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.
Surface characterization of modified aluminas. Part 4. Surface hydration and Lewis acidity of CeO2-Al2O3 systems
MORTERRA, Claudio;BOLIS, Vera Maria;MAGNACCA, Giuliana
1996-01-01
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.
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