Page 26 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey
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8 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters
Materials used in catalytic converters
A catalytic converter for an internal combustion engine comprises in general three main
elements: the ceramic or metallic support, the alumina washcoat (usually stabilized gamma
alumina y-AI203) and the noble metal (fig. 4). Single or double washcoats may be applied.
Double coats are used to enhance specific reactions and improve durability by separating
components of the catalyst [3]. The coated substrate is air dried and calcined to about 450-500
"C to ensure good adhesion. Washcoat thickness, which ranges from 20-60 pm, is engineered
for minimal diffusional resistance, so gases readily reach the catalytically active sites. It is also
configured for maximum resistance to contaminants, many of which deposit in the outer 10 to
15 pm of the washcoat. The washcoat is 5-1 5% of the converter weight and has a surface area
of I 00-200 m2/g.
The noble metals are impregnated into the highly porous alumina washcoat. Active catalytic
converters contain about 0.1 to 0.15% noble metals. The most commonly catalytic materials
(noble metals) used in the automotive industry are Platinum (Pt), Rhodium (Rh), Palladium
(Pd) and Ruthenium (Ru), which become efficient for temperatures over 140 "C. The quantity
of noble metals contained in a catalytic converter is 2-3 grams.
Recent trends in improvement of the catalyst relevant properties of the fbels available in the
U.S.A and Europe, along with the wide application of advanced engine management systems
with a capability for much tighter aidfuel control close to stoichiometry (aidfbel ratio=14.7) ,
lead to the use of tri-metal catalysts (Pt, Rh, Pd).
Other materials used are common metals like Nickel (Ni), Chromium (Cr) and Copper (Cu).
The disadvantage of these metals is that they become efficient at temperatures over 400 "C
([8l).
In oxidation catalytic converters for diesel engines, a silica washcoat is preferred over alumina
in order to minimize sulfate production. Also, Palladium (Pd) is preferred over Platinum (Pt)
for the same reasons.
A catalytic converter has an oxi~i~i?i~~4iiclioii to transform chemically the carbon monoxide
(CO) in carbon dioxide (COz) and the hydrocarbons (HC) to carbon dioxide (C02) and water
(HZO) according to the chemical reactions:
2 co + 0, + 2 (70,
A catalytic converter can also have a reducing jiiriclion by accelerating the reaction of
hydrogen (Hz) and carbon monoxide (CO) with nitrogen oxides (NO,) to produce nitrogen
(N2) and carbon dioxide (COZ) according to the chemical reaction:
