J . Phys. Chem. 1988, 92, 3965-3970 the original terminating layer with the underlying SLS layer, hoping thereby to alter the PEC properties. We also conducted PEC etching by exciting into the 575-nm band of Gap-terminated SLS’s, expecting to see a drop in photocurrent as the GaP layer gave way to the alloy underlayer. All these experiments were inconclusive, however, presumably because uniform etching on a 100-A scale is too formidable a task for these techniques. Despite this lack of success, we emphasize that the SLS’s do show evidence of terminating-layer-dependent electrooptical properties, making
3965
the development of layer-selective etchants of potential value for device fabrication.
Acknowledgment. Support of this research from the Office of Naval Research (A.B.E., P.B.J., S.P.Z.) is gratefully, acknowledged. Part of this work was supported by the U S . Department of Energy under Contract DEHAC04-76DP00789. Registry No. GeAso,,,Po,,,, 107103-01-5; GeP, 12063-98-8; Te2-, 22541-49-7; Se2-, 22541-48-6; KOH, 1310-58-3.
Solld-State NMR of Silicoaluminophosphate Molecular Sieves and Aluminophosphate Materials C. S. Blackwell* and R. L. Patton Union Carbide Corporation, Tarrytown, New York I0591 (Received: November 19, 1987)
The solid-state 27Al,31P,and 29SiNMR spectra of five silicoaluminophosphatemolecular sieves, SAPO-5, -1 1, -34, -35,and -37, are reported. The spectra of two aluminophosphate materials, AIPO1-quartz and A1P04-21, are also presented. The data for the silicoaluminophosphates are compared to the data for the aluminophosphates and for previously reported aluminophosphate-based molecular sieve spectra and known structural data. The 27AlNMR data are comparable to our previous AIPOl spectra, showing unique, nonzeolitic tetrahedral framework A1 coordination and in some cases secondary coordination by water and template. The 31PNMR spectra show both compositional and framework geometrical effects on the chemical shift. The 29Sichemical shifts for SAPO’Sare observed in the normal zeolite-aluminosilicate-silicate shift range. The observed 27AlNMR powder patterns of A1P04-quartz reported previously have been modeled and fitted for both static and MAS patterns to Qr= 4.05 MHz, 9 = 0.35, and b(isotropic) = 45.5 ppm,
Introduction Previous researchers have reported solid-state N M R data for nonmicroporous aluminophosphates’ and for A1P04-5.2 Studies of the aluminophosphate analogues of quartz and tridymite and for metavariscite, as well as of the aluminophosphate molecular sieves A1PO4-5, - 1 1, - 17, and -3 1 were reported3s4 previously. These materials have tetrahedral oxide frameworks with alternating AlO, and PO, tetrahedra; however, the framework of A1Po4-17 has some aluminum sites that, upon calcination and hydration, undergo additional secondary coordination to water that results in an 27A1N M R chemical shift in the octahedral A1 coordination range. The silicoaluminoph~sphates~ are one of the new generations of molecular sieves, discovered recently at Union Carbide Laboratories, which are based on the novel aluminophosphate family by incorporating additional elements, Si in this case, into the A1PO4 framework.6 Studies of one member, SAPO-5, have been previously reported by Appleyard et al.,’ who concluded that the silicon was tetrahedrally coordinated in the (Si,AI,P)02framework. Saldarriaga et aL8 have studied SAPO-37 and reached similar conclusions. We report herein additional studies of SAPO-5 and -37 as well as SAPO-1 1, -34, and -35. The 27AlN M R spectra of these types of materials can be made quite complex by second-order quadrupolar effects as we initially discussed in a prior papers3 The 27Al N M R MAS and static spectra of A1P04-quartz, which show multiple resonance maxima, can be shown to arise from one type of unique AI(OP), species. Experimental Section The N M R spectra were obtained on a Bruker CXP-200 solid-state and high-resolution N M R spectrometer operating at a field of 4.7 T with a standard 13Ccross-polarization magic angle spinning (CP-MAS) accessory probe. Additional spectra were taken on a Bruker AM-400 (9.4 T) a t Bruker and on a Bruker *Address correspondence to this author.
0022-365418812092-3965$01.50/0
MSL-400 at Union Carbide. The 31P N M R chemical shifts were referred to external H3P04 (85%), the 27AlN M R chemical shifts to external Al(H20)63+in A1(N03)3aqueous ~ o l u t i o nand , ~ the 29SiN M R chemical shifts to external T M S (tetramethylsilane). Recalibration for magnet drift was done daily; changes of only a few hertz back and forth were observed. The chemical shift calibration is reproducible to less than 0.1 ppm but when applied to samples is subject to bulk susceptibility effects. The Andrew-Beams rotors were made of Delrin and spun with dry nitrogen at approximately 3-4 kHz; if quadrupole effects were present extensive spinning side bands (ssb’s) were often observed. The magic angle was adjusted with KBr following the work of Frye and M a ~ i e l . A ~ typical 31PN M R spectrum was obtained from Bloch decay experiments of 20-1000 coadded FID’s (free induction decays) produced by 4-ps pulses followed by a 60-s relaxation delay. For 27AlN M R typically 500-1000 Bloch decay FID’s were accumulated by using 2-ps pulses and a 1-s delay between successive pulses. The spectra (sweep widths 40-1 25 KHz) generally showed spinning side bands and line shapes (1) Muller, D.; Grunze, I.; Hallas, E.; Ludwig, G. Z. Anorg. Allg. Chem. 1983, 500, 80.
(2) Muller, D.; Jahn, E.; Fahlke, B.; Ladewig, G.; Haubenreisser, U. Zeolites 1985, 5, 53.
(3) Blackwell, C. S . ; Patton, R. L. J . Phys. Chem. 1984, 88, 6135. (4) (a) Wilson, S. T.; Lok, B. M.; Messina, C. A,; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. SOC.1982, 104, 1146. (b) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. In Intruzeolite Chemistry; ACS Symposium Series 218; American Chemical Society: Washington, DC, 1983; p 79. (5) Lok, B. M.; Messina, C. A,; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. (a) U S . Patent 4440871, 1984; (b) J . Am. Chem. SOC. 1984, 106,6092. (6) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S. T. Pure Appl. Chem. 1986, 58, 1351. (7) Appleyard, I. P.; Harris, R. K.; Fitch, F. R. Chem. Lett. 1985, 1747. (8) Saldarriaga, L. S.; Saldarriaga, C.; Davis, M. E. J . Am. Chem. SOC. 1981, 109, 2686. (9) Frye, J. S.; Maciel, G. E. J. Mugn. Reson. 1982, 48, 125.
0 1988 American Chemical Society
3966 The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Blackwell and Patton
TABLE I 6(27AI),“
6(3’~),
6(29~i),
sample SAPO-5 (data from ref 7) SAPO-S(a)
PPm 33.5 38.2 7.7c
PPm -28.4 N.A.*
field, T 4.7 9.4
SAPO-S(b)
-30.4 N.A. N.A. N.A.
SAPO- 11(b)
32.3 36.7 N.A. 36.0 7.1‘ 33.3
PPm -92 -97 -102 -108 -98 -98 -93.4 -94.3
-30.9
SAPO-34
28.6
-28.1
-97 -108 -91.5 -108.9
N.A.
SAPO-34 (calcined) SAPO-35
34.5 3.4 38.2 -16.0 29.7
SAPO-S(c) SAPO- 1 1 (a)
SAPO-37
AlP04-quartz
AIP04-21
30 (sh) 0.8 33.5 2.4 34.1 19.9 -8.8 33.2 26.3 32.8 22.2 -12
mag
anal. composn wR(Si,AI,P,)O2 W
X
Y
z
(N.A.)
(0.071
0.47
0.46)
4.7 9.4 9.4 9.4
0.042
(0.093
0.48
0.43)
0.048 0.033
(0.043 (0.085
0.49 0.50
0.47) 0.41)
4.7
0.051
(0.120
0.45
0.43)
4.7
0.094
(0.132
0.50
0.37)
9.4
-27.4
-89.5
4.7
-27.4 -32.8 -26.1
-91.5
4.7
N.A.
(0.10
0.53
0.37)
-89.4 -106.5 -89.3
4.7
0.105
(0.125
0.51
0.36)
N.A.
9.4
-24.8
4.7 9.4
-14.8 -21.4 -26.4
4.7
“Pattern maxima are reported; the relationship of these to the isotropic shifts is discussed in the text. bN.A., not available. ‘Minor impurity.
suggestive of incomplete removal of second-order quadrupole coupling. Cross-polarization was only used for (‘H-27Al). For 29SiN M R typically 1000-7000 transients were coadded from data produced by 2-gs pulses followed by a 10-s relaxation delay. The (‘H-29Si] CP-MAS gives better S/N but causes changes in the resonance pattern artifically emphasizing low-field features; therefore, the Blcch decays were used for general characterization. The 9.4-T 29SiN M R MAS spectra has 90-s relaxation delays, which were necessary to assure quantitative peaks in these materials. The spectra of five silicoaluminophosphate (SAPO) molecular sieves, SAPO-5, - 1 1, -34, -35, and -37, in their “as-synthesized” or precursor states and some in the “calcined” states were obtained. Spectra were also obtained for two aluminophosphate materials, A1P04-quartz and AlP04-21. All samples were prepared at the Union Carbide Laboratories and shown to be essentially pure and fully crystalline by X-ray powder diffraction and for the molecular sieve species by McBain adsorption measurements. The as-synthesized SAPO’S were prepared by the methods of ref 5 and contain organic template and water molecules trapped within the microporous frameworks. The Si, Al, and P contents, determined by chemical analysis, are included in Table I. For the SAPO-34 sample the template and water were removed by calcination at 500 O C . a n d subsequent vacuum evacuation overnight. The dry sample was run in an O-ring sealed rotor. The sample was subsequently rehydrated with 100% water vapor for 1.5 days and another spectrum was obtained. SAPO-5 and -1 1 are novel structures with framework topologies analogous to those recently reported for A1PO4-5I1 and AlP04-11.” SAPO-34, SAPO-35, and SAPO-37 have framework topologies related to the zeolites chabazite, levynite, and faujasite, respectively. 5b (10) Ganapathy, S.; Schram, S.;Oldfield, E. J . Chem. Phys. 1982, 77,
4360. (1 1) Bennett, J. M.; Cohen, J. P.; Flanigen, E. M.; Pluth, J. J. Smith, J.
V. ACS Symp. Ser. 1983, 218, 109.
(12) Bennett, J. M.; Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V. Zeolites 1987, 7, 160.
,
1’’
‘ i ,
~ ~ 0m . 0.0 1m.o 0.0
-200.0
PPM
-
l ’ i 1.0
0.0
(Wll)
-1.0
1.0
0.0 (Wl.)
-1.0
-2.0
Figure 1. Observed and calculated 27AlNMR for A1P04-quartz. Top, observed spectra; bottom, calculated patterns; left, MAS; right, static.
Results The 29Si,”Al, and 31PNMR data are summarized in Table I. Unless otherwise noted the chemical shifts (6 values) are measured for the observed peak maxima for the stated magnetic field. These values will probably be shifted from the isotropic values for 6 in the case of 27A1because of second-order quadrupole shift effects. For 31Pand 29Si,both nuclei of spin 1/2 with no quadrupole moment, the observed maxima, under MAS conditions, should give a good approximation to the isotropic values for 6(29Si) and 6(31P).
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3961
Solid-state N M R of SAPO and AlPO, Materials
1
I 200
I
100
I
0
I
-1 00
I
-200
PPM
1
200
I
100
1
0
I
-100
I
-200
PPY
Figure 2. Left: 27A1N M R MAS for SAPO-34. Top, 4.7 T; bottom, 9.4 T. Right: 27A1NMR MAS for SAPO-37. Top, 4.7 T; bottom, 9.4 T.
27AlNMR. The use of 27AlN M R to characterize the coordination of A1 in molecular sieves is widespread. For each structurally unique kind of Al, there is, in principle, a unique 27Al chemical shift. It is tempting to interpret each maximum in the 27AlN M R MAS spectrum as characteristic of a distinct A1 environment. Such a practice can be very misleading. The spectrum of AlPO,-quartz, reported previ~usly,~ demonstrates the danger of such an assumption. The quartz structure with an aluminophosphate composition requires one and only one unique type of Al. The 27AlN M R static and MAS patterns at 4.7 and 9.4 T show multiple maxima. The change in position and narrowing of the pattern with increased field strength indicate that second-order quadrupole effects are d ~ m i n a n t . ~ To extract the isotropic chemical shift value of A1 in the AlP0,-quartz, the method of Ganapathy, Schramm, and Oldfieldlo has been applied for the static and MAS data. Expensive computing facilities are not required to perform the calculations; the Pascal programs were developed on an Apple 11+ and subsequently transferred to a DEC Rainbow, an IBM PC-AT, and a DEC MicroVax 11. The more powerful computers merely permitted finer digitization and much faster calculation. Figure 1 shows both the static (4.7 T) spectrum and its calculated fit as well as the MAS spectrum (9.4 T) and the corresponding calculated MAS pattern. Since the calculated patterns are plotted in a type of reduced coordinates, the peak maxima in the static and MAS patterns are used to extract Qf and G(isotropic) for 27Al. The shape of the pattern determines the value for 7,the nuclear asymmetry parameter. The best fit to our data was Qf= 4.05 MHz, 7 = 0.35, and 6(27Al) = 45.5 ppm. These values compare favorably with values calculated somewhat differently and reported by Muller et al.13 from MAS data. It is vital for understanding the spectra of AlPO, and SAPO samples to determine if the observed 27AlN M R MAS patterns are dominated by quadrupole effects or if pattern maxima can be related to unique A1 species. Pattern maxima may not be representative of the true isotropic 6(27Al),and this fact must be considered in interpreting the data. A striking example of this problem is demonstrated by the 27Al N M R MAS spectra of SAPO-34 and SAPO-37. The 27AlN M R MAS spectra of the as-synthesized samples were obtained at 4.7 and 9.4 T as shown in Figure 2. The rather amorphous, irregular patterns at 4.7 T resolve into two distinct, rather symmetrical lines at 9.4 T. The observed chemical shifts of the lines at 9.4 T occur
in ranges characteristic of tetrahedral and more highly (5 or 6) coordinated Al. A pattern with chemical shifts in both these 27Al N M R chemical shift ranges was previously observed for AlP04-17.3 In the case of AlP04-17 it was concluded that the 27Al N M R chemical shift indicative of higher coordination arose from an additional secondary interaction of A1 with extra-framework water, OH, and/or template in the sieve cavities3 Subsequently, X-ray single-crystal studies have confirmed the presence, in assynthesized AlPO,-17, of secondary coordination of some of the otherwise tetrahedral framework A1 with an additional nonframework oxygen species, most plausibly OH-, present to balance the template cations (piperidinium ions).14 This additional oxygen distorts the tetrahedral symmetry around the A1 toward a distorted trigonal bipyramid. It was postulated that, after the removal of the template and OH- by calcination and subsequent rehydration, adsorption of 1 or 2 H 2 0 molecules near the same framework A1 atom caused the additional N M R chemical shift found. For a sample of SAPO-34 the material was calcined at 500 “ C and rehydrated at room humidity, and the 27Al N M R MAS spectrum was obtained at 4.7 T. The previously irregular pattern broke up into two lines, 38.2 (tetrahedral range) and -16.0 ppm (octahedral range). The sample was dehydrated and run dry. The pattern collapsed into a single line with an irregular shape and a shift of 15.7 ppm. The low-field edge of this presumably quadrupole broadened pattern suggests an isotropic shift near 40 ppm, characteristic of tetrahedral A1 in the SAPO framework. Rehydration of the sample for 1.5 days with 100% water vapor resulted in restoration of the two lines, 36.4 and -14.7 ppm (see Figure 3). Only this latter line was cross-polarized to protons by (1H-27Al)CP-MAS, further indicating closer interaction of these Al’s with absorbed H 2 0 (see Figure 4). These data suggest a mechanism similar to that for AIP0,- 17 wherein secondary interaction of some of the tetrahedral framework A1 with adsorbed or occluded H 2 0and/or template produces a chemical shift indicative of higher coordination. The observed shift of the extra A1 line away from the tetrahedral A1 line was greater in the as-synthesized SAPO-34 and -37 than in the A1PO4-17. This may be related to structural differences, but it should also be noted that the apparent OH- species present in A1P04-17 to balance the template cation charge is not necessary or expected in the SAPO’S because their negative frameworks charge balance the template cations.
(13) Muller, D.; Jahn, E.; Ladwig, G.; Haubenreisser, U. Chem. Phys. Lett. 1984, 109, 332.
(14) Pluth, J. J.; Smith, J. V.; Bennett, J. M. Acta Crystallogr., Sect C: Cryst. Strucf. Commun. 1985, C42, 283.
3968
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988
Blackwell and Patton
I \
I
1
I 0.0
I
200.0
100.0 PPM
Figure 5. 27AlN M R MAS for A1P04-5, -1 1, SAPO-5(b), and SAPOl l ( b ) (4.7 T).
I
I
I
300.0
100.0
I
-100.0
I
-300.0
PPM
Figure 3. 27Al N M R MAS for calcined SAPO-34 (4.7 T). Bottom,
hydrated; middle, dehydrated; top, rehydrated.
I -50.0
I
-70.0
I
I
-110.0
-90.0
I
-130.0
A
-150.0
PPM
Figure 6. 29SiN M R MAS for SAPO-34 and -37 (4.7 T). Top, SAPO37; bottom, SAPO-34.
I
il
300.0
I
I
100.0
-100.0
I
-300.0
PPM
Figure 4. 27Al N M R MAS for SAPO-34 (calcined, hydrated). Top, Bloch decay 27AlNMR; bottom: ('H-27A1)CP-MAS.
The additional AI resonance is not always observed. The 27Al NMR MAS spectra of SAPO-5 and -1 1 are shown compared to those of A1P04-5 and -1 1 in Figure 5. The spectra are quite comparable and do not show any large apparent quadrupole effects
or additional water/template interactions as in SAPO-34 and -37. The A1 environment in these materials appears entirely tetrahedral, consistent with the published structures and very similar to that in the isostructural AIPO4 materials. 2qSi N M R . The 29SiNMR MAS spectra (Figure 6 ) of SAPO-34 and -37 show basically a single line pattern quite comparable with that reported for SAPO-5 by Appleyard et al.' The observed chemical shift, 6(29Si),was in the range -89 to -92 ppm. This is the area where, for aluminosilicate materials, the Si(4Al) and Si(3A1) resonance ranges overlap. The occasional appearance of minor resonances shown at -106 to -1 10 ppm in
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3969
Solid-state N M R of SAPO and AlP04 Materials
2
I
I
-50
-80
-70
-80
-90 . l o 0 PPM
-1 10
.120
-130
I 10.0
I
I
0.0
-10.0
I
-20.0
I
I
-30.0
PPM
Figure 7. 29SiN M R MAS for SAPO-5 and -1 1 (9.4 T). Top, SAPO1 l(a); middle and bottom, SAPO-5(c,a).
some SAPO-34 and -37 spectra indicates tiny amounts of highly siliceous framework region but, at the low levels observed, may simply represent small amounts of unreacted starting silica. The various possible modes of incorporation have been discussed by other^.^^,^ The single line -89 to -92 ppm patterns are consistent with one of the suggested types of Si incorporation into a hypothetical AlP04 framework, Si for P, which would create Si(OAI), species and one unit of negative framework charge per Si. Chemical analysis of our samples showing (Si P) N A1 is consistent with this picture. Another mechanism considered (ref 5b and 6), incorporation of Si into a hypothetical A1 site, would create unlikely Si(OP), groupings. There seems to be little or no information in the literature about the effect of such Si-0-P bonding on the 6(29Si). Simple arguments using electronegativity and diamagnetic shifts suggest a considerable upfield shift for a hypothetical Si(OP), species. None have been observed, and as will be discussed later, the ,IP N M R MAS evidence is also negative. For SAPO-5 and -1 1 the 29SiN M R data show more variability (see Figure 7). Some SAPO-5 and -11 spectra show a single dominant Si resonance (Figure 7 , top and middle), but others show additional resonance absorbance to high field (Figure 7 bottom). The S / N of these 9.4-T 29SiN M R spectra is relatively low. Small features on the general band outline should not be interpreted as reliable structural features. Such spectra indicate more variability for siting of Si in these novel frameworks than in those with zeolite structural analogues. Clearly more than one type of Si is present in some preparations. The higher field chemical shift is consistent with Si in a more Si-rich environment; apparently the second expected Si incorporation m ~ d e , ’ ~that , ~ of Si replacing equal amounts of P and A1 (2Si = P Al; no resulting charge) is operative together with the former Si for P mode in these samples. Such a combined mode would be expected to create Si-rich regions or islands in some SAPOS. The N M R pattern observed, and the relative proportion of the two Si incorporation modes, varies with the exact synthesis conditions for these SAPO’s. 31PNMR. The chemical shifts observed in 31PNMR MAS of these materials are all characteristic of tetrahedral phosphorus. Previous workers13have suggested a relationship between A1-0-P
Figure 8. ,IP N M R MAS for A1P04-21 (4.7 T).
+
+
SAPO-37
10.0
0.0
-10.0
-20.0
-30.0
PPM
Figure 9. lllP NMR MAS for SAPO’s (4.7 T). Top to bottom: SAPO-5(b), - l l ( b ) , -34, and -37.
angle and 6(,’P) in dense-phase AlPO, materials. The 31PN M R MAS spectrum of AIP04-21 (Figure 8) shows three distinct resonances (-14.8 ppm, -21.4 ppm, -26.4 ppm). The known crystallographic structure15 of this material requires three crys-
3970
J . Phys. Chem. 1988, 92, 3970-3974 1
L
1
I
10.0
0.0
-10.0
The 31PN M R MAS data for SAPO-5, -11, -34, and -37 are shown in Figure 9. The spectra each show a single symmetrical P line in the range observed for AlPO, molecular sieves indicative of one distinguishable type of tetrahedral phosphorus. The slight differences in chemical shift are presumeably structure specific. This hypothesis is further supported by the spectra of SAPO-37 and -35, shown in Figure 10. SAPO-37, with a faujasite-type framework, has one type of P in a double-six-ring (D6R). SAPO-35, with a levynite-type framework, has two possible P sites, one in a D6R and another in a single-six-ring (S6R). The structural distribution of the sites should be D6R:S6R of 2:l. The downfield resonance of the SAPO-35 spectrum (-27.4 ppm) clearly coincides with the single resonance of SAPO-37 (-26.1 ppm) and can be assigned to P in the D6R. The upfield SAPO-35 resonance (-32.8 ppm) must then be logically assigned to P in a S6R. The deconvoluted ratios are not quite 2:l (actually 2:0.9). The deviation from 2:l may indicate some ordering of Si in the P sites in the materials. All of the SAPO molecular sieve materials show 31PN M R shifts in the range previously observed for AIPO, molecule sieves. No new N M R features are observed that would support Si-0-P species, nor has there been any reported chemical evidence for Si-0-P in these types of materials.
I
-20.0
I
I
-30.0
PPM
Figure 10. 3'P NMR MAS for SAPO's (4.7 T). Top, SAPO-35;bottom,
SAPO-37. tallographically unique P positions in a ratio of 1:1:1. When the observed spectrum is deconvoluted to allow for differences in line widths, the areas of the lines are, indeed, 1:1:1. It is clear from these data that the three 31PN M R lines can be related to the three unequivalent P sites; therefore, one sees that the 31PN M R data are a useful structural tool of some considerable sensitivity. This is further demonstrated by the SAPO results (vide infra). (15) (a) Bennett, J. M.; Cohen, J. M.; Artioli, G.; Pluth, J. J.; Smith, J. V. Inorg. Chem. 1985, 24, 188. (b) Panse, J. B.; Day, C. S. Acla Crystallogr. Sect. C: Cryst. Struct. Commun. 1985, C41, 515.
Conclusions The data are consistent with a hypothetical Si for P incorporation mode in most SAPO materials, although some SAPO's appear to have Si-rich regions where additionally paired A1 and P sites are occupied. The N M R chemical shifts for 29Si, 27Al, and 3'P are observed in the zeolite and aluminophosphate molecular sieve ranges and are thus consistent with structures and bonding mechanisms similar to those materials. The NMR data are consistent with the published X-ray crystallographic results, wet chemical analysis, and infrared spectroscopic results. The 27AlN M R results for SAPO-34 and -37 are similar to those of AlPO,- 17 with shifts indicating secondary coordination of water to some of the tetrahedral framework 27Alspecies. The 31PN M R spectra of A1PO4-21and SAPO-35 clearly demonstrate structural and compositional effects on the 31PN M R chemical shift. Acknowledgment. We thank E. M. Flanigen for encouragement of this work, S. T. Wilson and R. T. Gajek for samples, and Union Carbide for permission to publish. We also thank Dr. Mark Mattingly of the Bruker Applications Laboratory, Billerica, MA, for running some of the 9.4-T spectra. Registry No. A1P04, 7784-30-7.
Hydrogen Interaction with Ni(100): A Static Secondary Ion Mass Spectroscopy Study X.-Y. Zhu and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: November 19, 1987)
The kinetics of hydrogen adsorption and desorption on Ni(100) was studied by static secondary ion mass spectroscopy (SSIMS). Hydrogen coverage on Ni(100) can be quantitatively followed by monitoring the SSIMS ion ratio (Ni,H+/Ni+), which varies linearly with hydrogen coverage over a broad range. Isothermal hydrogen uptake and decay curves are readily obtained, from which kinetic and thermodynamic parameters are extracted. The adsorption and desorption processes are adequately described by kinetic equations that are third-order in empty sites and second-order in hydrogen coverage, respectively. The calculated initial sticking coefficient (0.29) was independent of surface temperature. An activation energy of 22.7 f 0.2 kcal/mol and a preexponential factor of 0.09 f 0.04 cm2/(atom s) were obtained for hydrogen desorption from clean Ni( 100). The heat of adsorption calculated from steady-state coverage data was 22.7 f 0.4 kcal/mol.
Introduction The interaction of hydrogen with Ni( 100) has been the subject of many studies.'-5 However, there is still not a thorough and
consistent picture of the kinetics of this system. In recent SSIMS studies of ethylene6 and methyl iodide' in( 2 ) Christmann, K.; Schober, 0.;Ertl, G.; Neumann, M . J . Chem. Phys.
(1) Lapujoulade, J.; Neil, K. S. Surf. Sci. 1973, 35, 288.
0022-3654/88/2092-3970$01.50/0
1974, 60,4528.
0 1988 American Chemical Society
