Review of Recent After-Treatment Technologies for De-NOx Process in Diesel Engines

The removal of nitrogen oxides (NOx), which are major atmospheric pollutants, in exhaust gas after-treatment systems, has been studied extensively. The catalytic reduction of NOx in lean-burn environments is challenging in diesel after-treatment technology. The most relevant catalytic NOx reduction technologies for diesel engines are summarized in this review paper, focusing on current catalytic after-treatment systems for compliance with emission standards. Four promising after-treatment technologies for the abatement of tail-pipe NOx emissions are discussed: (i) lean NOx trap (LNT) and LNT combined with selective catalytic reduction (SCR) (LNT + SCR), (ii) Urea/NH3-SCR, (iii) Hydrocarbon-SCR (HC-SCR), and (iv) CO/H-2-SCR. The relevant reaction mechanisms and current major challenges are reviewed in detail. The effects of the active phases and support modifications of the respective catalysts are examined to evaluate their contribution to the after-treatment systems. Major issues in practical applications, such as hydrothermal aging, chemical poison, and metallic additives for improving de-NOx performance, are extensively reviewed in this paper.