The project presented here is the IGF follow-up project to “Catalytic NO_x removal by H₂ from the exhaust gas of GHG-neutral, lean H₂-DI combustion engines for stationary and mobile applications” (duration: 10/2022-12/2024, FKZ: 01IF22407N).

The aim of the project is/was to develop an H₂-deNO_x catalyst for the exhaust gas of lean H₂-DI combustion engines, which include trucks, heavy cars, ships and stationary applications, in particular CHP units. The engines mentioned are not yet available on the market, but require efficient exhaust gas aftertreatment for NO_x for future legal certification.

The novelty of this catalytic deNO_x approach was to use H₂ as reducing agent in the entire range of typical exhaust gas temperatures (250 - 450 °C). The hydrogen required for NO_x reduction is conceptually provided from the existing fuel tank. 

In the previous project catalyst formulations were found that were tested under realistic conditions in the laboratory and exhibit H₂-deNO_x activity in the “high temperature range”, whereby the catalyst tests were primarily carried out at a relatively high H₂ gas phase content of 1 vol.%. It is noteworthy, that the found catalysts do not only reduce NO_x to N₂, but also significantly to NH₃. By this fact the possibility opened to use the NH₃ formed for the reduction of still unconverted NO_x on a downstream SCR catalyst. In a powder scale experiment this possibility could be confirmed successfully. The NH₃ was used completely on a downstream Cu-Chabasite zeolite catalyst for the SCR reaction, which significantly increased the total NO_x conversion. This was the first time, that a significant NO_x reduction has been achieved without dosing NH₃ or a NH₃ precursor in a lean exhaust gas between 250 and 450 °C. It also could be shown, that the NO_x conversion is strongly dependent on the H₂ amount in the feed, i. e. in the presence of 0.2 and 0.5 % H₂, the NO_x conversion is correspondingly lower compared to 1% H₂. However, similar to the low-temperature H₂-deNOx reaction, N₂O is also formed in the high-temperature range.

The catalysts used to achieve these results will now be used as a starting point for in-depth investigations and optimizations with regard to their composition in order to specifically increase deNO_x activity and improve N₂ and H₂ selectivities, as well as for transfer to technically relevant honeycomb catalyst systems and pre-competitive evaluation. If this catalyst development is successful, H₂-deNO_x technology will be applicable not only in low-temperature but also in high-temperature ranges, meaning that lean H₂ engines will no longer require the entire AdBlue SCR infrastructure.

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