The Of Cerium Iron-based Composite Oxide Preparation And Catalytic Soot Combustion Properties

Nowadays, diesel engines have been paid more and more attention due to the strong power, high economy, good durability and low carbon dioxide emission. However, the soot emissions from diesel engins also lead to serious environmental problems. The small size of diesel soot particles may be linked to a number of health problems by its ability to penetrate the body through the respiratory system.The non-catalytic ignition temperature of soot oxidation generally exceeds 600℃, while the normal temperature of typical diesel exhaust temperatures is 150～450℃. The soot can not be removed by direct oxidation. The combination of diesel particulate filter (DPF) and oxidation catalysts appears to be the most plausible after-treatment technique to eliminate soot particles. The catalysts used in a catalytic DPF should display good activity in the temperature range of diesel exhaust, and good thermal stability under practical working conditions.In this paper, a series of Ce1-xFexO2(x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9 and 1) mixed oxides with different Ce/Fe molar ratios were prepared via hydrothermal method. The prepared catalysts were characterized by X-ray diffraction (XRD), BET specific surface area (BET), Raman spectroscopy (Raman) and H2 temperature-programmed reduction (H2-TPR) techniques. The performances of the catalysts in the soot oxidation were evaluated by temperature-programmed oxidation (TPO). We also investigated the correlation of catalysts surface states and bulk structures with catalytic properties, and find out the key factors affecting catalytic activity. The results showed that the strong synergetic effect between the Ce-Fe solid solutions and some highly dispersed Fe2O3, made the catalysts more active. The oxygen vacancies from the formation of Ce-Fe olid solutions can absorbe and activate oxygen, while the well dispersed surface Fe2O3 not only had a good reduction behaviour, but also can promote the activation of CeO2 lattice oxygen to improve the release rate of the lattice oxygen in CeO2. Among the series of Ce1-xFexO2 mixed oxides, the Ce0.8Fe0.2O2 and Ce0.4Fe0.6O2 two samples showed the best catalytic activity for soot combustion, and had the lowest Ti(262℃and 255℃, respectively, ignition temperature of soot oxidation) and Tp (both 314℃, temperature of maximum evolution of C02). Even after aging for long periods at high temperature, the Ti and Tp values for Ce0.8Fe0.2O2 and Ce0.4Fe0.6O2 samples were still relatively low. The Ti were 292 and 291℃, and Tp were 392 and 382℃, respectively.Ce1-xFexO2 mixed oxides had good catalytic activity, but their thermal stability should be further improved. So, in this paper, a series of zirconium doped Ce0.5Fe0.5-xZrxO2(X=0, 0.05,0.10,0.15,0.20,0.25,0.30,0.35,0.40 and 0.45) mixed oxides were also prepared via hydrothermal method. The characterization of XRD, BET, Raman and H2-TPR, and the TPO results showed that introduction of ZrO2 into the Ce1-xFexO2 mixed oxides can promote the formation of solid solutions to improve the catalytic activity for soot combustion. Moreover, it also results in the ability to improve thermal stability of the materials. Remarkably, ZrO2 can inhibit the growth of CeO2 crystallite size and the disappearance of solid solutions. Totally, Ce0.5Fe0.5-xZrxO2 ternary mixed oxides showed higher catalytic activity than Ce1-xFexO2 binary mixed oxides. Among the series of Ce0.5Fe0.5-xZrxO2 mixed oxides, the Ce0.5Fe0.30Zr0.20O2 catalyst showed the best catalytic activity for soot combustion, and had the lowest Ti (251℃) and Tp (310℃). Even after aging for long periods at high temperature, the Ti and Tp values for Ce0.5Fe0.30Zr0.20O2 sample were still relatively low. The Ti was 273℃, and Tp was 361℃, indicating high thermal stability.In this study, we finally chose the Ce0.5Fe0.30Zr0.20O2 sample which has the best catalytic activity as the research object, and prepared it via hydrothermal, physical mixture, impregnation, template, citric acid sol-gel and co-precipitation methods. The results showed that catalysts preparation methods have a great influence on structure and properties of catalysts. The catalyst prepared by hydrothermal method displayed the best catalytic activity and thermal stability.