Catalyst Book

Leaders in Diesel Catalyst Technology

Non-road machines represent a very different challenge to on-road vehicles. Indeed, it would be better to say that NRMM represents many thousands of different applications challenges.

Johnson Matthey has been supplying and applying retrofit aftertreatment systems to non-road machines for more than 15 years. Therefore, we have experience of the variety of operating requirements, engine duty cycles and packaging constraints found in non-road machines. We know the differences between a tractor and a tracked excavator. And we know the regulations. We are very well placed to understand the requirements on catalytic systems for Tier 4 compliance.

The non-road

Applications Challenge

An emission control catalyst is usually a catalytic coating applied to a ceramic or metallic substrate.

The catalyst has open channels and the gases react as they pass through. Some catalysts are designed to be able to store reagents (e.g. oxygen, ammonia) to enhance performance or even to act as a chemical trap, as in the case of NOx adsorbers.

A particulate filter is a physical trap. The most common type is a ceramic monolith like a catalyst substrate, but with channels closed at alternate ends so that the gases must pass through the channel walls, leaving the soot behind. These filters are nearly 100% efficient for solid particles. Partial filters, designed for a lower efficiency, are also available. A DPF does not always contain a catalyst, but catalysts are used to burn the accumulated particulate.

Catalyst: a substance which when present in small amounts increases the rate of a chemical reaction or process but which is chemically unchanged by the reaction.

Types of catalyst technology

When designing a catalyst, we make it active to give high conversions from smaller catalysts and to make the best use of precious metal.

We make it selective to ensure that it is active for the desired reactions and does not produce undesirable by-products.

And we make it durable to meet useful life requirements and to minimise the durability factors that must be applied to new systems. Designing durability requires an understanding of how the catalyst will be used in its application and the exhaust conditions that it will see throughout its life. It also requires knowledge of the deactivation mechanisms, both chemical and physical, of each type of catalyst. Nitrogen dioxide (NO2) and oxygen (O2) can both be used to combust the soot trapped in a filter. NO2 has the advantage that it reacts with soot at the temperatures found in diesel exhaust. The O2 reaction requires higher temperatures, but is much faster.

PM Control

NO2 reacts with soot trapped in a filter, making NO and CO2. This reaction can occur from 200°C, so the temperatures found in diesel exhaust streams are sufficient and no additional energy is required. Some of the NOX emitted by an engine is NO2 and more can be made by an oxidation catalyst designed to oxidise NO to NO2. This system -an oxidation catalyst to make NO2 followed by a particulate filter which is regenerated by the NO2 -is the CRT® system, a Johnson Matthey invention.

Using the CRT® effect to regenerate a filter has two great advantages: it is continuous (helping to maintain a low and even exhaust back pressure) and it requires no additional energy. Particulate matter collected in a filter can be oxidised very quickly using the oxygen (O2) in the exhaust gases. This reaction requires higher temperatures (>550°C) than are reliably found in most diesel engine applications, so some mechanism is required to raise the temperature of the exhaust periodically.

An oxidation catalyst placed before the filter is a very efficient way of doing this. When the filter needs regenerating, the hydrocarbon content of the exhaust stream is increased. The catalyst burns the hydrocarbon, producing the heat required. The catalyst can also be designed to make NO2 during normal operation to provide additional regeneration.

The hydrocarbon enrichment can be achieved in-cylinder using electronic fuel systems, or by injecting fuel directly into the exhaust system.

An SCR system reduces NOx to nitrogen (N2) and is capable of more than 90% conversion, depending on conditions. To achieve this reaction in normal, lean (i.e. high oxygen, low hydrocarbon) diesel exhaust, a chemical reductant is needed. In most systems, ammonia (NH3) is used, and this is delivered as an aqueous solution of urea ("AdBlue" in Europe). The urea decomposes in the exhaust stream to form ammonia, which reacts with the NOx to make nitrogen and water.

An SCR system requires a means of storing the urea and injecting the amount required to reduce the NOx emitted by the engine. Careful control is needed for transient cycles. The SCR catalysts usually have an oxidation catalyst , often called a slip catalyst, at the end as a guard to ensure that no ammonia is emitted.

Converting NOx from the engine to nitrogen requires a chemical reduction. Diesel exhaust is an oxidising, not a reducing, environment. There are two main strategies to achieve NOx reduction. NOx adsorber catalysts or NACs, also known as lean NOx traps (LNTs), operate in two modes. In normal diesel exhaust conditions, they adsorb the NOx from the exhaust gas, storing it chemically. In "rich" exhaust conditions, with little or no oxygen (O2) they release the stored NOx and react it with components of the rich exhaust gas -carbon monoxide (CO), hydrocarbons (HC) and hydrogen (H2) -to make nitrogen.

catalyic reduction

Like any trap, chemical or physical, NACs have a finite capacity so the engine or some auxiliary system must create rich exhaust conditions at intervals to regenerate the catalyst and this reduces the stored NOx to nitrogen. NACs also store sulphur and this reduces their capacity to trap NOx, so periodic "desulphations", requiring increased exhaust temperatures, are necessary. Test work starts in the laboratory using bottled gases to simulate exhaust gas with precise control of gas mix and temperature. Testing then moves to engines for performance and durability testing.

Catalyst

Technology

Johnson Matthey has its own test cells dedicated to diesel engine testing in Europe, the USA and Japan, many capable of transient operation. These cells are used to test and prove catalysts; for catalyst ageing and for collaborative development programmes using customer engines. Our facility in Detroit also performs contract testing (see www.jmtesting.com).

JM develops proprietary catalyst-specific accelerated ageing procedures for its own and customer testing. These are based on catalyst deactivation mechanisms and are validated against catalyst aged in real world operation on vehicles and machines.

Testing and ageing JM works to develop new aftertreatment systems and makes them available for new engines and retrofitting. This work covers not just the catalysts themselves, but the operating principles of the whole system. An example of this is the CRT® (Continuously Regenerating Trap) particulate filter system. Johnson Matthey developed its operating principle in the late 1980s. It has been used in the majority of retrofit particulate filter systems supplied around the world and has also been employed in many OEM applications.

JM's work in this area has moved beyond particulate filter systems to include systems for NOx and NO2 control and integrated 4-way systems, which simultaneously reduce emissions of PM and NOx as well as CO and HC.

and applications expertise

Systems technology

Johnson Matthey won the Royal Academy of Engineering MacRobert Award in 2000 for its CRT® technology, in recognition of an outstanding innovation of benefit to society Having worked on retrofit applications since the early 1990s, Johnson Matthey has direct, hands-on experience in the non-road market. This is based primarily on particulate filter systems for construction and materials handling machines, but we also have experience of other systems and of rail and agricultural applications. This long experience has given us an understanding of the particular challenges of non-road mobile machinery: the variable and sometimes extreme duty cycles, the physical demands placed on the exhaust system, the packaging constraints, heat rejection, performance monitoring and control.

Modelling JM builds computer models of catalyst performance, starting with the measured kinetics of individual reactions over real catalysts. These are built up into models capable of modelling fully transient operation. Models are important tools in JM's own catalyst development work but are also made available to customers for their own system development and calibration. This approach is particularly valuable in the non-road market, reducing the work required to check the performance of whole engine systems over different ratings and machine-specific duty cycles. Catalytic systems for PM or NOx control almost always contain more than one component. An SCR system will have an oxidation catalyst after the SCR catalyst and sometimes before. A particulate filter usually has an oxidation catalyst before the filter and a catalytic coating on the filter itself.

Systems to control PM and NOx are more than a simple combination of PM filter and NOx control system. It is best when the two are designed to work together: NO2 from a PM filter can help an SCR system downstream; NOx adsorber desulphation cycles can be used to regenerate particulate filters.

To assist packaging, the components themselves can be integrated, for example with SCR or NAC coatings on particulate filters.

Production

Johnson Matthey has ten TS16949certified manufacturing plants around the world, with two more under construction. JM's proprietary coating technology enables very precise control over the amount of washcoat and precious metal applied to each part, giving consistent high performance and efficient use of raw materials. It also allows different catalytic coatings to be applied to different areas of the substrate, further enhancing performance, efficiency and, in some cases, durability. It also enables more than one function to be built into a single component, reducing the complexity and cost of the finished system. JM is in full series production of diesel oxidation catalysts for active filter systems, catalysed particulate filters for light and heavy duty engines, SCR catalysts and NOx adsorbers.

Manufacturing technology

Most emission control catalysts for mobile applications contain precious metals. Johnson Matthey is an acknowledged authority on precious metal markets and sole marketing agent for Anglo Platinum Ltd, the world's largest producer of platinum group metals. JM is able to advise customers on the methods of sourcing, pricing and managing their precious metal requirements in the way that best suits their needs. For further information on the platinum group metals markets, visit www.platinum.matthey.com.

Johnson Matthey is also the world's largest refiner of secondary platinum group metals so, at the end of a machine's life, we are able to recover the precious metals from the exhaust catalysts and reuse them.

Johnson Matthey

Johnson Matthey is a speciality chemicals company focused on its core skills in catalysts, precious metals and fine chemicals.

The group's principal activities are the manufacture of autocatalysts and pollution control systems, catalysts and components for fuel cells, pharmaceutical compounds, process catalysts and fine chemicals, and the refining, fabrication and marketing of precious metals.