Low-Temperature IR Spectroscopy of CO Adsorption on Calcined Supported CeO2: 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.