(547c) Pt and Ba Impact the Effects of CO2 and H2O On the NOx Storage Capacity of Pt/BaO/Al2O3 Lean NOx Traps
Pt/BaO/γ-Al2O3 is the most common formulation of the lean NOx traps (LNTs, also known as NOx Storage Reduction (NSR) catalysts) studied. It is well known that CO2 and H2O, components typically present in the automotive exhaust, affect the NOx Storage Capacity (NSC) of these lean NOx traps. We ran performance tests on a set of seven monolithic samples containing combinations of 0.6, 2 and 6 wt. % Pt with 4, 8 and 20 wt. % Ba/ γ-Al2O3 using a flow reactor system capable of simulating the NSR operation. We also studied the samples using DRIFTS - with identical conditions as those used for the performance tests. The complementary information from these results gives a clearer understanding of the effects of CO2 and H2O on the NSC of these traps. It was found that (1) Pt, Ba loading and (2) presence of NO+O2 or NO2+O2 in lean feed at 300°C have significant impact on the effect of CO2 and H2O on ?*fast? and ?total? NSC (f-NSC and t-NSC, respectively) of the lean NOx traps. Thus, it became imperative that these influences be considered while drawing conclusions regarding the effects of CO2 and H2O on the NSC.
Using DRIFTS we have observed that, at 300°C, NOx can replace all the carbonates and carboxylates in the Ba phase in the absence of CO2 in the feed. Whereas in the presence of CO2 in feed, not all the BaCO3 can be converted into Ba(NO3)2. On traps with the highest Ba loading (20 wt. %), increasing Pt loading from 0.6 to 6 wt. % results in small increase in t-NSC and significant increase in f-NSC, suggesting that Pt affects the carbonate-nitrate ratio, especially for the fast NOx storage mechanism. Addition of water reduces the ionic bulk Ba-nitrate peak intensity in the DRIFTS spectrum for the trap containing 20 wt. % Ba. It also reduces the bidentate Ba-nitrate and nitrate on alumina peak intensities for traps containing 8 and 4 wt. % Ba at the same time it increases the ionic Ba-nitrate peak intensity on those samples. Correspondingly, the t-NSC on the trap containing 20 wt. % Ba decreased and that on traps containing 8 and 4 wt. % Ba increased. Thus, it is plausible that total NSC is most affected by changes in the ionic bulk nitrate phase, which increases in the presence of water on the low Ba loading materials but decreases on the 20 wt.% Ba samples. NOx storage on Ba with CO2 and H2O present in the feed is a synergistic process since CO2 and H2O each affect the storage differenctly and have different sensitivity to the the Pt ? Ba interaction.
Work is underway to use the trends above and models based on them discussed above to determine Pt and Ba loadings on the NOx traps which would be least affected by presence of CO2, H2O and NO/NO2 in the exhaust and which would be most efficient in utilizing the deposited materials (Pt, Ba) for maximizing NOx storage without compromising trap regeneration.
* fast NSC = time for 1% of the inlet NOx (NO + NO2) breakthrough x inlet [NOx]