(547g) “Fast” NOx Storage On Pt/BaO/Al2O3 with NO+O2 Vs. NO2+O2
At any given temperature, the NOx storage process on monolithic Pt/BaO/γ-Al2O3 lean NOx traps (LNTs, also known as NOx Storage Reduction ? NSR catalysts) has an intricate dynamics governed by kinetics of NO oxidation and NOx adsorption on Ba, NO oxidation equilibrium limitations, availability of Ba for NOx storage in presence of CO2 and H2O in the feed and Pt and Ba dispersion on the washcoat. It has been consistently reported in the literature that NOx trapping is most efficient when NO2 is the NOx source. Since NO is the primary component in the engine exhaust, NO oxidation and hence Pt play important roles during NOx storage. Our goal has been to investigate the differences in NOx Storage Capacities (NSC) due to presence of NO or NO2 in lean feed on NOx traps with various Pt, Ba loadings and to search for an optimum, if present. In this search we have examined the combined effects of Pt-Ba, Pt-Ba-CO2, Pt-Ba-H2O, and Pt-Ba-(CO2+H2O) on the NOx storage process on lean NOx traps.
We studied a set of seven monolithic samples containing combination of 0.6, 2 and 6 wt. % Pt with 4, 8 and 20 wt. % Ba/ γ-Al2O3 using a flow reactor setup capable of simulating the NSR operation. After several short NOx capture ? regeneration cycles to condition the trap to the desired feed conditions, long capture phases (1.5 hr for feed containing NO2 + O2 and 1 hr for NO + O2) were run on them with the same feed, and NO and NO2 breakthrough curves were recorded. All the experiments were performed at 300°C and 30000/hr space velocity in presence and absence of 7% CO2, 8% H2O. *Total NOx Storage Capacity (t-NSC) and **Fast NOx Storage Capacity (f-NSC) of these traps was measured.
In absence of CO2 and H2O, the t-NSC on three samples loaded with 0.6, 2 and 6 wt. % Pt/20 wt. % Ba dropped by 35 (±5)% with NO+O2 as compared to NO2 +O2 in feed; whereas the f-NSC decreased by only 10 (±2)%. On samples containing 2 wt. % Pt/8 wt. % Ba and 0.6, 2 and 6 wt. % Pt/4 wt. % Ba, the t-NSC and f-NSC remained the same irrespective of NO or NO2 in feed with O2.
In presence of 7% CO2 and 8% H2O, the t-NSC on three samples with 0.6, 2 and 6 wt. % Pt/20 wt. % Ba dropped by 55 (±5)% with NO+O2 as compared to NO2 +O2 in feed; whereas the f-NSC also dropped by 45 (±5)%. On samples containing 2 wt. % Pt/8 wt. % Ba the t-NSC dropped by 25% but the f-NSC remained unchanged with NO+O2 and NO2+O2. Lastly on the samples loaded with 0.6, 2 and 6 wt. % Pt/4 wt. % Ba, the t-NSC and f-NSC remained the same irrespective of NO or NO2 in feed with O2.
We observed a clear trend in f-NSC/Ba (in mol NOx/mol Ba) with exposed Pt/Ba (mol exposed Pt/mol Ba), on the samples with same Ba loading. Hence we conclude that Pt plays a significant role in ?fast? NOx storage with NO2+O2 as well as with NO+O2. These results clearly indicate that in addition to NO oxidation, Pt also assists the sorption of NO2. It can be envisaged that O2 is the source of O* spilled over from Pt, which is used as an oxidant during the fast NOx storage. Thus, Pt on samples with higher exposed Pt/Ba can cater to a larger fraction of the available Ba by increasing the source number of O* available per Ba, giving rise to higher f-NSC on those traps. Our DRIFTS spectra show that, at 300°C, most of the NOx is stored in the form of Ba(NO3)2 on these traps. Formation of barium nitrate from BaO and two NO molecules requires 3O* compared to 1O* if NO2 is used instead of NO. Direct storage of NO on Ba also contributes to fast NSC in addition to storage of NO2. Lower fast NOx storage/Ba (mol/mol) on traps containing higher Ba loadings might be due to this direct storage since reduces the amount of Ba that can be reached by O* spilled over from Pt by consuming more O* per barium nitrate formed. Furthermore, in presence of CO2 and H2O in the gaseous lean phase feed, a fraction of the fast NOx storing Ba to which Pt can provide O* might be blocked due to formation Ba-cabonate, carboxylate and hydroxide, resulting in lower f-NSC. Our DRIFTS studies show that the stability of these species depends on the gas phase feed composition. On traps with lower Ba loading, f-NSC is unaffected for feed containing NO2+O2 and NO+O2 simply because the amount of Pt present is sufficient to produce the required O*. This mechanism can also explain our results for experiments with 50 to 600 ppm NO +320 ppm NO2 + 10% O2 in lean feed for which a small drop in f-NSC is seen on traps with 20 wt. % Ba and negligible change on traps containing 8 and 4 wt. % Ba. The trend in f-NSC/Ba with exposed Pt/Ba under these conditions remained the same as expected.
We also observed that the time to breakthrough (1% of the inlet NOx) changed proportionally when 100 to 450 ppm NO2 + 10 % O2 + 7% CO2 +8% H2O + Ar (balance) was fed; keeping the f-NSC constant. Using this fact and fitting parameters for a power law fit of the ratio of ^ideal fast NSC /measured fast NSC (y) and exposed Pt on the trap (x), a simple empirical correlation was developed for time to breakthrough and PNO2 in the exhaust for a trap with known Pt dispersion and Ba loading. This correlation is specific to our samples, but it highlights a simple approach which can be applied to any set of traps to yield required parameters to predict their f-NSCs. The correlation is simple enough to be used in real time with real time measurements of PNO2 and temperature of the vehicle exhaust.
* total NSC = integrated area above the NOx breakthrough curve and under the inlet [NOx] level
** fast NSC = time for 1% of the inlet NOx (NO + NO2) breakthrough x inlet [NOx]
^ ideal fast NSC - assuming that all the Ba on the trap identically contributes to the fast NSC