Carbonate-Based Lean-Burn NOx Trap Catalysts Pt–K2CO3/ZrO2 with Large NOx Storage Capacity and High Reduction Efficiency

Abstract

A series of carbonate-based lean-burn NOx trap (LNT) catalysts Pt–K2CO3/ZrO2 with different K2CO3 loading were prepared by sequential impregnation, which show extremely good performance for lean NOx storage and reduction. The catalyst containing 15 wt % K2CO3 exhibits a large NOx storage capacity of 2.16 mmol/g and a very high NOx reduction percentage of 99%. Multiple techniques including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), temperature-programmed decomposition (TPD), Fourier-transform infrared spectroscopy (FT-IR), and in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) were employed for catalyst characterization. The results of XRD, FT-IR, and HR-TEM conformably show that, at room temperature, the K species exist as amorphous K2CO3; while at NOx storage temperature (350 °C), three kinds of K species including −OK groups, K2O, and K2CO3 are simultaneously present in the catalysts as revealed by in situ DRIFTS, TPD, and FT-IR results. Surface carbonates are identified as the most active species for NOx storage, showing the best NOx storage performance. Higher K2CO3 loading than 15 wt % leads to the formation of more bulk or bulk-like K2CO3 species, which are unfavorable to NOx storage. As K2CO3 loading is 10 wt % or less, the NOx is mainly stored as nitrates species such as monodentate nitrates, ionic nitrates, and bridging bidentate nitrates, while at higher K2CO3 loading, the NOx is only stored as bidentate nitrite species. The presence of excess amount of K2CO3 can decrease the ability of the catalysts for NO adsorption and oxidation, making the NOx oxidized only to nitrite species.