The aim of the project 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. 

The development of the novel H₂-deNO_x catalyst was carried out using a high-throughput screening, in which approx. 900 different powder catalyst formulations were designed. As a result, it was shown for the first time that some catalysts achieved promising NO_x conversion rates between 250 and 450 °C in realistic model exhaust gas.

It is to mention that the catalyst tests were mainly carried out in the presence of 1 vol% H₂, which is a relatively high gas phase concentration. The most active samples found are supported catalysts with binary and ternary metal oxide  supports and active precious metal components. 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 between 45 % (at 450 °C) and 100 % (at 250 °C) has been achieved without dosing NH₃ or a NH₃ precursor (e. g. urea) 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. In contrast to low-temperature active precious metal catalysts, the found catalysts only reveal small amounts of noble metals, which leads to the assumption that these catalysts will be less expensive than conventional diesel oxidation catalysts and three way catalysts. The results presented are an appropriate starting point for in-depth investigations and a targeted catalyst design toward the optimization of the deNO_x performance as well as the selectivities of N₂ and H₂. Moreover, the transfer of the optimised powder catalysts to technically relevant monolithic substrates is another important step to assess the full potential of the high-temperature H₂-deNO_x approach.

Research association:
Forschungskuratorium Maschinenbau e.V. 
Lyoner Straße 18 
60528 Frankfurt am Main

Further participating research institution:
Chemnitz University of Technology 
Institute of Chemistry 
Professorship of Chemical Technology 
09107 Chemnitz

Further information and the final report can be requested at info [at] fkm-net [dot] de (info[at]fkm-net[dot]de).

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