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Langmuir 1997, 13, 3162-3171
Adsorption and Decomposition of Methyl Iodide on Low Index Planes of NiAl Sanjay Chaturvedi†,‡ and Daniel R. Strongin*,§ Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, and Department of Chemistry, State University of New York, Stony Brook, New York 11794-3400 Received May 22, 1996. In Final Form: March 21, 1997X Surface chemistry of CD3I (CH3I) on the (111), (100), and (110) planes of NiAl has been investigated with X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). On the basis of XPS, adsorption of CD3I on the (110) and (111) planes is primarily associative at 120 K. In contrast, adsorption of the majority of the reactant on the NiAl(100) plane at 120 K is dissociative. This enhanced dissociation on NiAl(100) is attributed to its Al terminated surface. In addition to molecular CD3I desorption, TPD shows that CD4 and D2 are reaction products that desorb from the NiAl planes during the reaction of CD3I. TPD also suggests that methyl radical, CD3, may result during the thermal decomposition of CD3I on all three NiAl surfaces. Some C-C bond formation leading to the desorption of CD2CD2 is experimentally observed to occur on the (111) plane that exposes both Ni and Al atoms in the outermost surface.
1. Introduction Alkyl halides have recently received a great deal of attention, because they are important precursors of alkyl fragments.1 Since these fragments play an important role in Fischer-Tropsch synthesis, oil refining, and in the transformation of methane into other hydrocarbons,2-6 their surface chemistry on metals is of particular interest. Several recent studies have focused on the adsorption and subsequent decomposition of iodomethane on metals that include Ag(111),7 Au(100),8 Ni(100),9 Ni(111),10 Cu(110),11 Cu(111),12 Pt(111),13 Pd(100),14 Ru(001),15 and W(100).16 A fundamental difference between the chemistry of CH3I on a noble metal such as Cu and on a transition metal like Ni is that on the former metal C-C coupling gaseous products (e.g., CH3CH3) desorb. In contrast, prior studies of methyl iodide on Ni(100)16,9 indicate that CH4 and H2 are the only gaseous reaction products during the thermal reaction of CH3I. The relevance of these prior studies to our investigation of NiAl is that experimental and theoretical work suggests that NiAl has a filled band similar to that of Cu.17-19 In addition to the goal of understanding the thermal decomposition of CD3I on a * To whom correspondence should be addressed. Tel: (215) 2047119. Fax: (215) 204-1532. † State University of New York. ‡ Present address: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973. § Temple University. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Prior, W. A. Free Radicals; McGraw Hill: New York, 1966. (2) Rooney, J. J. J. Mol. Catal. 1985, 31, 147. (3) Biloen, P.; Sachtler, W. M. H. Adv. Catal. 1981, 30, 165. (4) Bell, A. T. Catal. Rev. Sci. Eng. 1981, 23, 203. (5) Somorjai, G. A. Catal. Rev. Sci. Eng. 1981, 23, 189. (6) Paal, Z.; Menon, P. G. Hydrogen effects in catalysis; Marcel Dekker: New York, 1988. (7) Zhou, X. L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci. 1989, 219, 294. (8) Yang, M. X; Jo, S. K; Paul, A.; Avila, L.; Bent, B. E Surf. Sci. 1995, 325, 102. (9) Tjandra, S.; Zaera, F. Langmuir 1992, 8, 2090. (10) Tjandra, S.; Zaera, F. J. Catal. 1994, 147, 598. (11) Chiang, C. M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836. (12) Lin, J. L.; Bent, B. E. J. Vac. Sci. Technol., A 1992, 10, 2202. (13) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 184, L325. (14) Solymosi, F.; Re’ve’sz, K. Surf. Sci. 1993, 280, 38. (15) Zhou, Y.; Henderson, M. A.; Feng, W. M.; White, J. M. Surf. Sci. 1989, 224, 386. (16) Zhou, X. L.; White, J. M. Surf. Sci. 1988, 194, 438. (17) Schultz, P. A.; Davenport, J. W. Scr. Metall. 1992, 27, 629.
S0743-7463(96)00503-3 CCC: $14.00
well-defined alloy system, a scientific objective of our study is to determine whether this electronic structure results in noble-metal-like chemistry for methyl iodide adsorbed on NiAl. Undoubtedly, in addition to the average electronic structure of the alloy, ensembles of particular surface atoms need to be present to allow reactions such as coupling to proceed. An attempt has been made to investigate how the surface composition influences the adsorption and the subsequent thermal reactions of CH3I/ NiAl. In the present research, we investigate three low Miller index planes of NiAl that exhibit well-defined surfaces. Prior research with other probe molecules such as CO,20,21 H2O,22,23 and CH3OH24,25 has indicated that the reactivity of the alloy is a strong function of the atomic composition and the structure of the surface. Stoichiometric NiAl crystallizes in an ordered CsCl structure. The (110), (100), and (111) planes of NiAl have all been shown by prior experimental work to exhibit relatively well-ordered surfaces. The (110) plane of NiAl has been shown to expose an outermost surface that is similar to what would be expected from an ideal termination of the bulk. The atomic composition of the surface is 50% Ni and Al,26,27 with the Al component sitting slightly above the Ni component in the outermost layer. Experiments show that the (100) plane of NiAl is terminated by a pure Al layer.28 More ambiguous than the structure of the (110) and (100) planes is the (111) surface. An ideal bulk termination of this plane may result in an outermost surface that is capped by either an Al or Ni layer. On the basis of prior structural studies, it is generally thought that this plane is terminated by 1 × 1 domains, which are (18) Lu, Z. W.; Wei, S. H.; Zunger, A. Acta Metall. Mater. 1992, 40, 2155. (19) Lui, S. C.; Davenport, J. W.; Plummer, E. W.; Zehner, D. M.; Fernando, G. W. Phys. Rev. B 1990, 42, 1582. (20) Mundenar, J. M.; Gaylord, R. H.; Liu, S. C.; Plummer, E. W.; Zehner, D. M. In Mater. Res. Soc. Symp. Proc.; Zehner, D. M.; Goodman, D. W., Eds.; Materials Research Society: Pittsburgh, PA, 1987; Vol. 83. (21) Mundenar, J. M. Ph.D. Thesis; University of Pennsylvania, 1988. (22) Gleason, N. R.; Chaturvedi, S.; Strongin, D. R. Surf. Sci. 1995, 326, 27. (23) Gleason, N. R. Ph.D Thesis; SUNY at Stony Brook: Stony Brook, NY. (24) Sheu, B. R.; Chaturvedi, S.; Strongin, D. R. J. Phys. Chem. 1994, 98, 10258. (25) Sheu, B.; Strongin, D. R. J. Catal. 1995, 154, 379. (26) Ross, P. N. J. Vac. Sci. Technol., A 1992, 10, 2546. (27) Yalisove, S. M.; Graham, W. R. Surf. Sci. 1987, 183, 556. (28) Davis, H. L.; Noonan, J. R. Mater. Res. Soc.; Pittsburg, PA, 1987; Vol. 83, p 3.
© 1997 American Chemical Society
Adsorption and Decomposition of Methyl Iodide
Figure 1. Ideal surface structures of the NiAl crystallographic planes used in this study. (a) NiAl(110); (b) NiAl(100), and (c, d) NiAl(111). The filled and open circles represent the Ni and Al atoms, respectively. In the case of NiAl(111), both Ni and Al terminated surfaces are thought to exist.
separated by monatomic steps, that are individually terminated by Ni and Al.29,30 This surface is quite rough compared to the other two faces, and surface reactions are expected to be influenced by the direct interaction of second and third layer surface atoms. Schematic representations of the three surfaces are shown in Figure 1, panels a-c. Figure 1a shows the top view of NiAl(110), where the black circles represent Ni and the white circles represent Al. Figure 1b shows the NiAl(100) surface that is Al terminated, and Figure 1, panels c and d, shows the Al and Ni terminated surfaces, respectively, of NiAl(111). Our investigation of the chemisorption and reaction of CD3I on the NiAl planes indicate that the mode of adsorption of CD3I and the nature of the gaseous products evolved during its thermally induced decomposition are greatly influenced by the structure of the alloy surface. Even the (110) and (111) surfaces, both of which expose Al and Ni in the outermost layer, exhibit very different reactivity toward CD3I. The close-packed (110) plane, compared to NiAl(111), is less reactive toward CD3I decomposition, since in contrast to NiAl(111) the majority of CD3I reversibly adsorbs on NiAl(110). CD3I undergoes the greatest amount of C-I bond cleavage on NiAl(100), and this is due to the Al terminated outermost layer. The thermal decomposition reactions of CD3I on NiAl(111) results in a small amount of ethylene, showing that C-C bond formation is facilitated to some extent on this crystallographic plane. We suspect, however, that this reaction channel on NiAl(111) is not related to noble metallike chemistry. No such C-C coupling reaction is observed on the (100) and (110) planes of NiAl. 2. Experimental Methods All the experiments presented in this contribution were performed in a bakeable ultrahigh vacuum (UHV) chamber that was evacuated by ion, turbomolecular, and titanium sublimation (29) Noonan, J. R.; Davis, H. L. Phys. Rev. Lett. 1987, 59(15), 1714. (30) Noonan, J. R.; Davis, H. L. J. Vac. Sci. Technol., A 1988, 6(3), 722.
Langmuir, Vol. 13, No. 12, 1997 3163 pumps. Working base pressures maintained during our experiments were typically below 4 × 10-10 Torr. Analytical equipment housed in the experimental apparatus consisted of a quadrupole mass spectrometer (QMS), low-energy electron diffraction (LEED) optics, double-pass cylindrical mirror analyzer (CMA), and X-ray source. Also available was an ion gun for sample cleaning. The NiAl single crystal ingot used in the study had a bulk composition of 50 atom % Ni and Al (supplier was General Electric Aircraft Engines). The crystallographic planes were prepared from this ingot by standard metallurgical methods (e.g., spark cut and polished with diamond paste) and were within 10 of the specified orientation. All the samples were approximately 8 mm on a side with a thickness of 2 mm. Four Ta support wires (0.25 mm diameter) were spot welded to each side of the individual samples, and the ends of the wires were spot welded to Ta tabs that were mechanically attached to a liquid nitrogen cryostat with cooling capability to 120 K. Heating of a particular sample was accomplished by passing current through the Ta wires. A chromel-alumel (type K) thermocouple was spot welded to the back of the sample for accurate temperature measurements. Each sample showed substantial carbon and oxygen contamination when introduced into the experimental chamber. Oxygen was removed from the near surface region of each sample by repeated 500 eV Argon ion sputter (20 min) and 950 °C anneal (10 min) cycles. Trace amounts of carbon left on the surface after these cycles were removed by heating the particular sample at 1000 K in oxygen (2 × 10-7 Torr) for times ranging from 2 to 10 min depending on the amount of carbon. This procedure removed the carbon without introducing oxygen contamination. Sharp (1 × 1) LEED patterns were observed for the individual samples after this cleaning procedure. Methyl iodide (CH3I, 99.5% purity; CD3I, 99.5 atom % D; Aldrich Chemicals) was used after several freeze-thaw-pump cycles. The purity of the reactant was confirmed by mass spectrometry. Iodomethane was admitted into the chamber through a 0.05 cm diameter dosing tube connected to a UHV compatible leak valve. The NiAl sample of interest was always held at 120 K during exposure to CH3I or CD3I. It is mentioned at this point that the vast majority of data presented in this contribution are for the adsorption and reaction of CD3I. The deuterium containing molecule was used to remove effects due to residual surface H and to allow us to be sensitive to abstraction reactions involving gaseous radicals desorbing from the surface and hydrogen on the apparatus walls during TPD experiments. The influence of this extraneous H will be discussed later. A complete set of experiments using CH3I was carried out in our experimental apparatus and its chemistry was very similar to that of CD3I (i.e., very small isotope shifts, for example, during TPD). Dosing of the sample was accomplished by back-filling the experimental chamber with the alkylhalide. Exposures quoted in this paper are presented in units of Langmuirs (L) (1 L ) 10-6 Torr s) and are not corrected for the sensitivity of the nude ionization gauge used the pressure measurement. Temperature programmed desorption (TPD) experiments were performed by heating the sample at a heating rate of 8 ( 1 K/s. Gases desorbing from the surface were analyzed by a multiplexed QMS capable of simultaneously measuring up to nine ions during a single TPD experiment. The mass spectrometer was housed in a gold-plated stainless shield having a 6.0 mm diameter aperture. Cooling of the shield to ∼100 K could be accomplished by passing liquid nitrogen through the interior of the shield. Experiments that utilize this cooling capability of the shield are discussed later. During TPD experiments the sample was placed within 2-3 mm of this aperture hole, thereby limiting the detection of gases evolving from the support wires. All the yields of products quoted in this paper have been corrected for the relative mass spectrometer sensitivities. XPS data presented in this paper were acquired with unmonochromatized MgKR (1253.6 eV) radiation. The pass energy of the CMA was set at 25 and 50 eV during the acquisition of the I 3d5/2 and C 1s data, respectively. The binding energy scale for the XPS was calibrated by aligning the Ni 2p3/2 of NiAl to 853.3 eV below the Fermi level (EF). XPS data, as a function of temperature, presented in this paper, were obtained by exposing the sample to iodomethane at 120 K and then heating the sample
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Figure 3. TPD of CD3I/NiAl(111) after a CD3I exposure at 120 K. The shield surrounding the mass spectrometer is cooled to ∼100 K, limiting extraneous reactions facilitated by this surface. In this experiment, the m/e 19 species that is associated with CD3H is suppressed, suggesting that this species is formed on the walls of the mass spectrometer shield.
Figure 2. TPD of CD3I/NiAl(111) after a CD3I exposure at 120 K. The inset shows selected desorption traces for CH3I/NiAl(111). With regard to CD4 product at 445 K, note that only a small amount of this product if formed (×4 factor). CH4 product does appear at a similar temperature in the m/e 15 and 16 traces of the inset, but due to the low yield it is partially obscured by the background. at a rate of 8 K/s to the desired temperature. XPS data were recorded after rapidly cooling the sample back to 120 K.
3. Results Results for CD3I adsorption and reaction on the (111), (100), and (110) planes of NiAl are presented in two sections. The first section contains results from TPD experiments. These data provide information about the various reactions that occur on the alloy surfaces. Results from XPS experiments are presented in the second section. These experimental results allow us to develop a picture that elucidates the nature of adsorption of CD3I and the subsequent stability of its decomposition fragments on the NiAl surfaces as a function of both temperature and coverage. 3.1. Temperature Programmed Desorption. Figure 2 shows thermal desorption traces for different products evolved after the (111) crystallographic plane of NiAl is exposed to CD3I at temperatures near 120 K. Inspection of the figure shows that CD3I (m/e 145), CD2CD2 (m/e 32), CD4 (m/e 20), and D2 (m/e 4) desorb from the (111) plane after an 8.0 L exposure of CD3I. The CD3I desorption feature with a peak temperature, TP, of 150 K is due to sublimation of the multilayer, since this feature grows indefinitely with increasing exposure. The CD3I trace shows a second feature with a Tp of 260 K that is due to molecular CD3I interacting directly with NiAl(111). XPS results presented below support this contention. A m/e 127 trace also is included, and the features below 400 K are attributed to the cracking of CD3I to I+ in the mass spectrometer. Near 900 K, there is a feature in the m/e 127 spectrum (and not in the m/e 145 spectrum) that is associated with the desorption of iodine from the alloy surface. In addition to the products just mentioned, it is inferred from TPD data that CD3H (m/e 19), with a Tp of 260 K, is detected by the mass spectrometer during the thermal
decomposition of CD3I on NiAl(111). Support for the m/e 19 feature at 260 K being due to CD3H comes from inspection of the inset to Figure 2 that exhibits CH3I TPD data. After removing the contribution of the CH3+ daughter ion fragment of CH3I to the m/e 15 feature at 260 K, the m/e 16:15 ratio is 1.2, consistent with CH4 product. The difficulty, however, is not in the identification of the species, but instead in determining whether the species is formed on the (111) plane of NiAl and not on extraneous surfaces in the experimental apparatus. CD3H product may be due to the reactions of carbonaceous fragments resulting from the thermal decomposition of CD3I with residual H on the NiAl(111) surface; the H resulting from the adsorption of background gas in the vacuum chamber. Experiments that have monitored H2 desorption from the surface resulting from residual surface H atoms, however, show that its yield is
