With steadily increasing emissions regulations being imposed by government agencies, automobile manufacturers have been developing technologies to mitigate NOX emissions. Furthermore, there has been increasing focus on CO2 emissions. An effective approach for CO2 reduction is using lean burn engines, such as the diesel engine. An inherent problem with lean-burn engine operation is that NOX needs to be reduced to N2, but there is an excess of O2 present. NOX storage and reduction (NSR) is a promising technology to address this problem. This technology operates in two phases; where in the lean phase, normal engine operation, NOX species are stored as nitrates, and in a reductant rich phase, relative to O2, the NOx storage components are cleaned and the NOX species reduced to N2. In this study, the effects of reductant type, specifically CO and/or H2, and their amounts as a function of temperature on the trapping and reduction of NOX over a commercial NSR catalyst have been evaluated. Overall, the performance of the catalyst improved with each incremental increase in H2 concentration. CO was found ineffective at 200°C due to precious metal site poisoning. The addition of the H2 to CO-containing mixtures resulted in improved performance at 200°C, but the presence of the CO still resulted in decreased performance in comparison to activity when just H2 was used. At 300-500°C, H2, CO, and mixtures of the two were comparable for trapping and reduction of NOX, although the mixtures led to slightly improved performance. Although NSR technology is very efficient in reducing NOX emissions, a significant challenge that questions their long-term durability is poisoning by sulfur compounds inherently present in the exhaust. Therefore, during operation, NSR catalysts require an intermittent high-temperature exposure to a reducing environment to purge the sulfur compounds from the catalyst. This desulfation protocol ultimately results in thermal degradation of the catalyst. As a second phase of this study, the effect of thermal degradation on the performance of NSR technology was evaluated. The catalyst performance between a 200 to 500°C temperature range, using H2, CO, and a mixture of both H2 and CO as reductants was tested before and after different high-temperature aging steps. Tests included water-gas shift (WGS) reaction extent, NO oxidation, NOX storage capacity, oxygen storage capacity (OSC), and NOX reduction efficiency during cycling. The WGS reaction extent was affected by thermal degradation, but only at low temperature. NO oxidation did not show a consistent trend as a function of thermal degradation. The total NOX storage capacity was tested at 200, 350 and 500°C. Little change was observed at 500°C with thermal degradation and a steady decrease was observed at 350°C. At 200°C, there was also a steady decrease of NOX storage capacity, except after aging at 700°C, where the capacity increased. There was also a steady decrease in oxygen storage capacity at test temperatures between 200 and 500°C after each increase in thermal degradation temperature, except again when the sample was degraded at 700°C, where an increase was observed. In the cycling experiments, a gradual drop in NOX conversion was observed after each thermal degradation temperature, but when the catalyst was aged at 700°C, an increase in NOX conversion was observed. These data suggest that there was redispersion of a trapping material component during the 700°C thermal degradation treatment while the oxygen storage capacity data indicate redispersion of oxygen storage components. It therefore seems likely that it is these oxygen storage components that are becoming ‘‘activated’’ as trapping materials at low temperature.

With steadily increasing emissions regulations being imposed by government agencies, automobile manufacturers have been developing technologies to mitigate NOX emissions. Furthermore, there has been increasing focus on CO2 emissions. An effective approach for CO2 reduction is using lean burn engines, such as the diesel engine. An inherent problem with lean-burn engine operation is that NOX needs to be reduced to N2, but there is an excess of O2 present. NOX storage and reduction (NSR) is a promising technology to address this problem. This technology operates in two phases; where in the lean phase, normal engine operation, NOX species are stored as nitrates, and in a reductant rich phase, relative to O2, the NOx storage components are cleaned and the NOX species reduced to N2. In this study, the effects of reductant type, specifically CO and/or H2, and their amounts as a function of temperature on the trapping and reduction of NOX over a commercial NSR catalyst have been evaluated. Overall, the performance of the catalyst improved with each incremental increase in H2 concentration. CO was found ineffective at 200°C due to precious metal site poisoning. The addition of the H2 to CO-containing mixtures resulted in improved performance at 200°C, but the presence of the CO still resulted in decreased performance in comparison to activity when just H2 was used. At 300-500°C, H2, CO, and mixtures of the two were comparable for trapping and reduction of NOX, although the mixtures led to slightly improved performance. Although NSR technology is very efficient in reducing NOX emissions, a significant challenge that questions their long-term durability is poisoning by sulfur compounds inherently present in the exhaust. Therefore, during operation, NSR catalysts require an intermittent high-temperature exposure to a reducing environment to purge the sulfur compounds from the catalyst. This desulfation protocol ultimately results in thermal degradation of the catalyst. As a second phase of this study, the effect of thermal degradation on the performance of NSR technology was evaluated. The catalyst performance between a 200 to 500°C temperature range, using H2, CO, and a mixture of both H2 and CO as reductants was tested before and after different high-temperature aging steps. Tests included water-gas shift (WGS) reaction extent, NO oxidation, NOX storage capacity, oxygen storage capacity (OSC), and NOX reduction efficiency during cycling. The WGS reaction extent was affected by thermal degradation, but only at low temperature. NO oxidation did not show a consistent trend as a function of thermal degradation. The total NOX storage capacity was tested at 200, 350 and 500°C. Little change was observed at 500°C with thermal degradation and a steady decrease was observed at 350°C. At 200°C, there was also a steady decrease of NOX storage capacity, except after aging at 700°C, where the capacity increased. There was also a steady decrease in oxygen storage capacity at test temperatures between 200 and 500°C after each increase in thermal degradation temperature, except again when the sample was degraded at 700°C, where an increase was observed. In the cycling experiments, a gradual drop in NOX conversion was observed after each thermal degradation temperature, but when the catalyst was aged at 700°C, an increase in NOX conversion was observed. These data suggest that there was redispersion of a trapping material component during the 700°C thermal degradation treatment while the oxygen storage capacity data indicate redispersion of oxygen storage components. It therefore seems likely that it is these oxygen storage components that are becoming ‘‘activated’’ as trapping materials at low temperature.

In this dissertation, density functional theory (DFT) calculations were used to investigate (1)NO2 adsorption on BaO in NOx Storage Reduction (NSR) catalyst affected by the morphology of BaO and the γ-Al2O3 support, (2) energy barrier of H2 dissociative adsorption over Mg clusters affected by its electronic structure, and (3) comparison of the activities of CeO2 clusters affected by two different supports--monoclinic ZrO2 and non-spinel γ-Al2O3. Our results showed that the electronic effect caused by the non-stoichiometry of the bare BaO clusters and surfaces improves their reactivities toward NO2 adsorption greatly, whereas the geometric structure of the catalyst has only minor effect on the activity; we also found that the γ-Al2O3 substrate improves the reactivities of the supported BaO clusters and at the same time the interface between BaO and γ-Al2O3 provided a unique and highly reactive environment for NO2 adsorption. Hydrogen dissociation barrier over pure Mg clusters is greatly affected by the electronic structure of the clusters--closed shell clusters such as Mg10 and Mg92- have higher energy barrier toward H2 dissociation; however, H2 dissociation over clusters that are two electrons shy from the closed electronic shell are relatively easier. As substrates, neither ZrO2(111) nor γ-Al2O3(100) affects the reactivity of the supported Ce2O4 toward CO2 adsorption and CO physisorption significantly; whereas the reactivity of Ce2O4 toward CO reactive adsorption were found to be affected by the two substrates very differently.

Lean burn engines are more fuel efficient than standard stoichiometric-burn engines but at the same time, the conventional three-way catalyst is not effective in reducing the NOx in oxygen-rich exhaust. One of the recent advancements in exhaust after treatment technologies for lean burn engines is the NOx storage and reduction (NSR) methodology. In this mechanism, NOx is stored on the storage component of a NSR catalyst during normal engine operation. However, before the catalyst reaches its saturation capacity, an excess of fuel is injected to the engine for a very short period resulting in reductant rich exhaust and during this period, NOx is released and subsequently reduced to N2, therefore, restoring the storage capacity of the catalyst. The operation is cyclic in nature, with the engine operating between an oxygen rich feed for long periods and a fuel rich feed for relatively shorter periods. To implement this technology in the most efficient way, a detailed understanding of the NSR chemistry under different operating conditions is required. For the past few years, several authors have studied the NSR systems using both experimental and modeling techniques. However, most of the models proposed in the literature were calibrated against the steady cyclic operation where the NOx profiles are similar for each cycle. In real life situations, the engine operation changes with different driving conditions, occurring due to sudden acceleration, roads in hilly areas, non-uniform braking, etc., which results in operation with a number of different transient cycle-to-cycle regimes depending upon the frequency with which the engine operation is altered. Due to such varying conditions, it is very important to investigate the significance of transients observed between the two different steady cycle-to-cycle operations for the optimization and control purposes. Also, the models in the literature are specific to the catalyst used in the study and therefore, their adaptation to other NSR catalysts is not straightforward. Therefore, one of the main motivations behind this research work is to develop a general approach to explain the storage dynamics. Moreover, the existing models have not studied the regeneration mechanisms, which is very important to explain the cyclic data in complete operation including both transients and steady state cycles. In this study, a pseudo one-dimensional model of a commercial NOx storage/release (NSR) catalyst is presented. The NOx storage is considered to be mass transfer limited, where as the storage proceeds, the barium carbonate particle is converted into the nitrate and for further storage, the NOx has to diffuse through this growing nitrate layer and a after certain depth, this penetration becomes nearly impossible. To explain the transient nature of the cyclic NOx profile, it is hypothesized that when incomplete regeneration occurs, only part of the nitrate is converted back to carbonate. Therefore, the nitrate layer increases in thickness with each cycle, thus making further storage increasingly more difficult. The shrinking core concept with incomplete storage in the lean phase followed by incomplete regeneration of the nitrate layer during the regeneration phase accounts for a net drop in storage capacity of the catalyst in each cycle, which continues decreasing until the amount of sites regenerated equal the amount used in NOx storage. The number of unknown parameters used for fitting were reduced by parameter sensitivity analysis and then fitted against a NOx profile at the reactor exit. The overall amount of NOx that can be stored in the lean phase of the cycle depends on the extent of regeneration that can be achieved during the previous rich phase, which in turn depends directly on the concentration of reductants in the feed. Therefore, there is a trade-off between the amount of fuel used and the NOx emissions. The proposed model can be potentially used to improve this trade-off by using model-based optimization techniques.

Lean burn engines are more fuel efficient than standard stoichiometric-burn engines but at the same time, the conventional three-way catalyst is not effective in reducing the NOx in oxygen-rich exhaust. One of the recent advancements in exhaust after treatment technologies for lean burn engines is the NOx storage and reduction (NSR) methodology. In this mechanism, NOx is stored on the storage component of a NSR catalyst during normal engine operation. However, before the catalyst reaches its saturation capacity, an excess of fuel is injected to the engine for a very short period resulting in reductant rich exhaust and during this period, NOx is released and subsequently reduced to N2, therefore, restoring the storage capacity of the catalyst. The operation is cyclic in nature, with the engine operating between an oxygen rich feed for long periods and a fuel rich feed for relatively shorter periods. To implement this technology in the most efficient way, a detailed understanding of the NSR chemistry under different operating conditions is required. For the past few years, several authors have studied the NSR systems using both experimental and modeling techniques. However, most of the models proposed in the literature were calibrated against the steady cyclic operation where the NOx profiles are similar for each cycle. In real life situations, the engine operation changes with different driving conditions, occurring due to sudden acceleration, roads in hilly areas, non-uniform braking, etc., which results in operation with a number of different transient cycle-to-cycle regimes depending upon the frequency with which the engine operation is altered. Due to such varying conditions, it is very important to investigate the significance of transients observed between the two different steady cycle-to-cycle operations for the optimization and control purposes. Also, the models in the literature are specific to the catalyst used in the study and therefore, their adaptation to other NSR catalysts is not straightforward. Therefore, one of the main motivations behind this research work is to develop a general approach to explain the storage dynamics. Moreover, the existing models have not studied the regeneration mechanisms, which is very important to explain the cyclic data in complete operation including both transients and steady state cycles. In this study, a pseudo one-dimensional model of a commercial NOx storage/release (NSR) catalyst is presented. The NOx storage is considered to be mass transfer limited, where as the storage proceeds, the barium carbonate particle is converted into the nitrate and for further storage, the NOx has to diffuse through this growing nitrate layer and a after certain depth, this penetration becomes nearly impossible. To explain the transient nature of the cyclic NOx profile, it is hypothesized that when incomplete regeneration occurs, only part of the nitrate is converted back to carbonate. Therefore, the nitrate layer increases in thickness with each cycle, thus making further storage increasingly more difficult. The shrinking core concept with incomplete storage in the lean phase followed by incomplete regeneration of the nitrate layer during the regeneration phase accounts for a net drop in storage capacity of the catalyst in each cycle, which continues decreasing until the amount of sites regenerated equal the amount used in NOx storage. The number of unknown parameters used for fitting were reduced by parameter sensitivity analysis and then fitted against a NOx profile at the reactor exit. The overall amount of NOx that can be stored in the lean phase of the cycle depends on the extent of regeneration that can be achieved during the previous rich phase, which in turn depends directly on the concentration of reductants in the feed. Therefore, there is a trade-off between the amount of fuel used and the NOx emissions. The proposed model can be potentially used to improve this trade-off by using model-based optimization techniques.

Le catalyseur quatre voies est destiné à diminuer simultanément les émissions d’hydrocarbures, de monoxyde de carbone, d’oxydes d’azote et de suies par l’intermédiaire d’un seul monolithe catalytique. Plusieurs études sur ce type de catalyseur ont montré que la présence d’oxydes d’azote entraîne une diminution de la température d’oxydation des suies. Cependant, l’effet de la présence d’eau sur l’adsorption des oxydes d’azote n’est pas encore clair, surtout en présence de suies. Les travaux présentés dans ce manuscrit ont pour but de mettre en évidence l’influence de la présence d’eau et/ou de suies sur le fonctionnement de catalyseur «piège à NOx». Les résultats obtenus montrent que la présence d’eau entraîne une inhibition de la fonction oxydante du catalyseur, une diminution de la quantité de stockage des oxydes d’azote, et une inhibition de la formation des espèces adsorbées de surface. Ces phénomènes ont été attribués à la voix réactionnelle spécifique en présence d’eau associée à l’adsorption des oxydes d’azote. Les résultats obtenus sur le mélange noir de carbone et catalyseur montrent que la présence de noir de carbone induit une diminution de stockage des oxydes d’azote. De plus, cet effet se trouve plus important en contact fort. Les expériences réalisées sur l’adsorption des oxydes d’azote en présence simultanée de noir de carbone et d’eau ont montré un effet non cumulé de l’eau et du noir de carbone. Ce phénomène a été attribué à une compétition entre l’action de l’eau qui favorise la formation de nitrate de cœur à partir des nitrates faiblement liés et l’action du noir de carbone qui tend à déstabiliser les nitrates faiblement liés pour former les carbonates. / The four ways catalyst is used to decrease the emissions of hydrocarbon, carbon monoxide, nitrogen oxides and soot by a monolithic catalyst. Several researches on this type of catalyst have shown that the presence of nitrogen oxides involves a decrease of the soot oxidation temperature. However, the effect of the presence of water on the nitrogen oxides adsorption is not clear yet, especially with the presence of soot. The works presented in this manuscript are intended to study the influence of the presence of water or/and soot on the performance of the catalyst NOx trap. The results showed that the presence of water involves an inhibition of the oxidation function of catalyst, a decrease of the capacity of the NOx storage, and an inhibition of the formation of the surface adsorbed species. It is attributed to a special reactive way in the presence of water linked to the adsorption of nitrogen oxides. The results obtained on the mix of carbon black and catalyst showed that the presence of carbon black induce a decrease of the NOx storage capacity of catalyst. Besides, this effect was more important with a tight contact between carbon black and catalyst. The experiments realized on the adsorption of nitrogen oxides with the presence of carbon black and water simultaneously showed an effect not accumulated of the water and the carbon black. This phenomenon is attributed to a competition between the action of water which favors the formation of the bulk nitrate from the weak-linked nitrates and the action of carbon black which tend to destabilize the weak-linked nitrates to form the carbonates.