An increasing number of heavy-duty vehicles are using liquefied natural gas (LNG) as a fuel due to the expanding refuelling station network for LNG and lower overall emissions compared to diesel vehicles. The latest EURO VI regulation or natural gas fuelled vehicles set a limit for NH 3 of 10 ppm, and N 2 O exhaust is expected to be restricted in Europe in the near future. Poisonous and corrosive NH 3 and the greenhouse gas N 2 O are formed as by-products in a three-way catalyst used to minimize the emissions of stoichiometric heavy-duty engines. In this work, we studied how high temperature NH 3 and N 2 O formed in modern, fresh and aged bimetallic Pd/Rh three-way catalysts in simulated exhaust gas. More precisely, the exhaust gas composition and temperature were examined. Decreases in NO concentration and increases in temperature lowered the formation of NH 3 and N 2 O, whereas a decrease in CH 4 concentration reduced only NH 3 formation. According to Raman and powder X-ray diffraction experiments, the structure of the catalyst changed during the ageing, and this reputedly affected the function of cerium-zirconium mixed oxides and thus the formation of NH 3 and N 2 O. Temperature programmed reduction (H 2 -TPR) measurements showed changes in cerium-zirconium mixed oxide performance after ageing supporting Raman spectroscopy findings. Catalyst ageing in oxidizing conditions increased the formation of N 2 O. This study showed that exhaust gas composition plays an important role in the formation of undesired NH 3 and N 2 O emissions.
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Nevalainen et al. (2017) studied this question.
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