Surface characterization of modified aluminas .5. Surface acidity and basicity of CeO2-Al2O3 systems

The addition of CeO2 to Al2O3 supports and Al2O3-based catalysts leads to improved performance, that can be explained by surface chemical rather than structural morphological effects. Surface characterization of CeO2-Al2O3 systems has been carried out by in situ FTIR spectroscopy. The adsorption-desorption of pyridine as a surface probe allowed us to test for Bronsted acidity, never found, and the total Lewis acidity, which is higher for CeO2-Al2O3 systems than for pure Al2O3 supports. Several types of surface centres of different acidity could be identified, mainly by testing the competitive adsorptivity towards CO after stepwise desorption of pyridine at various temperatures. The adsorption of CO2 was used to test the basicity of CeO2-Al2O3 systems. The presence of CeO2, a basic oxide, does not modify appreciably the basic properties of Al2O3, on which a low concentration of rather weak basic centres is present, whereas the presence of Al2O3 tends to inhibit the high basicity typical of CeO2.