CO adsorbed on the hydroxylated rutile (110) and anatase (101) surfaces: a quantum-mechanical study Jessica Scaranto and Santi Giorgianni Università Ca’ Foscari di Venezia – Dipartimento di Chimica Fisica, Calle Larga S. Marta 2137, I 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 represented by the (110) and the (101), respectively. Like the other metal oxides, TiO 2 adsorbs water when exposed at the atmosphere: the H 2 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 as consequence of a possible modification of the electrophilicity of the surface Lewis acid site represented by the Ti 4+ 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 which have no d electrons, e.g. Ti 4+, and the CO stretching frequency shifts toward higher wavenumbers with respect to the gas–phase (i.e cm -1 ); the greater is the electrophilicity of the surface Lewis acid site, the higher is the IR stretching frequency 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 groups: they derives from a dissociatevely adsorption of H 2 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 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. References References [1] Diebold, U. Surf. Sci. Rep. 2003, 48, 53. [2] Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem., 1971, 75, [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, [8] Muscat, J. PhD Thesis, University of Manchester, [9] Krishan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. Computational details Program CRYSTAL06 [6]Method DFT/B3LYP [7] Basis set Ti : DVAE (86-51G* ) [8] O : TVAE (8-411G) [8] CO, OH, H : standard 6-311G* [9] Study of the adsorption energetics Symbols mol: molecule sur: surface sys: adsorbate-substrate system E x : optimised energy of X E x|sys : energy of X at the geometry of the adsorbate-substrate system Interaction energy E P int = E sys - (E mol|sys + E sur|sys ) Distortion energy E P dis = (E mol|sys - E mol ) + (E sur|sys - E sur ) Binding energy BE P = E sys - (E mol + E sur ) Ti(5f) Ti(6f)O(3f)O(2f) Clean and hydroxylated rutile (110) surface Ti(5f)O(2f)O(3f)Ti(6f) Clean and hydroxylated anatase (101) surface CO = 2274 CO = 2274 CO on the clean rutile (110) CO = 2276 CO = 2276 CO on the clean anatase (101) CO = 2271 CO = 2271 CO on the hydroxylated anatase (101) [Structure I] CO = 2278 CO = 2278 CO = 2262 CO = 2262 CO on the hydroxylated rutile (110) [Structure I] CO on the hydroxylated rutile (110) [Structure II] CO = 2273 CO = 2273 CO on the hydroxylated anatase (101) [Structure II] Conclusion The presence of an hydroxyl group deriving from the adsorption of a proton on a two-fold coordinated oxygen ion (see structure I) gives rise to a decrease of the electrophilicity of the near Lewis acid site for both the rutile (110) and the anatase (101) surface. The effect is bigger for the rutile phase. The presence of an hydroxyl group deriving from the adsorption of a OH group on a five-fold coordinated titanium ion (see Structure II) gives rise to a increase and to a decrease of the near Lewis acid site for the rutile (110) and the anatase (101) surface, respectively.