Theoretical study of the adsorption of acids and bases
on TiO
2and MgO surfaces (*)
A. MARKOVITS, J. AHDJOUDJand C. MINOT
Laboratoire de Chimie Théorique, UPR 9070 CNRS, Tour 23-22, p. 114, Boîte 137 Université P. et M. Curie - 75252 Paris, Cédex 05, France
(ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — The adsorption of small molecules on TiO2and MgO is investigated by ab initio periodic Hartree-Fock calculations. The adsorption may be molecular or dissociative. This depends on their acidic and basic properties in gas phase. For the molecular adsorption, the molecules are adsorbed as bases on the metal sites and the adsorption energies correlate with the proton affinities. This also true on MgO, even if CO2 is ordinary considered as an acidic probe molecule. HSBA theory allows to compare the relative adsorption of molecules on TiO2and MgO. The dissociations on the surface correlate with the gas phase cleavages: basic cleavage for MeOH and acidic cleavage for MeSH. Another important factor is the adsorbate-adsorbate interaction: at high coverage, stabilization through H-bonds favors the vicinity of adsorbates on the surface (islands of adsorbates and coadsorption effects). Electro-static interactions explain the change of orientation (CO2) with the coverage. PACS 82.65.My – Chemisorption.
PACS 01.30.Cc – Conference proceedings.
1. – Introduction
The acidity or basicity of active sites of solid catalysts is of crucial importance to determine their adsorption properties [1]. On the surface of metal oxides, the exposed metallic cations are the acidic centers whereas the surface oxygen ions are the basic sites. In this paper, we present Hartree-Fock periodic calculations on the adsorption of various molecules (NH3, CO2, hydroxylic compounds) on perfect surfaces. Such
molecules have Lewis acidic and basic sites. The program, CRYSTAL-92 [2, 3], is presented in ref. [4]. We consider a slab and define a surface unit cell by two translation vectors. For MgO(100), we have used a single layer. The electron charge distribution at the surface is close to that in the bulk.
To represent the TiO2surfaces, we have used a polymer that is characteristic of the
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
rutile structure. It is a chain containing the sequence of titanium atoms and the equatorial oxygen atoms attached to them (see fig. 1) obtained by the cleavages of all the apical Ti-O bonds. It presents two sites for adsorption, the cation sites (Lewis acids) and bridging oxygen sites (Lewis bases). Calculations have also been done with a slab
BASIC SITE
ACIDIC SITE
Fig. 1. – The (110) face of rutile results from the coupling of TiO2polymers which contain the basic and acidic sites of adsorption. Small white circles are titanium atoms and large grey circles are oxygen atoms.
representing the (110) face that is the most stable face [5, 6]; the heats of adsorption are classified in the same order; the values are larger since the Ti sites from the (110) face are particularly acidic due to the donation from the polymer in the surface plane to the perpendicular polymer.
The adsorption energies are defined positive for a stable adsorbate/substrate system according to the expression: Eads4 Eadsorbate1 Esubstrate2 E( adsorbate 1substrate ),
where E( adsorbate 1substrate )is the energy of the adsorbate/substrate system, Esubstrateis the
energy of the substrate and Eadsorbate is the energy of the isolated adsorbate in its
equilibrium geometry. Our study concerns perfect surfaces; some studies point out that defects may be important, for example on MgO surfaces [7-9] and on TiO2[10].
2. – Molecular adsorption on the clean surfaces
On the titanium oxide, all the molecules that are adsorbed without dissociation are bound to the exposed metal centers and the heats of adsorption correlate with the gas phase proton affinities. Therefore, adsorbates must be considered as bases and the surface appears to be acidic. The extreme cases are ammonia and carbon dioxide that are experimentally [11, 12] used as probe molecules to test the surface basicity (CO2) or
acidity (NH3). At u 41/2, the most basic adsorbate is ammonia [13]. Ammonia behaves
as a base that donates its electron pair to the metal generating a dative Ti-N bond perpendicular to the surface polymer [13, 14]. The optimized Ti-N distance, 2.19 Å, is close to the sum of the covalent radii of Ti and N, 2.07 Å; the adsorption is clearly a
chemisorption. The lack of any appreciable charge transfer (0.012 at u 41) is also found with weaker bases [15-19]. The electron density along the Ti-N bond is very asymmetric, with a high density on the nitrogen; this is the reason why the Ti-N overlap population is weak, 0.043.
Ti Ti O O O O O O O O C Mg Mg Mg Mg Mg O O O O O C O A) B)
Fig. 2. – A) The perpendicular adsorption mode on TiO2 obtained at low coverage; at high coverage lateral interactions force the molecule to bend on the surface. B) The parallel adsorption mode on MgO above the Mg rows.
NH3 on MgO is also adsorbed on the acidic Mg centers. The adsorption is
exothermic for coverages 1/2 (Eads4 6.3 kcal/mol) or below. The heat of adsorption is
weaker since Mg2 1 is less acidic than Ti4 1. MgO is considered as a basic surface;
however, the adsorption on the basic center (the oxygen atom) through a hydrogen is weak (Eads4 3.1 kcal/mol).
CO2 appears on TiO2 as a weak base. This is unambiguous since in the best
adsorption mode it interacts with the metal center, perpendicular [20] or parallel to the surface (less favorable by 2.6 kcal/mol). The adsorption where the carbon atom of CO2
would be an electrophilic center attacked by an electron pair of a surface oxygen atom is poorly stabilizing relative to other modes, Eads4 6.3 kcal/mol. The large optimized
OQ Q QC distance, 2.4 Å, would correspond to a physisorption.
Auroux and Gervasini [11, 12] emphasized that the adsorption of CO2 on MgO was
larger than that of NH3, at variance with that on TiO2. For these authors, the large
heat of adsorption ECO2/MgOis due to an increased surface basicity, CO2behaving as the
acid. On the contrary, our calculations show that CO2 behaves as a base in both cases.
We compute the CO2adsorption on the clean MgO(100) surface and found, as Girardet
et al. [21-23] and Pacchioni [18], the adsorption mode parallel to the surface, CO2
bridging two magnesium atoms as shown in fig. 2B). At u 41/2, the perpendicular adsorption mode leads to a weakly negative Eads(21.7 kcal/mol) whereas the
adsorption on the oxygen atom that would imply the basicity of the surface is repulsive by 30 kcal/mol; we conclude that the O atoms from the MgO surface are not enough basic for the CO2 adsorption in agreement with a (OMg5)81/CO2cluster calculation [24].
In table I, the heats of adsorption of CO2and NH3on TiO2and MgO are displayed.
TABLEI. – Heats of adsorption on TiO2(polymer model) and MgO(100) in kcal/mol. Basis sets for the adsorbate are PS31G. For Ti and Mg the basis sets are from ref. [25] and [26] except for the value in parentheses when the “ionic” basis for MgO has been taken [27, 28].
Substrate Eads(CO2) Eads(NH3)
TiO2 MgO 19.0 7 (8.3) 47.6 6.3 (9.3)
Gervasini [11, 12]. The differential adsorption mode may be explained by the HSAB theory [29, 30]. The hard base, NH3, interacts preferentially on the hard acid, Ti4 1,
more strongly charged, whereas the soft base, CO2, interacts on the soft acid, Mg2 1.
According to Klopman, the hardness is measured by the E* energy value, the energy of an orbital under the influence of its partner; those for Ti4 1 and Mg2 1are 4.35 eV and
2.42 eV, respectively.
Without dissociation (the dissociation on the surface is favorable), H2O is adsorbed
on TiO2above Ti, fig. 3A). The four atoms TiQ Q QOH2are in a plane to allow a conjugation
of the p electron pair of the water with the d vacant orbitals of the titanium atom of the same symmetry. Since its interaction with Ti is weaker, H2S can more easily deviate
from the perpendicularity [31]. The sulfur atom is hybridized and H-bonds can be built with the surface oxygen. In the best geometry, H2S is nearly parallel to the surface
plane (the angle is 107) and Eads is improved by 2.5 kcal/mol.
On MgO(100) at u 41/2, the water appears rather as an acid than as a base. The best adsorption mode
(
fig. 3B))
uses both properties but primarily appears as an adsorption on an O site that benefits from a secondary MgQ Q QO interaction. Using the “ionic” basis set for MgO [27], Scamehorn [8, 32] has found that at u 41/4 H2O wasacidic and adsorbed through two HQ Q QO bonds
(
fig. 3C))
.It is surprising that MgO appears to be basic for the H2O adsorption and acidic for
the CO2adsorption. CO2is a stronger acid in the gas phase.
CO2+OH2KCO3H2(E4282.3 kcal/mol); H2O+OH2KOH2QQQOH2(E4230 kcal/mol).
OH2is a much stronger base than MgO; in the gas phase the formation of the
bicar-bonate corresponds to an activationless chemical reaction [33]. In aqueous phase, solvation effects decrease the nucleophilicity of OHsolvated and there is an energy
barrier. Under adsorption, a different mechanism occurs. The nucleophilic attack that would lead to the formation of a possible O2CQ Q QOMg precursor is not confirmed by the
calculations. The energy level of the LUMO of CO2is high, +0.177 a.u. CO2 becomes
acidic when distorted, bent with elongated CO distances. For CO=1.24 Å and OCO=1207, ELUMOis 20.046 a.u. The direct adsorption of such species (through C on a
O surface atom) still corresponds to an endothermic adsorption. Thus, the formation of the bicarbonate should not begin by a nucleophilic attack of the carbon atom of CO2
unless CO2 is first activated; this is done by the OCOQ Q QM adsorption. An argument
supporting this behaviour may be found in biochemically active species. For the mechanism of the hydration of CO2by carbonic anhydrase, two mechanisms have been
proposed; one [34, 35] corresponds to a nucleophilic attack of M-OH2groups to the CO 2
molecule, while the second one [36] involves the binding of CO2 to metallic center. In
agreement with our surface model, calculations [37] favor the second mechanism.
3. – Dissociative adsorptions
If the adsorption is strong enough, the adsorbate may be distorted or dissociated into fragments. Formally considering a heterolytic cleavage of the adsorbate, for instance H2O, one gets both an acidic and a basic fragment. The former adsorbs on the
surface oxygen and the latter on the titanium. Noguera et al. [38] have calculated the independent adsorptions of a proton and of an hydroxyl group. For TiO2, the
adsorption of OH2
(
Eads( OH2) 48.3 eV
)
is larger than that of H1 (Eads4 5.9 eV). Inspite of the extreme acidity of the proton, the adsorption of the basic species is the largest. The dissociative adsorption is energetically favorable [25, 39]. For MgO that is more basic, it is the other way round
(
Eads( OH2) 40.7 eV vs. Eads( H1) 48.5 eV). Thedissociation is not favorable [8, 38, 40-42]. It should happen on surface with defects [8, 9, 43]. H2S cleaves very easily relative to H2O, which is consistent with gas
phase cleavages.
For ROH and RSH, two cleavages, acidic or basic, may be distinguished. On the surface, they imply a different orientation of the fragments [31]. When ROH gives a basic cleavage, the fragment R is adsorbed on the bridging oxygen atom and the titanium center is hydroxylated (singly coordinated hydroxyl). On the contrary for an acidic cleavage, the OR fragment goes on the titanium center and a bridging hydroxyl group is formed. For MeOH the basic cleavage is obtained on the surface since, in the gas phase, it corresponds to the least endothermic cleavage. For MeSH, the result is reversed; in the gas phase the acidic cleavage is easier.
4. – Lateral and secondary interactions
At high coverages, the adsorbate molecules interact with each other. The main interactions are H-bonds and electrostatic interactions.
The adsorption of H2O at u 41 allows a sequence of H bonds (fig. 4) that does not
exist at lower coverage [25]. The heat of adsorption for u 41/2 K1 is larger than that for u 40 K1/2 by 9 kcal/mol. A similar effect explains a co-adsorption effect between H2O and NH3. The molecular adsorption of NH3 on a hydroxylated surface
(u 41/2) is more favorable than the adsorption on a bare surface (by 8 kcal/mol). Here again, there is a sequence of H-bonds between the NH3 and the adjacent hydroxyl
groups [13].
The best adsorption mode for carboxylic acids involves two adsorbed oxygen atoms. This favors the acidic cleavage vs. the basic cleavage. For the formic acid [44], both the O from the carbonyl group and the O from the hydroxyl group are adsorbed on adjacent metal centers (bidentate adsorption mode resulting from an acidic cleavage). For the oxalic acid [45], oxygens from each carboxyl group are involved. Ammonium can be adsorbed when bound to three surface oxygens by 3 H-bonds [13].
Electrostatic interactions can be seen for CO2/TiO2. At high coverage (u 41) the
optimization of the lateral interactions forces the adsorbed molecule to bend over the surface. The relative orientation of adjacent CO2is then close to the optimal geometry
of the dimer. Similar effects are also found for CO2/MgO; we calculated the interaction
between the adsorbates in the ordered CO2 monolayers; for the perpendicular mode
the CO2Q Q QCO2 interactions are repulsive by 2.8 kcal/mol whereas they become
attractive by 2 kcal/mol for the parallel mode shown in fig. 2B). The lateral interactions between the adsorbates thus contribute to favor the parallel mode; no such effect can easily be included in cluster calculations. Girardet et al. [21] proposed a large supercell were the relative orientations of parallel CO2alternate. Since the CO2Q Q QCO2interaction
is slightly more attractive than in the uniform orientation, this distribution should be the best.
The adsorption of CO on MgO is another example of the importance of lateral interactions between adsorbates [26]. At low coverages, the CO’s are vertical above the surface [46] and parallel to each other. At u 43/4, the molecules move to decrease their mutual repulsion. In the most favorable mode, two thirds of the CO molecules are tilted over the surface to decrease the repulsion between the adsorbates. They are above the Mg rows; the adsorption therefore still involves uniquely the cations sites of the surface.
5. – Conclusion
The most general adsorption mode on the clean TiO2and MgO surfaces implies the
exposed metal centers. These surfaces appear consequently as acidic species. H2O/MgO
is the main exception; the driving force for this adsorption is the HOHQ Q QOMg bond formation even though a weak H2OQ Q QMgO bond is also formed. The cleavage of the
molecules corresponds to the easiest cleavage in the gas phase. However, when the fragment adsorbed has several functional groups, the adsorption is determined by secondary interactions. For the carboxylic acid, the acidic cleavage is obtained since the formed ion is adsorbed by its two oxygen atoms. At high coverage, lateral effects are important. Stabilization through H-bonds favors the vicinity of adsorbates on the surface (islands of adsorbates, high coverage and coadsorption with ammonia). Electrostatic interactions explain the change of orientation between low and high coverages. The lateral interactions are more important on MgO than on TiO2. The
difference of reactivity on the clean surfaces between TiO2and MgO may be expressed
in terms of acid vs. basic behaviour for H2O adsorption and in terms of hard and soft
acidity for CO2adsorption.
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