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

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posted on 2013-11-14, 00:00 authored by Yuxia Zhang, Ming Meng, Fangfang Dai, Tong Ding, Rui You
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.

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