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Comment on the possibility of the CO chemisorption on alkali metal surface
L1015
Surface Science 177 (1986) LlOlS-L1020 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
COMMENT ON THE POSSIBILITY OF THE CO CHEMISORPTION ON ALKALI METAL SURFACE V. BONA&C-KOUTECK?, P. FANTUCCI H.-O. BECKMANN and J. KOUTECK?
*, J. KENDRICK
**,
Freie Uniuersitiil Berlin, Institut ftir Physikalische und Theoretische Chemie, 1000 Berlin 33, Germany Received
31 July 1986; accepted
for publication
28 August
1986
The investigation of the interaction between a CO molecule and Lis, Lig and Lise clusters representing parts of the fee and bee Li lattice employing ab initio CI procedure does not yield any significant indication for the chemisorption.
The intriguing conjecture that the non-transition metal surfaces can noticeably chemisorb carbon monoxide [l] is worthy of careful investigation using cluster models. In a previous paper [2], the interaction of a carbon monoxide molecule with Li, clusters modelling the “on top” and “hollow” chemisorption sites on the bee (100) and fee (110) Li surfaces, has been studied with ab initio SCF-CI procedure. No evidence has been found for an appreciable binding interaction between CO and alkali metal surface in contrast to ref. [l]. Nevertheless, it was suggested [2] that the interaction of the ltrger fee Li,(5, 4) cluster with the CO molecule should be studied. Since the central lithium atom in the first layer of the Li,(5, 4) cluster has all of its eight nearest neighbours (fig. l), the analysis of the chemisorption potential curve for this more consequent Li,-CO system can be very useful to show how large the cooperative effects of the second-layer atoms are and to estimate the influence of the crystal bulk on chemisorption. In this paper we first report the results of more accurate SCF-CI calculations than those given in ref. [2] in order to find out if the previous results are confirmed by calculations employing a much more flexible atomic basis set. Secondly, the larger systems Li,-CO and Li,,-CO have been studied in order * Permanent address: Universita di Milano, Dipartimento di Chimica Inorganica e Metallorganica, Centro CNR, I-20133 Milano, Italy. ** Permanent address: Imperial Chemical Industries PLC, New Science Group, Runcom, Cheshire, WA7 4QE, UK.
0039~6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L1016
V. Bona%;-Kouteckj
et al./ Possible CO chemisorption
Fig. 1. Li9(5, 4)-CO
on alkuli metals
cluster model
to show whether the main features of the chemisorption interaction curves are substantially modified by the presence of second and third layer atoms or not. Two atomic basis sets have been adopted. The first one (basis A) [3, 41 is of type (10s 2p/4s2p) for lithium and (11s 6p, ld/5s, 3p, Id) for carbon and oxygen. The second basis (basis B) is of split-valence type: (7s lp/3s, lp) for lithium and (7s 3p/3s, 2p) for carbon and oxygen atoms [3]. The CI calculations are carried out according to the MRD-CI procedure [5]. In order to reduce the computational costs of the CI treatment, a recently proposed 16) i~alization-selection procedure has been applied to occupied and virtual HF orbitals. The overall procedure can be briefly summarized in a few points. First, a “chemisorption molecule” is defined as composed of the adsorbate and the atoms of the cluster which are directly involved in the chemisorption process. Some relevant atomic basis functions centered on the che~so~tion molecule are chosen to build up a projector acting on the valence occupied MO’s. The occupie& MO’s projected on the chemisorption molecule are easily partitioned into two subsets: one relevant for the description of the chemisorption bond and the other describing a group of electrons only scarcely involved in the cluster-adsorption interaction. Only the first set of transformed occupied MO’s is considered in the CI treatment. Finally, the virtual HF orbitals are transformed in order to maximize their exchange interaction with the transformed occupied MO’s considered in the CI description of the interaction studied. The transfo~ation which causes the virtual orbitals to have a large amplitude in the same spatial region of the active transformed occupied MO, leads to an easy choice of the virtual orbitals which can be considered in the CI expansion. The Li,(5,0) and Li,(5,4) clusters (see fig. 1) are chosen to be a section of fee bulk lit~um, with a fixed nearest-neighbour distance equal to 5.9 au.
V. Bona%- Kouteck$ et al./ Possible CO chemisorption
on alkali metals
L1017
Two distinct SCF-CI calculations, carried out with basis sets A and B gave for the CO equilibrium bond distances equal to 2.121 and 2.186 a.u., respectively. The result obtained with basis A agrees well with the experimental value (2.132 a.u.) [7], while basis B overestimates the CO bond length by about 2.5%. This is an expected failure, due to the absence of polarization functions in the basis B. The two bond distances estimated for the free CO molecule, have been kept constant in the calculation of the cluster-CO interaction curve. The fee Li,-CO cluster model has been investigated using both basis sets A and B, with the localization-selection procedure described above: 5 doubly and 1 singly localized occupied MO’s have been correlated with 22 and 34 transformed virtual orbitals, in the case of basis A and B, respectively. The selection of occupied orbitals in both basis sets corresponds to five 2s electrons on five lithium atoms and 6 electrons on CO (5a*, 1r2, la). The SCF curve for the 2E lowest energy state employing the larger basis set shows even more pronounced repulsive Li,-CO interaction than the curve obtained with the smaller basis set (fig. 2). The same conclusion can be drawn from the analysis of the corresponding CI curves (fig. 2). The MRD-CT treatments using localized orbitals carried out with 11 and 5 reference configurations for basis sets A and B and selecting up to 10 000 configurations for the diagonalization do not exhibit even the weak minimum obtained in ref. [2], which led to an estimate of the chemisorption energy of only 1-2 kcal/moll’. A similar computational scheme has been applied to the Li,-CO system using the basis B only. Two different types of CI treatments have been carried out: (i) employing localization-selection technique so that 15 electrons and 29 MO’s are correlated and (ii) employing canonical HF orbitals so that only core orbitals can be discarded from the CI treatment and therefore 19 electrons and 61 MO’s must be correlated. MRD-CI calculations with 4 and 6 reference configurations respectively are performed. The cluster-CO interaction curve computed at SCF level shows a very shallow minimum lying, however, about 2 kcal/mol-’ above the dissociation limit, corresponding to a cluster surface-CO distance of 4.2 a.u. The energy position of the minimum is only slightly lowered when the correlation energy is considered. The CI treatments carried out with localized or canonical orbitals gave an estimate of a chemisorption energy not larger than l-2 kcal/mol-’ for a cluster-CO distance of 4.0-4.1 a.u. However, the existence of the very shallow minima in the SCF or CI curves can be a spurious effect due to the basis set superposition error (BSSE). In fact, when the SCF wavefunction of the isolated cluster is determined using also the cluster basis functions centered on CO (cluster + CO cluster-CO distance = 4.2 a.u.) the BSSE error is equal to 1.1 kcal/mol-‘. The corresponding calculation on the isolated CO molecule (CO + cluster) led to BSSE = 1.2 kcal/mol-‘. Even considering the sum of the two corrections as a
L1018
V. Bon&&Kouteckj
et al./ Possible CO chemisorption
on alkali met&
-1069.8
-LO74.5
\
6 > i z lL -4079 7
CI llel/27orb/llM
l\ l-~-----.
ZE
!
A0 Basis Li:lO,Z 1L.2 CO:11,6,115,3,1
\ a
ye1 91
l wLF
-4085 6 \ .
7
\ %
Cl llel/39orb/SM
\. -w I
I
I
I
I
I
3
1
5
6
7
8
R/au.
Fig. 2. SCF and CI interaction curves for the ground state of the Li,(5,0)-CO system. The results obtained with the smaller A0 basis set with only one p-function for Li atom (7, l/3, 1) and no polarization function for C and 0 (7, 3/3, 2) atoms are given in the upper part of the figure. After localization procedure 11 electrons have been chosen for the correlation treatment among 27 orbitals. MRD-CI procedure with 11 reference configurations (11M) have been employed. In the lower part of the figure the results obtained with a larger basis set are displayed. The additional polarization functions for Li atom (10, 2/4, 2) and for C and 0 atoms (11, 6, l/5, 3, 1) have been introduced. The correlation treatment with 5M configurations has been carried out for II electrons among 39 orbitals.
probably too severe estimate of the total BSSE, one can conclude that the minima‘ occurring in the Li,-CO potential curves are of the same order of magnitude as the BSSE and that the cluster-CO interaction is repulsive for every value of the chemisorption coordinate. On the other hand, the well with depth of 0.16 eV located at the CO-cluster distance of 4.2-4.3 a.u. on a generally repulsive interaction curve can be interpreted as an indication of a
V. BonoEiC-Kouiecki
et al./ Possible CO chemisorption
Fig. 3. Li,,(9,12,
9)-CO
on alkali metals
L1019
cluster model.
thermodynamically unstable “precursor”. Nevertheless, as already mentioned such a shallow minimum lies within the error limits of the method used. The Li,,-cluster which is a (9,12,9) section of bee lattice (see fig. 3) has been studied using the mixed basis set technique [8]. The contracted (2s lp) basis [9] on the central lithium atom of the first layer and a minimal basis (2s) on all other 29 lithium atoms have been used. A standard ST03-21G basis [lo] is used for C and 0 atoms. The Li,,-CO interaction curves computed for the ‘A, and 3E, (C,,) states exhibit minima of 8.9 and 8.6 kcal/mol-’ at a cluster-CO distance of 4.25 and 4.27 a.u., respectively. The computed BSSE correction for cluster + (CO) amounts to 5.5 kcal/mol-’ and for the CO + (cluster) is equal to 1.2 kcal/mol-‘. To summarize, the fee L&(5, 4) cluster model of the “on top” interaction of the CO moiety with the (100) fee Li surface, where all nearest neighbours of the attacked Li surface atom are included, supports the previous conjecture [2] that the bonding interaction of CO with Li surfaces is improbable. The SCF calculations carried out for the bee Li,,-CO system do not show noticeable bonding either. These results which contradict the conclusions of the Hartree-Slater studies [l] seem to be independent of the choice of the A0 basis set employed.
This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 6 “Structure and Dynamics of Interfaces” and by the Italian CNR.
L1020
V. Bona%-Kouteckj
et al./ Possible CO chemisorption
on alkali metals
References [I] D. Post and E. J. Baerends, Surface Sci. 109 (1981) 167. [2] J. Koutecky, U. Hanke, P. Fantucci, V. Bona&C-Koutecky and D. Papierowska-Kaminski, Surface Sci. 165 (1986) 161. [3] S. Huzinaga, J. Andzelm, M. Klobukowski, E. Radzio-Andzelm, Y. Saka and H. Tatewaki, Gaussian Basis Sets for Molecular Calculations (Elsevier, New York, 1984). [4] S. Huzinaga, J. Chem. Phys. 42 (1965) 1293; T.H. Dunning, Jr., J. Chem. Phys. 55 (1971) 716. [5] R.J. Buenker and S.D. Peyerimhoff, Theoret. Chim. Acta 35 (1974) 33; 39 (1975) 217: R. J. Buenker, S.D. Peyerimhoff and W. Butscher, Mol. Phys. 35 (1978) 771. [6] P. Fantucci, V. Bona&f-Koutecky and J. Koutecky, Phys. Rev. B. in press. [7] G. Herzberg, Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules (Van Nostrand, New York, 1950). (81 G. Pacchioni, J. Koutecky and H.-O. Beckmann, Surface Sci. 144 (1984) 602. (91 H.-O. Beckmann and J. Koutecky, Surface Sci. 120 (1982) 127. [lo] R. Ditchfield, W.J. Hehre and J.A. Pople, J. Chem. Phys. 54 (1971) 724.
Surface Science 177 (1986) LlOlS-L1020 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
COMMENT ON THE POSSIBILITY OF THE CO CHEMISORPTION ON ALKALI METAL SURFACE V. BONA&C-KOUTECK?, P. FANTUCCI H.-O. BECKMANN and J. KOUTECK?
*, J. KENDRICK
**,
Freie Uniuersitiil Berlin, Institut ftir Physikalische und Theoretische Chemie, 1000 Berlin 33, Germany Received
31 July 1986; accepted
for publication
28 August
1986
The investigation of the interaction between a CO molecule and Lis, Lig and Lise clusters representing parts of the fee and bee Li lattice employing ab initio CI procedure does not yield any significant indication for the chemisorption.
The intriguing conjecture that the non-transition metal surfaces can noticeably chemisorb carbon monoxide [l] is worthy of careful investigation using cluster models. In a previous paper [2], the interaction of a carbon monoxide molecule with Li, clusters modelling the “on top” and “hollow” chemisorption sites on the bee (100) and fee (110) Li surfaces, has been studied with ab initio SCF-CI procedure. No evidence has been found for an appreciable binding interaction between CO and alkali metal surface in contrast to ref. [l]. Nevertheless, it was suggested [2] that the interaction of the ltrger fee Li,(5, 4) cluster with the CO molecule should be studied. Since the central lithium atom in the first layer of the Li,(5, 4) cluster has all of its eight nearest neighbours (fig. l), the analysis of the chemisorption potential curve for this more consequent Li,-CO system can be very useful to show how large the cooperative effects of the second-layer atoms are and to estimate the influence of the crystal bulk on chemisorption. In this paper we first report the results of more accurate SCF-CI calculations than those given in ref. [2] in order to find out if the previous results are confirmed by calculations employing a much more flexible atomic basis set. Secondly, the larger systems Li,-CO and Li,,-CO have been studied in order * Permanent address: Universita di Milano, Dipartimento di Chimica Inorganica e Metallorganica, Centro CNR, I-20133 Milano, Italy. ** Permanent address: Imperial Chemical Industries PLC, New Science Group, Runcom, Cheshire, WA7 4QE, UK.
0039~6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L1016
V. Bona%;-Kouteckj
et al./ Possible CO chemisorption
Fig. 1. Li9(5, 4)-CO
on alkuli metals
cluster model
to show whether the main features of the chemisorption interaction curves are substantially modified by the presence of second and third layer atoms or not. Two atomic basis sets have been adopted. The first one (basis A) [3, 41 is of type (10s 2p/4s2p) for lithium and (11s 6p, ld/5s, 3p, Id) for carbon and oxygen. The second basis (basis B) is of split-valence type: (7s lp/3s, lp) for lithium and (7s 3p/3s, 2p) for carbon and oxygen atoms [3]. The CI calculations are carried out according to the MRD-CI procedure [5]. In order to reduce the computational costs of the CI treatment, a recently proposed 16) i~alization-selection procedure has been applied to occupied and virtual HF orbitals. The overall procedure can be briefly summarized in a few points. First, a “chemisorption molecule” is defined as composed of the adsorbate and the atoms of the cluster which are directly involved in the chemisorption process. Some relevant atomic basis functions centered on the che~so~tion molecule are chosen to build up a projector acting on the valence occupied MO’s. The occupie& MO’s projected on the chemisorption molecule are easily partitioned into two subsets: one relevant for the description of the chemisorption bond and the other describing a group of electrons only scarcely involved in the cluster-adsorption interaction. Only the first set of transformed occupied MO’s is considered in the CI treatment. Finally, the virtual HF orbitals are transformed in order to maximize their exchange interaction with the transformed occupied MO’s considered in the CI description of the interaction studied. The transfo~ation which causes the virtual orbitals to have a large amplitude in the same spatial region of the active transformed occupied MO, leads to an easy choice of the virtual orbitals which can be considered in the CI expansion. The Li,(5,0) and Li,(5,4) clusters (see fig. 1) are chosen to be a section of fee bulk lit~um, with a fixed nearest-neighbour distance equal to 5.9 au.
V. Bona%- Kouteck$ et al./ Possible CO chemisorption
on alkali metals
L1017
Two distinct SCF-CI calculations, carried out with basis sets A and B gave for the CO equilibrium bond distances equal to 2.121 and 2.186 a.u., respectively. The result obtained with basis A agrees well with the experimental value (2.132 a.u.) [7], while basis B overestimates the CO bond length by about 2.5%. This is an expected failure, due to the absence of polarization functions in the basis B. The two bond distances estimated for the free CO molecule, have been kept constant in the calculation of the cluster-CO interaction curve. The fee Li,-CO cluster model has been investigated using both basis sets A and B, with the localization-selection procedure described above: 5 doubly and 1 singly localized occupied MO’s have been correlated with 22 and 34 transformed virtual orbitals, in the case of basis A and B, respectively. The selection of occupied orbitals in both basis sets corresponds to five 2s electrons on five lithium atoms and 6 electrons on CO (5a*, 1r2, la). The SCF curve for the 2E lowest energy state employing the larger basis set shows even more pronounced repulsive Li,-CO interaction than the curve obtained with the smaller basis set (fig. 2). The same conclusion can be drawn from the analysis of the corresponding CI curves (fig. 2). The MRD-CT treatments using localized orbitals carried out with 11 and 5 reference configurations for basis sets A and B and selecting up to 10 000 configurations for the diagonalization do not exhibit even the weak minimum obtained in ref. [2], which led to an estimate of the chemisorption energy of only 1-2 kcal/moll’. A similar computational scheme has been applied to the Li,-CO system using the basis B only. Two different types of CI treatments have been carried out: (i) employing localization-selection technique so that 15 electrons and 29 MO’s are correlated and (ii) employing canonical HF orbitals so that only core orbitals can be discarded from the CI treatment and therefore 19 electrons and 61 MO’s must be correlated. MRD-CI calculations with 4 and 6 reference configurations respectively are performed. The cluster-CO interaction curve computed at SCF level shows a very shallow minimum lying, however, about 2 kcal/mol-’ above the dissociation limit, corresponding to a cluster surface-CO distance of 4.2 a.u. The energy position of the minimum is only slightly lowered when the correlation energy is considered. The CI treatments carried out with localized or canonical orbitals gave an estimate of a chemisorption energy not larger than l-2 kcal/mol-’ for a cluster-CO distance of 4.0-4.1 a.u. However, the existence of the very shallow minima in the SCF or CI curves can be a spurious effect due to the basis set superposition error (BSSE). In fact, when the SCF wavefunction of the isolated cluster is determined using also the cluster basis functions centered on CO (cluster + CO cluster-CO distance = 4.2 a.u.) the BSSE error is equal to 1.1 kcal/mol-‘. The corresponding calculation on the isolated CO molecule (CO + cluster) led to BSSE = 1.2 kcal/mol-‘. Even considering the sum of the two corrections as a
L1018
V. Bon&&Kouteckj
et al./ Possible CO chemisorption
on alkali met&
-1069.8
-LO74.5
\
6 > i z lL -4079 7
CI llel/27orb/llM
l\ l-~-----.
ZE
!
A0 Basis Li:lO,Z 1L.2 CO:11,6,115,3,1
\ a
ye1 91
l wLF
-4085 6 \ .
7
\ %
Cl llel/39orb/SM
\. -w I
I
I
I
I
I
3
1
5
6
7
8
R/au.
Fig. 2. SCF and CI interaction curves for the ground state of the Li,(5,0)-CO system. The results obtained with the smaller A0 basis set with only one p-function for Li atom (7, l/3, 1) and no polarization function for C and 0 (7, 3/3, 2) atoms are given in the upper part of the figure. After localization procedure 11 electrons have been chosen for the correlation treatment among 27 orbitals. MRD-CI procedure with 11 reference configurations (11M) have been employed. In the lower part of the figure the results obtained with a larger basis set are displayed. The additional polarization functions for Li atom (10, 2/4, 2) and for C and 0 atoms (11, 6, l/5, 3, 1) have been introduced. The correlation treatment with 5M configurations has been carried out for II electrons among 39 orbitals.
probably too severe estimate of the total BSSE, one can conclude that the minima‘ occurring in the Li,-CO potential curves are of the same order of magnitude as the BSSE and that the cluster-CO interaction is repulsive for every value of the chemisorption coordinate. On the other hand, the well with depth of 0.16 eV located at the CO-cluster distance of 4.2-4.3 a.u. on a generally repulsive interaction curve can be interpreted as an indication of a
V. BonoEiC-Kouiecki
et al./ Possible CO chemisorption
Fig. 3. Li,,(9,12,
9)-CO
on alkali metals
L1019
cluster model.
thermodynamically unstable “precursor”. Nevertheless, as already mentioned such a shallow minimum lies within the error limits of the method used. The Li,,-cluster which is a (9,12,9) section of bee lattice (see fig. 3) has been studied using the mixed basis set technique [8]. The contracted (2s lp) basis [9] on the central lithium atom of the first layer and a minimal basis (2s) on all other 29 lithium atoms have been used. A standard ST03-21G basis [lo] is used for C and 0 atoms. The Li,,-CO interaction curves computed for the ‘A, and 3E, (C,,) states exhibit minima of 8.9 and 8.6 kcal/mol-’ at a cluster-CO distance of 4.25 and 4.27 a.u., respectively. The computed BSSE correction for cluster + (CO) amounts to 5.5 kcal/mol-’ and for the CO + (cluster) is equal to 1.2 kcal/mol-‘. To summarize, the fee L&(5, 4) cluster model of the “on top” interaction of the CO moiety with the (100) fee Li surface, where all nearest neighbours of the attacked Li surface atom are included, supports the previous conjecture [2] that the bonding interaction of CO with Li surfaces is improbable. The SCF calculations carried out for the bee Li,,-CO system do not show noticeable bonding either. These results which contradict the conclusions of the Hartree-Slater studies [l] seem to be independent of the choice of the A0 basis set employed.
This work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 6 “Structure and Dynamics of Interfaces” and by the Italian CNR.
L1020
V. Bona%-Kouteckj
et al./ Possible CO chemisorption
on alkali metals
References [I] D. Post and E. J. Baerends, Surface Sci. 109 (1981) 167. [2] J. Koutecky, U. Hanke, P. Fantucci, V. Bona&C-Koutecky and D. Papierowska-Kaminski, Surface Sci. 165 (1986) 161. [3] S. Huzinaga, J. Andzelm, M. Klobukowski, E. Radzio-Andzelm, Y. Saka and H. Tatewaki, Gaussian Basis Sets for Molecular Calculations (Elsevier, New York, 1984). [4] S. Huzinaga, J. Chem. Phys. 42 (1965) 1293; T.H. Dunning, Jr., J. Chem. Phys. 55 (1971) 716. [5] R.J. Buenker and S.D. Peyerimhoff, Theoret. Chim. Acta 35 (1974) 33; 39 (1975) 217: R. J. Buenker, S.D. Peyerimhoff and W. Butscher, Mol. Phys. 35 (1978) 771. [6] P. Fantucci, V. Bona&f-Koutecky and J. Koutecky, Phys. Rev. B. in press. [7] G. Herzberg, Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules (Van Nostrand, New York, 1950). (81 G. Pacchioni, J. Koutecky and H.-O. Beckmann, Surface Sci. 144 (1984) 602. (91 H.-O. Beckmann and J. Koutecky, Surface Sci. 120 (1982) 127. [lo] R. Ditchfield, W.J. Hehre and J.A. Pople, J. Chem. Phys. 54 (1971) 724.