Characteristics of low platinum Pt–BaO catalysts for NOx storage and reduction

The characteristics of low platinum Pt–BaO catalysts for NOx storage and reduction (NSR) were studied over three series of samples that have similar barium loadings (ca. 15 wt% Ba), namely, Pt–BaO/Al2O3, CoOx promoted Pt–BaO/Al2O3 and Pt–BaO/ZrO2–Al2O3 catalysts. The catalysts were calcined at 800 °C and their NSR performances were evaluated under cyclic lean (1000 ppm NO and 10 vol% O2 in Ar)/rich (1 vol% H2 in Ar) conditions with particular attention paid to detect the possible harmful by-products, i.e., NH3 and N2O, during the reduction of the stored NOx under the rich condition. Lowering the Pt loading in the Pt–BaO/Al2O3 catalyst from 1.0 to 0.5 wt% resulted in a 15% decrease in the NOx storage capacity (NSC) under the lean condition but the N2 selectivity under the rich condition declined dramatically (from 85% to 53%), indicating that the NOx 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/Al2O3 at the Pt loading of 0.5 wt% further reduced the N2 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 ZrO2–Al2O3 as an alternative of the Al2O3 support was effective in lowering the dependence on Pt loading of the Pt–BaO for the NSR catalysis since the ZrO2–Al2O3 support could make the catalyst highly selective for the formation of N2 under the rich condition, at a lower Pt loading. The overall NSR performance of a Pt–BaO/ZrO2–Al2O3 catalyst with 0.5 wt% Pt was found comparable to that of its counterpart Pt–BaO/Al2O3 catalyst with 1.0 wt% Pt, demonstrating a viable way for developing low platinum NSR catalysts.