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Chemisorption of CO on defect sites of MgO
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Surface Science 275 (1992) 450-458 North-Holland
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Chemisorption of CO on defect sites of MgO Gianfranco
Paccbioni,
Tommaso
Minerva
Dipartimento di Chimica Inorganica e Metaliorgunicu, Urtkersit~ di Milano, Centro CNR, Via Venezicm 21. 201.73 M&no, It&
and Paul S. Bagus
Received 30 March 1992; accepted for publication 22 May 1992
Cb~misorption of a CO molecule on regular and defect sites of the ~g~l(~) surface has been ~nv~stig~t~(jby means of cluster model calculations. At all sites studied, CO bonds at the cation with the C atom closest to the surface, The bonding is considerably larger at a three-coordinated corner site than for a regular five-coordinated surface site. A hlueshift in the C-O stretching frequency, oc, of adsorbed CO compared to free CO is found; the shift is much higher for a corner than for a surface site because of the larger local electric Field for low-coordinated cations. Both the bond strength and the w shift ate largely due to electrostatic effects and not to the formation of a dative g-bond with the surface. Surface relaxation effects have also been considered.
It has been suggested that surface irregularities such as corners, edges, and kinks play an important role in the reactivity of oxide surfaces toward adsorbed species [1,21. Both theoretical [3-91 and experimenta 110-121 results indicate that the (100) surface of a simple oxide like MgO or NiO is rather unreactive when there are no defects present. The bonding of an adsorbed CO molecule is weak and dominated by electrostatic interactions [3-91, although some controversy exists in the literature about the extent of the chemical interaction and in particular of the CO r-donation to the surface [7,8,10,111; in some cases r-back donation from the surface to CO has also been claimed 1131. On the other hand, CO reacts rapidly with a rough MgO surface to produce CO:- and more complex surface species even at temperatures as low as 77 K [14,15]. It has been suggested that this occurs by nucle-
ophilic attack of an exposed surface 02- ion on a CO molecule bound to a Mg” ’ cation [ 151. One of the most commonIy employed techniques to characterize the chemisorption properties of an oxide surface is based on the infrared, IR, measurement of the vibratio~a1 shifts of a probe moIecule, Like CO [l&20]. The frequency shift, Aw, and the intensity change of the adsorbed molecule with respect to free CO have been used to extract information about the nature and number of cationic sites. including defect sites, on the surface [16]. Attempts have also been made to correlate the CO o shift with the local electric field at the surface cation 1171. Recently [7-91, we have theorcticahy analyzed the bonding and the w shift for the case of a single CO molecule adsorbed on regular MgO or NiO(100) surfaces. We found that, for an ideal MgO or Ni~(lOO) surface without defects, the positive u shift is largely determined by the fact that the CO mole&e stretches in the presence of
~~39-~~2~/9~/‘$~~S.~ 8 1992 - Elsevier Science Publishers B.V. All rights reserved
G. Pacchioni et al. / Chemisorption of CO on defect sites of MgO
the rigid surface, a wall effect, and that the electric field contribution to the shift is modest. We have also shown that a transition metal oxide like NiO behaves very similarly to MgO because the bonding is entirely electrostatic with little, if any, a-donation or r-back donation [7-91. In this work we extend our study to cluster models of surface irregularities, in particular edges and corners. We will consider three main points in our analysis. (1) The nature and the extent of surface relaxation for chemisorption sites where there are five-, four-, or three-coordinated Mg*+ surface cations, hereafter denoted as Mg::, Mgzz, and Mg;: 9 respectively. These differently coordinated Mg*+ cations model (i) a regular site of a flat surface, site MgzfMgJ:! cOrner an edge site, M$, or (iii> a (2) The’optimal geometry for a CO molecule interacting with these sites, i.e., perpendicular, bent, or flat. (3) The relative importance of electrostatic versus chemical bonding contributions in determining the bond strength and the CO w shift at the three sites. To this end, we have performed ab initio calculations on three cluster models, see fig. 1, simulating the various coordinative situations, and we have analyzed the corresponding wavefunctions.
2. Computational
details
Hartree-Fock SCF wavefunctions for the [MgO,, I”-/CO clusters were determined within the MO-LCAO approximation. The clusters used are [Mg0,]8-, [Mg0,16-, and [Mg0,14-, for five-, four-, and three-coordinated sites, respectively, see fig. I. The basis sets used are of triple-zeta quality [81. The rest of the ionic crystal has been simulated by a large set of +2 or -2 point charges, PCs, placed at the lattice positions. This array of point charges is large enough to reproduce the Madelung potential for a surface Mg2+ cation within reasonable accuracy [S]. As an alternative, one can determine the charges in a least square manner to fit the correct surface Madelung potential. This can be calculated by the Ewald
451
Fig. 1. Schematic representation of CO at surface, edge, and corner sites of MgO(100); the different equilibrium CO orientations, perpendicular at the surface site and bent at the edge and corner sites, are indicated. The atoms explicitly included in the cluster for the different sites are shown with hash marks.
method [21,22] or by other techniques [231. However, the analysis of the properties of interest in this study is rather insensitive to the detailed form of the potential and is practically independent of the point charge grid used [S]. Perfect ionicity is assumed for the MgO crystal [5,24-261. Surface relaxation effects of the Mg*+ cation and the equilibrium geometry of the adsorbed CO were determined by performing a geometry optimization for the SCF wavefunction by means of an analytical gradient technique using the HOND08 program [27]. The importance of correlation effects in determining the CO vibrational frequency and the bond strength has been considered by performing single-double configuration interaction, SD-CL calculations. For the determination of the CO binding energy we performed, for the SCF optimal geometry, a CI where the six 2p electrons on each O*ion plus the six lrr and 50 valence electrons of CO are correlated. Except for a small number of high-lying orbitals, excitations were made to all the SCF virtual orbitals. The CI wavefunction for the different clusters contains up to 56 000 configurations. The CO w, has been computed by polynomial fitting of five points around the minimum with
452
G. Pacchioni et al. / Chemisorption of CO OFIdefect sites of MgU
the distance between the Mg*+ cation and the CO center of mass fixed at a value close to 2,. Several effects can affect the absolute value of w, as well as Aw, e.g., the size of the substrate cluster, the basis set used, and the inclusion of correlation effects. These latter have been introduced by performing a SD-C1 calculation where only the CO 5u and lrr electrons are correlated. With this CI, the computed w, for free CO is 2067 cm-‘, while the SCF value is 2270 cm-’ (the experimental gas-phase value is 2143 cm-’ [ZS]). The computed vibrational frequencies and frequency shifts must be considered as qualitative. However, the trend in Aw shown by correlated and non-correlated wavefunctions is the same and allows one to identify the origin and the mechanisms contributing to different shifts for different surface sites.
relax vertically, but we kept the surrounding oxygen ions fixed. This results in a modest shrinking, - 0.04 bohr, of the distance of the Mg:: cation from the 02- anion below it, see table 1. This corresponds to a - 1% surface rumpling, in good agreement with experimental data 129-321 and periodic Hartree-Fock calculations [24]. The same vertical displacement is much more pronounced in the [MgO,l’- cluster model of an edge site. In this case, in fact, the Mgi: ion moves toward the 02- ion in the second layer by 0.20 bohr, a - 7% reduction of the first-second layer distance. This effect becomes very important for a Mgi,t cation at a corner site. With our cluster model, [Mg0,14-, we found that the Mg;,f cation moves down toward the O*- ions in the second layer by more than 0.5 bohr, with a contraction of N 20% of the unrelaxed distance, see table 1. A similar contraction has been predicted by means of non-empirical two-body potential calculations [33] but we are not aware of any other electronic structure study which includes surface displacements for this site. Moreover, there are no experimental estimates. The reason for this large relaxation is that at defect sites the Madelung potential is smaller than at a regular surface site 134). An inwards displacement of the
3. Surface relaxation It is well-known that the clean MgO(100) surface is almost unreconstructed and unrelaxed [24,29-321. For the [MgO,lsP cluster model of the regular surface, we allowed the Mgg,+ ion to
Table 1 SCF results equilibrium
for CO adsorbed C-end down on cluster models of surface, edge, and corner chemisorption sites on the MgO surface; distances, re in bohr, dissociation energies, D,, in eV; the basis set on CO does not include d functions Unrelaxed r(Mg
2+-02--j
Relaxed
substrate h)
r,(Mg-C)
MgO, Surface MgO,-CO Surface
3.976
MgO, Edge MgO,-CO Edge
2.811 2.811
4.618
MgO, Corner MgO,-CO Corner
2.296
_
2.296
4.468
3.976
4.915
,,(C-Of
D,
-
_
2.119
0.25 _
2.114
2.104
0.35
0.63
substrate
r,(MgZ+-O?-) 3.936 ( - 0.040) 3.975 (-0.001) 2.611 ( - 0.200) 2.728 ( - 0.083) 1.770 ( - 0.526) 1.797 ( - 0.499)
” h)
r,tMg-L)
r&C-O)
D,
_
_
4.915
2.119
0.25
_
_
_
4.619
2.112
0.32
_
_
_
4.472
2.fOfJ
0.58
‘) The position of the Mg’+ cation has been optimized with respect to the O-‘- ions and the point charges which are fixed at the regular crystal lattice positions. “) Height of the Mgzc cation from the second layer of the cluster: in parentheses is given the contraction with respect to the unrelaxed case.
G. Pacchioni et al. / Chemisorption
Mg2+ ion increases the electrostatic with its neighbors.
interaction
4. Optimal geometry of adsorbed CO The CO molecule is believed to bind perpendicularly to the surface with the C end down on ionic surfaces [l- 111. This is supported by theoretical calculations [3-91. However, the energy difference between C-down or O-down CO is small [8]. In fact, as we have shown in a previous paper IS], the SCF wavefunctions for a cluster model of CO/MgO(lOO) at a regular surface site lead to O-down CO being more strongly bound; for this calculation, d-polarization functions were not used for CO. The simultaneous inclusion of d-polarization functions for CO and of electron correlation effects with CI wavefunctions show that the C-end down CO is more stable ISI. We have found, in this work, that the behaviour at edge and corner sites is similar to that at the surface site discussed above. For SCF wavefunctions, O-end down CO is more strongly bound but once correlation effects and d functions on CO are included, the C-end down bonding is also preferred for these Mg$: and Mgs’ sites. The change in CO orientation as function of the level of theoretical treatment is entirely due to the different description of the CO multipole moments and not to a change in the surface-adsorbate charge transfer mechanisms [8]. For these reasons, we do not consider further O-end down orientations of CO/MgO. A simultaneous optimization of the positions of the Mg2+, C, and 0 atoms has been performed at the SCF level. Geometries where CO is bound C-end down correspond to local minima on the potential energy hypersurface. For the regular surface site, Mgz:, we confirm our earlier results IS] that the C-O internuclear axis is normal to the surface. For the edge and corner sites, CO is bent away from the surface normal, see fig. 1. The bend is such that the C-O axis is normal to the plane which would define an MgO(ll1) surface. At these sites, the CO bends away from the 02- anions on the terraces of the regular (100)
of CO on defect sites of MgO
453
surface thus reducing the Pauli repulsion, see below. For a corner, coordinatively unsaturated, Mg$z site, we explicitly considered several geometries were the CO molecule lies flat or almost flat on the surface (d-polarization functions on CO and correlation effects were included). However, these orientations do not correspond to local minima on the potential energy surface. The Mg’+-CO distance becomes shorter as one goes from a regular surface site, N 4.9 bohr, to an edge site, - 4.6 bohr, to a corner site, N 4.5 bohr. At the same time the C-O bond length decreases and the binding energy increases, table 1. The trend is the same when we use Yrnrelaxed” clusters, where the Mg2+ ion occupies the ideal position of a truncated cubic crystal, or clusters where the Mg2+ is free to change its position, see table 1.
5. Analysis of bonding Our calculations indicate that CO binds with different chemisorption energies on the various sites considered. In particular, the adsorption energy is considerably higher for an exposed cation at a corner site, modeled by [Mg0,14-, than for a regular surface site, modeled by [MgO,lK-. The estimated SCF bond strength for a regular surface site is De = 0.25 eV; this value is somewhat exagerated by the occurrence of basis set superposition errors, BSSE [35’]. The BSSE leads to calculated values of De which are too large; using the Boys and Bernardi counterpoise method [35], we estimate that the uncertainity in De for [MgO,lH- due to the BSSE is - 0.08 eV; this correction is probably an upper limit. Correlation effects increase the SCF D, = 0.25 eV by 0.05 to 0.3 eV, see table 2. Several experimental [lO,ll] and theoretical [3-91 estimates of De have been reported. These all range between 0.15 and 0.40 eV, suggesting that CO is rather weakly bound, or physisorbed, at MgO(lOO) surfaces. Experimenta1 measures of the chemisorption energy of CO at defect sites or on other MgO surfaces beside MgO(100) or thin films have not been reported so far.
454
G. Pacchioni et al. / Chemisorption of CO on defect sites of M,O
Table 2 Contributions, from SCF wavefunctions, from various bonding mechanisms to the interaction energy, Eint, in eV, for CO chemisorbed on cluster models of surface, edge and corner sites of the MgO surface; the analysis refers to r(C-0) = 2.15 bohr; the total binding energy, 0, in eV, is given for SCF and CI wavefunctions Cluster Site z(Mg-Cl
(bohr)
Electric field at Mg2+ (a.u.1 1. Electrostatic interaction 2. Pauli repulsion 3. MgO polarization 4. MgO + CO charge transfer 5. CO polarization 6. CO + MgO charge transfer Total Total
D, (SCF) D, (SD-CD
MOO,-CO Surface 4.90
MgO,-CO Edge 4.60
MgO,-CO Corner 4.50
0.019
0.033
0.062
+ 0.24 - 0.21 +0.02
+ 0.42 - 0.34 +0.01
+ 0.48 - 0.28 0.00
+ 0.06 + 0.06
+ 0.04 +0.12
+ 0.04 +0.27
+ 0.06
+ 0.07
+ 0.07
+ 0.23 + 0.32
+ 0.33 + 0.49
+ 0.59 + 0.83
The importance of electrostatic and chemical bonding contributions to the strength of the CO bond on the adsorption sites considered has been analyzed by determining the cluster wavefunction with contraints, according to the constrained space orbital variation, CSOV [36,37]. The results are for unrelaxed substrate geometries and for a CO geometry close to equilibrium. At each step of the CSOV, the interaction energy is defined as E,,,(CSOV step n) = E(CO1 + E(MgO1 E(MgO/CO; CSOV step n>. The change in Eint between CSOV step II and the preceding step n - 1 represents the energetic importance of the new variational freedom allowed at step n [36,371. The Eint for the CSOV variations for [MgO,ls-/ CO, [MgOJ-/CO, and [Mg0,14-/CO, are given in table 2. The first step of the CSOV consists in simulating the ionic surface by an array of PCs. The Mg2+ cation and the 02- anions that are normally included explicitly in the MgO cluster models are represented by PC = + 2 and PC = - 2, respectively. At this first step, the charge density of the CO molecule is fixed as it is for the SCF wavefunction of the isolated molecule for the
appropriate C-O bond distance. The value of E,,, at this step provides a measure of the importance of the pure electrostatic interaction. At step 1, Eint is considerably larger for corner and edge sites than for a surface site, table 2, because in the former cases the CO is adsorbed at a low-coordinated cation where the local electric field is stronger. While the absolute value of the electrostatic potential in the adsorption region is important for the CO binding energy, the shape and curvature of this potential are the features that are important for changing the C-O vibrational frequency. This is discussed further in section 6. In the second step of the CSOV we replace the few PCs defining the local chemisorption site with real Mg2+ and 02ions, but we do not allow the two charge densities, cluster and CO, to relax. The charge density for [MgO,] is fixed as that for the SCF wavefunction for the isolated cluster and the CO charge density is kept at that for the isolated molecule. However, at this step 2, the wavefunction formed from the superposition of the frozen charge densities of [MgO,] and CO, is normalized [36-381. This step measures the extent of Pauli repulsion originating from the interpenetration of electronic charge at equilibrium positions, see table 2. This energy contribution, which is purely repulsive, depends on the surface-CO distance but also on the number of 02- ions on the surface. In [MgO,l”-/CO there are four spatially extended 02anions which prevent the CO molecule from approaching too close to the surface; in [MgO,l”-/CO there are only two 0 2- ions on the first layer of the cluster, while in a corner site the 02ions are in the second layer, see fig. 1. The Pauli repulsion is smallest for the surface and largest for the edge, table 2, because it results from the balance of the two effects, i.e., the distance from the surface and the number of 02- ions which are nearest neighbors of the CO molecule. The substrate polarization, step 3, is measured by allowing the MgO cluster orbitals to vary. At this step the CO charge density remains frozen. This polarization, however, is negligible for all three sites. Also the MgO to CO charge transfer, measured at CSOV step 4, is very small, table 2,
455
G. Pacchioni et al. / Chemisorption of CO on defect sites of MgO
and in large part due to the BSSE [8,9]. In CSOV step 5, the CO molecule is free to polarize but the [MgO,] charge density is frozen as determined at the previous CSOV step. The CO polarization can be viewed as arising from two origins. The first origin reduces the Pauli repulsion arising from the interpenetration of the [MgO,] and CO charge distributions [39]. The second origin is the electric field or the electrostatic potential at the bonding site due to the Mg2+ and 02- ions. This potential causes the CO to polarize [7-9,401. The CO polarization is considerably larger for the corner site than for the regular surface site; the edge site is intermediate. The dominant effect determining the size of this polarization is the electric field associated with the different CO adsorption sites. The charge donation from the CO molecule to the surface, see CSOV step 6 in table 2, is small, similar for the three sites, and again exagerated by BSSE. Differently from what is often believed, there is little or no a-donation from CO to the ionic surface, even for a low-coordinated cation [7-91. The different binding energies computed for CO on the three sites arise principally from the
Table 3 Contribution chemisorbed Cluster Site z(Mg-C) 1. 2. 3. 4. 5. 6.
very different local electric field at the surface cation. For a regular, five-coordinated surface site this electric field is small, the initial electrostatic attraction is partially offset by the Pauli repulsion and the weak bonding is due to the electrostatic term plus the CO polarization; for a four- and a three-coordinated site, e.g., an edge or a corner site, the electric field becomes larger hence both the electrostatic attraction and the CO polarization increase. As a consequence, the chemisorption energy of a CO molecule on a corner site is almost three times larger than on a five-coordinated site; this is found for both the SCF and the correlated CI wavefunctions, see table 2.
6. Origin of the vibrational
The CO molecules adsorbed on ionic surfaces exhibit a o shift toward higher frequencies [16201. This provides a clear indication that CO is bound C end down because the reversed orientation gives negative w shifts, i.e., toward lower frequencies [S]. The positive w shift has been measured for CO adsorbed on polycrystalline
from various bonding mechanisms to the vibrational frequency, o,, and to the frequency on cluster models of surface, edge, and corner sites of the MgO surface
(bohr)
Field-dipole interaction Pauli repulsion (wall effect) MgO polarization MgO + CO charge transfer CO polarization CO + MgO charge transfer
SCF a) Change from Mg:: SD-C1 h’ Change from Mg,,z+ Experimental ‘) Change from Mg::
shift
shift, Ao, in cm-‘,
MgOs-CO Surface 4.90
MgO,-CO Edge 4.60
MgO,-CO Corner 4.50
UAW
%/A0
%/AU
2271/+ 2321/c 2321/t 2316/c 2312/c 2302/c
1 =) + 50) =) - 4) - 4) - 10)
2301/ + 31 0 21 lO/ + 43 0 2156/ + 13 0
a) Aw is computed with respect to free CO o,(SCF) = 2270 cm-‘. h, Aw is computed with respect to free CO w,(CI) = 2067 cm-t. ‘) From refs. [15,20]; Aw is computed with respect to free CO w,(exp.)
2293/ 2348/c 2348/c 2343/c 2334/c 2326/c
+ 23 a) + 55) =) - 5) - 9) - 8)
2356/ + 86 =) 2394/c + 38) 2394/c = ) 2391/c-3) 2375/c - 16) 2367/c - 8)
2325/ +24 2122/ +12 2164/ +8
+ 55
2367/ +66 2140/ +30 2203/ +47
+ 55 + 21
= 2143 cm-t.
+ 97 + 73 + 60
for CO
MgO [l&191, for CO on MgO smokes composed by perfect microcrystals [20], and for CO on MgO films grown on the Mo(100) surface [ll]. The measured shift ranges between approximately + 10 [l&20] and + 35 cm-’ [ 1l] in the limit of zero coverage. The shift has been found to depend on the degree of coverage for CO on MgO microcrystals 1201 and rough surfaces [18,19]. This is to be expected because of the lateral dipole-dipole interactions between coadsorbed CO molecules [l&20]. However, for CO/ MgO/ Mo(lOO), coverage dependence for w,(C-0) was not observed [ 111. The measured shift is in acceptable agreement with the computed Aw for [MgO,]sP/CO, 31 cm- ’ (SCF) or 43 cm ’ (CI), see table 3, given the simple model used. There are cluster size effects on AU but they are not large. A larger substrate cluster model for MgO, [Mg,,O,,]/CO, gives a SCF Aw = + 17 cm-‘. Experimentally it has been observed that the main line in the IR spectrum of CO on MgO microcrystals and on MgO powders, at 2156 cm- ‘, is accompanied by a low-intensity feature at 2164 cm ~ ’ [15,20] which has been attributed to CO molecules adsorbed on the edges of the crystallite [X,20]; the low intensity is consistent with the fact that the edge sites do not exceed the 5% of the total number of surface sites on the crystallite, the rest being regular, non-defect, surface sites. When CO is adsorbed polycrystalline MgO, which has an high concentration of defect sites, a third band is observed at 2303 cm- ’ [ 151. This band has been assigned to CO adsorbed on lowcoordinated Mg<: sites; an assignment which is fully supported by the present calculations, set table 3. The SCF w, for an edge Mg:: site, is 24 cm-’ larger than for the regular site, compared to the experimental assignement of the frequency shift being approximately + 10 cm ‘. For a corner, Mg:,’ , site, the SCF w, is 66 cm- ’ larger than for the regular, Mg::, site while the cxperilarger than for the mental w, is - 50 cm-’ regular site. A similar trend is found when o, (C-O) is computed with CI wavefunctions for the cluster. The CI w,, see table 3, for the edge site is 12 cm ’ larger than for the regular site and. for the corner site, it is 30 cm-’ larger; this is to be compared to the experimental changes of ap-
proximately + 10 and +50 crn~- ‘, respectively [ 15,201. The origin of this blueshift is matter of controversy. Several authors [lo,1 1,131 attribute the shift to the formation of a surface a-complex where charge flows from the CO 5~ orbital to the substrate. This interpretation is not supported by theoretical results. Both cluster model [S-9] and band structure [3,4] calculations indicate that the bonding does not arise from chemical interactions but is largely due to electrostatic effects. Even for the case of CO bound to a single metal cation it has been demonstrated that the bond is almost entirely electrostatic [41]. We have recently shown that this is also the origin of the w shift [7-91. In order to identify the origin of the different shifts for the three sites, we have performed a CSOV decomposition similar to that used to dctermine the bonding contributions. In practice, for each CSOV step we have computed the SCF C-O potential energy curve for the same grid of points around r,(CO). See refs. [8,Y,42] for a detailed description of this constrained dccomposition for the shifts in w,(C-0). The results arc given in table 3. The first term, see CSOV step 1 in table 3, is dctcrmincd by stretching the frozen orbital CO molecule in the presence of the PC’s simulating the ideal ionic surface; this term accounts for the electrostatic interaction between the non-uniform electric field and the C-0 dipole. The shift at this CSOV step is negligible. +1 cm-‘, for the regular surface site, modest, +23 cm- ‘, for an edge site, and large. + X6 cm -~I. for a corner site, see table 3. ‘I‘he diffcrences are consistent with the increasing strength of the local clcctric field. In the second CSOV step, or frozen orbital step. the CO molecule is stretched against the rigid surface which is now represented by frozen ions. Intra- or inter-unit polarization of the two fragments is not allowed at this step which thus measures the extent of the “wall effect”. The wall effect [42,43] occurs when the CO charge density penetrates the rigid surface during the stretching motion; the potential energy surface bccomcs more rcpulsivc as the surface-carbon distance becomes shorter. l’his effect always acts to incrcasc the (I)~ for CO. XC
G. Pacchioni et al. / Chemisorption
table 3. The trend of Aw at this step is again the result of a compromise between the surface-CO distance and the number of oxygens in the first layer of the cluster. Neither the MgO polarization nor the MgO to CO charge transfer contribute significantly to the w shift, table 3. The CO polarization lowers the frequency for a corner site more than for a surface site. However, this lowering is small for all three sites. The CO to MgO charge transfer step gives a negative shift which is almost the same for the three models, approximately - 10 cm-‘. For the traditional analysis of the shift of w,(C-0) based on effects due to u-donation and r-back donation, it is argued that the a-donation, augmenting the CO+ character of the adsorbate, leads to an increase in w,, i.e., to a positive contribution to AU [42]. This is because w,(C-0) is larger for CO+ (5~‘) [28]. Although the shift that we find to be associated with the CO udonation is small, it is actually a negative shift! The fact that the CO shift is negative shows that the traditional analysis is incorrect. The quite small a-donation which may occur actually reduces the Pauli repulsion between MgO and CO and, thus, reduces the wall effect and leads to a lowering of U, [42,44]. We note that negative w shifts are associated with the a-donation step of the constrained variation for CO on metal surfaces [42] as well as on oxide surfaces [8,9]. The net effect is a large positive w shift for a corner site and a modest shift for a surface site. The origin of these shifts, however, is not the same. For a regular surface site, the field-dipole interaction is small compared to the wall effect; it is the wall effect which largely contributes to the total positive shift. For a corner site, the field-dipole interaction is very large and dominates over the wall effect. In an edge site both mechanisms are important. Thus, as one goes from a high-coordinated to a low-coordinated site the importance of the wall effect is decreased and that of the local electric field is increased. Both effects contribute to increase the CO w, consistently with the observed blueshift in the IR spectrum. We should note that, although we use the phrase “field-dipole” to describe the effect of the electrostatic potential at the MgO site on
of CO on defect sites of MgO
451
w,(C-01, it is not the absolute value of the CO dipole moment, p, which determines the magnitude of the interaction. It is the shape of p(r), where r is the C-O distance, which is the important factor. In particular, the slope, dk/dr, and the curvature, d2p/dr2, are the parameters which determine how large the o shift due to the “field-dipole” interaction will be [8,9,40,43,44].
7. Conclusions
We have reported theoretical studies of the interaction of a CO molecule with Mg*+ cations at surface, edge, and corner sites of a MgO(100) surface. Surface relaxation effects have been taken into account; they are large for low-coordinated sites, and negligible for non-defect surface sites. However, the surface relaxation is not essential for determining the nature of the bonding of CO to the surface. This, in fact, originates from electrostatic interactions and not from the CO a-donation to the surface. Low-coordinated sites on ionic crystals generate higher local electric fields in the adsorption region. For this reason, CO is chemisorbed on a corner site with a binding energy of about three quarters of an eV, while it is weakly adsorbed, or physisorbed, on a five-coordinated surface site. The different sites also exhibit different vibrational shifts of the C-O stretching frequency with respect to free CO. A large blueshift, of approximately + 100 cm-‘, is computed for a corner site, compared to a moderate shift, approximately + 30 cm- ‘, for a surface site. The origin of these shifts, however, is not the same. The shift is basically due to two mechanisms: the interaction between the non-uniform electric field at the surface and the CO dipole, and the “wall effect”; both mechanisms act to raise 0,. For a coordinatively unsaturated corner site the field-dipole interaction is dominant with respect to the wall effect; for a five-coordinated surface site the field-dipole term is small and the shift is largely due to the wall effect.
458
G. Pacchioni et al. / Chemisorption of CO on defect sites of MgO
This work has been supported in part by a grant from Progetto Finalizzato Chimica Fine II of the Italian CNR. The support of NATO through the Collaborative Research Grant No. 900031 is gratefully acknowledged.
References [l] H.H. Kung, Transition Metal Oxides: Surface Chemistry and Catalysis (Elsevier, Amsterdam, 1989). [2] V.F. Kiselev and O.V. Krilov, Adsorption and Catalysis on Transition Metals and their Oxides (Springer, Heidelberg, 1989). [3] C. Pisani, R. Dovesi, R. Nada and S. Tamiro, Surf. Sci. 216 (1989) 489. [4] R. Dovesi, R. Orlando, F. Ricca and C. Roetti, Surf. Sci. 187 (1987) 267. [S] E.A. Colbourn and W.C. Mackrodt, Surf. Sci. 143 (1984) 391. [6] S.A. Pople, III. Hillier, M.F. Guest, E.A. Colbourn and J. Kendrick, Surf. Sci. 139 (1984) 299. [7] G. Pacchioni, G. Cogliandro and P.S. Bagus. Surf. Sci. 255 (1991) 344. [8] G. Pacchioni, G. Cogliandro and P.S. Bagus, Int. J. Quantum. Chem. 42 (1992) 1115. [9] G. Pacchioni and P.S. Bagus, in: Cluster Models for Surface and Bulk Phenomena, Eds. G. Pacchioni, P.S. Bagus and F. Parmigiani, NATO ASI Series B, Vol. 283 (New York, 1992) p. 305. [lo] C.R. Hemy, C. Chapon and C. Durez, J. Chem. Phys. 95 (1991) 700. 1111J.W. He, CA. Estrada, J.S. Corneille, M.-C. Wu and D.W. Goodman, Surf. Sci. 261 (1992) 164. [12] H. Kuhlenbeck, G. Odiirfer, R. Jaeger, G. Illing, M. Menges, Th. Mull, J.-H. Freund, M. Piilchen, V. Staemmler, S. Witzel, C. Scharfschwcrdt, K. Wennemann, T. Liedtke and M. Neumann, Phys. Rev. B 43 (1991) 1969. [13] E. Escalona-Platero, S. Coluccia and A. Zecchina, Surf. Sci. 171 (1986) 465. [14] M. Bailes and F.S. Stone, Mat. Chem. Phys. 29 (1991) 489. [I51 A. Zecchina, S. Coluccia, G. Spoto, D. Scarano and L. Marchese, J. Chem. Sot. Faraday Trans. 84 (1990) 703. [16] A.A. Davydov, Infrared Spectroscopy of Adsorbed
Species on the Surface of Transition Metal Oxides (Wiley, New York, 1990). [17] M.I. Zaki and H. Kniizinger, J. Catal. 119 (1989) 311. [IS] E. Escalona-Platero, D. Scarano, G. Spoto and A. Z&china, Faraday Disc. Chem. Sot. 80 (1985) 183. [19] S. Coluccia, in: Structure and Reactivity of Surfaces, Eds. C. Morterra, A. Zecchina and G. Costa (Elsevier, Amsterdam, 1989). [201L. Marchese, S. Coluccia, G. Martra, and A. Zecchina, Surf. Sci. 269/270 (1992) 135. [211P.P. Ewald, Ann. Phys. (Leipzig) 64 (1921) 253. L221D.E. Parry, Surf. Sci. 49 (1975) 433; 54 (1976) 195. [231 J.D. Levine and P. Mark, Phys. Rev. 144 (1966) 751. 1241 M. Causa, R. Dovesi, C. Pisani and C. Roetti, Surf. Sci. 175 (1986) 551. ]251 G. Pacchioni, C. Sousa and P.S. Bagus, to be published. 1261 S. Sasaki, K. Fujino, Y. Takeuchi, and R. Sadenaga, Acta Cryst. A 36 (1980) 904. 1271 M. Dupuis, A. Farazdel, S.P. Karna, S.A. Maluendes, in: Modern Techniques in Computational Chemistry, Ed. E. Clementi (Escom, Leiden, 1991). 1281 G. Herzberg, Molecular Spectra and Molecular Structure, Vol. 1 (Van Nostrand, Princeton, 1950). 1291CC. Kinniburgh, J. Phys. C 9 (1976) 2695. 1301 M.R. We&on-Cook and W. Berndt, J. Phys. C 15 (1982) 5691. [311 T. Urano, T. Kanaji and M. Kaburagi, Surf. Sci. 134 (1983) 109. 1321 P.A. Maksym, Surf. Sci. 149 (1985) 157. 1331 E.A. Colbourn, J. Kendrick and W.C. Mackrodt, Surf. Sci. 126 (1983) 550. 1341E. Giamello, P. Ugliengo and E. Garrone, J. Chem. Sot. Faraday Trans. I 85 (1989) 1373. 1351 S. Boys and F. Bernardi, Mol. Phys. 19 (1970) 553. [361 P.S. Bagus, K. Hermann and C.W. Bauschlicher. J. Chem. Phys. 810984) 1966. 1371P.S. Bagus, K. Hermann and C.W. Bauschlicher, J. Chem. Phys. 80 (1984) 4378. [381 P.S. Bagus, K. Hermann, W. Miiller and C.J. Nelin, Phys. Rev. Lett. 57 (1986) 1496. [391 K. Hermann, P.S. Bagus and C.J. Nelin, Phys. Rev. B 35 (1987) 9467. ]401 G. Pacchioni and P.S. Bagus, Phys. Rev. B 40 (1989) 6003. 1411A. Mauridis, I.F. Harrison, and J. Allison, J. Am. Chem. Sot. 111 (1989) 2482. 1421 P.S. Bagus and W. Miiller, Chem. Phys. Lett. 115 (1985) 540. [431 P.S. Bagus, C. Nelin. W. Miiller, M.R. Philpott and A. Seki, Phys. Rev. Lett. 58 (1987) 559. 1441P.S. Bagus and G. Pacchioni, Electrochim. Acta 36 (1991) 1669.
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Surface Science 275 (1992) 450-458 North-Holland
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Chemisorption of CO on defect sites of MgO Gianfranco
Paccbioni,
Tommaso
Minerva
Dipartimento di Chimica Inorganica e Metaliorgunicu, Urtkersit~ di Milano, Centro CNR, Via Venezicm 21. 201.73 M&no, It&
and Paul S. Bagus
Received 30 March 1992; accepted for publication 22 May 1992
Cb~misorption of a CO molecule on regular and defect sites of the ~g~l(~) surface has been ~nv~stig~t~(jby means of cluster model calculations. At all sites studied, CO bonds at the cation with the C atom closest to the surface, The bonding is considerably larger at a three-coordinated corner site than for a regular five-coordinated surface site. A hlueshift in the C-O stretching frequency, oc, of adsorbed CO compared to free CO is found; the shift is much higher for a corner than for a surface site because of the larger local electric Field for low-coordinated cations. Both the bond strength and the w shift ate largely due to electrostatic effects and not to the formation of a dative g-bond with the surface. Surface relaxation effects have also been considered.
It has been suggested that surface irregularities such as corners, edges, and kinks play an important role in the reactivity of oxide surfaces toward adsorbed species [1,21. Both theoretical [3-91 and experimenta 110-121 results indicate that the (100) surface of a simple oxide like MgO or NiO is rather unreactive when there are no defects present. The bonding of an adsorbed CO molecule is weak and dominated by electrostatic interactions [3-91, although some controversy exists in the literature about the extent of the chemical interaction and in particular of the CO r-donation to the surface [7,8,10,111; in some cases r-back donation from the surface to CO has also been claimed 1131. On the other hand, CO reacts rapidly with a rough MgO surface to produce CO:- and more complex surface species even at temperatures as low as 77 K [14,15]. It has been suggested that this occurs by nucle-
ophilic attack of an exposed surface 02- ion on a CO molecule bound to a Mg” ’ cation [ 151. One of the most commonIy employed techniques to characterize the chemisorption properties of an oxide surface is based on the infrared, IR, measurement of the vibratio~a1 shifts of a probe moIecule, Like CO [l&20]. The frequency shift, Aw, and the intensity change of the adsorbed molecule with respect to free CO have been used to extract information about the nature and number of cationic sites. including defect sites, on the surface [16]. Attempts have also been made to correlate the CO o shift with the local electric field at the surface cation 1171. Recently [7-91, we have theorcticahy analyzed the bonding and the w shift for the case of a single CO molecule adsorbed on regular MgO or NiO(100) surfaces. We found that, for an ideal MgO or Ni~(lOO) surface without defects, the positive u shift is largely determined by the fact that the CO mole&e stretches in the presence of
~~39-~~2~/9~/‘$~~S.~ 8 1992 - Elsevier Science Publishers B.V. All rights reserved
G. Pacchioni et al. / Chemisorption of CO on defect sites of MgO
the rigid surface, a wall effect, and that the electric field contribution to the shift is modest. We have also shown that a transition metal oxide like NiO behaves very similarly to MgO because the bonding is entirely electrostatic with little, if any, a-donation or r-back donation [7-91. In this work we extend our study to cluster models of surface irregularities, in particular edges and corners. We will consider three main points in our analysis. (1) The nature and the extent of surface relaxation for chemisorption sites where there are five-, four-, or three-coordinated Mg*+ surface cations, hereafter denoted as Mg::, Mgzz, and Mg;: 9 respectively. These differently coordinated Mg*+ cations model (i) a regular site of a flat surface, site MgzfMgJ:! cOrner an edge site, M$, or (iii> a (2) The’optimal geometry for a CO molecule interacting with these sites, i.e., perpendicular, bent, or flat. (3) The relative importance of electrostatic versus chemical bonding contributions in determining the bond strength and the CO w shift at the three sites. To this end, we have performed ab initio calculations on three cluster models, see fig. 1, simulating the various coordinative situations, and we have analyzed the corresponding wavefunctions.
2. Computational
details
Hartree-Fock SCF wavefunctions for the [MgO,, I”-/CO clusters were determined within the MO-LCAO approximation. The clusters used are [Mg0,]8-, [Mg0,16-, and [Mg0,14-, for five-, four-, and three-coordinated sites, respectively, see fig. I. The basis sets used are of triple-zeta quality [81. The rest of the ionic crystal has been simulated by a large set of +2 or -2 point charges, PCs, placed at the lattice positions. This array of point charges is large enough to reproduce the Madelung potential for a surface Mg2+ cation within reasonable accuracy [S]. As an alternative, one can determine the charges in a least square manner to fit the correct surface Madelung potential. This can be calculated by the Ewald
451
Fig. 1. Schematic representation of CO at surface, edge, and corner sites of MgO(100); the different equilibrium CO orientations, perpendicular at the surface site and bent at the edge and corner sites, are indicated. The atoms explicitly included in the cluster for the different sites are shown with hash marks.
method [21,22] or by other techniques [231. However, the analysis of the properties of interest in this study is rather insensitive to the detailed form of the potential and is practically independent of the point charge grid used [S]. Perfect ionicity is assumed for the MgO crystal [5,24-261. Surface relaxation effects of the Mg*+ cation and the equilibrium geometry of the adsorbed CO were determined by performing a geometry optimization for the SCF wavefunction by means of an analytical gradient technique using the HOND08 program [27]. The importance of correlation effects in determining the CO vibrational frequency and the bond strength has been considered by performing single-double configuration interaction, SD-CL calculations. For the determination of the CO binding energy we performed, for the SCF optimal geometry, a CI where the six 2p electrons on each O*ion plus the six lrr and 50 valence electrons of CO are correlated. Except for a small number of high-lying orbitals, excitations were made to all the SCF virtual orbitals. The CI wavefunction for the different clusters contains up to 56 000 configurations. The CO w, has been computed by polynomial fitting of five points around the minimum with
452
G. Pacchioni et al. / Chemisorption of CO OFIdefect sites of MgU
the distance between the Mg*+ cation and the CO center of mass fixed at a value close to 2,. Several effects can affect the absolute value of w, as well as Aw, e.g., the size of the substrate cluster, the basis set used, and the inclusion of correlation effects. These latter have been introduced by performing a SD-C1 calculation where only the CO 5u and lrr electrons are correlated. With this CI, the computed w, for free CO is 2067 cm-‘, while the SCF value is 2270 cm-’ (the experimental gas-phase value is 2143 cm-’ [ZS]). The computed vibrational frequencies and frequency shifts must be considered as qualitative. However, the trend in Aw shown by correlated and non-correlated wavefunctions is the same and allows one to identify the origin and the mechanisms contributing to different shifts for different surface sites.
relax vertically, but we kept the surrounding oxygen ions fixed. This results in a modest shrinking, - 0.04 bohr, of the distance of the Mg:: cation from the 02- anion below it, see table 1. This corresponds to a - 1% surface rumpling, in good agreement with experimental data 129-321 and periodic Hartree-Fock calculations [24]. The same vertical displacement is much more pronounced in the [MgO,l’- cluster model of an edge site. In this case, in fact, the Mgi: ion moves toward the 02- ion in the second layer by 0.20 bohr, a - 7% reduction of the first-second layer distance. This effect becomes very important for a Mgi,t cation at a corner site. With our cluster model, [Mg0,14-, we found that the Mg;,f cation moves down toward the O*- ions in the second layer by more than 0.5 bohr, with a contraction of N 20% of the unrelaxed distance, see table 1. A similar contraction has been predicted by means of non-empirical two-body potential calculations [33] but we are not aware of any other electronic structure study which includes surface displacements for this site. Moreover, there are no experimental estimates. The reason for this large relaxation is that at defect sites the Madelung potential is smaller than at a regular surface site 134). An inwards displacement of the
3. Surface relaxation It is well-known that the clean MgO(100) surface is almost unreconstructed and unrelaxed [24,29-321. For the [MgO,lsP cluster model of the regular surface, we allowed the Mgg,+ ion to
Table 1 SCF results equilibrium
for CO adsorbed C-end down on cluster models of surface, edge, and corner chemisorption sites on the MgO surface; distances, re in bohr, dissociation energies, D,, in eV; the basis set on CO does not include d functions Unrelaxed r(Mg
2+-02--j
Relaxed
substrate h)
r,(Mg-C)
MgO, Surface MgO,-CO Surface
3.976
MgO, Edge MgO,-CO Edge
2.811 2.811
4.618
MgO, Corner MgO,-CO Corner
2.296
_
2.296
4.468
3.976
4.915
,,(C-Of
D,
-
_
2.119
0.25 _
2.114
2.104
0.35
0.63
substrate
r,(MgZ+-O?-) 3.936 ( - 0.040) 3.975 (-0.001) 2.611 ( - 0.200) 2.728 ( - 0.083) 1.770 ( - 0.526) 1.797 ( - 0.499)
” h)
r,tMg-L)
r&C-O)
D,
_
_
4.915
2.119
0.25
_
_
_
4.619
2.112
0.32
_
_
_
4.472
2.fOfJ
0.58
‘) The position of the Mg’+ cation has been optimized with respect to the O-‘- ions and the point charges which are fixed at the regular crystal lattice positions. “) Height of the Mgzc cation from the second layer of the cluster: in parentheses is given the contraction with respect to the unrelaxed case.
G. Pacchioni et al. / Chemisorption
Mg2+ ion increases the electrostatic with its neighbors.
interaction
4. Optimal geometry of adsorbed CO The CO molecule is believed to bind perpendicularly to the surface with the C end down on ionic surfaces [l- 111. This is supported by theoretical calculations [3-91. However, the energy difference between C-down or O-down CO is small [8]. In fact, as we have shown in a previous paper IS], the SCF wavefunctions for a cluster model of CO/MgO(lOO) at a regular surface site lead to O-down CO being more strongly bound; for this calculation, d-polarization functions were not used for CO. The simultaneous inclusion of d-polarization functions for CO and of electron correlation effects with CI wavefunctions show that the C-end down CO is more stable ISI. We have found, in this work, that the behaviour at edge and corner sites is similar to that at the surface site discussed above. For SCF wavefunctions, O-end down CO is more strongly bound but once correlation effects and d functions on CO are included, the C-end down bonding is also preferred for these Mg$: and Mgs’ sites. The change in CO orientation as function of the level of theoretical treatment is entirely due to the different description of the CO multipole moments and not to a change in the surface-adsorbate charge transfer mechanisms [8]. For these reasons, we do not consider further O-end down orientations of CO/MgO. A simultaneous optimization of the positions of the Mg2+, C, and 0 atoms has been performed at the SCF level. Geometries where CO is bound C-end down correspond to local minima on the potential energy hypersurface. For the regular surface site, Mgz:, we confirm our earlier results IS] that the C-O internuclear axis is normal to the surface. For the edge and corner sites, CO is bent away from the surface normal, see fig. 1. The bend is such that the C-O axis is normal to the plane which would define an MgO(ll1) surface. At these sites, the CO bends away from the 02- anions on the terraces of the regular (100)
of CO on defect sites of MgO
453
surface thus reducing the Pauli repulsion, see below. For a corner, coordinatively unsaturated, Mg$z site, we explicitly considered several geometries were the CO molecule lies flat or almost flat on the surface (d-polarization functions on CO and correlation effects were included). However, these orientations do not correspond to local minima on the potential energy surface. The Mg’+-CO distance becomes shorter as one goes from a regular surface site, N 4.9 bohr, to an edge site, - 4.6 bohr, to a corner site, N 4.5 bohr. At the same time the C-O bond length decreases and the binding energy increases, table 1. The trend is the same when we use Yrnrelaxed” clusters, where the Mg2+ ion occupies the ideal position of a truncated cubic crystal, or clusters where the Mg2+ is free to change its position, see table 1.
5. Analysis of bonding Our calculations indicate that CO binds with different chemisorption energies on the various sites considered. In particular, the adsorption energy is considerably higher for an exposed cation at a corner site, modeled by [Mg0,14-, than for a regular surface site, modeled by [MgO,lK-. The estimated SCF bond strength for a regular surface site is De = 0.25 eV; this value is somewhat exagerated by the occurrence of basis set superposition errors, BSSE [35’]. The BSSE leads to calculated values of De which are too large; using the Boys and Bernardi counterpoise method [35], we estimate that the uncertainity in De for [MgO,lH- due to the BSSE is - 0.08 eV; this correction is probably an upper limit. Correlation effects increase the SCF D, = 0.25 eV by 0.05 to 0.3 eV, see table 2. Several experimental [lO,ll] and theoretical [3-91 estimates of De have been reported. These all range between 0.15 and 0.40 eV, suggesting that CO is rather weakly bound, or physisorbed, at MgO(lOO) surfaces. Experimenta1 measures of the chemisorption energy of CO at defect sites or on other MgO surfaces beside MgO(100) or thin films have not been reported so far.
454
G. Pacchioni et al. / Chemisorption of CO on defect sites of M,O
Table 2 Contributions, from SCF wavefunctions, from various bonding mechanisms to the interaction energy, Eint, in eV, for CO chemisorbed on cluster models of surface, edge and corner sites of the MgO surface; the analysis refers to r(C-0) = 2.15 bohr; the total binding energy, 0, in eV, is given for SCF and CI wavefunctions Cluster Site z(Mg-Cl
(bohr)
Electric field at Mg2+ (a.u.1 1. Electrostatic interaction 2. Pauli repulsion 3. MgO polarization 4. MgO + CO charge transfer 5. CO polarization 6. CO + MgO charge transfer Total Total
D, (SCF) D, (SD-CD
MOO,-CO Surface 4.90
MgO,-CO Edge 4.60
MgO,-CO Corner 4.50
0.019
0.033
0.062
+ 0.24 - 0.21 +0.02
+ 0.42 - 0.34 +0.01
+ 0.48 - 0.28 0.00
+ 0.06 + 0.06
+ 0.04 +0.12
+ 0.04 +0.27
+ 0.06
+ 0.07
+ 0.07
+ 0.23 + 0.32
+ 0.33 + 0.49
+ 0.59 + 0.83
The importance of electrostatic and chemical bonding contributions to the strength of the CO bond on the adsorption sites considered has been analyzed by determining the cluster wavefunction with contraints, according to the constrained space orbital variation, CSOV [36,37]. The results are for unrelaxed substrate geometries and for a CO geometry close to equilibrium. At each step of the CSOV, the interaction energy is defined as E,,,(CSOV step n) = E(CO1 + E(MgO1 E(MgO/CO; CSOV step n>. The change in Eint between CSOV step II and the preceding step n - 1 represents the energetic importance of the new variational freedom allowed at step n [36,371. The Eint for the CSOV variations for [MgO,ls-/ CO, [MgOJ-/CO, and [Mg0,14-/CO, are given in table 2. The first step of the CSOV consists in simulating the ionic surface by an array of PCs. The Mg2+ cation and the 02- anions that are normally included explicitly in the MgO cluster models are represented by PC = + 2 and PC = - 2, respectively. At this first step, the charge density of the CO molecule is fixed as it is for the SCF wavefunction of the isolated molecule for the
appropriate C-O bond distance. The value of E,,, at this step provides a measure of the importance of the pure electrostatic interaction. At step 1, Eint is considerably larger for corner and edge sites than for a surface site, table 2, because in the former cases the CO is adsorbed at a low-coordinated cation where the local electric field is stronger. While the absolute value of the electrostatic potential in the adsorption region is important for the CO binding energy, the shape and curvature of this potential are the features that are important for changing the C-O vibrational frequency. This is discussed further in section 6. In the second step of the CSOV we replace the few PCs defining the local chemisorption site with real Mg2+ and 02ions, but we do not allow the two charge densities, cluster and CO, to relax. The charge density for [MgO,] is fixed as that for the SCF wavefunction for the isolated cluster and the CO charge density is kept at that for the isolated molecule. However, at this step 2, the wavefunction formed from the superposition of the frozen charge densities of [MgO,] and CO, is normalized [36-381. This step measures the extent of Pauli repulsion originating from the interpenetration of electronic charge at equilibrium positions, see table 2. This energy contribution, which is purely repulsive, depends on the surface-CO distance but also on the number of 02- ions on the surface. In [MgO,l”-/CO there are four spatially extended 02anions which prevent the CO molecule from approaching too close to the surface; in [MgO,l”-/CO there are only two 0 2- ions on the first layer of the cluster, while in a corner site the 02ions are in the second layer, see fig. 1. The Pauli repulsion is smallest for the surface and largest for the edge, table 2, because it results from the balance of the two effects, i.e., the distance from the surface and the number of 02- ions which are nearest neighbors of the CO molecule. The substrate polarization, step 3, is measured by allowing the MgO cluster orbitals to vary. At this step the CO charge density remains frozen. This polarization, however, is negligible for all three sites. Also the MgO to CO charge transfer, measured at CSOV step 4, is very small, table 2,
455
G. Pacchioni et al. / Chemisorption of CO on defect sites of MgO
and in large part due to the BSSE [8,9]. In CSOV step 5, the CO molecule is free to polarize but the [MgO,] charge density is frozen as determined at the previous CSOV step. The CO polarization can be viewed as arising from two origins. The first origin reduces the Pauli repulsion arising from the interpenetration of the [MgO,] and CO charge distributions [39]. The second origin is the electric field or the electrostatic potential at the bonding site due to the Mg2+ and 02- ions. This potential causes the CO to polarize [7-9,401. The CO polarization is considerably larger for the corner site than for the regular surface site; the edge site is intermediate. The dominant effect determining the size of this polarization is the electric field associated with the different CO adsorption sites. The charge donation from the CO molecule to the surface, see CSOV step 6 in table 2, is small, similar for the three sites, and again exagerated by BSSE. Differently from what is often believed, there is little or no a-donation from CO to the ionic surface, even for a low-coordinated cation [7-91. The different binding energies computed for CO on the three sites arise principally from the
Table 3 Contribution chemisorbed Cluster Site z(Mg-C) 1. 2. 3. 4. 5. 6.
very different local electric field at the surface cation. For a regular, five-coordinated surface site this electric field is small, the initial electrostatic attraction is partially offset by the Pauli repulsion and the weak bonding is due to the electrostatic term plus the CO polarization; for a four- and a three-coordinated site, e.g., an edge or a corner site, the electric field becomes larger hence both the electrostatic attraction and the CO polarization increase. As a consequence, the chemisorption energy of a CO molecule on a corner site is almost three times larger than on a five-coordinated site; this is found for both the SCF and the correlated CI wavefunctions, see table 2.
6. Origin of the vibrational
The CO molecules adsorbed on ionic surfaces exhibit a o shift toward higher frequencies [16201. This provides a clear indication that CO is bound C end down because the reversed orientation gives negative w shifts, i.e., toward lower frequencies [S]. The positive w shift has been measured for CO adsorbed on polycrystalline
from various bonding mechanisms to the vibrational frequency, o,, and to the frequency on cluster models of surface, edge, and corner sites of the MgO surface
(bohr)
Field-dipole interaction Pauli repulsion (wall effect) MgO polarization MgO + CO charge transfer CO polarization CO + MgO charge transfer
SCF a) Change from Mg:: SD-C1 h’ Change from Mg,,z+ Experimental ‘) Change from Mg::
shift
shift, Ao, in cm-‘,
MgOs-CO Surface 4.90
MgO,-CO Edge 4.60
MgO,-CO Corner 4.50
UAW
%/A0
%/AU
2271/+ 2321/c 2321/t 2316/c 2312/c 2302/c
1 =) + 50) =) - 4) - 4) - 10)
2301/ + 31 0 21 lO/ + 43 0 2156/ + 13 0
a) Aw is computed with respect to free CO o,(SCF) = 2270 cm-‘. h, Aw is computed with respect to free CO w,(CI) = 2067 cm-t. ‘) From refs. [15,20]; Aw is computed with respect to free CO w,(exp.)
2293/ 2348/c 2348/c 2343/c 2334/c 2326/c
+ 23 a) + 55) =) - 5) - 9) - 8)
2356/ + 86 =) 2394/c + 38) 2394/c = ) 2391/c-3) 2375/c - 16) 2367/c - 8)
2325/ +24 2122/ +12 2164/ +8
+ 55
2367/ +66 2140/ +30 2203/ +47
+ 55 + 21
= 2143 cm-t.
+ 97 + 73 + 60
for CO
MgO [l&191, for CO on MgO smokes composed by perfect microcrystals [20], and for CO on MgO films grown on the Mo(100) surface [ll]. The measured shift ranges between approximately + 10 [l&20] and + 35 cm-’ [ 1l] in the limit of zero coverage. The shift has been found to depend on the degree of coverage for CO on MgO microcrystals 1201 and rough surfaces [18,19]. This is to be expected because of the lateral dipole-dipole interactions between coadsorbed CO molecules [l&20]. However, for CO/ MgO/ Mo(lOO), coverage dependence for w,(C-0) was not observed [ 111. The measured shift is in acceptable agreement with the computed Aw for [MgO,]sP/CO, 31 cm- ’ (SCF) or 43 cm ’ (CI), see table 3, given the simple model used. There are cluster size effects on AU but they are not large. A larger substrate cluster model for MgO, [Mg,,O,,]/CO, gives a SCF Aw = + 17 cm-‘. Experimentally it has been observed that the main line in the IR spectrum of CO on MgO microcrystals and on MgO powders, at 2156 cm- ‘, is accompanied by a low-intensity feature at 2164 cm ~ ’ [15,20] which has been attributed to CO molecules adsorbed on the edges of the crystallite [X,20]; the low intensity is consistent with the fact that the edge sites do not exceed the 5% of the total number of surface sites on the crystallite, the rest being regular, non-defect, surface sites. When CO is adsorbed polycrystalline MgO, which has an high concentration of defect sites, a third band is observed at 2303 cm- ’ [ 151. This band has been assigned to CO adsorbed on lowcoordinated Mg<: sites; an assignment which is fully supported by the present calculations, set table 3. The SCF w, for an edge Mg:: site, is 24 cm-’ larger than for the regular site, compared to the experimental assignement of the frequency shift being approximately + 10 cm ‘. For a corner, Mg:,’ , site, the SCF w, is 66 cm- ’ larger than for the regular, Mg::, site while the cxperilarger than for the mental w, is - 50 cm-’ regular site. A similar trend is found when o, (C-O) is computed with CI wavefunctions for the cluster. The CI w,, see table 3, for the edge site is 12 cm ’ larger than for the regular site and. for the corner site, it is 30 cm-’ larger; this is to be compared to the experimental changes of ap-
proximately + 10 and +50 crn~- ‘, respectively [ 15,201. The origin of this blueshift is matter of controversy. Several authors [lo,1 1,131 attribute the shift to the formation of a surface a-complex where charge flows from the CO 5~ orbital to the substrate. This interpretation is not supported by theoretical results. Both cluster model [S-9] and band structure [3,4] calculations indicate that the bonding does not arise from chemical interactions but is largely due to electrostatic effects. Even for the case of CO bound to a single metal cation it has been demonstrated that the bond is almost entirely electrostatic [41]. We have recently shown that this is also the origin of the w shift [7-91. In order to identify the origin of the different shifts for the three sites, we have performed a CSOV decomposition similar to that used to dctermine the bonding contributions. In practice, for each CSOV step we have computed the SCF C-O potential energy curve for the same grid of points around r,(CO). See refs. [8,Y,42] for a detailed description of this constrained dccomposition for the shifts in w,(C-0). The results arc given in table 3. The first term, see CSOV step 1 in table 3, is dctcrmincd by stretching the frozen orbital CO molecule in the presence of the PC’s simulating the ideal ionic surface; this term accounts for the electrostatic interaction between the non-uniform electric field and the C-0 dipole. The shift at this CSOV step is negligible. +1 cm-‘, for the regular surface site, modest, +23 cm- ‘, for an edge site, and large. + X6 cm -~I. for a corner site, see table 3. ‘I‘he diffcrences are consistent with the increasing strength of the local clcctric field. In the second CSOV step, or frozen orbital step. the CO molecule is stretched against the rigid surface which is now represented by frozen ions. Intra- or inter-unit polarization of the two fragments is not allowed at this step which thus measures the extent of the “wall effect”. The wall effect [42,43] occurs when the CO charge density penetrates the rigid surface during the stretching motion; the potential energy surface bccomcs more rcpulsivc as the surface-carbon distance becomes shorter. l’his effect always acts to incrcasc the (I)~ for CO. XC
G. Pacchioni et al. / Chemisorption
table 3. The trend of Aw at this step is again the result of a compromise between the surface-CO distance and the number of oxygens in the first layer of the cluster. Neither the MgO polarization nor the MgO to CO charge transfer contribute significantly to the w shift, table 3. The CO polarization lowers the frequency for a corner site more than for a surface site. However, this lowering is small for all three sites. The CO to MgO charge transfer step gives a negative shift which is almost the same for the three models, approximately - 10 cm-‘. For the traditional analysis of the shift of w,(C-0) based on effects due to u-donation and r-back donation, it is argued that the a-donation, augmenting the CO+ character of the adsorbate, leads to an increase in w,, i.e., to a positive contribution to AU [42]. This is because w,(C-0) is larger for CO+ (5~‘) [28]. Although the shift that we find to be associated with the CO udonation is small, it is actually a negative shift! The fact that the CO shift is negative shows that the traditional analysis is incorrect. The quite small a-donation which may occur actually reduces the Pauli repulsion between MgO and CO and, thus, reduces the wall effect and leads to a lowering of U, [42,44]. We note that negative w shifts are associated with the a-donation step of the constrained variation for CO on metal surfaces [42] as well as on oxide surfaces [8,9]. The net effect is a large positive w shift for a corner site and a modest shift for a surface site. The origin of these shifts, however, is not the same. For a regular surface site, the field-dipole interaction is small compared to the wall effect; it is the wall effect which largely contributes to the total positive shift. For a corner site, the field-dipole interaction is very large and dominates over the wall effect. In an edge site both mechanisms are important. Thus, as one goes from a high-coordinated to a low-coordinated site the importance of the wall effect is decreased and that of the local electric field is increased. Both effects contribute to increase the CO w, consistently with the observed blueshift in the IR spectrum. We should note that, although we use the phrase “field-dipole” to describe the effect of the electrostatic potential at the MgO site on
of CO on defect sites of MgO
451
w,(C-01, it is not the absolute value of the CO dipole moment, p, which determines the magnitude of the interaction. It is the shape of p(r), where r is the C-O distance, which is the important factor. In particular, the slope, dk/dr, and the curvature, d2p/dr2, are the parameters which determine how large the o shift due to the “field-dipole” interaction will be [8,9,40,43,44].
7. Conclusions
We have reported theoretical studies of the interaction of a CO molecule with Mg*+ cations at surface, edge, and corner sites of a MgO(100) surface. Surface relaxation effects have been taken into account; they are large for low-coordinated sites, and negligible for non-defect surface sites. However, the surface relaxation is not essential for determining the nature of the bonding of CO to the surface. This, in fact, originates from electrostatic interactions and not from the CO a-donation to the surface. Low-coordinated sites on ionic crystals generate higher local electric fields in the adsorption region. For this reason, CO is chemisorbed on a corner site with a binding energy of about three quarters of an eV, while it is weakly adsorbed, or physisorbed, on a five-coordinated surface site. The different sites also exhibit different vibrational shifts of the C-O stretching frequency with respect to free CO. A large blueshift, of approximately + 100 cm-‘, is computed for a corner site, compared to a moderate shift, approximately + 30 cm- ‘, for a surface site. The origin of these shifts, however, is not the same. The shift is basically due to two mechanisms: the interaction between the non-uniform electric field at the surface and the CO dipole, and the “wall effect”; both mechanisms act to raise 0,. For a coordinatively unsaturated corner site the field-dipole interaction is dominant with respect to the wall effect; for a five-coordinated surface site the field-dipole term is small and the shift is largely due to the wall effect.
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This work has been supported in part by a grant from Progetto Finalizzato Chimica Fine II of the Italian CNR. The support of NATO through the Collaborative Research Grant No. 900031 is gratefully acknowledged.
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