Understanding and improving the reactivity of low loaded noble metal three-way catalysts

Felix, Noelia Cortes (2014). Understanding and improving the reactivity of low loaded noble metal three-way catalysts. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f841

Abstract

Three-Way Catalysts (TWC) were created to control pollutants emissions from gasoline engines by performing simultaneous oxidation of CO and HC and reduction of NO_X. Over the last 20 years, the main focus on their development has been the optimisation of the oxygen storage capacity (OSC) component, which is based on CeO₂ materials. Improved thermal stability of these materials, in addition to improvements in fuel quality, has enabled the noble metal (NM) content of these catalysts to be reduced. As a result, the metal dispersion has improved and the interactions within the support become more important, making significant changes to the character of the noble metal, sometimes even leaving it fixed in a less reactive state. Conventional preparative routes to disperse low NM amounts (< 1wt %) have been successful in the past but this is reaching its limit. The proposed approach to disperse low NM contents on CeO₂ materials is based on the precipitation method to create a more intimate contact between the NM and the ceria, and improve in this way the oxygen mobility. The effect of the Ce-precursor was firstly evaluated. The co-precipitation of Pd²⁺ and Ce³⁺ led to a catalyst with a similar Pd surface area to the material prepared by impregnation but with an improved Pd-Ce interaction, due to the partial ceria decoration of Pd particles. The co-precipitation of Pd²⁺ and Ce⁴⁺ led to a catalyst with low Pd surface area content, partly due to some Pd leaching during the preparation, but also likely to be due to higher encapsulation with the ceria. 0.5Pt-CeO₂ and 0.5Rh-CeCO₂ catalysts were also studied. The co-precipitation of these NM with Ce³⁺ also led to products with an improved NM-Ce interaction. However, in this case, the co-precipitated catalysts contained a lower NM surface area compared to their reference catalysts prepared by impregnation. The improved NM-Ce contact improved their OSC and pollutants conversions, but the lower surface metal content did not allow an improvement in their light-off temperatures. Overall, the catalysts prepared by the co-precipitation method have increased the number of NM and Ce atoms in contact leading to higher OSC and pollutants conversions. This leads to a more efficient usage of noble metals and potentially cheaper catalyst.