Hydrothermal Synthesis and Low-Temperature Activity of Rare-Earth Metal Oxides intended for use as Automotive Three-Way Catalysts

Abstract

Ceria (cerium oxide or CeO₂) along with a few related oxides constitute an important class of catalysts that can rapidly pick up or release molecular oxygen (2CeO₂«Ce₂O₃+1/2O₂) under variable reducing and oxidizing conditions, mainly by way of a reversible change of the oxidation state of the cerium ions in combination with formation or elimination of oxygen vacancies. This property, characterized as the oxygen storage capacity (OSC), has made cerium oxide a material of considerable interest in applications such as in gas sensors, in fuel cell electrodes, and especially, in automobile exhaust clean-up. Three-way catalysts using cerium based rare-earth oxides systems are extensively used to drastically reduce pollutants (CO, NO_x, and hydrocarbons). The nearly unsurpassed performance of the cerium oxide redox system is however limited to rather high temperatures (>800 K) and/or low oxygen partial pressure, and by low thermal stability. To increase the performance of cerium oxide based catalytic converters during cold start conditions requires the development of catalysts that can operate at lower temperatures. It has been found that mixing ceria with other oxides (i.e., ZrO₂ and Al₂O₃) to form solid solution nanoparticles can enhance their oxygen storage capacity (OSC), increase the thermal stability of the materials, and also prevent sintering at higher temperatures. The mechanism by which the addition of other oxides enhances the properties is not yet well understood, but it likely involves the creation and stabilization of oxygen vacancies in the ceria lattice. Understanding the interactions between ceria and other oxides and their influence on crystal structure, defect structures, and transport properties is critical to further enhance the performance of CeO₂ based catalysts. To further optimize the CeO₂ based catalytic system, we expanded this strategy to other cerium-based Ce-M-O oxides (dopant metals M = Pr, Tb, Ti), and are planning to explore other rare-earth oxide systems (i.e. Pr- and Tb-based oxides) to elucidate the fundamental relationship between structure and chemistry of the nanoparticles and their low-temperature catalytic activity. High surface area Ce-M-O oxides (dopant metals M = Pr, Tb, Ti, Zr) samples were synthesized using hydrothermal method. Typically, Ce(NO₃)₃×6H₂O, the dopant metal nitrate, and NaOH mixtures heated to 150 °C and held for 48 hours in a sealed 100 mL Teflon-lined autoclave (~50 % filled). Then the autoclave was cooled to room temperature before the solid products were recovered by suction filtration. The materials were washed thoroughly with distilled water to remove any co-precipitated salts, then washed with ethanol to avoid hard agglomeration in the nanoparticles, and dried in air at 50°C for 12 hours. Transmission electron microscopy (TEM) characterization was performed using a JEOL2100 operated at 200 kV and equipped with an EDAX detector and annular dark-field detector. Hydrogen temperature programmed reduction (H₂-TPR) was used to test the redox activity of the synthesized rare-earth metal oxide materials in a Micromeritics AutoChem^TM II 2920. Our preliminary studies indicate that CeO₂-MO₂ oxides solid solutions can be readily obtained using hydrothermal synthesis methods at relatively low temperatures (e.g. 150°C). Figure 1 shows the XRD and TEM images for the synthesis of cerium-based mixed oxides (CeO₂-ZrO₂, CeO₂-TiO₂, and CeO₂-PrO₂), prepared by hydrothermal method. The XRD patterns of these mixed oxides did not show any diffraction peaks due to the doping oxides but their presence was evidenced by EDAX elemental analysis. The diffraction peaks were indexed as CeO₂ cubic fluorite structure, though the peak positions were shifted when compared to standard ceria (JCPDS file 34-0394).