Solid state reactions in highly dispersed single and mixed lanthanide oxide–SiO2 systems

This paper summarizes our previous studies on the interaction of highly dispersed lanthanide oxides (La, Ce, Pr, Nd and Lu) with amorphous silica and presents new data on the Ce0.5Yb0.5O1.75–SiO2 system. It is shown that at temperatures above 800 °C, chemical reaction between lanthanide oxide and silica may occur leading to formation of silicate phases with unusual structure not observed for bulk like oxide–SiO2 systems. For nano Ce0.5Yb0.5O1.75–SiO2 system results of TEM, SEM and XRD studies revealed that at or above 1000 °C, ytterbium is withdrawn from the mixed oxide and B-type polymorph of Yb2Si2O7 (Yb4[Si3O10][SiO4]) is formed. This polymorph, till now known as high pressure form, is unstable at 1100 °C and transforms into ordinary C-type Yb2Si2O7. Mechanism of silicate formation and its possible role in deactivation of supported catalysts containing lanthanide oxides as active phase or as promoters is discussed.