Low-Temperature IR Spectroscopy of CO Adsorption on Calcined Supported CeO 2 : Probing Adsorbed Species and Adsorbing Sites

Abstract

Carbon monoxide adsorption 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 CeO₂ and the individual support materials (SiO₂ and Al₂O₃). The adsorbents were prepared ex situ by heating at 770 K (3 h) in air, and pretreated in situ by heating at 720 K (1 h) in oxygen and then in vacuum. The results, as disclosed by v(OH) (3900–3300 cm⁻¹) and v(CO) (2250–2050 cm⁻¹) 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 SiO₂), hydrogen-bonded CO (on all of the adsorbents, but not CeO₂) and CO bound to electron-rich defect sites (only on unsupported CeO₂). It is concluded that the dispersion of ceria, particularly on alumina, is associated with a considerable development of the Lewis acidity of Ce⁴⁺ sites.