A combinatorial chemistry method for fast screening of perovskite-based NO oxidation catalyst

In-Sik  Nam, POSTECH

ORAL PRESENTATION: Selective catalytic reduction of NOx by urea (urea/SCR) and lean NOx trap (LNT) technologies are the two most efficient methods to remove NOx from diesel engine exhausts [1,2]. The important role of NO2 in achieving a high deNOx performance has been widely recognized for both technologies. For example, the low temperature deNOx performance (< 250 oC) of the urea/SCR can be significantly enhanced by the formation of NO2 over the preceding diesel oxidation catalyst (DOC), mainly by inducing the Fast-SCR reaction mechanism in the co-presence of NO and NO2 [3]. The oxidation of NO to NO2 is also an essential step for storing NOx on LNT catalysts under lean-conditions, especially for improving the catalysts’ low temperature activity below 250 oC [4]. Platinum among noble metals has been preferentially used in both DOC and LNT catalysts for oxidizing NO to NO2, owing primarily to its superior NO oxidation activity [4, 5]. Recently, however, the perovskite (ABO3) has been attracting growing attention from the auto industry as an inexpensive alternative catalyst for NO oxidation to replace the ever pricey Pt metal [6].
Developing a more active, new catalyst is almost always very time-consuming and costly, involving various procedures required for catalyst preparation, characterization and activity tests. Indeed, the situation could get much worse particularly with the perovskite catalysts due to the unique structure and chemical composition of perovskite catalysts, where a huge combinatorial variety of catalyst formulations is possible by various combinations of proper metals for A- and B-sites as well as by partial substitutions of both sites. Fortunately, however, there has recently come along a wide range of high throughput screening techniques such as colorimetric assay, fluorescence indicator, infrared (IR) thermography, laser induced fluorescence imaging (LIFI), etc. as a more efficient method for discovering a new, more active catalyst formulation [7].
In the present study, we have developed a fast screening method for perovskite-based NO oxidation catalysts. The screening system uses a high-throughput reactor system with parallel dual-bed reactors in a (10×10) array: Each dual-bed reactor consists of a perovskite-based NO oxidation catalyst in the front bed and a NOx storage catalyst (K/Al2O3) in the rear bed. The oxidation of NO to NO2 proceeds in the front bed, followed by the adsorption of NOx in the rear bed. The amount of NO2 produced over the perovskite catalyst can be determined by the aqueous extraction of NO2 stored on K/Al2O3, followed by the colorimetric assay based upon the Griess diazotization reaction [8]. The amount of NOx adsorbed on the NOx storage catalyst (K/Al2O3) well correlated with the NO oxidation activity directly measured using the conventional fixed-bed catalytic reactor system over a wide range of the reaction temperature (Fig.1).