Continuous Unsteady-State De-NO System via Tandem Water-Gas Shift, NH Synthesis, and NH-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 system using cyclic feeds of rich and lean gas mixtures with PGM-free catalysts. Initial catalyst screening tests showed that Cu/CeO with 5 wt % Cu loading was the most suitable for the water-gas shift reaction (WGS, CO + HO → CO + H), followed by the selective NH synthesis by the NO + H reaction. The unsteady-state system under alternating feeds of rich (0.1% NO + 0.5% CO + 1% HO) and lean (0.1% NO + 2% O + 1% HO) gas mixtures over a mixture of Cu/CeO and Cr-exchanged mordenite (CrMOR) showed higher NO conversion than the steady-state (0.1% NO + 0.35% CO + 0.6% O + 1% HO) reaction between 200 and 500 °C. The de-NO mechanism under periodical rich/lean conditions was studied by infrared (IR) experiments. In the rich period, the WGS reaction on the Cu/CeO catalyst yield H, which reduces NO to NH on the Cu/CeO catalyst. NH is then captured by the Brønsted acid sites of CrMOR. In the subsequent lean period, the adsorbed NH acts as a reductant for the selective catalytic reduction of NO 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 technology for automotive catalysis under periodic rich/lean conditions.