J . Phys. Chem. 1990, 94, 1953-1957 amount of ethylene desorption does not significantly change under isothermal temperature conditions for temperature values in the range between -236 and 274 K. The kinetic data demonstrate that FTMS is a sensitive and quantitative detection method and that the combination of laser desorption with the FTMS detection provides a very powerful tool for studying surface reaction kinetics. LITD/FTMS is capable of following surface reactions on a short time scale, and because a complete, high-resolution mass spectrum is obtained for each laser shot, the instrument can provide new insights on surface
1953
reactions that involve complex molecules or several surface species.
Acknowledgment. We thank Dirk Sander for developing the directed gas doser as well as for his experimental help in the early stages of this work. We would like to acknowledge financial support from the donors of the Petroleum Research Fund, administered by the America1 Chemical Society, from the National Science Foundation (CHE8511999), and from IBM Corporation Research Division. Registry No. C2H4,74-85-1; Pt, 7440-06-4; (CCHJ, 67624-57-1.
Interaction of Palladium Species with Hydrogen, Water, and Benzene on NaPd-Y and CaPd-Y Zeolites Studied by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies: Further Evidence for Migration of Palladium Species Ashim K. Ghosh and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: May 1 , 1989; In Final Form: July 31, 1989)
The generation of paramagnetic palladium species in NaPd-Y and CaPd-Y zeolites adsorbed with water, hydrogen, and benzene has been studied by electron spin resonance and electron spin echo modulation spectroscopies. The probable locations of the Pd species are assigned on the basis of Pd-adsorbate interaction distances. The Pd3+ions showing ESR signal A at gw= 2.23 in 02-pretreatedNaPd-Y zeolites occupy inaccessible sites (SI) in the hexagonal prism. In the presence of adsorbates, signal A decreases with the simultaneous appearance of various ESR signals due to Pd+ ions. The interactions of the Pd ions with adsorbates suggest that they migrate toward the zeolite supercage in order to interact with the adsorbates. In contrast to NaPd-Y, Pd3+ions in 02-pretreated CaPd-Y occupy relatively accessible sites in the @age (SII') and signal A disappears more rapidly in the presence of adsorbates with the appearance of signals due to Pd+ species.
Introduction It is known that the catalytic activity of cation-exchanged zeolites for nonacid catalysis is dependent on the type, amount, and location of active cations in the zeolite structure.I4 Previous studies5s6in this laboratory have demonstrated that the generation and location of transition-metal cations in the zeolite structure can be controlled by several parameters including the (a) size and charge of the coexchanged cation, (b) Si/AI ratio, (c) structural type of zeolite, (d) thermal pretreatment, and (e) presence of various adsorbates. In previous work,'+ we have reported results of ethylene dimerization catalyzed by palladium species on various palladium-exchanged X and Y zeolites. The reaction was found to be dependent on the (a) type of coexchanged cation, (b) Si/AI ratio, and (c) catalyst pretreatment. The results were interpreted as due to various site locations of the active palladium cations and their migration within the zeolite cages. We have shown that diand trivalent palladium ions in the zeolites are reduced by ethylene by an indirect electron-transfer mechanism to generate catalytically active Pd+ ions which migrate toward the supercage where reaction can occur. Electron spin resonance (ESR) coupled with electron spin echo modulation (ESEM) spectroscopies have proven to be useful techniques to characterize paramagnetic species in zeolites. ESEM can determine the number of adsorbate molecules with which a paramagnetic center is coordinated and the coordination distance. The location of paramagnetic palladium species generated in ( I ) Maxwell, 1. E. Adu. Catal. 1982, 31, 1. (2) Lunsford, J. H. Carol. Reu. 1975, 12, 137. (3) Ben Taarit, Y.; Che, M. In Catalysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 67. (4) Smith, J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A,, Ed.; American Chemical Society: Washington, DC, 1976; Chapter I . (5) Kevan, L. Acc. Chem. Res. 1987, 20, 1. (6) Kevan, L. Reu. Chem. Intermed. 1987, 8, 53. (7) Ghosh, A. K.; Kevan, L. J. Phys. Chem. 1988, 92, 4439. (8) Ghosh, A. K.; Kevan, L. J . Am. Chem. SOC.1988, 110, 8044. (9) Ghosh, A. K.; Kevan, L. J. Phys. Chem. 1989, 93, 3747.
0022-3654/90/2094-1953$02.50/0
NaPd-X and CaPd-X in the presence of various adsorbates has been studied in this laboratory.'OJ1 The current work extends those studies to Pd-exchanged Na-Y and Ca-Y zeolites in which the Si/AI ratio is about 2 times larger. It is found that palladium ions in NaPd-Y are reduced by adsorbate molecules by an indirect electron-transfer mechanism, migrate toward the supercage, and interact with adsorbates. In CaPd-Y the palladium ions are at sites accessible to the supercage where they are directly reduced by adsorbate molecules and interact with them.
Experimental Section Linde Na-Y zeolite was obtained from the Union Carbide Corp. Ca-Y zeolite was prepared from Na-Y by ion exchange with 0.1 M CaCIz solution at 80 "C for 1 week. Palladium was introduced into the zeolite as [Pd(NH3)4]2+cation by ion exchange with various amounts of 0.01 M palladium tetraammine chloride (Alpha) solution at room temperature for 24 h. Commercial atomic absorption was used to determine the palladium content. In this work samples of Na-Y, NaPdo,,-Y, NaPd2-Y, NaPd,-Y, NaPd14-Y, Ca-Y, CaPdo,,-Y, CaPd2-Y, CaPd,-Y, CaPd,,-Y, and CaPd14-Y zeolites were used where the subscript refers to the number of palladium ions per unit cell. A zeolite sample of 0.05 g was placed on a sintered glass disk inside a closed adsorption system made of glass. The zeolite was heated in oxygen flcw (-30 cm3/min) while slowly increasing the temperature to 500 OC at which heating was continued for 16 h. The sample was cooled to room temperature and excess O2was briefly pumped off before exposing to an adsorbate. All adsorptions were carried out at room temperature. The adsorbates were exposed at about 20 Torr. Deuterated compounds, Dz, and D 2 0 were obtained from MSD Isotopes and C6D6 was obtained from Stohler Isotope Chemicals. (10) Michalik, J.; Narayana, M.; Kevan, L. J . Phys. Chem. 1985,89,4553. (11) Michalik, J.; Heming, M.; Kevan, L. J . Phys. Chem. 1986, 90, 2132.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
1954
NoPd-Y/ H 2 0 n
Ghosh and Kevan NaPdlo- Y/Hz
02/500°C / l 6 h
XI
200 G
qien= 2.23
~
200 G U b XI
a XI
gko=2 23
- - -+8 x IO3
A, ,
'gDpp~zO/lh
v7
Ca Pd -Y/ H20
c 2x10~
02/500"/6 h
A
d 2x103
v
e xx800 4 H20/10 min
M
I
o
4
Figure 1. ESR spectra at 77 K of NaPd-Y and CaPd-Y: (a) and (c) after pretreatment under 0, flow at 500 "C, (b) and (d) with adsorbed water at 25
O C
The generation and migration of paramagnetic palladium species in the system was examined by ESR. The adsorption system was connected to a small quartz tube (3 mm 0.d.) so that the ESR spectrum could be recorded without exposure of the sample to air. Both before and after various adsorption times the sample was transferred in situ to the ESR tube. ESR spectra were recorded at 77 K with a Varian E-4 spectrometer. Electron spin echo spectra were recorded at 4.2 K with a home-built spect r ~ m e t e r . ' ~ *Deuterated '~ adsorbates were used for ESEM measurements in order to detect and analyze deuterium modulation.
Results Electron Spin Resonance Studies. N o paramagnetic species were observed in Na-Y and Ca-Y zeolites. As will be seen, various paramagnetic palladium species are observed in palladium-exchanged zeolites and their generation is dependent on the amount of palladium, type of coexchanged cation, catalyst pretreatment, and presence of adsorbate molecules. In previous an isotropic signal (A) at g = 2.23 assigned to Pd3+ ions has been observed in Pd-exchanged X zeolites. In this work, all O,-pretreated Pd-exchanged Na-Y zeolites show the signal A. i n contrast, at low palladium exchange the O,-pretreated CaPd-Y zeolites do not show signal A. However, this signal A is present in higher palladium-exchanged Ca-Y zeolites. The adsorption of water on O,-pretreated NaPd-Y and CaPd-Y zeolites resulted in relatively small changes in the ESR spectra (Figure I ) . Signal A, assigned to Pd3+ ions, decreases very slowly on adsorption of water. A number of weak signals in the region g = 2.2-1.95 are observed. When 0,-pretreated NaPd-Y zeolite was exposed to H, at 25 OC, the orange material became light brown and the ESR signal A decreased significantly. Various signals designated as B (gll = 2.85), C ( g , = 2.48), D (gl, = 2.33), and E (gi1= 2.68) all with the same g, value at 2.10 appeared (Figure 2). Species B and E were dominant. The decrease of species A and the growth of species B-E were enhanced with an increase of H2 pressure. On evacuation of the H2 adsorbed sample at 25 OC for different lengths of time, the ESR spectra did not change to an appreciable extent. The signals B, C, D, and E are assigned to the formation of Pd+ ions which are known to have large g anisotropy.I0 The Pd+ ions of signals B, C, D, and E are different due to locations (12) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979, 83, 3378. (13) Narayana, P. A.; Kevan, L. Magn. Reson. Reo. 1983, I , 234.
w
I/"
Figure 2. ESR spectra at 77 K of NaPd-Y: (a) after pretreatment under 0, flow at 500 OC, (b-d) with adsorbed H, for different times, (e) H,-adsorbed sample was evacuated. COPd, -Y/ H2 a XI
A f
g t o =2.23
b XI
0~/50O0C/16h
V
HD/25"C/I0 min
-
200
G
Figure 3. ESR spectra at 77 K of CaPd-Y: (a) after pretreatment under 0, flow at 500 O C , (b-d) with adsorbed H2 for different times.
at various sites in the zeolite structure or the formation of different Pd+-(H,)" species. The intensities of species B, C, and D further increase on admission of ethylene into the reactor containing NaPd-Y zeolite previously exposed to H2 and subsequently evacuated at 25 OC. In CaPd-Y zeolite the light brown 0,-pretreated zeolite turned dark brown on exposure to H2 at 25 "C. As observed in NaPd-Y zeolite, species A decreases with the simultaneous appearance of species B (gl = 2.76), C (SI, = 2.52), D (gll = 2.34), and E (g,, = 2.65) (Figure 3). A weak ESR signal at g = 2.05 and g = 2.00 attributed to 02-was observed in the spectra of CaPd-Y after exposure to H,. Figure 4 shows ESR spectra of various palladium-exchanged Na-Y zeolites after pretreatment in 0, and subsequent exposure to benzene. As mentioned, all 0,-pretreated Pd-exchanged Na-Y zeolites show an ESR signal A at gw = 2.23 assigned to Pd3+ions. On exposure of the 02-pretreated samples to benzene, signal A decreases. This decrease of signal A is due to reduction of Pd3+ ions by adsorbate molecules. The reduction is greatly dependent on the palladium content in the zeolite. As can be seen in Figure 4 no appreciable change was observed in signal A in NaPd,,,-Y in the initial 30 min but a slight decrease was observed overnight. A faster decay of signal A was observed with an increased Pd concentration. ESR signals at gll(B) = 2.85 and gll(C) = 2.52 both with g, = 2.10 appear with the decrease of signal A. No other paramagnetic species was observed even after overnight adsorption. It is noted that species C is found to be stronger in
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 1955
Paramagnetic Pd Species in Zeolites NaPd -Y/Benzene
'.Or A
1,
0.5 Pd2+/UC ~2 x io3 2x103
02/500°C/16 h
A
gkgE2.23",V
Benzene/30 min
V
5 Pd2+/UC x4
1 gDPPH
4-
x4 x40
0J50O0C/16h Benzene130min
NaPdIn-Y/ DaO 3 Pulse ESE-
r=0,27p,Hz2970G n = 2, r =0.37nm,a = O MHz 1-
-
XI0
2
= 1.96
U ,gL= 2.05
14 Pd2+/UC
I Benzenel30min
4
5
Figure 6. Experimental (-) and simulated (---) three-pulse ESEM spectra at 4 K of NaPd-Y with adsorbed D20recorded at a magnetic field set at gi,, = 2.23 (signal A). I.Or A
02/50OoC/l6h
-c
3 T, PS
Benzene130 min
?Cl,
X
I
0
v
v)
0.8
200 G I--+--c
Figure 4. ESR spectra at 77 K of NaPd-Y samples at various Pd loadings recorded after pretreatment under O2flow at 500 OC as well as after exposure of the 0,-pretreated sample to benzene at 25 "C.
CaPd-Y/D20 3 Pulse ESE
CaPd-Y/Benzene
= 0.27p, H = 2970 G n.2, r=0.33nm, a=O.IMHz T
0.5 Pd2+/UC
I
0
Benzene130 min giso=2.00
Benzene130 min g,, = 1.96
XI0
200 G
X2000 q,=2.05
5.0 Pd2+/UC
02/5000/I6 h BenzeneI30 min
10.0 Pd2+/UC
/Jlr
_c
XI
X
A
2
0
g,,=2.30
Benzene/30min
v
Figure 5. ESR spectra at 77 K of CaPd-Y samples at various Pd loadings recorded after pretreatment under 0 2 flow at 500 "C as well as after exposure of the O,-pretreated sample to benzene at 25 OC.
comparison to species B up to a palladium exchange level of around 10 Pd2+/unit cell, but above this exchange level species B became stronger. The signals B and C may be attributed to Pd+ species at two different locations in the zeolite structure. A signal at g,, = 1.96 and g, = 2.05 attributed to Pd2+-02-'0 is also observed. A strong isotropic signal at g = 2.00 is observed. Such an isotropic line in zeolites with adsorbed organic molecules is often assigned to organic free-radical formation, although the formation mechanism is unclear. As mentioned earlier, the 0,-pretreated CaPd-Y zeolites at low palladium exchange do not show signal (A) assigned to Pd3+ ions (Figure 5 ) . However, signals B and C due to Pd+ species appear immediately on exposure of the sample to benzene. This indicates that the Pd2+ ion is reduced to Pd+, in contrast to
I
1
I
2
I
3
4
,
5
T, PS
Figure 7. Experimental (-) and simulated (---) three-pulse ESEM spectra at 4 K of CaPd-Y with adsorbed D20recorded at a magnetic field set at giso= 2.23 (signal A).
NaPd-Y which did not show Pd+ species at low Pd exchange level when exposed to benzene. As observed in NaPd-Y, species C is stronger than B but the latter becomes dominant when the Pd content in the zeolite is above about 5 Pd2+/unit cell. In addition to species B and C, species D is observed at higher Pd exchange level. It is noted that species D was not observed in NaPd-Y samples when exposed to benzene. Electron Spin Echo Modulation Studies. The 0,-pretreated palladium-exchanged Y zeolites show too small an ESEM signal to analyze the modulation. On exposure of this O,-pretreated sample to deuterium, signal A decreases and various signals B, C, D, and E (Figure 2) due to Pd+ species appeared. This sample shows a spin echo signal but no deuterium modulation. This does not rule out the possible coordination of D, to Pd+ because the echo signal disappears in a time comparable to one period of deuterium modulation (0.5 M S ) . Signal A in 0,-pretreated NaPd-Y zeolites does not change appreciably on exposure of the sample to D,O. However, spin echo deuterium modulation was observed at a magnetic field set at gh = 2.23 (signal A) indicating coordination of Pd3+ ions with water. Simulation of the spectra indicate that two deuterium or one molecule of D 2 0 interacts with a Pd3+ion with a Pd3+-D (DzO) distance of 0.37 nm (Figure 6 ) . As in NaPd-Y, Pd3+ ions in CaPd-Y samples coordinated with two deuteriums or one molecule of D 2 0with a Pd3+-D (D,O) distance of 0.33 nm (Figure 7 ) , which is 0.04 nm shorter than that in NaPd-Y zeolite. As shown earlier, the ESR signal A in 0,-pretreated NaPd-Y zeolites decreases slowly on exposure to benzene. A spin echo signal with deuterium modulation was observed at a magnetic field set at giso= 2.23 for signal A (Figure 8). Simulation of this spectrum indicates that Pd3+ interacts with six deuterium or one molecule of benzene with a Pd3+-D (C6D6)distance of 0.50 nm. In NaPd-Y samples, with the decrease of signal A, signals B and C (Pd+ species) and signals at g,, = 1.96, g, = 2.05, and giso
1956 The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 10
Ghosh and Kevan NaPdla-Y/C& 3 Pulse ESE T = 0 . 2 7 ~ 5H, = 3 I 3 0 G n=6, r=035nm,a=0.15 MHz
LA 1 I :
NOP~S-Y/C~D~ 3 Pulse ESE ~ ' 0 . 2 7 p 5Hz2970G , n=6,r=0.50nm, a=OIMHz
I
0
I
1
I
2
3
I
I
4
5
T, PS
I
l
! l l 2700
l 1~ I 2900
/
I
I
I
I
I
I
I
2
3
4
5
T,ps
Figure 8. Experimental (-) and simulated (---) three-pulse ESEM spectra at 4 K of NaPd-Y with adsorbed C6D6recorded at a magnetic field set at g,, = 2.23 (signal A).
l l 2500
I 0
I
8
3100
1
I
I
)
3300
,
c
l
3500
FIELD (GAUSS)
Figure 9. Field-swept ESE spectrum at 4 K of NaPd-Y with adsorbed C,D6.The inset shows the ESR spectrum at 77 K.
= 2.00 are observed when the sample was exposed to benzene. Field sweep experiments show three peaks at g = 2.10, 2.05, and 2.00 (Figure 9). Spin echo spectra recorded at g = 2.05 and 2.00 show deuterium modulation too weak to analyze. However, spin echo with stronger deuterium modulation was recorded at g,(B,C) = 2.10 (Figure IO). It was not possible to distinguish the contributions of species B and C, since both have the same g, value and parallel components of B and C were not separated in the field sweep experiments. Simulation of the spectrum recorded at g = 2.10 suggests that species B and/or C are due to Pd+ ion interacting with six deuteriums or one molecule of benzene with Pd+-D (C&) distance of 0.35 nm. This Pd+-D (C6D6) distance is significantly shorter than Pd3+-D (C6D6) observed in NaPd-Y (see Figures 7 and 9). The results suggest that Pd3+ and/or Pd2+ after reduction to Pd+ ions migrate toward the supercage to interact with benzene molecules. At low Pd-exchange level CaPd-Y did not show any signal A from Pd3+ions. However, signals B, C, and D from Pd+ species are observed on exposure to benzene. The simulation of ESEM spectra recorded at g,(B,C,D) = 2.10 indicates that species B, C, and D are due to Pd+ ions interacting with one molecule of benzene with a Pd+-D (C6D6) distance of 0.36 nm (Figure 11). Discussion Zeolite Y is composed of alternating Si04and A104 tetrahedra with a Si/AI ratio of 2.4. These units are linked to form truncated octahedra called sodalite cages or @-cageswith an opening diameter of 0.22 nm. The unit cell of zeolite Y contains eight sodalite units (a total of 192 (Si,A1)04 tetrahedra) which are tetrahedrally bonded through a double six-ring unit called a hexagonal prism. This produces a larger cavity or supercage (a-cage) with 12-ring openings of 0.74 nm diameter. Zeolite Y has a three-dimensional channel network with large pore openings which allows easy diffusion of molecules through the internal volume. Due to the excess negative charge present on the A104 units, compensating cations are present to balance the charge in
Figure 10. Experimental (-) and simulated (---) three-pulse ESEM spectra at 4 K of NaPd-Y with adsorbed C6D6 recorded at a magnetic field set at g,(B,C) = 2.10.
the zeolite framework. Various cation site locations are designated as follow^:^ SI is at the center of the hexagonal prism, SI' is a displaced site from SI into the 0-cage, SI1 is at the center of the hexagonal window between an a-cage and @-cage,SII' and SII* are sites displaced from site SI1 into the P-cage and the a-cage, respectively, along an axis perpendicular to the hexagonal window. In this work, palladium was ion-exchanged into Na-Y and Ca-Y zeolites as [Pd(NH3),12+ ion. This cation initially can only occupy sites in the a-cage due to its bulky size and is decomposed into Pd2+and NH, during the thermal pretreatment. The Pd2+ ions so formed occupy sites in the @-cageand/or hexagonal prism. A small part (-4%) of the Pd2+ ions are oxidized to Pd3+ ions, giving signal A, during the 0, pretreatment.'] So, the palladiums in the 0,-pretreated sample remain mostly as Pd2+and partly as Pd3+ ions. Table I summarizes the paramagnetic palladium species generated in NaPd-Y and.CaPd-Y zeolites with adsorbates. The ESR signal A from Pd3+ ions, observed in the O,-pretreated NaPd-Y zeolite, decreases very slowly when the sample is exposed to water. In earlier work'] no appreciable change in signal A in NaPd-X exposed to water was also observed. The present ESEM results show that Pd3+ interacts with one molecule of DzO with a Pd3+-D (D,O) distance of 0.37 nm. This Pd3+-D distance is too long for direct coordination of water with Pd3+suggesting the location of Pd3+ in NaPd-Y at inaccessible sites in a hexagonal prism (SI) interacting with one molecule of D 2 0 in the @-cage. If Pd3+ occupy site SI in a hexagonal prism, their interaction with bulky molecules like benzene is unlikely since the adsorbate molecules cannot enter the @-cage or much less the hexagonal prism. The location of Pd3+ at S I is consistent with our earlier work in which we have observed no deuterium modulation associated with signal A due to Pd3+ when NaPd-X is exposed to C,D,. This suggests that the Pd3+-D distance for this adsorbate is longer than the distance of about 0.6 nm detectable by ESEM. In this work, however, we have observed deuterium modulation associated with signal A in ESEM studies of NaPd-Y adsorbed with C6D6 with a Pd3+-D distance of 0.50 nm. The Pd3+-D distance of 0.50 nm indicates the location of Pd3+ at sites in the @-cage (SI' or SII') at which an indirect coordination between Pd3+ (in @-cage) and the *-electron orbitals in the benzene molecule (in a-cage) occurs. The present result seems to suggest that Pd3+ ions in 0,-pretreated NaPd-Y at site SI migrate into a @-cage(SI') in the presence of benzene. The ESEM results show that Pd+ interacts with one molecule of benzene with a Pd+-D (C6D6) distance of 0.35 nm which is 0.15 nm shorter than the Pd3+-D (C6D) distance. This suggests that the palladium ions migrate to more accessible sites probably at SI1 or SII* for direct interaction with the adsorbate. This is in contrast to NaPd-X zeolites where only indirect interaction between Pd+ and C6D6is observed with a Pd+-D distance of 0.44 nm. Previously, we have observed migration of palladium ions during ethylene dimerization on Pd-exchanged X and Y zeolites.8~~ This cation migration is found to occur rapidly in zeolite Y in
The Journal of Physical Chemistry, Vol, 94, No. 5, 1990 1957
Paramagnetic Pd Species in Zeolites
TABLE I: Paramagnetic Palladium Species Generated in 02-Pretreated NaPd-Y and CaPd-Y Zeolites with Adsorbates zeolites/ads species obsd probable assignt Pd3+ions in hexagonal prism (SI) NaPd-Y A (giso= 2.23) CaPd-Y A (giso= 2.23)" Pd3+ions in &cage (SII') NaPd-Y / D 2 0 A (giso= 2.23) Pd3+-(D2O);Pd3+at SI CaPd-Y / D 2 0 A (gi, = 2.23)" Pd3+-(D20);Pd3+at SII' Pd3+-(C6D6);Pd3+at SI' or SII' NaPd-Y/C&e A (a, = 2.23) B (gll 2.85, g, = ESE spectrum recorded at g = 2.10, C (gl, = 2.85, g, = 2.10) 2.10) Pd+-(C&6), Pd+ at SI1 or sII* Pd2+-0; gil = 1.96, g, = 2.05 giso= 2.00 unknown free-radical CaPd-Y/C6D6 A (giso= 2.23)' disappears rapidly B (gll = 2.79, g, = 2.10) ESE recorded at g = 2.10, C (gli = 2.50, g, = 2.10) Pd+-(C6Da),Pd+ at SI1 or sII* D (gll = 2.30, g, = 2.10) Pd2+-0F gll = 1.96, g, = 2.05 giso= 2.00 unknown free-radical NaPd-Y/D, A (aso = 2.23) disappears rapidly B (gll= 2.85, g, = 2.10) C (gll = 2.48, g, = 2.10) Pd+, no D-mod observed: see text D (gll = 2.33, g, = 2.10) E (gll = 2.68, g, = 2.10) CaPd-Y /D2 A (gi, = 2.23)' disappears rapidly B (gll = 2.76, g, = 2.10) C (gS= 2.52, g, = 2.10) Pd+, no D-mod observed; see text D (gli = 2.34, g, = 2.10) E (gll = 2.65, g, = 2.10) gil = 2.05, g, = 2.00 02-
I
i
i t
" When Pd concentration is 3 5 Pd/unit cell.
-
1.0-
COPd,-Y/C& 3 Pulse ESE
r=0.27ps,H~3120G
fn
0.87'
n.6, r=0.36nm, a.O.1 MHz
= J1 I m E l 5 0.6 -. ,, I>. fn
5 0.4.I-
z
0
3 0.2W I
I
I
I
I
Figure 11. Experimental (-) and simulated (---) three-pulse ESEM spectra at 4 K of CaPd-Y with adsorbed C6D6recorded at a magnetic field set at g,(B,C) = 2.10.
comparison to zeolite X due to lesser cocation crowding in the former case. This may be the reason why we have observed coordination of Pd3+ions with benzene molecules in NaPd-Y in contrast to NaPd-X. In the presence of benzene, signal A from Pd3+ ions decreases with the simultaneous appearance of signals B and C due to Pd+ ions. Previously signals B and C were also observed in NaPd-X zeolites." The disappearance of signal A is apparently due to reduction of Pd3+ ions by benzene. Since Pd3+ ions occupy sites in a hexagonal prism where benzene cannot enter, the reduction of Pd3+ ions is not clearly understood but is s ~ g g e s t e d ' ~toJ occur ~ via an indirect electron transfer via the zeolite lattice. It is also possible that Pd3+ ions may migrate into accessible sites and are reduced by benzene. The details of this indirect electron-transfer process are unknown. Regardless of the mechanism of the reduction of the Pd ions, the present results clearly indicate that Pd ions migrate toward the a-cage for direct coordination with the adsorbate. This is supported by the fact that Pd+-D (C&) interaction distance is significantly shorter than the Pd3+-D (C&) distance. (14) Bergeret, G.; Tran Manh Tri; Gallezot, P. J . Phys. Chem. 1983.87, 1160. ( 1 5 ) Romanikov, V . N.; Ione, G. G.; Pedersen, L. A. J . Caral. 1980, 66, 121.
It has been reported that Ca2+ ions in CaPd-Y zeolite preferentially occupy sites SI' thus impeding Pd ions from entering the hexagonal prism (site SI). This forces the Pd ions to occupy relatively accessible sites, e.g., SI1 or SII'. In agreement with this the present ESE results show an interaction distance for Pd3+-D (D20) of 0.33 nm which is shorter than the 0.37 nm in NaPd-Y. Since Pd ions in CaPd-Y occupy relatively accessible sites in comparison to these in NaPd-Y the reduction of Pd3+ or Pd2+ ions to Pd+ ions occurs rapidly. ESE spectra could be obtained for CaPd-Y with adsorbed C6D6 at the initial stages of adsorption and simulation indicates that an interaction between Pd+ ion and one molecule of benzene occurred with a Pd+-D distance of 0.36 nm. This Pd+-D distance is shorter than the 0.41 nm observed
Conclusions
ESR signal A (giso= 2.23) assigned to Pd3+ cations in 02pretreated NaPd-Y zeolites does not change appreciably on exposure of the sample to water, but disappears or decreases substantially when exposed to hydrogen or benzene. It is found that Pd3+interacts with one molecule of water with a Pd3+-D (D20) distance of 0.37 nm. This Pd3+-D distance suggest an indirect coordination of water with the Pd3+cation at site SI in a hexagonal prism interacting with water at a distance in the @-cage. It seems that the Pd3+ cations migrate to relatively accessible sites probably at SI' or SII' in the @-cagein the presence of benzene, giving an indirect coordination with a Pd3+-D (C&) distance of 0.50 nm. Pd3+and Pd2+are reduced to Pd+ by benzene and migrate to sites SI1 or SII* for direct coordination as evidenced by the Pdt-D (C6D6) interaction distance of 0.35 nm. As in NaPd-Y, Pd3+ in CaPd-Y interacts with one molecule of D 2 0 with a Pd3+-D (D20) distance of 0.33 nm which is 0.04 nm shorter observed in NaPd-Y, indicating a relatively accessible location of Pd3+ probably at SII' in the Fcage. In the presence of H2 or benzene, signal A from Pd3+ disappears more rapidly in CaPd-Y due to the occupancy of Pd3+ at more accessible sites in comparison to the NaPd-Y zeolites. As in NaPd-Y, Pd+ interacts with one molecule of benzene with a Pd+-D distance of 0.36 nm, indicating migration to site SI1 to S I * .
Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Technology Research Program.
