Automotive-based exhaust is a major contributor to air pollution, and legislation controls the emission of several components. Catalytic converters are used to reduce these emissions to acceptable levels. As legislation becomes more stringent, better catalysts are required, and better kinetic models can accelerate the design of these models. This work considers the kinetics of the oxidation of CO and hydrogen on a Pt monolith diesel oxidation catalyst under lean conditions. The study of CO oxidation alone, while well documented in the literature, is a necessary step in the development of a complete model. CO oxidation is studied using both temperature ramp and concentration step experiments. These experiments are modelled and discussed. The selectivity of the catalyst toward CO is observed during light-off experiments of CO and hydrogen mixtures. Literature models are modified to correctly model this selectivity, improving the validity of the modified model. The hydrogen-promotion effect, whereby the presence of hydrogen promotes CO oxidation during light-off, is discussed and modelled. Experimental and numerical results are shown and compared, and modifications to the standard literature models are proposed. These modifications model the behaviour of the catalyst for mixtures of CO (up to 2000 ppm), hydrogen (up to 2000 ppm) and oxygen (6%) selectivity and the hydrogen promotion effect.

Emissions of methane to the atmosphere are deemed by many to pose environmental problems. Conversion of methane to carbon dioxide through combustion reduces greenhouse gasses and can provide a source of energy. Catalytic combustion is a viable option for oxidation of lean methane streams. Lean methane mixtures can be very difficult to oxidize due to the relative stability of methane, and high temperatures are usually required. One option for increasing reactor temperature is to use flow reversal to trap energy in the reactor. This thesis details the development, validation and use of a transient 2D model for a reverse flow catalytic reactor. It is demonstrated that a 2D model can show many dynamics in the system that cannot be accurately reproduced with a 1D model. The model is verified with experimental data using a pilot scale reactor. This research presents results from numerical simulations and experiments into the effects of operating parameters such as feed rate, methane concentration, and cycle time.

This paper reports the results of a study of the combined oxidation of hydrogen and carbon monoxide on a platinum diesel oxidation catalyst. Experimental results for the light-off curves obtained in a monolith supported catalyst are shown for different concentrations. The presence of hydrogen is shown to promote the oxidation of CO, with the largest effect shown for the first small addition of hydrogen, then a progressively decreasing effect as the hydrogen concentration is increased. The enhancement effect can be successfully simulated using a kinetic model in which the activation energy for the desorption of CO is decreased as a function of hydrogen adsorption. Hydrogen is allowed to adsorb on sites only accessible to hydrogen and not CO or oxygen.

Keywords: Platinum catalyst; Monolith; Carbon monoxide; Oxidation; Hydrogen promotion; Simulation

This paper presents experiments and model predictions for the oxidation of CO over a platinum catalyst in a monolith reactor. Experimental behaviour is broadly consistent with previously reported work on CO oxidation. Ignition-extinction (light-off) curves demonstrated the presence of multiple steady states with a hysteresis effect. The two branches corresponding to the two states of predominantly CO covered or oxygen covered. Admission of CO pulses to an oxygen covered surface results in reaction, indicating the occurrence of adsorption of CO on an oxygen covered surface. A model based on adsorption and surface reaction using the classical LHHW approach qualitatively was able to reproduce the light-off behaviour. The best model assumes dissociative chemisorption of oxygen on two surface sites. It was superior to a model proposing molecular adsorption of oxygen followed by rapid dissociation. Addition of steps allowing CO adsorption on an oxygen filled surface via an oxygen compression mechanism enable the qualitative description of the reactor response to step inputs of CO to an oxygen rich feed stream. Some parameter adjustment remains to allow a better fit between experiment and model predictions.

This paper presents experimental and modelling results for the oxidation of mixtures of hydrogen and carbon monoxide in a lean atmosphere. Transient light-off experiments over a platinum catalyst (80 g/ft^3 loading) supported on a washocated ceramic monolith were performed with a slow inlet temperature ramp. Results for CO alone agree with earlier results that predict self-inhibition of CO; that is an increasing light-off temperature with increasing CO concentration. Addition of hydrogen to the feed causes a reduction in light-off temperature for all concentrations of CO studied. The most significant shift in light-off temperature occurs with the addition of small amounts of hydrogen (500 ppm by volume) with only minor further enhancement occurring at higher hydrogen concentrations. Hydrogen alone in a lean atmosphere will oxidise at room temperature. In mixtures of hydrogen and CO, the CO was observed to react first until a conversion of about 50 of hydrogen rapidly went from zero to 100 Simulations performed using literature mechanistic models for the oxidation of these mixtures predicted that hydrogen ignites first, followed by CO, a direct contradiction of the experimental evidence. Upon changing the activation energy between adsorbed hydrogen and oxygen, the CO was observed to oxidise first, however, no enhancement of light-off was predicted. The effect can not be explained by the mechanistic model currently under discussion.

This paper describes the development and validation of a computer simulator for the modelling of a reverse flow catalytic reactor for the combustion of lean mixtures of methane in air. The simulator uses a heterogeneous two dimensional model for the reactor. The reactor uses a packed bed for catalytic sections and ceramic monoliths for inert sections, although the simulator is written in a general fashion so that any combination of packing can be used, in any desired variety. Validation is performed using a 200 mm internal diameter reactor over various flowrates and methane concentrations. The reverse flow reactor is observed to yield stable auto-thermal operation even for low methane concentrations. Higher methane concentrations are observed to give dual temperature peaks in the reactor. The transfer of energy is observed to be a significant factor in the reactor operation, which is shown by comparison of the heterogeneous model to a pseudo-homogeneous reactor model. The simulator can model the pilot reactor in real time for typical operating conditions.

Keywords: Catalytic combustion; Methane; Reverse flow

This paper describes an experimental investigation of a pilot scale reverse flow reactor for the catalytic destruction of lean mixtures of methane in air. It was found that using reverse flow it was possible maintain elevated reactor temperatures which were capable of achieving high methane conversion of methane in air streams at methane concentrations as low as 0.19 cycle time and feed concentration are all important parameters that govern the operation of the reactor. Control of these parameters is important to prevent the trapping of the thermal energy within the catalyst bed, which can limit the amount of energy that can be usefully extracted from the reactor.

Keywords: Catalytic combustion; Methane; Reverse flow

Automotive catalytic converters have been common for about thirty years, and have been very effective in reducing pollution caused by the automobile. Notwithstanding there evident success, there remain many unanswered questions about the reactions that occur. For a three way catalyst, there is the oxidation of carbon monoxide and hydrocarbons, as well as the reduction of oxides of nitrogen. For a lean oxidation catalyst, No is typically oxidized to NO2. Recently, much attention has been paid to the use of computer aided design methodologies for the improved design of catalytic devices. At the moment, most attention is devoted to lean oxidation catalysts, because of the increasing emphasis on the use of diesel fuel. In modelling studies, two kinetic approaches have been used. The first uses global rate expressions based on LHHW type models, whilst the second attempts to employ more mechanistically meaningful models. It is been shown that global models are able to predict some aspects of converter performance but are incapable of modelling all observed phenomena. On the other hand, the common mechanistic models most often cited in the catalytic converter literature have recently also been shown to be unable to quantify all observed performance. The key reaction set for the oxidative catalytic converter can be considered to be the system of carbon monoxide, hydrogen and water. Without an understanding of this system, any attempt at a more detailed mechanistic model must fail. This paper deals with a systematic study of this system in a real catalytic monolith with a washcoat containing a platinum catalyst. The experiments were transient in nature, and include ignition and extinction curves, concentration ramps and pulse responses. The system was modelled using multi-step mechanistic models using different hypotheses. We present the predictive ability of the common models from the converter literature, and highlight their weaknesses. New steps are proposed that can explain some of the observed behaviour.

Much work has been done to understand and model the nature of the reactions on a Pt surface under diesel oxidation conditions. The oxidation of CO and hydrogen has been studied for the preferential oxidation of CO in the production of clean hydrogen for use in fuel cells, however less discussion has taken place on the reaction under diesel exhaust gas conditions. Transient experiments (temperature ramps and concentration steps) under lean conditions (0-2000 ppm H2, 0-2000 ppm CO, 6 rest N2, SV 25000 hr-1, atmospheric pressure, 25-300°C) are presented and discussed. Simulations were performed using mechanistic models from the literature and compared to experimental results. CO is known to inhibit the oxidation of H2, whilst H2 promotes the oxidation of CO. This was experimentally verified, but in addition, it was found that a small amount of hydrogen had a large promotion effect, whereas the marginal effect of additional hydrogen did not have a proportional effect. This is shown in Figure 1, where CO conversion is shown as the temperature of the reactor is increased. The addition of 500ppm H2 shows a large promotion of the reaction. The additional benefit of 2000ppm H2 is smaller. Models in the current literature do not accurately model this phenomenon, and must be modified before agreement may be reached. Experimentally, the ignition of CO occurred before the ignition of hydrogen during temperature ramp experiments. Simulations using literature values were unable to predict the correct order of ignition. However, when the activation energy for the reaction between adsorbed hydrogen and oxygen be increased, the correct order of ignition was obtained.

There is a need for a CO-oxidation model for a Pt-based Diesel Oxidation Monolith Catalyst that is valid over a range of transient conditions. Such a model would assist in both simulations of catalyst performance and in optimizing the performance of a catalyst via an on-boad computer. The objective of this work was to validate experimentally some CO oxidation reaction mechanisms currently used in modelling platinum-based diesel oxidation monolith catalysts. Many steady-state and transient measurements of catalytic activity using Concentration Programmed Methods (CO ramps and step-functions) and Temperature Programmed Methods (lightoff and extinction curves) were made over the typical operating conditions of a diesel oxidation monolith catalyst (atmospheric pressure, 50-300 °C). Various mixtures of N2, O2, CO2, CO and H2O were used. These experiments were compared to numerical solutions of published mechanisms. The reactor simulator was developed by Mukadi and Hayes [1]. A generalized pattern search (GPS) algorithm was used for parameter estimation. Strengths and weaknesses of the various models are discussed and compared. The mechanisms for modelling CO oxidation on Pt used in this work were based on elementary reactions [2,3] and a global reaction mechanism [4]. None of the three mechanisms perfectly describes all experimental results over the full operating range, especially under transient conditions. Some evidence is found that the prolonged operation of the catalyst under reactive conditions leads to a reversible decrease in performance. This effect is often attributed to sub-surface oxidation of the platinum, and so far this effect is not taken into account in the mechanisms applied for modelling of diesel oxidation catalysts.

The oxidation of CO has been studied for many years both because it is an interesting reaction in its own right, as well as being an important reaction for automotive catalytic converters. The objective of this work was to validate experimentally some CO oxidation reaction mechanisms currently used in modelling platinum diesel oxidation catalysts. Many steady-state and transient measurements of catalytic activity involving CO ramps, step-functions and light-offs were made over the typical operating conditions of a diesel oxidation catalyst (atmospheric pressure, 50-300°C, with and without water). These experiments were compared to numerical solutions of published mechanisms using the reactor simulator developed by Mukadi and Hayes [1], coupled with a generalized pattern search (GPS) algorithm for parameter estimation. Strengths and weaknesses of the various models are discussed and compared. The mechanisms for modelling CO oxidation on Pt used in this work were based on elementary reactions [2,3] and a global reaction mechanism [4]. None of the three mechanisms perfectly describes all experimental results over the full operating range, especially under transient conditions. Some evidence is found that the prolonged operation of the catalyst under reactive conditions leads to a reversible decrease in performance. This effect is often attributed to sub-surface oxidation of the platinum, and so far this effect is not taken into account in the mechanisms applied for modelling of diesel oxidation catalysts.

As emission standards become more and more rigorous, the need for more efficient and effective catalytic converters and more detailed knowledge of the diesel automotive catalyst increases. While the oxidation of CO has been thoroughly studied, little is known about the combination of CO+H2, especially under lean conditions in a Pt-based monolith diesel oxidation catalyst. Currently, no published model accurately accounts for all phenomena in this system. An accurate model of this system would assist not only in the development of better on-board emission control systems for automobiles, but would also facilitate more effective design of new catalytic converters. The objective of this work was to experimentally validate some CO+H2 oxidation reaction mechanisms currently used in modelling platinum diesel oxidation catalysts. The mechanisms were first studied for CO oxidation alone, then for H2 alone, and then both in combination. By studying CO and H2 both alone and in combination, the effect of H2 on CO oxidation, and the effect of CO on H2 oxidation, has been observed. It is known that CO inhibits the ignition of H2, and H2 promotes the ignition of CO [1], however the interactions between kinetics, mass and energy transfer in a monolith are not well understood, especially under transient conditions. The presence of phenomena such as moving ignition and extinction fronts are difficult to accurately model. As the complexity of the system increases, the robustness of the models may be studied. Many steady-state and transient measurements of catalytic activity involving concentration ramps, step-functions and lightoffs were made over the typical operating conditions of a diesel oxidation catalyst (atmospheric pressure, 50-300 °C, with and without water). These experiments were compared to numerical solutions of published mechanisms using the reactor simulator developed by Mukadi and Hayes [2]. Strengths and weaknesses of the various models are discussed and compared. The mechanisms for modelling CO+H2 oxidation on Pt used in this work were based on elementary reactions [3, 4]. Neither of the mechanisms perfectly describes all experimental results over the full operating range, especially under transient conditions. References [1] Mingyong Sun, Eric B. Croiset, Robert R. Hudgins, Peter L. Silveston, and Michael Menzinger. Steady-state multiplicity and superadiabatic extinction waves in the oxidation of CO/H2 mixtures over a Pt/Al2O3-coated monolith. Industrial & Engineering Chemistry Research, 42(1):37-45, 2003. [2] L.S. Mukadi and R.E. Hayes. Modelling the three-way catalytic converter with mechanistic kinetics using the Newton-Krylov method on a parallel computer. Computers and Chemical Engineering, 26(3):439-455, 2002. [3] J.H.B.J. Hoebink, J.M.A. Harmsen, M. Balenovic, A.C.P.M. Backx, and J.C. Schouten. Automotive exhaust gas conversion: from elementary step kinetics to prediction of emission dynamics. Catalysis Today, 16/17(1-4):319-327, 2001. [4] Daniel Chatterjee, Olaf Deutschmann, and Jürgen Warnatz. Detailed surface reaction mechanism in a three-way catalyst. Faraday Discussions, 119:371-384, 2001.