The role of oxygen storage in NO conversion in automotive catalysts

An asymmetry in NO conversion efficiency is apparent during trans-stoichiom etric air-fuel ratio steps over a three-way catalyst. After a lean-to-rich step, NO conversion becomes high very rapidly and asymptotes to a higher va lue, followed by a reduction to a high, but reduced level. Following a :ich -to-lean step, conversion remains high for a period before gradually decrea sing to the low level seen at steady lean operation. This dynamic response in NO conversion can be modelled by a three-way catalyst model which includ es ceria reoxidation by way of NO dissociative adsorption. Five, periodic, different aspect triangular waveforms were tested to try to exploit the asy mmetry by waveform shaping. No strong trends in average NO conversion effic iency resulted; however, some shapes seemed to give a better overall conver sion.