A Highly Active and Stable Non-Platinic Lean NOx Trap Catalyst MnOx-K2CO3/K2Ti8O17 with Ultra-Low NOx to N2O Selectivity

A non-platinic lean NOx trap catalyst MnOx-K2CO3/K2Ti8O17 (K2CO3 loading: 25 wt %) was prepared via successive impregnation, which exhibits a large NOx storage capacity (3.21 mmol/g), a high NOx reduction percentage (98.5%) and an ultralow selectivity of NOx to N2O (0.3%). The catalyst was characterized by multiple techniques including XRD, SEM/HRTEM, EXAFS, FT-IR, CO2-TPD and in situ DRIFTS. Except for K2O, -OK groups, surface K2CO3 and bulk or bulk-like K2CO3, an unknown titanate phase with a K/Ti atomic ratio higher than 2/8 is identified, which is also active and regenerative for NOx storage and reduction. In-situ DRIFTS results reveal that NOx is mainly stored as bidentate nitrates and bidentate nitrite species in the catalyst. The appearance of negative bands around 1555 and 1575 cm(-1) (C=O stretching vibration in bidentate carbonates) suggests the involvement of carbonates in NOx storage. Based upon the characterization results, a carbonate-involved NOx storage/reduction mechanism was proposed.