CO adsorbed on the hydroxylated rutile (110) and anatase (101) CO adsorbed on the hydroxylated rutile (110) and anatase (101) surfaces: a quantum-mechanical studysurfaces: a quantum-mechanical study

Jessica Scaranto and Santi GiorgianniUniversità Ca’ Foscari di Venezia – Dipartimento di Chimica Fisica, Calle Larga S. Marta 2137, I-30123 Venezia, Italy

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 used are the rutile and the anatase ones, whose the most stable surfaces are 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 used are the rutile and the anatase ones, whose the most stable surfaces are represented by the (110) and the (101), respectively. Like the other metal oxides, TiOrepresented by the (110) and the (101), respectively. Like the other metal oxides, TiO

22 adsorbs water when exposed at the atmosphere: the H adsorbs water when exposed at the atmosphere: the H22O molecule can adsorb or molecularly or dissociatevely. Then, in the field of heterogeneous catalysis it is O molecule can adsorb or molecularly or dissociatevely. Then, in the field of heterogeneous catalysis it is

necessary to pre-treat the surface before performing the adsorption of the reagents. The pre-treated surface usually presents some isolated hydroxyl groups which can not be easily removed [2,3]. The presence of these OH groups may affect the adsorption necessary to pre-treat the surface before performing the adsorption of the reagents. The pre-treated surface usually presents some isolated hydroxyl groups which can not be easily removed [2,3]. The presence of these OH groups may affect the adsorption as consequence of a possible modification of the electrophilicity of the surface Lewis acid site represented by the Tias consequence of a possible modification of the electrophilicity of the surface Lewis acid site represented by the Ti 4+4+ ion. A widely employed technique to determine the surface Lewis acidity is the IR spectroscopy: among the possible probe basic ion. 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 [4]. The way in which this molecule interacts with the surface Lewis acid site of a metal oxide is well-know. CO is coordinated by a s–bond to metal cations molecules which can be used for this scope, the carbon monoxide represents the most useful one [4]. The way in which this molecule interacts with the surface Lewis acid site of a metal oxide is well-know. CO is coordinated by a s–bond to metal cations which have no d electrons, e.g. Tiwhich have no d electrons, e.g. Ti4+4+, and the CO stretching frequency shifts toward higher wavenumbers with respect to the gas–phase (i.e. 2143 cm, and the CO stretching frequency shifts toward higher wavenumbers with respect to the gas–phase (i.e. 2143 cm -1-1); the greater is the electrophilicity of the surface Lewis acid site, the higher is the IR stretching frequency ); the greater is the electrophilicity of the surface Lewis acid site, the higher is the IR stretching frequency of the adsorbed CO [5]. of the adsorbed CO [5]. Here we present the main results obtained from a periodic quantum-mechanical study of the adsorption of CO on the clean and the hydroxylated rutile (110) and the anatase (101) surfaces. In particular we have considered two kinds of isolated hydroxyl Here we present the main results obtained from a periodic quantum-mechanical study of the adsorption of CO on the clean and the hydroxylated rutile (110) and the anatase (101) surfaces. In particular we have considered two kinds of isolated hydroxyl groups: they derives from a dissociatevely adsorption of Hgroups: they derives from a dissociatevely adsorption of H

22O and are represented by a proton bound to a surface two-fold coordinated oxygen ion [O(2f)] and by a OH bound to a surface five-fold coordinated titanium ion [Ti(5f)]. Structures I and II O and are represented by a proton bound to a surface two-fold coordinated oxygen ion [O(2f)] and by a OH bound to a surface five-fold coordinated titanium ion [Ti(5f)]. Structures I and II

represent the situation in which the CO adsorb next to the former and the latter OH group, respectively. The calculations have been performed at DFT/B3LYP level using the CRYSTAL06 sofware package [6]. In order to isolate the two OH groups a represent the situation in which the CO adsorb next to the former and the latter OH group, respectively. The calculations have been performed at DFT/B3LYP level using the CRYSTAL06 sofware package [6]. In order to isolate the two OH groups a periodicity equal to (3x3) has been adopted. The adsorption energetics have been investigated in terms of interaction, distortion and binding energies. The effect on the electrophilicity of the surface Lewis acid site has been evaluated on the basis of both periodicity equal to (3x3) has been adopted. The adsorption energetics have been investigated in terms of interaction, distortion and binding energies. The effect on the electrophilicity of the surface Lewis acid site has been evaluated on the basis of both the interaction energy and of the shift of the CO stretching vibration.the interaction energy and of the shift of the CO stretching vibration.

ReferencesReferences[1] Diebold, U. Surf. Sci. Rep. 2003, 48, 53.[2] Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem., 1971, 75, 1216. [3] Lewis, K.E.; Parfitt, G.D. Trans. Faraday Soc., 1966, 62, 204. [4] Zecchina, A.; Lamberti C.; Bordiga, S. Catalysis Today 1998, 41, 169. [5] Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61 and references therein. [6] Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson C. M.; Pascale, F; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL06 User’s Manual, University of Torino (Torino, 2006).[7] Becke, A.D. J. Chem. Phys. 1993, 98, 5648.[8] Muscat, J. PhD Thesis, University of Manchester, 1999.[9] Krishan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650.

Computational detailsComputational details

ProgramProgram

CRYSTAL06 [6]

MethodMethod

DFT/B3LYP [7]

Basis setBasis set

Ti : DVAE (86-51G* ) [8]O : TVAE (8-411G) [8]

CO, OH, H : standard 6-311G* [9]

Study of the adsorption energeticsStudy of the adsorption energetics

SymbolsSymbolsmol: molecule

sur: surface

sys: adsorbate-substrate system

Ex: optimised energy of X

Ex|sys: energy of X at the geometry of the

adsorbate-substrate system

Interaction energyInteraction energy

EPint = Esys - (Emol|sys + Esur|sys)

Distortion energyDistortion energy

EPdis = (Emol|sys - Emol) + (Esur|sys - Esur)

Binding energyBinding energy

BEP = Esys - (Emol+ Esur)

Ti(5f) Ti(6f)O(3f) O(2f)

Clean and hydroxylated rutile (110) surfaceClean and hydroxylated rutile (110) surface

Ti(5f) O(2f)O(3f)Ti(6f)

Clean and hydroxylated anatase (101) surfaceClean and hydroxylated anatase (101) surface

COCO = 2274 = 2274

CO on the clean rutile (110)CO on the clean rutile (110)

COCO = 2276 = 2276

CO on the clean anatase (101)CO on the clean anatase (101)

COCO = 2271 = 2271

CO on the hydroxylated anatase (101) [Structure I]CO on the hydroxylated anatase (101) [Structure I]

COCO = 2278 = 2278

COCO = 2262 = 2262

CO on the hydroxylated rutile (110) [Structure I]CO on the hydroxylated rutile (110) [Structure I]

CO on the hydroxylated rutile (110) [Structure II]CO on the hydroxylated rutile (110) [Structure II]

COCO = 2273 = 2273

CO on the hydroxylated anatase (101) [Structure II]CO on the hydroxylated anatase (101) [Structure II]

ConclusionConclusion

The presence of an hydroxyl group deriving from The presence of an hydroxyl group deriving from the adsorption of a proton on a two-fold the adsorption of a proton on a two-fold

coordinated oxygen ion (see structure I) gives rise coordinated oxygen ion (see structure I) gives rise to a decrease of the electrophilicity of the near to a decrease of the electrophilicity of the near Lewis acid site for both the rutile (110) and the Lewis acid site for both the rutile (110) and the

anatase (101) surface. The effect is bigger for the anatase (101) surface. The effect is bigger for the rutile phase.rutile phase.

The presence of an hydroxyl group deriving from The presence of an hydroxyl group deriving from the adsorption of a OH group on a five-fold the adsorption of a OH group on a five-fold

coordinated titanium ion (see Structure II) gives coordinated titanium ion (see Structure II) gives rise to a increase and to a decrease of the near rise to a increase and to a decrease of the near

Lewis acid site for the rutile (110) and the anatase Lewis acid site for the rutile (110) and the anatase (101) surface, respectively.(101) surface, respectively.