642
Langmuir 1993,9, 642-645
IR Study of CO Adsorption at -77 K on ~t-AlzO3 C. Morterra,. G. Magnacca, and N. Del Favero Department of Inorganic, Physical and Materials Chemistry, University of Turin, via P. Giuria 7, I-101% Turin, Italy Received July 27, 1992. In Final Form: January 22, 1993 The low-temperature (-77 K)adsorption of CO onto a low surface area microcrystalline specimen of a-Aln03 has been studied by FTIR spectroscopy. CO uptake yields a broad complex absorption, in which (atleast)two poorlyresolved componenbare preeent, ascribed to the weak u-coordination onto coordinatively unsaturated AI ions in octahedral coordination,and to the interaction with surface OH groups. Though hardly observable in the VOH spectral region, OH groups are still present also after activation at hi& temperatures, and are perturbed by CO uptake.
Introduction a-Al2O3 possesses the corundum (trigonal) structure, and representsthe highest temperature step of the complex thermal phase transformation of AI hydroxides/~xides.~-~ Unlike transition aluminas, in the bulk a-Al203 phase the Al3+ ions occupy only octahedral interstices within the close-packed oxygen lattice (Avr). The surface of an oxide is normally thought of as a diffuse defect, in which the truncation of the crystallitesproduces coordinative unsaturations in the surface cations. These are partly saturated, upon exposure to the atmosphere, b y the formation of a surface h y d r a t e d layer, and the coordinative unaaturations are reproduced when the system is activated in vacuo, Le., when the oxide (in the present case a-Al203) is thermally dehydrated. a-A1203 is thus a suitable system to investigate the surface chemical features of coordinatively unsaturated (cus) A13+ions with octahedral coordination (Alvrcw), produced i n vacuo t h r o u g h the thermal surface dehydration process. Some work was proposed i n the past concerning some aspects of the surface chemistry of a-Al203 (e.g., see refs 4-6), but no detailed investigations have appeared y e t in the literature on the interaction of CO with a-Al203. The Lewis acidity of Alvrcwcenters is known to be fairly low; in fact, no CO adsorbs on a-A1203 at ambient temperature? whereas at low temperature a weak CO band was observed b y Zecchina et alS7at -2165 cm-l, Le., in the spectral position where, on other Al oxidic systems, bands ascribed to CO weakly interacting with Alvrcwcenters and/or with surface OH groups have been r e p ~ r t e d . ~ - l ~ The aim of the present paper is to understand whether, at the surfaceof a well-crystallizedmicrocrystalline a - A l 2 0 3 preparation, Alvrcussurface centers are actually strong enough to coordinate CO at -77 K and, if t h e y do so, (1) Lippens, B. C.; Steggerda,J. J. In Physical and Chemical Aspects Adsorbents and Catalysts; Limen, B. C., Fortuin, M. H., Okkersee, C., Steggerda, J. J., Eds.; Academic Press: London, 1970; p 171. (2) Stegmann, M. C.; Vivien, D.; Mazieres, C. J. Chim. Phys. Rays.Chim. Biol. 1974, 71, 761. (3) Baraton, M. I.; Quintard, P. J. Mol. Strut. 1982, 79, 337. (4) Morterra, C.; Chiotti, C.; Garrone, E.; Boccuzzi, F. J.Chem. SOC., Faraday Trans. 1 1976, 72, 2722. (5) Morterra, C.; Coluccia, S.;Chiotti, G.;Zecchina, A. 2.Phys. Chem., N.F., 1977, 104, 275. (6) Morterra, C.; Coluccia, S.;Chiorino, A.; Boccuzzi, F.J. Catal. 1978, of
54, 348.
(7) Zecchina, A.; Escalona Platero, E.; Otero Arean, C. J. Catal. 1987, 107, 244. (8) Morterra, C.; Emanuel, C.; Cerrato, G.,Magnacca, G.J. Chem. SOC.,Faraday Trans. 1992,88, 339. (9) Morterra,C.; Cerrato, G.; Cuzzato, P.; Masiero, A.; Padovan, M. J. Chem. SOC.,Faraday Trans. 1992,88, 2239. (10) Zaki, M. I.; Knhinger, H. Mater. Chem. Phys. 1987, 17, 201. (11) Zaki, M. I.; Knbzinger, H. Spectrochim. Acta 1987, 43A, 1455. (12) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041.
what the IR spectral features of the relevant CO adspecies are. This problem is of some interest, both per se and as a reference for the _controversialassignment of some CO bands observed at v I2170 cm-l at the surface of spinel transition aluminas, and assigned either to a-coordination onto AlVIcw sites7 or to the interaction with surface OH groups.10
Experimental Section a-A12O3specimens (Brunauer-Emmett-Teller, BET) surface area 4.5 m2 gl)were prepared by the oven thermal treatment a t 1523 K (in air) of self-supporting pellets of a pure boehmite preparation, whoae characteristics have been reported elsewhere.s X-ray diffraction (XRD) spectrograms and high-resolution transmission electron microscope ((HR)TEM) images indicated that the a-phase so obtained is crystallographically pure, and the crystallites are quite well formed (mainly hexagonal particles of 250-400 nm per side, characterized by sharp edges and terminating with extended patches of few regular low-index crystal planes.13 For in situ IR experiments, the thin a-AlzOs pellets (15-20 mg cm-2) were mounted in a thick gold frame, which guarantees a good thermal contact and leaves exposed to the IR beam a small circular portion of 12 mmz. The pellets were then transferred to a vacuum cell, containing CsI windows, and suitable for thermal treatments and for IR experiments at low temperatures (the cell has been described elsewhere"). The cell was attached to a conventional high-vacuum system (residual pressure
