Synthesis and catalytic properties of Ce0.6Zr0.4O2 solid solutions in the oxidation of soluble organic fraction from diesel engines

Ce0.6Zr0.4O2 solid solutions were synthesized by co-precipitation, sol–gel like method, solution combustion and surfactant-assistant approaches, respectively. The catalytic properties of bulk and γ-Al2O3 supported Ce0.6Zr0.4O2 solid solutions were studied for the oxidation of soluble organic fractions (SOF) from diesel engines by TG-DTA method. The physicochemical properties were characterized by XRD, BET surface area and pore distribution, SEM, TEM, and particle size distribution techniques. XRD and TEM results show that a Ce0.6Zr0.4O2 solid solution was formed for samples as-prepared and heat-treated at 900 °C for 2 h in air. The co-precipitation derived Ce0.6Zr0.4O2 has as high BET surface area as 153.71 m2/g by controlling preparation conditions. Notable is that the surface area and particle size for fresh Ce0.6Zr0.4O2 ignited at 350 °C decreased little after a thermal treatment in air at 900 °C for 2 h. Furthermore, its bulk density is lowest. The commercial engine oil (SJ5W/40) for FAW-VOLKSWAGEN, which was used by Bora 1.9 TDI diesel cars in China market was substituted for SOF. The catalytic activity was evaluated by normalized peak areas and extrapolated onset temperatures of DTA curves. A computer program was developed by direct non-linear regression model for simulation of TG/DTG curves to determine the thermal processes and kinetic parameters. It is found that lube evaporation/decomposition and thermal decomposition (pyrolysis) were observed under a nitrogen atmosphere. Lube evaporation fractions were inhibited by Ce0.6Zr0.4O2 and γ-Al2O3. While under an air atmosphere, namely, in the process of lube oxidation (combustion), evaporation/decomposition, low-temperature oxidation and high-temperature oxidation were distinguished. Ce0.6Zr0.4O2 solid solutions are active catalysts for lube oxidation, in which the sample prepared by solution combustion has the highest activity, mainly due to the maintenance of the surface area and particle size upon sintering and its lowest bulk density. However, γ-Al2O3 is more like a support. There exists synergism between Ce0.6Zr0.4O2 and γ-Al2O3: γ-Al2O3 adsorbs lube retaining it within its pore structure, whereas, Ce0.6Zr0.4O2 solid solutions initiate oxidation reactions when light-off temperatures reach. The application of CeO2-ZrO2 solid solution prepared by solution combustion at lower temperature would be promising in diesel oxidation catalysts.