New catalytic converter technology
Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2014
Micro-structured ceramic substrates with unique structure display promise for use in automotive catalysis applications.
KEY CONCEPTS
•
The standard catalytic converter designed in the 1940s has performance limitations.
•
A new catalytic converter technology developed from micro-structured ceramic substrates displays four main advantages, including a reduction in noble metals used.
•
The larger pore size and reduced requirement for a high SSA washcoat leads to the feeling that the impact of ZDDPs should be reduced.
AMONG THE MAIN CONCERNS with current automotive lubricants is finding ways to minimize their incompatibility with catalytic converters. It has been shown that the current three-way catalytic converter based on noble metals such as palladium, platinum and rhodium can be poisoned by lubricant additives such as zinc dialkyldithiophosphates (ZDDPs) via mechanisms that are not completely understood.
Research is underway to better understand how the noble metals used in catalytic converters act as catalysts. In a previous TLT article, a rhodium catalyst intermediate that is possibly involved in catalysis was identified (
1). The intermediate was characterized as a dirhodium-based species that was isolated from a chemical reaction between rhodium carboxylate catalysts and diazo esters.
This dirhodium species appears to be the key to the reactivity of the intermediate. One of the researchers indicated that a similar dirhodium intermediate may be produced on the surface of a catalytic converter.
The standard catalytic converter was designed in the 1940s and has not been changed much in composition over the past 70 years. Benjamin Kingsbury, research associate in the department of chemical engineering at Imperial College in London, UK, says, “A conventional catalytic converter consists of a ceramic substrate onto which a washcoat is deposited. The principal purpose of the washcoat is to provide a large specific surface area (SSA) to disperse the precious metal catalysts.”
Conventional catalytic converters have performance limitations due to a number of factors that are outlined by Kingsbury. “The particle size and pore size of the washcoat must be small, which means that there is resistance to diffusion of the exhaust gas into the washcoat. Designing substrates with large geometric surface areas (GSA) will enable the washcoat to spread more thinly and reduce the diffusion limitation,” Kingsbury says. “However, there is a limit to the GSA that can be achieved with the conventional honeycomb monolith substrates, both because of the production method and because the only way to increase GSA is to make the channels smaller, which increases the back pressure that is exerted on the engine.”
Kingsbury explains that anything that is placed in the exhaust system to restrict the flow of exhaust gases means that the engine has to work harder to push these gases through the system. This leads to a reduction in engine power and an increase in fuel consumption and carbon dioxide emissions. He adds, “There is always a pressure drop along a channel through which gas is flowing due to friction of the gas with the channel walls. The monolithic substrate currently used consists of thousands of tiny channels, so there is a significant pressure drop along the length of the ceramic brick.”
The extreme diffusion limitations and the problem with using the washcoat under the high temperature conditions found in the engine means that a large quantity of noble metals is required. Kingsbury says, “A large quantity of noble metals is required to compensate for the diffusion limitation in the washcoat and because the SSA of the washcoat is reduced over the time, as it is sintered due to the high temperatures encountered in the engine.”
There is a need for a new catalytic converter technology that can overcome these limitations and improve the overall operating efficiency in the automobile. Such a technology has now been developed.
MICRO-STRUCTURED CERAMIC SUBSTRATES
Kingsbury and his associates have developed ceramic substrates with very unique and highly ordered structures at the micro-level that exhibit ideal properties for automotive catalysis applications. He says, “The ceramic substrates are fabricated through the use of an extrusion technique that incorporates phase inversion of a ceramic-polymer-solvent mixture. Extrusion of the mixture in water causes the polymer phase of the mixture to precipitate. By controlling this aspect, we can create unique micro-structures spontaneously, which are generated chemically rather than mechanically.”
Figure 1 shows an image comparing the micro-structured ceramic substrate and the conventional monolithic substrate. There are at least four advantages in using the micro-structured ceramic substrates in a catalytic converter. Kingsbury says, “We can reduce the quantity of noble metals that are required to meet the emissions regulations as the microstructure allows us to utilize the catalyst more effectively. By implementing our new substrate technology, we can reduce the negative impact of the washcoat on catalyst aging, further increasing the effectiveness and durability of the catalyst.”
Figure 1. As compared to the conventional monolithic, the newly developed micro-structured ceramic catalytic converter substrate fabricated through an extrusion technique has the potential to provide more effective performance and be less vulnerable to ZDDPs. (Courtesy of Imperial College)
Results from carbon monoxide oxidation tests comparing the microstructured ceramic substrate versus a conventional catalyst support this last conclusion. Kingsbury says, “We evaluated our technology versus a state-of-the-art commercial monolith substrate-washcoat combination by examining the conversion efficiency for carbon monoxide oxidation, which is representative of the exhaust gas mixture when studying the effects of substrate and washcoat structure on catalytic activity. Comparable results were achieved with an 80 percent reduction in noble metal catalyst loading and a 75-90 percent reduction in the extent of catalyst aging over a simulated 100,000-mile test.”
The third advantage is the combination of a high GSA with large channel diameters. Kingsbury says, “The GSA for a current monolith catalyst is around 2,800 m
2/m
3, 3,500 m
2/m
3 and 4,300 m
2/m
3 for substrates with 400, 600 and 900 cells per square inch (cpsi), respectively. Our substrate not only has a GSA of approximately 32,000 m
2/m
3 but also offers far larger channel diameters than conventional monoliths, resulting in a 40-70 percent reduction in pressure drop along the length of the substrate.”
This pressure reduction may lead to a 2-3 percent reduction in fuel consumption. Kingsbury adds, “It has been shown that a 35 percent reduction in back pressure in a 12 cylinder heavy-duty diesel engine can lead to a fuel savings of 0.5- 2.0 percent for typical driving cycles using current technology. We believe that we can achieve greater fuel savings by further reducing the pressure drop.”
The fourth advantage concerns reducing the size of the substrate and fabricating the catalytic converter with exotic geometries. Kingsbury says, “This is of great benefit to engine and exhaust system designers as space is really at a premium these days, especially when the catalytic converter must be fitted in the close coupled position, which is more and more common these days.”
Kingsbury believes that ZDDPs acts to poison the monolithic catalyst by having a significant impact in reducing SSA in the washcoat, leading to pore blockage. He adds, “With the larger pore size present in our technology and the reduced requirement for a high SSA washcoat, we feel that the impact of ZDDPs on catalytic converter performance should be reduced.”
Future work will involve further bench testing to demonstrate the performance benefits of the new catalyst technology followed by vehicle testing to demonstrate durability. The researchers have formed MicroTech Ceramics Limited, a start-up company to develop and market the prototype catalytic converter.
Additional information on the new catalytic converter technology can be obtained by contacting Kingsbury at
ben.kingsbury06@imperial.ac.uk.
REFERENCE
1.
Canter, N. (2013), “Identification of a Rhodium Catalyst Intermediate,” TLT,”
69 (12), pp. 14-15.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.