Alkali and alkaline earth metal impurities found in diesel fuels
are potential poisons for diesel exhaust catalysts. Using an
accelerated aging procedure, a set of production exhaust systems
from a 2011 Ford F250 equipped with a 6.7L diesel engine have been
aged to an equivalent of 150,000 miles of thermal aging and metal
exposure. These exhaust systems included a diesel oxidation
catalyst (DOC), selective catalytic reduction (SCR) catalyst, and
diesel particulate filter (DPF). Four separate exhaust systems were
aged, each with a different fuel: ULSD containing no measureable
metals, B20 containing sodium, B20 containing potassium and B20
containing calcium. Metals levels were selected to simulate the
maximum allowable levels in B100 according to the ASTM D6751
standard. Analysis of the aged catalysts included Federal Test
Procedure emissions testing with the systems installed on a Ford
F250 pickup, bench flow reactor testing of catalyst cores, and
electron probe microanalysis (EPMA). The thermo-mechanical
properties of the aged DPFs were also measured.
EPMA imaging of aged catalyst parts found that both the Na and K
penetrated into the washcoat of the DOC and SCR catalysts, while Ca
remained on the surface of the washcoat. Bench flow reactor
experiments were used to measure the standard NOx conversion, NH₃
storage and NH₃ oxidation for each of the aged SCR catalysts. Flow
reactor results showed that the first inch of the SCR catalysts
exposed to Na and K had reduced NOx conversion through a range of
temperatures and also had reduced NH₃ storage capacity. The SCR
catalyst exposed to Ca had similar NOx conversion and NH₃ storage
performance compared to the catalyst aged with ULSD. Using a
chassis dynamometer, vehicle emissions tests were conducted with
each of the aged catalyst systems installed onto a Ford F250
pickup. Regardless of the evidence of catalyst deactivation seen in
flow reactor experiments and EPMA imaging, the vehicle successfully
passed the 0.2 gram/mile NOx emission standard with each of the
four aged exhaust systems. This indicates that total catalyst
volume is adequate to accommodate the catalyst activity loss
observed in the flow reactor experiments.