Experimental investigation of the reduction of NOx species by CO and H2 over Pt–Ba/Al2O3 lean NOx trap systems

In this paper the reactivity of NOx stored at 350 °C onto Pt–Ba/Al2O3 LNT catalyst in the reduction by H2, CO and CO/H2O mixture is investigated by means of transient experiments and complementary FT-IR analyses. Hydrogen is found to be the best reductant, being characterized by the highest NOx removal efficiency in the range 150–350 °C. It is proposed that the reduction by hydrogen proceeds according to a dual-steps mechanism in which NOx are first reduced to ammonia, which in turn further reacts with other nitrate species leading selectively to N2. Conversely, when CO is used as reducing agent under dry conditions only a part of the initially stored NOx is removed as N2, the other fraction being reduced to adsorbed cyanates/isocyanates species. It is proposed that the cyanate/isocyanate ad-species lead to the formation of nitrogen by oxidation involving other nitrate species; this reaction is self-poisoned by CO. In the presence of water, CO reduced nitrates into cyanates/isocyanates ad-species that were readily hydrolyzed to ammonia; this species is hence precursor in N2 formation upon reaction with residual nitrate species. Alternatively, stored nitrates are reduced by H2 formed via the water gas shift (WGS) reaction.