It is no laughing matter: nitrous oxide formation in diesel engines and advances in its abatement over rhodium-based catalysts

Abstract

Nitrous oxide (N₂O) appears as one of the undesired by-products in exhaust gases emitted from diesel engine aftertreatment systems, such as diesel oxidation catalysts (DOC), lean NO_x trap (LNT, also known as NO_x storage and reduction (NSR)) or selective catalytic reduction (NH₃-SCR and HC-SCR) and ammonia slip catalysts (ASC, AMOX, guard catalyst). N₂O acts as a greenhouse gas, damages the ozone layer and causes many environmental and human health problems. The catalytic N₂O decomposition (deN₂O) to nitrogen and oxygen serves as one of the available options for the removal of its emissions. In this Minireview, we focus on possible processes responsible for N₂O formation in the lean NO_x trap unit. Catalysts based on rhodium are the most active in deN₂O. Thus, we provide an overview on rhodium-based catalysts in metallic form, supported systems, modified zeolites, hydrotalcite derived mixed metal oxides and bimetallic (transition metal–rhodium) systems, as potential candidates for reducing N₂O emission. Finally, we propose future directions in developing efficient catalysts for deN₂O.