminescence properties of the higher gold content dicyanoaurates doped in zeolite varied with different excitations. For example, emission bands at 390-410 nm were obtained
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TABLE 3: Tentative Assignment of the Emission Bands Observed in [Ag(CN)2-] and [Au(CN)2-] Clusters Doped in A Zeolite Ag(CN)2- bands λmaxem,
340-360 375-390 410-445
250-270 275-290 300-330
νCN Raman modes
2153 2154 2159
ang-*[Ag(CN)2-]3 lin-*[Ag(CN)2-]3 *[Ag(CN)2-]4
Figure 3. Emission spectra of the dicyanoargentate ion doped in A zeolite at λex ) 280 nm and 77 K with silver content of (a) 0.22% (b) 2.2%, (c) 4.6%, and (d) 24.3%.
Figure 4. Raman spectra of [Ag(CN)2-] samples doped in A zeolite in the VCN stretching mode (left) and δMCN bending mode (right) with silver content of (a) 24.3%, (b) 4.6%, and (c) 2.2% along with the Raman spectrum of the KAg(CN)2 starting material (d).
upon excitation at 280-300 nm. The emission bands at 390410 nm diminish in intensity and another emission band in the low energy region appears when the luminescence spectra is monitored at λex> 300 nm. In contrast, the samples with low gold content (less than 5.0%) show a unique emission band at 390-400 nm with no change in the emission spectra at various excitation wavelengths or by varying the dopant concentration from 0.22 to 5.0% Au (spectrum not shown). An increase in the dopant concentration increases the probability of the formation of larger clusters. A similar result was reported for different concentrations of KAu(CN)2 doped in KCl.33,34 It was found that the emission intensity of the high energy (HE) luminescence bands decreased as the doping concentration increases. The observed reduction of the HE band intensities was found to be associated with a gradual increase in the intensity of the low energy bands (LE) (see Figure 5). To compare the luminescence bands for the gold system with the silver system we performed Extended Hu¨ckel calculations for dimers and trimers with gold and silver atoms. Molecular orbital calculations show that the main difference between the electronic structures of the monomeric forms of Au(CN)2- and Ag(CN)2- lie in the composition of the HOMO orbitals. Two theoretical models have been proposed in the literature to explain the ground-state Au(I)-Au(I) interaction. The first model
νCN Raman modes
Figure 5. Emission spectra of the dicyanoaurate ion doped in A zeolite with 42.3% Au at the indicated excitation wavelengths and 77 K.
attributes the gold-gold interactions to the hybridization of 5d orbitals with 6s and 6p orbitals.35 The second model attributes gold-gold interactions to correlation effects of the electrons, strengthened by relativistic effects that are much stronger in gold than in silver.36 Ground state extended Hu¨ckel calculations of Au(CN)2- show a gold contribution to the HOMO molecular orbitals of 73%, whereas the silver contribution in Ag(CN)2is only 33%. Relativistic effects are expected to destabilize the 5d orbitals of gold more than the 4d orbitals of silver. These effects result in a better match between the 6s and the 5d orbitals of gold than the 5s and 4d orbitals of silver. As a result, the HOMO of the Ag(CN)2- ion has less s and d hybridization compared to the HOMO of Au(CN)2- ion. The dimeric forms of [Au(CN)2-]2 and [Ag(CN)2-]2 show minimum potential energies at Au-Au and Ag-Ag separations of 3.48 and 3.57 Å, respectively. The overlap populations along the metal-metal bonds at distances that show minimum energies are 0.0225 and 0.0021, respectively. The M-M overlap populations in all dimers are small but positive showing the possibility of metal-metal covalent interactions. However, the M-M ground-state overlap population for the gold dicyanide dimer is 10 times greater than for the silver dicyanide dimer. Therefore, the Au-Au ground-state covalent bonding interaction is stronger than the corresponding Ag-Ag case and the stronger metal-metal interaction results in a smaller HOMO-LUMO gap. The HOMO-LUMO gap in metal clusters also depends on the number of metal atoms interacting at a particular site in addition to the individual metal-metal distances. The M-M ground state bonding mainly involves s and dz2 interactions, with little participation of pz orbitals. At short AuAu distances the contribution to the HOMO from the Au dz2 increases. Calculations were also carried out for the trimers of [Au(CN)2-]3 and [Ag(CN)2-]3 complexes. The Au 6s orbital contribution to the M-M interactions decreases while that from the dz2 orbital increases on going from the dimer to the trimer forms. Also the overlap population and the HOMO-LUMO gap decreases on going from dimer to trimer forms. Therefore, based on the theoretical results, we assign the luminescence bands at 390-410 to the presence of a linear *[Au(CN)2-]3
Silver and Gold Dicyanide Clusters
Figure 6. Raman spectra of the dicyanoaurate ion in the νCN frequency region (a) doped in zeolite with 42.3% Au (b) doped in A zeolite with 5.2% Au, and (c) in pure KAu(CN)2 gold and silver dicyanide samples doped in A zeolite along with the low-frequency region of spectrum for [Au(CN)2-]n sample with a highest gold loading (d).
exciplex, whereas, the band at 420-445 nm is assigned to the formation of an *[Au(CN)2-]4 exciplex. These assignments are in agreement with the observed luminescence bands for [Au(CN)2-] doped in alkali halide crystals.34 The samples with low gold loading show a unique band at 390-400 nm that is assigned to a linear *[Au(CN)2-]3 exciplex. This band is not sensitive to the temperature or to the excitation wavelength. Therefore, the luminescence data indicate the presence of one site in the low loading gold samples. The Raman spectrum for the pure dicyanoaurate starting material shows only one sharp band in the νCN region at 2163 cm-1 (see Figure 6c), whereas the Raman spectrum for the highest doped dicyanoaurate zeolite sample shows a strong band at 2176 cm-1 along with several weaker bands in the νCN stretching frequency region (see Figure 6a). In contrast, only one Raman band at 2176 cm-1 is observed for the samples that have a low dopant gold concentration of 0.22% (spectrum not shown). The observed luminescence band at 390-405 nm is associated with the observed νCN Raman stretching mode at 2176 cm-1. Extended Hu¨ckel calculations indicate that the Au-Au ground-state interaction increases as the excimer size increases. The stronger Au-Au interaction reduces the metal to ligand back-bonding and thus strengthens the CN bonding. Therefore, the highest Au(CN)2- dopant concentration doped in the zeolite moiety shows a luminescence band at 420-430 nm. The Raman spectrum for this sample (see Figure 6a) also shows several bands in the νCN region at 2164, 2176, as well as a weak shoulder at 2182 cm-1. The band at 2164 cm-1 is similar to the observed band for the pure potassium dicyanoaurate starting
J. Phys. Chem. B, Vol. 105, No. 39, 2001 9445 material (see Figure 6c). This band is not associated with any of the observed luminescence bands because the monomeric d10 systems do not luminescence. The highest gold loading sample shows a new luminescence band at 420-430 nm. This luminescence band is also associated with a weak Raman shoulder at 2182 cm-1. Further, evidence for the cluster formations in the zeolite host were achieved from Raman spectroscopy in the M-M stretching region. Figure 6d shows the Raman spectrum for [Au(CN)2-] doped in A zeolite in the low-frequency region with the highest gold loading. As shown in Figure 6d, bands at 62 and 88 cm-1 are observed and assigned to the νAu-Au stretching mode. Finally, KAu(CN)2 doped in nonporous silica and alumina gave rise to a single Raman band in the νCN stretching mode at 2163 cm-1 which is similar to the observed Raman band for the pure KAu(CN)2 starting material. KAg(CN)2 doped silica and alumina were also found to not be luminescent at 77 K indicating the formation of [M(CN)2-]n clusters in the zeolite channels rather than on the zeolite surface. 3. Temperature-Dependent Luminescence. The emission spectra of silver and gold dicyanide samples doped in A zeolite are strongly dependent on temperature. Figure 7a and b show the emission spectra of Au(CN)2- and Ag(CN)2- doped in A zeolite, respectively, as a function of temperature. The Au(CN)2doped zeolite sample shows a gradual decrease in the intensity of the high energy emission band (390 nm) and an increase in the intensity of the lower energy emission bands as the temperature increases. Similarly, two strong emission bands at 350 and 423 nm were observed upon the excitation of the Ag(CN)2- doped A zeolite at 280 nm and 77 K (see Figure 7b). At a temperature above 150 K, the intensity of the 350 nm emission band intensity is reduced, while the intensity of the 423 nm band is enhanced. This result indicates that energy transfer from smaller size clusters to larger size clusters is enhanced at higher temperatures. The emission bands for Au(CN)2- doped in A zeolite are also strongly dependent on temperature. At 77 K, the emission band at 390 nm was observed for Au(CN)2- doped in A zeolite. This band shifts to longer wavelengths as the temperature decreases. The observed red shift in the 390 nm band is explained in terms of the reduction in Au-Au separations with decreased temperature. A shift of the luminescence bands to lower energies upon cooling is a well-known spectroscopic feature for compounds with a layered structure. Yersin et al. have established that lowering the temperature results in thermal contraction of the in-plane M-M distances in low dimensional layered compounds.37 Also, a neutron diffraction study of Tl[Au(CN)2-] indicated that upon temperature reduction, the largest lattice parameter “b” becomes longer while the other lattice parameters “a and c” become shorter.38 The increase of the lattice parameter “b” upon cooling is considered as abnormal behavior due to the expected decrease in the cell volume upon cooling in layered solids such as KAu(CN)2 and TlAu(CN)2-.38,39 The b-parameter anomaly increase is mainly related to a linear reorientation of the Au(CN)2- ion, yielding a reduction of the tilt angle in the (a,b)-plane to the b-axis from 50.7 to 47.5 Å. With decreasing temperature, the Au(CN)2- orients more and becomes more parallel to the (a,b)-plane as it moves toward the b-axis. Thus, if the excitons responsible for the luminescence are localized along the lattice parameter that is lengthened upon cooling, then one would expect that the luminescence energy would undergo a blue shift as the temperature decreases. Therefore, the observed red shift of the low energy bands upon cooling result from delocalization of excitons along cell
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Figure 8. A plot of log[carbaryl] as a function of irradiation time (a) carbaryl-only, (b) carbaryl/Au(CN)2- doped in A zeolite, and (c) carbaryl/Ag(CN)2- doped in A zeolite.
Figure 7. Emission spectra of (a) high gold loading of Au(CN)2- ion doped in A-zeolite at λex ) 280 nm, shown in top figure, and (b) high silver loading of Ag(CN)2- doped in A-zeolite at λex ) 290 nm, shown in bottom figure (no luminescence properties are observed at RT).
directions that do not have an abnormal temperature dependence. Instead, the decrease of the M-M in plane bond distances cause the observed red shift of the high-energy bands as the temperature decreases. The temperature dependence of our energy transfer results in the dicyanoaurate system can also be explained in terms of
an exciton model. The equilibrium geometry of [Au(CN)2-]n in the first electronic excited state is different from the equilibrium geometry of the corresponding ground state. This difference is because the increase in the M-M bonding character in the first electronic excited state leads to a large reduction in the M-M distances in the first electronic excited state relative to the ground state. Therefore, the excitation energy of *[Au(CN)2-]n undergoes significant lattice relaxation before the energy transfer process. 4. Photodecomposition of Carbaryl in the Presence and Absence of M(CN)2- (M ) Ag and Au) Doped in A Zeolite Catalysts. The prepared dicyanoaurate and dicyanoargentate doped in A-zeolite samples demonstrated strong catalytic activity toward the photodecomposition of the carbaryl pesticide in aqueous solution and this activity can be linked to the formation of [M(CN)2-]n clusters. Carbaryl emits light at 330 and 334 nm upon excitation at 276 nm. A plot of ln[carbaryl] (measured from the peak areas under the carbaryl emission bands) as a function of irradiation time is linear as shown in Figure 8. Carbaryl decomposes slowly upon exposure to UV light at 254 nm at room temperature. The photodecomposition rate constant of an aqueous solution of carbaryl is 5.0 ( 0.13 × 10-5 s-1 at room temperature. The photodecomposition of carbaryl was dramatically increased in the presence of the metal dicyanide doped in A-zeolite catalysts. For example, the room-temperature photodecomposition rate constants of carbaryl in the presence of the M(CN)2- doped in A zeolite where M ) 24.2% Ag and 42.3% Au are 2.0 ( 0.1 × 10-3 and 3.0 ( 0.1 × 10-3, respectively. A summary of the photodecomposition rate constants of carbaryl in the presence and the absence of the M(CN)2- doped in the A zeolite catalyst is given in Table 4. Furthermore, the photodecomposition rate increases as the metal loading increases with higher reactivity in the presence of Ag(CN)2- doped in zeolite than Au(CN)2doped in zeolite catalyst. There is no change in the Ag and Au dicyanide ions doped in the zeolite as the Raman spectra of the catalysts recorded after irradiation show the presence of the νC-N stretching bands. GC-MS was used to identify photodecomposition products. GC-MS analysis for the photodecomposition products of irradiated carbaryl-only sample for 60 min indicates the formation of R-naphthol along with the carbaryl starting material. In contrast, the irradiated carbaryl in the presence of M(CN)2doped in A zeolite catalysts show three GC peaks with retention times of 12.63, 17.91, and 24.81 min. Figure 9a shows the gas chromatogram of carbaryl after UV irradiation for 60 min in
Silver and Gold Dicyanide Clusters
J. Phys. Chem. B, Vol. 105, No. 39, 2001 9447 SCHEME 1
Figure 9. Gas chromatogram of the irradiated carbaryl solution for 60 min in the presence of Ag(CN)2- doped in A zeolite (a) and the mass spectrum of the 24.81 min GC-band (b).
TABLE 4: Photodecomposition Rate Constants of Carbaryl in the Presence and Absence of the Metal Dicyanide Doped A-zeolite Catalysts catalyst
photodecomposition rate constants s-1 at 298 K
A-zeolite Au(CN)2-/A-zeolite 5.2% Au Au(CN)2-/A-zeolite 42.3% Au Ag(CN)2-/A-zeolite 4.6% Ag Ag(CN)2-/A-zeolite 24.2% Ag
5.00 ( 0.13 × 10-5 1.02 ( 0.15 × 10-3 3.00 ( 0.10 × 10-3 1.44 ( 0.12 × 10 -3 2.00 ( 0.10 × 10-3
the presence of the Au(CN)2- doped in A zeolite catalyst. The same photodecomposition products were identified in the presence of all catalysts. Mass spectra of these bands along with comparison with the GC-MS data of standard solutions of the identified products all indicate the presence of R-naphthol, phthalic acid, and quinone with no indication for the presence of the starting material. Figure 9b shows the mass spectrum of the GC band at 24.81 min, which identifies the presence of phthalic acid in the reaction mixture. Scheme 1 shows a proposed mechanism for the formation of the photodecomposition products. As indicated in Scheme 1, hydrolysis of the carbamate group produced R-naphthol, which is easily oxidized to produce quinone and phthalic acid. We believe that differences in both the rates of decomposition of carbaryl and the distributions of decomposition products between undoped and the Ag(CN)2- and Au(CN)2- -doped zeolites can be attributed to excimer and exciplex formation between the metal ions (Ag+ and Au+) and the carbamate nitrogen. To better understand the role of the catalyst in the photodecomposition of carbaryl, we performed extended Hu¨ckel calculations of Ag(CN)2- and Au(CN)2- bonded to the carbamate (H2NCO2H) nitrogen group by varying the metalcarbamate nitrogen bond distances. In all cases, in the first electronic excited state in comparison to the ground state the
M-N bond distance decreased while the M-N overlap population increased. The strength of the M-N bond in the excited state was also accompanied by a decrease in the bonding strength between the carbamate nitrogen and the carbonyl carbon. Therefore, excimer and exciplex formations are responsible for the observed catalytic reactivities toward the photodecomposition of carbaryl. Scheme 2 (top) shows a proposed mechanism for the photodecomposition of the carbaryl pesticide in the presence of the M(CN)2- doped A zeolite catalysts. Before excitation the carbamate nitrogen is weakly bonded to the silver or the gold metal ions of the catalyst; this bond becomes stronger upon photoexcitation. As a result, the carbamate carbonyl is easily hydrolyzed to produce R-naphthol and carbon dioxide. However, in the absence of the catalyst the carbamate group is hydrolyzed slowly due to stronger bonding between the car-
9448 J. Phys. Chem. B, Vol. 105, No. 39, 2001 bamate nitrogen and the carbonyl carbon as shown in Scheme 2 (bottom). Conclusions This study demonstrates the remarkably rich luminescent properties of [M(CN)2-] (M ) Au, Ag) doped in A-type zeolite. Luminescence results show several emission bands that depend on excitation wavelength, temperature, and the dopant concentration in the zeolite host. A temperature-dependent luminescence study of dicyanoaurate and dicyanoargentate doped in A zeolite shows a gradual decrease in the high energy band intensities while the low energy band intensities increases as the temperature is increased. This is due to the enhancement of energy transfer from monomers to dimers to trimers as the temperature increases. Raman studies of these systems show several stretching bands in the νC-N region indicating the presence of different cyanide sites in the prepared samples. The presence of several Raman modes in the νCN region along with the observed bands in the low-frequency region (νM-M stretching modes) are due to the presence of [M(CN)2-]n clusters rather than a change in symmetry. A zeolite doped with [M(CN)2-]n (M ) Au, Ag) can act as a photocatalyst to decompose carbaryl. The irradiated carbarylalone produces R-naphthol as well as the starting material, whereas, R-naphthol, phthalic acid, and quinone were the only identified products after the irradiation of carbaryl in the presence of M(CN)2- doped in A zeolite with M ) Ag or Au. Rates of carbaryl decomposition were significantly faster in the presence of M(CN)2- doped zeolites. The formation of *[M(CN)2-]n excimers and exciplexes in the zeolite channels as well as excimer formation between the silver or gold atoms and the carbamate nitrogen are responsible for the achievement of the observed catalytic activities. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administrated by the American Chemical Society, for the support of this research. References and Notes (1) Notari, B. AdV. Catal 1996, 253. (2) Corma, A. Chem. ReV. 1997, 97, 2373. (3) Petra´s, M.; Wichterlova´, B. J. Phys. Chem. 1992, 96, 1805. (4) Sass, C. E.; Chen, X.; Kevan, L. J. Chem. Soc., Faraday Trans 1990, 86, 189. (5) Karge, H. G. Stud. Surf. Sci. Catal 1994, 83, 135. (6) Selvam, T.; Vinod, M. P. Appl. Catal 1996, 134, 197. (7) Sen, T.; Ramasawamy, A. V.; Rajamohanan, P. R.; Sivasanker, S. J. Phys. Chem 1996, 100, 3809.
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