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Dissociative versus molecular chemisorption of oxygen on Cu(110)
L347
Surface Science 216 (1989) L347-L350 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
DISSOCIATIVE VERSUS MOLECULAR CHEMISORPTION OF OXYGEN ON Cu(ll0) A. WANDER University of Warwick,
Coventry,
Warwickshire
CV4 7AL, UK
Received 31 January 1989; accepted for publication 15 March 1989
The low temperature structure of oxygen on Cu(ll0) has recently been the subject of much controversy. Prabhakaran et al. used UPS, XPS, AES, HREELS and LEED to deduce that molecular chemisotption is the dominant process. Mundenar et al., using EELS and UPS, find no evidence of molecular chemisorption and propose atomic adsorption in a combination of four coordinated and long bridge sites. We calculate the total energies for both atomic and molecular chemisorption in all high symmetry sites on a Cut, cluster using a self-consistent Hartree-Fock scheme. No evidence of molecular chemisorption is found, while atomic oxygen is found to occupy a long bridge site forming Cu-0 bonds of about 1.8 A. The vibrational mode corresponding to vertical displacement of the oxygen atom about this position is found to have an energy of 52.9 meV, in agreement with Mundemar’s experimental value of 49 meV.
Two recent experimental studies have yielded conflicting evidence conceming the low temperature structure of oxygen on Cu(ll0). Prabhakaran et al. [1,2] observe a three-peaked UP spectrum which is thought to be characteristic of molecular oxygen chemisorption [3,4], in agreement with the earlier UPS work of Spitzer and Ltith on single-crystal Cu surfaces [5,6]. In addition, they observe an EELS signal at 660 cm-’ which they assign to an O-O vibrational stretch mode. However, Mundenar et al. present contradictory evidence [7,8]. They find no EELS signal which can be ascribed to an O-O mode, and find only an 0-Cu mode centred on 49 meV. They therefore conclude that the adsorption mechanism is dissociative in agreement with the EELS study of Wendelken [9]. Mundemar’s work also shows that the three-peaked structure in the UPS signal may be caused by small amounts of an impurity (either CO or H,O) coadsorbed on an oxygen predosed surface as proposed earlier by Prince and Paolucci [lo]. The aim of the present paper is to present results of calculations on both atomic and molecular chemisorption. We calculate total energies for both situations in all high symmetry sites on a Cu,, cluster (fig. la). The twelve Cu atoms are assumed to retain the unrelaxed Cu(ll0) surface structure (fig. lb) throughout the calculations. The use of this cluster allows all high symmetry 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L348
A. Wander / Dissociative versus molecular chemisorption of oxygen on Cu(Il0)
a
1”
Long Bridge
Short Bridge
AtOp
Atop 2”” Layer
1”’
Cl 0
Fig. 1. (a) The high symmetry
sites on the Cu(ll0)
surface.
Layer
cu
Layer
(b) The Cu,,
2nd Layer CU
oxygen
cluster.
sites to be investigated and hence, total energies are directly comparable. The total energy is minimized with respect to the surface adsorbate distance to yield the equilibrium geometry. The calculations were performed using the general atomic and molecular electronic structure systems (GAMESS) quantum chemistry package #’ and gaussian basis sets due to Huzinaga [ll]. The energies were calculated within a self-consistent restricted Hartree-Fock scheme (RHF) which is known to yield accurate geometries without the inclusion of electron correlation energies which were therefore neglected. The first set of calculations involved chemisorption of 0, with the O-O bond length set to the molecular value of 1.208 A. The calculations were performed with the O-O axis both parallel to, and perpendicular to, the surface normal. For the perpendicular case, the high symmetry configurations involving both 45 o and 90 o rotations of the O-O axis (fig. 2) about the surface normal were investigated. No evidence of a stable structure involving 0, chen&orption on the surface has been found. The second set of calculations involved atomic oxygen. This is found to form its most stable structure for chemisorption in a long bridge site forming Cu-0 bonds of 1.8 A with both the first- and second-layer Cu atoms. Table 1 summarises the relative stabilities of the various high symmetry sites. The energy of the Cu-0 vibrational mode was calculated by displacing the oxygen atom vertically about the equilibrium position (the frozen phonon approxima*I Thanks
are due to M.F. Guest
and J. Kendrick
for the use of this program.
A. Wander / Dissociative
versus molecular chemisorption
of oxygen on Cu(l10)
@B Cl 1st
45” Rotation
0’ Rotation
L349
Layercu
2” Layer cu
90” Rotation Fig. 2. Illustration Table 1 Summary
of atomic
Site Short Long Atop Atop
bridge bridge first layer second layer
a) 1.7 A with second
of 45 o and 90 o rotations
of the O-O
axis for the atop first layer site.
adsorption Bond length (A)
Relative
1.8 1.8 1.8 1.7/2.0
+ 0.55 0 + 1.20 +0.21
‘)
energy (ev)
layer, 2.0 A with first layer.
tion) while keeping the Cu atoms rigid. This mode is found to have an energy of 52.9 meV for the long bridge site, in agreement with Mundenar et al.‘s experimental value of 49 meV. In conclusion, RHF total-energy calculations using a Cu,, cluster show that the low temperature chemisorption of oxygen on Cu(ll0) is dissociative, in agreement with the experiments of Mundenar et al. I would like to thank B.W. Holland for several helpful discussions. This work was supported by the science and Engineering Research Council (UK).
References [l] [2] [3] [4]
K. Prabhakaran, P. Sen and C.N.R. Rao, Surface Sci. 177 (1986) L971. K. Prabhakaran and C.N.R. Rao, Surface Sci. 198 (1988) L307. C.N.R. Rao, P.V. Kamath and S. Yashonath, Chem. Phys. Letters 88 (1982) 13. P.V. Kamath and C.N.R. Rao, J. Phys. Chem. 88 (1984) 464.
L350
A. Wander / Dissociative
versus molecular r~em~sorption of oxygen on 0.41 IQ)
[5] A. Spitzer and H. Liith, Surface Sci. 118 (1982) 121. [6] A. Spitzer and H. Liith, Surface Sci. 118 (1982) 136. [7] J.M. Mundenar, A.P. Baddorf, E.W. Plummer, L.G. Sneddon, R.A. Didio and D.M. Zehner, Surface Sci. 188 (1987) 15. IX] J.M. Mundenar, E.W. Plummer. L.C. Sneddon, A.P. Baddorf, D.M. Zehner and CR. Gruzalski, Surface Sci. 198 (1988) L309. [9] J.F. Wendelken, Surface Sci. 108 (1981) 605. IlO] K.C. Prince and G. Paolucci, J. Electron Spectrosc. Related Phenomena 37 (1985) 181. [ll] S. Huzinaga , Gaussian Basis Sets for Molecular Calculations (Elsevier, Amsterdam, 1984).
Surface Science 216 (1989) L347-L350 North-Holland, Amsterdam
SURFACE
SCIENCE
LETTERS
DISSOCIATIVE VERSUS MOLECULAR CHEMISORPTION OF OXYGEN ON Cu(ll0) A. WANDER University of Warwick,
Coventry,
Warwickshire
CV4 7AL, UK
Received 31 January 1989; accepted for publication 15 March 1989
The low temperature structure of oxygen on Cu(ll0) has recently been the subject of much controversy. Prabhakaran et al. used UPS, XPS, AES, HREELS and LEED to deduce that molecular chemisotption is the dominant process. Mundenar et al., using EELS and UPS, find no evidence of molecular chemisorption and propose atomic adsorption in a combination of four coordinated and long bridge sites. We calculate the total energies for both atomic and molecular chemisorption in all high symmetry sites on a Cut, cluster using a self-consistent Hartree-Fock scheme. No evidence of molecular chemisorption is found, while atomic oxygen is found to occupy a long bridge site forming Cu-0 bonds of about 1.8 A. The vibrational mode corresponding to vertical displacement of the oxygen atom about this position is found to have an energy of 52.9 meV, in agreement with Mundemar’s experimental value of 49 meV.
Two recent experimental studies have yielded conflicting evidence conceming the low temperature structure of oxygen on Cu(ll0). Prabhakaran et al. [1,2] observe a three-peaked UP spectrum which is thought to be characteristic of molecular oxygen chemisorption [3,4], in agreement with the earlier UPS work of Spitzer and Ltith on single-crystal Cu surfaces [5,6]. In addition, they observe an EELS signal at 660 cm-’ which they assign to an O-O vibrational stretch mode. However, Mundenar et al. present contradictory evidence [7,8]. They find no EELS signal which can be ascribed to an O-O mode, and find only an 0-Cu mode centred on 49 meV. They therefore conclude that the adsorption mechanism is dissociative in agreement with the EELS study of Wendelken [9]. Mundemar’s work also shows that the three-peaked structure in the UPS signal may be caused by small amounts of an impurity (either CO or H,O) coadsorbed on an oxygen predosed surface as proposed earlier by Prince and Paolucci [lo]. The aim of the present paper is to present results of calculations on both atomic and molecular chemisorption. We calculate total energies for both situations in all high symmetry sites on a Cu,, cluster (fig. la). The twelve Cu atoms are assumed to retain the unrelaxed Cu(ll0) surface structure (fig. lb) throughout the calculations. The use of this cluster allows all high symmetry 0039-6028/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L348
A. Wander / Dissociative versus molecular chemisorption of oxygen on Cu(Il0)
a
1”
Long Bridge
Short Bridge
AtOp
Atop 2”” Layer
1”’
Cl 0
Fig. 1. (a) The high symmetry
sites on the Cu(ll0)
surface.
Layer
cu
Layer
(b) The Cu,,
2nd Layer CU
oxygen
cluster.
sites to be investigated and hence, total energies are directly comparable. The total energy is minimized with respect to the surface adsorbate distance to yield the equilibrium geometry. The calculations were performed using the general atomic and molecular electronic structure systems (GAMESS) quantum chemistry package #’ and gaussian basis sets due to Huzinaga [ll]. The energies were calculated within a self-consistent restricted Hartree-Fock scheme (RHF) which is known to yield accurate geometries without the inclusion of electron correlation energies which were therefore neglected. The first set of calculations involved chemisorption of 0, with the O-O bond length set to the molecular value of 1.208 A. The calculations were performed with the O-O axis both parallel to, and perpendicular to, the surface normal. For the perpendicular case, the high symmetry configurations involving both 45 o and 90 o rotations of the O-O axis (fig. 2) about the surface normal were investigated. No evidence of a stable structure involving 0, chen&orption on the surface has been found. The second set of calculations involved atomic oxygen. This is found to form its most stable structure for chemisorption in a long bridge site forming Cu-0 bonds of 1.8 A with both the first- and second-layer Cu atoms. Table 1 summarises the relative stabilities of the various high symmetry sites. The energy of the Cu-0 vibrational mode was calculated by displacing the oxygen atom vertically about the equilibrium position (the frozen phonon approxima*I Thanks
are due to M.F. Guest
and J. Kendrick
for the use of this program.
A. Wander / Dissociative
versus molecular chemisorption
of oxygen on Cu(l10)
@B Cl 1st
45” Rotation
0’ Rotation
L349
Layercu
2” Layer cu
90” Rotation Fig. 2. Illustration Table 1 Summary
of atomic
Site Short Long Atop Atop
bridge bridge first layer second layer
a) 1.7 A with second
of 45 o and 90 o rotations
of the O-O
axis for the atop first layer site.
adsorption Bond length (A)
Relative
1.8 1.8 1.8 1.7/2.0
+ 0.55 0 + 1.20 +0.21
‘)
energy (ev)
layer, 2.0 A with first layer.
tion) while keeping the Cu atoms rigid. This mode is found to have an energy of 52.9 meV for the long bridge site, in agreement with Mundenar et al.‘s experimental value of 49 meV. In conclusion, RHF total-energy calculations using a Cu,, cluster show that the low temperature chemisorption of oxygen on Cu(ll0) is dissociative, in agreement with the experiments of Mundenar et al. I would like to thank B.W. Holland for several helpful discussions. This work was supported by the science and Engineering Research Council (UK).
References [l] [2] [3] [4]
K. Prabhakaran, P. Sen and C.N.R. Rao, Surface Sci. 177 (1986) L971. K. Prabhakaran and C.N.R. Rao, Surface Sci. 198 (1988) L307. C.N.R. Rao, P.V. Kamath and S. Yashonath, Chem. Phys. Letters 88 (1982) 13. P.V. Kamath and C.N.R. Rao, J. Phys. Chem. 88 (1984) 464.
L350
A. Wander / Dissociative
versus molecular r~em~sorption of oxygen on 0.41 IQ)
[5] A. Spitzer and H. Liith, Surface Sci. 118 (1982) 121. [6] A. Spitzer and H. Liith, Surface Sci. 118 (1982) 136. [7] J.M. Mundenar, A.P. Baddorf, E.W. Plummer, L.G. Sneddon, R.A. Didio and D.M. Zehner, Surface Sci. 188 (1987) 15. IX] J.M. Mundenar, E.W. Plummer. L.C. Sneddon, A.P. Baddorf, D.M. Zehner and CR. Gruzalski, Surface Sci. 198 (1988) L309. [9] J.F. Wendelken, Surface Sci. 108 (1981) 605. IlO] K.C. Prince and G. Paolucci, J. Electron Spectrosc. Related Phenomena 37 (1985) 181. [ll] S. Huzinaga , Gaussian Basis Sets for Molecular Calculations (Elsevier, Amsterdam, 1984).