Advances in the Development of Filterless Soot Deposition Systems for the Continuous Removal of Diesel Particulate Matter

A model catalytic converter system has been developed to investigate and characterize novel catalyst structures for filterless diesel particulate matter deposition and oxidation in modern heavy duty vehicle diesel engine exhaust systems. The particulate traps are designed for low exhaust gas back pressures and to avoid the clogging effects observed in ceramic filters. In experiments under realistic flow conditions deposition efficiencies of up to 70% have been achieved for submicrometer particles in stacks of corrugated stainless steel foil with microsphere surface coating. The model catalytic converter system is also used to study the reaction kinetics of soot oxidation and volatilization by oxygen and nitrogen oxides under a wide range of reaction conditions, for real diesel engine soot, different model soot substances, and different types of converter surfaces.     

Advances in the development of filterless soot deposition systems for the continuous removal of diesel particulate matter

A model catalytic converter system has been developed to investigate and characterize novel catalyst structures for filterless diesel particulate matter deposition and oxidation in modern heavy duty vehicle diesel engine exhaust systems. The particulate traps are designed for low exhaust gas back pressures and to avoid the clogging effects observed in ceramic filters. In experiments under realistic flow conditions deposition efficiencies of up to 70% have been achieved for submicrometer particles in stacks of corrugated stainless steel foil with microsphere surface coating. The model catalytic converter system is also used to study the reaction kinetics of soot oxidation and volatilization by oxygen and nitrogen oxides under a wide range of reaction conditions, for real diesel engine soot, different model soot substances, and different types of converter surfaces.

     

Experimental study of a reverse flow catalytic converter for a dual fuel engine

Theperformance of a reverse flow catalytic converter for a methane/diesel dual fuel engine, a monolith honeycomb converter with palladium catalyst washcoat, was evaluated under steady and transient engine conditions. The reverse flow converter provided superior performance (that is, higher conversion of pollutants) for several steady engine operations, compared with unidirectional flow operation. For transient operation following a step change in engine operating conditions, reverse flow is better than unidirectional flow when the change in engine operation results in a reduction in exhaust gas temperature. For an increasing exhaust gas temperature, reverse flow decreased the rate of increase of reactor temperature. The reverse flow converter was tested using the transient Japanese 6‐Mode tests. Reverse flow operation gave higher conversions than unidirectional flow for this test, with a switch time of 5 s giving the best results.     

Transient simulation of a catalytic converter for a dual fuel engine

An experimental and modelling study was performed for a catalytic converter attached to a natural gas/diesel dual fuel engine. The catalytic converter was a ceramic monolith honeycomb substrate coated with a washcoat of catalyst. A multiple segmented design of converter was used. This paper describes the application of a one‐dimensional finite element model for the transient and steady state operation of this converter. The model is a single channel model. The laminar flow was approximated using a dispersed plug flow model. The chemical kinetics were simulated using LHHW type expressions. Comparison of simulated results are made with experimental results for heating and cooling cycles that resulted from speed and load changes on the engine. These comparisons showed a maximum difference between experimental and predicted emission levels of about 10%.     

Reaktionstechnische Aspekte der katalytischen Abgasreinigung bei Kraftfahrzeugen

Kinetic aspects of the catalytic detoxification of automobile exhaust . The technology of catalytic converters designed to reduce automobile exhaust emissions has developed considerably in recent years, as is well documented in the available literature. This review is intended to show how the methodology and philosophy of chemical reaction engineering have contributed to this development. A brief survey of possible reactions and actual catalysts is followed by a detailed account of the relevant basic principles of micro- and macrokinetics. Preliminary calculations of conversion of noxious substances in monolithic converters may be based upon the assumption that diffusion from the laminarly flowing gas phase to the catalytically active wall coating is the rate-determining step in a warmed-up converter. During the critical warm-up period, after a cold start, the chemical reaction steps offer the main resistance. The points discussed are the extent to which rate equations and kinetic parameters will be influenced by high degrees of dispersion of the noble metals, by catalyst deactivation, or by transient operation conditions. The structure of the highly porous catalyst support as well as metal impregnation profiles can be optimized so that poisoning is strongly retarded by in-pore diffusion, while the main reactions are affected as little as possible.     

Transient formation and reduction of sulfur trioxide in the presence of an unsupported platinum catalyst

The kinetics of sulfur dioxide oxidation with oxygen and the reduction of subsequently adsorbed sulfur trioxide with carbon monoxide were studied in transient performed experiments in the presence of an unsupported platinum catalyst. The heterogenity of the catalyst surface was taken into consideration in the mathematical modelling of the kinetics of the processes. One difficulty encountered in this modelling was the requirement that fractional coverages and amounts of various active sites be equal at the end of the oxidation and at the start of the reduction. This study is intended to be a part of an investigation concerned with suppressing the formation of sulfur trioxide from exhaust gas from cars with catalytic mufflers by cyclic oxidation and reduction.     

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.     

CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM-BASED CATALYSTS

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.     

Effect of perturbations in the exhaust gas composition on three-way catalyst light off

The influence of perturbations in the exhaust gas composition on the operation of three-way catalytic converters (3WCC) has been the subject of many research works. This paper aims to investigate the effect of such transients on the light-off temperature of a commercial 3WCC, by using a dynamic mathematical model for 3WCC simulation. This modeling approach embodies a comprehensive oxygen storage and release submodel into an existing 3WCC quasi-steady model. The dynamic model developed is validated against previously published experimental data, which imply that the presence of transients in the exhaust gas results in an improved low-temperature catalyst performance. Having validated the model, the improved light-off performance is investigated, and attributed primarily to exhaust gas stoichiometry and secondarily to the heat released in the catalyst by the oxidation reactions (self-acceleration). Finally, a parametric study is performed to assess the influence of different patterns of transient exhaust gas composition. The results obtained show that lean composition of the exhaust gas is more favorable during the light-off phase, while frequency and amplitude of the composition oscillation play only a minimal role. This investigation encourages further application of mathematical modeling in areas like lambda control strategy optimization, which were beyond the scope of earlier 3WCC models.     

Diesel particulate traps regenerated by catalytic combustion

The development of catalytic means for the regeneration of particulate-laden traps for diesel exhaust cleaning is the main topic of this paper. All the steps of the catalytic trap preparation are dealt with, including: the synthesis and choice of the proper catalyst and trap materials, the development ofin situ catalyst deposition, and the bench testing of the derived catalytic traps. Two different traps were considered (i.e., silicon carbide and cordierite wallflow monoliths operating via a shallow-bed filtration mechanism), whereas the best catalyst selected was the perovskite LaCr_0.9O₃. The filtration efficiency and the pressure drops of the catalytic and non-catalytic monoliths were evaluated on a diesel engine bench under various operating conditions. On the basis of the obtained results the catalysed SiC converter was found to be the most satisfactory converter to be placed on the exhaust line of the modern common rail diesel-engine cars.     

Modelling of transient kinetics in catalytic three-phase reactors: Enantioselective hydrogenation

Transient techniques are frequently used for catalytic gas-phase processes, but the application of transient techniques on catalytic three-phase systems is very scarce. Transient kinetic experiments provide valuable additional information about the behaviour of complex organic reaction systems, which was illustrated here with continuous enantioselective three-phase hydrogenation of ethyl benzoylformate over supported Pt catalyst particles in a fixed bed. The catalyst stability and the details of the adsorption–desorption behaviour of the reaction participants were revealed by transient experiments. Quantitative modelling of the data was based on kinetic experiments and characterisation of the reactor flow pattern by an inert tracer. Both liquid-phase species and adsorbed surface species were included in the modelling. The model predicted correctly the dynamic behaviour of the complex organic system under transient conditions. The approach is generally applicable to complex organic systems undergoing catalytic transformations.     

稀土在汽车排气催化净化中的应用

催化剂技术是治理汽车污染的重要措施之一，作为催化剂组分的稀土在其中起了重要的作用。本文讨论了稀土在汽车排气催化净化中的应用。其作用在于：提高了载体性能，储氧功能，提高催化剂的抗中毒性，提高催化剂热稳定性，作为助剂，应用在氧传感器中等。     

Behavior of three-way catalyst in a hybrid drive system

The dynamic behavior of a Pt-Rh/CeO₂-Al₂O₃ three-way catalyst (TWC) under pulsed flow operation conditions (intermittent mode), such as occur in a newly developed hybrid drive system, has been studied experimentally in a laboratory apparatus with simulated exhaust gas. The parallel hybrid system is based on the combination of an Otto-cycle engine with an electric motor and a flywheel providing a short-term energy storage. This configuration permits intermittent charging of the flywheel by the combustion engine which runs only during about 10% of the driving time. Each exhaust gas pulse (duration typically ca. 3 s) is proceeded by an air pulse, which results from the filling of the engine cylinders with air at start up and shut off. Experimental studies indicated that the air pulses have a negative impact on the performance of the catalytic converter, reducing the inherent benefits resulting from the intermittent operation mode of the combustion engine. Forced asymmetric λ-cycling during exhaust pulse was found to be most beneficial for improving catalyst performance. A simple reduced kinetic model derived from a Langmuir-Hinshelwood model for CO oxidation, which was extended by introducing CO and O₂ equivalents to mimic the complex exhaust gas, was used to describe the dynamic behavior of the TWC. The model proved to be useful for finding the optimal λ-cycling conditions. Experiments with the real Otto-cycle engine exhaust proved that the reduced kinetic model is suitable for use in a closed loop λ-control.     

速度分布对三效催化器性能的影响(Ⅰ)数理模型

描述了三效催化器的流动、传热传质和异相催化过程,分析了催化器的径向热导率的修正方法,比较了尾气分别作为双组分和多组分混合物时对其组分扩散系数的影响,集成了可用于模拟三效催化器的冷起动性能和暖机性能的二维轴对称瞬态模型,模型采用有限容积法求解.该模型可用于模拟和解释一维模型无法做到的流动分布等参数对催化器性能的影响.计算结果和分析在本研究中的第二部分叙述.     

Kinetic considerations of three-way catalysis in automobile exhaust converters

The activity of three-way catalysts is highly dependent on the reactants present in the automobile exhaust gases (CO, NO_x, HC, O₂, H₂O, CO₂, N₂) as well as their relative concentration. Thus, the influence of each reactant on the kinetic behavior of the whole mixture makes difficult to establish the accurate kinetics of the system.

Activity experiments carried out close to the real operation conditions (GHSV, concentration, etc.) with a Pt/CeO₂/Al₂O₃ catalyst supplied data on the CO and HC oxidation and NO reduction reactions in environments formed by different reactant combinations (from binary mixtures to the whole mixture simulating the real conditions at the automobile converter).

The obtained results have shown notable variations in the oxidation/reduction mechanisms depending on the presence (or absence) of components in the environment. The presence of water always promoted the three-way activity of the catalyst. The compensation effect applied to the CO, NO and HC conversions confirmed that kinetic expressions obtained with partial mixtures (not very close to the real converter environment) have only limited application for determining the whole kinetic scheme occurring in the automobile converters.     

Transient kinetics of n‐butane partial oxidation at elevated pressure

A transient Mars‐van Krevelen type kinetic model was developed for n‐butane partial oxidation over vanadyl pyrophosphate (VPP) catalyst. The model validity was verified over a relatively wide range of redox feed compositions as well as higher reactor pressure (410 kPa). Oxygen and n‐butane conversion increased with higher pressure while maleic anhydride (MA) selectivity decreased by as much as 20%. However, the overall MA yield was enhanced by up to 30%. High pressure maintains the catalyst in a higher oxidation state (as long as there is sufficient oxygen in the gas phase) and as a consequence, the catalytic activity is improved together with MA yield. High pressure also affects the redox reaction rates and activation energies. © 2012 Canadian Society for Chemical Engineering     

On the quantification of the controlling regimes in automotive catalytic converters

The conversion of pollutants in automotive catalytic converters is influenced by a number of physical and chemical processes that take place in the gaseous and solid phases as the exhaust gases flow through the converter. A detailed understanding of the complex processes involving flow dynamics, heat and mass transport and heterogeneous surface reactions is of crucial importance to improve the converter design. The main objective of the present study is to quantify the magnitudes of the external and internal mass transfer as well as chemical reaction limiting processes as a function of the converter operating temperature. To this end, experimental data, obtained for a three way catalyst (TWC) under real world operating conditions, are analyzed and compared against analytical expressions that allow for the quantification of the different limiting processes involved. The results demonstrate that (i) the external mass transfer resistance overlaps the reaction resistance only at moderate operating temperatures and not immediately above the ignition temperature as generally considered in the literature, (ii) the transport phenomena (external and internal mass transfer) represents 90% of the total resistance for temperatures higher than 792 K, (iii) the internal mass transfer in the porous washcoat presents a larger resistance than the external mass transfer from the bulk fluid to the washcoat wall even at high operating temperatures, and (iv) based on the quantification of the individual resistances as a function of the TWC operating temperature, it was demonstrated both the influence of the substrate cell density and of the effective diffusivity on the TWC conversions. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Some kinetic aspects of unsteady-state partial oxidation reactions. Dynamic processes on metal oxide surfaces

It is shown that steady-state kinetic data do not allow the discrimination between the redox and associated mechanisms of the partial oxidation reactions. A discrimination between these mechanisms was performed using transient experiments. The obtained rate expressions are in agreement with experimental kinetic data for catalytic partial oxidation of o-xylene.

An influence of the conjugate oxidation of a catalyst surface on dynamics and kinetics of the heterogeneous catalytic oxidative reactions is considered. Computing simulation of methane oxidative coupling of methane reaction at lowered temperature and elevated pressure has been performed. It showed that the reaction order with respect to oxygen exceeding unity is consistent with the chain branching mechanism of the reaction in the presence of TiSi₂ and TiB₂ and showed the important role of the branching chain cycles in the low-temperature OCM reaction at elevated pressure.     

Effect of Ba Addition on Catalytic Activity of Pt and Rh Catalysts Loaded on γ-Alumina

The catalytic activity of Pt catalyst loaded on -alumina was improved by Ba addition in simulated automotive exhaust gases. On the other hand, the result of Rh catalyst was the opposite. From the results of the partial reaction orders in C₃H₆–O₂ reaction and TPR, it was concluded that the Ba addition to Pt catalyst suppressed the hydrocarbon chemisorption on the Pt catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, Ba addition to Rh catalyst caused such a strong oxygen adsorption on Rh that rejected the hydrocarbon adsorption and suppressed the reaction.     

Performances of a catalytic foam trap for soot abatement

A catalytic trap for soot particles was prepared by deposition of Cu–V–K–Cl catalyst on a ceramic foam. Catalytic trap performances were evaluated by treating the exhaust of a gas oil burner under different operating conditions. The results obtained showed that ceramic foam is a particularly suitable support for this application since it yields low gas pressure drop, good soot collection efficiency (“deep bed” filtration mechanism), high thermal shock resistance and good contact throughout the filter between soot particles and catalyst surface. In addition, the catalytic foam trap is able to spontaneously regenerate at operating conditions comparable to those typical of diesel engine exhaust and after more than 70 test hours it retains its activity towards soot oxidation.