Lattice Oxygen Activity in Pr- and La-Doped CeO2for Low-Temperature Soot Oxidation

Abstract

Effects of Pr- and La-doped CeO2 on desorption of active lattice oxygen to soot oxidation were studied. Thermogravimetric analysis under N2 flow indicated that, despite the decrease in reducible Ce content, the amount of oxygen deficiency increases with increasing La content (x) to 15 mol % in Ce0.80–xPr0.20LaxO2. In inert atmosphere, the introduction of oxygen vacancies by doping La increased the available lattice oxygen for desorption. In the oxygen isotopic exchange, under 18O2 flow, desorption of oxygen molecules containing lattice oxygen atom (16O) was favored because of Pr and La doping in CeO2. These dopants increase the exchange rate constant between gas-phase and lattice oxygen atoms. The apparent activation energy, derived from an Arrhenius plot, for CeO2, Ce0.80Pr0.20O2, and Ce0.65 Pr0.20La0.15O2 were 311, 230, and 135 kJ/mol, respectively. The values indicated that Pr and La doping causes facile oxygen exchange between the gas phase and the solid. The comparison of the rate constants for 16O18O formation under 18O2 flow and 18O2/16O2 mixture flow showed that lattice oxygen predominantly contributes to the exchange reaction. Surface exchange coefficient (k) and diffusion coefficient (D) were obtained by secondary ion mass spectrometry (SIMS) depth analysis of the oxides after diffusion annealing in 18O2. The values of both k and D increase by doping Pr and La, and the surface exchange coefficient value, k, especially increases significantly. Hence, Pr and La doping enhances the oxygen exchange with lattice oxygen, and so high soot oxidation activity at low temperature could be related to the increased contribution of lattice oxygen.