A theoretical study of CO2 adsorption on TiO2

THEO CHEM ELSEVIER

Journal of Molecular Structure (Theochem) 371 (1996) 219-235

A theoretical study of CO2 adsorption on TiOT1 Alexis Markovits, Adil Fahmi2, Christian Minot* Laboratoire de Chimie ThPorique, UPR 9070 CNRS lJniversit6 P. et M. Curie., Boite 137, Tour 23-22 pike 75252 Paris CCdex OS, France

114, 4 Place Jussieu,

Received 2 January 1996; accepted 29 April 1996

Abstract This paper presents a theoretical study of the interaction of CO* with the rutile TiOz surface. The calculations are performed using the periodic Hartree-Fock CRYSTAL program. Several modes for the adsorption are investigated. On the bare surface at @=

l/2, the best adsorption mode is obtained when the CO* molecule is vertically adsorbed over a titanium atom; at saturation, the COz molecules are tilted toward the surface; then, the adsorption mode is also controlled by the adsorbate-adsorbate interactions. Another adsorption mode, competitive with the first one, corresponds to a parallel CO2 molecule over two titanium atoms. The adsorption over an oxygen atom of the oxide is weak. On the hydrated surface, the presence of hydroxyl groups favors the CO* adsorption and leads to the formation of adsorbed bicarbonate ions. Keywords:

Ab initio periodic Hartree-Fock

calculation;

Adsorption;

1. Introduction Whereas the fixation of carbon dioxide by green plants is the most important chemical process on earth [l], CO:! is not much used in the synthetic chemical industry [1,2]. However, the adsorption of carbon dioxide is involved in catalytic chemical syntheses such as methanol synthesis [3]. Activation of CO2 is also important because of air pollution since the industrial revolution. Several authors have focused on the adsorption on TiOz and PtKiOz [3,4]. The COz heat of adsorption has been estimated

* Corresponding author. ’ Dedicated to Professor Juan Bertran on his 65th birthday. * Present address: Eindhoven University of Technology, Department of Inorganic Chemistry and Catalysis, P.O. Box 513,560O MB Eindhoven, The Netherlands.

to be 15.1 kcal mol-’ on rutile (110) the most reactive face [5,6]. That on anatase is 10.8 kcal mol-’ [7]. CO2 adsorption is sensitive to lateral effects and not very sensitive to surface contaminants [8]. The CO2 adsorption on metal oxides has been the subject of only a few theoretical studies. Rodriguez [9] used semi-empirical calculations on ZnO. Pacchioni [lo] has performed ab initio all-electron calculations on MgO clusters. CO2 is an amphoteric molecule: it is used as a probe molecule to test the basicity of metal oxides [11,12] together with ammonia that tests their acidity. It is also used to investigate the surface acidity on anatase [8]. Linear CO2 adsorbs at the same sites as CO; CO is a ‘soft’ Lewis basis that is adsorbed perpendicularly to the surface over the titanium centers [13] and that is used to probe the acidic sites [14-161. From semiempirical calculations [9], it is concluded that CO*

0166-1280/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SO166-1280(96)04696-9

Titanium oxide; Carbon dioxide; Lateral interaction

A. Markovits et al.lJournal of Molecular Structure (Theochem) 371 (19%) 219-235

220 a) CO2

Coordinated

b)

species

C03H-

Bicarbonates

0

I

OAH

C

I

o’“>o ..

0

oHC\o

.’

‘\

*\

.’

\

Ti

‘\

Ti*’

I’

‘\

,’ ‘\

\

,’

LTi.S

lM-bidentate (chelating)

lM-monodentate

ZM-bidentate

lM-bidentate

o/c\o

O-T-0 I

0 2M-bidentate

IM-

C coordinated

I , I I I

Ti lM-monodentate

0-

C

coordinated

r12

coordinated Carbonates

ii lM-

ii

tM-bidentate

bidentate

Ti IM-monodentate

Fig. 1. The various adsorption

modes for COz C03HT and CO;.

A. Ma&wits

et aLlJournal of Molecular Structure (Theochem) 371 (1996) 219-235

221

is a net donor of electrons when adsorbed in a linear form and a net acceptor (u-acceptor r-donor) upon adsorption in a bent form.

surface of rutile does not take place at the oxygen vacancies but on the perfect surface. On ZnO, this adsorption mode (lM-bidentate) has been found more stable than the lM-monodentate [9].

2. Adsorption modes

2.2. The adsorption of carbonates and bicarbonates

The oxygen atoms of CO2 are weakly basic whereas the carbon atom is acidic. The adsorption of CO2 on oxide surfaces can use both properties. The different adsorption modes are shown in Fig. 1. When it is adsorbed as a base, the binding mode can involve one or two oxygen atoms of COz (monodentate or bidentate) and one or two metal centers (see Fig. 1; labels 1M or 2M refer to the number of metal atoms involved in the adsorption). As an acidic species, it should bind to an oxygen atom from the surface (O-C coordinated, see Fig. 1). The M-C coordination is only favorable when the metal donates electrons to the carbon of CO2 (the acidic site). The 1’ side-on mode is favorable when the metal donates electrons to the ?r*(CO) orbital. The coadsorption with water allows the formation of carbonates and bicarbonates. The formation of 1M or 2M-bidentate bicarbonates requires the presence of hydroxyl groups on the surface (one oxygen atom originates from a surface hydroxyl) and cannot be observed on the bare surface. The formation of 1M or 2M-bidentate carbonates should similarly appear on hydroxylated surfaces. However, a CO3 species can be formally generated from a CO* molecule and an oxygen atom from Ti02 in the case of a bare surface. This formal case will also be investigated in the paper.

On basic oxides CO2 is supposed to bind to an oxygen atom, that from the surface or that from an adsorbed hydroxyl group. This leads to monodentate or bidentate carbonates and to hydrogen carbonates (bicarbonates). The mechanism of CO* insertion in surface hydroxyl groups [18,19] that generates bicarbonates could involve both acidic and basic properties of CO*. Knozinger [19] proposed that the formation of hydrogen carbonates involves the reaction of CO2 already adsorbed at strong acidic sites (a mechanism of the Langmuir-Hinshellwood type), the acidity of the carbon being increased for adsorbed CO2 species. In contrast, Tsyganenko and Trusov [20], Lamotte et al. [21] and Hoggan et al. [22] proposed donation from a lone pair of surface hydroxyl to CO2 (a mechanism of the Eley-Rideal type). The calculated activation barrier is low, only 11.6 kcal mol-‘; this value supports the latter mechanism. A similar problem is found for the anhydrase-catalyzed hydration of COz. The nucleophilic attack from a Zn*‘-bound OH- to C proposed by Liang and Lipscomb [23] has low activation energy, while that from H20 has a large energy barrier [24]. The hydroxyl bound to a metal however, becomes a poor nucleophile with respect to isolated OH-. According to Lindskog [25], the hydration of CO2 by carbonic anhydrase (a hydroxyl bound to a Zn(I1)) is difficult without activation of the CO2 molecule. This model is supported by calculations from Jacob et al. [26] even if the presence of a water molecule facilitates the hydration process as shown by Sola et al. [27]. The Lindskog mechanism (the inner-sphere mechanism) [25], where the CO1 is bound to the Zn ion, is calculated to have a lower energy barrier than the Lipscomb mechanism (the outer-sphere mechanism) [23] where a hydroxyl group attached to the zinc reacts as a nucleophilic species on the C atom of COz. Monodentate carbonates have been postulated together with vertical CO2 [7]. On MgO [28] and yA1203 [29] the reaction with basic surface hydroxyl groups leads to the formation of bicarbonate ions HCO;. Similarly, CO;! adsorption has also been

2.1. CO z adsorption as a basic species An adsorption mode for CO* corresponds to upright molecules over unsaturated cations [7] (lM-monodentate). On Zr02 for instance, there are several adsorbed species, among them at least two corresponding to linearly adsorbed CO2 [17]. On the bare anatase Ti02 surfaces [8], CO2 also chemisorbs at the acidic sites. The adsorption mode implies an acid (the titanium centers from the substrate)-base (the adsorbate) interaction, even if the total charge transfer is not significant. Such interactions also favor the bidentate species. According to Gopel et al. [6] CO2 adsorption of lM-bidentate species on the (110)

222

A. Markovits et aLlJournal of Molecular Structure (Theochetn) 371 (1996) 219-235

X

“\ ,/O A

and that few adsorbed species are transformed into bicarbonates. The ratio of vertical CO* to other species depends on the sample, and whether it has been activated under air or under oxygen.

JT\O

2.3. The 7’ (M-C) and q2 side-on coordinated CO2

s/

0

I Y a)

the

polymer

model

_;_

Y b) the (110)

face

(3L model)

Fig. 2. The single polymer representing the rutile surface and the (110) slab. The (110) top layer is made of the coupling of alternate horizontal and vertical polymers; the 3L model includes a surface plane that contains the Ti and O1 atoms and two other planes with the oxygen atoms, 02.

In contrast to CO, COZ is not a common ligand in transition metal complexes. The coordination of CO2 with transition metals has long been thought to be the initial step in catalytic conversion. In the characterized complexes [l], CO* is v1 (M-C) or v2 side-on coordinated. These geometries are well explained by the interaction of frontier orbitals and require that one d orbital of the metal is occupied (a da for the 17’mode and a d?r for the v2 mode). IM-monodentate species have been proposed as first step of reaction but have not been characterized. The oxidation state of the metal in the complex differs from that in the oxide (the d-metal levels from the oxide are predominantly empty levels) and thus the coordination mode is not necessarily the same.

3. Computational

features

3.1. Method of calculation expected to generate HCO; ions on hydroxylated TiOz surfaces [30]. This adsorption increases with pressure and continues to increase when the COZ adsorption exceeds the amount of basic OH- ions on the surface. The adsorption is reduced when the surface is covered by phosphate ions. Broad IR bands suggest that the surface is heterogeneous for COZ adsorption [31], probably as CO;, CO3 and bicarbonates with differences between anatase and rutile. On hydroxylated anatase or rutile samples used by Primet et al. [32], CO2 forms bicarbonate species. The adsorption of COP on oxidized anatase produces a coordinated CO2 species which converts slowly into bicarbonate [33]. Monodentate carbonate with two equivalent bonds having a partial double-bond character, and bidentate carbonates with a unique bond and IR spectrum at high vibrational frequencies (double-bond character), are also postulated by Morterra et al. [8]. These characteristics are observed by Lamotte et al. [21] who conclude that the OH groups are weakly basic

Using the ab initio Hartree-Fock crystalline orbital program CRYSTAL [34,35], we performed effective core pseudopotential calculations. For the oxygen and the carbon atoms, the basis sets are the PS-31G basis sets [36]. For the titanium atoms, the basis set [37,38] consists of the d functions contracted to a (4/l) basis set and a single 4sp shell with an exponent of 0.484; the 4sp shell is almost vacant. The pseudopotentials are those from Durand-Barthelat [39]. Bulk Ti02 calculations for both rutile [37] and anatase [38], the adsorption of HZ0 on TiOz [40] and that of CO [13], have already been calculated with the same basis sets. Ti02 appears formally as made of Ti4’(do) cations and 02- anions even if the calculated structure is not completely ionic (the charge on Ti in the bulk is large, +2.6, and the Ti-0 overlap populations are weak, 0.080-0.096) [38]. COZ adsorbs either on the titanium atoms or on the oxygen atoms. The adsorption may be seen as resulting from Lewis acid-base reactions that do not affect the oxidation number of the atoms.

A. Ma&wits et aLlJournal of Molecular Structure (Theochem) 371 (1996) 219-235

3.2. The model for the rutile surfaces

Eads

=Eco~

+ETiO,

-E(CO,

+TiO*)’

% estimate the C, dispersion from the expression from Silvi [41] 3 (E* 2 (E’

-Ek02’(E*

- .qco,

coefficient

as 12.900 eV A”

-%i02 + (‘5’ - E)TiO,aCO’

is the total energy of the adsorbatei substrate system, Erioz is the total energy of the substrate and ECo2 is the total energy of the isolated adsorbate in its equilibrium geometry. A positive Eadsvalue corresponds to a stable adsorbate/substrate system. We have optimized the geometry of the adsorbate (CO2 or Hz0 + C02) assuming no reconstruction of the substrate. In our previous work [ 13,40,49,50], we have shown that the adsorption energies are overestimated by the calculations with the PS-31G basis set and the polymer model. They would be reduced if larger basis sets were used, mostly because of a better description of EcOz. The use of a slab would also contribute to a reduction of the adsorption energies. However the energy difference between the various adsorption modes should not be affected and our analysis should remain valid. We have also calculated the CO2 adsorption on the (110) face. The CO distances are transferred from the calculations on the polymer whereas the TiO distance is optimized. First, we have used a unit cell containing two TiOz units, periodically repeated in two dimensions. This model, labelled 3L, is built from the alternated coupling of the preceding polymers, one in the surface plane and the other one perpendicular (see Fig. 2(b)). Since the oxygen atoms of the vertical polymer are not in the plane that contains the titanium atoms, this model contains in fact two layers of oxygen atoms in addition to the main layer containing the titanium atoms. The adsorption of a Lewis base on the unsaturated titanium atom looks like that on the simple polymer model. The large positive charge developed on this titanium atom makes the adsorption stronger. The adsorption of a Lewis acid on an unsaturated oxygen atom from the surface looks like the adsorption in the plane of the polymer. Since the (110) face is the densest one with no dipole moment, it is the most stable face. The cohesive energy of the (110) face, calculated with 25 k-points in the irreducible Brillouin Zone, is 118 kcal mol-’ per Ti02 unit. Finally, we have also used slabs made of the stacking of two and three 3L-slabs; each 3L-slab is defined by two TiOz units and includes three layers, the plane that contains the titanium atoms and the triply-coordinated oxygen atoms and the two layers containing the doubly-coordinated oxygen atoms. For the 6L-model, COz is adsorbed on one face; for the where

Our polymer representing the rutile surface is a one dimensional planar structure containing the titanium atoms and equatorial oxygen atoms (see Fig. 2). It is made0 of rectangular pieces with a long edge (3.0065 A, the c parameter from the bulk) in the direction of the chain and a short edge (2.4872 A) in the perpendicular direction. The TiO distances are the equatorial distances from the bulk structure. This polymer is obviously far from being a complete representation of a crystallographic face. The electrostatic field arising from the bulk may be overlooked. Note, however, that since CO2 has no dipole moment, electrostatic interactions should be reasonably small in many cases. Dispersion interactions should also be small.3 The model contains titanium atoms with the oxidation number + IV and bridging oxygen atoms. The polymer can be recognized as a main feature of the (110) and (100) crystallographic faces. It contains the linearity of the sequence of titanium atoms that is specific to the rutile structure and that is part of the relative stability of the various TiOz surfaces [43]. In this model, the presence of tetracoordinated cations does not create states in the band gap and the electronic structure remains close to that of larger systems. The adsorption energies have been calculated according to the expression:

OLTQ

with (E**- E)co2 = 2.5 eV(the first bands in emission are observed at = 3.2 eV (the _Qand gap, see [38,43]) ~000 A [4?1), (E’ -E)TiOI ace, = 2.6 A [44,45] and orto, < 2.37 A (the value for MgO that should be an upper limit [46,47]). For a typical surface-adsorbate distance of 3 A, after a summation on the oxygen centers, the dispersion energy appears to be of the order of 475 meV A’, which is of the order of the different errors in the HF calculations due to approximations (cut-off, k-space integration...). This is less than for COz adsorption on Al or Ag by a factor 3 [48]. The dispersion energy may also be estimated after integration as C6r/Sz4 = 5 kcal mol.‘, where S = 15 A’ is the surface unit cell at 0 = l/2 and z = 3.5 A is the distance to the surface of the carbon of CO,. The variation of such a term is negligible with respect to the errors in our calculations.

223

&CO,

+ Ti02)

224

A. Markovits et aWourna1 of Molecular Structure (Theochem) 371 (1996) 219-235 -424

Fig. 3. Calculated

energies for the reaction involving

9L-model, the adsorption is made on both sides to take advantage of the horizontal symmetry. The cohesive energies per TiO2 unit for these two models, 174.4 and 184.3 kcal mol-’ respectively, approach that of the bulk-rutile, 207.5 kcal mol-‘.

4. Results and discussion

4.1. The CO2 molecule with the PS-31G basis set At the PS3;G level, the optimized CO distance for COZ is 1.162 A. The atomic charges, Qc = 0.953, are large compared with those of the carbon monoxide, Qc = 0.366. The C-O Mulliken overlap population is also large, 0.503. The binding energy at the SCF level is 176 kcal mol-‘. Correlation effects and large basis sets would be necessary to approach the experimental value, 382 kcal mol-‘. Then, the calculated values get close to the experimental one (386.1 and 385.3 kcal mol-’ at the G2-MP3 and G2-MP3 level respectively [51,52]). The correlation is more important for CO2 than for CO because of the difference of polarization. At the SCF level and with the PS31G basis set, the dissociation energy CO2 - CO + 0 is found to be too weak, 43.3 kcal mol-’ compared with an experimental value of 127 kcal mol-‘. Even larger basis sets [53] account for only 55% of the experimental value, i.e. 69.8 kcal mol-’ [54]. Again calculations including correlation effects [51,55] approach the experimental at the MP2/6-31G* 124.4 kcal mol-’ value: level, 127.1 kcal mol-’ at the G2-MP2 level and 126.8 kcal mol-’ at the G2-MP3 level. In this paper

the acidic and basic properties

of CO2 in the gas phase.

we deal with the molecular adsorption. On a clean surface of TiOZ, contrary to Pt/TiO*, CO2 does not dissociate [3]. This dissociation only occurs upon heating at 240 and 330 K and releases CO [7]. Our calculated energy for the reaction that leads to the coadsorption of CO and 0 on the polymer model is endothermic by 40.9 kcal mol-‘. Indeed, the adsorption energies for the two carbon oxides are not very different and that of the oxygen atom is slightly endothermic. At 8 = 1, E&O) = - 14.6 kcal mol-’ for the best adsorption mode (the oxygen atom bridging a Ti-0 bond from the polymer). For the coadsorption, the oxygen atom is as mentioned above and the CO is upright above the unsaturated titanium atom [13] (the unit cell contains two titanium atoms). As already mentioned, COP is an amphoteric compound. The main gas phase reactions involving the acid-base properties are shown in Fig. 3. When CO2 behaves as a Lewis basis, a u donation from the oxygen sp electron pair occurs. The reaction of CO* with a proton is very exothermic. Then, the electron transfer is large, 0.62 e; the polarization of COZ increases (the electronic density on the protonated oxygen atom increases by 0.128 e whereas that on the external atomic oxygen decreases by 0.289 e); the corresponding Lewis structure is: H-o-C&

1

The calculated value for the proton affinity at the PS31G level is close to that obtained with accurate calculations (128 kcal mol-’ [56]). This value is smaller than that of the C-protonation in carbon monoxide (138.9 at the PS-31G level, 140.5 at the G2 level

A. Markovits et al.lJournal of Molecular Structure (Theochem) 371 (1996) 219-235

[56]) but larger than that of the 0-protonation. It remains inferior to that calculated for more basic oxygen atoms (the protonation of the water molecule gives 182.5 kcal mol-‘). This order is well explained by an electron donation to the proton. It correlates the order of the energy levels of the u pairs and the magnitude of the electron transfers to the 1s H orbital. The interaction of CO2 with the hydronium ion gives only 19.8 kcal mall’ (the C02...+HOH;, distance is 1.55 A). Then, the electron transfer is weak, the donation to the hydronium ion represents only 0.04 e. CO*, however, is strongly polarized (the electronic density on the bound oxygen atom increases by 0.18 e whereas that on the external atomic oxygen decreases by 0.12 e). The reaction with the water molecule leads to a complex weakly bound through a hydrogen bond. Some IR experiments [57] and calculations [58-601 support the formation of such a complex. Note that other theoretical studies [61] have not always obtained this complex even if a shallow local minimum on the energy profile is not excluded. The electron transfer is weak: the donation to the water molecule represents only 0.007 e. COz is polarized: the electronic density on the bounded oxygen atom increases by 0.035 e whereas that on the external atomic oxygen decreases by 0.019 e. When CO2 reacts on different cations, the lMmonodentate mode is observed and calculated as linear [62,63] or bent [63-651. The 2M-bidentate mode is also calculated [63]. With the PS-31G basis set, the lM-bidentate mode for the alkali ions is unstable relative to the dissociation, when the CZV symmetry is imposed. We obtained the lM-bidentate structure only with Mg*‘, although this structure is less stable than the linear one, the lM-monodentate, by 68.5 kcal mol-‘. In this case, the bending of O-C-O (158.4”) is not large enough to make the Mg-0 distance, 2.64 A, smaller than the Mg-C distance, 2.60 A (the Mg-0 distant: is long relative to that of the monodentate, 1.99 A), so that the optimized geometry can also be considered as a C-coordinated species. When the lM-bidentate mode is found in matrices [66], it corresponds to a different electron count (the metal plus CO2 system is neutral). When the CO2 molecule interacts as an acid, it reacts through the carbon atom: the lowest unoccupied levels are the rr* orbitals that are mainly localized on the carbon atom. The reaction of CO2 with OHleads to the formation of a bicarbonate species,

225

CO&. The CO* fragment in C020H- is bent with an O-C-O tngle of 133”. The C-O bonds are elongated, 1.24 A, and the HOCO:! distance is 1.43 A. This geometry is similar to those with larger basis sets and correlation [67,68]. The reaction energy is -82.3 give -56 kcal mol-‘. More accurate calculations kcal mol-’ [67] and -51 kcal mol-’ [68]. The gas and aqueous phase estimated values based on experimental data are -49 kcal mol-’ [23] and -11.2 kcal mol-’ [67], respectively. The energy for the reaction with the water molecule, - 6.6 kcal mol-‘, is very weak as compared to that of OH- (see Fig. 3). The hydration of COZ by HZ0 involves a rather large activation energy in the gas phase, while hydration by OH- is activation less [24]. It leads to a planar H20-**C02 species with a T shape for the heavy atoms and an O..=C distance of 2.69 A; with the PS31G basis set, this species is slightly more stable than the bicarbonate molecule. At variance with the hydrogen-bonded complex, the T shape structure has been clearly determined by IR spectroscopy in matrices [69,70] and obtained by calculations [60,61]. In the gas phase CO* appears to be predominantly an acidic species, except when it interacts with H’, the strongest acidic agent. On the surface, we mentioned that it is used as a probe molecule to test the basicity of the oxides [11,12] as opposed to CO, which is used to probe the acidic sites. However the adsorption on unsaturated cations clearly uses the basicity of CO2 [7]. In solution, the formation of the bicarbonate is slightly endothermic whereas no mention appears of the protonation of the carbon dioxide [71]. CO2 + OHC03H-

+H+

-

C03H-

- 16.24 kcal mol-’

C03H2 - 1.8 kcal mol-’

The lateral interaction between two parallel CO;! molecules is repulsive when they approach each other without any shift. When one molecule is shifted by 1.96 A (see Fig. 4(a)), there is a stabilization by 2.9 kcal mol-‘. This shift places two adjacent CO2 molecules in a geometry represented in the rectangular frames of Fig. 4. The CO intermolecular distances are 2.92 A. 4.2. The perpendicular

orientation

In the perpendicular adsorption mode (lM-monodentate), COZ is adsorbed through the oxygen lone

A. Markovits et al./Journal of Molecular Structure (Theochem) 371 (1996) 219-235

226

,

I

h

Table 1 Heat of adsorption (kcat mol.t) for the perpendicular adsorption mode and proton affinities (last column) for carbon dioxide and carbon monoxide (O-down and C-down) at different coverages. Experimental values are taken from Ref. [5]. At 0 = l/2 the order of the heats of adsorption correlates with the proton affinities. At 6 = 1, the repulsion between adsorbates decreases the heat of adsorption for CO2 E ads

a>

co2

Co O-down CO C-down

. II

b)

3.0065 A

Fig. 4. (a) The geometry of the dimer; the distance, h, between parallel molecules is 2.81 A; the shift is 1.96 A. (b) The geometry of the adsorption perpendicular to the surface with successive short and long TiO distances; the shift is 2.22 A. (c) The geometry for the adsorption tilted over the surface; the shift is 1.46 A. For the adsorption geometries, h is fixed by the cell vector. The geometry of the dimer is shown in the frame.

pair. The adsorption takes place on the acidic center of the surface, the unsaturated titanium atoms. COZ is asymmetric with a shorter distance for the external CO bond; the overlap populations (Table 2) also shoy this trend. The Ti-0 distance is rather large, 2.236A

e=1 10.4 11.6 15.2

e = 112 19.0 16.5 20.0

experimental 15.14 19.09

H affinities 130.2 117.5 138.9

(the calculatedOTi-0 distan$es for the bulk-rutile structure are 1.951 A and 1.972 A). The adsorption energy is weak (Table 1). The net charge transfer is almost zero (Table 2). However, the adsorbate is strongly polarized; the charge on the oxygen bounded to the surface exceeds that on the other oxygen atom by 0.2 e. CO2 does not bend; however, a small bending would only induce a small destabilization: the derivative with respect to (Y, the OCO angle, is small, d2E/do2 = 0.015 kcal* degm2. At 8 = l/2, the heat of adsorption is intermediate between those of CO C-down and O-down (see Table 1). The perpendicular adsorption is slightly stronger than the CO adsorption with the TiOC orientation (Odown). The conjugation with the oxygen u. electron pair is slightly more important. For CO2 compared with CO, the u. pair is higher in energy. The conjugation of the co pair with the ac pair is reduced since the CO distance is indeed longer and since the uc pair participates to a uco bond. For CO, this conjugation is a stabilizing factor for the u. pair and therefore the u conjugation with the Ti( + IV) orbitals is small. In contrast, for C02, the u conjugation and the heat of adsorption are larger. This difference in reactivity between the two carbon oxides also appears in the gas phase proton affinities calculated with the PS31G basis set (see Table 1). The largest proton affinity corresponds to the highest u electron pair, the uc pair; it also corresponds to the largest adsorption energy. The rr pairs of the oxygen atom also conjugate with the titanium orbitals. The occupied molecular orbitals of CO2 are lowered in energy (lx, by 0.12-0.13 a.u. and la, by 0.09 a.u.). The conjugation of the CO orbitals with the metal orbitals does not induce any charge transfer (the

221

A. Markovits et al.lJournal of Molecular Structure (Theochemj 371 (19%) 219-235

Table 2 Mulliken atomic charges, distances (A) and Overlap populations (in parenthesis) for the different adsorption molecules are titlted over the surface whereas at 0 = l/2 they are perpendicular to the surface 19= 1 (polymer)

0 = l/2 (polymer)

6 = 1 (110)

e = l/2 (110)

modes of CO*. At 0 = 1, the CO2

free molecule

IM-monodentate

Qo,, QC

Qo.,, Ti-Oad, oads-c

c-o

- 0.639 1.082 - 0.442 2.24 (0.013) 1.160 (0.426) 1.151 (0.504)

- 0.638 1.047 - 0.403 2.225 2.14 (0.014) 1.167 (0.457) 1.149 (0.530)

- 0.693 1.144 - 0.427 2.11 (0.027) 1.160 (0.413) 1.151 (0.513)

- 0.703 1.117 - 0.378 (0.030) 1.167 (0.444) 1.149 (0.539)

- 0.476 0.952 - 0.476

1.162 (0.453) 1.162 (0.453)

2M-bidentate

Qo,

Qc Ti-O,ds c-o

- 0.517 1.000 2.395 (0.023) 1.16 (0.421)

- 0.500 1.044 2.414 (0.032) 1.162 (0.415)

- 0.509 1.001 2.40 (0.000) 1.16 (0.507)

- 0.477 0.933 2.92 (0.017) 1.162 (0.491)

- 0.476 0.952

1.162 (0.453)

O-C coordinated

Qo.,. QC

c-o,d,

c-o

absence of charge transfer does not result from a compensation of opposite u or rr charge transfers, both individual charge transfers being zero). The CO2 system, however, becomes strongly polarized, revealing a depopulation of the occupied orbitals, mainly the In, (the 7~system is more polarized than the u system) strictly compensated by a population of the empty ones (27rJ. More precisely, the 1~s and 27r, have to mix in the same crystalline orbitals to allow the loss of symmetry of the CO* adsorbed by one end. The phases on the bounded oxygen atom are determined by a conjugation with the metal as shown in Fig. 5. The delocalization on the metal is observed by the presence of a small amplitude on the titanium center in the crystal orbital. The density on the bounded oxygen increases by 0.127 e (0.089 rr + 0.038 s) and that on the external oxygen atom decreases by 0.073 e (0.082 u - 0.009 a). The absence of charge transfer does not support the description of the adsorption as an acid-base process

where CO* is the base and the metal center is the acid. Indeed, there is no donation unless compensated by an equivalent back-donation (within the same sets of MOs, r or a). This case is similar to that of the complex C02**.HOH (Section 4a). However, the shift of the MOs, the delocalization and the polarization are those predicted by considering a donation from CO;! to the metal. The use of language, ‘acidities’ and ‘basicities’, also remains efficient for select the adsorption site, for inferring the geometry of adsorption, and to compare the adsorption energy with other adsorbates whose acidic and basic properties are more pronounced. Under adsorption, the charge on the carbon atom increases (Table 2). Consequently, CO2 becomes much more acidic with the possibility of reaction with a base; the reaction with a hydroxyl ion would easily lead to a bicarbonate species. To estimate this activation, we have calculated the interaction between a water molecule and the adsorbed CO2 molecule,

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A. Markovits et al.IJournal of Molecular Structure (Theo&em) 371 (1996) 219-23.5

Fig. 5. The lx, mixes with the 2x, with the phase shown above imposed by the relationship of the bounded oxygen atom with the metal. This increases in the crystal orbital the density on the bounded oxygen and decrease that on the external oxygen atom. It also introduces an atomic contribution on the carbon that is in phase with the external oxygen. The polarization in the r system develops the electronic density on the bounded oxygen atom and the multiple bond character on the external CO.

transferring the geometry optimized in the complex Hz0.-.C02 (see Section 3.2). This geometry is shown at the left hand side of Fig. 6; the six atoms are coplanar to allow the conjugation of a p C orbital of CO* with the lb, orbital of H20. The interaction energy, 13.1 kcal mol-‘, is defined as follows,

This represents a doubling of the interaction of the complex, in spite of the absence of full optimization in the case of the adsorbed species and in spite of a small adsorbate-adsorbate repulsion, 0.6 kcal mol-‘. The CO2 activation has often been recognized as necessary for the conversion of COZ into useful forms of organic compounds [l]. Since CO* is a very stable molecule [3] and since it is a thermodynamic end product of many chemical processes such as combustion, it has to be activated to react. Such activation is proposed for the formation of bicarbonates [3,19]. The 6 = 1 coverage imposes a strong lateral interaction between the adsorbates. Each CO2 molecule is parallel to two CO* at 3A (the lattice parameter). The intermolecular interaction is repulsive; it can be estimated as 6.8 kcal mol-’ from a periodic calculation repeating the adsorbate alone with no substrate. This repulsion screens the increase of adsorption of COZ with respect to CO (the comparison is made with the Ti-O-C orientation). At 0 = l/2 coverage, this repulsion becomes negligible, 0.35 kcal mol-‘. The increase of the heat of

Fig. 6. The interaction of the water molecule (at 0 = l/2) with a COr activated by the adsorption. The perpendicular adsorption mode is represented at the left hand side of the figure and the parallel mode at the right hand side. The geometry of HrO is taken from the optimized complex, HrO..COr.

adsorption relative to 8 = 1, can be mostly attributed to the reduction of the C02-CO2 repulsion. The heat of adsorption, 19 kcal mol-‘, is larger than that of CO. The geometrical parameters are nearly the same as for 8 = 1 (see Table 2). 4.3. Lateral effects According to our calculations (see previous section), the CO2 adsorption Fnergy is weak. The rather long 0-Ti distance, 2.24 A, and the fact that the geometry of the adsorbed CO2 is nearly unperturbed, reveal the weakness of the adsorption. Since the adsorption is weak, it is sensitive to adsorbate-adsorbate interactions. Saturation of the polymer (6 = 1, all the Ti sites are covered) is reached despite a strong adsorbate-adsorbate repulsion of 3.7 kcal mol-‘.

co2(e=i/2) E(o_lp_~_l)

=

co2(e=1) -2.0 kcal mol

-1

When taken into account, the lateral interactions should increase this small energy difference. Lateral effects can be inferred from the consideration of the dimer structure: if a CO* molecule of the adsorbed layer stands at a larger Ti-0 distance than its two neighbours, reproducing locally the geometry of the dimer (see Fig. 4(b)), it would benefit from an attractive lateral interaction. On Fig. 7 the energy profile for the desorption of half of the CO2 molecules is represented. After passing a repulsive barrier, the system is stabilized.

A. Markovits et alSJourna1 of Molecular Structure (Theochem) 371 (1996) 219-235

3

1 .98

kcal/mol

adsorption

I 2

24

I 2.0

I

I

3.2

distance

I

36

to +- e

Fig. 7. The energy profile for the saturation

Therefore, half of the molecules are chemisorbed on the titanium atoms whereas the other half are bound to the chemisorbed COz molecules through van der Waals interactions. The Ti-0 distance for the farthest molecules from the surface is long, 4.45 A; thus these molecules cannot be considered as being directly bound to the surface. For this optimal geometry, the heat of adsorption per CO2 is increased from 10.4 to 12 kcal mol-’ and the energy for the saturation reaction becomes, E+112 _ ,+i) = -5 kcal mol-‘. Another geometry that fits the lateral constraints is simply obtained by a tilt of all the CO* molecules as represented in Fig. 4(c). In this case all the molecules remain parallel and equivalent. The tilt angle (angle

I

4

surface

44

I 48

52

(A)

of the surface CO2 (0 = l/2)

-

CO* (~9= 1).

with the z axis, perpendicular to the surface) is 29”; it is close to that of the CO2 dimer. The Ti-0 distance is 2.24 A. The heat of adsorption per CO* is increased to 15.5 kcal mol-’ and the enthapy for the saturation -1 reaction becomes E+112 _ 8_1j = -12.2 kcal mol , approacing the value at fl = l/2. This adsorption mode is therefore the best one obtained at saturation. Several authors [7,8,72] have shown that perpendicular CO2 chemisorption is sensitive to lateral effects. Lateral interactions have been already suggested to explain the ordering of the CO molecules on the surface. They are responsible for observed IR shifts [73-761. According to the nature of the oxide surface, these adsorbate-adsorbate interactions can be

Fig. 8. COr (acidic compound) bound to a basic center. Different orientations are presented. stable orientation is sketched at the right hand side of the drawing. The orientation at the left if one considers an oxygen atom from the polymer as also belonging to a CO3 species. At between an oxygen atom of COr and a titanium ion from the surface. A shift privileges a

The optimal O..COa distance is 2.4 A. The most hand side may be seen as a lM-bidentate carbonate the left hand side, there is a secondary interaction Ti-0 bond or a O-C bond.

230

A. Markovits et aLlJournal of Molecular Structure (Theochem) 371 (1996) 219-235

Fig. 9. The parallel adsorption. two surface cations.

CO2 is a basic compound

bound to

attractive [77,78] or repulsive [79,80]. A computer simulation by Tsyganenko et al. [81] also emphasizes the strong influence of repulsive interactions on the structure of the chemisorbed layer. 4.4. The parallel orientation When a CO2 molecule is parallel to the polymer plane, the optimized di$ance to the surface is large, in the range of 2.3-2.4 A or even longer according to the adsorption mode. Several orientations have been investigated for the situations where an oxygen atom of CO2 is above a titanium atom at 19= l/2. We first have oriented the COP molecule toward an oxygen atom from the surface to favor a C-e.0 bond formation (see the left hand side of Fig. 8). The optimization of the Ti-0 distance leads to a long distance, 2.9 A (Eads = 1 kcal mol-‘). Thus, this adsorption mode correspoqds rather to a physisorption of CO*. At Ti-0 = 2.4 A, the molecule tends to desorb (Eads = -6.4 kcal mol-‘). A small shift parallel to the surface would place the carbon atom above the oxygen privileging the TIO-Cop bond (see Section 2); then, the heat of adsorption becomes positive, 4.5 kcal mol-‘. On the contrary, the opposite shift leading to q* sideon species is not favorable. When the molecule is oriented along the Ti-Ti axis and forms two Ti.e.0 bonds (2M-bidentate Fig. 9) with the coverage 19 = l/2, the heat of adsorption, 16.35 kcal mol-‘, is close to that of the perpendicular adsorption mode. CO2 is undistorted, again linear, and lies at 2.37 A above the surface plane. This adsorption mode that preserves the symmetry between the two CO bonds uses the basic property of the CO2 molecule (formation of Tie..0 bonds). Semi-empirical potential [82] and ab initio calculations [lo] have shown that on MgO this orientation is slightly less stable than the perpendicular orientation. The acidity of the adsorbed molecule is increased. Though not revealed by the atomic charge, it is seen in

the decrease of C-O overlap population (from 0.441 to 0.421) and in the shift towards lower energies of the 2p C levels (the lowest unoccupied 2p C states are shifted from +0.177 a.u. in the molecule to -0.030 au. in the projected density of states (DOS) for the adsorbed molecule). As for the perpendicular adsorption, we have also calculated the energy of the interaction between the CO2 species and H20 to show the increase of acidity of the adsorbed molecule. The geometry is shown at the right hand side of Fig. 6. The interaction energy, 8.8 kcal mol-‘, represents an increase of 33% relative to the free H20*..C02 complex. The necessity of an activation of CO2 by a metal to allow a nucleophilic attack has already been outlined in studies of the role of zinc in carbonic anhydrases [26,27]. We have investigated the lM-bidentate species proposed by Gopel et al. [6]. This system is not stable, whatever the distance to the surface is. The O-C-O angle varies with the Ti-CO2 distance. When this distance is short, CO2 bends to give a lM-bidentate species. When this distance increases, CO2 becomes linear to become 1-M-C coordinated (Fig. 1) and also unstable relative to desorption. It is not surprising that the latter species is not stable since the acidic center of CO2 is bound to the acidic center of the surface. The repulsion between CO2 and the surface already appears in cluster models (02Ti...C02 or 04Ti..*C02). 4.5. Adsorption on the oxygen atom, Ti-O*‘.C02

The adsorption of CO2 on the basic oxygen atom of the polymer is shown in Fig. 8. The best interaction involves a u lone pair of the bridging oxygen (the carbon atom lies in the polymer plane). The optimized TiO**.C02 distance is very large, 2.40 A, and is associated with a small heat of adsorption, 6.3 kcal mol-‘. Since CO2 is far from the surface, it remains linear with unchanged bond lengths. The interaction is therefore mainly electrostatic. The rotation of the CO2 around the 0.s.C bond does not affect the heat of adsorption and the best geometry corresponds to the CO2 axis perpendicular to the polymer plane (see Fig. 8). The CO2 adsorption through an electron donoracceptor interaction between the oxygen from the surface and the carbon atom, as would be expected from the acidic property of CO*, is therefore not favorable. The adsorption can be described as physisorption (van

231

A. Markovits et al./Journal of Molecular Structure (Theochem) 371 (1996) 219-235

Table 3 Heat of adsorption (in kcal mol.‘) modes of CO2 at 0 = l/2

lM-monodentate ZM-bidentate O-C coordinated

for the different

polymer

(110)

19.0 16.4 8.0

36.1 20.0 1.7

adsorption

der Waals interactions) rather than chemisorption. From these results, we conclude that the oxygen atoms from Ti02 are not basic enough to be involved in the CO* adsorption. The bare TiOz surface appears to be a very acidic surface. An extreme case of basic oxygen atom may be found in the proposed polymer describing the anatase surface [40]. In this model the oxygen atoms are either singly (0 r) or triply (Oa) coordinated. The 0 1 atom is extremely basic [40,50]. Frontier orbitals show that the Or atom is more basic than the O3 atom in spite of the trend in electronic densities (the O3 atom has the largest negative charge; this charge is stabilized by the closeness of the titanium ions and is not an index of reactivity [50]). The projected density of the states is closer to the Fermi level for Ot than for 0s. When CO2 is adsorbed on Or, the heat of adsorption is 8 kcal mall’. It is slightly higher than the adsorption energy on the rutile doubly-coordinated anion. This value represents an upper limit for an interaction with a surfate oxygen. The O...C distance is still long, 2.048 A; it is, though, shorter than on the rutile case and the COZ molecule still remains linear (CO = 1.16 A). On 03, the least basic center, CO2 does not absorb. 4.6. Adsorption on the (1 IO) surface This surface appears to be more reactive by comparison with the results on the polymer (see Table 3). Indeed, when the 3L-model is used, all the adsorption energies are larger. The optimized Ti-OCO distances are smaller than for the polymer. The Ti-OCO overlap populations are larger (Table 2). Though small, the charge transfers become more important: the donation to the surface represents 0.036 e at 0 = l/2. The qualitative conclusions derived from the polymer model are justified by the study on the (110) face; the order of the heats of adsorption for the different

Table 4 Heat of adsorption for COz (in kcal mole’) on the polymer and on the different slab models describing the (110) face

polymer 3L-model 6L-model 9L-model

0 = 1 (tilted)

0 = 1 (perpendicular)

t9 = l/2

15.5 29.5 18.4 20.3

10.4 25.3 15.0 16.6

19.0 36.1

modes is confirmed. Note that the polymer model consumes much less space and time for a computer calculation. The high reactivity of the (110) face was already observed in the case of water adsorption [28]. It was attributed to the large acidic character of the unsaturated titanium atoms of the surface that bear large positive charges. On the contrary, the (110) face appears to be less basic. The heat of adsorption on the doubly-coordinated oxygen atoms is very small, 1.65 kcal mol-‘; the (110) face is made of the coupling of adjacent polymer, the polymers in the surface plane donating electrons to the polymers perpendicular to the plane. From the optimized values of the O-coordinated CO2 adsorption, this enrichment of the perpendicular polymer does not induce an enhancement of the basicity. The unsaturated titanium atom at the surface of the 6L- or 9L-model (see Section 3b) is pentacoordinated. This increase in the coordination is expected to reduce the heats of adsorption. At 19= 1 for the perpendicular mode, the heats of CO2 adsorption are 15 and 16.6 kcal mall’ respectively. These values are closer to the polymer model than to the 3L-model (see Table 4) and close to the experimental value found by Heinrich [5]. The optimized Ti-0 distances, 2.23 and 2.22 A, are also close to those calculated on the polymer model. Thus, by means of compensation of errors, the polymer model provides a reasonable description of the adsorption. 4.7. Coadsorption with water; the hydroxylated surface In Section 7, the TiO*..C02 interaction has been shown to be weak. The possibility of obtaining a TiO...CO* interaction remains as a secondary one in addition with a main Ti...OCO interaction; then the amphoteric properties of both the adsorbate and

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et al.IJournd

of Molecular Structure (Theochem) 371 (19%) 219-235

0

,ji ,qi O-------0

/”

/

H Fig. 10. Coadsorption on the surface.

of CO* (perpendicular)

and water dissociated

Fig. 12. Adsorption

substrate would be simultaneously used. This is obtained when the CO* is vertically adsorbed on a titanium atom and adjacent to a Ti-OH: the C atom of the CO2 molecule can interact with the oxygen atom of the hydroxyl (see Fig. 10). The adsorption energy, 74.1 kcal mol-‘, then exceeds the sum of the energies for the water and the COz molecule adsorbed independently at 19= l/2 (by 1.3 kcal mol-‘). The coadsorption is therefore slightly favorable despite the saturation of the surface. The TiHO**C02 distance is larger than 3 A. We have tested the possibility of tilting the CO2 to bring the carbon atom closer to the hydroxyl group. This is not favorable. The direct approach of CO* on a Ti-OH group is repulsive. It is not surprising since the oxygen atom from the hydroxyl group is less basic than the oxygen of the bare surface (TiOTi), since it is involved in a strong OH bonding. The repulsion is small, but it increases when the carbon dioxide app:oaches the hydroxyl grtup (6.8 kcal mall’ at 2.4 A, 2.3 kcal mol-’ at 3 A, 0 = l/2). On the contrary, we have mentioned earlier that the water adsorption on preadsorbed COz was favorable. Note that in this case CO2 must then be adsorbed first (a mechanism of the Langmuir-Hinshellwood type); as the adsorption energy of CO2 is weak compared to that of the water, CO;! would not displace water on the surface. Finally, we have tested the coadsorption of a flat o--c-O

Fig. 11. Coadsorption surface.

a)

of CO2 (parallel) and water dissociated on the

b) of the 2M-bidentate

bicarbonate

species.

COz molecule with an adjacent hydroxyl (see Fig. 11). The total adsorption energy drops to 47.5 kcal mol-’ and then the CO* coadsorption on the hydroxylated surface is not favorable. The distance between the proton of the hydroxyl group and the oxygen ftom from the carbon dioxide is too long, 2.49 A, to allow a hydrogen bonding interaction. 4.8. Adsorption of COJIWhen a CO2 molecule is adsorbed on two Ti of the surface, the carbon atom becomes very acidic. This allows the migration of a hydroxyl group leading to an adsorbed bicarbonate C03H- species. An alternate mechanism that generates C03H- is the reaction between a surface hydroxyl group and a CO,; though repulsive, this interaction is weak and does not lead to high energy barrier as shown by Hoggan et al. [22]. In both cases, the resulting system (see Fig. 12(a)) is remarkably stable: the C03H- adsorption energy, 86.0 kcal mol-’ 4 (89.7 kcal mol-’ relative to Hz0 and CO2 adsorptions) is larger than the value obtained for the coadsorption of water and carbon dioxide (by 15.6 kcal mol-‘; by 16.9 kcal mol-’ when the adsorption of Hz0 and COz are calculated independently& The geometry is that ofOthe bicarbonate ion at 1.8 A from the surface (1.84 A to the oxygen atoms, the 0 atoms are not exactly above the Ti atoms). The total charge of the bicarbonate, -0.59 e, is nearly compensated by the charge of the proton attached to the surface, +0.52 e. 4 The geometry of HCO; has been optimized for the isolated molecule and transferred to the adsorbed fragment. The distany from the C atom to the surface has been assumed to be 2.3 A. This system has the same number of atoms as for the coadsorption of COz and HzO: in addition to the adsorption of HCO; on Ti a proton is adsorbed on 0 to generate a neutral system.

A. Ma&wits

233

et aLlJournal of Molecular Structure (Theochem) 371 (1996) 219-235

O----O-----o Ti

/

Ti

0 _.... _____,? . . . . . . . . . . . o

Fig. 13. Adsorption of a 2M-bidentate carbonate species; two bridging oxygen atoms from the surface are protonated. The stoichiometry of the surface cell is similar to those of Fig. 10.

Another adsorption mode for C03H- is presented in Fig. 12(b). It involves the adsorption of a hydroxyl group and therefore is not as good as the previous one: the C03H- adsorption energy, 81.9 kcal mol-’ relative to HZ0 and CO2 adsorptions, is smaller by 7.8 kcal mol-‘. This situation, however, corresponds to the transition state of the Lindskog mechanism for the first step in the hydration of CO;! [25,27] where a hydroxyl adsorbed species is converted into a carbonyl adsorbed species. 4.9. Bide&ate CO, 4.9.1. Bide&ate CO3 on hydroxylated surface Finally, we have transferred the proton of the bicarbonate on an oxygen from the polymer; this leads to the adsorption of bidentate carbonate: CO; through two Ti-0 bonds [3]. To benefit from the symmetry, we have placed the two protons on opposite bridging oxygen atoms. The geometry of the CO3 group (see Fig. 13) changes: relative to the previous case, the two C-OTi distances are elongated while the outermost C-O distance is shortened. The O-C-O angle is smaller. The charges are Qc = 1.21, Q. = - 0.88 and Qo = - 0.52 on the bound and unbound oxygen atoms respectively; this corresponds to a unitary negative charge, - e, for the adsorbed CO; species; this charge is compensated by two positive 0.5 e charges on the adsorbed protons. The formation from the hydroxylated system (r3nz0 = l/2) and the CO* molecule is exothermic. The adsorption energy, 101.2 kcal mol-’ (104.9 kcal mol-’ relative to HZ0 and COZ adsorptions), is slightly larger than that of the carbonate; this is not surprising since the carbonate formation results from the migration of a proton from an oxygen atom to a more basic one: the oxygen from

/

Fig. 14. A distortion of the best adsorption mode from Fig. 8 that leads to a CO; bidentate species. An oxygen from the surface has moved and become symmetric to an oxygen from the adsorbate. This is not favorable.

the adsorbed carbonate, Ti2C03, is less basic than an oxygen atom from the surface; the HOMO of HZCOLl (a model for Ti2C03) lies at - 0.488 au.; i.e. at a lower level than the Fermi level of the bare polymer, -0.399 a.u. (the valence band is localized on the oxygen atoms). Therefore, the proton preferentially binds to the oxygen from the polymer. 4.9.2. Bidentate CO3 on bare surface A CO3 species can be formally generated from a CO2 molecule and an oxygen atom from Ti02 in the case of a bare surface. This formal case is investigated in this section. The first orientation that we have calculated for CO* parallel to the surface (see Section 6 and Fig. 8) was chosen to favor the formation of a C-O bond. If this distance were to be short, it would have led to a lM-bidentate carbonate species (counting the 0 atom from the surface inside the CO3 species). This did not happen and the CO formation went together with the breaking of the Ti-0 bond leading to a O-C coordinated species. We tried another possibility by moving an oxygen atom out of the surface plane to favor the formation of the carbonate (Fig. 14). The resulting system is not stable. From these test calculations, we conclude that the carbonate formation is difficult on the bare surface and should only occur when the surface is hydroxylated.

5. Conclusion In this work, we have investigated various adsorption modes of the carbon dioxide molecule on Ti02. We have mainly used a polymer chain as model for TiOz. On the (110) face, using the 3 layers model, the description of the adsorption remains the same though

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A. Markovits et al.lJournal of Molecular Structure (Theochem) 371 (1996) 219-235

this face is more reactive. When more layers are included in the slab (6L-9L models), the unsaturated titanium centers are penta-coordinated and less reactive. Then, the heats of the adsorption and the geometric parameters get closer those for the polymer model. The bare Ti02 surface is very acidic. At 8 = l/2, the best adsorption mode corresponds to a perpendicular CO2 molecule over an unsaturated titanium cation from the surface. Therefore, CO2 appears as a weak base interacting with the acidic center of the titanium oxide through the formation of a Ti-OCO bond. At saturation (0 = l), the adsorbate-adsorbate interaction forces the adsorbates to bend over the surface; the angle between the molecular axis and the direction perpendicular to the surface is 29” and the relative geometry of two adjacent CO;? molecules is close to that of the optimized dimer structure. At 19= l/2, the parallel CO2 (2M-bidentate) orientation is slightly less stable than the perpendicular one. In both orientations, the adsorbed CO2 species are activated molecules for which the carbon atom becomes very acidic and can react with a surface hydroxyl. Then carbonate and bicarbonate species are obtained. CO2 adsorption on the bare surface do not lead to carbonate species. Although CO2 is known usually as an acidic species, the interaction with a basic oxygen of the bare Ti02 surface is not favorable; this surface is too acidic. This interaction leads either to a small repulsion and desorption or to a weakly physisorbed species.

Acknowledgements We thank Professor Silvi for stimulating

discussions.

References [l] P. Braunstein, D. Matt and D. Nobel, Chem. Rev., 88 (1988) 747. [2] B. Denise and R.P.A. Sneeden, Chemtech, February (1982) 108. [3] K. Tanaka, K. Miyahara and I. Toyoshima, J. Phys. Chem., 88 (1984) 3504. [4] K. Tanaka and J.M. White, J. Phys. Chem., 86 (1982) 3977. [5] V. Heinrich, Rep. Prog. Phys., 48 (1985) 1481. [6] W. GBpel, G. Rocker and R. Feierabend, Phys. Rev. B, 28 (1983) 3427.

[7] G.B. Raupp and J.A. Dumesic, J. Phys. Chem., 89 (1985) 5240. [8] C. Morterra, G. Ghiotti, E. Garrone and E. Fisicaro, J. Chem. Sot. Faraday I, 76 (1980) 2102. [9] J.A. Rodriguez, Langmuir, 4 (1988) 1006. [lo] G. Pacchioni, Surf. SC., 281 (1993) 207. [ll] A. Auroux and A. Cervasini, J. Phys. Chem., 94 (1990) 6371. [12] A. Cervasini and A. Auroux, J. Therm. Anal., 37 (1991) 1737. [13] A. Fahmi and C. Minot, J. Organometall. Chem., 478 (1994) 67. [14] H. Knozinger, in K. Tanabe and H. Hattori (Eds.) Probing Acid Sites by Carbon Monoxide, 147: Acid-Base Catalysis, Kodansha, 1989. [15] K.M. Neyman, P. Strodel, S.P. Ruzankin, N. Schlensog, H. Kn ozinger and N. Rosch, Catal. L.&t., 31 (1995) 273. [16] P. Strode], K.M. Neyman, H. Kn6zinger and N. Rosch, Chem. Phys. Lett., 240 (1995) 547. [17] G. Busca, H. Saussey, 0. Saur, J.-C. Lavalley and V. Lorenzelli, Appl. Catal., 14 (1985) 245. [18] M. Bensitel, V. Moravek, 3. Lamotte, 0. Saur and J.C. Lavalley, Spectrochimica Acta, Part A, 43 (1987) 1487. [19] H. Knozinger, Adv. Catal., 25 (1976) 234. [20] A.A. Tsyganenko and E.A. Trusov, J. Phys. Chem., 59 (1985) 1554. [21] J. Lamotte, 0. Saur, J.C. Lavalley, G. Busca, P. Rossi and V. Lorenzolli, J. Chem. Sot. Faraday Trans. I, 82 (1986) 3019. [22] P.E. Hoggan, M. Bensitel and J.C. Lavahey, J. Mol. Struct. (Theochem), 320 (1994) 49. [23] J. Liang and W.N. Lipscomb, J. Amer. Chem. Sot., 108 (1986) 5051; Int. J. Quant. Chem., 36 (1989) 299. [24] K.M. Merz, R. Hoffmann and M.J.S. Dewar, J. Amer. Chem. Soc.,lll (1989) 5636. [25] S. Lindskog, in T.G. Spiro (Ed.) Zinc Enzymes, Wiley, New York, 1983, p. 77. [26] 0. Jacob, R. Cardenas and 0. Tapia, J. Amer. Chem. Sot., 112 (1990) 8692. [27] M. Sola, A. Lledos, M. Duran and J. Bertran, J. Amer. Chem. Sot., 114 (1992) 869. [28] J.V. Evans and T.L. Whateley, Trans. Farad. Sot., 63 (1967) 2769. [29] N.D. Parkyns, J. Chem. Sot. A, l-4 (1969) 410. [30] H.P. Boehm, Discuss. Faraday Sot., 52 (1971) 264. [31] D.J.C. Yates, J. Phys. Chem., 65 (1961) 746. [32] M. Primet, P. Pichat and M.-V. Mathieu, J. Phys. Chem., 75 (1971) 1221. [33] K. Tanaka and J.M. White, J. Phys. Chem., 86 (1982) 4768. [34] R. Dovesi, C. Pisani, C. Roetti and M. Causl, cRysT~L88, QCPE Program No. 577, Bloomington, IN, 1989. [35] C. Pisani, R. Dovesi and C. Roetti, in Hartree-Fock Ab Initio Treatment of Crystalline Systems, Lecture Notes in Chemistry 48, Springer, Heidelberg, 1988. [36] Y. Bouteiller, C. Mijoule, M. Nizam, J.C. Barthelat, J.P. Daudey, M. Pelissier and B. Silvi, Mol. Phys., 65 (1988) 295. [37] B. Silvi, N. Fourati, R. Nada and C.R.A. Catlow, J. Phys. Chem. Solids, 52 (1991) 1005. [38] A. Fahmi, C. Minot, B. Silvi and M. Causa, Phys. Rev. B, 47 (1993) 11717.

A. Markovits et al.lJournal of Molecular Structure (Theochem) 371 (1996) 219-235 [39] P. Durand and J.C. Barthelat, Theor. Chim. Acta, 38 (1975) 283. [40] A. Fahmi and C. Minot, Surf. Sci., 304 (1993) 343. [41] B. Silvi and V. Chandrasekharan, Mol. Phys., 48 (1983) 1053. [42] G. Herzberg, Electronic spectra of polyatomic molecules, in C. Litton (Ed.), Molecular spectra and molecular structure, Part III, Van Nostrand Reinhold, New York, 1966. [43] P. Reinhardt and B.A. Hess, Phys. Rev. B, 50 (1994) 12015. (441 R. Cambi, D. Cappelletti, G. Liuti and F. Pirani, .I. Chem. Phys., 95 (1991) 1852. [45] G.R. Alms, A.K. Bumham and V.H. Flygare, .I. Chem. Phys., 63 (1975) 3321. [46] P.W. Fowler and P.A. Madden, J. Phys. Chem., 89 (1985) 2581. [47] E.W. Pearson, M.D. Jackson and R.G. Gordon, J. Phys. Chem., 88 (1984) 119. [48] C. Girard, C. Girardet and B. Silvi, Chem. Phys., 125 (1988) 261. [49] A. Fahmi, C. Minot, P. Fourre and P. Nortier, Surf. Sci., 343 (1995) 261. [50] C. Minot, A. Fahmi and J. Ahdjoudj, in L.J. Farrugia (Ed.) Periodic HF Calculations of the Adsorption of Small Molecules on Ti02: 257, The Synergy Between Dynamics and Reactivity at Clusters and Surfaces, KIuwer, Dordrecht, The Netherlands, 1995. [51] L.A. Curtis& K. Raghavachari and J.A. Pople, J. Chem. Phys., 98 (1993) 1293. [52] L.A. Curtiss, K. Raghavachari and J.A. Pople, Chem. Phys. Lett., 214 (1993) 183. [53] T.H. Dunning, J. Chem. Phys., 53 (1970) 2823. [54] V. Kello, M. Urban, J. Noga and G.H.F. Diercksen, J. Amer. Chem. Sot., 106 (1984) 5864. [55] N. Hartz, G. Rasul and G.A. Olah, J. Amer. Chem. Sot., 115 (1993) 1277. [56] B.J. Smith and L. Radom, .I. Amer. Chem. Sot., 115 (1993) 4885. [57] T.L. Tso and E.K.C. Lee, J. Phys. Chem., 89 (1985) 1612. [58] P.A. Block, M.D. Marshall, L.G. Pedersen and R.E. Miller, J. Chem. Phys., 96 (1992) 7321. [59] J.R. Damewood, R.A. Kumpf and W.C. Miilhbauer, J. Phys. Chem., 93 (1989) 7640.

235

[60] J. Sadlej and P. Mazurek, P.; J. Mol. Struct. (Theochem). 337 (1995) 129. [61] J. Makarewicz, T.-K. Ha and A. Bauder, J. Chem. Phys., 99 (1993) 3694. [62] K.F. Willey, C.S. Yeh, D.L. Robbins and M.A. Duncam, Chem. Phys. Lett., 192 (1992) 179. [63] M. Sodupe, C.W. Bauschlisher and H. Partridge, Chem. Phys. Len., 192 (1992) 185. [64] R. Caballol, E. Sanchez Marcos and J.-C. Barthelat, J. Phys. Chem., 91 (1987) 1328. [65] J. Mascetti and M. Tranquille, J. Phys. Chem., 92 (1988) 2177. [66] Z.H. Kafafi, R.H. Hauge, W.E. Billups and J.L. Margrave, Inorganic Chem.. 23 (1984) 177. [67] B. Jdnsson, G. Karlstrom and H. Wennerstrom. J. Amer. Chem. Sot., 100 (1978) 1658. [68] Z. Peng and K.M. Merz, Jr., J. Am. Chem. Sot., 115 (1993) 9640. [69] L. Fredin, B. Nelander and G. Ribbegard, Chem. Ser., 7 (1975) 11. [70] T.-L. Tso and E.K.C. Lee, J. Phys. Chem., 89 (1989) 1618. [71] In Handbook of Chemistry and Physics, 58th Edn., CRC Press, Cleveland, OH, 1977,. [72] G. Ramis, G.Busca and V. Lorenzelli, V.; Mater. Chem. Phys.. 29 (1991) 425. [73] A.A. Tsyganenko, L.A. Denisenko, S.M. Zverev and V.N. Filimonov, J. Catalysis, 94 (1985) 10. [74] D. Scarano and A. Zecchina, Spectrochimica Acta, Part A, 43 (1987) 1441. [75] D. Scarano and A. Zecchina, Surf. Sci., 198 (1988) 11. [76] J.C. Lavalley, J. Saussey and A.A. Tsyganenko, Surf. Sci., 315 (1994) 112. [77] A. Lakhlifi and C. Girardet, Surf. Sci., 241 (1991) 400. [78] A. Lakhlifi and C. Girardet, J. Chem. Phys., 94 (1991) 688. [79] G.L. Griffin and J.T. Yates, J. Chem. Phys., 77 (1982) 3751. [80] C. Minot, M.A. Van Hove and J.P. Biberian, Surf. Sci.. 346 (1996) 283. [Sl] Y.A. Tsyganenko, V.A. Ermoshin, K.S. Smirnov and A.A. Tsyganenko, in R. Pick (Ed.), Europhysics Conference Abstracts, FrPcl2, ECOSS 15, EPS, 1995. [82] V. Panella, J. Suzanne, P.N.M. Hoang and C. Girardet, J. Phys. I. France. 4 (1994) 905.