Date of Award
Master of Science
Bruce G. Bunting, David K. Irick, J. Roger Parsons
The phosphorus poisoning of diesel oxidation catalysts (DOCs) by the lube-oil additive zinc dialkyldithiophosphate (ZDDP) is investigated in the present study. A 517 cc single-cylinder, naturally aspirated direct-injection diesel engine is used to accelerate the phosphorus poisoning of DOCs by artificially increasing the ZDDP consumption to approximately 700 times of that found during normal engine operation. Three methods of accelerating the ZDDP consumption rate are investigated, which have been shown in previous literature to cause phosphorus poisoning. These include the injection of high concentration ZDDP-doped lube-oil blended with diesel fuel though the fuel injector as well as injecting ZDDP-doped lube-oil directly into the intake manifold and exhaust manifold, respectively. Each method is shown to produce a different phosphorus poisoning behavior on automotive catalysts by creating unique poisoning exhaust environments causing different deactivation mechanisms; ZDDP passing through the combustion chamber results in phosphoric acid, ZDDP injected into the exhaust results in whole ZDDP molecules and their molecular fragments.
The deactivation resulting from each poisoning method is characterized using both total hydrocarbon (THC) and carbon monoxide (CO) light-off degradation as well as phosphorus adsorption and phosphorus chemistry identified within the DOC. Washcoat surfaces evaluated for lube-oil derived contamination using scanning electron microscopy with energy dispersive spectrometers (SEM-EDS) shows that topography depends on the method of ZDDP introduction. Exhaust manifold injection produces a zinc-phosphate glaze, which masks active sites and inhibits gaseous diffusion to the washcoat surface. Fuel and intake manifold injection methods produce chemically absorbed phosphorus, which poison active sites. THC and CO light-off performance degradation is also found to depend on the method of ZDDP introduction, with an increase in light-off temperature between 40 to 100oC. Total phosphorus, zinc, and sulfur accumulation within the DOCs is measured using X-ray fluorescence spectroscopy (XRF) and found to vary with both the ZDDP introduction method and the exhaust temperature during poisoning. Elemental (X-ray) maps and line-scans performed using electron probe microanalysis (EPMA) show a decreasing phosphorus concentration profile along the DOC length with phosphorus being confined to the uppermost layer of the washcoat.
Three high-mileage, two brick, field-deactivated DOCs were obtained from a bus fleet, which were removed from service due to a catastrophic event, to make comparisons in THC and CO light-off behavior as well as phosphorus poisoning with those undergoing accelerated ZDDP introduction methods. The field-deactivated DOCs are of similar formulation as those used during laboratory tests. It is shown that field- deactivated DOC THC and CO light-off behavior as well as phosphorus accumulation and surface contamination is reproduced using accelerated ZDDP introduction methods. Based on the surface characterization observations and light-off performance, the intake manifold injection method offers the best correlations between accelerated poisoning methods and field-deactivated passenger bus DOCs.
In order to accurately quantify the poisoning of DOCs by phosphorus, a bench- flow reactor system (BFR) is utilized to provide supplementary THC and CO light-off evaluations for more precise control of both DOC temperature and exhaust gases composition. It is found that light-off temperature measurements using the BFR are highly repeatable and show a correlation in the poisoning mechanisms between accelerated ZDDP introduction methods and field-deactivated DOCs. As a byproduct of the BFR evaluations, it is shown that DOC regeneration occurs in both the accelerated ZDDP injection methods and the field-deactivated DOCs by the high temperature oxidation and removal of soot and lube-oil contamination on washcoat surfaces. THC and CO light-off temperatures after regeneration are identical to those obtained for a fresh DOC. Subsequent EPMA and XRF analyses of regenerated DOCs reveal the presence of the phosphorus, sulfur and zinc within the washcoat, indicating that lube-oil derived poisons do not highly influence the THC and CO light-off behavior of DOCs, but rather, DOC performance is more susceptible to the presence of soot and lube-oil contamination on the washcoat surface.
Eaton, Scott Joseph, "Accelerated Poisoning of Diesel Oxidation Catalysts by Zinc Dialkyldithiophosphate-Derived Phosphorus. " Master's Thesis, University of Tennessee, 2006.