Deactivation of Diesel Oxidation Catalysts by Oil-Derived Phosphorus 2006-01-3422
The poisoning of diesel oxidation catalysts (DOCs) by the engine oil additive zinc dialkyldithiophosphate (ZDDP) is investigated in the present study. A 517cc single-cylinder diesel engine is used to accelerate the phosphorus poisoning of DOCs by artificially increasing the ZDDP consumption to approximately 700 times normal operation by three different methods. These include lube-oil doped fuel, intake manifold, and exhaust manifold injection with lube-oil containing an elevated level of ZDDP. The deactivation of DOCs under these conditions is characterized by a variety of physical and chemical techniques. Surface composition and structure of the poisoned catalysts analyzed with SEM-EDS show differences depending on the method of ZDDP introduction. Exhaust manifold injection produces a zinc phosphate glaze which masks the surface to species diffusion. Fuel and intake manifold injection methods produce chemically absorbed phosphorus on the catalyst washcoat surface.
Hydrocarbon and carbon monoxide light-off performance, used to quantify poisoning, is also found to depend on the accelerated poisoning method with an increase in light-off temperatures in the range of 40 to 100°C. Total phosphorus, zinc, and sulfur accumulated in the DOCs measured using X-ray fluorescence spectroscopy (XRF) are found to vary with the poisoning method and increase with increased exhaust temperature. Elemental (X-ray) maps and quantitative line-scans performed using electron probe microanalysis (EPMA) show a decreasing phosphorus concentration profile along catalyst length with phosphorus being confined to the uppermost layer of the washcoat. Catalyst deactivation of high-mileage passenger bus oxidation catalyst samples of the same formulation as those used in the engine bench is quantified for comparison purposes. Based on surface characteristic studies and light-off performance, the intake manifold injection offers the best correlations between engine bench-poisoned and in-service passenger bus catalysts