Effects of Fuel Additive MMT on Contaminant Retention and Catalyst Performance

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.