2.2. Structure of a three-way catalyst

Three-way catalysts have a honeycomb-like, monolithic structure as shown in Fig. 3. The monolith support is made either from metallic (stainless steel) or ceramic (cordierite) material. The structure of a metallic monolith is presented in Fig. 4. The monolith contains small channels, each about 1 mm in diameter (300-600 channels per square inch). The washcoat, which includes the active catalyst material, is impregnated on these channel walls. The washcoat consists of porous oxides, such as γ -Al₂O₃ and precious metals. The thickness of the washcoat layer is circa 20-60 µm and it has a large surface area of approximately 50-200 m²/g. Thus the diffusional resistance is minimal and gases easily reach the active surface sites, which allows close to 100% conversion with a high catalytic activity. Recently, the use of a layered washcoat has been applied in commercial three-way catalysts. Double-layered washcoats enhance specific reactions and improve the stability of the catalyst by separating the washcoat components. (Cybulski & Moulijn 1994, Heck & Farrauto 1996, Heck & Farrauto 2001, Lox & Engler 1997, Wan 1991)

The main compounds in the washcoat are base-metal oxides, such as aluminium, cerium and zirconium. In addition to these oxides, minor washcoat compounds are CaO and MgO as well as the oxides of rare earth elements, such as La₂O₃ (lanthana). These compounds are used as promoters or stabilizers (additives) in the washcoat to increase the catalytic activity or to stabilise the structure of the catalyst. Cerium is present in high quantities in the form of CeO₂ (circa 20 wt-% of washcoat Al₂O₃). Cerium has multiple functions. It is added to promote the low temperature water-gas shift reaction (WGSR), to store oxygen under lean (fuel deficient) conditions, to stabilize precious metal dispersion against thermal damage and to alter carbon monoxide oxidation kinetics (Lox & Engler 1997, Oh & Eickel 1988). Cerium is also known to minimize the thermally-induced sintering of an alumina washcoat (González-Velasco et al. 1994). The recent use of ceria-zirconia mixed oxides (Ce_XZr_1-XO₂) as catalytic washcoat materials has been promising due to the better thermal stability in closed-loop coupled applications. (Cuif et al. 1997, Fornasiero et al. 1996, Narula et al. 1996, Ozawa et al. 1993, Ozawa 1998)

The precious metals currently used in three-way catalyst applications are platinum, palladium and rhodium. These metals are well-known catalysts with high activities for controlling the exhaust emissions, and they are also preferred because they are less prone to poisoning compared to metal oxide catalysts, such as CuO (Shelef et al. 1978). The amount of the active metals in the catalyst is normally circa 1-2 wt-% of the washcoat. Precious metals are used to reduce the emissions of exhaust gases in the presence of reducing or oxidizing agents, such as hydrocarbons, CO and hydrogen, and oxygen and NO_X respectively. Rhodium has proven to be an efficient catalyst for NO_X reduction (Taylor 1993), whereas palladium and platinum metals are used in CO and hydrocarbon oxidation reactions (Armor 1992), in particular during cold start. Therefore, commercially-used three-way catalysts for gasoline engines are often a bimetallic combination of the precious metals, such as Pt-Rh or Pd-Rh. (Becker & Watson 1998, Koltsakis & Stamatelos 1997)