K-supported catalysts for diesel soot combustion: Making a balance between activity and stability

• Activity and stability are two sides of the coin for K-containing catalysts.
• The active sites are the free K species.
• The immobilized K+ ions in the lattice can seriously reduce the activity.
• Balancing of activity and stability was realized by using the tunneled cryptomelane.

Two typical K-supported oxides (K/Al2O3 and K/TiO2) were selected as catalysts for diesel soot combustion in order to evaluate the relationship between activity and stability. The surface species were first characterized by X-ray powder diffraction (XRD), X-ray absorption fine-structure (XAFS), infrared analysis (IR), X-ray photoelectron spectroscopy (XPS), in situ IR of CO2 adsorption and temperature-programmed desorption of CO2 (CO2-TPD). Then the activity was studied by temperature-programmed oxidation (TPO), temperature-programmed reduction with soot (Soot-TPR) and in situ IR of soot combustion reactions. Lastly, the stability of the catalyst was checked by washing with water and analysis for K. This showed that K/TiO2 displayed negligible activity due to the immobilization of K+ ions in TiO2 to form K2Ti6O13. However, K/Al2O3 exhibited superior activity owing to the presence of active free K+ ions on the surface. The downside is the leaching of K+ ions into the effluent containing some water vapor, resulting in its poor stability – the other side of the coin for the highly active free K species. One of the suggested strategies for making a balance between activity and stability is encapsulating active K species in the channel of the catalyst as demonstrated by the tunneled cryptomelane.

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