Structure evolution of nanocrystalline CeO2 and CeLnOx mixed oxides (Ln = Pr, Tb, Lu) in O2 and H2 atmosphere and their catalytic activity in soot combustion

Abstract

This paper reports results of studies on structure and activity in soot combustion of nanocrystalline CeO2 and CeLnOx mixed oxides (Ln=Pr, Tb, Lu, Ce/Ln atomic ratios 5/1). Nano-sized (4–5nm) oxides with narrow size distribution were prepared by a microemulsion method W/O. Microstructure, morphology and reductivity of the oxides annealed up to 950°C in O2 and H2 were analyzed by HRTEM, XRD, FT-IR, Raman spectroscopy and H2-TPR. Obtained mixed oxides had fluorite structure of CeO2 and all exhibited improved resistance against crystal growth in O2, but only CeLuOx behaved better than CeO2 in hydrogen.The catalytic activity of CeO2, CeLnOx and physical mixtures of CeO2+Ln2O3 in a model soot oxidation by air was studied in “tight contact” mode by using thermogravimetry. Half oxidation temperature T1/2 for soot oxidation catalysed by nano-sized CeO2 and CeLnOx was similar and ca. 100°C lower than non-catalysed oxidation. However, the mixed oxides were much more active during successive catalytic cycles, due to better resistance to sintering. Physical mixtures of nanooxides (CeO2+Ln2O3) showed exceptionally high initial activity in soot oxidation (decrease in T1/2 by ca. 200°C) but degraded strongly in successive oxidation cycles. The high initial activity was due to the synergetic effect of nitrate groups present in highly disordered surface of nanocrystalline Ln2O3 and enhanced reductivity of nanocrystalline CeO2.