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Worldwide emission standards are now required to cover engine operation under extreme ambient conditions, which affect the raw emissions and the efficiency of the exhaust aftertreatment systems. These regulations also target new combustion technologies for decarbonization, such as neat hydrogen ( H 2 ) combustion or dual-fuel strategies, which involve a challenge to the analysis of exhaust aftertreatment system requirements and performance. This work addresses the impact of high altitude and low ambient temperature conditions on the reactivity of an oxidation catalyst in the presence of H 2 . A reaction mechanism is proposed to cover the main conversion paths of CO, HC, and H 2 , including the formation and consumption of high-energy surface reaction intermediates. The mechanism has been implemented into a faster-than-real-time reduced-order model for multi-layer washcoat honeycomb catalytic converters. The model was utilized to investigate the effect of H 2 concentration on the reactivity of CO and HC within the catalyst under various operating and ambient conditions. By applying the model and examining the selectivity towards different reaction pathways in the presence of H 2 , insights into surface intermediates and reactivity across different cross-sections of the monolith were obtained. This analysis discusses the underlying causes of reactivity changes promoted by H 2 and its relative importance as a function of driving boundary conditions. [ABSTRACT FROM AUTHOR]

Motor vehicle exhaust is an important cause of atmospheric pollution. Nowadays, mainstream exhaust emission aftertreatment technologies, such as TWC, DOC, SCR, and DPF, usually require sufficient temperature to perform good purification or maintain normal working conditions. Compared with exhaust gas heating technologies such as engine enrichment and fuel injection, electric heating technology can quickly increase the temperature of exhaust gas aftertreatment devices without adverse effects on engine operating conditions. This article introduces the research and progress of electric heating technology combined with traditional aftertreatment devices on major types of vehicles, such as gasoline vehicles, diesel vehicles, motorcycles, and hybrid vehicles, to improve exhaust purification efficiency and its accompanying fuel consumption impact. In addition, the common structure and characteristics of electric heaters, as well as the current status and development trend of electric heating unit technologies such as electric heating power supply are introduced. [ABSTRACT FROM AUTHOR]

Future passenger cars must comply with whatever regulations will be created to mitigate greenhouse gas emissions (GHG) and pollution. Regarding the pollutants of an internal combustion (IC) engine, they are dealt with by using an exhaust after-treatment system (EATS). Yet, catalytic converters encounter light-off problems after cold start: their after-treatment efficiency is unsatisfactory until their temperature reaches a threshold, usually called the light-off temperature. Consequently, in this phase, important levels of pollutants are produced, being therefore paramount to achieve the catalyst light-off as fast as possible. Passing to EURO7, which is expected to happen in 2025 means to have the EATS ready for efficient treatment “instantaneously” after cold starting. The solutions usually used to generate more heat at the EATS level are not considered sufficient for the coming EURO7 regulation. Consequently, they must be complemented with other systems, such as the afterburner, which is an extra-combustion device. Accordingly, this paper is focused on the challenges that come with the use of such an afterburner and it presents the following: (1) a proposal for a layout and operation strategy of such a device and (2) an experimental investigation of the effect an afterburner has on the light-off performance of a usual three-way catalyst. On the latter, the cold start exhaust emissions were analyzed with and without using this extra-combustion device, thus being able to conclude on its effectiveness: full light-off of the pre-catalyst thanks to burner’s operation up to 1000 °C and, consequently, an efficient treatment of the pollutants. Equally, the paper presents the impact of the afterburner on fuel consumption and, therefore, on GHG it generates, given the fact that it burns carbon-based fuel as the engine.

Urban air pollution is partly due to exhaust emissions from road transport. Vehicle emissions have been regulated for more than 30 years in many countries around the world. Each motor type is equipped with a specific emission control system. In gasoline vehicles, a three-way catalytic converter (TWC) is implemented to remove at the same time hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). However, TWCs are only efficient above 200 °C and at a stoichiometric air-to-fuel ratio in the exhaust. However, deviations from stoichiometry occur during fast accelerations and decelerations. This study reports the analysis of unregulated VOCs commercial mini-TWC fed by model gasoline gas mixtures. A synthetic gas bench was used to control the model exhaust containing two model hydrocarbons (propene and propane) to identify the conditions at which VOCs are created under non-optimal conditions. Most of the pollutants such as N2O and VOCs were emitted between 220 and 500 °C with a peak at around 280 °C, temperature which corresponds to the tipping point of the TWC activity. The combination of different mass spectrometric analysis (online and offline) allowed to identify many different VOCs: carbonated (acetone, acetaldehyde, acroleine), nitrile (acetonitrile, propanenitrile, acrylonitrile, cyanopropene) and aromatic (benzene, toluene) compounds. Growth mechanisms from propene and to a lesser extend propane are responsible for the formation of these higher aromatic compounds that could lead to the formation of secondary organic aerosol in a near-field area. [ABSTRACT FROM AUTHOR]

To develop a high-performance three-way catalyst, it is necessary to construct an accurate catalytic reaction model. Previously, we have developed the three-way catalyst model under dry simulated exhaust gas conditions. Water, which is contained in the real exhaust gas, however, complicate the phenomenon encountered in the three-way catalyst. Thus, in this study, the effect of water on the catalytic reaction on a palladium-based three-way catalyst was investigated. Moreover, a catalytic reaction model was developed based on the experimental results. As a results, it is found that the constructed model well predicted the light-off behavior of the palladium-based three-way catalyst.

Urban air pollution is partly due to exhaust emissions from road transport. Vehicle emissions have been regulated for more than 30 years in many countries around the world. Each motor type is equipped with a specific emission control system. In gasoline vehicles, a three-way catalytic converter (TWC) is implemented to remove at the same time hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). However, TWCs are only efficient above 200 °C and at a stoichiometric air-to-fuel ratio in the exhaust. However, deviations from stoichiometry occur during fast accelerations and decelerations. This study reports the analysis of unregulated VOCs commercial mini-TWC fed by model gasoline gas mixtures. A synthetic gas bench was used to control the model exhaust containing two model hydrocarbons (propene and propane) to identify the conditions at which VOCs are created under non-optimal conditions. Most of the pollutants such as N2O and VOCs were emitted between 220 and 500 °C with a peak at around 280 °C, temperature which corresponds to the tipping point of the TWC activity. The combination of different mass spectrometric analysis (online and offline) allowed to identify many different VOCs: carbonated (acetone, acetaldehyde, acroleine), nitrile (acetonitrile, propanenitrile, acrylonitrile, cyanopropene) and aromatic (benzene, toluene) compounds. Growth mechanisms from propene and to a lesser extend propane are responsible for the formation of these higher aromatic compounds that could lead to the formation of secondary organic aerosol in a near-field area.

In an ideal conditions, three-way catalytic converters have a structure that will convert almost all of the harmful exhaust emissions. However, these conditions are not valid for cold start and idle conditions. Especially in cold weather, people wait for a while at the idle speed even though it is not a good way to warm up the engine. In this period, since the required temperature to reach light-off values are not provided, the emission released to the environment is at the highest level. The effects of preheating of the catalytic converter have been investigated in order to minimize the exhaust gases emitted under cold and idle working conditions and to meet the Euro standards as soon as possible after the emission release starts. In this experimental study, preheating of 1060, 1253 and 1493 Watt energy was applied to the exhaust line before the three-way catalytic converter at the cold start and idle conditions. Preheating was started when the engine started running. In the experiments, temperature and emission values were obtained at the catalytic converter inlet and outlet at idle conditions (1000 rpm) for about 430 seconds. The effects of inlet temperatures of exhaust gases on HC and CO emission behavior and converter conversion efficiency were investigated. When 1060, 1253 and 1493 Watt preheating are applied to the exhaust line, the times for reaching the light-off for HC and CO emissions occur at 395, 295, 220 and 369, 277, 200 seconds, respectively. However, when preheating is not applied, the catalytic converter has not been able to reach the light-off within the specified time. Finally, the EURO VI standard could not be achieved for the specified period without preheating. However, when the preheating load was set to 1493 Watt, EURO VI standards were achieved at the end of 310 and 285 seconds preheating times for both HC and CO emissions. [ABSTRACT FROM AUTHOR]

More efficient lean-burn, natural gas engines are limited by greenhouse gas emissions due to methane oxidation catalysts (MOC) that suffer from water inhibition and high temperature activation. Herein, we report that the addition of Mg to supported 1 wt% Pd MOCs improved hydrothermal stability even after severe hydrothermal aging. The superior methane oxidation activity compared to the corresponding Mg-free catalyst was attributed to (1) influence of Mg during surface roughening and restructuring at 700 °C on metal-support interaction, (2) reducibility of PdOx sites and (3) preferential stabilization of active Pd (1 0 0) facets in the sample as was evidenced by H 2 TPR and CO TPD characterization experiments. Methane conversion under synthetic exhaust conditions relevant to natural gas, lean-burn engines were investigated. BET, TPR, CO pulse chemisorption followed by TPD provided valuable insights into the surface area, pore volume, reducibility, Pd dispersion and Pd particle size of the selected catalyst samples. [Display omitted] • Mg modified Pd MOCs increase hydrothermal stability and inhibit PdOx agglomeration. • Mg presence during aging preferentially exposed CH 4 oxidation active PdOx facets. • Route of Pd and Mg introduction into SSZ-13 impacted catalyst activity. • Pd+Mg supported on Al 2 O 3 exhibited better low temperature activity than SSZ-13. • Increased TOF with PdOx particle size consistent with Mars-van Krevelen mechanism. [ABSTRACT FROM AUTHOR]

The increasingly stringent emission limits require high performances also at high mileage, increasing the need for reliable catalysts characterization during their whole useful life. Thermal ageing is the main problem: high temperatures, variable environment and the presence of water lead to a progressive deactivation of the components. Due to the cost of ageing a converter on-road or on an engine bench, much more rapid laboratory aging methods are now becoming very interesting, rising the problem of the correlation between real and accelerated ageing conditions. In this work a series of identical catalysts aged in different conditions are characterized in order to understand the correlation between the real and accelerated ageing protocols. Applying a wide spectrum of techniques (XRD, SEM, TEM, BET) a complete morphological characterization is performed and interlaced with functional testing, in order to understand the link between the catalysts modification due to thermal effects and their deactivation, and to allow the design of more realistic laboratory ageing protocols. The correlation between real samples, coming from an ageing on an engine, and laboratory ageing is good for the medium-high mileage sample, which results very similar in terms of performance and morphology with the sample aged at hydro-thermal condition. Some differences remain due to the chemical ageing, which was not possible to replicate on the laboratory scale. [ABSTRACT FROM AUTHOR]

Worldwide emission standards are extending their requirements to cover engine operation under extreme ambient conditions and fill the gap between the type-approval and real driving conditions. The new ambient boundaries affect the engine performance and raw emissions as well as the efficiency of the exhaust aftertreatment systems. This study evaluates the impact of high altitude and low ambient temperature on the light-off temperature and conversion efficiency of an oxidation catalyst. The results are compared in a common range of exhaust mass flow and temperature with the baseline sea-level operation at 20 °C. A reduction of CO and HC conversion efficiencies was found at 2500 m and −7 °C, with a relevant increase of the light-off temperature for both of the pollutants. The analysis of the experimental data was complemented with the use of a catalyst model to identify the causes leading to the deterioration of the CO and HC light-off. The use of the model allowed for identifying, for the same exhaust mass flow and temperature, the contributions to the variation of conversion efficiency caused by the change in engine-out emissions and tailpipe pressure, which are, in turn, manifested in the variation of the reactants partial pressure and dwell time as governing parameters. [ABSTRACT FROM AUTHOR]

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This study investigates geometric parameters of commercially available or recently published models of catalyst substrates for passenger vehicles and provides a numerical evaluation of their influence on heat-up behavior. Parameters considered to have a significant impact on the thermal economy of a monolith are: internal surface area, heat transfer coefficient, and mass of the converter, as well as its heat capacity. During simulation experiments, it could be determined that the primary role is played by the mass of the monolith and its internal surface area, while the heat transfer coefficient only has a secondary role. Furthermore, an optimization loop was implemented, whereby the internal surface area of a commonly used substrate was chosen as a reference. The lengths of the thin wall and high cell density monoliths investigated were adapted consecutively to obtain the reference internal surface area. The results obtained by this optimization process contribute to improving the heat-up performance while simultaneously reducing the valuable installation space required.

Worldwide emission standards are extending their requirements to cover engine operation under extreme ambient conditions and fill the gap between the type-approval and real driving conditions. The new ambient boundaries affect the engine performance and raw emissions as well as the efficiency of the exhaust aftertreatment systems. This study evaluates the impact of high altitude and low ambient temperature on the light-off temperature and conversion efficiency of an oxidation catalyst. The results are compared in a common range of exhaust mass flow and temperature with the baseline sea-level operation at 20 °C. A reduction of CO and HC conversion efficiencies was found at 2500 m and −7 °C, with a relevant increase of the light-off temperature for both of the pollutants. The analysis of the experimental data was complemented with the use of a catalyst model to identify the causes leading to the deterioration of the CO and HC light-off. The use of the model allowed for identifying, for the same exhaust mass flow and temperature, the contributions to the variation of conversion efficiency caused by the change in engine-out emissions and tailpipe pressure, which are, in turn, manifested in the variation of the reactants partial pressure and dwell time as governing parameters.

A global kinetic model is used to study the light-off behavior of a mixture of CO and HCs ( C 2 H 4 , C 7 H 8 , C 6 H 14 & C 2 H 6 ) on Pt, Pd and combined catalysts. The light-off curves are clustered around 550 K on Pt whereas species light-off temperatures range from 400–600 K on Pd. The self poisoning of CO is more pronounced on Pt than on Pd. As long chain HCs such as hexane have a better light-off performance on Pt than on Pd, it is beneficial to use dual layer or dual brick catalysts for optimising HC conversion on a C1 basis. Dual brick catalyst with Pd upstream showed superior performance compared to the configuration with Pd downstream. Pt in 50% of washcoat and Pd in the remaining 50% of washcoat has a better overall light-off performance than other fractions of Pt, Pd. It is found that the light-off behavior of all the species is nearly same for both dual layer and dual brick configurations at low space velocities. However, the strong diffusional limitations in the washcoat at high space velocities result in improved light-off performance of dual layer configuration. The more practical C1 basis conversion, which is the weighted average conversion of HCs, is investigated for different configurations. It is also shown that the exotherm generated by the oxidation of CO can be used to improve the C 1 basis conversion of HCs. [ABSTRACT FROM AUTHOR]

The light-off (ignition) and steady-state behavior for individual oxidation and co-oxidation of CO and propylene under near-stoichiometric conditions was studied using Pd/Al 2 O 3 and Pd/CeO 2 -ZrO 2 monolith catalysts. CO and propylene are shown to be self- and mutually-inhibiting, with inhibition mitigated by the promotional effect of CeO 2 -ZrO 2 . Oxidation is enhanced at low temperatures for CO, and at intermediate and high temperatures for propylene. Light-off behavior during CO and propylene co-oxidation is similarly improved at low and high temperatures. Steady-state differential kinetics measurements using Pd/Al 2 O 3 show reaction orders of ∼−1 with respect to CO and propylene. Using Pd/CeO 2 -ZrO 2 , the reaction order with respect to CO shifts towards zero and the activation energy decreases, suggesting an alternate reaction mechanism for CO oxidation when enough ceria is present. Mechanisms for CO and propylene oxidation that are consistent with the kinetics and inhibition trends are presented. [ABSTRACT FROM AUTHOR]

In many laboratory catalytic reactors light-off curves describing the exit conversion versus inlet gas temperature do not follow the same path during ramp up and ramp down and this leads to dynamic hysteresis. This dynamic hysteresis, which may occur even under ideal conditions (e.g. plug flow and negligible heat effects) is fundamentally different from the steady-state hysteresis which is due to heat effects and thermal feedback. We present expressions for the width of the dynamic hysteresis loop as a function of the ramp rate, solid to gas heat capacity ratio, space time and the heat Peclet number, for the limiting cases of pseudo-homogeneous and two phase plug flow models with negligible heat effects ( Δ T ad = 0 ). We review the conditions under which steady-state hysteresis can exist for a finite Δ T ad and demonstrate the combined impact of steady-state and dynamic hysteresis effects. We study the light-off behavior and dynamic hysteresis during the co-oxidation of CO + C 2 H 6 mixture on Pt / Al 2 O 3 . It is found that 1% of CO alone ignites at 473 K whereas 500 ppm of C 2 H 6 alone ignites at 723 K. The exotherm generated by the CO oxidation can reduce the ignition temperature of C 2 H 6 during the co-oxidation of CO and C 2 H 6 . Bifurcation diagrams of solid exit temperature versus fluid inlet temperature for different inlet compositions illustrate the simultaneous as well as separate ignitions for CO and C 2 H 6 . Each 1% increase in the CO feed mole percentage decreases C 2 H 6 ignition temperature by about 80 °C. A feed of 3% CO is required for simultaneous ignition of a mixture of CO and 500 ppm of C 2 H 6 . [ABSTRACT FROM AUTHOR]

Monolith reactors used in catalytic after-treatment systems (e.g. TWC, DOC, LNT and SCR) are often studied in laboratory experiments using monolith samples of the same cell density, washcoat thickness, and catalyst loading but with a smaller length compared to the full-scale reactor. The flow rate in the laboratory scale operation is selected so that the same space velocity is maintained as in the full-scale system. However, matching space velocity between the laboratory-scale and full-scale reactor leads to similar performance only under ideal conditions such as isothermal or adiabatic operation with negligible heat and mass dispersion (i.e. plug flow conditions) and small adiabatic temperature rise. In this work, it is shown that when thermal effects are significant (or adiabatic temperature rise is large) and heat and mass transfer and dispersion effects are not negligible, similarity may not exist and the ignition/extinction behavior of the two systems can be qualitatively different. Further, when laboratory-scale data is used for kinetic parameter estimation, failure to consider these effects in the reactor model can lead to false kinetic parameters and inaccurate prediction of reactor performances. Another major reason for possible discrepancy between laboratory-scale and full-scale reactors, external heat exchange with furnace (surroundings), is also investigated. We present analysis, simulations, and analytical results quantifying the impact of these effects on the scale-up of monolith reactors. [ABSTRACT FROM AUTHOR]

Mathematical models of automotive exhaust aftertreatment catalysts are often based on reaction kinetics developed using laboratory reactor light-off data. This paper investigates an effect observed while developing CO oxidation kinetics for a Diesel Oxidation Catalyst: it was found that while several different sets of kinetic parameters would give an equivalent prediction of the reactor data, they gave different predictions over a vehicle drive cycle. This represents a major problem when developing an aftertreatment catalyst model. This effect may be observed with any reaction which is strongly inhibited by one of the reactants and which can become external transport limited. The main cause of this effect is that the reaction exhibits hysteresis; kinetic parameters which predict the same behaviour when the temperature is increasing, give different predictions when the temperature is decreasing. The size of the hysteresis increases with the magnitude of the self-inhibition constant in the rate equation and with reactant concentration. A second cause for this effect is that if lower concentrations are encountered in the drive cycle than were used for kinetic development; kinetic parameters which give equivalent predictions at higher concentration may give different predictions at lower concentration. To avoid this problem, reactor data during both temperature ramp up and ramp down should be used for developing kinetics. However, it should be noted that it has been found that (i) not all experiments exhibit hysteresis and (ii) that discretisation of the catalyst into an exceptionally large number of elements is required for accurate prediction during ramp down. [ABSTRACT FROM AUTHOR]

Transient two-dimensional simulations of fuel-lean H 2 /air combustion were performed in a 2-mm-height planar channel coated with platinum, using detailed hetero-/homogeneous chemistry and transport as well as heat conduction in the solid wall. The developed model resolved, for the first time, all relevant spatiotemporal scales in a practical channel-flow reactor configuration. A parametric study was carried out to investigate the effects of wall material, inlet velocity, and inlet temperature on the fundamental catalytic and gas-phase combustion processes. Computational singular perturbation (CSP) analysis identified the key catalytic reactions affecting light-off and homogeneous ignition. Homogeneous ignition crucially depended on the OH desorbing fluxes from the catalyst, while flame propagation and stabilization involved time scales of a few milliseconds. During the short duration of the light-off event, the ensuing Stefan velocity appreciably altered the flow field. Predictions of time accurate numerical simulations were further compared against those of a code relying on the quasisteady state assumption, and the specific conditions under which the latter was invalidated were identified. Finally, CSP analysis unraveled the reasons for the high computational cost of the fully transient 2-D simulations. The surface reaction mechanism exhibited a high stiffness with fastest time scales of the order of 10 - 12 s, pertaining to the hydrogen adsorption and to the H(s) + O(s) = OH(s) + Pt(s) reactions. These time scales were in turn six orders of magnitude shorter than the ones associated with gas-phase chemistry or with a simplified single-step catalytic reaction. [ABSTRACT FROM AUTHOR]

One of the challenges of full-scale computer simulation of a catalytic reactor is to consider the different scales involved in the problem in a practical fashion. In a monolith catalytic converter, these scales range from the molecular scale for the reactions, through the pore scale, washcoat scale, channel scale, and finally the full converter scale. This paper describes the implementation of a model reduction methodology using look-up tables to perform a consistent comparison of six different catalytic converters used for the catalytic combustion of methane. A detailed mechanistic model for methane combustion is used. Diffusion in the non-uniform washcoat is considered. The converters have different cell densities and wall thicknesses. Steady state and transient light-off simulations are performed. Efficient computational speed is achieved by successive model reduction, which allows the preservation of detailed small-scale information. The results obtained show that there is a non-intuitive relationship between the various operating parameters, which can only be deduced from a comprehensive model. [ABSTRACT FROM AUTHOR]

Highlights: [•] Temperature step experiments are used to compare the dynamic cold-start behaviour. [•] A significantly delayed light-off is observed for the particulate filter. [•] A numerical simulation model for the gasoline particulate filter is presented. [•] The experimentally observed delayed light-off is well predicted by the model. [•] Sensitivity analyses identify the physical effects contributing to this delay. [ABSTRACT FROM AUTHOR]

Highlights: [•] We couple the 2-D flow model of catalytic converter with the reaction model. [•] The detailed surface reaction mechanism over Rh is employed instead of the global one. [•] Increasing substrate length ratio of dual-substrate converter improves flow uniformity. [•] Light-off process is accelerated as the content of precious metal increases. [ABSTRACT FROM AUTHOR]

Abstract: We study the effect of the substrate material (ceramic versus metallic) on the steady-state and transient performance of monolith reactors using a one-dimensional two-phase model with position dependent transfer coefficients. When the operation of the reactor is on the ignited branch, it is shown that monoliths with metallic substrate clearly lead to a superior steady-state performance compared to those with ceramic substrate. In such cases, the ignited branch extends to lower inlet gas temperatures (typically 60–100°C in after-treatment applications) for the same catalyst loading. For transient operation where the time to light-off is important, it is shown that for the case of back-end ignition (corresponding to low inlet temperatures or low catalyst loading), metallic substrates are again superior. However, for the case of front-end ignition, ceramic converters may lead to lower cumulative emissions at lower inlet gas velocities while the converse is true at higher velocities. It is shown that the transient heating time and hence the cumulative emissions decrease with decrease in the channel hydraulic diameter and thermal capacitance of the substrate but are not monotonic with the inlet gas velocity. We present mathematical analysis and simulations to support these conclusions. Some novel results on the effect of substrate conductivity on the number of steady-states and upstream propagation of temperature fronts are also presented. [Copyright &y& Elsevier]

A typical monolithic catalyst consists of long, narrow, square channels containing a washcoat of catalytic material. While this geometry is the most common, other shapes may be better suited for particular applications. Of interest are hexagonal, triangular, and circular channel geometries. This paper provides a succinct review of these channel shapes and their associated heat and mass transfer correlations when used in a one plus one-dimensional model including diffusion in the washcoat. In addition, a summary of the correlations for different mechanical and thermal stresses and strains are included based on channel geometry. By including the momentum equation in the model formulation with geometry specific friction factors, this work illustrates a unique optimization procedure for light off, pressure drop, and lifetime operation according to a desired set of catalyst specifications. This includes the recalculation of washcoat thickness and flow velocity through the channels when cell density changes. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes a computational study of a full size catalytic converter. The simulations are validated against experimental data obtained from the literature. It is seen that the flow distribution under both reacting and non-reacting conditions is a strong function of the converter properties, including such factors as the monolith cell density, the diffuser angle and the aspect ratio. The mass flow rate of the feed also has a strong influence. Monolith substrates with a high cell density showed faster light-off for a model reaction. A dual zone monolith consisting of a high cell density core and a low cell density outer annulus showed worse performance in the light-off compared to all other configurations tested. The overall execution time required for a two dimensional simulation required anything from under a minute for a steady state cold flow test, to 2h for a transient light-off with reaction. [Copyright &y& Elsevier]

A methodology was developed, which helped to investigate the 'light-oft' characteristics of a real diesel oxidation catalyst (DOC) while it was connected to a diesel engine. As a demonstration of the technique, trials were performed on a Ford 2.0 L engine, with a Pt on -γalumina DOC (o.d.=106 mm; length=114 mm). By operating the engine at a constant speed (e.g. 2000 rpm), it was shown that when the engine torque was varied (e.g. from 5 to 47 N m), the gas inlet temperature to the DOC could also be varied (e.g. from 146 to 285 °C), while the gas composition and the mass flow of gas remained relatively constant-this enabled more scientific studies of catalyst 'light-off' to be performed on a real exhaust system. Experiments were also performed with higher background levels of a reactant (e.g. CO concentrations = 3000, 4000 and 5000 ppm), and hysteresis between 'light-off and 'light-down' was observed. It was also shown how valuable additional information may be obtained from experiments using a thin-slice DOC (o.d. = 106 mm; length = 5 mm), and the CO inhibition effect was demonstrated on a real engine exhaust. [ABSTRACT FROM AUTHOR]

Abstract: A microkinetic model is developed to simulate the promoting effect of H2 on the light-off of CO oxidation on platinum. It is shown that the coupling of the CO and the H2 oxidation sub-mechanisms, via the hydroxyl intermediate and its reaction with CO to carboxyl, is crucial to describe the process. This carboxyl-path was not considered in previous modeling studies. [Copyright &y& Elsevier]

Abstract: The effects of the length of the gas flow path from the exhaust outlet in the cylinder head to the catalyst inlet in the exhaust line, the ignition timing and the engine idle speed on the three-way catalyst light-off behavior in an electronically controlled inlet port LPG (liquefied petroleum gas) injection SI (spark-ignition) engine during cold start were investigated experimentally. The results showed that these factors affect the catalyst light-off behavior significantly during cold start. The reduction of the gas flow path length upstream the catalyst reduces the heat loss from the exhaust gases, increases the temperature of the catalyst, and results in faster catalyst light-off. Retarding the ignition timing from 0 to 15°CA ATDC decreases 22 and 8 s catalyst light-off time for HC and CO respectively. Increasing the engine idle speed from 1400 to 1800 rpm decreases 19 and 15 s the light-off time for HC and CO respectively. [ABSTRACT FROM AUTHOR]

Abstract: Nanocalorimeters based on a micromachined hotplate platform were characterized for potential applications in catalyst screening and reaction mechanism studies. The nanocalorimeters were designed to limit heat loss to the surroundings, resulting in a small heat capacity (50nJK−1) and thermal time constant (1ms). This greatly increases the sensitivity of the device, allowing reaction heat flows on the order of <1μW to be measured in air. The Pt catalyzed combustion of hydrogen in air was used as a model reaction. A relatively-simple heat and mass transfer model was developed which accurately described the data from the microscale system, and the rate-limited and mass transfer-limited regimes of the ignition traces were clearly distinguished. An average activation energy of 48±4kJmol−1 was determined for the Pt catalyzed combustion of hydrogen in air. However, unique rapid temperature programmed calorimetry methods convincingly demonstrate that the activation energy is influenced by kinetic phase transitions which also result in reaction rate bistabilities and oscillations. [ABSTRACT FROM AUTHOR]

Abstract: It is well known that the performance of a catalytic monolith is bounded by two limits: the kinetic regime at low temperatures (or before ignition for the case of exothermic reactions) and the external mass transfer controlled regime at sufficiently high temperatures (or after ignition). The washcoat diffusional resistance can also be significant over an intermediate range of temperatures. The transition temperatures at which the controlling regime changes from kinetic to washcoat diffusion to external mass transfer depend on the various geometric properties of the monolith, flow properties, the catalyst loading and washcoat properties. We present analytical criteria for determining these transition temperatures. These are derived using the recently developed low-dimensional model and the concepts of internal and external mass transfer coefficients. The criteria are more general than those in the literature and are useful in analyzing the experimental data. Further, we present an explicit expression for the experimentally measurable dimensionless apparent mass transfer coefficient () in terms of individual transfer coefficients in each phase. It is shown that can be lowered by orders of magnitude compared to the theoretical upper bound obtained in the limit of external mass transfer control. Low values of are obtained due to a small value of effective diffusivity in the washcoat, low catalyst loading or a reaction with low activation energy. The analytical criteria may be used for the design of monolith properties and experimental conditions so that the performance of the monolith approaches the upper limit defined by the external mass transfer controlled limit. [Copyright &y& Elsevier]

In this work we report how the use of low-sulfur gasoline decreases formation of N2O as a by-product during light-off of commercial Pd-only three-way catalytic converters (TWC). Our results also show that even in the absence of sulfur a large quantity of N2O (reaching instantaneous concentrations up to 700 ppm) is produced at low temperature, between 200° and 400°C. At high temperature (400° to 600°C) and lean conditions we found an unexpected emission of N2O (200 ppm), which is apparently produced by reduction of NO with C3H8 on the TWC. The emission of N2O during light-off is practically independent of the air-to-fuel ratio but depends strongly of the sulfur content in gasoline. Under rich and stoichiometric conditions, the presence of SO2 promotes N2O formation (concentrations from 700 to 900 ppm) via NO reduction by CO. The presence of SO2 in a lean environment has no effect upon formation of N2O. [ABSTRACT FROM AUTHOR]

Abstract: A minimization of the maximum diesel particulate filter (DPF) wall temperature and fast light-off during regeneration are targets for a high durability of the DPF and a high efficiency of soot regeneration. A one-channel numerical model has been adopted in order to predict the transient thermal response of the DPF. The effect of the ratio of the length to diameter (L/D), cell density, the amount of soot loading on the temporal thermal response and regeneration characteristics have been numerically investigated under two representative running conditions: city driving mode and high speed mode. The results indicated that the maximum wall temperature of the DPF increased with increasing ‘L/D’ in ‘high speed mode’. On the contrary, the maximum wall temperature decreases with increasing ‘L/D’ in the range of ‘L/D ⩾0.6’ in ‘city driving mode’. The maximum temperature decreased with increasing cell density because heat conduction and heat capacity were increased. Before commencing soot regeneration, the maximum allowed soot loading for retaining DPF durability was about 140g (5.03kg/m3) under ‘city driving mode’ and about 200g (7.19kg/m3) under ‘high speed mode’ in this study. The effect of the amount of soot loading on light-off time was negligible. [Copyright &y& Elsevier]

Abstract: The selective catalytic reduction of NO x by propene under lean-burn conditions was investigated over Pd/Ce0.66Zr0.34O2/TiO2/Al2O3 catalysts supported on wire-mesh honeycomb and was compared with pellet catalysts and ceramic monolithic catalysts. The wire-mesh honeycomb possessed high geometric surface area (GSA) of 43.0in.2 in.−3 and large open frontal area (OFA) of 86.6%. The wire-mesh honeycomb catalyst exhibited high NO x conversion at lower temperatures and could be stabilized in shorter time after temperature elevation than that of ceramic monolithic catalyst in the unsteady-state experiment. The wire-mesh honeycomb catalyst also exhibited characteristics of fast light-off. [Copyright &y& Elsevier]

Abstract: The effects of adding small amounts of palladium to Ni/γ-Al2O3 catalysts for the autothermal reforming of methane, in terms of activity, reducibility, capacity of repeated ignition and temperature profile of the reactor are described. The effect of different Pd sources was also studied. The Pd addition favors nickel reduction at lower temperatures. When the palladium is added as PdCl2 (PdNiAl-Cl) it exhibits a higher reduction temperature than when Pd(NO3)2 (PdNiAl-N) is used, an this can be attributed to the formation of Pd x Cl y O z species. Palladium strongly increases the activity of the Ni catalyst in autothermal reforming of methane, which is proportional to an increase in metal surface area. The addition of palladium to the catalyst also leads to a flatter temperature profile through the catalytic bed in the autothermal reforming of methane, and this is assigned to the high surface metal area of the catalyst. Only PdNiAl-N catalyst catalyzes the autothermal reforming of methane without previous reduction, while the PdNiAl-Cl catalyst only catalyzes the methane combustion and the unpromoted catalyst was inactive. [Copyright &y& Elsevier]

Abstract: This paper presents experiments and model predictions for the oxidation of CO over a platinum catalyst in a monolith reactor. Experimental behavior 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 Langmuir–Hinshelwood (LH) approach qualitatively was able to reproduce the light-off behavior. 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. [Copyright &y& Elsevier]

High-throughput and combinatorial approaches have been applied to the discovery of catalysts for selective low temperature CO oxidation/VOC removal using mixed CO/propylene feeds, and for the water–gas shift (WGS) reaction using real post-reformer feeds containing CO, CO2, H2O and H2. The screening approach was based on a hierarchy of qualitative and semi-quantitative primary screens for the discovery of hits, and quantitative secondary screens for hit confirmation, lead optimization and scale-up. For WGS, primary screening was carried out using scanning mass spectrometry. For CO oxidation and VOC removal, parallel IR thermography was the primary screen. Multi-channel fixed bed reactors equipped with imaging reflection FTIR spectroscopy or GC were used for secondary screening. Novel RuCoCe compositions were discovered and optimized for CO oxidation/VOC removal and the effect of doping was investigated for supported and bulk mixed oxide catalysts. For WGS, noble metal-free and Pt-doped CoFeRu mixed oxides as well as Pt on CeO2 and Pt on CeO2/ZrO2 were investigated and a new synergistic PtFeCe ternary composition was discovered. In these cases oxygen vacancies in the ceria lattice are believed to play a key role in the strong and synergistic Pt–Ce interaction. Alkaline metal doping was found to enhance the selectivity towards WGS by suppressing the unselective methanation side reaction and to increase the low temperature catalytic activity. [ABSTRACT FROM AUTHOR]

Offers a light-off criterion for a first-order reaction to more general kinetics and illustrates a new and general light-off criterion by using an example of carbon monoxide oxidation. Background on a mathematical model used in the criterion; Assumptions made in deriving the light-off criterion; Analysis of light-off behavior of carbon monoxide oxidation using the ignition criterion.

Abstract: This paper describes the evolutions of the distributions of temperature and concentration in the monolith during the cold-start period and the effects of the flow distributions in the monolith on the performance of a catalytic converter using numerical simulation. The effects of the flow distributions on the light-off, warm-up performance and the temporal variation of the solid temperature were studied through numerical simulation. The following conclusions were obtained from the predicted light-off curves and the steady conversion efficiency: (1) The light-off time increases and the total outlet conversion efficiency reduces as the flow uniformity index decreases, which is in good agreement with experimental results. (2) The conversion efficiency decreases due to the reduction of residence time caused by the increase of the velocity as the radius is reduced. The analyses on the distribution of temperature in the monolith show that the temperatures of the solid in the area near the front face of the substrate rise as the radius increases, and the radial gradient of the temperature of the solid becomes greater as the flow uniformity index decreases. An experiment on the flow distribution in a catalytic converter with a honeycomb spherical arc was conducted. Experimental results show that the flow distribution in the monolith can be improved greatly by installing the honeycomb spherical arc in the tapered inlet header with a penalty of a relatively small increase of pressure drop. [Copyright &y& Elsevier]

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O2 concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature.As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO2 production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO2 peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO2 production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO2 production rate drops abruptly as the reaction “quenches”.Catalyst layer thickness was changed over the range 0.06–1.0mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO2 production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness. [Copyright &y& Elsevier]

This paper presents the results of two-dimensional studies on a range of different catalyst substrates warmed by exhaust front a four-cylinder gasoline engine. Engine speeds investigated were 2000 and 3000 r/min, corresponding to frequencies of 67 and 100 Hz. Of the nine substrates investigated, four were washcoated but non-reactive and four were washcoated and reactive. The ninth substrate was a hybrid sample with its front end inactive and its downstream end active: this is analogous to an aged catalyst. The temperatures at 16 locations within the substrates were measured. All the substrates were warmed by the pulsating exhaust flow from an engine running lean of stoichiometric. The reactive substrates were warmed through light-off and both substrate temperatures and hydrocarbon conversion were monitored. The latter was measured with very high response time fast flame ionization detectors. Predicted temperatures and conversion were obtained from a two-dimensional computational fluid dynamics (CFD) model. The model was based on the porous medium approach and incorporated a simple chemical scheme for oxidation under lean conditions. Standard values for reaction rate constants were found to be accurate for the lean conditions studied. Comparison was made of measurements of temperature and conversion fraction with predictions, with particular reference to the time taken to achieve light-off. Steady flaw CFD predictions were found to be in good overall agreement with engine test data. Pulsing flow CFD predictions were found, however, to improve on the degree of agreement for warm-up of non-reactive samples, but at the expense of increased computation time. The performance of the reactive hybrid sample was accurately predicted by adjustment of the parameter for active area per unit volume, providing the basis for a technique for prediction of performance for aged catalysts. [ABSTRACT FROM AUTHOR]

We have combined the use of a molecular beam reactor and in situ spectroscopy (XPS) in order to correlate changes in the rate of CO oxidation and the CO–NO reaction with the coverages of the adsorbates and intermediates on the surface. In the reactor, both reactions exhibit an isothermal “light-off” phenomenon in which the rate autocatalytically increases with time. In the case of the CO oxidation reaction this is due to the desorption of CO which releases extra sites for O2 dissociation which, in turn, removes more CO, and hence the acceleration. In effect the reaction can be written as 2COa + O2g + 2S → 2CO2g + 4S, the acceleration coming from the release of extra adsorption sites S, which are involved in the reaction itself. “Fast XPS”, carried out in situ during the course of the reaction, shows domination of the surface by COa below 390 K and domination by Oa above that temperature, with a rapid change in surface coverage over a very narrow temperature window. On high surface area samples this acceleration is further reinforced due to a rapid temperature increase because of the highly exothermic nature of the overall reaction. The situation for the CO–NO reaction is broadly similar, except that the surface is dominated by NO at low temperature, not CO which tends to be displaced from the surface by NO. “Light-off” is dictated by the onset of the dissociation of NOa, which occurs at ∼400 K. Once Na and Oa are formed, N2O production is immediate and accelerates due to the creation of vacant sites for both NO and CO adsorption, the latter removing Oa as CO2g. Again, the reaction self-accelerates and there is a rapid change of surface coverage from NOa to Oa at ∼450 K. The overall self acceleration is due to the following overall reaction, 2NOa + COg + S → N2Og + CO2g + 3S, again producing more adsorption sites (S) in carrying out the reaction step. The rate is reduced at high temperature due to domination of the surface by Oa and to the reduced coverages of the molecular species. [ABSTRACT FROM AUTHOR]

This paper presents the results of studies on a range of different catalyst substrates warmed by engine exhaust. Engine speeds were in the range 1200–3000 r/min. One substrate was non-washcoated, four were washcoated but non-reactive and four were washcoated and reactive. The temperature at four locations within the non-reactive substrates was measured. The reactive substrates were warmed to light-off by the pulsating exhaust flow from an engine running fuel rich of stoichiometric. Both substrate temperatures and hydrocarbon conversion were measured. Predicted temperatures and conversion were obtained from a one-dimensional computational fluid dynamics (CFD) model. The model was based on the porous medium approach and incorporated a simple three-way chemical scheme. Comparison was made of measurements with predictions, with particular reference to the time taken to achieve light-off. Pulsing flow CFD predictions were found to be almost identical to steady-flow predictions for the conditions investigated. The CFD predictions were found to be in fair agreement with the engine test results, but using kinetic rate constants higher than previously reported values. [ABSTRACT FROM AUTHOR]

A one-dimensional two-phase model is used to derive an analytical light-off criterion for a straight channeled catalytic monolith with washcoat, in which the flow is laminar. For the case of uniform catalyst loading and a first order reaction, the light-off criterion is given by1/f(δckv(Tf,in)/De)ERΩδc(ΔTad)kv(Tf,in)/Tf,in2Rgg(Peh)L/RΩ2u¯+4eρfcpf/kfNuH,∞) ERΩδc(ΔTad)kv(Tf,in)/Tf,in2Rgg(Peh)L/RΩ2u¯+4eρfcpf/kfNuH,∞g(Peh)L/RΩ2u¯+4eρfcpf/kfNuH,∞+4eρfcpf/kfNuH,∞>1.Here, Tf,in is the inlet fluid temperature, ΔTad is the adiabatic temperature rise, RΩ is one-half the channel hydraulic radius (RΩ=AΩ/PΩ, AΩ, PΩ=channel cross-section area, perimeter), L is the channel length, u¯ is the fluid velocity, De is the reactant effective diffusivity in the washcoat, δc is the effective washcoat thickness, kf is the fluid thermal conductivity and kv(Tf,in) is the first order rate constant per unit washcoat volume at the inlet fluid temperature. NuH,∞ is the asymptotic Nusselt number in the channel. The function f accounts for diffusional limitations in the washcoat and is given by f(ϕ)=1 for ϕ<0.5 and f(ϕ)=2ϕ for ϕ>0.5. The factor g(Peh) depends on the solid conductivity, or more precisely, the heat Peclet number, Peh=u¯LρfcpfRΩ/δwkw, where δw(kw) is the effective wall thickness (thermal conductivity). The function g(Peh) decreases monotonically from 2.718 for Peh=0 to unity for Peh=∞. We also show that if the second term is negligible and the first exceeds unity, then ignition occurs at the back-end. If the second term exceeds unity then ignition occurs at the front-end. If the sum exceeds unity with the second term less than unity and not negligible compared to the first term then ignition occurs in the middle of the channel. This analytical ignition criterion is verified by numerical simulations using an accurate transient model that uses position dependent heat and mass transfer coefficients.We show that the plot of exit concentration versus time consists of two regions: kinetically controlled transient region and the mass transfer controlled steady-state asymptote. For the case of high solid conductivity, we present an analytical expression for the transient time at which the monolith shifts from the kinetically controlled to the mass transfer controlled regime. We also determine the influence of various parameters such as the washcoat thickness, channel dimensions, catalyst loading and initial solid temperature on this transient time and the cumulative emissions. Examination of the influence of solid conduction and channel geometry on cumulative emissions showed that designs that are optimum for steady-state operation lead to higher transient emissions and vice versa. Finally, we discuss the transient and steady-state behavior of the monolith for the special case of Lef<1 (hydrogen oxidation). [Copyright &y& Elsevier]

A multi-channel model is used to study the impact of flow non-uniformity during cold-start transient operations of a catalytic converter. It is seen that inlet zone recirculation can lead to significant non-uniformity of the flow in the monolith, and this non-uniformity can lead to significant differences in ignition characteristics among the channels. These ignition differences are especially pronounced at lower exhaust temperatures, where the axial location of ignition can vary from one channel to another. It is suggested that this strong effect of temperature on ignition may explain some of the apparently contradictory conclusions about the impact of flow non-uniformities in the literature. The simulations here show that the index of non-uniformity, as defined in many past studies, is an inadequate measure of the full impact on ignition characteristics. For the same index of non-uniformity, the non-uniformity effects on ignition become less significant with increasing exhaust flow rate. This implies that more detailed simulations of flow and temperatures non-uniformities caused by the recirculation zones, heat losses at the boundaries and insufficient mixing upstream of the monolith can be relevant to practical applications. [Copyright &y& Elsevier]

Two types of catalysts with the same palladium loading, palladium-substituted perovskite (LaFe0.77Co0.17Pd0.06O3) and perovskite-supported palladium catalyst (Pd/LaFe0.8Co0.2O3) were prepared, and the effects of the preparation methods and reactant gas composition on the catalytic behaviors were investigated in detail. The catalyst structure was characterized by X-ray diffraction (XRD), transmission electron micrograph (TEM), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) techniques. It was found that the excellent three-way catalytic activities could be obtained under slight rich conditions due to hydrocarbon steam reforming. The activity performance of Pd/LaFe0.8Co0.2O3 was higher than that of LaFe0.77Co0.17Pd0.06O3 and this was owing to the ease of reduction of palladium in the former. [Copyright &y& Elsevier]