Facile carbon monoxide dissociation on copper: promotion by aluminum

J . Phys. Chem. 1993,97, 2701-2710

2707

Facile Carbon Monoxide Dissociation on Copper: Promotion by Aluminum M. L. Colaianni, J. G. Cben,+and J. T. Yates, Jr.' Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: October 19, 1992; In Final Form: January 4 , 1993

High-resolution electron energy loss spectroscopy (HREELS) and Auger electron spectroscopy (AES) have been utilized to study C O adsorption and decomposition on copper supported on an A l ( l l 1 ) single crysta1. CO adsorption sites are created upon the evaporation of copper onto an atomically clean Al( 1 11) surface a t 95 K. Exposing a Cu/Al( 111) interface, possessing approximately one-half monolayer of copper, to C O produces a species which exhibits a weak vibrational mode with an unusually low C O stretching frequency of 1260 cm-I. The low-frequency of this C O mode indicates that this species possesses a severely weakened C - 0 bond. This is verified by its thermal decomposition upon heating to 348 K and the coincident formation of adsorbed carbon and oxygen on the surface, as observed spectroscopically. The thermal instability and the vibrational spectra of this C O species suggest that the C O is bound in a di-a structure, where both the C and 0 atoms are chemically bonded to metal atoms (Le., Cux-C-0-AI,). This structure is proposed to be a precursor state to C O dissociation on the Cu/Al( 11 1) interface. Thus, aluminum atoms, as neighbors to copper atoms, can provide a route to CO dissociation which is not available to either copper or aluminum alone.

Introduction This is a preliminary report showing that CO chemistry on Cu surfaces is strongly modified when the Cu atoms are supported on an Al( 111) surface. The nonconventional bonding of carbon monoxide to metal surfaces has been observed in several recent studies.'-8 On clean and K-promoted single-crystal metal surfaces, CO has been reported to bond in structures which are inclined (C-O molecular axis tilted toward the surface plane) or side-on (C-0 molecular axis parallel to the surface plane). Such structures have been observedvibrationally on K-promoted Ru(001)' and on theclean surfaces of Fe(100),2.3Cr(110),4 Mo(~OO),~ and Mo(110).6 On bimetallic surfaces and in one single-crystal study,6CO has been observed to bond in di-u structures, where carbon and oxygen each bond chemically to the surface (Le., MX-C-O-M',).9 These di-u bound CO species have been identified vibrationally on Nicovered AI( l l l),' Al-covered Ni and Cu foils,8 and Mo( l A common characteristic of C O bound in these nonconventional structures is their unusually low C O stretching frequency. In the studies mentioned above, CO stretching frequencies were observed in the range 1060-1600 cm-I:l-8 far below the gas-phase CO stretching frequency of 21 4 3 cm-I and below the C-0 stretching frequency observed for conventionally bonded CO. These low frequencies are believed to be due to the increased overlap of the CO 2r* and surface d-band orbitals which occur favorably for the tilted CO structures. This increased orbital overlap allows larger electron donation from the surface d-band into the 277* antibonding CO molecular orbital than occurs in traditional carbonyl surface bonding via the carbon moiety alone.' The weakening of the C-0 bond may result in rupturing of this bond. For this reason, these tilted' and di-u bonded9 species have been referred to as stable surface precursors to C O dissociation. This paper utilizes high-resolution electron energy loss spectroscopy (HREELS) and Auger electron spectroscopy (AES) to provide evidence for the existence of a CO precursor species (to dissociation)on Cu-covered AI( 1 11) at 95 K. This CO precursor most likely possesses a di-u structure, Cu,-C-O-Al,, which is consistent with its low-frequency C-0 stretch at 1260 cm-1 and its dissociation upon heating to 348 K. CO dissociation on copper

' Permanent address: Exxon Research and Engineering Co., Rte 22, Annandale, NJ 08801. * To whom correspondence should be addressed. 0022-3654/93/2097-2107%04.0010

covered aluminum clearly distinguishes this CO species from CO observed on clean copper, which is known to only weakly bond CO, as indicated by its low desorption temperature (240 K).lO In addition clean aluminum has been shown to be unreactive toward CO. Al( 111) does not even adsorb CO at 95 K,' and Al(100) has been shown to desorb CO at 125 K."

Experimental Methods The experiments reported here were performed in a three-level stainless steel ultrahigh-vacuum (UHV) chamber with a typical base pressure below 1 X 1O-Io mbar. A detailed description of the chamber, along with the cutting, mounting, and cleaning procedures for the Al( 11 1) crystal, has been given previously.12 HREELS was performed using a Leybold-Heraeus ELS-22 spectrometer. The incident elastic beam energy was between 3.0-3.5 eV with a typical full width at half-maximum (fwhm) of 70-90 cm-I for the specular beam. Elastic peak intensities of approximately 2 X lo5 counts/s (cps) were typically obtained from a clean and well-annealed Al(111) surface at a total scattering angle of 120". Upon the evaporation of Cu on Al(1 l l ) , however, the elastic peak intensity decreased by 1-2 orders of magnitude, and the fwhm degraded to 90-120 cm-I. This is probably due to surface roughening by the copper overlayer. A single-pass Perkin-Elmer Auger electron spectrometer was employed with a beam energy of 3 keV, a modulation voltage of 6 eV peak-to-peak, and a total electron beam current at the crystal of 3.0 MA. The Al(111) crystal was cleaned by repeated cycles of Ar+ sputtering followed by annealing for 1-2 h at 700 K. Carbon, oxygen and copper contaminants were always reduced to less than 0.5 atom % in the sampling depth of Auger spectroscopy prior to performing experiments. Copper deposition was carried out with a shielded metal evaporation source. It consisted of 0.25-mm-diameter copper wire (99.999% purity) wrapped around a resistively heated tungsten wire of 0.5-mm diameter. This thin tungsten wire was mounted on thicker tungsten wire hairpin support loops (1.O-mm diameter) which were independently degassed in a vacuum prior to copper deposition. This arrangement results in low outgassing rates during copper deposition. Copper coverages reported in this paper are given in monolayers (ML). The monolayer calibration was achieved by using AES to monitor the deposition of copper on an A1203 substrate in work to be published e1~ewhere.l~A reproducible distinct break in the copper Auger 0 1993 American Chemical Society

Colaianni

The Journal of Physical Chemistry, Vol. 97,No. 1 1 , 1993

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CO Adsorption on a Cu/AI(III) Interface a s a Function of Cu Coverage I

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Figure 1. HREELS measurements of a saturated C O layer on Cu/AI( I 1 1) interfaces as Cu coverage is increased up to one monolayer. The copper deposition and HREEL measurements were all carried out at an Al(111) crystal temperature of 95 K.

intensity versus deposition time indicated the completion of the first copper monolayer. The C O gas (99.99% purity) was obtained from Scientific Gas Products in a break-seal flask and was used without further purification. C O was introduced onto the surface through a carefully calibrated molecular beam doser containing an internal -2-pm-diameter orifice, to limit the gas flow rate, and a glass capillary array ~ o l l i m a t o r . ' ~The ~ ~ 5 total C O flux which was emitted from the collimator was calculated to be 6 X l o t 3C O s-I Torr-'. This was determined from accurate measurements of the gas depletion rate from the gas storage vessel behind the orifice. The actual C O exposure to the Al( 11 1) crystal was estimated by incorporating a correction factor (0.28) which accounted for our doser-sample geometry.I6

Results

Low TemperatureCOAdsorption. Figure 1 shows the HREEL spectra obtained from saturation-CO coverages on Cu/Al( 11 1) interfaces at 95 K as a function of copper coverage. As previously reported,' and as shown in Figure la, CO does not adsorb on a clean Al( 111) crystal at 95 K. However, CO readily adsorbs on copper deposited onto a Al( 111) surface at 95 K (Figure lb-d). Figure 1b displays the C-0 and the Cu-CO stretching modes at 21 15 and 355 cm-1, respectively, on a copper coverage of 0.2 ML. These two vibrational features, due to a terminally bound CO species, are also the dominant modes a t higher copper coverages (Figure lc,d). A more interesting observation in Figure 1 is seen on a copper layer at a coverage of 0.4 ML of Cu (Figure IC). An additional relatively weak vibrational feature is observed at 1260 cm-1. This 1260-cm-' vibrational mode was best observed on interfaces possessing roughly one-half monolayer of copper on the AI( 11 1 ) substrate. As discussed in the Introduction, C-0 stretching frequencies between 1100 and 1600 cm-l have been associated

1000

97

2000

Energy Loss (cm-l) Figure 2. Thermal behavior of a CO/Cu/Al( 11 1) layer possessing 0.4 M L of Cu and a saturation exposure of CO. The layer was prepared at 95 K, momentarily heated to the indicated temperature and then cooled a t 95 K prior to the collection of spectral data.

with tilted or di-a bonded CO The low u(C0) mode at 1260 cm-' in Figure 1 can be assigned to CO bound in either a tilted or di-a structure. The assignment of this species will be discussed later. Thermal Stability of CO on Cu/Al(lll). HREELS Results. To further investigate the origin and nature of the 1260-cm-1 vibrational mode, the thermal behavior of a CO/Cu/Al( 111) interface containing 0.4 ML of Cu has been studied. Figure 2a is a HREEL spectrum recorded at 97 K prior to heating. Figure 2b-f was obtained after controlled heating of the layer (dT/dt = 2 K/s) to the indicated temperatures; all HREELS measurements were then performed after cooling to 95-100 K. After heating the layer to 148 K (Figure 2b) the feature at 1260 cm-1 remained unchanged, while the CuC-0 stretch at 2155 cm-1 and theCu-COstretchat 355 cm-1 werebothattenuated. Thisresults from the onset of terminal-CO desorption from copper, as previously reported.I0 Continued heating of this layer to 199 K (Figure 2c) causes an almost total depletion of the t e r m i n a l 4 0 feature and the elimination of the distinct copper-carbon stretch at 355 cm-I. With the removal of the terminal-CO species, a new mode at -600 cm-I is observed in the spectrum. By 250 K all terminal-CO is removed while the 1260 cm-I feature is still observed and the - 6 0 0 - ~ m - ~peak appears to intensify. Heating to 348 K (Figure 2e) causes the removal of the 1260-cm-1 feature, and the development of two new vibrational features at -630 and -860 cm-I. These two vibrational modes have been shown toresult from theonset of Al( 111) oxidation.'* Thus, theabsence of the 1260-cm-1 mode after heating to 348 K and thedevelopment of a layer showing vibrational features similar to those of aluminum oxide is indicative of C-0 bond dissociation by 348 K. After further heating to 600 K (Figure 2f) the HREEL spectrum shows only two aluminum oxide related vibrational features at 670 and 860 cm-I, respectively. Auger SpectroscopicResults. An Auger analysis of the surface, shown in Figure 3, indicates carbon and oxygen remain on the surface after heating a CO/Cu/Al( 1 11) interface to 400 K. Figure

Facile Carbon Monoxide Dissociation on Cu Auger Study o f

CO

The Journal of Physical Chemistry, Vol. 97, No. 1I , 1993 2709

Adsorptlon on a C u / A I ( i i i I Interface

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3a shows a clean Al( 111) surface after sputtering and annealing as described in the Experimental Methods section. Upon the deposition of copper at 95 K (Figure 3b), small carbon and oxygen Auger signals are detected.I7 However, increased carbon and oxygen Auger signals are recorded after the Cu/Al( 111) interface is exposed to a saturation-CO coverage and then heated to 400 K (Figure 3c). This increase in the adsorbed carbon and oxygen must result from CO dissociation upon heating the Cu/Al( 111) interface.

Discussion From all the results presented above, one can conclude that the behavior of CO either on Al( 111) or on a copper surface is dramatically different from our observed behavior of C O on a Cu/Al( 111) interface. While terminal-COadsorbs on thecopperaluminum interface a t all coverages at 95 K, as also observed on a clean copper surface? copper coverages close to one-half of a monolayer produce an additional CO-related feature yielding a weak stretching mode a t 1260 cm-1 which has not been observed on copper alone. The appearance of this low-frequency C-0 stretching mode results from the influence of aluminum on surface copper atoms. This interaction can occur in one of two possible ways: either (1) aluminum chemically modifies the copper overlayer, causing it to bind C O in a new manner, or (2) aluminum directly interacts with the adsorbed CO molecule. The appearance of the unusually low C O frequency at 1260cm-l, when the exposed copper and Al(111) surface sites are approximately equal in number suggests direct A1 bonding to adsorbed CO. At one monolayer copper coverage, the 1260-cm-I mode is not observed due to the depletion of bare aluminum sites. Also, ultraviolet photoelectron spectroscopy (UPS)studies of copper overlayers deposited on an Al( 111) substrate have observed that the valence band of the copper remained unaffected for copper coverages between 0.2 and 2.3 ML.I8 Copper coverages of 10 ML were required before bulklike copper valence structures were obtained. This indicates that an identical electronic modification of the valence band of the copper exists from the submonolayer to multilayer range.I8 However, the 1260-cm-l CO species is not observed on copper coverages of 1 ML (Figure Id), but only on a layer which exposes both copper andaluminum sites. Therefore, a direct interaction of aluminum with the adsorbed C O is the favored interpretation from the results obtained in this study, and we can discount the idea that an electronic modification of copper atoms by aluminum is the dominant factor here. Also, a di-ostructure for CO(a) seemsvery plausible when oneconsiders the large affinity of A1 for ~ x y g e n . ~

-

Additional support for the identification of the 1260-cm-1 CO as a di-o bound species is obtained from an analysis of the vibrational spectra prior to the rupture ofthis bond. The 600-cm-1 loss resolved a t 199 K (Figure 2c) can be assigned to an Al-0 bond in the Cu,-C-&AI, species by comparison with a similar A1-0 stretching frequency (655 cm-I) which was observed from an AI-O-CH, species on Al( 11l).I9 Upon heating chemisorbed CO to 348 K, the 1260-cm-l band disappears as the 600-cm-I mode shifts to 630 cm-I and becomes a shoulder on the newly emerged 860-cm-l peak. These two frequencies are very similar to A1203phonon modes which have been assigned to surface (645 cm-1) and subsurface oxygen (845 cm-l) modes in the A1203 lattice on an AI( 111) crystal.20 It should also be pointed out that Figure 2f could also contain spectral contributions from the u(Al-C) stretching modes. In a previous investigation, a u(A1-C) mode appeared near 720-820 cm-I, although its intensity was much weaker than those of the u(A1-O) vibrational modes.21The simultaneous appearance of A1-0 modes as the 1260-cm-I CO(a) species disappears suggests that this species is the precursor to C O dissociation. Thus, metallic aluminum is a strong promoter for C O dissociation on copper since aluminum supplies a receptor site for the oxygen moiety from CO.

Conclusions

In summary, we have observed the decomposition of C O on a Cu/Al( 111) interface at 348 K. CO dissociation results from a chemisorbed CO species which has a very low C-0 stretching frequency at 1260 cm-I. This precursor species dissociates on heating to 348 K, leaving C O decomposition products on the surface as observed by A S . The 1260-cm-' CO species is believed to be bound in a di-astructure, in which both C and 0 are directly bonded to metal atoms, Le., Cu,-C-O-Al,. This view is supported by the following observations. First, the 1260-cm-1 species on a Cu/AI( 111) interface a t 95 K is most intense when the number of exposed sites of both copper and aluminum are approximately equal. Second, a vibration at 600 cm-I is detected prior to the thermal dissociation of the 1260-cm-I CO species, which we assign to the AI-0 bond of the Cu,-C-O-Al, species. In addition to strongly bonded C O on this bimetallic surface, conventional terminal bonding of C O is also observed on certain Cu sites which presumably do not possess nearby A1 neighbor sites properly situated to bond Cu,-C-0-AI, species. More detailed studies are needed to answer questions about the role of C O coverage and Cu coverage on the dissociation of C O on this bimetallic surface. Acknowledgment. We acknowledge with thanks the support of this work by the Department of Energy, Office of Basic Energy Sciences. References and Notes ( I ) Hoffmann, F. M.; dePaola, R. A. Phys. Rev. Lerr. 1984, 52, 1697. (2) Moon, D. W.; Bernasek, S. L.; Lu, J . P.; Gland, J. L.; Dwyer, D. J. Surf. Sci. 1987, 184, 90. (3) Benndorf, C.; Kriiger, B.; Thieme, F. Surf. Sci. 1985, 163, L675. (4) Shinn, N. D.; Madey, T. E. J. Chem. Phys. 1985.83, 5928. (5) Zaera, F.;Kollin, E.; Gland, J. L. Chem. Phys. Leff. 1985,121,464 (6) Chen, J . G.; Colaianni, M. L.; Weinberg, W. H.; Yates, J. T., Jr. Chem. Phys. Left. 1991,177, 113. Colaianni, M. L.;Chen, J.G.; Weinberg, W. H.; Yates, J. T., Jr. J. Amer. Chem. Soc. 1992, 114, 3735. ( 7 ) Chen, J. G.; Crowell, J. E.; Ng, L.; Basu, P.; Yates, J. T.. Jr. J. Phys. Chem. 1988, 92, 2574. (8) Rao, C. N . R.; Kajumon, M. K.; Prabhakaran, K.; Hegde, M. S.; Kamath, P. V . Chem. Phys. Left. 1986, 129. 130. (9) Sachtler. W. M. H.; Shriver, D. F.; Hollenberg, W. B.; Lang, A. F. J . Cafal. 1985, 92, 453. (IO) Harendt, C.; Goshnick. J.; Hirschwald, W. Surf.Sci. 1985,152/153, 453. (11) Paul, J.; Hoffmann, F. M. Chem. Phys. Letf. 1986, 130, 160. (12) Crowell. J. E,: Chen. J. G.; Yates. J. T.. Jr. Surl. Sci. 1986, 165, 37. (13) Chen, J: G.; Colaianni, M. L.; Weinberg, W.-H.; Yates, J. T., Jr. Surf. Sci. 1992, 279, 223. ~I

2710 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 (14) Bozack, M. J.; Muehlhoff, L.; Russell, J. N., Jr.; Choyke, W. J.; Yates, J. T., Jr. J. Vac. Sci. Technol. 1987, AS, 1. (15) Smentkowski, V. S.; Yates, J. T., Jr. J. Vac.Sci. Techno/. 1989, A7, 3325. ( I 6) Campbell. C. T.; Valone, S.M. J . Vac. Sci. Technol. 1985, A3,408. (17) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Raich, G. E.;

Weber, R. E. HandbookofAuger EIectronSpectroscopy; Perkin-Elmer: Eden Prairie, MN, 1976.

Colaianni (18) Barnes, C. J.; Asonen, H.; Salokatve, A,; Pessa, M. Sur/. Sci. 1987, 184. 163. (19) Chen, J. G.;Basu, P.; Ng, L.; Yates, J. T., Jr. Sur/. Sci. 1988, 194, 397.

(20) Erskine, J. L.; Strong, R. L.; Phys. Reu. 1982, E25, 5547. (21) Chen, J. G.; Beebe, T. P., Jr.; Crowell, J. E.; and Yates, J. T., Jr. J . Am. Chem. SOC.1987, 109. 1726.