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The deactivation of automotive catalysts by engine oil-derived components of phosphorus and zinc can occur by the formation of an amorphous zinc pyrophosphate (Zn2P2O7) that is impervious to gas diffusion. The catalyst poison, derived from antiwear oil additive zinc dialkyl dithiophosphate (ZDP) in low-temperature exhaust environments, appears as glassy, amorphous deposits on catalysts as shown by scanning electron microscopy (SEM). Laboratory studies were performed to understand the effects of exhaust stoichiometry, temperature, rate of oil burn, and chemical form of P and Zn compounds on glaze formation. The formation of the amorphous deposits using a laboratory pulsator apparatus showed that noncombusted ZDP causes the glaze formation. Electron microprobe studies indicated the association of P with Zn on precious metal films exposed to ZDP combustion products. Secondary ion mass spectrometry (SIMS) confirmed a similar P to Zn correspondence on the vehicle-aged catalysts.

Post-mortem analyses of 50,000 mi (80,000 km) vehicle-aged catalysts revealed that the use of 0.125g Mn/gal (33 mg/L) as MMT (methyl-cyclopentadienyl manganese tricarbonyl) significantly reduces phosphorus and zinc retention levels at the catalyst inlets by ~20-fold and ~5-fold, respectively. In subsequent laboratory pulsator experiments the presence of 0.016 to 0.157g Mn (as MMT)/gal (4 to 41 mg/L) isooctane fuel containing a 10-fold excess of ZDP (zinc dialkyldithiophosphate, source of oil P and Zn) similarly reduced the retention of P and Zn on TWCs by proportional amounts, while the TWCs maintained significantly higher 3-way conversions than in the absence of MMT. The combustion of Mn from MMT to very stable Mn3O4 probably serves as a scavenger in the exhaust for transporting away fuel- and oil-derived catalyst poisons such as P, Zn, and Pb. The utility of the laboratory results will require verification in vehicle studies.

On a global basis there is a resurgence of interest in the use of palladium in automotive emission control catalysts because of cost, availability and performance advan­tages under certain operating con­ditions relative to more expensive noble metals. This paper reviews a variety of potential vehicle applic­ations for the use of palladium containing catalysts. Included in the study are for the replacement of platinum by palladium in conventional platinum/rhodium systems, palladium-only three-way catalysts, palladium-only dual bed catalysts and two-stroke and lean-burn engine applications.