Adsorption and Decomposition of Methanol on NiAl (110)

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J. Phys. Chem. 1994,98, 10258-10268

10258

Adsorption and Decomposition of Methanol on NiAI(110) Bor-Ru Sheu, S. Chaturvedi, and D. R. Strongin* Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794 Received: June 4, 1994@ Methanol adsorption and reaction on NiAl( 110) have been investigated with temperature-programmed desorption, X-ray and ultraviolet photoelectron spectroscopy, and high-resolution electron energy loss spectroscopy. Methanol chemisorbs associatively (at least at relatively high coverage) on NiAl(110) at 120 K and transforms into surface methoxy by 200 K. Decomposition of the methoxy overlayer leads to the evolution of gaseous Hz, CO, C h , and CH3 radicals, in addition to the deposition of surface oxygen and carbonaceous species. The formation of CzH4 is also observed after a high coverage of methoxy decomposes on NiAl(110). Methyl radical desorption occurs near 570 K and C h desorbs near 350 K from NiAl(110). The low-temperature CH4 desorption peak indicates that the C - 0 bond of a fraction of surface methoxy is cleaved below 350 K on NiA1( 1lo), in contrast to monometallic A1 and Ni, emphasizing the unique reactivity of this alloy surface.

Introduction The unique chemical properties of alloy surfaces are of fundamental importance in many technologically important areas, which include catalysis, microelectronics fabrication, electrochemistry, corrosion passivation, and materials One such alloy is NiAl, which exhibits weight and strength characteristics that make this material a candidate for many structural application^.^ In addition to being of technological importance, the NiAl alloy offers electronic and structural properties that are interesting on a fundamental level. With regard to structural properties, the NiAl alloy (CsC1 structure) shows strong ordering tendencies, exemplified by the fact that the low-index planes of this alloy exhibit surfaces that have a geometric structure and stoichiometry similar to those of the bulk. For example, the NiAI(110) surface, which is used in our research, has been shown in prior research to have a surface composition (50% Al) and structure that are similar to those of the bulk, except that A1 atoms in the surface layer are displaced toward the vacuum, relative to the Ni With respect to the electronic structure of NiAl, both experiment and theory suggest that the hybridization between Ni and A1 electronic levels results in a filled alloy d The filled d band might imply that the surface reactivity of NiAl would be similar, for example, to that of Cu. This correlation between electronic structure and surface reactivity may be consistent with the experimental observation that the desorption temperature of CO is lower on NiAl( 110) than on monometallic Ni, but it is similar to the noble metal C U . ~ ~On , ' the ~ NiAl(1 11) surface, however, a fraction of CO desorbs at a temperature that is similar to monometallic Ni, while the remainder of the CO undergoes dissociation (in contrast to Ni)." Dissociation of CO also occurs on the (100) plane of NiA1,12 and all these results taken together suggest that the reactivity of this alloy is a strong function of the atomic composition and structure of the surface. More studies, however, are needed to develop a better understanding of the apparently diverse surface reactivity of NiAl and of how this reactivity is related to the atomic composition (and electronic structure thereof) of the alloy surface. Research presented in this paper addresses the surface reactivity of NiAl further, by investigating the chemisorption

* To whom correspondence should be addressed. @

Abstract published in Advance ACS Abstracts, September 1, 1994.

0022-365419412098- 10258$04.50/0

and reaction of CH30H on NiAl(110). The reasons for choosing CH30H as the reactant in this study are at least two-fold. First, the CH30H molecule should be a useful probe for characterizing the alloy surface reactivity, since its decomposition reactions and the type of gaseous products evolved during these reactions are very sensitive to the nature of the substrate surface.13 For example, on All4-'' and single crystal planes CH30H transforms into a surface methoxy species, CH30(ad), and on the sp-metal A1 this species undergoes CO bond cleavage, ultimately leading to the formation of gaseous CHq near 450 K.14-17 In contrast, the thermal chemistry of CH30(ad) on Ni does not include C - 0 bond cleavage but instead involves dehydrogenation reactions at -260 K19-24that lead to the formation of adsorbed CO product, which then desorbs near 400 K. It is expected that this sensitivity of the thermal chemistry of CH30H to the nature of the substrate (Le., sp versus transition metal) will help to characterize the chemical reactivity of NiAl( 110) in terms of its surface composition and electronic structure. Also, results mentioned in the preceding paragraph suggest that the surface reactivity of NiAl( 110) for CO appears to be similar to that of Cu. We feel that it is of interest then, to determine whether NiAl( 110) exhibits a similar noble-metallike surface reactivity toward an adsorbate, such as CH30H, that will presumably interact strongly with the sp band of the alloy (given that monometallic A1 is highly reactive in chemisorbing and decomposing CH30H). Second, on a more applied level, research of the surface reactivity of NiAl may have relevance to the understanding of the surface reactivity of catalysts consisting of Ni supported on aluminum oxide, since it has been suggested that under catalytic conditions Ni-A1 intermetallic and Ni-AI-0 phases may exist.36 With this possibility in mind our results for the chemisorption and reaction of the catalytically important CH3OH molecule on NiAI are compared to relevant results obtained previously by other researchers for the Ni/Al203 catalyst at the later part of this paper. We show in this research with X-ray and ultraviolet photoelectron spectroscopy ( X P S and UPS) and high-resolution electron energy loss spectroscopy (EELS) that CH30H associatively adsorbs on NiA1( 110) at 120 K and transforms into surface methoxy upon heating to 200 K. Temperatureprogrammed desorption (TPD) studies show that gaseous H2, CO, CHq, CH3 radical, and C 2 b are evolved from NiAl(110) during CH30H decomposition, in addition to the deposition of 0 1994 American Chemical Society

J. Phys. Chem., Vol. 98, No. 40, 1994 10259

Adsorption and Decomposition of Methanol on NiAl( 110) extensive amounts of surface oxygen and carbon. The products that desorb from NiAl are, for the most part, a combination of those species that desorb from monometallic Ni (CO and H2) and A1 (CH4 and H2) during methanol decomposition. The details of the thermal chemistry of methanol on NiA1(110), however, are markedly different than on either Ni or Al. Methoxy undergoes C-0 bond scission at temperatures lower than pure A1 (-350 K compared to -450 K), and this results in part to C&(g) production with a peak maximum temperature (Tp) of 350 K. Also, gaseous CO (Tp 440 K) appears to be a reaction limited product on NiA1( 1lo), in contrast to pure Ni where methoxy decomposition results in an adsorbed CO species.

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Experimental Section Experiments were performed in a two-level stainless steel ultrahigh-vacuum (UHV) chamber with a base pressure of 2 x Torr. The chamber was equipped with a quadrupole mass spectrometer (QMS), low-energy electron diffraction (LEED) optics, double-pass cylindrical mirror analyzer (CMA), X-ray source, differentially pumped ultraviolet source, and an ion sputtering gun. The lower level housed a EEL spectrometer with a single-pass electron monochromator and analyzer. The QMS, CMA, and EEL spectrometer were controlled with computer-automated measurement and control (CAMAC) modules interfaced to an IBM-386 computer. The stoichiometric NiAl( 110) crystal (8 mm x 8 mm, 2 mm thick) used in this research was spark-cut from a singlecrystalline ingot, and it was subsequently polished by standard procedures. Three Ta support wires (0.025 cm diameter) were spot-welded to each side of the sample, and the ends of the support wires were spot-welded to Ta tabs that were mechanically attached to a liquid nitrogen cryostat with cooling capabilities to 120 K. Only carbon and oxygen contamination were detected by AES and XPS when initially introduced into the UHV chamber. Cleaning of NiAl(110) was accomplished by repeated cycles of 500 eV argon ion sputter and anneal (950 "C for 20 min) cycles. Trace amounts of carbon was removed by heating NiAl( 110) Crystal in 1 x Torr of oxygen at 1000 K. A sharp, rectangular (1 x 1) LEED pattern was obtained for NiA1( 110) after this cleaning procedure. Methanol (CH30H, HPLC grade, Fisherchemical; CH30D, 299.5 atom % D, Sigma Chemical Co.; CD30D, 299.8 atom % D, Cambridge Isotope Laboratories; 13CH30H, '99 atom %; Sigma Chemical Co.) was purified by numerous freezepump-thaw cycles. Methanol was admitted into the UHV chamber through a 0.05 cm diameter dosing tube connected to a leak valve. During dosing, the opening of the tube was typically 5-8 mm away from the front face of NiAl( 110) crystal. The methanol exposures quoted in this paper are in langmuirs (1 langmuir = Torr s), and they are corrected for the lineof-sight dosing configuration and for the sensitivity of the ionization gauge (used for pressure measurements) for CH3OH. The heating rate for TPD experiments was 7 f 1 IUS.The QMS was multiplexed so that nine ions could be monitored during TPD. All the yields of products quoted in this paper have been corrected for the sensitivity of the mass spectrometer. The mass spectrometer was housed in a gold-plated stainlesssteel enclosure with a 5 mm diameter aperture, which was designed to limit the detection of gases evolving from the sample holder and support wire. A chromel-alumel (type K) thermocouple wire was spot-welded to the top fo the NiA1( 110) sample for accurate temperature measurements. Mg K a (1253.6 eV) and He I (21.2 eV) radiation were used in XPS and UPS experiments, respectively. The pass energy

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multilayer 3

4

5

6

7

Methonol E x p o s u r e (L)

Figure 1. Plot of the work function change (A$), as a function of metahnol exposure at a crystal temperature of 120 K. The work function drops sharply up to an exposure of 2.4 langmuirs and then shows only a small change at higher exposures. These data are taken to infer that exposures less than 2.4 langmuirs result in a submonolayer coverage of adsorbate and higher exposures lead to the formation of CH30H multilayers. The inset to the figure shows that CH3OH (mle = 32) desorption from a condensed layer (peak at -150 K) occurs after methanol exposures greater than 2.4 langmuirs consistent with the Ab data.

of photoelectrons through the CMA was set at 50 eV for X P S and 5 eV for the UPS measurements. The binding energy scale for XPS was calibrated by aligning the 2~312level of a pure Fe sample to 706.8 eV below the Fermi level (EF).Work function changes (A4) reported in this paper were obtained by analyzing the change in the secondary electron cutoff during He I UPS measurements, as a function of methanol exposure at 120 K. The sample was typically biased 3 V above ground potential during these experiments. The work function measurements have an error of f 0 . 3 eV. EELS spectra were acquired at a specular detection angle (t9i = Of = 60"). An incident beam energy of 3.5 eV was used in all the EELS experiments. A typical counting rate of lo4 Hz and a resolution of 96 cm-* was measured for the elastically scattered electrons from NiAl( 110). The resolution and counting rate of the primary beam varied from 100 to 140 cm-I and from 5 x lo2 to 5 x lo3 Hz, respectively, after exposure of NiAl(110) to methanol. There is an uncertainty of f 1 0 cm-l in all the vibrational frequencies reported in this paper.

Results and Interpretation Methanol Coverage As Estimated from Work Function Measurements. As mentioned in the preceding section, CH3OH exposures are expressed in langmuirs. It is important to relate this quantity to the resulting adsorbate coverage on the NiAl(110) crystal, since this relationship is needed for the interpretation of many of our TPD and electron spectorscopic results presented later. An estimate of the adsorbate coverage, as a function of methanol exposure, can be obtained by examining Figure 1, which exhibits a plot of the work function change (A4 = @dosed - &lean) of the sample (held at 120 K) as a function of methanol exp~sure.~'These data show that A 4 decreases monotonically to a plateau value of -1.6 eV, at a CH30H exposure close to 2.4 langmuirs. Electron spectroscopy results presented later show that CH30H adsorbs associatively on NiAl( 110) at 120 K, but regardless of the exact nature of the adsorbate at this point in the paper, these data suggest that an exposure of about 2.4 langmuirs leads to a saturation layer

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10260 J. Phys. Chem., Vol. 98, No. 40, 1994

3,

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Figure 3. (a) Desorption traces for mle = 18, 19, and 20 ions for CD30D/NiAl(llO) TF’D. These mle values are attributed to the CD3+, CD3H+, and CD4+ ions. The desorption feature near 400 K is due to CDI product that is evolved from the NiAl(Il0) surface. Desorption of CD3 radical is responsible for the 570 K feature in the mle = 18

570K

( d i f f . spectrum

CH,OH

32

100 200 300 400 500 600 700 BOO 900 100011001200

Temperature ( K )

Figure 2. TPD spectra of products desorbing from NiA1( 110) after a 9 langmuir exposure of CH3OH. The daughter ion, CH3+,contribution of CHq has been removed from the CH3 (mle = 15) spectrum, so that this deconvoluated peak at 570 K is due only to the desorption of methyl radicals from the NiAI(110) surface. Note that mle = 27 corresponds to the daughter ion, CzH3+, of C2Hq product. The parent ion, Cz&+, is overwhelmed by the mle = 28 intensity due to CO product.

of adsorbate on NiAl( 110). It is noted that a work function change of -1.6 eV is in good agreement with previous research that has obtained similar values for various transition metals (e.g, R u , Cu,39340 ~~ and Pd41.42)and A143with a saturation layer of adsorbed methanol. The relative insensitivity of the work function change to higher CH30H exposures (>2.4 langmuirs) is attributed to the formation of a methanol condensed layer. TPD spectra shown in the inset to the figure are consistent with the conclusions arrived at from the work function data, since the desorption traces show that no methanol is evolved from the surface after a 2.4 langmuir exposure, but methanol desorption from a condensed layer (the peak at -150 K grows indefinitely with increasing exposure) is observed after a 3.6 langmuir exposure. TPD Studies. Figure 2 displays TPD spectra of all the species that desorb from NiAl( 110) after exposure to 9 langmuirs of CH30H. Note that the mle = 27 spectrum corresponds to the daughter ion (CzH3+) of CzHq product.@ The difference spectrum shown in the figure is obtained by removing the C& daughter ion contribution to a mle = 15 TPD trace, and the peak in this spectrum at 570 K is attributed to the desorption of methyl radicals. In support of this assignment, two additional pieces of information are offered. First, the ratio of the mle = 14-15 signal (after removal of the CHq daughter ion contributions) is 0.46, which falls in the range of literature values (0.29,45 0.42,460.54,47 and 0.6548) for the cracking pattern of the CH3 radical. In contrast, the corresponding ratio for the mle = 14 and 15 daughter ions of C& has been determined to be 0.17 for our mass spectrometer settings. Second, we examine TPD spectra obtained from CD30D adsorbed on NiAl( 110) that are presented in Figure 3. The mle = 18, 19, and 20 desorption features above 300 K49 in the figure

spectrum. A significant fraction of the methyl radicals that desorb from NiAl(110) undergo a hydrogen abstraction reaction with the walls of the vacuum chamber to yield CD3H (mle = 19) prior to detection. are attributed to the CD3+, CD3H+, and CD4+, ions, respectively. Perhaps, the most noticeable aspect of the mle = 20 TPD trace is that there is no CD4 desorption near 570 K. Therefore, the peak centered at 570 K in the mle = 18 spectrum cannot be due to the cracking of CD4 product, and instead the bulk of the mle = 18 intensity is attributed to the presence of gaseous CD3 radicals. Furthermore, the 570 K peak in the mle = 19 spectrum is thought to be due to the presence of CD3H, formed by the interaction of methyl radicals with the walls of the vacuum chamber (Le., CD3 H(ad) CD3H) before detection by the mass s p e c t r ~ m e t e r . On ~ ~ ~the ~ ~basis of this premise, it is suspected that the 570 K peak in the mle = 16 spectra of Figure 2 is due to a similar type of wall reaction, which converts CH3 into CH4 (Le., CH3 H(ad) CHq), and therefore it is concluded that only methyl radicals desorb from NiAl( 110) near 570 K, during the decomposition reactions of CH30H. While the mle = 20 trace, presented in Figure 3, shows no product desorption near 570 K, it does show that CD4 desorbs from NiAl( 110) near 400 K. We infer from this result that the formation of CHq between 300 and 400 K, which is observed to occur during CH30H TPD (see Figure 2), is a process that is taking place on the NiA1( 110) crystal (i.e., this is not a “wall” reaction), during the decomposition reactions of CH30H. An explanation for the differences in the Tp of CHq and CD4, however, cannot be ascertained from the data presented in this paper. Yields of the products that desorb from NiAl( 110) vary with coverage, and this dependence is summarized in Table 1 (uncertainty in these values is about &lo%). The methyl radical yields, which are exhibited in the table, are viewed as lower limits, since it is likely that some of the CH3 is converted to chemical forms other than CH4 that go undetected in our experiment. Figure 4 graphically illustrates the product yields of the carbon-containing reaction species, as a function of CH3OH exposure. At least four general comments can be made about the trends in the coverage dependence for these products. First, at low exposure CO is the primary gaseous product that desorbs from NiAl( 110). Second, the yield of CH3 is insignificant at the lowest CH30H exposure, but the yield of this product rises rapidly with increasing CH30H exposure. At the highest exposures the yield of CH3 becomes greater than the

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Adsorption and Decomposition of Methanol on NiAl( 110)

J. Phys. Chem., Vol. 98, No. 40, 1994 10261

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traces for CD30D/NiAl(110) and CH30H/NiAl(110). No significant kinetic isotope effect is observed for CO production. CO yield. Third, the yield of CHq at 350 K (Tp is invariant with coverage) is less than the yield of CH3 product at all CH3OH exposures. Finally, C2H4 product (TP = 490 K at all CH3OH exposures) is observed, but only after the highest CH30H exposures, and even then, this product accounts for only about 1-3% of the total carbon-containing product. Nevertheless, the presence of C2H4 indicates that some C-C bond formation does occur on the NiAI( 110) surface. Some further information about the evolution of these products can be inferred from closer inspection of their desorption traces and isotopic labeling experiments, and this is discussed below. Figure 5a displays TPD spectra of CO from NiA1( 110) as a function of CH30H exposure. At the lowest CH30H exposure the peak temperature of CO desorption is -425 K. With increasing exposure the Tp progressively shifts to higher temperature, and at the highest exposure of 9 langmuirs the maximum rate of CO desorption occurs at 447 K. Figure 5b

and C& product for CH3OD/NiAl(110). The desorption of CH3D shows that a fraction of the deuterium (originally in the hydroxyl group of methanol-dl) is incorporated into the methane product at 350 K. shows CO TPD spectra for NiAl( 110) after exposure to 3.6 langmuirs of CH30H and 4 langmuirs of CD30D. The similarity of the peak maxima of CO desorption for both spectra suggest that there is little, if any, isotope effect in the CO production step. Previous research has shown that CO desorbs from clean NiAl(110) in the temperature range of 290-310 K12 (data are not shown, but we have found that CO desorbs from our NiAl(110) sample with a peak maximum of 330 K in agreement with the prior study). These results suggest that CO desorption from CH30WNiA1(110) is rate limited by the decomposition reactions of CH30H and not by the desorption kinetics of an adsorbed CO species. Furthermore, examination of the TPD spectra of H2 from NiA1( 1lo), which are presented in Figure 6a, is consistent with this contention. These data show that H2 desorbs from CH30WNiAl( 110) in a broad temperature range, 300-700 K, but a revealing aspect of the desorption traces is that a peak is present at 440 K (most prominent at 3.6 langmuirs and below), similar to the peak maximum temperature of CO. We believe that the concurrent desorption of CO and H2 is further evidence for a reaction limited desorption step. Spectroscopic data presented later suggest that it is likely that the dehydrogenation of methoxy, CH30(ad), results in the CO and HZproduction at 440 K. It is not likely that the 440 K H2 desorption is due to the combination of surface hydrogen that had been produced by a surface process at temperatures much lower than 440 K. Atomic hydrogen is expected to be stable on NiAl(110) only at temperatures lower than 350 K. This conjecture is based on the CH30D TPD spectra presented in Figure 6b. These data show that the desorption of D2 occurs with a peak maximum of 350 K. Spectroscopic data presented in the next section show that the 0-D bond of CH30D is broken at temperatures at 200

Sheu et al.

10262 J. Phys. Chem., Vol. 98, No. 40, 1994

TABLE 1: Yields (Arbitrary Units) of Gaseous Products Resulting from Thermal Decomposition of CHJOH on NiAI(110) exposure (langmuirs) CH30H (TP 150 K) H2 (300-800 K) CO (TP 440 K) CH4 (TP 350 K) CH3 (TP 570 K) C2& (TP 490 K)

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2935 (2200) -(2063) 1450 1155 1020 (985) 540 (540)

Corresponding frequencies for CD30D and CD30(ad) are shown in parentheses.

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TABLE 2: Vibrational Fre uencies (cm-') for Condensed CH30H and Submonolayer CHJOH and I3CH30(ad)on NiAl(110)" condensed 1.5 langmuirs of 1.5 langmuirs of modes CH3OH I3CH3OH(ad)(120 K) 13CH30(ad)(200 K)

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E n e r g y Loss ( c m - ' )

Figure 7. Vibrational spectra recorded by EELS after exposure of NiAl(110) to 1.5 langmuirs of 13CH30Hand CD3OD at 120 K. The spectrum is mode at 750 cm-' (575 cm-') in the L3CH30H(CD30D) due to the bending of the 0-H (0-D) group and indicates that at least some fraction of l3CH30H(CD3OD)associatively adsorbs on NiAl(110) at 120 K. The spectrum obtained after heating 13CH30W NiAl(110) to 200 K shows that the 0-H bending mode is eliminated and a new mode at 540 cm-I appears, which is attributed to the v(M-0) mode of CH3O(ad). K or lower, suggesting that the behavior of the D2 peak at 350 K is controlled by the desorption kinetics of adsorbed deuterium. It is also mentioned that a fraction of the surface deuterium does not desorb as D2 or HD but is instead incorporated into the methane product as evidenced by the peak at 350 K in the CH3D desorption trace. Also, the desorption of HD and C h product, between 300 and 400 K, suggests that some C-H bond breaking has occurred near 300 K. However, the H2 trace shows that the majority of C-H bond cleavage occurs at higher temperatures (i.e., between 390 and -600 K). EELS of CHJOH Adsorption at 120 K and Heating to 600 K. Figure 7 shows EELS spectra of 1.5 langmuirs of 13CH30H,and 1.2 langmuirs of CD30D adsorbed on NiAl( 110) at 120 K. On the basis of the work function data, the CH30H exposures used in these experiments are expected to produce an adsorbate coverage, which is nearly 0.5 of a saturation layer. The top spectrum is obtained by heating the 1.5 langmuir 13CH30WNiA1(110) surface to 200 K. Assignments for the EEL features in the spectra are exhibited in Table 1, and it is important to mention that all the mode assignments except for the 500 and 540 cm-' loss features, which are addressed below,

have been verified with spectra obtained for condensed CD3OD and CH30H (also exhibited in Table 2).51 Perhaps the most noteworthy aspect of the 120 K spectra is that the mode assigned to the bending of the 0-H group is present, suggesting that at least some CH30H associatively adsorbs on NiAl(110) at 120 K. Furthermore, these spectra show a EEL feature at 500 cm-', and we assign this to the v(M-0) mode of methanol, consistent with those mode being relatively insensitive to isotopic substitution. Absent in all the 120 K spectra is a well-resolved mode that can be attributed to the v(0-H) or v(0-D) mode of associatively adsorbed CH3OH or CD30D. We feel that the absence of the 0-H stretching mode is probably due to a low scattering cross section or that this mode is not dipole allowed due to a chemisorbed CH30H species with a 0-H bond parallel to the surface plane. Heating the adsorbed CH30H to 200 K eliminates the 0-H bending mode and results in the appearance of a new EEL mode at 540 cm-'. It is also noted that the broad shoulder at 3230 cm-' in the 120 K spectrum is eliminated upon heating to 200 K, suggesting that this spectral feature at 120 K may be the 0 - H stretching mode. However, we infer from these data that CH30H transforms into CHsO(ad) upon heating and assign the 540 cm-' feature in the 200 K spectrum to the v(M-0) mode of surface methoxy. Figure 8 displays EELS spectra of NiAl( 110) after exposure to 6 langmuirs of CH30H at 120 K and after subsequent heating to various temperatures. The loss features present in the 120 K spectrum are due to condensed CH30H, and those in the 200 K spectrum, as established by the argument above, are due to the presence of a methoxy adlayer. Heating the overlayer to 300 K results in no noticeable spectral changes, indicating that the methoxy overlayer is stable, at least, up to 300 K. By 400 K the intensity of the v(C0) mode at 1040 cm-' decreases, and a new spectral feature appears at 780 cm-'. Loss of the v(C0) mode intensity is attributed to the decomposition of some methoxy, which results, at least in part, in the formation of gaseous methane and hydrogen (as shown by TPD). The mode at 780 cm-' is believed to be due to an A1-0 vibration, based on prior research that shows that the corresponding mode for chemisorbed oxygen on Al(111) appears near 800 cm-1.s2 Prior research in our laboratory has shown that a loss feature also appears at 780 cm-' in the EEL spectrum of partially

J. Phys. Chem., Vol. 98,No. 40, 1994 10263

Adsorption and Decomposition of Methanol on NiAl( 110)

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Energy Loss (cm-')

Figure 8. EELS spectra obtained as a function of temperature after a multilayer of CH3OH is adsorbed on N M ( 110) at 120 K. The spectra are recorded near 120 K after heating the surface to the indicated temperatures.

decomposed CH30(ad) on FeAl(1 consistent with this mode being due to an Al-0 vibration. Also, this prior research showed that the v(M-0) mode of CH30(ad) on FeAl(110) appears at 540 cm-', similar to the position of the corresponding mode on NiAl( 1lo), suggesting that methoxy interacts strongly with the A1 component of the aluminide. We note that this is the fiist instance of us comparing the structure of methoxy on NiAl(110) to FeAl(110). It is emphasized that this, and the comparisons presented later in this paper between these surfaces, should be viewed as being qualitative in nature. While the bulk structure of FeAl and NiAl are similar, recent research54 has shown that, unlike NiA1(110), the surface of FeAl(110) may not be commensurate with the bulk. The intensities of the vibrational modes attributed to methoxy are markedly reduced upon heating from 400 to 500 K. Concurrent with this reduction of the methoxy modes is the evolution of intense features in the region between 500 to 900 cm-'. These features become sharper and more intense upon heating to 600 K, and the frequencies and relative intensities of these modes agree well with those of aluminum oxide.52,55*56 The C-H stretching and deformation (bending) features in the 600- K spectrum are attributed to the presence of adsorbed carbonaceous (i.e., CH,) species. UPS Studies of CH30H Chemisorption and Reaction on NiAl(110). The bottom panel of Figure 9 displays U P S spectra of clean NiAl( 1lo), after exposure to 1.2 langmuirs of CH30H and after subsequent heating to various temperatures. Difference spectra, which are obtained by subtracting the clean NiAl( 110) spectrum from the CH30H/NiAl( 110) data, are presented in the top panel of Figure 9. On the basis of the work function data presented in Figure 1, the 1.2 langmuir spectrum is taken to be representative of a submonolayer (-0.5 of saturation) coverage of CH30H. The vertical lines at the top of the figure are the ionization potentials of the valence levels of gas-phase CH3-

6

4

2

0

Binding Energy (eV)

Figure 9. Ups spectra obtained as a function of temperature after NiAl(110) is exposed to 1.2 langmuirs of CH30H at 120 K. The bottom panel exhibits raw data while the top panel shows difference spectra. The 120 K spectrum shows valence band features that are characteristic of an associatively adsorbed CH3OH. The spectra are recorded near 120 K after heating the surface to the indicated temperatures.

OH?7 We assign the features at 12.3 and 10.3 eV to the 5a' and (la'' 6a') levels of methanol and have subsequently aligned the corresponding vertical lines to these positions. The tacit assumption in aligning the gas-phase and chemisorbed levels in this manner is that the more deeply bound 5a' and (la" 6a') levels are least perturbed upon chemisorption. The remaining two features at 8.1 and 5.8 eV in the 1.2 langmuir spectrum are assigned to the 7a' and 2a" levels of methanol. Note that the 4a' level lies deep in the valence band and is not accessible using the 21.2 eV photons in our experiments. These UPS data, therefore, support the EELS results that suggest that at least a fraction of methanol adsorbs associatively on NiAl(110) at 120 K. Examination of the 1.2 langmuir spectrum shows that the 2a" and 7a' levels of adsorbed methanol are shifted from the corresponding gas-phase levels by about 0.2 and 0.8 eV, respectively, suggesting that these orbitals are involved in the chemisorption bond. (We emphasize, however, that this shift is the result of our choosing to align the 5a' and (la" 6a') levels of gas-phase CH30H to the corresponding orbital of the chemisorbed methanol.) The 2a" molecular orbital is nonbonding and localized on the oxygen atom, and the 7a' molecular orbital is localized along the C-0-H bond in the CH30H molecule. 16~18,57We postulate, largely from the binding energy shift of the 2a" orbital, that associatively adsorbed CH3OH bonds to the substrate via the oxygen atom. The shift of the 7a' orbital may be due to the direct chemisorption interaction with the hydroxyl-oxygen and/or to a distortion of the H-0-C bond angle or 0-H bond length, with respect to the gas-phase molecule. A chemisorbed CH30H species with the oxygen atom interacting strongly with the NiAl surface is consistent with the observed decrease in work function, and with the assignment of the 500 cm-* mode in the EELS spectra of Figure 7 to the v(M-0) mode of chemisorbed CH30H.

+

+

+

Sheu et al.

10264 J. Phys. Chem., Vol. 98, No. 40, 1994

Upon heating to 200 K, the spectral weight at 12.3 and 8.1 eV, attributed to the 5a' and 7a' orbital features of CH30H, is removed, consistent with the formation of surface methoxy, as has been shown by previous r e ~ e a r c h . ~ ~ , ' *Perhaps ~ ~ * - ~the ~ only noticeable change that occurs upon heating from 200 to 300 K is that a spectral feature becomes resolved at 5.3 eV. Heating to 400 K results in the elimination of this 5.3 eV feature, and in a decrease of the intensity of the (la" 6a') feature. Also, this latter feature undergoes an apparent shift of about 0.3 eV to higher binding energy, upon heating from 300 to 400 K. These spectral changes are attributed to the decomposition of a fraction of methoxy, which results in the deposition of surface oxygen, carbonaceous species, and in the desorption of C I 4 and Hz. By 500 K the methoxy valence features are further reduced in intensity, and upon heating to 800 K the methoxy overlayer has completely decomposed, and all that remains in the valence band spectrum is a broad peak due to the presence of surface oxygen. The reason for the binding energy shifts of the methoxy features when the surface is heated from 300 to 400 K is not entirely clear. One possible explanation may be that the surface has become oxidized changing the electronic structure of methoxy . Another potential explanation is that the valence features in the 300 K (andor 200 K) spectrum are due to the overlapping molecular orbitals of more than one methoxy species with dissimilar electronic structures. On the basis of this contention, it is argued that upon heating to 400 K, one or more of these species decomposes, thus resulting in an apparent shift. We favor this latter explanation, since if oxidation of the surface were an important factor, then one might expect further binding energy changes to occur in the valence levels of methoxy as the surface is heated to 500 K. Instead, the energy positions of the methoxy levels in the 400 and 500 K spectrum are similar, even though oxidation of the surface is more severe at 500 K, as evidenced by the prominence of the A1 oxide features in the 500 K EELS spectrum in Figure 8. XPS Studies. EELS and UPS data have already established most of the details of methanol chemisorption and reaction on NiAl( 110). XPS data presented in this section not only support the surface picture that has been developed from these spectroscopic techniques but also allow the concentration of surface methoxy and carbonaceous species on NiAl(110) to be estimated, as a function of temperature. In addition, these XPS data will be used to estimate the amount of gaseous product that desorb from NiAl( 1lo), after exposure to methanol at 120 K and heating to 600 K. Figure 10 exhibits O(1s) and C(1s) spectra of NiAl(110) after exposure to 6 langmuirs of CH30H at 120 K and stepwise heating to the indicated temperatures. On the basis of the EELS and U P S results we assign the O(1s) and C(1s) features at 532.4 and 286.4 eV, respectively, in the 200 K spectra to methoxy. These binding energies for methoxy are consistent with those obtained in previous research for methoxy on various transition m e t a l ~ and ~ ~ A1.16 , ~ ~ Perhaps, it is of importance to mention that the O( 1s) and C( 1s) binding energy values for methoxy on NiAl(110) agree to within 0.2 eV of the corresponding values for CHsO(ad) on FeA1(110), and polycrystalline NiAl and TiAl.@ This similarity is interpreted as meaning that CH30(ad) interacts strongly with the Al component of the aluminides. The spectral features present in the 200 K spectrum persist in both position and intensity upon heating to 300 K, but by 400 K spectral changes become evident. A shoulder appears at 531.4 and a peak develops at 282.8 eV in the O(1s) and C(1s) spectra, respectively. Both of these newly developed features are accompanied by a decrease in the intensity of the 532.4

8 L CH OH/NiA1(110) 3 C(1S) (x3.7) 5 3 2 . 4 531.4 286.4

283.0

+

20OK 300K 400K 500K

BOOK BOOK

288 8 l

1

1

I

L

283.8

'

536 5 3 4 5 3 2 530 5 2 8 5 2 6

288 286 2 8 4 2 8 2 2 8 0 271

Binding Energy (eV)

Figure 10. O(1s) and C(1s) XPS spectra obtained as a function of temperature after a multilayer of CH30H is adsorbed on NiAI(110) at 120 K. The spectra are recorded near 120 K after the surface is heated to the indicated temperatures.

and 286.4 eV peaks, which are attributed to surface methoxy. These changes are taken to indicate that a fraction of methoxy has decomposed by 400 K. New features in the 400 K spectra are believed to be due to the presence of surface oxygen and carbonaceous species, and they continuously grow in intensity, at the expense of the surface methoxy features, as the surface is heated to 500 and then to 600 K. We note that the C(1s) feature, which is attributed to methoxy in the spectra obtained at 400 K and above, appears to be shifted by about 0.2 eV to higher binding energy (from 286.4 to 286.6 eV), with respect to the corresponding peaks in the 200 and 300 K spectrum. These changes in the core level spectra are consistent with the valence band spectra presented earlier, which also show spectral changes in this same temperature range. As postulated before, as a possible explanation for the valence band changes, we believe that the changes in the core level spectra are due to the decomposition of a electronically unique methoxy species between 300 and 400 K. Heating the surface from 600 to 800 K results in some significant spectral changes; the C(1s) feature shifts from 283.0 to a broad peak centered at 283.8 eV and the O(1s) feature develops more spectral weight on its high binding energy side, compared to the 600 K spectrum. TPD experiments show that there is some product desorption (e.g., Hz, CO, and CH3) between 600 and 800 K, but the amount that desorbs in this region would seem to be too small to result in the observed spectral changes. It is likely that these shifts are due to a change in the surface structure of the alloy, possibly due to extensive oxidation of the A1 component. This type of process may lead to the surface segregation of Al, and a change in the chemical environment of the carbonaceous species. More experiments are needed to address the validity of this possibility. Table 3 displays the integrated areas of each O(1s) and C( 1s) peak displayed in Figure 10. The integrated O( 1s) and C( 1s)

Adsorption and Decomposition of Methanol on NiAl( 110)

TABLE 3: Peak Areas (Arbitrary Units) of O(1s) and C(1s) Core Levels for Methoxy Decomposition(Saturation Laver) on NiAI(110) at Various Annealing Temperatures area

200K

400K

100 28

65

C( 1s) peak 1,286.4 eV ueak 2. 283.0 eV

500K

600K 14

54 68

81

88

peak areas for the 200 K spectra have been set to a value of 100, so that the values quoted in the table for the 400, 500, and 600 K spectra are just percentages of the total amount of oxygen and carbon, present on NiAl( 110) at 200 K. All these peak area values have uncertianties of about &lo%. Also, we do not attempt to deconvolute the O(1s) feature, so the relative amount of surface methoxy and oxygen is not given in the table. Instead, only the total area of the O(1s) feature is given for selected temperatures. Analysis of these data indicate that upon heating the NiAl( 110) surface to 400 K that 35% of the initial methoxy overlayer decomposes, and 7% of the carbon resulting from this decomposition is lost from the surface. On the basis of the TPD results, this decrease in C( 1s) peak area is attributed to the desorption of C& (TP 350 K). Further heating to 500 K results in a 13% loss in O(1s) total peak area, and this decrease is most likely due to the desorption of CO, the only oxygenated product that is found to desorb from CH30H/NiAl(110). Finally, analysis of the data shows that 25% of the C(1s) peak area is lost when the surface is heated from 400 to 600 K. After accounting for the contribution of CO to this decrease, we estimate that 13% of the carbon is removed from NiAl(110) as methyl radical product. It is emphasized that these values are only estimates, since diffusion of carbon, for example, into the bulk at elevated temperatures or the formation of an aluminum oxide layer over the carbonaceous species can lead to a decrease in the C(1s) intensity. Comparison of these XPS and the TPD results that were presented earlier, however, suggest that these estimates are reasonable. For example, examination of the TPD results presented in Table 1, for a 7.2 langmuir CH30H exposure, shows that CO accounts for 36% of the carbon-containing gaseous reaction product that desorbs from NiAl( 110). Analysis of the X P S data shows that the desorption of CO accounts for 39% of the C(1s) area decrease, consistent with the TPD result. A somewhat bigger discrepancy, however, is found in the CH3 circumstance, where X P S and TPD data suggest that this product accounts for 39 and 49%, respectively, of the carbon-containing reaction product.

-

Discussion On the basis of our EELS, UPS, XPS, and TPD results, the major aspects of the chemisorption and reaction steps of CH3OH on NiAl(110) are summarized as follows: CH,OH(g) CH,OH(ad)

-

-

-

CH,OH(ad)

CH30(ad)

+ H(ad)

[120 K]

[200 K]

[300-800 K, H(ad) resulting 2H(ad) H2(g) from 0 - H bond scission desorbs as H2 with a Tp of 350 K] CH,O(ad)

+ H(ad) - CH,(g) + O(ad) [Tp(CH,) = 350 K]

J. Phys. Chem., Vol. 98, No. 40, 1994 10265

CH,O(ad)

-

CH30(ad)

+

[Tp(CO) = 450 K, CO(g) xH2(g) stoichiometric coefficient, x, is not known] CH,(g)

+ O(ad)

[T,(CH,) = 570 K]

Examination of the reaction steps shows that the surface reactivity of NiAl(110) for methanol is quite diverse, and perhaps this is best exemplified by the variety of gaseous products, which include H2, CO, CH4, and CH3 radicals, that desorb from the alloy surface during CH30H decomposition. The desorption of C2& is also found to desorb after a high coverage of CH30H is adsorbed on NiAl( 110). As mentioned in the introduction, the decomposition of CH30H on transitionmetal surfaces, such as Ni results in the desorption of CO and H2 product, while on AI the thermal decomposition of CH30H results in gaseous CHq and H2. In a zeroth-order approximation, therefore, the product distribution for NiAl( 110) is a combination of the product distribution for monometallic Ni and Al. Not unexpectedly, however, the details of the surface chemistry that leads to the formation of these products on the NiA1( 110) alloy is quite different than on the corresponding monometallic surfaces. The remainder of this section is used to discuss some of the surface processes that may lead to these products. Methane Evolution between 300 and 400 K. Methane is observed desorbing from NiAl(110) at a peak temperature of 350 K. Our spectroscopicresults indicate that surface methoxy is stable on NiA1( 110) up to 300 K and that -35% of the initial methoxy adlayer decomposes by 400 K, suggesting that the CHq evolved in this same temperature range is produced concurrently or immediately after methoxy decomposition. Cleavage of the C-0 bond of methoxy and the desorption of methane near 350 K appears to be a unique property of the NiAl( 110) surface. For example, C-0 bond scission on A1 during methanol decomposition, occurs at a significantly higher temperature (-450 K) than on NiAl(110). The reason for the increased reactivity of the alloy for C-0 bond cleavage is not obvious, considering that the filled hybridized d band of NiAl is not expected to be ideal for a C-0 bond-breaking step. This point is well established by prior research that has shown that scission of the C-0 bond does not occur during the decomposition reactions of CH30H on Cu surface^.^^^^ We believe, therefore, that there must be a strong interaction of the CH30(ad) moiety with the sp states of the alloy, presumably with a strong contribution from the A1 component. Certainly, the similarity of the vibrational and electronic structure of CHsO(ad) on NiA1( 110) and FeAl( 1lo), as already alluded to in the preceding sections, supports this picture that has CH30(ad) interacting strongly with the A1 component of the aluminides. The hydrogenation reactions, however, that produce the methane might be expected to be occumng on sites with strong contributions from the Ni component, when one considers prior research, that has shown that hydrocarbon fragments, such as CH2 and CH3, are readily hydrogenated by surface hydrogen on monometallic Ni67,68to gaseous methane, at temperatures near 250 K. It is tempting to speculate that on NiA1(110), the presence of both Ni and A1 facilitates cleavage of the C-0 bond of methoxy through a heterolytic bond-breaking step. Prior research has shown, for example, that the dissociation of CO occurs on Ni supported on Al, and it was hypothesized that a Nix-C-O-Aly species exists,69but in this case EELS shows a distinct vibrational mode which can be associated with the complex. The presence of an analogous type complex aiding the breaking of the C-0 bond on methoxy cannot be directly supported by our EELS results, due to the lack of any readily identifiable mode. U P S does raise the possibility, however, that

10266 J. Phys. Chem., Vol. 98, No. 40, 1994 at least two types of methoxy species, with distinct electronic structures, are present on NiAl( 110) at 300 K and that one of these species disappears between 300 and 400 K. While this result is by no means conclusive, it is consistent with there being a unique binding site(s) for methoxy on the NiAl( 110) surface that facilitates cleavage of the C - 0 bond of methoxy. One might also come to this same conclusion by noting that scission of the C - 0 bond of methoxy does not occur at such low temperatures on FeAl (C-0 bond cleavage occurs at temperatures greater than 400 K on FeAl),64even though this alloy might have been expected to be more reactive than NiAl, due to the presence of an unfilled alloy d band.8 CO Evolution in the Temperature Range of 400 and 500 K. Heating the surface between 400 and 500 K further decomposes the methoxy overlayer, resulting in the formation of surface oxygen and carbonaceous species, in addition to the evolution of gaseous CO and H2. All our techniques indicate that the production of the CO is reaction limited, presumably by the decomposition of methoxy, since this is the only identifiable intermediate at the temperature at which CO is produced. It is probably reasonable to assume that the presence of the Ni component is responsible for the reactions (i.e., dehydrogenation of methoxy) that lead to CO production, since this product is formed via methoxy decomposition on monometallic Ni, while its formation is unprecedented on monometallic Al. We emphasize, however, that the details of the CO production are clearly a result of the unique chemical properties of NiA1, since unlike monometallic Ni,24this species does not exist as a stable adsorbed species on NiAl( 110). Also, while almost all the methoxy adsorbed on monometallic Ni decomposes to CO, only about 13% of the initial saturated adlayer of methoxy on NiAl( 110) decomposes to form CO. Prior research has suggested that the surface oxygen that results from methoxy decomposition is bound to the A1 component, and it is likely then that the strong affinity of A1 for oxygen limits the amount of gaseous CO production on the aluminide. The decomposition of CH30H on polycrystalline NiAl was also found in the previous study to yield much less CO than our present study of NiA1(110), suggesting that the formation of this product is sensitive to the atomic composition of the reacting surface. We suspect that the presence of Ni in the outer surface is necessary for CO production, since recent work in our laboratory70has found that CH30H decomposition on the (100) plane of NiA1, which is terminated by an A1 does not yield CO. One must keep in mind that extensive decomposition of methoxy and partial oxidation of the A1 component is occurring prior to and during the CO desorption step. It is probably reasonable to assume that restructuring of the surface, due to this bond formation, results in a modification of the Ni-A1 bonding and electronic structure. (This is most certainly the driving force for the formation of an aluminum oxide l a y e P on the aluminides after exposure to molecular oxygen or even CH30H at elevated temperatures.) The important point here is that it is not known to what extent the geometric and electronic structure of the surface has changed, due to these stoichiometric reactions, and how these changes may contribute to reactions leading to the formation of CO. Methyl Radical Formation above 500 K. Continued heating of the remaining methoxy species results in the desorption of CH3 radicals, at a peak temperature of 570 K. The generation of gaseous methyl radicals from surface methoxy is not unprecedented, and this process has been shown to occur, with a peak temperature of 590 K, from oxygen-covered Mo(1 Theoretical calculations have shown that the C - 0 bond strength of methoxy on O/Mo(llO) is close to 1.5 eV,

Sheu et al. consistent with a peak temperature near 590 K.74 We suspect that the preadsorbed oxygen in the case of Mo( 110) and the extensive amount of surface oxygen and carbonaceous species on NiAl( 110) stabilizes a fraction methoxy (by preventing surface mediated decomposition processes) so that the moiety is able to exist up to temperatures where homolytic bond cleavage can occur. A similar reaction scheme also exists for FeA1( 1lo), where methyl radicals, and a smaller amount of methane, desorb with a peak temperature of 540 K, during the decomposition reactions of CH30H.53 Given the similarity of this methyl radical production step on FeAl and NiA1, it is reasonable to assume that the methoxy species responsible for these products are bound primarily on the A1 component. Support for this surface picture comes from theoretical calculation^^^ that have suggested that the C - 0 bond of CHsO(ad) on partially reduced alumina may be weak enough so that methyl ejection andor methane production (via the abstraction of surface hydrogen by the methyl radical) become energetically favorable reaction channels at elevated temperatures. Possible Relevance of the NiAl Alloy to the Ni/A1203 System? In this section some of our results for NiAl(110) are compared to selected results for the Ni/A1203 catalyst. Prior research has shown with temperature-programmed reaction studies that CH30H decomposition on Ni/Al2O3 results, in the order of decreasing yield, the formation of gaseous CO, H2, C&, (CH3)20, CO2, and H z O . ~In~ other s t ~ d i e s , ~ ~which -~O investigated the CO hydrogenation reaction on Ni/Al2O3, it was shown that heating the supported catalyst with adsorbed CO in a flow of HZresulted in two CHq desorption peaks, with maxima at 443 and 520 K. Isotopic labeling experimentss0showed that the lower temperature (-440 K) methane peak resulted from the hydrogenation of CO adsorbed on the nickel component and that the desorption of CHq at the higher temperature (-520 K) resulted from decomposition of methoxy on the alumina support. We are rather cautious in making any detailed comparison between NiAl and the supported catalyst at this time, due to the differences in the relative amounts of Ni and A1 and in the experimental conditions, but at least two general similarities appear to be present. First, CH30H decomposition on the supported catalyst and NiAl single crystal yields CO, C&, and H2 product, suggesting that similar decomposition steps are occurring on both surfaces. Second, the latter study of the supported catalyst suggests that the evolution of CHq is due to the decomposition of methoxy on the A1203 component. Likewise, our results for NiA1(110), in addition to those for FeAl( 110) and TiA1, also suggest that CH30(ad) decomposition on the A1 component results in gaseous C& (and CH3) producti~n.~~.~ Finally, we mention that both studies of the Ni/A1203catalyst conclude that methoxy is a mobile intermediate and that the diffusion of CH30(ad) from the A1203 to Ni component results in the production of CO product. This conclusion may be useful for interpreting some of our results, which pertain to CO production on NiAl( 110). We have shown that the kinetics of methoxy formation to form CO on NiAl(110), as judged by TPD, is very similar whether CH,O(ad) or CDsO(ad) is the decomposing intermediate. It is instructive to compare this behavior to the analogous process on Fe( 100). On this monometallic surface it has been shown that methoxy decomposition results in the desorption of gaseous CO near 400 K and that the evolution of this product is reaction limited by the decomposition of methoxy,81similar to what we observe for NiAl(l10). In this prior research,

Adsorption and Decomposition of Methanol on NiAl( 110) however, it was shown that the peak temperature of CO desorption from CDsO(ad) decomposition on Fe( 100) occurs 15 K higher than when CH30(ad) is the decomposing intermediate, suggesting that C-H bond breaking is involved in the rate-limiting step on this surface. Our results do not exhibit such an isotope effect, and they therefore suggest that dehydrogenation of the methoxy species on NiA1( 110) to form CO may not be rate-limiting. On the basis of the results obtained for the supported catalyst, it may be that the diffusion of methoxy from A1 (or oxide thereof) to sites with strong Ni character may possibly be the slow step. It is noted that CO production from NiAl( 110) is occumng in parallel with the oxidation of the A1 component near 450 K, and this process may be important for forcing methoxy, which is interacting strongly with the A1 component, onto “Ni“ sites where dehydrogenation and CO desorption can occur. This dehydrogenation step would be expected to occur immediately at temperatures near 450 K, since CH30(ad) dehydrogenates to adsorbed CO and H below room temperature on monometallic Ni.19-27 Furthermore, TPD experiments show that the peak maximum of CO desorption increases slightly in temperature with adsorbate coverage, suggesting that there is an increase in the activation energy for the evolution of CO. This experimental observation may mean that there is a decreased ability for surface diffusion and/or that the creation of active sites, needed for the dehydrogenation of CH30(ad), is becoming more difficult as the adsorbate coverage is increased. More experiments are needed to address these possibilities.

Summary The adsorption and thermal decomposition of CH3OH on NiAl(110) have been investigated with TPD, XPS, U P S , and EELS. Methanol, to a large extent, associatively chemisorbs on NiAl( 110) at 120 K and transforms into surface methoxy by 200 K. Subsequent C-H and C - 0 bond breaking steps at higher temperatures lead to the evolution of gaseous H2, CO, C&, CH3 radical, and C2& in addition to the extensive deposition of surface oxygen and carbonaceous species. A unique aspect of the surface reactivity of NiAl(110) is its ability to break the C-0 bond of methoxy in an unusually low temperature range (-300-400 K), compared to monometallic A1 or transition metals. A fraction of the methoxy that decomposes in this temperature range leads to the desorption of C& with a peak temperature of 350 K. The desorption of CO is found to occur at higher temperatures (between 400 and 500 K), and the invariance of its peak temperature to isotopic substitution (CH30H opposed to CD30D) suggests that this product may not be rate limited by the dehydrogenation of methoxy, but instead by the diffusion of methoxy species from A1 to Ni sites (where dehydrogenation to CO can occur). Continued heating in the temperature range of 500 and 600 K results in the desorption of methyl radicals.

Acknowledgment. Partial support of this research by the National Science Foundation through a NSF Young Investigator Award (NYI) is appreciated (Grant DMR-9258544). We also appreciate Dr. Ram Darolia, at General Electric Aircraft Engines, for supplying a single-crystal ingot of NiAl. References and Notes (1) Sinfelt, J. H. Bimetallic Catalysis-Discoveries, Concepts, and Applications; Wiley: New York, 1983. (2) Liu, C. T. In Matr. Res. SOC.Symp. Proc.; Material Research Society: Pittsburgh, 1993; Vol. 288, pp 3-19. (3) Rodriguez, J. A.; Goodman, D. W. J . Phys. Chem. 1991,95,4196. (4) Campbell, C. T. Annu. Rev. Phys. Chem. 1990, 41, 775.

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