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Modern emission control catalysts utilize monolithic flow-through supports coated with high surface area inorganic oxides and, in most cases, precious metals. After the method of its application, the refractory oxide layer is called the washcoat. It is a porous, high surface area layer bonded to the surface of the support. The main function of the washcoat is to provide high surface area needed for the dispersion of catalytic metals. Additionally, the washcoat can physically separate and prevent undesired reactions between components of a complex catalytic system. The exact role of the washcoat—clearly very important for many aspects of the catalyst activity and durability—is not always understood or explained.
Washcoat materials include inorganic base metal oxides such as Al2O3 (aluminum oxide or alumina), SiO2, TiO2, CeO2, ZrO2, V2O5, La2O3 and zeolites. Some of them are used as catalyst carriers, others are added to the washcoat as promoters or stabilizers, still others exhibit catalytic activity of their own. Good washcoat materials are characterized by high specific surface area and thermal stability. The specific surface area is typically determined by nitrogen adsorption measurement technique in conjunction with mathematical modeling known as the BET (Brunauer, Emmet, and Teller) method. Thermal stability is evaluated by exposing samples of given material to high temperatures in a controlled atmosphere, usually in the presence of oxygen and water vapor. The loss of BET surface area, which is re-measured at different time intervals during the test, indicates the degree of thermal deterioration of the tested material. Standardized BET procedures exist, such as the German Industrial Standard DIN 66131.
The washcoat is applied to the catalyst support from a water based slurry. The wet washcoated parts are then dried and calcined at high temperatures. An electron microscope image of the washcoat surface of a commercial diesel oxidation catalyst is shown in Figure 1.
Figure 1. SEM Photograph of Catalyst Washcoat
Commercial diesel oxidation catalyst (mid-1990s)
Precious metal catalyst(s) may be either present in the washcoat slurry, or else are applied in a second step called impregnation. During the impregnation, the washcoated monolith is exposed to a water-based solution containing catalytic precursors. The supported catalyst is then dried and calcined to its final form. During the calcination, the catalyst precursors decompose to form the final catalyst, usually a metal or a metal oxide. The most common catalysts are platinum group metals (PGM) such as platinum itself (Pt), palladium (Pd) and rhodium (Rh).
Many base metal oxides, such as V2O5 or CeO2, also exhibit catalytic activity. Some of the base metal catalysts (e.g., Ce) are applied through the washcoating process, while others (e.g., V) may be impregnated from aqueous solutions of their precursors.