Infra-Red thermography and spatially-resolved capillary inlet mass spectrometry (SpaciMS) have been used to characterize propylene oxidation along a Pt/Al2O3 monolith-supported catalyst, before and after heterogeneous deactivation. The combined techniques clearly show reaction location, and therefore catalyst use, and how these change with thermal and sulphur degradation. Following the heterogeneous thermal aging, the reaction zones at steady state were broader and located farther into the catalyst relative to those observed with the fresh catalyst. As well, the time for the temperature and concentration waves to travel through the catalyst during back-to-front ignition increased. These effects were more pronounced with 1500 ppm propylene relative to 4500 ppm propylene. Such trends could not be detected based on standard catalyst-outlet measurements. The light-off behaviour was also impacted by the aging, resulting in complex changes to the temperature front propagation, depending on the propylene concentration. With each sulphur exposure step, light-off temperatures increased and the time for back-to-front ignition during temperature programmed oxidation changed pattern. With 1500 ppm propylene fed, the reaction zones established during steady-state operation shifted farther into the catalyst and increased slightly in width following sulphur treatment; at very high temperature and for 4500 ppm propylene, the reaction zones were very close to the catalyst inlet and virtually indistinguishable between catalyst sulphation states. However, at lower steady-state temperatures for the higher propylene concentration, the catalyst did experience delays in reaction light-off and light-off position moved downstream in the catalyst with sulphur damage.

The Diesel Oxidation Catalyst (DOC) is a key component in the exhaust after-treatment system of diesel engines. In this study two aspects of a DOC were investigated: catalyst distribution and reactant species interactions. In the first part, the effect of an axial Pt distribution along a DOC was investigated by comparing a standard sample, with a homogeneous Pt distribution along the length, with a zoned sample, where the Pt was non-homogeneously distributed along the length. Temperature-programmed oxidation (TPO) and spatially-resolved gas-phase concentration measurement experiments were used to compare the CO, C3H6 and NO oxidation performance of the standard and zoned catalysts. Both catalyst types had the same total amount of Pt but different distributions. The zoned catalyst, with more Pt located in the upstream portion, showed better performance than the standard catalyst, especially at high total flow rate and when a mixture of the reactants were used. The superior performance of the zoned sample is due to a larger, localized exotherm in the upstream region, where more Pt is located, and a decrease in the self-poisoning effect downstream, where reaction light-off occurs. In addition, catalyst durability against thermal degradation was tested by exposing the whole catalyst (homogeneous aging) and part of the catalyst (heterogeneous aging) to high temperatures. In general, the zoned catalyst showed better performance than the standard catalyst after thermal aging, especially after heterogeneous aging. The reason for the superior performance of the zoned catalyst, especially after heterogeneous aging, is that the back of the catalyst, which is exposed to higher temperature, contains less Pt than the front; therefore, most of the Pt particles in the zoned catalyst were not affected by thermal aging. However, after homogeneous aging, the performance of the standard catalyst was better than the zoned catalyst at higher flow rate and temperature most likely due to the different sintering rates in the zoned sample compared to the standard one. In the second part of this research, the interactions between CO, C3H6, H2, and NO were tested over a commercial Pt/Al2O3 monolith sample by studying these reactions during ignition and extinction (warm-up and cool-down). Results showed that CO, C3H6, and NO inhibit their own oxidation and each other’s oxidation due to the self-poisoning effect and competitive adsorption over active sites. In the case of a CO + C3H6 mixture, interesting CO and C3H6 oxidation trends were observed during the extinction phase. As the C3H6 concentration increased in the mixture, the catalytic activity of CO oxidation during the extinction phase decreased until it was actually poorer than that during the ignition phase. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed different C3H6 oxidation intermediates during the extinction phase on the catalyst surface, thus blocking active sites and lowering catalyst activity.

Infra-Red thermography and spatially-resolved capillary inlet mass spectrometry (SpaciMS) have been used to characterize propylene oxidation along a Pt/Al2O3 monolith-supported catalyst, before and after heterogeneous deactivation. The combined techniques clearly show reaction location, and therefore catalyst use, and how these change with thermal and sulphur degradation. Following the heterogeneous thermal aging, the reaction zones at steady state were broader and located farther into the catalyst relative to those observed with the fresh catalyst. As well, the time for the temperature and concentration waves to travel through the catalyst during back-to-front ignition increased. These effects were more pronounced with 1500 ppm propylene relative to 4500 ppm propylene. Such trends could not be detected based on standard catalyst-outlet measurements. The light-off behaviour was also impacted by the aging, resulting in complex changes to the temperature front propagation, depending on the propylene concentration. With each sulphur exposure step, light-off temperatures increased and the time for back-to-front ignition during temperature programmed oxidation changed pattern. With 1500 ppm propylene fed, the reaction zones established during steady-state operation shifted farther into the catalyst and increased slightly in width following sulphur treatment; at very high temperature and for 4500 ppm propylene, the reaction zones were very close to the catalyst inlet and virtually indistinguishable between catalyst sulphation states. However, at lower steady-state temperatures for the higher propylene concentration, the catalyst did experience delays in reaction light-off and light-off position moved downstream in the catalyst with sulphur damage.

The Diesel Oxidation Catalyst (DOC) is a key component in the exhaust after-treatment system of diesel engines. In this study two aspects of a DOC were investigated: catalyst distribution and reactant species interactions. In the first part, the effect of an axial Pt distribution along a DOC was investigated by comparing a standard sample, with a homogeneous Pt distribution along the length, with a zoned sample, where the Pt was non-homogeneously distributed along the length. Temperature-programmed oxidation (TPO) and spatially-resolved gas-phase concentration measurement experiments were used to compare the CO, C3H6 and NO oxidation performance of the standard and zoned catalysts. Both catalyst types had the same total amount of Pt but different distributions. The zoned catalyst, with more Pt located in the upstream portion, showed better performance than the standard catalyst, especially at high total flow rate and when a mixture of the reactants were used. The superior performance of the zoned sample is due to a larger, localized exotherm in the upstream region, where more Pt is located, and a decrease in the self-poisoning effect downstream, where reaction light-off occurs. In addition, catalyst durability against thermal degradation was tested by exposing the whole catalyst (homogeneous aging) and part of the catalyst (heterogeneous aging) to high temperatures. In general, the zoned catalyst showed better performance than the standard catalyst after thermal aging, especially after heterogeneous aging. The reason for the superior performance of the zoned catalyst, especially after heterogeneous aging, is that the back of the catalyst, which is exposed to higher temperature, contains less Pt than the front; therefore, most of the Pt particles in the zoned catalyst were not affected by thermal aging. However, after homogeneous aging, the performance of the standard catalyst was better than the zoned catalyst at higher flow rate and temperature most likely due to the different sintering rates in the zoned sample compared to the standard one. In the second part of this research, the interactions between CO, C3H6, H2, and NO were tested over a commercial Pt/Al2O3 monolith sample by studying these reactions during ignition and extinction (warm-up and cool-down). Results showed that CO, C3H6, and NO inhibit their own oxidation and each other’s oxidation due to the self-poisoning effect and competitive adsorption over active sites. In the case of a CO + C3H6 mixture, interesting CO and C3H6 oxidation trends were observed during the extinction phase. As the C3H6 concentration increased in the mixture, the catalytic activity of CO oxidation during the extinction phase decreased until it was actually poorer than that during the ignition phase. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed different C3H6 oxidation intermediates during the extinction phase on the catalyst surface, thus blocking active sites and lowering catalyst activity.

Zone-coated diesel oxidation catalysts (DOCs) can be used to obtain overall improved performance in oxidation reaction extents. However, why this occurs and under what conditions an impact is expected are unknown. In order to demonstrate why these catalysts work better than their standard counterparts and how significant the improved performance is, the CO oxidation performance over a series of Pt−Pd/Al2O3 catalysts, each with a different distribution of precious metal down the length, while maintaining equivalent totals of precious metal, was modeled. Simulations with different flow rates, ramp rates, steady-state temperatures at the end of the ramp rate, different total precious metal loadings, and CO inlet values were compared. At conversions less than 50%, the most significant differences were noted when the temperature was ramped to just at the CO oxidation light-off point (a typical measure of 50% conversion/oxidation), with catalysts containing more precious metal at the downstream portions leading to better light-off conversion performance. However, in terms of cumulative emissions over a long period of time, a “front-loaded” design proved best. These results are readily explained by decreased CO poisoning and the propagation of the heat derived from the exotherm from the front to the rear of the catalyst. Also, although the trends were the same, regardless of change in the parameter, the impact of different distributions was more apparent under conditions where a catalyst would be challenged, i.e., at low temperature ramp rates, higher CO inlet concentrations, and lower amounts of total catalyst used. At higher ramp rates, the input heat from the entering gas stream played an increasingly important role, relative to conduction associated with the exotherm, dampening the effects of the catalyst distribution. Therefore, although catalysts that are zone-coated with precious metals, or any active sites, could prove better in terms of performance than homogeneously distributed active site catalysts, this improvement is only significant under certain reaction conditions. In a mixture of three reactants, CO, C3H6 and NO oxidation, it was found that a loading a larger amount of active sites in the catalyst middle, maintained better CO and C3H6 oxidation but not NO oxidation, which required the whole catalyst length. A faster light-off conversion was also related to higher amount of precious metal at the catalyst outlet. The CO conversion performance for a variety of distributed precious metal designs was evaluated as a function of exposure time to sulphur and the spatial accumulation profile of sulphur along the monolith length was predicted. The results illustrate that the sulphur accumulates near the catalyst inlet and decreases toward the outlet, resulting in shifting the reaction zones further toward the catalyst outlet. With sulfation, light-off temperatures (T50) increased and the time for back to front reaction propagation also increased. A back loaded catalyst resulted in the best light-off conversion compared to the other catalyst designs and a middle loaded catalyst maintained a higher overall conversion if sulphur poisoning takes place. These catalyst designs were also tested under thermal aging conditions by using a second order sintering model integrated with the CO oxidation reaction model. The spatial normalized dispersion profiles along the monolith showed that the catalyst outlet experienced significant damage relative to the inlet due to sintering. A front loaded catalyst design had the highest catalytic activity due its resistance to sintering.

Zone-coated diesel oxidation catalysts (DOCs) can be used to obtain overall improved performance in oxidation reaction extents. However, why this occurs and under what conditions an impact is expected are unknown. In order to demonstrate why these catalysts work better than their standard counterparts and how significant the improved performance is, the CO oxidation performance over a series of Pt−Pd/Al2O3 catalysts, each with a different distribution of precious metal down the length, while maintaining equivalent totals of precious metal, was modeled. Simulations with different flow rates, ramp rates, steady-state temperatures at the end of the ramp rate, different total precious metal loadings, and CO inlet values were compared. At conversions less than 50%, the most significant differences were noted when the temperature was ramped to just at the CO oxidation light-off point (a typical measure of 50% conversion/oxidation), with catalysts containing more precious metal at the downstream portions leading to better light-off conversion performance. However, in terms of cumulative emissions over a long period of time, a “front-loaded” design proved best. These results are readily explained by decreased CO poisoning and the propagation of the heat derived from the exotherm from the front to the rear of the catalyst. Also, although the trends were the same, regardless of change in the parameter, the impact of different distributions was more apparent under conditions where a catalyst would be challenged, i.e., at low temperature ramp rates, higher CO inlet concentrations, and lower amounts of total catalyst used. At higher ramp rates, the input heat from the entering gas stream played an increasingly important role, relative to conduction associated with the exotherm, dampening the effects of the catalyst distribution. Therefore, although catalysts that are zone-coated with precious metals, or any active sites, could prove better in terms of performance than homogeneously distributed active site catalysts, this improvement is only significant under certain reaction conditions. In a mixture of three reactants, CO, C3H6 and NO oxidation, it was found that a loading a larger amount of active sites in the catalyst middle, maintained better CO and C3H6 oxidation but not NO oxidation, which required the whole catalyst length. A faster light-off conversion was also related to higher amount of precious metal at the catalyst outlet. The CO conversion performance for a variety of distributed precious metal designs was evaluated as a function of exposure time to sulphur and the spatial accumulation profile of sulphur along the monolith length was predicted. The results illustrate that the sulphur accumulates near the catalyst inlet and decreases toward the outlet, resulting in shifting the reaction zones further toward the catalyst outlet. With sulfation, light-off temperatures (T50) increased and the time for back to front reaction propagation also increased. A back loaded catalyst resulted in the best light-off conversion compared to the other catalyst designs and a middle loaded catalyst maintained a higher overall conversion if sulphur poisoning takes place. These catalyst designs were also tested under thermal aging conditions by using a second order sintering model integrated with the CO oxidation reaction model. The spatial normalized dispersion profiles along the monolith showed that the catalyst outlet experienced significant damage relative to the inlet due to sintering. A front loaded catalyst design had the highest catalytic activity due its resistance to sintering.

In this thesis, the experimental test facilities consisted of a well instrumented live Ford 2.0 litre turbocharged diesel engine connected to a specially made exhaust can, which contained a diesel oxidation catalyst (DOC). Experiments were performed on DOCs, which were specially prepared by Johnson Matthey, and had thermocouples mounted in their walls to measure axial temperature profiles. These DOCs consisted of a Pt catalyst dispersed in an alumina washcoat on a cordierite monolith supports, and were representative of a commercial application. Experiments were performed on Full-scale DOCs (o.d. = 106 mm, length = 114 mm), and also on Thin-slice DOCs (length = 5 and 10 mm), which generate some interesting data, and enabled a technique that is normally only used in laboratory bench-top experiments to be applied to a live engine. A number of different methodologies were developed based on (a) the operation of the engine at pseudo-steady-state operating conditions, and (b) transient experiments (e.g. a pulse of CO was injected into the exhaust gas just before the DOC). For example, it was shown how experiments on a live engine can be used to explore: (a) the hysteresis between light-off and extinction curves, (b) how catalyst temperature rise during warm-up of a DOC, (c) the promotion effect that hydrogen has on the conversion of CO, (d) the extent of competition for active catalytic sites, e.g. between CO, THCs, propane or hydrogen. The main findings are: (a) the hysteresis between light-off and extinction curves are mainly caused by CO inhibition, (b) the promotion effect of hydrogen on CO oxidation is largely attributed to thermal effect, (c) LHHW form rate expression is not adequate for catalytic converter modelling under transient conditions, (d) the competition for active catalytic sites is not apparent at the test conditions performed in this thesis. Moreover, a number of case studies were also used to illustrate how the experimental results/techniques developed in this thesis, may be used to support modelling studies. iii

There has been a growing interest in using lean-burn engines due to their higher fuel economy and associated lower CO2 emissions. However, there are challenges in reducing NOX in an O2-rich (lean-burn) exhaust, and in low temperature soot oxidation. NOX storage/reduction (NSR) and selective catalytic reduction (SCR) are commercial NOX reduction technologies, and both are more efficient with levels of NO2 that are higher than those that are in engine exhaust (engine-out NO2 levels are ~10% of the total NOX). Therefore diesel oxidation catalysts are installed upstream of these technologies to provide NO2 through NO oxidation. The motivation behind this research project was two-fold. The first was to gain a better understanding of the effect of hydrocarbons on NO oxidation over a monolithic diesel oxidation catalyst. The second was to spatially resolve competitive oxidation reactions as a function of temperature and position within the same diesel oxidation catalyst (as that used in the first part). A technique known as spatially resolved capillary-inlet mass spectrometry (SpaciMS) was used to measure the gas concentrations at various positions within the catalyst. Diesel engine exhaust contains a mixture of compounds including NO, CO and various hydrocarbons, which react simultaneously over a catalyst, and each can influence the oxidation rates of the others. While studying the effect of hydrocarbons on NO oxidation in this project, propylene was found to have an apparent inhibition effect on NO oxidation, which increased with increasing propylene concentration. This apparent inhibition is a result of the NO2, as a product of NO oxidation, reacting with the propylene as an oxidant. Experiments with NO2 demonstrate a significant temperature decrease in the onset of NO2 reduction when propylene was present, which decreased further with increasing amounts of propylene, verifying NO2 as an oxidant. Similar results were observed with m-xylene and dodecane addition as well. The results also demonstrate that NO2 was consumed preferentially relative to O2 during hydrocarbon oxidation. With low inlet levels of O2, it was evident that the addition of NO2 had an apparent inhibition effect on propylene oxidation after the onset of NO2 reduction. This subsequent inhibition was due to the NO formed, demonstrating that C3H6 results in reduced NO2 outlet levels while NO inhibits C3H6 oxidation. The development of new models as well as validation of existing models requires the ability to spatially resolve oxidation reactions within a monolith. Spatially-resolved data will also give catalyst manufacturers insight into the location of active fronts, thereby directing the design of more efficient catalysts. In this research project, spatially resolving the oxidation reactions demonstrated that H2 and CO are oxidized prior to C3H6 and C12H26 and clearly show back-to-front ignition of the reductant species. An enhancement in NO oxidation was observed at the same time as dodecane oxidation light off, likely related to dodecane partial oxidation products.

There has been a growing interest in using lean-burn engines due to their higher fuel economy and associated lower CO2 emissions. However, there are challenges in reducing NOX in an O2-rich (lean-burn) exhaust, and in low temperature soot oxidation. NOX storage/reduction (NSR) and selective catalytic reduction (SCR) are commercial NOX reduction technologies, and both are more efficient with levels of NO2 that are higher than those that are in engine exhaust (engine-out NO2 levels are ~10% of the total NOX). Therefore diesel oxidation catalysts are installed upstream of these technologies to provide NO2 through NO oxidation. The motivation behind this research project was two-fold. The first was to gain a better understanding of the effect of hydrocarbons on NO oxidation over a monolithic diesel oxidation catalyst. The second was to spatially resolve competitive oxidation reactions as a function of temperature and position within the same diesel oxidation catalyst (as that used in the first part). A technique known as spatially resolved capillary-inlet mass spectrometry (SpaciMS) was used to measure the gas concentrations at various positions within the catalyst. Diesel engine exhaust contains a mixture of compounds including NO, CO and various hydrocarbons, which react simultaneously over a catalyst, and each can influence the oxidation rates of the others. While studying the effect of hydrocarbons on NO oxidation in this project, propylene was found to have an apparent inhibition effect on NO oxidation, which increased with increasing propylene concentration. This apparent inhibition is a result of the NO2, as a product of NO oxidation, reacting with the propylene as an oxidant. Experiments with NO2 demonstrate a significant temperature decrease in the onset of NO2 reduction when propylene was present, which decreased further with increasing amounts of propylene, verifying NO2 as an oxidant. Similar results were observed with m-xylene and dodecane addition as well. The results also demonstrate that NO2 was consumed preferentially relative to O2 during hydrocarbon oxidation. With low inlet levels of O2, it was evident that the addition of NO2 had an apparent inhibition effect on propylene oxidation after the onset of NO2 reduction. This subsequent inhibition was due to the NO formed, demonstrating that C3H6 results in reduced NO2 outlet levels while NO inhibits C3H6 oxidation. The development of new models as well as validation of existing models requires the ability to spatially resolve oxidation reactions within a monolith. Spatially-resolved data will also give catalyst manufacturers insight into the location of active fronts, thereby directing the design of more efficient catalysts. In this research project, spatially resolving the oxidation reactions demonstrated that H2 and CO are oxidized prior to C3H6 and C12H26 and clearly show back-to-front ignition of the reductant species. An enhancement in NO oxidation was observed at the same time as dodecane oxidation light off, likely related to dodecane partial oxidation products.

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