Reduction of NOx by H2 over Pt-K/Al2O3 Lean NOx Trap catalysts

Introduction Lean NOx Trap (LNT) catalysts are viable catalysts for the abatement of NOx emissions from lean–burn engines [1]. These systems work under periodic cyclic conditions, alternating lean (NOx storage) and rich conditions during which the stored NOx are reduced by a reductant. Although PtBa/Al2O3 is a typical model catalyst for this application, systems containing K have also been proposed. In previous studies the reduction with hydrogen of nitrates stored on a model PtBa/Al2O3 catalyst has been investigated [2]. It has been shown that the formation of N2 occurs via a 2steps in series molecular pathway involving at first the formation of ammonia upon reaction of nitrates with H2 (step 1), followed by the reaction of NH3 with residual nitrates to give N2 (step 2). Step 1 (i.e. the reduction of stored nitrates to NH3 ) is faster than step 2, the subsequent reaction of NH3 with nitrates. Hence this step is rate determining in the formation of N2, and high NH3 formation is observed at low temperatures where the reactivity of NH3 towards nitrates is low. Besides, due to the high reactivity of H2 towards nitrates, and due to the integral nature of the reactor, an H2 front develops so that NH3 which is formed at the front reacts with nitrates stored downstream the H2 front, if the temperature is high enough. This explain the typical product sequence observed upon regeneration of LNT catalysts, with NH3 evolution following that of N2 [2]. Objective of this work is to investigate mechanistic aspects of the reduction process when a Pt-K/Al2O3 system is employed, i.e. to analyze the effect of the storage component on the reduction of the stored NOx.