New insights in the NOx reduction mechanism with H2 over Pt–Ba/γ-Al2O3 lean NOx trap catalysts under near-isothermal conditions

Mechanistic aspects of the reduction of stored NOx with hydrogen were analyzed over model Ba/γ-Al2O3 and Pt–Ba/γ-Al2O3 LNT trap catalysts and over a Ba/γ-Al2O3–Pt/γ-Al2O3 physical mixture. NOx species were stored and stabilized at 350 °C under controlled conditions, and their stability/reactivity was then analyzed under different atmospheres (He and hydrogen) either at constant temperature or with temperature programming under nearly isothermal conditions, that is, in the absence of significant thermal effects. The collected data indicate that over the Pt–Ba/Al2O3 catalyst, the reduction of stored NOx occurs at temperatures below those corresponding to their thermal stability. This indicates that the reduction of stored nitrates occurs in typical NSR catalysts according to a Pt-catalyzed surface reaction, which does not involve the thermal decomposition of the adsorbed NOx species as a preliminary step. The occurrence of such a pathway also requires the co-presence of the storage element and of the noble metal on the same support, because this route does not operate when they are dispersed on different support particles.