The Role of Hydrogen in Exhaust Gas Aftertreatment Systems of Hydrogen Combustion Engines

Hydrogen internal combustion engines (H2-ICE) do not emit any fuel-borne carbon emission species. Nitrogen oxides are the remaining raw emission species at significant levels. However, the exhaust aftertreatment system is exposed to a different exhaust matrix, including unburned hydrogen. This raises the question of the role of hydrogen emissions for the aftertreatment system. Extensive synthetic gas bench (SGB) test campaigns address the role of hydrogen in several production catalyst components. Starting with selective catalytic reduction (SCR) systems, a systematic variation of the hydrogen concentration shows rather small effects on the NOX reduction performance. A change in selectivity results in increased secondary N2O emissions for a copper-zeolite system, whereas a vanadium-based SCR catalyst is unaffected. However, both SCR types are highly sensitive to the NO2/NOX ratio in the raw emission. Therefore, an upstream oxidation catalyst remains important for cold start performance. Investigations of oxidation catalysts with varying Platinum loadings show increased oxidation performance with higher hydrogen content. This effect is attributed to the accelerated heating of the catalytic centers due to the exothermic hydrogen reaction. In parallel however, secondary N2O emissions increase during light-off, speaking against a post-oxidation-based catalyst heating strategy. The strongest sensitivity to hydrogen is found in lambda sensors. Fast hydrogen diffusion through the zirconia distorts the signal towards rich mixtures. Overall, the results emphasize the important role of hydrogen, especially with respect to secondary N2O emissions, requesting H2-ICE-specific operating strategies to achieve zero-impact tailpipe emissions.