An AB initio CI study on the possibility of the CO chemisorption on Alkali metal surfaces: Interaction of the CO molecule with a Li atom and with Li5 clusters modelling the chemisorption sites at the (100) surface of the bcc and fcc Li lattice

Surface Science 165 (1986) 161-178 North-Holland, Amsterdam

161

A N AB I N I T I O CI S T U D Y O N T H E P O S S I B I L I T Y O F T H E C O CHEMISORPTION ON ALKALI METAL SURFACES: INTERACTION O F T H E C O M O L E C U L E W I T H A Li A T O M A N D W I T H Li 5 C L U S T E R S M O D E L L I N G T H E C H E M I S O R P T I O N S I T E S A T T H E (100) S U R F A C E O F T H E bcc A N D fcc Li L A T T I C E J. K O U T E C K Y , U. H A N K E , P. F A N T U C C I *, V. B O N A ( ~ I ( ~ - K O U T E C K ~ ( a n d D. P A P I E R O W S K A - K A M I N S K I Institut ff~r Physikalische Chemie, Freie Univeriti~t Berlin, D - 1000 Berlin 33, Germany

Received 14 February 1985; accepted for publication 20 August 1985

The interaction between the CO molecule and Li 5 cluster models of the chemisorption sites on (100) bcc and fcc lithium surfaces is studied employing the ab initio multireference doubly excited configuration interaction (MRD-CI) method. The Li pentamer models do not yield an indication of an appreciable chemisorption of CO on alkali metals. This conjecture is rationalized in terms of generally repulsive o- and attractive rr-interactions of CO with metal atoms. The discrepancy between these results and those obtained from a local potential approach (the Hartree-Fock-Slater method) has been pointed out.

I. Introduction T h e investigation of the i n t e r a c t i o n b e t w e e n c a r b o n m o n o x i d e a n d Li clusters is of large interest because of at least three different reasons: (i) T h e i n t e r a c t i o n of a C O molecule with the transition metals [1] involving electronic o - d o n a t i o n a n d ~r-backdonation [2] has the simplest a n a l o g u e in the i n t e r a c t i o n of C O with the Li atom. Therefore, the c o n s i d e r a t i o n of the system L i - C O can yield an insight in the relative role of the o- a n d ~r-interactions. (ii) T h e question can b e answered if involvement of d o r b i t a l s in the C O - m e t a l b o n d is a necessary c o n d i t i o n for the strength of this b o n d . Or alternatively, if r e p l a c e m e n t of d orbitals b y a p r o p e r c o m b i n a t i o n of s a n d p o r b i t a l s from several metal a t o m s at the c h e m i s o r p t i o n site is sufficient for the b i n d i n g i n t e r a c t i o n with 5o a n d ~r* M O s of CO. T h e p o s s i b i l i t y of a strong i n t e r a c t i o n b e t w e e n the C O molecule and an s - p metal surface is of general interest. (iii) In fact, such strong b i n d i n g between C O a n d Li5 clusters has b e e n f o u n d * Permanent address: Department of Inorganic and Organometallic Chemistry, University of Milano, Centro CNR, Via Venezian 21, 1-20133 Milan, Italy. 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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[3] employing the Hartree-Fock-Slater (HFS) approach [4]. The local potential methods enjoy large popularity for calculation of cluster properties and for consideration of chemisorption models involving clusters. Therefore, it is challenging to verify the interesting results obtained from the HFS method for the interaction of CO with Li 5 clusters, employing an ab initio method free from the assumption upon a local potential. In this work we do not address all aspects of the important question to which extent cluster models are reliable for predictions of processes on surfaces. Evidently, such models with a relatively small number of atoms do miss inevitably some features of an interaction involving a metal surface. In particular, different charges at different centres in a cluster which should mimic equivalent surface atoms seem to limit seriously the reliability of cluster models for chemisorption. Therefore, also the problem of atomic charges in Li 5 clusters will be discussed in this work. On the other hand, appropriate cluster models contribute towards an understanding of the local "chemical" properties of chemisorption sites. The usefulness of cluster models for the description of chemisorption phenomena has already been demonstrated on the example of alkali metals [5,6]. In fact, the interaction energy between an appropriately chosen Li cluster and a hydrogen or oxygen atom is ~lmost independent on the detailed cluster shape. The question remains open how important are these local properties for the chemisorption. However, a strong binding interaction between an alkali metal cluster model and the CO molecule (as has been found with the HFS method) could certainly be considered at least as a strong hint for CO chemisorption on alkali metal surfaces. In this paper, we employ the ab initio multireference doubly excited configuration interaction ( M R D - C I ) method [7] (section 2) to determine the interaction of CO with Li atom and with Li 5 clusters. The results for the L i - C O system are briefly described in section 3. The main part of this work, given in section 4, is the study of the interaction between CO and Li pentamers (Lis(5, 0), Lis(1, 4) and Li5(4, 1), compare figs. 3 and 7) modelling the "on top" and "hollow" chemisorption sites on bcc and fcc (100) surfaces. The emphasis is put on the detailed interpretation of the theoretical results for the sake of understanding the general features of the investigated interaction.

2. Method and geometries of investigated systems The M R D - C I procedure [7] includes all singly and doubly excited configurations with respect to the chosen set of reference configurations. If the CI spaces are too large for diagonalization, only those singly and double excitations with respect to the reference configurations are explicitly considered, which contribute more to the energy lowering of the given state than the chosen energy selection threshold T. An extrapolation technique is then used to estimate the energies for T = 0 corresponding to the full M R D - C I space.

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For L i s - C O systems the AO basis employed in this work is of double-zeta quality proposed by Huzinaga [8]: Li ( 7 s l p / 4 s l p ) , C (7s3p/4s2p), O (7s3p/4s2p). The basis set superposition error is negligible ( - 0.2 kcal/mol). Although the results are independent of enlargement of the AO basis set beyond double-zeta quality, in the case of the L i - C O system the results obtained with a slightly larger basis set Li (8s2p/6s2p) [9] and (12s7p/5s3p) [10] for C and O are discussed. Several states of different symmetry and different multiplicity have been considered for each system. First, the. corresponding H F states have been determined. For the correlation treatment in the framework of the M R D - C I technique modified virtual orbitals (MVOs) have been used for building up the CI space [11]. In this procedure the virtual orbitals are obtained as eigenfunctions of the exchange operator E~= l k j where j labels the occupied MOs of the HF configuration. In this manner the virtual orbitals are localized (with respect to the leading configuration). Therefore, there is a substantially smaller number of configurations contributing to the lowering of the energy in the truncated CI wavefunction for the given choice of the energy selection threshold T. In other words: the CI spaces for given T are considerably smaller in comparison with the CI spaces spanned by the usual canonical H F orbitals. Such a truncation of a correlated wavefunction makes it also possible to use a lower energy selection threshold T to ameliorate the correlation treatment. Also, after localization of virtual orbitals has been carried out, the new choice of reference configurations improves substantially the quality of correlated wavefunctions, because the reference configurations can easily contribute up to 95% in the expansion of the correlated wavefunction. In this expansion the reference set (M = number of reference configurations) for each state considered consists of the leading SCF configuration with very large weight and of several configurations with considerably smaller weights but > 0.05. They represent either multiple tr-lr* excitation within the CO molecule, or the excitation from bonding to antibonding C O - L i s MOs, or a double excitation within the Li cluster. In our correlation treatments of L i - C O and of the L i s - C O system, 9 electrons (4a2o5Oc2o~r~oZSaLi)and 15 electrons ( " l s " electrons are not included), respectively, are considered explicitly in the CI. The size consistency error is negligible because the energies of the composite system for CO at a distance 1000 a.u. from Li 5 are in a good agreement with energies obtained for CO and Li 5 as two separate systems. The nature of SCF and CI wavefunctions of different states considered for the L i - C O and Li~-CO systems is discussed in sections 3 and 4, respectively. The approach of the CO molecule towards the Li atom with the carbon end directed towards the metal atom has been considered for three different CO distances, r = 2.13, 2.18 and 2.23 a.u. In the case of interaction of CO with Li 5 clusters, the carbon atom is directed toward the Li atom placed in the centre of a square for Li5(5, 0), towards the "empty" middle of the square for Li5(4, 1)

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J. Kouteckj~ et al. / Chemisorption of CO on alkali metals

and towards the single lithium atom in the first plane the Lis(1, 4). The symbol Lis(ml, m2) means that m k atoms are situated in the k t h plane (k = 1, 2) perpendicular to the direction of CO approach. The CO distance is kept constant (r = 2.186 a.u., corresponding to the calculated equilibrium distance) in all calculations reported in this paper. Calculations with other CO bond lengths have not given qualitatively different results. This is due to the prevalently non-bonding interactions in the ground states of the L i - C O and L i s - C O systems (cf. sections 3 and 4). Li5(5, 0) with interatomic distance of 6.6 and 5.9 a.u. represents the model for the "on top" chemisorption site at the (100) surface of the bcc and fcc Li crystal, respectively ("bcc" Li5(5, 0) and "fcc" Li5(5, 0)). In " b c c " Li5(4, 1) the distance between next neighbours in the first layer is 6.6 a.u. and the distance between these atoms and the Li atom of the second "layer" is 5.74 a.u. In "fcc" Li5(4, 1) the distances between neighboring atoms in the first layer and between these atoms and the single Li atom in the second "layer" are equal to 5.9 a.u. " b c c " Li5(4, 1) and "fcc" Li5(4, 1) are the models for the "hollow" chemisorption sites at the (100) surface of the bcc and fcc Li crystal lattices. In the " b c c " Lis(1, 4) cluster the distances between the Li atom in the first plane and the four nearest neighbours in the second plane is 5.9 a.u. Therefore, Lis(1, 4) is an additional cluster model for "on top" chemisorption in which all nearest neighbours of the Li atom approached by CO are present.

3. Li-CO system The collinear approach of the Li atom towards the carbon end of the CO molecule yields the repulsive ground state 2~ and the attractive lowest excited state 2/-/ (fig. 1). The crossing between two potential curves occurs for a L i - C distance R which increases with elongation of the CO bond length. The minimum of the 2H excited state lies higher than the dissociation limit in the ~ ground state of CO and into the ground state (2S) of the isolated Li atom for all C - O distances considered. The state of 2Z nature, in which an interaction analogous to o-donation in the transition metal-CO interaction prevails, is clearly of repulsive nature. In contrast, the 2/7 state, in which interaction in the ~r-region analogous to ~r-backdonation dominates, is of binding nature. As the 2Z and 2H states obviously correlate in the asymptotic limit with the 2S and 2p states of a Li atom, respectively, our results for the L i - C O system can be summarized in the following manner: The o-interaction between the 2s (and po) orbital of the Li atom and the 50 orbital of CO is repulsive while the interaction between the Li 2p= orbital and the CO ~r* orbital is strongly attractive. This situation remains unchanged after inclusion of correlation effects and it is demonstrated by the difference one-electron density maps (fig.

J. Kouteck~ et aL / Chemisorption of CO on alkali metals

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A(@IzI~> ~ ~ Izl ~b>~.9- <~lzl@>~. Because the electronic d i s t r i b u t i o n s resulting f r o m the CI wavefunctions a n d f r o m the c o r r e s p o n d i n g l e a d i n g H F c o n f i g u r a t i o n s are very similar, the wavefunctions in the following expressions { ~k I z I@ >" are s i m p l y the H F c o n f i g u r a -

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J. Kouteck~ et al. / Chemisorption of CO on alkaB metals

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tions. The value - 1 . 3 9 a.u. of the A(2ZI z lZZ) a corresponds to the enhancement of o electrons at the Li side and the depletion of o electrons at the CO moiety in the M R D - C I one-electron density map for the 2Z state. On the other hand, the distribution of the ~r-electrons in the 2Z leading configuration does not change considerably ( A ( Z ~ l z l 2 Z ) ~' = -0.31). On the contrary, the ~r-electron shift in the 2/7 configuration is very large ( A ( 2 H I z I Z / 7 ) ~ = 2.11 a.u. and it is parallel to the "back-donation" shown in fig. 2b. The shift of o electrons in the 2/7 leading configuration is negligibly small ( A ( 2 H I z l 2 H ) a = - 0.04 a.u.). The analogy with the Sc-CO, P d - C O , R h - C O , N i - C O and P t - C O interactions is striking: the states which in the asymptotic limit correlate with transition metal atom states with the maximum occupancy for the ns atomic orbitals are more repulsive than the states which correlate with transition metal atom states of higher (n - 1)d AO occupancy (cf. refs. [5, 1]). In general, the o-interaction is clearly repulsive for s-p metals as well as for transition metals and the ~r-interaction attractive [lk]. If the magnitude of the promotion energy from an s AO to a p AO or to a d AO in the case of an s-p metal or a transition metal, respectively, is too large, the H-state minimum does not lead to the formation of a stable M e - C O complex. This is the case in the L i - C O system. 4. L i s - C O systems 4.1. Li~(5, O)-CO

We consider first bare Li5(5, 0) clusters. The Hartree-Fock ground state of the " b c c " Li5(5, 0) cluster is a 4B2g state with electronic configuration: (...(3alg)Z(2eu)Z(2blg)). The SCF energy of the 4B2g state ( - 3 7 . 0 9 9 a.u.) is only slightly lower than the SCF energy of the 2E u state ( - 3 7 . 0 8 6 a.u.) with electronic configuration (...(3alg)2(2e,)3). 2Alg lies energetically ( - 3 7 . 0 3 3 a.u.) considerably above the 4B2g and 2E u states. For the bare "fcc" Li5(5, 0) cluster (with shorter interatomic distances than " b c c " Li5(5, 0)) the H F procedure yields a 2E u ground state, although the 4B2g state is energetically very close ( E R r ( E E l ) = -37.104 a.u., EHF(4B2g)= --37.101 a.u., EHF(2Alg) ---- --37.043 a.u.) (cf. refs. [11,12]). The SCF energies of all three considered states for "fcc" Li5(5, 0) are lower than the energies of t h e corresponding states for the " b c c " Lis (5, 0) cluster. The reason for the low energy of the 4B2g H F state of " b c c " Li5(5, 0) is the relatively small antibonding interaction between the peripheral Li AOs due to their large interatomic distances. The antibonding interactions between the central atom and the peripheral atoms present in the 4alg MO are removed in the 2big MO due to symmetry. Therefore, the 4alg MO of " b c c " Li5(5, 0) is energetically less favourable than the 2big MOs.

168

J. Kouteckj, et al. / Chemisorption of CO on alkali metals

In the M R D - C I procedure configuration interaction lowers the energies of the 2E u states more than the energies of the 4B2g states so that the CI ground state of both " b c c " and "fcc" bare Li5(5, 0) clusters are doublets of E u symmetry (cf. ref. [12]). An important feature of the leading configuration in t h e 2E u and in t h e 4Bzg CI wavefunctions is the double occupancy of the 3alg MOs. In contrast, the leading configuration of the 2Alg wavefunctions has the 3alg MO only singly occupied. Consequently, the contribution from the 2s AO of the middle Li atom (to which the CO has the shortest distance in the Li5(5, 0 ) - C O system) is higher in the 2E u and 4Bzg states than in the 2Alg state. Moreover, the 2p AOs of the Li atom have large weights in the 2eu MOs of bare Li5(5, 0) clusters. Therefore, when Li5(5, 0) clusters interact with the CO molecule it is to be expected that the 3alg and 2eu MOs in the Li5(5, 0 ) - C O system will have a similar role as the 2s and 2p AOs of the Li atom in the L i - C O system. The only difference being that in the separate Li and CO system the 2p and 2s orbitals at the Li atom are empty in the ground and excited state, respectively. The Mulliken population analysis of the M R D - C I 2E wavefunction for the free Li5(5, 0) cluster exhibits relatively small charge shifts at the peripheral Li atoms (the total atomic charges are 3.03). For the Li5(5, 0 ) - C O system at R = 4 the depletion of electrons from the central atom is more pronounced but remains relatively small (Q = 2.70). Drawing the analogy between M - C O systems (with M = Li, Na, Sc, Rh, Ni, Pd and Pt) and the L i s - C O system, the following conjectures upon the form of the potential curves for different states can be made: (i) The 2A~ state of the Li5(5, 0 ) - C O system should exhibit a minimum due to the lower occupancy of the 2s AO at the central atom in the Li5(5, 0) cluster. (ii) T h e 4B2 state should be repulsive due to the higher occupancy of the 2s AO at the central Li atom and due to the nodal properties of the big MO which are not adequate for bonding with the CO molecule (analogously to the d8 AOs of the transition metal atom). (iii) The 2E state should exhibit mixed bonding and antibonding features because the eg-type MOs of the Li5(5, 0) moiety; with considerable weight of 2p-type orbitals at the central Li atom, can interact with the 7r* orbital of the CO molecule, but the double occupancy of the 3a~ MO gives rise to repulsive interaction. Indeed, the potential energy curves for the L i s - C O system obtained from the SCF and the M R D - C I treatment exhibit the predicted shapes (figs. 3-5). A pronounced minimum for the 2A~ state, especially for the "fcc" Li~(5, 0 ) - C O system lying high above the dissociation limit in the ground states of separate systems, a more or less repulsive curve for t h e 4B2 and a complicated behaviour for the 2E states can be seen in figs. 3-5. For the "fcc" system both SCF and M R D - C I procedures yield the 2E state as the ground state (cf. figs. 3 and 5). Similarily as for the bare Li5(5, 0) " b c c "

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cluster, the SCF ground state of the composite Li5(5, 0 ) - C O " b c c " system is 4B2, while the correlation effect in the M R D - C I procedure lowers 2E substantially below the 4 B 2 s t a t e (cf. figs. 3 and 4). The energy curve of the CI 2E ground state does not become repulsive unless R decreases below 4.2 a.u. and a very shallow minimum has been found at 4.25 and at 5.0 a.u. for " b c c " and "fcc" Li5(5, 0 ) - C O systems, respectively (cf. figs. 4 and 5) only when the extrapolation towards the full M R D - C I space ( T ~ 0) has been employed. The energy values at these minima are only slightly lower than the energy of the separated systems ( - 2 . 0 and - 1 kcal/mol for the " b c c " and "fcc" systems). Therefore it is doubtful that the shallow minimum of the ground-state potential curve can be taken as an indication of a very weak binding between the Li 5 cluster and the CO molecule with a binding energy < 2 kcal/mol.

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T h e 4B 2 p o t e n t i a l c u r v e for e x t r a p o l a t e d C I e n e r g i e s of the " b c c " s y s t e m e x h i b i t s also a s h a l l o w m i n i m u m b u t t h e o p t i m a l C I d e s c r i p t i o n o f this state is v e r y d i f f i c u l t to r e a c h a n d t h e r e f o r e , the s h a p e of this p o t e n t i a l c u r v e d e p e n d s s t r o n g l y o n the d e t a i l s o f the C I p r o c e d u r e .

4.2. Lift4, 1)-CO T h e H F a n d M R D - C 1 t r e a t m e n t s yield the 2E g r o u n d state for b a r e " b c c " a n d " f c c " Li5(4, 1) clusters. T h e l o w e s t e x c i t e d state 2A 1 lies s u b s t a n t i a l l y a b o v e the g r o u n d state. T h e a~ M O of the Li5(4, 1) c l u s t e r has a l o w e r e n e r g y t h a n the b~ M O b e c a u s e the b i n d i n g i n t e r a c t i o n b e t w e e n the Li a t o m s p l a c e d at the c o r n e r s of the s q u a r e is m u c h s t r o n g e r t h a n for the Li 5(5, 0) g e o m e t r y .

J. Kouteck~ et al. / Chemisorption of CO on alkali metals

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Fig. 5. The MRD-CI potential energy curves of the 2 E , 4B2 a n d 2A1 states for the "on top-fcc'" Li5(5, 0)-CO system as a function of the distance R. Analogous treatments as in fig, 4 have been employed. Plotted are variational (-- -- --) and extrapolated ( ) energies.

Consequently, the 4B 2 state has a higher energy than the 2A t state of the Li5(4, 1) cluster. In the bare Li5(4, 1) cluster with bcc interatomic distances the differences between the Mulliken charges at the apical a t o m and on the basis centres are relatively small (Qapic = 2.92 for the M R D - C I 2E state and Qapic = 2.87 for the M R D - C I 2A 1 state). This difference is deminished for the Li5(4, 1 ) - C O system. For the composite " b c c " Li5(4, 1 ) - C O system the 2E state is the g r o u n d state of repulsive nature and the high-lying first excited state 2A 1 exhibits a very flat R-dependence with an indication of a shallow m i n i m u m (figs. 6 and 7). Different shapes of the 2A 1 and 2E potential energy curves for the Li5(4, 1 ) - C O system from those obtained for Lis(5, 0 ) - C O can be rationalized by the different position of the "central" Li a t o m on the C 4 s y m m e t r y axis. In the Li5(4, 1) cluster the "central" Li atom is placed below the plane of the other four Li atoms. Therefore, the p A O of the "central" Li a t o m is not sufficiently close to form the binding e u M O with the proper c o m b i n a t i o n of 2s orbitals of the other four Li atoms and consequently does not contribute to the 7r-type of interaction between Li 5 cluster and 7r* orbital of the C O molecule.

172

J. Kouteck~ et aL / Chemisorption of C O

on

alkali metals

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Indeed, the m i n i m u m of the 2A 1 state is very shallow and the potential curve is very flat even for very small distances R between CO and the Li5(4, 1) cluster. Similarily, for small R, 0 _< R < 2 a.u., the energy of the repulsive state 2E does not increase very steeply. The behavior of both states 2A] and 2E for short distances R can be explained by the fact that it is energetically not so i n c o n v e n i e n t for the 9" M O of the CO m o l e c u l e to approach the plane of four Li atoms because of the possible binding interaction with the p orbital of the Li atom in the second "layer" of the Li5(4, 1) cluster. The different shapes of the energy curves for the "fcc" and "bcc" Li5 (4, 1 ) - C O systems (figs. 6 - 8 ) can be understood easily as due to differences in the geometries of both systems as well. The L i - L i distances are smaller in

J. Kouteck]~ et al. / Chemisorption of CO on alkali metals

173

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Fig. 7. The MRD-CI potential energy curves of the 2E and 2A1 states for (I) the "hollow-bcc" Lis(4, b-CO and (II) the "on top-bcc" Lis(1, 4)-CO system (2E and 2A1) as a function of the distance R. Similar treatments as in fig. 4 have been employed (7M/1R for the 2E state and 3M/1R for the 2A1 state with energy selection threshold T = 0.6/~h). Plotted are variational (---) and extrapolated ( ) energies. -

-

the c o m p a c t "fcc" than in the " b c c " Li5(4, 1) cluster. T h e fifth Li a t o m lies deeper below the "surface plane" (b = 4.17 a.u.) in the "fcc" cluster than in the " b c c " Li5(4, 1) cluster (b = 3.3 a.u.). F o r small distances, 0 < R < 2 a.u., the energy of the repulsive state 2E increases steeply because the p orbital of the Li a t o m in the second layer is still too far to interact sufficiently with the 7r* M O of CO. Therefore, the repulsion originating from the s AOs localized at the Li centres in the first layer dominates. T h e 2E energy curve shows also a

J. Kouteck~ et a L / Chemisorption of CO on alkali metals

174

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Fig. 8. The MRD-CI potential energy curves of the 2E and 2A! states for the "hollow-fcc" Li5(4, I)-CO system as a function of the distance R. Analogous treatments as in fig. 7 have been employed. Plotted are variational ( . . . . ) and extrapolated ( ) energies. small m a x i m u m at R = 0 due to the attractive i n t e r a c t i o n with the p o r b i t a l of the Li a t o m in the second layer for R = 0 (cf. figs. 6 a n d 8). Similarily, the m i n i m u m of the 2A t curve can develop b e t t e r in the case of the " f c c " Li5(4, 1 ) - C O system than for " b c c " Li5(4, D - C O . These qualitative c o n s i d e r a t i o n s s u p p o r t e d b y the q u a n t i t a t i v e results o b t a i n e d from the ab i n i t i o - C I m e t h o d offer a global u n d e r s t a n d i n g of the L i , - C O i n t e r a c t i o n from which the following conclusion can safely be d r a w n : the b i n d i n g c a n n o t occur between the " b c c " or " f c c " Li5(4, 1) cluster a n d the C O molecule.

4.3. Lis(1, 4)-C0 T h e Li 5 cluster with one Li a t o m p l a c e d out of the p l a n e c o n t a i n i n g the o t h e r four Li a t o m s can serve also as a m o d e l for " o n t o p " c h e m i s o r p t i o n sites

J. Kouteckj, et al. / Chemisorption of CO on alkali metals

175

on the bcc (100) surface when this out-of-plane central Li atom is approached by the CO molecule (Lis(1, 4)-CO). In this case all four nearest neighbours of the central Li atom in the (100) bcc surface are considered. The M R D - C I atomic charges at the Li atoms in the Lis(4, 1)-CO system differ only in the third decimal place from the corresponding Mulliken charges in the free Lis(1, 4) cluster. The ground state of the Lis(1, 4)-CO system is again a 2E state exhibiting a very shallow minimum and 2A~ with more pronounced minimum lies high above the ground state as depicted in fig. 7. As expected, the shapes of the energy curves for the 2E and 2A1 states are very similar to those obtained for the Li5(5, 0)-CO "on-top" model (cf. fig. 4). Obviously, the in-plane or out-of-plane position of four Li atoms with respect to the central Li atom approached by CO, does not influence the general features of the considered energy curves. Conclusions which have been drawn for the Li5(5, 0)-CO composite system are valid also for the Lis(1, 4)-CO system: no appreciable bonding can be predicted with this cluster model. 5. Comparison of the MRD-CI and HFS method

Post and Baerends [3] have obtained appreciable binding energies for the L i s - C O system employing the Hartree-Fock-Slater method (HFS) [4]. The AO basis sets employed in this work are different but of similar quality as those used in Post and Baerends' paper [3]. In comparison with the ab initio HF method, the HFS procedure clearly overestimates the binding between the ~r* MO of CO and the eu MOs of Li 5 clusters and the repulsive interaction between s-type AOs. For this reason, in the HFS method the attractive nature of the 2E and 2A~ states with doubly and singly occupied 3alg one-electron functions for the composite Lis-CO system is overestimated. The M R D - C I method yields a nonattractive 2E state (with a doubly occupied 3alg MO in the leading configuration of the correlated wavefunction) lying energetically below the attractive 2A1 state. Even when correlation effects are very important, especially for the energy sequence of the close-lying states of the Li5(5, 0)-CO system (where CI inverses the sequence of 2E and 4B2 states), the general features of MOs obtained from the one-electron HF approximation remain present in the natural orbitals obtained from the correlated M R D - C I wavefunctions. The main discrepancy between the results obtained with the two types of methods appears for the Li5(4, 1)-CO system in which the interplay between ~r-attraction and o-repulsion is even more delicate. In general, the sequences and behaviour of the states of L i s - C O systems differ completely in M R D - C I and HFS methods. The decomposition of the HFS "adsorption" energy presented in the work of Post and Baerends [3] (cf. fig 3b in ref. [3]) yields o-bonding and 7r-backbonding of comparable strength for both states 2E and ZA1. Although we have not tried to separate the contribution due to steric repulsion (cf. also ref. [3]) all M R D - C I results of our

176

J. Kouteck9 et al. / Chernisorption of CO on alkali metals

work, which are in details internally consistent, contradict the assumption of o-bonding. The general concept of antibonding o-interaction agrees also with recent results on transition metal clusters, interacting with a CO molecule (cf. ref. [1]). The example discussed in this work shows that local potential methods should be applied with caution for modelling the complicated chemisorption phenomena because they can overestimate the interaction energies between clusters and molecules.

6. Conclusions The ab initio M R D - C I investigation of the interaction between CO and Li pentamers of different geometries does not yield any appreciable binding interaction between these two subsystems. In analogy to the L i - C O system the crossing between the repulsive 2~ ground state and the attractive excited state 21i occurs also for the N a - C O system. The minimum of the Eft/ state lies above the dissociation limit in the ground state of the separate systems. The existence of a conical intersection, and general features of the states involved, were successfully applied to explain the energy transfer process, i.e. the quenching of the excited Na* atom by a CO molecule observed in the crossed atomic, molecular and laser beam experiment [13]. These experimental results represent an independent confirmation of the theoretical prediction about the general form of the relevant potential curves for the composite system in which an alkali metal atom interacts with the CO molecule. The results of the present work can be understood from the general form of the wavefunctions of the interacting moieties if the o-interaction is viewed as repulsive and the 7r-interaction as attractive for systems of the type (metal)-CO. A high occupancy of the unfavourable o-type one-electron functions in the leading configuration of the correlated wavefunction does not allow for a considerable binding between Li5 (or Li) and the CO molecule. Therefore, the low-lying states of L i s - C O systems which involve an interaction between doubly occupied o-type MOs of the Li 5 cluster and the doubly occupied 5o MO of the CO molecule have the best chance to be repulsive. The binding character of the interaction becomes larger for the states with the lowest occupancy of closed-shell o-subsystems. For the L i s - C O composite systems, the energy minima of the states which exhibit pronounced binding interaction lie close to the dissociation limit of the repulsive ground state due to the relatively large excitation energies from 2E to 2A of bare Li 5 clusters. As shown in sections 4.1 and 4.3 the ground states of the Li5(5, 0 ) - C O and Li5(4, 1)-CO composite systems exhibit such shallow minima in the same R interval that the conjecture upon chemisorption of the CO molecule on the

J. Kouteck~ et al. / Chemisorption of CO on alkali metals

177

(100) surface of alkali metals based on the considerations of cluster models for the on-top chemisorption sites cannot be drawn. In connection with the general problem of applicability of duster models, the question can be raised if the binding interaction between CO and the central Li atom will be considerably stronger due to the simultaneous presence of the nearest neighbours in the surface and in the subsurface layer. For example, the central Li atom in the Li9(5 , 4) cluster model of the " o n top" chemisorption site of the (100) fcc surface contains all eight nearest neighbours. The SCF treatment for the 2E state of the bare fcc Li9(5, 4) cluster yields larger population of the p AOs at the "central" atom than in the case of the Li5(5, 0) and Li5(4, 1) clusters. Therefore, investigation of the composite system Li9(5, 4 ) - C O (being in progress) is desirable in order to find out to which extent the presence of the second-layer neighbours in the cluster might favour the possibility of chemisorption of CO on (100) fcc surfaces of the alkali metals. It is not to be expected that the inclusion of polarization functions at the CO molecule will change the overall description of the L i s - C O interaction. According to the cluster models studied, chemisorption on "hollow" sites is even more unprobable than on "on top" sites. The ground states of these cluster models are clearly repulsive and the excited states exhibiting the minima lie too high in energy to allow the conjecture that CO chemisorption at these "hollow" sites on alkali metal surfaces might take place. These results contrast with the conclusions of Post and Baerends [3]. Furthermore, the main features of the cluster models for " b c c " and "fcc" chemisorption sites are qualitatively the same. This is also understandable because the " b c c " and "fcc" cluster models differ only slightly in the interatomic distances of the Li atoms in clusters. The fact that the charge shifting effect in Li 5 clusters and in L i s - C O systems is not pronounced in both H a r t r e e - F o c k and M R D - C I treatments represents further support for the relative reliability of the cluster models for the interaction between CO and alkali metal surfaces. The investigated cluster models of CO chemisorption on Li crystal surfaces show that a combination of s and p orbitals of a few metal atoms at the chemisorption site is not capable to take over the role of d orbitals for the binding interaction with the 50 and 7r* MO of CO. Therefore, we can draw the conclusion (based on consideration of models involving pentamer clusters and the CO molecule) that chemisorption of CO on the (100) surface of bcc and fcc alkali metal crystals is not likely to occur.

Acknowledgement This work was supported by the " D e u t s c h e Forschungsgemeinschaft" SFB 6 "Structure and Dynamics of Interfaces" and by the Italian National Research Council (CNR).

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J. Kouteckj: et al. / Chemisorption of CO on alkali metals

References [1] (a) (b) (c) (d) (e) (0 (g) (h) (i) O)

[21 [3]

[4] [5] [6] [7]

[8] [9] [10] [11] [12] [13]

P.S. Bagus, C.J. Nelin and C.W. Bauschlicher, Jr., J. Vacuum Sci. Technol. A2 (1984) 905; S.P. Walch and W.A. Goddard III, J. Am. Chem. Soc. 98 (1976) 7908; A.B. Rives and R.F. Fenske, J. Chem. Phys. 75 (1981) 1293; P.S. Bagus and B.O. Roos, J. Chem. Phys. 75 (1981) 5961; H. Itoh and G. Ertl, Z. Naturforsch. 37a (1982) 346; G. Pacchioni, J. Kouteck~, and P. Fantucci, Chem. Phys. Letters 92 (1982) 486; H. Basch and D. Cohen, J. Am. Chem. Soc. 105 (1983) 3856; P.S. Bagus, C.J. Nelin and C.W. Bauschlicher, Jr., Phys. Rev. B28 (1983) 5423; P.S. Bagus and K. Hermann, Surface Sci. 89 (1979) 588; P.S. Bagus, K. Hermann and C.W. Bauschlicher, Jr., Ber. Bunsenges. Phys. Chem. 88 (1984) 302; (k) P.S. Bagus, C.W. Bauschlicher, Jr., C.j. Nelin and C.B. Laskowski, J. Chem. Phys. 81 (1984) 3599, and references therein. (a) M.J. Dewar, Bull. Soc. Chim. France 18 (1951) C71; (b) J. Chatt and L.A. Duncanson, J. Chem. Soc. 3 (1953) 2939. D. Post and E.J. Baerends, Surface Sci. 109 (1981) 167. (a) J.C. Slater, Advan. Quantum Chem. 6 (1972) 1; (b) E.J. Baerends, D.E. Ellis and P. Ros, Chem. Phys. 2 (1973) 52. J. Kouteck~, G. Pacchioni and P. Fantucci, Chem. Phys. 99 (1985) 87. K. Hermann and P.S. Bagus, Phys. Rev. B17 (1977) 4082. (a) R.J. Buenker and S.D. Peyerimhoff, Theoret. Chim. Acta 35 (1974) 33; (b) R.J. Buenker and S.D. Peyerimhoff, Theoret. Chim. Acta 39 (1975) 217; (c) R.J. Buenker, S.D. Peyerimhoff and W. Butscher, Mol. Phys. 35 (1978) 771. S. Huzinaga, J. Andzelm, M. Klobukowski, E. Radzio-Andzelm, Y. Sakai and H. Tatewski, Gaussian Basis Sets for Molecular Calculations (Elsevier, Amsterdam, 1984). H.-O. Backmann, J. Kouteck~, and V. Bona~i6-Kouteck2;,, J. Chem. Phys. 73 (1980) 5182. B.O. Roos and P. Siegbahn, Theoret. Chim. Acta 17 (1970) 209. P. Fantucci, V. Bona~i6-Kouteck~, and J. Kouteck~, J. Comp. Chem., to be published. H.-O. Beckmann and J. Kouteck~¢, Surface Sci. 120 (1982) 127. (a) W. Reiland, H.V. Tittes, I.V. Hertel, V. Bona6i6-Kouteck~, and M. Persico, J. Chem. Phys. 77 (1982) 1908; (b) D. Papierowska-Kaminski, M. Persico and V. Bona~i6-Kouteck2;,, Chem. Phys. Letters 113 (1985) 264.