Characterization of Phosphates Found in Vehicle-Aged Exhaust Gas Catalysts: A Raman Study

Phosphorus contamination from engine oil additives has been associated with reduced performance of vehicle-aged exhaust gas catalysts. Identifying phosphorus species on aged catalysts is important for understanding the reasons for catalytic performance degradation. However, phosphorus is present only in small quantities, which makes its detection with bulk analytical techniques difficult.

Raman microscopy probes small regions (a few microns in diameter) of a sample, and can detect both crystalline and amorphous materials. It is thus ideal for characterizing phosphates that may have limited distribution in a catalyst. However, suitable Raman spectra for mixed-metal phosphates that might be expected to be present in contaminated catalysts are not generally available. To provide reference data for characterizing phosphates in exhaust-gas catalysts, selected single- and mixed-metal phosphates were synthesized using compounds containing calcium, zinc and/or magnesium (found in engine oil additives) and/or barium (found in washcoats of three-way catalysts and NO_x traps). Raman spectra of the synthesized phosphates were obtained and compared with spectra from vehicle-aged catalysts. A number of matches were found. X-ray powder diffraction was used to confirm the identity of the phases.