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Observation of muonium in zeolites
Physica B 326 (2003) 64–67
Observation of muonium in zeolites D.J. Arseneaua,*, D.G. Flemingb, C.A. Fyfeb, M. Senbac b
a TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2A3 Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1 c Department of Physics, Dalhousie University, Halifax, NS, Canada
Abstract Muonium has been directly detected over a range of temperatures and fields by transverse field mSR in different zeolites: 3A, 13X, USY, ZSM-5, and S-115 (a high-silica form of ZSM-5), as well as in silica gel. The polarizations determined from data at 75 and 150 G were independent of both field and temperature. The amounts of Mu seen vary from B20% to 40%, with a large missing fraction seen in every case, which may be partly due to slow Mu formation. There is also a fast Mu relaxation rate seen in all samples. This is the first direct observation of Mu in zeolites. r 2002 Elsevier Science B.V. All rights reserved. PACS: 36.10.Dr; 82.75.Mj Keywords: Zeolite; Muonium; Polarization; Relaxation
1. Introduction Zeolites are microcrystalline alumino-silicate structures, incorporating extra-framework cations, which have a long history as molecular sieves and heterogeneous catalysts in chemical industry [1]. Nevertheless, even today, relatively little is known about the microscopic interactions of molecules in different zeolite frameworks, particularly in the case of neutral free radicals that could be formed by H-atom addition reactions involving catalytically active sites. The Mu-cyclohexadienyl radical (MuC6H6) in particular has been identified and characterized by both TF and ALCR mSR spectroscopy, in USY [2], NaY [3], in silicious ZSM-5 [4], in Cu-ZSM-5 [5], and in HZSM-5 [6]. *Corresponding author. Tel.: +604-222-1047; fax: +604222-1074. E-mail address: [email protected] (D.J. Arseneau).
Other Mu-radicals have also been identified in different frameworks by TF-mSR alone [7]. In all of these studies, the implicit assumption [3] is that the radicals are formed by Mu addition reactions. As a necessary precursor to a mechanism in which Muradicals are formed by Mu addition (in zeolites), it is important to first identify the presence of Mu itself in unloaded samples, which has not been previously achieved. This was the motivation for the present study, in which Mu was sought by TFmSR in zeolites 13X, USY, ZSM-5, 3A, S-115 and also in silica gel, which has long been recognized as a good source for Mu formation [9].
2. Zeolite samples The basic structure of all zeolites is a tetrahedrally bonded alumino-silicate lattice, wherein
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 5 7 6 - 4
D.J. Arseneau et al. / Physica B 326 (2003) 64–67
Al has replaced Si in the elementary SiO2 bond, leaving a net negative charge which must be compensated by ‘extra-framework’ cations, Na+, H+, Ca2+, etc. The tetrahedral sites are bonded in a 3D framework which contains a regular arrangement of channels and cavities of uniform pore size. These channels have dimensions that make them size- and shape-selective towards small to medium size molecules. Zeolites are all microcrystalline in nature, with B1 mm grain sizes, of well-known geometries [10], determined primarily by powder X-ray and neutron diffraction and particularly 29Si NMR [11]. (Some few singlecrystal cases have also been characterized.) We have investigated Mu formation in a number of different structures, supplied by Linde (Union Carbide), Aldrich Chemicals, or ChemieUtikon (Zurich), . whose (Na-cation) generic molecular formulae are given in Table 1. The labels (LTA, etc.) designate the framework type [10] and quoted as well is the largest pore size, indicated in the last column. Two very common zeolites are X and Y, which have the ‘Faujasite’ (FAU) framework and differ only in their Si/Al ratios. They have among the most open geometries, with large internal ‘super ( diameter, accessed by ‘window’ cages’ of E12 A ( diameter, the latter defining the sites of about 7 A pore size. In its acidic form, zeolite Y is an important industrial catalyst for hydrocarbon cracking in petroleum production. The present study employed a variant of Y (USY, ‘ultra-stable Y’) which has a much-reduced Al content (Si/ AlE50). The zeolite ZSM-5 (‘M’ for Mobil Oil) has an even more complex geometry, consisting of ( both straight and zig-zag channels of about 5 A diameter, which intersect. In a recent study of the MuC6H6 radical in HZSM-5, the Si/Al ratio was 25 1
65
[6]. This ratio is very much enhanced in high-silica zeolites, even approaching 1000 1 ; called ‘silicalite’ for ZSM-5 and ‘DAY’ (De-aluminated Y) for the FAU framework. A form of silicalite, developed by Union Carbide, called ‘S-115’, was utilized in the present study, along with silica gel, which is pure SiO2, but of non-uniform pore size. It is worth noting that zeolite 3A in the first row of Table 1 has the smallest pore size (used primarily as a drying agent for organic solvents).
3. Measurements Powdered samples of zeolites were placed in (316) stainless steel cells, with interior diameter of 32 mm and depth 9 mm, and a 0.025 mm thick stainless steel foil window. Some were heated to 520 K under vacuum to remove adsorbed material and sealed by crimping a copper tube. Others were heated to 450 K in vacuum while being measured, to examine the effect of the baking procedure. Time-differential mSR spectra were measured at several transverse magnetic field values (from 7 to 150 G) and temperatures (from 300 to 450 K), although zeolites USY and ZSM-5 were measured only at room temperature. The results were independent of temperature, as it turned out, although the initial bake-out of the samples was important. Muonium was observed in all samples, and found to undergo quite rapid relaxation. The accompanying diamagnetic signal was observed to have relaxing and non-relaxing components. Sample spectra are shown in Fig. 1 for zeolites 3A and S-115 measured at 75 G, with the proportions of Mu and diamagnetic signals seen for all samples listed in Table 2, along with the Mu relaxation
Table 1 Representative zeolite types and their formulae Zeolite
Type
Generic molecular formula
Si/Al
( Pore diameter (A)
3A 13X USY ZSM-5 S-115
LTA FAU FAU MFI MFI
K9Na3(AlO2)12(SiO2)12 Na86(AlO2)86(SiO2)106 Nan(AlO2)n(SiO2)192n Nan(AlO2)n(SiO2)96n Nan(AlO2)n(SiO2)96n
1 1.2 >100 B30 >100
3 7 7 5 5
D.J. Arseneau et al. / Physica B 326 (2003) 64–67
66
rates. The absolute polarizations are normalized to the signal measured in a copper sample of the same geometry as the zeolite samples ðAmax ¼ 0:260 for the first four entries in Table 2 and 0.234 for the last two). Also listed are relative fractions fM ¼ PM =ðPM þ PD Þ and fD ¼ PD =ðPM þ PD Þ: Due to the fast Mu relaxation, the Mu hyperfine couplings were not precisely determined, with only zeolite 3A showing a significant reduction below the vacuum Mu value. The diamagnetic signal was found to exhibit both relaxing and non-relaxing components, and the amplitudes of both are included in the PD non values ðPD ¼ Prel D þ PD Þ; the fraction of the relaxing component is listed as fDrel ¼ Prel D =PD in Table 2. The fast-relaxing D component was
largest in zeolite 3A, moderate in 13X, and insignificant in others. In both zeolites 13X and 3A, measurements made during the bake-out showed that baking increased the relaxation rate of Mu, decreased the amplitude of the relaxing diamagnetic component and had little or indeterminate effect on other parameters.
4. Discussion The measured polarizations, relative fractions, and relaxation rates for Mu and Diamagnetic muons recorded in Table 2 do not suggest any obvious correlations with sample properties. Notably, the (fast) Mu relaxation rates are not correlated with the prevalence of nuclear moments—as determined by the Al content (Table 1). While about 50% Mu is seen in Si gel, consistent with early measurements giving 45% [9], it is not clear why the relaxation rate should be so fast. Perhaps this is an indication of random hyperfine anisotropies due to collisions in a constrained geometry, but that is unlikely [12], especially since the smallest value of lM is found in the 3A sample, which has the smallest pore diameter. More likely is that Mu relaxation is due to paramagnetic sites, whether impurities or dangling oxygens, which could be important in all samples, and indeed previous measurements of Mu relaxation rates in different silica powders do demonstrate a strong sample dependence due to impurities (e.g., Fe3+) [12]. ESR measurements are planned. The large differences in Mu fractions seen between the Si gel and the S-115 zeolite may point to impurities
Fig. 1. mSR Signals for zeolites 3A and S-115 measured at 75 G TF. The data represent summed runs at different temperatures, since there was little or no temperature dependence. The difference in Mu relaxation and amplitude is apparent. Not obvious over this time-range is the difference in diamagnetic signals (with relaxing and non-relaxing components).
Table 2 Polarization fractions and their relaxations Zeolite
PM
PD
PL
fM (%)
fD (%)
fDrel (%)
lM (ms1)
lD (ms1)
Si Gel 3A 13X USY ZSM-5 S-115
0.396(27) 0.168(08) 0.192(17) 0.316(34) 0.402(51) 0.074(12)
0.396(06) 0.446(10) 0.576(07) 0.436(06) 0.402(17) 0.560(04)
0.208(28) 0.386(13) 0.229(18) 0.248(35) 0.197(54) 0.366(12)
50 27 25 42 50 12
50 73 75 58 50 88
3 31 3 0 0 9
26(3) 6(1) 17(2) 7(2) 10(2) 11(3)
3(5) 9(1) 7(4) — — 11(2)
D.J. Arseneau et al. / Physica B 326 (2003) 64–67
rather than dangling -O sites since both are essentially SiO2 environments. The identity of the diamagnetic and lost fractions has not be established by the present studies, but the fast-relaxing D component can be tentatively attributed to slow Mu formation. It is noteworthy that the largest and next-largest relaxing fractions (3A, S-115; Table 2) are associated with the two greatest PL values. If Mu is formed by recombination of m+ with radiolysis electrons, then the size of the relaxing component is determined by the competition between Mu formation and electron removal; somewhat faster, but not prompt, Mu formation would contribute to PL : Measurements under electric field would be useful. The lost polarization of delayed and dephased Mu might be partially recovered by epithermal addition reactions forming Mu-radicals in zeolites, as recently determined for the MuCH2CH2 radical in the gas phase [8]. LF (repolarization) studies carried out here some years ago indicated two components, one fast and one slow, but were inconclusive. Since Mu has clearly been identified in all the zeolite samples we have looked at, it is reasonable to assume that this is indeed the precursor for the formation of muoniated radicals in loaded zeolites. The generally small amplitudes seen suggest small radical fractions, consistent with radical measure-
67
ments [3]. Nevertheless, Mu formation may well be enhanced when a zeolite is loaded with an organic compound, as is the case for previous radical studies, and, indeed, for industrial use.
References [1] D.W. Breck, Zeolite Molecular Sieves, Krieger, Malabar, FL, 1973. [2] M. Shelley, et al., Stud. Surf. Sci. Catal. 94 (1994) 469. [3] D.G. Fleming, M.Y. Shelley, D.J. Arseneau, M. Senba, J.J. Pan, E. Roduner, J. Phys. Chem. B 106 (2002) 6395; D.G. Fleming, et al., Physica B 603 (2000) 289. [4] E. Roduner, M. Stolm!ar, H. Dilger, I.D. Reid, J. Phys. Chem. 102 (1998) 7591. [5] M. Stolm!ar, E. Roduner, J. Am. Chem. Soc. 120 (1998) 583. [6] E. Roduner, H. Dilger, J. Am. Chem. Soc. 123 (2001) 7717. [7] C.J. Rhodes, T.C. Dintinger, C.A. Scott, Magn. Reson. Chem. 38 (2000) 62. [8] P.W. Percival, J.-C. Brodovitch, D.J. Arseneau, M. Senba, D.G. Fleming, Formation of the muoniated ethyl radical in the gas phase, Physica B, these proceedings. [9] G.M. Marshall, J.B. Warren, D.M. Garner, G.S. Clark, J.H. Brewer, D.G. Fleming, Phys. Lett. A 65 (1978) 351. [10] Ch. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Structure Types, Elsevier, Amsterdam, 2001. [11] C.A. Fyfe, Y. Feng, H. Grondey, G.T. Kokotalio, H. Gies, Chem. Rev. 91 (1991) 1525. [12] J.R. Kempton, R.F. Marzke, D.G. Fleming, A.C. Gonzalez, S.K. Leung, J.J. Pan, P.W. Percival, M. Senba, A. Tempelmann, Hyperfine Int. 65 (1990) 811.
Observation of muonium in zeolites D.J. Arseneaua,*, D.G. Flemingb, C.A. Fyfeb, M. Senbac b
a TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, Canada V6T 2A3 Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1 c Department of Physics, Dalhousie University, Halifax, NS, Canada
Abstract Muonium has been directly detected over a range of temperatures and fields by transverse field mSR in different zeolites: 3A, 13X, USY, ZSM-5, and S-115 (a high-silica form of ZSM-5), as well as in silica gel. The polarizations determined from data at 75 and 150 G were independent of both field and temperature. The amounts of Mu seen vary from B20% to 40%, with a large missing fraction seen in every case, which may be partly due to slow Mu formation. There is also a fast Mu relaxation rate seen in all samples. This is the first direct observation of Mu in zeolites. r 2002 Elsevier Science B.V. All rights reserved. PACS: 36.10.Dr; 82.75.Mj Keywords: Zeolite; Muonium; Polarization; Relaxation
1. Introduction Zeolites are microcrystalline alumino-silicate structures, incorporating extra-framework cations, which have a long history as molecular sieves and heterogeneous catalysts in chemical industry [1]. Nevertheless, even today, relatively little is known about the microscopic interactions of molecules in different zeolite frameworks, particularly in the case of neutral free radicals that could be formed by H-atom addition reactions involving catalytically active sites. The Mu-cyclohexadienyl radical (MuC6H6) in particular has been identified and characterized by both TF and ALCR mSR spectroscopy, in USY [2], NaY [3], in silicious ZSM-5 [4], in Cu-ZSM-5 [5], and in HZSM-5 [6]. *Corresponding author. Tel.: +604-222-1047; fax: +604222-1074. E-mail address: [email protected] (D.J. Arseneau).
Other Mu-radicals have also been identified in different frameworks by TF-mSR alone [7]. In all of these studies, the implicit assumption [3] is that the radicals are formed by Mu addition reactions. As a necessary precursor to a mechanism in which Muradicals are formed by Mu addition (in zeolites), it is important to first identify the presence of Mu itself in unloaded samples, which has not been previously achieved. This was the motivation for the present study, in which Mu was sought by TFmSR in zeolites 13X, USY, ZSM-5, 3A, S-115 and also in silica gel, which has long been recognized as a good source for Mu formation [9].
2. Zeolite samples The basic structure of all zeolites is a tetrahedrally bonded alumino-silicate lattice, wherein
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 5 7 6 - 4
D.J. Arseneau et al. / Physica B 326 (2003) 64–67
Al has replaced Si in the elementary SiO2 bond, leaving a net negative charge which must be compensated by ‘extra-framework’ cations, Na+, H+, Ca2+, etc. The tetrahedral sites are bonded in a 3D framework which contains a regular arrangement of channels and cavities of uniform pore size. These channels have dimensions that make them size- and shape-selective towards small to medium size molecules. Zeolites are all microcrystalline in nature, with B1 mm grain sizes, of well-known geometries [10], determined primarily by powder X-ray and neutron diffraction and particularly 29Si NMR [11]. (Some few singlecrystal cases have also been characterized.) We have investigated Mu formation in a number of different structures, supplied by Linde (Union Carbide), Aldrich Chemicals, or ChemieUtikon (Zurich), . whose (Na-cation) generic molecular formulae are given in Table 1. The labels (LTA, etc.) designate the framework type [10] and quoted as well is the largest pore size, indicated in the last column. Two very common zeolites are X and Y, which have the ‘Faujasite’ (FAU) framework and differ only in their Si/Al ratios. They have among the most open geometries, with large internal ‘super ( diameter, accessed by ‘window’ cages’ of E12 A ( diameter, the latter defining the sites of about 7 A pore size. In its acidic form, zeolite Y is an important industrial catalyst for hydrocarbon cracking in petroleum production. The present study employed a variant of Y (USY, ‘ultra-stable Y’) which has a much-reduced Al content (Si/ AlE50). The zeolite ZSM-5 (‘M’ for Mobil Oil) has an even more complex geometry, consisting of ( both straight and zig-zag channels of about 5 A diameter, which intersect. In a recent study of the MuC6H6 radical in HZSM-5, the Si/Al ratio was 25 1
65
[6]. This ratio is very much enhanced in high-silica zeolites, even approaching 1000 1 ; called ‘silicalite’ for ZSM-5 and ‘DAY’ (De-aluminated Y) for the FAU framework. A form of silicalite, developed by Union Carbide, called ‘S-115’, was utilized in the present study, along with silica gel, which is pure SiO2, but of non-uniform pore size. It is worth noting that zeolite 3A in the first row of Table 1 has the smallest pore size (used primarily as a drying agent for organic solvents).
3. Measurements Powdered samples of zeolites were placed in (316) stainless steel cells, with interior diameter of 32 mm and depth 9 mm, and a 0.025 mm thick stainless steel foil window. Some were heated to 520 K under vacuum to remove adsorbed material and sealed by crimping a copper tube. Others were heated to 450 K in vacuum while being measured, to examine the effect of the baking procedure. Time-differential mSR spectra were measured at several transverse magnetic field values (from 7 to 150 G) and temperatures (from 300 to 450 K), although zeolites USY and ZSM-5 were measured only at room temperature. The results were independent of temperature, as it turned out, although the initial bake-out of the samples was important. Muonium was observed in all samples, and found to undergo quite rapid relaxation. The accompanying diamagnetic signal was observed to have relaxing and non-relaxing components. Sample spectra are shown in Fig. 1 for zeolites 3A and S-115 measured at 75 G, with the proportions of Mu and diamagnetic signals seen for all samples listed in Table 2, along with the Mu relaxation
Table 1 Representative zeolite types and their formulae Zeolite
Type
Generic molecular formula
Si/Al
( Pore diameter (A)
3A 13X USY ZSM-5 S-115
LTA FAU FAU MFI MFI
K9Na3(AlO2)12(SiO2)12 Na86(AlO2)86(SiO2)106 Nan(AlO2)n(SiO2)192n Nan(AlO2)n(SiO2)96n Nan(AlO2)n(SiO2)96n
1 1.2 >100 B30 >100
3 7 7 5 5
D.J. Arseneau et al. / Physica B 326 (2003) 64–67
66
rates. The absolute polarizations are normalized to the signal measured in a copper sample of the same geometry as the zeolite samples ðAmax ¼ 0:260 for the first four entries in Table 2 and 0.234 for the last two). Also listed are relative fractions fM ¼ PM =ðPM þ PD Þ and fD ¼ PD =ðPM þ PD Þ: Due to the fast Mu relaxation, the Mu hyperfine couplings were not precisely determined, with only zeolite 3A showing a significant reduction below the vacuum Mu value. The diamagnetic signal was found to exhibit both relaxing and non-relaxing components, and the amplitudes of both are included in the PD non values ðPD ¼ Prel D þ PD Þ; the fraction of the relaxing component is listed as fDrel ¼ Prel D =PD in Table 2. The fast-relaxing D component was
largest in zeolite 3A, moderate in 13X, and insignificant in others. In both zeolites 13X and 3A, measurements made during the bake-out showed that baking increased the relaxation rate of Mu, decreased the amplitude of the relaxing diamagnetic component and had little or indeterminate effect on other parameters.
4. Discussion The measured polarizations, relative fractions, and relaxation rates for Mu and Diamagnetic muons recorded in Table 2 do not suggest any obvious correlations with sample properties. Notably, the (fast) Mu relaxation rates are not correlated with the prevalence of nuclear moments—as determined by the Al content (Table 1). While about 50% Mu is seen in Si gel, consistent with early measurements giving 45% [9], it is not clear why the relaxation rate should be so fast. Perhaps this is an indication of random hyperfine anisotropies due to collisions in a constrained geometry, but that is unlikely [12], especially since the smallest value of lM is found in the 3A sample, which has the smallest pore diameter. More likely is that Mu relaxation is due to paramagnetic sites, whether impurities or dangling oxygens, which could be important in all samples, and indeed previous measurements of Mu relaxation rates in different silica powders do demonstrate a strong sample dependence due to impurities (e.g., Fe3+) [12]. ESR measurements are planned. The large differences in Mu fractions seen between the Si gel and the S-115 zeolite may point to impurities
Fig. 1. mSR Signals for zeolites 3A and S-115 measured at 75 G TF. The data represent summed runs at different temperatures, since there was little or no temperature dependence. The difference in Mu relaxation and amplitude is apparent. Not obvious over this time-range is the difference in diamagnetic signals (with relaxing and non-relaxing components).
Table 2 Polarization fractions and their relaxations Zeolite
PM
PD
PL
fM (%)
fD (%)
fDrel (%)
lM (ms1)
lD (ms1)
Si Gel 3A 13X USY ZSM-5 S-115
0.396(27) 0.168(08) 0.192(17) 0.316(34) 0.402(51) 0.074(12)
0.396(06) 0.446(10) 0.576(07) 0.436(06) 0.402(17) 0.560(04)
0.208(28) 0.386(13) 0.229(18) 0.248(35) 0.197(54) 0.366(12)
50 27 25 42 50 12
50 73 75 58 50 88
3 31 3 0 0 9
26(3) 6(1) 17(2) 7(2) 10(2) 11(3)
3(5) 9(1) 7(4) — — 11(2)
D.J. Arseneau et al. / Physica B 326 (2003) 64–67
rather than dangling -O sites since both are essentially SiO2 environments. The identity of the diamagnetic and lost fractions has not be established by the present studies, but the fast-relaxing D component can be tentatively attributed to slow Mu formation. It is noteworthy that the largest and next-largest relaxing fractions (3A, S-115; Table 2) are associated with the two greatest PL values. If Mu is formed by recombination of m+ with radiolysis electrons, then the size of the relaxing component is determined by the competition between Mu formation and electron removal; somewhat faster, but not prompt, Mu formation would contribute to PL : Measurements under electric field would be useful. The lost polarization of delayed and dephased Mu might be partially recovered by epithermal addition reactions forming Mu-radicals in zeolites, as recently determined for the MuCH2CH2 radical in the gas phase [8]. LF (repolarization) studies carried out here some years ago indicated two components, one fast and one slow, but were inconclusive. Since Mu has clearly been identified in all the zeolite samples we have looked at, it is reasonable to assume that this is indeed the precursor for the formation of muoniated radicals in loaded zeolites. The generally small amplitudes seen suggest small radical fractions, consistent with radical measure-
67
ments [3]. Nevertheless, Mu formation may well be enhanced when a zeolite is loaded with an organic compound, as is the case for previous radical studies, and, indeed, for industrial use.
References [1] D.W. Breck, Zeolite Molecular Sieves, Krieger, Malabar, FL, 1973. [2] M. Shelley, et al., Stud. Surf. Sci. Catal. 94 (1994) 469. [3] D.G. Fleming, M.Y. Shelley, D.J. Arseneau, M. Senba, J.J. Pan, E. Roduner, J. Phys. Chem. B 106 (2002) 6395; D.G. Fleming, et al., Physica B 603 (2000) 289. [4] E. Roduner, M. Stolm!ar, H. Dilger, I.D. Reid, J. Phys. Chem. 102 (1998) 7591. [5] M. Stolm!ar, E. Roduner, J. Am. Chem. Soc. 120 (1998) 583. [6] E. Roduner, H. Dilger, J. Am. Chem. Soc. 123 (2001) 7717. [7] C.J. Rhodes, T.C. Dintinger, C.A. Scott, Magn. Reson. Chem. 38 (2000) 62. [8] P.W. Percival, J.-C. Brodovitch, D.J. Arseneau, M. Senba, D.G. Fleming, Formation of the muoniated ethyl radical in the gas phase, Physica B, these proceedings. [9] G.M. Marshall, J.B. Warren, D.M. Garner, G.S. Clark, J.H. Brewer, D.G. Fleming, Phys. Lett. A 65 (1978) 351. [10] Ch. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Structure Types, Elsevier, Amsterdam, 2001. [11] C.A. Fyfe, Y. Feng, H. Grondey, G.T. Kokotalio, H. Gies, Chem. Rev. 91 (1991) 1525. [12] J.R. Kempton, R.F. Marzke, D.G. Fleming, A.C. Gonzalez, S.K. Leung, J.J. Pan, P.W. Percival, M. Senba, A. Tempelmann, Hyperfine Int. 65 (1990) 811.