Researchers at Waseda University in Japan, with a colleague from Isuzu Central Research, are proposing electric field-assisted lean NO_x reduction for lean-burn engines. An open-access paper on their work is published in the RSC journal Chemical Communications.

Lean-burn engines offer lower CO₂ emissions, higher thermal efficiency, and better fuel economy than stoichiometric combustion engines. However, classical three-way catalysts (TWCs) are ineffective at reducing NO_x in lean burn engines because of the presence of residual oxygen in the exhaust gases. To overcome this difficulty, NO_x storage reduction (NSR) technology has attracted attention as a viable approach to remove NO_x from lean-burn engine exhaust.

AN NSR catalyst combines nobles metal active sites with alkali or alkaline earth metal oxides as NO_x storage sites. Under lean burn conditions, NO is oxidized by O₂ and is stored on the NSR catalyst surface in the form of nitrate (NO₃^-). NO_x released from the surface in rich conditions )oxygen-deprived) is reduced to nitrogen (N₂) by reductants such as hydrogen (H₂), carbon monoxide and hydrocarbons.

… conventional NSR technology requires complex cyclic operation with frequent switching between lean and rich states, resulting in unavoidably lower fuel efficiency.

For this study, a new catalytic NSR process is proposed to reduce the adsorbed NO_x and to improve engine efficiency using H₂ in a lean burn state without switching to a rich combustion state after NO_x adsorption. This new concept is designated as electric field-assisted lean NO_x reduction, as an electric field is applied to the NSR catalyst during lean NO_x reduction.

With the introduction of this technology, surplus electricity in the vehicle is useful for exhaust gas purification, which is part of energy management. Furthermore, the application of an electric field activates the reduction function of the supplied H₂ and simultaneously promotes hydrogen transfer. This effect promotes NO_x reduction, even at low temperatures.

—Shigemoto et al.

. Assumed reaction model for electric field-assisted lean NO_x reduction over 3wt%Pt-16wt%BaO/CeO₂. Shigemoto et al.

Tests with a 3wt%Pt-16wt%BaO/CeO₂ catalyst demonstrate markedly higher NO_x conversion to N₂ (13.1% vs. 2.9% without an electric field).

Resources

• A. Shigemoto, Y. Inoda, C. Ukai, T. Higo, K. Oka and Y. Sekine (2024) “Electric field-assisted NSR process for lean NO_x reduction at low temperatures” Chem. Commun. doi: 10.1039/D3CC05189G.