Decomposition of water on clean and oxidized aluminum (100)

J. Phys. Chem. 1986, 90, 5321-5324

5321

Decomposition of H,O on Clean and Oxidized Ai( 100) J. Paul* and F. M. Hoffrnann Corporate Research Science Laboratories, EXXON Research and Engineering Company, Annandale. New Jersey 08801 (Received: March 27, 1986)

This paper presents electron energy loss spectra (EELS) and thermal desorption (TDS) traces following the adsorption and subsequent annealing of water on A1(100), with and without the presence of a surface oxide. A water molecule will in both cases either (1) reversibly bind at a hydrogen-bonded site or (2) decompose,preferentially to a surface-bound hydroxyl species. The H 2 0 dissociation occurs via H2 evolution only on the bare surface and also via hydrogen abstraction in the presence of a surface oxide. Neither dihydrogen nor water desorption are observed as the hydroxyl species diminish during annealing. Instead, the aluminum hydroxide transforms into aluminum oxide and “trapped” hydrogen atoms, depicted as an Al/A10, hydride.

Introduction This study is part of an ongoing effort to make the surfaces of free-electron metals and oxides available for surface studies under ultra-high-vacuum (UHV) conditions. Aluminum is of particular interest because of its high electron density, surpassed only by beryllium, and the intense use of alumina both in catalysis and as an adsorbent. The large sp electron density of the metal gives an effective screening, i.e., a short range of perturbations and a pronounced electrostatic barrier at the surface. Furthermore, an increased electron density markedly alters the chemisorption potential of, e.g., an oxygen atom without invoking hybridization with “localized” d states. The first part of this study presents data on the adsorption of water onto clean Al(100). Previous experiments showed not only the formation of hydroxyl species but also the reversible adsorption at the surfaces of relatively low electron density metals like potassium (rs/ao= 4.86)’ and sodium (rs/ao= 3.93).2 The present results will show that increasing the electron density from rJa0 = 4.86 (K) to 2.07 (Al) means passing the limit for any reversible adsorption of water on a metal surface. Water adsorption on the surface of bulk lithium (rs/ao= 3.25) and TDS measurements on Na/H,O are relevant future projects. We note that there is no substitute for UHV experiments on free-electron metal surfaces. Oxygen adsorption and the formation of oxide overlayers on aluminum have been subject to extensive studiesS3 Nevertheless, UHV studies devoted to the interaction between adsorbates and these oxide overlayers are rare. We present in the second part of this paper data on water decomposition on Al/A10, as a test example of such studies. The formation and stability of hydroxyl species on alumina is of obvious relevance, and in situ data for bulk samples are a c c ~ m u l a t i n g . ~ ~ ~ Experimental Section Experiments were performed in a UHV chamber equipped with a Leybold EELS spectrometer, a single-pass cylindrical mirror analyzer (Physical Electronics), a multiplexed mass spectrometer (UTI lOOC), and a Varian low-energy electron diffraction (LEED) unit. The Al( 100) crystal6 was “spotwelded” to two tantalum rods and a W-5%Re/W-26%Re thermocouple. We utilized a daily 3-h cleaning procedure which included (1) a 5-min Ar+ sputtering (1) Thiel, P. A.; Hrbek, J.; DePaola, R. A.; Hoffmann, F. M. Chem. Phys. Lett. 1984, 108, 25. (2) Paul, J. Surf. Sci. 1985, 160, 599. (3) Batra, I. A,; Kleinman, L. J . Electron. Spectrosc. Relat. Phemm. 1984, 33, 175. (4) Knozinger, H. Adu. Catal. 1976, 25, 184. (5) Boehm, H. P.; Knozinger, H. Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlin, 1983); Vol. 4, p 39. (6) The crystal was prepared by B. Addis, TOL, Cornel1 University. Diameter: 11 mm. Thickness: 2 mm.

0022-3654/86/2090-5321$01.50/0

at 300 K, 0.5-1.5 kV, and 15 HA cm-2, (2) a 45-min annealing at 650-700 K, (3) a 5-min Ar+ sputtering at 500 K, 0.5 kV, and 10 pA cm-2, (4) a 45-min annealing at 650-700 K, and (5) steps 3 and 4 repeated. This procedure provided a highly reflecting surface without most of the “grayishness” experienced after prolonged roughening of A1 and Ag surfaces. Auger and EELS spectra show little evidence of contaminants (Figure 1). Oxide overlayers were prepared by exposing the bare surface to dioxygen at 575 K followed by annealing to 650 K. This again produced a highly reflecting substrate with only about 50% lower absolute intensity in the elastic beam (EELS) compared to the unexposed surface (Figure 1). The nature of each vibrational band will be discussed below. We note that some local order must persist, even though no diffraction pattern was observed with LEED, because of the intensity of the electric as well as the inelastic EELS beams. Water was adsorbed at 80 K, commonly via a beam doser. Figures on surface coverages are not easily accessible since no ordered LEED patterns in addition to the attenuated (1 X 1) structure were observed. We give when relevant the ratios of the Auger signals from oxidic vs. metallic aluminum, A15 1/A168 (Figure 1A) or from oxygen vs. metallic aluminum, 0503/A168 (Figure 1A) as measures of dissociated water. Doses are given in approximate backfilling equivalents. EELS spectra were obtained at 60-cm-’ resolution and 5-eV primary beam energy, and Auger spectra were obtained at 2-eV modulation amplitude and 3-keV beam energy. Finally, 10 A gave a heating rate of 1.3 K s-’ during TDS measurements, and desorbing species were sampled with the crystal positioned 5 mm from the front aperture of the mass spectrometer.

Results and Discussion AI(100)/H20.Figure 2a shows EELS obtained after adsorption and subsequent annealing of about a monolayer of H2’*0on clean Al(100). The 80 K spectrum shows predominantly molecular adsorption with significant intermolecular bonding, characterized by the perturbed scissor mode at 1650 ~m-’.’-’~ The low intensity of the intermolecular band at around 200 cm-I indicates adsorption in the monolayer range, but the overall spectrum is neither very surface specific nor representative of monomer a d s o r p t i ~ n . ~ ~ . ’ ~ (7) Ibach, H.; Lehwald, S. Surf.Sci. 1980, 91, 187. (8) Fisher, G. B.; Sexton, B. A. Phys. Reu. Lett. 1980, 44, 683. (9) Sexton, B. Surf. Sci. 1980, 94, 435. (10) Thiel, P. A,; Hoffmann, F. M.; Weinberg, W. H. J . Chem. Phys. 1981, 75, 5556. (11) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1981, 111, 11. (12) Thiel, P. A.; DePaola, R. A,; Hoffmann, F. M. J. Chem. Phys. 1984, 80, 5326. (13) Stuve, E. M.; Jorgensen, S. W.; Madix, R. J. Surf. Sci. 1984, 146, 179. (14) Andersson, S.; Nyberg, C.; Tengstll, C. G. Chem. Phys. Lett. 1984, 104, 305. (1 5 ) Ibach, H.; Miiller, J. E. Catalyst Characterization Science, Surface and Solid State Chemistry; Deviney, M. L., Gland, J. L., Eds.; American Chemical Society: Washington, DC, 1985; p 392.

0 1986 American Chemical Society

5322 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986

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Figure 1. (A) Auger spectra of clean and oxidized Al(100). The oxide overlayer was prepared by exposing the clean surface to approximately 250 L of 0, at 575 K followed by annealing to 650 K. (B) EELS analogous to Auger spectra in A. Frequencies are summarized in Table I. We assign the peaks above 825 cm-' to multiple losses because the energy of each peak can be deconvoluted as a sum of the energies of the single losses and because of the analogy with other oxide surfaces.40

Heating to around 150 K activates the formation of additional aluminum-bound hydroxyl speciesI6 and the desorption of some excess water. Further annealing gave birth to a new band around 1900 cm-I at the expense of the hydroxyl band around 3700 cm-l. This transformation occurred gradually between 300 and 500 K and coincided with marked changes in the metal-oxygen frequency range 350-1000 cm-l. Analogous experiments with D2I60 gave a hydroxyl stretch band around 2700 cm-' (Figure 2B). The band at 1900 cm-l shifts upon deuteration to around 1400 cm-l and must be assigned to a hydrogen-containing species. It has been observed previously and interpreted as an aluminum/alumina interfacial but never before shown to be a product of hydroxyl decomposition. We have no geometrical model for the site of the hydrogen atom and are unaware of any "well-known" molecule with absorption (16) Szalkowski, F. J. J . Chem. Phys. 1982, 77, 5224. (17) Igalson, J.; Adler, J. G. Phys. Reu. B: Condens. Matler 1983, 28, 4970.

(18) Gauthier, S.; decheveigne, S.; Klein, J.; Belin, M. Phys. Reo. B Condens. Matter 1984, 29, 1748. (19) Thiry, P. A.; Pireaux, J. J.; Liehr, M.; Caudano, R. J . Vac. Sci. Techno!., A 1985, 3, 1439.

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Figure 2. (A, B) EELS following adsorption of (A) H2"0 or (B) D2l60 onto Al(100) at 80 K. Annealing temperatures as indicated in the figures. Frequencies are summarized in Table I. Each surface was exposed to approximately 2 L of water. A151/A168 = 0.1 5 after annealing to 650 K. (C) TDS following the adsorption of approximately 1 L of HzO (DzO) on Al(100) at 80 K. A151/A168 rr 0.05 after flashing to 650 K. Note the change of the scale on the horizontal axis at ~ 3 2 K. 5 The mass 2 signal for A1(100)/H20 was not magnified because of an unfortunate high background pressure.

Decomposition of H 2 0 on AI( 100) in this range. The intensity of this hydride band relative to the double loss peak at 1700 cm-I supports the previous interpretation of a species trapped at the metal/oxide interface. Obviously we cannot indisputably rule out the possibility of a yet unknown, highly distorted, and very stable hydroxyl group embedded in bulk alumina. Reported 0-H stretch frequencies of aluminum hyCore level photoemission droxide range from 3700 to 3800 ~m-l.49~ spectra may provide complementary information about the high-temperature stability of the hydroxyl groups.16,20,21 Figure 2C shows the desorption of water and dihydrogen following the adsorption of H2I80(D2I60)on Al(100) at 80 K. We note that a significant amount of dihydrogen but almost no water desorbs from a clean surface exposed to low quantities of water. Higher exposures led to the desorption of water at around 165 K, which is typical for hydrogen-bonded molecules (often synonymous with multilayer adsorption; see following section and ref 1, 10-13, 22-24). We also note a 40 K higher desorption temperature for D, compared to H2. Correspondingly, EELS in Figure 2, parts A and B, show a higher temperature for maximum intensity of the deuterated hydroxyl band. Finally, no water or dihydrogen desorption correlates with the disappearance of the hydroxyl band around 3700(2700) cm-l. Our conclusions for water adsorption on clean Al(100) must be the following: (1) a water molecule coordinated to the metal will decompose with almost unity probability following the reaction H 2 0 O H (ads.) + 1/2H2(8) and (2) virtually all hydroxyl groups will transform into interface "hydrides". Step 1 is thermodynamically favorable but also believed to involve more than one molecule.2s The high electron density and consequently large ~ prohibit any reversible mobinding energies of 0 2 p orbitals lecular adsorption of water on the aluminum surface. The driving force for the hydroxyl to hydride transformation and the assigning of remaining vibrational bands will be discussed in connection with the results on water adsorption onto an oxide overlayer. Our results are in full accordance with and complementary to results obtained by ESDIAD (electron stimulated desorption ion angular distribution) for AI( 11 1)/H20.26 A1(100)/AIO,. We suggest that AIO, on AI(100) may serve as a surface-science substitute for macroscopically sized single crystals of a l ~ m i n a . ~All ' vibrational spectra of alumina or oxide overlayers on aluminum show the same basic features. Minor deviations are observed with altered thickness of the oxide film, different annealing temperatures, and, for oxides on single-crystal surfaces, different modes polarized normal to the surface plane. Strong and Erskine observed bands similar to those in Figure 2 - ~ ~ assigned after extensive oxidation of an AI( 111) ~ u r f a c e . ~ *They these bands to calculated eigenmcdes of a face-centered cubic unit cell modified by oxygen atoms.32 Following their work we name the peaks as shown in Table I. Adsorption studies33as well as detailed studies of the onset of ~ x i d a t i o n ~support ~ z ~ the assignment of the band at 640 cm-l to surface oxygen. Further studies are under way to elucidate the homogeneity of adsorption sites and

The Journal of Physical Chemistry, Vof. 90, No. 21, 1986 5323

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Eberhardt, W.; Kunz, C. Surf. Sci. 1978, 75, 709. Akimov, A. G.; Makarychev, Yu. B. Pouerkhnost 1983, 5, 88. Madey, T. E.; Yates, J. T., Jr. Chem. Phys. Lett. 1977, 51, 77. Fisher, G. B.; Gland, J. L. Surf. Sci. 1980, 94, 446. Wittrig, T. S.; Ibbotson, D. E.; Weinberg, W. H. Surf. Sci. 1981, 102, Muller, J. E.; Harris, J. Phys. Reo. Lett. 1984, 53, 2493. Netzer, F. P.; Madey, T. E. Surf. Sci. 1983, 127, L102. Dubois, L. H.; Hansma, P. K.; Sornorjai, G. A. Appl. SurJ Sci. 1980,

( 2 8 ) Erskine, J. L.; Strong, R. L. Phys. Reo. B Condens. Matter 1982, 25, 5547. (29) Strong, R. L.; Firey, B.; deWette, F. W.; Erskine, J. L. Phys. Reu. E : Condens. Matter 1982, 26, 3483. (30) Strong, R. L.; Firey, B.; deWette, F. W.; Erskine, J. L. J. Electron. Spectrosc. Relat. Phenom. 1983, 29, 187. (31) Strong, R. L.; Erskine, J. L. J . Vac. Sci. Technol., A 1985, 3, 1428. (32) Barker, A. S., Jr. Phys. Reu. 1963, 232, 1474. (33) Potassium adsorption on Al/AlO, at 80 K selectively suppresses the peak assigned to surface oxygen (Paul, J. J . Vac. Sci. Technol. A, submitted). (34) Chen, J. G.; Crowell, J. E.; Yates, J. T., Jr. Phys. Reo. E : Condens. Matter 1986, 33, 1436. (35) Crowell, J. E.; Chen, J. G.; Yates, J. T. Surf. Sci. 1986, 165, 37.

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Figure 3. (A, B) EELS analogous to parts A and B of Figure 2 but with an oxide overlayer on the aluminum surface. The overlayer was prepared by oxidation in 0,. A151/A168 = 0.12 before water adsorption and 0.62 after flashing to 650 K. (C) TDS analogous to Figure 2C but in the presence of an oxide overlayer. Each surface was exposed to 10 L of H,O at 80 K. Note that the mass 2 signal tracking the molecular mass signal is a spectrometer effect and not true desorption.

Paul and Hoffmann

5324 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 I

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the growth mode of the oxide ~ v e r l a y e r . ~ ~ As shown in the previous section, EELS strongly suggest that high temperature annealing of water deposited onto Al( 100) produces an oxide overlayer with "trapped hydrides" (Figures 1B and 2A,B). An obvious question was whether the actual formation of the "ordered" oxide film is a prerequisite for the encapsulation of the hydride. We address that issue in the following section and show data on water decomposition as our first adsorption study on Al/AIO,. AI/A10,/H20. An annealed oxide overlayer (Figure 1B) was completely covered with water at 80 K (Figure 3A). The onset of ice formation is indicated by intermolecular bands (hydrogen bonds, hindered translations, 0-0 vibrations, etc.) around 200 cm-l. Excess water desorbed and hydroxyl species formed upon flashing to 300 K. Further annealing again converted hydroxyl groups into hydrides (cf. top of Figure 3A and Figure IB). The process was accompanied by an increase of the ratio between the Auger signals of oxidic and metallic aluminum. Figure 3B shows a sequence analogous to that of Figure 3A but with D2I60adsorbed onto AI/A1I60, rather than H2I80adsorbed onto Al/Al'*O,. Parts A and B of Figure 3 as well as adsorption studies with thick water overlayers on the bare metallic surface revealed that the intensity of the hydride peak was not proportional to the oxide bands but rather constant. This suggests that the hydride is associated with the metal/oxide interface. Since the band is dipole-active and thus not limited by mean free paths as opposed to impact scattering, it is not surprising that such an interface band is observed (see Appendix). Dipole-excited vibrations of hydrogen atoms trapped in bulk aluminum3' are not likely to be observable because of the effective screening of this metal. Assigning metal-oxygen bands at intermediate annealing temperatures is nontrivial. A minor fraction of the impinging water molecules may instantaneously convert to adsorbed oxygen atoms and thus give rise to intensity interfering with the AI-OH stretching and AI-0-H bending mode^.^^-^^,^^^^ We note, however, that the hydroxyl band around 3700(2700) cm-' always is accompanied by additional intensity around 750 cm-I (Figures 2A and 3A). This is obvious also from studies on bulk alumina ~ ~ presumably caused by internal after H+ b ~ m b a r d m e n tand

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AlOH modes. All bands assigned with confidence are summarized in Table I. TDS following the adsorption of water on Al/AIO, provide little evidence for dihydrogen desorption (Figure 3C and ref 26). The mass 2(4) peak overlapping the water molecular mass peak is a spectrometer effect and thus irrelevant. Again we note that no H 2 (D2) or H 2 0 (D20) desorption is observed above the temperature for desorption of hydrogen-bonded water molecules from ice. Our conclusions regarding water adsorption on Al/A10, are (1) all water molecules more tightly bound than to ice will decompose, this time following the different reaction H 2 0+ A10, OH(ads.) + AI0,-H, and (2) again virtually all hydroxyl groups will transform into hydrides. Although slightly different, the hydroxyl to hydride transformation temperatures on both AI and Al/AlO, do coincide with the temperature range for the formation of bulk-like alumina on aluminum (Figures 2A and 3A and ref 39). This suggests that the driving force for the transformation is aluminum-oxygen energetics.

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Acknowledgment. We acknowledge D. Schmeisser for discussions about preparations of aluminum surfaces, P. Nordlander for comments on binding sites of hydrogen atoms in bulk alum i n ~ m , ~and ' J. L. Robbins for providing facts about the interaction of water and bulk alumina. Appendix Figure 4 shows the absolute intensities of certain absorption bands as a function of the detection angle relative to the specular direction. We have chosen the system A1(100)/H2I80and the annealing temperatures 200 and 575 K as representative of the hydroxyl and hydride states. We conclude from Figure 4 that the hydroxyl band is the only band which shows significant deviation from the intensity vs. angle dependence of the elastic beam and consequently the only band dominated by impact ~cattering.~' Registry No. H20, 7732-18-5; AI, 7429-90-5. (38) Chatelet, J.; Claassen, H. H.; Gruen, D. M.; Sheft, I.; Wright, R. B. Appl. Spectrosc. 1975, 29, 185. (39) Flcdstrorn,S.A,; Martinsson, C. W. B.; Bachrach, R. 2.; Hagstrom, S. B. M.; Bauer, R. S. Phys. Rev. Lett. 1978, 40, 907. (40) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic: New York, 1982.