J. Phys. Chem. 1995,99, 10024-10034
10024
Spectroscopic Studies of Electron Trapping in Zeolites: Cation Cluster Trapped Electrons and Hydrated Electrons Xinsheng Liu, Guohong Zhang, and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received: March 8, 1995; In Final Form: April 10, 1 9 9 9
The trapping of electrons produced in pulsed electron radiolysis and steady-state y -radiolysis, by cation clusters in dehydrated zeolites NaX and N a y , and in these samples containing Tl+ and Cd2+, as well as by water molecules in hydrated zeolites was studied using time-resolved transient absorption spectroscopy and electron paramagnetic resonance spectroscopy. For fully hydrated NaX and Nay, water molecules, rather than cation clusters, trap electrons, giving rise to hydrated electrons, whereas for the fully hydrated NaA and NaZK-4, water molecules cannot trap electrons, and the cation clusters act always as the trapping centers for electrons. The G value of the hydrated electrons in NaY was measured to be 8.3 f 0.2, indicating that the charge generation and separation processes in these zeolites are efficient. The spectra of Na32+ and N a 3 + trapped electrons in the dehydrated NaX and NaY were measured at time scales as short as 20 ns. The broad N a 3 + absorption band was found to be composed of more than one type of N a 3 + trapped electrons which exhibited different decay rates, indicating that the environments experienced by the N a 3 + trapped electrons are varied. Introduction of TIC into NaX and NaY modifies the systems, increases the yields of both N a 3 + trapped electrons and holes, and generates a different type of N a 3 + trapped electron having an absorption at -450 nm. At high concentrations, T1+ competes with the N a 4 + cation clusters for the hyperthermal electrons, leading to a static quenching of the N a 3 + trapped electrons, and it also reacts with the N a 3 + trapped electrons, leading to a dynamic quenching of these trapped electrons.
Introduction The trapping of electrons by charge-balancing cation clusters of zeolites such as Mnn+(M = Na, K, or Ag; n = 6, 5,4, 3, or 2) is an intrinsic property of the zeolite structures, and this area of research has become active in recent years'-I3 due to the interests in metal-to-insulator transition^,^.'^ charge transfer and stabilization,' solar energy conversion,I2 and catalytic reactiom6 The phenomenon of electron trapping by cation clusters in zeolites was first found by Kasai' in 1965 for a zeolite Nay, which became pink after y-irradiation. Electron paramagnetic resonance studies of the sample gave a spectrum with hyperfine structure of 13 lines, indicating that the trapped electrons are interacting with four Na+ ions in the zeolite cavity, Le., Na3+. Later, similar trapped electrons were observed by Barrier et a1.,2 Edwards et al.,3 and other^^,^^^,'^ through alkali metal vapor deposition into zeolites such as sodalite, A, X, and Y. Thereafter, the studies in this area not only established methods for preparations of the trapped electrons such as chemical decomposition of NaN3,6 exchange of solvated electrons in liquid a m m ~ n i aexchange ,~ of alkali metal in organic ~ o l v e n t , ~ and far-UV light irradiation'If but also identified many new types of trapped electrons in cation c l u ~ t e r s . ~ . ~The - ~ - trapped l~ electrons were also found in the zeolites containing arene molecules on laser photolysis.' Recently, we examinedI4 factors such as temperature, framework SUA1 ratio, oxygen quenching, metal ions, and solvent molecules on the electron trapping following the far-UV light excitation, y-irradiation, and fast electron radiolysis of the zeolites. For the many metal ions studied,I4 we found that the T1+ ion behaves differently to the others. The Tl' ion increases the initial yield of the Nu3+ trapped electrons and also simultaneously quenches the trapped
'
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Absrracfs, May 15, 1995.
0022-365419512099-10024$09.00/0
electrons. In the present paper, we present our detailed studies on the electron trapping by cation clusters in NaX and NaY in the presence of Tl+ cations and discuss the functions that T1+ plays in the electron trapping process. We also report, for the first time, results of studies on electron trapping by water in the fully hydrated zeolites.
Experimental Section Zeolites and Chemicals. Zeolites NaA, NaX, and NaY with SilAl ratios of 1.0, 1.4, and 2.5, respectively, were obtained from Aldrich. ZK-4 was synthesized following the literature proceduresI5 using tetramethylammonium (TMA+) as a template. The sample was examined with X-ray diffraction and chemically analyzed to give framework SUA1 ratio, 1.4. The as-synthesized ZK-4 was calcined at 500 "C for 8 h and followed by ion exchange with 1 M NaCl aqueous solution for 24 h at room temperature. The sample was then washed with distilleddeionized water and dried at 60 "C. Cd(N03)y4H20 and CuCly2H20 were certified Fisher Scientific products. TlCl was a product of A.D. Mackay, Inc. Methylviologen dichloride hydrate (98%) was purchased from Aldrich and used as received. Preparation of M Zeolites (M = Cu2+, CdZ+,and Tl+). A 6.5 g sample of hydrated NaX or NaY was added while stirring to a 100 mL aqueous solution Containing the calculated amounts of M2+ or M+ ions from 0.25 to 41uc (uc = unit cell). The suspension was stirred overnight at ambient temperature. The corresponding supernatant was tested using a Na2S or NazSO4 aqueous solution to see whether any M2+or M+ ions remain in the solution. Under the above conditions, all M2+ and M+ ions were introduced into the zeolites. The samples were thoroughly washed with distilled-deionized water and dried at 60 "C. For -90% ion-exchangedT E samples, a 0.1 M solution of T1+ was used. Diffuse Reflectance Spectroscopic Measurements. For diffuse reflectance measurements, the zeolite samples were 0 1995 American Chemical Society
Electron Trapping in Zeolites dehydrated at 500 "C under vacuum for 5 h in quartz tubes and sealed. After cooling to room temperature, the samples were further cooled to 77 K by liquid nitrogen in a Dewar and then subjected to y-irradiation for 20 min (200 h a d ) at 77 K. The diffuse reflectance spectra were recorded at 77 K with a specially designed quartz Dewar on a Cary 3 UV-vis spectrophotometer equipped with an integrating sphere. A plain zeolite Y sample was taken as a reference. Electron Paramagnetic Resonance (EPR). The samples were held in 5 mm quartz EPR tubes and dehydrated at 500 "C under vacuum for 6 h and then sealed with a flame torch. The sample tubes were cooled to 77 K with liquid NZand y-irradiated at 77 K for 20 min. The EPR spectra were recorded at 77 K on a Varian E-lines Century Series electron spin resonance spectrometer operated in the X-band frequency region with 100 kHz field modulation. DPPH was used as a standard. Time-Resolved Transient Absorption Measurements. A pulsed fast electron beam delivered from a Febetron 706 (Field Emission Corp.) with a pulse width of -2 ns and a dose of -2 x lo5 radpulse was used as the excitation source in the pulse radiolysis. Transient species produced by the high-energy radiation in the zeolite samples were monitored using conventional nanosecond transient absorption spectroscopy. A pulsed Xe lamp (450 W, Oriel) generating a short intense white flash (width -500 ps) covering the wavelength region 300-800 nm was used as the analyzing light. The cross configuration of excitation and probing was used, and the transients were measured in the diffuse transmittance mode. The time response of the measuring system was -15 ns. Transient spectra were collected by assembling the measurement at different wavelengths with a bandwidth of -3 nm and corrected against the Febetron outputs which were monitored by splitting -10% of the analyzing light (after passing through the sample disc) into a separate PMT-oscilloscope system. The major transient absorption band of the sample was monitored at a fixed wavelength for the whole run of the spectral measurement. The sample compartment consists of a stainless steel chamber with two quartz windows for the analyzing light and a thin titanium metal film for the electron beam. The windows were glued onto the chamber using vacuum-proof epoxy resin. The sample disc was held by a brass sample holder in the chamber which was arranged with the disc surface normal 45" to the electron beam. The disc was heated in situ in the chamber up to 300 "C with two tube heaters built in the sample holder. The whole chamber was evacuated with a vacuum line to a vacuum about Torr. To avoid the accumulation of permanent products (long-lived trapped electrons) in the samples during the run, a high temperature (200 "C) was used for the spectral collection. The use of high temperature also facilitates the measurements of the slow decayed transient species within the time windows used. For fully hydrated samples, however, the spectra were recorded at room temperature (22 "C) under similar vacuum conditions under which 15-20% of the total content of water was removed from the samples. The G value of hydrated electrons generated in the electron pulsed hydrated NaY was measured against pure waterI6 by using G(eaq-) = 3.0 on a nanosecond time scale. Data Analysis. Following the physical significance of the different systems, two models, the Gaussian distribution and electron tunneling, were used for the nonlinear least-squares curve fitting. The Gaussian distribution model, developed by Albery et al. for inhomogeneous reaction kinetics in colloid systems" and subsequently applied successfully by othersl8-I9 to solid systems such as titania,'* silica, alumina, clays, and zeolites,I9 describes the natural logarithm of observed rate
J. Phys. Chem., Vol. 99, No. 24, 1995 10025
-
constants as a Gaussian distribution with two parameters: k, a mean rate constant, and y , a distribution parameter. The dispersion in the first-order rate constants for -00 5 x I 00 becomes
+
ln(k) = ~n(k) yx
(1)
The observed decay profile is composed of the summation of the contributions from each microscopic species, Z(t) -_
exp[-kt exp(yx)l dX
- J-:exP(x2)
I(0)
(2) j-:exP(-xz)
dX
where J"_exp( - xz) dx = nilz and x is the integration range. After transformation of the variable, x = In@) for x < 0 and x = -In@) for x > 0, the integration of the above equation by the extended second-order Simpson expansionIgagives
+
+
+
02 = -{ 2[g(O.1) g(0.3) g(0.5) g(0.7) 3n'I2 g(O.9)] g(0.2) g(0.4) g(0.6)
+
+
+
+
+ g(0.8) + exp(-it)}
(3)
where
+
g(A) = A-' exp{-[~n(~)l~}{exp(-kt~~) exp(-ktil-Y)) (4) The electron tunneling modelz0describes the system via the following equation:
+
Z = I, exp{ -A[(ln(~t))~ 1.73l(ln(vt))2
+
5.934(ln(vt))
+ 5.4451)
(5)
where Z and IOare the intensity at time t and time 0, respectively, v is the vibrational frequency in a square potential well, and t is the time. A is a factor that depends linearly on the quencher concentration n
A = nu = (4/3)nu 3n
(6)
In the expression of A, v is the electron transfer volume and a is the attenuation length of the wave function, describing how fast the rate constant, k(r), of electron transfer changes with the distance r via the equation
k(r) = Y exp(-r/a)
(7)
Results and Discussion Before giving the results, it is pertinent to describe the structures of zeolites used in this study and the processes induced in the zeolites by high-energy radiation. Both zeolites X and Y, having different framework Si/Al ratios (> 1.5 for zeolite Y and < 1.5 for zeolite X), are isostructural to the natural mineral, faujasite.21 The structure is cubic and built from sodalite cages connected via the double-six-membered ring (D6R) of Si04 and A104- in a manner of tetrahedron. The structure contains three types of cages: D6R, sodalite cage, and supercage. The Na+ cations, which compensate the negative charges of the framework due to isomorphous substitution of Si by Al, are distributed among at least four kinds of sites: site I in the center of the D6R cage, site I' inside the sodalite cage, site I1 on the wall of the supercage, and site I11 above the four-membered ring in the ~ u p e r c a g e . ~ ~Zeolite . ' ~ A is a synthetic material with no natural
Liu et al.
10026 J. Phys. Chem., Vol. 99, No. 24, 1995 SCHEME 1: Schematic Drawing of Framework Structures of Zeolites Used in This Study and Their Possible Cation Locations (a)
B
W ~ o u n t e r p a r t .The ~ ~ structure is also composed of sodalite cages similar to faujasite but connected through double-four-membered rings (D4R) of Si04 and A104-. By this connection, three cages are present: D4R, sodalite cage, and a-cage. The structure of zeolite A is also cubic, having a SUA1 ratio of 1.0. The cation locations are found to be on the site A in the center of the sixmembered ring of sodalite cage and the site E on the window of the eight-membered ring. ZK-4 is also a synthetic zeolite formed in the presence of tetramethylammonium ion and isostructural to zeolite A but of higher framework Si/Al ratio.I5 Scheme 1 shows a schematic drawing of the framework structures together with the possible cation sites. The occupation of the cation sites depends on the nature of the cations and on the conditions under which the samples are treated. Foreign cations present in NaY occupy preferentially the cation sites; for example, Cu2+ and Cd2+ prefer the site 1',25,26 while T1+ prefers the site III.27 On hydration, water molecules fill the spaces of the cages, generally on average a total of -4 water molecules in the sodalite cage, -24 in the a-cage of NaA, and 28-32 in the supercage of NaY and NaX.28 The water molecules are randomly located in the supercage of NaX and .Nay but structured (pentagonal dodecahedron) in the a-cage of NaA.29 The following processes occur on subjection of zeolites to high-energy radiation: -+
h+
+ eh+
h+
-
-
Z*+
(hole trapping)
etrap- Q
+
(recombination)
etrap (electron trapping)
Z*+ etrap- Z
+
(zeolite framework ionization)
+ e- -nil or hv e-
+
Na,'"-
e-
eaq- (hydrated electron formation)
eA
Z
+ (H,O),-
e- + Nann+
Q-
(8) (9) (10) (1 1)
(trapped charge recombination) (12) (quenching trapped electron) (1 3)
Process 11 can be further detailed into electron trapping by different species
)+
+ Q - Q-
(cation cluster electron trapping)
(1 1')
(electron scavenging)
where 2 denotes zeolite framework and z'+the trapped hole in the zeolite framework. e- denotes the hyperthermal electron, Nann+the cation cluster capable of trapping an electron, and (H20), the liquidlike water cluster in the zeolite cavities. Q is the electron quencher or scavenger including foreign molecules, metal ions introduced by ion exchange, and impurities and defect sites in the zeolites. Hydrated Electrons in Fully Hydrated Zeolites. Fully hydrated NaX and NaY (just evacuated to Torr at room temperature), when irradiated by the fast electron pulse, show transient absorption spectra significantly different from those of their corresponding dehydrated samples.' l e Figure 1 gives the transient absorption spectra of the fully hydrated NaX and NaY recorded 20 ns after the electron pulse. The broad absorption covers the entire spectral range examined, with a maximum centered at -740 nm for NaY and -720 nm for NaX. The match of the spectra with that reported for bulk water in the 1iterature3O suggests that the absorption originates from hydrated electrons. This assignment is confirmed by the short lifetime of the species which distinguishes it from the cation cluster trapped electrons and by the quenching studies using electron scavengers (see below). We therefore conclude that, instead of the cation clusters, water molecules, present in the cages, trap electrons, giving rise to hydrated electrons. The competition of Na+ cation clusters with water for trapping electrons electrons is present, as exemplified by the presence of the long-lived transient signal of N a 3 + monitored at 550 nm (Figure 2). Oxygen (gas) quenches the signal of hydrated electrons and leaves intact that of the Na43+ trapped electrons (Figure 2). The unquenchable nature of the N a 3 + trapped electrons and the quenching of the hydrated electrons by 0 2 clearly demonstrate that, in the hydrated zeolite, the Na43+ trapped electron is located in the sodalite cage where 0 2 cannot access, while the hydrated electron is formed in the supercage. Monitoring the signal at 700 nm under the same conditions shows no residual signals, indicating that the N a 4 + cation clusters are the only species competing with water for electrons in the sample. However, the content of the N a 3 + trapped electrons in the hydrated sample is not high and only contributes about 10% of the total intensity of the 550 nm absorption, as seen from Figures 1 and 2. The low yield of the N a 3 + trapped electrons is due to inefficient competition of the N a 4 + sites with water for trapping electrons and/or to the decrease of the number of the N a 4 + sites by destruction of the sites via changing the cation locations on hydration. To understand how many water molecules present in the zeolite cages are essential for electron trapping, we examined the fully hydrated NaA. This zeolite has a slightly smaller a-cage (-1 1 A in diameter29)compared to the supercage of faujasite (-13 8, in diameter28). Figure 3a shows the transient absorption spectra of the hydrated and dehydrated NaA recorded 20 ns after the electron pulse. The spectrum of the hydrated NaA is very similar to that of the dehydrated NaA. In both cases, long-lived Na+ cluster trapped electrons are observed. In contrast to the NaX and Nay, the water molecules present in the a-cage of the hydrated NaA seem not capable of trapping electrons. To clarify whether this is due to the specific arrangement of water molecules (pentagonal dodecahedron) in the a-cage of NaA revealed by the X-ray structural studies,29 we examined ZK-4, a zeolite which is synthesized in the
Electron Trapping in Zeolites
J. Phys. Chem., Vol. 99, No. 24, 1995 10027 0.4
0.3 -
0.3
1
I
la)
NaA
4 0.2 J
al
u c
E
5:
n
0.2
0.1
a
CI
._ s
0 300
E e
0.1
400
500
600
700
800
900
800
900
Wavelength (nm)
7
04
,
350
,
450
650
550
850
750
Wavelength (nm)
Figure 1. Transient absorption spectra of fully hydrated NaX (in stars) and NaY (in circles) recorded 20 ns after the electron pulse. The spectrum of NaY is offset by +0.04 absorbance unit. Inset: decay curves monitored at 700 nm for NaX (dash line) and for NaY (solid line). 0.36
0.27
-
4
,f 0.18
0.09
I\--‘
A I!,
lal
~
4
8
12
16
20
T h tmicml.condrl
0.15
(bl h = 550nm
4 0
O.1°
!,
t
+
0.05
.... ...,.. .........................., .
0.00
0
4
8
12
500
600
700
Figure 3. (a) Transient absorption spectra of fully hydrated (circles, offset +0.04 for clarity) and dehydrated (triangles) NaA recorded 20 ns after the electron pulse. (b) Transient absorption spectra of fully hydrated (circles, offset +0.04 for clarity) and dehydrated (triangles) NaZK-4 recorded 20 ns after the electron pulse.
.......... .... \ .................... 0
400
Wavelength (nm)
1 = 720nm
0.00
300
16
20
Time Imkrowcondal
Figure 2. Time-resolved absorption signals monitored at 550 and 720 nm of hydrated NaY in vacuum and under 20 mbar of oxygen. The fast component of the decay signal is due to hydrated electrons, while the long-lived component under oxygen is due to N a 3 + trapped electrons.
presence of tetramethylammonium ions and structurally similar to zeolite A but of a higher Si/Al ratio, 1.4. The ZK-4 sample was calcined at 500 “C to remove the organic molecules and subsequently ion-exchanged with Naf to finally obtain a sample with a structure of NaA and a framework SUA1 ratio of NaX. These attempts ensure the random distribution of water molecules in the a-cage. Examination of this sample gives the spectra shown in Figure 3b. The spectra before and after
dehydration look very similar to that of NaA and are not affected by the dehydration process. This demonstrates that no matter how the water molecules are arranged, structurally or randomly in the a-cage, no hydrated electrons are generated. We therefore conclude that, when the cage of the zeolite becomes smaller or equal to 11 A, the water molecules in the cage can no longer compete with the cation clusters for trapping electrons. Using the electron spin echo technique to determine the structure of hydrated electron, Kevan3’ has found that six water molecules are coordinated to the electron. Theoretical studies of the solvation structure of a hydrated electron in water by Rossky et aL3*have shown that the electron is surrounded by six water molecules. A study of hydrated electrons in the water pools of inverted micelles in heptane by Wong et al.33has shown that no hydrated electrons can be observed when the ratio of the number of water molecules to the number of sodium diisooctyl sulfosuccinate molecules reaches four. In other words, the water which participates in the solvation of Na+ ions is not available to solvate electrons. In comparison with these literature reported results, we conclude that at least -6 water molecules are “free” or liquidlike in each supercage of NaX and NaY for the formation of a hydrated electron and that there are no or insufficient “free” water molecules in the a-cage of NaA and ZK-4. The water in a zeolite cavity is found to be present in several forms as cation solvation water, similar to that seen in the inverted micelle systems, as hydrogen-bonded water to the framework atoms, and as bridging water in between pairs of cations or between cations and other water or framework oxygens. The number of water molecules directly coordinated to each cation depends on the nature of the cation, the location of the cation in the cavities, and the coordination of the cation to the framework oxygen. In general, the smaller the cage, the fewer water molecules are coordinated to the cations. In NaA, for instance, only four water molecules are found in each sodalite
Liu et al.
10028 J. Phys. Chem., Vol. 99, No. 24, 1995 cage.29 If we assume that there are four Na+ ions in each sodalite cage, then the number of Na+ in each a-cage would become eight, and the ratio of the number of water molecules to the number of Naf in the a-cage (nH20/nNa+) would be 2418 = 3, which is conspicuously smaller than the value found in the inverted micelle systems.33 For NaZK-4, even though its Si/Al ratio is 1.4, the nH20/nNa+ratio estimated is only -4 if the same situation in its sodalite cage as in NaA is assumed. On the basis of these estimations and a comparison with the inverted micelle systems, one would not expect to observe hydrated electrons in NaA and NaZK-4, which is in agreement with the experimental observations. The same estimation may be performed on NaX and NaY using the literature data.34%35 If we assume that each Na+ ion on the site I1 is coordinated only by two water molecules from the supercage due to its coordination to the framework oxygens and that there are four additional Naf ions in each supercage of NaX and one in each supercage of Nay, then this gives a nH20/nNa-ratio in the supercage of -6 for NaX and -20 for Nay. This predicts “free” water in these systems, and consequently the hydrated electrons are expected in these zeolites, which is in accord with the experimental observations. If we also assume that the Na+ ions in the supercages are similar to those in the water pool of the inverted micelle and each is coordinated by four water mole c u l e ~ then , ~ ~ the number of “free” water molecules estimated i s 4 x 6 - 4 x 4 = 8forNaXand 1 x 20 - 1 x 4 = 16for Nay. Even though 15-20% of the total water can be removed by vacuum, the number of “free” water molecules in a supercage of these zeolites is still 6.4 for NaX and 12.8 for Nay. From this estimation and in comparison with the experimental observations, it appears that a minimum of six water molecules are required for the formation of hydrated electrons. As more “free” water is present in the supercage of NaY compared to NaX, then the hydrated electron is more confined by its environment in NaX than in Nay. The slight blue shift of the absorption band of the hydrated electrons in NaX compared to that in NaY (720 vs 740 nm) may be a consequence of this effect. Similar phenomena have been observed in inverted micelle systems33where the spectra of the hydrated electrons becomes narrower and blue-shifted when the water pools become smaller and rather tightly confined by their environment. Figure 1 also gives the decays of the hydrated electrons in NaX and NaY (see inset in Figure 1). From the decay curves, the average lifetimes of the hydrated electrons were measured to be 0.46ps in NaX and 3.61 ps in Nay. The shorter lifetime of the hydrated electrons in NaX compared to that in NaY reflects the difference in environment experienced by the hydrated electrons in these two zeolites, which is parallel to what were seen for the cation cluster trapped electrons. Quenching studies of the hydrated electrons using electron scavengers give information about mobility of hydrated electrons in the zeolite systems. Figure 4, a and b, shows the decay of the hydrated electron and the concomitant formation of the reduced methyl viologen (MV’+) in zeolite Y, monitored at 700 and 390 nm, respectively, as a function of the concentration of methyl viologen (MV2+). The reactions involve
+
M V ~ + eMV*+
-
MV*+
+ eaq- - MV’+
(14)
k ,5
(15)
The first equation is the reaction of MV2+ with the quasifree electrons, which contributes to the instantaneous rise of the M V ” in Figure 4b, and the second is the reaction of MV2+ with the hydrated electrons over microseconds to form M V + , which contributes to the slow growth of MV’+ seen in Figure
I
1
0
5
10
15
20
15
20
Time lmicrosecondrl
5
0
10
Time (microseconds)
- 6E+6 I
T a
3 4E+6
E 2E+6
f OE+O
0
p .c
0.04
0.01 0.02 0.03 [Methyl Viologenl (MI
0.04
0.02 0.03 [Methyl Viologonl IM)
1.00-
2
0.75
.
0.50
1
3
f
0.01
1 > 0.251
0.00 / 0
Figure 4. (a) Quenching of the hydrated electrons in hydrated NaY by MV?+ monitored at 700 nm. The buildup of the long-lived component is due to M V + , which increases in yield with the concentration of MV2+ in the order of 0, 3.2, 6.4, 12.8, and 25.6 mM. (b) Formation of MV’+ monitored at 390 nm at different concentrations of MV? 3.2, 6.4, 12.8, and 25.6 mM. (c) Plots of decay rates of hydrated electrons (circles) and of MV’+ (triangles) in hydrated NaY versus the concentration of MV”. The solid line is the fitting with a linear function. (d) Plot of the yield of the static formation of M V ” versus the concentration of MV?+. The solid line is the fitting using the Perrin model.
Electron Trapping in Zeolites
J. Phys. Chem., Vol. 99, No. 24, 1995 10029
4b and the dynamic quenching of the hydrated electrons seen in Figure 4a. The presence of the growing part of the M V + signal in Figure 4b unambiguously demonstrates that the second reaction (eq 15) indeed takes place. Computer fitting of the decay curves of the hydrated electrons and of the slow growing part of the M V f using the Gaussian distribution model17 (see Experimental Section) gives the quenching rate constant, kls, for hydrated electrons reacting with MV2+ in NaY to be 1.26 x lo8 M-’ s-l (Figure 4c). The exact match of the decay rate of the hydrated electrons and the formation rate of MV” confirms the mechanism for the dynamic quenching of the hydrated electrons that occurs solely between MV2+ and hydrated electrons (see Figure 4c). Due to its size and charges, the mobility of MV2+ in the cavities could be neglected, and the diffusion coefficient of the hydrated electrons (De)in NaY is estimated using the Smoluchowski relation36
k,, = 4xRDeN
a = (3/4)7cRC3N
la)
0
500
1000
1500
ZOO0
2500
BO
100
Time Inenoeaconds) 1E+7,
i
(16)
where N is the Avogadro constant and R is the effective reaction radius which includes the electrostatic interaction between MV2+ and hydrated electron and was in this case taken roughly as 16 8, on the basis of the rate constant measured in aqueous s~lution,~’ k = 7.5 x 1Olo M-’ s-’. The De value estimated then is 1.2 x IO-’ cm2 s-l, which is about 2 orders of magnitude smaller than that of hydrated electrons in pure water, 4.9 x cm2 s-1.38 The slow mobility of the hydrated electrons in the cavities of zeolites reflects confinement of the hydrated electrons by the zeolite structures. The instantaneous formation of the M V f in Figure 4b, which corresponds to the static quenching of the hydrated electron signals, originates from the reaction of MV2+ with e- (eq 14). The change in this part of the MV’+ yield which is formed on increasing the MV2+ concentration is plotted in Figure 4d. Computer curve fitting using the Perrin gives the parameter a, which relates the radius of “active” sphere, R,, of MV2+ in the zeolite for reacting with electrons.
YIY, = 1 - exp[-a[MV2+]]
0.30 I
(17) (17’)
where Y, is the yield of M V + at the highest MV2+concentration and N is the Avogadro constant. The R, obtained from this equation is 30.3 8,. Figure 5a gives the decay curves of oxygen quenching of hydrated electrons in the fully hydrated zeolites, and Figure 5b shows the plots of the corresponding rate data using the Gaussian distribution model.” The quenchable feature of the hydrated electrons by oxygen indicates that the cavities of NaX are not completely blocked by the adsorbed water molecules. This is consistent with the removal of 15-20% of the total amount of water from the cavities. The oxygen molecules can penetrate through the free volume into the cavities to quench the hydrated electrons.
Loading large molecules such as tris(2,2’-bipyridine)ruthenium(11) ions, Ru(bpy)32+, onto the external surface of the zeolite particles does not lead to observable quenching of eaq-, confirming the penetration of oxygen into the supercages. The apparent 0 2 quenching rate constants for the hydrated electrons in NaX and Nay, kl8, are 5.50 x lo4 and 6.98 x lo4 mbar-I s-I, respectively (see Figure 5b). The difference in the 0 2 quenching rate may just reflect the different degree of
OE+O
4 0
20
40
60
Oxygen Pressure (mbar)
Figure 5. (a) Oxygen quenching of hydrated electrons in NaX monitored at 700 nm, and smooth lines are the fitting using the Gaussian distribution model. The oxygen pressures from top to bottom lines are 0, 8, 16, 33, and 66 mbar. (b) Plot of decay rates of hydrated electrons in hydrated NaX as a function of oxygen pressure.
blocking in these two zeolites due to their different surface polarities or hydrophilicities for water molecules. The quenching involves two mobile species: the hydrated electrons and the 02 gas molecules. To understand the movement of 0 2 in the zeolites, the 0 2 quenching of pyrene singlet in the hydrated NaX was examined under conditions identical to those used in the eaq- studies. Such comparison was justified by the fact that both pyrene and eaq- are located in the supercage. Due to the relative immobility of pyrene molecule in this system, only 0 2 molecule moves to quench the excited pyrene singlet via a diffusion-controlled quenching mechanism. The 0 2 quenching rate obtained for the hydrated Py-NaX is 5.34 x lo4 mbar-’ s-I, which is very close to the value given above for the hydrated electrons. This result therefore indicates that 0 2 quenching of the hydrated electrons is mainly due to diffusion of 0 2 through the free space. However, the movement of 0 2 in the fully hydrated zeolites is obstructed severely by the water molecules compared with the dehydrated sample. This can be seen by a comparison of the 02 quenching rate of the excited pyrene singlet in the dehydrated NaX. The 0 2 quenching rate for ‘Py*measured in the dehydrated case is 1.2 x lo7 mbar-’ s-I, which is about 200 times larger than that in the hydrated case. Under the condition of complete water blocking of the cavities (equilibrated with 15 mbar of water vapor), the 0 2 quenching rate becomes very small, 3.56 x lo2 mbar-l SKI,which corresponds to 0 2 diffusion through the water in zeolites. The G value of the hydrated electrons in NaY was measured in electron pulse radiolysis, taking pure water as a standard, to be 8.3 f 0.2, -3 times that of hydrated electrons (3.0) in pure water.I6 This indicates that charge generation efficiency in the zeolites is high.
Liu et al.
10030 J. Phys. Chem., Vol. 99, No. 24, 1995 0.6
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Figure 7. (a) Transient absorption spectra of dehydrated CdxNass-&Y ( x = 0-2.0) recorded 50 ns after the electron pulse. Inset: plot of yield of N q 3 + trapped electrons as a function of concentration of Cd?+. (b) Decay curves of N u 3 + trapped electrons monitored at 500 nm in the NaY samples containing Cd?+ ( x = 0.5, 1 .O, and 2.0). The smooth
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400
500
000
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Figure 6. (a) Transient absorption spectra of dehydrated NaY recorded 20 ns and 5 ps after the electron pulse. (b) Decay curves of cation cluster trapped electrons at the wavelengths of 410, 500, and 650 nm. (c) Transient absorption spectrum of dehydrated NaX recorded 50 ns after the electron pulse.
Cation Cluster Trapped Electrons in Dehydrated NaX and Nay. Figure 6a shows the transient absorption spectra of dehydrated NaY recorded 20 ns and 5 pus after the electron pulse. The 5 ps spectrum is in accord with those reportedfiepreviously where only one band centered at 500 nm (2.48 eV) was observed. However, the 20 ns spectrum exhibits different features compared to the one taken 5 p s after the pulse: the band is broader (measured by the full width at half-maximum), -0.87 eV for the 20 ns spectrum versus -0.60 eV for the 5 ps one; an absorption band at above 600 nm (2.07 eV) is observed. Monitoring the intensity decays at the wavelengths 410 nm (3.02 eV), 500 nm (2.48 eV), and 650 nm (1.91 eV) shows different rates (Figure 6b), the decays being faster at 410 and 650 nm than at 500 nm. This suggests that more than one type of trapped electron is produced in the sample and can be resolved on the short time scales. The absence of the fast decaying species in NaY in the previous studies" is simply due to the
long time scales used, which is exemplified by the 5 p s spectrum in Figure 6a of the present study. On the basis of the knowledge gained from the previous studies," we assign the band around 650 nm to Na3*+ trapped electrons which involve the Na+ cations located in site I11 (see Scheme 1). This assignment is supported by the results obtained from examination of NaX. The intensity of the 650 nm band is enhanced (comparing parts a and c of Figure 6) due to more Na+ cations on site I11 of NaX. The presence of Na32+ trapped electrons in NaX has previously been observed in the studies using a far-UV light.lIf Similar to the 500 nm band, the fast decaying band monitored at 410 nm is also assigned to N a 3 + trapped electrons. The negatively charged nature of the species is supported by 0 2 quenching. The differing decay rates compared to that of the main band can be understood from the different environments experienced by the trapped electrons in the supercages of the zeolite. The presence of different supercage environments has been demonstrated in the laser photolysis studies of zeolites using pyrene molecule as a probe.40 Electron Trapping in Cd*+-ContainingZeolites. Transient absorption spectra of the NaY samples containing Cd2+recorded 50 ns after the electron pulse show that the N a 3 + trapped electrons were quenched by Cd2+(Figure 7a). The initial yield of the trapped electron decreases with increasing the Cd2+ concentration, and a plot of this relation is given as an inset in
Electron Trapping in Zeolites
J. Phys. Chem., Vol. 99, No. 24, 1995 10031
Figure 7a. This feature, as seen for many other metal ions in the diffuse reflectance studies of zeolites following y-irradiation,I4 reflects the competition of the metal ion with the cation clusters for the hyperthermal electrons. Computer fitting the curves using the Perrin in a fashion similar to the treatment for the MV2+-NaY system (see above), gives an "active" radius, R, = 13.3 A, which is smaller than the value obtained for MV2+, suggesting that the reactivity of Cd2+with the hyperthermal electrons is lower than that of MV2+. Besides the static quenching, a dynamic quenching of the cation cluster trapped electrons is also observed from the timeresolved decay traces as the Cd2+ concentration is increased. This observation suggests that Cd2+ions interact not only with e- (eq 19) but also with the cation cluster trapped electrons (eq 20): Cd2+
+ e- - Cd+
(19)
Because the zeolites are dehydrated under vacuum, the reactions between the metal ions and the trapped electrons are unlikely to occur through a diffusion but rather an electron tunneling mechanism. If we take the decay function of N a 3 + in NaY to be Ao(t) and assume that the intrinsic decay process of Nq3+ is independent of Cd2+ ions, the observed decay of Nq3+ in the presence of Cd2+ ions can be written as A ( t ) = Ao(t)Aet(t).Act( t ) is the decay of N a 3 + due to reaction 20 via electron tunneling. Prior to fitting, the decay data, A(t), was transformed to A,,(t) to remove the contribution of the intrinsic decay of the N a 3 + trapped electrons, Le., Aet(t) = A(t)/Ao(t). The A,,(t) curves thus obtained for different Cd2+loading were then fitted with the electron tunneling model described by eq 5 (see Experimental Section). Figure 7b shows the decay curves of N a 3 + , monitored at 500 nm, in the samples containing Cd2+ for 0.5, 1.0, and 2.0 per unit cell. Fitting the decay curves to eq 5 gives the parameters A and Y (see Figure 7b). Taking the value of Y from the slope of A [Cd2+]plot (eq 6), ZI = 8.8 f 0.9 A3, and a is estimated to be 1.28 & 0.04 A. The average Y obtained is (2.02 f 0.16) x lo8 s-l. The rate constant k2o(r) of electron transfer from the cation cluster trapped site to Cd2+ is then given by
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k20(1)= (2.02 f 0.16) x 108exp(-r/l.28 f 0.04) (21) Electron Trapping in TI+-ContainingZeolites. Figure 8a shows the transient absorption spectra of Tl,Na55-,Y ( x = 0, 0.25, 0.5, 1.0, 2.0, and 4.0) recorded 50 ns after the electron pulse, and Figure 8b gives the plot of intensity change monitored at 450,500, and 650 nm as a function of the T1+ concentration. Introducing Tl+ ions into the zeolite modifies the zeolite system. Several new features of the spectra compared to the original zeolite are observed: (1) The intensity of the 500 nm band ( N a 3 + trapped electrons) increases when 0.25 Tl+/uc is introduced, then decreases on further increase of the T1+ concentration, and almost completely disappears at a concentration 4.0 Tl+/uc. This change is consistent with the results obtained from the studies of the same samples using the 193 nm f a r - W light as an excitation source.14 (2) A new band splits out from the 500 nm band as the T1+ concentration is increased. The maximum position of the band shifts toward shorter wavelength and finally reaches 450 nm, as clearly seen from the samples having higher T1+ concentrations. The intensity of the 450 nm band then decreases as the T1+ concentrtion is further increased (compare the spectra of Tl+ = 2.0 and 4.0/
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2
4
6
0
10
lime (microseconds)
Figure 8. (a) Transient absorption spectra of Tl,Na55-xY (n = 0-4.0) recorded 50 ns after the electron pulse. (b) Intensity change of different bands monitored at 450, 500, and 650 nm as a function of TI+ Concentration. (c) Decay curves of the N q 3 + trapped electrons in Tl,Na55-xY ( x = 0.25, 0.5, 1.0, and 2.0) monitored at 500 nm and computer-fitted curves (smooth lines) using the electron tunneling model.
uc). (3) The intensity of the band around 650 nm decreases monotonously with increasing the T1+ concentration. The low-temperature diffuse reflectance spectroscopic studies of the samples y-irradiated at 77 K for 20 min gave similar spectra (Figure 9), suggesting that at 77 K almost all the species observed in the transient studies are stabilized. The EPR technique distinguishes the trapped electrons from holes and facilitates the assignment for the bands observed in
10032 J. Phys. Chem., Vol. 99, No. 24, 1995
Liu et al.
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0 0.6
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Figure 9. Low-temperature (77 K) steady-state diffuse reflectance spectra of TlCNa55-,Y( x = 0-4.0) y-irradiated at 77 K for 20 min.
the transient absorption spectra. Figure 10a shows a typical EPR spectrum of y-irradiated Tl,Na55-,Y recorded at 77 K. Three species were observed in the spectrum: trapped holes (the broad band near the center of the spectrum), Nan("-')+ trapped electrons (n = 4 and 3) (the spectrum with the hyperfine structure), and atomic hydrogen (the sharp peak on each side of the spectrum). Figure 10b gives the plot of the intensity changes of the Nan("-')+trapped electrons and the trapped holes as a function of the T1+ concentration. The intensity change of the trapped electrons was monitored using the area of one of the least perturbed hyperfine bands by other peaks in the spectrum. It is seen from the plot that the intensity change of the trapped electrons with T1+ is similar to that observed from the transient absorption spectra (Figure 8b), while the change of the trapped holes is different from that of the trapped electrons (see Figure lob). The intensity of the trapped holes increases when the T1+ concentration reaches 0.25/uc and remains constant at the TI+ concentration higher than this value. To identify the band at 450 nm, we conducted experiments of oxygen quenching on the transient species and performed a direct comparison of the transient absorption data with those of EPR on the basis of the similarity of the transient spectra with the low-temperature diffuse reflectance spectra. The oxygen quenching studies support the concept that the species is negatively charged, suggesting that the 450 nm band is due to trapped electrons. The estimation of the intensity ratios for the samples containing 0.25 and 4.0 Tl+/uc using the data in Figures 10b and 8b gives 1 0 2 5 TIC/uc/140TI+/"^ = 5.3 for the absorption data monitored at 500 nm and 3.7 for the EPR data. Due to the contribution of the intensity at 500 nm from the band of 450 nm in the absorption spectrum of 4.0 Tl+/uc the actual value for the absorption data is larger than 5.3. The difference from the comparison suggests that the 450 nm absorption band observed in the absorption spectra is also due to N a 3 + trapped electrons. The absence of other EPR signals, except for those assigned to Nw3+ and Na32+,positively charged hole, and atomic hydrogen, excludes the possibility of assignment of the 450 nm band to the species T1° or T12+. We therefore conclude that the 450 nm band which survives at rather higher concentrations of T1+ is due to Nw3+ trapped electrons. The difference (-50 nm) in the maximum position of the absorption compared to the 500 nm Nw3+ band is due to the modification of the environment of the cation clusters by the T1+ ions. The difference in the absorption maximum of Na43+ to -50 nm is not unusual and has been observed for NaX and NaY due to their different SUA1 ratio and to the different distributions of Si and A1 among their frameworks. The
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Figure 10. (a) A typical EPR spectrum of y-irradiated TlrNa55-,Y at 77 K for 20 min. The spectra were recorded at 77 K. (b) Plot of yields of the N a 3 +trapped electrons (squares) and the electronic holes (circles) in the Tl,Na55-
