Surface characterization of modified aluminas. Part 4.—Surface hydration and Lewis acidity of CeO2–Al2O3 systems

Abstract

In catalysis, it has been suggested that the addition of CeO₂ and other rare-earth metal oxides to alumina can stabilize the surface area and the spinel transition phases of the alumina support used in automotive catalysis. As this structural stabilizing effect by CeO₂ was recently observed to occur only to a very limited extent, it was thought that the beneficial effect of CeO₂ addition to Al₂O₃ supports would be useful for surface chemistry rather than structurally and morphologically. The surface characterization of several CeO₂–Al₂O₃ systems has been carried out mainly by in situ FTIR spectroscopy and adsorption microcalorimetry. Specific OH groups bound to Ce surface cations are difficult to identify, but the usual OH profile of aluminas is modified by the presence of CeO₂ to an extent that depends on CeO₂ content and on activation temperature. Using CO as a surface probe molecule the surface Lewis acidity of the CeO₂–Al₂O₃ mixed systems was characterized and it was revealed that CeO₂ accumulates preferably on the flat patches of low-index crystal planes of the spinel structure, and that the presence of Ce cations stabilizes, also at high temperatures, the most acidic Lewis centres. The latter are constituted by coordinatively unsaturated Al ions with (incomplete) tetrahedral coordination and are probably located in crystallographically defective situations. Moreover, the presence of CeO₂ brings about the formation of a family of Lewis-acid sites of medium strength, which are not present at the surface of bare (transition) aluminas.