Effect of Dithering on Post-Catalyst Exhaust Gas Composition and on Short Time Regeneration of Deactivated PdO/Al ₂ O ₃ Catalysts under Real Engine Conditions

Fossil fuels such as natural gas used in engines still play an important role worldwide which however is also exacerbating climate change as a result of carbon dioxide emissions. Although natural gas engines show an overall low pollutant emissions level, methane slip due to incomplete combustion occurs, causing methane emissions with a more than 20 times higher global warming potential than CO₂. Additionally, further tightening of emissions legislation is to be expected bringing methane emissions even more into focus making exhaust gas aftertreatment issues remain relevant. For lean gas applications, (Pd)-based catalysts turned out to convert CH₄ most efficiently usually being supported by metal oxides such as aluminium oxide (Al₂O₃). Water (H₂O) contained in the exhaust gas causes strong inhibition on Pd catalysts. In real exhaust gases, not only water vapour but also pollutants and sulphur-containing compounds such as hydrogen sulphide (H₂S) or sulphur oxides (SO_x) are poisoning the catalytic converter. Rich pulses decomposing sulphur species adsorbed on Pd-Pt methane oxidation catalysts, enable efficient regeneration of heavily poisoned catalysts. A strategy similar to operation with rich pulses, but with a different motivation, is the use of high-frequency oscillations between lean and rich exhaust gas, so-called dithering, to improve pollutant conversion. A combination of a stoichiometric pulse while simultaneously dithering shows better results in recovery as well as emissions during regeneration than a pure rich pulse.