Bonding and vibration of CO molecules adsorbed on low-coordinated surface sites of MgO: a LCGTO-LDF cluster investigation

...... ...

,...A. :.:.::::~:::::~~~:~:~.:~~:::~,:~::~,~:::~ :::,:., ..I.... . . . . . . . . . ...i.._n...n_.nr.. . . .. . . . . . . ..~.....~...... .. . . . . .. . . . . . ..A. . ..A . . .../._ :,,, ,,,,,,,

.““““.‘..“‘i ““““‘.‘j.,...‘.

~~:~.~~,~~,~~:~:~~~:::.:.:.:,

. . ...... . . . . . . .... .

‘.‘. . ...:

y:<:::....

Surface Science 297 (1993) 223-234 North-Holland

.,):.:

‘surface science

... ,... :.:.:::g# .‘.....:,:.:. :~:~::~,.~~,~.y ““‘“‘5: -:”‘.X” “‘:‘i.‘..A ..Y... .,., ........_.....~.,.,,,~,_,,:,~,~ ‘*.....,.... ......r,~.n,,m,,,, ,:;;...“” “‘-““-.‘:. ‘A”“” ‘A ..A ,,,,,,,,:,., ““‘::‘V.“Cf.. ,,.> “‘A.’ “C....‘. ..A., .,.,.,:.,,, _, “‘.‘;‘.‘.:.::.~::.::::::::,:~::: :.:,);,,, :,., : :,,, _ ,,_

Bonding and vibration of CO molecules adsorbed on low-coordinated surface sites of MgO: a LCGTO-LDF cluster investigation Konstantin

M. Neyman * and Notker Riisch

Lehrstuhl fiir Theoretische

Chemie, Technische Universitiit M&hen,

D-85747 Garching,

Germany

Received 17 May 1993; accepted for publication 26 July 1993

The adsorption of CO molecules on low-coordinated edge and comer cationic centers of a MgO surface is investigated theoretically by the LCGTO-LDF cluster method. Internuclear distances as well as frequencies and absolute IR intensities of the MgO-CO and C-O vibrational modes are calculated for model clusters embedded in large arrays of point charges. The MOO-CO bond is found to be predominantly electrostatic. But there is also a minor redistribution of the surface electronic charge similar to that occurring for the adsorption at Mg ‘+ sites of a perfect MgO@Ol) surface. For the C-end-bound CO the main contribution to the increased blue shift of the stretching frequency at the more open sites comes from a change of the electrostatic field near the center of coordination. For CO molecules bound via the oxygen atom to the same sites a red shift is calculated which should provide a facile way to distinguish O-end bound species from the moiety coordinated via carbon. Surface relaxation is found to have only a negligibly small effect on the MgO/CO vibrational frequencies and intensities, even for the most relaxed corner site. The cluster models predict an approximate doubling of the C-O absolute IR intensity upon the CO adsorption C-end down. The intensity shows only a moderate dependence on the coordination of the adsorption center. This enhancement derives to a large extent from a change of the rr component of the CO dynamical dipole moment due to interactions between the adsorbate at the cation-atop position and the nearest-neighbor surface anions. For the CO molecule adsorbed via the oxygen atom the intensity enhancement is mainly of electrostatic origin. The LCGTO-LDF characteristics are compared to the available data obtained at the Hartree-Fock level of theory for non-stoichiometric cluster models of MgO. The effect of the composition and charge of the model cluster on calculated vibrational parameters and on the adsorption energy is also analyzed.

1. Introduction

A characteristic feature of solid catalysts is their large specific surface area. These materials usually exhibit different types of coordinatively unsaturated centers. In powder magnesium oxide, which is a catalyst for the oxidative conversion of methane [l], surface ions may occupy the following positions: five-fold-coordinated Mg,, and O,, on perfect MgNOOl) faces, four-fold-coordinated Mg,, and O,, on the edges and three-fold-coordinated Mg,, and O,, at the corners of rock-salt structure microcrystallites (see fig. 1). The elec-

* To whom correspondence 0039-6028/93/$06.00

should be addressed.

tronic state of the ions depends noticeably on the coordination number [2] and the coordinatively unsaturated centers play an important role in the surface chemistry of MgO [3,4]. Manifestations of the electronic inequivalence of the anions are found, for example, in the photoluminescent and reflectance spectra of polycrystalline MgO. Several bands of the spectra were assigned to electronic transitions involving the oxygen ions at sites of low coordination [5-71. The infrared (IR) spectroscopy of adsorbed carbon monoxide molecules as a probe is utilized to investigate the nature of cationic centers on surfaces of metal oxides [8]. For low coverage values at 77 K, the IR spectra of CO on MgO samples with a large surface area exhibit absorption maxima at 2157,

0 1993 - Elsevier Science Publishers B.V. All rights reserved

224

KM. Neyman, N. R&K% / CO on h&O: a LCGTO-LDF clusterinvestigation

Fig. 1. Sketches of the cluster models for CO adsorption on the five-, four- and three-fold-coordinated cationic sites of MgO: (a) Mg,O,-CO (Mgs,); (b) Mg,O,-CO CM&); (c) Mg,O,-CO (Mg,,). The main rotational axis af the substrate clusters coincides with the z direction along which the CO adsorbate is assumed to approach the surface either C-end or O-end down. Only a small fraction of the surrounding point charges (small spheres) is shown.

2170 and 2203 cm-l that have been attributed to Mg,,-CO, Mg,,-CO and Mg,,-CO complexes, respectively 191. The chemisorption of CO molecules on Iowcoordinated sites of MgO has been recently studied with help of the Hartree-Fock self-consistent field (HF-SCF) cluster approach. It was found [lo] that CO binds considerably stronger at the corner Mg, cations than at the regular Mg,, sites. The nature of the CO bonding to the surface, the adsorbate-substrate and C-O distances as well as the adsorption energy were only slightly affected by surface relaxation. The MgO-CO bond was characterized as being purely electrostatic, without any o--donation to the surface.

Also, for a comer site a much larger blue shift of the C-O frequency of adsorbed CO was calculated than for a perfect surface site, in agreement with experiment [91. This was rationalized as a consequence of the larger local electric field at the low-coordinated centers [lo]. Previously, we have investigated the adsorption of isolated (low-coverage) 1111 and interacting (high-coverage) [12] CO molecules at a perfect MgO(OO1) surface using the linear combination of the Gaussian-type orbitals local density functional (LCGTO-LDF) cluster method [13151. The present LCGTO-LDF study deals with the adsorption of CO on the inherent surface irregularities of MgO, focusing the discussion on ~brational observable& Supplementing the investigations of ref. [lo] we shall address the following questions: (i) the adsorption-induced change of the absolute IR intensity of the C-O mode; (ii) the effect of the orientation of the adsorbed molecule on the C-O frequency (C-end versus O-end adsorption); (iii) the effect of surface relaxation on ~brational characteristics of MgO/ CO complexes; (iv> the dependence of calculated observables on the constitution and on the size of the substrate model clusters. The present results will also be compared to those of the HF-SCF studies [10,16,17]. Tlsis comparison is of a special interest because the extended stoichiometric LCGTO-LDF cluster models of the CO adsorption on a perfect Mg~~l~ surface [11,12] resulted in a picture of the adsorption bond which differs somewhat from that obtained at the HF-SCF level of theory for minimal non-stoichiometric clusters [l&17]. It seems also worth pointing out that a quite different description of the MgO-CO bonding, invoking dispersive forces, was obtained in a recent accurate post-HF study which was based on large charged cluster models [18]. An unexpectedly low adsorption energy of 0.04 eV was calculated, the measured value is 0.16 eV 1191. The disagreement even of this accurate electronic structure method with experiment indicates that further insight may be necessary into the following two aspects of the problem: (v) how should one properly construct surface cluster models of MgO, and (vi>, may the experimental adsorption energy of CO be due to

KM. Natal

N. R&h

/ CO on MgQ: a LCGTO-LDF cluster ~nuest~gat~n

interactions with surface defects? We will comment on these questions as well.

2. Compu~tional details and cluster models The “first principles” LCGTO-LDF cluster method [13-151 was used in the Xa approximation (a = 0.7). It has been shown to be adequate for the description of the MgO/CO adsorption complexes [11,12]. The construction of the orbital basis started from basis sets 1171which had been optimized for Mgt and O- ions. These basis sets were extended by d-functions (Mg: 1.1765, 0.36891, 0.14371 [ZOl; 0: 1.154 [213) and were slightly less contracted (Mg (13s, 8p, 3d) + [7s, 4p, Id] and 0 (lls, 7p, Id) -+ [6s, 4p, Id]) [ill. For the adsorbate atoms C and 0, basis sets of the type (9s, 5p, Id) -+ [7s, 4p, Id] [22] were used with Cs-exponents taken from ref. [21]. It should be recalled that the LCGTO-LDF results for the free CO molecule obtained with these basis sets are in good agreement with experimental data as are those for diatomic MgO [ll]. The correct sign and value of the CO electric dipole moment and its displacement derivative is of special importance for the accuracy of the calculated C-O vibrational intensity. The two auxiliary basis sets used in the LCGTO-LDF method to fit the electronic charge density and the exchange-correlation potential were constructed from the orbital exponents in a standard fashion 114,233. The basic stoichiometric cluster models for the MgO/CO complexes utilized in the present investigation are displayed in fig. 1 (see also section 5). Only two of the four possible vertical adsorption situations at on-top sites, namely the C-end and O-end bindings of CO to cationic centers, are discussed because the interaction with the oxygen anions was found to be repulsive 111,241. A MgSc site on a perfect crystahine (001) surface of MgO is surrounded by five nearest-neighbor oxygen anions. It is represented by the cluster Mg,O, of CsV symmetry (fig. la). The clusters Mg,O, (C,; fig. lb) and Mg,O, (C,,; fig. Ic) are chosen to describe low-coordinated cations on edges (Mg,) and at corners (Mg,,) of microcrys-

225

tallites, respectively. The central position of these clusters is occupied by a magnesium cation. All atomic clusters are embedded in electroneutral matrices of point charges (PC) to reproduce the Madelung field at the site of the central cation. The PC arrays were constructed in the form of parallelepipeds, using 17 X 17 x 6 = 1734 (Mg,O,), 17 x 10 x 10 = 1700 (Mg,O,) and 10 X 10 X 10 = 1000 (Mg,O,) point charges of q = t-2 a.u. situated at the atomic positions of the rocksalt crystal (lattice parameter 4.21 A [25]). Of course, those ions treated explicitly were deleted from the regular PC arrays. In some cases, the value of the point ions was reduced from its nominal value to + 1.5 a.u. for comparison with the standard models. Simplified substrate clusters, PC, consisting only of a matrix of the point charges, were used to simulate the electrostatic adsorbate-adsorbent interaction. If not specified otherwise (see section 4), results are presented for models which are based on the appro~mation of an unrelaxed substrate. The following procedure was used to search for the equilibrium geometry of adsorption complexes where the CO molecule was assumed to be oriented upright along the z axis (fig, l), either C-end or O-end down. First, the Mg-C (Mg-0) distance z(Mg-X) w$s optimized for a fixed C-O bond length of 1.13 A and then the C-O distance r(C-0) was varied, keeping the CO center of nuclear charge (and of mass) fixed as calculated in the previous step. Test calculations showed that a further reoptimization of the z(Mg-X) parameter with this new C-O distance is not necessary. Equilibrium internuclear distances z, and r,, anha~onic ~brational frequencies (energies of the 0 + 1 transition between the vibrational levels) w, and the minimum of total energy were computed by fitting a polynomial to five (Mg-CO) and seven (C-O) points of the potential curve located near the minimum. The normal modes of CO on MgO have been approximated by the Mg-CO and C-O internal modes, The absolute IR intensities have been cdculated in the double-harmonic approach 1261 according to which an IR intensity is proportional to the square of the dynamical dipole moment of the normal mode, i.e. to the square of the partial

226

KM. Neyman, N. R&h

/ CO on MgO: a LCGTO-LDF

derivative of the total dipole moment p with respect to the corresponding nuclear displacement. The dynamical dipole moment it+/& was calculated from a parabolic approximation defined by three points near the equilibrium.

3. MgO-CO intensities

bonding, frequencies and IR of the C-O and Mg-CO modes

At low temperatures, CO molecules are usually considered to be adsorbed vertically on the (001) plane of ionic rock-salt-type materials with the carbon atom directed to a surface cation (ref. [27], and references therein). However, there is no direct experimental confirmation for this orientation of the CO molecule. Rather, this geometry is chosen based on the assumed analogy with metal-carbonyl compounds and with chemisorption complexes at metal surfaces. A study of the CO adsorption on MgO(OO1) by means of the high-resolution LEED method [28] was unable to ascertain either the molecular orientation or the height of the CO molecules above the surface. The measured nearest-neighbor CO-CO distances are in agreement with a model of CO molecules standing nearly perpendicular to the surface. The results of cluster model calculations for MgO(OOl)/CO [11,12,16,17] are in line with the C-down orientation of CO molecules as the computed adsorption-induced blue shift of the C-O frequency agrees qualitatively with the measured IR data [9,29,30]. For the O-down orientation a red shift of the frequency has been predicted [11,16,17]. The HF-SCF models furnish rather similar adsorption energies for C-end and O-end CO on MgO(001). They also give preference to the oxygen-down coordination on regular and stepped LiF(001); this unexpected result may in part be due to the lack of correlation in the HF method [31,32]. Indeed, although the present LCGTO-LDF results also show a noticeable attraction of the oxygen-down CO to MgO, it is smaller in size than that for the carbon-down orientation (table 1). The reasons, why the overestimated LDF binding energies must be taken with a caveat only in cases of weak interactions, are discussed in ref.

cluster investigation

Table 1 Equilibrium distances, dipole moment Ak = &ubstrate-CO) - &ubstrate) of the adsorbed CO and binding energies for various MgO/CO models Model

&k-CO PC-CO Mg,O,-CO Mg,c-CO PC-CO Mg,O, -CO Mgsc -CO PC-CO Mg,O, -CO Mg,O,R-CO b, CM@- )-CO (Mg,O;+ )-CO Mgsc - OC PC-oc Mg,O,-OC Mg,,-OC PC-oc Mg,O,-OC Mg,,-OC PC-oc Mg,O, -0C

z,(Mg-XI

r,(C-0)

ALL

D,

(‘Q

(Al

(D)

(eV)

2.22 =r 2.22

1.132 1.124

+0.90 + 1.45

0.62 0.97

2.18 a1 2.18

1.131 1.124

+ 1.26 +2.05

0.92 0.94

2.23 ‘) 2.23 2.21 2.20 2.26

1.128 1.121 1.120 1.120 1.125

+1.97 + 2.88 +2.75 +2.91 +1.78

1.32 1.07 1.13 1.25 0.74

2.19 a) 2.19

1.132 1.137

+0.15 +1.1s

0.24 0.51

2.07 a) 2.07

1.136 1.139

+0.55 + 1.56

0.48 0.49

2.05 a) 2.05

1.146 1.143

+ 1.33 +2.31

0.82 0.68

a) Distance fixed as calculated for the corresponding cluster. b, Relaxed substrate cluster.

Mg,O,

[ll]. We would like to emphasize that several corrections have to be applied to the LDF values 0, presented in table 1 before they may be compared to the experimental adsorption energies: corrections (i) for the basis set superposition error [33]; (ii) for the effect of the reduced substrate ionicity below the nominal value Mg’+02-, according to which the surrounding atomic cluster point charges should be chosen smaller than + 2 a.u. [11,34]; and (iii) for deficiencies of the local density approximation [35]. Each of the contributions may reduce the adsorption energy by several tenths of an electron volt, while leaving the other adsorption-induced calculated parameters essentially unchanged [ 111. Further studies of all these corrections at a rather high degree of accuracy are necessary before the adsorption energy can be reliably estimated within the present theoretical approach.

KM. Neyman, N. R&h

/ CO on MgO: a LCGTO-LDF cluster investigation

All theoretical models support the contention that in principle one should be able to observe the anomalous O-end-oriented CO on ionic surfaces under favorable conditions, e.g. in cases of a stronger electrostatic attraction to the low-coordinated edge and corner cationic sites. These circumstances provide one of the motivations for the present study. It seemed interesting to calculate the spectroscopic characteristics for such unusual moieties. We shall first discuss the results for the Mg-CO complexes and then comment on the Mg-OC species. 3.1. C-end down coordination of the CO adsorbate The geometrical parameters and binding energies as summarized in table 1 indicate a rather weak interaction of CO with the cationic sites of MgO via carbon. The calculated C-O distcnce is only slightly compressed by 0.006-0.010 A comTable 2 IR frequencies a) and intensities for the MgO/CO System

w(Mg-CO) (cm-‘)

221

pared to the bond length of 1.130 A [ll] calculated for the free CO molecule. As measured by the binding energy which is overestimated significantly in the LDF models [11,12], the interaction becomes stronger for low-coordinated centers in agreement with the experiment [29]. This trend is mainly due to a stronger electrostatic attraction of carbonyl in the vicinity of the more exposed cations at edges and corners. Similar results were obtained by the HF-SCF cluster models [lo] where a noticeably shortening of the M@-C distance from 2.60 A (Mg,,-CO) to 2.36 A (Mgsc-CO) was found. This is at variance with the present LDF data, the ,Mg-C bond length changes only by about 0.05 A. The HF-SCF Mg-C distances are substantially longer than the LDF ones as has been already discussed for the models of MgO(OOl)/CO [ill. The shorter LDF bond distances are in line with the overestimation of the adsorption energy.

complexes and CO molecule A&C-O) (cm-‘)

b,

ICC-0) &m/m00

&Jar (au.) ‘)

k.Jar (a.u.1 ‘)

40 124

0.54 0.50

- 1.07 - 1.42

M&-CO PC-CO d, Mg,O,-CO Exp. e, JW&-CO PC-CO d, Mg,O,-CO Exp. e, &,,-CO PC-CO d’ Mg,O,-CO Mg,O,R-CO (MgO,4-)-CO ;$$_,-co

-8( 183 (158)

-

o

0)

54 (16) 14

198 (185)

-6t 7) 56 (33) 27

28 125

0.58 0.46

- 1.02 - 1.39

197 (184) 201(180) 218 179

25 (32) 85 (59) 84 (56) 99 32 60

12 88 94 74 357

0.59 0.48 0.50 0.48 0.62

-

0.37

- 1.05

0.88 1.27 1.31 1.21 2.21

co co Exp. h,

-

2169 @ 2143 g,

66 60.5

a) The data obtained with surrounding point charges of f 1.5 a.u. are shown in parentheses. b, Shift of the C-O frequency relative to that of free CO. Cl For +/ar: 1 au. = 4.80 D/A. d, r (surface-C) fixed (see table 1). e, Ref. [9]. n Relaxed substrate cluster. g, Reference value for the determination of A&C-O). h, Refs. [36,37].

228

KM. Neyman, N. R&h

/ CO on A4gO: a LCGTO-LDF

From the results of table 1 one deduces that the purely electrostatic interaction as modelled by the corresponding PC-CO clusters provides a substantial part of the MgO-CO bonding, increasing for the complexes with more exposed cations. The electrostatic contributions are counteracted by the effect of Pauli repulsion. The small donor-acceptor component of the Mg,,CO bond, caused by a charge redistribution of about 0.1 a.u. in the (T channel from the adsorbate to the surface [11,12], is not decreased in the Mg,-CO and Mg,,-CO complexes as deduced from the calculated adsorption-induced shift of the Mg 1s core level of the ion nearest to the adsorbate. The calculated vibrational parameters for the MgO/CO complexes are shown in table 2. Adsorbate-substrate frequencies of about 200 cm-’ reflect a rather weak interaction. Very small calculated absolute IR intensities of these modes (5 0.5 km/mol) may provide a rationalization why the corresponding peaks have not yet been detected experimentally. The calculated C-O frequencies agree qualitatively with the HF-SCF data computed for the minimal cluster models [lo]; they support an assignment of the high-energy features in the carbonyl region of the IR spectra of CO on polycrystalline MgO to the adsorption on the coordinatively unsaturated cations located in the edge and corner positions [9,291. The “chemical” component of the C-O frequency shift on the low-coordinated Mg,, and Mgdc centers is close to that on Mg,, (- + 17 cm-’ [111X However, the electrostatic contributions to the blue shift Aw(C-0) estimated from the PC-CO models exhibit a strong dependence on the cation coordination: the lower the coordination, the larger the blue shift. A noticeable overestimation of the calculated frequency shift Ao(C-0) should be mentioned in the models with the surrounding point charges of nominal ionic@ & 2.0 a.u. As discussed elsewhere 111,341 this model exaggerates the Madelung field on Mg0(001). The MgO/CO frequency for the clusters embedded in arrays of smaller charges 4 = k 1.5 a.u. (table 2) are in significantly better, certainly fortuitous, agreement with the experiment. Although these results support a model where the ionicity of

cluster investigation

MgO is lower than that described by the nominal charges of the ions, 4 = f2.0 a.u., the value of the charge reduction by 25% is too large and in part compensates deficiencies of the boundary conditions and possibly of the LDF approach. A change in the observables similar to that found in the cases of the point charge reduction will be achieved if the Pauli repulsion at the border between a cluster and its solid environment is taken into account (see section 5). It is worth mentioning here that the rigorous analysis of the LDF charge distribution in MgO(001) slabs, based on the topological atom approach or on the Madelung field generated above a slab, led to effective atomic charges of + 1.6 and + 1.8 a.u., respectively [34]. The accuracy of the computed absolute IR intensity for free CO (table 2) encourages investigations of the intensity for MgO/CO complexes. The calculated IR intensity of the C-O mode for adsorption on MgO(001) [11,12] does not show a decrease relative to the value for gas-phase CO. This change might have been expected from the analysis of the dynamic contribution to the coverage dependence of the C-O frequency shift [29] with the help of the modified Hammaker equation [38]. Arguments have been given elsewhere [ 111 that this indirect procedure underestimates the “experimental” C-O intensity [29]. Here, we demonstrate that an enhancement of the C-O mode intensity also accompanies the adsorption at edge Mg,, and corner Mg,, cations. For clusters of C,, symmetry (fig. 1) the calculated intensity enhancement can be rationalized in terms of u and n- components of the dynamical dipole moment $~/ar(C-o) which coincides here with the electronic dynamical dipole moment (the center of nuclear charges remains fixed when the C-O bond is stretched). In this approach +,/ar and &Jar should be identified with the “oscillating” charge density of (+ and rr orbitals. An increase of +/ar concomitant with the C-O bond elongation corresponds to a displacement of the electronic density “down” to the surface, a decrease indicates an “upward” displacement. This displacement may be caused by field effects, by Pauli repulsion and by charge transfer.

KM. Neyman, N. R&h

/ CO on MgO: a LCGTO-LDF cluster investigation

The dynamical dipole moment of CO is the sum of a small (by the absolute value) u component and a large r component of opposite signs. Inspection of table 2 (PC-CO models) reveals that the electrostatic field causes a decrease of the intensity ICC-0) due to a polarization of the CO u charge directed to the surface. The la orbital of CO at the face and at the edge sites is only slightly affected by the field and the u channel is responsible for the reduction of the absolute value of ap/ar and thus for the smaller intensity. For CO at the corner position, the field attraction of the 1~ orbital electron density (increase of the negative value of +.L,/&) is more pronounced; it leads to a further decrease of ICC-01. In the presence of the substrate atoms (models Mg,O,-CO) which allow additional interaction mechanisms to occur the prevailing effect, responsible for the overall intensity enhancement, is a decrease of the negative ?&Jar component. It is mainly due to the interaction of the CO la orbital with the nearest-neighbor oxygen anions [ll]. The change of the QTcomponent is larger for the models Mg,O,-CO and Mg,O,-CO where the adsorbate interacts more strongly with the neighboring substrate anions. This leads to a net enhancement of the absolute IR C-O intensity

Table 3 IR frequencies and intensities for the MgO/OC Model

o(Mg-CO) (cm-‘)

229

upon adsorption on MgO. The smaller intensity calculated for the Mg,=-CO species may be rationalized as a consequence of the reduced polarization of the CO r system because the surface oxygen ions surrounding the corner Mg,, center are situated at a larger distance from the adsorbate. 3.2. O-end down coordination of the CO adsorbate As can be seen from table 1, the adsorption energies for the Mg,,-OC models are smaller than those for the Mg,,-CO clusters. The main driving force for the oxygen-down CO adsorption is also the electrostatic field. Again, its contribution increases for the low-coordinated cations. Possible reasons for such an unusual orientation of the CO molecule may derive from its very low static dipole moment [36] and from important polarization contributions to the adsorbate-substrate interaction. Indicative for the latter is a significant value of the dipole moment of adsorbed CO (table 1). The Mg-X (X = CO or 00 distances are quite similar for the MgSc-CO and Mg,,-OC models, but they become shorter for the Mg,-OC and Mg,,-OC clusters. The longer C-O distances in the Mg,,-OC models compared to free CO molecule reflect a weakening of

complexes and CO molecule AotC-0) (cm-‘)

a)

ICC-0) (km/mol)

+,/ar (a.u.) b,

apL,/ar (au.) b,

Mg,-OC PC-oc c, Mg,O,-OC Mg,c-OC PC-oc c, Mg,O,-OC Mg,,-OC PC-oc =) Mg,O,-OC oc oc a) b, ‘) d,

122

-18 -60

99 201

- 0.26 -0.17

1.09 1.34

176

-49 -76

133 196

-0.19 -0.18

1.16 1.35

197

-124 -98

201 269

-0.13 -0.12

1.32 1.49

66

-0.37

1.05

2169 d’

Shift of the C-O frequency relative to that of free CO. For a@/&: 1 a.u. = 4.80 D/A. r (surface-O) fixed (see table 1). Reference value for the determination of AotC-0) (see table 2).

230

KM. ~eyma~,

N. R&ch

/ CO on MgO: a LCCTO-LDF

the intra-adsorbate bond and are in line with a lowering of the C-O force constants and vibrational frequencies (table 3). According to these results one should expect to detect an oxygenbound terminal CO molecule on MgO or on similar ionic surfaces by an IR band red-shifted relative to the value of free CO. The calculated shifts of - 60 to - 98 cm-i are probably overestimated in our models just as those for the C-endbound CO (table 2). Nevertheless, the data demonstrate an obvious trend of increasing shifts along the series MgSc-OC + Mg,,-OC + Mg,,OC due to a stronger Coulomb field. Therefore, an IR band shifted to lower energies by several tens of wavenumbers compared to the position for free CO might well be a manifestation of an anomalous carbonyl complex where the CO adsorbate is bound to a cation via its oxygen atom. Favorable experimental conditions for the formation and detection of such complexes should comprise a very low temperature and a strong electrostatic field (e.g. a cation in a coordination as low as possible). The opposite sign of the shift A&C-O) for Mg,,-CO and Mg,,-OC moieties can be rationalized in a similar way (except for a small correction for the MgO/CO charge transfer in Mg,,-CO) as it has been done for the threeatomic clusters Mg2+-CO and Mg2+-OC [171: the electrostatic effect on WCC--0) depends on the slope of the dipole moment curve and the sign of the derivative of the dipole moment is reversed when the orientation of CO with respect to Mg,, is changed (cf. tables 2 and 3). An analysis of the adsorption-induced IR C-O intensity change in terms of oscillating LTand 7 effective charges, similar to that given in the preceding section, may be carried out for the O-end coordinated CO (table 3). Here, in contrast to the Mg,O,-CO systems (table 2), the calculated intensity is the largest for the Mg,,-OC complex with a cation in the lowest coordination. For CO with the O-end down the enhancement is mainly caused by the electrostatic field (cf. the results for the PC-OC clusters) which suppresses the small negative CTcomponent &Jar, but simultaneously enlarges the sizable positive r component +..,/ar. Therefore, the strongest field

cluster ~v~stigat~~

effect in the vicinity of an Mg,, site produces the largest enhancement of the IR intensity. It is interesting to note that for the Mg,O,-OC complexes the u charge transfer, as measured by a Mulliken population analysis, takes place predominantly via the CO4cr orbital. The T charge displacement, also in the direction toward the substrate, is essentially due to changes in the CO 1~ orbital.

4. Effects of surface relaxation The models presented so far correspond to adsorption on the unrelaxed sites of MgO, but a displacement of the surface ions from their positions in the terminated ideal bulk might affect the properties of the adsorption complexes. The problem of relaxation of metal oxide surfaces is discussed in a recent review [27]. According to HF-SCF calculations [lo] on minimal clusters of CO adsorbed on relaxed MgO only minor differences in the geometry and the binding energy should occur compared to the unrelaxed surface. No quantum-chemical study of the relaxation effect on the vibrational characteristics, which are the most sensitive and most easily measured observables, has yet been reported for the MgO/CO complexes. Two different procedures have been applied in the present work to calculate relaxed atomic positions for the perfect (001) face model Mg,O, and for the edge cluster Mg,O,, on the one hand, and for the corner model Mg,O, on the other hand (see fig. 1). In the first case, all top-layer Mg and 0 atoms of the clusters were initially allowed to move by the same amount along the z axis against the lower-layer atoms keeping the point charges fixed (relaxation). Then the z coordinate of the central Mg,, (or Mg,,) atom only was reoptimized (~~~~~~g~. For clusters that include the adsorbate all the top-layer ions and point charges were moved from the terminated bulk positions vertically by the distances calculated in the relaxed substrate models (table 4) for the central cation and the nearest-neighbor anions of this upper layer and then were kept frozen. In the alternative case of the corner site, the MgO,

KM. Neyman, N. R&h

/ CO on MgO: a LCGTO-LDF

Table 4 Calculated outward/inward displacement ‘) of the surface magnesium and oxygen atoms relative to their positions in the terminated bulk MgO; the data obtained with surrounding point charges + 1.5 a.u. are shown in parentheses

M&S% (Mg,,) Mg,O, Mg,O,

(‘Q

d(O) 6)

- 0.09 (0.02) 0.07 (0.11) 0.40 (0.45)

0.01 (0.05) 0.13 (0.14) 0.31 (0.28)

d(Mg)

Model

(Mg,,) (MgA

a) Positive values correspond to inward displacements.

fragment of the Mg,O, cluster was moved first vertically as a whole followed by a reoptimization of the z coordinate of Mg,,. The point charges were kept fixed in the corner clusters both without and with the adsorbed CO. The resulting vertical atomic displacements d(Mg) and d(O) from the terminated bulk MgO positions for the face, edge and corner sites are shown in table 4. These data are in line with the experimental and other theoretical results [27]: five-coordinated atoms on Mg0(001) relax only slightly, but the relaxation increases noticeably for more open Mg sites. In models of the MgO(001) both relaxation and rumpling do not exceed 4% of the first-to-second layer distance, while th,e displacement of the corner Mg,, atom by 0.4 A comprises a significant fraction of the MgO(lll) interlayer separation (1.2 A). For CO adsorption even at the most relaxed Mg,, center the geometric parameters and binding energy are very close to those calculated without substrate relaxation (table 1, cluster Mg,O,R-CO versus Mg,O,-CO), a fact that has been noted previously [lo]. It is an important result of the present study that the Mg-CO and C-O frequencies and IR intensities (table 2) are also rather insensitive to the site relaxation. Still smaller effects than those displayed in the tables 1 and 2 are expected if the substrate cluster geometry were reoptimized taking into account the presence of the adsorbate. Negligible changes of the vibrational parameters due to the surface relaxation are calculated for the clusters Mg,O,-

cluster investigation

231

CO and Mg,O,-CO which represent adsorption at ideal (001) surfaces and at edges. Two conclusions can be drawn from this study of the surface relaxation in MgO/CO. First, an approximation of the unrelaxed substrate is accurate enough for studying the geometrical and vibrational characteristics of adsorbed CO. Second, the rather small influence of the surface site structure on the intramolecular vibrations of the adsorbate will render it difficult to detect a displacement of the surface ions (or a surface roughness) by means of vibrational spectroscopy of probe molecules. We believe that these findings are valid not only for the system MgO/CO, but also for all weakly bound molecular adsorption complexes on other ionic crystal surfaces.

5. Effects of cluster composition on the adsorption properties All models that have been discussed so far contain a stoichiometric (and consequently electroneutral) atomic fragment of the MgO substrate. However, charged cluster models of ionic crystals are also quite common [27]. As a rule, they consist of less atoms than the electroneutral clusters and therefore require less computational effort. The loss of accuracy due to the smaller cluster size usually is not considered to be significant. This is in line with our conclusion that a propagation of electronic effects in good insulators like MgO is limited to very short distances [12]. A tacit assumption underlying the use of a charged cluster model is that uncompensated electronic and nuclear charges have negligible effects on the cluster properties. Also, in this case, one has to ascribe a certain number of electrons to each of the cluster ions, usually based on the nominal ionicity of the substrate atoms. To the best of our knowledge, the last two approximations have not yet been studied in detail for adsorption complexes at ionic surfaces. But their effects are not negligible as we will show in the following discussion. An alternative and rather straightforward way to construct model clusters for metal oxide surfaces has been suggested recently [18]. The idea is

232

KM. Neyman, N. R&h

/ CO on MgO: a LCGTO-LDF

to take into account a major missing component of the Pauli repulsion between the model cluster and the surrounding bulk by requesting that all cluster anions at the “bulk” edges have a complete shell of nearest-neighbor cations which are to be treated either at the all-electron or at a pseudo-potential level. Applications of this scheme have been presented for the adsorption at MgO [181 and at CaO [39]. We carried out LCGTO-LDF calculations for clusters of a corner Mg,, site of C,, symmetry, (MgOi-)-CO and (Mg,0,8’>-CO, in order to examine the outlined technique and its efficiency in accounting for the Pauli repulsion effect on the adsorption characteristics and other cluster properties. The first cluster contains a Mg0,(1,3) moiety carrying a surplus charge of 4 electrons and is embedded in the PC array as described in section 2. (The notation (n,, Q, etc.) indicates n, substrate atoms in the top layer, n2 atoms in the next layer down, etc.) In this cluster, as an artifact, the electrons of the three oxygen atoms in the second layer are strongly polarized by the attraction to the positive point charges beneath them. In order to follow the construction principle outlined above all six third-layer neighbors of the three oxygen anions are taken into account in the second cluster (Mg,Ot+)-CO (substrate moiety Mg,O,(l, 3, 6)). The electroneutrality of these clusters is achieved by means of the embedding point charges. Computed equilibrium distances and spectroscopic and electrostatic properties of the charged clusters are shown in tables 1 and 2. Note, that a change of the calculated observables accompanying the Pauli repulsion between the oxygen anions and the layer of six explicit cations is similar to the tendency observed when the surrounding point charges are decreased from A 2.0 to + 1.5 a.u.: the adsorption bond becomes weaker and the C-O bond strengthens with corresponding changes of the equilibrium distances and the vibrational frequencies. The calculated ionization potential for the cluster MgOi-, 11.17 eV, decreases to 8.44 eV for Mg70t+, a trend just like that found for the Mg,O, cluster model of MgO(001) at q pc = k2.0 and f 1.5 a.u. (10.95 and 9.09 eV, respectively) 1111. An analogous

cluster investigation

tendency for energy level shifts, binding energies and equilibrium distances as a consequence of presence of the outer cations has been pointed out in refs. [18,39], where the calculated data were compared to the only experimental parameter - the energy of CO adsorption on MgO(OO1). No agreement had been found for this observable [181 as discussed in the introduction. We can compare the LCGTO-LDF charged-cluster results also to the known experimental CO stretching frequency shift of 60 cm-’ [9], caused by the adsorption at Mg,, site of MgO (table 2). It is seen that the C-O frequency shift is overestimated for the negatively charged cluster (MgOi->-CO (99 cm -I), underestimated for the positively charged cluster (Mg,O!+)-CO (32 cm-‘) and that the result for the stoichiometric model Mg,O,-CO (85 cm-i) is in between with a slight overestimation. The most interesting result is that, for (Mg,Ot+)-CO, the C-O frequency shift is computed too small compared to experiment. An overbinding between the carbon and oxygen in the adsorbed carbonyl molecule (or an overestimation of Aw(C-0)) might be easily explained either by the peculiarities of the LDF approach or by the exaggerated value of the point charges in the approximation of nominal ionicity (see the discussion in refs. [11,34]). But there is an underestimation of the frequency shift Aw(C0) not observed so far in the LCGTO-LDF investigations [11,12,40], some of which utilized rather extended stoichiometric models of MgO/CO. It is not clear, which of the following two reasons are more important for the large decrease of Au(C-0) by 67 cm-’ when going from (MgOi-)-CO to (Mg,Ot+)-CO models: (i) the fact that the nuclear charge does not compensate the electron charge of the substrate clusters or (ii) an unbalanced description of the Pauli repulsion introduced by properly surrounding only the frontier anions. (The cluster size effect on the C-O frequency is expected to be much smaller than the shifts found in the present work, based on the study of electroneutral MgO/CO clusters [ill.) A detailed analysis of the two likely reasons for deficiency of the charged-cluster models is out of the scope of this investigation. However, it

KM. Nepan,

N. R&h

/ CO on MgO: a LCGTO-LDF

is worth mentioning that a charged cluster model of CO adsorption at Mg0(001), (Mg,,0,(13, 5,1)‘8+)-C0 [181, with all the anionic centers totally surrounded by the nearest-neighbor cations to account for the Pauli repulsion, furnishes at the LCGTO-LDF level of theory a red shift -20 cm-’ of the C-O frequency [40] in disagreement with the experimental blue shift + 14 cm-’ [9] for MgO(OOl)/C-0. On the other hand [40], a large stoichiometric cluster Mg,,0,,(21,5)-CO [121 with optimization of the adsorbate-substrate distance reproduces reasonably well the measured MgO(OOl)/C-0 frequency shift with a usual, but minor overestimation, A&C-O) = + 23 cm- ‘. The present discussion about the merits and the demerits of the charged cluster models of MgO and related materials, with the Pauli repulsion exerted by the frontier anions taken into account, may be summarized as follows. The Pauli repulsion between the cations and anions on the boundaries of the cluster models significantly affects their adsorption and electronic properties. A partial account for this repulsion, e.g. by surrounding only the anions with explicit cations, may lead to an unbalanced description. Inclusion of the peripheral cations in the cluster causes a noticeable distortion of the electronic structure, similar to, but stronger than that found when the value of the external point charges was reduced below the nominal value. The calculated LCGTO-LDF results for the MgO/CO cluster models constructed in this way agree worse or even disagree with the available experimental data for the C-O frequency shift, unlike the data for the extended stoichiometric cluster models. This indicates that the electroneutral models are preferable to the charged ones, at least at the LCGTO-LDF level of approximation. Based on this experience one may suggest the following strategy for designing optimal cluster models of ionic surfaces: (1) choose an extended stoichiometric fragment of the substrate for description at the all-electron level; (2) surround it by all external nearest neighbors represented as frozen ions; (3) embed this electroneutral moiety in an extended array of (reduced) point charges. A similar approach has already been tested for MgO and showed encouraging results [41].

cluster investigation

233

6. Conclusions The present investigation demonstrates that extended stoichiometric LCGTO-LDF cluster models are adequate to describe rather sensitive properties of weakly bound molecular complexes on the surface of magnesium oxide, a typical representative of ionic crystals. The results agree with the interpretation [9] of the blue-shifted maxima in the IR spectra of various Mg,,-CO moieties and predict an adsorption-induced red shift of the C-O frequency for the hypothetical iso-carbonyls Mg,,-OC bounded via the oxygen atom. A noticeable enhancement of the dynamical dipole moment for the C-end-adsorbed CO molecule is also predicted, which, to a significant degree, is related to an adsorption-induced change of the CO lrr orbital. This change in the rr component of the dipole moment derivative is rationalized as a polarization effect. Changes of the CO dynamical dipole moment in the case of the O-end adsorption are attributed to the electrostatic interaction with the substrate. The surface relaxation, which is rather pronounced in the case of three-coordinated corner sites, may nevertheless, to a very good approximation, be neglected in the cluster calculations of all considered characteristics of the adsorption complexes MgO/CO, including the vibrational frequencies and intensities. The results computed for the examined non-stoichiometric models of MgO/CO fit the experimental data significantly worse than those for the models with a stoichiometric composition.

Acknowledgements

Financial support by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Industrie is gratefully acknowledged. K.M.N. also thanks the Alexander von Humboldt Foundation for a research fellowship. References 111T. Ito, J.-X. Wang, C.-H. Lin and J.H. Lunsford, J. Am. Chem. Sot. 107 (1985) 5062.

234

KM. Neyman, N. R&h

121 A.G. Anshits,

[3] [4] [S] [6] [7] [8]

[9] [lo] [ll] [12] [13]

[14] [15]

[16] [17] [18] [19]

/ CO on h&O: a LCGTO-LDF

E.N. Voskresenskaya, V.V. Rivanenkov, V.A. Nasluzov and K.M. Neyman, React. Kinet. Catai. Lett. 46 (1992) 285. M. Anpo, Y. Yamada, S. Coluccia, A. Zecchina and M. Che, J. Chem. Sot. Faraday Trans. I, 85 (1989) 609. A. Zecchina, S. Coluccia, G. Spoto, D. Scarano and L. Marchese, J. Chem. Sot. Faraday Trans. I, 86 (1990) 703. S. Coluccia, A.J. Tenth and R.L. Segal, J. Chem. Sot. Faraday Trans. I, 79 (1979) 1769. E. Garrone, A. Zecchina and F.S. Stone, Philos. Mag. B 42 (1980) 683. W.W. Duley, Philos. Mag. B 49 (1984) 159. A.A. Davydov, Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides (Wiley, Chichester, 1990). D. Scarano, G. Spoto, S. Bordiga, S. Coluccia and A. Zecchina, J. Chem. Sot. Faraday Trans. 88 (1992) 291. G. Pacchioni, T. Minetva and P.S. Bagus, Surf. Sci. 275 (1992),450. K.M. Neyman and N. Rosch, Chem. Phys. 168 (19921267. K.M. Neyman and N. Rosch, Ber. Bunsenges. Phys. Chem. 96 (1992) 1711. N. Rosch, P. Knappe, P. Sandl, A. Gijrling and B.I. Dunlap, in: The Challenge of d and f Electrons. Theory and Computation, Eds. D.R. Salahub and M.C. Zerner, ACS Symp. Ser. 394 (ACS, Washington, DC, 1989) p. 180. B.I. Dunlap and N. Rosch, Adv. Quantum Chem. 21 (1990) 317. N. Rdsch, in: Cluster Models for Surface and Bulk Phenomena, Eds. G. Pacchioni, P.S. Bagus and F. Parmigiani, NATO AS1 Series B, (Plenum, New York, 19921,p. 251. G. Pacchioni, G. Cogliandro and P.S. Bagus, Surf. Sci. 225 (1991) 344. G. Pacchioni, G. Cogliandro and P.S. Bagus, Int. J. Quantum Chem. 42 (1992) 1115. M.A. Nygren, L.G.M. Pettersson, Z. Barandiarin and L. Seijo, to be published. E.A. Paukshtis, R.I. Soltanov and E.N. Yurchenko, React. Kinet. Catal. Lett. 16 (1981) 93.

cluster investigation

[20] C.W. Bauschlicher, Jr., B.H. Lengsfield and B. Liu, J. Chem. Phys. 77 (1982) 4084. [21] S. Huzinaga, Ed., Gaussian Basis Sets (Elsevier, Amsterdam, 1984). [22] F.B. van Duijneveldt, IBM Res. Rep. RJ 945 (1971). [23] H. J&g, N. R&h, J.R. Sabin and B.I. Dunlap, Chem. Phys. Lett. 114 (1985) 529. [24] E.A. Colbourn and WC. Mackrodt, Surf. Sci. 143 (1984) 391. [25] R.W.G. Wyckoff, Crystal Structures, Vol. 1 (Interscience, New York, 1963). [26] R.D. Amos, Adv. Chem. Phys. 67 (1987) 99. [27] E.A. Colbourn, Surf. Sci. Rep. 15 (1992) 281. [28] P. Audibert, M. Sidoumou and J. Suzanne, Surf. Sci. Lett. 273 (1992) L467. [29] E. Escalona Platero, D. Scarano, C. Spot0 and A. Zecchina, Faraday Disc. Chem. Sot. 80 (1985) 183. [30] L. Marchese, S. Coluccia, G. Martra and A. Zecchina, Surf. Sci. 269/270 (1992) 135. [31] C. Pisani, F. Cot%, R. Orlando and R. Nada, Surf. Sci. 282 (1993) 185. [32] M. Causl, R. Dovesi and F. Ricca, Surf. Sci. 280 (19931 1. [33] S.F. Boys and F. Bernardi, Mol. Phys. 19 (1970) 553. [34] N. RGsch, K.M. Neyman and U. Birkenheuer, in: Adsorption on Ordered Surfaces of Ionic Solids and Thin Films, Eds. H.-J. Freund and E. Umbach (Springer, Berlin, 1993). [35] T. Ziegler, Chem. Rev. 91 (1991) 651. [36] K.P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure. Constants of Diatomic Molecules, Vol. 4 (Van Nostrand Reinhold, New York, 1979). [37] M.F. Weisbach and C. Chackerian, Jr., J. Chem. Phys. 59 (1973) 4272. [38] G.D. Mahan and A.A. Lucas, J. Chem. Phys. 68 (1978) 1344. [39] D. Stromberg, Surf. Sci. 275 (1992) 473. [40] K.M. Neyman and N. Rosch, unpublished. [41] V.A. Nasiuzov, G.L. Gutsev, V.V. Rivanenkov, K.M. Neyman and A.G. Anshits, Sov. J. Struct. Chem. 33 (1992) 157.