Deterioration of Automotive Catalytic Converters: Physical Catalyst Characterisation

The degree of physical deterioration of catalytic converters removed from two groups of motor vehicles with low and high odometer readings have been studied. The changes in the physical and chemical properties between the two catalyst groups were investigated using the XRD, BET and PIXE/PIGE techniques. Thermal damage was the main catalyst deterioration mechanism in both odometer groups. The low odometer group showed near-uniformity in both surface area loss (average 45 %) and degree of CeO₂ sintering representing the baseline thermal deterioration from normal vehicle operation. High odometer catalysts displayed more complex deactivation mechanisms involving both chemical contamination and thermal deactivation such as support phase transformation, internal “hot zones” and contaminant-support interactions.

The linear relationship between the contamination (increased) and kilometres travelled in the low odometer group (extrapolated to the kilometres travelled in the high odometer group) showed less contamination than predicted. There was no evidence of crystalline contaminant compounds in the low odometer catalysts and contamination concentrations greater than 10.5 wt. % resulted in pore mouth blocking. High odometer catalysts showed considerable chemical contamination, in both crystalline and amorphous forms, and increased Pb concentrations.

An induced thermal deactivation treatment was conducted on a contaminated catalyst, showing a major surface area loss, contaminant vaporisation and formation of CePO₄ and CeAlO₃.