The role of the 2π level in CO chemisorption on metal surfaces

Surface Science 176 (1986) L847-L851 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS T H E R O L E O F T H E 2~r L E V E L I N C O C H E M I S O R P T I O N ON METAL SURFACES J. R O G O Z I K a n d V. D O S E Max-Planek-lnstitut fftr Plasmaphysik, D-8046 Garching bei Mfinchen, Fed. Rep. of Germany

Received 10 June 1986; accepted for publication 27 June 1986

An inverse photoemission study of CO adsorption on stepped Pd(lll)6°(011) and on Ru(001) is presented. In both cases a difference of 0.8 eV between the more tightly- and the more loosely-bound species is observed. The CO-induced emission of the more tightly-bound species lies in both cases closer to the Fermi energy, which favors enhanced backdonation. This verifies the predominant importance of 2rr backdonation in CO chemisorption, predicted in recent cluster calculations.

D e s p i t e the e n o r m o u s n u m b e r of p a p e r s on C O c h e m i s o r p t i o n the study of the C O - m e t a l b o n d still a p p e a r s to be a live issue in surface science. C h e m i s o r p t i o n of C O on metal surfaces is usually discussed in the f r a m e w o r k of B l y h o l d e r ' s d o n a t i o n - b a c k d o n a t i o n m o d e l [1]. W i t h i n this model, the c h e m i s o r p t i o n b o n d is established b y a charge d o n a t i o n from the highest o c c u p i e d m o l e c u l a r o r b i t a l 50 ( H O M O ) to the metal a n d charge b a c k d o n a t i o n into the lowest u n o c c u p i e d m o l e c u l a r o r b i t a l 2Tr ( L U M O ) . U l t r a v i o l e t p h o t o electron s p e c t r o s c o p y ( U P S ) studies of the valence levels of a d s o r b e d C O reveal a shift of the 5o-derived level relative to the l~r a n d 40 levels when c o m p a r e d to their gas phase values. This shift of the 50 level has been taken as evidence for its m a j o r c o n t r i b u t i o n to the C O - m e t a l b o n d [2]. On the other hand, it is well k n o w n that the energetic p o s i t i o n of the 50 level of chemis o r b e d C O exhibits o n l y m i n o r variations for different substrates a n d shows a l m o s t no sensitivity to the s u b s t r a t e c r y s t a l l o g r a p h i c o r i e n t a t i o n [3]. This o b s e r v a t i o n casts some d o u b t on the i m p o r t a n c e of the 5a level for the f o r m a t i o n of the c h e m i s o r p t i o n bond. In fact, recent cluster calculations by Bagus et al. [4] d e m o n s t r a t e that the 27r L U M O is b y far the m o r e essential i n g r e d i e n t in a b o n d formation. If so, we expect sizable variations of the 2~r-affinity level of c h e m i s o r b e d C O u p o n variations of s u b s t r a t e m a t e r i a l a n d / o r orientation. In o r d e r to verify this view experimentally, we chose two systems which are k n o w n to a c c o m m o d a t e C O in two different b i n d i n g states as d e m o n s t r a t e d b y thermal d e s o r p t i o n ( T D S ) e x p e r i m e n t s for an inverse p h o t o e m i s s i o n (IPE) s t u d y of the 2Tr*-affinity level. 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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d Rogoz/k, V. Dose / The 2~r level in CO chemisorption on metals

Davies and Lambert [5] have investigated CO adsorption on a stepped Pd surface. By comparison of TD spectra from Pd(331) and a faceted Pd surface they were able to correlate a TDS peak at 500 K with CO molecules adsorbed at step sites and a second peak at 410 K with CO molecules desorbing from terrace sites. Chemisorption of CO on Ru(001) also has been studied by a variety of methods [6]. Again, TDS shows two distinct desorption peaks. A single peak at 450 K shows up for coverages up to 0 = 0.33 where low energy electron diffraction (LEED) reveals a (~/3 x ~ ) R 3 0 ° pattern. High-resolution electron energy loss spectroscopy (HREELS) has identified the associated bonding site as an on-top position [7]. When the CO coverage is increased beyond 0 = 0.33 a new TDS peak emerges at 380 K. Vibrational spectroscopy indicates a slight displacement from the ideal on-top position and supports the conclusion from TDS that the binding energy of the CO decreases for 0 > 0.33 probably due to adsorbate lateral interaction. The inverse photoemission experiments were performed in a standard ultra-high vacuum (UHV) system with a base pressure of 2 × 10 -8 Pa. The IPE spectrometer employed in this work has been described in detail earlier [8]. The IPE spectra were recorded for normal electron incidence with a Geiger-Mi~ller photon detector of 9.7 eV mean detection energy. The samples could be cooled to 100 K and heated by either electron bombardment or ohmic heating via the sample support wires. Argon ion bombardment and annealing in oxygen were used to prepare clean and well-ordered sample surfaces. Surface order and cleanliness were monitored by LEED and Auger electron spectroscopy (AES). The LEED pattern of the Ru(001) surface showed a sharp (1 × 1) structure with low background. Upon CO adsorption, the sequence of LEED patterns reported in the literature was observed [6]. The LEED pattern from the clean Pd surface showed the energy-dependent splitting of diffraction spots characteristic for a stepped surface. The step direction was identified as (011) [9]. Fig. 1 shows a series of IPE spectra from a clean and CO-covered stepped Pd(111) surface. The spectrum from the clean surface (a) is very similar to that from a smooth Pd(111) surface. A prominent peak just above the Fermi energy arises from transitions into empty Pd d-states. The peak shows some tailing at its high-energy side which is not observed on the (001) surface [10]. From spectrum (b) in fig. 1 it is evident that this tail is quenched by exposure to 0.5 L CO. This suggests a surface state or resonance just above the d-bands. Such a surface state is well known from PES on A g ( l l l ) [11] and has also been identified in IPE on N i ( l l l ) [12]. A further feature of the clean sample spectrum is the image potential emission near 4.5 eV which is somewhat less pronounced than on the smooth surface [13]. The exposure of the clean sample to 0.5 L CO does not only quench the surface state and image potential emission, but a new CO-induced emission

J. Rogozik, V. Dose / The 2~r level in CO chemisorption on metals

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..t-.,

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.6 >.-

Z IdA I-Z

0

2

z.

6

8

(E-E F ) / e V Fig. 1. Inverse photoemission (isochromat) spectra from a stepped Pd surface with and without adsorbed CO.

peak emerges at 3.8 eV above E F. Further CO exposure leads to significant enhancement of the CO-induced feature and to a peak shift to 4.6 eV. The dashed line in spectrum (c) indicates the previous CO-induced peak at 0.5 L exposure. The final spectrum (d) was obtained after raising the sample temperature to 450 K. This leads to desorption of the more loosely-bound CO. Since spectrum (d) is identical to (b), we conclude that the peak in (b) and (d) arises from CO bound on step sites while the peak in (c) is a composite of emission from step- and terrace-bound CO. Fig. 2 shows IPE spectra from clean and CO-covered Ru(001). The clean surface spectrum exhibits two bulk transitions, B1 and B2, and surface states, S1 and $2. $2 is probably again the image potential emission, Exposure of the clean surface to 6 L CO at 100 K quenches the surface state emissions and leads to a new prominent CO-induced emission feature at 4.9 eV above E F. The LEED pattern associated with this adsorbate state is a (2f3- × 2VC3)R30 °

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J. Rogozik, Id Dose / The 2~ level in C O chemisorption on metals I

I

,CO/Ru

I

I

I

I

I

I

I

(001)

i

6L t-

O

4.10 K

>.-

Z ILl I-Z

clean

B1

0

" " $2

2

/-,

6

8

(E - E F) / eV Fig. 2. Inverse photoemlssion (isochromat) spectra from clean and CO-covered Ru(001).

superstructure. Raising the sample temperature to 410 K leads to incomplete CO desorption. The remaining CO overlayer is characterized by a (v~ × ~/-3)R30 ° superstructure. The IPE spectrum corresponding to this state shows a reduced emission from the CO-induced state, but, more important, a shift to a final state energy of 4.0 eV above E v. These IPE data on both Ru and Pd surfaces covered by CO demonstrate that the energetic position of the CO 2~r-derived electronic state responds sensitively to the binding state and energy of the molecule. In the case of stepped Pd the difference in bonding arises from step and terrace sites. On the smooth Ru(001) surface the molecules are first adsorbed on the energeticallyfavored on-top sites up to 0 = 0.33. Higher coverage results in displacement from these optimum sites as a consequence of lateral interaction. In both cases we find that the 2~r*-affinity level is closer to the Fermi energy by 0.8 and 0.9 eV respectively for the more tightly-bound species. In a

J. Rogozik, V. Dose / The 2~r level in CO chemisorption on metals

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simple h y b r i d i z a t i o n picture, the smaller separation from the metal d - b a n d s causes increased mixing between metal d-states a n d the C O 2~" state. This m a y in t u r n be interpreted as e n h a n c e d b a c k d o n a t i o n associated with the stronger bond. A direct c o m p a r i s o n of the I P E data with U P S m e a s u r e m e n t s is, unfortunately, not possible since our a p p a r a t u s lacks a U P S facility. D a t a available in the literature suggests only m i n o r variations of the b i n d i n g energy of the valence 4a, 5o a n d l~r levels for different a d s o r p t i o n states of CO or C O adsorbed o n different low-index crystal faces [3]. With respect to the Blyholder model, this would m e a n a less significant c o n t r i b u t i o n of the valence levels to the c h e m i s o r p t i o n b o n d c o m p a r e d to the 2 v level. This is perfectly in line with cluster calculations by Bagus et al. In view of the difficulties e n c o u n t e r e d so far in the i n t e r p r e t a t i o n of molecular chemisorption, the study of affinity levels by I P E seems to offer a new possibility to a refined u n d e r s t a n d i n g . This work has been financially supported by the Deutsche Forschungsgemeinschaft. W e are i n d e b t e d to J. Meier a n d K. H o r n for preparation of the Pd crystal.

References [1] [2] [3] [4] [5] [6] [7]

G. Blyholder, J. Phys. Chem. 68 (1964) 2772. C.L. Allyn, T. Gustafsson and E.W. Plummer, Chem. Phys. Letters 47 (1977) 127. S. Ishi, Y. Ohno and B. Viswanathan, Surface Sci. 161 (1985) 349. P.S. Bagus, C.J. Nelin and C.W. Bauschlicher, Jr., Phys. Rev. B28 (1983) 5423. P.W. Davies and R.M. Lambert, Surface Sci. 111 (1981) L671. H. Pfniir and D. Menzel, Surface Sci. 148 (1984) 411, and references therein. H. Pfniir, D. Menzel, F.M. Hoffmann, A. Ortega and A.M. Bradshaw, Surface Sci. 93 (1980) 431. [8] K. Desinger, V. Dose, M. Gl~Sbland H. Scheidt, Solid State Commun. 49 (1984) 479. [9] M. Henzler, J. Appl. Phys. 9 (1976) 11. [10] J. Rogozik, J. Kiippers and V. Dose, Surface Sci. 148 (1984) L653. [11] H.F. Roloff and H. Neddermeyer, Solid State Commun. 21 (1977) 561. [12] G. Borstel, G. Th~Srner, M. Donath, V. Dose and A. Goldmann, Solid State Commun. 55 (1985) 469. [13] P.D. Johnson, D.A. Wesner, J.W. Davenport and N.V. Smith, Phys. Rev. B30 (1984) 4860; P.D. Johnson, H.H. Farell and N.V. Smith, Vacuum 33 (1983) 775.