377 Low-temperature IR Spectroscopy of CO Adsorption on Calcined Supported Ce0 2 : Probing Adsorbed Species and. Adsorbing Sites Gamal A.H. Mekhemer' and Mohamed I. Zaki> (1) ChemistryDepartment, Faculty of Science, Minia University. El-Minia 61519. Egypt. (2) ChemistryDepartment, Faculty of Science, Kuwait University, P.O. Box 5969 Safar. 13060 Kuwait. (Received 3 August 1996; accepted 30 October 1996) ABSTRACT: Carbon monoxideadsorption on ceria dispersed on silica (CeSi) and alumina (CeAl) at 300-80 K was observed by in-situ IR spectroscopy. For control purposes,CO adsorption was also observed on unsupported Ce0 2 and the individual support materials (Si0 2 and AlPJ)' The adsorbents were prepared ex situ by heating at 770 K (3 h) in air, and pretreated in situ by heating at 720 K (I h) in oxygen and then in vacuum.The results, as disclosed by v(OH) (3900-3300 cm') and v(CO) (2250-2050 em:') spectra taken before and after CO adsorption, reveal that CO adsorbs on all of the test adsorbents at temperatures < 300 K only. The resulting adsorbed species include CO coordinated to Lewis acid sites (on all of the adsorbents, but not Si0 2 ), hydrogen-bonded CO (on all of the adsorbents, but not Ce0 2 ) and CO bound to electron-rich defect sites (only on unsupported Ce0 2 ). It is concluded that the dispersion of ceria, particularly on alumina, is associated with a considerabledevelopment of the Lewis acidity of Ce 4+ sites. INTRODUCTION Ceria, Ce0 2 , is a competent catalytic material in redox reactions (Taylor 1984; Zaki and Sheppard 1983). This arises from its tolerance to perfectly reversible oxygenation/deoxygenation cycles (Ostuka et al. 1985) without disruption of the fluorite geometry of the lattice-structure (Brauer et al. 1960). In fact, ceria has been found to exhibit extended defect structures (Ce0 2 _. ) prior to conversion into Ce,O, (Laachir et al. 1991). The ease of electron migration across the surface, which is a prime requirement of redox catalysis, is facilitated by the availability of interactive 'Ce 4+D' defect sites or Ce 4+-Ce3+ pair-sites, whether exposed on Ce0 2 particles or included in CeOx monolayer structures (Yao and Yu-Yao 1984; Kaufherr et al. 1985). Hence studies focusing on bulk ceria (Zaki and Sheppard 1983; Mekhemer 1995) have found that its redox catalytic prop- erties are dependent on the particle size of the material. This may be understood in terms of the particle size dependence of the n-semiconductive properties of ceria (Tandon and Gupta 1970; Sundaram and Wahid 1990). For these reasons, we were stimulated to carry out surface and bulk studies of ceria dispersed on high surface-area support materials, an area of work which has not hitherto received much re- search attention (Shyu et at. 1988; Haneda et al. 1993; Bensalem et al. 1995). In a recent communication (Zaki et al. 1996), we reported the results of X-ray diffractometry (XRD) and *Author to whom all correspondence should be addressed.