Observation of cerium phosphate in aged automotive catalysts using Raman spectroscopy

Phosphorus is well-known to deactivate automotive catalysts and degrade emissions performance over time. Most phosphorus deposits have been previously identified as compounds of Zn, Ca, Mg, and Al. In this work, cerium phosphate is clearly identified using visible and UV Raman spectroscopy in various phosphorus-contaminated automotive catalysts having ceria as a washcoat component. Catalysts examined were either customer- or engine-dynamometer aged, with phosphorus introduced using zinc dialkyldithiophosphate (ZDDP) or an organic phosphorus compound either through normal oil consumption or doped fuel. The monazite form (CePO4) of cerium phosphate is observed in all cases and is unaffected by different aging conditions and precious metal loading. However, in Pd-containing catalysts, the P–O stretch frequencies were sometimes red-shifted ∼5 cm−1 and had linewidths ∼30% broader than usual. The relative intensities of the Raman spectra showed different amounts of CePO4 formed along the length of the catalyst and throughout the washcoat layer. The formation of aluminum phosphate and zinc phosphate species was also observed. Lastly, UV Raman is particularly useful for detecting CePO4 in customer-aged samples showing large fluorescent backgrounds under visible excitation.