Evidence for Discrepancy between the Surface Lewis Acid Site Strength and Infrared Spectra of Adsorbed Molecules: The Case of Boria-Silica A. Travert,* ,† A. Vimont, † J.-C. Lavalley, † V. Montouillout, †,‡ M. Rodrı ´guez Delgado, § J. J. Cuart Pascual, § and C. Otero Area ´ n § Laboratoire Catalyse et Spectrochimie, CNRS-ENSICAEN, UniVersite ´ de Caen, 6 BouleVard du Mare ´ chal Juin, 14050 Caen Cedex, France, and Departamento de Quı ´mica, UniVersidad de las Islas Baleares, 07122 Palma de Mallorca, Spain ReceiVed: May 14, 2004; In Final Form: July 23, 2004 The acidity of amorphous B 2 O 3 -SiO 2 has been investigated by infrared spectroscopy using the following three probe molecules presenting a wide range of basic strength: pyridine, acetonitrile, and carbon monoxide. The results are compared to those obtained on γ-Al 2 O 3 . No coordination of carbon monoxide is observed on B 2 O 3 -SiO 2 even at low temperatures, whereas strongly coordinated CO species are formed on γ-Al 2 O 3 under such conditions. Coordinated pyridine and acetonitrile show important infrared frequency shifts on both metal oxides, indicating strong charge transfer from the probe molecules to the surface Lewis acid centers. However, the thermal stability of coordinated species is much lower on B 2 O 3 -SiO 2 than on γ-Al 2 O 3 , which suggests that there is no direct correlation between charge transfer and adsorption energy. Density functional theory (DFT) calculations on the interaction of these probe molecules with simple models representing Al 3+ and B 3+ Lewis acid sites adequately reproduce experimental observations. The main difference between Al 3+ and B 3+ results from the higher energy required to convert boron from a trigonal planar conformation to a tetrahedral conformation upon adsorption of the probe molecule. Despite strong charge transfer, this leads to a weaker adsorption of pyridine and acetonitrile on B 3+ as compared to Al 3+ Lewis acid sites. Carbon monoxide is not basic enough to compensate for the energy required for the conformational change of the B 3+ Lewis acid center. 1. Introduction Infrared spectroscopy of adsorbed basic probe molecules is one of the most often used technique for characterizing the acidic properties of solid surfaces, particularly those of metal oxides. 1 Provided that the molecular probe has been well chosen, its infrared spectrum shows absorption bands that are characteristic of its interaction with the surface (hydrogen bonding, protona- tion, or coordination) and allows the nature (Brønsted or Lewis acid) of the surface adsorption sites to be determined. The number of surface acid sites of each type can then be assessed by measuring the intensities of the IR bands of the adsorbed probe molecules, provided that their molar absorption coef- ficients are known. More problematic is the determination of the strength of surface acid sites. Infrared spectroscopy could allow this measurement to be performed, provided that correlations between the IR spectra and adsorption heats can be established. The Brønsted acid sites of metal oxides present the advantage of being directly observable by infrared spectroscopy through the ν(OH) absorption bands when H-bonded complexes are formed. By analogy with liquid solutions, 2 empirical relation- ships between ν(OH) frequency shifts and the adsorption heats of H-bonded basic probe molecules could be established; 3,4 the higher the ν(OH) shift, the higher the adsorption heat. When spectra in the ν(OH) range are too complex to be accurately analyzed, which is often the case for many metal oxides, frequency shifts of specific IR absorption bands of the adsorbed probe molecule can also give useful information on surface acid strength; referring to the infrared spectrum of the probe molecule in a liquid or gas phase, shifts of specific IR bands can thus be correlated with interaction energy. 5-7 On the other hand, Lewis acid sites which are formally electron lone pair acceptors cannot be directly detected by infrared spectroscopy. Hence, the strength of these sites can only be assessed by the infrared spectra of molecules interacting with them. Charge transfer occurring from the adsorbed probe molecule leads to important modifications of its electron density, which in turn is reflected in frequency shifts of characteristic infrared absorption bands. 8 Carbon monoxide, 1,9-12 acetonitrile, 1,13-16 and pyridine 1,17-21 are the most frequently used molecules for probing surface Lewis acidity; their ν(CO), ν(CtN), and ν(CdC) (ν 8a and ν 19b ring vibrations) bands are very sensitive to the charge transfer to the adsorption site: Despite their very different basicities, these probe molecules give rise to similar trends for the surface Lewis acidity of metal oxides showing a wide range of acid strength. 1,16 In particular, the observed frequency shifts were directly correlated to the * Corresponding author. Phone: +33(0)2 31 45 28 23. Fax: +33(0)2 31 45 28 21. E-mail: arnaud.travert@ismra.fr. † Universite ´ de Caen. ‡ Present address: CNRS-CRMHT, 1D Avenue de la Recherche Scien- tifique, 45071 Orleans Cedex 2, France. § Universidad de las Islas Baleares. 16499 J. Phys. Chem. B 2004, 108, 16499-16507 10.1021/jp0479365 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/23/2004