Enhanced Methanol Dissociation on Nanostructured 2D Al Overlayers

Aug 22, 2007 - D. Petti , A. Torti , F. Damin , L. Sola , M. Rusnati , E. Albisetti , A. Bugatti , R. Bertacco , M. Chiari. Sensors and Actuators B: C...
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J. Phys. Chem. C 2007, 111, 13524-13530

Enhanced Methanol Dissociation on Nanostructured 2D Al Overlayers Zhen Zhang, Qiang Fu,* Hui Zhang, Yong Li, Yunxi Yao, Dali Tan, and Xinhe Bao* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, P.R. China ReceiVed: May 11, 2007; In Final Form: June 25, 2007

Three well-defined two-dimensional (2D) Al overlayers including the Al/Si(111) R-phase with Al coverage of ∼0.25 ML, the Al/Si(111) γ-phase with Al coverage of ∼0.8 ML, and the bulk Al film with Al coverage of 16 ML on Si(111) have been prepared using the molecular beam epitaxy technique. Their surface morphology and electronic structure were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The 16 ML Al film presents the metallic Al character, while the nanostructured Al/Si(111) R-phase and γ-phase are semiconducting due to space confinement of the surface Al atoms. The interfacial Al-Si interaction results in electron deficiency in surface Al atoms which follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. Methanol dissociation reactions on the Al surfaces at room temperature were studied by XPS and high-resolution electron energy loss spectroscopy (HREELS). The Al/Si(111) R-phase presents the highest activity for the dissociation of CH3OH and CH3Oads and the lowest activity for the bulk Al film. The exceptional activity of the nanostructured 2D Al surfaces for O-H and C-O bond scission has been attributed to their unique geometric structures as well as their semiconducting and electron-deficient characters.

1. Introduction One main goal of fundamental catalysis research is to understand the relationship between structure and reactivity of catalysts.1,2 The unambiguous identification of the structurereactivity relationship relies on preparation of catalyst systems, whose structures can be well controlled and understood.3,4 Twodimensional (2D) metal films may serve as an ideal model catalyst system. Therein, the surface electronic and atomic structure can be tuned by sophisticated film growth techniques. Two-dimensional metal overlayers grown on semiconductors have been studied intensively in past decades. Many welldefined metal ultrathin films or metal superstructures can be prepared on semiconductor surfaces and may be used as the 2D film catalysts. The present work concentrates on Al/Si(111) model systems. Recently, the Al/Si(111) structures have attracted extensive research effort.5-19 It has been shown that various surface phases may be obtained, such as R-7×7, x3×x3, x7×x7, and γ-phase, depending on growth temperature and Al coverage. The atomic structure of the Al phases has been clearly illustrated (e.g., ref 8), and their electronic structures are found to be quite unique because of the space confinement of the surface Al atoms in these 2D superstructures.20-22 These well-defined Al/Si(111) structures offer us good model catalysts with which to study the structure-reactivity relationship. Methanol (CH3OH) is often used as a probe molecule due to its simple and well-understood structure. The interaction of CH3OH with metal surfaces has been of considerable interest for a long time since it plays an important role in elementary catalytic reactions and many industrial processes.23-34 Generally, two competing pathways of CH3OH decomposition on transitionmetal surfaces exist: (i) O-H bond scission with dehydroge* Corresponding authors. Tel: +86-411-84686637. Fax: +86-41184694447. E-mail: [email protected] (Xinhe Bao), [email protected] (Qiang Fu).

nation producing the main products of CO and H2,28-30,32-37 and (ii) C-O bond scission resulting in CHx (x ) 0-4), OH, and H2O.24-27,29-31,33,34,36,37 Whether CH3OH decomposition occurs by O-H bond scission or by C-O bond scission is still in debate. However, the surface chemistry of CH3OH on simple metals, such as Al, is different and straightforward due to its strong affinity for oxygen. The CH3OH reaction with bulk Al surfaces, such as Al(111) and polycrystalline Al, results in the surface methoxy intermediate (CH3Oads) via O-H bond dissociation, and further decomposition of the CH3Oads produces products of CH4(g), H2(g), surface-adsorbed O and C via C-O and C-H bond scission.38-43 This paper presents a study of the surface chemistry of CH3OH on various well-defined Al/Si(111) structures. Using the molecular beam epitaxy (MBE) technique, we prepared three different Al/Si surfaces: R-7×7, γ-phase, and bulk Al films as the model catalysts. The surface atomic and electronic structures were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). Reaction of CH3OH on the Al surfaces was explored by XPS and high-resolution electron energy loss spectroscopy (HREELS). The aim is to identify the relationship between the surface chemistry of CH3OH and the structural properties of the Al surfaces. 2. Experimental All experiments were carried out in an Omicron multiprobe surface analysis system, which consists of three ultrahigh vacuum chambers: preparation, spectroscopic, and microscopic chambers. The system has been described elsewhere.44,45 XPS data were acquired with Mg KR (hν ) 1253.6 eV) radiation. The pass energy for O 1s and C 1s is 30 eV, and 50 eV for Al 2p. The spectra were calibrated with the Ag 3d5/2 peak at 368.1 eV. UP spectra were recorded at normal emission with He I radiation (hν ) 21.2 eV), and the spectra were calibrated with

10.1021/jp073607b CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

CH3OH Dissociation on Nanostructured 2D Al Layers

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Figure 1. STM images of (a) a clean Si(111)-7×7 surface (40 nm × 40 nm), (b) an ordered Al nanocluster array on Si(111) (80 nm × 80 nm), (c) a quasi-periodic Al structure on Si(111) (50 nm × 50 nm), and (d) a 16 ML Al film on Si(111) (100 nm × 100 nm).

respect to the Si(111)-7×7 Fermi level (EF). The STM measurement was carried out at room temperature (RT) with a constantcurrent mode using a homemade W-tip. In HREELS, the monochromatized electrons were incident at an angle of 55° with respect to the surface normal of the sample at primary energy (Ep) of 7.2 eV. A Si(111) sample (P-doped) was cleaned by flashing the sample to 1200 °C for several cycles. A boron nitride crucible was used to produce Al (purity 99.9999%) atomic beams. The Al flux is calibrated by STM, and the Al coverage at the formation of the perfectly ordered Al/Si R-phase is 0.24 ML.11,18,19 The Al flux is about 0.012 monolayer (ML)/ min (1 ML ) 7.8 × 1014 atoms/cm2). The methanol has been purified by several freeze-pump-thaw cycles before use. It was introduced onto sample surfaces by backfilling the chamber via a leak valve at RT. In the text, the gas exposure will be given in Langmuir (L), 1 L ) 1 × 10-6 Torr s. 3. Results and Discussion 3.1. Preparation of Different Al/Si(111) Structures. By controlling the Al coverage and substrate temperature, various well-defined Al overlayers have been grown on Si(111)-7×7 surfaces. Figure 1 shows STM images of the different surfaces, which include (a) a clean Si(111)-7×7 surface, (b) an ordered

Al nanocluster array on Si(111), (c) a quasi-periodic Al structure on Si(111), and (d) an Al film on Si(111). A perfectly ordered 2D array of identical Al nanoclusters could be obtained by depositing Al on a Si(111)-7×7 surface above 500 K. The STM image shows that all the Al clusters reside at the center of each half unit cell (HUC) of Si(111)7×7 and form a honeycomb structure (Figure 1b). This 2D Al cluster array presents the same periodicity as that of Si(111)7×7 and is also called the R-7×7-Al phase.8,9,18,19 In the text, it is referenced as the Al/Si(111) R-phase. The atomic structure of the Al cluster has been resolved by high-resolution STM combined with first-principles calculation, which indicates that each cluster contains six Al atoms and removes six dangling bonds from each HUC of Si(111)-7×7.18,19 This means that one unit cell of Si(111)-7×7 contains 12 Al atoms and the Al coverage of the Al/Si(111) R-phase should be ∼0.24 ML (12/ (7×7) ) 0.24). It should be mentioned that the preparation conditions such as the Al coverage and growth temperature are very critical for fabrication of perfectly ordered Al clusters with large areas. For example, corner Si adatoms could also be replaced by Al atoms and some x3×x3 and x7×x7 phases can form at higher temperature (∼850 K) and higher Al coverage (∼0.35 ML).8,9 Deposition of ∼0.8 ML Al onto a Si(111)-7×7 surface above 900 K results in the formation of the Al/Si(111) γ-phase. This

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Zhang et al.

Figure 2. He I (21.2 eV) valence band spectra of the different Al/Si(111) structures: (a) clean Si(111)-7×7 surface, (b) Al/Si(111) R-phase, (c) Al/Si(111) γ-phase, and (d) bulk Al film on Si(111).

2D quasi-periodic Al surface is built of triangle-shaped subunits, which have a close but not identical size (Figure 1c). The surface structure of the γ-phase has been explained to be that Al substitutes for the Si atoms in the top layer of the bulk-like Si(111) surface and the misfit between the Al-incorporated surface layer and the Si bulk leads to the formation of domain walls.6,13,46 The detailed structures of the Al/Si(111) γ-phase and domain walls were discussed earlier.8,12 A ∼16 ML Al film was deposited on the Si(111)-7×7 surface at RT. It is known that Al films can be epitaxially grown on Si(111) with the epitaxial relationships of Al(111)||Si(111) and Al(100)||Si(111).10,47,48 Our STM result shows that deposition of 16 ML of Al at RT results in the growth of big and flat Al(111) and Al(100) islands, which present the bulk-like Al surface structure and are regarded as bulk Al film (Figure 1d). The valence band structure of the different Al/Si structures was investigated by UPS, and the spectra are shown in Figure 2. For the clean Si(111)-7×7 surface (Figure 2a), there are two peaks at 0.15 and 0.85 eV below EF, which are labeled as S1 and S2, respectively. They are attributed to the surface electronic states that originated from the dangling bonds of adatoms and rest atoms on the Si(111)-7×7 surface.49,50 On the Al/Si(111) R-phase and γ-phase surfaces, the two surface states, in particular, the S1 state, diminish and the density of states (DOS) at EF also decreases largely such that an energy gap near EF can be observed (Figure 2b,c). The local electronic structure of the Al/Si(111) R-phase has been explored by Narita et al. using scanning tunneling spectroscopy and current imaging tunneling spectroscopy.11 They found that there is an energy gap for the Al clusters. An energy gap of 0.2 eV for the Al clusters at the Al/Si(111) R-phase was also confirmed by density functional theory calculations.21 In Figure 2d, the UP spectrum of the bulk Al film shows the characteristic features of metallic Al and a high Fermi edge intensity can be observed.38 The Al 2p core level spectra were recorded from the different Al/Si(111) structures, which were demonstrated in Figure 3. The binding energy (BE) of Al 2p of the bulk Al film is located at 73.0 eV, the same as the bulk Al value.51 As the coverage decreases, the BE value increases from 73.15 eV for the Al/ Si(111) γ-phase to 73.3 eV for the Al/Si(111) R-phase. According to the structural models of the Al/Si(111) R-phase18,19 and the Al/Si(111) γ-phase,8,12,13 Al atoms in the two surfaces bond with neighboring Si atoms by covalent bonds. The electronegativity of Al (1.61) is smaller than that of Si (1.90).52

Figure 3. Al 2p XP spectra of (a) Al/Si(111) R-phase, (b) Al/Si(111) γ-phase, and (c) bulk Al film on Si(111).

Thus, it is reasonable to suggest that partial electron-transfer could occur from Al to Si, resulting in positive Al 2p BE shifts for the two Al/Si(111) structures. The positive BE shifts may indicate electron deficiency of the Al atoms at the two Al/Si(111) structures. The XPS results combined with the UPS data suggest that the electron deficiency of Al atoms at the three surfaces follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. The DOS near EF in Figure 2 also confirms the results. 3.2. Methanol Adsorption on the Al/Si(111) Surfaces. Various amounts of CH3OH were exposed to the surfaces of the Al/Si(111) R-phase, the Al/Si(111) γ-phase, and the bulk Al film at RT. The surface reactivity was investigated by XPS and HREELS. Figure 4 gives the Al 2p XP spectra from the three Al/Si(111) surfaces before and after exposure to 3.5 L CH3OH at RT. The spectra after CH3OH exposure were deconvoluted into two peaks at ∼73.0 and ∼74.8 eV for comparison. It can be seen that there is significant difference in the Al 2p XPS data. For the bulk Al film surface, the Al 2p spectrum is characteristic of metallic Al and hardly changes after the CH3OH exposure. However, for the Al/Si(111) γ-phase, exposure of the surface to 3.5 L CH3OH results in the appearance of a new peak at 74.6 eV. The intensity of the new peak is about 1.5 times that of the metallic signal (calculated from areas of the two deconvoluted peaks). The change in Al 2p occurring on the surface of the Al/Si(111) R-phase is even more significant. After exposure of the surface to 3.5 L CH3OH, the Al 2p spectrum is almost dominated by the new feature at 74.9 eV, which is about 2.3 times that of the metallic signal. The Al 2p XPS results clearly indicate that CH3OH adsorption induces some changes in the chemical state of the surface Al atoms. It has been well established that dissociative adsorption of CH3OH occurred on bulk Al surfaces at RT. Generally, the reaction of CH3OH produces surface methoxy intermediates (CH3Oads) on Al surfaces. The methoxy species can further decompose, and CH4 and H2 may desorb leaving O species adsorbed on the surfaces (Oads) and maybe also C.38-43 The interaction of the Oads species with surface Al atoms could cause the new features at high BE observed in Al 2p spectra (Figure 4). On the other hand, surface-adsorbed OH species that resulted from C-O scission, if any, will react with surface Al atoms

CH3OH Dissociation on Nanostructured 2D Al Layers

Figure 4. Al 2p XP spectra from surfaces of (a) Al/Si(111) R-phase, (b) Al/Si(111) γ-phase, and (c) bulk Al film on Si(111) before and after exposure to 3.5 L CH3OH at RT.

and also produce the high BE features. The stronger peak at 74.8 eV indicates the stronger dissociative reactions of CH3OH and CH3Oads on Al surfaces. Accordingly, we can infer that CH3OH reacts more strongly on the surface of the Al/Si(111) R-phase than on the surface of the Al/Si(111) γ-phase, and least strongly on the bulk Al film. More evidence could be derived from O 1s XPS results. Figure 5 displays the O 1s XP spectra from the three Al/Si(111) surfaces exposed to 3.5 L of CH3OH at RT. All the spectra can be deconvoluted into two peaks at ∼533.2 and ∼531.7 eV. The peak at 533.2 eV may be ascribed to O in CH3Oads, and that at 531.7 eV can be attributed to O bonded to Al (AlOx). The two surface oxygen species in CH3Oads and AlOx have been also suggested in the cases of CH3OH adsorption on clean Al surfaces.40,43,53 Close investigation of the O 1s XP spectra showed that the peak area ratio of 531.7 to 533.2 eV changed drastically with the Al/Si structure. The ratios are 0.67, 0.4, and 0.2 for the Al/Si(111) R-phase, the Al/Si(111) γ-phase, and the bulk Al film, respectively. As discussed above, the surface O species that peaked at 531.7 eV in O 1s spectra result from the C-O bond dissociation in CH3OH and CH3Oads. A higher ratio of 531.7 to 533.2 eV indicates a stronger surface reaction. The Al 2p and O 1s results are consistent with each other, and both of them demonstrate that the Al surface activity to CH3OH follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. The CH3OH adsorption on the three surfaces was investigated at different CH3OH exposures. Figure 6, parts A and B, plots the O intensities at 531.7 eV (I(O531.7)) and 533.2 eV (I(O533.2)) as a function of methanol exposure. Similar to the result shown in Figure 5, the signal from O bonded to Al, I(O531.7), is strongest on the Al/Si(111) R-phase and weakest on the bulk Al film over the CH3OH exposure range. This again clearly indicates the highest activity of the Al/Si(111) R-phase even though this surface has the smallest surface Al coverage (∼0.25 ML). At

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Figure 5. O 1s XP spectra from the surfaces of (a) Al/Si(111) R-phase, (b) Al/ Si(111) γ-phase, and (c) bulk Al film on Si(111) exposed to 3.5 L CH3OH at RT.

Figure 6. Plots of O 1s intensity at 531.7 eV, I(O531.7) (A), O 1s intensity at 533.2 eV, I(O533.2) (B), and C 1s intensity (C) as a function of CH3OH exposure.

very low CH3OH exposure (below 0.5 L), the signal from O in CH3Oads, I(O533.2), on the Al/Si(111) R-phase surface is also the highest, suggesting the quick dissociation reaction of CH3OH on the surface. It almost saturates over 0.5 L. However, I(O533.2) of the Al/Si(111) γ-phase and the bulk Al film keep increasing over the exposure range. At 33.5 L, I(O533.2) values of the Al/Si(111) γ-phase and the bulk Al film are larger than that of the Al/Si(111) R-phase (Figure 6B). It can be understood that on the former two surfaces there are more surface Al atoms interacting with CH3OH, producing more CH3Oads. The plots of the C 1s peak area (Figure 6C) are more closely correlated

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Figure 7. HREELS spectra of the surfaces of (a) Al/Si(111) R-phase, (b) Al/Si(111) γ-phase, and (c) bulk Al film on Si(111) exposed to 4.1 L CH3OH at RT.

with the I(O533.2) plots (Figure 6B). The result suggests that the C signals are mainly associated with CH3Oads. Decomposition of CH3Oads produces methyl species, which may desorb from the surfaces in the form of CH4 as suggested by Tindall et al.40 or eject directly from the surface due to the strong Al-O bonding energy.53 It should be mentioned that dissociation of CH3OH on clean Si(111)-7×7 occurs at RT. CH3OH molecule dissociation necessitates the adatom-rest atom pair site to form Si-OCH3 and Si-H.54,55 In the Al/Si(111) R-phase, all rest atoms and half of the adatoms of the Si(111)-7×7 surface are saturated by Al atoms and formation of the Al/Si(111) γ-phase removes all the rest atoms and adatoms. Therefore, the direct reaction of CH3OH with Si in the Al/Si(111) structures should be excluded. Furthermore, CH3OH adsorption on the clean Si(111)7×7 surface does not produce any features lower than 532 eV in O 1s (results not shown here). The O 1s feature at 531.7 eV could only be due to CH3OH reactions with surface Al atoms. HREELS was applied to study the surface-dissociated products. Figure 7 displays HREEL spectra recorded from the three Al/Si(111) surfaces exposed to 4.1 L CH3OH at RT. On all the surfaces, three main losses at ∼610, ∼1440, and ∼2930 cm-1 have been observed, which can be assigned to ν(Al-OCH3), δ(CH3), and ν(CH3) vibrational modes, respectively.42 Meanwhile, some differences can be seen in the spectra. A feature with the characteristic loss peak of surface-adsorbed OH (OHads) at 3730 cm-1 42 can be clearly observed on the Al/Si(111) R-phase and γ-phase surfaces but not on the bulk Al film. HREEL spectra of the Al/Si(111) surfaces at various CH3OH exposures (not shown here) indicate that the feature from surface OHads appears on the two surfaces with CH3OH exposure ranging from 0.2 to 34.1 L while it does not exist even after exposing 34.1 L CH3OH to the bulk Al film. Tindall et al.40

Zhang et al. proposed that surface OHads forms on Al surfaces by reaction of the hydroxyl proton of CH3OH with surface Oads, but this happens only at large CH3OH exposure (>5 L). Therefore, we suggest that the OHads loss peaks may be from surface OHads originating from C-O bond scission in CH3OH. The pathway of C-O bond scission producing surface OHads is active only for CH3OH decomposition on the two nanostructured Al surfaces. Another difference is the broad peak located in the range of 1000-1200 cm-1, which may be deconvoluted to ν(C-O) and γ(CH3) vibrational modes peaked at 1025 and 1170 cm-1.42 For the bulk Al film, the peak at 1027 cm-1 is dominant; on the Al/Si(111) R-phase and γ-phase surfaces, the 1027 cm-1 peak is strongly attenuated and the main peak is at 1120 cm-1. That means that on the Al/Si(111) R-phase and γ-phase surfaces strong C-O bond dissociation decreases the intensity of ν(CO) relative to that of γ(CH3), which is consistent with the XPS results. 3.3. Effect of Al/Si(111) Structures on Methanol Dissociative Reaction. On the basis of the above XPS and HREELS results, we can conclude that the Al surface structure strongly influences the dissociative reactions of CH3OH. The reactivity of the Al surfaces to CH3OH follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. Moreover, the CH3OH reaction pathway varies with the Al surface structure. On the bulk Al film, the CH3OH decomposition proceeds through O-H bond scission followed by C-O bond scission, which is typical for CH3OH reactions on bulk Al surfaces.38-43 On the Al/Si(111) R-phase and γ-phase surfaces, besides the O-H bond scission mechanism C-O bond scission could take place in parallel producing surface OH species, for which the mechanism is only found be active in CH3OH reactions on transition-metal surfaces.24-27 There are some arguments about what determines the dissociative reaction of CH3OH on metal surfaces. For example, Winograd et al.25,27,31 found that the thermal decomposition pathways of CH3OH adsorbed at 110 K are strongly dependent on the initial coverage of CH3OH on Pd(111) surfaces. This is clearly not the case for our results. Schauermann et al.37 have studied CH3OH adsorption on Al2O3-supported Pd particles. Using molecular-beam and reflection-adsorption infrared spectroscopy techniques, they concluded that the step and edge sites at Pd particles are crucial for the C-O bond breakage. However, the defect sites observed in supported nanoparticles do not apply to the 2D Al model systems we used. Other experiments demonstrate that the surface chemistry of CH3OH depends on the surface structure of the metal substrate. For CH3OH adsorption on Pt(110), the rate of C-O bond scission varies very strongly with the Pt surface reconstruction. On Pt(110)-(1×1), a C-O bond breaks to yield H2O and a mixture of CHx intermediates, while no H2O or CHx species were detected upon CH3OH adsorption on Pt(110)-(1×2).26 The surface chemistry of CH3OH is found to be different on Cu(110) and Cu(111); CH3OH adsorbed on Cu(110) at 100 K can decompose to CH3O and CH2O upon annealing to 200 K,56 but the Cu(111) surface is inert to CH3OH at this temperature.57 The surface geometric structure varies with the metal-surface crystallography, which affects the reaction activity and selectivity. Similar to these cases, the three Al/Si(111) surfaces present quite different atomic structures, which may contribute to the different CH3OH dissociative reactions. In the Al/Si(111) R-phase, 6 Al atoms form an isolated cluster in which the distance between two Al atoms is around 4.6 Å.18,19,21 The distance between the neighboring Al atoms in the quasi-periodic

CH3OH Dissociation on Nanostructured 2D Al Layers γ-phase Al is 4.19 Å.12 Both of them are much larger than that in the bulk Al(111) surface (2.86 Å). Undoubtedly, the different surface Al geometric structures will influence the adsorption site and reaction path of a surface CH3OH molecule. Furthermore, the electronic structure of the Al phases could also play a critical role in the surface reactions. Chaturvedi et al.53 investigated the C-O bond strength of CH3Oads on Alterminated NiAl(100), FeAl(100), and TiAl(010) surfaces. They found that the transition metal underneath the topmost Al layer affects the surface EF position and it is the alloy EF determining the C-O bond cleavage. Koch et al.34 also found that the Rh(111)/V subsurface alloy is more reactive to CH3OH than the Rh(111) surface due to the slightly different adsorption configuration caused by the changes in the electronic structure. As we showed above, the electronic structure of the Al surfaces was studied by UPS and XPS (Figures 2 and 3). The valence band spectra suggest that the Al/Si(111) R-phase and γ-phase are nonmetallic and the bulk Al film has the bulk metallic Al character. Core-level spectra revealed that Al-Si interaction at interfaces results in the electron transfer from Al to Si and the electron deficiency in surface Al atoms follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. The reaction activity follows the same order. It is known that adsorption of alcohols on metal surfaces generally occurs via donation of a lone pair of electrons from the oxygen to the surfaces.38,58 The more semiconducting and electron-deficient characters of surface Al atoms will strengthen the Al-O bonding and, thus, favor dissociation of O-H and/or C-O bonds. Reactions of other molecules on the three Al surface phases are still in progress, and we may see a similar influence of the Al surface geometric and electronic structure on the surface reactions. 4. Conclusions Three 2D Al overlayers, the Al/Si(111) R-phase, the Al/Si(111) γ-phase, and the bulk Al film, have been grown on Si(111) surfaces. These Al/Si(111) phases present well-defined surface structures and have different electronic structures. The Al/Si(111) R-phase and γ-phase are semiconducting because of space confinement of the surface Al atoms in the nanostructured Al overlayers, while the bulk Al film shows the character of metallic Al. Al-Si interaction at the interfaces results in electron transfer from Al to Si, and electron deficiency in Al atoms at the three surfaces follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. CH3OH molecules dissociatively adsorb on the three Al/Si(111) surfaces at RT, producing CH3Oads and surface-adsorbed O species. The Al/Si(111) R-phase presents the highest activity to the dissociation of CH3OH, while that for the bulk Al film is the lowest. On the Al/Si(111) R-phase and γ-phase surfaces, besides the O-H bond scission, C-O bond scission could take place in parallel producing surface OH species whose mechanism is only found be active in CH3OH reactions on transitionmetal surfaces. The unique reaction activity and pathway on the two Al/Si(111) surfaces have been attributed to their specific surface Al geometric structures, in particular, the Al-Al distance as well as semiconducting character and the electron deficiency at the surface Al atoms. This result may give us a hint that a simple metal such as Al can catalyze a surface reaction as a transition metal if the metal structure is carefully fabricated. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 90206036,

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