Platinum Group Elements in the Environment: Emissions and Exposure - Reviews of Environmental Contamination and Toxicology - page 115

The modern three-way catalysts consist of a ceramic monolith in the form of a
honeycomb. A washcoat is applied onto the monolith. The monolith contains small
channels, each about 1 mm in diameter (300-600 channels in −2 ). The washcoat,
which includes the active catalyst material, is impregnated on these channel walls.
The washcoat consists of porous oxides, such as Γ-Al 2 O 3 , and precious metals
(Mitzukami et al. 1991).
Today, there is a wide range of possible combinations and concentrations of Pt,
Pd, and Rh required by car manufacturers that are used to achieve different catalyst
performance features. In the various Pd-Rh, Pt-Rh, Pt-Pd, Pt-only, or Pt-Pd-Rh
catalyst combinations, the percentage of PGEs constitutes 0.1 wt% of the catalysis
support material. Other elements present in the catalyst, such as Ce (cerium),
Zr (zirconium), and rare earth metals, are used as components of the washcoat to
increase specified properties, such as catalyst PGE impregnation, oxygen storage
capability, and chemical inertness (Palacios et al. 2000a; Cuif et al. 1996, 1997).
The three-way catalytic converters are able to remove more than 90% of CO and
NO x s from exhaust gases, and 80% of unburned HCs (Onovwiona and Uqursal
2006). The automotive CO and NO x emission-control catalysts used in catalytic
converters are usually guaranteed for 40,000-80,000 km, depending on the quality
of the product (Shams and Goodarzi 2006).
Moreover, beginning in 1989, an increasing number of diesel passenger cars in
Europe were equipped with precious metal-based oxidation catalysts (Van den
Tillaard et al. 1996); Pt was and is usually the first choice for these, although recent
developments in catalyst research indicate that a partial substitution of Pt by Pd is
possible (Heck and Farrauto 2001; Moldovan 2007). These catalysts oxidize gase-
ous pollutants, such as CO and HCs; in addition, such catalysts oxidize the HC
component adsorbed on particulates, thereby reducing the emission of particulates
from auto exhaust (Van den Tillaard et al. 1996). The three-way catalytic-converter
technology is not applicable to diesel engines because conversions of NO x to N 2 and
CO and of HCs to CO 2 and H 2 O will not take place when an excessive amount of
air exists. In contrast, oxidation catalysts enable the oxidation of CO and HCs to
CO 2 and H 2 O, in the presence of excessive oxygen. CO and nonmethane HC con-
version levels of 98-99% are achievable, and methane conversion may reach levels
of 60-70% (Energy Nexus Group 2002).
When exhaust fumes are released from an engine, chemical and physical reac-
tions occur on the surface of the catalyst carrier as a result of rapidly changing
redox conditions, high temperature, and mechanical friction (Bencs et al. 2003).
These reactions result in PGE emissions (bound to carrier material) into the envi-
ronment at ng km −1 rates (Moldovan et al. 1999; Ek et al. 2004). Pt, emitted from
catalytic converters, exists as surface-oxidized metal nanoparticles, and is bound
to larger Al 2 O 3 particles (Rühle et al. 1997). Based on the distribution size of
particles freed from a catalytic converter, it was shown that large particles (>10
µm) dominate, reaching 62-67% of the total number of emitted particles,
medium-size particles (3.1-10 µm) constitute 21%, and small ones (<3.1 µm)
13% (Artelt et al. 1999b; Ravindra et al. 2004). Particles with a diameter of >10
µm have the highest Pt content (Alt et al. 1997; Moldovan et al. 2002).