On-Board Diagnostics for emissions-related components require the monitoring of the catalytic converter performance. Currently, the dual Exhaust Gas Oxygen (EGO) sensor method is the only proven method for monitoring the catalyst performance for hydrocarbons (HC). The premise for using the dual oxygen sensor method is that a catalyst with good oxygen storage capacity (OSC) will perform better than a catalyst with lower OSC. A statistical relationship has been developed to correlate HC performance with changes in OSC. The current algorithms are susceptible to false illumination of the Malfunction Indication Light (MIL) due to: 1. The accuracy with which the diagnostic algorithm can predict a catalyst malfunction condition, and 2. The precision with which the algorithm can consistently predict a malfunction. A new algorithm has been developed that provides a significant improvement in correlation between the EGO sensor signals and hydrocarbon emissions.
Three-way catalysts (TWC) to remove the HC, CO, and NOx pollutants from the exhaust of gasoline powered vehicles employ rhodium in combination with platinum and palladium. Of these precious metals, rhodium is by far the most expensive. Since it is so heavily used for its NOx reduction capabilities, the amount per vehicle approaches and sometimes exceeds the naturally occurring mine ratio. A program was conducted to determine the feasibility of a non-rhodium TWC catalyst. It showed that Pt and Pd in conjunction with other washcoat support materials exhibited relatively good TWC characteristics compared to a Pt/Rh catalyst after engine dynamometer aging. In FTP evaluations this new REDOX type catalyst gave comparable HC and CO efficiency and 85% of the NOx efficiency of a Pt/Rh-containing catalyst. Presently the operating window is being defined but comparisons to conventional Pt/Pd and Pt/Rh catalysts have been made under a number of conditions.
A new technique has been developed to assess the deterioration of hydrocarbon lightoff during FTP Phase 1. This method maps the dynamic real time data onto the hydrocarbon conversion/gas phase temperature plane to show the instantaneous hydrocarbon activity as a function of the exhaust temperature. Clearly, the map exhibits an apparent hysteresis bifurcation. The bifurcation is thought to be predominantly related to catalyst surface temperature. The hysteresis envelope of the map expands with aging severity. Therefore, the map can be used as a measure of catalyst deactivation.
The technical issues related to NOx abatement for diesel applications are summarized. Data on improved catalysts and a novel approach which involves temporarily trapping of NOx before reduction are presented. New high temperature lean NOx materials have been identified which have better hydrothermal stability than the state of the art Cu/ZSM-5. One of these materials, Catalyst A, was shown to reduce the NOx emitted from a 2.5 L diesel engine at temperatures ≥ 350°C using injected diesel fuel as a reductant. Catalyst A also showed reasonably good durability after aging for 500 h at ca. 500°C on a 14 L diesel truck engine. Pt/Al2O3, a low temperature lean NOx reduction catalyst (200-300°C), demonstrated fairly good performance after 125 h of aging on a 4 L diesel truck engine, however sulfate make and N2O formation are high on this material. New low temperature NOx traps show promise for transient removal of NOx below 200-400°C.
The plasma assisted catalytic reactor (PACR) approach to lean NOx abatement is a two step process. The non-thermal plasma oxidizes the engine out NO to NO2, which is then reduced to N2 over a catalyst using a hydrocarbon reductant. Whereas it was once believed that the plasma itself directly reduces NOx to N2, it has been shown that the plasma's principle function is to oxidize NO to NO2. This is accomplished without oxidizing SO2 to SO3, resulting in lower sulfate particulate when compared to standard lean NOx catalysis using platinum or reducible oxide catalysts. We have performed reactor studies comparing the relative reducibility of NO2 and NO in a synthetic diesel exhaust using diesel fuel as the hydrocarbon reductant, with attention to time-on stream behavior and determination of NOx reversibly adsorbed on the catalyst. We find that at 200°C, 50% of the NO2 disappearance over Na-ZSM5 is attributable to reversible adsorption on the catalyst.
The desire for improved fuel economy, and lower emissions of green house gases, such as CO2, is projected to increase the demand for diesel and lean-burn gasoline engines throughout the world. Several commercial diesel oxidation catalysts (DOCs) were developed in the last 3-4 years to reduce hydrocarbon, CO, and particulates emitted from the exhaust of diesel passenger cars and trucks. To meet future U.S. and European NOx standards, it is essential to develop catalyst technology that will allow NOx reduction in addition to the other three pollutants. Two materials that attracted great attention as lean NOx catalysts are the Cu/ZSM-5 and Pt based. Cu containing ZSM-5 are active for lean-NOx reduction at temperatures above 350°C, provided sufficient hydrocarbons are present as reductants.
Close coupled catalysts represent a solution being pursued by automotive engineers to meet stringent LEV and ULEV emission standards. Close coupled systems provide fast light-off by utilizing the energy in the exhaust gas rather than energy supplied by an auxiliary source such as an electrically heated catalyst or a burner in the exhaust. Previous close coupled catalyst designs were limited by the temperature capability of the catalyst coatings. A successful close coupled catalyst technology has been developed 'that is resistant to higher temperature deactivation. This technology is able to function well at low temperature during the vehicle cold start when light-off is critical. The close coupled catalyst technology has approached ULEV emission levels after aging at 1050°C for 24 hours. This study will present experimental results for a close coupled catalyst including the selection of catalyst volume, cross sectional area and combination of catalyst technologies.
The revisions in the United States Clean Air Act of 1990 and recent regulatory actions taken by the California Air Resources Board and European Economic Community require the development of automobiles with much lower tailpipe emissions. A significant portion of the total pollutants emitted to the atmosphere by motor vehicles occurs immediately following the startup of the engine when the engine block and exhaust manifold are cold, and the catalytic converter has not yet reached high conversion efficiencies. An effective, energy efficient strategy for dealing with cold start hydrocarbon using carbon-free hydrocarbon traps and heat exchange related TWC catalyst beds has been successfully tested on a wide variety of current model vehicles. In each case U.S. FTP 75 total hydrocarbon emissions were reduced between 45 - 75% versus the vehicle's stock exhaust system.
Motorcycle exhaust emission standards throughout the world are becoming more stringent. Emission control systems utilizing the catalytic converter are already in production in Taiwan for 2-stroke engine motorcycles. Catalysts designed for 2-stroke engines encounter a more severe exhaust environment than do those designed for 4-stroke engines. The two aspects of increased severity are the higher temperatures and higher stresses due to engine vibrations. Precious metal catalysts have been designed to operate in the thermal environment of 2-stroke engines and such catalysts have been successfully applied to both metal and ceramic substrates. However, until now, only the metal substrate catalysts have been utilized in motorcycle application. Ceramic based catalysts have not been considered because the mounting material that holds the catalyst substrate in place did not have enough durability to withstand the thermal/vibrational forces encountered in 2-stroke engine exhaust.
A common practice to improve vehicle fuel economy is to employ a fuel cut-off strategy on deceleration. This practice exposes the TWC exhaust catalyst to varying concentrations of oxygen depending on the vehicle control strategy. Since it is well known that exposure to oxygen at high temperature is deleterious to long term catalyst durability, it is important to understand the impact of oxygen concentration and temperature on catalyst performance. Simulated fuel cut agings at about 1%, 3%, and 9% oxygen concentration were compared to a full fuel cut aging (21% oxygen concentration). It was found that even small concentrations of oxygen at high temperature damaged catalyst performance. Deactivation increased with increasing oxygen concentration and increasing temperature.
The challenge to substitute less expensive Pd for Pt in TWC catalysts is complicated by the fact that Pd is susceptible to fuel poisons. Laboratory studies indicate that while the precious metal support plays an important role in the CO-NOx reaction, sulfur poisoning dominates. In a reaction to probe selectively the Rh metal function within a washcoat, it was found that small levels of Pd can have a deleterious impact on the performance of the Rh metal. Engine aging studies corroborate the work of recent publications showing that conventional Pd/Rh TWC catalysts exhibit poorer performance than standard Pt/Rh catalysts. The more stringent TLEV and LEV emission standards require more robust catalysts than are currently available. To obtain faster light-off in close coupled positions, the catalyst will experience higher exhaust temperatures. A Pd/Rh catalyst, with an engineered washcoat to minimize alloying, can exceed the performance of a current Pt/Rh commercial catalyst.
Close-coupled catalysts are being actively pursued by automotive engineers in order to meet stringent LEV/ULEV emission standards. However, future applications of close coupled catalyst will be exposed to 50 to 100°C higher operating temperatures with elimination of fuel enrichment to cool the catalyst. A successful close coupled catalyst technology must then be resistant to even higher temperature deactivation and yet continue to function at low temperature during the vehicle cold start. A close coupled catalyst technology is formulated through advanced catalyst design to meet LEV and ULEV emission standards after high temperature aging at 1050°C. This paper will show the inherent stability of the close coupled catalyst for both light-off temperature and steady state performance for aging temperatures up to 1100°C.
The Federal Test Procedure (FTP) test contains an initial period, prior to the catalyst becoming fully activated, during which hydrocarbons escape the vehicle. These hydrocarbons constitute 60-80% of the total emitted over the entire FTP test. To meet future emission levels mandated by the California Air Resources Board, alternate technologies must be created that deal effectively with these cold start hydrocarbons. This paper describes an adsorbent bed/catalyst system that can trap approximately 70% of the available nonmethane hydrocarbons over the first two minutes of the FTP test. Importantly, the trap does not require bypass valves because of a unique heat exchange approach to catalytically consuming the trapped hydrocarbons, and because the trapping materials are unaffected by engine exhaust temperatures below 800°C. Experiments with a prototype system demonstrate that LEV emissions are possible.
A novel process for coating and assembling metal converters utilizing precoated foil as building blocks has been developed which yields a converter capable of withstanding typical industry specified hot vibration protocols. The precoating process used here results in uniform catalyst coating distributions with coating adhesion to the foil on a par with the coatings' adhesion to ceramic substrates. FTP and MVEG vehicle emission performance of this unique precoated metal converter design versus a more conventional dip-coated metal monolith (parts with the same volume, cell density, and tri-metal catalyst coating), exhibited improved catalyst emission breakthrough efficiencies with respect to HC, CO, and NOx after two different engine-aging protocols. These advantages were observed on three different test vehicles across most phases of these driving cycles.
Because of their relatively low particulate make, lean burn natural gas vehicles (NGV's) are a viable approach to meeting the ULEV particulate standards in urban environments where NGV's are substituted for diesel powered buses and other fleet vehicles. Our experience with oxidation catalyst technology for natural gas vehicle emissions abatement has been consistent: that palladium based catalysts maintain excellent NMHC activity and particulate reduction, but methane activity, while initially very high, decreases within the first 50 hours of operation. This paper will show that sulfur oxides at sub-ppm concentrations diminish catalyst methane activity, and that inorganic ash components from the lubricating oil (P, Zn, Ca) do not significantly contribute to the initial catalyst deactivation. Using laboratory simulations, we explore systems approaches to increasing catalyst life.
Catalytic reduction of NOx from heavy duty diesel engines via addition of reductant to the exhaust is accompanied by a substantial exotherm in the catalyst bed which does not occur, for example, in a diesel oxidation catalyst. Engine tests show that thermal management in the aftertreatment system is required for optimum reductant use and maximum NOx conversion by the low-temperature (200-300°C) catalyst NSP-5, but of less importance with the high temperature (> 350°C) Catalyst A. Understanding thermal effects is also important for reconciling test results in the near-adiabatic environment of a full-sized catalyst on an engine with the near-isothermal one of a test piece in a laboratory reactor. The effects of reductant type and concentration on NOx conversion on NSP-5 were shown to result in part from non-steady state behavior of the catalyst during steady state engine operation.
Classical approaches to pollution control have been to develop benign non-polluting processes or to abate emissions at the tailpipe or stack before emitting to the atmosphere. A new technology called PremAir™ Catalyst Systems takes a different approach and reduces the existing ground level ozone. For the automotive application, the new systems involve placing a catalytic coating on a car's radiator and air conditioner condenser. As air which contains ozone passes over the radiator and condenser, the catalyst converts the ozone into oxygen. Tests conducted on a 1991 full size passenger vehicle showed that the PremAir™ Catalyst System could convert up to 90% of the ozone passing over the radiator during a driving cycle lasting 5840 kilometers (3650 miles). The effect of ozone concentration and flow rate were determined as well as the ozone destruction rate over the coated radiator. During the 5840 kilometers of driving, the catalyst exhibited steady ozone conversion.
Base metal oxide diesel oxidation catalyst technology having low sulfate making tendencies was evaluated using the ECE R-49 Test procedure on medium and heavy duty diesel engines and found to achieve substantial reduction of particulate, gas phase HC and CO emissions. Although the engines met the current European standards, further reduction in these emissions for vehicles operated in congested urban areas, such as buses, would have a positive impact on general air quality. A study of varying fuel sulfur levels (110-770 ppm S) showed that the catalyst was effective for control of sulfate-make such that overall particulate removal in the test was not compromised. However, it was found that lower fuel sulfur levels (< 550 ppm S) gave the best results for the ECE R-49 test which places emphasis on test modes yielding the highest exhaust temperatures.
Traditional approaches to pollution control have been to develop benign, non-polluting processes or to abate emissions at the tailpipe or stack before release to the atmosphere. A new technology called PremAir® Catalyst Systems1 takes a different approach and directly reduces ambient, ground level ozone. For mobile applications, the new system involves coating a heat exchange device in a vehicle, such as the radiator or air conditioning condenser. The catalyst converts ozone to oxygen as ozone-containing ambient air passes over the coated surface of the radiator. The technology is relatively simple and provides a positive benefit to the environment while being totally passive to the end user application. Volvo Car Corporation was the first automobile manufacturer to voluntarily introduce the technology on their S80 luxury sedan. Nissan Motor Corporation is also using the technology on their new Sentra CA (Clean Air) certified PZEV vehicle for California.