Platinum Metals Rev., 1991, 35, (4), 178
Flow-Through Catalysts for Diesel Engine Emissions Control
Platinum Coated Monoliths Reduce Particulates
The treatment of vehicle emissions with an exhaust catalyst in order to reduce the level of three major air pollutants, namely carbon monoxide, unburnt hydrocarbons and nitrogen oxides, is becoming a world-wide requirement for gasoline fuelled engines (1, 2). First standards were introduced in the U.S.A. and Japan during the early 1970s (3). Similar standards will be enforced in Europe beginning in 1992/93 (4, 5). Diesel engine exhaust poses an additional challenge for emission control because, as compared to exhaust from gasoline engines, it also contains particulate matter. Early attempts at particulate control utilised catalytic trap oxidisers to filter the exhaust and oxidise the particulates (6–8). Catalytic trap oxidisers have the disadvantage of requiring an active regeneration mechanism to remove particulate build-up. Recent advances in diesel engine technology have greatly reduced the formation of particulates, widi the result that air quality standards may now be met by the use of catalysed flow-through monoliths (9–11). These catalysts have the advantage of being passive systems which do not require regeneration.
Catalysts for diesel engines must function differently to those for gasoline engines because of differences in the chemical composition of the exhaust gas. Modern spark-ignition gasoline powered automobiles, with emission control devices, operate near stoichiometry (at an ainfuel ratio of ~ 14.7:1) and under closed-loop electronic control. The catalyst typically operates in the temperature range of 300 to 900°C and functions by oxidising carbon monoxide and unburnt hydrocarbons to carbon dioxide and water, while oxides of nitrogen are reduced to nitrogen gas (12–16). The standard practice for gasoline exhaust treatment is to utilise a noble metal catalyst supported on a flow-through ceramic or metallic monolith, typically having 300 to 400 axial channels per square inch of frontal area. The monolith walls are coated with a thermally durable, high surface area oxide on which are supported the active catalyst components, generally platinum, palladium and rhodium.
Combustion of diesel fuel occurs by high pressure ignition, rather than by spark ignition, and at air:fuel ratios greater than 20:1. The exhaust temperatures are typically cooler, 150 to 450°C, and are always oxidising. The diesel engine generates intrinsically low emission levels of gas phase hydrocarbons and carbon monoxide, and when fitted with a conventional oxidation catalyst there is little trouble meeting regulated standards for these pollutants. The use of typical three-way catalysts to control nitrogen oxide emissions is impractical due to the oxidising nature of the exhaust. Therefore, nitrogen oxide emissions must be controlled by engine design and calibration, often at the expense of increased particulate emissions. As a result, the major challenge to diesel exhaust emission control by catalytic means remains one of particulate removal. Additionally, in Europe, where there is a combined hydrocarbon + nitrogen oxide standard, removal of low temperature gas phase hydrocarbons is beneficial to meeting the overall design requirements.
There are a variety of diesel engine designs and these may be classified according to the method of fuel and air injection, as well as by the end-use application. Engines with indirect injection of fuel are typically utilised for passenger cars and light-duty trucks, while direct injection engines have inherently better fuel economy and are utilised in heavy duty vehicles. Many manufacturers are developing high speed direct injection engines for use in light-duty applications. Air is added to the combustion chamber by natural aspiration or under pressure by turbocharging, and turbocharging is often accompanied by intercooling. World-wide emissions standards for diesel fuelled engines will finally require the use of catalytic converters in the 1992–94 time frame, see Table I.
|Market||Test||Class||HC||CO||NOx||HC + NOx||Particulate|
|U.S. 1987||FTP g/mile||LD||0.41||3.4||1.0||0.20|
|California 1989||FTP g/mile||LD||0.39*||7.0||0.4||0.08|
|U.S. 1991||HDT g/hph||HD||1.3||15.5||5.0||0.25|
|U.S. 1994||HDT g/hph||HD||1.3||15.5||5.0||0.10|
|Japan 1990||10 Mode g/km||LD||0.62||2.70||0.70||None|
|Japan 1994||10 Mode g/km||LD||0.62||2.70||0.50||0.2|
|EC 1992||ECE+EUDC g/km||Auto||2.72||0.97||0.14|
|Stage 1 1992/93||EC steady state test g/kWh||HD||1.1||4.5||8.0||0.36|
|Stage 2 1995/96||EC steady state test g/kWh||HD||1.1||4.0||7.0||0.15|
Diesel Particulate Catalysis
Diesel particulate standards are measured via the weight increase of a fibre filter placed in the diesel exhaust. The material collected consists of graphitic “hard” carbon or “soot”, a soluble organic fraction (SOF), water, sulphuric acid and an inorganic ash residue. Typical particulate compositions are given in Figure 1. The soluble organic fraction consists of hydrocarbons derived from both fuel and lubricating oil which condense as the exhaust cools, or adsorb during collection. The sulphuric acid/water fraction arises from oxidation of sulphur dioxide to sulphur trioxide and condensation with water vapour.
Importance of Sulphur in Diesel
Sulphur is present to some extent in all diesel fuel and, as a result of the combustion process, is emitted in the exhaust as sulphur dioxide. A small fraction, typically about 2 per cent, is further oxidised to sulphur trioxide which condenses with water in the exhaust as sulphuric acid, and is then absorbed on carbonaceous soot particles, thus contributing to the total measured mass of particulate emissions. The use of a catalyst can increase the fraction of fuel sulphur converted to sulphate, and even a modest increase in this fraction will result in the “manufacture” of significant particulate. For example for a 0.05 weight per cent sulphur fuel, 100 per cent conversion of the sulphur in the fuel to sulphuric acid would by itself result in a particulate emission five times higher than the 1994 U.S. heavy duty truck standard.
Particulate Control Strategy
A “flow-through” monolithic reactor, as opposed to a particulate trap, achieves particulate reduction by catalytic oxidation of the soluble organic fraction. However, this will result in minimal conversion of the hard carbon, and therefore, the catalyst must be able to keep itself free from particulate fouling. During low temperature operation particulate increase via sulphuric acid formation is not significant, but under these low temperature conditions it is necessary for the catalyst to remove the soluble organic fraction and the gas phase hydrocarbons. During high temperature operation the oxidation of hydrocarbon is facile, but the increase of particulates due to sulphate formation must be minimised or eliminated.
Catalytic Carbon Oxidation
It has been reported previously that the removal of carbon requires reaction with nitrogen dioxide, which is formed by the oxidation of nitrogen monoxide over a platinum catalyst (17). Palladium and rhodium are much less effective for the oxidation of nitrogen monoxide at low temperatures, and their inclusion with platinum in a catalyst formulation results in de-activation of the platinum. In the absence of platinum an increase in back pressure in a flow-through system has been observed as a function of time, see Figure 2. This emphasises the requirement for platinum in the catalyst formulation in order to prevent fouling by carbon particulate.
Catalytic Sulphur Oxidation
Similar to carbon oxidation, it has been demonstrated that platinum is the most effective catalyst for sulphur dioxide oxidation, that palladium and rhodium are less active, and that the alloying of palladium or rhodium with platinum results in a compromise activity. Sulphur dioxide adsorbs strongly on platinum at room temperature and inhibits carbon monoxide, nitrogen monoxide and alkene oxidation. Sulphur dioxide oxidation is kinetically limited at low temperature and thermodynamically limited at high temperature, but near 500 to 600°C can reach 80 to 90 per cent conversion. Factors which can limit sulphur dioxide oxidation activity in this temperature range include oxygen concentration, space velocity (Figure 3), and intrinsic catalytic activity.
Removal of the Soluble Organic Fraction
Hydrocarbon species in diesel exhaust consist of gaseous compounds as well as heavy, condensable hydrocarbons (soluble organic fraction) which can be solubilised from the diesel particulate. The soluble organic fraction is both fuel and oil derived, and has been characterised by gas chromatography-mass spectroscopy (18, 19). As with the oxidation of sulphur dioxide and nitrogen monoxide, platinum is generally considered the best low temperature hydrocarbon oxidation catalyst, with palladium or rhodium being both less effective and/or poisons for platinum. At high temperatures complete oxidation is favoured.
Emission Characteristics of Diesel Engines
To demonstrate the variability in emission composition with engine type and operating conditions, test results for two diesel engines are given in Table II (20). The engines were characterised using the U.S. cold start Federal Test Procedure, the European hot start Extra Urban Driving Cycle and a hot steady-state test with inlet temperature in the range of 400 to 420°C. The emitted gaseous components and particulates measured without a catalyst fitted to the engines are given.
|Hot SS g/test||A||1.87||5.72||12.2||2.79|
The gaseous hydrocarbon fraction resulting from incomplete fuel combustion was analysed in further detail and, as shown in Table III, was composed primarily of methane, and unsaturated, cyclic and oxygenated hydrocarbons. The aldehyde/ketone components are generally considered to be responsible for the offensive odour associated with diesel engine exhaust.
The composition of the particulate component of diesel emissions is dependent on the combustion process, as controlled by engine design, the lubricant and the fuel used. The sulphate content was analysed by ion chromatography, the soluble organic fraction was extracted and subsequently characterised into fuel derived and oil derived fractions by gas chromatography, and the carbon content was quantified by thermogravimetric analysis. The remaining component has not been fully characterised, but represents the inorganic zinc, calcium, and iron compounds derived from lubricants and from engine wear. The particulate analyses, for all three vehicle test cycles, are shown in Table IV, and illustrate some specific engine characteristics which are important when designing a catalyst for particulate attenuation control.
Engine A showed a consistently higher soluble organic fraction and would be considered to produce a “wet particulate”, with low carbon content and high soluble organic fraction. While Engine B could be described as forming a “dry particulate”, low in soluble organic fraction and high in carbon content. The differences in emissions can be partially explained by the differences in operating temperature, with the hotter engine resulting in a “dryer” particulate. Thus Engine B represents the more difficult system to treat catalytically, and would present the greatest challenge.
The challenge for diesel catalyst development is to reduce the gaseous hydrocarbons, carbon monoxide and absorbed soluble organic fraction of the exhaust without increasing the mass of the particulate emissions arising from the formation of sulphates.
Catalyst development was conducted using Engine B fitted to a stand dynamometer (20). The catalysts were evaluated under steady state conditions, using engine load to control the operating temperature. This test was conducted at 2500 RPM using fuel with 0.16 weight per cent sulphur. The system was stabilised at each test point prior to exhaust sampling. Baseline emissions were measured at each temperature, with an uncatalysed substrate in the exhaust system to equalise back-pressure on the engine.
The data in Figure 4 represents the baseline particulate analyses as a function of engine load. For Engine B, which generates a “dry” particulate, with increasing load the soluble organic fraction remains approximately constant while the carbon and sulphate fractions increase. Figure 5 gives the particulate formation rate when a standard platinum oxidation catalyst is placed in the exhaust system. Excellent removal of the soluble organic fraction is observed with little impact on the “hard” carbon particulate, however, above 300°C progressively larger quantities of sulphate are formed. In spite of the removal of the soluble organic fraction over the catalyst there is actually a large net increase in particulate formation, due to the oxidation of sulphur dioxide and the formation of sulphuric acid.
In view of the clear requirement for reduced sulphur dioxide oxidation, new platinum-based diesel catalyst formulations were developed to reduce the oxidation of sulphur dioxide to sulphur trioxide and to limit the degree of sulphur storage on the catalyst surface. Initial evaluation of developmental catalysts was performed utilising a synthetic gas mixture. Since hydrocarbons in the soluble organic fraction are typically of high molecular weight, decane (C10H 22) was added to the feed-gas to model the hydrocarbon oxidation requirements of a diesel catalyst. The relevant experimental conditions are given in Table V. Hydrocarbon conversion was measured by flame ionisation detection while sulphur dioxide conversion was monitored by gas chromatography. The test results are given in Figure 6 and show a decrease in sulphur dioxide conversion with the new catalyst formulations. This lower activity is accompanied by a decrease in hydrocarbon oxidation activity.
|Space Velocity:||42,000 per hour|
|Catalyst Size:||1.0″ dia × 2.33″ long|
|Gas Flow Rate:||20.0 SLPM|
|C10H22||1000 ppm (C1)|
These new catalyst formulations were then evaluated on Engine B utilising the steady-state diesel exhaust test. An obvious improvement in higher temperature particulate control was observed. However, as predicted in the synthetic gas test, low temperature hydrocarbon activity was somewhat reduced. Dynamometer data for particulate and hydrocarbon conversions are given in Figures 7 and 8, respectively. Significant removal of particulate can be achieved by designing the catalyst to minimise sulphate emissions and prevent fouling by rapid carbon accumulation. Some compromise must be made for gas phase hydrocarbon activity. Although, meeting standards for gas phase hydrocarbon emissions is typically not difficult, some further improvement may be desirable for European light-duty vehicles which must meet a hydrocarbon + nitrogen oxide standard. This is the focus of future efforts in this area.
The development of platinum-based catalysts for the control of diesel engine exhaust requires a knowledge of the exhaust temperature and the particulate composition. Although these factors are dependent upon individual engine design, the catalyst may be tailored to meet specific requirements.
To meet new regulatory standards a reduction in exhaust particulates is required and the key to achieving this is by the control of sulphur dioxide oxidation. Some compromise in gas phase hydrocarbon activity may be required to obtain the necessary reduction in the formation of sulphuric acid. New platinum-based catalysts have been developed to lower sulphur dioxide oxidation while at the same time minimising fouling of the catalyst due to carbonaceous soot. Thus particulate reduction is achieved primarily by efficient catalytic removal of the soluble organic fraction of the particulate.
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