Page 578 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 31 DEVELOPMENT OF EXHAUST CATALYST
experimentally proving the applicability of this method
to the slurry spray formation.
The outline of the newly developed nozzle-free inkjet
technology has been introduced. This technology has a
potential to extend the areas of printing and formation
to the use of materials such as suspensions, which has
been difficult by the conventional inkjet technology.
The extension of this nozzle-free inkjet technology to
new applications is looked forward to in the near future.

References
[1] S.A. Elrod, B. Hadimioglu, B.T. Khuri-Yakub, E. Richley,
C.F. Quate, N.N. Mansour and T.S. Lundgren: J. Appl.
Phys., 65, 3441–3447 (1989).
[2] Technical information: The Micromeritics, published
Figure 30.7 by Hosokawa Powder Technology Research Institute,
Forming state of slurry droplets (7 vol.% PS slurry). 48, 91 (2004).

APPLICATION 31
31 DEVELOPMENT OF EXHAUST CATALYST

Three-way catalysts that are developed by using new On the other hand, the three-way catalyst must
technology for nanolevel powders are explained in clean up HC and CO by oxidation and must clean up
this section. NO by reduction at the same time. Simultaneous
x
cleaning by the reduction and oxidation can be
1. Supported metal catalyst achieved in an operating window of A/F around the
stoichiometric composition of reductive components
The temperature of an automobile exhaust reaches as and oxidative composition in the three-way catalyst
high as 1,000 C under a high-load driving condition. (Fig. 31.1). For keeping up the stoichiometric com-
Therefore, exhaust catalyst must endure such a high position of an exhaust gas, the three-way catalyst is
temperature and must keep up its catalytic activity used in the three-way catalyst system, which is com-
for cleaning up automobile exhaust throughout a posed of not only a catalyst itself but also electroni-
car’s life. For the purpose of keeping up the high cat- cally controlled fuel injector, air flow meter, oxygen
alytic activity of a fresh catalyst, the particles of plat- sensors and computer to control their working. An
inum group element (mainly Pt, Rh and Pd), which oxygen sensor set just upstream of a catalyst sends a
are loaded in the catalyst, must be kept at single- signal of oxygen concentration in an exhaust gas to
nanometer level. Because these noble metal particles the electronically controlled fuel injector for control-
play a role of active sites of the catalyst, the smaller ling A/F (air to fuel ratio) at the stoichiometric com-
size of the noble metal particle means that a larger position. However, there is a small delay between the
number of active sites exist in the catalyst. The noble signal of oxygen sensor and the response of control-
metals are scarce and expensive resources; hence, ling A/F; an exhaust gas composition fluctuates from
more effective utilization of them is strongly needed. fuel rich to fuel lean especially in the transient
For keeping up such a high dispersion state of the condition, acceleration or deceleration. The A/F
noble metals, they must be loaded on a catalyst deviance from the stoichiometric composition makes
support, which has a high specific surface area. Their the conversions of HC, CO and NO worse. For solv-
x
high dispersion state enables the components of ing this problem, catalyst itself has to have a function
exhaust gas, hydrocarbons (HC), carbon monoxide of buffer of oxygen to keep the stoichiometric
(CO) and nitrogen oxides (NO ), to collide more composition in the transient condition in which the
x
frequently and efficiently with active sites in a high- mechanical and electronic control cannot follow.
velocity stream of exhaust gas. Oxygen-storage materials in a three-way catalyst
550