I. Introduction
With fuel consumption and emission standards becoming more tightly regulated, lean-burn gasoline direct injection (GDI) engines are capturing the interests from the automotive industry due to 10-15% fuel saving benefits over stoichiometric gasoline engines [1]. However, cost-effective and efficient removal of NOx in lean exhaust condition remains a significant challenge to overcome. Low-cost three-way catalysts (TWCs), which are capable of reducing hydrocarbon (HC), CO and NOx emission from stoichiometric gasoline engines at high efficiencies, demonstrated limited deNOx capabilities in the presence of excess oxygen. The Diesel engine industry has adopted lean NOx traps (LNTs) and selective catalytic reduction (SCR) systems for NOx reduction. However, LNTs contain precious group metals (PGMs), leading to a high cost. In addition, LNT are subject to sulfur poisoning, which leads to reduced emission control performance. Urea-based SCR systems have become standard for medium-to-heavy duty applications but are less accepted for passenger car applications due to the requirements of the consumable urea solution and complex dosing systems.