Sulfuric Acid and Nitrate Emissions from Oxidation Catalysts 750091

The authors have investigated the emission of sulfuric acid and nitrates from a 1973 Ford 351C V-8 engine equipped with prototype 1975 emission controls and mounted on a dynamometer test stand. Tests were performed on unleaded fuel at 60 mph road-load conditions, using monolithic noble-metal catalysts and no catalyst, and at 60 and 30 mph road-load conditions using a pellet-bed noble-metal catalyst.

Sampling for SO₂ and H₂SO₄/SO₃ measurement was effected at several points within the exhaust system as well as from a dilution tunnel where the exhaust was diluted ten-fold with filtered air. Samples collected in the dilution tunnel were analyzed for total particulate mass, sulfate, condensed water, and pH. Ammonia injection into the dilution tunnel on selected runs permitted collection of nitrate and HNO₃ on the filters as NH₄NO₃.

Nearly all of the fuel sulfur was emitted as SO₂, and less than 1% as sulfate, in the absence of a catalyst. With the catalyst, about 43% of the emitted sulfur was in the form of sulfate at 60 mph (monolith or pellet), and 86% at 30 mph (pellet catalyst). Sulfate storage in the monolithic catalyst at 60 mph road load was negligible. Some sulfate storage was noted in the pellet-bed catalyst at 60 mph road load. Afterwards, substantial storage occurred in the pellets at 30 mph; the storage rate was >50% of the sulfur consumed during the first 2 h of 30 mph operation; with further cumulative storage the storage rate declined to ∼15% of the consumption rate after 1000 miles at 30 mph, at which time the pellets contained 3% sulfate. Catalyst-produced particulate emissions were composed almost entirely of sulfuric acid and associated water. About 95% of the H₂SO₄ mass (with catalyst) was composed of droplets of less than 0.3 μm aerodynamic diameter. The amount of nitrate present in the engine exhaust was very low (≲300 μg/mi), and was not perceptibly enhanced by the presence of a catalyst.