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J. Phys. Chem. C 2009, 113, 4560–4565
How Does Percolation Behavior Influence Binding? A Comparison of 2,4-Xylidine and 2,4-Dichlorophenol at/in Ruthenium(II)-tris-Bipyridine/Titanium Dioxide Co-doped Zeolite Y Megh Raj Pokhrel,† Katharine Janik,‡ and Stefan H. Bossmann*,‡ Department of Chemistry, TribhuVan UniVersity, Kathmandu, Nepal, and Department of Chemistry, Kansas State UniVersity, 111 Willard Hall, Manhattan, Kansas 66506-3701 ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: January 19, 2009; In Final Form: January 21, 2009
Ruthenium(II)-tris-bipyridine/titanium dioxide co-doped zeolite Y (Ru(bpy)32+/TiO2/Y) are efficient and longterm stable heterogeneous photocatalysts in solar Fenton-type oxidation reactions. However, these catalysts have exhibited an inverse dependency of the oxidation efficiency on the concentration when aromatic substrates were treated. Several mechanistic reasons had previously been invoked, such as competitive absorption of H2O2 and the substrates at the nanocrystalline TiO2 centers or parasitic light absorption by oxidation intermediates. Here, we present luminescence depolarization studies, indicating that the adsorption of the substrate, which can occur either at the surface or in the interior of the heterogeneous photocatalyst, may play an important role in the observed photocatalytic reactivity. Whereas the neutral 2,4-dichlorophenol is mainly adsorbed on the zeolite’s surface, 2,4-xylidine is able to percolate into the zeolite. We attribute the observed low reactivity at high 2,4-dichlorophenol concentrations to the formation of a hydrophobic substrate layer at the outside of the zeolite, which prevents H2O2 from entering the zeolite’s framework. TiO2
Introduction
• Ru(bpy)2+* + H2O2 98 Ru(bpy)3+ 3 3 + HO + HO (1)
Zeolite photocatalysts utilizing the principle of titanium dioxide sensitization are enjoying a renewed interest for solar hydrogen generation, as well as the oxidation of pollutants in water.1-4 Ruthenium(II)-tris-bipyridine/titanium dioxide/ zeolite Y photocatalysts ([Ru(bpy)3]2+/TiO2/ zeolite Y) can be regarded as one example for heterogeneous AOP possessing nanoscale components, which could be suceessful not only in the laboratory, but also on a technical scale.5-10 The development of long-term stable heterogeneous reaction systems, which can be integrated more easily into technical remediation processes than homogeneous processes, is of a high priority. However, [Ru(bpy)3]2+/TiO2/zeolite Y photocatalysts are known to lose their effectiveness when treating high concentrations of aromatic amines and chlorophenols.10 In our previous studies, we have identified several potential mechanistic reasons for this inverse dependence of the photocatalytic activity on the subtrates’ concentration: (a) azo-dyes were discovered as light-absorpting intermediates during the oxidation of 2,4-xylidine;11 (b) competitive displacement of hydrogen peroxide by the substrates 2,4xylidine and 2,4-dichlorophenol on the catalytic TiO2nanocenters in the zeolite Y’s interior and on its surface.10 The decrease of H2O2 on the catalytic TiO2 centers leads to inefficient electron transfer from the excited Ru(bpy)3]2+* sensitizers to H2O2 according to eq 1 and, therefore, to a reduced concentration of TiO2-bound hydroxyl radicals.7,9
Since the manifold of advanced oxidation processes (AOPs) is initiated by hydrogen abstraction, addition, or electron transfer reactions of the hydroxyl radical, the AOP efficiency drastically decreases with a lower concentration of hydroxyl radicals.11 The intermittent appearance of a parasitic absorption has been confirmed by vis spectroscopy when degrading aromatic amines.11 However, it does not account for the reactivity of chlorinated phenols in this heterogeneous AOP.10,11 Therefore, it would be highly desirable to either prove or disprove whether competitive adsorption at TiO2 really exists. One experimental strategy is to determine the concentration of the substrate INSIDE the zeolite photocatalyst. In this report, we describe the use of luminescence-depolarization spectroscopy to gain insight into the mobility of individual Ru(bpy)32+ complexes inside the supercages of zeolite Y as a factor of solvent polarity and especially when loaded with the substrates 2,4-xylidine and 2,4-dichlorophenol. At pH 3, 2,4-xylidinium should be is able to percolate into the zeolite, whereas the (at this pH) neutral dichlorophenol may have difficulties to enter into the zeolite Y’s network and consequently may form a hydrophobic layer at the exterior thus hindering the access of hydrogen peroxide. The formation of hydrobobic substrate layers may explain the observed photocatalytic reactivity.7,10,11 Methods
* To whom correspondence should be addressed. E-mail: sbossman@ ksu.edu. Phone: 785-532-6817. Fax: 785-532-6666. † Tribhuvan University. ‡ Kansas State University.
All reagents and solvents used were of the highest purity available and were purchased from Fisher Scientific. H2O was of bidestilled quality. The heterogeneous photolysis experiments
10.1021/jp809952q CCC: $40.75 2009 American Chemical Society Published on Web 02/25/2009
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were carried out in a batch reactor (V ) 0.50 L, optical path length ) 2.54 cm, diameter ) 7.65 cm, cutoff of the pyrex sleeve ) 325 nm). The reactor was equipped with a medium pressure mercury lamp (Hanovia, 100 W). The synthesis of the [Ru(bpy)3]2+/TiO2/zeolite Y photocatalysts (23% ( 0.5% (wt) [Ru(bpy)3]2+, 34% ( 0.5% (wt) TiO2) is described in detail in ref 7. The reaction system was intensively purged by compressed air. The analysis was performed immediately after taking the samples (2.0 mL) from the reactor, adding 1.0 mL of 0.50 M Na2SO3 solution to consume all residual H2O212,13 and filtering the suspension using Nylon Luer-Lock membrane filters (Fisher, 0.22 × 10-6 m). The Langmuir isotherms were recorded following the same technique, except that Na2SO3 was omitted. For quantitative HPLC analysis an HP Series II 1090 liquid chromatograph, equipped with a diode-array detector (DAD) and a LiChrospher-100 RP 18 column and precolumn, were used (mobile phase: (0.10 mol L-1 (C2H5)3N/H3PO4 (pH 7.0)/ acetonitrile, 75:25 v/v)) for the determination of 2,4-xylidine and reaction intermediates. The UV/vis absorption of the photolysis suspension and the filtered solutions was monitored by using a HP 8342A diode-array spectrophotometer. The maximal experimental error, determined by three consecutive photolysis experiments under identical conditions, was (6 rel. % of the determined substrate concentrations as a function of time. Polarized luminescence spectra were recorded by using a single-photon counter (Edinburg Analytical Instruments, EAIFS/FL900). Results and Discussion The first question that will be addressed here is Where is the substrate? It is obvious that the framework of zeolite Y should permit the percolation of both substrates because the window regions between the supercages are not completely blocked by TiO2 when optimal loadings ( 0, because NaY has been employed as starting material.7 On the other hand, 2,4-dichlorophenol should be neutral and more hydrophobic.11 Binding of 2,4-Xylidine and 2,4-Dichlorophenol to the Photocatalysts. The Langmuir isotherm (eq 2), although originally developed for the adsorption on surfaces featuring discrete adsorption sites, is a simple and straightforward method to compare the absorption of our substrates.11,14,15
cabs(t) )
cmaxKc(t) 1 + Kc(t)
(2)
where cabs(t) is the concentration that is absorbed by the photocatalyst, cmax is the maximal concentration that is absorbed by the photocatalyst, c(t) is the concentration that is added to the heterogeneous system at the time t (all concentration are in mg of carbon per kg (mg C kg-1), K is the absorption constant (kg (mg C)-1 g-1), and t is time (s).
TABLE 1: Maximal Concentrations That Are Absorbed by the Photocatalyst (cmax) and Absorption Constants K from Aqueous Solution at pH 3.0 2,4-xylidine 2,4-dichlorophenol
cmax (mg C kg-1)
K (kg (mg C)-1 g-1)
32.8 ( 1.0 43.7 ( 1.2
4.1 ( 0.3 14.2 ( 0.4
TABLE 2: Apparent Rates of 2,4-Xylidine and 2,4-Dichlorophenol Oxidation As a Function of Substrate Concentration
500 400 300 200 100
mg mg mg mg mg
C C C C C
kg-1 kg-1 kg-1 kg-1 kg-1
apparent rates of 2,4-xylidine removal, s-1 g-1
apparent rates of 2,4-dichlorophenol removal, s-1 g-1
5.75 × 10-3 6.02 × 10-3 7.51 × 10-3 8.45 × 10-3 8.20 × 10-3
1.41 × 10-3 1.72 × 10-3 2.23 × 10-3 3.28 × 10-3 3.88 × 10-3
The results from the Langmuir isotherms, as summarized in Table 1, are described in detail in ref 11. 2,4-Dichlorophenol, possessing an (estimated) octanol-water partition coefficient (log P)16 of 2.76, is 3.5 times more strongly absorbed by the zeolite-based photocatalyst than the more hydrophilic 2,4-xylidine (log P ) 2.21 for the neutral molecule and 1.88 for the 2,4-xylidinium cation). However, this classic measurement does not provide us with any information about the location of the matter. Is it at the zeolite Y’s surface? Does it percolate into the zeolite? According to conventional wisdom, equilibria between the organic compound in the water and the zeolite’s surface, as well as between the zeolite’s surface and interior, can be expected. Furthermore, 2,4-dichlorophenol is more hydrophobic and should display a stronger tendency toward the formation of hydrophobic layers or clusters than 2,4xylidine. Luminescence Depolarization. When linearly polarized light is used to excite the luminophore Ru(bpy)32+ and linearly polarized components of its emission are detected, information on the molecular movements of this metal complex that take place during its luminescence lifetime can be obtained.17 In an earlier paper, we determined the luminescence lifetime of Ru(bpy)32+* in zeolite Y containing 34 wt % of TiO2 to be 274 ns;7 therefore, we have an observation window of ∼400 ns to observe the motion of Ru(bpy)32+ within the supercage of zeolite Y. In the case of linearly polarized excitation, those complexes whose absorption oscillators are oriented parallel to the direction of polarization will be preferentially excited. This will result in highly polarized fluorescence if the complexes do not rotate during the interval between the absorption and emission of light. If the complexes are free to rotate, the orientation of the absorbing molecules will be partially or completely randomized. This results in partial or total depolarization. The degree of luminescence polarization, P is defined as the difference between the luminescence intensities detected with a polarization parallel and perpendicular to the excitation polarization.18
P) c(t) 1 c(t) + ) cabs(t) cmax Kcmax
(Ip - Ipp) (Ip + Ipp)
(4)
(3)
The Langmuir linear regression method eq 2 was proposed by Irvin Langmuir in 1918: The plot of c(t)/cabs(t) vs c(t) yields a slope of (cmax)-1 and an intercept of (Kcmax)-1.
where P is the degree of luminescence polarization, Ip is the luminescence intensity that is polarized parallel to the absorbed plane-polarized radiation, and Ipp is the luminescence intensity that is polarized perpendicular to the absorbed plane-polarized radiation
4562 J. Phys. Chem. C, Vol. 113, No. 11, 2009
Pokhrel et al.
Figure 1. Fundamental components of the [Ru(bpy)3]2+/TiO2/zeolite Y photocatalysts. Percolation behavior is observed in the oxidative degradation of 2,4-xylidine (c0 ) 1.87 × 10-3 mol L-1; 180 mg C kg-1) in the presence of H2O2 (c0 ) 5.5 × 10-2 mol L-1) and 0.30 g L-1 of photocatalyst, pH 3.0.
Figure 2. Degree of luminescence polarization P of 1.0 g kg-1 of the photocatalyst [Ru(bpy)3]2+/TiO2/zeolite Y (23 ( 0.5% (wt) [Ru(bpy)3]2+, 34 ( 0.5% (wt)) in water (pH 3.0), methanol and ethanol; λex ) 460 nm. The solid sample was dried at 100 °C for 12 h prior to the measurement.
SCHEME 1: Mechanistic Scheme for the Oxidative Degradation of Organic Substrates within the [Ru(bpy)3]2+/TiO2/Zeolite Y Photocatalystsa
a
The reaction proceeds at the nanoscopic TiO2 centers.
If the emission is completely polarized in the parallel direction, then P ) 1. For natural or unpolarized light, Ip ) Ipp and P ) 0. If the light is totally polarized in the perpendicular direction, then P ) -1. The theoretical limits of polarization are thus +1 to -1. In solution, these limits are very seldom realized. Luminescence is usually seen as only partially polarized or completely unpolarized. Any P less than 1 is referred to as fluorescence (here luminescence) depolarization. However, Ru(bpy)32+* within the supercage of zeolite Y under certain conditions provides an exception, as is shown in Figure 2.
It is noteworthy that when the [Ru(bpy)3]2+/TiO2/zeolite Y photocatalysts are suspended in ethanol (dielectric constant (Dc) ) 24)19 a very high degree of luminescence polarization of Pmax ) 0.96 is observed between λem ) 594-642 nm. Under these conditions, P is higher than for the solid sample (Pmax ) 0.82 at λem ) 550 nm). When suspended in methanol (Dc ) 33) Pmax decreases to 0.33 at λem) 608 nm. In water (Dc ) 80), Pmax drops to -0.15 at λem ) 601 nm, indicating that the Ru(bpy)32+* complexes can almost tumble freely within the supercages. In sharp contrast, the complexes almost do not exhibit any motion when ethanol is used as a solvent. The motion in methanol is intermittent. The comparison of all four emission polarization spectra is pointing to the strong influence of the solvent. It is obvious that the solvent can percolate through the window regions, which interconnect the supercages. Otherwise, the high polarization (P > 0.6) observed above 680 nm would also be discerned when the solvents are used. Although the main axes of the zeolite particles possess diameters between 500 and 1000 nm, the light-absorbing layer near the surface of these particles should be of the order of 100 nm.20 This means that all the experimental observations discussed here aresstrictly speakingsonly valid for this outer layer. However, since all the photochemical activity occurs here, these observations are of great importance for the reactivity of the photocatalysts. We have established that P can only be in the interval between -1 and +1. The more the tumbling motion of the photoexcited Ru(bpy)32+* complexes within the supercages is hindered, the higher the value of P will be. We can now discern the influence of both oxidation substrates: the higher the relative concentration of 2,4-xylidine or 2,4-dichlorophenol together with the metal complex in the supercage is, the bigger the measured value of P will be. Ru(bpy)32+ possesses a diameter of 1.03 nm,21 2,4xylidine 0.62 nm, and 2,4-dichlorophenol 0.56 nm. The maximal diameter of the supercage is 1.3 nm, but both organic compounds fit through the window regions of 0.74 nm between the supercages.21 When adding 2,4-xylidine, the degree of luminescence polarization increases from P ) -0.15 at λem ) 601 nm (0 mg C kg-1) to P ) -0.12 at λem ) 613 nm (100 mg C kg-1) to P ) -0.045 at λem ) 618 nm (300 mg C kg-1). At 500 mg C kg-1, which is located in the plateau region of the Langmuir isotherm, P increases to 0.13 at λem ) 603 nm. On the other hand, when 2,4-dichlorophenol is adsorbed, the
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J. Phys. Chem. C, Vol. 113, No. 11, 2009 4563
Figure 3. Degree of luminescence polarization P of 1.0 g kg-1 of the photocatalyst [Ru(bpy)3]2+/TiO2/zeolite Y (23 ( 0.5% (wt) [Ru(bpy)3]2+, 34 ( 0.5% (wt)) in water (pH 3.0), and after the addition of 100, 300, and 500 mg C kg-1 of 2,4-xylidine (A) and 2,4-dichlorophenol (B); λex ) 460 nm.
Figure 4. (Left) Plot of the concentration of substrate (A, 2,4-xylidine; B, 2,4-dichlorphenol) vs the degree of luminescence polarization P of Ru(bpy)32+* as a function of total concentration. (Right) Plot of the concentration of substrate (A, 2,4-xylidine; B, 2,4-dichlorophenol), which is adsorbed on or absorbed in the zeolite vs the degree of luminescence polarization P of Ru(bpy)32+* as a function of total concentration.
SCHEME 2: Space-Filling Model of the Zeolite Y Lattice with TiO2 Nanoparticles (Yellow), 2,4-Xylidinium (Red), and Ru(bpy)32+ (Blue)a
a (Left) Interconnecting network of a faujasite. (Right) Comparison of the sizes of lattice-embedded TiO2 nanoparticles (from TEM20) and 2,4xylidinium and Ru(bpy)32+ with the two-dimensional periodic building unit of the faujasite structure.
maximal increase of P reaches only -0.088 at λem ) 640 nm (500 mg C kg-1). Apparently, the luminescence polarization of Ru(bpy)32+ within the supercages of the hydrated zeolite Y can be used to estimate its mobility. As the concentration of substrate increases
(which is percolating into the zeolite’s framework), the interior of the supercages becomes more “crowded”, and therefore, P increases. We have attempted to use the classic Perrin equation22 to calculate approximate lifetimes of the reorientational motion of Ru(bpy)32+*, but the results were not meaningful due to a
4564 J. Phys. Chem. C, Vol. 113, No. 11, 2009
Figure 5. Plot of the apparent rates of 2,4-xylidine (A) and 2,4dichlorophenol (B) removal vs the degree of luminescence polarization P of Ru(bpy)32+*.
change of the presign of P during the addition of substrate. Furthermore, we are aware that a linear dependence of P from the substrate concentration has not been established. However, we have shown that P increases linearly with increasing substrate concentrations and, therefore, can be used for the purpose of a relative comparison. It is noteworthy that the polarization P for both, 2,4-xylidine and 2,3-dichlorophenol increases steadily with increasing concentration, as Figure 4, left, indicates. However, when the zeolite-adsorbed/absorbed concentration of the substrates is plotted vs the luminescence polarization P instead of the total concentration, the differences become more discernible: at pH 3, 2,4-xylidinium enters the zeolite and leads to a remarkable increase in P. In distinct contrast, 2,4-dichlorophenol does not cause a big increase in P, although its total absorbed concentration is higher, according to the observed Langmuir adsorption isoterms.11 From the comparison of this data, it is apparent that more 2,4-xylidine is absorbed in the zeolite Y’s interior, although more 2,4-dichlorophenol is bound to the zeolite. Therefore, the latter must be rather adsorbed at the surface region of zeolite Y. It is known from our earlier work7,9 that the photocatalysis reaction proceeds at the zeolite-incorporated TiO2 nanoparticles (d
