A quantum-mechanical study of CO adsorbed on TiO2: A comparison of the Lewis acidity of the rutile (1 1 0) and the anatase (1 0 1) surfaces

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Abstract

The adsorption of carbon monoxide on the rutile (1 1 0) and the anatase (1 0 1) surfaces has been investigated by a periodic approach using hybrid-exchange density functional theory; the quantum-mechanical study on the CO adsorption on the anatase surface has been carried out for the first time. The full optimization of the systems has been performed by considering different surface coverages and periodicities. The adsorption energetics have been investigated, on the basis of a recently proposed model, in terms of interaction, distortion and binding energies and the lateral effects have been analyzed using a simple model of nearest and next-nearest neighboring molecules. The experimental adsorbed CO stretching frequencies have been compared with the computed ones obtained by considering the highest investigated periodicity. The Lewis acidity of the two surfaces is comparable and the rutile (1 1 0) acid site seems to be slightly more electrophilic than that of the anatase (1 0 1) one.

Introduction

Titanium dioxide represents one of the most interesting metal oxides as it is widely employed in the field of heterogeneous catalysis [1]. The two natural allotropic forms mainly considered are the rutile and the anatase ones, whose the most stable surfaces are represented by the (1 1 0) and the (1 0 1), respectively. These two surfaces have similar structure and are constituted by fivefold and sixfold coordinated titanium cations (indicated as Ti(5f) and Ti(6f)) and twofold and threefold coordinated oxygen anions (indicated as O(2f) and O(3f)). The Ti(5f) and the O(2f) ions are the most active because the lower coordination of these ions bind more strongly with an adsorbed molecule, i.e. they represent the surface Lewis acid and basic sites, respectively. A significant difference between the two surfaces is that the rutile one is flat with the O(2f) anions bound to the Ti(6f) cations and projected out of the surface plane, while the anatase one has a sawtooth profile in which the O(2f) anions are directly bound to the Ti(5f) cations. A comparison on the electrophilicity of the two Lewis acid sites can be useful to understand the possible different behaviour of the two surfaces with respect to the same adsorbate. A widely employed technique to determine the surface Lewis acidity is the IR spectroscopy: among the possible probe basic molecules which can be used for this scope, the carbon monoxide represents the most useful one [2]. The way in which this molecule interacts with the surface Lewis acid site of a metal oxide is well-known. CO is coordinated by aσ-bond to metal cations which have no d electrons, e.g. Ti4+, and the CO stretching frequency shifts toward higher wavenumbers with respect to the gas-phase (i.e. 2143 cm−1); the greater is the electrophilicity of the surface Lewis acid site, the higher is the IR stretching frequency of the adsorbed CO [3]. From the experimental point of view, the adsorption of CO on powdered rutile and anatase has been widely investigated [3], [4], [5], [6], [7], [8], [9]. In general, it has been found that the CO stretching occurs at ∼2190 cm−1 for the rutile, and at ∼2190 and 2210 cm−1 for the anatase. A possible explanation given for the presence of two absorptions in the latter form is that the weaker (∼190 cm−1) occurs on a Ti(5f) cation while the stronger (∼2210 cm−1) is related to a Ti(4f) cation [3]. The determination of the heat of adsorption has been subject of different works [5], [6], [7] in which a correlation between the adsorption energy and the CO stretching mode has also been found: as the binding energy increases the CO stretching absorption moves towards higher wavenumbers. In general, it has been deduced that the absorption at 2210 cm−1 is associated to the heat of adsorption of about 70 kJ mol−1 while the one at 2190 cm−1 is connected with the value of about 50 kJ mol−1. From the computational point of view, the CO-rutile (1 1 0) system has been investigated by several authors and the obtained results largely different depend on the employed computational conditions, e.g. the approach adopted to simulate the surface, the method and the basis set; in general, the binding energy and the blue shift of the CO stretching vibration can be found in the ranges 22–110 kJ mol−1 and 28–140 cm−1, respectively [10], [11], [12], [13], [14], [15], [16], [17]. One of the most important common results is that the adsorption through the C atom is energetically much more favorable than that through the O one [11], [16], [17].

It is important to notice that, to our knowledge, no works related to the simulation of the CO-anatase (1 0 1) system are reported in the literature.

The aim of this work is to study the adsorption of CO on the rutile (1 1 0) and the anatase (1 0 1) surfaces at the same time in order to compare the electrophilicity of the two surface Lewis acid sites. The calculations are performed at DFT/B3LYP level by using a periodic approach and the adsorption is investigated by considering different surface coverages and periodicities. The energetics are studied in terms of interaction, distortion and binding energies according to a recent scheme [18], [19] in which a simple model of nearest and next-nearest neighboring molecules has been adopted. The calculated vibrational frequency of the adsorbate is chosen in correspondence of the highest considered periodicity, i.e. when the lateral effects are negligible.

Section snippets

Computational details

The calculations have been performed at DFT level using the CRYSTAL03 and CRYSTAL06 software packages [20], [21], based on the expansion of the crystalline orbitals as a linear combination of a basis set consisting of atom centred Gaussian orbitals.

The titanium and oxygen atoms have been described by a double valence all-electron basis set (an 86-51 G∗ contraction: one s, three sp and one d shell) and a triple valence (an 8-411G contraction: one s and three sp shell), respectively [22]; the most

Results and discussion

The rutile and the anatase phases belong to the P42/mnm (D4h14) and I41/amd (D4h19) tetragonal space group, respectively. The unit cell is defined by the lattice vectors aB and cB (the subscript B denotes the bulk phase) and the fractional coordinate u. The rutile (1 1 0) and the anatase (1 0 1) surfaces are depicted in Fig. 1a. They have been cut from the bulk whose optimized parameters are reported in Table 1, along with the structural parameters defining the primitive surface cell, i.e. the

Conclusions

This work reports the first quantum-mechanical study on the adsorption of carbon monoxide on the anatase (1 0 1) surface. Moreover, it makes a comparison between the electrophilicity of the Lewis acid site of this surface with that of the rutile (1 1 0) one. The simulation of the adsorbate-substrate systems has been performed at DFT level using the hybrid exchange-correlation functional B3LYP. The two surfaces have been modeled by using a slab thickness which converges with respect to both the

Acknowledgment

Financial support by PRIN funds is gratefully acknowledged.

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