States and Function of Potassium Carbonate Species in the Polytitanate Nanobelt Supported Catalysts Used for Efficient NOx Storage and Reduction

A series of polytitanate nanobelt supported lean-burn NOx trap catalysts Pt-xK₂CO₃/K₂Ti₈O₁₇ with different weight loading of K₂CO₃ (x = 0%, 5%, 15%, 20%, 25%, or 30%) were synthesized by successive impregnation. The nanobelt support K₂Ti₈O₁₇ displays a specific surface area as high as 302 m²/g, and the corresponding catalysts Pt-xK₂CO₃/K₂Ti₈O₁₇ show excellent NOx storage performance. As K₂CO₃ loading increases from 5% to 30%, the NOx storage capacity (NSC) exhibits a volcano-type altering tendency with the maximum appearing at 25% (2.68 mmol/g); the highest NOx reduction efficiency of 99.2% was also achieved over this catalyst in cyclic alternative lean/rich atmospheres. Further increase of K₂CO₃ loading induces the formation of more bulk or bulk-like K₂CO₃ species, decreasing the performance of the catalysts for NOx storage and reduction. HR-TEM and FT-IR results indicate that the K species exist as highly dispersed phases including K₂O, K₂CO₃, and −OK groups, which are undetectable by X-ray diffraction (XRD) even at the K₂CO₃ loading of 30%. Several carbonate species with different thermal stability and reactivity are identified by FT-IR and CO₂-TPD. In situ diffuse reflectance FT-IR (DRIFTS) reveals that at low K₂CO₃ loading (<20%) NOx is mainly stored as monodentate nitrates and monodentate nitrites, while at higher K₂CO₃ loading NOx is mainly stored as bidentate nitrite species, which results from the decrease of oxidation ability of the catalysts due to the potential covering of K₂CO₃ on Pt sites.