THE EFFECTS OF CERIA ADDITION ON AGING AND SULFATION OF LEAN NOx TRAPS FOR STAND ALONE AND LNT-SCR APPLICATIONS Model powder and fully formulated monolithic lean NOx trap (LNT) catalysts were used to investigate the effect of ceria on desulfation behavior. Temperature-programmed reduction (TPR) experiments (model catalysts) showed each of the oxide phases present is able to store sulfur and possesses distinct behavior (temperature at which desulfation occurs). La-CeO2 or CeO2-ZrO2-containing samples (monoliths) showed a greater resistance to deactivation during sulfation and required lower temperatures to restore the NOx storage efficiency to its pre-sulfation value. Fully formulated monolithic LNT catalysts containing varying amounts of Pt, Rh and BaO were subjected to accelerated aging to elucidate the effect of washcoat composition on LNT aging. Elemental analysis revealed that residual sulfur, associated with the Ba phase, decreased catalyst NOx storage capacity and that sintering of the precious metals resulted in decreased contact between the Pt and Ba phases. Spatially-resolved inlet capillary mass spectrometry (SpaciMS) was employed to understand the factors influencing the selectivity of NOx reduction in LNT catalysts degreened and thermally aged) containing Pt, Rh, BaO and Al2O3, and contained La-stabilized CeO2. Stretching of the NOx storage and reduction zone (NSR) zone resulted in increased selectivity to NH3 due to the fact that less catalyst was available to consume NH3 by either the NH3-NOx SCR reaction or the NH3-O2 reaction. Additionally, the loss of oxygen storage capacity (OSC) and NOx storage sites, along with the decreased rate of NOx diffusion to Pt/Rh sites, led to an increase in the rate of propagation of the reductant front after aging, in turn, resulting in increased H2:NOx ratios at the Pt/Rh sites and consequently increased selectivity to NH3. Finally, a crystallite scale model was used to predict selectivity to NH3 from the LNT catalysts during rich conditions after a fixed amount of NOx was stored during lean conditions. Both the experimental and model predicted data showed that the production of NH3 is limited by the rate of diffusion from the Ba storage sites to the Pt particles at 200 °C. At 300 °C, the process is limited by the rate at which H2 is fed to the reactor.

The ability to resolve reactions within a monolith spatially and temporally is key in developing reliable kinetic models, as well as in validating proposed reaction mechanisms. In this work, two techniques, IR-thermography and spatially-resolved capillary inlet mass spectrometry (SpaciMS), were used to measure temperature and gas-phase concentrations. Specifically, they were applied to monitor the axial distribution of temperature and concentration profiles during propylene oxidation over a Pt/Al2O3 monolith-supported catalyst. Also, the effect of thermally aging the catalyst on the temperature and concentration patterns observed was investigated. During temperature programmed oxidation experiments, the data show that conversion of propylene began at the outlet, and a reaction front generated at the rear of the monolith traveled upstream, as a moving reaction zone, thereby creating a temperature wave pattern since the reaction is exothermic. The conversion was always complete downstream of this reaction zone at any point along the catalyst. When the reactor was cooled, the conversion of propylene started to drop, accompanied by a similar temperature wave pattern that traveled in the opposite direction (from upstream to downstream) and was attributed to a phenomenon known as wrong-way behavior. Finally, thermally aging the catalyst led to a slower and more localized moving hot zone.

The ability to resolve reactions within a monolith spatially and temporally is key in developing reliable kinetic models, as well as in validating proposed reaction mechanisms. In this work, two techniques, IR-thermography and spatially-resolved capillary inlet mass spectrometry (SpaciMS), were used to measure temperature and gas-phase concentrations. Specifically, they were applied to monitor the axial distribution of temperature and concentration profiles during propylene oxidation over a Pt/Al2O3 monolith-supported catalyst. Also, the effect of thermally aging the catalyst on the temperature and concentration patterns observed was investigated. During temperature programmed oxidation experiments, the data show that conversion of propylene began at the outlet, and a reaction front generated at the rear of the monolith traveled upstream, as a moving reaction zone, thereby creating a temperature wave pattern since the reaction is exothermic. The conversion was always complete downstream of this reaction zone at any point along the catalyst. When the reactor was cooled, the conversion of propylene started to drop, accompanied by a similar temperature wave pattern that traveled in the opposite direction (from upstream to downstream) and was attributed to a phenomenon known as wrong-way behavior. Finally, thermally aging the catalyst led to a slower and more localized moving hot zone.