Effect of reductive treatments on Pt behavior and NOx storage in lean NOx trap catalysts

Lean NOx trap (LNT) catalysts represent a promising approach to meet increasingly stringent NOx emission regulations on diesel and other lean-burn engines. Pt material properties, including dispersion and particle size, are known to be important factors in determining NOx uptake performance, since Pt provides active sites for NO oxidation to NO2 necessary for storing NOx as nitrates, and for the reduction of nitrates to N2. In this work, the physicochemical properties of Pt in Pt–BaO/Al2O3 LNT catalysts, such as the Pt accessible surface area and particle size, were investigated by using various tools, such as irreversible volumetric H2 chemisorption, high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD), following successive reductive treatments at elevated temperatures. NOx uptake activities were also measured to establish a relationship between the properties of Pt and NOx storage following identical high-temperature reductive treatments. We find that the reductive treatments of Pt–BaO/Al2O3 lean NOx trap catalysts at temperatures up to 500 °C promote a significant increase in NOx uptake explained, in part, by an induced close interaction between Pt and BaO phases in the catalyst, thus enabling facilitation of the NOx storage process.

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► Pt accessible area and Pt particle size in Pt/BaO–Al2O3 catalysts are determined by H/Pt ratios, TEM and XRD, and then correlated with measured NOx uptakes after the successive reductive treatments at elevated temperatures.
► Reductive treatments of Pt–BaO/Al2O3 lean NOx trap catalysts at temperatures up to 500 °C promote a significant increase in NOx uptake.
► Close interactions between Pt and BaO phases in the catalyst are proposed to improve the efficient use of barium species in the NOx storage process.