SURFACE CHARACTERIZATION OF MODIFIED ALUMINAS .4. SURFACE HYDRATION AND LEWIS ACIDITY OF CEO2-AL2O3 SYSTEMS

In catalysis, it has been suggested that the addition of CeO2 and othe r rare-earth metal oxides to alumina can stabilize the surface area an d the spinel transition phases of the alumina support used in automoti ve catalysis. As this structural stabilizing effect by CeO2 was recent ly observed to occur only to a very limited extent, it was thought tha t the beneficial effect of CeO2 addition to Al2O3 supports would be us eful for surface chemistry rather than structurally and morphologicall y. The surface characterization of several CeO2-Al2O3 systems has been carried out mainly by in situ FTIR spectroscopy and adsorption microc alorimetry. Specific OH groups bound to Ce surface cations are difficu lt to identify, but the usual OH profile of aluminas is modified by th e presence of CeO2 to an extent that depends on CeO2 content and on ac tivation temperature. Using CO as a surface probe molecule the surface Lewis acidity of the CeO2-Al2O3 mixed systems was characterized and i t was revealed that CeO2 accumulates preferably on the flat patches of low-index crystal planes of the spinel structure, and that the presen ce of Ce cations stabilizes, also at high temperatures, the most acidi c Lewis centres. The latter are constituted by coordinatively unsatura ted Al ions with (incomplete) tetrahedral coordination and are probabl y 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.