Characterization of Surface Structure Evolution in Ni3Al Foil Catalysts

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Characterization of Surface Structure Evolution in Ni3Al Foil Catalysts by Hard X-ray Photoelectron Spectroscopy Ya Xu,*,† Hideki Yoshikawa,‡ Jun Hyuk Jang,†,§ Masahiko Demura,† Keisuke Kobayashi,‡ Shigenori Ueda,‡ Yoshiyuki Yamashita,‡ Dang Moon Wee,§ and Toshiyuki Hirano† Fuel Cell Materials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, NIMS Beamline Station at SPring-8, National Institute for Materials Science, Sayo, Hyogo 679-5148, Japan, and Department of Materials Science and Engineering, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea ReceiVed: December 8, 2009; ReVised Manuscript ReceiVed: February 11, 2010

We had, in a previous study, found that flat cold-rolled Ni3Al foil is spontaneously activated during the initial stage of catalytic methanol decomposition, and further, this is accompanied by the gradual formation of fine Ni particles. In this study, we investigate the evolution of the foil surface structure at the beginning of the spontaneous activation by using hard X-ray (hν ) 5.95 keV) photoelectron spectroscopy. The core level spectra of Ni 2p, Ni 3p, Al 2p, O 1s, Al 1s, and C 1s have been analyzed in detail. Ni in the Ni3Al foil remained in the metallic state during the reaction, and neither Ni oxide nor Ni hydroxide was formed. In contrast, Al was found to react with the gaseous products of methanol decomposition to form two compounds in succession. At the beginning of the reaction, Al was oxidized to form an Al2O3 layer on the surface. The outer surface of the Al2O3 layer then hydroxylated to Al(OH)3, thereby forming an Al(OH)3/Al2O3 two-layer structure. Thus, it was found that an Al(OH)3/Al2O3 two-layer structure with metallic Ni particles evolves during the initial stage of catalytic methanol decomposition. 1. Introduction The intermetallic compound Ni3Al is known to be a promising high-temperature structural material because of its excellent strength at high temperatures and corrosion/oxidation resistance.1–3 Its poor room-temperature ductility was a serious problem previously, but we have overcome this problem and successfully developed a thin foil of Ni3Al with a thickness of 23 µm by cold rolling unidirectionally solidified ingots.4–6 Recently, we found that such a flat Ni3Al foil shows high catalytic activity and selectivity for methanol decomposition into H2 and CO (CH3OH f 2H2 + CO) despite its small surface area. Unusually and interestingly, the catalytic activity increased with time during the initial stage of the reaction. This spontaneous catalytic activation was attributed to the gradual formation of fine Ni particles on the foil surface during the initial stage of the reaction.7,8 In our previous study, the formation of fine Ni particles was observed after 1 h of the reaction at 793 K by scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).7,8 Laboratory X-ray photoelectron spectroscopy (XPS) analyses revealed that the Ni-particle formation was accompanied by the formation of Al2O3 and/or Al(OH)3. This indicates that Al atoms on the surface layer of the Ni3Al foil were selectively oxidized and/or hydroxylated. Therefore, we proposed that Ni atoms remained on the foil surface as a result of the selective oxidation and/or hydroxylation of Al and aggregated into fine Ni particles. However, the surface structure evolution is still to be clarified * Corresponding author. E-mail: [email protected]. † National Institute for Materials Science, Tsukuba. ‡ National Institute for Materials Science, Sayo. § Korea Advanced Institute of Science and Technology.

in detail. In particular, the formation of the Ni particles and Al2O3 and Al(OH)3 species at the beginning of the spontaneous activation is still not understood. It is known that laboratory XPS using Al KR (1486.6 eV) or Mg KR (1253.6 eV) line as an excitation source is surface sensitive. This is because the kinetic energy of the excited photoelectrons is low as a result of the low photon energy, and hence, their inelastic mean free path (IMFP) values are very small. The probing depth is typically less than 4 nm. Therefore, elaborate surface cleaning procedures by an ion beam sputtering are essential for the analysis of subsurface structures, and this has made it difficult to analyze the buried structure nondestructively. Recently, high-resolution photoelectron spectroscopy using hard X-rays from a synchrotron radiation source (hν ≈ 6 keV) has been developed.9,10 By using such high photon energies, the probing depth is greater than 15 nm. Namely, this technique is suitable for analyzing the buried structure. In this study, we use this hard X-ray photoelectron spectroscopy (HXPS) technique and investigate the surface structure evolution during the initial 1 h of methanol decomposition at 793 K in Ni3Al foils in order to elucidate the spontaneous activation process. 2. Experimental Section A Ni3Al foil with a thickness of 30 µm was fabricated by cold rolling a single crystal plate of Ni3Al (Ni-24 at% Al). The details of the foil fabrication procedure have been described elsewhere.5,6 The methanol decomposition was performed over the cold-rolled Ni3Al foil in a conventional fixed-bed flow reactor, as described in refs 7 and 8. Before the methanol decomposition, the foil was treated at 773 K for 1 h in flowing hydrogen (30 mL (STP)/min), and subsequently, nitrogen flow

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was used to remove the hydrogen. The temperature of the reactor was then increased to 793 K, and gasified methanol was introduced at a gas hourly space velocity (GHSV) of 0.6095 m3 h-1 m-2 (defined as the volume of gasified methanol passed over the unit geometrical surface area of the foil per hour). After the reaction occurred at 793 K for 3, 20, 40, and 60 min, the foil was cooled to room temperature in nitrogen atmosphere in the reactor. Surface analysis of the foil before and after the reactions was performed at room temperature by using a HXPS (hν ) 5.95 keV) apparatus installed at the undulator beamline BL15XU of SPring-8. The experimental configuration has been reported in detail previously.9,10 The sample size for the measurement was 5 × 5 × 0.03 mm3, and the measuring area was 0.2 × 1.2 mm2. The base pressure of the chamber was maintained below 6 × 10-8 Pa during the measurements. The energy scale was calibrated by using the peak position of the Au 4f core level and the Fermi level (Ef) of an evaporated Au thin film. The total energy resolution was set to 260 meV. The core level spectra of Al 1s, Al 2p, Ni 2p, Ni 3p, O 1s, and C 1s were then measured. In order to obtain the depth profile of the surface structure, the measurements were carried out at two takeoff angles (TOA) from the surface of 85 and 40°. In order to ensure that the original surface states of each foil remain for the XPS analysis, no surface sputtering process was used before the measurement. For comparison, the measurement was also carried out for a pure Ni foil having a thickness of 50 µm (The Nilaco Co.). The obtained spectra were fitted by a XPS analysis software (UNIFIT 2007) using a Gaussian-Lorentzian peak model.11 The background was subtracted according to the Shirley method.12 3. Results 3.1. Ni 2p Core Level. Figure 1 shows the Ni 2p core level measured at TOA ) 85° for the Ni3Al foil samples before and after the reactions at 793 K for 3, 20, 40, and 60 min. For comparison, the Ni 2p core level of the pure Ni foil is also shown in Figure 1. Before the reaction, a 2p3/2 peak at a binding energy of 852.5 eV and a 2p1/2 peak at 869.8 eV were detected; they were accompanied by satellite peaks. These peaks corresponded to the metallic Ni.13 After reactions at 793 K for various periods of time, the shape and position of each main Ni 2p peak showed no obvious change, which indicates that Ni in the foil remained in the metallic state during the reaction. When compared with the Ni 2p core level of pure Ni, it can be seen that the main 2p1/2 and 2p3/2 peaks of Ni3Al show very little differences with those of pure Ni; this is consistent with the previous reports.14–17 3.2. Ni 3p and Al 2p Core Levels. Figure 2a shows the Ni 3p and Al 2p spectra obtained at TOA ) 85° for the samples before and after the reactions. Before the reactions, the Ni 3p spectra agreed well with the Ni 3p spectra of metallic Ni.18–21 After the reactions, the Ni 3p spectra showed no obvious changes in shape and position. As previously reported by us, it is difficult to detect the Ni 3p spectra after 1 h reaction by laboratory XPS.7 This implies that the metallic Ni is buried deep below the surface. Because of the large probing depth of HXPS, we can detect the Ni 3p signals. The Al 2p core level showed two chemical states before the reaction, as shown in Figure 2a. The major peak located at 72.3 eV corresponds to the binding energy of metallic Al,18 whereas the minor peak around 74.1 eV corresponds to Al oxide (probably amorphous such as AlOx).22 This indicates the presence of Al native oxide on the surface. After the reaction

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Figure 1. Ni 2p spectra of the Ni3Al foils before reaction (BR) and after exposure to methanol at 793 K for 3, 20, 40, and 60 min (TOA ) 85°); the Ni 2p spectra of the pure Ni foil is inserted for comparison.

proceeded for 3 min, the intensity of the metallic Al peak decreased, and a new peak appeared at around 75 eV instead. This new peak can be fitted by two components at 75.4 and 74.1 eV. The first component corresponds to Al2O3 according to the previous reports.13,22–24 The second component corresponds to AlOx. This means that the Al in the surface layer was mostly oxidized to Al2O3 in the first 3 min of the reaction. After the reaction proceeded for 20 min, the Al 2p spectrum significantly increased in intensity. This is due to the increase in the amount of Al2O3 and, in addition, to the new formation of Al(OH)3. In fact, the Al 2p peak at 75.4 eV represents a mixture of the components of Al2O3 and Al(OH)3. Because the binding energies of Al 2p for Al(OH)3 and Al2O3 are close to each other,13 it is difficult to separate both components in the Al 2p spectra. A clear separation of the chemical states of Al is performed by using the intense O 1s and Al 1s spectra, and it is described in the following section. After 40 min, the intensity of Al2O3 and Al(OH)3 decreased. Both the Ni 3p and Al 2p spectra were also measured at TOA ) 40°, as shown in Figure 2b. The spectra showed the same binding energies and spectral shapes as those at TOA ) 85°. The spectral intensity ratio of Al 2p relative to Ni 3p at TOA ) 40° is higher than that at TOA ) 85°. From the obtained spectra, we will discuss the evolution of Al2O3 and Al(OH)3 with the reaction time in the following section. 3.3. O 1s Core Level. Figure 3 shows the O 1s core level measured at TOA ) 85° before and after the reactions for 3, 20, 40, and 60 min. Before the reaction, the O 1s spectrum was broad and fitted by three components at 530.3, 532.5, and 533.4 eV with a full width at half maxima (fwhm) of 1.7 eV. The first component corresponds to AlOx, whereas the other two probably correspond to the adsorbed oxygen atoms in a hydrated

Surface Structure Evolution in Ni3Al Foil Catalysts by HXPS

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Figure 2. Al 2p and Ni 3p spectra of the Ni3Al foils before and after exposure to methanol at 793 K for 3, 20, 40, and 60 min. (a) TOA ) 85°; (b) TOA ) 40°.

Figure 3. O 1s spectra of the Ni3Al foils before and after exposure to methanol at 793 K for 3, 20, 40, and 60 min (TOA ) 85°).

form (OsH) and a carbon-oxygen bond (OsC), respectively. After the reaction proceeded for 3 min, the O 1s spectral width became narrower. A main component was located at 531.8 eV

with an fwhm of 1.7 eV, whereas three minor components were located at 530.3, 532.6, and 533.6 eV with the same fwhm of 1.7 eV. The major component at 531.8 eV corresponds to Al2O3, which indicates that Al2O3 was formed during the reaction. The first minor one at 530.3 eV corresponds to the pre-existent AlOx, whereas the second minor one at 532.6 eV corresponds to Al(OH)3.24–28 The third minor component at 533.6 eV, which was very weak during the reaction, probably corresponds to the adsorbed oxygen atoms. All four components, Al2O3, Al(OH)3, AlOx, and adsorbed oxygen, were detected after the reaction proceeded for more than 20 min. It must be noted that the intensity of the Al(OH)3 component increased with time, whereas that of the Al2O3 component decreased. In other words, Al oxide (Al2O3) formed first on the surface (within the first 3 min of the reaction), whereas Al hydroxide (Al(OH)3) formed on Al2O3 subsequently. 3.4. Al 1s Core Level. The intensity of the Al 1s spectra is considerably stronger than that of the Al 2p spectra at the photon energy of 5.95 keV because the atomic photoionization crosssection of the Al 1s orbital is considerably larger than that of the Al 2p orbital.29 By measuring the Al 1s spectra, it is possible to distinguish Al2O3 from Al(OH)3 in the surface layer. Figure 4a,b shows the Al 1s spectra at TOA ) 85 and 40°, respectively, before and after the reaction at 793 K for 3, 20, 40, and 60 min. Before the reaction, the Al 1s spectra showed two peaks: the major peak at the binding energy of 1558.9 eV can be ascribed to the metallic Al, whereas the smaller peak at 1560.9 eV can be ascribed to the native oxide (AlOx). After 3 min of the reaction, the relative intensity of metallic Al peak (Almet.) significantly decreased, and a strong peak appeared at a binding energy higher than that of the AlOx peak. This strong peak was well fitted by a major component at 1561.9 eV and two minor components at 1560.9 and 1562.6 eV (each with an fwhm of 1.8 eV). The major component was ascribed to Al2O3, whereas the minor ones at 1560.9 and 1562.6 eV were ascribed to AlOx and Al(OH)3, respectively, from the analyses

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Figure 4. Al 1s spectra of the Ni3Al foils before and after exposure to methanol at 793 K for 3, 20, 40, and 60 min. (a) TOA ) 85°; (b) TOA ) 40°.

of Al 2p and O 1s spectra (Sections 3.2 and 3.3). It must be noted that the relative intensity of the Al(OH)3 component increased with time, which indicates that the amount of Al(OH)3 increased during the reaction. We will discuss the evolution of each component along the depth direction with reaction time in Section 4. 3.5. C 1s Core Level. It has been previously shown that carbon was deposited in a fibrous form on the Ni3Al foil after the reaction for 1 h.7,8 Carbon fibers can serve to support active Ni particles. In this study, the behavior of carbon deposition at the initial stage of the reaction was investigated. Figure 5 shows the C 1s spectra (TOA ) 85°) of the samples before and after the reaction. Before the reaction, a broad peak of the C 1s spectrum was detected at 285.8 eV along with a long tail at the higher-binding-energy side. This peak originated in the CsH and CsO bonds. This result indicates the presence of carbon contamination on the surface.30 After the reaction proceeded for 3 min, a peak appeared at 284.5 eV; this is considerably stronger than that due to carbon contamination. The intensity of this peak increased with time, although the shape and position showed no obvious change. This peak is ascribed to graphite because it is accompanied by a broad peak at a binding energy higher than 6.5 eV, as shown in the inset in Figure 5. The broad peak was confirmed as a π-π* shakeup satellite peak of the 1s of the graphite.13 Thus, it became clear that the formation of graphite had already begun in the first 3 min of the reaction, and it continued during the reaction. 4. Discussion The above results show that no Ni oxide and hydroxide formed on the Ni3Al foil during the first 1 h of the reaction at 793 K. In contrast, Al was oxidized to Al2O3 immediately after the reaction started (within the first 3 min), and Al(OH)3 formed as the reaction progressed. The evolution of the surface structure during the initial period is discussed below. The spectral intensity ratio of Al 2p to Ni 3p shown in Figure 2 can be accessed by a curve fitting method. It is possible to

determine the position of Al2O3(Al(OH)3), metallic Al and Ni components along the depth direction, by analyzing the change of the intensity ratios. Because the components of Al2O3 and Al(OH)3 were not separable in this case, both were treated together as Al2O3(Al(OH)3). Figure 6a shows the intensity ratio of the Al2O3(Al(OH)3) component in the Al 2p (IAl2O3(Al(OH)3)) to the Ni 3p spectra (INi) as a function of the time on stream. During the first 20 min, IAl2O3(Al(OH)3)/INi at both TOA ) 40 and 85° increased with time, which suggests an increase in the amount of Al2O3 and Al(OH)3) species. Conversely, with further increase in time, IAl2O3(Al(OH)3)/INi decreases, which indicates an increase in the relative intensity of the Ni 3p peak. This can be ascribed to the formation of metallic Ni and carbon deposition on the foil surface for the following reasons. In our previous researches, it was confirmed that the metallic Ni particles were formed accompanied by the formation of Al oxide and hydroxide.7,8 At the beginning of the reaction, these Ni particles are supposed to be buried below Al2O3 and Al(OH)3, which results in a increasing trend of IAl2O3(Al(OH)3)/INi during the first 20 min, as shown in Figure 6. The Al2O3 and Al(OH)3 layers are supposed to be not dense and do not prevent CH3OH and CO molecules from penetrating through them to Ni particles. As shown in Section 3.5, the formation of graphite begun in the first 3 min of the reaction. It is considered that the deposited graphite accumulated near the Ni particles during the first 60 min of the reaction because the Ni particles acted as the catalyst for the carbon deposition (2CO f C + CO2) as well as the methanol decomposition, as described in ref 8. The accumulated graphite may decrease the intensity of Ni 3p signal, which also contribute to the increase of IAl2O3(Al(OH)3)/INi during the first 20 min. As the reaction progressed, the amount of deposited graphite increased, as described in Section 3.5. The accumulated graphite lifted the Ni particles up to the surface, which may affect the relative intensities of Ni 3p and Al 2p of the Al2O3(Al(OH)3) component, resulting in the decrease of IAl2O3(Al(OH)3)/INi after 20 min. In addition, from Figure 6a, it can be seen that IAl2O3(Al(OH)3)/INi at TOA ) 40° is larger than that at



Figure 5. C 1s spectra of the Ni3Al foils before and after exposure to methanol at 793 K for 3, 20, 40, and 60 min (TOA ) 85°). The inset shows the magnification of the π-π* shakeup satellite peak of graphite.

TOA ) 85° during the reaction. This result indicates that Al2O3(Al(OH)3) was formed on the foil surface. Figure 6b shows the intensity ratio (IAl/INi) of the metallic Al component in Al 2p to Ni 3p as a function of the time on stream. Before the reaction, IAl/INi at TOA ) 85° had few differences with that at TOA ) 40°. This result indicates that there is no Al enrichment near the foil surface before the reaction. However, after the reaction, IAl/INi at TOA ) 40° was higher than that at TOA ) 85°, which suggests that metallic Al might segregate to form an Al-enriched layer on the outmost surface of Ni3Al foil during the reaction. It is noted that the IAl/INi at TOA ) 40° increased more than that at TOA ) 85° with the progress of the reaction. We suggest the following possible reasons for this behavior. The Al segregation to the surface of Ni3Al foil probably increased with the progress of the reaction, resulting in an increase of IAl. Furthermore, the contribution to Ni 3p signal from the Ni3Al foil decreased more at TOA ) 40° than that at TOA ) 85° with the increase of the surface products (carbon, Al2O3, and Al(OH)3), resulting in a strong decrease of INi after 40 or 60 min at TOA ) 40°. Some researches have reported that annealing Ni3Al above 600 K at a low partial pressure of oxygen resulted in Al segregation close to the surface of Ni3Al, thereby forming an Al-enriched interfacial layer between the Al oxide film and Ni3Al substrate.23–25 This segregated metallic Al layer was reported in the case of one or two Al atomic thickness. It was considered that the formation of Al2O3 and Al(OH)3 layers might protect such an Al-enriched interfacial metallic layer on Ni3Al from internal oxidation.

Figure 6. (a) Intensity ratio of Al2O3(Al(OH)3) component in Al 2p (IAl2O3(Al(OH)3)) to Ni 3p (INi) as a function of time on stream; (b) intensity ratio of metallic Al component in Al 2p (IAl) to Ni 3p (INi) as a function of time on stream.

Figure 7. Intensity ratio of the Al2O3 component (IAl2O3) to the total Al 1s spectra (Itot.) and intensity ratio of the Al(OH)3 component (IAl(OH)3) to Itot. as a function of time on stream.

Because the Al2O3 and Al(OH)3 species were distinguished from each other on the Al 1s spectra (Figure 4), we now determine their position along the depth direction. Figure 7 shows the intensity ratios of each component (IAl2O3, IAl(OH)3) to the total Al 1s spectra (Itot.) as a function of the time on stream. After 3 min of the reaction, IAl2O3/Itot. was considerably higher than IAl(OH)3/Itot., which clearly shows that Al2O3 was mainly

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Figure 8. Schematic representation of the surface structure evolution of Ni3Al foil at the initial stage of methanol decomposition.

formed at the beginning of the reaction. In addition, as seen in Figure 7, a small amount of Al(OH)3 was formed after 3 min. The formed Al(OH)3 must be present on the surface of Al2O3, considering that the contribution of the Al(OH)3 component to the Al 1s spectra (IAl(OH)3/Itot.) at TOA ) 40° is larger than that at TOA ) 85°. After 20 min, IAl2O3/Itot. decreased, whereas IAl(OH)3/Itot. increased. This indicates that the Al(OH)3 component increased during the reaction. It is seen that both IAl2O3/Itot. and IAl(OH)3/Itot. show little difference between TOA ) 85° and TOA ) 40° after 20 min of reaction. This means that the Al(OH)3 and Al2O3 species did not separate into two layers along the depth direction. As the reaction progressed further, IAl(OH)3/Itot. at TOA ) 40° became larger than at TOA ) 85°, whereas IAl2O3/Itot. at TOA ) 40° became smaller than that at TOA ) 85°. These results strongly suggest that Al(OH)3 becomes a surface species compared to Al2O3 as the reaction progressed. On the basis of all the above analyses, a model is schematically drawn in Figure 8 to show the evolution of the Ni3Al foil surface during the reaction. Before the reaction, a thin layer of native Al oxide (AlOx) with a small amount of carbon contamination existed on the surface of the cold rolled Ni3Al foil (Figures 3-5). The surface structure before the reaction is illustrated in Figure 8a. After 3 min of the reaction, Al2O3 was formed, and this was accompanied by the formation of Ni particles and carbon by the following reaction (eq 1).

2Ni3Al + 3H2O S 6Ni + Al2O3 + 3H2

thicknesses of the surface layers (Å) Ni3Al Al2O3 Al(OH)3

IMFPs (Å)

before reaction

after 3 min

after 1 h

54 82 101

13

94 9

129 45

The thicknesses of the native Al oxide and formed Al2O3 and Al(OH)3 layers were estimated from the intensity ratio between the intensities of the oxide and hydroxide Al 1s components and the values of the IMFPs of photoelectrons in Al(OH)3, Al2O3, and Ni3Al, based on a uniform two-layer model.28,31,32 In this study, we have neglected the effect of deposited carbon, produced Ni particles, and Al segregation. The values of the IMFPs of photoelectrons obtained from the TPP-2M equation33,34 have been estimated and summarized in Table 1. On the basis of the above calculations, the thickness of the surface layers was estimated, as summarized in Table 1 and Figure 8. Before the reaction, the thickness of the native oxide layer was estimated to be 13 Å. Here, the value of the IMFP of Al2O3 was used to approximate the value of the native Al oxide. After 3 min of the reaction, the thickness of Al2O3 layer was estimated to be 94 Å, and that of Al(OH)3 was estimated to be 9 Å. After 1 h of the reaction, the thickness of the Al2O3 layer was estimated to be 129 Å, and that of Al(OH)3 was estimated to be 45 Å.

(1) 5. Conclusions

In our previous paper, we found that a small amount of H2O was produced at the beginning of methanol decomposition.8 It is considered that the produced H2O oxidized Al in Ni3Al, in accordance with eq 1, leading to the formation of Ni particles. The surface structure after 3 min is illustrated in Figure 8b. In addition, as shown in Figure 7, a small amount of Al(OH)3 species was confirmed as a surface species formed on Al2O3. The Al(OH)3 species is considered to be formed when the foil is exposed to ambient conditions after the reaction. After 20 min of the reaction, the amount of Al(OH)3 species increased and coexisted with Al2O3 species. Al(OH)3 was considered to be formed in accordance with the following reaction (eq 2).

Al2O3 + 3H2O S 2Al(OH)3

TABLE 1: Calculated IMFPs in Ni3Al, Al2O3, and Al(OH)3 by Using the TPP-2M Equation and Estimated Thicknesses of Al2O3 and Al(OH)3 Layers During the Reaction

(2)

It is considered that the Al(OH)3 species was preferentially formed near the Ni particles because H2O was produced on the surface of the Ni particles. The surface structure after 20 min is schematically shown in Figure 8c. As the reaction progressed further, the amount of Al(OH)3 species increased, and a surface layer was formed after 1 h, as shown in Figure 8d.

The evolution of the surface structure in the cold rolled Ni3Al foil during the initial 1 h of methanol decomposition at 793 K was investigated by using HXPS. The results obtained are summarized as follows. The spectra of Ni 2p, Ni 3p, and O 1s show that Ni remained in the metallic state during the reaction. The C 1s spectra show the formation of graphite at the beginning of the reaction. It is believed that the deposited graphite lifts the metallic Ni to the outermost surface during the reaction. The spectra of Al 1s, Al 2p, and O 1s show that Al in the surface layer of the Ni3Al foil was oxidized to form an Al2O3 layer in the first 3 min of the reaction. As the reaction progressed, Al2O3 was hydroxylated to Al(OH)3, and the amount of Al(OH)3 significantly increased during the reaction. This resulted in an Al(OH)3/Al2O3 two-layer structure, in which Al(OH)3 was a surface species and Al2O3 was a subsurface species. The thickness of Al2O3 and Al(OH)3 layers during the reaction was estimated. After 3 min of the reaction, the Al2O3 layer was approximately 92 Å. After 1 h, the thicknesses of Al2O3 and Al(OH)3 layers were 129 and 45 Å, respectively. Acknowledgment. The authors would like to thank D. Nomoto (SPring-8 Service Co. Ltd.) for his assistance on HXPS



measurement. The authors are grateful to HiSOR, Hiroshima University and JAEA/SPring-8 for the development of HXPS at BL15XU of SPring-8. The HXPS measurements were performed under the approval of NIMS Beamline Station (Proposals no. 2006B4602 and 2007A4600). This work was partially supported by two Grant in Aids for Scientific Research (C-no.19560774 and B-no.19360321) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT).

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