J. Phys. Chem. 1994,98, 9175-9181
9175
Activation of Carbon Monoxide and Carbon Dioxide at Cesium-Promoted Cu( 110) and Cu( 110)-0 Surfaces A. F. Carley, M. W.Roberts,’ and A. J. Strutt School of Chemistry and Applied Chemistry, The University of Wales, Cardiff CFl 3TB, United Kingdom Received: February 23, 1994; In Final Form: May 10, 1994”
Through a combination of X-ray photoelectron and vibrational spectroscopies the interaction of carbon monoxide and carbon dioxide with cesiated C u ( l l 0 ) and C u ( l l 0 ) - 0 surfaces has been investigated. At 80 K carbon monoxide is adsorbed a t an atomically clean C u ( 110) surface with a characteristic loss feature a t 2085 cm-1 and O( 1s) and C ( 1s) core-level binding energies a t 533.8 and 290 eV, respectively. Exposure of the C O adlayer to cesium a t 80 K effects a decrease in the frequency of the loss feature to 1730 cm-’ and on warming to 298 K this is replaced by two loss features a t 1450 and 1960 cm-’. The O(1s) peak has simultaneously shifted to 532 eV. The interaction of C 0 2 with Cu( 1 10)-Cs a t 80 K is characterized by loss features assigned to the ionic COz&(a) and physically adsorbed CO2 species; on warming to 298 K the spectrum simplifies to generate two strong losses a t 1380 cm-’ and 1500 cm-1 and weaker losses a t 1050,600, and 350 cm-l and assigned to carbonate formation. Both chemisorbed and coadsorbed oxygen a t a Cu( 1 10)-Cs surface are highly reactive to C O with both core-level and vibrational spectroscopy providing evidence for low-energy pathways to surface carbonate via C02& as an intermediate. These pathways are also available at atomically clean Cu( 1 10) surfaces; Le., in the absence of cesium. In this case when CO2-rich COz-dioxygen mixtures are coadsorbed reactive oxygen transients participate in the chemistry.
Introduction Studies of the interaction of carbon monoxide with metal surfaces have been central to the development of surface science over the last 2 decades. The distinction between dissociative and molecular regimes of chemisorption and the development of current views on CO bonding were the result of the application of both photoelectron and vibrational spectroscopies.’ Correlations were shown to exist2 between the O(1s) binding energy of CO(a), the strength of the chemisorption bond, and the tendency for C-0 bond cleavage, with the early views of Blyholder and those implicit in the Dewar-Chatt model for bonding in metalcarbonyl complexes being vindicated by the surface science approach. More recently much attention has, however, been given to the influence of surface additives-particularly alkali metals-on CO bonding, mainly at transition-metal surface^.^ That additives other than alkali metals influence the surface chemistry of CO has also been recognized-sulfur inhibiting CO dissociationat iron surfaces4 and aluminum promoting dissociation at nickel5 and copper surfaces.6 The latter observation, reported by Rao and his colleagues5at Bangalore, went largely unnoticed until Yates et a1.6 published analogous observations of very low (1200 cm-1) loss peaks for CO in the Al( 1 1 1)-Cu system. That it might be possible to “tune” s- or p-metal surfaces to exhibit a chemistry more characteristic of the transition or d metals was indeed proposed by Mason,’ who simultaneously drew attention to the lack of real distinction between sp- and d-metal surface chemistry. In this paper we report investigations of the adsorption and reactivity of carbon monoxide and carbon dioxide at cesiated Cu( 110) and Cu( 1 10)-0 surfaces using a combination of in situ XPS (X-ray and photoelectron spectroscopy) and VEELS (vibration electron energy loss spectroscopy). The advantages of this combination is that core-levelspectra provide both quantitative surface concentration data and through shifts in binding energies chemical identificationof species,whereas VEELS can distinguish between structural aspects of the same species. We report experimental data for the chemisorption of both carbon monoxide and carbon dioxide at cesium-modified Cue Abstract published in Aduonce ACS Abstracts, August 15, 1994.
0022-3654/94/2098-9 175$04.50/0
(110) surfaces over a wide temperature range (80-360 K). In addition the role of cesium in the chemistry of both carbon monoxide and carbon dioxide at Cu( 110)-0 surfaces is investigated. Emphasis is given to low-temperature chemistry in order to search for direct spectroscopic evidence for the participation of transient species and in this sense these studies extend those reported by Campbell et a1.8 using XPS, Auger electron spectroscopy, and temperature programmed desorption.
Experimental Section The experimental strategy involved the use of a specially designed ultra-high-vacuum spectrometer incorporating photoelectron, electron energy loss, and low-energy electron diffraction facilities. The XPS,UPS,and LEED components were supplied by VG Scientific (East Grinstead) and the HREELS (or VEELS) by VSW (Manchester). Cu( 110) single crystals were supplied by Metal Crystals & Oxides Ltd. (Cambridge). Data analysis, and in particular the estimation of surface concentrations from the intensities of core-level spectra, has been described el~ewhere.~ Spectroscopic pure gases, further purified by passing through cold traps, were obtained from P.J. Mason and cesium was deposited from a well outgassed SAES getter. We stress the particular advantages in obtaining both corelevel and vibrational spectra for each of the systems we have investigated. Vibrational spectra, without the relevant surface concentration data (from XPS),can be misleading, strong loss peaks not necessarily implying the presence of a high coverage species. Results
Adsorption of Carbon Monoxide at Cu(110)-Cs Surfaces in the TemperatureRange 80-360 K. The O(Is) and VEEL spectra for the adsorption of carbon monoxide on an atomically clean Cu( 1 10) surface at 80 K are shown in Figure 1. The O(1s) and C(ls) spectra consist of broad peaks centred at 533.5 and 290 eV, respectively, while the characteristic loss feature YCO is 2085 cm-1 with also a Cu-CO stretch at 250 cm-1. The loss at 750 cm-l we assign to 6(H20) present at very small concentrations with no evidence for VOH at 3600 cm-I. On depositing cesium on 0 1994 American Chemical Society
9176
Carley et al.
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
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Figure 1. (a) O(1s) and C(1s) spectra for the adsorption of CO(g) at a Cu(l10) surface at 80 K followed by exposure to Cs atoms at the same temperature and warming to 298 K. Also shown are the atom concentrations ( u ) for oxygen and carbon calculated from the O(1s) and C( Is) intensities. (b) CorrespondingVEELspectra taken with an electron beam energy of 5 eV for CO(a) at 80 K, under the influence of Cs at 80 K, and after warming to 298 K.
to the CO adlayer a t the same temperature, there is an immediate downward shift in YCO by about 350 cm-l to 1730 cm-l. The O(1s) peakincreases slightly in intensity due to C O pickupduring Cs deposition and also shifts to lower binding energy. On warming the composite Cu(1 10)-Cs-CO adlayer to 298 K the O(1s) intensity decreases and the binding energy shifts to 531.9 eV. Most significantly, however, there are now two distinct loss features at 1450 and 1960 cm-l in the VEEL spectrum. The O(1s) intensity at 53 1.9 eV indicates an "oxygen" surface concentration of 1.6 X 1014atoms cm-2 and the C(1s) intensity at 289.4 eV indicates approximately the same carbon atom concentration (- 1.5 X 1014 carbon atoms cm-2). The accuracy with which we can estimate the surface concentration of carbon atoms from C ( 1s) spectra is not as high as that for oxygen from O(1s) spectra, but the results are consistent with just CO(a) being present at the Cu(ll0)-Cs surface at 298 K. The loss spectra, however, indicate that there are two distinct states of carbon monoxide. It is well established that adsorption of carbon monoxide is relatively weak at copper surfaces, the maximum heat of adsorption being -80 kJ mol-' and desorption being complete
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Figure 2. (a) O(1s) and C(1s) spectra for CO chemisorption at a clean and cesiated Cu( 110)surface at 360 K. Also indicated is uothe "oxygen" atom concentration ( 2 X l O I 4 cm-2) calculated from the O( Is) intensity at 360 K. ucsestimated from the Cs(3d) intensity was 3.6 X 10'' cm-2. (b) VEEL spectra for CO adsorption at a clean Cu( 110) and cesiated Cu(ll0) surface at 360 K.
below 200 K. This is consistent with the observation that when a C u ( l l 0 ) surface was exposed (20 klangmuirs) to carbon monoxide at 360 K no loss features were observed in the VEEL spectrum (Figure 2b). However, when cesium was deposited in the presence of CO(g) at this temperature a distinct loss peak appears immediately at 1430 cm-1 and with increasing exposure to C O two other losses emerge at 1600 and 1800 cm-l. This is compatible with other observations of the influence of alkali metals on CO chemisorption at transition metals-loss peaks moving to higher frequencies with increasing CO coverage. Therecent FTIR data10 for CO chemisorption at Pt(1 11)-K surfaces illustrate this but with the advantage of the higher resolution available compared with VEELS. Core level spectra in the O(1s) and C ( 1s) regions also indicate (Figure 2a) that the cesiated surface at 360 K is active in CO chemisorption with intensities present at -532and287eV,respectively. AnalysisoftheO(1s) spectrum establishes the presence of a total of 2 X 1014 "oxygens" cm-2. We therefore interpret our data simply as indicative of a shortrange interaction between CO and Cu( 110)-Cs sites involving charge transfer from Cs to "adjacent" copper atoms and to the 2n* orbitals of the adsorbed carbon monoxide. Consequently the molecule is adsorbed more strongly than on the clean Cu(110) surface. The question as to whether or not surface
Activation of CO and C02
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9177
compounds are formed in similar systems (e.g., C O adsorption at Cu( 110)-K) has recently beendiscussed but rejected by Bonze1 and Pirug." Clearly there are distinct sites present at the Cu(1 10)-Cs surface capable of bonding carbon monoxide in states where the carbon-oxygen bond is considerably reduced in strength compared with that for CO bonding a t atomically clean Cu(1 10). The shifts in the O( 1s) binding energy of CO(a) to lower values (e.g., Figure l a ) in the presence of cesium (533.5 to 53 1.9 eV) also support this view, indicating an increase in the electron density at the oxygen atom through back-donation to the 2?r* orbitals and consequent weakening of the carbon-oxygen bond. The carbon monoxide is therefore in a potentially more reactive state than when present at the clean Cu( 1 10) surface. Using the correlation between O( 1s) binding energies and the heat of CO adsorption suggests that accompanying weakening of the carbonoxygen bond there is a concomitant increase in the heat of adsorption2 from about 80 kJ mol-' (clean surface) to about 150 kJ mol-' at the Cu( 110)-Cs surface. It follows that a given CO coverage can be sustained to a higher temperature at the cesiated surface than at an atomically clean Cu( 1 10) surface. Adsorption of Carbon Monoxide at Cesium-Modified Cu(110)-0 Surfaces at Low Temperatures. Subsequent to the chemisorption of oxygen at a Cu( 1 10)-Cs surface at 298 K the composite surface was cooled to 80 K and exposed to carbon monoxide at this temperature (Figure 3). Chemisorbed oxygen is characterized by an O(1s) peak at 529.8 eV (Figure 3a) and a loss feature at 350 cm-1 (not shown). The electron energy loss (VEEL) spectrum (Figure 3b) after exposure to carbon monoxide at 80 K had intense features at 660, 1300, and 1400 cm-' with weaker losses at 1640,2080, and 2345 cm-l. The corresponding O(1s) spectrum had an intense peak at 535.9 eV, a peak a t 529.8 eV characteristic of chemisorbed oxygen, and a broad band of intensity from 531 to 533 eV. The peak a t 535.9 eV is unambiguously assigned to physically adsorbed C02, which is also characterized by the presence of C( 1s) intensity at 292.1 eV. The O(1s) intensity in the 531-533 eV range we assign to a second more strongly chemisorbed C02&(a) species. The clear implication is therefore that carbon monoxide hasceacted at the cesium-modified Cu( 110)-0 surface to generate at 80 K carbon dioxide in two forms; one weakly adsorbed species CO2(a) the other more ionic COf(a). The O(1s) spectral region is curve fitted (Figure 3c) to reveal three distinct components with binding energies of 529.9,532.4, and 536.0 eV and FWHM values of 2.4, 2.8, and 2.3 eV, respectively. The ratio of the carbon atoms (calculated from the C( 1s) intensity) to the total oxygen atom concentrationscalculatedfrom the t w o 0 ( 1s) componentsat 532.4 and 536.4 eV is close to 1:2 (uo = 1.03 X 10'5 atoms cm-2; u, = 4.7 X 10i4 atoms cm-z), establishing the presence of adsorbed C02 clearly in two different states C02"(a) and C02 physically adsorbed. The VEEL spectra (Figure 3b) support this. We have reported elsewherei2 electron energy loss spectra for weakly adsorbed COz at a Bi(OOO1) surface at 80 K; for CO2 present at 0 = 0.5 there is a very intense loss peak at 670 cm-' assigned to S(OC0) and two much weaker features at 1360 and 2390 cm-' assigned to u,(COO)/ZS(OCO) and v,(COO), respectively. We have in addition to these (Figure 3b), losses at 1400 and 1640 cm-I, which we assign to v,(OCO) and v,(OCO) of the more strongly chemisorbed CO+- species, and a very weak feature at 2080 cm-*,which we assign to ucopresent at an unmodified copper site. On warming to 298 K the VEEL spectrum simplifies to give strodg loss peaks at 380,600,1380, and 1500 cm-I and a weaker feature at 1050 cm-l. The O(1s) spectrum (Figure 3a) develops slight asymmetry on the high binding energy side and the C(1s) region shows some intensity at 289 eV. The latter corresponds to a carbon concentration of 1 X loi4atom cm-2, which we suggest is associated with surface carbonate. The VEEL loss features (Figure 3b) confirm this assignment in that the 1500
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Figure 3. (a) O(1s) and C(1s) spectra for the reactive chemisorption of carbon monoxide at a Cu( 1 lO)-Cs-O surface at 80 K and after warming to 298 K. Also shown are uoand ucvalues for some spectra. (b) VEEL
spectra for reactive chemisorption of CO at a Cu(1 lO)-Cs-O surface at 80 K followed by warming to 298 K. (c) Curve fitting of the O(1s) spectral region 529-536 eV (see part a) revealing three distinct species: chemisorbed oxygen O"(a), chemisorbedanioniccarbon dioxideCO*&(a), and physically adsorbed COz(a) at 80 K. and 1380 cm-l losses correspond to V(CO) of monodendate and bidendate carbonate species, 1050 cm-' corresponds to u,(OCO), and 600 cm-' corresponds to 6, (OCO) of the carbonate species. If a Cu(ll0)-Cs surface (ucS= 3.7 X 10i4 cm-2) is exposed to dioxygen a t 298 K (Figure 4a) followed by carbon monoxide at 360K, theO(1s) peakat 529.5 eV,characteristicofchemisorbed oxygen, is replaced with increasing CO exposure by intensity a t 53 1.2 eV. The C(1s) region shows intensity peaking at 289.2 eV but with asymmetry to lower binding energy. The electron energy lossspectrum (Figure4b) showsfeaturesat 335 (vcUo),605 (UWO), 1045 (~,oco), 1500 ( Y C ~ ) and , 1840 cm-i ( V C O ) . There is also a weak feature at 1300 cm-1. Changes in the O( 1s) spectral region suggest that chemisorbed oxygen present a t the Cu( 1 10)-Cs surface is very reactive and is transformed on exposure to carbon monoxide a t 298 K directly to carbonate. The corresponding VEEL spectrum (Figure 4b, ucs = 3.7 X 1014 cm-2) confirms this conclusion-the only difference between it and that observed in the low-temperature reaction of CO with the cesiated Cu( 110)-0 surface (Figure 3b) is that the 1380-cm-1v,(OCO) loss feature is much weaker and that chemisorbed carbon monoxide is present with a characteristic vco loss feature at 1840 cm-1. The O(1s) binding energy of a
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Carley et al.
9178 The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 (a)
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Figure 4. (a) O(1s) and C(1s) spectra for the reactive chemisorption of CO(g) at an oxidized Cu(1 10)-Cs surface at 360 K. om = 3.7 X 1014 (b) VEEL spectra for CO adsorption at 360 K, for u- = 3.7 X 10'4 cm-2 (see 4a above) and for a higher cesium coverage om = 4.3 X
1014 cm-2 (a separate experiment). surface carbonate species is likely to be similar (53 1.5 eV) to that observed (see later). Whether or not a loss feature was observed at -1800 cm-I depended on the Cs coverage. At high ,a values (e.g., 4.3 X lOI4 atoms cm-2 or Bcs = 0.86) the VEEL spectrum shows intense losses at 315 and 1500 cm-1 with very weak features a t 700 and 1050 cm-I (Figure 4b). The O(1s) component at 531 eV corresponded to an oxygen atom concentration of 6.0 X 1014 cm-2, while the C ( 1s) peak at 289 eV corresponded to 2.2 X lOI4 carbon atoms cm-2, providing further support for the formation of surface carbonate, as indicated by the VEEL spectra and the O(1s) and C(1s) binding energies. Coadsorptionof Carbon Monoxideand Dioxygen at a Cu( 110)Cs Surface. When a Cu( 110)-Cs surface was exposed (200 langmuirs) to a mixture of C O and 0 2 a t 80 K the O(1s) and VEEL spectra shown in Figure 5 were observed. The O(1s) spectral region peaks at 53 1 eV but with also significant intensity in the region 531-534 eV and a smaller peak at 535 eV. The corresponding VEEL spectra exhibit all the loss features associated with physically adsorbed COz and its anionic form C02&-(a). TheselossfeatureswerealsoobservedwhenacesiatedCu(llO)-O surface was exposed to either CO(g) (Figure 3) or a cesiated C u ( l l 0 ) surface was exposed to CO2(g) a t 80 K (Figure 6). On warming the adlayer to 298 K the VEEL spectra indicate the
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Figure 5. VEEL and O(1s) spectra when a mixture of CO and 0 2 were coadsorbed at a Cu( 1lO)-Cs surface (0- = 3.5 X 1014cm-2) at 80 K and the adlayer was warmed to 298 K.
formation of surface carbonate is similar to that observed in both the COz-Cu( 110)-Cs (Figure 6) and the CO-Cu( 110)-Cs-0 systems at the same temperature. The O(1s) spectra at 298 K is confined to a single much narrower peak with a binding energy of 531 eV, also indicating the presence of carbonate (Figure 5). Adsorption of Carbon Dioxide at a Cesium-Modified Cu( 110) Surface at 110 K. When a Cu(1 lO)-Cs surface was exposed to 150 langmuirs of carbon dioxide at 80 K and then warmed to 1 10 K the O(1s) peak was broad and centered at 532 eV (Figure 6a). The corresponding C ( 1s) peak is at 289.0 eV. There is also very weak O(1s) intensity a t 536.0 eV associated with physically adsorbed carbon dioxide. The corresponding electron energy loss spectrum (Figure 6b) has strong losses at 295, 660, 1460, and 1660 cm-l with a weaker feature at 1975 cm-I. In view of the weak O(1s) intensity at 536 eV, we estimate that the surface concentration of physically adsorbed COz is no more than a few percent of a monolayer. Previous VEELS studies2 of weakly interacting CO2 with a Bi(OOO1) surface indicated a particularly strong loss peak at 670 cm-1 and we assign the feature at 660 cm-I (Figure 6b) to ba(0co)of COz(a). The losses a t 1460 and 1660 cm-1 we assign to U,(OCO) and v a ( ~ oof) the more ionic COzb(a) species, characterized also by the broad O( 1s) spectral region centered a t 532 eV and the C ( 1s) feature centered at 289 eV. The losses a t 1975 and 295 cm-l we assign to ~ C O and ) vcU-cofor CO(a) present in relatively small (0 < 0.1) concentration. On warming the adlayer from 110 K to 298 K the O(1s) and C(1s) intensities decrease with peaks at 531 and 289 eV (Figure 6a), while the electron energy loss spectrum simplifies to give strong losses a t 350 and 1510 cm-I and weaker losses at 700 and 1060 cm-I (Figure 6b). The assignments of these are as follows: 1510 cm-I (v-), 1060 cm-I (Y,OCO), 700 cm-l (~oco),and 350 cm-l (vcUa),all associated with a surface carbonate species. Although COz is unreactive a t a clean Cu(ll0) surface, when it is coadsorbed with oxygen in a COz-rich mixture, surface carbonate is formed with a characteristic O(1s) binding energy of 531 eV (Figure 7). Reactive oxygen surface transients are
Activation of C O and COz
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9179
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Figure 7. O(1s) and C(1s) spectra for a clean Cu( 110) surface at 83 and
295 K subsequent to the surface being exposed to a CO2-rich carbon dioxide-clioxygenmixture at 83 K and then warmed to 295 K. No spectra were recorded at 83 K after exposure to the mixture. This ensures that carbonate formation occurs without the possibility of an X-ray-induced reaction (see text and ref 12). 1
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Figure 6. (a) O(1s) and C(1s) spectra for the adsorption of COz (1.50 Langmuirs) at a cesiated Cu(ll0) surface at 80 K followed by warming (in the absence of X-rays) to 110 K and then to 298 K. Also shown are u0 and uc values calculated from the O(1s) and C(1s) intensities. The ua value was 3.8 X l O I 4 cm-2, (b) VEEL spectra at 110 K and 298 K (see part a).
suggested to provide the low-energy pathway in this case, a feature characteristic of the chemistry of gas mixtures where dioxygen is one component.I3 This is also analogous to our earlier observations'4of carbonate formation with CO-dioxygen mixtures at aluminum surfaces when oxygen in the "final chemisorbed state" is unreactive. Thus oxygen transients present under dynamic reaction conditions exhibit the same high chemical reactivity associated with chemisorbed oxygen present at a cesium-modified Cu( 110) surface.
Discussion Although the adsorption of carbon monoxide at atomically clean copper surfaces, of any crystal orientation, is relatively weak (AH< 80 kJ mol-'), the presence of cesium results in a number of distinct, more strongly chemisorbed, CO states at Cu(1 10) surfaces. For a given cesium coverage, CO states with characteristic loss features emerge with both increasing temperature and C O coverage. These vibrational loss features vco are in the range 1400-1960 cm-1, i.e., up to about 600 cm-l less than that characteristic of the atomically clean Cu( 110) surface (see Table l ) , reflecting variations in C - 0 bond strength and hence the bonding site, bonding configuration, and bond length. Most other analogous studies have to date been reported" for the Pt( 111)-K system and in this case the vco losses observed are in the range 1400-2100 cm-l. There are three examples where
~
TABLE 1: Assignments of Spectral Features for C02(a), C026(a),C03(a),CO(a),and W ( a ) Observed at Cu(llO), Cu(ll0)-Cs,and Cu(110)-OKsSurfaces' C02(a) COz'(a) Coda)
O(ls), eV C(ls), eV VEEL spectra, cm-l 660,1300,2350 536 292 532 289 700,1460, 1660 350,600,1050,1380, 531 289 1500 529.8 435 -533 289.4 250,2085
@-(a) CO(a)-Cu(1 10) CO(a)-Cu(l lO)-Cs 533 290 305,1730 80 K 298 K 531.9 1450,1960 a Assignments: 350 cm-I vm-C03;435 cm-' v m a ; 600 cm-' 60~0; 660 o ; cm-I v,(OCO); 1300 cm-1 260~0;1380 c m - l b ~ ~ 700 o ; c m - l b ~ ~1050 and 1500 cm-* vc3.0 for bidendate and monodendate surface carbonates; 1460 cm-I v,(OCO); 1660 cm-l v.(OCO); 2350 cm-I va(OCO). Losses at 1450, 1730, and 1960 cm-I are assigned to CO(a) at cesium-modified copper sites; the values observed are dependent on both Cs coverage and temperature (Figures 1 and 2). appreciably lower frequency vibrational losses have been observed for carbon monoxide: the A1(100)-K system studied by Paul and Hoffmann15 with losses a t 1060, 1250, 1750, and 1910 cm-1, the aluminum-promoted nickel system studied by Rao et al.5 with a loss at 1300 cm-1, and the aluminum-promoted copper system (Yates et a1.6) with a loss at 1260 cm-1. Adsorption states are suggested for chemisorbed carbon monoxide which are precursors to bond cleavage and dissociative chemisorption. The recent STM studies16 of the K-Cu(ll0) system are particularly significant in that they provide direct evidence that surface structural changes are a function of potassium coverage. A variety of missing-row-type reconstructions are observed where a single K atom removes on average 2.5 Cu atoms out of a (1 10) string of the unreconstructed (1 X 1) substrate and then fills the resulting "hole". Various phase (1 X 3) a t 6~ = 0.13, and a (1
9180 The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 X 2) missing row structure a t 6~ = 0.2 are observed. These clearly provide the special sites for strongly chemisorbed carbon monoxide. There is no analogous experimental data available for the Cu( 110)-Cs system but a LEED intensity analysisI7 of the Ru(0001)-Cs system was interpreted in terms of dipoledipole repulsions between Cs adatoms with initially (2 X 2) structures (6 = 0.23) followed by a series of structures with rotated unit cells and a ( d 3 X d3)R3Oo structure a t around 6 = 0.33. Whether modification of Cu( 1 10) by cesium rather than potassium generates very different structures from those observed by STM for the Cu( 110)-K system remains to be established. Only then will it be possible to define the nature of the surface sites responsible for the spectral features observed in the present work. What is, however, unambiguous from the present studies is that carbon monoxide present a t Cu( 1lO)-Cs-O surface is highly reactive and via a low-energy pathway involving reactive chemisorbed oxygen O*”(a) forms a surface carbonate
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Both the VEEL and XP spectra indicate the presence of a reactive Cob@) species which is readily converted to C0Z6(a), even at 80 K, at the cesiated Cu( 110)-0 surface. Similar chemistry is observed when CO(g) is coadsorbed withdioxygen at 80 K (Figure S), suggesting that subsequent to oxygen bond cleavage the reactive oxygen adatoms O*b(a) are largely scavenged by COb(s) to form COz(a) at 80 K and carbonate at 298 K. In the absence of surface oxygen the reactive Cob@) species form a stable adlayer (even at 360 K) and may reflect the formation a t high C O exposures of ethenedione COb(s) + CO(g) CZOz”(a). This reaction is thermodynamically favorable in the gas phase and has some support from recent FTIR studies as occurring a t oxide surfaces.’* In the case of the CO-Cu( 1 10)-Cs-0 system we have isolated the ionic C026(a) species as an intermediate in surface carbonate formation through the observation of characteristic loss features at low temperatures, supported by core-level O(1s) and C ( 1s) spectra (Figure 3). The spectra are similar to those reported by Freund and his colleagues for the N i ( l l 0 ) system,19 SolymosiZo for the Rh( 111)-K system, and also from this laboratory for CO2 interaction with aluminum, copper, and magnesium surfaces.21 Although Paul and his colleaguesz2 have presented strong arguments for surface oxalate ( C Z O ~ ~species -) as intermediates in carbonate formation, we are of the view that the ionic COz”(a) species are equally plausible. The fact that we observe (Figure 3) COzb(a) associated with physisorbed COz(a) also supports the Freund-Messmer dimer disproportionation mechanism COzb-C02 C03(a) + CO(g) as a low-energy pathway to carbonate formation. The experimental and theoretical evidence for the participation of COZ”(a) in the chemistry of COzat metalsurfaces has been reviewedl3vZ3with the conclusion that its formation is favored a t low temperatures, a t high COz pressures, and with atomically rough metal surfaces and especially those that have been modified by alkali metals and therefore of low work function. Evidence for UV-induced formation of COz”(a) has been reported recently by Solymosi and Kilivenyiz4 a t a Rh( 1 11)-K surface a t 90 K. They assign features in VEEL spectra at 1620,1320, and 830 cm-1 to the asymmetric and symmetric stretches and the bending mode of the anionic COz6 species. These observations are reminiscent of earlier reportsI2 of X-ray-induced C02” formation at copper surfaces. Furthermore the COz” species is chemically reactive, in that when coadsorbed with ammonia it
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Carley et al. forms a carbamateZSa t Cu( 110) and acetal and formate species in the presence of CHs radicals and hydrogen at N i ( l l 0 ) surfaces.23 Formate species also formz6when COZand HzO are coadsorbed at Mg(0001) and Al(11 l ) , C H stretch loss peaks being observed at 3000 cm-I. What is also clear from the present study is that discussions of the mechanism of methanol synthesis whether from C O or C02 cannot be progressed from arguments based solely on the established chemistry of atomically clean copper surfaces. Although there is now available extensive evidence for the role of surface oxygen in both adsorbate activation and opening up low-energy reaction pathways at metal surfaces, its role in methanol synthesis is still controversial. In a recent paper Mukhler et al.z7concluded that “methanol is formed on an essentially 0-*free copper metal surface”. On the other hand, Fujitani et a1.z8established that oxygen activation was coverage specific, activation occurring provided eoxygen < 0.16. For Boxygcn > 0.18 the methanol synthesis activity decreased and the authors drew attention to the promoting role of specific Cu+-CuO sites at the surface. That surface oxygen can act as both a promoter and an inhibitor (poison) of surface-catalyzed reactions at metal surfaces is well-known13J9 and illustrated recently by the oxydehydrogenation reaction of ammonia a t single crystal copper surfaces.30 Imide species are formed through a low-energy pathway under conditions where both the “oxide overlayer” and the clean metal are unreactive. Furthermore there is no evidence from XPS or VEELS for any surface oxygen present (6 < 0.05) during the reaction. Nevertheless it is oxygen that plays the key role in the catalysis, and recent theoretical calculations by van Santen et ala3’emphasize the participation of oxygen transients with very short surface lifetimes in the oxydehydrogenation reaction. There has been a tendency to oversimplify the role that oxygen and oxides can have in catalytic chemistry largely due to the development of experimental techniques that are sensitive more to the static than the dynamic aspects of the system. LEED has, for example, suffered from its insensitivity to structural anarchy and provided information only on ordered structures. However, chemical reactivity is more likely to be related to the local arrangement of atoms a t surfaces (defects) and establishing their significance requires an experimental approach that is sensitive to the local order. In this paper we illustrate the advantages of combing XPS and VEELS a t low temperatures to isolate the reactive species-it is an approach that we have found valuable in making distinctions between the chemistry associated with chemisorbed oxygen, perfect and defective oxide overlayers, and oxygen transients. The chemistry of the Cu( 110)-Cs surface not only illustrates this approach but also highlights factors that favor the formation of the highly reactive COzb(a), the anionic form of chemisorbed carbon dioxide, likely to be central to the mechanism of methanol synthesis. Acknowledgment. We are grateful to the British Petroleum Research Laboratories as Sunbury-on-Thames (SERC CASE Award) and the EC for their support. Note Added in hoof. Since this work was submitted further evidence for both the anionic COz-(a) and carbonate species has been reported by Thomsen et al.32for the Cu( 1lO)-K-COz system using XPS and TPD. References and Notes (1) See, for example: van Santen, R. A. Faraday Symp. Chem. SOC. 1986,83, 1915. Roberts, M. W. Adu. Coral. 1980,29, 55. (2) Joyner, R. W.; Roberts, M. W. Chem. Phys. Lett. 1974, 29, 447. (3) Kiskinova, M. J . Vac. Sci. Technol. 1987,AS, 852. Bonzel, H.P. Surf. Sci. Rep. 1988,8, 43. (4) Kishi, K.;Roberts, M. W. J . Chem. Soc., Faraday Trans. I1975,71, 1715.
Activation of CO and COz ( 5 ) Rao, C. N. R.; Rajumon, M.K.; Prabhakaran, K.; Hegde, M. S.; Kamath, P. V. Chem. Phys. Lett. 1986, 129, 130. (6) Colliani, M. L.; Chen, J. G.;Yates, J. T. J . Phys. Chem. 1993, 97, 2707.
(7) Mason, R. Catal. Today 1992, 12, 409. (8) Rodriguez, J. A.; Glendening, W. D.; Campbell, C. T. J . Phys. Chem. 1989, 93, 5238. (9) Carley, A. F.; Roberts, M. W. Proc. R. SOC.London 1978, A363, 403. Roberts. M. W. Adv. Catal. 1980. 29, 55. (10) Tushaus, M.; Gardner, P.; Bradshaw, A. M. Surf.Sci. 1993, 286, 212. ( 1 1 ) Bonzel, H. P.; Pirug, G.In The Chemical Physics of Solid Surfaces; King, D. A., Woodruff, D. P. Eds.; Elsevier: New York, 1993; Vol. 6. (12) Browne, V. M.; Carley, A. F.; Cooperthwaite, R. G.; Davies, P. R.; Moser, E. M.; Roberts, M. W. Appl. Surf.Sci. 1991, 47, 375. (13) Roberts, M. W. Chem. SOC.Rev. 1989,18,451;3. Mol. Catal. 1992, 74, 1 1 ; Surf.Sci. 1994, 2991300, 769. (14) Carley, A. F.; Roberts, M. W. J . Chem. SOC.,Chem. Commun. 1987, 355.
(15) Paul, J. Nature 1986,323,701. Paul, J.; Hoffmann, F. M. J. Chem. Phys. 1987,86, 5188. (16) Schuster, R.; Barth, J. V.; Ertl, G.;Behm, R. J. Phys. Rev. E 1991, 44, '1 3689. (17) Over, H.; Bludau, H.; Skottke-Klein, M.; Ertl, G.;Moritz, W.; Campbell, C. T. Phys. Rev. E 1992, 45, 8638. (18) Giamello, E.; Murphy, D.; Marchese, L.; Marta, G.;Zechina, A. J . Chem. SOC.,Faraday Trans. 1993, 21, 3715. (19) Iling, G.;Heskett, D.; Plummer, E. W.; Somers, J.; Lindner, Th.; Bradshaw, A. M.; Buskotte, U.; Neumann, M.;Starke, U.; Heinz, K.; de Andres, P. L.; Saldin, D.; Pendry, J. B. Surf.Sci. 1988, 206, 1 .
The Journal of Physical Chemistry, Vol. 98, No. 37, I994 9181 (20) Berko, A.; Solymosi, F. Surf.Sci. 1986,201, L489; J. Catal. 1986, 104,458. Solymosi, F.; Bugyi, L. J. Chem. Soc., Faraday Trans. I1987,83, 2015. (21) Carley, A. F.; Gallagher, D. E.;Roberts, M. W. Spectrochim. Acta Sect. A 1987,43A, 1447; Surf. Sci. 1986,183, L263. Cooperthwaite, R. G.; Davies, P. R.; Morris, M.A.; Roberts, M. W.; Ryder, R. A. Catal. Lett. 1988, I , 11. (22) Paul, J.; Hoffmann, F. M. Catal. Lett. 1988, I, 445. (23) Wambach, J.; Freund, H.-J., private communication to M. W. R. (24) Solymosi, F.; Klivenyi, G.Catal. Lett. 1993, 22, 337. (25) Carley, A. F.; Davies, P. R.; Roberts, M. W. J. Chem. SOC.,Chem. Commun. 1989. 677. (26) Carley,'A. F.; Moser, E. M.; Davies, P. R.; Read, S.; Gallagher, D.; Zeini-Isfan, A. Unpublished results. (27) Muhler, M.; TBrenqvist, E.; Nielsen, L. P.; Clausen, B. S.; Top0e, H. Cafal.Lett. 1994, 25, 1. (28) Fujitani, T.; Saito, M.; Kanai, Y.; Kakumoto, T.; Watanabe, T.; Nakamura. J.: Tuchiima. T. Catal. Leu. 1994. 25. 271. (29) S&, Au, C. T:; Pashuski, A.; Read, S.;Roberts, M. W.; Zeini-Isfahan, A. (and references therein) In Adsorptionon OrderedSurfacesofIonicSolids and Thin Films; Freund, H.-J., Umbach, E., Eds.; Springer Series in Surface Sciences 33; Springer Verlag: New York, 1993; p 241. (30) Afsin, B.; Davies, P. R.; Pashuski, A.; Roberts, M. W. SurJ Sci. Lett. 1991, 259, L724. Boronin, A.; Pashuski, A.; Roberts, M. W. Caral. Lett. 1992,16,345. Afsin,B.;Davies,P.R.;Pashuski,A.;Roberts,M. W.;Vincent, D. Surf.Sci. 1993, 284, 109. (3 1) van Santen, R. A.; Neurock, M.; Biemolt, W.; Jansen, A. P. J. J . Am. Chem. SOC.,in press. (32) Thomsen, E. V.; Jargemen, B.; Onsgaard, J. Surf.Sci. 1994, 304, 85.
