13720
J. Phys. Chem. 1994,98, 13720-13728
Electron Trapping by Sodium Cation Clusters in Zeolites: Effects of SUA1 Ratio and Foreign Species Xinsheng Liu, Kai-Kong Iu, and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received: August 24, 1994; In Final Form: October 15, 1994@
The effects of the SUA1 ratio, electron scavenging ions such as Cu2+, Co2+, Ni2+, Zn2+, Pb2+, and T1+, and n-pentane on electron trapping by sodium cation clusters in zeolite faujasite under y-irradiation at room temperature are studied. The results show that as the SUA1 ratio increases, the absorption band of N q 3 + trapped electrons shifts toward shorter wavelengths and becomes narrower, the oxygen quenching rate constant for the Nq3+ trapped electrons decreases, and the activation energy increases. n-Pentane interacts with the electronic hole and enhances the yield of Nq3+ trapped electron. The electron scavenging ions compete with the Nq4+ electron trapping sites, and the quenching of the Nq3+ trapped electrons is static in nature except for TI+. Here the T1+ ion increases the initial yield of the Nq3+ trapped electrons and concomitantly fosters a faster decay of the trapped electrons. According to the quenching kinetics, these ions are grouped into four categories: those reacting with thermalized electrons (Cu2+,Cd2+,Ni2+),those with quazifree electrons (Zn2+, Pb2+), those with both states of the electrons (Co2+), and those with trapped electrons (Tl+).
Introduction
UOP. The compositions of the zeolites were analyzed using wet chemical analysis and gave SUA1 ratios of 1.8 and 2.1, Electron transfer in zeolites is important for the understanding respectively. Cd(N0&4H20, CoC126H20, Pb(N03)2, BaC12. of mechanisms of catalytic reactions that occur in zeolite 2H20, CuCly2H20, and NH&1 were certified Fisher Scientific cavities. As well as the normal Lewis acid sites which accept products. Ni(N03)26H20 was a Mallinckrodt product, and TlCl electrons, cation clusters such as Nann+(n = 2, 3, or 4), in the was from A. D. Mackay. n-Pentane (HPLC grade, '99%) was sodium form of zeolites such as sodalite, A, X, and Y, may further dried with the thermally activated molecular sieve 3A also act as electron traps and form the corresponding cation pellets and degassed three times using freeze-pump-thaw clusters, The existence of these trapping sites method prior to adsorption. depends on the structure of the zeolite, the temperature, and Sample Preparation for Diffuse Reflectance Measurethe conditions of sample preparation. A number of electron ments. The zeolite samples were dehydrated at 500 "C under preparation routes such as y-irradiation,'~~,~ chemical reactions vacuum for 5 h in quartz cuvettes. After cooling to room with strong reducing agents? and alkali metal vapor 1 0 a d i n g ~ - ~ ~ ~ ~ ~ temperature, the samples were further cooled to 77 K by liquid have been used. Recently, we have s t ~ d i e dthe ~ ~formation .~ nitrogen in a dewar and then subjected to y-irradiation for 20 and transformation of cation cluster trapped electrons in a series min at 77 K. The diffuse reflectance spectra were recorded at of zeolites under y-irradiation and fast electron radiolysis and 77 K. realized that the electron trapping in zeolites is affected by Preparation of M Ion-Exchanged Zeolites (M = Cu2+, foreign species. In this report, we present results of electron Ni2+,Co2+,Zn2+, Cd2+,Pb2+,Ba2+,N&+, and T1+). A 6.5 trapping by sodium cation clusters in faujasites with different g sample of NaY (Aldrich, hydrated, SUA1 = 2.5) was added SitA1 ratios and the effects of foreign species such as electron while stimng to 100 mL of aqueous solution containing the scavenging ions and n-pentane molecule on the electron trapping calculated amounts of M2+ or M+ ions from 0.25 to 2 Cu2+/uc following y-irradiation. The questions posed are whether the (uc = unit cell). The suspension was stirred ovemight at electron trapping by Na+ clusters is affected by the Si/A1 ratio ambient temperature. The corresponding supematant was tested of the framework, whether it is a competitive or a quenching using a Na2S or Na2S04 aqueous solution to see whether any process when cations other than Na+ are present, and how the M2+ ions remained in the solution. With the above conditions, guest molecules in the supercage affect the electron trapping. all M2+ and T1+ ions were introduced into the zeolite. The To answer these questions, synthetic zeolites X and Y were samples were thoroughly washed with distilled-deionized chosen because they readily give N a 3 + electron trapping under water. y-irradiation. Sample Preparation for EPR Measurements. Typically, zeolite Y (Si/Al = 2.5) in an EPR Suprasil quartz tube was Experimental Section heated to 500 "C under vacuum Torr) for 6 h and then Zeolites and Chemicals. Zeolites X and Y with Si/Al ratios cooled to room temperature. The degassed n-pentane vapor was of 1.4 and 2.5 were obtained from Aldrich and UOP Co., introduced via a vacuum line and allowed to reach equilibrium respectively. The zeolites Y with SUA1 ratios of 1.8 and 2.1 (-17 wt% adsorption). were synthesized following the procedures of ref 10. The Si/ Instruments. EPR spectra were recorded on a Varian E-lines A1 ratio of the starting gel was varied to synthesize the zeolites Century Series electron spin resonance spectrometer operated with required SUA1 ratio. The products were examined using in the X-band frequency region with 100 kHz field modulation. X-ray powder diffraction and showed that they were zeolites Y DPPH was used as a standard. Fourier-transform infrared of crystallinities similar to those obtained from Aldrich and spectra were recorded on a Perkin-Elmer 1600 FT-IR instrument, using self-supporting discs of the samples. Steady-state lu* To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, December 1, 1994. minescence and excitation spectra of zeolite Y containing @
0022-3654/94/2098-13720$04.5010
0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 51, 1994 13721
Electron Trapping by Na+ Clusters in Zeolites
a
1 *
Figure 1. Schematic representation of structure of zeolite faujasite. The Roman number denotes the possible locations of cations.
1Cu2+/uc(CulNa53Y) were recorded on a Perkin-Elmer spectrofluorimeter also using self-supporting discs. Steady-state diffuse reflectance spectra were collected on a Cary 3 UV-vis spectrophotometer equipped with an integrating sphere. The zeolites were held in the 2 mm thick quartz cuvettes and subjected to evacuation at 500 "C. Time-resolved high-energy electron pulse radiolysis studies were performed on a pulsedelectron beam device (Febetron, Model 706, Field Emission Corp.) having a 2 ns electron pulse width of -lo6 raapulse. Experimental details can be found elsewhere.gd The 193 nm laser is a Lambda Physik ArF excimer laser (10 ns pulse width and -50 mJ/cm2 per pulse).
Results and Discussion I
I
1
I
I
I
300 400 500 600 700 800 Before giving the results, it is pertinent to describe the faujasite structure. Faujasite is a natural mineral, and its Wavelength (nm) synthetic counterpart is called zeolite X or Y, depending on their Si/A1 ratio (Si/Al < 1.5 for zeolite X and > 1.5 for zeolite Y; see ref 11, p 92). The structure of faujasite is cubic and built from sodalite cages connected via the double-six-membered ring (D6R) of Si04 and AlO4- in a manner of tetrahedron. The structure contains three kinds of cages: a D6R cage, a sodalite cage, and a 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 three kinds of sites: site I in the center of the D6R cage, site I' inside the sodalite cage, and site 11' on the wall of the s ~ p e r c a g e . ~ ~ J ~ The occupation of the sites depends on the nature of the cations 0 10 20 30 40 ! and on the conditions under which the samples are treated. Figure 1 shows the structure of faujasite together with the r-lrradlatlon Tlme (minutes) possible cation sites. Figure 2. (a) A typical EPR spectrum of 20 min y-irradiated zeolite y-Irradiation of Nay. y-Irradiation of zeolites produces Y at room temperature. The asterisk denotes the signal of electronic electrons and positive holes. It was found almost 30 years ago1 hole. The sharp center line is due to the defects created in the quartz that electrons produced by y-irradiation of NaY were trapped tube used. The arrow denotes the position of the DPPH standard. (b) Top: steady-state diffuse reflectance spectrum of the zeolite NaY by sodium cation clusters having the form Na4+. The trapped y-irradiated for 20 min at room temperature. Bottom: plot of yield of electrons, Na3+, exhibit a characteristic electron spin resonance Na3+ as a function of y-irradiation time. spectrum of 13 lines, corresponding to interactions of the electron with the four Na+ nuclei (I = 3/2), and a broad indicates that the trapping of electrons by Na4+ clusters absorption band around 500 nm in its UV-vis absorption stabilizes the charges. The bottom plot in Figure 2b gives s~ectrum.~3~ Figure 2, a and b (top spectrum) shows respectively changes in intensity of the 500 nm band as a function of a typical EPR and a UV-vis absorption spectrum of y-irradiated y-irradiation time. Within the time period studied, a linear NaY at room temperature. The 13-line hyperfine structure of increase of the yield is observed which corresponds to a the trapped electrons interacting with the four Na+ ions is clearly mol/(L min). The formation rate of Na3+ of 2.45 x seen, g = 1.999 and A = 30 G. In addition to the trapped quantum yield (G value) calculated from this curvegdis 2.76, electrons, the electronic holes are also observed (signal labeled which is comparable to that of hydrated electrons in water and with asterisk, g = 2.0034). The sharp signal around the center is indicative of an efficient radiolytic process.14 of the spectrum originates mainly from defects of the quartz Effects of SUA1 Ratio. Figure 3 shows the diffuse reflectance tube created by the y-ray, which is supported by comparison spectra of faujasites with different Si/Al ratios y-irradiated at with signals from irradiation of the empty sample tube. The 77 K for 20 min. Low temperature was used because of the copresence of trapped electrons and holes in the NaY sample
Liu et al.
13722 .IPhys. . Chem., Vol. 98, No. 51, 1994 0.4
1
400
300
500
600
700
800
Wavelength (nm)
Figure 3. Diffuse reflectance spectra of faujasites with different Sil
I
A1 ratios y-irradiated at 77 K for 20 min. The number denotes the SUA1 ratio of the sample.
TABLE 1: Absorption Maximum Position, Bandwidth and Oxygen Quenching Rate Constants of Na3+ Trapped Electrons in Zeolites with Different SUA1 Ratios absorption band zeolite NaX NaY NaY NaY
SUA1 1.4 1.8 2.2 2.5
position (nm) 546 514 508 505
width (nm) 166 137 122 127
oxygen quenching kq (Torr-' s-l) (3.2 f 0.3) x (4.9 f 1.1) x (4.6 f 3.4) x (2.2 f 0.5) x
104 104 104 104
instability of the trapped electrons in NaX at room temperature. It is clearly seen from the Figure that (1) all the samples give an absorption band maximum around 500-550 nm, characteristic of Nq3+trapped electron^;^,^ (2) the maximum absorption position of the N a 3 + cluster shifts toward shorter wavelength as the SUA1 ratio of the sample increases; (3) the shift from zeolite X to zeolite Y (-50 nm) is significant compared to that among the zeolites Y; and (4) the absorption band becomes narrower as the S U M ratio increases. The zeolite X (SUA1 = 1.4) exhibits the broadest absorption and the zeolite Y (SUA1 = 2.1-2.5) the narrowest. Table 1 summarizes the data obtained from Figure 3 together with the oxygen quenching rate constants (k,) for these samples. The spectra observed for zeolites X (1.4) and Y (2.5) are consistent with those of previous The shift observed here reflects the effects of framework SUA1 ratio of the zeolites on the trapped electrons. The jump of the maximum absorption position of N a 3 + from zeolite X to Y parallels the discontinuity of the unit cell dimensions from X to Y found from X-ray studies." Although both zeolites X and Y have the same framework topology, the detailed distribution of Si and A1 in their frameworks is different. The distinct difference in their physical and chemical properties of zeolites X and Y have already been documented from studies such as surface polarities,l52l6a d ~ o r p t i o n , ' ~and ~ ' ~oxidizing properties.17 The narrowing of the absorption band, as the SUA1 ratio of the samples increases, may be a reflection of the presence of a distribution of Na3+ clusters with different environments. The varied local environments in the zeolites have been shown by using photophysical techniques in studies taking pyrene molecules as probes where distinct local environments for concomitantly stabilizing pyrene cation and anion radicals in zeolites X and Y were observed.18 Figure 4 shows the ln(decay rate constant of the Na3+) in these samples as a function of 1/T (T = temperature in kelvin). The Arrhenius activation energies and the Arrhenius preexponential factors for electron-hole recombination reaction in these
Figure 4. Arrhenius plot of the ln(decay rate constant of the Na43+at 520 nm) as a function of l/T (T = temperature in kelvin): (1.4); *, NaY(1.8); x, NaY(2.2); 0, NaY(2.5).
+, NaX-
TABLE 2: Activation Energy of Na3+ Trapped Electrons in Zeolites with Different SUA1 Ratio temp range zeolite NaX(1.4) NaY(1.8) NaY(2.2) NaY(2.5)
(K) 300-365 340-420 350-455 430-470
preexponential factor (s-') (3.9 f 0.2) (1.5 f 0.2) (7.2 f 0.7) (5.8 f 0.3)
x lo8 x lo7
x lo7 x lo9
E, (kcdmol) 7.1 i 0.7 7.0 f 1.5 7.7 f 1.4 12.9 f 1.1
samples are obtained respectively from the slope and intercept of the linear fit and given in Table 2. The data exhibited higher activation energies for the zeolites with higher SUA1 ratios. The thermal stability of the trapped electron (Na3+) follows the sequence of X(1.4) < Y(1.8) < Y(2.1) < Y(2.5) and can also be visualized from the different fading rates of the pink color of the zeolites after being y-irradiated. To correlate the maximum absorption position of the trapped electron (Na3+) with the framework composition, a plot of the Sanderson electronegativityZoof the zeolites against the maximum absorption position is given. A nearly linear correlation among the zeolites Y is observed, as shown in Figure 5 . The plot demonstrates that the electron trapping by a sodium cation cluster in a supercage is governed by the entire structure. The long-range effects of the Na3+ tapped electron by its surrounding have been illustrated from the theoretical calculation of the absorption spectrum of the trapped electrons in zeolite sodalite.21 In addition to the features mentioned above, a weak absorption band (shoulder) around 620 nm which overlaps with the main band is also seen from Figure 3. This absorption band is more intense in zeolite X when the sample is excited with farUV light (193 nm).I9 A detailed discussion of this band was given elsewhere.lg Oxygen Quenching. Table 1 lists oxygen quenching rate constants for the samples. The data exhibit a trend that the oxygen quenching rate constant becomes smaller as the SUA1 ratio increases. The oxygen quenching rate constant usually reflects the mobility of oxygen molecules in a system. The largest oxygen quenching rate observed15 in zeolite faujasites is lo7 Tom-' s-l for the singlet excited pyrene in NaX. However, the oxygen quenching rate constants listed in Table 1 are 3 orders of magnitude smaller than that for lpV* in NaX. The higher SUA1 ratio of zeolite Y, compared to that of zeolite X, leads to a shrinkage of the supercage due to the difference in the Si-0 bond (0.162 nm) and the A1-0 bond (0.175 nm) and as a consequence may hinder the diffusion of 0 2 . The large
J. Phys. Chem., Vol. 98, No. 51, 1994 13723
Electron Trapping by Na+ Clusters in Zeolites
NaX,NaY-Gamma-POmins at 77K
"1"
, 2.4
2.5 Sanderron
2.6 Electronegatlvlty
2.7
Figure 5. Plot of maximum absorption position vs the Sanderson
electronegativity of the faujasites.
-
Figure 6. EPR spectrum of NaY sample adsorbed 17% of n-pentane vapor, y-irradiated 20 min at room temperature.
difference in oxygen quenching rate compared to the diffusioncontrolled limit (lo7 Torr-' s-') indicates that the oxygen quenching of the N a 3 + trapped electrons in zeolites X and Y is less effective. Effects of nPentane Molecule. The effects of n-pentane molecule on electron trapping by Na4+ clusters in zeolites were studied using the EPR technique. Figure 6 shows the EPR spectrum of NaY sample containing -17% of n-pentane y-irradiated for 20 min at room temperature. By comparison with the plain zeolite (Figure 2a), the yield of N a 3 + trapped electrons observed is doubled that in the absence of pentane. In addition, the positively charged hole which was observed in the plain sample does not appear in this sample. The increase of the yield of the trapped electrons and the disappearance of the positive charged holes imply that the n-pentane molecule interferes with the electron-hole recombination process. The interactions of the alkane molecules with the positively charged holes in zeolite systems lead to an increase of the yield of the trapped electrons (Ge- = 5.52). The absence of any EPR signal for the pentane molecules is consistent with the observations that the cation radicals of alkane molecules react to give products.22 Effects of Electron Scavengers. The presence of electron scavenging ions such as Cu2+,Ni2+,Cd2+,Co2+,Zn2+,and Pbz+
Wnvenumbers (em-*)
Figure 7. Infrared spectrum of the sample CulNas3Y
in the NaY zeolite leads to a decrease in the yield of the N a 3 + trapped electrons. The decrease is expected, due to the electron scavenging properties of these ions and to the destruction of the Na4+ trapping sites by the ions themselves and by the protons produced during the heat treatment of the samples. FTIR studies of these samples dehydrated at 400 "C under vacuum reveal that the protons are indeed produced.23 A typical FT-IR spectrum in the region 3900-3500 cm-' is given in Figure 7. The CulNa53Y sample shown in the figure was calcined at 400 Torr. Three absorption "C for 2 h under a vacuum of 1 x bands at 3737, 3694, and 3646 cm-' are clearly seen from the spectrum. The band at 3737 cm-' is assigned to vibrations of nonacidic OH groups ( E S ~ - O H ) ~and ~ the band at 3646 cm-' to those of acidic OH groups inside the super cage^,^^ while the band at 3694 cm-' is assigned to those of A1-OH groups overlapping with HZO-Na+ vibration^.^^ From the IR studies, it is clear that (1)calcination of Cu2+ion-exchanged NaY causes framework dealumination which creates =Si-OH and =AlOH groups, while under identical conditions the starting material NaY does not show spectra of =Si-OH and =AI-OH groups; (2) dehydration of the Cu2+ion-exchanged NaY produces acidic OH groups in the sample; and (3) the acidic OH groups produced are located in the supercages with a characteristic absorption band at 3646 cm-' but are not located in the sodalite cages which would correspond to an absorption band at 3540 ~ 1 2 1 - I . ~ Similar ~ results were observed for other samples containing electron scavenging ions, but the amount of protons produced varies. To study the effects of the protons that are concomitantly produced with the ions during dehydration on the electron trapping by the Na4+ clusters in the zeolites, a series of NaY samples with varying proton content from 0 to 2Iuc were prepared via ion exchange followed by calcination at 400 "C under vacuum. Examination of these samples under the same conditions of y-irradiation as for other samples shows a decrease in the yield of the N a 3 + trapped electrons (Figure 8). The curve in Figure 8 sets the upper limit for the effect of protons present in the samples. Figure 9a shows examples of steady-state diffuse reflectance spectra of y-irradiated (20 min) NaY samples containing different amounts of Mz+ (M = Cd, Zn, and Co), and Figure 9b gives plots of the relative yield of N a 3 + (normalized to the sample NaY with no M2+ ions) as a function of the M2+ concentration. The relative intensity of the N a 3 + in each individual system decreases with increasing the Mz+ concentration. To understand the reaction mechanism and to know whether the electron scavenging ions react with the electrons prior to being trapped by the N a 4 + trapping sites or react with the Nq3+ trapped electrons, we examined in detail the sample
m+
Liu et al.
13724 J. Phys. Chem., Vol. 98, No. 51, 1994 100
c, 0.1 5
:I
I
200 ~
~
~
1
1 .5
300
,
500
400
600
700
800
700
800
700
800
Wavekngth (nm)
0 0
0.5
2
Ht/u.c
Figure 8. Change of the yield of the N a 3 + trapped electron as a function of concentration of H+ in zeolite Y.
CulNa53Y after thermal treatment and subsequent y-irradiation using a combination of techniques such as luminescence and excitation spectroscopies, time-resolved luminescence, and highenergy electron pulse radiolysis. Figure 10a,b shows the steady-state excitation (monitored at 535 nm) and luminescence (excited at 320 nm) spectra of the CulNa53Y sample after being calcined at 200 "C under vacuum for 2 h. The excitation spectrum of the sample exhibits a band with maximum at 300 nm which is associated with the charge transfer band of 0 Cu+,24,25 while the luminescence spectrum shows a broad band centered at 535 nm, which is the luminescence characteristic of the Cu+ ions located at sites I' in the sodalite cages (see Figure 1).26 In the absence of Cu+, no bands could be observed from both excitation and luminescence spectra. Figure 11 shows the time-resolved luminescence decay curves (monitored at 535 nm) of the sample (heated at 200 "C under vacuum for 6 h (solid line)). Computer fitting of the experimental curve shows that the luminescence decay does not follow a single exponential, indicating that the local environment of each individual Cu+ ions is nonuniform. This is likely to be due to different distributions of Cu2+ions around the Cu+ ions. The decay can be fitted with a biexponential (57% vs 43%) which gives lifetimes 10 and 42 ,us, respectively (curves not shown). The shorter lifetime compared to that in the literature, z = 120 ps,27could be due to quenching of Cu' luminescence by Cu2+ ions (energy transfer).27 The fact that 0 2 does not disturb the decay (see Figure 11, dotted line) provides strong evidence for the location of copper ions in the sodalite cages. From these results we conclude that the Cu2+ ions, originally ion-exchanged into the zeolite, undergo reduction during dehydration and are partially reduced to Cu+ and that the copper ions are located inside the sodalite cages. Reduction of Cu2+ to Cu+ leads to the formation of acidic OH groups in the supercages and to dealumination of the framework (see IR studies). A similar phenomenon has been observed for NaY and other zeolites containing Cu2+ ions.28,29 y-Irradiation of the CulNas3Y sample for different periods of time produces further Cu+ ions, as seen in Figure loa. Irradiation for 20 min increases the intensity of the Cu+ band about 3-fold. Further irradiation of the sample (70 min) causes the intensity of the band to decrease, indicating that Cu+ is further reduced to Cuo which has absorption bands in the nearIR region and cannot be seen in the UV-vis spectrum.30 Increasing the time of irradiation causes a similar trend in the luminescence spectrum of the sample (see Figure lob). Figure 12 shows the time-resolved absorption decay curves of high-energy electron pulse radiolysis (monitored at 500 nm for Nq3+) of the zeolite samples containing 0 and 0.5 Cu2+/uc.
-
0 o.2 .15
8
1
1
0
A '
0.25
0.1
E 0.05 0 200
300
400
500
600
Wawkngth (nm) 0.25 1 0.2
1 0
CONaY
1
0
Opl:
0.05 0 200
300
500
400
600
Wavelength (nm)
b
100
90 80
70 +.
3
60
s
-
1
50 40
30 20 10
-.i \
-
0 0
0.5
LX
1
I
1.5
2
M(Z+)/u.c.
Figure 9. (a) Steady-state diffuse reflectance spectra of y-irradiated NaY samples containing different amounts of M (M = Cd2+, ZnZ+, and Co2+). The numbers denote the cofitents of M2+ per unit cell. (b) Plots of the relative yield of N a 3 + (normalized to the sample NaY without M2+ ions) vs the M2+ concentration from 0 to 2 M2+/uc.
The intensity decays were recorded 4 ps after the electron pulse. A static quenching of N a 3 +by Cu2+is observed from the initial Nq3+ intensity, and the decay rate is not disturbed by the Cu2+
Electron Trapping by Na+ Clusters in Zeolites g*"
=
J. Phys. Chem., Vol. 98, No. 51, 1994 13725
1w
; ,t
; )a, 14
gal"
!e',,
D
30
W a v ~ l * n p t h (nml
WaVelonglh (nm)
Figure 10. Excitation (A) and luminescence (B) spectra of the sample CulNassY y-irradiated at room temperature for different periods of time: (a) before y-irradiation, (b) after 20 min y-irradiation, and (c) after 70 min y-irradiation. The excitation spectra were monitored at 535 nm, while the luminescence spectra were excited at 320 nm. The samples were heated at 200 "C under vacuum for 2 h.
The above experiments clearly demonstrate that the scavenging of electrons by Cu2+ ion occurs before the electrons are trapped by the N a 4 + trapping sites. Now we consider the electrons prior to the trapping process. It is known from above that y-irradiation of zeolites produces electrons and positive charged holes (see Figure 2a). The electrons are expected to undergo many processes such as scattering, thermalization, and geminate recombination prior to being trapped by the N a 4 + trapping sites. The solvation process of electrons, which normally occurs in solutions, does not occur in the dry zeolites. Just before the electrons are trapped by the Na4+ trapping sites, two kinds of electrons may possibly be present: thermalized and quazifree electrons. These electrons can react with scavengers, leading to a decrease in the yield of N a 3 + trapped electrons. Following Freeman,31the probability that electrons are trapped by the N u 4 + trapping sites can be approximately described as
where Itlo is the absorbance ratio of the N a 3 + trapped electrons with and without presence of an electron scavenger, Ns is the molar fraction of the scavenger, fh and fqf describe the reaction efficiencies of the thermalized and quasifree electrons with the scavengers, respectively, n is a parameter that relates to the separation distance between electron and ion and the diffusion jump distance of the species, and p represents the probability that electrons do not become trapped. The above equation can be simplified if one of the two reactions dominates
I
2. 1 - 0.711 ti
ffl ? ' Ll I
c Z
+. 0.m
hi
3. H
c-
;
0 . n
I/Zo = (1 -f,N,)"
u1 IT
0.00
-
I
I
40
no
TIME
L 120
SW
200
(m i c r o s e c o n d s l
Figure 11. Time-resolved luminescence decays of the sample CUITorr) Nas3Y after treatment at 200 "C for 6 h under vacuum (2 x (solid line) and in the presence of 300 mbar of
0 2
(dotted line).
if the reaction of thermalized electrons with scavenger ions dominates, and
(3) if the reaction of quazifree electrons with scavenger ions dominates. Equation 2 can be further reduced to
1.00
(4) > t-
H
0.m
m z
W
I-
Z
0.-
w
>
t i
I-
5
0.25
W
! I
0.00
.,..Ad---'---
uc-
.a
l.P
S.6
( m i 11i s e c o n d s ) Figure 12. Transient absorption signal decays of N a 3 + produced in the NaY with Cuz+ (0.5 Cu2+/uc, dashed line) and without Cuz+ (solid line) by the high-energy electron pulse. The signals were recorded 4 p s after the electron pulse. TIME
ions. The intensity observed 1 ns after the electron pulse gives the same feature as those taken at longer times (spectra not shown). We therefore conclude that electrons produced from the zeolite framework react directly with the Cu2+ions and that the process does not undergo a Nq3+ cation cluster intermediate.
if fdS
