Characteristics of low platinum Pt–BaO catalysts for NO x storage and reduction

Abstract

The characteristics of low platinum Pt–BaO catalysts for NO x storage and reduction (NSR) were studied over three series of samples that have similar barium loadings (ca. 15 wt% Ba), namely, Pt–BaO/Al 2O 3, CoO x promoted Pt–BaO/Al 2O 3 and Pt–BaO/ZrO 2–Al 2O 3 catalysts. The catalysts were calcined at 800 °C and their NSR performances were evaluated under cyclic lean (1000 ppm NO and 10 vol% O 2 in Ar)/rich (1 vol% H 2 in Ar) conditions with particular attention paid to detect the possible harmful by-products, i.e., NH 3 and N 2O, during the reduction of the stored NO x under the rich condition. Lowering the Pt loading in the Pt–BaO/Al 2O 3 catalyst from 1.0 to 0.5 wt% resulted in a 15% decrease in the NO x storage capacity (NSC) under the lean condition but the N 2 selectivity under the rich condition declined dramatically (from 85% to 53%), indicating that the NO x reduction performance under the rich condition is the key to low platinum NSR catalysts. The cobalt oxides was identified as an unsuitable promoter for lowering the Pt loading; its addition into Pt–BaO/Al 2O 3 at the Pt loading of 0.5 wt% further reduced the N 2 selectivity under the rich condition although significant improvement in the NSC under lean condition could be observed. It was found instead that the use of ZrO 2–Al 2O 3 as an alternative of the Al 2O 3 support was effective in lowering the dependence on Pt loading of the Pt–BaO for the NSR catalysis since the ZrO 2–Al 2O 3 support could make the catalyst highly selective for the formation of N 2 under the rich condition, at a lower Pt loading. The overall NSR performance of a Pt–BaO/ZrO 2–Al 2O 3 catalyst with 0.5 wt% Pt was found comparable to that of its counterpart Pt–BaO/Al 2O 3 catalyst with 1.0 wt% Pt, demonstrating a viable way for developing low platinum NSR catalysts.