Feature Article

Advanced Three-Way Catalyst Technology

Dr. Joseph Kubsh, MECA

One of the most important technology bases that have developed within the automotive industry in the past fifty years is the development, introduction, and continued evolution of automotive emission control technology. The centerpiece of this technology base is the three-way catalyst used on gasoline, spark-ignited vehicles in all major world markets today. The name three-way catalyst was applied to catalytic controls that were capable of reducing all three criteria pollutants: carbon monoxide (CO), oxides of nitrogen (NOx), and volative organic compounds (VOCs) within a narrow range of inlet exhaust gas compositions that corresponded to approximately the stoichiometric air/fuel ratio of the engine. Today, more than 90% of the new automobiles sold around the world are equipped with catalytic converters, adding to the more than 600 million vehicles worldwide have been equipped with catalysts since their first introduction in the U.S. in 1975.

Automotive catalytic emission controls were pioneered in the United States in response to public health concerns associated with elevated ambient ozone levels stemming, in part, from automotive tailpipe emissions of hydrocarbons and oxides of nitrogen. These public health concerns were translated into emission control regulatory programs by both the United States federal government and the state of California. On the federal level, the Clean Air Act Amendments of 1970 mandated significant reductions in automobile tailpipe emissions of CO, NOx , and volatile organic compounds starting in 1975. These federal standards led to the introduction of oxidation catalysts on automobiles starting with the 1975 model year to control CO and VOCs, and the use of three-way catalysts to control CO, NOx, and VOC tailpipe emissions starting in 1981. California, with severe smog problems in its large metropolitan areas, was provided with its own authority to set automobile emission standards and has typically led the U.S. federal government and the world with the tightest standards requiring the best available emission control technology for automobiles.

Since the mid-1970s, U.S. federal and California light-duty motor vehicle tailpipe emission regulations have been continually pushed to lower levels in response to air quality concerns. At the forefront of these new waves of regulatory programs aimed at significantly reducing emissions from light-duty vehicles are the U.S. Environmental Protection Agency?s (EPA) Tier 2 and the California Air Resource Board?s (ARB) Low Emission Vehicle II (LEV II) programs. California acted first, adopting their LEV II program in late 1998, followed by EPA finalizing the Tier 2 regulations in December 1999. Both the ARB LEV II regulations and the EPA Tier 2 regulations began their phase-in with the 2004 model year. In a parallel or slightly delayed timeframe relative to these U.S. initiatives, Europe (Euro 3 and Euro 4 regulations), Japan (Japan Low Emission Vehicle regulations), and Korea (Korea Low Emission Vehicle regulations) also established new, more severe light-duty emission regulations during the 1990s. Emission regulations for new vehicles based on the use of three-way catalyst technologies are now being implemented in almost every world market including large emerging markets in India and China. The introduction of catalytic converters in the U.S. and other world markets also required these countries to introduce unleaded gasoline since vehicle operation on leaded fuel results in dramatic deactivation of the active catalytic materials present in three-way catalytic converters.

All of these current U.S. light-duty vehicle emission programs require significant reductions in hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx) emissions relative to vehicle emission requirements associated with the regulations that precede each of these new emission programs (e.g., EPA?s Tier 1 or California LEV I regulations). The LEV II regulations, for example, maintain tight hydrocarbon emission levels established in the LEV I program (adopted in 1990; implementation began with the 1994 model year), but significantly reduce NOx emission requirements compared to LEV I requirements. The Tier 2 program draws from both the California LEV I and LEV II programs in significantly tightening both HC and NOx tailpipe emissions relative to Tier 1 regulations that were first implemented with the 1994 model year. An important input into each of these regulatory processes was the ability of emission control technologies to meet these increasingly tighter tailpipe emission standards in a cost effective manner. MECA provided important technical inputs into the EPA Tier 2 and California LEV II rulemaking process by completing a successful test program in the late 1990s that demonstrated that advanced three-way catalysts were capable of significantly reducing exhaust emissions from four different Tier 1-compliant passenger cars and trucks. Details of this test program were reported in a Society of Automotive Engineers technical paper published in 1999 (SAE paper no. 1999-01-0774). Compared to pre-controlled vehicles sold in the U.S. prior to 1975, today?s Tier 2 and LEV II cars and trucks are meeting emission standards that require reductions of up to 98+% with respect to VOCs, 96% for CO and 98% for NOx.

Some additional details concerning the EPA Tier 2 and California LEV 2 emission regulations are provided here since each program includes the tightest light-duty emission certification categories currently in place in the world today. Full useful life tailpipe emission standards for the fully phased-in U.S. EPA Tier 2 and California LEV II programs are summarized in Tables 1 and 2, respectively. Each of these programs provides auto manufacturers with several different certification categories to choose from for their light-duty vehicle fleet. Tailpipe emissions are measured on a chassis dynamometer using the U.S. Federal Test Procedure (FTP, a vehicle speed vs. time driving cycle). The concept of multiple certification categories was first introduced with California?s LEV I program with Transitional Low Emission Vehicle (TLEV), Low Emission Vehicle (LEV), and Ultra-Low Emission Vehicle (ULEV) certification options that varied by vehicle weight class (e.g., passenger car and light-duty truck weight classes). The EPA Tier 1 light-duty emission regulations also had weight class specific emission regulations but only one set of emission standards for each gasoline vehicle weight class. The Tier 2/LEV II programs have several common features that are also significant changes from either Tier 1 or LEV I requirements: 1) fuel neutral requirements (emission standards are equivalent for gasoline and diesel-fueled vehicles); 2) 120,000 mile full useful life durability; and 3) a single set of standards that does not vary with light-duty vehicle weight class (up to 8500 lb. gross vehicle weight for all passenger cars and light-duty trucks; up to 10,000 lb. for medium-duty passenger vehicles [MDPVs]). Treating passenger cars and light-duty trucks on an equivalent emissions basis is an important focus for both the Tier 2 and LEV II programs. Both of these programs place a premium on cold-start emission performance and high emission system efficiencies with respect to NOx emissions.

Table 1. California LEV II 120,000 mile tailpipe emission limits
 
Certification Level NMOG (g/mi) CO (g/mi) NOx (g/mi)
LEV-2 0.090 4.2 0.07
LEV-2/LDT2* 0.090 4.2 0.10
ULEV-2 0.055 2.1 0.07
SULEV 0.010 1.0 0.02
* the LEV-2/LDT2 certification category is limited to no more than 4% of the LDT2 light-duty truck production for a given manufacturer

 

Table 2. U.S. EPA Tier 2 120,000 mile tailpipe emission limits
 
Certification Level NMOG (g/mi) CO (g/mi) NOx (g/mi)
Bin 1 0.0 0.0 0.0
Bin 2 0.010 2.1 0.02
Bin 3 0.055 2.1 0.03
Bin 4 0.070 2.1 0.04
Bin 5 0.090 4.2 0.07
Bin 6 0.090 4.2 0.10
Bin 7 0.090 4.2 0.15
Bin 8 0.125 4.2 0.20

 

Reaching the tailpipe emission levels associated with the Tier 2 and LEV II regulations summarized in Tables 1 and 2 requires a concerted systems approach that includes the use of advanced spark-ignited engines, advanced engine control strategies, clean fuels, clean lubricants, and advanced emission control technologies. Both ARB and EPA have included the clean fuel component in their LEV II and Tier 2 regulatory programs with respect to gasoline sulfur levels. ARB established a 30 ppm sulfur average for gasoline as a part of their California Phase II reformulated gasoline requirements. This sulfur level was further reduced to an average of 15 ppm sulfur starting in 2004 with the introduction of California Phase III reformulated gasoline regulations. Similarly, the EPA included gasoline sulfur level regulations as an integral part of their Tier 2 regulatory package with the phase-in of 30 ppm average S levels started in 2005. Lubricant constituents such as phosphorus and inorganic elements such as Zn and Ca have also been shown to act as catalyst poisons or catalyst masking agents driving lubricant producers to optimize lubricant formulations to insure adequate engine lubrication characteristics with minimal impacts on catalyst performance and driving engine designers to minimize engine oil consumption characteristics of advanced engines. Clean fuels and clean lubricants are a necessary pre-requisite for maintaining the high performance levels of the advanced engine and emission systems required for Tier 2/LEV II compliance.

The three-way catalytic converter (TWC) has been the primary emission control technology on light-duty gasoline vehicles since the early 1980s. The use of TWCs, in conjunction with an oxygen sensor-based closed-loop fuel delivery system, allows for simultaneous conversion of the three criteria pollutants, hydrocarbons, CO, and NOx, produced during the combustion process of an internal combustion, spark ignited engine. Figures 1 and 2 depict a cut-away drawing and a cut-away photo of typical three-way catalytic converters, one using a ceramic substrate and one using a metallic substrate. The active catalytic materials are present as a thin coating of precious metals (e.g., Pt, Pd, Rh), and oxide-based inorganic promoters and support materials on the internal walls of the honeycomb substrate. The substrate typically provides a large number of parallel flow channels to allow for sufficient contacting area between the exhaust gas and the active catalytic materials without creating excess pressure losses.

Figure 1. Three-way catalytic converter with ceramic substrates
Figure 1. Three-way catalytic converter with ceramic substrates

Catalytic materials are typically applied by contacting the substrate with a water-based slurry containing the active inorganic catalyst materials. The coated substrate is contained within an outer metal-based shell that facilitates connection of the converter to the vehicle?s exhaust system through flanges or welds. The honeycomb-based substrates are typically either ceramic or metal foil-based. Cordierite, a magnesium alumino-silicate compound, is the preferred ceramic substrate material due to its low coefficient of thermal expansion, good mechanical strength characteristics, and good coating adhesion properties. The ceramic substrate is formed as a single body using an extrusion process followed by high temperature firing. Metal-foil based substrates are made from thin ferritic-based specialty stainless steel foils brazed together to form the parallel flow passages. The ferritic foil alloy provides good oxidation resistance in the exhaust environment, good mechanical strength, and an oxidized surface that promotes good adhesion of the catalytic coating to the foil. In the case of ceramic substrates, a special oxide fiber-based mounting material (typically referred to as a "mat") is used between the substrate and the metal outer shell to hold the substrate in place, provide thermal insulation, and cushion the ceramic body against the shell. The outer metal shell or mantle is an integral part of the metal substrate production scheme and no additional mounting materials are generally required. As shown in Figures 1 and 2, in some cases the converter housing or "can" can be surrounded by a second metal shell with an annular gap between these two metal shells. This type of arrangement provides additional heat insulation to the converter. The annular region between the two shells may be left as an air gap or filled with an insulating material such as an inorganic fiber-based material.

Figure 2. Three-way catalytic converter with metallic substrates
(photo courtesy of Emitec GmbH)
Figure 2.  Three-way catalytic converter with metallic substrates

Although the primary components and function of a three-way catalytic converter has remained relatively constant during its more than twenty years of use on light-duty gasoline vehicles, each of the primary converter components (catalytic coating, substrate, mounting materials) has gone through a continuous evolution and redesign process aimed at improving the overall performance of the converter while maintaining a competitive cost effectiveness of the complete assembly. The performance-based catalytic converter re-engineering effort has had three main focuses: (1) wide application of close-coupled converters mounted near the exhaust manifold of engines for improved performance following a cold engine start; (2) the development of thin-wall, high cell density substrates for improved contacting efficiency between the exhaust gas and the active catalyst, and lowering the thermal mass of the converter; and (3) the design of advanced, high performance TWCs for both close-coupled and underfloor converter applications that emphasize excellent thermal durability and efficient use of the precious metals platinum, palladium, and rhodium.

These advanced TWC formulations often utilize multi-layer architectures and/or axial placement of different catalyst materials along the length of the substrate that allow for the optimization of specific catalytic functions (e.g., improved light-off characteristics or improved overall efficiency for reducing hydrocarbons, CO, and/or NOx). These advanced catalysts also utilize a variety of advanced materials (in addition to the active precious metals) that promote the oxidation and reduction reactions associated with three-way catalysts and allow these catalysts to maintain activity in severe thermal exhaust environments. Catalyst substrate channel or cell densities as high as 1200 cells/in2 have been used on production catalytic converters with 600 cells/in2 substrates used in many late model vehicle applications. A similar re-engineering effort has occurred with other exhaust system components such as exhaust manifolds and exhaust pipes that complement improvements in catalytic converter technology. The focus of these manifold and other exhaust component improvements has been exhaust system thermal management and heat conservation through the use of low thermal mass, air gap insulated components or other heat insulation strategies.

MECA recently completed a second light-duty gasoline vehicle test program in 2006 that demonstrated that advanced three-way catalytic converter systems allow even the heaviest light-duty gasoline trucks (e.g. SUVs and larger pick-up trucks) to achieve very low exhaust emissions of hydrocarbons and NOx. Results of this test program are available in SAE paper no. 2007-01-1261. Additional information on advanced three-way catalytic converters, including a summary of advanced exhaust emission systems used in MECA?s two light-duty vehicle test programs and the emission results from these MECA test programs, is available in MECA?s report "Tier 2/LEV II Emission Control Technologies for Light-Duty Gasoline Vehicles" found on the MECA website: www.meca.org (under the "Resources" tab found on the home page).