Reducing Emissions from Lean-Burn Hydrogen Combustion Engines Using a State-of-the-Art Oxidation Catalyst and a VWTi-Based SCR Catalyst: Potentials and Challenges

Hydrogen (H₂) is commonly considered as one of the most promising carbon-free energy carriers allowing for a decarbonization of combustion applications, for instance by retrofitting of conventional diesel internal combustion engines (ICEs). Although modern H₂-ICEs emit only comparably low levels of nitrogen oxides (NOx), efficient catalytic converters are mandatory for exhaust gas after-treatment in order to establish near-zero emission applications. In this context, the present study evaluates the performance of a commercial state-of-the-art oxidation catalyst (OC) and of a catalyst for selective catalytic reduction (SCR) that are typically used for emission reduction from diesel exhausts under conditions representative for H₂-fueled ICEs, namely oxygen-rich exhausts with high water vapor levels, comparably low temperatures, and potentially considerable levels of unburnt H₂. Herein, the OC is supposed to convert H₂ slippage, which can occur due to incomplete combustion, and to oxidize NO to NO₂, which enables an efficient NOx removal over the SCR catalyst. While the vanadia-based SCR catalyst was barely affected by high water vapor levels, the presence of H₂, or hydrothermal aging, H₂O inhibited NO to NO₂ oxidation over the OC and hydrothermal aging with 20 vol.-% H₂O resulted in significant deactivation of the OC. At the cost of producing the inhibitor H₂O and the greenhouse gas N₂O, the presence of H₂ facilitates a fast light-off due to temperature generation. These results underscore the importance of developing suitable catalyst operation strategies that account for efficient pollutant conversion and avoid secondary emissions formation.