Rare-Earth Doped CeO2-ZrO2 Nanodispersions: Material Characterisation and Oxygen Storage Capacity Evaluation

Gasoline direct injection (GDI) technology has become increasingly popular for passenger vehicle in recent years owing to its improved performance, fuel economy, and CO2 emissions in comparison to the traditional port fuel injection. However it has been observed that this technology leads to the formation of large quantities of ultrafine soot particles being emitted from the vehicles exhaust. In response to the rising popularity of GDI technology and the health hazards associated with the ultrafine soot particles that it produces, many emissions regulations globally have introduced limits on the number of particulates (PN) allowable to be emitted from passenger vehicles. Such legislation has led to the development of gasoline particulate filters (GPF) in order to allow GDI vehicles to operate within the PN limits.

Commonly GPFs are coated in a catalytic wash-coating analogous to that of the catalytic converter or three way catalyst (TWC). This coating serves a dual-purpose: firstly to eliminate pollutants such as CO, NOx, and hydrocarbons substituting for TWC volume; and secondly to allow for control over the filtration efficiency of the filter. A major component of these coatings is the oxygen storage material, most commonly ceria-zirconia that helps to maintain optimal oxidising/reducing conditions for pollutant elimination by storing and releasing oxygen through the redox couple Ce3+ ↔ Ce4+. In this work a number of rare-earth doped ceria-zirconia nanodispersions were characterised and evaluated for their application as oxygen storage materials in GPFs. The materials synthesised were quarternary ceria-zirconia doped with La3+ and Y3+, Nd3+, or Pr3+ as well as a five component mixed oxide doped with La3+, Y3+, and Nd3+. The prepared materials were then studied for their colloidal and physicochemical properties.

Stable nanodsipersions were obtained that were found to be polydisperse with average particle sizes of approximately 200nm by transmission electron microscopy and dynamic light scattering. Acoustic attenuation spectroscopy however gave smaller particle sizes of 19-60nm. This indicates that the forces holding together the aggregated particles are relatively weak and are readily re-distributed. The stabilities of the nanodsipersions were assessed by ζ-potential titration vs pH. It was found that approximately at pH 2-6 the nanodispersions had high ζ-potentials of 20-40mV and that the rare earth dopants caused the isoelectric point to shift to higher pH values (>pH 9) improving their stability.

The crystal structure of the dried and thermally aged materials was investigated using x-ray diffraction (XRD) and Raman spectroscopy. All samples were initially single phase solid solutions of the cubic fluorite or t’’ structure with the exception of the 30% Ce undoped sample which was likely tetragonal. The XRD patterns of the thermally aged materials revealed the thermal stability of the solid solutions. The La/Y and La/Nd/Y doped compositions had the greatest thermal stability remaining single phase even after thermal aging at 1150°C. Raman spectroscopy of the dried materials revealed that the rare-earth doped samples had highly defective structures with oxygen vacancies and were in fact of the t’’ crystal structure except from the sample doped with 5%La and 5%Y.

Low temperature N2-adsorption analysis using BET and BJH models showed high initial surface areas of between 180-217 m2/g and pore volumes between 0.20-0.25 cm3/g for the rare-earth doped samples. The doped samples showed improved thermal stability in relation to surface area and pore volume loss in comparison to the pure ceria-zirconia samples. The adsorption-desorption isotherms showed that the materials were mesoporous and had non-uniform “ink bottle” shaped pores except the Pr containing sample which had more uniform cylindrical pores.

Evaluation of the catalytic activity of the materials using H2-temperature programmed reduction (H2-TPR) and oxygen pulse chemisorption techniques. The rare-earth doped samples had reduction temperatures 20-25°C lower than the pure ceria-zirconia samples. The total hydrogen consumption however was dependent on the total Ce content of the material. Oxygen pulse chemisorption of the reduced materials showed a clear difference between the pure and doped samples. At 50°C the La/Y had the greatest oxygen storage capacity (OSC) of 124.9 μmol/g whereas the pure ceria-zirconia samples were not oxidised at all. At 100°C the La/Nd/Y had the greatest OSC of 131.6 μmol/g. Normalising the oxygen uptakes to Ce content showed that the values were not dependant on Ce content, indicating that the rare-earth dopants play a major role in determining the oxidation behaviour. This is likely due to the improved oxygen mobility associated with the doped materials.