J . Phys. Chem. 1989, 93, 2563-2569 of PtC16’-. When corrected for the disappearance of ZnO,, we find that Pt, quenches the ZnO, emission; the fluorescence quenching rate constant measured k, = 2.1 X 10l2and 1.4 X 10l2 M-I s-l for [Pt,] = 4 X lod and 1 X M, respectively (in terms of Pt atoms). Thus, the Pt, particles quench the ZnO, emission effectively, showing that the Pt, is deposited on the ZnO, surface. The photodeposition method has been used previously by several groups to prepare Pt, deposited on Ti02 colloids.21*22 Upon addition of excess acid, the ZnO dissolves and Pt, stabilized by PB remains as the only colloid particles in solution. Such solutions show typical light scattering and are stable for at least several months. Since it is easy to control the number of Pt atoms deposited on each ZnO, particle by controlling the [PtC162-]and [ZnO,], it is possible to use the method described here to produce various colloid particles with controlled sizes. Conclusions
The positive polymer, polybrene, strongly interacts with ZnO, (21) Kraewutler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 4318. (22) Nakahira, T.; Gratzel, M. J . Phys. Chem. 1984, 88, 4006.
2563
at pH 11.7 and facilitates reactions of light-induced electrons and positive holes with negatively charged scavengers such as ferriand ferrocyanide ions and PtCl2-. In the absence of polybrene, no quenching of the ZnO, emission is observed in these cases. The effects of [ZnO,], [PB], and [Q] enable the calculation of the average charge of a ZnO colloidal particle. PtC162- ions are photolytically reduced, and Pt, is formed on the ZnO, surface. Pt, quenches the emission of ZnO, apparently by electron scavenging. Cu(OH), and “Fe(OH),” (iron(II1) mixed salts and hydrated oxides) can form a “sandwich colloid” together with ZnO, and quench the ZnO, visible emission.
Acknowledgment. We are indebted to Dr. S.Gershuni for assistance in synthesis and helpful discussions. This research was supported by the Israel-USA BNSF and by the Balfour Foundation, Tel Aviv, Israel. Registry No. PB, 28728-55-4; ZnO, 1314-13-2; Fe(CN):-, 1340862-3; Fe(CN)64-,13408-63-4; CU(OH)~, 20427-59-2; Fe(OH),, 130933-7; PtCIz-, 16871-54-8; Pt, 7440-06-4.
Application of 12’Xe NMR to the Study of Ni2+Y Zeolites A. Gedeon, J. L. Bonardet, T. Ito, and J. Fraissard* Laboratoire de Chimie des Surfaces associt au CNRS, U.A. 870, UniversitP Pierre et Marie Curie, 4 place Jussieu 75252, Paris CPdex 05, France (Received: June 22, 1988)
Nickel-exchanged sodium Y zeolites have been studied by using 129XeNMR spectroscopy. In spite of the paramagnetism of the Ni2+cations, it is possible to determine quantitatively the electric and magnetic effects for different samples at various levels of exchange (A = 15, 35, 58%) and hydration (26 I Tt O C I 500) as well as to locate the Ni2+cations in the zeolite. The application of the strong chemisorption model and the Simplex procedure gives similar results for the magnetic and electric perturbations.
Introduction
Several have shown that the 129 isotope of xenon is an ideal probe for investigating the environment and the location of cations within a zeolite and the dimensions of the void volume therein as well as the distribution and the size of metal particles when they are too small to be detected by electron microscopy. The aim of the present work is to confirm that Iz9XeN M R can be extended to the study of solids containing paramagnetic species. We have chosen as the adsorbate a Na-Y zeolite partially exchanged with Ni2+cations, and we have studied the effect of the extent of exchange and the dehydration temperature on the chemical shift of the adsorbed xenon. Experimental Section
Sample Preparation. The initial Na-Y zeolite used is LZY52 from Union Carbide Co. (Si/Al = 2.4). Sodium-nickel exchange is achieved by treatment with a 0.1 M aqueous solution of nickel nitrate under reflux at 80 OC for 12 h. The solution is then filtered; the solid is washed with distilled water to complete elimination of nitrate ions and then dried in an oven. The Ni2+ and Na+ cations are quantitatively analyzed for by atomic absorption spectroscopy. The samples prepared correspond to cation exchange levels of 15, 35, and 58%. (1) Fraissard, J.; Ito, T.; de Menorval, L. C. Proc. 8rh Inr. Congr. Caruly. 1984, 3, 25.
(2) Springuel-Huet, M. A,; Demarquay, J.; Ito, T.; Fraissard, J. Proceedings of the International Symposium on Innovation in Zeolite Materials Science; Nieuwport: Belgium, 1987. (3) Fraissard, J.; Ito, T.Zeolites 1988, 8, 350.
0022-3654/89/2093-2563$01.50/0
Treatment of the Solid. A known amount of the product is placed in an N M R tube closed by a tap. The tube is evacuated to Torr at 26 “ C and then heated with a gradient of 24 O C h-’ to the desired treatment temperature (26 ITt I500 “C). The sample is held at this temperature for 12 h and then brought back slowly to 26 “C. The loss of mass is determined by weighing (to ~ 0 - g). 4 Nomenclature. Samples treated at temperature Tt are denoted NAY-T,, where X represents the extent of Na+ exchanged in percent (A = 15, 35, 58). These samples therefore contain 4, 10, and 16 NiZf per unit cell. Xenon Adsorption. Xenon is always adsorbed at 26 OC, the temperature of the N M R probe. The amount of xenon adsorbed is generally expressed as the number of Xe atoms, N , per gram of anhydrous solid. 129XeN M R . The N M R spectrum of adsorbed xenon is generally recorded at 26 OC on a Bruker CXP 100 Fourier transform instrument at 24.9 MHz; the duration of recycle delay is 0.5 s, and the number of scans is between 500 and 50000. Results
Effect of the Extent of Exchange X on the Residual Water Concentration [ H 2 0 ](Table Z). The relative water concentration, [H20], is referenced to that of NaY containing 260 water molecules per unit cell: 0 I[H20] Il . [ H 2 0 ] = l corresponds to 26% w/w of hydrated solid as is shown in Table I . The water concentration decreases monotonically with increase in the treatment temperature. However, for the same value of Tt, [H20] increases in proportion to the extent of exchange (Figure 1). From the value of A, we have deduced, by studying more or less hydrated
0 1989 American Chemical Society
2564
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
TABLE I: Values of Residual Water Concentration and Free Volume for NaY and NIXY-Tt TI, "c nH20/UC nH,o/Nilr [H,O] f'f,Cm3 g-' Ni 1 5Y - TI not treated 298 74.5 0 26 0.6 1 160 40 0.12 50 81 20.25 0.3 1 0.27 53 13.25 100 0.20 0.29 21 5.25 150 0.08 0.3 250 0.02 6 1.5 0.3 350 0.6 0.15 0.002 0.3 500 0 0 0 0.3
Gedeon et al.
Nt
Ni3 5Y - T,
not treated 26 50 100 150 250 350 500
312 190 105 67 27 8.4 1 0
31.2 19 10.5 6.7 2.7 0.84 0.1 0
not treated 26 50 100 150 250 350 500
342 220 139 84 31 11.6 11.1
Ni58Y- T, 21.4 13.75 8.68 5.25 1.93 0.725 0.69
not treated 26
260 146
0
0
0.73 0.40 0.26 0.10 0.033 0.004 0
0 0.07 0.24 0.28 0.3 0.3 0.3 0.3
0.85 0.54 0.32 0.12 0.045 0.042 0
0 0.05 0.17 0.26 0.3 0.3 0.3 0.3
2 Xe atomsicavity
- 5
NaY 50 100 150 250 350 500
1 0.56
0
13.5
0.05
0.3
9
0.03
0.3
0
0
0.3
0.19
'nH20/UC: number of water molecules per unit cell. nH20/Nir1: number of water molecules per cation Ni". [H,O]:relative water 5 1. V,: Volume of the free spaces capable concentration 0 5 [H20] of adsorbing xenon (V, cm3 g-l).
1101
.,
u 54d0 10"
Xe atoms,g 10"
Figure 3. 129Xe N M R chemical shift versus number of xenon atoms per gram for Nil5Y sample pretreated at the following TI ("C): 0, 26; @, 50; 0,100;
10
-s
2,-
the volumes, Vf, of the free spaces capable of adsorbing xenon. Whatever A, V, is greatest when TI 1 150 "C for the NiXY samples and when TI 2 100 OC for N a y . Irrespective of the water distribution in the solid, Table I also presents the average number of water molecules per unit cell (uc), nH2O/Uc, and per cation, nH20/Ni2+. Xenon Adsorption Isotherms. For samples degassed under vacuum at 26 O C the xenon adsorption isotherms all have the same appearance regardless of the extent of exchange, in particular, (4) Gedeon, A.; Ito, T.; Fraissard, J. Zeolites 1988, 8, 376
150; A, 250;
X,
350;
+, 500.
a plateau indicates that the supercages are saturated as soon as they contain an average of two xenon atoms (corresponding to N = lo2'). For the NiAY-T, samples where 50 ITt I500 "C, the isotherms, expressed as log N = f(log P ) , are linear in the pressure range considered (IO IPx, I1000 Torr). They depend very little on TI and the nickel concentration. For this reason we give only the curves for Ni35Y-Tt (Figure 2). Iz9XeNMR. In what follows we shall use the following notation: 6S,NaYis the value of 6 at zero concentration for fully dehydrated N a y . 6S,NaYhere represents the effect of the faujasite structure. 6,, is the value of 6 obtained by extrapolation of the experimental curves to N = 0. The error on 6, depends very much on the form of the 6-N curve. Naturally, it is least when this latter is a straight line. NilJY-TI Samples. ( a ) Chemical Shift (Figure 3). The spectra of xenon adsorbed on NilSY-TI samples measured at 26 "C all have a single component regardless of the xenon concentration Nand the pretreatment temperature Tt. However, Figure 3 shows that the 6-N variation depends greatly on TI and, therefore, among other things on the residual water concentration [H20] detailed in Table I. For TI = 26 or 50 O C , that is, [ H 2 0 ] = 0.61 or 0.31, 6 varies linearly with N. The 6 values at zero concentration,,,,,a are 130 and 112 ppm, respectively. For comparison, they are 108 and 82 ppm for equally hydrated NaY sample^.^
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2565
129XeN M R Study of NiZ+YZeolites
Xe atomslcovity
i
190
i
\
400-
L
A '
200:
lozo
L
S
--n-g-o-j
&&d*Z
5.1020 102' Xe atms/g
Figure 4. '29Xe NMR chemical shift versus number of xenon atoms per gram for Ni35Y sample pretreated at the following: T, ("C): 0, 26; 0 ,
50; 0, 100; m, 150; A, 250; X, 350; +, 500.
The 6-N variation begins to depart from linearity at low N for T1 = 100 O C . For 150 I T, I350 OC the curves have the following characteristics: They show a minimum the depth of which increases with Tl and, therefore, when the water concentration is lowest; the values of 6 for low N are very high but decrease also with [HzO],at least when the signal can be detected; for example, for N = 1.8 X lo", Le., about 0.5 atom/supercage, 6 falls from 194 to 128 ppm when T, goes from 150 to 350 O C ; therefore, [HzO] decreases from 0.08 to practically zero. In this range of [ H 2 0 ] values V, is constant and maximal (0.3 cm3 g-1).5 The 6-N curve of the Ni15Y-500 sample is like the previous ones but it is slightly below that for Tl = 350 OC. ( b ) Line Width. For T, = 26 and 50 O C the line widths, AH, are independent of N and equal to 8 and 9 ppm, respectively. However, for T, 1 100 O C at low N values, AH increases with the extent of dehydration. For example, for N = 1.8 X lozo,AH goes from 17 to 27 ppm when T, is raised from 150 to 350 O C . Finally, for any value of T, the width decreases when N increases. Ni35Y-Tl Samples. ( a ) Chemical Shift (Figure 4 ) . The &N curves are very similar to those above, particularly the following: for T, = 26 OC,that is, [HzO] = 0.73, the variation is linear. The is 148 ppm, greater than that for NaY value of 6 at N = 0,,,6, at the same [HzO], 121 ppm. However, beyond T, = 50 O C ([H,O] = 0.40) the curve departs from linearity. For 100 C T, C 350 O C , there is a minimum whose depth increases with T, and, therefore, as [HzO] decreases. The curve for Tt = 500 OC is above that for T, = 350 "C. However, particularly for the last two temperatures, the slope, which is negative near the minimum, appears to be much more marked than for the Nil5Y samples. Moreover, for the same T, value, 6(Ni35Y) is always much greater than 6(Ni15Y), especially at low N values. Nevertheless, this difference is reduced when T, increases. ( b ) Line Width. Apart from a tendancy toward higher AH values, the line width varies with T, in much the same way as for the Nil 5Y sample at the same dehydration temperatures. For example, for N = 1.8 X 1020, AHgoes from 30 to 53 ppm when T, rises from 100 to 500 "C. Ni58Y-Tt Samples. ( a ) Chemical Shvt (Figure 5 ) . The NMR spectra of xenon adsorbed on this sample still consist of a single line. As in the previous cases, the chemical shifts for T, = 26 and 50 O C are linear with N . The corresponding 6, values are 200 and 180 ppm, respectively, much higher than the corresponding values for Nil5Y and Ni35Y. ( 5 ) Breck, D.
W. Zeolite Molecular
Sieves; Wiley: New York, 1974.
Figure 6. Variation of the chemical shift 6 and line width AH against the reciprocal temperature of NMR experiment l/TE: m, Ni15Y-350; 0,Ni35Y-350.
Generally speaking, for T, > 100 OC: The chemical shifts are particularly high. For example, for T, = 300 O C and N = 1.8 X lozo, 6 = 1700 ppm. The values decrease monotonically as N increases. The minimum found previously is not reached even for two Xe atoms per supercage. The spectra of xenon adsorbed on Ni58Y-Tt > 350 "C are so broad that it is impossible to detect a signal even at high xenon concentration. ( b ) Line Width. The lZ9XeN M R signals are much broader than in the previous cases. For example, for N = 1.88 X lom and T , = 150 O C , AH increases from 17 ppm for Nil5Y to 38 ppm for Ni35Y and to 100 ppm for Ni58Y. Effect of the N M R Experiment Temperature TE(Figure 6). Because of the very broad spectra for Ni58Y we have studied the effect of the N M R experiment temperature from 26 to -80 O C for the Ni15Y-350 and Ni35Y-350 samples only, at the concentration N = 5.1 X 1020(at 26 " C ) . The dead volume of the sample tube being very small, N hardly changes when the temperature is reduced. The signal width (Figure 6) increases as TE decreases, and the shift 6 increases linearly with l/TE. Determination of the Chemical Shifts Characteristic of the Various Factors Affecting Xenon Adsorbed on the NiXY-T, Samples The spectrum of adsorbed xenon consists of a single line,
whatever the sample. At a given moment Xe atoms are adsorbed
2566
The Journal of Physical Chemistry, Vol. 93, No. 6 , 1989
on various sites characterized by different components. However, because of rapid exchange at 26 “ C of the atoms adsorbed on the sites, the signal detected is due to coalescence of the components associated with the various adsorbed states. We must therefore attempt to determine the corresponding chemical shifts. Strong Chemisorption: Theoretical Outline. In the study of chemisorption by NMR, the N M R problem is to determine the chemical shift hC of the chemisorbed molecules, difficult to measure directly because of the broadness of the corresponding component. Long before the appearance of the MAS-NMR technique, Bonardet et al. showed that one can use the fast exchange between molecules chemisorbed and physisorbed on diamagnetic solids.6 This method was then developed for paramagnetic systems by Borovkov et ai.’ and Enriquez et a].* and then expressed in the following mathematical form by Pfeifer et consider the equilibrium K
P+A=C (1) where P represents the physisorbed molecules (number N p ) ,A the active chemisorption sites (total number N A ) , and C the chemisorbed complex (number N c ) . The equilibrium constant is given by
--
K=
N P ( N A- N c )
NC ( N - N c ) ( N A- N c )
(2)
N is the total number of adsorbed molecules: N = Np $. N c and NA - Nc is the number of unoccupied sites. In the case of fast exchange, with N INc, the chemical shift of the single line observed is given by Nc 6=-& N+ -
N - NC N 6P
Gedeon et al. studied here in which the xenon is subjected to purely physical but very different interactions. Fraissard et al.” have shown that the chemical shift of xenon in a zeolite is the sum of terms corresponding to the various perturbations to which this probe is subjected. The equation can be written
where 6o is the reference (gas at zero pressure), 6S,NaYcorresponds to the effect of the structure alone ( N a y ) , and 6xe expresses the increase in 6 due to Xe-Xe collisions. It is proportional to the local xenon density, p X e , and can be written 6xe = kN if the free volume, V f ,where the xenon is adsorbed is constant. Jameson in the case of gases1*and Bansal and Dybowsky for that of slightly dehydrated NaNiYI3 have shown that at high densities it is necessary to introduce a correction term, k’M to take into account triple collisions. C6iexpresses the sum of the other perturbation effects within the zeolite, such as electric B E and magnetic 6mag fields due to cations, the effects of other adsorbates (water, for example), supported metal particles, etc. In the case considered, x6,must be put in the form (6, + 6mg)Nc/N33139’4 and must tend toward a finite value, 6c, given by experiment when the number of xenon atoms adsorbed is very small. Determination of the Various Factors. ( a ) Calculation of 6c. Let 6c be the chemical shift of the xenon when it is closest to a Ni2+ cation more or less surrounded by water molecules or O H groups. In this case, 5 becomes
where 6S,NaY is characteristic of the effect of the void space wall, more or less covered with water depending on the value of Tt:
(3)
Generally, for light nuclei, the chemical shift of the physisorbed phase, bP, is negligible. bP N 6ref,gas 0. Equation 3 becomes (3’)
Considering
we obtain
where When N obtains1°
- 0, A
A=
6/6C
= Nc/N
Amo = 66/,c. X Y = - - N Amo
After various calculations one 1 A
Amo2
(4)
where
The dependence of Y on X leads to Amo, to hC and NA. This method generally gives acceptable results for hC but sometimes appears unreliable for the determination of the number of sites, NA. NAY-T, Samples. Theory. In what follows we shall show that the previous method of calculation is still applicable to the system (6) Bonardet, J. L.; Fraissard, J. Jpn. J . Appl. Phys. Suppl. 2 1974,2,319. (7) Borovkov, V . Yu.; Zhidomirov, G. M.; Kazansky J . Strucr. Chem. 1975, 16, 308. ( 8 ) Enriquez, M. A,; Fraissard, J. J . Catal. 1982, 77, 96. (9) Michael, A.; Meiler, W.; Michel, D.; Pfeifer, H.; Hoppach, D.; Delmau, J. J . Chem. Soc., Faraday Trans. I 1986, 82, 3053. (IO) Michel, D.; Germanus, A,; Pfeifer, H. J . Chem. Soc., Faraday Trans. I 1982, 78, 231.
or
In the system studied the interaction of the xenon atoms with the more or less hydrated Ni2+cations is not, a priori, always much greater than that of these atoms with the surface. However, the magnetic effect of these cations is very much greater when the Xe atoms are close to them. Equation 8 shows that the formalism developed in the case of strong chemisorption can be adopted for the calculation of aC, provided that 6NaY is taken as the reference for each value of N . The difference compared to the classical chemisorption-physisorption equilibrium is that in the present case the reference aNaY depends on N: Let (9)
( 1 1) Ito, T.; Fraissard, J . J . Chem. Phys. 1982, 76, 5225. (12) Jameson, A. K.; Jameson, C. J.; Gutowski, H. S. J. Chem. Phys. 1973, 59, 4540. (13) Bansal, N.; Dybowski, C. J . Phys. Chem. 1988, 92, 2333. (14) Cheung, T. T. P.; Fu, C. M.; Wharry, S. J . Phys. Chem. 1988, 92, 5 170.
129XeNMR Study of Ni2+Y Zeolites
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2567
a
P
Or
TABLE 11: Values of Calculated Chemical Shifts
b
TI, O C
/
Q20,
.
.5.1020
PPm
amin
and b 2
6C,l
6C,l
Ni 15Y - Tt xelg
,
A
0
2 Xelcavity
1
6,0>
Xelg
26 50 100 150 250 350 500
130 f 0.5 112 f 0.5 121 f 0.5 202 f 1 190 f 5 175 f 5 1 9 0 f 10
26 50 100 150 250 350 500
148.2 f 0.5 134 f 0.5 266 f 2 285 f 3 205 f 5 155 f 10 172 f 5
26 50 100 150 250 350 500
198.6 f 0.5 180.3 f 0.5 325 f 2 800 f 20 1700 f 30 1960 f 30
121 193.5 152.5 118 132
130 f 0.1 112f 1 123 f 1 229 f 2 189 f 10 165 k 10 170 f 20
130 111.7 123.8 229 192 168 175
148.2 f 1 139.5 f 1 268 f 4 290 f 6 198 f 10 215 f 10 232 f 10
148.2 135 270.1 294.2 201.4 216.5 238
198.6 f 1 181 f 1 321 f 4 750 f 40 1660 f 60 2020 f 60
198.6 181.1 318 746.4 1659.3 2070
Ni35Y - Tt
Figure 7. Determination of 6c for NiX15Y-150 (application of the method detailed in eq 6-14): (a) 6 against xenon concentration; (b) 6 - ~ N against xenon concentration; (c) variation of Y against X (Y and X defined in the text).
~ Y
Ni58Y-TI
I 55
h.I d
1
Figure 8. Variation of 6' = 6 - dNaY with the reciprocal xenon concentration 1 / N for Ni35Y-250.
We have observed that eq 4 is still valid provided that 6 and 6,, are replaced by 6' and Ym0. The quantity Y ' = N(
&)(
1
- &)-I
is linearly dependent on
Y' = f ( X 9
of the NaY zeolite nor even sometimes a straight line parallel to the latter. This difference between the asymptote and the line for NaY proves that there is a residual value of the magnetic and electric interactions that is nonzero even when the xenon is fairly distant from the cation. The mean value is more or less large depending on the values of X and Tt. To take this effect into account, we have used the calculation method proposed by Bansal and DybowskyI3 for the same type of study but for much smaller nickel concentrations and levels of dehydration. The Simplex program used allows the evaluation of four terms of the chemical shift: 6E + amag are electric and magnetic contributions of the cations; are contribution due to collisions between the xenon and the more or less hydrated walls, increased by the average effect in space of the previous contributions. 6S,AS is the intersection of the straight line asymptote with the axes of 6(N=0); k and k'are coefficients for double and triple Xe-Xe collisions. The values obtained are given in Table
a = 1/ Atmo=
6'C/6',o
(12)
whence 6 ' ~= 6',o/A'mo
aC,
(13)
sought is 6C.l
= 6'C
+ 6S,NaY
(14)
Figure 7 presents for Ni15Y-150 the measured values of 6, and those of calculated from eq 12 and listed in Table 11. Remark: It is observed that the plot of 6' = 6 - dNaY = f l l / N ) often has a linear section when 1 / N is sufficiently small. The However, in slope of this is therefore equal to NC(GCthe present case, one cannot deduce 6C since Nc is equal to the number (often unknown) of Ni2+ cations in the supercages weighted by the probability of the presence (equally unknown) of Xe atoms in their vicinity. Conversely, for large values of 1 / N , 6 tends towards a finite limit, ich it is sometimes easier to obtain by extrapolation of the 6 - 1 / N curve than from the 6-N curve 0, e.g., Figure 8. for N (6) Calculation of the Other Parameters. Figures 1-3 and the curves defined previously 6' = 6 - BNaY (eq 8) reveal that when N is sufficiently large the shift 6 very often shows as an asymptote a straight line that is neither the linear variation, 6NaY= f ( N ) .
h
Finally, it should be pointed out that 6c can be calculated by the method described in eq 6-14 but by taking the asymptote defined above, JAS = bSAS kN, as the reference instead of bNaY. The corresponding values, 6C,2 (Table 111) are very close to the previous ones,
+
is
-
355 490
111.
The slope of
and the value,
134 264 259 171.5' 122 133
Discussion Dehydration. Table I shows that for each value of the dehydration temperature, T,, the residual water concentration, [H20], increases linearly with the cation-exchange level, X. The excess of water molecules compared to NaY is therefore due to the presence of nickel. This would suggest that some of these molecules are directly bonded to Ni2+. At high [H20] the supercages must contain the [Ni(H20)6]2+complex as they contain the [Ni(NH3)6]2+complex after high adsorption of ammonia on dehydrated NiXY ~amp1es.l~ Iz9Xe N M R . First of all we observed that the chemical shift b of xenon adsorbed on the three NiXY-350 samples increases linearly with l/TE, where TE is the temperature of the NMR experiment. This result demonstrates the effect of the paramagnetism of the NiZ+cations on 6. This effect', as that of the electric field created by the 2+ charges is proportional to f 3 , where r is the cation-xenon distance. It therefore decreases very rapidly as r increases. This is the main reason for the wide variations of the 6 = f ( N ) curves with Tt (or [H2Q]) and A. ~~~~
~
(15) Gedeon, A . Thesis, University Pierre et Marie Curie-Paris,
1987.
2568 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Gedeon et al.
TABLE 111: Values of Various Factors in Fraissard's Equation from the Simplex Procedure Tt, "C
26 130
111.7
0
0
k k'
17.8 0.361
13.39 0.253
100 Ni 15Y- T, 114.9 1.17 12.19 0
&,AS
148.2 0 23.8 0.41
122.2 2.47 14.59 0.311
Ni3 5Y- Tt 248.3 226.2 3.83 18.61 11.63 14.44 0 0
146.6 18.72 8.7 0
111.2 7.12 6.88 0
88.7 28.4 17.7 0
198.6 0 33.78 0.464
180.3 0.026 23.7 0
Ni58Y-T, 272.4 503.6 15.12 90.1 3.105 1.23 0 0
329.8 112.3 1.1 0
470.0 111.6 60.4 0
0
8~ +
Lag
6~ + h a g k k' k A S
6~ + h a g k k'
50
The effect of the degree of exchange is first revealed by the
dC values, which increase with A, for each value of TI.It should be pointed out, however, that for X = 15 and 35 the variations of bC with [ H 2 0 ] may seem small compared to the error on 6c due to the experimental error on.,6 On the other hand, the form of the curves, the existence of a minimum, and finally the position of the asymptotes characterized by the value 6S,AS appear to be particularly important. In what follows we shall discuss the results for each value of X in turn. NilSY Samples. In the Ni15Y-26 sample, with high water concentration, [ H 2 0 ] = 0.61 (Figure 3), the Ni2+ cations are surrounded by a large number of water molecules. The Xe atoms cannot get close to these cations; the electric and magnetic Ni-Xe interactions are relatively weak. So we are not able to reason in terms of the model with two types of centers having very different physical effects. The 6-N variation is then imposed solely by the Xe-Xe collisions, which depend on the volume, V , left free in the supercages by the water molecules, and where the xenon can be adsorbed. It is therefore linear with a slope similar to that of the NaY sample ( [H20] = 0.61) which has much the same degree of dehydration. However, for the same value of N the shift 6N,y([H20] = 0.61) is less than that of the sample considered. In particular, for N
=o
6s,As(NilSY,[H20]=0.61)(130 ppm) > 6S,NaY([H201=0.61) (lo8 ppm) This difference of 22 ppm expresses the average effect of the magnetic and electric fields in the supercage due to the Ni(H20):+ cations. By comparison with the results obtained for diamagnetic cations Mg2+and Ca2+,the electric effect is negligible compared to the magnetic effect.'(' When the temperature Tt increases slightly, the water concentration falls but remains large enough for the 6-N variation to be linear. As in the case of N a y 4 the shift and the slope decrease with [ H 2 0 ] or VF1. At the same time the difference with respect to the NaY reference of the same water concentration rises slightly and expresses either a small increase in the average magrletic field in the supercage due to the fact that the Xe atoms can get a little closer to the Ni2+ cations or a different distribution of the water molecules. For example, for [ H 2 0 ] = 0.31 6s,~s(Nil5Y,[H,O]=0.31)(112 ppm) > &,NaY([H20]=0.31) (82 ppm) The difference, 30 ppm, is somewhat greater than that of 22 ppm for the previous case ( [ H 2 0 ] = 0.61). Finally, it should be noticed that for TI I 50 "C the coefficient k'for triple Xe-Xe collisions is nonzero. This is, of course, related
150
250
350
500
180.4 3.68 9.1 0
136.8 7.3 1 8.93 0
97.9 11.5 9.75 0
102.9 20.3 10.95 0
to the fact that for a given value of N the probability of such multiple collisions is inversely proportional to the volume V, and therefore to the extent of dehydration. The 6-N variations begin to depart from linearity at low N values for Tt = 100 O C or [H20] = 0.20. For this water concentration the volume Vfof the supercages is almost maximal and the coefficient k'is zero, at least in the N concentration range used. The presence of even a shallow = 114.9 minimum associated with the fact that the value of ppm is somewhat greater than that for Ni15Y-50 proves that one can now use the model of two well-differentiated types of adsorption center. More exactly, when Tt goes from 100 to 150 "C the number of water molecules per unit cell falls from 60 to 24 (that is, from 15 to 6 water molecules per Ni2+ cation). The Xe atoms do not therefore come into direct contact with Ni2+ (if this were not the case, 6 would be much greater, as will be seen for the Ni58Y sample). The 6-N curve is displaced toward higher 6 but always has a shallow minimum. For Tt = 150 "C, the values of 6c = 229 ppm,,,6 = 193.5 ppm, and ,,6 = 180.4 ppm are maximal. The form and the 6-N dependence, particularly at values relatively close to these three shifts, can be explained only by the existence of many Ni(Hz0);+ adsorption sites in the supercages: the chemical shift of xenon in contact with these centers is not very large, but the average field created by them inside the supercage is high because there are so many of them. From TI = 150 "C onward, dehydration leads to the formation of complex ions, Ni(OH)+.'7$'8 When Tt increases from 150 to 350 "C, 6c and 6,,, decrease from 229 to 168 ppm and from 193.5 to 118 ppm, respectively. This proves that the number of Xe atoms in contact with the Ni(OH)+ sites decreases and, therefore, that these latter migrate toward the prisms and sodalite cages. This migration is confirmed by a displacement of the asymptote toward smaller 6, resulting in a decrease of from 180.4 to 97.9 ppm, which proves that the average magnetic field in the supercages has considerably diminished. For Tt = 500 "C the shifts 6c = 175 ppm, = 103 ppm, and 6,, = 132 ppm are slightly higher than the analogous shifts for Ni15Y-350. In our opinion this small increase in these 6 values expressed an increase in the effect of the mean electric field caused by the Ni2+cations after decomposition of the Ni(OH)+ complexes at about 500 "C. It can also correspond to the Ni2+cations being located somewhat closer to the supercages. It should be pointed out lastly that there are never any Ni2+cations in the supercages of the Nil5Y samples (6, would be much greater; cf. the Ni58Y samples) and, therefore, that all the nickel cations in the Ni(OH)+ or Ni2+ form are outside the supercages when the treatment temperature is 350 O C or more. However, these 6c values and the form of the 6-N curves shows that these cations remain, at least in part, sufficiently close to the supercages to markedly (17) Ward, J. W, J . Phys. Chem. 1968, 72, 238.
(16) Ito, T.;Fraissard, J. J . Chem. Soc., Faraday Trans. I 1987, 83, 451.
( 1 8 ) Guilleux, M . F.; Tempere, J. F. C.R. Acad. Sci. 1971, 272, 2105.
IzgXe N M R Study of Ni2+Y Zeolites polarize the Xe atoms that approach them and, therefore, that they are located preferentially at the SI{ sites. Ni35Y Samples. The dependence of the 6-N curves on the treatment temperature is very similar to that of the Nil5Y sample. The curves are simply shifted toward greater 6 because of the high nickel concentration. For the same reason the most significant variations mentioned previously occur at Tt values several tens of degrees lower. The evolution of this sample is briefly as follows: for 26 5 Tt 5 50 O C , 6-N is a straight line whose position depends on the high water concentration and the mean magnetic field created by the Ni2+ cations more or less surrounded by H 2 0 molecules. The value 6c decreases from 148.2 to 135 ppm between 26 and 50 O C and then increases sharply to 265 ppm for Tt = 100 O C . The supercages of this sample (Ni35Y-100) contain nickel cations, mainly in the complex form Ni(H20):+. When T, goes from 100 to 150 OC the number of residual water molecules falls from 78 to 31 per unit cell (or from 7.8 to 3.1 H 2 0 molecules/Ni2+), which explains the increase of 6c from 270 to 294 ppm. At the same proves that these Ni(H20):+ time the decrease in aminand cations are beginning to migrate toward sites outside the supercages. This migration continues beyond 150 OC,as is shown by the fall in 6c. Finally, the Ni(OH)+ formed in the last phase of dehydration are decomposed between 350 or 500 O C , as is indicated by the small shift toward higher 6 of the corresponding curve. Inspection of the 6 values, compared to these of the Ni58Y sample, demonstrates that all the Ni2+ cations have migrated from the supercages when the temperature is 250 OC or more. Ni58YSample. As for the NilSY and Ni35Y samples the 6 N plots are straight lines when the water concentration is large and their gradients and decrease with [H20]. For a given [H,O] value they are, of course, displaced toward high 6 as compared to the corresponding curves for the other samples, because of the greater X value. For T, = 100 O C , 6c = 318 ppm, and we find again a curve with a shallow minimum corresponding to the presence of many Ni(H20):+ cations in the supercages, (7 H20/Ni2+on average), so that the average field they create is relatively homogeneous. For treatment temperatures about 100 O C the chemical shifts become very large and, for each T, value, decrease monotonically with N . When T, goes from 100 to 150 OC,the number of H 2 0
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2569 molecules falls from 110 to 41 per unit cell (or from 6.8 to 2.5 H20/Ni2+). The rise in 6c from 318 to 746 ppm can then be attributed to the fact that most of the nickel cations are in the supercages in the form Ni(H20):+ where n is small. However, some Ni(OH)+ may already be present. When Tt reaches 250 "C, [H20] = 0.045,6, is extremely high (1659.3 ppm), showing that all the cations are in the Ni(OH)+ form and even that some of those have already been decomposed and occur in the supercages as Ni2+. The Xe atoms can then be adsorbed on these latter, the distance Ni-Xe is minimal and the pseudocontact interaction particularly large. These exceptional hC values can also be explained by a contact interaction. Buckingham et aI.l9 have shown that for xenon diluted in gaseous O2 or N O there can be. 02-Xe or NO-Xe contact interactions despite the fact that there is no chemical bond between these species. However, the slope of the 6-N curve for Ni58Y-250 is much greater than that for Ni58Y-150. This proves that some of the Ni(OH)+ complexes formed have migrated from the supercages. When T, = 350 OC the value of 6c = 2070 ppm indicates clearly that there are many Ni2+ cations in the supercages. Finally, for T, = 500 O C the very large signal width makes it impossible to obtain a plausible estimate of the chemical shift, but this evolution of the width demonstrates that all the Ni(OH)+ complexes are not decomposed at Tt = 350 O C .
Conclusion These results confirm that the NMR-detected xenon probe can be used to study zeolites (and probably any other solid of large specific area) containing paramagnetic centers, at least as long as they are not too numerous or too close to the Xe atoms. The chemical shift of adsorbed 129Xefor the NiXY-Tt samples is mainly attributable to the paramagnetism of these Ni2+ cations and, if there is any, to residual water. By following the variation of the chemical shift with the number of atoms adsorbed, the evolution of the environment of these cations can be studied as a function of the pretreatment temperature, for each level of exchange, and their migration from the supercages can be studied. Registry No. Ni, 7440-02-0; '29Xe, 13965-99-6. (19) Buckingham, A.
D.;Kollman, P. A. Mol. Phys. 1972, 32, 65.
