Effect of reductive treatments on Pt behavior and NO x storage in lean NO x trap catalysts

Abstract

Lean NO x trap (LNT) catalysts represent a promising approach to meet increasingly stringent NO x 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 NO x uptake performance, since Pt provides active sites for NO oxidation to NO 2 necessary for storing NO x as nitrates, and for the reduction of nitrates to N 2. In this work, the physicochemical properties of Pt in Pt–BaO/Al 2O 3 LNT catalysts, such as the Pt accessible surface area and particle size, were investigated by using various tools, such as irreversible volumetric H 2 chemisorption, high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD), following successive reductive treatments at elevated temperatures. NO x uptake activities were also measured to establish a relationship between the properties of Pt and NO x storage following identical high-temperature reductive treatments. We find that the reductive treatments of Pt–BaO/Al 2O 3 lean NO x trap catalysts at temperatures up to 500 °C promote a significant increase in NO x uptake explained, in part, by an induced close interaction between Pt and BaO phases in the catalyst, thus enabling facilitation of the NO x storage process.