Continuous Unsteady-State De-NO x System via Tandem Water-Gas Shift, NH 3 Synthesis, and NH 3 -SCR under Periodic Lean/Rich Conditions.

The development of urea-free and platinum group metal (PGM)-free catalytic systems for automotive emission control is a challenging task. Herein, we report a new de-NO x system using cyclic feeds of rich and lean gas mixtures with PGM-free catalysts. Initial catalyst screening tests showed that Cu/CeO 2 with 5 wt % Cu loading was the most suitable for the water-gas shift reaction (WGS, CO + H 2 O → CO 2 + H 2 ), followed by the selective NH 3 synthesis by the NO + H 2 reaction. The unsteady-state system under alternating feeds of rich (0.1% NO + 0.5% CO + 1% H 2 O) and lean (0.1% NO + 2% O 2 + 1% H 2 O) gas mixtures over a mixture of Cu/CeO 2 and Cr-exchanged mordenite (CrMOR) showed higher NO x conversion than the steady-state (0.1% NO + 0.35% CO + 0.6% O 2 + 1% H 2 O) reaction between 200 and 500 °C. The de-NO x mechanism under periodical rich/lean conditions was studied by operando infrared (IR) experiments. In the rich period, the WGS reaction on the Cu/CeO 2 catalyst yield H 2 , which reduces NO to NH 3 on the Cu/CeO 2 catalyst. NH 3 is then captured by the Brønsted acid sites of CrMOR. In the subsequent lean period, the adsorbed NH 3 acts as a reductant for the selective catalytic reduction of NO x catalyzed by the Cr sites of CrMOR. This study demonstrates a new urea-free and PGM-free catalytic system that can provide an alternative de-NO x technology for automotive catalysis under periodic rich/lean conditions.