Surface Characterization of γ-Ga 2 O 3 : A Microcalorimetric and IR Spectroscopic Study of CO Adsorption M. Rodrı ´guez Delgado, † C. Morterra,* ,‡ G. Cerrato, ‡ G. Magnacca, ‡ and C. Otero Area ´n † Departamento de Quı ´mica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain, and Dipartimento di Chimica IFM, Universita ` di Torino and Consorzio INSTM, Unita ` di Ricerca di Torino, 10125 Torino, Italy Received August 6, 2002. In Final Form: October 16, 2002 Strong Lewis acidity of a phase-pure γ-Ga2O3 was studied by the adsorption of CO at ambient temperature, with the combined use of adsorption microcalorimetry and in situ Fourier transform infrared spectroscopy. The concentration of strong Lewis acid sites turned out to be quite low, but it grew fast with increasing surface dehydration upon thermal treatments in the 573-773 K range. Two main families of Lewis acid sites were observed, characterized by rather different molar adsorption heats and CO stretching frequencies. These acid sites have been ascribed to coordinatively unsaturated Ga 3+ ions located in defective (higher νCO) and regular (lower νCO) crystallographic sites, respectively. The possibility of extracting, for the CO/ γ-Ga2O3 system, quantitative information from the IR spectroscopic data is discussed in some detail. 1. Introduction Gallium(III) oxide, also known as gallia, presents a polymorphism analogous to that of alumina. 1 The only thermodynamically stable polymorph is -Ga 2 O 3 , which has a monoclinic structure. However, several metastable (low temperature) forms of Ga 2 O 3 exist, which are similar to the well-known transition aluminas; 2 depending on the preparation conditions, they can appear as single or as mixed crystal phases. 3 Among these metastable poly- morphs, γ-Ga 2 O 3 has a defective spinel-type structure analogous to that of γ-Al 2 O 3 . Preparation of this polymorph as a pure crystalline phase has recently been reported, 3 as well as a preliminary characterization of its surface structure. The present work is aimed at a further and detailed study of the surface chemistry of phase-pure γ-Ga 2 O 3 , which is accomplished by using Fourier transform infrared (FTIR) spectroscopy and microcalorimetric mea- surements of adsorbed carbon monoxide. Understanding of the surface chemistry of γ-Ga 2 O 3 is relevant in view of (i) the possible presence in the surface layers of γ-Ga 2 O 3 of both tetrahedral and octahedral cationic sites, which can be studied without the perturba- tions deriving from the presence of mixed phases; (ii) the use of gallium-containing materials (mainly zeolites) as industrial catalysts, for example, for hydrocarbon dehy- drogenation and cyclization; 4,5 (iii) the recently claimed potential use of Ga-containing oxidic systems as catalysts for the abatement of nitrogen oxides 6 and for hydrocarbon isomerization; 7 and (iv) the fact that in these catalytic processes both structural features and the role played by gallium surface centers are still far from being understood. The combined use of FTIR spectroscopy and adsorption microcalorimetry affords characterization of the strong Lewis acidity of γ-Ga 2 O 3 , that is, the acidic character of coordinatively unsaturated cations as monitored by interaction (at ambient temperature) with a weak Lewis base such as CO. FTIR spectroscopy of adsorbed CO gives information on the types of acidic sites present and on their relative proportions, while adsorption microcalo- rimetry helps to quantify the interaction energy and the amount of adsorbed CO. 2. Experimental Section 2.1. Materials. Phase-pure γ-Ga2O3 was prepared starting from an ethanolic solution of high-purity (hydrated) gallium nitrate (Avocado Italia, s.r.l.) and concentrated aqueous ammonia diluted in ethanol, as described in detail elsewhere. 3 The hydrated gallia gel thus obtained was thoroughly washed, filtered, and vacuum-dried, and the resulting xerogel was calcined in air at 573 K (30 min) leading to a single-phase γ-Ga 2O3 polymorph having the spinel-type structure, as revealed by powder X-ray diffraction (XRD); 3 this material is hereafter referred to as the “starting γ-Ga2O3”. From N2 adsorption-desorption isotherms at 77 K, the Brunauer-Emmett-Teller (BET) surface area of the starting γ-Ga2O3 was found to be SBET ) 191 m 2 g -1 , and the pore volume VP ) 0.16 cm 3 g -1 , as reported in the first row of Table 1. For both IR and microcalorimetric adsorption measurements, γ-Ga2O3 required a proper in situ vacuum activation, which was carried out as follows. The starting γ-Ga2O3 underwent a first activation in vacuo and oxidation (40 Torr O2) in the vacuum system at 673 K, to eliminate the impurities present on the sample (basically nitrates, carbonates, and carbonaceous residues, deriving from the early steps of preparation, calcination, and exposure to the atmosphere); this step was followed by a thorough rehydration (in the vacuum system, with saturated water vapor at ambient temperature) and a final in situ vacuum activation (2 h, residual pressure < 10 -5 Torr) and oxygen treatment at one of the selected temperatures: 573, 673, and 773 K. The * Corresponding author. Prof. Claudio Morterra, Dipartimento di Chimica IFM, via P. Giuria 7, 10125 Torino, Italy. Tel: +39 011 670 7589. Fax: +39 011 670 7855. E-mail: claudio.morterra@ unito.it. † Universidad de las Islas Baleares. ‡ Universita ` di Torino and Consorzio INSTM. (1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, 1984. (2) Lippens, B. C.; Steggerda, J. J. Physical and Chemical Aspects of Adsorbents and Catalysts; Linsen, B. G., Fortuin, M. H., Okkersee, C., Steggerda, J. J., Eds.; Academic Press: London, 1970; p 171. (3) Otero Area ´n, C.; Lo ´pez Bellan, A.; Pen ˜ arroya Mentruit, M.; Rodrı ´guez Delgado, M.; Turnes Palomino, G. Microporous Mesoporous Mater. 2000, 40, 35. (4) Inui, T.; Matsuda, H.; Yamase, O.; Nagata, H.; Fukuda, K.; Ukawa, T.; Miyamoto, A. J. Catal. 1986, 40, 491. (5) Khodakov, A. Yu.; Kustov, L. M.; Bondarenko, T. N.; Dergachev, A. A.; Kazansky, V. B.; Minachev, Kh. M.; Borbely, G.; Beyer, H. K. Zeolites 1990, 10, 603. (6) Haneda, M.; Kintaichi, Y.; Hamada, H. Appl. Catal., B 1999, 20, 289. (7) Moreno, J. A.; Poncelet, G. J. Catal. 2001, 203, 453. 10255 Langmuir 2002, 18, 10255-10260 10.1021/la026362x CCC: $22.00 © 2002 American Chemical Society Published on Web 11/21/2002