Unusually low stretching frequency for CO adsorbed on Fe(100)

Surface Science 163 (1985) L675-L680 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS UNUSUALLY LOW STRETCHING FREQUENCY FOR CO ADSORBED ON Fe(100) Carsten BENNDORF, Bernd KROGER and Fritz THIEME Physical Chemistry Department, University of Hambur~ Laufgraben 24, D-2000 Hamburg 13, Fed. Rep. of Germany Received 4 March 1985; accepted for publication 9 July 1985

For CO adsorption on Fe(100) different adsorption species are detected with high resolution EELS (electron energy loss spectroscopy) which sequentially fill in with increasing coverage. Up to - 350 K and low CO exposure ( < 1 L), a predominant molecular species with an unusually low stretching frequency, 1180-1245 cm-1, is detected. This unusual CO bond weakening is consistent with a "lying down" binding configuration of CO. For higher CO coverages at 110 K, further CO adsorption states with vibrational frequencies of 1900-2055 cm-1 are populated which are due to CO bound with the molecular axis perpendicular to the surface.

From IR studies of metal carbonyls it is well known that the C - O stretching frequency can be correlated with (a) the strength of the C-O bond, (b) the population of antibonding CO 2~r* orbitals and finally (c) with the metal-CO bond strength [1]. Generally, for metal carbonyls the CO stretching frequency of CO bound to a single metal atom is found above 2000 cm-1; it decreases to 1850-2000 cm-1 for CO bridge bonded to two metal atoms and further below 1850 cm -t foi CO with threefold coordination [1]. This behavior is observed, also, for CO adsorbed on transition metal surfaces [2]. In most cases, on transition metal surfaces evidence was found that CO bonds via the C atom with the CO axis perpendicular to the surface. "Inclined" CO has only been reported occasionally for stepped or rather "open" surfaces [2-4]. "Inclined" CO species are characterized by CO stretching frequencies below 1650 cm-1 [2,3]. Recently, Seip et al. reported for C O / F e ( l l l ) a C-O stretching mode as low as 1530 cm -1 [5], which they attributed to CO adsorbed in "deep hollow" sites of Fe(lll). CO with a "lying down" binding configuration has been postulated very recently by Shinn and Madey for CO/Cr(ll0) [6]. They found on clean Cr(ll0) a stretching frequency of 1150-1330 cm -1 and concluded that the substantial CO bond weakening (which requires a large electron donation into the antibonding CO 2~r* orbitals) is due to "side-on" bonded CO [6]. In this letter we report on electron energy loss measurements for CO/Fe(100) which show the existence of unique and previously unreported behavior of CO 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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at low coverage on an iron surface. This adsorption state is characterized by an anomalously low CO stretching frequency of 1180-1245 cm -1 and is predominately populated at low CO coverages. By analogy with recent conclusions for C O / C r ( l l 0 ) [6] we suggest that this CO adsorption state on Fe(100) has a "lying down" binding configuration. At higher CO coverages further CO binding modes fill in which are associated with CO stretching modes between 1900-2055 cm -~. These adsorption states are identified as terminally bound CO with the molecular axis perpendicular or nearly perpendicular to the surface. The experiments were performed in a stainless steel ESCALAB U H V system (Vacuum Generators) equipped with a hemispherical electron energy analyzer and an electron monochromator (EMU, Vacuum Generators). The Fe(100) crystal was cut by spark erosion and prior to polishing was treated in a H 2 atmosphere (24 h at 1020 K and then for 48 h at 1120 K) to remove bulk carbon contamination. During this and the following U H V procedures the Fe(100) crystal was kept always below the phase transition temperature from a-Fe to y-Fe. Inside the UHV system the Fe(100) crystal was cleaned in the usual way by repeated cycles of Ar+-ion sputtering and annealing. Surface cleanliness and orientation were probed with AES and LEED, respectively. The only contamination which could not completely be removed from the Fe(100) surface was oxygen. The residual oxygen surface concentration on the "clean" Fe(100) was determined with AES to be 0(0) < 0.05, as follows. The oxygen AES signal from O/Fe(100) was measured during 02 dosing onto the Fe(100) surface. Following ref. [7], the saturation of chemisorbed oxygen on Fe(100) is associated with a (1 x 1) LEED pattern which corresponds to 8 ( 0 ) = 1. This oxygen signal for 0 ( O ) = 1 was used for the determination of the residual oxygen on the "clean" Fe(100) surface. The Fe(100) crystal was mounted on an X Y Z - r o t a r y manipulator, which allowed temperature control between 110 and 900 K. The temperature was measured with a thermocouple spotwelded to the backside of the crystal. The EELS measurements were performed in specular reflection using a primary electron energy of 6 eV. The count rate for the elastic peak was in the order of 1 x 105 counts/s with a typical F W H M of 110 cm -~. However, the background tail near the elastic peak precluded well resolved measurements below - 500 cm -1. EELS measurements were complemented b y ESDIAD (electron stimulated desorption ion angular distribution), LEED, XPS (X-ray induced photoelectron spectroscopy) and TDS (thermal desorption spectroscopy) experiments; these results will be published in detail in a separate report [8]. Fig. 1 shows a series of EELS spectra measured for different CO coverages at 110 K. For the calibration of the CO coverage we used TDS. The present TDS spectra measured for CO/Fe(100) are in good agreement with data reported by Benziger and Madix for the same system [7]. We observed three molecular desorption states, which are denoted after ref. [7] as a 3 ( Tm - 4 2 0

C Benndorf et al. / Low stretching frequency for CO on Fe(100)

CO/Fe(IO0) a-CO

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|N1 -

b-CO ~.6o

(J • .4n. CO/Fe(IO0)

,~)-

xl

CO Exposure (L)

CO-Exposure 6L

2.3 L

1.8 L

1.1L

0.38

1180

I

I

0

1000

I

I

L

I

2000

Loss Energy (cm -1) Fig. 1. High resolution EELS for CO/Fe(100) at 110 K and different CO exposures. Ep = 6 eV in specular reflection. (A weak loss feature which may be attributed to the metal-CO vibration is visible as a shoulder on the background tail at - 500 cm -1 and is indicated by an arrow.) Insert: Relative CO coverage as function of exposure as derived from TDS experiments.

K), o/2 ( T m - 270 K) a n d a I ( Tm - 190 K). T h e d e s o r p t i o n m a x i m a at 780 K ( d e n o t e d as fl) is believed to be due to the r e c o m b i n a t i o n of dissociation fragments, C(ad) a n d O(ad). W i t h EELS we f o u n d evidence that for T < 110 K all C O a d s o r p t i o n states are molecular. This is consistent with our XPS results a n d the XPS data reported b y Benziger a n d M a d i x [8,7]. I n fig. 1 u p to 1.1 L C O (1 L = 10-1 T o r r s), EELS is d o m i n a t e d b y a loss peak, d e n o t e d as a-CO, which is f o u n d at low C O coverage at - 1180 c m -~ a n d which shifts to - 1210 cm -~ at 1.8 L C O exposure. This C O a d s o r p t i o n

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C. Benndorf et al. / Low stretching frequency for CO on Fe(lO0)

states is the predominant CO species at low CO coverages and, as we will show later leads to the et3 + fl TDS states upon heating. For higher CO coverages this low coverage loss peak (a-CO) decreases in intensity and a broad peak develops between 1900 and 2055 cm -1, denoted as b-CO. This peak grows further in intensity with increasing CO coverage and merges into a relatively sharp loss peak at - 2055 cm-1 at the saturation coverage, - 6 L. For exposures near and above 1.1 L a further weak peak is detected near 1500 cm -1, shifting to 1610 cm -1 for exposures - 6 L. A loss peak at 1530 c m - 1 was also seen by Seip et al. [5] for C O / F e ( l l l ) . At this surface, however, CO with stretching frequencies of - 1800 and 2000 cm -1 was populated first and the 1530 cm -1 mode was filled only for exposures > 2.5 L. On F e ( l l l ) the 1530 cm-1 loss was interpreted as due to CO adsorbed in deep hollow sites of the F e ( l l l ) surface. On Fe(100) we suggest that the weak loss intensity with maxima between 1500 and 1610 cm -1 is due to CO adsorbed at step sites of the Fe(100) crystal. A deviation of 2 °, for example, of the surface normal from the [100] direction is associated with - 3% step sites. Surprisingly, we did not find a well-resolved m e t a l - C O vibration loss, which is usually detected for terminally bound CO on other transition metals with high intensity at energies of about 500 cm-1. This might be due to a low cross section of the vibrational mode for C O / F e ( 1 0 0 ) or to our limited resolution and high inelastic background near the elastic peak. A weak intensity of the m e t a l - C O stretching mode is reported also for C O / C r ( l l 0 ) , where the CO is suggested to be bound in a "lying down" configuration [6]. Further, similar low intensities have been found for CO + K on Ru(001), where a side-on bonding configuration is believed to be induced by K [9]. Only for the highest CO exposures, 2.3 and 6 L, there is a weak shoulder near 500 cm -1 (indicated in fig. 1 by an arrow), which could be due to the m e t a l - C O vibrational loss. Instead of a well-resolved m e t a l - C O vibrational mode, we always observed - even from the "clean" Fe(100) surface - a weak loss peak at 320 cm-1. This loss peak is suggested to be due to residual oxygen from the "clean" Fe(100) surface. To exclude that the CO adsorption behavior at low 8 is disturbed by the residual oxygen, several experiments were performed on oxygen predosed Fe(100). With increasing 8 ( 0 ) we found with T D S a decrease of the a 3 ( - 420 K) and fl ( - 780 K) desorption states and a decrease of the a-CO loss intensity. This demonstrates that the 1180-1245 cm -1 loss is not due to a O - C O interaction. The vibrational modes of b-CO with loss frequencies of 1900-2055 c m - 1 are consistent with a two-fold bridging and single bonded CO mixture. The -contribution of different terminally bound CO to the b-CO loss is suggested from the F W H M of this peak, which is - 250 cm-1 at 1.8 L. The unusually low stretching frequency of 1180-1245 cm -~ for the low coverage CO state (a-state) requires a different interpretation. Our high resolution EELS results suggest that the vibrational mode at 1180-1245 cm -1 is due to molecular CO.

C. Benndorf et al. / Low stretching frequency for CO on Fe(l O0)

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This interpretation is consistent with our XPS measurements [8] and the reported XPS data of Benziger and Madix [7]. They concluded for CO/Fe(100) that at 150 K CO was not dissociated. But apparently, dissociation occurs at higher temperatures ( > 300 K) and gives rise to C ls and O ls XPS peaks from atomic C(ad) and O(ad) and to the fl desorption state [7,8]. The unusual low vibrational mode of a-CO on Fe(100) suggests a substantial weakening of the C - O bond due to a high population of the 2¢r* orbital of CO. A significant overlap of the CO 2~r* orbitals, which is necessary for a large charge transfer from the metal valence electrons into unoccupied CO states should be possible with a CO species oriented with the molecular axis parallel to the surface. Side-on bonded F e - C O complexes are known to exist with very low stretching frequencies (for example 1301-1312 cm -1 for [,15CsHsFe(CO)]4BBr 3 [10]). The interpretation of a-CO (vibrational frequency of 1180-1245 cm -~) on Fe(100) as a side-on bonded species is consistent with ESDIAD results, which will be discussed in detail elsewhere [8]. In brief, coverage dependent ESDIAD measurements reflect also the sequential filling of two different CO modes. For CO exposures up to 0.8 L only a vanishingly weak O + emission is observed. The superpression of O ÷ ESD emission from a-CO on Fe(100) supports the interpretation of a strongly inclined (angle between CO axis and surface normal > 60 °) or lying down CO. At higher CO exposure, with the observation of b-CO with vibrational frequencies of 1900-2055 cm -~, intense O ÷ emission in narrow directions ( + 20 °) of the surface normal is observed. This result for b-CO is consistent with normal bonded or slightly inclined CO. EELS measurements performed during thermal annealing indicate that a-CO is a precursor state for the CO dissociation on Fe(100). After saturation of the Fe(100) surface at 110 K the crystal was successively heated to higher temperature and cooled again to 110 K to perform EELS. These measurements, which will be discussed in detail in a separate paper [8], reveal that the loss intensity of the 2055 cm -1 peak (b-CO) decreases near 250 K and completely disappears at < 370 K. With TDS the desorption of the al-CO desorption state starts at 170 K and the et2-CO state at - 250 K. We conclude that b-CO is associated with a~ + et2 desorption states. A different behavior is observed for the 1180-1245 cm -~ a-CO mode. During annealing the intensity of this peak first increases up to 370 K and then rapidly decreases and vanishes at - 4 5 0 K. In this temperature range, 300-450 K, dissociation of some of the CO was suggested from XPS data [7]. We conclude that the disappearance of a-CO (1180-1245 cm -1) for T > 370 K is partly due to a desorption of the ct3 state and partly due to dissociation into C(ad) and O(ad), which recombine to CO at higher temperature and desorb as fl CO. The behavior of a-CO on Fe(100) is remarkable different from CO on F e ( l l l ) and F e ( l l 0 ) surfaces, where a C - O stretching mode in the range - 1 2 0 0 cm -1 was not detectable. For example, Seip et al. [5] reported for

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Fe(111) as the lowest C - O stretching mode on this surface a vibrational frequency of 1530 cm -1, attributed to CO in "deep hollow" sites of F e ( l l l ) . During thermal annealing this 1530 cm -1 adsorption mode converts into normal bond CO with stretching frequencies of 1800-2030 cm-1. This behavior demonstrates that the 1530 cm -1 adsorption state on F e ( l l l ) is not a precursor state for CO dissociation. In summary, a sequential population of different adsorption states was detected for CO/Fe(100) with EELS. Up to - 1 L CO exposure a predominant molecular adsorption state with an unusually low CO stretching frequency of 1180-1245 cm -1 is found which is stable up to - 350 K. This significant C - O bond weakening is suggested to be due to CO in a "lying down" binding configuration. The a-CO adsorption state on Fe(100) with 1180-1245 cm-1 is remarkable different from CO on F e ( l l l ) and F e ( l l 0 ) where no vibrational mode in the range of 1200 cm-1 was detected [5,11]. Our results demonstrate that various Fe planes differ considerably in their reactivity towards CO. For CO exposures > 1 L adsorption states with vibrational modes between 1900-2055 cm -1 fill in. These loss frequencies are consistent with CO terminally bound to Fe(100). The unusual bond weakening of the low coverage a-CO state on Fe(100) is important for understanding the mechanism of the Fischer-Tropsch synthesis of hydrocarbons from CO + H 2 on Fe surfaces. In this synthesis, the dissociation of CO is one of the fundamental steps. Our results suggest that the "lying down" CO on Fe(100) is a precursor state for the CO dissociation into C(ad) and O(ad) on iron surfaces. The authors acknowledge with pleasure valuable discussions with Drs. T.E. Madey and M. Kiskinova.

References [1] N. Sheppard and T.T. Ngnyen, in: Advances in Infrared and Raman Spectroscopy, Vol. 5, Eds. R.J.H. Clark and R.E. Haster (Heyden, London, 1978). [2] W. Erley, H. Ibach, S. Lehwald and H. Wagner, Surface Sci. 83 (1979) 585. [3] H. Hopster and H. Ibach, Surface Sci. 77 (1978) 109. [4] N.D. Shinn, M. Trenary, M.R. McClellan and F.R. McFeely, J. Chem. Phys. 75 (1981) 3142. [5] U. Seip, M.-C. Tsai, K. Christmann, J. Ktippers and G. Ertl, Surface Sci. 139 (1984) 29. [6] N.D. Shinn and T.E. Madey, Phys. Rev. Letters 53 (1984) 2481. [7] J. Benziger and R.J. Madix, Surface Sci. 94 (1980) 119. [8] C. Benndorf, B. Kri~ger and T.E. Madey, to be published. [9] F.M. Hoffmann and R.A. de Paolo, Phys. Rev. Letters 52 (1984) 1697. [10] C.K. Rofer-DePoorter, Chem. Rev. 81 (1981) 447. [11] W. Erley, J. Vacuum Sci. Technol. 18 (1981) 472.