12494
J. Phys. Chem. 1996, 100, 12494-12502
Polarization Modulation Infrared Reflection Absorption Spectroscopy of CO Adsorption on Co(0001) under a High-Pressure Regime Gerhard A. Beitel,* Amir Laskov,† Heiko Oosterbeek, and Edgar W. Kuipers‡ Shell Research and Technology Centre, Amsterdam, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands ReceiVed: January 2, 1996; In Final Form: April 12, 1996X
The adsorption of CO on Co(0001) has been investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-RAIRS), which has been applied for the first time in a study of a model system for a heterogeneous catalyst. The CO/Co(0001) system was studied in the pressure range from 10-10 to 600 mbar at temperatures between 300 and 550 K, showing the in situ potential of PM-RAIRS and the significant scope of this method for catalysis research. Linearly and bridge-bonded CO species could be distinguished on well-annealed surfaces. High-pressure RAIRS experiments done at room temperature were in agreement with previous low-energy electron diffraction (LEED) investigations in ultrahigh vacuum (UHV) at 100 K,3,4 indicating a transition in the CO layer from a (x3 × x3)R30° to a (2x3 × 2x3)R30° structure with increasing CO coverage. By comparison of well-annealed and Ar-sputtered (defective) surfaces, we could identify, at a high frequency of around 2080 cm-1, a CO species attached to defect sites. It is shown that annealing at 450-490 K at 100 mbar of CO pressure leads to the creation of defects at the cobalt surface. The defects influence the structure of the CO overlayer. The nature of this “defect”-bound CO is discussed. Postreaction X-ray photoelectron spectroscopy (XPS) showed the development of surface carbide upon annealing in CO, which is in good agreement with the vanishing of the RAIRS signal of adsorbed CO at temperatures above 520 K.
1. Introduction The surface chemistry of CO on cobalt is of considerable interest in relation to the commercial importance of cobalt-based catalysts in the Fischer-Tropsch reaction.1,2 Long, straightchain hydrocarbons are formed from CO and H2 (synthesis gas) at temperatures between 450 and 570 K and pressures in the 5-30 bar range. In spite of this, only a small part of the quite considerable amount of research spent on CO chemisorption on transition metal surfaces has dealt with the CO/cobalt system. These few CO/cobalt studies mostly deal with the hexagonal closed-packed Co(0001) surface. Adsorption of CO on this surface has been studied by low-energy electron diffraction (LEED), photoelectron spectroscopy (UPS, ARUPS, XPS), auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), electron energy loss spectroscopy (EELS), and work-function measurements (∆φ).3-6 LEED measurements revealed several CO superstructures,3,4 depending on the dose of CO or CO partial pressure. LEED, AES, UPS, XPS, TDS, and EELS studies have also been published for three other lowindex cobalt single-crystal surfaces, namely, Co(101h2), Co(112h0), and Co(101h0).7-11 Vibrational spectroscopy on Co(0001) and Co(101h2) surfaces has been done by Geerlings et al.,12,13 who applied high-resolution electron energy loss spectroscopy (HREELS). After dosing 10 langmuirs of CO (1 langmuir ) 10-6 Torr‚s) on a clean Co(0001) surface, they obtained a loss signal centered at 2000 cm-1, which was attributed to linearly bound CO. In a very recent reflection absorption infrared spectroscopy (RAIRS) study14 on Co(101h0) in combination with LEED and TDS, several ordered phases were found upon increasing the CO pressure up to 10-7 * Corresponding author. † Present address: School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel. ‡ Present address: Shell Research and Technology Centre, Thornton, P.O. Box 1, Chester, CH1 3SH, United Kingdom. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00045-7 CCC: $12.00
mbar at a surface temperature of 300 K or upon lowering the surface temperature to 150 K at a constant CO pressure of 10-8 mbar. All of the above-mentioned studies were performed under ultrahigh vacuum (UHV) conditions at CO pressures below 10-6 mbar. Hence, there is a huge pressure gap of at least 10 orders of magnitude between these studies and the conditions of the Fischer-Tropsch process. The pressure dependence of surface morphology and surface atomic structure, surface coverages, the nature of surface-adsorption sites, and surface reactions makes conclusions drawn from results obtained under UHV conditions of unknown relevance to industrial catalytic practice.15-17 Surface morphology and surface atomic structure can have a large impact on catalytic properties.16,18 Surface defects in particular can influence sticking behavior and can act as centers for dissociation in such a way that catalytic behavior is influenced. Changes in surface morphology upon high-pressure treatment have been observed on Pt(110) surfaces in oxygen, hydrogen, and carbon monoxide atmospheres by using an in situ scanning tunneling microscope (STM).19 Changes in the surface structure of Co(0001) upon synthesis gas treatment in a high-pressure reactor (520 K, 4 bar), observed by ex situ UHVSTM investigations, have been reported by Wilson and De Groot.20 An island structure was shown to develop on the initially atomically flat surface under Fischer-Tropsch synthesis conditions. The development of these islands was believed to be caused by an etch-regrowth process, whereby cobalt was removed from edge and kink sites and mobile cobalt species, which nucleate after reaching a critical density in islands on the terraces, were created. The mobile species were assumed to be cobalt subcarbonyls Co(CO)x (x ) 1-3). The presence of such species is known from supported rhodium catalysts21 and was also postulated on single-crystalline rhodium surfaces at CO pressures between 1 and 150 mbar,22 as well as on Rh field-emitter tips at 10-6 mbar of CO pressure.23 © 1996 American Chemical Society
CO Adsorption on Co(0001) For an investigation of weakly bound adsorbate layers and surface reactions, in situ methods have to be used. Unfortunately there are only a few in situ analytical techniques available that can be applied to characterize adsorbates under highpressure reaction conditions. Reflection absorption infrared spectroscopy (RAIRS) can provide such information,24-27 but at pressures above 10-3 mbar the contribution of the gas-phase CO to the absorption spectra obscures the weak surfaceabsorption signal of CO.28 It has been shown that RAIRS can be used to study CO adsorption at relatively high pressures (e10 mbar) by normalizing the desired spectrum against a spectrum taken at the same pressure at high temperatures (1000 K), where hardly any CO is adsorbed at the surface.29,30 However, it is also possible to account for gas-phase absorption in a more straightforward manner by coding the surfaceabsorption signal via the application of a polarization modulation (PM) technique to a conventional RAIR spectrometer. The efficiency of polarization modulation RAIRS in discriminating near-surface absorptions from the isotropic stray absorptions occurring in the gas phase has been demonstrated for both a setup using a dispersive infrared spectrometer and a setup using a Fourier transform (FT) spectrometer.31,32 The adsorption of CO and NO on polycrystalline Pt foils has been investigated by PM-RAIRS under CO pressure of up to 50 mbar,33,34 but in principle there is no limitation to applying higher pressures. Briefly, the basic principle of the PM-RAIRS method used in the present study comprises the combination of conventional Fourier transform RAIRS (grazing incidence of linearly polarized light) with a fast modulation of the polarization state of the incident electric field (ideally between p and s linear states) by using a photoelastic modulator (PEM). While p-polarized light produces a net surface electric field and thus can interact with adsorbed molecules, the net surface electric field in the case of s-polarized light vanishes. Both polarization states are, however, equally sensitive to gas-phase absorption because gasphase molecules are randomly oriented. By electronic filtering and demodulation, a differential reflectivity spectrum is computed that does not show contributions from the gas phase and, compared to the absolute measurement of a conventional RAIRS setup, has a much higher surface sensitivity. In the present study, the polarization modulation RAIRS technique (PM-RAIRS), based on a Fourier transform (FT) spectrometer,35,36 was applied for the first time in a detailed investigation of model systems for heterogeneous catalysts. We present results of the adsorption of CO on Co(0001) in a pressure range from 10-10 up to 600 mbar. The adsorption behavior of CO under the high-pressure regime (1-600 mbar) at temperatures between 300 and 520 K was investigated. We show that CO can be used as a probe molecule to investigate changes in surface defect structure and morphology. The influence of surface defects on the adsorption behavior was investigated by comparing an annealed and a sputtered cobalt surface. Postreaction X-ray photoelectron spectroscopy (XPS) was used to check a change in carbon species present at the surface that was caused, for example, by the dissociation of CO. The influence of hydrogen on the adsorption behavior of CO will be discussed in another paper.37 2. Experimental Section The experiments were performed in a modified MT 500 UHV system (base pressure ) 1 × 10-10 mbar) designed by Fisons Instruments. The UHV analysis chamber is equipped with an X-ray photoelectron spectrometer (Clam 2 analyzer), an electron energy loss spectrometer (LK 2000, LK Technologies), a
J. Phys. Chem., Vol. 100, No. 30, 1996 12495 quadrupole mass spectrometer for residual gas analysis, and an ion gun for sample sputtering. The sample can be heated in situ by radiation to 1200 K and cooled by liquid nitrogen (lN2) to below 100 K. Infrared spectroscopy was performed in a highpressure cell, which is connected via a transfer chamber to the analysis chamber. In the high-pressure cell, samples can be heated in CO/H2 gas mixtures to 650 K in a 10-10 mbar to 1 bar pressure range. The PM-RAIRS setup is described in detail elsewhere.28 The main parts are a BOMEM 100 series FTIR spectrometer, a photoelastic modulator (PEM), which enables us to switch the polarization of the incident beam, and a broadband lN2-cooled mercury cadmium telluride (MCT) detector. The polarized IR light of the incident and reflected beams is adjusted by parabolic and flat mirrors to the sample (grazing incidence of 82°) and to the detector with an overall optical path length through air of about 1 m. The high-pressure cell is connected to the optical beam via calcium fluoride windows. The signal from the detector is split and directed to either a high-pass filter (10 kHz) and a digital lock-in amplifier (output time constant ) 30 µs) to derive a difference spectrum (Rp Rs) or to a low-pass filter (6 kHz) to derive a sum spectrum (Rp + Rs). The spectra shown are the ratios (Rp - Rs)/(Rp + Rs), which are baseline corrected. Data acquisition and evaluation were done by BM Grams software (Galactic) installed on a commercial PC. Infrared spectra were taken with a resolution of 4 cm-1. For the difference spectrum, 250 scans were accumulated, and for the sum spectrum, 50 scans were accumulated, which required a total time of 3 min. The mirror speed of the IR spectrometer was 1 cm/s to obtain 100 scans/ min at 4 cm-1 resolution. For investigation of the region of the stretch frequency of adsorbed CO, a typical PEM setting of 1500 cm-1 was chosen, which provides optimum surface sensitivity in the regions of interest (i.e., 1600-2200 and 28003200 cm-1). The gases used were research-grade hydrogen 6.0 further cleaned by a Pd purifier and carbon monoxide 4.7. Carbonyls such as Ni(CO)4, which were present in the CO gas because of the contact with the stainless steel walls in the gas handling, were decomposed before reaching the high-pressure cell in a trap consisting of a copper pipe heated to 570 K containing zeolite 3A. The Co(0001) crystal (8 × 2 mm circular disk), with a purity of 99.999%, was purchased from Metal Crystals Ltd. The crystal was polished down to 0.25 µm with a maximum misorientation of (0.5°. The crystal was mounted on a heatable molybdenum sample holder (ESCA stub, Fisons Instruments) with clamps of polycrystalline cobalt. The temperature was measured directly at the crystal by a type N thermocouple (NiCrSi/NiSiMg). The crystal was prepared in UHV by subsequent Ar sputtering at 620 K (1.0 kV, 5 × 10-6 mbar of Ar, sample current ) 1 µA) and annealing at 620 K. The annealing temperature for cobalt is limited by the hcp-fcc phase transition at around 700 K.10 The procedure mentioned earlier is known to produce a stepped surface with atomically flat terraces.20 Residual carbon of about 2-4% of a monolayer could be further decreased below 1% of a monolayer by subsequent annealing in 1 × 10-7 mbar of oxygen atmosphere, in 1 × 10-7 mbar of hydrogen atmosphere, and in a vacuum. However, after this procedure a slight oxygen contamination (1-2% of a monolayer) remained at the surface. This surface was used for the experiments on an “annealed” surface with low defect concentration. By argon sputtering the annealed surface at room temperature for 1 h (1.75 kV, 2 × 10-6 mbar of Ar, sample current ) 1 µA), a highly defective surface was produced for further investigation of the influence of defects on CO adsorption.
12496 J. Phys. Chem., Vol. 100, No. 30, 1996
Beitel et al.
Figure 1. Series of PM-RAIR spectra taken on an annealed Co(0001) surface at room temperature at subsequently higher CO pressures from UHV (10 langmuirs of CO dose) to 300 mbar. Intensity and peak position of the signal of linearly bound CO change continuously with pressure. Above 10-3 mbar of CO, species attached to 3-fold hollow sites and bridged sites develop.
3. Results and Discussion 3.1. Pressure Dependence of CO/Co(0001) Adsorption. Figure 1 illustrates a typical series of infrared spectra of the CO layer adsorbed at room temperature on an annealed Co(0001) surface at successively higher CO ambient pressures. After dosing 10 langmuirs of CO, only one signal can be observed at 2012 cm-1. This signal can be attributed to the CO stretch frequency of linearly bound “on top” CO, which has been observed on metal surfaces in the 2000-2130 cm-1 range.38,39 The observed spectrum is in good agreement with previous HREELS investigations,12 which, under similar conditions, also revealed only one broad CO absorption signal centered around 2000 cm-1. Upon increasing the dynamic pressure of CO up to 1 mbar, there is a continuous shift of 35 cm-1 to higher frequencies; a further pressure increase to 300 mbar causes a shift back of about 20 cm-1 to lower frequencies. A second weak signal at 1850 cm-1 starts to develop at 10-3 mbar and shifts to 1900 cm-1 at a CO pressure of 10-300 mbar simultaneously with the lowering of the frequency of the linearly bound CO. Because of its characteristic frequencies between 1800 and 1950 cm-1, these signals must be attributed to multiple-bonded CO species.38,39 CO attached to 3-fold bridges (hollow sites) shows an absorption between 1800 and 1850 cm-1 as on Ni(111) or Pd(111). CO bound to 2-fold bridges (bridgebonded) on the (111) surfaces of Ni, Pd, Pt, and Rh shows an absorption between 1870 and 1950 cm-1. The pressure dependence of the peak shift and intensity of the linearly bound CO is summarized in Figure 2. On a logarithmic pressure scale, an almost linear increase in the frequency with pressure up to 1 mbar, followed by a linear decrease in the 1-300 mbar range, is visible (Figure 2a). The intensity increases from 10 langmuirs of dosing to 10-7 mbar of CO pressure, but then decreases if the CO pressure is increased to 10-3 mbar (Figure 2b). At pressures above 1 mbar, the decrease continues but with a considerably steeper slope. Repetitive experiments under the same pressure conditions revealed errors of (10% for the signal intensity and (3 cm-1 for the frequency. To understand the changes in peak frequency and intensity, comparison with previous LEED investigations on CO/Co(0001) surfaces3,4 is useful. The CO overlayer structures observed in these studies are summarized in Table 1. Upon dosing of 1.2 langmuirs of CO at either room temperature or 100 K, a (x3 × x3)R30° superstructure is observed with a surface coverage
Figure 2. Change in (a) peak position and (b) intensity of the infrared signal of linearly bound CO on a logarithmic pressure scale. The marked changes at 1 mbar indicate a structural transition in the CO adlayer.
of CO of 1/3 ML with the CO molecules situated on top, as shown in Figure 3a.4 At room temperature this is identical with the saturation coverage under UHV conditions. This explains the RAIRS results obtained in the present study, which also showed only linearly bonded CO species after dosing 10 langmuirs of CO at room temperature (Figure 1). An increase in the CO coverage beyond 1/3 ML is possible upon further CO dosing at low temperatures or by increasing the CO background pressure. At 100 K the LEED overlayer spots first become diffuse, indicating a disordered structure, while after dosing of 2.2 langmuirs a newly ordered (2x3 × 2x3)R30° superstructure with a CO coverage of 7/12 ML could be observed. The CO molecules are hexagonally close-packed in this structure and are either on top or nearly bridge bonded, which is shown in Figure 3b. At 286 K, increasing of the CO background pressure leads to a splitting of the overlayer spots, which was explained by continuous compression and rotation of the CO overlayer.4 Finally, at a pressure of 2 × 10-6 mbar, a newly ordered structure was observed, which is attributed to a (x7/3 × x7/3)R10.9° overlayer with CO in on top and in 3-fold hollow sites and a CO coverage of 3/7 ML (Figure 3c). From LEED results obtained at 300 K,3 a compression toward a (x7 × x7)R19.2° overlayer was deduced, with CO in on top and bridge-bonded sites and a higher coverage of 4/7 ML, which is almost equal to the close-packed CO (2x3 × 2x3)R30° structure mentioned earlier. However, the (x7 × x7)R19.2° structure could not be obtained on the entire surface because the achievable CO pressure was limited in the LEED experiments. The frequency shift and the change in intensity and adsorption sites observed by PM-RAIRS as a function of increasing CO
CO Adsorption on Co(0001)
J. Phys. Chem., Vol. 100, No. 30, 1996 12497
TABLE 1: Structures and Structural Transitions of CO Adsorption Layers on Co(0001) at 100, 286, and 300 K Obtained by LEED Investigations3,4 surface coverage temperature (K)
θe
1/ 3
ML
θ>
1/ 3
ML
θ ) 3/7 ML
θ ) 4/7 ML4 (7/12 ML)3
1003
dosage of 1.2 langmuirs (x3 × x3)R30° on top sites
2863 and 3004
increasing CO equilibrium 2 × 10-6 mbar3 dosage of 1.2 langmuirs in >10-6 mbar4 (x7 × x7)R19.2° (?) pressure, splitting of spots on top and bridged sites UHV (x3 × x3)R30° on top (x7/3 × x7/3)R10.9° w continuous compression sites (saturation coverage) on top and 3-fold sites and rotation
dosage of 1.6 langmuirs diffuse pattern w structural transition
dosage of 2.2 langmuirs (2x3 × 2x3)R30° on top and bridged sites
Figure 3. Structural models of several CO adlayers on Co(0001); the large, open circles indicate the close-packed cobalt surface atoms, the small, filled circles indicate the adsorbed CO molecules, and the unit cells are indicated by solid lines. (a) (x3 × x3)R30° structure, θ ) 1/ ML; (b) (2x3 × 2x3)R30° structure, θ ) 7/ 3 12 ML; (c) (x7/3 × x7/3)R30° structure, θ ) 3/7 ML.
pressure (Figures 1 and 2) can also be explained in terms of structural changes in the CO layer. The frequency shift as a function of coverage can be caused by two effects:38,40,41 firstly, there is a chemical shift, which is due to changes in the metaladsorbate interaction. The chemical shift can be explained in terms of the Blyholder model,42 which describes the metaladsorbate interaction in terms of a donation of electrons from the 5σ orbital of the CO to the metal and a back-donation of electrons into the 2π* orbital of the CO. Secondly, there is a coupling shift due to dipole-dipole interactions of adsorbed CO molecules. Intensity changes of the CO absorption signal reflect, in the first instance, the amount of CO present on the surface, but the intensity can be strongly modified by depolarization effects38 and intensity transfer between absorption signals.40 In particular, depolarization leads to a decrease in the intensity with increasing CO coverage due to the reduction in the intermolecular distance of the CO adsorbate structure. For pressures up to 1 mbar, the continuous 25 cm-1 upward shift of the peak attributed to linearly bound CO is most probably due to continuous compression and rotation of the (x3 × x3)R30° structure, which was also observed by LEED.3,4 The resulting CO layer consists of linearly or (because of the compression) almost linearly bound CO. In the case of linearly bound CO, the observed upward shift should be attributed to dipole-dipole coupling rather than to chemical effects. Depolarization caused by a continuous compression of the CO layer also explains the observed decrease in signal intensity at CO pressures higher than 10-7 mbar. This interpretation is in agreement with the results of RAIRS investigations under UHV conditions on systems such as CO/ Pt(111)43,44 and CO/Ru(0001),45 where coupling-induced shifts of about 20 cm-1 and similar changes in signal intensity caused by depolarization effects45 have been observed. At CO pressures beyond 1 mbar, the RAIRS experiments clearly indicate a rearrangement of the CO layer. The presence of on top and bridge-bonded sites can be attributed to the formation of a structure similar to the close-packed (2x3 × 2x3)R30° overlayer (Figure 3b) observed by LEED under
Figure 4. Change in the full width at half-maximum (FWHM) of the absorption signal of linearly bound CO with increasing CO pressure, showing a considerable broadening of the signal around 10 mbar.
UHV conditions at low temperatures. In this structure, the number of on top CO is reduced by 75% compared to the (x3 × x3)R30° structure. Hence, the structural transition is accompanied by a change of adsorption sites from on top (and 3-fold hollow sites) to bridge-bonded sites. This “dilution” of linearly bound CO explains the larger decrease in intensity of the absorption signal in the pressure range between 1 and 300 mbar shown in Figure 2b. In this pressure range, continuous compression and rearrangement of the CO layer obviously take place, with on top and bridge-bonded sites being occupied. The frequency shift of the linearly bound CO signal back to lower wavenumbers is probably also caused by the dilution of linearly bound CO in bridge-bonded CO. The reason for this is the big difference in the vibrational frequencies (>100 cm-1) of these CO species, resulting in a very small dipole-dipole coupling. Therefore, the signal can shift to lower wavenumbers despite the further increase in CO coverage. RAIRS investigations on the system CO/Pt(111) at low temperatures in UHV44 have shown similar results upon changing the CO overlayer structure from (x3 × x3)R30° to (2x3 × 2x3)R30°. Further insight into the adsorbate structure is obtained by inspection of the development of the full width at half-maximum (FWHM) of the absorption signal of linearly bound CO shown in Figure 4. The FWHM stays constant over the whole pressure range below around 10 mbar. At 10 mbar, a big increase from
12498 J. Phys. Chem., Vol. 100, No. 30, 1996 20-25 to 39 cm-1 is visible. This indicates that there is a higher degree of disorder at the surface, resulting in considerable inhomogeneous line broadening.38 Note that this coincides with the structural rearrangement from linearly (and 3-fold hollow sites) to bridge-bonded CO, which starts at 10 mbar of pressure. The subsequent narrowing of the signal at pressures above 10 mbar shows the development of a new ordered structure, in agreement with the data shown in Figures 1 and 2. These results show that at room temperature major changes in the overlayer structure of CO occur in the “high”-pressure range above 1 mbar. The successively higher pressures and subsequent higher CO coverage lead to a rearrangement of CO via a continuous compression, with a resultant change in the type of adsorption sites. Results previously obtained under UHV conditions and low temperatures (100 K) on cobalt3,4 extrapolate remarkably well into the high-pressure, hightemperature regime. At identical coverages, very similar surface structures obviously exist that are independent of surface temperature. This conclusion is in good agreement with previous RAIRS studies of CO adsorption on Cu(100) and Pd(111) surfaces.29,30 However, the correspondence of low-pressure LEED data3,4 with the PM-RAIRS data is not perfect. LEED experiments show that compression of the (x3 × x3)R30° superstructure at room temperature results in a (x7/3 × x7/3)R30° structure at 2 × 10-6 mbar of CO pressure. In this structure the linearly bonded CO species is reduced to 41% compared to the (x3 × x3)R30° structure and diluted in CO attached to 3-fold hollow sites (the ratio between CO in 3-fold hollow sites relative to linearly bound CO is 2) (Figure 3a,c). This is not consistent with the RAIRS results, because a dilution of the linearly bound CO should cause a shift back to lower wavenumbers as seen earlier and not the observed further upward shift. Furthermore, the signal observed at 1850 cm-1 is very weak despite the fact that the majority of CO is located in 3-fold hollow sites in this structure. Thus, from a RAIRS perspective, it seems unlikely that a (x7/3 × x7/3)R30° structure develops over the entire surface. However, some CO attached to 3-fold hollow sites could exist close to surface imperfections or domain boundaries in the CO adlayer. An inspection of the FWHM (Figure 4) shows no broadening in the 10-6-10-3 mbar range where the first change in adsorption sites takes place according to LEED. This also suggests that there is no real structural change in this pressure range. The reason for this apparent contradiction is unclear. A similar observation has been made for CO adsorption on Ru(0001) at low temperatures,45 which was explained by the assumption that despite the geometrically different adsorption sites the vibrational character of the CO can remain unchanged. As a reason for this phenomenon, a stronger interaction between the molecules of the CO layer than the COplatinum interaction was assumed, which preserves uniform vibrational behavior of the CO layer.45 3.2. CO Adsorption on Defects. The influence of surface defects on CO adsorption (Figure 5) has also been investigated in this study. To this end, an annealed cobalt surface with large, atomically flat terraces was compared to a sputtered unannealed surface with a high concentration of (point) defects. While the annealed surface showed only one symmetric CO signal after a 10 langmuir dose, the sputtered surface showed, in addition, a pronounced shoulder to this peak (Figure 5a). A decomposition of the signal reveals two main peaks at 2016 and 2055 cm-1, respectively. The new signal at 2055 cm-1 is attributed to CO attached to defect sites, while the signal at 2016 cm-1 again is due to linearly adsorbed CO at well-ordered terrace sites. It is difficult to derive the concentration of defects at the surface
Beitel et al.
Figure 5. Influence of surface defects on the CO-adsorption layer by comparison of a sputtered (defective) and an annealed (defect-free) surface; PM-RAIR spectra showing the CO-absorption region (a) after 10 langmuirs of CO dosing at room temperature in UHV, (b) at the CO pressure of 100 mbar at room temperature, and (c) at a CO pressure of 100 mbar at 490 K.
from the peak intensity ratio because of additional effects such as intensity transfer from the lower to the higher frequency signal, which is a well-known consequence of lateral interaction.40 However, it is reasonable to assume that there are defect concentrations of at least 10% of a monolayer or more after sample sputtering. We also know from STM measurements that the terraces of the annealed surfaces used in this study are atomically flat and have an average diameter of 100 nm.20 By assuming that defect sites exist only at steps, we calculate a defect concentration of
