- Email: [email protected]
Quantum chemical study of the bond activation for NO adsorbed on a Cu surface
SOLID STATE
Solid State Ionics 63-65 (1993) 777-780 North-Holland
IOHICS Quantum chemical study of the bond activation for NO adsorbed on a Cu surface M. F e r n g m d e z - G a r c i a , J.C. C o n e s a lnstituto de Catdlisis y Petroleoquimica (C.S.L C.). Campus Universidad AutOnoma, Cantoblanco. 28049 Madrid, Spain
and F. Illas Departament de Quirnica Fisica, Facultat de Qulmica, Universitat de Barcelona, C/Martl i Franquks 1, 28028 Barcelona, Spain
Pseudopotential SCF molecular orbital calculations are performed on a Cus-NO cluster modelling the adsorption of NO on Cu ( 111) surfaces. In all three adsorption sites studied, tilted geometries and strongly ionic adsorption states (i.e. describable as
NO--Cu~- ) are found. The vibration frequencies predicted are compared with the experimental data, and their rather different values interpreted as due to geometric factors ("surface wall effect") since the extent of charge transfer from Cu to NO is similar in all cases.
1. Introduction The adsorption of NO on metals is of large importance in decontamination processes, especially for automobile exhaust purification [1 ]. The understanding of this system is less advanced than for other adsorbates: the difficulties relate not only to the wider reactivity of NO, but also to the structural assignation of the experimentally observed NO species [2,31. Although the main catalysts for this process are based on precious metals, activity has been reported also for other less noble elements such as Cu [ 1 ]. Theoretical studies on the N O / C u system have been available only in the last few years [ 4,5 ]. All of them show that the N O - C u bond is essentially ionic, with charge transfer from the metal to the 2n* N O orbitals, yielding an adsorbed N O - . The ionic nature o f the bond has several consequences: first, a non directional bond favours a tilted situation for the adsorbed species [ 5 ], and second, the electronic properties of the bond are little dependent on the Cu
crystal face or position of adsorption [4,5]. While the N O - C u interaction seems to be identical in all of the cases theoretically studied, the experimentally observed frequencies of the N - O stretching [2,3] and the N - O distances obtained from the calculations [4,5] change from one adsorption position to another, suggesting some kind o f difference in the interaction. Then, with these considerations in mind, it seems appropriate to start this study by checking whether the different adsorption positions of the ( 11 1 ) face (which, as we will show, have very different NO stretching frequencies) produce distinct NO adsorbed molecules, and by relating the data with the reactivity of these NO species. In the present work the ground states arising from the interaction of NO with different adsorption positions of the Cu ( 11 1 ) surface have been studied at the SCF level of theory. The frequencies of the most significant vibration modes for these adsorbed states have also been obtained, trying to assign the experimentally observed species and to relate the bond properties to the reactivity of these species.
Author to whom correspondence should be addressed. 0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
778
M. Ferndmdez-Garcia et aL / NO adsorbed on a Cu surface
2. Computational details
3. Results and discussion
The Cu ( 111 ) surface has been modeled by a small cluster including four atoms of the surface layer and one of the second layer, using geometry (fcc) and distances (dcu_cu=2.54 A) as those in the bulk material. This cluster was chosen because it allows the study of bent three-fold, bridge and on-top NO (Ndown) positions, keeping the Cs symmetry throughout the work (see fig. 1 ). For this Cu5(4,1 ) cluster model the inner (18-electron) cores have been replaced by a non-empirical pseudopotential [ 6,7 ], and the basis set is of ( 3 s 3 p 5 d / 2 s l p l d ) quality. For NO a (4s4p/2s2p) basis set is used. The wavefunctions are obtained at SCF level of theory. Open-shell restricted Hartree-Fock calculations have been done using the coupling-operator formalism of Carb6 et al. [8] implemented in the H O N D O computer program [9-11 ]. Although correlation effects have not been included, limiting the accuracy of the numeric results, it is well known [ 12,13 ] that SCF or MCSCF wavefunctions give adequate qualitative information on the bond nature and on the different contributions to the bonding energy or to the vibrational frequencies, allowing a proper discussion of the results in terms of bonding nature, frequency shifts respect to free NO or N O and other qualitative information extracted from the wavefunctions. Due to the large differences in the numerical value of the frequencies of the several vibrational modes, they have been obtained by an approximate separation of high and low frequencies [ 14 ]. It is worth noting that this method yields very good approximations (especially for vibration such as the N - O stretch) for adsorbate-surface systems.
For the Cu5(4,1) cluster the three lowest electronic states obtained were respectively of 2A' (ground state), 4A" (4 Kcal/mol above the ground state) and 2A' (6 Kcal/mol) symmetry. From the results obtained with the NO-Cu5 system, the equilibrium distance above the surface plane, N - O distance, Cu-NO angle and binding energy in the ground state (found to be ofSA ' symmetry in all cases) for the NO in interaction with the three-fold, bridge and on-top positions of the Cu( 111 ) surface are reported in table 1. As previously mentioned, these states have identical bonding properties; the analysis of the charge received by NO and of the dipole moment curves [5] points out that the bond is ionic, with negligible involvement of the C u d orbitals in direct bonding or back-bonding effects, and it could be shown that these states correlate, at large separation, with Cu~- (in the 3A" state generated from the 4 A " Cu5 state) and N O - (3Z- state). Note that, because t h e 4 A " state is not the ground state of Cus, the interaction could be interpreted by adopting the idea of bond preparation given by Panas et al. [ 15 ]. Anyway, the point to be stressed here is that the NO molecule adsorbs as N O - (3E-) independently of the adsorption position; comparison with other results published in the literature [4] suggests that this behaviour is characteristic of the N O / C u interaction irrespectively of the surface plane involved. With the aim of establishing a connection to the experimental results recently reported in ref. [ 3 ], we show in table 2 the vibration frequencies ( C u - N O and N - O stretchings and C u - N O bending) calculated for the optimum configurations of adsorption described in table 1, together with the frequencies that So et al. [3 ] obtained for the N O / C u ( 111 ) sysTable 1 Results of optimal geometries and interaction energies obtained for different adsorption sites. The C u - N O angle 0 is defined in fig. 1.
4~
~5
Fig. 1. Geometric arrangement used in the calculations.
Adsorption position
r± (A)
rNo (A)
0cu-No (°)
BE (Kcal/mol)
three-fold bridge on-top
1.75 1.873 1.90
1.24 1.28 1.24
58 11 50
-9.8 - 13.3 - 12.4
M. Fern(mdez-Garcia et al. / NO adsorbed on a Cu surface
779
Table 2 Results on vibrational frequencies (in cm-~ ) obtained from ab-initio calculations on the NO-Cu5 system. Adsorption site
/JINX)
VCu_NO
V & (Cu_NO)
three-fold
1680 ( + 265 ) a) [1535] b),c) (+200) ") 1420 (+5) a) [1040] b) (--300) ~) 1655 (+240) *~
450 [350] b)x) _d)
150 _d)
660
110
bridge on-top
a) Values in parenthesis indicate the shifts from the appropriate value for free (gas phase) NO-: either theoretically computed ( 1415 cm- 1, this work ) or experimentally measured ( 1340 cm- t ). b) Values in brackets are from experiment (ref. [3] ); the assignation of them implied in this table is proposed by comparison with the theoretical shift values, and differs in some cases from that in ref. [ 3 ]. c) Main v(NO) vibrations observed in ref. [3] at low NO coverage. d) Due to the strong coupling of these modes of vibration in the bridge geometry, the separation of high and low frequencies does not yield reliable results for them. tern. On the basis o f these data, it is p r o p o s e d that at low coverages N O would adsorb mainly as threefold coordinated species, whereas at coverages greater than 0.05 L a n g m u i r three N O species could be present on the surface: the three-fold c o o r d i n a t e d species, the bridged one and another one (showing a frequency o f 1825 c m - 1), not assigned by us, which probably corresponds to a molecular N O instead o f a N O - species; the lack o f charge transfer in this case could be explained if these species were located near an electronegative atom, e.g. oxygen [ 16 ] which can be p r o d u c e d upon N O dissociation [3 ]. It is particularly worth noting that even though the C u - N O b o n d i n g is nearly equally ionic in all the adsorption sites considered in our calculations, the N O stretch frequencies o f the species occurring on the surface m a y present rather different numerical values. This can be u n d e r s t o o d considering the results reported by Bagus et al. [ 17,18 ], which show that there are three m a i n contributions to the vibrational frequency and to the N - O b o n d distance: the extent o f charge transfer, which as discussed above is practically a constant in the N O / C u system (obviously, if this contribution were the only one, the N - O frequency and distance would correspond to that in free N O - ) ; the "surface wall effect", which energetically is repulsive and occurs when the N O charge density penetrates the rigid surface during the stretching motion; a n d the electric field i n d u c e d by the positively charged metal. Note that, as pointed out in refs. [ 17 ] and [ 18 ], both the surface wall effect and the electric field work in the same direction (increasing the
frequency and decreasing the N - O distance, i.e. in opposite direction as charge transfer) and d e p e n d on the orientation o f the a d s o r b e d molecule. Therefore, due to these reasonings and to the nearly equal charge received by N O in the three adsorption positions studied, a qualitative justification o f frequency shifts (respect to N O - ) and N - O distances reported in table 1 and 2 should be based on the degree o f tilting. G i v e n that the 5A' ground state, and therefore the Cu5 and N O charge density distributions, are nearly identical in all the positions studied, the surface wall effect should be similar in the only moderately tilted three-fold and on top positions, whereas for the highly inclined bridge position this effect will hinder the vibration m o v e m e n t in a much lower, probably negligible, extent. On the other hand, the electric field effect must be relatively small; in fact, a calculation o f the N - O stretch frequency shift and equilibrium distance in the presence o f a ( + 1 ) point charge located colineafly with the molecule at an image charge distance gives values ( + 3 0 cm - I and 1.28 A respectively) which may account for the magnitude o f the values obtained for the highly tilted bridge situation, but not for those in the on-top and three-fold sites. We therefore conclude that the intrinsic chemical characteristics o f the N O - C u b o n d are little dependent on the crystal face or adsorption position, yielding N O - species with similar electronic properties. The analysis o f the "corresponding orbitals" (which are the linear c o m b i n a t i o n s o f the occupied molecular orbitals o f the N O - C u 5 system that are as close
780
M. b\erndndez-Gareia et al. / N O adsorbed on a Cu surface
as possible to those o f the isolated N O or Cu5 fragm e n t s [19] ) in fact shows that the orbitals o f the a d s o r b e d N O do n o t change practically (respect to free N O - ) u p o n i n t e r a c t i o n with a n y o f the three positions o f the C u ( 111 ) surface here studied. Some observables, as v i b r a t i o n a l frequencies, m a y however display a wide range of values that could be err o n e o u s l y i n t e r p r e t e d on the basis o f intrinsically " d i s t i n c t " N O a d s o r b e d molecules. A n y differences in reactivity o f the different C u - a d s o r b e d N O species is therefore expected to be m a i n l y a f u n c t i o n o f geometric effects, i.e. the angle o f tilting. This will be p r o b a b l y a factor o f i m p o r t a n c e in electrophilic attacks a n d m a y also influence the ability o f the surface to dissociate NO; this latter reaction, which implies the e s t a b l i s h m e n t o f a direct o x y g e n - m e t a l bond, will be probably easier for the most highly tilted configuration, i.e. in the bridge site. Apart form that, the ionic b o n d is n o t particularly directional a n d there is p r o b a b l y little energetic cost in m o v i n g the N O molecule across the surface to a site f a v o u r a b l e for reaction (this c o n f i r m e d by the small differences for the i n t e r a c t i o n energies reported in table 1 ), resulting in a high reactivity for this molecule. O f course, the N O - character of these species i m p l i e s that repulsive a d s o r b a t e - a d s o r b a t e interactions could be i m p o r t a n t at m o d e r a t e N O coverages, which m a y limit the validity o f these c o n c l u s i o n s to the low-coverage range.
Acknowledgements F i n a n c i a l s u p p o r t by C I C Y T projects PB89-0648CO2-01 a n d M A T 9 1 - 1 0 8 0 - C O 3 - 0 2 , a n d by EC Science project SCI 0 0 2 2 / C / T T is gratefully acknowledged. O n e o f us ( M . F . - G . ) t h a n k s the M i n i s t e r i o de E d u c a c i 6 n y C i e n c i a o f S p a i n for a P h D fellowship.
We also express our t h a n k s to Dr. P.S. Bagus for s t i m u l a t i n g discussions.
References [ 1 ] T.J. Truex, R.A. Searles and D.C. Sun, Platinum Met. Rev. 36 (1992) 2. [2 ] J.F. Weldelken, Appl. Surface Sci. 11/ 12 ( 1983 ) 172. [3] S.K. So, R. Franchy and W. Ho, J. Chem. Phys. 95 ( 1991 ) 1385. [4] P.S. Bagus, C.J. Nelin and Ph. Avouris, J. Vac. Sci. Technol. A5 (1987) 701. [5 ] M. Fern~,ndez-Garcia,J.C. Conesa and F. lllas, Surface Sci., to be published. [6] Ph. Durand and J.C. Barthelat, Theoret. Chim. Acta 38 (1975) 283. [ 7 ] M. Pelissier and Ph. Durand, Theoret. Chim. Acta 55 ( 1980 ) 43. [ 8 ] R. Carb6 and J.M. Riera, in: A General SCF Theory, Lecture notes in Chemistry (Springer, Berlin, 1975 ). [9] M. Dupuis, J. Rys and H.F. King, HONDO-76, QCPE Program 338, Universityof Indiana, Bloomington, IN47401. [ 10] M. Pelissier and J.P. Daudey, Pseudopotential Adaptation of Program used in [9], (Universit6 Paul Sabatier, Toulouse, France, 1986). [11 ] R. Caballol and J.P. Daudey, General Open-Shell Adaptation of Program used in refs. [9,10], (Universit6 Paul Sabatier, Toulouse, France, 1987 ). [ 12] P.S. Bagus, C.J. Nelin and C.W. Bauschlicher Jr, J. Vac. Sci. Technol. A2 (1984) 905. [13] K. Hermann and P.S. Bagus, Appl. Surface Sci. 33/34 (1988) 252. [ 14] E.B. Wilson Jr., J.C. Decius and P.C. Cross, in: Molecular Vibrations. The Theory of Infrared and Raman Vibrational Spectra (Dover, New York, 1980). [ 15 ] 1. Panas, J. Schule, P.E.M. Siegbahn and U. Walgren, Chem. Phys. Lett. 149 (1988) 265. [ 16 ] J.G. Chen, W. Erley and H. Ibach, Surface Sci. 224 ( 1989 ) 215. [ 17 ] P.S. Bagus and G. Pacchioni, in: Cluster Models for Surface and Bulk Phenomena, Eds. G. Pacchioni, P.S. Bagus and F. Parmigioni (Plenum Press, New York, 1992). [ 18] P.S. Bagus, private communication. [ 19] A.T. Amos and C.G. Hall, Proc. Roy. Soc. London A263 (1961) 483.
Solid State Ionics 63-65 (1993) 777-780 North-Holland
IOHICS Quantum chemical study of the bond activation for NO adsorbed on a Cu surface M. F e r n g m d e z - G a r c i a , J.C. C o n e s a lnstituto de Catdlisis y Petroleoquimica (C.S.L C.). Campus Universidad AutOnoma, Cantoblanco. 28049 Madrid, Spain
and F. Illas Departament de Quirnica Fisica, Facultat de Qulmica, Universitat de Barcelona, C/Martl i Franquks 1, 28028 Barcelona, Spain
Pseudopotential SCF molecular orbital calculations are performed on a Cus-NO cluster modelling the adsorption of NO on Cu ( 111) surfaces. In all three adsorption sites studied, tilted geometries and strongly ionic adsorption states (i.e. describable as
NO--Cu~- ) are found. The vibration frequencies predicted are compared with the experimental data, and their rather different values interpreted as due to geometric factors ("surface wall effect") since the extent of charge transfer from Cu to NO is similar in all cases.
1. Introduction The adsorption of NO on metals is of large importance in decontamination processes, especially for automobile exhaust purification [1 ]. The understanding of this system is less advanced than for other adsorbates: the difficulties relate not only to the wider reactivity of NO, but also to the structural assignation of the experimentally observed NO species [2,31. Although the main catalysts for this process are based on precious metals, activity has been reported also for other less noble elements such as Cu [ 1 ]. Theoretical studies on the N O / C u system have been available only in the last few years [ 4,5 ]. All of them show that the N O - C u bond is essentially ionic, with charge transfer from the metal to the 2n* N O orbitals, yielding an adsorbed N O - . The ionic nature o f the bond has several consequences: first, a non directional bond favours a tilted situation for the adsorbed species [ 5 ], and second, the electronic properties of the bond are little dependent on the Cu
crystal face or position of adsorption [4,5]. While the N O - C u interaction seems to be identical in all of the cases theoretically studied, the experimentally observed frequencies of the N - O stretching [2,3] and the N - O distances obtained from the calculations [4,5] change from one adsorption position to another, suggesting some kind o f difference in the interaction. Then, with these considerations in mind, it seems appropriate to start this study by checking whether the different adsorption positions of the ( 11 1 ) face (which, as we will show, have very different NO stretching frequencies) produce distinct NO adsorbed molecules, and by relating the data with the reactivity of these NO species. In the present work the ground states arising from the interaction of NO with different adsorption positions of the Cu ( 11 1 ) surface have been studied at the SCF level of theory. The frequencies of the most significant vibration modes for these adsorbed states have also been obtained, trying to assign the experimentally observed species and to relate the bond properties to the reactivity of these species.
Author to whom correspondence should be addressed. 0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
778
M. Ferndmdez-Garcia et aL / NO adsorbed on a Cu surface
2. Computational details
3. Results and discussion
The Cu ( 111 ) surface has been modeled by a small cluster including four atoms of the surface layer and one of the second layer, using geometry (fcc) and distances (dcu_cu=2.54 A) as those in the bulk material. This cluster was chosen because it allows the study of bent three-fold, bridge and on-top NO (Ndown) positions, keeping the Cs symmetry throughout the work (see fig. 1 ). For this Cu5(4,1 ) cluster model the inner (18-electron) cores have been replaced by a non-empirical pseudopotential [ 6,7 ], and the basis set is of ( 3 s 3 p 5 d / 2 s l p l d ) quality. For NO a (4s4p/2s2p) basis set is used. The wavefunctions are obtained at SCF level of theory. Open-shell restricted Hartree-Fock calculations have been done using the coupling-operator formalism of Carb6 et al. [8] implemented in the H O N D O computer program [9-11 ]. Although correlation effects have not been included, limiting the accuracy of the numeric results, it is well known [ 12,13 ] that SCF or MCSCF wavefunctions give adequate qualitative information on the bond nature and on the different contributions to the bonding energy or to the vibrational frequencies, allowing a proper discussion of the results in terms of bonding nature, frequency shifts respect to free NO or N O and other qualitative information extracted from the wavefunctions. Due to the large differences in the numerical value of the frequencies of the several vibrational modes, they have been obtained by an approximate separation of high and low frequencies [ 14 ]. It is worth noting that this method yields very good approximations (especially for vibration such as the N - O stretch) for adsorbate-surface systems.
For the Cu5(4,1) cluster the three lowest electronic states obtained were respectively of 2A' (ground state), 4A" (4 Kcal/mol above the ground state) and 2A' (6 Kcal/mol) symmetry. From the results obtained with the NO-Cu5 system, the equilibrium distance above the surface plane, N - O distance, Cu-NO angle and binding energy in the ground state (found to be ofSA ' symmetry in all cases) for the NO in interaction with the three-fold, bridge and on-top positions of the Cu( 111 ) surface are reported in table 1. As previously mentioned, these states have identical bonding properties; the analysis of the charge received by NO and of the dipole moment curves [5] points out that the bond is ionic, with negligible involvement of the C u d orbitals in direct bonding or back-bonding effects, and it could be shown that these states correlate, at large separation, with Cu~- (in the 3A" state generated from the 4 A " Cu5 state) and N O - (3Z- state). Note that, because t h e 4 A " state is not the ground state of Cus, the interaction could be interpreted by adopting the idea of bond preparation given by Panas et al. [ 15 ]. Anyway, the point to be stressed here is that the NO molecule adsorbs as N O - (3E-) independently of the adsorption position; comparison with other results published in the literature [4] suggests that this behaviour is characteristic of the N O / C u interaction irrespectively of the surface plane involved. With the aim of establishing a connection to the experimental results recently reported in ref. [ 3 ], we show in table 2 the vibration frequencies ( C u - N O and N - O stretchings and C u - N O bending) calculated for the optimum configurations of adsorption described in table 1, together with the frequencies that So et al. [3 ] obtained for the N O / C u ( 111 ) sysTable 1 Results of optimal geometries and interaction energies obtained for different adsorption sites. The C u - N O angle 0 is defined in fig. 1.
4~
~5
Fig. 1. Geometric arrangement used in the calculations.
Adsorption position
r± (A)
rNo (A)
0cu-No (°)
BE (Kcal/mol)
three-fold bridge on-top
1.75 1.873 1.90
1.24 1.28 1.24
58 11 50
-9.8 - 13.3 - 12.4
M. Fern(mdez-Garcia et al. / NO adsorbed on a Cu surface
779
Table 2 Results on vibrational frequencies (in cm-~ ) obtained from ab-initio calculations on the NO-Cu5 system. Adsorption site
/JINX)
VCu_NO
V & (Cu_NO)
three-fold
1680 ( + 265 ) a) [1535] b),c) (+200) ") 1420 (+5) a) [1040] b) (--300) ~) 1655 (+240) *~
450 [350] b)x) _d)
150 _d)
660
110
bridge on-top
a) Values in parenthesis indicate the shifts from the appropriate value for free (gas phase) NO-: either theoretically computed ( 1415 cm- 1, this work ) or experimentally measured ( 1340 cm- t ). b) Values in brackets are from experiment (ref. [3] ); the assignation of them implied in this table is proposed by comparison with the theoretical shift values, and differs in some cases from that in ref. [ 3 ]. c) Main v(NO) vibrations observed in ref. [3] at low NO coverage. d) Due to the strong coupling of these modes of vibration in the bridge geometry, the separation of high and low frequencies does not yield reliable results for them. tern. On the basis o f these data, it is p r o p o s e d that at low coverages N O would adsorb mainly as threefold coordinated species, whereas at coverages greater than 0.05 L a n g m u i r three N O species could be present on the surface: the three-fold c o o r d i n a t e d species, the bridged one and another one (showing a frequency o f 1825 c m - 1), not assigned by us, which probably corresponds to a molecular N O instead o f a N O - species; the lack o f charge transfer in this case could be explained if these species were located near an electronegative atom, e.g. oxygen [ 16 ] which can be p r o d u c e d upon N O dissociation [3 ]. It is particularly worth noting that even though the C u - N O b o n d i n g is nearly equally ionic in all the adsorption sites considered in our calculations, the N O stretch frequencies o f the species occurring on the surface m a y present rather different numerical values. This can be u n d e r s t o o d considering the results reported by Bagus et al. [ 17,18 ], which show that there are three m a i n contributions to the vibrational frequency and to the N - O b o n d distance: the extent o f charge transfer, which as discussed above is practically a constant in the N O / C u system (obviously, if this contribution were the only one, the N - O frequency and distance would correspond to that in free N O - ) ; the "surface wall effect", which energetically is repulsive and occurs when the N O charge density penetrates the rigid surface during the stretching motion; a n d the electric field i n d u c e d by the positively charged metal. Note that, as pointed out in refs. [ 17 ] and [ 18 ], both the surface wall effect and the electric field work in the same direction (increasing the
frequency and decreasing the N - O distance, i.e. in opposite direction as charge transfer) and d e p e n d on the orientation o f the a d s o r b e d molecule. Therefore, due to these reasonings and to the nearly equal charge received by N O in the three adsorption positions studied, a qualitative justification o f frequency shifts (respect to N O - ) and N - O distances reported in table 1 and 2 should be based on the degree o f tilting. G i v e n that the 5A' ground state, and therefore the Cu5 and N O charge density distributions, are nearly identical in all the positions studied, the surface wall effect should be similar in the only moderately tilted three-fold and on top positions, whereas for the highly inclined bridge position this effect will hinder the vibration m o v e m e n t in a much lower, probably negligible, extent. On the other hand, the electric field effect must be relatively small; in fact, a calculation o f the N - O stretch frequency shift and equilibrium distance in the presence o f a ( + 1 ) point charge located colineafly with the molecule at an image charge distance gives values ( + 3 0 cm - I and 1.28 A respectively) which may account for the magnitude o f the values obtained for the highly tilted bridge situation, but not for those in the on-top and three-fold sites. We therefore conclude that the intrinsic chemical characteristics o f the N O - C u b o n d are little dependent on the crystal face or adsorption position, yielding N O - species with similar electronic properties. The analysis o f the "corresponding orbitals" (which are the linear c o m b i n a t i o n s o f the occupied molecular orbitals o f the N O - C u 5 system that are as close
780
M. b\erndndez-Gareia et al. / N O adsorbed on a Cu surface
as possible to those o f the isolated N O or Cu5 fragm e n t s [19] ) in fact shows that the orbitals o f the a d s o r b e d N O do n o t change practically (respect to free N O - ) u p o n i n t e r a c t i o n with a n y o f the three positions o f the C u ( 111 ) surface here studied. Some observables, as v i b r a t i o n a l frequencies, m a y however display a wide range of values that could be err o n e o u s l y i n t e r p r e t e d on the basis o f intrinsically " d i s t i n c t " N O a d s o r b e d molecules. A n y differences in reactivity o f the different C u - a d s o r b e d N O species is therefore expected to be m a i n l y a f u n c t i o n o f geometric effects, i.e. the angle o f tilting. This will be p r o b a b l y a factor o f i m p o r t a n c e in electrophilic attacks a n d m a y also influence the ability o f the surface to dissociate NO; this latter reaction, which implies the e s t a b l i s h m e n t o f a direct o x y g e n - m e t a l bond, will be probably easier for the most highly tilted configuration, i.e. in the bridge site. Apart form that, the ionic b o n d is n o t particularly directional a n d there is p r o b a b l y little energetic cost in m o v i n g the N O molecule across the surface to a site f a v o u r a b l e for reaction (this c o n f i r m e d by the small differences for the i n t e r a c t i o n energies reported in table 1 ), resulting in a high reactivity for this molecule. O f course, the N O - character of these species i m p l i e s that repulsive a d s o r b a t e - a d s o r b a t e interactions could be i m p o r t a n t at m o d e r a t e N O coverages, which m a y limit the validity o f these c o n c l u s i o n s to the low-coverage range.
Acknowledgements F i n a n c i a l s u p p o r t by C I C Y T projects PB89-0648CO2-01 a n d M A T 9 1 - 1 0 8 0 - C O 3 - 0 2 , a n d by EC Science project SCI 0 0 2 2 / C / T T is gratefully acknowledged. O n e o f us ( M . F . - G . ) t h a n k s the M i n i s t e r i o de E d u c a c i 6 n y C i e n c i a o f S p a i n for a P h D fellowship.
We also express our t h a n k s to Dr. P.S. Bagus for s t i m u l a t i n g discussions.
References [ 1 ] T.J. Truex, R.A. Searles and D.C. Sun, Platinum Met. Rev. 36 (1992) 2. [2 ] J.F. Weldelken, Appl. Surface Sci. 11/ 12 ( 1983 ) 172. [3] S.K. So, R. Franchy and W. Ho, J. Chem. Phys. 95 ( 1991 ) 1385. [4] P.S. Bagus, C.J. Nelin and Ph. Avouris, J. Vac. Sci. Technol. A5 (1987) 701. [5 ] M. Fern~,ndez-Garcia,J.C. Conesa and F. lllas, Surface Sci., to be published. [6] Ph. Durand and J.C. Barthelat, Theoret. Chim. Acta 38 (1975) 283. [ 7 ] M. Pelissier and Ph. Durand, Theoret. Chim. Acta 55 ( 1980 ) 43. [ 8 ] R. Carb6 and J.M. Riera, in: A General SCF Theory, Lecture notes in Chemistry (Springer, Berlin, 1975 ). [9] M. Dupuis, J. Rys and H.F. King, HONDO-76, QCPE Program 338, Universityof Indiana, Bloomington, IN47401. [ 10] M. Pelissier and J.P. Daudey, Pseudopotential Adaptation of Program used in [9], (Universit6 Paul Sabatier, Toulouse, France, 1986). [11 ] R. Caballol and J.P. Daudey, General Open-Shell Adaptation of Program used in refs. [9,10], (Universit6 Paul Sabatier, Toulouse, France, 1987 ). [ 12] P.S. Bagus, C.J. Nelin and C.W. Bauschlicher Jr, J. Vac. Sci. Technol. A2 (1984) 905. [13] K. Hermann and P.S. Bagus, Appl. Surface Sci. 33/34 (1988) 252. [ 14] E.B. Wilson Jr., J.C. Decius and P.C. Cross, in: Molecular Vibrations. The Theory of Infrared and Raman Vibrational Spectra (Dover, New York, 1980). [ 15 ] 1. Panas, J. Schule, P.E.M. Siegbahn and U. Walgren, Chem. Phys. Lett. 149 (1988) 265. [ 16 ] J.G. Chen, W. Erley and H. Ibach, Surface Sci. 224 ( 1989 ) 215. [ 17 ] P.S. Bagus and G. Pacchioni, in: Cluster Models for Surface and Bulk Phenomena, Eds. G. Pacchioni, P.S. Bagus and F. Parmigioni (Plenum Press, New York, 1992). [ 18] P.S. Bagus, private communication. [ 19] A.T. Amos and C.G. Hall, Proc. Roy. Soc. London A263 (1961) 483.