Quantum chemical study of carbon monoxide adsorption at the MgO(100) surfaceKarl Jug and Gerald Geudtner Citation: The Journal of Chemical Physics 105, 5285 (1996); doi: 10.1063/1.472368 View online: http://dx.doi.org/10.1063/1.472368 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/105/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mixed quantum/classical simulation of the photolysis of HCl on MgO(001) J. Chem. Phys. 105, 11347 (1996); 10.1063/1.472924 Comparing ab initio computed energetics with thermal experiments in surface science: CO/MgO(001) J. Chem. Phys. 105, 9339 (1996); 10.1063/1.472724 Ammonia adsorption on MgO(100): A density functional theory study J. Chem. Phys. 105, 7753 (1996); 10.1063/1.472558 Embedding procedure for cluster calculations of ionic crystals J. Chem. Phys. 105, 6395 (1996); 10.1063/1.472492 Supported nickel and copper clusters on MgO(100): A firstprinciples calculation on the metal/oxide interface J. Chem. Phys. 104, 7329 (1996); 10.1063/1.471400

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Quantum chemical study of carbon monoxide adsorptionat the MgO(100) surface

Karl Jug and Gerald GeudtnerTheoretische Chemie, Universita¨t Hannover, Am Kleinen Felde 30, 30167 Hannover, Germany

~Received 20 February 1996; accepted 21 June 1996!

Semiempirical molecular orbital calculations are performed to study the physisorption of carbonmonoxide molecules at the MgO~100! surface. This surface is simulated by clusters of differentsurface and layer sizes. Besides the submonolayer coverage four different types of monolayers areinvestigated. The stability and structure of the adsorbate system are determined. An adsorptionenergy per unit cell of the substrate is defined for a comparison of the stability of differentmonolayers with varying coverages. Agreement is found between the theoretical and experimentaloverlayer structure. ©1996 American Institute of Physics.@S0021-9606~96!02536-6#

I. INTRODUCTION

Studies of adsorption of small molecules on ordered sur-faces of ionic solids have become a popular subject of in-creasing importance. This is manifested in a book edited byFreund and Umbach1 which contains a variety of theoreticaland experimental articles on this subject based on the pro-ceedings of a conference. Among the systems which are re-peatedly mentioned is the CO adsorption on an MgO surface.Experimental investigations allow to determine the overlayerstructure of a monolayer of CO molecules2 or use CO as auseful probe for the nature of the coordination site~acidstrength!.3 Theoretical studies have focused on the changesof frequency of the adsorbed CO molecule4 and on the bond-ing as an electrostatic effect.5 It predicted that for the bond-ing of CO via the C atom a blueshift should observed,whereas the bonding via the O atom should yield aredshift.4,6,7 This can be explained as an effect of the inter-action of the positive charge of the Mg atom with the dipoleof the CO molecule. From diffusion experiments for COmolecules on a clean MgO surface covered with Pd particlesan adsorption energy between 6.9 to 9.2 kcal/mol was esti-mated via a kinetic model.8 More specific information on themonolayer structure could be obtained from low energy elec-tron diffraction ~LEED!9 and polarization infrared~PIR!10

spectroscopy. The adsorption of CO molecules on MgO canlead to a variety of structures due to the competition betweenadsorbate–substrate and adsorbate–adsorbate interaction.LEED pattern exhibit a~432! structure atT539 K and a~332! structure atT545 K. These measurements weresupplemented with semiclassical calculations. Gerlachet al.11 found ac(4 3 2) structure with helium atom scatter-ing ~HAS!. The same structure was deduced from the PIRexperiments.10 From both the LEED experiments and thePIR experiments it was concluded that the (432) unit cellcontains six CO molecules with different orientations. In theformer case the authors assume that two molecules are per-pendicular and two are parallel to the surface, whereas theother two are tilted. In the latter case the authors ruled outthe parallel orientation from the number of different frequen-cies detected fors and p polarization and postulated fourtilted CO molecules instead.

In the present work we wish to contribute to this discus-sion by semiempirical SINDO112–14 calculations on the sta-bility of various adsorbate structures. We determine orienta-tion and adsorption energy of submonolayer and monolayercoverage by cluster model studies.

II. SUBMONOLAYER COVERAGE

Ion crystal clusters have structures very similar to thecuboidal form of bulk fragments. They are therefore particu-larly suitable for adsorption studies, because surface andlayer size can be varied in a very simple way. In order tostudy the properties of clusters in comparison to the bulk, wehave recently introduced a relative average coordinationnumberk for atoms in ion crystal clusters.15 For each atomthe number of formal bonds is counted and the average num-ber of bonds in the cluster is calculated. This number isdivided by the number of formal bonds in the bulk. For ex-ample, the cube Mg4O4 has four Mg and O atoms with co-ordination number 3. Hencek 5 0.5 in this case. To reachk5 0.9 one needs a~10310310! cube of 1000 atoms, andonly an infinite number of atoms with six neighbors wouldyield k51. We could show that a linear relationship existsfor MgO clusters between the binding energy per molecularunit and the coordination numberk. Such a relationship wasalso found for the bond distance of an MgO bond. This re-lationship allows an extrapolation from the clusters to thebulk. Fork 5 1 we would find 2.122 Å instead of the experi-mental 2.105 Å for the Mg–O distance in the bulk. Clusterswith structures close to cubes are most suitable. We havetherefore chosen a~53534! Mg50O50 cluster with an opti-mized distance of 2.021 Å for the study of submonolayeradsorption. This cluster has a large enough surface and layersize so that border effects are avoided. The CO molecule wasplaced above the central Mg atom and its geometry was op-timized. Both C adsorption and O adsorption were consid-ered. In both cases the molecule was oriented perpendicularto the surface on top of an Mg atom. The C adsorption ismore stable with an adsorption energy of 8.1 kcal/mol, com-pared to 5.0 kcal/mol for the O adsorption. The value for theC adsorption is within the range of the experimental estimateof 6.9–9.2 kcal/mol.8 If we use the scaling procedure, which

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we recently introduced for SINDO1 calculation of adsorbedmolecules,16 for the adjustment of the SINDO1 frequency ofthe CO vibration of the free molecule to the experimentalvalue, we find a blue shift of 11 cm21 for the C adsorptionand a redshift of 26 cm21 for the O adsorption. The calcu-lated C–O distance for the free molecule is 1.142 Å17 com-pared to the experimental 1.128 Å. The unscaled CO dipolemoment of 0.53 D17 and the scaled dipole moment of 0.13D18 are in the same direction as the experimental dipole mo-ment of 0.11 D with the negative end of the dipole at thecarbon atom.

To study the effect of surface buckling on the adsorptionenergy, we let the planes of the O atoms relax with respect toplanes of the Mg atoms for all planes from the surface planedown to the bottom of the cluster. Only the bottom plane wasnot relaxed. This procedure resulted in a buckling of the Mgatoms out of the plane, different from the results from ex-periments where the O atom was buckled out.19–21 Fortu-nately, the effect on the geometry is relatively small amount-ing to 1%–2% of the bond distance. The energy change ofthe adsorption energy due to buckling is negligible with 0.1kcal/mol increase for the C adsorption and 0.2 kcal/mol in-crease for the O adsorption. All further studies of adsorptionwere therefore performed without surface relaxation. To ex-tend and complement the SINDO1 results, we used alsoabinitio22 and DFT23 methods and compared the results withcalculations from otherab initio24 and DFT25,26 methods.There are also periodic HF calculations27 which show a de-crease in adsorption energy with increasing coverage. Thecollected data are in Table I. For our ownGAUSSIAN92 anddeMon calculations we used an Mg9O9 cluster with experi-mental Mg–O distance of 2.105 Å as substrate. The size of

this cluster was limited by the available computer time. Thecluster was chosen as stoichiometric and not embedded for asuitable comparison with the semiempirical calculations onMg50O50 and Mg150O150. To get an unbiased view, SINDO1calculations on the Mg9O9 cluster with the optimized Mg–Odistance of 1.980 Å and the experimental Mg–O distance of2.105 Å were added. We see from the results that in all casesthe C adsorption is more stable and the CO is perpendicularto the surface except for the O adsorption with SINDO1 onthe Mg9O9 cluster without cluster geometry optimization~method B!. We kept this angle at 90°. The results for theadsorption energy vary considerably. Theab initio value~method C! is much too small, whereas the DFT results~method D! seem much improved, probably due to the in-cluded correlation energy. The dependence of the SINDO1adsorption energy on the cluster size is small for the opti-mized structures. It is clear from SINDO1 calculations on theMg9O9 cluster that the Mg–O distance must be optimized,before adsorption is studied. Otherwise binding capacity notused in the substrate is converted to increased binding fromthe surface atoms to the adsorbate molecules. The SINDO1value for the adsorption energy is substantially increased, ifthe experimental Mg–O distance is used instead of the opti-mized value. This can be explained in the following way.Optimization of bond distance increased the bonding withinthe cluster so that the bonding capacity to the adsorbent isreduced. Due to the decreased Mg–O distance there is moreelectrostatic repulsion between the carbon lone pair and theoxygen anions of the cluster. A similar effect is observed forthe ab initio calculations, if the Mg–O distance for theMg9O9 cluster is optimized. The adsorption energy decreasesfurther. It decreases also for a split valence basis, but in-

TABLE I. Geometries@R(Å),q(°)# and adsorption energyEad ~kcal mol21) of CO at MgO with variousmethods.

Type of adsorptionMethod

C adsorption

A B C D E F G

RCO/surface 2.550 2.558 3.372 2.282 '2.90 2.22~2.543! ~2.441! ~2.906! ~2.32!

qCO/surface 90.0 90.0 90.0 90.0 90.0 90.0Ead 8.1 8.5 1.7 5.8 1.6–2.1 22.4 6.9–9.2

~8.3! ~14.0! ~2.3! ~6.2!

O adsorption

A B C D E F G

RCO/surface 2.410 2.411 3.445 2.342 2.19~2.271! ~3.053!

qCO/surface 90.0 90.0 90.0 90.0 90.0Ead 5.0 5.0 1.1 3.9 11.8

~8.6! ~1.2!

A SINDO1, Mg50O50 , opt. Mg–O distance~Mg150O150, opt. Mg–O distance!.B SINDO1, Mg9O9, opt. Mg–O distance~Mg9O9 , exp. Mg–O distance!.C GAUSSIAN 92 ~Ref. 22!, 6-31G, Mg9O9, opt. Mg–O distance~Mg9O9, exp. Mg–O distance!.D deMon ~Ref. 23!, DZVP, Mg9O9 , exp. Mg–O distance.E MCPF ~Ref. 24!, MgO5

82 .F LCGTO LDF ~Ref. 25!, Mg9O9 , exp. Mg–O distance@gradient corrected~Ref. 26!#, Mg9O9, exp. Mg–Odistance.G Exp. ~Ref. 8!.

5286 K. Jug and G. Geudtner: Carbon monoxide adsorption at MgO

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creases for polarization functions. Altogether the adsorptionenergy remains, however, much too small.

It is also remarkable that in SINDO1 the O–Mg distanceis smaller than the C–Mg distance, whereas theab initio andDFT methods show the opposite trend. If we include polar-ization functions in theab initio basis set, we obtain also asmaller O–Mg distance in theab initio calculations.

Previousab initio calculations by Nygrenet al.24 showalso a much too small adsorption energy. Here, an MgO5

82

cluster was embedded in anab initio model potential and aproper Madelung potential, and a basis set superposition er-ror was considered. The authors showed that the inclusion ofcorrelation effects is indispensable inab initio calculationsfor CO on MgO with their model systems. In contrast, semi-empirical calculations include part of the correlation energyusually via parametrization already on the SCF level. Quitedifferent are the adsorption energies for a LCGTO LDF cal-culation in theXa approximation.25 Here, an Mg9O9 clusteris embedded in an electroneutral point charge field of up to1700 charges with values of62. The adsorption energy isoveremphasized and the C–Mg distance is quite short. Agradient corrected calculation increases the C–Mg distanceto 2.32 Å and decreases the adsorption energy to 6.2kcal/mol,26 both steps in the right direction.

III. MONOLAYER COVERAGE

Since the experiments are really based on monolayercoverage, a simulation of this situation in the cluster modelwas necessary. Previous DFT calculations28 used anMg13O13 cluster embedded in more than 1700 point chargesof values62. The surface of the cluster consisted of 9 Mgand 12 O atoms. A central Mg atom was the adsorption siteof a perpendicular CO molecule. This central molecule wassurrounded by four other CO molecules which adsorbed atMg sites which were separated by a single O atom in eachcase. This corresponds to a (A23A2) R 45° overlayerstructure with coverageu50.5. A blueshift of 19 cm21 cal-culated with respect to the free CO molecule.

We started with an Mg54O54 cluster of~63633! shape.On this cluster eight CO molecules can be placed on top ofthose Mg surface atoms with five adjacent O atoms. TheMg–O cluster distance was optimized. The results for the~131! and~231! overlayer structures are in Table II. In thiscase the coverageu is equal to 1. The carbon adsorption isagain more stable than the oxygen adsorption. The adsorp-tion energy is reduced compared to the submonolayer ad-sorption. This is due to the repulsive interactions between thelarge local atomic dipoles of C and O atoms of the CO mol-

ecules. In the~231! overlayer structure the CO moleculesappear tilted. This reduced the CO lateral repulsion and sta-bilizes the~231! overlayer structure slightly compared to the~131! overlayer structure.

From experiments9,10 a c(432)overlayer structure isobserved. To understand this observation we performed ad-sorption calculations on a further enlarged Mg150O150 withthree layers and a 10310 surface. The Mg–O distance wasagain optimized. On the basis of the results on the Mg54O54

cluster we could exclude the oxygen adsorption from furtherconsideration. We chose four different overlayer structures:(A23A2) R 45°,(432),(131),(231). The results are inTable III and Fig. 1. It is important to realize that the twonew overlayer structures in Fig. 1~a! and 1~b! have coveragesof u50.5 andu50.75, respectively. The largest adsorptionenergyEad per CO molecule is found for the lowest cover-age. This value differs slightly from the value for the sub-monolayer coverage, because there are different nonequiva-lent adsorption sites involved, whereas only a centraladsorption site was considered for the submonolayer. Weconclude that the lateral interactions between the CO atomsare small in this case. The~432! structure has a smalleradsorption energyEad due to increased lateral repulsion. Inthis case the energetic equivalence of the CO molecules,present in the other three overlayer structure, is no longermaintained. We have two kinds of CO molecules, which areindependently optimized. CO molecules denoted by CO(1)

are perpendicular, whereas the CO(2) molecules are tiltedwith respect to the surface. The first number of the corre-sponding column in Table III refers to CO(1), the second toCO(2). Both the~131! and ~231! overlayer structures havelower adsorption energiesEad than the other two, because thelateral repulsion is again increased. To get a better view ofthe underlying experimental overlayer structure, we must,however, consider the adsorption energyEad8 per unit cell ofthe substrate which is defined as the product of the average

TABLE II. Geometries@R(Å),q(°)# and adsorption energyEad ~kcal mol21! for monolayer adsorption of COat Mg54O54 .

~131!-overlayer structure adsorption ~231!-overlayer structureC adsorption O adsorption C adsorption O adsorption

RCO/surface 2.58 2.48 2.59 2.51qCO/surface 90 90 77 78Ead 5.2 1.9 5.4 2.0

TABLE III. Geometries@R(Å),q(°)#, adsorption energyEad~kcal mol21)

per molecule and adsorption energyEad8 ~kcal mol21) per unit cell for mono-layer adsorption of CO at Mg150O150.

CoverageuOverlayer structure

16 CO0.5

(A23A2)R45°

24 CO0.75

~432!

32 CO1.0

~131!

32 CO1.0

~231!

RCO/surface 2.53 2.55/2.57 2.60 2.58qCO/surface 90 90/66 90 80Ead 8.6 7.3 4.9 5.0Ead8 4.3 5.5 4.9 5.0

5287K. Jug and G. Geudtner: Carbon monoxide adsorption at MgO

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adsorption energyEad per molecule and the coverageu. Thisquantity takes into account that the stability of the wholestructure depends on how much energy is released, if allmolecules on the surface are adsorbed. If the coverage is lowmore molecules can be adsorbed until a saturation for amonolayer is reached due to increasing lateral repulsion ofthe adsorbate molecules. The most favorable overlayer struc-ture is one for whichEad8 has a maximum value. In this senseEad8 in Table III describes the most stable monolayer struc-ture under consideration of varying coverage of the surface.Here the~432! overlayer structure turns out to be the ener-getically most preferable structure.

This view is supported also from the fact that there aretwo ir frequencies found29 for the adsorbed CO molecules at2147 and 2136 cm21. If we use the scaling procedure foradsorbed molecules in the SINDO1 method16 we obtainn~CO~1!!52145 cm21 and n(CO(2))52138 cm21, in goodagreement with the experimental results.

IV. CONCLUSION

A cluster model has been successfully used for the studyof adsorption of CO on a clean MgO surface. The SINDO1method provides a reliable basis for the calculation of clusterstructures and cluster–adsorbate complexes. The results forthe adsorbate structure both with submonolayer and mono-layer coverage agree qualitatively and quantitatively verywell with the experimental data. A~432! overlayer structurewith two kinds of adsorbed CO molecules is found to be themost stable arrangement. This supported by an analysis ofthe ir frequencies of the adsorbed CO molecules.

ACKNOWLEDGMENTS

This work supported by Deutsche Forschungsgemein-schaft and Fonds der Chemischen Industrie. The calculationswere performed on a Siemens S400/40 at Universita¨tHannover and on an IBM RS6000/340.

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FIG. 1. Monolayer adsorption of CO/MgO simulated on a (1031033) Mg150O150 cluster.~a! (A23A2) R 45°, ~b! ~432!, ~c! ~131!, ~d! ~231! overlayer.

5288 K. Jug and G. Geudtner: Carbon monoxide adsorption at MgO

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5289K. Jug and G. Geudtner: Carbon monoxide adsorption at MgO

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