Characterization of La2O3/SiO2 mixed oxide catalyst supports

Thermal analysis, energy dispersive spectroscopy, X-ray diffraction, high-resolution electron microscopy, X-ray photoelectron spectroscopy, and CO2 adsorption have been applied to study a series of La2O3/SiO2 samples with lanthana loadings ranging from 1.6 to 75 wt%. The results are interpreted on the basis of a lanthana-silica interaction model which assumes that patches of an amorphous La-Si-O phase, embedded in the silica matrix, modify the support surface in a progressive way, leading to a full coverage for 37.5% La2O3. This mixed silicate is soluble in acid media and its properties can be satisfactorily explained considering a structure which has a short-range order resembling that of crystalline La2Si2O7. This behaviour differs from that described in the La2O3/Al2O3 system, more extensively studied in the literature, and from previous studies on La2O3/SiO2 in which the formation of La2O3 particles on the silica surface was proposed. This contribution serves as a basis for understanding the additional promoting effect of lanthana in catalysts consisting of metals dispersed on modified silica supports. (C) 1999 Academic Press.