3970
J . Phys. Chem. 1988, 92, 3970-3974 1
L
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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.
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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 (1) Lapujoulade, J.; Neil, K. S. Surf. Sci. 1973, 35, 288.
0022-3654/88/2092-3970$01.50/0
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. 1974, 60,4528.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 13, 1988 3971
Hydrogen Interaction with Ni( 100)
i.0 d
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Figure 2. Calibration curve: SSIMS signal ratio versus hydrogen coverage (relative TPD area). The inset shows TPD spectra corresponding to data points a, b, c, and d. Hydrogen was dosed at 150 K (circles) and 200 K (squares).
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Figure 1. Static SIMS spectra of clean (upper) and hydrogen-covered (lower) Ni(100) at 200 K. The Ar+ beam used was 1 keV, 2 nA/cm* (same for all the following figures). The upper curve is offset by 2000 counts and multiplied by 10 for clarity.
100
200 300 400 500 Temperatyre/K
teractions with Ni(100), we observed an intense Ni;H+ cluster ion signal from surface hydrogen, which can provide information on surface processes involving H(a) and motivated this study. The appearance of M,H+ (n = 1,2) ions in SSIMS has been reported by different However, no attempt has been made to use these to obtain kinetic information. In this paper, which amplifies a preliminary rep0rt,l4 we present a detailed study of the kinetics of hydrogen adsorption and desorption on Ni( 100) using SSIMS.
Experimental Section All experiments were performed in a stainless steel ultrahighTorr by a 450 L/s vacuum (UHV) chamber pumped to 3 X turbomolecular pump. The system was equipped with a double-pass cylindrical mirror analyzer for Auger electron spectroscopy (AES) and a quadrupole mass spectrometer for SIMS, temperature-programmed desorption (TPD), and residual gas analysis. The quadrupole mass spectrometer was interfaced to an IBM-PC computer for data collection and analysis. (3) Andersson, S . Chem. Phys. Lett. 1978, 55, 185. (4) Karlsson, F.-A,; Martensson, A.-S.; Andersson, S.; Nordlander, N. Surf. Sci. 1986, 175, L159. (5) Hamza, A. V.; Madix, J. J . Phys. Chem. 1985, 89, 5381. Castro, M. E.;Akhter, S.;White, J. M.; Houston, J. E., (6) Zhu, X.-Y.; to be published. (7) Zhou, X.-L.; White, J. M. Surf. Sci., in press. ( 8 ) Benninghoven, A,; Beckmann, P.; Greifendorf, D.; Muller, K.-H.; Schemmer, M. Surf. Sci. 1981, 107, 148. (9) Benninghoven, A.; Muller, K.-H.; Schemmer, M. Surf. Sci. 1978.78, 565. (10) Barber, M.; Bordoli, R. W.; Vickerman, J. C.; Wolstenholme, J. In Proceedings of the 7th International Vacuum Congress and the 3rd International Conference on Solid Surfaces, Vienna, 1977; p 983. (11) Marlen, J.; De Pauw, E.; Pelzer, G. J . Phys. Chem. 1983,87, 4344. (12) Benninghoven, A,; Muller, K.-H.; Schemmer, M.; Beckmann, P. In Proceedings of the 7th International Vacuum Congress and the 3rd International Conference on Solid Surfaces, Vienna, 1977; p 1063. (13) Benninghoven, A,; Beckmann, P.; Greifendorf, D.; Schemmer, M. Surf. Sei. 1982, I 1 4, L62. (14) Zhu, X.-Y.; Akhter, S.; Castro, M. E.; White, J. M., Surf. Sci., submitted.
i 0 0 1 200
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Figure 3. Calibration sensitivity factor ( p ) versus temperature. The five closed circles are the slopes of isothermal calibration curves at 150, 200, 300, 320, and 330 K. The solid curve is the ratio of TPSSIMS and integrated TPD (inset). TPD and TPSSIMS were done at a temperature ramp of 6.3 K/s after 5-langmuir dose of hydrogen.
The Ni( 100) crystal was cleaned by oxidation (2 X lo-* Torr of 02,1000 K, 3 min) and reduction (5 X lo-' Torr of H2, 1100 K, 6 min) cycles. The cleanliness was monitored by AES. The sample could be cooled to 90 K and resistively heated to 1400 K. The temperatures were measured by a chromel-alumel thermocouple spot-welded to the back of the crystal. H2 was dosed by back-filling. Dosing pressures were measured with an ion gauge using a relative sensitivity factor (SHz/SN2)of 0.44. Static SIMS spectra were generated by using an Ar+ beam (1 keV, 2 nA/cm2) rastered over the Ni(100) crystal face. TPD and TPSSIMS were done with a temperature ramp of 6.3 K/s.
Results Calibration. Hydrogen adsorbs dissociatively on Ni( loo), and H(a) is believed to be bonded to the fourfold s i : e ~ . l ~ , Figure '~ 1 shows the SSIMS spectra (50-150 amu) of a clean (upper panel) and a hydrogen-covered (lower panel) Ni( 100) surface at 200 K. The major secondary ions of a hydrogen-covered surface were Ni2H+and Ni+, while those of a clean Ni( 100) surface were Ni+, NiH20+, Ni(H20)2+,and Ni2+. The water-containing ions arise from tiny amounts of background H 2 0 (
