Bonding and vibrations of CO molecules adsorbed at transition metal impurity sites on the MgO (001) surface. A density functional model cluster study

Chemical Physics 177 (1993) 561-570 North-Holland

Bonding and vibrations of CO molecules adsorbed at transition metal impurity sites on the MgO (00 1) surface. A density functional model cluster study K.M. Neyman * and N. Riisch Lehrstuhlfilr TheoretischeChemie, TUMiinchen, D-85747 Garching,Germany Received 26 May 1993

First principles density functional cluster investigations of adsorption at the (001) surface of pure and doped magnesium oxide are carried out to characterize and compare the interaction of CO molecules with main group (Mg’+ ) and d metal (Coz+, Ni*+, Cu’+) surface cationic centers of the ionic substrate. The geometry of the adsorption complexes, the binding mechanism and spectroscopic manifestations of the surface species are analyzed. Special attention is payed to vibrational frequencies and intensities. The calculations qualitatively reproduce observed trends in the adsorption-induced frequency shifts for the series of the surface aggregates Mg,,-CO+Ni,,-CO+Co5,-CO and the corresponding change of the infrared intensities of the C-O vibrational mode. For the transition metal impurity sites these results are rationalized in terms of a small, but notable Md,-COn interaction.

1. Introduction In the past decade a growing number of investigators have turned their attention to the surface chemistry and physics of ionic solids, especially that of metal oxides. Enhanced interest in this field is related to the discovery of new properties of these materials and of new catalytic reactions on their surfaces [ l-3 1. These experimental and theoretical studies have demonstrated that the chemical bonds and the structure of adsorption complexes at insulating surfaces may differ significantly from those found at metals and semiconductors and that the methods of quantum chemistry provide a useful tool for rationalizing the specific properties of the ionic systems [ 3-

51. The adsorption of CO molecules is commonly used as a probe for detecting Lewis acid sites at various metal oxide surfaces with the help of infrared (IR) spectroscopy [ 61. On the (001) crystal plane of the pure magnesium oxide, CO molecules are found to adsorb reversibly and only at low temperatures [ 79 1. For sintered polycrystalline MgO samples [ 8 ] or l

Corresponding author.

0301-0104/93/$06.00

MgO smoke [ 91, a blue-shift Ao(C-0) of the C-O frequency by 14 cm-’ (relative to its value in the gas phase) is observed in the zero-coverage limit. When the coverage increases the frequency gradually moves to smaller values as a consequence of the lateral adsorbate-adsorbate interactions. The adsorption of CO on low-coordinated Mg cations is accompanied by larger values of Aw( C-O), 27 cm- ’ for the fourfold coordinated Ma, step or edge sites and 60 cm-’ for the Mg,, corner sites [ 8 1. These and other IR observations were recently analyzed [ lo- 16 ] by means of model cluster calculations in the frameworks of the Hartree-Fock selfconsistent field (HF-SCF) method and of the linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) technique. The bonding of CO to the MgO substrate was classified as almost purely electrostatic in the HF-SCF models [ lo- 121 or, at the LDF level of theory, as mainly electrostatic accompanied by a minor, but not negligible o charge rearrangement [ 13- 16 1. The adsorption-induced blue-shift Aw( C-O) of the C-O frequency has been rationalized [ 10,111 as a manifestation of the Pauli repulsion of a CO molecule vibrating against the rigid MgO( 001)’ surface. The LDF results [ 131 indicate

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

562

KM. Neyman, N. R&h /Chemical Physics 177 (1993) MI-570

that the charge rearrangement also contributes to the frequency shift. Both theoretical approaches agree that an increase of the Ao( C-O) value for the more exposed edge and comer adsorption sites of MgO is caused predominantly by a stronger electrostatic field in the vicinity of these centers [ 12,161. A comparative LDF study of the MgO (00 1) /CO complexes for the low-coverage limit and for the monolayer [ 141 led to a new explanation of the static partial red shift of the C-O frequency that occurs for an increasing surface coverage. This red shift is rationalized as a “solvent” effect that is related to the small adsorbateto-substrate o charge redistribution which modifies the electrostatic field on the surface. No transmission of the donated charge through the MgO substrate has been found. It is worth mentioning that all LCGTO-LDF models [ 13-161 predict an enhancement of the absolute IR intensity of the C-O mode (O-+ 1 vibrational transition) upon CO adsorption on MgO. However, the value of the molar IR absorption coefficient for CO on MgO (00 1) derived from the measured change of the C-O dynamic frequency shift with coverage is about half of that of free CO [ 781. Arguments have been given elsewhere [ 13 ] that a probable reason for the disagreement may be connected to the indirect way [ 781 in which this “experimental” molar IR absorption is derived. No HF-SCF results on the adsorption-induced change of the IR intensity of the C-O mode for MgO(001 )/CO have been reported so far. Summarizing the recent HF-SCF [ 10-l 21 and LCGTO-LDF [ 13-161 studies on the adsorption system MgO/CO one notes that in general HF results, compared to those based on the LDF method, tend to ascribe a larger ionicity to the substrates and a smaller covalency to the weak molecular adsorption at their surface. This characteristic feature of the HF-SCF approach might explain a failure [ lo] to reproduce the observed difference between the adsorption complexes MgO(OOl)/CO and NiO(OOl)/CO. In particular, no contribution of the Ni 3d orbitals to the adsorption bonding has been found in the cluster HF models [ lo] (see also ref. [ 17 ] ). This bonding aspect is usually assumed to cause a diversity of the adsorption parameters in oxides of alkaline-earth and of transition metals [ 7,8]. Experimental studies re-

veal a somewhat stronger interaction of the adsorbed CO with NiSc than with Mg,, cations in the corresponding oxides. It is evident from an increased heat of adsorption [ 18,19 1, a lower frequency and a noticeably larger intensity of the C-O mode for N&,CO species compared to Mg,,-CO [ 7 1. A special IR study [ 8 ] of CO adsorbed on the (00 1) faces of sintered polycrystalline MgO and NiO/MgO and COO/ MgO solid solutions, containing 10% of Ni and Co atoms, has been undertaken to eliminate the consequences of a possible difference in the substrate ionicity on the vibrational parameters of the MS,-CO complexes (M = Mg, Ni, Co) and to focus on the role played by the d-rc overlap in the bonding between the transition metal cations and the CO molecules adsorbed on them. The observed difference in the measured C-O stretching frequency and intensity of these systems has been ascribed to a small d-x: overlap contribution which is present in addition to the electrostatic adsorbate-substrate interactions [ 8 1. One of the goals of the present work is to examine if the LCGTO-LDF cluster models are capable of reproducing and rationalizing the observed differences between the vibrational characteristics of the CO adsorption at pure MgO (00 1) and on the transition metal impurity defects. CO molecules are usually considered to be oriented vertically with the carbon atom directed to a surface cation [ 3,7-9 1. However, there is no direct experimental confirmation for this orientation of the CO molecule on MgO and a different structure of the CO monolayer on the similar ionic crystal NaCl(O0 1) surface has been reported [ 20,2 11. A high resolution low-energy electron diffraction (LEED) study of the CO adsorption on MgO (001) [ 22 ] was unable to ascertain either the molecular orientation or the height of the CO molecules above the surface. The LEED data are in agreement with a model of CO molecules standing nearly perpendicular to the surface. The results of cluster model calculations for MgO ( 00 1) /CO [ 10, 1 1,13,14] are in line with the C-down orientation of CO molecules as the computed adsorptioninduced blue shift of the C-O frequency agrees qualitatively with the measured IR data. For the O-down orientation on MgO a red-shift of the C-O frequency has been predicted [ 10, 1 1,13 1. These and other theoretical models support the contention that one should be able to observe the anomalous O-end ori-

KM. Neyman, N. Rikch /Chemical Physics177 (1993) Ml-570

ented CO on ionic surfaces under favourable conditions. These circumstances provide a second motivation for the current study. It seemed interesting to calculate the spectroscopic characteristics for such unusual iso-carbonyl moieties, M-OC, coordinated to d metal cations and to determine how these hypothetical complexes could be distinguished from the common M-CO ones. The present LCGTO-LDF investigation is also aimed at ascertaining how sensitive the vibrational parameters of the M-CO adsorption complexes are to the position of a transition metal impurity in the matrix of MgO( 001). It is unclear whether it is possible to derive information about the substrate structure from the IR studies of adsorbed probe molecules at the oxide materials under consideration.

2. Computational details The fivefold coordinated (M*+ ) 5cadsorption sites (M = Mg, Co, Ni and Cu) on a (00 1) surface of MgO are represented by the stoichiometric cluster models MMg,09 of C,” symmetry (fig. 1). The substrate

Fig. 1.Sketch of the cluster model MMOg,O,-CO (M = Mg, Co, Ni, Cu) for CO adsorption at the fivefold coordinated cationic sites (M2+)sEon MgO(001) surface. The main rotational axis of the substrate cluster coincides with the z direction along which the CO adsorbate is assumed to approach the surface either Cend or O-end down. Only a fraction of 17 16 surrounding point charges (small spheres) is shown.

563

clusters are embedded in the electroneutral array of 17x17x6-18=1716pointcharges(PC)ofq=f2 au located at the atomic positions of the rock-salt crystal to reproduce the Madelung field at the site of the central cation. The lattice parameter is 4.21 8, [ 23 1. If not specified otherwise, the results are displayed for the models which are based on the approximation of an unrelaxed substrate with the impurity cations substituting the (Mg2+)Sc center (for a validation of this approach for the models of CO adsorption on a pure surface of MgO see refs. [ 15,241). In section 3.2 we also consider the CO adsorption on a Cu*+ impurity ion located in its equilibrium position above the (00 1) plane as calculated from the cluster model CuMg,O,. Two of the four conceivable vertical adsorption situations at on-top sites, namely the C-end and Oend binding of CO to cationic centers, are discussed. The distances M-CO and C-O were computed for the complexes where the CO molecule was assumed to be oriented upright along the main (z) axis of the cluster models, either C- or O-end down. First, the M-C (M-O) distance z( M-X) was optimized for a fixed C-O bond length of 1.13 A and then the C-O distance was varied, keeping the CO center 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. The normal modes of Mg(M*+ )O/CO have been approximated by the Mg(M*+)O-CO and C-O internal modes. Equilibrium internuclear distances z= and r,, anharmonic vibrational frequencies (energies of the O--+1 transition between the vibrational levels) w and the minimum of total energy were computed by fitting a polynomial to five (M-CO) or seven (C-O) points of the potential curve located near the minimum. The absolute IR intensities have been calculated in the double-harmonic approach [ 25 ] according to which an IR intensity is proportional to the square of the dynamical dipole moment of the ith normal mode, ap/ar,. The dynamical dipole moment was calculated from a parabolic approximation defined by three points near the equilibrium. The LCGTO-LDF cluster method [ 26-281 is employed in the Xa approximation ((Y= 0.7 ). This approach has proven adequate for the description of the various MgO/CO adsorption complexes [ 13- 16 1. To

K.M. Neyman, N. R&h / Chemical Physics 177 (I 993) 561-5 70

564

contracted (Mg ( 13s8p3d) -, [ 7s4pld] and 0 (lls7pld)+ [6s4pld]) [13].Fortheadsorbateatoms C and 0, basis sets of the type (9s5pld) -+ [ 7s4p Id] [ 321 were used with d exponents taken from ref. [ 3 11. It should be recalled that the LCGTOLDF results for the free CO molecule obtained with these basis sets are in good agreement with experimental data as are those for the diatomic MgO [ 13 1. The correct sign and value of the CO electric dipole moment and its displacement derivative are of special importance for assessing the accuracy of the calculated C-O vibrational intensity in the present cluster models. The orbital basis sets ( 14s9pSd) for Co [ 331, Ni and Cu [ 341 were extended each by one d exponent [ 351, one s-exponent (Co: 0.321752; Ni: 0.3460; Cu: 0.3305) and by two p-exponents (Co: 0.2603, 0.1084; Ni: 0.2640, 0.1040; Cu: 0.2650, 0.0991). They were contracted to [6s5p3d] using LDF atomic eigenvectors [ 361. The two auxiliary basis sets used in the LCGTO-LDF method to represent the electron charge density and the exchangecorrelation potential were constructed from the orbital exponents in a standard fashion [ 27,371.

probe any dependence of the results on the form of the exchange-correlation potential, we also utilized a more elaborate functional (VWN) [ 291 (see section 3.1). Spin polarization has been taken into account for the open-shell models of the substitutional defect sites of MgO. The following electronic configurations correspond to the ground states of the adsorption models VII-IX (tables 1 and 2): I-V and 27a: 4a: 1Ob: 1Ob: 23e4 VII); (Mg so: I, 30a] 4a: 1 lb] 1 lb:25e4 (Ni di2d~2_y2d&,d~Zd&: II, VIII); 30af4a: 1 lb; 1 lb:25e4 (Co d$di2_,,2di,,d&d&: III, IX); 30at 4a: 1 lb: 1 lb:25e4 (Cu df2d$_,2d&,d~Zd&: IV, V). Thus they may be classified as singlet, triplet, quartet and doublet, reThe spectively. electronic configuration 30aT4a: 1 lb] 1 lb:25e4 VI, cu (model d$d:l_,j?d&,d&d$) corresponds to the lowest excited state of this impurity cluster. The construction of the orbital basis for MgO started from basis sets [ 111 which had been optimized for Mg+ and O- ions. These basis sets were extended by d functions (Mg: 1.1765, 0.36891, 0.14371 [30]; 0: 1.154 [31]) and were slightly less

Table 1 Calculated and experimental [ 81 (in parentheses) parameters for MgO ( Mz+ ) /CO complexes and for the CO molecule: equilibrium height z, of the nearest adsorbate atom above the central cation at the surface, C-O distance r,, energy w(M-CO) of the substrateadsorbate stretching fundamental vibrational transition, shift Aw(C-0) of the C-O vibrational frequency (with respect to the value calculated for the CO molecule) and the corresponding absolute IR intensity I(C-0), induced dipole moment By= p(M.O,CO) -p(M.O.) -p(CO), derivatives of the dipole moment apc/ilz and ap/ar along with the o and I[ components a&&- and &/ar Model

I II III IV V VI

Mg,O,-CO NiMg,Og-CO CoMgsO,-CO CuMgsOs-CO CuMg*O,-CO CuMgsOs-CO

=) d,

z, (A)

w(M-CO)

ap/az=)

r,

Au(C-0)

(cm-‘)

(au)

(A)

(cm-‘)

2.22 1.88 1.89 1.85 1.82 2.03

183 327 333 342 352 237

0.12 0.41 0.44 0.28 0.39 0.23

1.124 1.126 1.127 1.125 1.124 1.125

co VII VIII IX

Mg,O,-OC NtMg,O,-OC CoMgsO,-OC oc

2.19 2.06 2.10

122 202 191

-0.17 -0.19 -0.11

54 (14) 45 (1) 38 (-25) 60 58 50

1.130

2169”

1.137 1.136 1.137

-60 -42 -57

1.130

2169”

I(C-0) (km/mol)

ap/ara’

ap_b/a+

ak,larQ

AP

(au)

(au)

(au)

(D)

124 (1 b)) 283 (2 b)) 330 (4b’) 220 262 186

-0.93 - 1.41 - 1.52 -1.24 - 1.36 -1.14

0.50 0.72 0.73 0.75 0.70 0.51

- 1.42 -2.15 -2.27 -2.01 -2.06 -1.63

1.19 1.67 1.22 0.84 0.44 1.35

-0.68

0.37

-1.05

0.26 f,

66

(-60)

201 206 153

1.18 1.20 1.04

-0.17 0.09 0.03

1.34 1.10 1.00

66

0.68

-0.37

1.05

a) For ap/ar: 1 auz4.80 D/A. b, Experimental IR intensity of the C-O mode relative to that in the MgO/CO complex. ‘) Cu2+ in the relaxed position 0.24 8, above the (001) plane. d, Excited state of the cluster IV (see section 2). ‘) Reference value for the determination ofAo(C-0). f, Dipole moment for free CO or OC.

1.44 1.92 1.15 -0.26

f,

565

KM. Neyman. N. R&h /Chemical Physics177 (1993)561-570

Table 2 Parameters for MgO ( M2+ )/CO complexes and for the free CO molecule calculated with the Xa! and VWN [ 291 exchange-correlation potential: equilibrium height z. of the nearest adsorbate atom above the surface, C-O distance r, energy w(M-CO) of the substrateadsorbate stretching fundamental vibrational transition, shift &(C-0) of the C-O vibrational frequency (with respect to the value calculated for the CO molecule) and the corresponding absolute IR intensity Z(C-0), induced dipole moment A+p(M,O”CO) - p(M.0.) - JI( CO), derivatives of the dipole moment +/i_t.z and +/ar along with the o and I( components &/fIr and +,Jar Exchangecorrelation

.z.

McO,-CO M&O%-CO NiMg,O,-CO NiMgsOg-CO

Xa VWN Xa VWN

2.22 2.18 1.88 1.86

co co

xo VWN

-

Model I I II II

U(M-CO)

(A) (cm-‘) 183 221 327 342

ap/a2’)

r,

(au)

(A)

0.12 0.10 0.41 0.43

1.124 1.124 1.126 1.128

-

Aw(C-0)

(cm-‘) 54 60 45 46

1.130 2169b’ 1.131 2174b’

Z(C-0) (hm/mol)

ajiiar.1

a.4Jar.j

apJar.1

dp

(au)

(au)

(au)

(D)

124 135 283 298

-0.93 -0.97 -1.41 -1.45

0.50 0.50 0.72 0.74

- 1.42 - 1.46 -2.15 -2.20

1.19 1.12 1.67 1.62

66 67

-0.68 -0.69

0.37 0.37

-1.05 -1.06

0.26 =’ 0.25 ”

a1 For aplar: 1 auz4.80 D/A. b, Reference value for the determination of Aw( C-O). c, Dipole moment for free CO.

3. Adsorption of CO molecules at transition metal

impurity sites on MgO(OO1) 3.1. Adsorbed CO at Ni2+ and Co’+ impurities The LDF results for the various cluster models obtained with Xa potential are given in table 1. Let us start with the C-end down adsorption and compare the data for the NiSc and Cost sites to those for the Mg,, one (models II and III versus I ). Shorter Ni-CO, Co-CO distances and higher frequencies o( Ni-CO), o( Co-CO) indicate a stronger interaction of CO with the nickel and cobalt sites than with a magnesium site. The charge rearrangement between CO and the transition metal cations is more intense than that in model I for Mg’+. This can be seen both from the increase of the dynamical dipole moment ap/az (a measure of the total charge moved during a vertical displacement of CO against the substrate) and from the electron density difference plots in figs. 2-4. For the transition metal ions it seems to be a classical charge transfer of o and SCtype where the CTchannel dominates in magnitude. Mulliken populations, which can be taken only for a very rough evaluation of the charge redistribution, indicate that the partial occupation of the antibonding CO 27~~orbital is accompanied by a notable depletion of the CO lx orbital. Both changes are of the order of 0.1 au with a net increase of the CO II population by about 0.05-o. 1 au; the overall decrease of the CO o popu-

lation can be estimated to be 0.3-0.4 au. The reduction of the C-O frequency by 9 and 16 cm-’ in the clusters NiMg,Og-CO, CoMgaOg-CO (II, III), respectively, relative to that of Mg,09-CO (I ) is mainly due to the K interaction. These differences are in line with the values of 13 and 39 cm- ’ measured for the solid solutions [ 8 1. They should be compared to those detected for CO on pure NiO( 001) and on CoO(OOl), 5 and 21 cm-’ [S]. The agreement of the observed and calculated frequencies is reasonable, keeping in mind the rather small size of the frequency shifts and the model character of the present study (see also ref. [ 13 ] ) . A further argument in favour of a non-negligible K contribution to the N&CO and Co,,-CO bonds, an intrinsic feature of d metal complexes, can be gleaned from the intensity I( C-O) of the C-O mode. The intensity enhancement in the models II and III compared to I is calculated in good agreement with experiment [ 8 ] (see table 1). The computed variation of the intensity may be rationalized in terms of the o and n: components of the dynamical dipole moment ap/ar( C-O) which coincides here with the electron dynamical dipole moment. (The center of nuclear charges remains fixed when the C-O bond is stretched.) In this approach a,u,lar and a&& should be identified with the “oscillating” charge density of the o and n: orbitals. An increase of the dynamical dipole moment as a consequence of the C-O bond elongation corre-

566

K.M. Neyman, N. Rdsch /Chemical Physics 177 (1993) 561-570

b

Fig. 2. Contour maps of the electron density difference Ap=p(Mg909-CO) -p( MgsOs) --&CO) in the yz plane which contains Mg2+, 02-, C and 0 centers (black circles): (a) contribution of o orbitals; (b) contribution of n orbitals. The contour values are ? 3.16 x 10e4, k lo-‘, + 3.16~ lo-‘, 5 lo-‘, * 3.16~ lO-2, + 1.0 e/bohr-‘. Dashed lines indicate negative values.

b

Fig. 3. Contour maps of the electron density difference &=p(NiMgaOs-CO) -p(NiMg,09) -p(CO) in the yz plane which contains Ni’+, Mg’+, O*-, C and 0 centers (black circles): (a) contribution of o orbitals; (b) contribution of z orbitals. The contour values are the same as in fig. 2.

sponds to a displacement of the electronic density “down” to the surface, a decrease indicates an “upward” displacement away from the surface. The mechanism of the intensity enhancement is similar for all considered carbonyl complexes with the d metal impurities. A larger o donation to the substrate increases the small positive ap.Jar component, but the back donation into the A channel has obviously a stronger effect on the intensity, decreasing the large absolute value of the negative ap,Jar component for

NiMg,09-CO (and CoMg,O,-CO ) by a factor of 1.5 (1.6) compared to that in the Mg,09-CO model. As can be seen from table 1, the computed intensities I(C-0) reasonably fit the experimental trend [ 81, predicting however a significantly smaller difference between the nickel and the cobalt complexes. At first glance this appears to be a substantial inconsistency which warrants further discussion. The experimental intensities in table 1 [ 81 are in fact the integrals of the IR peaks which are not nor-

K.M. Neyman, N. Riisch /Chemical Physics I77 (1993) 561-570

561

b

Fig. 4. Contour maps of the electron density difference &=p(CoMg,09-CO) -p(CoMg,O,) -p(CO) in the yz plane which contains Co*+, Mg2+1O*- >C and 0 centers (black circles): (a) contribution of o orbitals; (b) contribution of n orbitals. The contour values are the same as in fig. 2.

malized to the actual number of vibrating species, in spite of an equal concentration of the impurities in the various samples. The adsorbate-adsorbate repulsion restricts the maximum coverage of CO on MgO (00 1) to 1CO : 2Mg2+ [ 7 1, i.e. on the average all the nearest neighbour cationic positions are blocked for adsorption, but the next-nearest neighbours are free to take up further adsorbates. For the solid solutions with two kinds of the cations, a direct proportionality between the intensities and the oscillator strength of the MO (00 1 )/C-O vibrations will hold only in the case of a statistical distribution of the adsorbed molecules among the two types of centers [ 81. If a preferential adsorption at the minority d metal cations took place, up to twice of the amount of these specific oscillators may be present depending on how different the sticking probabilities are. To derive a value of the oscillator strength one has to normalize the measured IR peak integral with respect to the number of the impurity complexes which is unknown in general. These considerations allow us to suggest that at equal conditions the scaling factor, i.e. the number of the Co2+-CO moieties, should be somewhat larger than that of the Ni2+-CO complexes, because the former aggregates are found to be more stable than the latter ones [ 8 1. Therefore, one may expect that after a correction as outlined above the “true” intensities I( C-O) measured for the Ni2+CO and Co2+-CO complexes will differ relative to

each other by less than a factor of two [ 81 (table 1 ), in a better agreement with the calculated quantity of 1.2. The present density functional results are to some extent at variance with those of previous HF calculations [ lo]. There, essentially no difference was found for CO adsorption at the Nisc and the Mg,, oxide centers. However, important experimental evidence [8] of specific interactions with the d metal sites are qualitatively reproduced and rationalized at the LCGTO-LDF level of theory. Further investigations should be carried out to explain why the HF and LDF cluster results for CO adsorption on alkalineearth and d metal oxides exhibit such notable differences. In this context we should add that the computed LDF observables are not affected significantly by the type of the exchange-correlation approximation, Xa or VWN (table 2). One may wonder about adsorption energies for the carbonyl complexes formed at the impurity sites. Although the computed adsorption energies reflect the tendency of a CO molecule to bind stronger to the impurities ( 1.66 eV Xcr, 1.82 eV VWN for N&,-CO) than to the regular Mg cations (0.97 eV Xcy, 1.09 eV VWN ) [ 13,16 ] the values themselves are markedly exaggerated, not only for the d metal sites, but for all MgO/CO species considered so far [ 13-l 6 1. One should recall that several corrections have to be applied to the LDF values for the adsorption energy be-

568

KM. Neyman, N. R&h /Chemical Physics 177 (1993) 561-570

fore they may be compared to the experimental data: corrections (i) for the basis set superposition error [ 381; (ii) for the effect of the reduced substrate ionicity below the nominal value Mg*+-O*-, according to which the surrounding atomic cluster point charges should be chosen smaller than + 2 au [ 13,16 ] ; (iii) for deficiencies of the local density approximation [ 391. Each of the contributions may reduce the adsorption energy by some tenths of an electron volt, while leaving the other adsorption-induced calculated parameters essentially unchanged [ 131. Further studies of all these corrections at a rather high level of accuracy are necessary before the adsorption energy can be reliably estimated within the present theoretical approach. 3.2. Adsorption at Cuz+ impurity: effect of electronic excitation and substrate relaxation A the characteristic feature of very Mg( Cu*+ )0( 00 1) system is the location of the impurity ion well above the top crystal plane [ 40,4 11. It may be compared to a negligibly small displacement from this plane (about 0.0 1 A) computed for the Ni*+ impurity. In this respect it is interesting to examine the sensitivity of the vibrational parameters of the probe CO adsorbed on the Cu*+ to the position of the impurity center. To this end, we compare the idealized geometry of a copper defect at the (00 1) surface (table 1, model IV) to model V where a vertical relaxation of the impurity is taken into account for the substrate cluster. The dependence of the vibrational and other observables on the electronic state of the surface carbonyls is also illustrated by the copper complexes (table 1, models IV and VI ). In agreement with the data of the previous LDF investigations for the CO adsorption at various sites of pure MgO microcrystallites [ 16,241, even such sensitive parameters as the vibrational properties of the adsorbed CO are affected only slightly by a significant vertical displacement (relaxation by 0.24 A) of the Cu cation in the MgO matrix (cf. models IV and V). The largest, but still rather moderate differences are found for the intensity I( C-O) (or the dynamical dipole moment i&/ar) and for the induced dipole moment A,u. The changes of both parameters may be rationalized by the variation of the electrostatic field in the vicinity of the CO moiety when the center of

coordination is located at different heights above the (00 1) plane. Therefore, the conclusion that a relaxation of the surface cations of MgO can hardly be detected by means of IR studies of the probe molecules [ 16,241, is also valid for the transition metal impurity sites. The results in table 1 demonstrate that a variation of the electronic configuration from the ground state (model IV) to the low-lying excited state (model VI) does not have a pronounced effect. Both electronic configurations are doublets (section 2)) but the latter is about 0.3 eV higher in energy. The excitation is accompanied by a weakening of the adsorbate-substrate bond and results in a slightly longer Cu-C distance and a smaller Cu-CO frequency. No principal difference is found for the copper carbony1 complexes compared to the nickel or cobalt systems as far as the Md,-COrt overlap contribution to the adsorption bond is concerned. For the Cu-CO species this interaction causes an enhancement of the computed intensity I( C-O) which is comparable to that already discussed for the Ni and Co defects; it may be rationalized similarly. Unfortunately, we have not found any experimental data for a comparison of the calculated vibrational characteristics for CO adsorbed on Cu*+ impurities of MgO (001) 3.3. Iso-carbonyl species on d metal impurities In the IR spectra of CO on the solid solutions NiO/ MgO and CoO/MgO [ 8 ] one finds several characteristic peaks around 2 100-2060 cm-‘, some ofwhich may well be a manifestation of CO adsorbed O-down at NiSo CoSc or other Ni and Co sites. Most maxima were assigned to ketenic products of CO reactions with coordinatively unsaturated oxygen anions [ 8 1. The stretching mode of the adsorbed “C-0 isotope also contributes to this spectral region. The most probable candidate to be identified with a vibration of an iso-carbonyl species bound via the oxygen atom is a rather intensive peak at 2083 cm-’ in the IR spectra of the CoO/MgO solid solution samples. To predict vibrational parameters of iso-carbonyl species bound to a d metal cation in oxide matrices we investigated the models NiMg,09-OC (VIII) and CoMg,09-OC (IX ) as representative examples. One has to point out, however, that the experimental conditions (foremost the quality of the sintered poly-

KM. Neyman, N. R&h /Chemical PhysicsI77 (1993) 561470

crystalline samples [ 8 ] ) are not sufficiently well defined and the structure of the relevant region of the IR spectra is very complicated [ 8 1. Thus, without a thorough computational analysis of alternative structures like adsorbates Iying flat, surface ketenic species, etc., any conclusions concerning the presence of the O-end down CO will only be of preliminary nature. In table 1 the results for the models VIII and IX are compared to those of the Mg’+-OC cluster (model VII). These complexes exhibit only moderate differences. The most important finding is that the C-O vibration is predicted to be red shifted relative to that of free CO for the OC adsorption at the transition metal cations, just like that for the main group Mg2+ ion [ 13,15,24]. The frequency shift Ao(C-0) = - 57 cm-’ calculated for model IX fits well the measured position of the IR peak at 2083 cm-‘,Aw(C-O)=-60cm-* [8], thusprovidinga strong argument in favour of the Co’+-OC origin of this experimental feature. An analysis of the adsorption-induced IR intensity change I( C-O) in terms of oscillating o and A effective charges, similar to that given above for CO in the usual coordination, may be carried out for the O-end coordinated CO. Here, the intensity enhancement is mainly caused by the electrostatic field (cf. the results in ref. [ 131) which simultaneously enlarges both the small negative o component a,u,/ar and the sizeable positive ICcomponent &u&b-. The field effect on the ap,/ar component is compensated to a certain extent by a small x charge redistribution from the d metal impurities to the adsorbate. It is interesting to note that for the MO-OC complexes the o charge transfer, as measured by a Mulliken population analysis, takes place predominantly via the CO 40 orbital.

569

CO Ni2+-CO and Co’+-CO. The calculated data sho& a notable Md,-COn component of the adsorbate-substrate interaction in the impurity situations. This contribution plays a major role in the explanation of various vibrational parameters for a CO probe adsorbed at the main group, Mg’+, and at d metal cations of the oxide matrix surface. A displacement of the impurity cation from the position of a regular Mg2+ center (relaxation) causes only a rather small effect on the frequency and the intensity of adsorbed CO. Thus it will be difftcult to obtain information on the quality of ionic surfaces via IR spectroscopy of probe molecules. Unusual iso-carbonyl M-OC species are expected to show up through a C-O frequency red shifted by about 50 cm-‘. One of the features of the IR spectrum of the polycrystalline Mg(Co’+)O/CO sample, namely a peak at 2083 cm-’ [ 81, is a likely candidate for assignment to the iso-carbonyl adsorption complex at a Co2+ center with a CO ligand coordinated via the oxygen end. However, more experimental and computational efforts are necessary for a confirmation of this hypothesis.

Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft (SFB 338 ) and by the Fonds der Chemischen Industrie is gratefully acknowledged. KMN also thanks the Alexander von Humboldt Foundation for a research fellowship.

References [ 1] H.H. Kung, Transition metal oxides: surface chemistry and

4. Conclusion The present study demonstrates that the LCGTOLDF method is adequate to describe very sensitive parameters of weakly bound adsorption complexes of CO molecules with d metal impurity cations at the MgO( 001) surface. The observed trends in the adsorption-induced frequency shift, Ao(C-0), and in the IR intensity, I(C-0), are qualitatively reproduced by the cluster models for the moieties Mg2+-

catalysis (Elsevier, Amsterdam, 1989). [2] J.S. Lee and ST. Oyama, Catal. Rev. Sci. Eng. 30 (1988) 249. [ 31 E.A. Colbourn, Surface Sci. Rept. 15 ( 1992) 28 1. [4] J. Sauer, Chem. Rev. 89 (1989) 199. [ 51H.-J. Freund and E. Umbach, eds., Adsorption on ordered surfaces of ionic solids and thin films (Springer, Berlin), in press. [ 61 A.A. Davydov, Infrared spectroscopy of adsorbed species on the surface of transition metal oxides (Wiley, New York, 1990). [ 7 ] E. Escalona Platero, D. Scarano, G. Spot0 and A. Zecchina, Faraday Discussions Chem. Sot. 80 ( 1985) 183.

570

K.M. Neyman. N. R&h /Chemical Physics 177 (1993) 561-570

[8] D. Scarano, G. Spoto, S. Bordiga, S. Coluccia and A. Zecchina, J. Chem. Sot. Faraday Trans. 88 ( 1992) 29 1. [9]L. Marchese, S. Coluccia, Cl. Martra and A. Zecchina, Surface Sci. 269/270 (1992) 135. [lo] Cl. Pacchioni, G. Cogliandro and P.S. Bagus, Surface Sci. 225 (1991) 344. [ 111 Cl. Pacchioni, G. Cogliandro and P.S. Bagus, Intern. J. Quantum Chem. 42 ( 1992) 1115. [ 121 G. Pacchioni, T. Minerva and P.S. Bagus, Surface Sci. 275 (1992) 450. [ 131 KM. Neyman andN. R&h, Chem. Phys. 168 (1992) 267. [ 14 ] KM. Neyman and N. R&ch, Ber. Bunsenges. Physik. Chem. 96 (1992) 1711. [ 151 KM. Neyman and N. R&ch, J. Mol. Struct. 293 (1993) 303. [ 161 N. Rosch, KM. Neyman and U. Birkenheuer, in: Adsorption on ordered surfaces of ionic solids and thin films, eds. H.-J. Freund and E. Umbach (Springer, Berlin), in press. [ 171 M. Piihlchen and V. Staemmler, J. Chem. Phys. 97 (1992) 2583. [ 181 E.A. Paukshtis, RI. Soltanov and E.N. Yurchenko, React. Kinet. Catal. Letters 16 (1981)93. [ 191 K. Klier, Collect. Czech. Chem. Commun. 28 (1963) 2996. [20] J. Heidberg, E. Kampshoff and M. Suhren, J. Chem. Phys. 95 (1991) 9408. [21] D. Schmicker, J.P. Toennies, R. Vollmer and H. Weiss, J. Chem. Phys. 95 (1991) 9412. [22] P. Audibert, M. Sidoumou and J. Suzanne, Surf. Sci. Letters 273 (1992) L467. [23] R.W.G. Wyckoff, Crystal structures, Vol. 1 (Interscience, New York, 1963).

[24] K.M. Neyman andN. Riisch, Surface Sci. 297 (1993) 223. [25] R.D. Amos, Advan. Chem. Phys. 67 (1987) 99. [ 261 N. R&h, P. Knappe, P. Sandl, A. Giirling and B.I. Dunlap, in: The challenge of dand f electrons. Theory and computation, ACS Symp. Ser., Vol. 394, eds. D.R.Salahub and M.C. Zemer (Am. Chem. Sot., Washington, 1989) p. 180. [27] B.I. Dunlap and N. Rosch, Advan. Quantum Chem. 21 (1990) 317. [28] N. Rissch, in: Cluster models for surface and bulk phenomena, NATO AS1Ser. B, eds. G. Pacchioni, P.S. Bagus and F. Parmigiani (Plenum Press, New York, 1992) p. 25 1. [ 291 S.H. Vosko, L. Wilk and M. Nusair, Can. J. Phys. 58 ( 1980) 1200. [ 301 C.W. Bauschlicher Jr., B.H. Lengsfield and B. Liu, J. Chem. Phys. 77 ( 1982) 4084. [ 3 1 ] S. Huzinaga, ed., Gaussian basis sets (Elsevier, Amsterdam, 1984). [ 321 F.B. van Duijneveldt, IBM Res. Rept. No. RJ 945 (1971). [33] A.K. Rappe, T.A. Smedley and W.A. Goddard, J. Phys. Chem. 85 (1981) 2607. [34] A.J.H. Wachters, J. Chem. Phys. 52 (1970) 1033. [35] P.J. Hay, J. Chem. Phys. 66 (1977) 4377. [36] R.C. Raffenetti, J. Chem. Phys. 58 (1973) 4452. [ 37 ] H. J&g, N. Riisch, J.R. Sabin and B.I. Dunlap, Chem. Phys. Letters 114 (1985) 529. [ 381 S.F. Boys and F. Bemardi, Mol. Phys. 19 ( 1970) 553. [39]T.Ziegler,Chem.Rev.91 (1991)651. 1401 E.A. Colboum and W.C. Mackrodt, Solid State Ion. 8 (1983) 221. [41] N.C. Bacalis and A.B. Kunz, Phys. Rev. B 32 (1985) 4857.