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 NOX. 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.

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