‘Fast’ NOx storage on Pt/BaO/γ-Al2O3 Lean NOx Traps with NO2 + O2 and NO + O2: Effects of Pt, Ba loading

NOx storage on Pt/BaO/γ-Al2O3 Lean NOx Traps (LNTs) has been studied with a particular focus on the NOx storage capacity (NSC) of these traps for the time up to which 1% of the inlet [NOx] escapes the trap. This complete or ‘fast’ NOx sorption capability of LNTs for appreciable amounts of time is what makes these catalysts useful for automotive NOx emission abatement. The fast NSC for LNT formulations with combinations of a range of Pt (0.6–6.3 wt.%) and Ba (4–20 wt.%) loadings was measured under various lean feed compositions including NO, NO2 and NO + NO2 as the NOx sources and presence and absence of 7%CO2 and 7%H2O. All the measurements were performed at 300 °C and at a space velocity of 30,000 h−1. The complex trends in the fast NSC due to various Pt, Ba loading combinations are explained with the help of a phenomenological model. The model addresses the trends in fast NSC primarily through combinations of NOx storage contributions in parallel pathways on Ba vicinal to Pt and Ba uninfluenced by Pt. We attribute the influence of Pt to the spillover of dissociated oxygen atoms from Pt to the vicinal Ba sites. This Pt–Ba synergy was found to play a dominating role in governing the fast NSCs of all the LNT samples especially in presence of CO2, H2O and CO2 + H2O in the lean feed. It was also found to be the prominent factor in limiting the fast NSC when NO and NO + NO2 (with low NO2/NO) were the NOx sources rather than NO2. We propose that the NOx storage process on Ba vicinal to Pt involves a localized reaction front of NOx that travels through the catalyst bed with saturation of those sites. This process has no preference between NO and NO2 as a precursor. CO2 and H2O affect through competition for both types of Ba sites available for NOx storage.