- Email: [email protected]
Nuclear relaxation rates for 3He adsorbed on zeolite
NUCLEAR
Sandia
RELAXATION RATES FOR ADSORBED ON ZEOLITE<’ Laboratories.
Abstract -Transient nuclear tion times have been carried types of commercial zeolite. determined by spin counting. tions among helium spins and
H. T. WEAVER Albuquerque, New
1. INTRODUCTION
2.
magnetic resonance (NMR) methods provide detailed information relating to the atomic environment. One class of problems for which such studies are of considerable use is the behavior of atoms confined to surfaces. However, the intrinsic sensitivity of NMR techniques restricts such applications to a few materials. Gases adsorbed on molecular sieves[ 11 or zeolites constitute a convenient and interesting system which meets this restriction. Of the NMR investigations of surface nuclei, helium adsorbed on zeolite has received the most study[2-41. Generally. these works were directed toward finding an ordered nuclear spin configuration and consisted mainly in studies of resonance line shapes. Although NMR results indicate that ordering does not occur. there remains the problem of investigating the nature of the two-dimensional helium spin system. To this end we have carried out transient NMR measurements in the l-2-4 K range on “He adsorbed on two types of zeolite. These measurements include spin counting of the adsorbed nuclei, spin-lattice (T,) and spin-spin (T,) relaxation times. work was supported Commission.
X7 1 IS. U.S.A.
magnetic resonance measurements of spin-lattice and spin-spin relaxaout as a function of temperature and pressure on :‘He adsorbed on two In addition. the number of atoms adsorbed on unit weights of zeolite was Mechanisms for spin-spin relaxation were provided by dipole interacspin-lattice relaxation was probably due to atomic motion.
NUCLEAR
“This Energy
Mexico
:3He
by the United
States
Atomic
EXPERIMENTAL
(a) Appararus Relaxation time measurements were accomplished using a conventional phase coherent transient NMR spectrometer. A teebridge cable arrangement [5] with a single coil serving as transmitter and receiver was employed. Experiments were carried out at 12 MHz with rotating fields of about 25 G. Spinlattice relaxation times were determined by monitoring the recovery of a spin echo produced by a 3 pulse sequence. Spin-spin relaxation time measurements were conducted using an echo[6] produced by two equal width pulses. For spin counting[71 the sample consisted of a mixture of zeolite and Pt metal powder. Free induction decays of :rHe were compared to the ‘“sPt signal from spins contained in the metal. In all cases, signal averaging was carried out using a 102s Biomation high speed digitizer and standard cryogenic methods were used to achieve low temperatures. Our samples were placed in a Pyrex tube and inserted into the r.f. coil. The tube was connected through a needle valve to a pumping system shown schematically in Fig. I. An experiment was begun by closing valve V 1 and opening V2 and V3. The sample S was then heated to about 300°C under a vacuum main-
H.
P
Fig.
T.
iS
IT
I. Schematic
ofgas
handling
system
tained by a pump connected at V2. The time for this activation process was varied from about l/2-8 hr with no detectable effects on our measurements. After activation, V2 is closed and S is inserted into the spectrometer. The “He gas was contained in a sealed pump (P) and a small storage tank (T). The pump was valved so that it could pump both into and from S in order to return the gas to the storage space. By pumping through a regulator(R) the gas pressure above the sample could be held constant at a value measured by gauge G. (b) Procedure and results Figure 2 shows the 4K concentration of helium spins contained on the surface of two types of zeolite as a function of pressure. The 0.07 1
Fig. and
2. Number 13X per
I
I
I
of”He spins adsorbed on zeolite types unit weight of adsorbents vs pressure helium at 4°K.
I 1
3A of
WEAVER
commercial designations 3A and 13X refer to the cage (or pore) size which varies from about 3A” for type 3A to IOA” for type 13X. However, helium easily moves into either size pore. Notice that much more helium can be adsorbed on to the 13X and that a residual film remains on this type sorbent even after the pressure has been dropped to zero. This residual layer remained after several hours of pumping at low temperature. Undoubtedly, the large variation in the amount of adsorbed gas with pressure for type 3A and the ability to pump away all the gas reflect a weaker bond to the surface for 3A than for 13X. In order to make sure that the resonance signals for 3A originated with spins on the zeolite rather than the gas. we made up a sample with only half as much zeolite by diluting with NaCI. The data denoted by A in Fig. 2 were taken using this sample. That no appreciable change was detected in the number of spins per unit weight of sorbent was taken as evidence that the “He signals were from adsorbed atoms. As regards the actual coverage for the surfaces, we are unable to give precise values since the surface area per gram is only known to about ? 50 per cent. Howerver, for an area [8] of 800 m’/gm and an assumed helium atomic radius of 0.9 A”, we have a maximum of about 70 per cent of a monolayer coverage on type 13X. The 3A surface area is less than this, but probably not by an order of magnitude, so that the coverage is again less than one monolayer. Relaxation time data were taken as a function of both pressure and temperature. For the 13X zeolite no pressure dependence of either T, or T, was found, but for 3A the spin-lattice relaxation time decreased considerably as the pressure decreased (Fig. 3). For the temperature studies we maintained type 3A at 30 torr and set the pressure at 1 torr for the 13X material. No major changes in the signal, aside from the Boltzmann factor, occur with temperature. indicating the coverage to remain roughly constant. Figure 4 shows the variation of T, with tem-
NUCLEAR
RELAXATION
RATES
FOR
>He
ADSORBED
ON
ZEOLITE
P(Torr1 Fig.
3. Relaxation
times for :‘He vs helium type 3A zeolite.
pressure
over
I/T(K)
-c : E
Fig. 5. Temperature dependence for spin-lattice relaxation time of :rHe on type 3A (A) and type 13X (01 zeolites. The solid lines are sketched into the figure to indicate the possibility of thermally activated processes.
1.0 -
P 0.5
3.
-
0
I I
I 2
I 3
I 4
I
T(K) Fig.
4. Spin-spin relaxation adsorbed on type 3A (Al
time vs temperature for and type 13X (0) zeolites.
:‘He
perature for both types of material. The change in T, for either sample is small. However, the loss of phase memory displayed a gaussian shape for 3A and exponential for 13X. This provides a second indication that the nature of the binding to the two surfaces is different. The temperature dependence of the spinlattice relaxation time is shown in Fig. 5. Unlike the number of spins and T2 data, these measurements are similar for the two zeolites. If we regard the differences in the former two sets of data as being a result of different surface interactions, we are lead to assume that the rate determining process for T, is controlled by the helium spin system.
DISCUSSION
Spin-spin relaxation for the helium is probably accomplished by nuclear dipole interactions between the “He spins contained on a two-dimensional lattice. We can estimate the effectiveness of this mechanism by calculating the second moment of the SHe resonance line from the Van Vleck relation[7,91
Mr= 3/5y2h21(1+1) 2 (l/ri6)y I
(1)
where y and I are the gyromagnetic ratio and spin, respectively, and the sum runs over all occupied sites. There is some ambiguity in the calculation of M,. This arises in the assignment of a helium atom configuration for the determination of ri. For example, if we assume that theEsHe spins lie on a close packed plane where the atomic diameter of helium is the near neighbor distance, we find T, values of 1IS and 256 psec for a 50 and 10 per cent coverage, respectively. On the other hand, if we use an average interatomic distance such that a uniform layer is present, T, is found to be 194 and 2 I70 psec for the respective coverages of 50
424
H.
T.
WEAVER
and 10 per cent. Experimentally we find T, to vary from about 500 to 1000 +ec (Fig. 4). Consequently, the simplest explanation for the magnitudes of T, is found by assuming that the spins tend to randomly fill a lattice that is not close packed. In this case the concentration dependence varies as the square root of the spins tier unit area which roughly gives a factor of two difference for the two materials. As regards the line shapes, our arguments are somewhat speculative. We note that exponential decay for the echo (Lorentzian shape) is normally associated with rapid motion of the spins. However, the temperature dependence for both T, and T2 as well as the magnitude of T2 do not indicate such motion. Furthermore, the binding to the surface is stronger for 13X than for 3A, yet the 13X shows the exponential behavior. A second situation for which exponential spin-spin relaxation occurs is in the random filling of only a fraction of lattice positions [7]. Spins which reside in clusters have a large line width and are not observed. The remaining atoms yield the narrowed center to the resonance line which is approximately Lorentzian in shape. The implication of this argument is that helium atoms randomly occupy sites on the 13X zeolite but are arranged in an ordered manner on the 3 A. A third, and perhaps the most likely, cause for the Lorentzian shape is exchange narrowing[lO] which occurs in solid helium. If this is the phenomenon responsible for line shapes, the Gaussian line for 3He on 3A zeolite indicates a lower packing density than for 13X. The small pore size for 3A is consistent with this explanation. The spin-lattice relaxation time data do not unambiguously indicate a single mechanism. There does appear to be an activated process of some type (Fig. 5) which probably reflects either atomic motion or spin diffusion. The possibility of atomic motion is strengthened by the work of Monod and Cowen[ 1 l] who find T, minima and motional narrowing at temperatures just above 4K for “He adsorbed
onto 13X. T, values in fact show an increase at 4K consistent with a narrowing of the line. Furthermore, the decrease in T, with pressure for :‘He on 3A indicates an increased mobility for the atoms in support of a motion mechanism. The lack of pressure dependence for T, of “He on 13X probably means that the zeolite pores remained quite filled so that the atomic mobility was unchanged. We note that atomic motion as a T, mechanism does not contradict the rigid lattice explanation for the T, values. In order for the NMR data to have rigid lattice properties, the atomic jump rate must be roughly less than the homogeneous line width. Even this slow motion can produce spin-lattice relaxation. A change in behavior for the :jHe on both zeolites is indicated by the break in the data of Fig. 5 near l/T = O-6 K-‘. Again, Monod and Cowen [ 1 I] offer an attractive explanation. They suggest that atoms in the center of the pore and cages of the zeolites are able to move more easily than the ones trapped at the walls. As the temperature lowers the atoms immediately adjacent to the walls become rigid allowing spin-lattice relaxation to be produced only by the motions of atoms in the center. Regarding this explanation, the observation of experimentally equal spin-lattice relaxation times for both zeolites is somewhat puzzling. Since considerably more helium can be absorbed by 13X than 3A zeolite, one might expect a reduced spin-lattice interaction at low temperatures for 3A relative to 13X. That this does not occur might indicate wall effects to persist to near 1K. Acknowledgements-We Barry Narath
express our Hansen for technical assistance for the use of the helium pumping
appreciation and to Dr. system.
to A.
REFERENCES 1. HERSH C. K.. Molecular Sieves, Reinhold. (1961). 2. SANTINI M., In Low Tempernture Physics by R. 0. Davies), LT8. p. 59. Butterworths, (1963). 3. CARERI G.. SANTINI M. and SIGNORELLI In Low Tctnperctrure Pk.vsics (Edited by J. G.
London (Edited London G.. Daunt,
NUCLEAR
4. 5. 6. 7.
RELAXATION
RATES
D. 0. Edwards. F. J. Milford and M. Yayub): LT9, p. 364. Plenum Press, New York ( 1965). SANTINI M., GIRERA M. andCARERl G..P/tys. Lerr. 4.64 ( I963 ). LOWE I. J. and TARR C. E..J. scient. Instrum. 1, 320 ( 1968). HAHN E. L..Pltys. Rev. 80.580(1950). ABRAGAM A.. Principles of Nuclear Mugnrristn Oxford University Press, New York ( I96 I ).
FOR
:‘He
ADSORBED
ON
ZEOLITE
425
8. This area is a rough average of the values provided by F. L. Cummingham of Union Carbide Corporation, the manufacturer of the zerolites. 9. VAN VLECK J. H.,Phys. Rev. 74, I168 (1948). IO. REICH H. A.,Phys.Reu. 129,630(1963). I I. MONOD P. and COWEN J. A., Internal Report, Service de Physique du Solide et de Resonance Magneetique, Centre d’Etudes Nucleaires de Saclay (1972).
Sandia
RELAXATION RATES FOR ADSORBED ON ZEOLITE<’ Laboratories.
Abstract -Transient nuclear tion times have been carried types of commercial zeolite. determined by spin counting. tions among helium spins and
H. T. WEAVER Albuquerque, New
1. INTRODUCTION
2.
magnetic resonance (NMR) methods provide detailed information relating to the atomic environment. One class of problems for which such studies are of considerable use is the behavior of atoms confined to surfaces. However, the intrinsic sensitivity of NMR techniques restricts such applications to a few materials. Gases adsorbed on molecular sieves[ 11 or zeolites constitute a convenient and interesting system which meets this restriction. Of the NMR investigations of surface nuclei, helium adsorbed on zeolite has received the most study[2-41. Generally. these works were directed toward finding an ordered nuclear spin configuration and consisted mainly in studies of resonance line shapes. Although NMR results indicate that ordering does not occur. there remains the problem of investigating the nature of the two-dimensional helium spin system. To this end we have carried out transient NMR measurements in the l-2-4 K range on “He adsorbed on two types of zeolite. These measurements include spin counting of the adsorbed nuclei, spin-lattice (T,) and spin-spin (T,) relaxation times. work was supported Commission.
X7 1 IS. U.S.A.
magnetic resonance measurements of spin-lattice and spin-spin relaxaout as a function of temperature and pressure on :‘He adsorbed on two In addition. the number of atoms adsorbed on unit weights of zeolite was Mechanisms for spin-spin relaxation were provided by dipole interacspin-lattice relaxation was probably due to atomic motion.
NUCLEAR
“This Energy
Mexico
:3He
by the United
States
Atomic
EXPERIMENTAL
(a) Appararus Relaxation time measurements were accomplished using a conventional phase coherent transient NMR spectrometer. A teebridge cable arrangement [5] with a single coil serving as transmitter and receiver was employed. Experiments were carried out at 12 MHz with rotating fields of about 25 G. Spinlattice relaxation times were determined by monitoring the recovery of a spin echo produced by a 3 pulse sequence. Spin-spin relaxation time measurements were conducted using an echo[6] produced by two equal width pulses. For spin counting[71 the sample consisted of a mixture of zeolite and Pt metal powder. Free induction decays of :rHe were compared to the ‘“sPt signal from spins contained in the metal. In all cases, signal averaging was carried out using a 102s Biomation high speed digitizer and standard cryogenic methods were used to achieve low temperatures. Our samples were placed in a Pyrex tube and inserted into the r.f. coil. The tube was connected through a needle valve to a pumping system shown schematically in Fig. I. An experiment was begun by closing valve V 1 and opening V2 and V3. The sample S was then heated to about 300°C under a vacuum main-
H.
P
Fig.
T.
iS
IT
I. Schematic
ofgas
handling
system
tained by a pump connected at V2. The time for this activation process was varied from about l/2-8 hr with no detectable effects on our measurements. After activation, V2 is closed and S is inserted into the spectrometer. The “He gas was contained in a sealed pump (P) and a small storage tank (T). The pump was valved so that it could pump both into and from S in order to return the gas to the storage space. By pumping through a regulator(R) the gas pressure above the sample could be held constant at a value measured by gauge G. (b) Procedure and results Figure 2 shows the 4K concentration of helium spins contained on the surface of two types of zeolite as a function of pressure. The 0.07 1
Fig. and
2. Number 13X per
I
I
I
of”He spins adsorbed on zeolite types unit weight of adsorbents vs pressure helium at 4°K.
I 1
3A of
WEAVER
commercial designations 3A and 13X refer to the cage (or pore) size which varies from about 3A” for type 3A to IOA” for type 13X. However, helium easily moves into either size pore. Notice that much more helium can be adsorbed on to the 13X and that a residual film remains on this type sorbent even after the pressure has been dropped to zero. This residual layer remained after several hours of pumping at low temperature. Undoubtedly, the large variation in the amount of adsorbed gas with pressure for type 3A and the ability to pump away all the gas reflect a weaker bond to the surface for 3A than for 13X. In order to make sure that the resonance signals for 3A originated with spins on the zeolite rather than the gas. we made up a sample with only half as much zeolite by diluting with NaCI. The data denoted by A in Fig. 2 were taken using this sample. That no appreciable change was detected in the number of spins per unit weight of sorbent was taken as evidence that the “He signals were from adsorbed atoms. As regards the actual coverage for the surfaces, we are unable to give precise values since the surface area per gram is only known to about ? 50 per cent. Howerver, for an area [8] of 800 m’/gm and an assumed helium atomic radius of 0.9 A”, we have a maximum of about 70 per cent of a monolayer coverage on type 13X. The 3A surface area is less than this, but probably not by an order of magnitude, so that the coverage is again less than one monolayer. Relaxation time data were taken as a function of both pressure and temperature. For the 13X zeolite no pressure dependence of either T, or T, was found, but for 3A the spin-lattice relaxation time decreased considerably as the pressure decreased (Fig. 3). For the temperature studies we maintained type 3A at 30 torr and set the pressure at 1 torr for the 13X material. No major changes in the signal, aside from the Boltzmann factor, occur with temperature. indicating the coverage to remain roughly constant. Figure 4 shows the variation of T, with tem-
NUCLEAR
RELAXATION
RATES
FOR
>He
ADSORBED
ON
ZEOLITE
P(Torr1 Fig.
3. Relaxation
times for :‘He vs helium type 3A zeolite.
pressure
over
I/T(K)
-c : E
Fig. 5. Temperature dependence for spin-lattice relaxation time of :rHe on type 3A (A) and type 13X (01 zeolites. The solid lines are sketched into the figure to indicate the possibility of thermally activated processes.
1.0 -
P 0.5
3.
-
0
I I
I 2
I 3
I 4
I
T(K) Fig.
4. Spin-spin relaxation adsorbed on type 3A (Al
time vs temperature for and type 13X (0) zeolites.
:‘He
perature for both types of material. The change in T, for either sample is small. However, the loss of phase memory displayed a gaussian shape for 3A and exponential for 13X. This provides a second indication that the nature of the binding to the two surfaces is different. The temperature dependence of the spinlattice relaxation time is shown in Fig. 5. Unlike the number of spins and T2 data, these measurements are similar for the two zeolites. If we regard the differences in the former two sets of data as being a result of different surface interactions, we are lead to assume that the rate determining process for T, is controlled by the helium spin system.
DISCUSSION
Spin-spin relaxation for the helium is probably accomplished by nuclear dipole interactions between the “He spins contained on a two-dimensional lattice. We can estimate the effectiveness of this mechanism by calculating the second moment of the SHe resonance line from the Van Vleck relation[7,91
Mr= 3/5y2h21(1+1) 2 (l/ri6)y I
(1)
where y and I are the gyromagnetic ratio and spin, respectively, and the sum runs over all occupied sites. There is some ambiguity in the calculation of M,. This arises in the assignment of a helium atom configuration for the determination of ri. For example, if we assume that theEsHe spins lie on a close packed plane where the atomic diameter of helium is the near neighbor distance, we find T, values of 1IS and 256 psec for a 50 and 10 per cent coverage, respectively. On the other hand, if we use an average interatomic distance such that a uniform layer is present, T, is found to be 194 and 2 I70 psec for the respective coverages of 50
424
H.
T.
WEAVER
and 10 per cent. Experimentally we find T, to vary from about 500 to 1000 +ec (Fig. 4). Consequently, the simplest explanation for the magnitudes of T, is found by assuming that the spins tend to randomly fill a lattice that is not close packed. In this case the concentration dependence varies as the square root of the spins tier unit area which roughly gives a factor of two difference for the two materials. As regards the line shapes, our arguments are somewhat speculative. We note that exponential decay for the echo (Lorentzian shape) is normally associated with rapid motion of the spins. However, the temperature dependence for both T, and T2 as well as the magnitude of T2 do not indicate such motion. Furthermore, the binding to the surface is stronger for 13X than for 3A, yet the 13X shows the exponential behavior. A second situation for which exponential spin-spin relaxation occurs is in the random filling of only a fraction of lattice positions [7]. Spins which reside in clusters have a large line width and are not observed. The remaining atoms yield the narrowed center to the resonance line which is approximately Lorentzian in shape. The implication of this argument is that helium atoms randomly occupy sites on the 13X zeolite but are arranged in an ordered manner on the 3 A. A third, and perhaps the most likely, cause for the Lorentzian shape is exchange narrowing[lO] which occurs in solid helium. If this is the phenomenon responsible for line shapes, the Gaussian line for 3He on 3A zeolite indicates a lower packing density than for 13X. The small pore size for 3A is consistent with this explanation. The spin-lattice relaxation time data do not unambiguously indicate a single mechanism. There does appear to be an activated process of some type (Fig. 5) which probably reflects either atomic motion or spin diffusion. The possibility of atomic motion is strengthened by the work of Monod and Cowen[ 1 l] who find T, minima and motional narrowing at temperatures just above 4K for “He adsorbed
onto 13X. T, values in fact show an increase at 4K consistent with a narrowing of the line. Furthermore, the decrease in T, with pressure for :‘He on 3A indicates an increased mobility for the atoms in support of a motion mechanism. The lack of pressure dependence for T, of “He on 13X probably means that the zeolite pores remained quite filled so that the atomic mobility was unchanged. We note that atomic motion as a T, mechanism does not contradict the rigid lattice explanation for the T, values. In order for the NMR data to have rigid lattice properties, the atomic jump rate must be roughly less than the homogeneous line width. Even this slow motion can produce spin-lattice relaxation. A change in behavior for the :jHe on both zeolites is indicated by the break in the data of Fig. 5 near l/T = O-6 K-‘. Again, Monod and Cowen [ 1 I] offer an attractive explanation. They suggest that atoms in the center of the pore and cages of the zeolites are able to move more easily than the ones trapped at the walls. As the temperature lowers the atoms immediately adjacent to the walls become rigid allowing spin-lattice relaxation to be produced only by the motions of atoms in the center. Regarding this explanation, the observation of experimentally equal spin-lattice relaxation times for both zeolites is somewhat puzzling. Since considerably more helium can be absorbed by 13X than 3A zeolite, one might expect a reduced spin-lattice interaction at low temperatures for 3A relative to 13X. That this does not occur might indicate wall effects to persist to near 1K. Acknowledgements-We Barry Narath
express our Hansen for technical assistance for the use of the helium pumping
appreciation and to Dr. system.
to A.
REFERENCES 1. HERSH C. K.. Molecular Sieves, Reinhold. (1961). 2. SANTINI M., In Low Tempernture Physics by R. 0. Davies), LT8. p. 59. Butterworths, (1963). 3. CARERI G.. SANTINI M. and SIGNORELLI In Low Tctnperctrure Pk.vsics (Edited by J. G.
London (Edited London G.. Daunt,
NUCLEAR
4. 5. 6. 7.
RELAXATION
RATES
D. 0. Edwards. F. J. Milford and M. Yayub): LT9, p. 364. Plenum Press, New York ( 1965). SANTINI M., GIRERA M. andCARERl G..P/tys. Lerr. 4.64 ( I963 ). LOWE I. J. and TARR C. E..J. scient. Instrum. 1, 320 ( 1968). HAHN E. L..Pltys. Rev. 80.580(1950). ABRAGAM A.. Principles of Nuclear Mugnrristn Oxford University Press, New York ( I96 I ).
FOR
:‘He
ADSORBED
ON
ZEOLITE
425
8. This area is a rough average of the values provided by F. L. Cummingham of Union Carbide Corporation, the manufacturer of the zerolites. 9. VAN VLECK J. H.,Phys. Rev. 74, I168 (1948). IO. REICH H. A.,Phys.Reu. 129,630(1963). I I. MONOD P. and COWEN J. A., Internal Report, Service de Physique du Solide et de Resonance Magneetique, Centre d’Etudes Nucleaires de Saclay (1972).